准同型相界钛酸铋钠基铁电固溶体的电卡效应研究

唐　瑾, 王飞飞, 李　岩, 田承越, 汪　青, 石旺舟

(上海师范大学 数理学院,上海 200234)

准同型相界钛酸铋钠基铁电固溶体的电卡效应研究

唐瑾, 王飞飞, 李岩, 田承越, 汪青, 石旺舟

(上海师范大学 数理学院,上海 200234)

摘要:利用铁电材料的电卡效应实现致冷成为当前国内外的研究热点之一.基于Maxwell 关系,围绕着准同型相界组分的Bi0.5Na0.5TiO3基无铅铁电固溶体,研究了其电卡效应,结果表明:该体系在5 kV/mm的外电场下,显示出优异的致冷特性,温度的变化量可达到1.15 K,相应的致冷强度可达0.23 K·mm/kV,在全新的固态致冷器件中显示出应用前景.

关键词:电卡; 无铅; 铁电致冷; 准同型相界

0引言

电卡效应是热释电效应的逆效应,它是指在极性材料中由于外加电场的改变从而引起系统的极化状态的变化,从而产生绝热温变或等温熵变[1].电卡效应提供了一种全新的固态制冷的方法,与传统的压缩致冷方法相比,铁电致冷具有高效率、环保、健康等优点;与磁致冷相比较,外电场的调控更加容易,有利于设计及器件的小型化,因此近来受到国内外研究人员的广泛关注,在一些固态的设备制冷上有着非常重要的应用前景[2].

电卡效应的研究最早可以追溯到1930年,Kobeco 和 Kurtschatov在罗息盐中发现了电卡效应[3].但在当时由于效应比较弱,没有进行定量表征,直到1956年才首次报道了SrTiO3具体的数值[4].1960~1970年左右,掀起了电卡效应的研究热潮,但是由于铁电材料性能的限制,电卡效应都相对较弱,测得的绝热温变大都在1 K以下.2006年,国际上Mischenko等人率先在PbZr0.95Ti0.05O3(PZT) 薄膜中报道了较大的绝热温变,在居里温度附近,电卡效应达到0.48 K/V[5].随后,围绕薄膜体系的电卡效应的研究取得了进一步突破,温变可达到十几K[6-8].

1实验

采用传统的固相烧结方法,制备了准同型相界组分Mn改性的0.895 Bi0.5Na.5TiO3-0.04Bi0.5K0.5TiO0.3-0.065BaTiO3(BNKBMT) 陶瓷,具体过程可参考本文作者前期研究工作[18].利用X射线衍射仪 (D8 Focus,Bruker,Germany) 对样品的结构进行分析,利用阻抗分析仪 (Agilent HP4294A,Santa Clara,CA) 测试样品的介电性能,采用铁电分析仪(TF2000 analyzer,Aixacct,Aachen,Germany)测试了不同温度的电滞回线,采用DSC (DSC-2C,Perkin Elmer,America)测试了样品的热容.

2结果与讨论

根据XRD结果,从慢扫的 (111) 和 (200) 两个特征衍射峰(分别对应三方相与四方相)中可以看出,BNKBMT陶瓷处于准同型相界组分,室温下铁电三方相和四方相共存[18].图1(a) 是BNKBMT陶瓷极化后的介电常数和损耗随温度的变化曲线,测试温度为25~400℃,测试频率为100 Hz~100 kHz.从图1(a) 可以看出,样品具有较宽的介电峰,显示出典型的弥散性相变过程,并且有两个特征温度,一个对应退极化温度Td,通常由介电损耗的峰值温度来确定,另一个是介电常数的峰值温度Tm.图1(a)表明,对于极化后的样品,当温度低于Td时,介电常数几乎不随测试频率的变化而变化,表明在外加电场的作用下,铁电微畴结构转化为宏畴结构.当温度超过Td时,因为温度扰动使得铁电宏畴结构被破坏,介电常数显示出强的色散现象.Tm附近有很宽的介电峰,表明了BNKBMT的弥散性相变特征,这主要是因为体系中A位有多种离子(Bi3+,Na+,Ba2+,K+) 存在,引起微区间成分的不均匀而造成的[18].

图1(b) 是BNKBMT不同温度下的电滞回线,测试电场 5 kV/mm,频率为10 Hz.从图1(b)中可以看出,随着温度不断升高,剩余极化和矫顽场均不断减小,当温度接近退极化温度 (70 ℃) 时,电滞回线呈现明显的双电滞回线的特点.对出现双电滞回线的原因,早期认为是由于陶瓷从三方铁电相到反铁电相的相变引起的,但目前仍然存在争议,没有有力的证据报道.当温度超过180 ℃,电滞回线表现一般电介质的响应.

