Electrocaloric and energy storage properties of lead-free Na0.5Bi0.5Ti0.6Hf0.4O3 ferroelectric ceramics for sustainable energy solutions are affected by the sintering temperatures

Electrocaloric and energy storage properties of lead-free Na0.5Bi0.5Ti0.6Hf0.4O3 ferroelectric ceramics for sustainable energy solutions are affected by the sintering temperatures | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Electrocaloric and energy storage properties of lead-free Na 0.5 Bi 0.5 Ti 0.6 Hf 0.4 O 3 ferroelectric ceramics for sustainable energy solutions are affected by the sintering temperatures Mohan Babu Kuppam, Vijaya Lakshmi K, SreenivasaRao Pandi, Sobhanachalam P, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8021474/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Dec, 2025 Read the published version in Journal of Materials Science: Materials in Electronics → Version 1 posted You are reading this latest preprint version Abstract Ferroelectric materials are widely regarded as promising candidates for sustainable energy technologies, particularly in solid-state cooling and energy-storage applications. In this work, the microstructural, dielectric, electrocaloric, and energy-storage properties of sol–gel-derived Na 0.5 Bi 0.5 Hf 0.4 Ti 0.6 O 3 (NBHT) ceramics were systematically investigated. X-ray diffraction (XRD) analysis confirmed the coexistence of monoclinic and tetragonal phases, with an enhanced tetragonal contribution observed at higher sintering temperatures. Increasing the sintering temperature facilitated grain growth, improved densification, and significantly enhanced the dielectric and ferroelectric responses. The NBHT ceramic sintered at 1100 °C exhibited a maximum electrocaloric temperature change (ΔT) of 0.35 K and a recoverable energy density (W rec ) of 1.52 J cm -3 with an energy efficiency of approximately 80% at 120 °C. In contrast, the sample sintered at 1200 °C demonstrated a W rec of 1.02 J cm -3 and high efficiency of 79.13% under an applied electric field of 40 kV cm -1 . These findings reveal that the NBHT ceramics exhibit a strong electrocaloric effect coupled with excellent energy-storage performance, underscoring their potential as lead-free ferroelectric materials for next-generation solid-state cooling and energy-storage devices. Energy storage Electrocalorics Dielectrics Lead-free ceramics Polarization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction A major challenge in the modern energy sector is the rising power consumption associated with air-conditioning systems. With the steady increase in global temperatures and the widespread use of cooling technologies, energy demand has surged dramatically. Conventional air conditioners primarily operate on vapor-compression refrigeration, which not only requires substantial electrical energy but also releases fluorinated greenhouse gases, contributing to environmental degradation [ 1 – 4 ]. This growing concern has driven the development of sustainable and energy-efficient cooling alternatives. One promising avenue lies in electrocaloric (EC) materials, which exhibit a reversible temperature change under an applied electric field due to the entropy variation caused by ferroelectric domain reorientation [ 5 – 6 ]. This EC effect forms the foundation for environmentally friendly, solid-state cooling devices, offering an alternative to traditional refrigerant-based systems. In parallel, ferroelectric materials have gained attention for their dual potential in energy storage and solid-state cooling applications [ 7 – 8 ]. Among them, Na 0.5 Bi 0.5 TiO 3 (NBT) based lead-free ceramics stand out owing to their excellent ferroelectric properties, making them ideal candidates for exploring both energy storage performance and electrocaloric effects in next-generation sustainable energy technologies. Ferroelectric ceramics based on Na 0.5 Bi 0.5 TiO 3 (NBT) have garnered considerable attention for energy storage and related multifunctional applications. With the growing demand for high-efficiency energy systems to power portable electronics, stabilize electrical grids, and manage the intermittency of renewable energy sources, the development of advanced dielectric materials has become essential [ 9 – 10 ]. Among these, ferroelectric ceramics are particularly promising owing to their ability to store and release electrical energy efficiently, facilitated by their high dielectric permittivity and low dielectric loss. The perovskite-structured NBT exhibits strong intrinsic ferroelectricity, making it a potential candidate for both energy storage and electrocaloric (EC) cooling applications. Furthermore, recent studies indicate that elemental substitution such as the incorporation of hafnium (Hf) can effectively modify the NBT lattice, enabling precise tuning of its structural, dielectric, and ferroelectric properties to achieve enhanced overall performance. As hafnium (Hf) is incorporated at the B-site of the NBT lattice, the crystal structure undergoes notable modifications that influence the ferroelectric phase transition behavior and overall structural stability. With increasing Hf concentration in the Na 0.5 Bi 0.5 Ti 1− x Hf ₓ O 3 (NBTH) system, the depolarization temperature (T d ) markedly decreases from approximately 190°C to around 110°C or lower, facilitating the coexistence of multiple crystallographic phases cubic, tetragonal, and orthorhombic near or even at room temperature. This phase coexistence effectively reduces the energy barrier for polarization rotation, thereby inducing a pronounced electrocaloric (EC) effect, enhanced dielectric constant, improved piezoelectric response, and superior energy storage capability. Despite these promising features, only a limited number of studies have systematically examined the energy storage potential of Hf-substituted NBT ceramics. In addition to chemical modification, extrinsic factors such as synthesis route, processing parameters, and particularly the sintering temperature play a crucial role in dictating the electrical and physical characteristics of such materials [ 11 – 12 ]. Among these, sintering temperature is a decisive parameter governing densification, grain growth, and defect evolution, which in turn strongly influence the microstructural, dielectric, and ferroelectric properties of the ceramics [ 13 – 14 ]. By tuning the sintering temperature, it is possible to control grain size, domain configuration, and dopant distribution, enabling the optimization of material performance. In this study, the influence of sintering temperature on the dielectric, electrocaloric (EC), energy storage, and structural properties of Na 0.5 Bi 0.5 Hf 0.4 Ti 0.6 O 3 (NBHT) relaxor ferroelectric ceramics is systematically investigated. This particular composition was selected because its depolarization temperature (T d ) lies close to ambient conditions, which is highly desirable for practical EC and energy storage applications. Ferroelectric materials typically exhibit maximum EC temperature change (ΔT) and high recoverable energy density (W rec ) near their T d [ 15 – 16 ]. From an application standpoint, achieving a large EC temperature variation and high energy storage density near room temperature is a key objective. Hence, this work provides valuable insights into how sintering temperature governs the structure–property correlations in NBHT ceramics, thereby offering guidance for designing environmentally benign, high-performance materials for solid-state cooling, capacitive energy storage, and next-generation electronic systems. 2. Experimental section Eco-friendly Na 0.5 Bi 0.5 Hf 0.4 Ti 0.6 O 3 (NBHT) ceramics were synthesized via the conventional solid-state reaction route. High-purity analytical reagent (AR)-grade raw materials were used as starting precursors: Bismuth(III) oxide (Bi 2 O 3 , 99.9%, Sigma-Aldrich, USA), Titanium(IV) oxide (TiO 2 , 99.8%, Merck, India), Hafnium(IV) oxide (HfO 2 , 99.8%, Alfa Aesar, UK), and Sodium carbonate (Na 2 CO 3 , 99.9%, Sigma-Aldrich, USA). These oxides and carbonate powders were weighed in stoichiometric proportions according to the nominal composition and ball-milled in ethyl alcohol for 10 h at 300 rpm using zirconia balls to ensure uniform mixing and homogeneity. The dried powders were calcined at 800°C for 3 h in air to promote solid-state reaction and phase formation. The calcined powders were again milled for 10 h at 300 rpm and subsequently mixed with 4 wt% polyvinyl alcohol (PVA, Merck, India) binder solution (prepared by dissolving 4 g of PVA in 100 mL of deionized water) to improve compactness during pelletization. The obtained powders were pressed into circular pellets (10 mm diameter and ~ 1.5 mm thickness) using a uniaxial hydraulic press. The green pellets were sintered at 1100°C, 1150°C, and 1200°C for 3 h in air with a controlled heating rate of 5°C min⁻¹. The phase formation, crystal structure, and chemical purity of the sintered NBHT ceramics were verified using X-ray diffraction (XRD, PANalytical X’Pert PRO, Cu Kα radiation, λ = 1.5406 Å). The microstructural features were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-7900F) equipped with energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, UK) to confirm elemental composition and homogeneity. For ferroelectric and strain measurements, the sintered pellets were polished to a thickness of approximately 0.3 mm, and silver electrodes were deposited on both faces using an ion-sputter coater (Quorum Q150T ES, UK). The polarization–electric field (P–E) and strain–electric field (S–E) hysteresis loops were recorded using a Precision Premier II Ferroelectric Tester (PolyK Technologies, USA) at a frequency of 1 Hz. The temperature- and frequency-dependent dielectric properties were measured using an impedance analyzer (KEYSIGHT E4990A) in the frequency range 20 Hz–2.2 MHz. In addition, temperature-dependent dielectric measurements were performed between 30°C and 500°C at 10 kHz using an Agilent E4980A LCR meter (Agilent Technologies, Palo Alto, CA, USA). 