Recent Advances in Dielectric and Ferroelectric Behavior of Ceramic Nanocomposites: Structure Property Relationships and Processing Strategies

Abstract

In the race toward next-generation electronics and energy systems, ceramic nanocomposites have taken center stage due to their remarkable dielectric and ferroelectric functionalities. By pushing the boundaries of nanoscale engineering, recent studies have shown how microstructural control and interfacial design can unlock unprecedented levels of polarization, permittivity, and frequency stability. This review presents a critical and up-to-date synthesis of the last decade’s progress in ceramic-based nanocomposites, with a special focus on the structure property processing nexus. Diverse processing techniques ranging from conventional sintering to advanced spark plasma sintering and scalable wet-chemical methods are analyzed for their influence on phase purity, grain boundary behavior, and interfacial polarization. The review also explores breakthroughs in lead-free and eco-friendly systems, flexible ferroelectric nanocomposites, and high-k dielectrics suitable for miniaturized devices. By identifying both the scientific opportunities and persistent challenges in this rapidly evolving field, this work aims to guide future innovations in material design, device integration, and sustainable performance.

1. Introduction

Ceramic nanocomposites have become pivotal in advanced materials, offering multifunctionality across electronics, energy storage, sensing, and environmental applications [1]. Early breakthroughs, such as low-temperature sol–gel and nonhydrolytic routes for aluminum titanate fibers, highlighted the importance of interface-controlled dielectric behavior [2]. As wireless technologies advanced, dielectric composites were optimized for wearable devices. Research on low-SAR, high-efficiency tri-band watch antennas [3] was complemented by multi-antenna 5G MIMO arrays [4] and base-station radome composites [5,6]. These developments underscore the demand for permittivity-engineered materials in communication systems [7]. Driven by sustainable energy goals, catalytic nanocomposites with nanoconfined active sites within carbon matrices have delivered enhanced conversion performance [8]. Techniques like dendritic microstructure control in Ti alloys [9] and advanced solvent engineering for CO₂ capture [10] highlight the intersection of environmental strategy and material design [11,12]. Biomaterials also inform composite design: proteomic diagnostics [13,14], bioinformatics-driven saliva screening [15,16], and machine-learning enhanced surgical platforms [17] all reflect multifunctional interface principles. Notably, research into oral/gastrointestinal biomarkers [18,19] and neuropeptide modulation [20,21,22] provides tangible parallels in interfacial property optimization. Traditional medicine-inspired formulations from herbal tinctures [23,24] and brain-activated compounds [25,26,27] to cerebral neuronal studies [28,29,30] reinforce the role of structural tailoring for targeted response. Meanwhile, fundamental studies in deposition chemistry, such as sodium-source impacts on β″-Al₂O₃ channels [31], provide insight into ion-transport-relevant processing strategies. Concurrently, work on magnetic perovskites [32], Alzheimer’s model neural regulation [33,34,35], and bioactive pathway mapping [36,37,38] underscores the consistent theme: structure–function control via processing. Additionally, phase-aware photon-laser network signal recovery using HCMPE-Net [39,40] highlights data-driven diagnostic parallels.

Environmental governance and material sustainability are also guiding research trajectories, as seen in studies on green industrial optimization [41], forest-carbon sequestration models [42], and macro-level carbon-neutral frameworks [43,44,45]. Within ceramics, these same sustainability drives lead to green synthesis, energy-efficient processing, and lead-free materials. Despite this, ceramics still face challenges under dynamic and harsh conditions such as corrosion in structural joints [46], nighttime pedestrian distraction in lighting systems [47], and multi-phase mechanical fatigue in grinding systems [48,49]. These performance demands echo the environment and stress-resilience requirements of dielectric nanocomposites. Advances in diagnostic and holographic imaging [50,51,52], crowd counting analytics [53,54], and fractional Fourier-based systems [55] reflect the growing overlap between electrical, structural, and computational disciplines. These overlaps inform modern ceramic nanocomposite design approaches that blend structural tuning, interface control, and dielectric functionality. The influence of nanostructuring on functional ceramics is also evident in optical and photoluminescent applications. Researchers have investigated multicomponent rare-earth phosphors with tunable emissions, such as Ba₉Lu₂Si₆O₂₄ Eu²⁺, Mg²⁺ [56], and lanthanide-substituted CaZnOS systems [57], to improve lighting technologies and wavelength conversion efficiency. Thermal and morphological control in AlN-based composites [58] and Al₂O₃/Ti₃AlC₂ microwave absorbers [59] has further demonstrated how microstructural refinement affects dielectric behavior under extreme conditions.

These insights parallel the development of Al₂O₃@YAG:Ce phosphors with high luminous efficiency [60], as well as advanced LED packages using rare-earth-doped host lattices for stability and brightness tuning [61]. Such studies highlight the importance of phase compatibility and dopant integration, themes also vital in ferroelectric nanocomposite design. Broader societal and environmental motivations are pushing innovation toward carbon-neutral, circular economy materials. Strategies in green governance, such as forest carbon stock optimization [62], urban sustainability frameworks [63], and hybrid low-carbon development models [64], are influencing materials design philosophies. In parallel, dynamic systems modeling [65] and intelligent planning algorithms [66] are increasingly employed to guide the development of eco-efficient fabrication routes and device integration. Marine environment monitoring technologies [67], blockchain-supported agriculture traceability systems [68], and global emission target compliance frameworks [69] further demonstrate the push for multifunctional, intelligent, and sustainable systems. These intersect with recent efforts in ceramic nanocomposites to eliminate lead, reduce processing energy, and implement recyclable or bio-derived fillers. The global push for carbon neutrality by 2060 has intensified these efforts [70], demanding not only technical performance but full lifecycle awareness in material design.

Recent advances in electronic ceramics are propelled by developments in multiferroics, neuromorphic computing, and dielectric elastomers. Wang et al. [71] showcased a wireless multiferroic memristor with giant impedance modulation and synaptic behavior, illustrating magnetic electric coupling at the nanoscale. Xiao et al. [72] enabled low-temperature MgO ceramic sintering using MgF₂ and Al₂O₃, advancing cost-efficient processing. Electric-field control of interfacial effects, such as the Dzyaloshinskii–Moriya interaction in Pt/Co/Pt, allows tunable magnetic anisotropy for next-gen nanoelectronics [73]. In energy storage, Yu et al. [74] developed a prestretch-free dielectric elastomer achieving record energy and power densities through polarization and strain engineering. Wu et al. [75] introduced an Au/Ni-assisted etching method, enhancing structural control in silicon-based nanodevices. Ceramic nanocomposites also enable multifunctional memory. Yang et al. [76] applied relaxor antiferroelectric behavior for neuromorphic computing, while Li et al. [77] created a 2D heterointerface floating gate memory integrating sensing, memory, and computation. Zhao et al. [78] emphasized ceramic–metal interface design in laminated composites for improved dielectric and mechanical performance in armor systems.

The growing demand for high-performance electronic and energy-storage devices has accelerated research into materials that offer superior dielectric and ferroelectric properties. Among these, ceramic nanocomposites have emerged as a versatile and promising class of materials due to their ability to combine the intrinsic advantages of ceramics, such as high permittivity, thermal stability, and mechanical strength, with the tunability offered by nanoscale engineering [79]. By incorporating nanosized fillers or phases into ceramic matrices, researchers have achieved significant enhancements in electrical, structural, and functional performance, opening new possibilities for applications in capacitors, actuators, sensors, and flexible electronics [80]. In recent years, advances in nanotechnology and processing techniques have facilitated unprecedented control over grain size, morphology, phase distribution, and interfacial structures within ceramic composites. These microstructural modifications have been directly linked to improved dielectric constants, reduced losses, and enhanced ferroelectric polarization especially in systems designed with optimized interface chemistry and controlled defect structures. The interdependence of structure, processing, and properties has thus become a central theme in guiding material innovation in this field [81].

