Bimodal-Structured 0.9KNbO₃-0.1BaTiO₃ Solid Solutions with Highly Enhanced Electrocaloric Effect at Room Temperature.

The electrocaloric effect (ECE) is a phenomenon where an electric field induces a temperature change in a material. While ECE has been observed in various dielectrics, achieving strong effects at room temperature remains a challenge. Previous studies have explored perovskite oxides, including potassium niobate (KNbO₃) and barium titanate (BaTiO₃), for their potential in electrocaloric cooling. However, the ECE in these materials is typically small and occurs near phase transitions that are not at ambient conditions. This gap motivated the search for new synthesis methods that could enhance ECE at room temperature. Prior research has shown that grain size and microstructure significantly influence dielectric and electrocaloric properties. That uncertainty drove the investigation into bimodal grain structures as a means to manipulate the ECE response. No prior work had resolved how abnormal grain growth could be leveraged to improve electrocaloric performance in KNbO₃-based systems.

This study aimed to develop a novel ceramic material with a bimodal grain size distribution to enhance the electrocaloric effect at room temperature. The specific problem addressed was the weak ECE observed in conventional KNbO₃-based ceramics. The motivation stemmed from the need for efficient cooling materials that operate at ambient conditions. The researchers proposed that introducing a bimodal grain structure could influence the material’s polarization behavior and, consequently, its electrocaloric response. The study sought to confirm whether abnormal grain growth could be used to create such a structure. Additionally, the team aimed to evaluate the material’s ECE performance using direct measurements of temperature and entropy changes. The goal was to demonstrate a practical method for synthesizing high-performance electrocaloric materials. The researchers also intended to analyze the microstructure using high-resolution transmission electron microscopy to understand the underlying mechanisms.

The researchers used an induced abnormal grain growth (IAGG) method to synthesize 0.9KNbO₃-0.1BaTiO₃ ceramics. The process involved combining micron-sized filler powders with a sol precursor matrix. During sintering, the filler powders formed extra-large grains (~10-50 μm), while the precursor matrix produced fine grains (~0.05-0.35 μm). This bimodal grain size distribution was achieved at a relatively low sintering temperature. The team then characterized the microstructure using high-resolution transmission electron microscopy. They analyzed polar nanodomain regions to confirm the material’s relaxor-like behavior. To assess the electrocaloric effect, the researchers applied an electric field and measured adiabatic temperature changes and isothermal entropy changes. They used direct measurements at an electric field of 1.0 MV·m⁻¹. The study also evaluated the ECE strengths by calculating ΔT/ΔE and ΔS/ΔE values.

The 0.9KNbO₃-0.1BaTiO₃ ceramics exhibited a bimodal grain size distribution with extra-large and fine grains. The material displayed relaxor-like behavior with a diffused phase transition near room temperature. High-resolution transmission electron microscopy revealed polar nanodomain regions, indicating structural heterogeneity. The electrocaloric effect was significantly enhanced, with an adiabatic temperature drop (ΔT) of 1.5 K observed. The isothermal entropy change (ΔS) reached 2.48 J·kg⁻¹·K⁻¹. The ECE strengths were measured at |ΔT/ΔE| = 1.50 × 10⁻⁶ K·m·V⁻¹ and ΔS/ΔE = 2.48 × 10⁻⁶ J·m·kg⁻¹·K⁻¹·V⁻¹. These values were directly measured at an electric field of 1.0 MV·m⁻¹. The researchers confirmed that the bimodal grain structure played a key role in enhancing the ECE. The results suggest that the IAGG method is effective in creating high-performance electrocaloric materials.

The study demonstrated that the IAGG method successfully produced 0.9KNbO₃-0.1BaTiO₃ ceramics with a bimodal grain size distribution. The material’s relaxor-like behavior and polar nanodomain regions were confirmed through microscopy. The researchers observed a large electrocaloric effect at room temperature, with ΔT and ΔS values significantly higher than previously reported. The ECE strengths were among the highest measured for this class of materials. The findings suggest that the bimodal grain structure enhances the electrocaloric response. The authors propose that the abnormal grain growth method is a promising approach for synthesizing high-performance electrocaloric materials. The study highlights the importance of microstructure in influencing ECE performance. The results support the potential of these ceramics for use in efficient cooling devices.