3. Results and discussion Figure 1 shows the room-temperature XRD patterns of NBHT bulk ceramics sintered at 1100°C, 1150°C, and 1200°C. All compositions exhibit single-phase perovskite structures without any detectable secondary or impurity phases, confirming successful phase formation across the studied temperature range. Although the overall diffraction profiles remain similar, subtle variations in peak intensity, sharpness, and position indicate a strong dependence of crystallinity and phase composition on the sintering temperature. With increasing temperature, the diffraction peaks become narrower and more intense, signifying improved crystallinity and an increase in average crystallite size due to enhanced grain growth and lattice ordering. A structural transformation is evident from the deformation of the (211) PC reflection with rising sintering temperature, suggesting that multiple adjacent peaks merge into a single, well-defined peak at higher temperatures. This merging behavior points to the coexistence of monoclinic (Cc) and tetragonal (P4mm) phases, which could be responsible for the non-monotonic variation observed in the 54ᵒ peak positions. In particular, samples sintered above 1200°C or those containing trace secondary phases exhibit more pronounced tetragonal characteristics, indicating a temperature-driven phase transition from the monoclinic to the tetragonal symmetry. To gain detailed insight into the phase composition and lattice parameters, Rietveld refinement of the XRD data was carried out using the Full Prof software package. As depicted in Fig. 2 (a) and 2(b) , the refined profiles show an excellent match between the observed and calculated patterns, with low residual values Rp and R wp < 7% and χ² values close to unity, confirming the reliability of the fitting. The refined structural parameters summarized in Table.1 reveal a progressive decrease in the c/a ratio with increasing sintering temperature, indicating a gradual suppression of the monoclinic distortion and stabilization of the tetragonal phase. Moreover, the bulk density of the NBHT ceramics increases from 5.219 to 5.467 g·cm − 3 as the sintering temperature rises from 1100°C to 1200°C, further evidencing improved densification and microstructural compactness at elevated temperatures. Figure 3 presents the surface morphology of NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C. The micrographs reveal well-defined grains with distinct boundaries and a uniform distribution across the surface, indicating effective densification. Only a few residual pores are visible, contributing to a high relative density (~ 92%), as confirmed by comparison with the theoretical density calculated using Archimedes’ principle. Importantly, the absence of secondary phase segregation along the grain boundaries further confirms phase purity and compositional uniformity. The grain size distribution, evaluated using a Gaussian fitting approach and verified through the line-intercept method (ASTM standards [ 17 – 18 ]), reveals average grain sizes of 1.95 µm, 2.66 µm, and 2.96 µm for ceramics sintered at 1100°C, 1150°C, and 1200°C, respectively. The slight yet systematic increase in grain size with rising sintering temperature reflects enhanced atomic mobility and lattice diffusion at elevated thermal conditions. This temperature-driven grain coarsening suggests improved crystallinity and domain alignment, contributing to the observed densification behavior. In the NBHT lead-free ceramics, sintering temperature plays a pivotal role in controlling the microstructural evolution. Higher sintering temperatures promote accelerated grain growth and densely packed morphologies, while excessive thermal exposure may induce abnormal grain coarsening or non-uniform microstructures, which can negatively impact the electrical and functional performance. Figure 4 illustrates the temperature-dependent variation of the dielectric constant (ε′) and loss tangent (tan δ) for NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C, measured at 1 MHz. All samples exhibit broad dielectric maxima, characteristic of diffuse phase transitions commonly observed in lead-free disordered systems. The dielectric constant shows a slight enhancement with increasing sintering temperature, suggesting improved structural ordering and polarization behavior. Two distinct dielectric anomalies are observed: the first, corresponding to the ferroelectric–antiferroelectric transition, occurs at the depolarization temperature (T d ), while the second anomaly, associated with the antiferroelectric–paraelectric transition, appears at the Curie temperature (T C ), where ε′ attains its maximum value. The relaxation behavior near T d can be attributed to cationic disorder arising from the random distribution of ions, a well-known feature of relaxor ferroelectrics [ 19 ]. For NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C, the dielectric anomaly temperature (T d ) is located around 219°C, with only slight variation in ε′ among the compositions. The sintering temperature has a pronounced influence on the dielectric response. As the sintering temperature increases, grain growth and densification are enhanced due to improved atomic diffusion, leading to reduced porosity and stronger domain polarization both of which contribute to an elevated dielectric constant. However, temperatures exceeding the optimal sintering range may promote abnormal grain growth or secondary phase formation, which can disrupt the microstructure and degrade dielectric performance. Therefore, maintaining an appropriate sintering temperature is critical for achieving superior dielectric properties. Within this optimal range, the overall dielectric constant rises as the proportion of low-ε r grain boundaries diminishes relative to the bulk grains. Consequently, the reduction in grain boundary contribution and enhanced domain wall activity collectively improve the dielectric performance of NBHT ceramics. For NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C, the bipolar strain–electric field (S–E) and polarization electric field (P–E) responses were evaluated at 1 Hz and room temperature to investigate the dipole behavior under an applied electric field. As illustrated in Fig. 4 (b) , all samples exhibit well-saturated hysteresis loops with relatively high coercive fields (EC), confirming their intrinsic ferroelectric nature. The ferroelectric polarization of lead-free NBHT ceramics is highly sensitive to the sintering temperature. At 1100°C, limited densification and smaller grain size hinder domain alignment, resulting in low remanent polarization and weak strain response. The sample sintered at 1150°C exhibits the highest polarization and maximum bipolar strain, attributed to enhanced grain growth, improved densification, and facilitated domain wall mobility. However, at 1200°C, abnormal grain growth and the emergence of secondary phases impede uniform domain switching, leading to a decline in ferroelectric polarization and strain despite the overall high density. This behavior indicates that 1150°C represents the optimal sintering temperature for achieving superior ferroelectric performance in NBHT ceramics. The microstructural evolution strongly influences ferroelectric switching behavior. In fine-grained ceramics sintered at lower temperatures, a high grain boundary density induces internal stress and lattice distortion, disrupting long-range ferroelectric domain ordering and restricting polarization rotation. Conversely, larger grains formed at elevated sintering temperatures promote the development of long-range domains and thinner grain boundaries, thereby enhancing ferroelectric properties. Nonetheless, the sample sintered at 1200°C exhibits localized voids that reduce the effective density and limit polarization, even though its microstructure appears denser overall. In NBHT ceramics, temperature-dependent polarization–electric field (P–E) hysteresis loops were recorded from 400°C to 1200°C to evaluate the electrocaloric (EC) and energy-storage behaviors at sintering temperatures of 1100°C, 1150°C, and 1200°C. The EC response was estimated using the indirect method, which derives the entropy change (ΔS) and adiabatic temperature change (ΔT) from Maxwell’s relations based on the temperature-dependent polarization data. The calculations incorporated the pyroelectric coefficient (dP/dT), bulk density (ρ), and specific heat capacity (Cp), with Cp taken as 0.41 J g⁻¹ K⁻¹, consistent with values reported in the literature [E1, E2]. The