Review on Thermal Properties with Influence Factors of Solid–Liquid Organic Phase-Change Micro/Nanocapsules

Abstract

Solid–liquid organic phase-change micro/nanocapsules are potential candidates for energy storage. Recently, significant progress has been made regarding phase-change micro/nanocapsules in terms of their synthesis, properties, and applications. Extensive research has been conducted to enhance their thermal properties, such as thermal storage capacity, thermal conductivity, and thermal reliability. However, factors that influence the thermal properties of micro/nanocapsules have received little attention. This study presents a comprehensive review of phase-change micro/nanocapsules focusing on their thermal properties and their influencing factors. In addition, the thermal properties of the major solid–liquid organic pure phase-change materials are summarized. Furthermore, common micro/nanoencapsulation methods and their influence on the thermal properties were analyzed. Finally, the potential applications of these phase-change micro/nanocapsules were also investigated. This study was devoted to enhancing the thermal properties of micro/nanocapsules, which play a crucial role in their practical applications.

1. Introduction

There is rising interest in developing and implementing practical processes for storing or transporting thermal energy that may contribute to harnessing renewable energy sources such as solar energy, making better use of non-renewable energy sources, and regulating temperature in certain applications [1,2]. This energy can be stored as sensible or latent heat in materials for thermal energy storage (TES) systems [3,4]; in particular, latent heat is more efficient for TES because the material can maintain a constant temperature during energy storage/release [5,6,7].

Phase-change materials (PCMs) are the primary carriers for thermal energy storage in a latent heat TES system because PCMs can store a considerable amount of heat under nearly isothermal conditions. Phase transition, such as solid-to-liquid, enables materials to store thermal energy without significant changes in volume and/or pressure, making solid–liquid PCMs suitable for various applications [3,4,6,8]. Thus, solid–liquid PCMs are the most prevalent thermal energy storage materials. Solid–liquid PCMs typically include organic, inorganic, and eutectic substances [9,10,11,12,13]. Paraffins, fatty acids, and alcohols are among the frequently used solid–liquid organic PCMs. Solid–liquid inorganic PCMs are classified into metals, salts, and hydrates. Eutectics are a combination of two or more chemical compounds, such as capric–palmitic acid. Among these, solid–liquid organic PCMs are the most commonly studied for low-temperature (below 100 °C) applications, as they have the potential for numerous applications [14,15,16,17] such as solar energy to make better use of residual thermal energy or renewable energy sources, food industry and textile production to regulate temperature changes [18,19,20,21,22,23], buildings with refrigeration and heating systems to ensure energy-saving, and thermal management of electronic devices [24,25]. In practical applications, phase-change capsules formed by encapsulation technology outperform those formed directly from PCM because encapsulation can prevent morphological changes during phase transitions, increase the specific surface area with small particles, prevent the loss of liquid PCM, limit the exposure of the core PCM to the environment, and increase thermal conductivity by employing a thermally conductive shell. According to their size, encapsulated PCMs can be classified into macroencapsulated PCMs (>1000 μm), microencapsulated PCMs (1–1000 μm, MEPCMs), and nanoencapsulated PCMs (<1 μm, NEPCMs) [26,27]. There is significant interest in the synthesis of micro/nanocapsules with larger specific surface areas to facilitate heat transfer. These micro/nanocapsules are favored for use in working fluids that can not only store heat but also achieve rapid heat transfer.

Thermal properties, such as thermal storage capability, thermal conductivity, and thermal reliability, are the most important characteristics of phase-change micro/nanocapsules [28]. The thermal storage capability is generally evaluated by the heat stored per unit mass of micro/nanocapsules, also known as latent heat (ΔH). A high ΔH indicates a high thermal storage density, which is desirable for practical applications. Thermal conductivity represents the heat transfer capacity of the micro/nanocapsules and is an important performance indicator for determining whether the micro/nanocapsules can rapidly release or absorb heat. Therefore, thermally conductive micro/nanocapsules are ideal for most applications. The thermal reliability of micro/nanocapsules is a practical performance parameter that represents their service life. Generally, good thermal reliability is expected if there is little loss of ΔH after extensive thermal cycling. Recent reviews have focused on the thermal properties of micro/nanocapsules. For instance, Leong et al. [29] discussed the phase-change temperatures, latent heat, thermal conductivity, and phase-change duration of nanoparticles. Regarding the thermal storage capacity, they indicated the requirement for additional investigation into the primary elements influencing this property. For thermal conductivity, they summarized the factors influencing the thermal conductivity of nano-enhanced PCM. In addition, various researchers [30,31,32,33] have reviewed methods for improving PCM thermal conductivities. Lin et al. [30] compared these methods and concluded that adding thermally conductive fillers was the most effective way to improve the thermal conductivity of PCMs. In the duration section, references were briefly presented to illustrate the changes in the phase-change temperature, thermal conductivity, and thermal storage capacity prior to and following the thermal cycling test. Felipe et al. [34] examined the influence of micro/nanocapsule composition on their thermal properties. They demonstrated that employing appropriate synthetic methods and strategies to control the composition (such as altering the core or shell composition) enabled the preparation of MEPCMs and NEPCMs with better performance.

Notably, based on the aforementioned reviews, these studies rarely involved systematic summaries and analyses of the three key thermal properties of micro/nanocapsules. For example, numerous reviews on thermal conductivity are primarily concerned with a summary of enhancement methods; nonetheless, the factors affecting thermal conductivity must also be analyzed. Various studies have been devoted to developing micro/nanocapsules with good thermal storage capacity; however, the thermal storage capacity is affected by the core latent heat, shell density, shell thickness, particle size, etc. Thus, it is necessary to systematically analyze the factors influencing the thermal storage capacity of micro/nanocapsules. In addition, thermal reliability is an important performance parameter accompanying the preparation of micro/nanocapsules; however, there have been few reviews on the thermal reliability of micro/nanocapsules to date, preventing the evaluation of performance integrity in the future development of capsules. Accordingly, this study aims to gain insight into the thermal properties and factors influencing the utilization of micro/nanocapsules for diverse applications.

The workflow distribution of this study consists of five major sections in addition to the conclusion. In Section 2, the properties of solid–liquid organic PCM and their challenges are introduced; in particular, paraffin and fatty acid-based PCMs are summarized considering their nature and thermal properties. In Section 3, micro/nanoencapsulation of solid–liquid organic PCMs is elaborated with fabrication methods and thermal properties. The size and encapsulation analysis of micro/nanocapsules with different methods, including the preparation trend over the past years, help to understand the development of the ideal thermal performance evaluation with current studies. Section 4 summarizes the research on the thermal properties of solid–liquid organic PCM micro/nanocapsules, including thermal storage capacity, thermal conductivity, and thermal reliability. Section 5 presents the factors that influence these thermal properties. Section 6 introduces the possible applications of solid–liquid organic phase-change micro/nanocapsules.

2. Solid–Liquid Organic PCMs as Core Materials of Micro/Nanocapsules

The selection of the core PCM for micro/nanocapsules is the key to their thermal storage performance. Solid–liquid organic PCMs are suitable for various applications with temperatures below 100 °C [31]. Owing to their excellent properties, such as consistent melting, narrow phase transition temperature range, little or no supercooling, self-nucleation, high thermal energy density, and lack of phase segregation, they are preferred for energy-efficient buildings, solar energy storage, working fluids, thermal management of electronic devices, batteries, etc. [35].

2.1. Paraffin-Based PCMs

As a mixture of n-alkanes with straight chains, paraffin waxes have the general formula CH₃–(CH₂)_n–CH₃, which can be classified based on the number of carbon atoms and their physical state. Specifically, n-alkanes with no more than four carbon atoms include pure alkanes in the gas phase, carbon atoms between 17 and 45 are liquid paraffins, and carbon atoms greater than 45 are solid waxes at room temperature.

Paraffin-based PCMs refer to solid paraffin waxes, which are mixtures of saturated hydrocarbons including linear, isomeric, hyperbranched, and naphthenic hydrocarbons [36]. Commercial paraffin waxes contain mixtures of isomers whose melting temperatures vary and increase with the chain length. Paraffin waxes commonly exhibit a high thermal storage capacity, although this value is not proportional to increasing chain length (

Table 1. Physical properties of some paraffin waxes [39].

Paraffins	Tm (°C)	ΔHm (J/g)	λ (W/m·K)
n-Tetradecane (C14)	6	228–230	0.14
n-Pentadecane (C15)	10	205	0.2
n-Hexadecane (C16)	18	237	0.2
n-Heptadecane (C17)	22	213	0.145
n-Octadecane (C18)	28	245	0.148
n-Nonadecane (C19)	32	222	0.22
n-Eicosane (C20)	37	246
n-Henicosane (C21)	40	200, 213
n-Docosane (C22)	44.5	249	0.2
n-Tricosane (C23)	47.5	232
n-Tetracosane (C24)	52	255
n-Pentacosane (C25)	54	238
n-Hexacosane (C26)	56.5	256
n-Heptacosane (C27)	59	236
n-Octacosane (C28)	64.5	253
n-Nonacosane (C29)	65	240
n-Triacontane (C30)	66	251
n-Hentriacontane (C31)	67	242
n-Dotriacontane (C32)	69	170
n-Triatriacontane (C33)	71	268	0.2
Paraffin C16-C18	20–22	152
Paraffin C13-C24	22–24	189	0.21
RT 35 HC	35	240	0.2
Paraffin C16-C28	42–44	189
Paraffin C20-C33	48–50	189
Paraffin C22-C45	58–60	189	0.2
Paraffin C21-C50	66–68	189
RT 70 HC	69–71	260	0.2
Paraffin natural wax 811	82–86	85	0.72

). Generally, paraffin waxes with a phase-change temperature within 100 °C are inexpensive, safe, non-irritating, and reliable. Considering the chemical point and economics, most industrial-grade waxes are inactive and stable and can be used in latent heat TES systems for lengthy crystallization-melting cycles owing to their high stability [37]. Table 1 lists certain thermal properties of paraffins as low-to-medium temperature (within 100 °C) thermal storage materials. In addition to its favorable properties, paraffin also exhibits some undesirable properties, such as low thermal conductivity and moderately high flammability [38], which can be partially eliminated using additives or paraffin compounds.

2.2. Fatty Acid

Fatty acids are carboxylic acids composed of a long hydrocarbon chain (possibly saturated or unsaturated) and a terminal carboxyl group. The majority of naturally occurring fatty acids have an even number of carbon atoms and lack branched chains (CH₃(CH₂)_2nCOOH, n (4–28)). Saturated fatty acids lack a C=C double bond [40]. They share the same formula of CH₃(CH₂)_nCOOH, with only a difference in “n”. The most commonly employed saturated fatty acids (SFAs) are lauric acid (LA), palmitic acid (PA), capric acid (CA), stearic acid (SA), and myristic acid (MA). Different fatty acids or fatty acids with other solid–liquid organic PCMs can favorably form eutectic mixtures or synthetic esters, allowing the adjustment of the phase transition temperature.

Fatty acids have gained considerable interest as potential TES candidates among solid–liquid organic PCMs. Around 1989, D. Feldman evaluated the thermal properties of CA, LA, MA, PA, SA, and their binary blends [41]. These fatty acids exhibited a melting temperature range of 30–65 °C and a melting latent heat of 153–182 J/g. Feldman et al. [42] extensively studied the applications and thermal stability of fatty acids. The thermal properties of SA, PA [43], and MA [44] have also been analyzed. Cedeno et al. [45] obtained descriptive data regarding the melting temperature and heat of fusion of PA, SA, oleic acid, and their binary and ternary blends. Gbabode [46] characterized the thermal properties of the crystal components of odd-numbered fatty acids, from tridecanoic acid to tricosanoic acid.

Currently, fatty acids and their derivatives are recognized as attractive PCMs that could serve as energy-retaining compounds in solar energy heat utilization systems and buildings owing to their relatively efficient thermal, physical, and chemical properties and their easy incorporation into composite materials [47]. The properties of fatty acid-based PCMs are summarized in

Table 2. Thermal properties of some fatty acids [5,48,49].

