n-Octadecane/Fumed Silica Phase Change Composite as Building Envelope for High Energy Efficiency

A novel n-octadecane/fumed silica phase change composite has been prepared as a building envelope with a high content of phase change material and improved energy efficiency. With a high porosity (88 vol%), the fumed silica provided sufficient space to impregnate a high quantity of n-octadecane (70 wt%). The composite exhibited high latent heat storage capacity (155.8 J/g), high crystallization fraction (96.5%), and a melting temperature of 26.76 °C close to that of pure n-octadecane. A 200 accelerated thermal cycle test confirmed good thermal reliability and chemical stability of the phase change composite. The thermal conductivity of n-octadecane was reduced by 34% after impregnation in fumed silica. A phase change composite panel was fabricated and compared to a commercial polystyrene foam panel. When used as the roof of a test room, the phase change composite panel more efficiently retarded heat transfer from a halogen lamp to the room and delayed the increase in the indoor temperature than that by the polystyrene panel. The indoor temperatures of the room with the phase change composite panel roof were 19.8 and 22.9 °C, while those with the polystyrene panel roof were 29.9 and 31.9 °C at 2200 and 9000 s after lamp illumination.


Introduction
With improved living standards, the rising population, as well as worldwide urbanization, the demand for energy in buildings has continuously increased. Globally, buildings account for up to 40% of total energy consumption, which results in problems such as environmental pollution and energy shortages [1,2]. Energy conservation in buildings, therefore, has become an important issue in energy research. Using phase-change materials (PCMs) as building envelopes is one of the most effective methods for energy conservation and human thermal comfort [3]. The first PCM applications in buildings were for thermal energy storage, and the materials were further developed as components of composites with construction materials such as concrete and cement [1,4]. PCMs can store large quantities of thermal energy in the form of latent heat during their phase transition from solid to liquid, and the temperature of PCMs is constant during this process. Owing to these characteristics, PCMs have also been studied in the form of wallboards [5] or panels [6] to increase the thermal mass of lightweight buildings because they tend to have high temperature variations. During the daytime, the PCMs melt and absorb heat; at night, they solidify and release the absorbed heat [7]. PCM building envelopes can impede the heat of the PCC were thoroughly characterized. A large amount of PCM (70 wt%) was impregnated in the FS without leakage, and with a high heat storage capacity (155.8 J/g), and reduced thermal conductivity. In addition, the thermal performance of the PCC was evaluated and compared to that of a commercialized polystyrene (PS) foam panel, when constructed as the roof of a test room. The PCC could reduce the indoor peak temperature and delay the indoor temperature rise, compared to the PS material.

Preparation of n-Octadecane/Fumed Silica (FS) Phase-Change Composites (PCC)
The FS was heated at 200 • C for 24 h to remove physically adsorbed water. Subsequently, n-octadecane/FS composites were prepared by an evaporative solution impregnation method [28] with various contents of n-octadecane (60−75 wt%). Typically, a predetermined amount of n-octadecane was dissolved in hexane, and a pre-calculated amount of FS was introduced into the above solution. The mixture was magnetically stirred for 5 h at 25 • C and subsequently heated to 60 • C until the solvent evaporated. The as-obtained white powder was dried in an oven at 60 • C overnight for complete solvent evaporation.