electrocaloric properties were analyzed by extracting polarization versus temperature (P–T) data from the measured P–E loops at various applied electric fields. The corresponding ΔS and ΔT values were computed by numerical integration of the derived Maxwell relations, as detailed in Refs. [ 20 – 21 ]. Figures 6 (a–c) display the evolution of the P–T behavior for NBHT ceramics sintered at different temperatures, while Fig. 6 (d–f) present the resulting electrocaloric temperature change (ΔT) as a function of operating temperature and applied electric field. These results reveal distinct EC responses correlated with sintering-induced microstructural variations, highlighting the influence of densification and phase composition on the functional thermal behavior of NBHT ceramics. $$\:\varDelta\:T=-\frac{1}{\rho\:{C}_{p}}{\int\:}_{{E}_{1}}^{{E}_{2}}T\frac{dP}{dT}dE$$ The specific heat capacities of NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C were determined from standard literature [ 22 ] to be 0.540 J·g⁻¹·K⁻¹, 0.531 J·g⁻¹·K⁻¹, and 0.532 J·g⁻¹·K⁻¹ respectively, measured at room temperature (RT). In this context, P denotes polarization, T represents temperature, and E 1 and E 2 correspond to the initial and final applied electric fields used in the indirect calculation of the electrocaloric parameters. The bulk densities of the NBHT ceramics sintered at 1100°C, 1150°C, and 1200°C were found to be 5.92 g·cm − 3 , 5.85 g·cm − 3 , and 5.82 g·cm − 3 , respectively, confirming excellent densification with slight variation at higher sintering temperatures. Figure 6 (d–f) presents the calculated electrocaloric temperature change (ΔT) as a function of temperature for NBHT ceramics subjected to various applied electric fields. A pronounced electrocaloric effect (ECE) is observed near the ferroelectric-to-antiferroelectric transition temperature (T d ), which represents the region of maximum polarization change. Under an applied electric field of 40 kV·cm⁻¹, the maximum ΔT values recorded for the ceramics sintered at 1100°C, 1150°C, and 1200°C are 0.35 K, 0.30 K, and 0.21 K, respectively. The reduced ΔT in the sample sintered at 1200°C is attributed to its relatively lower density and the presence of crystal imperfections, resulting from non-uniform grain growth and partial structural disorder at elevated temperatures. The variations in EC response among the samples arise from multiple factors, including grain size, phase coexistence, electric field strength, sintering conditions, and intrinsic compositional effects. The superior ΔT observed in the 1100°C and 1150°C samples reflects the optimized balance between grain size and densification, which enhances dipole reorientation and entropy change under the applied field. For practical applications in solid-state cooling, achieving a high ΔT at low electric fields near ambient temperature is crucial. The measured ΔT values in this study are comparable to or higher than those reported in previous literature, underscoring the strong electrocaloric performance of NBHT ceramics. Their substantial ΔT near room temperature and operation under moderate electric fields make these materials promising candidates for environmentally friendly electrocaloric cooling devices. Polarization–electric field (P–E) hysteresis loops were recorded at a frequency of 1 Hz over a temperature range of 40°C to 120°C in 5°C intervals, as shown in Fig. 7 (b) , to assess the energy storage characteristics of the NBHT ceramics. As the temperature increases, both the coercive field (EC) and remanent polarization (P r ) exhibit a gradual decrease, indicating thermally induced softening of ferroelectric domains and a reduction in switching barriers. The distinct separation between the charging and discharging branches of the P–E loops reflects the material’s ability to store and release electrical energy. The positive quadrant of the P–E loop is typically employed to determine the recoverable energy storage density (W rec ), energy loss density (W loss ), and energy storage efficiency (η) of the ceramics. The recoverable energy density (W rec ) for dielectric capacitors is evaluated from the area enclosed between the polarization and electric field curves using the following relation: $$\:{W}_{rec}={\int\:}_{{P}_{r}}^{{P}_{max}}Edp$$ where Pₘₐₓ and P r represent the maximum and remanent polarizations, respectively. The total stored energy density ( Wₜₒₜ ) corresponds to the area under the charging curve, while the energy storage efficiency (η) is given by: This analytical approach provides a quantitative measure of the energy storage capability and thermal stability of NBHT ceramics under varying operational temperatures. Where P represents the saturated polarization, P r denotes the remanent polarization, and Pₘₐₓ is the maximum polarization attained under a given electric field (E). A larger difference between Pₘₐₓ and P r (i.e., Pₘₐₓ – P r ), together with a higher dielectric breakdown strength (EB), is crucial for achieving enhanced energy storage capability. While improvements in sintering optimization, doping, and microstructural control can significantly increase the EB value, certain intrinsic dielectric breakdown mechanisms remain fundamentally dependent on the chemical composition and defect structure of the material. The energy loss and storage efficiency (η) were calculated using the standard procedures reported in [ 23 ]. $$\:{W}_{loss}={\int\:}_{0}^{{P}_{max}}Edp$$ $$\:\eta\:=\frac{{W}_{rec}}{{W}_{rec}+{W}_{loss}}\times\:100\%$$ Notably, the recoverable energy density (W rec ) values obtained from the room-temperature polarization loops were 0.40 J/cm 3 , 0.95 J/cm 3 , and 1.02 J/cm 3 for NBHT ceramics sintered at 1100°C, 1250°C, and 1200°C, respectively. These samples exhibited relatively low energy loss, achieving an energy storage efficiency of approximately 80%. The corresponding polarization loops for the NBHT ceramics sintered at 1200°C are presented in Fig. 7 (c–d) , which were used to estimate the variation of W rec with temperature. Despite remarkable progress, the thermal stability of energy storage performance continues to be a critical challenge for advanced dielectric capacitors. To address this issue, the polarization–electric field (P–E) behavior and energy storage characteristics of NBHT ceramics sintered at 1200°C were systematically examined across a temperature range of 30°C to 120°C, with 5°C increments. As shown in Fig. 7 (c–d) , the specimens exhibit both high energy storage density and excellent thermal stability. The remarkable retention of polarization behavior at elevated temperatures indicates that the typically competing characteristics of high energy density and temperature stability have been effectively decoupled. The NBHT ceramics sintered at 1200°C deliver a maximum energy storage density of 1.52 J/cm 3 and an efficiency of 80.13% under an applied field of 40 kV/cm, marking a significant advancement among lead-free ferroelectric ceramics. These performance metrics remain stable throughout the 40°C–120°C temperature range, with minimal variation within ± 5% for W rec and ± 3% for η. The presence of dense grain boundaries effectively restricts domain wall motion and hinders complete polarization alignment, causing the induced polarization to rapidly decay after the removal of the external field. This results in low remnant polarization (P r ), thereby enhancing the energy storage capability. In contrast, BHT ceramics sintered at higher temperatures tend to develop larger grains and higher P r values, associated with long-range ferroelectric domains and easier polarization orientation [ 23 – 25 ]. Overall, the energy storage performance of lead-free ceramics is strongly influenced by factors such as density, grain size, and sintering temperature. In this study, the optimized NBHT ceramics demonstrate superior energy storage density, efficiency, and thermal reliability, establishing them as a promising lead-free ferroelectric system for next-generation sustainable energy storage applications. 4. Conclusion The study underscores the sintering temperature as a decisive parameter governing the structural, microstructural, and functional characteristics of NBHT ceramics. XRD analysis confirmed the formation of a single-phase structure with no secondary phases up to 1200°C, accompanied by progressive improvement in crystallinity and grain growth at elevated temperatures. The optimum dielectric, ferroelectric, and electrocaloric responses were achieved at 1150°C, attributed to enhanced densification and a well-balanced grain size distribution. Although higher densification occurred at 1200°C, irregular grain growth and the possible emergence of structural defects contributed to reduced polarization and a decline in electrocaloric efficiency. A maximum recoverable energy density of 1.52 J/cm 3 and an energy efficiency of approximately 80% were recorded for the sample sintered at 1200°C, indicating a significant improvement in energy storage capability with increasing sintering temperature. Overall, the NBHT ceramics demonstrated superior electrocaloric and energy storage performance at optimized sintering conditions. These findings highlight the promise of NBHT as a lead-free ferroelectric material for the realization of high-performance solid-state cooling and next-generation energy storage devices, paving the way toward environmentally sustainable alternatives to conventional lead-based systems. Declarations ACKNOWLEDGMENTS Kuppam Mohan Babu thanks to Dr. A Chittibabu, department of physics, C. R. Reddy College of Engineering, is thanked for his assistance with structural refinement. Data Availability The authors declare that all the data generated or analyzed during this study are included in this manuscript Conflict of Interest The authors declare that they have no conflict of interest. CRediT authorship contribution statement: Kuppam Mohan Babu: Project administration, Conceptualization, Methodology, Writing- Original draft, Data curation, K. Vijaya Lakshmi: Visualization, Software, Review, PandiSreenivasaRao: Visualization, Software, Review, P. 