Fatty Acid	Tm (°C)	ΔHm (J/g)	λ (W/m·K)
Enanthic	−7.4	107
Butyric	−5.6	126
Caproic	−3	131
Propyl palmiate	10	186
Pelargonic	12.3	127
Isopropyl stearate	14–18	140–142
Phenylacetic	16.7	102
Caprylic	16	149	0.149
Butyl stearate	19	140
Dimethyl sabacate	21	120~135
Undecylenic	24.6	141
Vinyl stearate	27~29	122
Undecylic	28.4	139
Capric	31.5	153	0.153
Tridecylic	41.8	157
Methyl-12 hydroxy-stearate	42~43	120~126
Lauric	45	193	0.150
Elaidic	47	218
Myristic	56	212	0.150
Pentadecanoic	52~53	178
Margaric	60	172
Palmitic acid	62	218	0.162
Stearic acid	71	237	0.172
Nonadecylic	67	192
Arachidic	74	227
Heneicosylic	73~74	193
Acetamide	81	241

.

2.3. Other Organic Solid–Liquid PCMs

In addition to paraffins and fatty acids, other types of solid–liquid organic PCMs also exist, including sugar alcohols, esters, and glycols. Sugar alcohols, also called polyalcohols, are regarded as medium temperature PCMs (90–200 °C) and have not received much attention. Hormansdorfer first mentioned that polyalcohols can be used as PCMs [5]. Subsequently, Talja and Roos [50] and Kaizawa et al. [51] studied their phase-change behavior and concluded that certain polyalcohols exhibit a latent heat nearly twice that of other organic PCMs. However, they exhibit a large degree of supercooling, which hinders the TES efficiency. For instance, A. Sari [52,53] indicated that erythritol exhibited a melting/solidifying temperature of 118.4/36.22 °C with a corresponding latent heat of 379.57/255.95 J/g. Evidently, the latent heat value for solidification is approximately 32.6% lower than that of melting, which indicates that only 67.4% of the stored thermal energy could be used. This mismatch leads the TES to deviate from the original path, which is unacceptable for practical applications.

Esters, which are derived from acids by substituting one hydroxyl group (–OH) with an alkyl (–O) group, exhibit a narrow temperature range during the solid–liquid transformation, and eutectics can be formed with minimal or no supercooling. In addition, Nikolic’ et al. [54] used a differential scanning calorimeter (DSC) to examine the solid–liquid phase-change process of five fatty acid esters. The samples included methyl palmitate, methyl stearate, hexadecyl palmitate, hexadecyl stearate, and their eutectic mixtures, and 50 thermal cycles were conducted between temperatures of −10 and 60 °C. The tests were repeated after storing these samples for 18 months, and the results demonstrated no changes in their thermophysical properties.

Polyethylene glycol (PEG, POE, and PEO), which consists of dimethyl ether chains (HO–CH₂–(CH₂–O–CH₂–)_n–CH₂–OH), has been extensively and experimentally investigated and found to exhibit chemical and thermal stability, nonflammability, non-corrosiveness, nontoxicity, and low price [55,56,57,58]. Notably, its phase-transition temperature can be adjusted by adjusting the molecular weight. Sarier and Onder [59] demonstrated that the melting temperature and latent heat increase with an increase in the molecular weight of PEG. For example, for PEG 400, PEG 2000, and PEG 20000, the melting temperatures were 3.2, 51, and 68.7 °C, respectively, with latent heat values of 91.4, 181.4, and 187.8 J/g, respectively. However, PEG with molecular weights exceeding 10,000 would exhibit decreased latent heat because the PEG crystallinity decreases with long molecular chains. The thermal properties of sugar alcohols, esters, and glycol PCMs are listed in

Table 3. Thermal properties of sugar alcohol, ester, and glycols.

	Tm (°C)	ΔHm (J/g)	λ (W/m·K)	Reference
Erythritol	118.4	379.6		[52,53]
Butyl stearate	23.7	121	0.230	[60]
Isopropyl stearate	22.1	113.1	0.150
Glycerol tristearate	63.5	149.4	0.170
Erythritol tetrapalmitate	21.9	201.1	0.250	[44]
Erythritol tetrastearate	30.4	208.8	0.260
PEG400	3.2	91.4		[59]
PEG1000	34.8	154.4	0.280	[61]
PEG1500	47.2	161.4	0.310
PEG2000	50.8	165.4	0.310
PEG4000	56.0	173.6	0.330
PEG6000	59.5	179.7	0.340
PEG8000	59.7	177.5	0.330
PEG10000	58.0	182.9	0.330
PEG12000	60.9	173.4	0.320
PEG20000	62.3	168.5	0.320

.

2.4. Challenges of Solid–Liquid Organic PCMs

Over the past years, more and more studies have focused on paraffins, PEG, and fatty acids, owing to their adjustable phase transition temperature and high economic benefits (Figure 1). Among paraffins, n-octadecane, n-eicosane, n-hexaoxane, and n-octadecane are popular medium- and low-temperature PCMs for TES. Meanwhile, SA, PA, and LA are the most commonly studied fatty acid PCMs.

However, the properties of these PCMs may be unsatisfactory. Paraffin has low thermal conductivity and flammability. Its volume fluctuates drastically during the phase change. Fatty acids are unstable at high temperatures and have low thermal conductivities [62]. In addition, the leakage problem of the liquid state restricts the application of these pure PCMs [31].

3. Micro/Nanoencapsulation of Solid–Liquid Organic PCMs

It is widely considered that encapsulating PCMs is an effective means for preventing problems associated with solid–liquid phase transitions. In addition to preventing liquid leakage when combining a PCM with other materials, encapsulation is generally beneficial for protecting the PCM by forming capsules that take the PCM as the core, thereby ensuring that the encapsulated PCM has enhanced thermal reliability, controlled thermal energy release, reduced corrosion, increased heat transfer efficiency, heat transfer area, and protection against degradation during endothermic/exothermic cycling. Therefore, encapsulation is an effective method for promoting the application of solid–liquid PCMs as it employs solid material as a shell that envelops the PCM core so that the phase transition is restricted within the shell, thereby maintaining the solid state of the particles [63].

3.1. Encapsulation Methods of Solid–Liquid Organic PCMs

Microencapsulation produces capsules with a size below 1000 μm, which are called microcapsules, and capsules with a size below 1000 nm are regarded as nanocapsules. To date, the most commonly used microencapsulation methods include in situ polymerization, interfacial polymerization, suspension polymerization, emulsion polymerization, miniemulsion polymerization, and sol–gel [64]. Based on these methods, the capsule sizes have evolved from microcapsules to nanocapsules, and the organic shell materials have been replaced by inorganic ones.

3.1.1. In Situ Polymerization

In situ polymerization is a PCM encapsulation method. This process begins by mixing two immiscible liquids, such as water and an organic solvent that contains complementary and direct-acting organic intermediates, which then mutually react to form solid pre-condensates [65]. Melamine–formaldehyde (MF) and urea–formaldehyde (UF) are two widely used shell materials because of their chemical stability and high mechanical strength. Figure 2 depicts a schematic of the in-situ polymerization process [66]. Using the situ polymerization process, Rao et al. [67] prepared an n-docosane/melamine resin MEPCM with particle sizes of 5–50 μm. At a stirring rate of 6000 rpm, the prepared MEPCM with a particle size of 10 μm exhibited an encapsulation ratio of 60% and a melting latent heat of 150 J/g.

Nanocapsules can also be synthesized using an in situ polymerization method. Du et al. [68] fabricated NEPCMs with n-octadecane as the core and phosphorus–nitrogen diamine (PNDA)-modified MF as the shell via in situ polymerization. During this process, the PNDA-modified MF prepolymer was carefully dropped into the previously prepared n-octadecane emulsion with continuous stirring. After filtering, the prepared sample was washed using a 50 wt.% aqueous ethanol solution to remove the impurities, and unencapsulated n-octadecane flame-retardant NEPCMs were obtained after drying in a vacuum environment for 24 h. The diameters of the prepared NEPCMs ranged from 80 to 140 nm, and their latent heats of fusion ranged from 110.8 to 141.3 J/g.

In situ polymerization is widely used to prepare micro-/nanoencapsulated PCMs. As reported [69], the in situ method can produce the best quality microcapsules in terms of the diffusion tightness of their shells and a size range of 5–100 μm. This method is beneficial owing to its cost-effectiveness, ease of automation, and compatibility with the environment. However, residual formaldehyde may be unavoidable after shell formation, leading to environmental and health problems during polymerization [70].

3.1.2. Interfacial Polymerization

The interfacial polymerization method involves dispersing the organic phase (including multifunctional monomers and/or oligomers) together with the core material, which is encapsulated in an aqueous phase [71]. The interfacial polymerization method can also be employed to prepare microcapsules and nanocapsules. Figure 3 illustrates a schematic of micro/nanocapsules formed by interfacial polymerization [72]. Zou et al. [73] fabricated hexadecane/polyurea microcapsules via the interfacial polymerization method; the microcapsules with a size of 2.5 μm possessed a latent heat of 66 J/g at a melting point of 18 °C. Shi et al. [72] devised a one-step interfacial polymerization method to prepare paraffin/polymethyl methacrylate (PMMA) nanocapsules. The results indicated that the nanocapsules with particle sizes ranging from 200 to 400 nm had a spherical shape with a homogeneous, smooth, and compact surface.

Interfacial polymerization is commonly employed to synthesize microcapsules. Commercially produced microcapsules exhibit sizes of 20–30 μm and 3–6 μm, where the former are employed in pesticidal and herbicidal applications and the latter for carbonless paper ink [69]. This method is advantageous as it delivers a high encapsulation ratio and can protect active substances. However, the encapsulated material must be acid- and alkali-resistant to react with the monomer, and it also exhibits the problem of dealing with excess monomers [70].

3.1.3. Suspension Polymerization

The suspension polymerization method involves suspending the water-immiscible reaction mixture as droplets in an aqueous continuous phase [74]. These droplets were formed on vigorously stirring the mixture, and the stabilizer was dissolved in the aqueous phase. Subsequently, polymerization was initiated at the desired temperature until completion. Figure 4 depicts a schematic of the formation process of the suspension polymerization method. The suspension polymerization method can be employed for both microcapsules and nanocapsules. You et al. [75] fabricated n-octadecane/styrene (St)–divinylbenzene (DVB) copolymer microcapsules via suspension polymerization. The microcapsules exhibited an average diameter of 80 μm and a melting fusion of 126 J/g. Zhang et al. [76] used the suspension polymerization method to fabricate n-octadecane/poly(2,3,4,5,6-pentafluorostyrene) nanocapsules, and results demonstrated that nanocapsules with an average size of 490 ± 16 nm exhibited good morphology, high latent enthalpy (171.8 ± 2.2 J/g), and excellent thermal stability.

This method is advantageous because it has a high encapsulation efficiency, the prepared capsules exhibit a smaller particle size, and the process is smooth. However, auto-acceleration is unavoidable in this method, and dispersants are required during polymerization [70].

3.1.4. Emulsion Polymerization

In emulsion polymerization, the initiator should first be dissolved in the aqueous phase; monomers should be emulsified in the polymerization medium and distributed among the droplets, surfactant micelles, and aqueous phase by employing surfactants, as depicted in Figure 5. The commencement of the polymerization reaction lies in the aqueous phase because the initiator can only exist in this phase, which later extends to the micelles. The proportion of monomer molecules dissolved in the aqueous phase is crucial to the size of the obtained particles. In addition, the concentration of the emulsifier, initiator, and polymerization temperature also influence the size [78]. Emulsion polymerization is viable for both microcapsules and nanocapsules. Sari et al. [79] adapted an emulsion polymerization method to fabricate micro/nanocapsules with an MA–PA eutectic mixture as the hybrid core and PMMA as the shell. Microcapsules with particle sizes of 0.1–70 μm possess a latent heat of 68–100 J/g at the phase-change temperature. Using the same method, Baek et al. [80] prepared n-hexadecane nanocapsules coated with the poly(glycidyl methacrylate) macromolecule poly(methylmethacrylate-co-glycidyl methacrylate). The size of the prepared nanocapsules was 260 nm, with the melting/solidifying temperatures and latent heats being 17.23/14.85 °C and 148.05/147.63 J/g, respectively; thus, the encapsulation ratio was 62.46%.

This method proves beneficial as it possesses a fast polymerization rate, a high molecular weight, and allows direct use of an emulsion. However, the polymer separation process is complex, and the product involves large quantities of residual impurities [70].