(1)

图1(d) 为计算得到的不同电场下BNKBMT陶瓷的ΔT随温度的变化关系.从图1(d)中可以看出,与介温频谱相似,相应的ΔT随温度增加也呈现宽的峰,对应的峰值温度位于Td附近(100 ℃ @ 1 kV/mm).随着电场的增加,ΔT的峰值不断增大,这是由于在高电场下,BNKBMT的有序程度增加,相应的熵减少的缘故.在5 kV/mm的电场下,ΔT的最大值可达到1.15 K.

表1小结了近期报道的相关铁电单晶、陶瓷体系的电卡效应,从表1中可以看出,与其他无铅体系相比,准同型相界BNKBMT体系呈现较高的电卡强度,达到0.23 K·mm/kV,与铅基体系也相接近,显示了BNKBMT在制冷器件应用中具有一定的优势.

3结论

本研究利用Maxwell关系研究了准同型相界组分BNKBMT陶瓷的电卡效应,结果表明,在退极化温度附近,具有最大电卡效应,5 kV/mm的电场下,ΔTmax值达到1.15 K,相应的致冷强度可达0.23 K·mm/kV,在新型环境友好的铁电制冷器件中显示出一定的应用前景.

参考文献:

[1]SCOTT J F.Electrocaloric materials[J].Annual Review of Materials Research,2011,41:229-240.

[2]LU S G,TANG X G,WU S H.Large electrocaloric effect in ferroelectric materials[J].Journal of Inorganic Materials,2014,29(1):06-07.

[3]KOBECO P,KURTCHATOV I V.Dielectric properties of rochelle salt crystal[J].Zeitschrift für Physikalische,1930,66:192-205.

[5]MISCHENKO A S,ZHANG Q,SCOTT J F,et al.Giant electrocaloric effect in thin-film PbZr0.95Ti0.05O3[J].Science,2006,311(5765):1270-1271.

[6]FENG Z,SHI D,DOU S.Large electrocaloric effect in highly (001)-oriented 0.67PbMg1/3Nb2/3O3-0.33PbTiO3thin films[J].Solid State Communications,2011,151(2):123-126.

[7]FENG Z,SHI D,ZENG R,et al.Large electrocaloric effect of highly (100)-oriented 0.68PbMg1/3Nb2/3O3-0.32PbTiO3thin films with a Pb(Zr0.3Ti0.7)O3/PbOxbuffer layer[J].Thin Solid Films,2011,519(16):5433-5436.

[8]CHEN H,REN T L,WU X M,et al.Giant electrocaloric effect in lead-free thin film of strontium bismuth tantalite[J].Applied Physics Letters,2009,94(18):182902.

[10]VALANT M,DUNNE L J,AXELSSON A K,et al.Electrocaloric effect in a ferroelectric Pb(Zn1/3Nb2/3)O3-PbTiO3single crystal[J].Physical Review B,2010,81(21):214110.

[11]GOUPIL F L,BERENOV A,AXELSSON A K,et al.Direct and indirect electrocaloric measurements on (001)-PbMg1/3Nb2/3O3-30PbTiO3single crystals[J].Applied Physics Letters,2012,111(12):124109.

[12]HAGBERG J,UUSIMKi A,JANTUNEN H.Electrocaloric characteristics in reactive sintered 0.87Pb(Mg1/3Nb2/3)O3-0.13PbTiO3[J].Applied Physics Letters,2008,92:132909.

[13]BAI Y,HAN X,QIAO L J.Optimized electrocaloric refrigeration capacity in lead-free (1-x)BaZr0.02Ti0.08O3-xBa0.7Ca0.3TiO3ceramics[J].Applied Physics Letters,2013,102(25):252904.

[14]Directive 2002/95/EC,On the restriction of the use of certain hazardous substances in electrical and electronic equipment[S].European Union:Official Jounal of the European Communities,2003.

[15]TAKENAKA T,NAGATA H.Current status and prospect of lead-free piezoelectric ceramics[J].Journal of the European Ceramic Society,2005,25(12):2693-2700.

[16]SUN R,ZHANG Q,FANG B,et al.Dielectric,electromechanical coupling properties of Mn-doped Bi0.5Na0.5TiO3-BaTiO3lead-free single crystal[J].Applied Physics A,2011,103(1):199-205.