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Table Table 1: The structural refined parameters and lattice constants of NBHT ceramic sintered at 1200˚C. Cc NBHT-1200 0 C Positions x y z Na/Bi 0.0000 0.2517 0.0000 Ti/Hf 0.0000 0.2517 0.0000 O 0.1982 0.2521 0.5107 a =9.4460Å, b= 5.4783Å, c = 5.6120Å, P4mm NBHT-1200 0 C Positions x y z Na/Bi 0.0010 0.49381 0.2500 Ti/Hf 0.0000 0.2517 0.0000 O 1 -0.0572 0.0020 0.2510 O 2 0.2130 0.2870 0.0267 a =3.8844Å, b= 3.8844Å, c = 3.9449Å, Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 17 Dec, 2025 Read the published version in Journal of Materials Science: Materials in Electronics → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8021474","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":549343859,"identity":"777d2aec-fb7b-469b-a81e-b25507e1a9d7","order_by":0,"name":"Mohan Babu Kuppam","email":"","orcid":"","institution":"Sri Venkateswara College of Engineering","correspondingAuthor":false,"prefix":"","firstName":"Mohan","middleName":"Babu","lastName":"Kuppam","suffix":""},{"id":549343860,"identity":"fb23ebef-3c50-475d-b06b-7652214b656b","order_by":1,"name":"Vijaya Lakshmi K","email":"","orcid":"","institution":"Government College For 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04:28:06","extension":"xml","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":92079,"visible":true,"origin":"","legend":"","description":"","filename":"442fa4b130a44410bc6ffd5ab34c2a4d1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/b68b77ed0374d52f75557646.xml"},{"id":96711339,"identity":"f1319f8a-5a39-41e9-9e8f-8417d5476450","added_by":"auto","created_at":"2025-11-25 10:11:53","extension":"html","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":101673,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/060706a98264dbbd754fcbf3.html"},{"id":96683987,"identity":"ec90311a-8f27-45b4-936a-66fd2b22fdc9","added_by":"auto","created_at":"2025-11-25 04:28:05","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":843811,"visible":true,"origin":"","legend":"\u003cp\u003eRoom temperature X-ray diffraction patterns of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C. The magnified window of (110)\u003csub\u003ePC\u003c/sub\u003e and (211)\u003csub\u003ePC\u003c/sub\u003e Brags reflection shown beside.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/1bf46a29dc9db88c615d0c0d.png"},{"id":96710350,"identity":"1c00b64e-529e-4c05-b28e-cd0ba7223584","added_by":"auto","created_at":"2025-11-25 10:10:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":9437476,"visible":true,"origin":"","legend":"\u003cp\u003eThe structural refinement of NBHT ceramic sintered at 1150˚C and 1200˚C, along refined parameters.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/7b3081c1c9a7fed7b5ee5d3b.png"},{"id":96683989,"identity":"8037cdcc-e1a2-45e8-91fd-363b88a20d37","added_by":"auto","created_at":"2025-11-25 04:28:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":3962324,"visible":true,"origin":"","legend":"\u003cp\u003eThe surface morphology and average grain size of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C, along with average grain size distribution for respective specimens showed.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/df2ef18692ad2165664f2c4b.png"},{"id":96683990,"identity":"2ecc3f37-0013-4544-bcc3-4aa03865ee2e","added_by":"auto","created_at":"2025-11-25 04:28:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1797916,"visible":true,"origin":"","legend":"\u003cp\u003e(a). The dielectric constant and loss curves at frequency 1MHz of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/0255e3681d41c430d2767bde.png"},{"id":96683998,"identity":"1ca44c0f-8334-40b5-8b77-388930efa345","added_by":"auto","created_at":"2025-11-25 04:28:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":8709625,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c). The P-E and S-E curves of loopsof NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C. (d-e). Temperature dependent P-E loops of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/e309a1ecf2f14362009ee5dc.png"},{"id":96710441,"identity":"bae38d14-a2d8-40e9-be5d-ac471dab3a1c","added_by":"auto","created_at":"2025-11-25 10:10:39","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":5409311,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c). The polarization behaviour vs specific electric fields of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C. (d-e).Adiabatic temperature change (ΔT) of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/542feedc89cb88f4be9216b1.png"},{"id":96684000,"identity":"040b3b56-93c6-42e1-bca9-4e9f8ad9943d","added_by":"auto","created_at":"2025-11-25 04:28:06","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5217538,"visible":true,"origin":"","legend":"\u003cp\u003e(a). The P-E loops of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C measured 40˚C. (b). Recoil Energy density W\u003csub\u003erec, \u003c/sub\u003eEnergy loss W\u003csub\u003eloss \u003c/sub\u003eand efficiency η of NBHT ceramic sintered at 1100˚C, 1150˚C and 1200˚C measured 40˚C. (c).The temperature dependent polarization of NBHT ceramic sintered at 1200˚C. (d).\u0026nbsp;\u0026nbsp; Recoil Energy density W\u003csub\u003erec, \u003c/sub\u003eEnergy loss W\u003csub\u003eloss \u003c/sub\u003eand efficiency η of NBHT ceramic sintered at 1200˚C.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/742d5311bee2e6b43fb0d4f2.png"},{"id":98814942,"identity":"4e308bc0-2aed-485d-ae98-1d1196032903","added_by":"auto","created_at":"2025-12-22 16:13:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":36310915,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8021474/v1/bef82dd6-6c80-48f9-954c-179c65e86bf1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eElectrocaloric and energy storage properties of lead-free Na\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.5\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eBi\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.5\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eTi\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.6\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eHf\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.4\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3 \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eferroelectric ceramics for sustainable energy solutions are affected by the sintering temperatures\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eA major challenge in the modern energy sector is the rising power consumption associated with air-conditioning systems. With the steady increase in global temperatures and the widespread use of cooling technologies, energy demand has surged dramatically. Conventional air conditioners primarily operate on vapor-compression refrigeration, which not only requires substantial electrical energy but also releases fluorinated greenhouse gases, contributing to environmental degradation [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. This growing concern has driven the development of sustainable and energy-efficient cooling alternatives. One promising avenue lies in electrocaloric (EC) materials, which exhibit a reversible temperature change under an applied electric field due to the entropy variation caused by ferroelectric domain reorientation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. This EC effect forms the foundation for environmentally friendly, solid-state cooling devices, offering an alternative to traditional refrigerant-based systems. In parallel, ferroelectric materials have gained attention for their dual potential in energy storage and solid-state cooling applications [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Among them, Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e (NBT) based lead-free ceramics stand out owing to their excellent ferroelectric properties, making them ideal candidates for exploring both energy storage performance and electrocaloric effects in next-generation sustainable energy technologies.