Moreover, the transition toward lead-free and environmentally benign materials has added a new dimension to ceramic nanocomposite research. The quest for sustainable alternatives to traditional lead-based ferroelectrics (such as PZT) has stimulated interest in systems based on barium titanate, potassium sodium niobate, and other perovskite or non-perovskite architectures [82]. In parallel, the integration of ceramic nanocomposites into flexible and wearable electronic platforms has further expanded their application landscape, demanding new approaches in materials processing and interface compatibility [83,84]. This review provides a comprehensive overview of the recent developments (approximately the last decade) in the dielectric and ferroelectric behavior of ceramic nanocomposites. Emphasis is placed on understanding how processing strategies, such as sol–gel synthesis, spark plasma sintering, and microwave-assisted methods, influence microstructure and functional properties. Special attention is also given to the role of interfacial polarization, phase synergy, and structural heterogeneity in enhancing overall performance. By mapping out current achievements and identifying persistent challenges, this review aims to support the rational design of next-generation ceramic nanocomposites for emerging energy and electronic applications.

2. Fundamental Concepts in Dielectric and Ferroelectric Ceramic Nanocomposites

2.1. Dielectric Behavior in Ceramic Nanocomposites

Dielectric ceramics exhibit the ability to store electrical energy through the alignment of internal dipoles when subjected to an external electric field. This behavior arises from several types of polarization mechanisms, namely electronic, ionic, dipolar, and interfacial (Maxwell–Wagner) polarization [85,86]. In conventional ceramics, the dielectric constant (ε_r) is primarily dictated by intrinsic lattice contributions and extrinsic effects such as grain boundaries and defects. However, in ceramic nanocomposites, the situation is markedly different. The high surface-to-volume ratio and the introduction of distinct phases dramatically enhance interfacial polarization effects. Interfaces between dissimilar phase such as ceramic–metal or ceramic polymer act as internal capacitive elements, where trapped charges or conductivity mismatches create localized electric fields [87]. These fields contribute significantly to the overall permittivity and frequency response. Interfacial effects dominate especially when conductive fillers (e.g., CNTs, graphene, or metal oxides) are embedded in insulating ceramic matrices. As a result, careful tuning of the volume fraction, dispersion state, and compatibility between the matrix and fillers become essential in optimizing the dielectric performance of nanocomposites for applications in energy storage, embedded capacitors, and radio frequency devices. In

Table 1. Summary of polarization mechanisms in dielectric nanocomposites, including their physical origin, active frequency range, and typical influence on dielectric behavior [88,89,90,91,92,93,94,95].

Mechanism	Origin	Frequency Range	Typical Role
Electronic	Displacement of electron clouds in atoms	Optical (1015 Hz)	Minor, fast response in high-frequency fields
Ionic	Relative motion of positive and negative ions	Infrared (1013 Hz)	Moderate contributor to polar ceramics
Dipolar (Orientational)	Rotation of permanent dipoles (e.g., in polymers)	Microwave (109–1011 Hz)	Strong effect on polymer-based composites
Interfacial (MWS)	Charge accumulation at interfaces in heterogeneous materials	Low frequency (<106 Hz)	Dominant in nanocomposites with mixed phases

, we discussed the different polarization phenomena in dielectric nanocomposites.

Sahu et al. [96] explored the dielectric properties of Ag and GO-based poly (vinyl alcohol) (PVA) nanocomposite films by analyzing how GO concentration, ionic liquid (IL) presence, and temperature variations influence dielectric permittivity (ε′), as illustrated in Figure 1a–c. In Figure 1a, the dielectric response of virgin PVA, Ag-PVA, and GO-Ag-PVA films demonstrated a marked enhancement in ε′ with increasing GO content, especially at lower frequencies, owing to stronger interfacial polarization and the Maxwell–Wagner–Sillars effect. This enhancement is attributed to the high surface area of GO, facilitating effective dipole interaction under an external electric field. Figure 1b highlighted the role of IL, where the dielectric permittivity was further amplified due to improved filler dispersion and the introduction of additional charge carriers by the polar nature of ILs. These effects were particularly pronounced at low frequencies, emphasizing IL’s ability to enhance interface polarization. Finally, Figure 1c showed that rising temperatures (40–150 °C) significantly increased ε′ due to enhanced dipole mobility and segmental motion of polymer chains, which reduced the resistive gaps between conducting channels. Collectively, these observations underline the synergistic impact of GO, IL, and temperature on the dielectric performance of GO-Ag-PVA nanocomposites, making them attractive candidates for high-performance dielectric and charge storage applications.

Tayari et al. [97] conducted a detailed investigation of the dielectric properties of Bi_0.75Ba_0.25(FeMn)_0.5O₃ ceramics synthesized via the sol–gel method, focusing on the temperature and frequency dependence of the real part of permittivity (ε′), as shown in Figure 2. Their findings revealed a clear increase in ε′ with rising temperature, especially at frequencies below 10³ Hz, a trend commonly associated with interfacial polarization and the accumulation of space charges at electrode interfaces. At lower frequencies, charge carriers have sufficient time to align with the applied electric field, leading to higher ε′ values due to polarization at grain boundaries. However, as the frequency increases, dipole reorientation becomes limited, resulting in a decline in ε′. This behavior of dispersion points to dielectric relaxation dominated by electronic transport processes rather than ionic contributions, which typically manifest at higher temperatures and lower frequencies. Figure 2 also illustrates how both ε′ and AC conductivity is governed by similar mechanisms such as hopping conduction and charge mobility. The observed dielectric enhancement at room temperature highlights the material’s suitability for capacitive and transducer applications, while its high permittivity and temperature-dependent response make it a promising candidate for energy storage and microwave device integration.

Faouzia Tayari et al. [98] synthesized Ag-doped Sr(NiNb)_0.5O₃ ceramic using a sol–gel method and explored its dielectric properties over a frequency range of 10³ to 10⁶ Hz and temperatures between 260 K and 340 K. The study demonstrated that the dielectric constant decreases with increasing frequency, a trend observed across all measured temperatures, which is attributed to reduced polarization efficiency at higher frequencies. This behavior is explained by Koop’s theory and the Maxwell–Wagner model, suggesting the material comprises conductive grains separated by insulating grain boundaries. At low frequencies, space charge polarization and grain boundary effects dominate, leading to higher ε′ and energy dissipation, while at high frequencies, the grain response prevails, reducing both permittivity and dielectric loss (tan δ). The dissipation factor also declines with increasing frequency, especially at elevated temperatures, signifying better energy retention. These characteristics point to the material’s suitability for dielectric applications requiring low power loss. The frequency-dependent variations of ε′ and tan δ across different temperatures are illustrated in Figure 3a,b, emphasizing the influence of grain and grain boundary dynamics on dielectric behavior.

Faouzia Tayari et al. [99] conducted a comprehensive investigation into the dielectric behavior of Fe-doped Ba_0.67Ni_0.33Mn_1−xFe_xO₃ ceramics (x = 0 and 0.2), synthesized via conventional solid-state sintering. Their study primarily focused on the influence of Fe substitution at the Mn-site on the dielectric response of the material over a range of frequencies and temperatures. As shown in Figure 4a, the dielectric constant of the doped sample (x = 0.2) exhibits a pronounced decrease with increasing frequency, a typical behavior attributed to the inability of electric dipoles to keep pace with the alternating electric field at higher frequencies. The permittivity increases with temperature, indicating enhanced dipolar and interfacial contributions. This trend is characteristic of perovskite ceramics, where space charge accumulation at grain boundaries or interfaces become significant at lower frequencies. To directly compare the effect of Fe doping, Figure 4b presents the frequency-dependent dielectric permittivity for both undoped and doped samples at room temperature.

A clear enhancement in ε′ is observed for the Fe-doped composition, suggesting that Fe³⁺ ions may facilitate increased polarization by influencing grain boundary conductivity and defect structures. This result highlights the beneficial role of Fe substitution in optimizing the dielectric storage capability of the material, which is a critical parameter for capacitive energy storage devices. In addition to permittivity, the dielectric loss factor was examined across different temperatures and frequencies, as shown in Figure 4c. At low frequencies, the dielectric loss is considerably high, attributed to space charge polarization and Maxwell–Wagner interfacial effects. These mechanisms are prominent when free charges accumulate at internal interfaces or at the electrode/sample interface, leading to greater energy dissipation. As frequency increases, the dielectric loss decreases due to the reduced response time of slower polarization mechanisms, such as dipolar and ionic contributions. The dielectric loss behavior indicates that while the material exhibits high permittivity, careful tuning is required to minimize energy dissipation for practical device applications. Overall, the results reported by Tayari et al. demonstrate that Fe doping enhances the dielectric constant while slightly increasing dielectric loss at lower frequencies. These findings position Ba_0.67Ni_0.33Mn_1−xFe_xO₃ ceramics as strong candidates for use in energy storage, dielectric resonators, and electronic devices where high permittivity and controlled loss are desired.

