Effect of CuO/Li₂CO₃ on Electrocaloric Effect of Barium Titanate-Based Ferroelectric Ceramics

Refrigeration technology is widely used in military, commercial, industrial, and civilian fields. However, the existing mechanical compression refrigeration technology faces limitations such as large volume, high energy consumption, low cooling efficiency, and environmental unfriendliness, making it unable to meet the growing demands of emerging technological fields. The electrocaloric effect (ECE), which refers to the alignment of dipoles in polar materials under an electric field, leading to changes in entropy (ΔS) and temperature (ΔT), offers a promising alternative. Solid-state electrocaloric coolers based on the ECE are directly driven by electrical energy, which can achieve heat transfer and cooling. They exhibit some advantages such as high energy efficiency, ease of miniaturization and integration, and environmental friendliness, making them a highly potential and efficient refrigeration technology. Barium titanate (BaTiO₃)-based ceramics have attracted extensive research attention in the electrocaloric field due to their excellent polarization strength and rich phase structures. However, some challenges such as low breakdown electric fields, high sintering temperatures, and the inability to simultaneously optimize working temperature ranges and polarization strength hinder their practical applications. To address these issues, this work was to incorporate sintering additives CuO/Li₂CO₃ into lead-free Ba_0.97Ca_0.03Sn_0.1Ti_0.9O₃ ceramics to promote dense grain growth and optimize the breakdown electric field. The dielectric constant was improved, while maintaining the broad temperature range and continuous phase transition characteristics of BCST ceramics, ultimately achieving a large electrocaloric temperature change of 3.37 K (@55 ℃, 160 kV·cm^-1).

Ba_0.97Ca_0.03Sn_0.1Ti_0.9O₃ powder was synthesized by a conventional solid-state reaction sintering method at 1200 ℃ with BaCO₃, CaCO₃, SnO₂, and TiO₂ as raw materials in a stoichiometric ratio. The ceramic powder was then pressed into disks with the dimensions of ϕ13 mm×1 mm and sintered at 1350–1500 ℃ for 3 h to obtain dense ceramic samples. After sintering, the ceramic samples were polished, and gold electrodes were sputtered onto their surfaces. The polarization-electric field hysteresis loops (P−E loop) at room temperature were recorded by a model FETS-2000 ferroelectric tester (Wuhan Yanhe Technology Co., Ltd., China) under a frequency of 10 Hz. The values of ΔT were tested through a model RdF P/N 27134-3 heat flux sensor. The dielectric properties were obtained by a model PK-CPT1705 test system (PolyK Technologies Co., USA) with a testing temperature range from 25 to 150 ℃ and a frequency range from 100 Hz to 100 kHz. The microstructure of the grain was determined by a model GeminiSEM 300 scanning electron microscope (SEM) (Zeiss Co., Germany) and a model 7000 S/L X-ray diffractometer (XRD) (Shimadzu Corp., Japan).

The XRD patterns reveal that all ceramic samples exhibit a pure perovskite structure, indicating that Cu²⁺ and Li⁺ ions diffuse into the BCST lattice to form a solid solution. The addition of CuO/Li₂CO₃ causes the XRD diffraction peaks shifting to lower angles, indicating that Li⁺(the radius of 0.73 Å) and Cu²⁺(0.71 Å) substitute the B-site Ti⁴⁺(0.61 Å) and Sn⁴⁺(0.71 Å), leading to a lattice expansion. The SEM images demonstrate that the incorporation of CuO/Li₂CO₃ results in significant grain growth and well-developed, highly dense microstructures in the ceramic samples. The grain size increases from 7 μm to 46 μm due to the low melting points of CuO/Li₂CO₃, which induces liquid-phase sintering effects during the early stages of the sintering process, thus promoting mass transfer and facilitating grain growth and ceramic densification. Dielectric temperature spectra indicate that the addition of CuO/Li₂CO₃ significantly enhances the dielectric constant of BCST ceramics. This improvement can be attributed to two factors, i.e., the increase in grain size; second, the aliovalent acceptor doping of Cu²⁺ and Li⁺ substituting Ti⁴⁺ and Sn⁴⁺, which generates lattice defects and oxygen vacancies V_O^... These defects coupled to form dipole clusters, thereby enhancing the dielectric constant. Also, the local random electric fields caused by the charge mismatch between Cu²⁺/Li⁺ and Ti⁴⁺/Sn⁴⁺, as well as the average ionic size effect, due to the reduction in the phase transition temperature. Polarization–electric field (P–E) loops show a significant increase in the polarization strength of the ceramic samples with the addition of CuO/Li₂CO₃, which can also be attributed to the role of dipole clusters. The electrocaloric performance of the material is greatly enhanced, with the BCST-CL ceramic achieving a giant electrocaloric temperature change of ΔT = 3.37 K (i.e., at 55 ℃, E = 160 kV·cm^–1).

The Ba_0.97Ca_0.03Sn_0.1Ti_0.9O₃ (BCST) ceramic samples with varying amounts of CuO/Li₂CO₃ sintering additives exhibited a single perovskite structure, characterized by large, well-developed grains and a dense microstructure. The addition of CuO/Li₂CO₃ sintering agents enhanced the dielectric constant of the material, while preserving the broad continuous phase transition temperature range of BCST. During the sintering process, Cu²⁺ and Li⁺ ions entered the grains, leading to lattice expansion and increased ferroelectric displacement. Also, as acceptor dopants, they formed defect dipole clusters with oxygen vacancies, thereby improving the material's polarization strength. The direct measurement tests demonstrated a significant enhancement in the electrocaloric temperature change (ΔT) of the ceramic samples, while maintaining excellent operational stability. The BCST-CL composition achieved a maximum ΔT of 3.37 K (at 55 ℃, 160 kV·cm^–1), providing a feasible strategy for further improving the performance of electrocaloric ceramic materials.