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在传统铁电材料中，电场可以诱导无序的自发极化沿外场方向排列，伴随温度升高，极化熵降低；撤去电场后温度下降，极化熵增大。电卡制冷采用循环的方式实现对热量的“搬运”。但在PbZrO3等反铁电材料中，可以观测到施加电场温度降低的现象。这种与电卡效应相反的“负”电卡效应可以和正电卡效应结合，设计循环制冷器件，大幅提高电卡制冷效率。但在微观尺度，反铁电材料负电卡效应的起源仍存在争议，缺乏系统的机理研究。此外，低电卡强度和窄工作温区仍是制约负电卡制冷应用的两大难点。
来自北京理工大学前沿交叉科学研究院的黄厚兵教授团队，采用相场法从畴尺度揭示了PbZrO3反铁电材料负电卡效应的产生机理。提出通过引入高密度极化界面的方式，提升负电卡效应的工作温区。
他们建立了原型PbZrO3的温度-电场相图，发现在反铁电和铁电相界处存在两相共存的区域。这种逐步相变而非瞬态相变的特征能够提升负电卡效应的工作温区。相场模拟表明，反铁电→铁电和铁电→反铁电逆相变的形核位点均倾向于畴壁或局域非公度界面处。因此，通过朗道自由能势函数的涨落诱导出反铁电纳米畴，实现高密度极化界面的引入，能够将负电卡工作温区提升至75 K(电场强度42 kV/cm)，最大绝热温变约-13 K(电场强度84 kV/cm)。
Fig. 1 | Schematic diagrams of polar boundaries modulating the negative ECE properties of PZO-based AFE materials.
Fig. 2 | Temperature-electric field phase diagram of PZO polymorphic domain.
该文近期发表于npj Computational Materials 10:150 2024，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Design of polar boundaries enhancing negative electrocaloric performance by antiferroelectric phase-field simulations
Ke Xu, Xiaoming Shi, Cancan Shao, Shouzhe Dong & Houbing Huang
Electrocaloric refrigeration which is environmentally benign has attracted considerable attention. In distinction to ferroelectric materials, which exhibit an extremely high positive electrocaloric effect near the Curie temperature, antiferroelectric materials represented by PbZrO3 have a specific negative electrocaloric effect, i.e., electric field decreases the temperature of the material. However, the explanation of the microscopic mechanism of the negative electrocaloric effect is still unclear, and further research is still needed to provide a theoretical basis for the negative electrocaloric effect enhancement. Herein, the antiferroelectric phase-field model was proposed to design polar boundaries enhancing antiferroelectric negative electrocaloric performance in PbZrO3-based materials. Based on this, we have simulated the polarization response and domain switching process of the temperature and electric field-induced antiferroelectric - ferroelectric phase transition. It is shown that the temperature range tends to increase as the density of polar boundaries increases from the antiferroelectric stripe domain, polymorphic domain to the nanodomain. Among them, the peak adiabatic temperature change of antiferroelectric nanodomains can reach -13.05 K at 84 kV/cm, and a wide temperature range of about 75 K can be realized at 42 kV/cm. We expect these discoveries to spur further interest in the potential applications of antiferroelectric materials for next-generation refrigeration devices.
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