\u003c/p\u003e\u003cp\u003eFerroelectric ceramics based on Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e (NBT) have garnered considerable attention for energy storage and related multifunctional applications. With the growing demand for high-efficiency energy systems to power portable electronics, stabilize electrical grids, and manage the intermittency of renewable energy sources, the development of advanced dielectric materials has become essential [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Among these, ferroelectric ceramics are particularly promising owing to their ability to store and release electrical energy efficiently, facilitated by their high dielectric permittivity and low dielectric loss. The perovskite-structured NBT exhibits strong intrinsic ferroelectricity, making it a potential candidate for both energy storage and electrocaloric (EC) cooling applications. Furthermore, recent studies indicate that elemental substitution such as the incorporation of hafnium (Hf) can effectively modify the NBT lattice, enabling precise tuning of its structural, dielectric, and ferroelectric properties to achieve enhanced overall performance.\u003c/p\u003e\u003cp\u003eAs hafnium (Hf) is incorporated at the B-site of the NBT lattice, the crystal structure undergoes notable modifications that influence the ferroelectric phase transition behavior and overall structural stability. With increasing Hf concentration in the Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eTi\u003csub\u003e1\u0026minus;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eHf\u003cem\u003eₓ\u003c/em\u003eO\u003csub\u003e3\u003c/sub\u003e (NBTH) system, the depolarization temperature (T\u003csub\u003ed\u003c/sub\u003e) markedly decreases from approximately 190\u0026deg;C to around 110\u0026deg;C or lower, facilitating the coexistence of multiple crystallographic phases cubic, tetragonal, and orthorhombic near or even at room temperature. This phase coexistence effectively reduces the energy barrier for polarization rotation, thereby inducing a pronounced electrocaloric (EC) effect, enhanced dielectric constant, improved piezoelectric response, and superior energy storage capability. Despite these promising features, only a limited number of studies have systematically examined the energy storage potential of Hf-substituted NBT ceramics.\u003c/p\u003e\u003cp\u003eIn addition to chemical modification, extrinsic factors such as synthesis route, processing parameters, and particularly the sintering temperature play a crucial role in dictating the electrical and physical characteristics of such materials [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Among these, sintering temperature is a decisive parameter governing densification, grain growth, and defect evolution, which in turn strongly influence the microstructural, dielectric, and ferroelectric properties of the ceramics [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. By tuning the sintering temperature, it is possible to control grain size, domain configuration, and dopant distribution, enabling the optimization of material performance.\u003c/p\u003e\u003cp\u003eIn this study, the influence of sintering temperature on the dielectric, electrocaloric (EC), energy storage, and structural properties of Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eHf\u003csub\u003e0.4\u003c/sub\u003eTi\u003csub\u003e0.6\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (NBHT) relaxor ferroelectric ceramics is systematically investigated. This particular composition was selected because its depolarization temperature (T\u003csub\u003ed\u003c/sub\u003e) lies close to ambient conditions, which is highly desirable for practical EC and energy storage applications. Ferroelectric materials typically exhibit maximum EC temperature change (ΔT) and high recoverable energy density (W\u003csub\u003erec\u003c/sub\u003e) near their T\u003csub\u003ed\u003c/sub\u003e [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. From an application standpoint, achieving a large EC temperature variation and high energy storage density near room temperature is a key objective. Hence, this work provides valuable insights into how sintering temperature governs the structure\u0026ndash;property correlations in NBHT ceramics, thereby offering guidance for designing environmentally benign, high-performance materials for solid-state cooling, capacitive energy storage, and next-generation electronic systems.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cp\u003eEco-friendly Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eHf\u003csub\u003e0.4\u003c/sub\u003eTi\u003csub\u003e0.6\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (NBHT) ceramics were synthesized via the conventional solid-state reaction route. High-purity analytical reagent (AR)-grade raw materials were used as starting precursors: Bismuth(III) oxide (Bi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, 99.9%, Sigma-Aldrich, USA), Titanium(IV) oxide (TiO\u003csub\u003e2\u003c/sub\u003e, 99.8%, Merck, India), Hafnium(IV) oxide (HfO\u003csub\u003e2\u003c/sub\u003e, 99.8%, Alfa Aesar, UK), and Sodium carbonate (Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, 99.9%, Sigma-Aldrich, USA). These oxides and carbonate powders were weighed in stoichiometric proportions according to the nominal composition and ball-milled in ethyl alcohol for 10 h at 300 rpm using zirconia balls to ensure uniform mixing and homogeneity. The dried powders were calcined at 800\u0026deg;C for 3 h in air to promote solid-state reaction and phase formation. The calcined powders were again milled for 10 h at 300 rpm and subsequently mixed with 4 wt% polyvinyl alcohol (PVA, Merck, India) binder solution (prepared by dissolving 4 g of PVA in 100 mL of deionized water) to improve compactness during pelletization. The obtained powders were pressed into circular pellets (10 mm diameter and ~\u0026thinsp;1.5 mm thickness) using a uniaxial hydraulic press. The green pellets were sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C for 3 h in air with a controlled heating rate of 5\u0026deg;C min⁻\u0026sup1;.\u003c/p\u003e\u003cp\u003eThe phase formation, crystal structure, and chemical purity of the sintered NBHT ceramics were verified using X-ray diffraction (XRD, PANalytical X\u0026rsquo;Pert PRO, Cu Kα radiation, λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;). The microstructural features were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-7900F) equipped with energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, UK) to confirm elemental composition and homogeneity. For ferroelectric and strain measurements, the sintered pellets were polished to a thickness of approximately 0.3 mm, and silver electrodes were deposited on both faces using an ion-sputter coater (Quorum Q150T ES, UK). The polarization\u0026ndash;electric field (P\u0026ndash;E) and strain\u0026ndash;electric field (S\u0026ndash;E) hysteresis loops were recorded using a Precision Premier II Ferroelectric Tester (PolyK Technologies, USA) at a frequency of 1 Hz. The temperature- and frequency-dependent dielectric properties were measured using an impedance analyzer (KEYSIGHT E4990A) in the frequency range 20 Hz\u0026ndash;2.2 MHz. In addition, temperature-dependent dielectric measurements were performed between 30\u0026deg;C and 500\u0026deg;C at 10 kHz using an Agilent E4980A LCR meter (Agilent Technologies, Palo Alto, CA, USA).\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the room-temperature XRD patterns of NBHT bulk ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C. All compositions exhibit single-phase perovskite structures without any detectable secondary or impurity phases, confirming successful phase formation across the studied temperature range. Although the overall diffraction profiles remain similar, subtle variations in peak intensity, sharpness, and position indicate a strong dependence of crystallinity and phase composition on the sintering temperature. With increasing temperature, the diffraction peaks become narrower and more intense, signifying improved crystallinity and an increase in average crystallite size due to enhanced grain growth and lattice ordering. A structural transformation is evident from the deformation of the (211)\u003csub\u003ePC\u003c/sub\u003e reflection with rising sintering temperature, suggesting that multiple adjacent peaks merge into a single, well-defined peak at higher temperatures. This merging behavior points to the coexistence of monoclinic (Cc) and tetragonal (P4mm) phases, which could be responsible for the non-monotonic variation observed in the 54ᵒ peak positions. In particular, samples sintered above 1200\u0026deg;C or those containing trace secondary phases exhibit more pronounced tetragonal characteristics, indicating a temperature-driven phase transition from the monoclinic to the tetragonal symmetry. To gain detailed insight into the phase composition and lattice parameters, Rietveld refinement of the XRD data was carried out using the Full Prof software package. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(a) and 2(b)\u003c/b\u003e, the refined profiles show an excellent match between the observed and calculated patterns, with low residual values Rp and R\u003csub\u003ewp\u003c/sub\u003e \u0026lt; 7% and χ\u0026sup2; values close to unity, confirming the reliability of the fitting. The refined structural parameters summarized in Table.1 reveal a progressive decrease in the c/a ratio with increasing sintering temperature, indicating a gradual suppression of the monoclinic distortion and stabilization of the tetragonal phase. Moreover, the bulk density of the NBHT ceramics increases from 5.219 to 5.467 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e as the sintering temperature rises from 1100\u0026deg;C to 1200\u0026deg;C, further evidencing improved densification and microstructural compactness at elevated temperatures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the surface morphology of NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C. The micrographs reveal well-defined grains with distinct boundaries and a uniform distribution across the surface, indicating effective densification. Only a few residual pores are visible, contributing to a high relative density (~\u0026thinsp;92%), as confirmed by comparison with the theoretical density calculated using Archimedes\u0026rsquo; principle. Importantly, the absence of secondary phase segregation along the grain boundaries further confirms phase purity and compositional uniformity. The grain size distribution, evaluated using a Gaussian fitting approach and verified through the line-intercept method (ASTM standards [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]), reveals average grain sizes of 1.95 \u0026micro;m, 2.66 \u0026micro;m, and 2.96 \u0026micro;m for ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C, respectively. The slight yet systematic increase in grain size with rising sintering temperature reflects enhanced atomic mobility and lattice diffusion at elevated thermal conditions. This temperature-driven grain coarsening suggests improved crystallinity and domain alignment, contributing to the observed densification behavior. In the NBHT lead-free ceramics, sintering temperature plays a pivotal role in controlling the microstructural evolution. Higher sintering temperatures promote accelerated grain growth and densely packed morphologies, while excessive thermal exposure may induce abnormal grain coarsening or non-uniform microstructures, which can negatively impact the electrical and functional performance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the temperature-dependent variation of the dielectric constant (ε\u0026prime;) and loss tangent (tan δ) for NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C, measured at 1 MHz. All samples exhibit broad dielectric maxima, characteristic of diffuse phase transitions commonly observed in lead-free disordered systems. The dielectric constant shows a slight enhancement with increasing sintering temperature, suggesting improved structural ordering and polarization behavior. Two distinct dielectric anomalies are observed: the first, corresponding to the ferroelectric\u0026ndash;antiferroelectric transition, occurs at the depolarization temperature (T\u003csub\u003ed\u003c/sub\u003e), while the second anomaly, associated with the antiferroelectric\u0026ndash;paraelectric transition, appears at the Curie temperature (T\u003csub\u003eC\u003c/sub\u003e), where ε\u0026prime; attains its maximum value. The relaxation behavior near T\u003csub\u003ed\u003c/sub\u003e can be attributed to cationic disorder arising from the random distribution of ions, a well-known feature of relaxor ferroelectrics [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. For NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C, the dielectric anomaly temperature (T\u003csub\u003ed\u003c/sub\u003e) is located around 219\u0026deg;C, with only slight variation in ε\u0026prime; among the compositions. The sintering temperature has a pronounced influence on the dielectric response. As the sintering temperature increases, grain growth and densification are enhanced due to improved atomic diffusion, leading to reduced porosity and stronger domain polarization both of which contribute to an elevated dielectric constant. However, temperatures exceeding the optimal sintering range may promote abnormal grain growth or secondary phase formation, which can disrupt the microstructure and degrade dielectric performance. Therefore, maintaining an appropriate sintering temperature is critical for achieving superior dielectric properties. Within this optimal range, the overall dielectric constant rises as the proportion of low-ε\u003csub\u003er\u003c/sub\u003e grain boundaries diminishes relative to the bulk grains. Consequently, the reduction in grain boundary contribution and enhanced domain wall activity collectively improve the dielectric performance of NBHT ceramics.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C, the bipolar strain\u0026ndash;electric field (S\u0026ndash;E) and polarization electric field (P\u0026ndash;E) responses were evaluated at 1 Hz and room temperature to investigate the dipole behavior under an applied electric field. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e, all samples exhibit well-saturated hysteresis loops with relatively high coercive fields (EC), confirming their intrinsic ferroelectric nature. The ferroelectric polarization of lead-free NBHT ceramics is highly sensitive to the sintering temperature. At 1100\u0026deg;C, limited densification and smaller grain size hinder domain alignment, resulting in low remanent polarization and weak strain response. The sample sintered at 1150\u0026deg;C exhibits the highest polarization and maximum bipolar strain, attributed to enhanced grain growth, improved densification, and facilitated domain wall mobility.\u003c/p\u003e\u003cp\u003eHowever, at 1200\u0026deg;C, abnormal grain growth and the emergence of secondary phases impede uniform domain switching, leading to a decline in ferroelectric polarization and strain despite the overall high density. This behavior indicates that 1150\u0026deg;C represents the optimal sintering temperature for achieving superior ferroelectric performance in NBHT ceramics. The microstructural evolution strongly influences ferroelectric switching behavior. In fine-grained ceramics sintered at lower temperatures, a high grain boundary density induces internal stress and lattice distortion, disrupting long-range ferroelectric domain ordering and restricting polarization rotation. Conversely, larger grains formed at elevated sintering temperatures promote the development of long-range domains and thinner grain boundaries, thereby enhancing ferroelectric properties. Nonetheless, the sample sintered at 1200\u0026deg;C exhibits localized voids that reduce the effective density and limit polarization, even though its microstructure appears denser overall.\u003c/p\u003e\u003cp\u003eIn NBHT ceramics, temperature-dependent polarization\u0026ndash;electric field (P\u0026ndash;E) hysteresis loops were recorded from 400\u0026deg;C to 1200\u0026deg;C to evaluate the electrocaloric (EC) and energy-storage behaviors at sintering temperatures of 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C. The EC response was estimated using the indirect method, which derives the entropy change (ΔS) and adiabatic temperature change (ΔT) from Maxwell\u0026rsquo;s relations based on the temperature-dependent polarization data. The calculations incorporated the pyroelectric coefficient (dP/dT), bulk density (ρ), and specific heat capacity (Cp), with Cp taken as 0.41 J g⁻\u0026sup1; K⁻\u0026sup1;, consistent with values reported in the literature [E1, E2]. The electrocaloric properties were analyzed by extracting polarization versus temperature (P\u0026ndash;T) data from the measured P\u0026ndash;E loops at various applied electric fields. The corresponding ΔS and ΔT values were computed by numerical integration of the derived Maxwell relations, as detailed in Refs. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Figures\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a\u0026ndash;c) display the evolution of the P\u0026ndash;T behavior for NBHT ceramics sintered at different temperatures, while Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003cb\u003e(d\u0026ndash;f)\u003c/b\u003e present the resulting electrocaloric temperature change (ΔT) as a function of operating temperature and applied electric field. These results reveal distinct EC responses correlated with sintering-induced microstructural variations, highlighting the influence of densification and phase composition on the functional thermal behavior of NBHT ceramics.\u003c/p\u003e\u003cp\u003e\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\varDelta\\:T=-\\frac{1}{\\rho\\:{C}_{p}}{\\int\\:}_{{E}_{1}}^{{E}_{2}}T\\frac{dP}{dT}dE$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe specific heat capacities of NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C were determined from standard literature [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] to be 0.540 J\u0026middot;g⁻\u0026sup1;\u0026middot;K⁻\u0026sup1;, 0.531 J\u0026middot;g⁻\u0026sup1;\u0026middot;K⁻\u0026sup1;, and 0.532 J\u0026middot;g⁻\u0026sup1;\u0026middot;K⁻\u0026sup1; respectively, measured at room temperature (RT). In this context, P denotes polarization, T represents temperature, and E\u003csub\u003e1\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003e correspond to the initial and final applied electric fields used in the indirect calculation of the electrocaloric parameters. The bulk densities of the NBHT ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C were found to be 5.92 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, 5.85 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, and 5.82 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, respectively, confirming excellent densification with slight variation at higher sintering temperatures.