Characterization Methods
Scanning electron microscopy (SEM) was performed using a JSM 6701 instrument (JEOL, Tokyo, Japan) with a beam energy of 5 kV. The nitrogen adsorption-desorption isotherms were recorded using a BELSORP-Max instrument (MicrotracBel, Osaka, Japan) at the temperature of liquid nitrogen (−196 • C). The surface area was obtained using the Brunauer-Emmett-Teller theory. The pore size distribution was calculated from the adsorption branch using the non-local density functional theory. The porosities of the materials were analyzed by mercury intrusion porosimetry, using a Microactive Autopore V 9600 (Micromeritics, Norcross, GA, US). The measurements were conducted at 20 • C in the pressure range of 0.1 to 61,000 psi.
The chemical compositions of the materials were evaluated using a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer (Thermo Scientific, Waltham, MA, US) in the transmittance mode.
The crystallinity of the materials was characterized by X-ray diffraction (XRD) using a powder X-ray diffractometer (Rigaku Miniflex, Tokyo, Japan) with Cu-Kα radiation. The testing current and voltage were 15 mA and 40 kV, respectively. The scanning rate was 5 • /min in the 2θ range 5-50 • .
The phase change properties of the materials were investigated using a DSC 4000 differential scanning calorimeter (Perkin Elmer, Waltham, MA, US). The measurements were performed at a temperature ramp rate of 5 • C/min with N 2 purge at 20 mL/min and a temperature range of −10-45 • C. The phase change temperatures were regarded as the onset temperatures. Each measurement was performed for two cycles, and the first cycle was excluded to eliminate the thermal history.
The thermal stabilities of the materials were tested by thermogravimetric (TG) analysis using a 4000 thermogravimetric analyzer (Perkin Elmer, Waltham, MA, US). The temperature range was 30-500 • C, accompanied by a ramp rate of 10 • C/min and N 2 purge of 20 mL/min. The decomposition temperatures were calculated at the onset temperatures.
The thermal reliability of the composite was examined by a 200 thermal cycle test (1 • C ↔ 60 • C) under ambient atmosphere, using two temperature-controlled water baths. The temperature of the cool bath (1 • C) was controlled by ice, while that of the hot bath (60 • C) was controlled by a hot plate. The composite (1 g) was placed in a glass vial and then shifted between the two baths, where it was kept for 5 min.
Thermal conductivities were determined by the transient plane source method at room temperature, using a thermal constant analyzer (TPS 3500, Hot-Disk AB, Goteborg, Sweden). Subsequently, 11.6 g of PCCs was compressed into two round blocks with a diameter of 30 mm and a depth of 10 mm with a homemade mold. For comparison, the pure PCM was melted and poured into the mold to obtain two round blocks of the same size as the PCC blocks. The thermal conductivity was measured for four cycles to obtain an average value.

Evaluation of the Thermal Performance of n-Octadecane/FS PCC
To evaluate the smoothing performance of the temperature fluctuation in a lab scale, the thermal performances of PCC panels in previous reports were tested in small test rooms. A polystyrene panel or a gypsum panel, conventionally employed as building envelopes, were used in small room tests [9,29]. We hereby follow the methodology to evaluate the thermal performance of our n-octadecane/FS PCC.

Characterization of n-Octadecane/FS PCCs
The SEM image of FS (Aerosil 200, Evonik Korea, Seoul, Korea) in Figure 2a show that the nanoparticles formed an interconnected porous network with a high proportio of macropores, allowing impregnation of the PCM. The N2 sorption isotherm of the F was measured, as shown in Figure 3a. A type II isotherm was obtained, but the m cropore volume of the FS was not well developed. The interconnected silica nanoparticl formed nanosized pores in the range of 50-150 nm [23]. Due to the limited pore size di

Thermal Performance Evaluation
The experimental setup for examining temperature variations in a test room is shown in Figure 1c. The test room (200 × 200 × 200 mm) was built with five PS panels. One panel (300 × 300 × 50 mm) was employed as the base, and four panels (250 × 220 × 50 mm) were used as the walls. The as-fabricated PCC and PS panels were utilized as roofs in the room. A halogen tungsten lamp (Philip PAR38, Amsterdam, The Netherlands) was employed as the solar simulator and hung 130 mm above the roof of the test room. The ends of a T-type thermocouple connected to a data acquisition unit (Yokogawa, MV200, Tokyo, Japan) were positioned at the inner surface of the test panel and the center of the room. The test began as the lamp was switched on, and the temperature fluctuations at the two positions were recorded. After 12,600 s, the lamp was switched off and the temperature variations during cooling were measured.