The morphology of nanofillers plays a critical role in determining dielectric behavior. Spherical nanoparticles (e.g., BaTiO₃ spheres) tend to provide isotropic permittivity enhancement but may aggregate if surface-modification is inadequate. Rod-like fillers (e.g., TiO₂ nanorods, ZnO nanowires) often create anisotropic conduction pathways that can raise dielectric constant but also risk increased loss tangent at high loadings. Layered fillers, such as graphene oxide or MXenes, provide large interfacial areas and can improve breakdown strength while enabling tunable permittivity. Comparative studies have shown that rod-like morphologies typically yield higher ε_r enhancement at low volume fractions, while layered structures excel in thermal stability due to their barrier effect on charge migration. These differences underscore the importance of tailoring filler morphology to the target application.

2.2. Ferroelectric Behavior and Domain Switching Dynamics

Ferroelectricity in ceramics arises from the presence of spontaneous polarization, which is switchable under an applied electric field. This phenomenon is a direct consequence of non-centrosymmetric crystal structures, such as the tetragonal phase in BaTiO₃ or rhombohedral structure in BiFeO₃. The defining feature of ferroelectric materials is their hysteresis loop in the polarization electric field curve [100,101]. In nanocomposites, these properties are highly sensitive to grain size, interface strain, and defect density. The incorporation of secondary phases or dopants can alter domain wall mobility, induce local lattice strain, and generate space charge fields, all of which influence ferroelectric switching [102,103]. For instance, adding rare-earth dopants like Gd³⁺ or La³⁺ can shift the Curie temperature (T_C), flatten hysteresis loops, and enhance energy storage efficiency. Recent studies have also explored ferroelectric–metal oxide heterostructures that exploit internal electric fields to control domain configurations at the nanoscale. The optimization of such ferroelectric behavior is particularly important for applications in non-volatile memory, sensors, actuators, and piezoelectric energy harvesting systems [104].

Denis Alikin et al. [105] investigated the competition between ferroelastic and ferroelectric domain wall dynamics in (111)-oriented rhombohedral PMN-PT single crystals using Piezoresponse Force Microscopy under varying relative humidity (RH) conditions. As illustrated in Figure 5, the switching behavior is significantly influenced by the ambient RH in the SPM chamber. At low RH (<4%), ferroelastic switching dominates. This is evidenced by the extended, irregular domain structures characteristic of ferroelastic walls, visible in the left panel of the figure. The corresponding electric field distribution shows a symmetric dipolar pattern, where local field gradients favor stress-driven switching mechanisms. In contrast, at high RH (70%), the behavior shifts to ferroelectric switching, as shown in the right panel. The domain shape is more circular and localized, with stronger field concentration near the probe tip. This indicates that water adsorption at higher humidity enhances surface screening and modulates the spatial electric field distribution, suppressing ferroelastic domain growth. The simulation of the out-of-plane electric field component explains this transition: at high RH, enhanced screening reduces the lateral component of the electric field, which diminishes ferroelastic activity and favors purely ferroelectric domain formation. This observed shift reflects the kinetic nature of domain pattern formation in multiaxial ferroelectrics, suggesting that environmental conditions like humidity can be used as a tunable parameter to engineer domain configurations in piezoelectric devices.

2.3. Interfacial Polarization and Maxwell–Wagner Effects

Interfacial polarization, also known as Maxwell–Wagner–Sillars (MWS) polarization, becomes a dominant mechanism in heterogeneous systems like nanocomposites. It occurs due to the accumulation of charge carriers at the interfaces between phases with contrasting permittivity or conductivity [106]. In ceramic nanocomposites, especially those composed of insulating grains and conductive or semi-conductive phases (e.g., graphene, CNTs, or metal oxides), MWS polarization leads to significant enhancements in dielectric constant, particularly at low frequencies [107]. The extent of this effect depends on the mismatch in conductivity, filler dispersion, and interface compatibility. For example, in BaTiO₃-CNT composites, CNTs form conductive networks that facilitate interfacial charge accumulation but may also increase leakage current if not properly controlled [108].

Table 2. Interfacial polarization in selected ceramic nanocomposites and its effect on performance [109,110,111,112,113].

Composite System	Filler Type	Interfacial Polarization Effect	Performance Impact
BaTiO3–CNT	1D conductive	High MWS polarization; may increase leakage at high loading	↑ εr ↓ breakdown strength if CNTs percolate
PVDF–BaTiO3	Ceramic–polymer	Strong dipolar + interfacial effects	↑ flexibility and dielectric constant
BaTiO3–Graphene Oxide	2D conductive	Enhanced interface area; tailored permittivity	↑ εr with controlled loss tangent
KNN–SiO2	Ceramic boundary	Space charge modification via SiO2 additives	↑ breakdown strength; ↓ dielectric loss

Note: ↑ indicates an increase; ↓ indicates a decrease.

shows the polarization in some nanocomposites. Therefore, the percolation threshold must be optimized to maximize interfacial polarization without compromising dielectric loss. Surface modifications of fillers (e.g., functionalization or coating with insulating layers) are often employed to control the interfacial chemistry and prevent undesired conductive pathways.

2.4. Grain Size Effects, Doping Strategies, and Sintering Behavior

The dielectric and ferroelectric performance of ceramic nanocomposites is strongly influenced by grain size, dopant chemistry, and sintering conditions [114,115]. Finer grains increase the density of grain boundaries, which enhances dielectric breakdown strength but may also increase dielectric loss due to space charge accumulation. Dopants, especially rare-earth or transition metal ions, are commonly used to tailor the lattice structure, reduce defect formation, and tune phase transitions. For instance, Gd³⁺ or La³⁺ doping in BaTiO₃ can shift the Curie point and improve thermal stability. Meanwhile, sintering techniques determine the final microstructure [116,117].

Table 3. Effect of Sintering Techniques on Ceramic Microstructure [119,120,121,122,123,124,125].

Sintering Technique	Temp (°C)	Grain Control	Key Benefits	Challenges
Conventional Sintering	1200–1400	Coarse grain growth	Simple, widely used	High porosity, energy intensive
Spark Plasma Sintering	800–1100	Excellent grain retention	Fast densification, fine grains	Expensive equipment
Cold Sintering	<300	Nanograin preservation	Eco-friendly, low cost, fast	Limited to specific material systems
Sol–gel + Annealing	600–900	Moderate grain control	Molecular-level mixing and uniform composition	Agglomeration if not well controlled

illustrates the effect of sintering techniques on ceramic microstructure. Traditional sintering often leads to grain growth and porosity, while advanced methods like spark plasma sintering or cold sintering allow low-temperature densification with grain size retention. These techniques are especially valuable for maintaining nanoscale features that are critical for high-performance dielectric behavior. Xiao et al. [118] investigated the dielectric performance of cold-sintered ZnO ceramic composites modified with polytetrafluoroethylene (PTFE) and metal oxides (CoO and Mn₂O₃). The study aimed to manipulate grain boundary structures through compositional engineering to enhance electrical properties. As shown in Figure 6a, the relative permittivity of the samples decreases as frequency increases across the range of 10 Hz to 10⁶ Hz. This behavior aligns with interfacial polarization mechanisms that diminish at higher frequencies. Due to the inherently low permittivity of PTFE (ε_r ≈ 2.1), the overall dielectric constant of the composites doped with PTFE and metal oxides was reduced in comparison to undoped ZnO ceramics. Figure 6b illustrates that dielectric losses (tanδ) sharply decrease at low frequencies (below 1000 Hz), where dc conductivity predominantly contributes to loss. Notably, the sample labeled S1 exhibited high tanδ values (~2.437 at 50 Hz), while the S4 sample containing both PTFE and oxides demonstrated a significantly lower loss (~0.028), attributed to suppression of charge conduction across grain boundaries. Furthermore, high-frequency dielectric loss behavior exhibited relaxation peaks in some doped samples (S2–S4), which are attributed to electronic relaxation associated with intrinsic oxygen vacancy defects.