3.1.5. Miniemulsion Polymerization

In this technique, polymerization occurs within the tiny droplets, which are dispersed while maintaining their size at the nanoscale under the action of large shear forces, usually involving emulsifiers, monomers, water, and initiators [81]. A schematic of the mini-emulsion polymerization process is shown in Figure 6 [77]. Miniemulsion polymerization is a common technique employed to produce nanocapsules. Rezvanpour et al. [82] used the mini-emulsion polymerization process to prepare nanocapsules where n-eicosane served as the PCM core and PMMA served as the shell. The mean diameter of the n-eicosane/PMMA nanocapsules prepared was 135 nm, and under a weight ratio of 50/50, the melting/solidifying temperatures were 34.66/32.92 °C, and the corresponding latent heats were 124.7/119.13 J/g, respectively.

Presently, the common shell materials for nanocapsules prepared using this method are styrene (St)-methylmethacrylate copolymer, polyurea, polystyrene (PS), PMMA, and styrene-butyl acrylate (SBA) [81]. This method is advantageous because the reaction temperature is stable and does not involve volatile solvents. However, it requires a high-energy mixing process [70,83].

3.1.6. Sol-Gel

The sol-gel method [70] is a wet chemical process for producing inorganic oxide materials. Highly chemically active precursors are used to obtain a stable and transparent solution via hydrolysis and condensation, and a shell can be obtained following the gel process. A schematic illustration of the formation process via the sol-gel method is depicted in Figure 7 [84]. The sol-gel method can be employed to prepare both microcapsules and nanocapsules. Wang et al. [85] studied the sol-gel method to produce a silica shell by combining the oil/water (O/W) emulsion technique for the first time, and the resulting spherical microcapsules exhibited a size range of 4–8 μm. Fang et al. [86] and Zhang et al. [87] also prepared the microcapsules using paraffin as the core and SiO₂ as the shell by employing the sol-gel method and obtained an encapsulation ratio exceeding 85%. Later, Latibari et al. [88] fabricated nanocapsules using the sol-gel method with PA as the core and SiO₂ as the shell. By adjusting the solvent pH value from 11 to 12, the size was enhanced from 183.7 to 722.5 nm, and the encapsulation ratio was enhanced from 83.25 to 89.55%. Subsequently, they synthesized SA/TiO₂ (titania) nanocapsules using the same method [89], and the prepared nanocapsules exhibited sizes of 583.4–946.4 nm and presented an encapsulation ratio of 30.36–64.76%.

The sol-gel process is a method for preparing micro/nanocapsules with inorganic shells such as SiO₂, TiO₂, and ZrO₂ [90]. This method is advantageous because it possesses a high reaction efficiency and the mixing process is uniform. However, the reaction is time-consuming, and the raw material is relatively expensive [70].

3.2. Analysis of the Encapsulation Methods of Solid–Liquid Organic PCMs

The synthesis method is pivotal for obtaining micro/nanocapsules with good thermal properties because it determines the shell material, size, and properties of the capsules. The methods of interfacial, in situ, emulsion, suspension, and miniemulsion polymerizations are employed to prepare micro/nanocapsules with organic shells (such as PMMA), whereas the sol–gel method is employed to prepare micro/nanocapsules with inorganic shells (like SiO₂, TiO₂, etc.).

The major disadvantage of organic shells is their extremely low thermal conductivity, which retards heat exchange with the environment during heat absorption/release cycles, supercooling, and overheating [6]. Compared to organic shells, inorganic shell materials exhibit a relatively high thermal conductivity, which renders them favorable for micro/nanocapsule applications requiring fast heat transfer. Although the inorganic shells could improve the thermal conductivity, their enhancing effect was insignificant owing to their moderate thermal conductivity. Laghari et al. [91] used TiO₂ as the shell material to obtain a paraffin-based nano-enhanced PCM with improved thermal conductivity, and the results indicated that using TiO₂ (4.8–11.8 W/(m·K)) improved the thermal conductivity of the paraffin-based nano-enhanced PCM from 0.2 W/(m·K) to 0.3690 W/(m·K). However, a minor improvement in thermal conductivity does not significantly enhance their application utility. Therefore, further research is necessary to enhance the thermal conductivity.

In addition to the shell material, the size and encapsulation of the micro/nanocapsules produced using different methods over the previous decades were analyzed (as shown in Figure 8a). The methods of interfacial polymerization, in situ polymerization, emulsion polymerization, suspension polymerization, and sol-gel involve phase-change capsules from nano to micro size, whereas the miniemulsion polymerization method is the only method for preparing nanocapsules. In situ polymerization is predominantly used to prepare microcapsules, and the sol-gel method focuses on preparing nanocapsules between 100 and 1000 nm. The latent heat and encapsulation ratio (Figure 8b and Figure 8c, respectively) of micro/nanocapsules prepared using different methods can all achieve high values because the performance can be adjusted by varying the preparation conditions. Recently, other synthesis methods have gradually occupied a larger proportion of micro-nanocapsules, and related research has dropped drastically in the previous year (Figure 8d and Figure 8e, respectively), indicating that researchers are working on expanding the research of shell materials to further strengthen the performance of micro/nanocapsules, such as using highly thermally conductive metal shell materials to enhance the thermal conductivity of micro/nanocapsules [92].

4. Thermal Properties of Solid–Liquid Organic Phase-Change Micro/Nanocapsules

In applications involving encapsulation, the important aspects of the performance of encapsulated phase-change materials (EPCMs) include thermal storage properties, thermal conductivity (λ), and thermal reliability. Generally, thermal storage properties are evaluated using the ΔH. The following equations are commonly used to analyze the thermal storage properties:

$$R=\frac{{∆H}_{m,EPCMs}}{{∆H}_{m,PCMs}}\times 100\%$$

(1)

$$E=\frac{{∆H}_{m, EPCMs}+{∆H}_{c,EPCMs}}{{∆H}_{m,PCMs}+{∆H}_{c,PCMs}}\times 100\%$$

(2)

ΔH_m,PCMs, ΔH_c,PCMs, ΔH_{m, EPCMs}, and ΔH_{c, EPCMs} indicate the melting latent heats of pure PCMs, solidifying latent heats of pure PCMs, melting latent heats of EPCMs, and solidifying latent heats of EPCMs, respectively. R represents the encapsulation ratio of the core materials for the EPCMs, and E reflects the encapsulation efficiency of the PCM. R and E were both positively associated with the core/shell mass ratio. R is typically employed for evaluating EPCM encapsulation owing to the fact that the melting and solidifying latent heats often exhibit minor differences for the solid–liquid EPCMs. The thermal storage properties of the EPCMs were analyzed using a DSC device. Thermal conductivity is an important thermal performance parameter for EPCM applications, particularly for applications in a working fluid, as it affects the heat transfer of the TES system. Typically, EPCM thermal conductivity can be tested using a thermal conductivity analyzer with a laser flash or steady-state plate method. Thermal reliability is also crucial for practical applications, as it could evaluate the EPCM performance after long-term service. Typically, thermal reliability is tested via thermal cycling by heating/cooling the as-prepared samples for a large number of cycles.

4.1. Thermal Storage Properties Overview of Various Micro/Nanocapsules

Thermal storage properties are key performance metrics for micro/nanocapsules applications. A large thermal storage capacity and encapsulation ratio are the expected objectives of EPCM synthesis, because EPCMs with a high thermal storage capacity can absorb and release a large amount of heat in a smaller mass, which is highly efficient for applications. Early research was primarily devoted to the preparation of phase-change microcapsules and their thermal storage properties. Sari et al. [93] prepared the microencapsulated CA, LA, and MA with polystyrene (PS) by employing the emulsion polymerization method, and they obtained microcapsules with mean diameters of 7.7–42.0 μm and latent heats of 87–98 J/g for melting and 84–96 J/g for solidifying, corresponding to the R range of 43.56–48.7%. Moreover, the melting and solidifying phase-change temperature ranges are 22–48 °C and 19–49 °C, respectively. Later, they [94] fabricated n-nonadecane/PMMA microcapsules using the same method, and microcapsules were produced with a mean diameter of 8.18 μm and melted at 31.23 °C, with the latent heat increasing to 139.20 J/g and the R increasing to 60.3%. Further, they [95] synthesized a series of micro/nanocapsules (0.01–115 μm) with a eutectic mixture of tetracosane (C24)-octadecane (C18) as the core and PS as the shell using an emulsion polymerization process. With a shell/core ratio of 2:1, 1:1, and 1:2, the melting latent heats of the PS/(C24-C18) micro/nanocapsules were observed as 71.73, 116.32, and 156.39 J/g, respectively, which correspond to the encapsulation ratio of 29.6, 48.0, and 64.4%, respectively. Figure 9 depicts the morphology and DSC results for the fatty acid/PS, n-nonadecane/PMMA, and C₂₄-C₁₈ eutectic mixture/PS microcapsules.

Zhang et al. [96] employed a suspension-like polymerization method to obtain paraffin wax/PMMA microcapsules and micro/nanocapsules, both of which have a high core content. By increasing the core/shell ratio, the paraffin contents in microcapsules (94 μm) could approach 89.5% with a corresponding latent heat of 137.2 J/g, and for the prepared micro/nanocapsules (0.1–19 μm), the paraffin contents approached 80.2% with a latent heat of 123.0 J/g.

Using an in-situ polymerization method, Yu et al. [97] employed an M/F resin shell to synthesize n-dodecanol microcapsules. The obtained microcapsules (30.6 μm in diameter) melt at 21.5 °C, and the latent heat and R of the microcapsules were as high as 187.5 J/g and 93.1%, respectively, when the emulsifier/n-dodecanol ratio was 4.8%.

Rao et al. [67] prepared the n-docosane (C₂₂H₄₆)/melamine resin microcapsules via the in-situ polymerization method. The prepared microcapsules (approximately 10 μm) exhibited an R of 60% and demonstrated a high melting latent heat of 150 J/g.

Fang et al. [98] fabricated n-tetradecane/UF resin nanocapsules (average particle size of 100 nm) via in situ polymerization. By increasing the C₆H₆O₂ (resorcin, modifier) concentration from 0.25 to 5%, the R increased from 30.4 to 61.8%, and the melting and crystallization latent heat increased from 66.01 and 66.39 J/g to 134.16 and 134.5 J/g, respectively.

Existing studies show that encapsulation ratio and phase-change enthalpy are commonly used parameters to characterize thermal energy storage performance. Besides, the volumetric energy storage density, ΔH_v, may also be used to characterize thermal storage capacities as it also considers the density of phase-change micro/nanocapsules, which is an important property for the micro/nanocapsules.

Therefore, we derived the ΔH_v based on the enthalpy ΔH_m, encapsulation ratio R, and density $\mathit{\rho }$ of EPCMs.

$${\Delta H}_V=\mathit{\rho }{∆H}_{m,PCMs}$$

(3)

$$\mathit{\rho }={\mathit{\rho }}_{shell}\left({1-R}_V\right)+{\mathit{\rho }}_{core}{R}_V$$

(4)

$$R_V=\frac{1}{1+\frac{{\mathit{\rho }}_{core}}{{\mathit{\rho }}_{shell}}(\frac{1-R}{R})}$$

(5)

where ΔH_v indicates the volumetric energy storage density, (J/g), $\mathit{\rho }$ indicates the density of phase-change micro/nanocapsules, R_v indicates the volumetric encapsulation ratio of the micro/nanocapsules, and ρ_shell and ρ_core indicate the density of the shell and core of the micro/nanocapsules, respectively.

With the research mentioned above, we calculated the ΔH_v of the corresponding micro/nanocapsules. The results are shown in

Table 4. Volumetric energy storage density of the phase-change micro/nanocapsules.