Characterization of n-Octadecane/FS PCCs
The SEM image of FS (Aerosil 200, Evonik Korea, Seoul, Korea) in Figure 2a shows that the nanoparticles formed an interconnected porous network with a high proportion of macropores, allowing impregnation of the PCM. The N 2 sorption isotherm of the FS was measured, as shown in Figure 3a. A type II isotherm was obtained, but the micropore volume of the FS was not well developed. The interconnected silica nanoparticles formed nanosized pores in the range of 50-150 nm [23]. Due to the limited pore size distribution obtained from the isotherm, mercury intrusion porosimetry was performed ( Figure S1). The pore size distributions show that the FS possesses a wide range of mesoand macroporosities. The porosity obtained from mercury intrusion was 88 vol%. The pore volumes and surface areas were 1.0 cm 3 /g and 185 m 2 /g, respectively, as calculated from the adsorption branch of the N 2 sorption isotherm and 17 cm 3 /g and 205 m 2 /g, respectively, as calculated from mercury intrusion porosimetry.
The SEM images of the PCCs with various n-octadecane mass fractions are shown in     The chemical compatibility of the composites was evaluated by FT-IR spectroscopy, as shown in Figure 4. In the IR spectrum of FS, the broad band centered at 3425 cm −1 was due to the overlapping stretching vibration of surface silanol groups (−Si−OH) and surface-adsorbed water molecules [30]. The adsorbed water was observed with a bending vibration at 1627 cm −1 . Meanwhile, the bands at 1095, 802, and 462 cm −1 were assigned to the Si-O-Si asymmetric stretching vibration, symmetric stretching vibration, and bending mode, respectively. In the case of n-octadecane, the bands at 2915 and 2854 cm −1 were attributed to the stretching vibration of −C−H bonds, while those at 1465 and 1377 cm −1 were assigned to the −C−H bending vibration. The peak at 725 cm −1 belonged to the in-plane rocking mode of the -CH2− groups. For the 70 wt% PCC, the characteristic peaks of n-octadecane overlapped with those of the SiO2 matrix. Furthermore, no additional peaks were observed. These results indicated that the composite components were physically compounded with no chemical reactions, and thus possessed good chemical compatibility. The crystallization behaviors of the pristine materials and their composites were characterized by XRD, as shown in Figure 4b. The FS showed a broad peak in the 2θ range of 15-30°, typical of an amorphous SiO2 structure. The pure n-octadecane was characterized by a mixture of two crystal phases [31]: the peaks at 2θ = 7.5, 11.5, 15.  To evaluate the anti-leakage ability of the prepared composites, a leakage test was conducted. The pure n-octadecane and the composites were placed on filter papers and kept in an oven at 50 • C (~20 • C higher than the melting point of n-octadecane) for 60 min. The results are shown in Figure S2. While n-octadecane was completely liquidized, the composites with 60 wt% and 70 wt% PCM were maintained without leakage owing to the capillary and surface tension forces in the FS. However, the sample with 75 wt% of PCM showed slight leakage; thus, it could not be used as a PCC. TG analysis was conducted to further confirm the anti-leakage ability of the 70 wt% composite, as shown in Figure S3. No significant difference in the weight losses could be observed between the samples before and after the leakage test, demonstrating that the PCCs were capable of maintaining good shape stability with up to 70 wt% of n-octadecane. A higher content of the PCM would benefit the thermal storage capacity of the composite. Therefore, 70 wt% PCM was selected as the optimal mass fraction for the PCC.
The chemical compatibility of the composites was evaluated by FT-IR spectroscopy, as shown in Figure 4. In the IR spectrum of FS, the broad band centered at 3425 cm −1 was due to the overlapping stretching vibration of surface silanol groups (−Si−OH) and surface-adsorbed water molecules [30]. The adsorbed water was observed with a bending vibration at 1627 cm −1 . Meanwhile, the bands at 1095, 802, and 462 cm −1 were assigned to the Si-O-Si asymmetric stretching vibration, symmetric stretching vibration, and bending mode, respectively. In the case of n-octadecane, the bands at 2915 and 2854 cm −1 were attributed to the stretching vibration of −C−H bonds, while those at 1465 and 1377 cm −1 were assigned to the −C−H bending vibration. The peak at 725 cm −1 belonged to the inplane rocking mode of the -CH 2 − groups. For the 70 wt% PCC, the characteristic peaks of n-octadecane overlapped with those of the SiO 2 matrix. Furthermore, no additional peaks were observed. These results indicated that the composite components were physically compounded with no chemical reactions, and thus possessed good chemical compatibility.
The crystallization behaviors of the pristine materials and their composites were characterized by XRD, as shown in Figure 4b. The FS showed a broad peak in the 2θ range of 15-30 • , typical of an amorphous SiO 2 structure. The pure n-octadecane was characterized by a mixture of two crystal phases [31]: the peaks at 2θ = 7.5, 11.5, 15. attributed to the stretching vibration of −C−H bonds, while those at 1465 and 1377 cm −1 were assigned to the −C−H bending vibration. The peak at 725 cm −1 belonged to the in-plane rocking mode of the -CH2− groups. For the 70 wt% PCC, the characteristic peaks of n-octadecane overlapped with those of the SiO2 matrix. Furthermore, no additional peaks were observed. These results indicated that the composite components were physically compounded with no chemical reactions, and thus possessed good chemical compatibility. The crystallization behaviors of the pristine materials and their composites were characterized by XRD, as shown in Figure 4b. The FS showed a broad peak in the 2θ range of 15-30°, typical of an amorphous SiO2 structure. The pure n-octadecane was characterized by a mixture of two crystal phases [31]: the peaks at 2θ = 7.5, 11.5, 15.