The fabrication route of these ceramic composites is presented in Figure 7, which schematically outlines the cold sintering process (CSP). In this method, ZnO powder was mixed with PTFE suspension and the selected oxides, followed by a ball milling step, low temperature pressing, and brief thermal exposure at 300 °C. This process enabled the densification of ceramic composites at significantly reduced temperatures compared to traditional sintering. The CSP-assisted integration of polymer and metal oxides effectively refined microstructure reducing average grain size from ~526 nm in pure ZnO to ~338 nm in doped samples and enhanced grain boundary resistance. These modifications contributed to higher Schottky barrier heights and improved dielectric characteristics. Overall, the study demonstrates a feasible and energy-efficient pathway to optimize ceramic nanocomposites for electronic insulation or varistor applications by simultaneously controlling composition and processing strategy.

2.5. Relaxor Ferroelectrics: Structure and Energy Storage Potential

Relaxor ferroelectrics represent a class of materials with diffuse phase transitions and high dielectric constants that vary with frequency. Unlike classical ferroelectrics with sharp Curie transitions, relaxors possess polar nanoregions (PNRs) that fluctuate dynamically [126]. This results in broad dielectric peaks, low remanent polarization, and slim P–E loops ideal traits for energy storage devices where low hysteresis loss and high recoverable energy density are crucial. Examples include materials like Pb(Mg₁/₃Nb₂/₃)O₃ (PMN), Ba(Zr_xTi_1−x)O₃ (BZT), and modified BiFeO₃ systems [127]. Nanocomposite strategies that embed relaxor ceramics into flexible or polymeric matrices have shown promising results, especially in achieving high energy densities above 10 J/cm³. Furthermore, dopants or strain-engineered interfaces can stabilize the relaxor phase, providing enhanced breakdown strength and better thermal stability [128].

Pattipaka et al. [129] explored the energy storage capabilities of lead-free (1 − x)Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-xBi_0.2Sr_0.7TiO₃ (BNKT-BST) relaxor ferroelectric ceramics, demonstrating a domain engineering strategy to enhance dielectric performance. As illustrated in Figure 8a, the recoverable energy density W_rec is derived from the area between the polarization (P)–electric field (E) hysteresis loop, particularly from the difference between maximum polarization (Pmax) and remnant polarization (Pr). The incorporation of BST into the BNKT matrix disrupted long-range ferroelectric order and promoted the formation of highly dynamic polar nano-regions (PNRs), key to relaxor behavior. These PNRs exhibit reversible transformation into long-range ferroelectric domains under strong electric fields, allowing for a large ΔP = P_max − P_r, and hence higher energy density and efficiency.

Additionally, as shown in Figure 8b, the phase evolution from ferroelectric (FE) to relaxor ferroelectric (RFE) states was achieved by increasing BST content, leading to a mixed rhombohedral–tetragonal structure and finer grains. These structural modifications resulted in a lower dielectric maximum temperature (T_m), enhanced breakdown strength (EBD), and suppressed hysteresis loss, culminating in a high recoverable energy density of 0.81 J/cm³ and an energy efficiency of 86.95% for the x = 0.45 composition under 90 kV/cm. This demonstrates that well-tailored relaxor behavior and domain structure significantly contribute to boosting energy storage performance in lead-free dielectric ceramics, offering an environmentally friendly alternative to lead-based systems.

The dielectric and ferroelectric behavior of ceramic nanocomposites is governed by a complex interplay of polarization mechanisms, interfacial effects, grain size, and processing strategies. Understanding and tailoring these parameters is essential for optimizing performance in advanced applications such as energy storage, sensors, and flexible electronics.

Beyond the BNKT–BST relaxors, BNT-based lead-free relaxor composites have emerged as some of the most promising alternatives to PZT due to their ability to combine high energy-storage density with excellent strain response under low electric fields. These materials benefit from the coexistence of long-range ferroelectric domains and dynamic polar nanoregions (PNRs), which produce slim hysteresis loops and high recoverable energy efficiency. For example, Khaliq et al. demonstrated that incipient piezoelectrics integrated with relaxor ferroelectrics can exploit stress fields at the ferroelectric/relaxor interfaces to achieve electrostrains exceeding 0.45% under modest fields [130]. Similarly, Sheeraz et al. reported that BNT–BKT-based relaxor composites exhibit large recoverable energy densities and superior electrostrain owing to enhanced interfacial compatibility and defect-mediated polarization mechanisms [131]. These findings suggest that interfacial stress engineering in BNT-derived relaxors provides a scalable pathway to realize both high strain and high efficiency, attributes that are vital for advanced actuators and energy storage devices. Moreover, when coupled with dopants or secondary phases such as BaZrO₃ or SrTiO₃, BNT-based composites display broadened relaxor transitions and improved breakdown strengths, further reinforcing their status as environmentally friendly contenders for next-generation dielectric and electromechanical applications.

3. Techniques and Microstructural Control in Nanocomposites

Having introduced the key material systems and their dielectric/ferroelectric behaviors in Section 2, we now turn to the synthesis and processing strategies that govern microstructural evolution and, consequently, the material properties. The dielectric and ferroelectric performance of ceramic nanocomposites is intimately linked to their microstructure, which is, in turn, governed by the processing route. Processing not only defines the grain size, phase purity, and porosity, but also critically shapes the interfacial structures, domain configurations, and defect distributions factors that control polarization dynamics, permittivity, and dielectric loss. This section presents a comparative discussion of the most prominent processing techniques used in recent ceramic nanocomposite research: solid-state sintering, sol–gel synthesis, spark plasma sintering, cold sintering, and hydrothermal methods. Each offers unique advantages and limitations in terms of temperature, grain refinement, scalability, and compatibility with complex architectures. Spark Plasma Sintering shows the highest dielectric constant (~3000) due to dense packing and refined grains (~300 nm), while traditional solid-state Sintering exhibits larger grain size (~4000 nm) and lower dielectric response. Sol–gel and cold sintering techniques balance grain size and performance, demonstrating the role of nanoscale control in dielectric behavior.

3.1. Solid-State Sintering

Solid-state sintering remains one of the most widely adopted methods for ceramic fabrication due to its simplicity and scalability. However, it typically requires high sintering temperatures (1200–1500 °C) and results in coarse grains (~4 µm), as shown in

Table 4. Comparative summary of processing techniques for selected dielectric and ferroelectric nanocomposites, including typical compositions, processing temperatures, grain sizes, dielectric constants, and key advantages [134,135,136,137,138,139,140,141,142,143,144,145].

Method	Typical Material Examples	Temp (°C)	Grain Size (nm)	Dielectric Constant (εr)	Key Advantages
Solid-State Sintering	BaTiO3, K0.5Na0.5NbO3 (KNN), Pb(Zr,Ti)O3 (PZT)	1300–1500	~4000	~1200	Simple, scalable, suitable for bulk fabrication
Sol–Gel	BaTiO3, BiFeO3, PbTiO3	600–900	~50	~1800	Fine grains, low porosity, precise stoichiometry control
Spark Plasma Sintering	BaTiO3–CNT, Ba(Zr,Ti)O3 (BZT), CaCu3Ti4O12 (CCTO)	900–1100	~300	~2500	High density, rapid sintering, nanoscale grain retention
Cold Sintering	ZnO–PTFE, BaTiO3–PVDF	<300	~200	~1400	Low-energy, compatible with polymers, eco-friendly
Hydrothermal	BaTiO3, SrTiO3, TiO2 nanostructures	100–250	~70	~1600	Morphology control, selective crystallization, high purity

These large grains reduce grain boundary area, limiting interfacial polarization and resulting in moderate dielectric properties Table 4, which compares different techniques used in the preparation of nanocomposites. While this method is cost-effective for bulk ceramics, it often suffers from compositional inhomogeneities, high porosity, and limited control over nanostructure, making it less suitable for high-performance ferroelectric devices [132,133].

