(111)取向无铅K_(0.5)Na_(0.5)NbO_(3)外延薄膜的相变和电卡效应:外应力与错配应变效应  

Phase transitions and electrocaloric effects of(111)-oriented K_(0.5)Na_(0.5)NbO_(3) epitaxial films:effect of external stress and misfit strains

作　　者：白刚 韩宇航 高存法[2] Bai Gang;Han Yu-Hang;Gao Cun-Fa(College of Integrated Circuit Science and Engineering,Nanjing University of Posts and Telecommunications,Nanjing 210026,China;State Key Laboratory of Mechanics and Control of Mechanical Structures,Nanjing University of Aeronautics and Astronautics,Nanjing 210016,China)

机构地区：[1]南京邮电大学集成电路科学与工程学院,南京210026 [2]南京航空航天大学,机械结构力学及控制国家重点实验室,南京210016

出　　处：《物理学报》2022年第9期341-348,共8页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:51602159)资助的课题。

摘　　要：无铅K_(1–x)Na_(x)NbO_(3)薄膜作为传感器以及机电和电卡冷却装置的候选者越来越受到关注.但是(111)取向K_(1–x)Na_(x)NbO_(3)薄膜的相变与电卡效应的内在关联还并不清楚.本文首先推导出基于八阶朗道自由能多项式的(111)取向铁电薄膜的热力学势,并在此基础上建立了K_(0.5)Na_(0.5)NbO_(3)薄膜温度-错配应变相图和室温错配应变-面外应力相图.重点研究了(111)取向K_(0.5)Na_(0.5)NbO_(3)薄膜的室温电卡效应的应变和取向控制,这对于实际的电卡制冷应用至关重要.研究发现,在无面外应力和零错配应变下,三方铁电-顺电相变附近,30 MV/m电场下K_(0.5)Na_(0.5)NbO_(3)薄膜在居里温度附近(约673 K)最大电卡绝热温变ΔT可高达18 K.施加约-6.7 GPa的面外应力可以有效地将居里温度降低至室温,但代价是最大绝热温变ΔT降低至7.5 K.本工作为应变和取向工程调控K_(1-x)Na_(x)NbO_(3)基薄膜的相变和电卡性能提供了理论指导.Lead-free K_(1–x)Na_(x)NbO_(3)thin films,as a candidate for sensors and electromechanical and electrocaloric cooling devices,have increasingly received attention.However,for(111)-oriented films,the relation between phase transitions and electrocaloric effect is not clear.Here,we derive the thermodynamic potential of(111)-oriented thin film ferroelectrics K_(1–x)Na_(x)NbO_(3)based on the 8^(th) order polynomial function,and then establish the temperature-misfit strain and out-of-plane stress-in-plane misfit strain phase diagrams and calculate electrocaloric(EC)entropy changes ΔS and temperature changes ΔT.This study focuses on mechanical and orientation controls of room-temperature EC effect of K_(0.5)Na_(0.5)NbO_(3)films,which is critical for environmentally friendly electrocaloric refrigeration applications in practice.Under the stress-free and zero misfit strain conditions,the(111)-oriented K_(0.5)Na_(0.5)NbO_(3)film in an electric field of 30 MV/m has a maximum EC ΔT of~18 K near the rhombohedral ferroelectric-paraelectric phase transition temperature(about 673 K).However,an out-of-plane stress of about-6.7GPa can reduce the optimal operating temperature to room temperature where the K_(0.5)Na_(0.5)NbO_(3)film has the EC ΔT of~7.5 K under the action of applied electric field of 30 MV/m.The present work provides theoretical guidance for exploring the strain engineering and orientation engineering of K_(1–x)Na_(x)NbO_(3)-based thin films with optimized electrocaloric and electromechanical properties.

关 键 词：铁电薄膜 电卡效应 取向 应变 

分 类 号：TB383.2[一般工业技术—材料科学与工程]