Phase Change Properties of n-Octadecane/FS PCC
The phase change properties of pure n-octadecane and PCC were investigated by DSC, as shown in Figure 5. The melting/solidifying temperatures (T M /T S ) and the melting/solidifying phase change enthalpies (∆H M /∆H S ) obtained from the DSC curves are summarized in Table 1. The pure n-octadecane showed a single endothermic peak (28.81 ± 0.12 • C) during melting and exothermic peak (28.21 ± 0.14 • C) during solidification. Both the endothermic and exothermic peaks were asymmetric, suggesting the presence of more than one crystalline phase. Indeed, the n-octadecane crystal consisted of two crystalline phases, αand β-, resulting from heterogeneous and homogeneous nucleations, respectively, and the β-crystal phase was dominant [31]. Under certain conditions, the two crystalline phases showed very close crystallizing points, thus overlapping with each other. For the 70 wt% PCC, T M and Ts were 26.76 ± 0.10 • C and 26.69 ± 0.37 • C, respectively, slightly lower than those of pure n-octadecane. The decreased phase change temperature of n-octadecane in the FS was a typical phenomenon due to confinement effects and has been observed in other PCCs [32]. During melting, the PCC showed one peak (Figure 5a), and its solidifying process was quite different as compared to the pristine PCM (Figure 5b). The αand β-crystalline phases were separated due to a shift in the β-phase peak to a lower temperature than that of pure n-octadecane (see details in Table 1). As n-octadecane was confined in the porous network of the FS, the homogeneous nucleation of the β-phase was limited, causing a shift in the crystallization temperature. Simultaneously, the α-crystal peak exhibited a higher intensity than the β-crystal peak, revealing that heterogeneous nucleation was dominant in the PCC. showed mixed patterns of PCM and SiO2, further confirming that the composite components were physically combined without chemical reactions. The crystallinity of the PCC is discussed in Section 3.2.

Phase Change Properties of n-Octadecane/FS PCC
The phase change properties of pure n-octadecane and PCC were investigated by DSC, as shown in Figure 5. The melting/solidifying temperatures (TM/TS) and the melting/solidifying phase change enthalpies (ΔHM/ΔHS) obtained from the DSC curves are summarized in Table 1. The pure n-octadecane showed a single endothermic peak (28.81 ± 0.12 °C) during melting and exothermic peak (28.21 ± 0.14 °C) during solidification. Both the endothermic and exothermic peaks were asymmetric, suggesting the presence of more than one crystalline phase. Indeed, the n-octadecane crystal consisted of two crystalline phases, αand β-, resulting from heterogeneous and homogeneous nucleations, respectively, and the β-crystal phase was dominant [31]. Under certain conditions, the two crystalline phases showed very close crystallizing points, thus overlapping with each other. For the 70 wt% PCC, TM and Ts were 26.76 ± 0.10 °C and 26.69 ± 0.37 °C, respectively, slightly lower than those of pure n-octadecane. The decreased phase change temperature of n-octadecane in the FS was a typical phenomenon due to confinement effects and has been observed in other PCCs [32]. During melting, the PCC showed one peak (Figure 5a), and its solidifying process was quite different as compared to the pristine PCM (Figure 5b). The αand β-crystalline phases were separated due to a shift in the β-phase peak to a lower temperature than that of pure n-octadecane (see details in Table  1). As n-octadecane was confined in the porous network of the FS, the homogeneous nucleation of the β-phase was limited, causing a shift in the crystallization temperature. Simultaneously, the α-crystal peak exhibited a higher intensity than the β-crystal peak, revealing that heterogeneous nucleation was dominant in the PCC.