3.2. Sol–Gel Processing

Sol–gel synthesis offers excellent control over stoichiometry and grain size. Through molecular-level mixing of precursors, this method produces ultrafine ceramic powders (~50 nm) at significantly lower calcination temperatures [134,146,147]. As a result, sol–gel derived nanocomposites often show improved permittivity and lower dielectric loss due to better phase uniformity and dense packing of grains. The nanoscale grain structure boosts interfacial polarization and domain wall mobility, contributing to high dielectric constants (~1800) and improved frequency stability. However, the process is sensitive to hydrolysis conditions and drying kinetics, which can affect reproducibility. Ahmed et al. [148] developed a novel electrochemical sensing platform for investigating the interaction between the anticancer drug capmatinib and double-stranded DNA (dsDNA) using a disposable pencil graphite electrode (PGE) modified with cerium oxide-decorated carbon nanofiber ceramic films (CeNPs@CNF-CF). The sensor was fabricated through a multi-step sol–gel approach, where a ceramic precursor solution containing MTMOS, HCl, and methanol was combined with carbon nanofibers and cerium oxide nanoparticles to form a stable nanocomposite dispersion. Pencil graphite rods (PGRs) were then flame-treated, cleaned via sonication in acetone, and coated with the CeNPs@CNF sol–gel through dip-coating using a Teflon holder. This process, illustrated in Figure 9, resulted in the formation of CeNPs@CNF-CF/PGRs with a uniform, conductive, and stable surface suitable for electrochemical applications. The interaction between capmatinib and dsDNA was monitored using cyclic and square wave voltammetry, revealing that the presence of dsDNA significantly suppressed the oxidation peak of capmatinib, indicating a strong electrostatic binding mechanism. The sensor exhibited high sensitivity (LOD = 5 × 10⁻⁸ M) and a notable binding constant, confirming its potential for DNA–drug interaction studies.

3.3. Spark Plasma Sintering (SPS)

Spark plasma sintering is an advanced processing technique in which pulsed direct current and uniaxial pressure are simultaneously applied to ceramic powders [149]. This method enables rapid densification at relatively low temperatures (900–1100 °C), thereby preserving nanoscale grain sizes (~300 nm) while achieving near-full density. Composites fabricated via SPS exhibit excellent dielectric properties, with dielectric constants exceeding 2500 [150]. These enhancements are primarily attributed to reduced porosity, refined grain structures, enhanced interfacial polarization, and minimal grain growth. SPS is particularly effective for the processing of doped perovskites, high-k dielectric materials, and composites containing metallic or conductive phases. Figure 10 illustrates a schematic diagram of the SPS setup used to prepare the composite, along with an example of a resulting microstructure [151].

3.4. Cold Sintering Process (CSP)

Cold sintering is an emerging green technique that enables densification of ceramics at sub −300 °C, using a transient aqueous phase. This method is ideal for integrating ceramics with polymeric or temperature-sensitive substrates. CSP can produce dense ceramics with grain sizes of ~200 nm, though dielectric performance is typically moderate (~1400). Despite this, the sustainability and low energy consumption of CSP are unmatched. CSP is gaining traction for applications in flexible electronics, IoT sensors, and hybrid multilayer structures [152,153]. A schematic representation of the cold sintering process and the thermocompression apparatus employed is shown in Figure 11 [154].

3.5. Hydrothermal Synthesis

Hydrothermal methods offer a low-temperature route (100–250 °C) for producing highly crystalline nanostructures (~70 nm grain size) with controlled morphology. The aqueous environment allows selective phase formation and minimizes defects. Hydrothermally synthesized dielectrics exhibit dielectric constants in the range of 1600–1700, with low leakage and good breakdown strength. However, post-synthesis processing (e.g., annealing) is often needed to enhance densification and tailor domain structures [155,156]. Hayfa et al. [157] reported the hydrothermal synthesis of multifunctional bimetallic silver–copper oxide (Ag-CuO) nanohybrids with tunable Ag content, which were evaluated for their antimicrobial, antibiofilm, and antiproliferative properties.

The nanohybrids (Ag-C-1 to Ag-C-4) were synthesized using varying concentrations of silver nitrate (0.05–0.5 g), resulting in nanostructures with pleomorphic morphology, predominantly spherical in shape, as confirmed by SEM and TEM analyses. The particle sizes ranged from 20 to 35 nm, and EDX spectra confirmed the presence of Ag, Cu, and O, verifying the formation of bimetallic hybrids. The nanohybrids exhibited strong antimicrobial activity against E. coli and C. albicans, with MIC and MBC values in the range of 4–12 mg/mL and 2–24 mg/mL, respectively. Moreover, the Ag-CuO nanohybrids induced dose-dependent cytotoxicity and apoptosis in human colon cancer (HCT-116) cells, indicating their potential as antiproliferative agents. The synthesis strategy is illustrated in Figure 12, which shows a schematic representation of the one-step hydrothermal route employed to obtain these functional nanohybrids.

4. Interfacial Engineering and Polarization Mechanisms in Ceramic Nanocomposites

4.1. Role of Interfaces in Dielectric Enhancement

Interfacial engineering is a cornerstone in optimizing dielectric and ferroelectric behavior in ceramic nanocomposites. As grain sizes are reduced to the nanoscale, the volume fraction of grain boundaries and phase interfaces increase significantly, introducing unique charge accumulation sites and affecting local electric fields [158]. These interfaces, when properly engineered, can enhance the dielectric constant, reduce dielectric loss, and introduce relaxor-like behavior. In ceramic–polymer nanocomposites, interfaces play a dual role: they act as mechanical barriers and as charge trapping sites. The quality of the interface between ceramic fillers and polymer matrices (e.g., epoxy) governs polarization, charge mobility, and dispersion uniformity. For instance, ceramic particles with tailored surface functionalization (e.g., via silanes, phosphonic acids, or polymers) can enhance interfacial compatibility and promote stronger interfacial polarization through Maxwell–Wagner–Sillars effects. In all-ceramic systems (e.g., core–shell or multilayered structures), internal interfaces lead to localized field enhancement, beneficial for achieving high dielectric performance [159,160]. The internal barrier layer capacitor effect, seen in BaTiO₃-based systems, is a prominent example where insulating grain boundaries trap free carriers and promote charge accumulation, significantly boosting dielectric constant [161].

4.2. Polarization Mechanisms and Frequency Response

Polarization in ceramic nanocomposites arises from several fundamental mechanisms, each governed by the material’s structure, composition, and operating frequency. These mechanisms include electronic, ionic, dipolar, interfacial (Maxwell–Wagner-type), and ferroelectric polarization [162,163]. At the atomic scale, electronic polarization results from the displacement of electron clouds relative to the nuclei within atoms or ions, and it typically dominates at very high frequencies (in the terahertz range). Similarly, ionic polarization arises due to the relative displacement of positive and negative ions in a crystal lattice under an external field [164,165]. Both electronic and ionic contributions are extremely fast and largely independent of frequency across most usable ranges, forming the baseline response in all dielectric ceramics such as Al₂O₃, MgO, or ZrO₂ [166,167]. In contrast, dipolar polarization is associated with the orientation of permanent dipoles within a material and becomes significant at moderate frequencies (kHz–MHz). Dipoles can arise from intrinsic lattice asymmetries (as in perovskite structures) or be introduced through aliovalent doping that creates defect dipoles. For example, in doped BaTiO₃ or KNbO₃, substitutional ions (such as La³⁺ or Mn²⁺) generate localized asymmetries that enhance dipolar alignment under an applied field [168,169].

Interfacial polarization, also known as Maxwell–Wagner–Sillars polarization, becomes highly significant in composite systems, particularly those with heterogeneous interfaces such as ceramic–polymer nanocomposites or grain boundary-rich ceramic systems. It arises due to the accumulation of charges at interfaces between materials with contrasting conductivities or permittivities. In systems like ZnO epoxy, or TiO₂–PMMA, interfacial polarization enhances the overall dielectric response at low frequencies (Hz–kHz) by acting as capacitive layers where charge carriers are slowed or trapped [170,171]. Similarly, in all-ceramic heterostructures, such as core–shell BaTiO₃@SiO₂ or layered BiFeO₃ systems, charge buildup at grain boundaries or shell interfaces contributes to elevated permittivity and tunability. A particularly interesting case is ferroelectric polarization, which involves the reversible reorientation of spontaneous electric dipoles within ferroelectric domains under an external electric field [172]. This mechanism is central to memory, sensor, and actuator applications. Materials like BaTiO₃, Pb(Zr,Ti)O₃ (PZT), and Na_0.5Bi_0.5TiO₃ (NBT) are classical ferroelectrics that exhibit hysteresis behavior and remnant polarization, making them suitable for applications in non-volatile memory and high-strain actuators. In nanocomposites, ferroelectric polarization is often preserved or even enhanced through nanoscale grain control, strain engineering, and interfacial design [172,173,174,175].

