铁性体中畴结构产生的热力学描述

在批判平衡态热力学局限性的基础上,采用不可逆过程热力学对铁性体中畴结构的产生做了动态描述.注意到了铁电与铁磁两种情况下的相似与区别.在铁弹情况下,出于客观的必要性,类比地引入了铁弹相变“分子场”的概念.明确了体系的有限性、过程的不可逆性与畴结构的产生的内在关联. 关键词： 铁性体 畴结构 不可逆性 对称性     

冲击加载下PZT-95/5陶瓷铁电－反铁电相变实验研究

 用石英计测量了PZT-95/5陶瓷在冲击波作用下发生铁电－反铁电一级相变时产生的双波结构，相变起始压力约为0.5 GPa。对不同状态下的PZT-95/5陶瓷材料进行了扫描电镜电畴分析，结果表明，在该压力区域陶瓷发生了铁电－反铁电相变。     

钙钛矿结构中的简谐子软模

利用复合空间型方法,在自由边界条件下解出了钙钛矿结构有限尺寸晶体的简谐振动方程.计算结果给出了收敛的简谐子谱分布,并发现了许多简谐子软模.用这些软模花样说明了钛酸钡晶体冷却时发生具有a畴和c畴结构的铁电相变.理论表明铁电相变过程涉及屏蔽电荷的激发及其在界面的缓慢扩散,以最后得出各个电畴内部的均匀自发极化 关键词： 晶格动力学 简谐子 软模 钛酸钡     

PbLa(Zr,Sn,Ti)O_3反铁电陶瓷在脉冲电场下的极化与相变行为

在实际应用中,反铁电陶瓷常处于快速变化的脉冲电场下,而传统电滞回线测量时所施加的电场变化速率较慢,并不能真实反映反铁电陶瓷实际应用时的极化和相变行为.本研究建立了反铁电陶瓷脉冲电滞回线测试平台,研究了Pb_(0.94)La_(0.04)[(Zr_(0.52)Sn_(0.48))_(0.84)Ti_(0.16)]O_3反铁电陶瓷在微秒级脉冲电场下的极化和相变行为.研究结果表明,反铁电陶瓷在微秒级脉冲电场下可以发生相变,但其极化强度降低,正向相变电场变高,反向相变电场变低,从而导致其储能特性发生了显著的变化.因此,低频电滞回线并不能真实反映反铁电陶瓷在脉冲电场下的性能,脉冲电滞回线对其应用具有更重要的参考价值.     

铁电材料中的极性拓扑结构

调控磁性材料中的自旋拓扑结构(流量闭合型、涡旋、半子(meron)、斯格明子(skyrmion)等自旋组态)可以改进材料的磁性和电磁性能,因而引起了学术界的广泛关注.最近研究表明,在尺寸效应、界面耦合及其相互作用、外延应变等作用下,铁电材料中也会出现自发的极性拓扑畴结构,同时表现出新的铁电相结构和丰富的物理性能.本文总结了铁电纳米结构、铁电薄膜和铁电超晶格中的极性拓扑畴结构类型及其形成机理,分析了这些极性拓扑结构与铁电、压电、介电、光电性能之间的关联,并分别讨论了铁电材料中极性拓扑结构的整体拓扑相变调控和单个极性拓扑结构的外场调控,最后展望了极性拓扑结构未来的可能研究方向.     

铌酸钡钠晶体中铁弹相变的实时观察

用锥光干涉图实时观察铌酸钡钠(BNN)晶体中的铁弹相变,实验表明从室温至300℃范围内相变是连续变化的,且呈弥散型转变,较快的区域在250±20℃范围内,升、降温曲线存在热滞约20—40℃,在相变过程中实时观察到孪晶(铁弹畴)的消失和再现均与相变同步,还发现孪晶的存在对相变转折点T_s有很大影响,它使T_s降低,热滞变大,缺陷和局域的组份不均匀是造成相变弥散型的主要原因。 关键词：     

Si掺杂HfO_2薄膜的铁电和反铁电性质

通过改变Si掺杂量制备出了具有显著铁电和反铁电特征的HfO2纳米薄膜,对其电滞回线、电容-电压和漏电流-电压特性以及物相温度稳定性进行了对比研究.反铁电薄膜的介电系数大于铁电薄膜,在电场加载和减载过程中发生的可逆反铁电-铁电相变导致了双电滞回线的出现,在室温至185℃的测试温度范围内未出现反铁电→顺电相变.在电流-电压特性测量时观察到的负微分电阻效应归因于极化弛豫等慢响应机理的贡献.     