Sample T M ( • C) ∆H M (J/g) T S,α ( • C) Ts, β ( • C) ∆H S (J/g) F (%)
n-octadecane 28 As for the phase change enthalpies, pure n-octadecane exhibited ∆H M and ∆H S of 230.5 ± 6.0 J/g and 229.4 ± 3.7 J/g, respectively. They are in good agreement with data of other literature in which the ∆H M and ∆H S of pure n-octadecane were between 214.6 and 236.6 J/g [14,33]. The 70 wt% PCC exhibited ∆H M and ∆H S of 155.8 ± 2.5 J/g and 154.7 ± 1.2 J/g, respectively. For a PCC, the crystallinity could be evaluated by calculating the crystallization fraction (F), as described in the following equation (Equation (1)) [34]; where ∆H M,PCC and ∆H S,PCC are the melting and solidifying enthalpies of the PCC, respectively, and ∆H M,PCM and ∆H S,PCM are the melting and solidifying enthalpies of the pure PCM, respectively. x is the relative mass proportion of the PCM in the composite. The crystallized fraction of the 70 wt% n-octadecane/FS PCC was calculated to be 96.5 ± 3.4% according to Table 1, suggesting that confinement in the porous framework of the FS did not affect the crystallinity of n-octadecane. As a result, the PCC could exhibit a high crystallized fraction, resulting in a high latent heat storage capacity (155.8 ± 2.5 J/g). The PCM content and thermal energy storage capacity of the n-octadecane/FS PCC were compared to those of previously reported PCCs that had melting points suitable for building applications, as shown in Table 2. While the n-octadecane/FS PCC in this work had a PCM content of 70 wt%, most of the reported PCCs exhibited a lower PCM content (≤60 wt%) owing to the low impregnation capacity of their porous supports. As shown in Table 2, exfoliated graphite exhibited 80 wt% loading of fatty acid ester PCCs. In comparison, the n-octadecane/FS composite in this work possessed excellent thermal energy storage capacity.

Thermal Stability of n-Octadecane/FS PCC
Thermal stability is an important property for building applications of PCCs. Herein, the thermal stability was characterized by TG analysis, as shown in Figure 6. The pure PCM presented onset and endset decomposition temperatures of 168 and 230 • C due to the volatilization of n-octadecane, while the as-prepared PCC exhibited values of 192 and 240 • C. The PCC showed improved thermal stability over the pure PCM; this was ascribed to the interfacial interactions (capillary and surface tension forces) between the FS Nanomaterials 2021, 11, 566 9 of 14 and PCM, which could inhibit the spilling of the PCM out of the FS porous network, thus improving the thermal stability of the PCC [38,39]. The onset decomposition temperature of the PCC (192 • C) was higher than its targeted working temperature (ambient temperature). Therefore, the n-octadecane/FS composite possesses high thermal stability for building envelope applications.

Thermal Reliability and Chemical Stability of n-Octadecane/FS PCC
To determine the long-term working ability of a PCC, its phase change temperature and enthalpy after 200 thermal cycles were recorded. In previous works, the thermal reliability of the PCC was tested using DSC under an inert atmosphere [37]. In this work, to evaluate the thermal reliability of the PCC under practical conditions, a 200 cycle test was conducted under an ambient atmosphere using a temperature-controlled water bath (1 °C ↔ 60 °C). The DSC curves and the detailed phase change properties of the samples after the test are shown in Figure 7a and Table 1, respectively. Compared to the as-prepared composite, the composite subjected to multiple thermal cycles exhibited a comparable DSC curve and phase change temperature. In addition, the phase-change enthalpies were almost unchanged (Table 1). To confirm the chemical stability of the PCC after the thermal cycle test, FT-IR spectra were measured (Figure 7b). No remarkable difference in the shape, intensity, and wavenumber of the absorbed bands could be detected compared to those of the original one, indicating that the PCC had good chemical stability after multiple thermal cycles. All the above results demonstrate that the PCC possesses excellent thermal reliability and chemical stability for long-term utilization.