Moreover, relaxor ferroelectrics, such as Pb(Mg₁/₃Nb₂/₃)O₃–PbTiO₃ (PMN–PT), and Pb(Sc₁/₂Nb₁/₂)O₃ (PSN) systems, show a broad, frequency-dependent dielectric peak, attributed to the presence of polar nano-regions [176,177]. These regions fluctuate dynamically, resulting in diffuse phase transitions and enhanced dielectric response over a wide temperature and frequency range. In ceramic nanocomposites, relaxor-like behavior can be induced or amplified by nanoscale particle dispersion, interfacial strain, and dipole-dipole interactions across matrix/filler boundaries. For instance, PMN–PT epoxy composites or BaTiO₃–PVDF–TrFE films demonstrate a tunable dielectric relaxation that is ideal for tunable capacitors and flexible devices. Ultimately, understanding and manipulating these polarization mechanisms is essential for optimizing dielectric and ferroelectric behavior in ceramic nanocomposites. The interplay of frequency, microstructure, and interfacial characteristics allows for the engineering of materials with tailored response for specific applications such as energy storage, sensing, and high-frequency electronics [178,179].

4.3. Influence of Interface Type (Core–Shell vs. Heterojunctions)

The interface type in ceramic nanocomposites plays a crucial role in defining their dielectric, ferroelectric, and multifunctional properties. Among the most studied interfacial architectures are core–shell structures and heterojunction interfaces, both of which leverage nanoscale interfacial phenomena to tailor material performance. Core–shell nanostructures are characterized by a central ceramic core encapsulated by a secondary phase shell, which may be another oxide, polymer, or even metallic layer. These architectures enable precise control of interface strain, charge distribution, and polarization alignment, which are essential for optimizing dielectric response and reducing leakage currents. For example, BaTiO₃@SiO₂, BaTiO₃@Al₂O₃, or ZnO@TiO₂ systems exhibit enhanced dielectric strength and long-term stability due to the insulating shell layer, which suppresses conduction pathways and stabilizes grain boundaries [180,181,182]. In particular, the SiO₂ shell acts as a barrier to grain growth during sintering, preserving nanoscale dimensions and enhancing surface-to-volume ratio, which intensifies interfacial polarization.

In ferroelectric core–shell composites, the shell can also induce compressive or tensile strain on the ferroelectric core, modifying the Curie temperature and spontaneous polarization. For instance, in BaTiO₃@ZrO₂ or BiFeO₃@TiO₂, strain-induced lattice distortion at the interface can stabilize the ferroelectric phase over a wider temperature range, improving performance in energy harvesting and memory applications [183,184]. The thickness of the shell also plays a pivotal role; thin shells (~1–5 nm) can maintain high dielectric constants, while thicker shells may reduce permittivity due to dilution effects, though often improving dielectric loss behavior. On the other hand, heterojunction interfaces involve the combination of two or more different ceramic or hybrid phases joined together, often with a discontinuous or semi-coherent interface. These can include oxide–oxide junctions (e.g., TiO₂–SnO₂, ZnO–NiO), which promote Maxwell–Wagner-type interfacial polarization [184,185]. In such systems, charge carriers accumulate at the phase boundaries due to differences in electrical conductivity or permittivity, leading to significant enhancement in low-frequency dielectric response. For example, TiO₂/SrTiO₃ multilayers and Nb-doped BaTiO₃/ZnO junctions show large dielectric permittivity due to internal barrier layer capacitor effects [186,187,188,189,190].

The nature of the interface, whether abrupt, graded, or chemically reactive, also influences polarization switching and dielectric relaxation. Reactive interfaces can introduce interfacial dipoles or defect complexes that modify the local electric field and enhance the response under external stimuli. This is particularly important in applications such as tunable dielectrics, where rapid field-induced polarization changes are desirable. Furthermore, core–shell structures often offer better thermal stability and mechanical integrity compared to random heterojunctions, making them suitable for high-temperature or harsh environment applications. However, heterojunction-based systems can provide higher flexibility and tunability by combining materials with vastly different properties, especially in printed electronics and flexible capacitors. In recent advances, engineered gradient interfaces have emerged as a bridge between core–shell and heterojunction designs. These use gradual compositional or structural transitions to minimize interfacial stress while enhancing charge separation and dielectric uniformity. Functionally graded materials and multilayered superlattices are examples where gradient interfaces significantly improve energy storage density and polarization switching speed.

Table 5. Comparative Summary of Core–Shell and Heterojunction Interfaces in Ceramic Nanocomposites.

Aspect	Core–Shell Structures	Heterojunction Interfaces
Interface Type	Conformal, typical uniform around core	Discontinuous or planar, between distinct phases
Polarization Mechanism	Enhanced interfacial and ferroelectric polarization	Dominantly Maxwell–Wagner interfacial polarization
Permittivity Behavior	High dielectric constant with improved breakdown strength	Strong low-frequency dielectric dispersion
Thermal Stability	High, due to encapsulation and grain growth suppression	Moderate; depends on phase interaction
Energy Storage	High energy density in dense, well-aligned systems	Moderate; often limited by interface defects
Examples	BaTiO3@SiO2, BiFeO3@TiO2, ZnO@Al2O3	BaTiO3–PVDF, TiO2–SnO2, BaTiO3–Nb:SrTiO3
Applications	Capacitors, energy harvesters, high-temperature dielectrics	Flexible electronics, tunable dielectrics, EMI shielding

summarizes the key differences and applications of core–shell versus heterojunction interfaces.

4.4. Dielectric Relaxation and Energy Storage Behavior

Dielectric relaxation is a central phenomenon in ceramic nanocomposites, governing how materials respond to alternating electric fields across different frequencies and temperatures. It refers to the time dependent response of dipoles and interfacial charges that do not immediately align with the external field. This delayed polarization creates frequency-dependent dielectric properties, which are crucial for applications such as capacitors, pulse power devices, and high-energy density storage systems. The performance of dielectric materials in energy storage is thus closely linked to the relaxation dynamics of the different polarization mechanisms present in the system. In ceramic nanocomposites, several types of polarization contribute to dielectric relaxation, each operating on different time scales. Electronic and ionic polarization are the fastest and occur almost instantaneously. These mechanisms are primarily active at high frequencies (in the MHz to GHz range) and contribute modestly to the overall dielectric constant. Dipolar polarization, on the other hand, arises from orientation of permanent or induced dipoles. This mechanism is more sensitive to temperature and defects, and its contribution becomes significant in the lower frequency range.

However, in complex systems such as ceramic polymer composites or multiphase nanocomposites, interfacial polarization also known as Maxwell–Wagner polarization plays a dominant role at low frequencies (Hz to kHz). This type of polarization is due to charging accumulation at the interfaces between materials with contrasting electrical properties, such as between conductive ceramic grains and insulating polymer matrices or secondary phases. In such systems, the nature and quality of the interfaces become critical design variables that influence both relaxation and energy storage performance [191,192]. Another major contributor in many advanced ceramic systems is ferroelectric polarization, which is associated with the reversible switching of spontaneous dipoles under an external electric field. Ferroelectric materials, such as BaTiO₃ or lead-free alternatives like BZT–BT and KNN, exhibit strong hysteresis behavior and high dielectric constants. However, their energy storage efficiency is often limited by the energy lost during polarization reversal, manifested as the area enclosed in the polarization electric field (P–E) loop [193].

Relaxor ferroelectrics, a class of disordered ferroelectrics, provides a compromise between high permittivity and low hysteresis. Materials such as PMN–PT, BZT–BT, or doped BaTiO₃ demonstrate diffuse phase transitions and frequency-dependent dielectric peaks due to the presence of polar nano-regions (PNRs) [194]. These PNRs facilitate local field fluctuations, resulting in a broad range of dielectric response. In nanocomposites, this behavior is further tuned by particle size distribution, interface coupling, and defect dipole dynamics. This makes relaxor systems particularly attractive for high-temperature and high-frequency dielectric applications. In the context of energy storage, the key metrics are energy density and efficiency. High dielectric constants and breakdown strength are desirable for storing more energy in each volume [195]. However, these need to be balanced against dielectric losses and hysteresis, which reduce overall efficiency. Fine-grained microstructures and tailored doping can suppress domain wall motion and minimize losses, while advanced sintering methods like spark plasma sintering improve grain boundary quality, resulting in enhanced breakdown strength.