用Weiss分子场理论对有外电场时铁电体系相变特征的研究

Weiss分子场理论(WMFT)对晶体中顺磁-铁磁和顺电-铁电相变特征的定量描述是相当成功的. 由于是平均场理论,又可作为初步分析结构无序体系和复杂组分体系相变行为的理论依据. 但是迄今为止,并没有对有外场时WMFT的相变特征进行详细研究. 而对铁电体系,仅仅对分子取向为两个状态时WMFT的相变特征进行了研究. 另外,虽然铁磁与铁电体系的WMFT描述极为相似,但是由于两种体系中微观磁化和极化的单元不同,导致相应的数学描述与结果也有所不同. 本文首先对外电场中分子取向包含任意状态的铁电体系的WMFT相变特征, 包括自发极化、内能和比热以及静态极化率随温度变化进行严格推导, 然后对相变特征随外电场的演变进行了研究.结果表明: 1)无外场时,体系发生二级顺电-铁电相变,且随状态数的增加,相变温度减小, 这是与铁磁体系不同的地方,同时单分子的平均极化强度减小,而内能、比热和极化率增大; 2)外场的存在,使得体系原有的二级相变转化为弥散相变,且外场越强,弥散温区越大. 上述结果对深入研究铁电体系的相变,特别是弥散相变无疑是有益的.     

具有形状记忆效应的反铁电材料的制备及其相变行为的研究

研究表明，铁电与反铁电材料在形状记忆方面与合金具有相似的性质，用变温Ｘ射线衍射来研究Ｐｂ０．９７Ｌａ０．０２（Ｚｒｏ０．６５Ｔｉ０．１０Ｓｎ０．２５）Ｏ３在升温过程中的相变并测得在相变的同时晶面间距ｄ有明显的跳变。还用相变潜热测量（ＤＳＣ）与电滞回线测量证实了在各个温区的相     

弛豫铁电体弥散相变与热滞效应的伊辛模型

弛豫铁电体材料在通讯、传感、超声、能量转换、航空航天等领域具有重要的应用.与正常铁电体不同,弛豫铁电体在冷却过程中出现弥散相变,体系的宏观极化不会突然产生,而是出现纳米极性微区,体系的宏观晶体对称性没有明显的变化.如何理解弥散相变及其与内部机制之间的相互影响是一个重要的问题.本研究基于伊辛模型(Ising model),对自旋变量(在研究中视为电偶极子)引入能量势阱的作用,并计算了这一系统的相变过程.结果表明这一改进的伊辛模型使极化率的相变曲线显著变缓,呈现出具有弥散相变的弛豫体特性.研究显示,弛豫体现象出现的一个重要原因是系统内部偶极子受到势阱限制而出现反转受阻,从而使极化率偏离常规铁电体.利用这一改进的伊辛模型进一步研究了弛豫铁电体的热滞效应,分析了热滞的起源,并与实验结果进行了对比分析,明确了弛豫体弥散相变和热滞的物理机制.     

High-resolution calorimetric study of pb(mg_{1/3}nb_{2/3})o_{3} single crystal

Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, we performed a high-resolution heat capacity and polarization study of the field-induced phase transition in the relaxor ferroelectric single crystal Pb(Mg_{1/3}Nb_{2/3})O_{3} (PMN) oriented along the [110] direction. We show that the discontinuous evolution of polarization as a function of the electric field or temperature is a consequence of a true first order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. We also find that in a zero field there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures.     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15 K 到室温范围的介电特性及热电特性的研究. 分析其介电特性和热电特性与极化电场的关系, 用x 射线粉末衍射进行佐证. 证实在50 一70 K 的范围内, SBN , PBN , KNSBN 三类铁电钨青铜铌酸盐晶体均存在着一个新的相变. 相变是由点群4mm铁电相到点群mm2 铁电相的转变. 铁电自发极化方向由四方晶胞的c轴方向转变到正交晶胞的b 轴方向.介电特性的高频及低频测量表明该相变具有扩散(或称弥散)型特征. 比热的实验结果证明相变是属于高于一阶相变的高阶相变. 对相变前后的晶胞结构提出了一个模型解释. 关键词：     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15K到室温范围的介电特性及热电特性的研究。分析其介电特性和热电特性与极化电场的关系,用X射线粉末衍射进行佐证。证实在50—70K的范围内,SBN,PBN,KNSBN三类铁电钨青铜型铌酸盐晶体均存在着一个新的相变。相变是由点群4mm铁电相到点群mm²铁电相的转变。铁电自发极化方向由四方晶胞的e轴方向转变到正交晶胞的b轴方向。介电特性的高频及低频测量表明该相变具有扩散(或称瀰散)型特征。比热的实验结果证明相变是属于高于一阶相变的高阶相变。对相变前 关键词：     