EPCMs	R (%)	ΔHm, PCMs (J/g)	$\mathit{\rho }$ (g/cm3)	ΔHv (J/cm3)	Reference
CA/PS	45.40	86.45	0.97	83.86	[93]
LA/PS	43.56	87.21	0.96	83.72
MA/PS	48.70	98.26	0.94	92.36
C19/PMMA	60.30	139.20	0.91	126.67	[94]
PS/(C24-C18)(2:1)	29.60	71.73	0.96	68.86	[95]
PS/(C24-C18)(1:1)	48.0	116.32	0.91	105.95
PS/(C24-C18)(1:2)	64.4	156.39	0.87	136.06
Paraffin wax/PMMA (94 μm)	89.5	137.20	0.92	126.22	[96]
Paraffin wax/PMMA (0.1–19 μm)	80.2	123.00	0.94	115.62
n-dodecanol/MF	93.1	187.50	0.85	159.38	[97]
C12/MF	60.0	150.00	0.87	130.50	[67]
C14/UF	61.8	134.16	0.88	118.06	[98]

. Due to the density, the calculated ΔH_v is relatively smaller than ΔH_{m, PCMs}. This is because of the small density of the core material and shell material. If the micro/nanocapsules show a large density, the value of ΔH_v should be increased accordingly, which means that the micro/nanocapsules present a good energy storage density per unit volume. Details are discussed in Section 5.1.4. Since most of the micro/nanocapsules studied show a relatively small density (less than 1 g/cm³), we still use phase-change enthalpies and R to characterize the thermal storage capacity.

To date, an increasing number of micro/nanocapsules exhibiting good thermal storage capacities have been developed.

Table 5. Research on the phase-change micro/nanocapsules with thermal storage properties.

Shell	Core	Melting		Solidifying		R (%)	D (nm)	Reference
		Tm (°C)	ΔHm (J/g)	Tc (°C)	ΔHc (J/g)
UF	C14	9.01	134.16	2.81	134.50	61.80	100.00	[98]
		8.49	131.09	2.19	129.83	60.30
		7.19	103.57	1.78	103.48	47.70
		5.57	66.01	2.40	66.39	30.40
PNDA (modified MF resin)	C18	32.60	126.10	18.90	124.30	87.00	109.60	[68]
		32.50	125.40	19.40	123.50	86.40	109.30
		32.00	126.50	19.70	123.70	86.90	108.80
		32.40	124.50	19.50	123.40	86.10	110.20
		32.10	125.90	19.70	124.10	86.80	111.70
P (MMA-co-AMA)	C18	24.70	129.00	23.80	155.00	58.11	692.80	[99]
		24.60	140.00	25.3	165.00	63.06
		25.20	140.00	25.7	156.00	63.06	614.00
		27.40	151.00	27.2	166.00	68.02
		26.50	141.00	26.6	161.00	63.51	577.50
MF resin	C18	34.70	195.00	32.10	182.00	87.00	840.00	[99]
		34.40	97.00	15.50	101.00	45.00
MF resin	C18	28.06	137.62	25.82	137.93	59.58	484.00	[100]
		28.14	145.26	25.32	147.01	62.88	636.00
		28.22	137.07	25.26	137.86	59.34	630.00
		27.88	117.12	25.51	115.51	50.70	684.00
		27.62	111.36	25.15	109.78	48.21	723.00
PNDA (modified MF resin)	Paraffin	28.00	83.46	NA	NA	62.00	50.00	[101]
MUF	Paraffin	27.50	49.40	30.10	46.00	22.80	413.30± 1.30	[102]
		27.40	69.20	29.90	67.50	31.60	340.40 ± 3.20
		27.10	80.30	30.20	75.10	36.70	370.40 ± 6.40
		27.30	62.00	29.90	56.90	28.30	368.00 ± 2.30
		28.30	24.90	29.70	24.50	14.90	455.70 ± 25.10
UF	Paraffin	64.30	74.20	NA	NA	48.12	256.00	[103]
PU and PS	C18	28.00	122.00	22.00	125.00	54.00	410.00	[104]
PU	C18	23.70	123.40	28.20	124.10	78.10	200.00	[105]
PU	C19	30.54	92.85	30.78	91.86	44.70	103.00	[106]
PMMA SiO2	Paraffin	26.80	69.90	19.80	71.00	57.40	120.00	[107]
		26.60	57.50	19.60	57.50	46.90
		27.10	53.50	19.50	53.50	43.60
		28.00	62.10	19.80	62.40	50.70
AlOOH	PA	12.70	19.00	NA	NA	10.19	200.00	[108]
		13.80	22.60	NA	NA	12.12
		16.00	27.80	NA	NA	14.91
PMMA	C22	41.00	54.60	40.60	48.70	28.00	160.00	[109]
P (MMA-co-GMA)	C16	17.23	148.05	14.85	147.63	62.46	260.00	[80]
PS	Hexadecanol	52.68	124.85	NA	NA	c48.72	120.00	[110]
PS-co-EA	C14	7.97	182.68	−0.18	184.94	79.26	50–200	[111]
	C15	11.60	121.83	5.20	127.89	69.16
	C16	20.38	196.09	12.78	201.70	87.09
	C17	24.04	140.51	17.23	149.60	81.59
PMMA	C16	17.34	145.61	14.85	128.19	61.42	220	[112]
PMMA	C20	35.20	84.20	34.90	87.50	35	700	[113]
PS-co-EA	Paraffin	42.39	49.03	37.41	49.05	32.12	165	[114]
	PA	62.66	97.93	56.22	97.21	47.79	265
PMMA	C19	31.23	139.20	31.03	142.39	60.30		[94]
PS	C24-C18	25.96	156.39	26.04	152.83	64.40		[95]
UF	C16	16.15	143.70	16.04	134.30	68.90	253.00	[115]
		16.26	121.00	16.32	112.50	58.06	259.00
		16.27	119.40	15.88	112.30	57.29	268.00
		16.36	114.60	16.07	98.99	54.99	285.00
PS	C18	27.00	44.00	23.00	43.00	20.47	31.00	[116]
SiO2	C18	27.50	108.60	5.60	99.80	53.80	335.00	[117]
PHEMA-SiO2		27.90	104.30	8.60	104.80	51.60	449.00
PS-SiO2		27.90	101.10	7.00	97.30	50.10	289.00
PMMA	C18	32.60	110.10	19.50	108.30	91.00	104.00	[118]
		33.50	111.20	18.30	110.00	92.20	101.00
		32.70	108.90	18.80	107.20	90.10	109.00
		33.20	110.50	19.00	108.60	91.30	112.00
		32.00	109.10	18.80	107.20	90.10	106.00
PEMA	C18	32.20	198.50	29.80	197.10	89.50	140	[119]
PMMA		31.90	208.7	30.2	205.9	94.7	119
St-MMA	C18	30.20	97.90	25.90	93.80	40.90	152.00 ± 12.00	[120]
		28.90	74.40	25.60	73.80	31.10	236.00 ± 13.00
		28.40	85.90	25.30	82.90	35.90	146.00 ± 51.00
		29.50	107.90	24.60	104.90	45.10	102.00 ± 11.00
		30.00	117.30	26.70	113.80	49.00	127.00 ± 30.00
PS	C18	20–25	124.40	NA	NA	53.55	100–123	[121]
PS	C14	4.04	98.71	−3.43	91.27	89.00	132.00	[122]
PS-SiO2	C14	2.13	83.38	0.39	79.37	42.56	120.00	[123]
PS		2.59	82.35	0.008	81.46	42.04	80.00
PMMA	C20	34.66	124.70	32.92	119.13	62.00	135.00	[82]
PS	C32	70.90	174.80	61.80	177.1	61.23	168.00	[124]
St-co-MMA	Paraffin	61.50	140.30	NA	NA	60.70	439.40	[125]
		60.40	79.75	NA	NA	34.50	274.80
		60.40	44.21	NA	NA	19.10	118.80
		61.40	17.20	NA	NA	7.50	74.00
Styrene–butyl	RT80	78.40	4.90	54.10	6.50	3.04	52.00	[126]
acrylate		81.80	16.60	55.00	18.90	10.3	58.00
copolymer		79.80	12.00	54.40	14.60	7.45	62.00
PMMA	n-dodecanol	18.20	98.80	NA	NA	82.20	150.00	[127]
PMMA	n-dodecanol	18.40	109.3	NA	NA	44.07	50–100	[128]
MMA	C18	33.20	116.00	15.70	112.00	52.90	466.00	[129]
PUT	C18	31.30	35.20	19.50	34.80	55.00	601.00	[130]
		30.10	67.70	20.00	65.40	74.00	698.00
		30.80	81.80	19.90	80.10	70.00	723.00
		28.40	36.40	18.10	35.40	70.00	432.00
		29.50	48.20	19.30	47.70	62.00	598.00
		30.10	73.70	20.30	72.80	71.00	654.00
		25.30	18.50	15.40	18.20	72.00	401.00
		29.00	39.60	18.30	38.60	76.00	432.00
		31.90	58.50	17.90	57.50	75.00	497.00
Acrylic resin copolymers	n-dodecanol	22.26	93.31	NA	NA	43.00	933.91	[131]
Organosilica	C18	28.44	113.30	23.76	108.80	55.40	NA	[132]
SiO2	C18	27.35	116.00	24.17	109.00	NA	563.00	[133]
Ag-coated SiO2	C18	28.51	126.90	22.19	120.60	62.83	274.00	[134]
		28.27	106.58	22.13	104.80	52.77
		28.03	71.95	22.30	71.65	35.62
		28.05	49.30	22.03	50.60	24.41
		27.77	33.19	22.21	34.00	16.43
		27.51	26.13	22.26	26.87	12.93
Organosilica	C18	27.92	107.50	24.58	102.00	51.26	385.00	[135]
		27.92	98.41	24.94	92.32	46.93	346.00
		27.69	105.50	25.12	102.40	50.30	624.00
		27.35	93.20	24.16	91.73	44.44	693.00
		26.51	95.18	23.31	92.98	45.39	200.00
SiO2	PEG	3.19	62.27	2.61	57.53	55.20	528.00	[136]
SiO2	PA	61.60	180.91	57.08	181.22	89.55	722.50	[88]
		60.92	172.16	56.80	173.40	85.22	466.40
		61.06	168.16	57.62	170.23	83.25	183.70
SiO2	LA	36.70	165.60	34.30	152.50	85.90	210–460	[137]
SiO2	LA	34.50	40.60	29.40	32.50	21.10	60.00	[138]
		37.30	90.40	32.80	82.30	46.90	104.00
		36.80	116.00	34.80	94.00	60.20	221.00
		38.80	160.00	32.90	154.80	83.00	357.00
		36.70	120.50	34.30	119.40	62.50	615.00
		38.80	7.20	30.20	4.70	3.70	750.00
		40.60	4.30	28.80	2.10	2.20	828.00
TiO2	SA	59.14	123.96	50.54	109.43	64.76	946.40	[89]
		58.72	99.02	50.20	84.93	51.73	620.10
		58.46	87.95	49.97	74.76	45.94	583.40
		58.23	58.12	49.80	50.59	30.36	317.60

NA: no data.

presents detailed information on the micro/nanocapsules.

4.2. Thermal Conductivity Overview of Solid–Liquid Micro/Nanocapsules

Thermal conductivity (λ) is an important property for phase-change micro/nanocapsules in various applications because λ affects the heat transfer during the heat absorption and release processes [139]. Generally, a larger λ improves the charge/discharge rate during energy storage/release. For heat dissipation applications in electronic devices, a higher λ is necessary to eliminate excess heat as rapidly as possible.

4.2.1. Thermal Conductivity Enhancement by Additives

Most solid–liquid organic PCMs possess extremely low λ, and the shell materials employed to encapsulate the PCM core predominantly include organic materials, such as PMF resin [140], PUF resin [141], polyurethane (PU) [142], PS [143], and PMMA [144], which also suffer from low λ. Both of these factors limit the practical applications of micro/nanocapsules in heat dissipation systems. Therefore, the λ of the micro/nanocapsules must be improved to achieve a fast heat transfer rate. Several studies have been conducted to overcome the issue of low λ of micro/nanocapsules, and the major techniques adopted to enhance the λ of micro/nanocapsules have been embedding/grafting thermally conductive inorganic materials into the shell. Thus far, numerous studies on improving the λ of micro/nanocapsules have been reported [30]. Jiang et al. [145] synthesized microcapsules with a PMMA shell. After the shell was inlayed by nano-Al₂O₃, the modified microcapsules (16 wt.% nano-Al₂O₃) exhibited a higher λ of 0.314 W/(m·K) than the unmodified ones (approximately 0.243 W/(m·K)). Yang et al. [144] prepared n-octadecane microcapsules with an PMMA shell, and silicon nitride (Si₃N₄) was employed to modify the shell, as depicted in Figure 10. The results indicated that λ was significantly enhanced by adding the highly thermally conductive Si₃N₄. Wang et al. [146] prepared n-octadecane microcapsules containing PMF and shells. The addition of 7% nano-SiC could increase the thermal conductivity of the PMF–SiC microcapsules by 60.34% compared to that of PMF microcapsules. Further detailed thermal conductivity data with related references are presented in

Table 6. Summary of micro/nanocapsules for thermal conductivity enhancement.