Core–shell architectures, such as BaTiO₃@SiO₂ or BZT@Al₂O₃, have emerged as promising structures for balancing high permittivity with reduced leakage current. The insulating shell layer suppresses intergranular conduction and enhances reliability under high electric fields. Similarly, combining high-k ceramics with flexible polymers such as PVDF, P(VDF-TrFE), or epoxy resins has led to nanocomposites with both mechanical flexibility and high recoverable energy [196]. These composite systems benefit from interfacial polarization and high field tolerance, though their overall permittivity is usually lower than that of pure ceramics [197]. Notably, materials like CaCu₃Ti₄O₁₂ (CCTO) have attracted attention due to their colossal dielectric constants, but their high dielectric losses and low breakdown strengths limit their practical utility in energy storage [198]. On the other hand, layered structures or heterostructures that combine materials like SrTiO₃ with ZrO₂ or BiFeO₃ allow for field-tuned behavior and enhanced recoverable energy density by controlling the orientation and interaction of interfaces at the nanoscale. Ultimately, the dielectric relaxation and energy storage behavior of ceramic nanocomposites are not governed by a single factor but result from the interplay of material composition, defect chemistry, grain structure, phase interfaces, and processing history. Therefore, rational design strategies require multi-scale control from atomistic doping and defect engineering to mesoscale interface tuning and macroscale densification to fully exploit the potential of these materials [199].

4.5. Temperature and Frequency Dependence of Dielectric Performance

The dielectric behavior of ceramic nanocomposites is significantly influenced by temperature and frequency, both of which dictate the activation and relaxation of polarization mechanisms. Understanding this dependence is crucial for engineering materials for real-world applications such as capacitors, actuators, sensors, and energy storage devices that must function reliably under varying thermal and signal conditions.

4.5.1. Temperature Dependence

As temperature increases, atomic vibrations intensify, which influences dielectric constant (ε′), dielectric loss (tan δ), and conductivity. In most dielectric ceramics, permittivity rises with temperature up to a critical point, typically associated with a phase transition, such as the Curie temperature in ferroelectrics. For instance, in BaTiO₃, a sharp increase in permittivity is observed near its Curie point (~120 °C), where the material transitions from the ferroelectric to paraelectric phase [200]. In relaxor ferroelectrics like PMN–PT or BZT–BT, instead of a sharp transition, the dielectric peak is broad and shifts with frequency a behavior that stems from the dynamic nature of polar nano-regions (PNRs). This diffuse transition makes relaxors more thermally stable, a desirable feature for devices operating over wide temperature ranges [201]. Ceramic–polymer nanocomposites, due to the polymer matrix, often show smoother temperature dependence. However, their performance can degrade near the polymer’s glass transition or melting temperature. Hence, high-temperature polymers like polyimides or fluorinated systems (e.g., PVDF-TrFE) are often used to maintain dielectric performance at elevated temperatures [202].

4.5.2. Frequency Dependence

Frequency influences which polarization mechanisms are active. At low frequencies (Hz to kHz), interfacial and dipolar polarization dominate. These mechanisms are slower and more susceptible to the material’s microstructure, interfaces, and defects. As the frequency increases to the MHz–GHz range, only electronic and ionic polarization remain active because they are fast enough to respond to the rapidly oscillating field. This results in a characteristic dielectric dispersion, a decrease in permittivity with increasing frequency. Simultaneously, dielectric loss often increases at lower frequencies due to conduction and interfacial effects. This is especially true in composites or multi-phase ceramics where charge carriers may become trapped at grain boundaries or phase interfaces. The Maxwell–Wagner–Sillars effect is commonly observed in such systems, where interfacial polarization becomes prominent at low frequencies due to charge accumulation at heterointerfaces. In ceramics like CCTO or doped TiO₂ systems, very large permittivity values are seen at low frequencies, though accompanied by high loss due to leakage conduction. In contrast, nanostructured materials with core–shell architectures, or those processed via high-density methods like spark plasma sintering, often exhibit more stable frequency-dependent behavior, suppressing space charge accumulation and reducing dielectric loss [203].

5. Emerging Applications of Dielectric and Ferroelectric Ceramic

The continuous evolution of multifunctional materials has positioned ceramic nanocomposites at the forefront of modern applications spanning electronics, energy, sensing, and sustainability. Owing to their intrinsic dielectric properties, mechanical resilience, and adaptability in microstructural design, these materials offer a unique advantage in scenarios that demand both performance and miniaturization. Their integration into hybrid systems enables synergistic enhancements where single-phase materials would fall short. In this section, we explore key areas where dielectric and ferroelectric ceramic nanocomposites are becoming indispensable. A summary of the diverse and emerging applications enabled by ceramic nanocomposites is illustrated in Figure 13, highlighting their multifunctional roles across electronics, energy, sensing, and biomedical platforms.

5.1. Energy Storage Devices: Capacitors and Hybrid Supercapacitors

One of the most impactful applications of dielectric ceramic nanocomposites is in high-performance energy storage, particularly in capacitors and hybrid supercapacitors. The need for rapid charging and discharging, along with long-term stability, has driven innovations in nanostructured dielectric layers that possess both high permittivity and breakdown strength [204]. Traditional bulk ceramics such as BaTiO₃ and Pb(Zr,Ti)O₃ have been foundational in capacitor technology, but by integrating them at the nanoscale with polymers or hybrid matrices, their energy density can be drastically enhanced. For instance, core–shell nanostructures like BaTiO₃@SiO₂ exhibit efficient electric field distribution and minimal leakage, which is crucial for pulsed power applications. Moreover, multilayer nanocomposites using PVDF as the matrix with high-aspect-ratio ceramic fillers like TiO₂ nanowires or BaTiO₃ nanosheets demonstrate anisotropic dielectric behavior, contributing to superior energy densities often exceeding 20 J/cm³ in optimized systems. Such devices are also being tailored for grid-level energy buffering, electric vehicle systems, and drone applications, where fast response and thermal robustness are mandatory [205,206,207].

5.2. Flexible and Wearable Electronics

The demand for stretchable, conformable, and skin-integrated electronics has opened new opportunities for ceramic nanocomposites in wearable technologies. While ceramics are inherently brittle, embedding ceramic nanoparticles or nanowires into elastomeric or piezoelectric polymer matrices transforms them into mechanically flexible yet electrically responsive systems. For example, composites of BaTiO₃ or ZnO with PDMS, PU, or PVDF yield piezoelectric nanogenerators that can harvest biomechanical energy from human motion [208,209,210]. In wearable sensing platforms, these composites enable real-time pressure, strain, or temperature monitoring with high sensitivity and durability. To further enhance flexibility and mechanical resilience, researchers have introduced design strategies such as wrinkled structures, stretchable interconnects, and gradient composite layering, mimicking biological tissues. Additionally, advanced processing like inkjet printing or electrospinning allows scalable manufacturing of patterned, flexible devices suitable for integration into smart textiles, e-skins, and next-generation health monitors.

5.3. Neuromorphic Systems and Smart Memory Devices

The increasing push toward artificial intelligence and brain-like computing has spurred interest in neuromorphic devices that emulate synaptic behavior. Ceramic nanocomposites, particularly those containing ferroelectric phases, play a critical role in this domain by offering nonvolatile memory characteristics, multilevel data retention, and dynamic switching capabilities. Ferroelectric tunnel junctions (FTJs), based on ultrathin layers of HfO₂, BiFeO₃, or doped BaTiO₃, exhibit programmable resistance states that can mimic synaptic plasticity [211]. When embedded in composite architectures with oxide semiconductors or conductive polymers, these materials can be tuned to exhibit spike-timing-dependent plasticity, a hallmark of neuromorphic computation. Furthermore, ceramic-based resistive switching materials in memristors offer fast switching speeds, high endurance, and low power consumption, making them suitable for edge computing and neuromorphic accelerators. Recent advances also include heterostructured devices that combine ferroelectric switching with photoresponsivity or electrochemical modulation, enabling multifunctional logic-in-memory systems.

5.4. High-Frequency Communication and RF Devices

In the realm of RF communications and high-frequency signal processing, ceramic nanocomposites are being engineered to offer controlled dielectric constants and low loss tangents over broad frequency ranges. Materials such as BST, LaAlO₃, and AlN have emerged as reliable candidates for GHz–THz applications. When structured at the nanoscale or combined with low-permittivity polymers, they allow fine-tuning of electromagnetic properties required in reconfigurable antennas, tunable filters, and dielectric resonators. For example, BST–MgO composites provide a balance between tunability and loss, suitable for phase shifters and tunable capacitors in 5G systems. Moreover, the development of ceramic–polymer gradient refractive index materials has enabled flexible waveguides and beam-steering devices, expanding applications into mm-wave and sub-THz domains [212,213]. Through interface control and anisotropic particle alignment, researchers have further reduced frequency-dependent losses, enabling the deployment of these materials in miniaturized devices like RF MEMS, wearable antennas, and adaptive radars.