Thermal Reliability and Chemical Stability of n-Octadecane/FS PCC
To determine the long-term working ability of a PCC, its phase change temperature and enthalpy after 200 thermal cycles were recorded. In previous works, the thermal reliability of the PCC was tested using DSC under an inert atmosphere [37]. In this work, to evaluate the thermal reliability of the PCC under practical conditions, a 200 cycle test was conducted under an ambient atmosphere using a temperature-controlled water bath (1 • C ↔ 60 • C). The DSC curves and the detailed phase change properties of the samples after the test are shown in Figure 7a and Table 1, respectively. Compared to the as-prepared composite, the composite subjected to multiple thermal cycles exhibited a comparable DSC curve and phase change temperature. In addition, the phase-change enthalpies were almost unchanged (Table 1). To confirm the chemical stability of the PCC after the thermal cycle test, FT-IR spectra were measured (Figure 7b). No remarkable difference in the shape, intensity, and wavenumber of the absorbed bands could be detected compared to those of the original one, indicating that the PCC had good chemical stability after multiple thermal cycles. All the above results demonstrate that the PCC possesses excellent thermal reliability and chemical stability for long-term utilization.

Thermal Reliability and Chemical Stability of n-Octadecane/FS PCC
To determine the long-term working ability of a PCC, its phase change temperature and enthalpy after 200 thermal cycles were recorded. In previous works, the thermal reliability of the PCC was tested using DSC under an inert atmosphere [37]. In this work, to evaluate the thermal reliability of the PCC under practical conditions, a 200 cycle test was conducted under an ambient atmosphere using a temperature-controlled water bath (1 °C ↔ 60 °C). The DSC curves and the detailed phase change properties of the samples after the test are shown in Figure 7a and Table 1, respectively. Compared to the as-prepared composite, the composite subjected to multiple thermal cycles exhibited a comparable DSC curve and phase change temperature. In addition, the phase-change enthalpies were almost unchanged (Table 1). To confirm the chemical stability of the PCC after the thermal cycle test, FT-IR spectra were measured (Figure 7b). No remarkable difference in the shape, intensity, and wavenumber of the absorbed bands could be detected compared to those of the original one, indicating that the PCC had good chemical stability after multiple thermal cycles. All the above results demonstrate that the PCC possesses excellent thermal reliability and chemical stability for long-term utilization.

Thermal Conductivity of n-Octadecane/FS PCC
Thermal conductivity is one of the crucial parameters affecting the thermal transfer rate of building envelope materials. In this work, the thermal conductivities were measured using the transient plane source method at room temperature. As shown in Figure 8, the thermal conductivity of n-octadecane was 0.281 ± 0.025 W/m K, which is consistent with previous reports showing that the thermal conductivity of pure n-octadecane is between 0.12 and 0.4 W/m K [40]. Meanwhile, the thermal conductivity of PCC was 0.184 ± 0.010 W/m K. Apparently, the PCC showed a thermal conductivity approximately 34% lower than that of pure n-octadecane. The low thermal conductivity (~0.05 W/m K) of the FS is attributed to the absence of a convection factor because the pore size is of the same order as the mean free path (70 nm) of free air at atmospheric pressure [23]. After the impregnation of n-octadecane, the mesopores of the FS were filled with the PCM (Figure 3b), but macropores larger than 300 nm were partially retained ( Figure S1 and Figure 2c). Assuming that 100% of the FS pores are filled with n-octadecane, the theoretical content of the PCM would be 95 wt%, calculated with 88 vol% porosity of the FS. The thermal conductivity of the composite was intermediate to those of the two components. The lower conductivity of the PCC than that of the pristine PCM is a positive parameter, which retards the heat transfer through the PCC.

