Large Electrocaloric Effect in (Bi_0.5Na_0.5)TiO₃-Based Relaxor Ferroelectrics

Area of Science:

• Materials Science
• Solid State Physics
• Thermodynamics

Background:

• The electrocaloric effect (ECE) is crucial for developing environmentally friendly cooling technologies, offering an alternative to traditional vapor compression refrigeration.
• Lead-free bismuth sodium titanate-based ferroelectrics show promise for high ECE due to their depolarization process, but their high depolarization temperature (Td) limits room temperature (RT) applications.
• Previous attempts to shift Td towards RT via chemical doping often compromise essential ferroelectric properties.

Purpose of the Study:

• To design a lead-free ferroelectric ceramic with a depolarization temperature (Td) shifted closer to room temperature (RT).
• To achieve a high electrocaloric effect (ECE) in the designed material at temperatures near RT.
• To investigate the underlying mechanisms responsible for the enhanced ECE performance.

Main Methods:

• Development of a novel relaxor ferroelectric ceramic system: {[Bi0.5(Na0.72K0.18Li0.1)0.5]1-xSrx}TiO3 (BNKLSxT).
• Characterization of the electrocaloric effect, including adiabatic temperature change and electrocaloric coefficient measurements.
• Utilizing piezoresponse force microscopy (PFM) to analyze domain structure and polarization characteristics.

Main Results:

• The synthesized BNKLSxT ceramic system exhibits a decreased depolarization temperature (Td) approaching RT.
• A significant ECE was achieved, with an adiabatic temperature change of 2.51 K and an electrocaloric coefficient of 0.386 K mm kV−1 near Td.
• The enhanced ECE is attributed to increased maximum polarization (Pmax), resulting from Sr addition and the depolarization process, evidenced by smaller domain sizes and higher domain density observed via PFM.

Conclusions:

• The developed lead-free BNKLSxT ferroelectric ceramic offers a promising material for efficient electrocaloric (EC) cooling applications near room temperature.
• The study demonstrates a viable strategy for designing advanced EC materials by tuning depolarization temperatures and enhancing polarization.
• This research paves the way for novel lead-free ferroelectric materials tailored for next-generation, sustainable cooling technologies.