5.5. Sustainable and Environmentally Friendly Technologies

Sustainability is becoming a core driver in materials development, and ceramic nanocomposites are at the heart of green innovations in electronics, energy, and sensing. Efforts to replace toxic or resource-scarce elements, such as lead or rare earths, have led to the rise of lead-free ferroelectrics like (K,Na)NbO₃ and BiFeO₃ [214,215,216]. These materials are being incorporated into composites with natural polymers like chitosan, cellulose, or gelatin, yielding biodegradable sensors and capacitors suitable for eco-electronics. Furthermore, green processing techniques such as low-temperature sintering, use of deep eutectic solvents, and additive manufacturing are reducing the environmental footprint of ceramic synthesis. Transient electronics, which physically degrade after usage, also leverage water-soluble or thermally degradable ceramic–polymer systems for medical implants, environmental sensors, and military devices. Additionally, by integrating carbon-neutral production pathways and enabling end-of-life recyclability, ceramic nanocomposites are contributing to the circular economy, supporting both sustainability and high-performance functionality in advanced devices.

A particularly important direction in sustainable ceramic nanocomposites is the replacement of lead-based systems with BNT-based and relaxor–ferroelectric composites, which not only mitigate toxicity but also offer competitive or superior functional performance. Unlike traditional Pb(Zr,Ti)O₃ systems, these lead-free ceramics leverage local disorder and interfacial strain to generate enhanced dielectric tunability and electrostrain. For instance, BNT–BaTiO₃ relaxor composites provide high efficiency and reduced hysteresis losses, while BNT–KBT systems demonstrate domain-engineering strategies that yield high recoverable energy storage capabilities at relatively low fields [130,131]. The dynamic relaxor state allows for reduced energy dissipation during repeated cycling, which is crucial for eco-friendly capacitors and wearable devices requiring long-term operational stability. Furthermore, the integration of these lead-free relaxors into hybrid ceramic polymer architectures (e.g., BNT–PVDF or BNT–epoxy systems) has shown promise for flexible and biodegradable energy devices, where mechanical adaptability is combined with high dielectric performance. These advances underscore that the shift toward BNT-based lead-free composites is not only environmentally motivated but also technologically advantageous, marking a paradigm change in the roadmap for sustainable dielectric materials.

6. Future Approaches for Next-Generation Ceramic

The future development of ceramic nanocomposites hinges on the convergence of sustainable processing, advanced interface design, and digital innovation. One of the primary goals for the coming decade is the shift toward energy-efficient, scalable, and environmentally benign synthesis methods. Traditional sintering and solid-state routes, while reliable, often demand high temperatures and long processing durations, leading to significant energy consumption and environmental burden. As such, hybrid techniques such as spark plasma sintering combined with microwave or cold sintering routes are increasingly being explored to enable densification at reduced thermal budgets. Furthermore, the design of sol–gel and ink-based methods for low-temperature deposition of dielectric and ferroelectric films is being prioritized to align with the needs of flexible substrates and printed electronics. Some of these solutions are already demonstrated in recent studies, while others remain forward-looking proposals intended to guide future research efforts.

Table 6. Key challenges in ceramic nanocomposites and corresponding existing or proposed solutions, based on current literature and emerging research directions.

Key Challenge	Proposed Solution
Scalability of processing techniques	Adoption of low-temperature scalable sintering (e.g., SPS, cold sintering)
Interfacial engineering limitations	Advanced core–shell and surface functionalization strategies
Long-term stability and reliability	Improved encapsulation and barrier coatings
Eco-toxicity of additives and solvents	Development of green synthesis and biodegradable matrices
Integration with flexible substrates	Engineering polymer–ceramic hybrids with elastic interfaces
Limited AI-guided material discovery	Use of machine learning for structure–property predictions
Cost and energy consumption	Optimization of processing-energy tradeoffs using LCA tools

shows the different challenges in ceramic nanocomposites and the proposed solutions.

Another vital area of focus is the use of lead-free and bio-derived materials. With regulatory pressure mounting against lead-containing ceramics like PZT, researchers are intensively studying alternatives such as BaTiO₃-based solid solutions, potassium sodium niobate, and bismuth-based systems. However, many of these alternatives suffer from trade-offs between piezoelectric coefficient, phase stability, and fatigue resistance. Addressing these challenges requires not only improved synthetic strategies but also precise control of doping, crystallographic orientation, and interface coherence. Moreover, replacing organic solvents with eutectic or bio-based alternatives could significantly reduce toxicity and improve life-cycle sustainability, a growing concern in both academic and industrial sectors. At the microstructural level, interface engineering remains a cornerstone for performance optimization. Interfacial polarization, grain boundary stability, and local electric field enhancements are critical in determining dielectric and ferroelectric responses. Core–shell architectures, gradient interfaces, and dopant segregation strategies are being refined to mitigate space charge accumulation and enhance dielectric breakdown strength. Particularly in heterophase nanocomposites, controlling the mismatch in dielectric permittivity and thermal expansion coefficients is essential to reduce stress-induced failure and ensure long-term reliability under cyclic loading or high-voltage operation. Simultaneously, the integration of ceramic nanocomposites into flexible, wearable, or neuromorphic systems demands a rethinking of material substrate compatibility. In Figure 14, we illustrate the various challenges and strategic solutions for future ceramic nanocomposite applications.

Traditional ceramics are inherently brittle and incompatible with flexible substrates, but advances in nanocomposite design such as embedding ceramics in polymer matrices or developing thin-film conformal coatings are helping overcome these barriers. For instance, dielectric inks that can be screen-printed or inkjet-printed onto PET or PI substrates are enabling the realization of flexible capacitors, sensors, and energy harvesters. The development of low-curing-temperature formulations is particularly critical for CMOS-backend integration, where thermal budgets must remain below 400 °C. Looking forward, the field must embrace computational and data-driven strategies to accelerate material discovery and device optimization. Machine learning (ML) algorithms trained on large datasets of composition structure property relationships are already assisting in predicting dielectric behavior and processing windows for complex oxides. High-throughput experimentation, when coupled with AI models, allows researchers to explore vast compositional spaces and identify high-performance candidates in a fraction of the time compared to traditional methods. Furthermore, materials informatics tools can guide the optimization of grain boundary chemistry, defect energetics, and sintering parameters to yield tunable, application-specific nanocomposites. Future research must address not only the performance optimization of ceramic nanocomposites but also their reliability under operational stressors such as thermal cycling, humidity exposure, and electrical fatigue. Sustainability goals should drive the development of lead-free compositions, low-energy processing, and recyclable systems. Furthermore, successful integration into commercial devices will require standardization of processing routes, scalability studies, and life-cycle assessments to ensure economic and environmental viability.

7. Conclusions

In this comprehensive review, we have explored the significant advancements in ceramic nanocomposites, focusing on their dielectric and ferroelectric behavior, structure–property relationships, and the wide range of processing strategies that enable tailored performance. By integrating insights from conventional sintering to spark plasma sintering and advanced wet-chemical routes, we observed how microstructural control particularly at grain boundaries and interfaces plays a critical role in dictating electrical responses. The multi-scale polarization mechanisms, including dipolar, interfacial, and ferroelectric contributions, are shown to be highly sensitive to interface design (core–shell vs. heterojunction) and the nanoscale morphology of the ceramic matrix. Innovative structural configurations have not only enhanced dielectric constant and polarization but also paved the way for new applications across flexible electronics, high-energy storage devices, and neuromorphic systems.

Emerging trends in material design increasingly emphasize sustainability, flexibility, and integration into multifunctional platforms. Lead-free systems, eco-friendly fabrication techniques, and AI-assisted materials discovery are all shaping the future trajectory of this field. Yet, challenges remain ranging from interface instability and defect control to scalability and integration into commercial systems. To overcome these barriers, a synergistic effort involving materials scientists, chemists, physicists, and engineers is essential. Future research should prioritize high-throughput synthesis, atomic-level interface engineering, and device-level validation to bring ceramic nanocomposites closer to real-world applications.