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003cb\u003e(d\u0026ndash;f)\u003c/b\u003e presents the calculated electrocaloric temperature change (ΔT) as a function of temperature for NBHT ceramics subjected to various applied electric fields. A pronounced electrocaloric effect (ECE) is observed near the ferroelectric-to-antiferroelectric transition temperature (T\u003csub\u003ed\u003c/sub\u003e), which represents the region of maximum polarization change. Under an applied electric field of 40 kV\u0026middot;cm⁻\u0026sup1;, the maximum ΔT values recorded for the ceramics sintered at 1100\u0026deg;C, 1150\u0026deg;C, and 1200\u0026deg;C are 0.35 K, 0.30 K, and 0.21 K, respectively. The reduced ΔT in the sample sintered at 1200\u0026deg;C is attributed to its relatively lower density and the presence of crystal imperfections, resulting from non-uniform grain growth and partial structural disorder at elevated temperatures.\u003c/p\u003e\u003cp\u003eThe variations in EC response among the samples arise from multiple factors, including grain size, phase coexistence, electric field strength, sintering conditions, and intrinsic compositional effects. The superior ΔT observed in the 1100\u0026deg;C and 1150\u0026deg;C samples reflects the optimized balance between grain size and densification, which enhances dipole reorientation and entropy change under the applied field. For practical applications in solid-state cooling, achieving a high ΔT at low electric fields near ambient temperature is crucial. The measured ΔT values in this study are comparable to or higher than those reported in previous literature, underscoring the strong electrocaloric performance of NBHT ceramics. Their substantial ΔT near room temperature and operation under moderate electric fields make these materials promising candidates for environmentally friendly electrocaloric cooling devices.\u003c/p\u003e\u003cp\u003ePolarization\u0026ndash;electric field (P\u0026ndash;E) hysteresis loops were recorded at a frequency of 1 Hz over a temperature range of 40\u0026deg;C to 120\u0026deg;C in 5\u0026deg;C intervals, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e, to assess the energy storage characteristics of the NBHT ceramics. As the temperature increases, both the coercive field (EC) and remanent polarization (P\u003csub\u003er\u003c/sub\u003e) exhibit a gradual decrease, indicating thermally induced softening of ferroelectric domains and a reduction in switching barriers. The distinct separation between the charging and discharging branches of the P\u0026ndash;E loops reflects the material\u0026rsquo;s ability to store and release electrical energy. The positive quadrant of the P\u0026ndash;E loop is typically employed to determine the recoverable energy storage density (W\u003csub\u003erec\u003c/sub\u003e), energy loss density (W\u003csub\u003eloss\u003c/sub\u003e), and energy storage efficiency (η) of the ceramics. The recoverable energy density (W\u003csub\u003erec\u003c/sub\u003e) for dielectric capacitors is evaluated from the area enclosed between the polarization and electric field curves using the following relation:\u003c/p\u003e\u003cp\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:{W}_{rec}={\\int\\:}_{{P}_{r}}^{{P}_{max}}Edp$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere Pₘₐₓ and P\u003csub\u003er\u003c/sub\u003e represent the maximum and remanent polarizations, respectively. The total stored energy density (\u003cem\u003eWₜₒₜ\u003c/em\u003e) corresponds to the area under the charging curve, while the energy storage efficiency (η) is given by: This analytical approach provides a quantitative measure of the energy storage capability and thermal stability of NBHT ceramics under varying operational temperatures. Where P represents the saturated polarization, P\u003csub\u003er\u003c/sub\u003e denotes the remanent polarization, and Pₘₐₓ is the maximum polarization attained under a given electric field (E). A larger difference between Pₘₐₓ and P\u003csub\u003er\u003c/sub\u003e (i.e., \u003cem\u003ePₘₐₓ \u0026ndash; P\u003csub\u003er\u003c/sub\u003e\u003c/em\u003e), together with a higher dielectric breakdown strength (EB), is crucial for achieving enhanced energy storage capability. While improvements in sintering optimization, doping, and microstructural control can significantly increase the EB value, certain intrinsic dielectric breakdown mechanisms remain fundamentally dependent on the chemical composition and defect structure of the material. The energy loss and storage efficiency (η) were calculated using the standard procedures reported in [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{W}_{loss}={\\int\\:}_{0}^{{P}_{max}}Edp$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$\\:\\eta\\:=\\frac{{W}_{rec}}{{W}_{rec}+{W}_{loss}}\\times\\:100\\%$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eNotably, the recoverable energy density (W\u003csub\u003erec\u003c/sub\u003e) values obtained from the room-temperature polarization loops were 0.40 J/cm\u003csup\u003e3\u003c/sup\u003e, 0.95 J/cm\u003csup\u003e3\u003c/sup\u003e, and 1.02 J/cm\u003csup\u003e3\u003c/sup\u003e for NBHT ceramics sintered at 1100\u0026deg;C, 1250\u0026deg;C, and 1200\u0026deg;C, respectively. These samples exhibited relatively low energy loss, achieving an energy storage efficiency of approximately 80%. The corresponding polarization loops for the NBHT ceramics sintered at 1200\u0026deg;C are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(c\u0026ndash;d)\u003c/b\u003e, which were used to estimate the variation of W\u003csub\u003erec\u003c/sub\u003e with temperature. Despite remarkable progress, the thermal stability of energy storage performance continues to be a critical challenge for advanced dielectric capacitors. To address this issue, the polarization\u0026ndash;electric field (P\u0026ndash;E) behavior and energy storage characteristics of NBHT ceramics sintered at 1200\u0026deg;C were systematically examined across a temperature range of 30\u0026deg;C to 120\u0026deg;C, with 5\u0026deg;C increments. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(c\u0026ndash;d)\u003c/b\u003e, the specimens exhibit both high energy storage density and excellent thermal stability. The remarkable retention of polarization behavior at elevated temperatures indicates that the typically competing characteristics of high energy density and temperature stability have been effectively decoupled. The NBHT ceramics sintered at 1200\u0026deg;C deliver a maximum energy storage density of 1.52 J/cm\u003csup\u003e3\u003c/sup\u003e and an efficiency of 80.13% under an applied field of 40 kV/cm, marking a significant advancement among lead-free ferroelectric ceramics. These performance metrics remain stable throughout the 40\u0026deg;C\u0026ndash;120\u0026deg;C temperature range, with minimal variation within \u0026plusmn;\u0026thinsp;5% for W\u003csub\u003erec\u003c/sub\u003e and \u0026plusmn;\u0026thinsp;3% for η. The presence of dense grain boundaries effectively restricts domain wall motion and hinders complete polarization alignment, causing the induced polarization to rapidly decay after the removal of the external field. This results in low remnant polarization (P\u003csub\u003er\u003c/sub\u003e), thereby enhancing the energy storage capability. In contrast, BHT ceramics sintered at higher temperatures tend to develop larger grains and higher P\u003csub\u003er\u003c/sub\u003e values, associated with long-range ferroelectric domains and easier polarization orientation [\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Overall, the energy storage performance of lead-free ceramics is strongly influenced by factors such as density, grain size, and sintering temperature. In this study, the optimized NBHT ceramics demonstrate superior energy storage density, efficiency, and thermal reliability, establishing them as a promising lead-free ferroelectric system for next-generation sustainable energy storage applications.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe study underscores the sintering temperature as a decisive parameter governing the structural, microstructural, and functional characteristics of NBHT ceramics. XRD analysis confirmed the formation of a single-phase structure with no secondary phases up to 1200\u0026deg;C, accompanied by progressive improvement in crystallinity and grain growth at elevated temperatures. The optimum dielectric, ferroelectric, and electrocaloric responses were achieved at 1150\u0026deg;C, attributed to enhanced densification and a well-balanced grain size distribution. Although higher densification occurred at 1200\u0026deg;C, irregular grain growth and the possible emergence of structural defects contributed to reduced polarization and a decline in electrocaloric efficiency. A maximum recoverable energy density of 1.52 J/cm\u003csup\u003e3\u003c/sup\u003e and an energy efficiency of approximately 80% were recorded for the sample sintered at 1200\u0026deg;C, indicating a significant improvement in energy storage capability with increasing sintering temperature. Overall, the NBHT ceramics demonstrated superior electrocaloric and energy storage performance at optimized sintering conditions. These findings highlight the promise of NBHT as a lead-free ferroelectric material for the realization of high-performance solid-state cooling and next-generation energy storage devices, paving the way toward environmentally sustainable alternatives to conventional lead-based systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKuppam Mohan Babu thanks to Dr. A Chittibabu, department of physics, C. R. Reddy College of Engineering, is thanked for his assistance with structural refinement.