唐　瑾, 王飛飛, 李　巖, 田承越, 汪　青, 石旺舟

(上海師范大學 數(shù)理學院,上海 200234)

準同型相界鈦酸鉍鈉基鐵電固溶體的電卡效應研究

唐瑾, 王飛飛, 李巖, 田承越, 汪青, 石旺舟

(上海師范大學 數(shù)理學院,上海 200234)

摘要:利用鐵電材料的電卡效應實現(xiàn)致冷成為當前國內外的研究熱點之一.基于Maxwell 關系,圍繞著準同型相界組分的Bi0.5Na0.5TiO3基無鉛鐵電固溶體,研究了其電卡效應,結果表明:該體系在5 kV/mm的外電場下,顯示出優(yōu)異的致冷特性,溫度的變化量可達到1.15 K,相應的致冷強度可達0.23 K·mm/kV,在全新的固態(tài)致冷器件中顯示出應用前景.

關鍵詞:電卡; 無鉛; 鐵電致冷; 準同型相界

0引言

電卡效應是熱釋電效應的逆效應,它是指在極性材料中由于外加電場的改變從而引起系統(tǒng)的極化狀態(tài)的變化,從而產生絕熱溫變或等溫熵變[1].電卡效應提供了一種全新的固態(tài)制冷的方法,與傳統(tǒng)的壓縮致冷方法相比,鐵電致冷具有高效率、環(huán)保、健康等優(yōu)點;與磁致冷相比較,外電場的調控更加容易,有利于設計及器件的小型化,因此近來受到國內外研究人員的廣泛關注,在一些固態(tài)的設備制冷上有著非常重要的應用前景[2].

電卡效應的研究最早可以追溯到1930年,Kobeco 和 Kurtschatov在羅息鹽中發(fā)現(xiàn)了電卡效應[3].但在當時由于效應比較弱,沒有進行定量表征,直到1956年才首次報道了SrTiO3具體的數(shù)值[4].1960~1970年左右,掀起了電卡效應的研究熱潮,但是由于鐵電材料性能的限制,電卡效應都相對較弱,測得的絕熱溫變大都在1 K以下.2006年,國際上Mischenko等人率先在PbZr0.95Ti0.05O3(PZT) 薄膜中報道了較大的絕熱溫變,在居里溫度附近,電卡效應達到0.48 K/V[5].隨后,圍繞薄膜體系的電卡效應的研究取得了進一步突破,溫變可達到十幾K[6-8].

1實驗

采用傳統(tǒng)的固相燒結方法,制備了準同型相界組分Mn改性的0.895 Bi0.5Na.5TiO3-0.04Bi0.5K0.5TiO0.3-0.065BaTiO3(BNKBMT) 陶瓷,具體過程可參考本文作者前期研究工作[18].利用X射線衍射儀 (D8 Focus,Bruker,Germany) 對樣品的結構進行分析,利用阻抗分析儀 (Agilent HP4294A,Santa Clara,CA) 測試樣品的介電性能,采用鐵電分析儀(TF2000 analyzer,Aixacct,Aachen,Germany)測試了不同溫度的電滯回線,采用DSC (DSC-2C,Perkin Elmer,America)測試了樣品的熱容.

2結果與討論

根據(jù)XRD結果,從慢掃的 (111) 和 (200) 兩個特征衍射峰(分別對應三方相與四方相)中可以看出,BNKBMT陶瓷處于準同型相界組分,室溫下鐵電三方相和四方相共存[18].圖1(a) 是BNKBMT陶瓷極化后的介電常數(shù)和損耗隨溫度的變化曲線,測試溫度為25~400℃,測試頻率為100 Hz~100 kHz.從圖1(a) 可以看出,樣品具有較寬的介電峰,顯示出典型的彌散性相變過程,并且有兩個特征溫度,一個對應退極化溫度Td,通常由介電損耗的峰值溫度來確定,另一個是介電常數(shù)的峰值溫度Tm.圖1(a)表明,對于極化后的樣品,當溫度低于Td時,介電常數(shù)幾乎不隨測試頻率的變化而變化,表明在外加電場的作用下,鐵電微疇結構轉化為宏疇結構.當溫度超過Td時,因為溫度擾動使得鐵電宏疇結構被破壞,介電常數(shù)顯示出強的色散現(xiàn)象.Tm附近有很寬的介電峰,表明了BNKBMT的彌散性相變特征,這主要是因為體系中A位有多種離子(Bi3+,Na+,Ba2+,K+) 存在,引起微區(qū)間成分的不均勻而造成的[18].

圖1(b) 是BNKBMT不同溫度下的電滯回線,測試電場 5 kV/mm,頻率為10 Hz.從圖1(b)中可以看出,隨著溫度不斷升高,剩余極化和矯頑場均不斷減小,當溫度接近退極化溫度 (70 ℃) 時,電滯回線呈現(xiàn)明顯的雙電滯回線的特點.對出現(xiàn)雙電滯回線的原因,早期認為是由于陶瓷從三方鐵電相到反鐵電相的相變引起的,但目前仍然存在爭議,沒有有力的證據(jù)報道.當溫度超過180 ℃,電滯回線表現(xiàn)一般電介質的響應.

(1)

圖1(d) 為計算得到的不同電場下BNKBMT陶瓷的ΔT隨溫度的變化關系.從圖1(d)中可以看出,與介溫頻譜相似,相應的ΔT隨溫度增加也呈現(xiàn)寬的峰,對應的峰值溫度位于Td附近(100 ℃ @ 1 kV/mm).隨著電場的增加,ΔT的峰值不斷增大,這是由于在高電場下,BNKBMT的有序程度增加,相應的熵減少的緣故.在5 kV/mm的電場下,ΔT的最大值可達到1.15 K.

表1小結了近期報道的相關鐵電單晶、陶瓷體系的電卡效應,從表1中可以看出,與其他無鉛體系相比,準同型相界BNKBMT體系呈現(xiàn)較高的電卡強度,達到0.23 K·mm/kV,與鉛基體系也相接近,顯示了BNKBMT在制冷器件應用中具有一定的優(yōu)勢.

3結論

本研究利用Maxwell關系研究了準同型相界組分BNKBMT陶瓷的電卡效應,結果表明,在退極化溫度附近,具有最大電卡效應,5 kV/mm的電場下,ΔTmax值達到1.15 K,相應的致冷強度可達0.23 K·mm/kV,在新型環(huán)境友好的鐵電制冷器件中顯示出一定的應用前景.

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(責任編輯：顧浩然)

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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:王飛飛,中國上海市徐匯區(qū)桂林路100號,上海師范大學數(shù)理學院光電子材料與器件重點實驗室,郵編200234,E-mail:ffwang@shnu.edu.cn;石旺舟,中國上海市徐匯區(qū)桂林路100號,上海師范大學數(shù)理學院光電子材料與器件重點實驗室,郵編200234,E-mail:wzshi@shnu.edu.cn

基金項目:國家自然科學基金(11204179,61376010);上海市教委“晨光計劃”(11CG49)

收稿日期:2014-09-02

中圖分類號:TB 34

文獻標志碼:A

文章編號:1000-5137(2015)02-0127-05