固体制冷用铋基钙钛矿无铅弛豫铁电陶瓷的电卡效应及其机理研究

Compared to the magneto-caloric cooling that needs lots of accessories, the technology of electric field induced cooling has attracted attentions in recent years as a result of its simple structure and low cost. In view of the present problems concerning to the solid-state cryogenic technologies, we use the lead-free Bi-based relaxor ferroelectric ceramics with perovskite structure as the research object to explore: 1) The evolution of electro-caloric effect (ECE) in the vicinity of ferroelectric-relaxor phase transition boundary; 2) Uncover the relationship between the relaxor properties and ECE as well as the its dependence of temperature, besides, the difference of ECE between morphotropic phase boundary and the ferroelectric-relaxor phase boundary is also investigated; 3) Clarify the intrinsic mechanism for the giant ECE in order to construct the interrelation among composition-phase structure-relaxor characteristics-ECE; 4) Modulating the ferroelectric-relaxor transition temperature and the design of the multi-layer ceramic device for realization of the giant ECE in a wide temperature range. What’s more, the direct method of adiabatic calorimetry is adopted to precisely characterize the temperature change to reflect the intrinsic ECE evolution. Through the above study, we should clear the relationship between the ferroelectric-relaxor phase transition and ECE, the relaxor properties and the temperature stability of ECE, and aim to explore the giant ECE in Bi-based relaxor ferroelectric ceramics, thus making a contribution to the advancement of environmental-friendly solid-state cryogenic technologies.

相比磁卡制冷需要庞大的附加机构，电场诱发的制冷技术由于其结构简单、成本低等特点受到广泛关注。针对目前固态制冷材料中所存在的问题，本项目拟以铋基钙钛矿无铅弛豫铁电陶瓷为研究对象，主要研究内容包括：1)探究在铁电-弛豫相转变温度附近电卡效应的演变行为；2)揭示弛豫行为与电卡效应、温度稳定性之间的关系以及在准同型相界和铁电-弛豫相界处电卡效应的异同；3) 阐明大电卡效应产生的本征机理，建立材料组成-相结构-弛豫特征-电卡效应之间的相互联系；4)通过对铁电与弛豫相转变温度的调控和多层结构陶瓷器件的设计制备，实现宽温区和大电卡效应。拟采用高精度绝热量热仪来直接测量体系的温度变化，旨在探究电卡效应的本征演变规律。通过以上的研究，明确铁电-弛豫相转变与电卡效应演变以及弛豫行为与电卡效应、温度稳定性之间的关系，旨在寻找一种宽温区、大电卡效应的铋基钙钛矿弛豫铁电材料，以此推动我国固态环保制冷材料的发展。

环保型无铅电卡材料的实际应用高度依赖于大的制冷性能和宽的工作温区，所以对电卡效应和服役温区的优化十分迫切和重要。基于这两种挑战，本项目利用铋基钙钛矿弛豫铁电陶瓷通过组分调控和结构设计，成功研制出具有宽工作温区和优异电卡性能的BNT基陶瓷，并系统地分析了介电、压电、铁电、极化电流、局域结构、相结构、畴结构的演化规律以及与温度、电场的依赖关系，取得以下代表性的研究结果：1. 对钛酸铋钠/钛酸铋钾引入掺杂组分或改变温度来诱导铁电-弛豫相变，优化电卡性能，明确了组分/温度在调制过程中的作用；2. 对于钛酸铋钠钾体系，利用本征的Na+/K+离子比例调控，进行电卡性能优化，同时揭示了本征优化机制；3. 构造A位和B位局域缺陷场调控BNT基钙钛矿结构，利用缺陷偶极子增大电场施加和撤销过程中偶极熵的变化，进而提高电卡响应；4. 多层三明治正反梯度结构设计有效地拓宽了电卡效应的工作温区。这些研究结果可以为电卡材料在结构设计提供指导路线和理论依据，推动电卡陶瓷材料的应用进程。