Thermal Conductivity of n-Octadecane/FS PCC
Thermal conductivity is one of the crucial parameters affecting the thermal transfer rate of building envelope materials. In this work, the thermal conductivities were measured using the transient plane source method at room temperature. As shown in Figure  8, the thermal conductivity of n-octadecane was 0.281 ± 0.025 W/m K, which is consistent with previous reports showing that the thermal conductivity of pure n-octadecane is between 0.12 and 0.4 W/m K [40]. Meanwhile, the thermal conductivity of PCC was 0.184 ± 0.010 W/m K. Apparently, the PCC showed a thermal conductivity approximately 34% lower than that of pure n-octadecane. The low thermal conductivity (~0.05 W/m K) of the FS is attributed to the absence of a convection factor because the pore size is of the same order as the mean free path (70 nm) of free air at atmospheric pressure [23]. After the impregnation of n-octadecane, the mesopores of the FS were filled with the PCM (Figure  3b), but macropores larger than 300 nm were partially retained ( Figure S1 and Figure 2c). Assuming that 100% of the FS pores are filled with n-octadecane, the theoretical content of the PCM would be 95 wt%, calculated with 88 vol% porosity of the FS. The thermal conductivity of the composite was intermediate to those of the two components. The lower conductivity of the PCC than that of the pristine PCM is a positive parameter, which retards the heat transfer through the PCC.

Properties of PCC Panel
Some thermophysical properties of the PCC and PS panels were measured, as shown in Table 3. The PS panel possessed zero latent heat at ambient temperature, a very low density of 20 ± 0.5 kg/m 3 , and thermal conductivity of 0.045 ± 0.007 W/m K. In comparison with the PS panel, the PCC panel exhibited a latent heat of 155.8 ± 2.5 J/g, revealing its excellent ability to store thermal energy. The PCC panel had a density of 700 ± 18 kg/m 3 , which is 35-fold higher than that of the PS panel. Although the thermal conductivity of the PCC panel was 0.184 ± 0.010 W/m K, approximately 4 times higher than that of the PS panel, it was still at a lower level.  3.6. Thermal Performance of PCC Panel 3.6.1. Properties of PCC Panel Some thermophysical properties of the PCC and PS panels were measured, as shown in Table 3. The PS panel possessed zero latent heat at ambient temperature, a very low density of 20 ± 0.5 kg/m 3 , and thermal conductivity of 0.045 ± 0.007 W/m K. In comparison with the PS panel, the PCC panel exhibited a latent heat of 155.8 ± 2.5 J/g, revealing its excellent ability to store thermal energy. The PCC panel had a density of 700 ± 18 kg/m 3 , which is 35-fold higher than that of the PS panel. Although the thermal conductivity of the PCC panel was 0.184 ± 0.010 W/m K, approximately 4 times higher than that of the PS panel, it was still at a lower level. To investigate the thermal performance of the PCC panel in a test room in comparison to the commercialized PS panel, the two panels were utilized as the roofs of the test room. A halogen tungsten lamp was used as the solar simulator. During the tests, the temperature variations at the inner surface of the test panels and the center of the test room (indoor) were recorded (see Figure 1 for details). The results of these tests are shown in Figure 9. Based on the tangential method, the temperature rising curve at the inner surface of the PCC panel during illumination can be divided into three steps: 0-2200 s, 2200-9000 s, and 9000-12,600 s. The first step (0-2200 s) and the last step (9000-12,600 s) showed a temperature rise before and after the melting of the PCC, respectively, and were developed by the absorption of sensible heat. The middle step (2200-9000 s) presented the temperature rise during the melting of the PCC, developed by both sensible heat and latent heat absorptions. The slope of the middle step was relatively lower than those of the other two steps. A PCM generally has higher latent heat than the sensible heat. While the sensible heat directed the first and last steps, the latent heat governed the second step, thus leading to the slow temperature rise of the second step. As a result, the inner surface of the PCC panel maintained a low temperature (<27 • C) until 9000 s. In contrast, the temperature at the inner surface of the PS panel rapidly increased to 39.4 • C after 2000 s and reached a peak at approximately 42 • C after 4000 s.
ison to the commercialized PS panel, the two panels were utilized as the roofs of the test room. A halogen tungsten lamp was used as the solar simulator. During the tests, the temperature variations at the inner surface of the test panels and the center of the test room (indoor) were recorded (see Figure 1 for details). The results of these tests are shown in Figure 9. Based on the tangential method, the temperature rising curve at the inner surface of the PCC panel during illumination can be divided into three steps: 0-2200 s, 2200-9000 s, and 9000-12,600 s. The first step (0-2200 s) and the last step (9000-12,600 s) showed a temperature rise before and after the melting of the PCC, respectively, and were developed by the absorption of sensible heat. The middle step (2200-9000 s) presented the temperature rise during the melting of the PCC, developed by both sensible heat and latent heat absorptions. The slope of the middle step was relatively lower than those of the other two steps. A PCM generally has higher latent heat than the sensible heat. While the sensible heat directed the first and last steps, the latent heat governed the second step, thus leading to the slow temperature rise of the second step. As a result, the inner surface of the PCC panel maintained a low temperature (<27 °C) until 9000 s. In contrast, the temperature at the inner surface of the PS panel rapidly increased to 39.4 °C after 2000 s and reached a peak at approximately 42 °C after 4000 s.
The indoor temperature slightly increased from room temperature to 19.6 °C with the PCC panel roof, while it rose to 29.6 °C with the PS panel roof after 2000 s of illumination. The indoor temperature peaked at approximately 31.5 °C after 4000 s with the PS panel roof, and at ~25 °C after 12,600 s with the PCC panel roof. The PCC panel played a role in reducing the indoor peak temperature and delaying the indoor temperature rise. The PCC panel could decrease the temperature swings and maintain the room temperature closer to a level suitable for human comfort than the PS panel. Although the PCC panel had a higher thermal conductivity than the PS panel, it effectively retarded heat transfer from the illumination of the halogen lamp to the room and maintained the room temperature within the human comfort range. The indoor temperature slightly increased from room temperature to 19.6 • C with the PCC panel roof, while it rose to 29.6 • C with the PS panel roof after 2000 s of illumination. The indoor temperature peaked at approximately 31.5 • C after 4000 s with the PS panel roof, and at~25 • C after 12,600 s with the PCC panel roof. The PCC panel played a role in reducing the indoor peak temperature and delaying the indoor temperature rise. The PCC panel could decrease the temperature swings and maintain the room temperature closer to a level suitable for human comfort than the PS panel. Although the PCC panel had a higher thermal conductivity than the PS panel, it effectively retarded heat transfer from the illumination of the halogen lamp to the room and maintained the room temperature within the human comfort range.