Core	λPCM (W/(m·K))	Shell	D	R (%)	λmicro/nanocapsules (W/(m·K))	Enhancement (%)	Reference
n-octadecane		PMF		77.00	About 0.1300		[146]
		PMF + nano-SiC (3 wt%)		72.90		18.31
		PMF + nano-SiC (5 wt%)		72.80		41.36
		PMF + nano-SiC (7 wt%)		72.30		60.34
n-dodecanol		PMF			0.1157	0	[147]
		PMF+GO (0.2 wt%)			0.1297	12.1
		PMF+GO (0.4 wt%)			0.1396	20.65
		PMF+GO (0.6 wt%)			0.1586	37.07
		PMF+GO (0.8 wt%)			0.1663	43.73
		PMF+GO (1 wt%)			0.1738	50.21
		PMF+GO (2 wt%)			0.1897	63.95
		PMF+GO (3 wt%)			0.1918	65.77
		PMF+GO (4 wt%)			0.1924	66.29
Paraffin	0.220	Polyurea	400~600 nm	66.56	0.2250	102	[148]
		Polyurea + nano-Fe3O4 (3.1 wt%)		62.11	0.2330	106
		Polyurea + nano-Fe3O4 (5.7 wt%)		56.42	0.3350	152
		Polyurea + nano-Fe3O4 (6.6 wt%)		54.83	0.3420	155
Paraffin	0.220	PMMA	0.5~2 μm	63.59	0.2442	111	[145]
		PMMA + nano-Al2O3 (5 wt%)		60.77	0.2786	127
		PMMA + nano-Al2O3 (17 wt%)		53.81	0.3104	141
		PMMA + nano-Al2O3 (27 wt%)		48.70	0.3409	155
		PMMA + nano-Al2O3 (33 wt%)		43.92	0.3591	163
		PMMA + nano-Al2O3 (38 wt%)		43.43	0.3816	173
n-octadecane	0.150	PMMA	4~11 μm	73.14			[144]
		PMMA + Si3N4 (5 wt%)		88.00	0.2315	154
		PMMA + Si3N4 (10 wt%)		81.92	0.2753	184
		PMMA + Si3N4 (15 wt%)		73.78	0.2918	195
		PMMA + Si3N4 (20 wt%)		66.13	0.2997	200
		PMMA + Si3N4 (30 wt%)		63.61	0.3630	242
Paraffin		UFR	13.08 μm	37.38	0.0820		[149]
		UFR + CNT (1 wt%)	0.15 μm	23.14	0.0950
		UFR + CNT (2 wt%)			0.1170
		UFR + CNT (3 wt%)			0.1330
		UFR + CNT (4 wt%)			0.1460
n-octadecane	0.150	SiO2	7~16 μm		0.6213	414	[87]
n-octadecane	0.150	CaCO3	5 μm	21.89	1.6740	1120	[150]
n-eicosane	0.161	TiO2	1.5~2 μm	49.90	0.8650	540	[151]
n-octadecane	0.153	SiO2	274 nm	62.83	0.2460	161	[134]
		SiO2 + PDA	300 nm	52.77	0.1980	129
		SiO2 + PDA + Ag (8 g/L)	500 nm	35.62	0.5360	350
		SiO2 + PDA + Ag (16 g/L)	550 nm	24.41	0.5180	339
		SiO2 + PDA + Ag (32 g/L)	650 nm	16.43	0.8400	549
		SiO2 + PDA + Ag (48 g/L)	600 nm	12.94	1.3460	161
Paraffin	0.300	SiO2	30 μm	50.80	1.0310	344	[152]
		SiO2 + GO		49.60	1.1620	387
RT28	0.289	CaCO3			0.7590	262	[153]
RT28+RT42		CaCO3			0.7390
RT42	0.388	CaCO3			0.9360	241
n-docecane	0.152	ZrO2	1~1.5 μm	64.4	0.906	596	[154]
PA	0.226	Ag	100~600 nm	34.30	4.0720	1800	[92]

.

4.2.2. Thermal Conductivity Enhancement by Inorganic Shells

To improve the λ of micro/nanocapsules, various researchers have focused on encapsulation using inorganic shells. Zhang et al. [87] fabricated n-octadecane microcapsules using an inorganic SiO₂ shell to improve λ via a sol-gel process. Tetraethyl orthosilicate (TEOS) was used as the SiO₂ precursor. When the n-octadecane/TEOS mass ratio was set to 50/50, the λ of the microcapsules was enhanced to 0.6213 W/(m·K). Eventually, the λ of the microcapsules improved significantly compared with that of the pure PCM, owing to the thermally conductive SiO₂ shell. Yu et al. [150] synthesized n-octadecane/calcium carbonate (CaCO₃) microcapsules using a self-assembly method. The n-octadecane/CaCO₃ with a mass ratio of 30/70 exhibited a remarkable improvement in λ, which equaled 1.674 W/(m·K), and correspondingly, the enhancement ratio approached 726%. Chai et al. [151] fabricated n-eicosane/TiO₂ microcapsules using a sol-gel process. During the process, tetra butyl titanite (TBT) was employed as the precursor of TiO_2. With an n-eicosane/TBT weight ratio of 40/60, the microcapsules exhibited a λ of 0.865 W/(m·K) corresponding to an enhancement of 18.6% compared to n-eicosane. To further improve the λ of micro/nanocapsules, some researchers have focused on integrating thermally conductive fillers into inorganic shells. Zhu et al. [134] prepared n-octadecane/silica nanocapsules, and by grafting Ag nanoparticles into the silica shell (as depicted in Figure 11), the apparent λ of the nanocapsules increased significantly from 0.246 to 1.346 W/(m·K) on changing the AgNO₃ concentration from 8 to 48 g/L. Yuan et al. [152] employed graphene oxide (GO) to modify paraffin/SiO₂ microcapsules, and they discovered that the λ increased from 1.03 to 1.162 W/(m·K). They also dispersed the two types of microcapsules in a water fluid and determined that the λ of the slurry increased from 0.703 W/(m·K) containing 10 wt.% paraffin/SiO₂ microcapsules to 0.713 W/(m·K) containing 10 wt.% paraffin/SiO₂–GO microcapsules.

4.2.3. Thermal Conductivity Enhancement by Metal Shell

To further improve the thermal conductivity of micro/nanocapsules, a method was developed in a recent study to fabricate phase-change micro/nanocapsules employing a continuous metal (Ag) shell coating [92] (Figure 12). In this process, AgBr ultrafine solid particles were introduced as emulsifiers to improve the rigidity of the emulsion droplet during the solid-liquid transition, and hydroquinone was subsequently used as a reducing agent to adhere to the emulsion after repeated reduction. The AgBr on the droplet surface eventually formed an Ag shell. Palmitic acid (PA) was employed as the core material. The PA/Ag nanocapsules (100–600 nm in diameter) exhibited excellent thermal conductivity up to 4.072 W/(m·K), indicating that the Ag shell significantly improved the thermal conductivity of the nanocapsules.

4.3. Thermal Reliability Overview of Solid–Liquid Micro/Nanocapsules

In practical applications, PCMs or micro/nanoencapsulated PCMs must possess the ability to store and release heat for numerous cycles without significant changes in the latent heat and phase-change temperatures. Therefore, thermal reliability is crucial for a wide range of applications, and particular studies have also considered this aspect. Abhat [155] considered PCMs within the temperature range of 0–120 °C and indicated that studies of thermal reliability are necessary to evaluate their long-term and short-term usage. Salunkhe and Krishna [156] indicated that PCMs should satisfy both primary and secondary criteria before they are considered for applications and that thermal reliability is crucial for efficient latent heat storage usage.

Thermal reliability is often tested using thermal cycling tests, where the core PCM is heated to a liquid state at a temperature above the melting point and then cooled down to a solid state at a temperature below the freezing point, which represents one cycle for the test. Following a large number of cycles, the samples were tested using DSC to compare the changes in the phase transition temperature and latent heat. Typically, if a micro/nanocapsule can withstand numerous thermal cycles (N) with only a minor change in the heat storage performance, particularly the latent heat, after thermal cycling, this indicates that the micro/nanocapsule possesses good thermal reliability. Several review articles have focused on the thermal reliability of PCMs. Sharma and Sagara [157] presented a list of 250 PCMs considering their melting temperature and latent heat, and they discussed long-term stability studies related to the thermal cycling of these particular PCMs. Rathod and Banerjee [4] presented a detailed review of the thermal reliability of latent-heat-storage materials. They discovered that paraffins were stable after several heating/cooling cycles. They also recommended investigating fatty acids in relation to thermal cycling. Ferrer et al. [158] reported that common standards for performing thermal reliability tests are unavailable.

In addition, various studies on organic solid–liquid phase-change micro/nanocapsules also conducted thermal reliability tests. Thermally enhanced n-octadecan/SiO₂–BN nanocapsules were prepared by Lan et al. [159] using a miniemulsion polymerization method. These nanocapsules underwent 500 heating/cooling cycles in the thermal reliability assessment experiment. By analyzing the latent heat and phase-transition temperature changes of the nanocapsules under different cycles, the results indicate that the phase-change temperature fluctuation was within 1.5 °C, implying a minor change in the phase-change temperature following the thermal cycling test. In addition, the latent heat of n-octadecane/SiO₂-BN nanocapsules exhibited a variation below 4% after 500 thermal cycles, which nearly remained constant. Zhang et al. [76] produced n-octadecane/poly(2,3,4,5,6-pentafluorostyrene) phase-change nanocapsules with a diameter of 490 ± 16 nm using the suspension polymerization method, and little change in latent heat was observed after 200 thermal cycles for the nanocapsules. Nikpourian et al. [160] prepared paraffin wax/PU nanocapsules and evaluated their thermal reliability via thermal cycling tests, as shown in Figure 13. The results indicated no obvious leakage outside the nanocapsules following 50 and 100 thermal cycles. Detailed thermal reliability data with related references are presented in

Table 7. Thermal reliability data with the related references about solid–liquid organic micro/nanocapsules.

Core	Shell	D	Tm (°C)	ΔHm (J/g)	Tc (°C)	ΔHc (J/g)	N	Reference
ES-EP	SiO2-Ag	510~580 nm	20.2	108.5	15.0	107.8	0	[161]
			20.1	108.4	14.9	107.7	200
			20.1	108.4	14.9	107.6	500
			20.0	108.3	14.8	107.6	1000
	SiO2-Cu	510~580 nm	20.6	109.7	15.6	108.8	0
			20.5	109.7	15.6	108.8	200
			20.5	109.6	15.5	108.7	500
			20.4	109.6	15.5	108.7	1000
LA	SiO2	131 nm	35.9	36.8	33.7	48.6	1200	[137]
		142 nm	37.0	81.9	33.1	76.1
		174 nm	37.2	107.5	33.0	89.3
		311 nm	36.9	165.1	36.1	150.5
		408 nm	35.8	133.4	33.4	117.4
		510 nm	37.0	115.2	33.5	112.3
		474 nm	35.1	130.7	33.2	127.4
		250 nm	36.8	120.3	34.5	112.7
		402 nm	37.9	143.5	33.3	120.6
		215 nm	36.5	115.5	33.0	103.8
		122 nm	37.5	27.5	31.1	25.5
n-octadecane	PPFS	490 nm	31.2	171.8	23.0	169.3	200	[76]
PA	SiO2	474 nm	62.9	109.9	57.8	100.1	0	[162]
			62.8	109.2	57.9	98.6	100
PA	Ag	100–600 nm	56.40	107.87	57.46	105.22	2000	[92]
D-mannitol	silica/GO	100–400 nm	166.2	216.7	122.3	174.4	1	[163]
				205.6		176.7	10
				214.6		181.1	20
				196.0		166.6	30
				199.5		169.9	40
				206.7		169.2	50
Paraffin wax	PU	25~185 nm	62.4	153.9	66.9	142.3	0	[160]
			63.0	152.2	66.8	142.4	50
			63.0	151.2	66.8	141.5	100
SA	Ag	167~252 nm	46.13	117.73	66.13	116.61	2000	[164]
SA	TiO2	946 nm	59.14	123.95	50.54	109.43	0	[89]
			59.41	119.23	50.01	105.68	2500
PA	SiO2	221 nm	38.8	160.0	32.8	154.8	0	[138]
			38.9	157.3	32.9	152.5	3000
		357 nm	36.7	120.5	34.3	119.4	0
			36.8	118.4	34.3	116.7	3000
		615 nm	36.8	116.0	34.8	94.0	0
			36.9	114.0	34.7	92.8	3000
Dimethyl adipate	MF	900 nm	6.4	80.2			100	[165]
C14	MUF	209 nm	7.7	140.3	−0.76	142.2	100	[166]
	MUF-TiO2	156.2 nm	8.4	156.2	−4.32	156.0	100
n-octadecane	SiO2	346 nm	27.92	98.4	24.94	92.3	500	[135]
				99.1		92.3	100
				101.6		94.4	300
				101.6		94.9	500
n-octadecane	SiO2	624 nm	27.69	105.5	25.12	102.4	0	[135]
				107.6		104.9	100
				105.1		102.9	300
				102.7		100.7	500
n-octadecane	ST-MMA	102 nm	24.9	107.9	27.3	104.9	0	[120]
			24.8	106.7	27.3	105.9	90
			24.4	105.1	27.5	100.5	180
			24.9	105.4	27.8	101.4	270
			24.1	105.1	27.1	103.7	360
n-nonadecane	PMMA	8.18 μm	31.2	139.2	31.0	142.4	0	[94]
			31.2	132.7			5000
CA	PS	7.7 μm	21.89	80.56			0	[93]
			17.22	80.28			5000
LA	PS	13.2 μm	21.89	80.56			0
			17.22	80.28			5000
MA	PS	42.0 μm	47.12	92.12			0
			48.74	88.29			5000
n-octadecane	SiO2	563 nm	27.35	109.5	24.17	98.85	0	[167]
				109.4		99.09	1
				118.2		107.3	100
				115.3		103.4	300
				114.4		102.4	500