On the dielectric and thermal behaviour of the ferroelectric NaNO2

The temperature dependence of the spontaneous polarization of the uniaxial ferroelectric NaNO₂ is measured. The results are discussed in connection with the known behaviour of the dielectric constant and of the specific heat. They can be represented by a Landau expansion of the thermodynamic potential describing a phase transition of the first order. The coefficients of this expansion could be uniquely determined from the data of the ferroelectric phase.     

X-ray study of the phase transition in RbHSeO4

The ferroelectric phase transition in RbHSeO₄ has been examined by precise measurements of the temperature dependence of the unit-cell parameters using the Bond technique. The transition, at T_c = 370.6 K, is of first order and the triclinic unit cell of the ferroelectric phase transforms to an equitranslational monoclinic cell in the paraelectric phase. In a temperature region of several K below T_c reflections belonging to both phases have been observed.     

Probing the material properties and phase transitions of ferroelectric liquid crystals by determination of the Landau potential

The full Landau potential of several, widely varying ferroelectric liquid-crystalline materials has been experimentally determined. Tilt angle and polarisation data is analysed across the SmA to SmC transition for varying applied electric-field amplitudes, allowing the determination of all the coefficients of the generalised Landau model of ferroelectric liquid crystals. The materials investigated encompass different materials, including low-polarisation mixtures to high-polarisation single-component materials. The materials also possess a variation in the order of the SmA to SmC phase transition from strongly first order to strongly second order. The effects of both the polarisation and order of phase transition of the system are discussed with respect to the various terms of the generalised Landau model. Further, the mechanisms behind the difference between a first- and second-order phase transition are discussed with respect to the Landau potential and the second Landau coefficient b .     

Symmetry breaking at the phase transitions in ferroelectric pyridinium salts

The phase transitions in ferroelectric pyridinium tetrafluoroborate (PyBF₄) and pyridinium perchlorate (PyClO₄) have been characterized by structural studies. The continuous ferroelectric phase transition at 238.7 K in PyBF₄ appears to be a unique case for multiaxial ferroelectrics, while the first order phase transition in PyClO₄ at 248 K is consistent with the Landau theorem for deducing the character of phase transitions from symmetry considerations. The phase transition in both materials is caused by ordering of the pyridinium cation as well as tetrahedral anions.     

Phase transitions in barium–strontium titanate films on MgO substrates with various orientations

A comparative study of the lattice dynamic upon phase transitions in a polycrystalline Ba_0.8Sr_0.2TiO₃ (BST) film on a Pt substrate and in epitaxial BST films grown on various sections of an MgO substrate has been performed by Raman spectroscopy. It has been found that different sequences of phase transitions take place in these films. The BST/Pt films demonstrate the same sequence of phase transitions that is observed in the bulk ceramics. The hardening of a soft mode in BST/(001)MgO and BST/Pt films shows that the transition from the tetragonal ferroelectric phase to the paraelectric phase has features of the displacement-type phase transition and also the order–disorder phase transition. When approaching the ferroelectric transition temperature, the soft mode in the BST/(111)MgO film is softened, following the Cockran law, which indicates the displacement-type phase transition.     

On the mechanism of the increasing phase transition point in ferroactive nanocomposites

The numerical analysis of a nonlinear equation set has revealed a possible increase in the ferroelectric phase transition point at the presence of the intermediate layer at the ferroelectric–dielectric interface. The order parameter distribution in the ferroelectric particles, as well as the effect of the intermediate layer thickness on the Curie temperature, has been studied.     

A New Model of Ferroelectric Phase Transition with Neglectable Tunneling Effect

Due to the obvious deviations of the existing theoretical models from the experimental results of ferroelectric phase transition, a new model is proposed on the basis of the coupling between spontaneous polarization and spontaneous strain in ferroelectrics. The spontaneous polarization and specific heat of ferroelectric phase transition predicted by the model are in better agreement with the corresponding data of triglyceride sulfate, a typical ferroelectric.In addition, the model predicts a new type of ferroelectric in which a phase transition and a phase-like transition coexist.