Conclusions
A novel phase change composite, consisting n-octadecane as a PCM and low-cost fumed silica as a supporting material, was prepared by the evaporative solution impregnation method. FS exhibited an interconnected porous structure with high porosity (88 vol%), supplying sufficient space to impregnate n-octadecane, which was firmly confined in pores by capillary and surface tension forces. The FS had a high adsorption capacity for n-octadecane (70 wt%), and the latent heat storage capacity of the PCC was up to 155.8 J/g, superior to previously reported PCCs. The melting temperature of the PCC was 26.76 • C, close to those of the pure n-octadecane (28.21 • C). The thermal stability of n-octadecane improved after its incorporation into FS. Good thermal reliability and chemical stability of the PCC were also proved by a 200 accelerated thermal cycle test. The thermal conductivity of the PCC was only 0.184 W/m K which was reduced by 34% compared to that of pure n-octadecane. The PCC was fabricated into a panel to evaluate its thermal performance in a test room and compared to a commercialized PS panel. The two panels were employed as roofs in a test room. It was found that the PCC panel could reduce the indoor peak temperature and delay the heat flux entering the test room, compared to the PS panel. With the low-cost and aforementioned excellent thermal performance, the n-octadecane/FS PCC has potential applications as a building envelope for energy conservation. For future work, the PCC panel should be applied in an actual building to evaluate the practical thermal performance, heat storage stability, and mechanical durability.
Supplementary Materials: The following are available online at https://www.mdpi.com/2079-499 1/11/3/566/s1, Figure S1: Mercury porosimetry intrusion curves of the pristine FS and the 70 wt% n-octadecane/FS PCC, Figure S2: Photographs of pure n-octadecane and PCCs with various noctadecane mass fractions from the leakage test, Figure S3: Thermogravimetric analysis (TGA) curves of the 70 wt% n-octadecane/FS PCC before and after the leakage test. Relative mass proportion of the PCM in the composite SEM Scanning electron microscopy FT-IR Fourier transform infrared spectroscopy DSC Differential scanning calorimetry TGA Thermogravimetric analysis