.

From the aforementioned studies, although substantial research involved evaluating the thermal reliability using thermal cycling tests, the number of thermal cycling tests in any particular research does not indicate the maximum number of cycles that the prepared micro/nanocapsules can undergo. In addition, the results clearly indicate the absence of a general standard that dictates the number of cycles the micro/nanocapsules should undergo. However, as reported on a daily basis [168], the latent heat TES will experience heat absorption at least once during the daytime and heat release once during the night. Consequently, for each day, there would be a thermal cycle, and within one year, 365 thermal cycles would be performed by the latent heat TES system. Therefore, 1200, 3650, and 10,000 thermal cycles correspond to operational performances equivalent to approximately 4, 10, and 30 years, respectively. This condition can be simulated using an accelerated thermal cycling test in the laboratory, which can be performed by repeating melting/solidification cycles for the PCM under controlled conditions, which would reproduce the long-term performance of the TES system within a relatively short time.

5. Influence Factors on the Thermal Properties of Micro/Nanocapsules

5.1. Influence Factors on the Thermal Storage Properties

For micro/nanocapsules, their morphological, physico-chemical, and thermal properties rely on various synthetic parameters, such as pH, mixing rate, content and type of core PCM, emulsifier, core/shell ratio, reaction time, and temperature [6]. It is difficult to predict the most appropriate preparation recipe for a particular system, and several experiments are required to produce the micro/nanocapsules with highly favorable properties. Among the various synthetic conditions, a few major factors influence the thermal storage properties of micro/nanocapsules, which are summarized in this study.

5.1.1. Mass Ratio of Core/Shell

The mass ratio of the core/shell materials added during the preparation process is a key factor that affects the thermal storage properties. Generally, under suitable preparation conditions, if the additional core material is used, high thermal storage properties and a large encapsulation ratio of micro/nanocapsules can be achieved. Recently, Hu et al. [169] prepared methyl laurate/PU microcapsules using an interfacial polymerization method. They studied the preparation conditions of the microcapsules and discovered that by adjusting the mass ratio of core/shell from 1:1 to 4:1, the melting enthalpies, freezing enthalpies, and encapsulation ratio of microcapsules increased from 79.03 J/g, 70.63 J/g, and 40.71% to 155.6 J/g, 136.4 J/g, and 85.28% (as shown in Figure 14), respectively. Latibari et al. [89] fabricated SA/TiO₂ nanocapsules via a sol-gel process. During the process, titanium tetraisopropoxide (TTIP) was used as the precursor of TiO₂, and with different SA/TTIP ratios of 50/50, 60/40, 70/30, and 80/20, the obtained nanocapsules exhibited an increased melting latent heat of 58.12, 87.95, 99.02, and 123.96 J/g, respectively, and an encapsulation ratio of 30.36, 45.94, 51.73, and 64.76%, respectively (Figure 15).

5.1.2. pH

In reality, synthesis conditions with a large core/shell mass ratio do not always produce micro/nanocapsules with high latent heat and encapsulation ratios. Liu et al. [170] fabricated n-eicosane/PF microcapsules using an in situ polymerization method and studied the effect of the core/shell mass ratio on thermal storage (Figure 16). With the addition of n-eicosane of 1, 2, 3, 4, and 5 g, the latent heat of the prepared microcapsules changed to 94.86, 99.50, 173.83, 143.94, and 155.36 J/g, respectively, which indicated that the latent heats of microcapsules did not increase with the core/shell mass ratio, and other factors also affected the thermal storage of the microcapsules. Their study remarked that during the fabrication process, phenol and formaldehyde monomers could be polycondensed directly into a solid layer in the polycondensation process instead of forming a shell on the surface of n-eicosane emulsion droplets, resulting in a reduction in the actual amount of n-eicosane within the microcapsules. Therefore, shell-forming conditions are also important for thermal storage capacities. In our previous study [138], the effect of alkaline pH on LA/SiO₂ nanocapsule preparation and thermal storage capacities was explored. We had fixed the core/shell ratio at a certain value and adjusted the pH of the shell precursor solution in the sol-gel process to study the formation of nanocapsules and thermal storage properties. Eight samples were prepared and investigated. The results demonstrated that the reasonable pH range for nanocapsule formation lies between 9.9 and 10.9, within which the nanocapsules obtained presented an encapsulation ratio as high as 83.0%, and correspondingly, the latent heats approached 160.0 J/g, implying a large thermal storage capacity. When the pH was lower than 9.9, the nanocapsules exhibited low latent heat (less than 100 J/g), whereas when the pH was higher than 10.9, nanocapsules with core-shell structures could not be produced. Therefore, controlling shell formation is also crucial for the heat storage capacity of nanocapsules, which is predominantly due to its influence on the shell thickness or particle distribution of the capsules.

5.1.3. Emulsion Droplet Size

Furthermore, the formation condition of the core is also highly important for the thermal storage capacities. Theoretically, the micro/nanocapsule size relies on the core PCM emulsion droplets, which form a solid-liquid organic PCM that can be dispersed in the solution and subsequently coated with thin shell materials. Large emulsion droplets tend to produce large micro/nanocapsules with high latent heat because large cores may occupy high mass ratios in the final micro/nanocapsules. However, the latent heat may also be affected by other factors during the preparation process. In one of our studies [137], we studied the effect of particle size on thermal storage capacity. We prepared LA/SiO₂ nanocapsules under different conditions; these nanocapsules exhibited varying latent heats with their corresponding mean diameters (Figure 17). On analyzing the relation of the latent heat with particle size, it was observed that an optimum particle size of 340 nm in diameter corresponds to the highest latent heat. For particle sizes below 340 nm, the latent heat increased with the increase in particle size. In contrast, for particle sizes exceeding 340 nm, the latent heat exhibited a decreasing trend with increasing particle size for the nanocapsules. This is primarily related to the relationship between the cumulative percentage of droplets and the generation rate of silica clusters. When the cumulative percentage of the droplets increased faster than the generation of the corresponding clusters, the shell tended to be thinner and the core tended to be larger, which led to an increased thermal storage capacity with particle size and encapsulation ratio (zone I in Figure 18a). When the clusters are generated faster than the increase rate of the core particle size, the shell thickening results in a decreased thermal storage capacity with particle size. When the cumulative percentage of the droplet size is gradually balanced by that of the cluster generation rate, the relative ratio of the core to shell attains a maximum value, and thus the optimum particle size, corresponding to the highest latent heat, is obtained.

5.1.4. Summary of the Influence Factors on Thermal Storage Capacity

From the aforementioned analysis, it is observed that regardless of the factors affecting the latent heat of micro/nanocapsules, they all contribute to their final core/shell mass ratio. Generally, micro/nanocapsules with larger cores and thinner shells exhibit good thermal storage capacity. This is an inference based on the premise that the shell material density is relatively small (such as SiO₂, approximately 2.2 g/cm³). If the shell density significantly exceeds that of the core (such as Ag shell, 10.5 g/cm³), the measured latent heat of the nanocapsules will still be small, primarily owing to the larger mass fraction of the shell material occupied within the micro/nanocapsules. Theoretically, the latent heat is obtained according to the mass fraction of the capsule occupied by the core with the identical amount of core PCM; the latent heat calculated using the shell of a small density is relatively higher than that of a large density, which creates the illusion that the micro/nanocapsules with heavy shells show poor thermal storage capacity (as shown in Figure 19a), whereas the energy stored in the core is the same. Thus, the evaluation by latent heat is not appropriate, and a new evaluation parameter should be derived. R_V, defined as the volume ratio of the core occupying the whole micro/nanocapsule, was proposed in our previous study [92] to evaluate the encapsulation efficiency of the micro/nanocapsules, considering R and the difference in density (k = $\frac{{\mathit{\rho }}_{core}}{{\mathit{\rho }}_{shell}}$) between the core and shell materials. Figure 19b depicts a comparison of R and R_v for different k levels. From the structural comparison, we observe that different k values lead to the visual miscalculation of R, whereas R_V can eliminate this miscalculation to present the real core-shell structure state. Nevertheless, micro/nanocapsules having a ‘light’ shell are the first choice for the application.

5.2. Influence Factors on Thermal Conductivity

In the present study, the methods to enhance the thermal conductivity of micro/nanocapsules mainly included embedding/grafting thermally conductive particles into the shell and encapsulating PCMs with high thermal conductivity shells. Both methods could effectively improve the λ of the phase-change micro/nanocapsules, whereas some influencing factors are important in the enhancement.

For thermally conductive particles and enhancement of the micro/nanocapsules, sufficient addition of nanoparticles is required to develop a network that is conducive to thermal conduction [171], which is supported by Zhang et al. [172] in their experimental study on the thermal properties of short carbon fiber/erythritol PCMs. Based on this, the λ of the phase-change micro/nanocapsules relies on the particle concentration, particle dispersion, and interface interaction between the particle and shell.

5.2.1. Thermal Conductive Nanoparticle Content

Various studies have reported that the λ of enhanced micro/nanocapsules increases with increasing nanoparticle content [134,144,145,146,147,148,149]. Li et al. [149] added up to 4 wt.% of carbon nanotubes to the urea formaldehyde resin (UFR) shell of paraffin/UFR microcapsules, and an increase of λ from 0.095 W/(m·K) (1 wt.% GO) to 0.146 W/(m·K) (4 wt.% GO) was observed. Park et al. [148] studied the fabrication of paraffin/polyurea-(nano Fe₃O₄) nanocapsules. They confirmed that the λ of nanocapsules with 6.6 w.t% nano Fe₃O₄ increased by 155% (0.342 W/(m·K)). Chen et al. [147] studied the λ of GO-enhanced microcapsules by increasing their GO content. When the GO content was below 1 wt.%, an increase in λ was obtained from 0.1157 to 0.1738 W/(m·K). When the GO content was 1 wt.%, the λ of the GO-enhanced microcapsules increased by 50.21%. When the GO content was 4 wt.%, the λ of the GO-enhanced microcapsules was enhanced by 66.29%. For this enhancement, the oxygen-containing functional groups on the GO surface enabled GO to disperse well in the MF prepolymers, and the GO sheet network had a positive effect on the λ of the GO-enhanced microcapsules. Hence, a minor amount of GO can significantly improve the λ of the GO-enhanced microcapsules. However, the networks became saturated as the GO content increased to 4 wt.%; thus, there was an insignificant increase at the end.