[17]ZHANG Q H,ZHANG Y Y,WANG F F,et al.Enhanced piezoelectric and ferroelectric properties in Mn-doped Bi0.5Na0.5TiO3-BaTiO3single crystals[J].Applied Physics Letters,2009,95(10):102904.

[18]XU M,WANG F F,WANG T,et al.Phase diagram and electric properties of the (Mn,K)-modified Bi0.5Na0.5TiO3-BaTiO3lead-free ceramics[J].Journal of Materials Science,2011,46(13):4675-4682.

[19]BAI Y,ZHENG G P,SHI S Q,et al.Abnormal electro caloric effect of Bi0.5Na0.5TiO3-BaTiO3lead-free ferroelectric ceramics above room temperature[J].Materials Research Bulletin,2011,46:1866-1869.

[20]TANG J,WANG F F,SHI W Z.Influence of the composition-induced structure evolution on the electrocaloric effect in Bi0.5Na0.5TiO3-basedsolid solution[J].Ceramies International,2015,41:3888-3893.

[21]LIU X Q,CHEN T T,WU Y J,et al.Enhanced electrocaloric effects in spark plasma-sintered Ba0.65Sr0.35TiO3-based ceramics at room temperature[J].Journal of the American Ceramic Society,2013,96(4):1021-1023.

[22]BAI Y,HAN X,DING K,et al.Combined effects of diffuse phase transition and microstructure on the electro caloric effect in Ba1-xSrxTiO3ceramics[J].Applied Physics Letters,2013,103(16):162902.

[23]WANG J F,YANG T Q,CHEN S C,et al.Nonadiabatic direct measurement electro caloric effect in lead-free Ba,Ca(Zr,Ti)O3[J].Journal of Alloys and Compounds,2013,550:561-563.

[24]LUO L H,CHEN H B,ZHU Y J,et al.Pyroelectric and electrocaloric effect of (111) oriented 0.9PMN-0.1PT single crystal[J].Journal of Alloys and Compounds,2011,509(32):8149-8152.

[25]WANG J F,YANG T Q,WEI K,et al.Temperature-electric field hysteresis loop of electrocaloric effect in ferroelectricity-direct measurement and analysis of electrocaloric effect.[J].Applied Physics Letters,2013,102(15):152907.

[26]SHEBANOVS L,BORMAN K,LAWLESS W N,et al.Electrocaloric effect in some perovskite ferroelectric ceramics and multilayer capacitors[J].Ferroelectrics,2002,273(1):137-142.

(责任编辑：顾浩然)

[27]LUO L H,DIETZE M,SOLTERBECK C H,et al.Orientation and phase transition dependence of the electrocaloric effect in 0.71PbMg1/3Nb2/3O3-0.29PbTiO3single crystal[J].Applied Physics Letters,2012,101(6):062907.Electrocaloric materials[J].Annual Review of Materials Research,2011,41:229-240.

The electrocaloric effect of Bi0.5Na0.5TiO3-based solid solution withthe composition around the morphotropic phase boundaryTANG Jin, WANG Feifei, LI Yan, TIAN Chengyue, WANG Qing, SHI Wangzhou

(College of Mathematics Science,Shanghai Normal University,Shanghai 200234,China)

Abstract:Recently,the electrocaloric effect of ferroelectric materials has attracted much attention at home and abroad due to its potential applications in solid-state refrigeration.In this paper,based on Maxwell relations,we study the electrocaloric effect of Bi0.5Na0.5TiO3-based lead-free ferroelectric quasi-solid solution with the composition focusing around the morphotropic phase boundary.The results show that the system has excellent refrigeration characteristics under external electric field of 5 kV/mm and the amount of change in temperature reaches 1.15 K.The corresponding refrigeration strength reaches 0.23 K·mm/kV,showing the prospect of applications in new environmental-friendly solid-state cooling devices.

Key words:electrocaloric effect; lead-free; ferroelectric refrigeration; morphotropic phase boundary

通信作者:王飞飞,中国上海市徐汇区桂林路100号,上海师范大学数理学院光电子材料与器件重点实验室,邮编200234,E-mail:ffwang@shnu.edu.cn;石旺舟,中国上海市徐汇区桂林路100号,上海师范大学数理学院光电子材料与器件重点实验室,邮编200234,E-mail:wzshi@shnu.edu.cn

基金项目:国家自然科学基金(11204179,61376010);上海市教委“晨光计划”(11CG49)

收稿日期:2014-09-02

中图分类号:TB 34

文献标志码:A

文章编号:1000-5137(2015)02-0127-05