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that all the data generated or analyzed during this study are included in this manuscript\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eKuppam Mohan Babu:\u003c/strong\u003eProject administration, Conceptualization, Methodology, Writing-\u003c/p\u003e\n\u003cp\u003eOriginal draft, Data curation, \u003cstrong\u003eK. Vijaya Lakshmi:\u003c/strong\u003e Visualization, Software, Review, \u003cstrong\u003ePandiSreenivasaRao:\u003c/strong\u003eVisualization, Software, Review, \u003cstrong\u003eP. Sobhanachalam:\u003c/strong\u003e Resources, \u003cstrong\u003eSanthosh Kumar R:\u003c/strong\u003e Resources, \u003cstrong\u003eM. GnanaKiran:\u0026nbsp;\u003c/strong\u003eMethodology, Review, \u003cstrong\u003eRajesh. V:\u0026nbsp;\u003c/strong\u003eResources, Auditing Manuscript, \u003cstrong\u003eB. Venkateswarlu:\u0026nbsp;\u003c/strong\u003eVisualization, Software, Review, \u003cstrong\u003eP.Mohan babu:\u003c/strong\u003e Resources, \u003cstrong\u003eRamanaiahMalla:\u003c/strong\u003e Conceptualization, Writing-original draft, Review.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFunding Support\u003c/strong\u003e: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eP. E. Ohenhen, O. Chidolue, A. A. Umoh, B. Ngozichukwu, A. V. Fafure, V. I. Ilojianya\u0026amp; K. I. 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Negi, Influence of sintering temperature on electrocaloric and energy storage properties of sol-gel derived lead-free BaHf0. 20Ti0.80O3 ferroelectric ceramics for sustainable energy solutions. \u003cstrong\u003e\u003cem\u003eMaterials Chemistry and Physics\u003c/em\u003e\u003c/strong\u003e, 319, 129374(2024).\u003c/li\u003e\n \u003cli\u003eJ. Shi, R. Dong, J. He, D. Wu, W. Tian\u0026amp; X. Liu, Regulating ferroelectric polarization and dielectric properties of BT-based lead-free ceramics. \u003cstrong\u003e\u003cem\u003eJournal of Alloys and Compounds\u003c/em\u003e\u003c/strong\u003e, 933, 167746(2023).\u003c/li\u003e\n \u003cli\u003eO. A. Ramdasi, P. S. Kadhane, Y. D. Kolekar, V. R. Reddy \u0026amp; R. C. Kambale, Nonlinearities in ferroelectric, piezoelectric, and dielectric behavior of Hf incorporated BaTiO3 nontoxic electroceramics. \u003cstrong\u003e\u003cem\u003eJournal of Materials Science: Materials in Electronics\u003c/em\u003e\u003c/strong\u003e, 31, 18803-18815(2020).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003e\u003cstrong\u003eTable 1:\u0026nbsp;\u003c/strong\u003eThe structural refined parameters and lattice constants of NBHT ceramic sintered at 1200˚C.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" align=\"\" width=\"505\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCc\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 80.9901%;\"\u003e\n \u003cp\u003eNBHT-1200\u003csup\u003e0\u003c/sup\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003ePositions\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e\u003cem\u003ex\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e\u003cem\u003ey\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e\u003cem\u003ez\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eNa/Bi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.2517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eTi/Hf\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.2517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.1982\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.2521\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.5107\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\" valign=\"top\" style=\"width: 100%;\"\u003e\n \u003cp\u003e\u003cem\u003ea\u003c/em\u003e =9.4460\u0026Aring;,\u003cem\u003e\u0026nbsp;b=\u003c/em\u003e 5.4783\u0026Aring;, c\u003cem\u003e\u0026nbsp;=\u003c/em\u003e 5.6120\u0026Aring;, \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eP4mm\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 80.9901%;\"\u003e\n \u003cp\u003eNBHT-1200\u003csup\u003e0\u003c/sup\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003ePositions\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e\u003cem\u003ex\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e\u003cem\u003ey\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e\u003cem\u003ez\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eNa/Bi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0010\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.49381\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.2500\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eTi/Hf\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.2517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0000\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eO\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e-0.0572\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.0020\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.2510\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 19.0099%;\"\u003e\n \u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.2130\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 29.1089%;\"\u003e\n \u003cp\u003e0.2870\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 25.9406%;\"\u003e\n \u003cp\u003e0.0267\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\" valign=\"top\" style=\"width: 100%;\"\u003e\n \u003cp\u003e\u003cem\u003ea\u003c/em\u003e =3.8844\u0026Aring;,\u003cem\u003e\u0026nbsp;b=\u003c/em\u003e 3.8844\u0026Aring;, \u0026nbsp;c\u003cem\u003e\u0026nbsp;=\u003c/em\u003e 3.9449\u0026Aring;, \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Energy storage, Electrocalorics, Dielectrics, Lead-free ceramics, Polarization","lastPublishedDoi":"10.21203/rs.3.rs-8021474/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8021474/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFerroelectric materials are widely regarded as promising candidates for sustainable energy technologies, particularly in solid-state cooling and energy-storage applications. In this work, the microstructural, dielectric, electrocaloric, and energy-storage properties of sol–gel-derived\u003cstrong\u003e \u003c/strong\u003eNa\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eHf\u003csub\u003e0.4\u003c/sub\u003eTi\u003csub\u003e0.6\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (NBHT) ceramics were systematically investigated. X-ray diffraction (XRD) analysis confirmed the coexistence of monoclinic and tetragonal phases, with an enhanced tetragonal contribution observed at higher sintering temperatures. Increasing the sintering temperature facilitated grain growth, improved densification, and significantly enhanced the dielectric and ferroelectric responses. The NBHT ceramic sintered at 1100 °C exhibited a maximum electrocaloric temperature change (ΔT) of 0.35 K and a recoverable energy density (W\u003csub\u003erec\u003c/sub\u003e) of 1.52 J cm\u003csup\u003e-3\u003c/sup\u003e with an energy efficiency of approximately 80% at 120 °C. In contrast, the sample sintered at 1200 °C demonstrated a W\u003csub\u003erec\u003c/sub\u003e of 1.02 J cm\u003csup\u003e-3\u003c/sup\u003e and high efficiency of 79.13% under an applied electric field of 40 kV cm\u003csup\u003e-1\u003c/sup\u003e. These findings reveal that the NBHT ceramics exhibit a strong electrocaloric effect coupled with excellent energy-storage performance, underscoring their potential as lead-free ferroelectric\u003cstrong\u003e \u003c/strong\u003ematerials for next-generation solid-state cooling and energy-storage devices.\u003c/p\u003e","manuscriptTitle":"Electrocaloric and energy storage properties of lead-free Na0.5Bi0.5Ti0.6Hf0.4O3 ferroelectric ceramics for sustainable energy solutions are affected by the sintering temperatures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 04:28:01","doi":"10.21203/rs.3.rs-8021474/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0eebaed4-ddde-4abb-a9cb-d464ab18eda7","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-22T16:08:39+00:00","versionOfRecord":{"articleIdentity":"rs-8021474","link":"https://doi.org/10.1007/s10854-025-16400-8","journal":{"identity":"journal-of-materials-science-materials-in-electronics","isVorOnly":false,"title":"Journal of Materials Science: Materials in Electronics"},"publishedOn":"2025-12-17 15:57:55","publishedOnDateReadable":"December 17th, 2025"},"versionCreatedAt":"2025-11-25 04:28:01","video":"","vorDoi":"10.1007/s10854-025-16400-8","vorDoiUrl":"https://doi.org/10.1007/s10854-025-16400-8","workflowStages":[]},"version":"v1","identity":"rs-8021474","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8021474","identity":"rs-8021474","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}