5.2.2. The Distribution and Interface Interaction of Particles in the Shell

The λ of micro/nanocapsules with high thermal conductivity nanoparticles added to the shell cannot always exhibit larger values than those with low thermal conductivity nanoparticles [29]. The dispersion state of the highly conductive nanoparticles (fillers) in the shell and the interfacial interaction between the nanoparticles (fillers) and shell majorly influence the λ of micro/nanocapsules. The former is related to the thermal resistance of the interface, and the latter involves the formation of a thermal conduction path.

Liu et al. [173] prepared dodecanol/MF resin microcapsules and modified the shell with a GO–CNT hybrid filler to improve λ. Herein, by adding GO and CNTs, the λ of the microcapsules could be enhanced, and the GO–CNT hybrid filler was preferable to that of individual GO or CNT. Specifically, the λ of the microcapsule was enhanced by 195% when the hybrid filler content reached 0.6 wt.%. This illustrated that GO and the polymer shell exhibited a good interface interaction, whereas CNT presented a poor interface interaction with the shell. To study the dispersion of the fillers in the polymer shell, they poured the prepolymer of the shell with brown GO or black CNTs into a cylinder mold and then cured and dried it, as shown in Figure 20. The results indicate that GO exhibits a better dispersion in the polymer shell than CNT. In addition, the GO–CNT hybrid filler can be dispersed uniformly in the polymer shell because the presence of CNT can promote the dispersion of GO in the shell, leading to homogeneous dispersion of the filler network.

5.2.3. High Thermal Conductivity Shell

In fact, it is difficult to ensure a uniform distribution of heat conduction enhancement particles within the shell, and long-term use will also lead to adverse results such as precipitation and aggregation, thus reducing λ and deteriorating the temperature uniformity of the heat storage system [30]. In addition, this type of enhanced micro/nanocapsule may work in the stacking form, whereas when they are used in the working fluid, the effect of λ enhancement is limited because the filler particles are distributed discontinuously in the shell. In contrast, a continuous shell with a high-λ coating on the PCM core surface is better because it can exhibit heat transfer enhancement in all applications, especially in the working fluid. However, for a continuous shell, there are also factors that influence the λ enhancement of the microcapsules. The λ value of the shell is important for the heat transfer process. Zhang et al. [87] and Yu et al. [150] fabricated n-octadecane microcapsules with inorganic shells; when SiO₂ was used as the shell with a tested λ of 1.296 W/(m·K), the λ of the obtained microcapsules was 0.6213 W/(m·K); when CaCO₃ was used as the shell with a λ of 2.467 W/(m·K), the λ of the obtained microcapsules was 1.674 W/(m·K). Therefore, a highly thermally conductive shell tends to enhance thermal conductivity. In addition, the λ of the microcapsules was also restrained by the shell thickness. Zhang et al. [87] prepared n-octadecane microcapsules with a silica shell (TEOS as the precursor) via a sol-gel process, and the λ of the microcapsules was determined to be 0.6213 W/(m·K) with an n-octadecane/TEOS mass ratio of 50/50, whereas λ was only tested to be 0.3751 W/(m·K) with an n-octadecane/TEOS mass ratio of 70/30, which indicates that the thin and porous shell material is not conducive to heat transfer.

5.2.4. Mass Ratio of Core/Shell

As discussed in Section 5.1.1, the mass ratio of the core/shell materials affects the thermal storage properties, where a high mass ratio often leads to a large encapsulation ratio. In our previous research [164], the relationship between the encapsulation ratio and thermal conductivity was discussed. Three SA/Ag nanocapsules with a SA content of 0.4 g (S1), 0.8 g (S2), and 1.2 g (S3) were prepared via a chemical reduction method, and the encapsulation ratio was increased from 15.3% to 31.7% to 71.4%. The effective thermal conductivity (λ_eff) of these three samples was also tested, and the results varied from 6.020 to 0.974 W/m·K. This indicates that the thermal conductivity showed the opposite trend with the variation in R. It is easy to understand that a higher R leads to a lower shell mass ratio. While the shell generally has a relatively higher thermal conductivity than the core PCM, it contributes most to the thermal conductivity of micro/nanocapsules. Therefore, micro/nanocapsules prepared by a high mass ratio of the core/shell show a large encapsulation ratio, which is not beneficial to the thermal conductivity.

5.2.5. Summary of the Influence Factors on Thermal Conductivity

Although both highly thermally conductive particles and shells can improve the thermal conductivity of micro/nanocapsules, their limitations are also inevitable. With an increase in the content of the conductive particles or the high conductive shell thickness, the proportion of core to micro/nanocapsules will be reduced, resulting in a reduction in the thermal storage capacities and encapsulation ratio. The higher the particle (or shell) content, the lower the thermal storage capacity and encapsulation ratio. In addition, for high-density materials such as shells or fillers, such as Ag, the high density will not only lead to a reduction in the thermal storage capacities and encapsulation ratio but may also cause deposition problems when used within the working fluid; therefore, the suspension problem should be discussed. In our previous research [164], we indicated that to maintain a good suspension of Ag-coated micro/nanocapsules, the particle size should be controlled below the theoretical suspension critical diameter (TSCD). Therefore, to enhance thermal conductivity, both methods need to be improved. It is likely that encapsulating PCMs with shell materials of high thermal conductivity and low density is an ideal choice at present, while the search for this material and the development of synthetic technology are the main directions.

5.3. Influence Factors on Thermal Reliability

In the studies at present, most of the preparation of micro/nanocapsules has mentioned the test of thermal reliability, based on which the factors affecting thermal reliability can be summarized.

5.3.1. Compactness of the Shell Material

The compactness of the shell material greatly affects the thermal reliability of the micro/nanocapsules. The latent heat loss after the thermal cycling test relies on the core PCM escaping through the pore structure of the shell material. The more mesopores in the shell structure, the better the core content escapes during the heating/cooling process, which eventually leads to poor thermal reliability. Lan et al. [117] performed shell modification of n-octadecane(n-OD)/SiO₂ nanocapsules with poly(hydroxylethyl methacrylate) (PHEMA) and PS with 500 thermal cycling tests. After 500 thermal cycles for n-OD/SiO₂ nanocapsules, the melting and solidifying latent heats decreased by 9.0 and 8.7%, respectively, whereas for the n-OD/PS–SiO₂ and n-OD/PHEMA–SiO₂ nanocapsules, the latent heats decreased much less, indicating that polymer–SiO₂ hybrid shell material is more suitable than SiO₂ for nanocapsules to repeatedly store/release heat in applications. Liu et al. [174] fabricated uniform n-eicosane/meso-silica nanospheres and nanocapsules using a self-templating strategy (Figure 21). Thermal reliability analysis showed that the n-eicosane/meso-silica nanospheres had poor coincidence in the DSC curves, and the altitudes of crystallization and melting peaks gradually decreased during the repeated phase transition. In contrast, the n-eicosane/meso-silica nanocapsules showed good consistency in phase-change performance because their DSC curves almost overlapped with each other during the repeated phase transition process, and no shift was observed in their melting and crystallization peaks. Besides, the melting and solidifying latent heats of the nanospheres indicated a downward trend with thermal cycles and were reduced by 31.1 and 30.7% after 1000 thermal cycles, respectively. However, the melting and solidifying latent heats showed a stable trend with the thermal cycles and only varied by 0.25 and 0.2 J/g, respectively, after 1000 thermal cycles, which illustrated that silica shell could protect the encapsulated n-eicosane for the nanocapsules. In addition, the freezing and melting temperature fluctuations of the nanocapsules with thermal cycles were 0.19 and 0.36 °C, respectively, which was a narrower range than that of the nanospheres. These results confirmed that the nanocapsules demonstrated better thermal reliability than the nanospheres.

5.3.2. The Size and Shell Thickness

In addition to the intrinsic factors of the shell structure, frequent contact between the micro/nanocapsule particles during extensive heating/cooling cycles can also cause the destruction of the capsule structure and the escape of the core PCM, resulting in the loss of latent heat. The size and shell thickness of the micro/nanocapsules are critical for the destruction of the capsule structure. Sukhorukov et al. [175] studied polyelectrolyte capsules with sizes of 10 nm and 10 mm by applying the same force and found that capsules with a size of 10 nm showed a substantially smaller deformation than that of 10 mm capsules. This study verified that nanocapsules are more stable in structure than macro/microcapsules. Theoretically, smaller-sized capsules have better freedom of contact between particles, whereas larger-sized capsules are more likely to be worn out during the heating/cooling process. For larger-sized capsules, a thick shell may be important to compensate for their structural stability. Melanie et al. [176] conducted a mechanical property test with capsule sizes of 10–50 μm and shell thicknesses of 50–200 nm and found that the stiffness of the capsule increased with the shell thickness. Therefore, a small size and thick shell are favorable factors for promoting the thermal reliability of phase-change capsules. From the above analysis, it appears that capsules with a smaller size would show less latent heat loss after extensive thermal cycling processes. However, when the size is sufficiently small, such as nanocapsules, the effect of size on thermal reliability changes. In our previous study [137], we prepared LA/SiO₂ nanocapsules of different sizes, and their thermal reliability was tested over 1200 thermal cycles. The results indicated that the percentage of latent heat reduction after the thermal cycling test decreased significantly with increasing particle size (Figure 22), which was mainly due to the decreased specific surface area of the nanocapsules reducing the volatilization of the encapsulated LA. Therefore, nanocapsules with relatively large sizes are the optimum choices for achieving thermal reliability.

5.3.3. Summary of the Influence Factors on Thermal Reliability

From the above analysis, we can conclude that all these factors can be attributed to strengthening the shell structure and improving the freedom of the particles. However, some of the above influencing factors may weaken the thermal storage capacity, such as shell thickness. As discussed previously, thick shells are beneficial to thermal reliability, but thin shells can improve thermal storage capacity. Therefore, there should be a shell balance when considering thermal storage capacity and thermal reliability. Furthermore, although we summarized some intrinsic factors, there are still various factors, such as the application environment, application temperature, and application form. Further research is necessary to determine its thermal reliability.

6. Application Prospects of Solid–Liquid Organic Phase-Change Micro/Nanocapsules

Various applications of phase-change micro/nanocapsules have been studied. For instance, phase-change micro/nanocapsules used in buildings to save energy, in the food industry and textiles to reduce temperature fluctuations, in the thermal management of electronics to cool the device, in solar systems, and in working fluids to improve heat storage or heat transfer efficiency [29,177]. In particular, for phase-change micro/nanocapsules with good thermal performance, such as large latent heat, high thermal conductivity, and long-term cycling performance, they are crucial to the applications of working fluids for solar systems and thermal management of electronics, and other applications depend on the application form because these thermal performances are key factors affecting the thermal charging/discharging rate.

6.1. Applications in Working Fluids

EPCMs can be dispersed in a fluid to form a multiphase medium that integrates heat storage and heat transfer, which are known as latent functional thermal fluids (LFTF) [178]. Figure 23 shows the microcapsule slurry and an SEM image of the microcapsule particles [179]. For this working fluid, water is commonly used as the carrier fluid, and surfactants are used to help micro/nanocapsules disperse well into the carrier fluid. With the large latent heat and high λ of micro/nanocapsules, the working fluid presents good heat transfer and thermal storage performance in various applications.

6.1.1. Air Conditioning, Ceiling Cooling, and Energy Storage

Wang et al. [179] used a C₁₆H₃₄/amino plastic LFTF combined with evaporative cooling technologies to propose a low-energy air-conditioning strategy applied in a cooling ceiling system. The results indicated that applying the new system would have an energy-saving potential as high as 80% in the northern China climate and 10% in the southern China climate. Wang and Niu [180] conducted a further study and designed a new air-conditioning system under the climatic conditions of Hong Kong by combining a cooled ceiling and a C₁₆H₃₄/UF LFTF storage tank. When the system was installed in an office room, the electricity demand during daytime could be reduced by approximately 33%. Concurrently, by utilizing the high-temperature operation of ceiling panels for sensible space cooling load removal, energy savings and cooling demand shifting could be achieved. The results indicated that the small LFTF storage tank could transfer a portion of the cooling load from daytime to nighttime, and the proposed system is regarded as an energy-saving and economical air-conditioning system. The n-paraffin wax microcapsules (2 μm) with a melting point of 8 °C and latent heats of 75.9 J/g dispersed in water were used for cooling at Narita Airport (Tokyo, Japan) [181]. The LFTF was applied to compensate for the decrease in energy capacity when changing refrigerants for environmental reasons. The storage tank has a height of 24.7 m and a diameter of 7.4 m. Cold energy can be stored at night and released during the day. Compared with the old thermal storage system that used external ice melting, the thermal storage density of the LFTF system was reduced by approximately 60%, but the operating cost was reduced by 32%.

