弛豫型铁电单晶0.76PMN-0.24PT的铁电相变特征

采用介电、铁电与热释电表征法,结合电畴结构的实时观察,对０．７６ＰＭＮ－０．２４ＰＴ单晶的相变特征进行了研究,发现其铁电性质介于典型弛豫铁电体与正常铁电体之间．该单晶极化前具有一种处于微畴至典型宏畴之间的过渡型电畴,其（００１）晶片在极化后发生场致相变,并在其后的升温程序中表现出两个特征温度Ｔ_ｄ与Ｔ_ｍ．Ｔ_ｄ为电容率随温度下降而开始急剧下降的温度,对应于零场升温程序中的由电场诱导的典型宏畴－过渡型宏畴的转化;在Ｔ_ｄ～Ｔ_ｍ温区,晶体持续发生铁电－顺电相变或退极化作用,以宏畴态存在的铁电相与顺电相共存;在略高于Ｔ_ｍ的温度,晶体转化为宏观顺电相,铁电相以微畴态存在．由于场致相变与晶体取向有关,相对于同成分的单晶来说,多晶陶瓷在同样的温度一电场历程下显示不出特征温度Ｔ_ｄ．     

Ferroelectric phase transition in relaxor ferroelectric single crystals 0.76PMN-0.24PT

The features of the single crystals 0.76PMN-0.24PT in dietectric, ferroetectric, pyroetectric properties and domain structures indicate that they are located between typical ferroetectrics and normal ones. The unpoled crystals present a transitional domain configuration between microdomains and typical macrodomains while the crystals on (001) cuts undergo fietd-induced phase transition under poling, showing two special temperature points T_d andT _m during the succedent heating procedure. The dietectric constant starts to decrease drastically at T_d during cooling, or the transformation from induced macrodomain to transitional domain takes place atT _d during heating. Ferroetectric-paraetectric phase transition or depolarization continues within the whole temperature range ofT _d-T _m, where ferroetectric phase in the form of transitional macrodomains coexists with paraetectric phase. Then the crystals macroscopically transoform into paraetectric phase containing ferroetectric microdomains at a temperature aboveT _m. However, owing to the influence of crystallite orientation on fietd-induced phase transition, the temperatureT _d does not appear in the same temperature-etectric fietd history in multicrystal ceramics with the same composition as the above single crystals.     

新型弛豫型铁电单晶PMNT的铁电与压电性能

用改进的Bridgman法生长出尺寸达40 mm×80 mm的新型弛豫型铁电单晶PMNT.对其铁电、压电性能的表征研究表明,该晶体的性能随组分与切型变化.(001)切型的PMNT76/24单晶的介电常数ε为3400左右,介电损耗tanδ<0.7%,压电常量d33达到980 pC/N,机电耦合因数kt为0.55,Tc约为110℃.(001)切型的PMNT 67/33 单晶的ε达5300左右,tanδ＜0.6%,d33最大达3000 pC/N,kt达0.64,k33达0.93,Tc约为150℃.(110)与(111)切型的压电性能远低于(001)切型.用此法生长的三方相的PMNT单晶的压电性能超过传统的高温溶液法,且其kt值亦大于三方相的PZNT单晶.PMNT单晶的铁电与压电性能尚存在一定的波动,这与晶体成分均匀性与单畴化程度等因素有关.     

Ferroelectric and piezoelectric properties of novel relaxor ferroelectric single crystals PMNT

Relaxor ferroelectric single crystals PMNT with the size of 40 mm×80 mm have been grown by a modified Bridgman method and their ferroelectric and piezoelectric properties have been characterized.The properties varied with the compositions and cut types.On the (001) cut,PMNT76/24 single crystals exhibited a dielectric constant of about 3 400,a dielectric loss of tan＜0.7%,a piezoelectric constant d33 of 980 pC/N,an electromechanical coupling factor kt of 0.55 and Tc of about 110℃.Whereas the properties of PMNT67/33 single crystals on (001) cut were better: ε of about 5 300,tanδ＜0.6%,d33 up to 3 000 pC/N,kt 0.64,k33 0.93 and Tc of about 150℃.The piezoelectric properties on other cuts such as (110) and (111) were much lower than those on the (001) cut.The rhombohedral PMNT crystals grown by this method showed more excellent piezoelectric properties than those grown by high temperature solution method and higher value of kt than the rhombohedral PZNT single crystals.It has also been found that the fluctuation in ferroelectric and piezoelectric properties was related to such factors as composition uniformity and poling degree.     

新型弛豫铁电单晶体Pb(Mg1/3Nb2/3)O3-PbTiO3的形貌与缺陷结构

采用改进的Bridgman法生长出了尺寸达 2 5mm× 2 5mm× 5 0mm的透明的、压电性能十分优异的弛豫铁电单晶体Pb(Mg1/ 3 Nb2 / 3 )O3 PbTiO3 ,其为纯钙钛矿相的三方或四方结构 .这些单晶主要显露 {0 0 1 }面 ,而 [1 1 1 ]方向的生长速度相对较快 .可以利用负离子配位多面体生长基元理论模型解释PMNT单晶的形貌特征与生长习性 .在光学显微镜和SEM下观察到了散射颗粒、气泡及负晶结构等宏观缺陷 .在对结构缺陷形成机制研究的基础上 ,通过调节生长参数 ,可以减少或消除这些缺陷 .用光学显微镜对三方相单晶的 71°或 1 0 9°电畴、四方相单晶的 90°电畴进行了观察 ,发现微畴 宏畴转变可由成分诱导并存在过渡区 ,分析了电畴结构的形成机理及与铁电相变的关系 .