6.1.2. Thermal Management of Electronic Devices

LFTF is preferable for the thermal management of electronic devices because it can store and transfer large amounts of heat at near-constant temperatures [182]. Several studies have reported the use of LFTFs in the thermal management of electronic devices.

Ye et al. [183] studied the thermal management of LED with eicosane/polyurea-polyurethane microcapsules (5–30 μm, 37.0 °C, 180.5 J/g) LFTF as a coolant. The experimental results indicated that a system with LFTF as a coolant has a larger cooling capacity than that with water. In particular, at high ambient temperatures, the effect was more significant when using LFTF instead of water, owing to the phase change of the microcapsules. Li et al. [184] established a model with an LFTF as a coolant to study a battery thermal management system. They observed that the maximum temperature of the battery could be further reduced by using LFTF instead of water as the coolant. In addition, the LFTF flow rate, mass fraction, latent heat of microcapsules, and range of melting point significantly affected the performance. Deng et al. [185] established mathematical models to study the heat transfer performance of an LFTF. When an LFTF is used in microchannels for chip cooling, the heat transfer can be enhanced compared with the single-phase fluid used. Bai et al. [186] studied a thermal management system for batteries with an LFTF and minichannel cooling plates by establishing a three-dimensional thermal model. The results illustrated that the LFTF containing 20 wt.% n-octadecane microcapsules and 80 wt.% water presented better performance than pure water, mineral oil, and glycol solution. Zhang et al. [187] thermodynamically assessed the active cooling/heating methods for batteries with an LFTF cycle. The results indicated that the LFTF cycle method exhibited a higher energy efficiency than the refrigerant circulation method. They also conducted a thermodynamic assessment of active cooling/heating methods under extreme conditions for lithium-ion batteries in electric vehicles. Pakrouh et al. [188] conducted a numerical study on battery thermal management with the application of LFTF, and they observed that the maximum temperature and its difference were reduced by 9.21 and 3.07 K compared with that of pure water, whereas the pressure drop and power consumption increased. Sabbah et al. [189] studied the effect of the LFTF on the heat transfer coefficient of a microchannel radiator by simulation, and using the LFTF as a coolant instead of water, they observed an even distribution of temperature and a significant improvement in performance.

In the present study, the LFTF demonstrated a significant role in the thermal management of electronic devices. In particular, for micro/nanocapsules with good thermal performance, large latent heat could promote the micro/nanocapsules to absorb a large amount of heat from the device and maintain its temperature stability, and high λ can enhance the thermal response by fast heat transfer from the device to the core PCM, which avoids the heat absorption delay caused by the slow heat transfer and good thermal reliability, which can ensure a long service life of the micro/nanocapsules. Therefore, micro/nanocapsules with good thermal performance are ideal candidates for the thermal management of electronic devices.

6.2. Applications in Bulk Form

6.2.1. Green Energy-Saving Building

PCMs excel at increasing the energy storage capacity of the building envelope and regulating the air temperature distribution to improve occupancy comfort.

Buildings account for 40% of global energy consumption and 38% of greenhouse gas emissions. Improving building energy efficiency to mitigate global warming is crucial [190]. PCMs can improve the thermal storage capacity of a building enclosure and adjust the air temperature distribution for living comfort [191,192]. Phase-change micro/nanocapsules are also good candidates for this application because the existence of a shell could prevent the interaction between the core PCM and the building matrix material, and the shell can also prevent the leakage of the liquid core PCM.

Shossig et al. [193] studied the incorporation of paraffin microcapsules into gypsum wallboards. The microcapsules had an average diameter of 8 μm and were homogeneously dispersed between gypsum crystals. Compared to the walls without microcapsules, the indoor temperature fluctuations showed a reduction of approximately 2 °C. Arce et al. [194] integrated phase-change microcapsules into the concrete walls of cubicles and studied the effect of awnings added to the cubicles. They observed that the peak temperatures showed a reduction of approximately 6%, and the comfort time was increased by at least 10% with awnings.

Other researchers have designed and developed a range of ceilings, floors, roofs, and walls incorporating PCM for greater energy savings [177,195], which relies on latent heat, λ, the loading amount of micro/nanocapsules, and the thickness of building decoration materials. Moreover, economic evaluation should consider the applications of PCMs and micro/nanocapsules in buildings [196].

6.2.2. Food Industry

Preserving foods under refrigeration and maintaining low temperatures is essential to avoid/delay microbial, physical, and chemical changes; considering this, phase-change micro/nanocapsules may help prevent temperature fluctuations [197].

Pérez-Masiá et al. [198] produced dodecane/zein micro/nanocapsules using electrospinning. They proposed that the produced structures could act as new smart packaging materials to improve the maintain/control temperature. Unal et al. [199] studied the packaging design with the thermal buffering effect using octanoic acid/styrene polymer microcapsules (ΔH_m = 42.9 J/g). The results indicated that using microcapsules or PCMs could provide a thermal buffering effect as long as 8.8 or 6 h for 160 g of chocolate compared with that without PCMs.

In the food industry, PCMs are commonly used in heat storage and transport systems, such as refrigeration, heat treatment sections, and packaging applications, and thus, phase-change micro/nanocapsules show promising prospects for this application.

6.2.3. Temperature-Controlled Textiles

PCMs can be used in temperature-controlled textiles to achieve automatic temperature regulation by storing and releasing thermal energy. Organic PCMs incorporated into textiles provide a comfortable microclimate environment for the human body and thus improve their work efficiency and quality of life [200,201]. In the application of PCM technology in clothing and household products, phase-change microcapsules are combined with acrylic fibers, polyurethane foams, and coating compounds for topical application to fabrics or foams [202,203]. There are various outdoor apparel products with phase-change microcapsules on the market, such as ski suits, hunting suits, boots, gloves, and earmuffs, under the trade names Outlast and Comfor Temp [204].

Shin et al. [204] developed thermoregulating textile materials by adding the prepared eicosane/MF microcapsules to polyester knit fabrics. The strengthening of the microcapsules was sufficient to ensure their stability under stirring in hot water and alkaline solutions. The thermal storage capacity of the temperature-regulating fabric was 0.91–4.44 J/g, which depended on the concentration of microcapsules. After five times of launderings, the treated fabric retained 40% of its heat storage capacity. Paula et al. [205] studied thermo-regulating textiles with paraffin wax microcapsules, and the results indicated that the coating fabric showed a thermal storage capacity of 7.6 J/g with 35% of microcapsules added. They also found that after washing, rub fastness, and ironing treatments, the coating fabric could still maintain high durability and adequate stability. Koo et al. [206] applied phase-change microcapsules to waterproof nylon fabrics using a dual-coating method.

Most organic-based microcapsules can be used to fabricate temperature-controlled textiles; however, it should be noted that their components are environmentally friendly. Some heat-volatile substances, such as formaldehyde, glutaraldehyde, and ether, are harmful to human health [207]. The application of environment-friendly cross-linking agents or shell materials such as chitosan and polyaniline in the preparation of micro/nanocapsules is encouraged [208].

6.2.4. Solar System

Solar energy is regarded as one of the most important sources of renewable energy. Solar water heating (SWH) systems are simple, reliable, and cost-effective for the use of solar energy in hot water production. Energy can be collected and stored so that it can be used when there is no sunlight. SWH systems can be active or passive, depending on the availability of electric pumps in the system. These SWH modes are available in vacuum, flat plates, or centralized collectors. Integrating the PCM in the upper part of the solar water heater tank improves the functionality of the system, as it can enhance the stratification effect. The supply of hot water was from the upper part, where the temperature was the highest (Figure 24). Connecting the inlet of the solar collector to the lower temperature bottom of the tank would produce better collector efficiency. Several factors affect the thermal energy storage system; in particular, the PCM plays an important role in the system.

Su et al. [209] prepared paraffin wax/MF phase-change microcapsules for SWH systems. A theoretical evaluation of the thermal properties of the microcapsules with the highest latent heat (126 J/g) was conducted. Compared with water-based systems, the microcapsule-applied SWH system has a larger TES density and a correspondingly smaller physical storage size. In addition, although the λ of the microcapsules is slightly lower, it is still approximately twice that of the most common storage units with PCM. Furthermore, it compresses the entire system volume using phase-change microcapsules.

In addition, the production of phase-change micro/nanocapsules is beneficial for increasing the efficiency of solar TES by converting solar radiation into thermal energy. Wang et al. [146] reported that n-octadecane/poly(melanine-formaldehyde)-SiC microcapsules (3 μm) have a good storage capacity (167–168 J/g), and they can efficiently perform photothermal conversion and storage of solar energy, especially with 7% of SiC nanoparticles. The λ of the microcapsules was improved by 60.34%, which would result in a temperature of approximately 10 °C higher than that without SiC after irradiating the same time with an infrared lamp.

7. Further Outlook

Phase-change micro/nanocapsules have excellent potential in various fields. The thermal properties of micro/nanocapsules have made significant progress in the past few decades, and some micro/nanocapsules have been applied in industries and households. Although micro/nanocapsules have excellent advantages, a few challenges must be considered.

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Although most of the preparation of micro/nanocapsules has mentioned the thermal reliability test, the thermal cycling performance only involved representing the evaluation under a certain number of thermal cycles, whereas the actual service life of the micro/nanocapsules cannot be estimated based on the data. Therefore, finding a unified standard to evaluate the thermal reliability of different phase-change micro/nanocapsules is a problem that needs to be solved.

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The preparation technology for micro/nanocapsules is mostly concentrated in the laboratory stage; realizing large-scale production and promoting technology requires further research.

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In addition, micro/nanoencapsulation of PCM has several limitations, such as the selection of the correct material and the high cost of the process. Therefore, simplification of the production methods, improvement in stability, and reduction in encapsulation costs are crucial.

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In addition, the development of bifunctional/multifunctional phase-change micro/nanocapsules will significantly expand their scope of application beyond the traditional application field.

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Finally, a database of PCMs must be established to facilitate the selection of production methods as well as shell materials.

8. Conclusions

Overall, this study reviews the thermal properties and factors of organic solid–liquid phase-change micro/nanocapsules. The following conclusions were drawn from this study:

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The thermal properties of solid–liquid PCMs with paraffin-based PCMs, fatty acids, and other PCMs are summarized. In the past years, n-octadecane, n-eicosane, n-hexaoxane, and n-octadecane have been the most commonly used PCMs among paraffins, with SA, PA, and LA being the most commonly studied fatty acid PCMs.

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The methods of micro/nanoencapsulation of organic solid–liquid PCMs and their effects on the shell material, size, and properties of the micro/nanocapsules were analyzed. The sol-gel method is used to prepare micro/nanocapsules with organic shells, whereas other methods are mostly used to prepare organic shells. However, all methods can be used to prepare micro/nanocapsules with high thermal storage capacity because they can be adjusted by the preparation conditions. Recently, other synthesis methods have gradually occupied a larger proportion of micro/nanocapsules, indicating that researchers are working on expanding the research on shell materials to further strengthen the performance of micro/nanocapsules.

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The thermal properties, including the thermal storage capacity, thermal reliability, and thermal conductivity, of various micro/nanocapsules and their influencing factors were reviewed.

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The core/shell mass ratio, shell-forming conditions, and core-formation conditions during the preparation process are the main factors affecting the thermal storage properties. In addition, considering the density difference between the core and shell materials, R_v should be used to fairly assess the encapsulation effect of the micro/nanocapsules.

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Nanoparticle content, interface interaction, and dispersion state are key factors affecting the enhanced λ of micro/nanocapsules with additives, whereas a continuous shell with a high λ is better. However, the additive amount of conductive particles is negatively correlated to the thermal storage capacities; this problem requires attention, including the deposition problem when used in the working fluid.

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The compactness of the shell, mesopores in the shell structure, and size of the micro/nanocapsule particles are important factors that affect the thermal reliability of micro/nanocapsules.

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Finally, the application prospects were presented for phase-change micro/nanocapsules with good thermal performance, which are crucial to the applications of solar systems, working fluids, and thermal management of electronics.