晶体中负离子配位多面体结晶方位、形变与晶体压电、铁电性

本文研究了压电、铁电晶体中负离子配位多面体的结晶方位与形变,提出了压电晶体中同一种负离子配位多面体的结晶方位是一致的.在铁电晶体中,负离子配位多面体发生形变,伴随着晶体发生顺电-铁电相变,并从这一基本过程出发,对铁电体相变的形成机理进行了讨论.     

利用静电聚苯乙烯纳米微粒对LiNbO3铁电畴结构的观测

利用静电聚苯乙烯纳米微粒缀饰技术,在铁电晶体铌酸锂(LiNbO3)的极性表面观察到铁电畴表面束缚电荷吸附带异性电荷纳米小球所形成的衬度图像,正负电畴衬度图像清晰稳定.这种对铁电晶体表面电畴结构进行表征的新方法不会对样品造成任何程度的损伤和破坏,操作简便易行,并具有较高的分辨率.     

福建物构所分子基相变晶体材料研究获新进展

由于分子运动引起的相变晶体材料具有广泛的应用,尤其是介电可调的相变化合物可用于数据通信、信号处理和传感、可擦写的光学数据储存等。最近虽然基于电荷转移的有机共轭发色团基元被报道用于构筑可逆相变化合物和铁电晶体材料,但是其中的微观相变机制和伴随的固?固相变机理还有待进一步研究。在科技部973计划、863计划、国家自然科学基金、中科院"百人计划"等项目的支持下,福建物构所中科院光     

铁电晶体KNbO3的研究进展

本文评述了近年来国内外对铁电晶体KNbO3在激光倍频和声表面波应用研究方面取得的显著进展,简述了新近生长该晶体的几种方法.最后对KNbO3晶体产业化前景作出了评估.     

AgGaGeS_4晶体介电性能的初步探讨

利用本实验室生长的红外非线性晶体材料AgGaGeS_4(AGGS),常温下进行腐蚀实验并观察畴结构,测试了不同频率、电压下晶体的电滞回线以及同一电压下不同频率介质的电容值.腐蚀图像显示出畴结构,畴尺寸5～10 μm左右,证实AGGS为一热释电晶体.然而,室温下的电滞回线变形为一近似椭圆,介质电容与电场频率关系表现出强色散特性.本文对这一现象进行了系统分析,最后提出了进一步探索AGGS铁电性质的具体建议.     

坩埚下降法在新材料探索及晶体生长中的应用

坩埚下降法是一种重要的晶体生长技术,成功用于生长闪烁晶体锗酸铋(Bi4Ge3O12)、声光晶体氧化碲(TeO2)、压电晶体四硼酸锂(Li2B4O7)以及新型弛豫铁电晶体等材料,并实现了产业化.坩埚下降法在层状结构晶体、异型晶体、高通量生长等新材料探索中也有巨大的潜力.本文主要介绍我们团队近年来在坩埚下降法生长硒化锡(SnSe)晶体、全无机铅卤基钙钛矿晶体、高温合金、硅酸铋晶体高通量筛选等方面的研究结果.     

新型有机热释电晶体——酒石酸铵系列晶体研究

本文从晶体结构对称性的观点出发,寻找新型有机热释电晶体材料,选取了具有P21空间群结构的酒石酸铵系列晶体作为研究对象,用水溶液缓慢降温法生长了酒石酸铵及其掺质酒石酸铵晶体,掺质分别为尿素、溴化铵和甘氨酸,研究了晶体生长形态并作了比较.测定了酒石酸铵系列晶体的红外光谱、晶胞参数和主要热释电性能.发现酒石酸铵系列晶体的热释电性能,其品质因子具有与TGS晶体相同的数量级.     

非水解溶胶-凝胶法低温制备铁稳定钛酸铝粉体

本文以无水二氯化铝和四氯化钛为前驱体,以无水乙醇为氧供体,以铁粉、三氯化铁和乙醇铁为稳定剂,采用非水解溶胶-凝胶法制备铁稳定钛酸铝粉体.运用XRD、FT-IR和热膨胀仪等测试手段研究和探讨了铁稳定剂对低温合成钛酸铝的效果、稳定作用及其机理.结果表明:引入铁粉既不能提高钛酸铝低温合成效果也不能改善其热稳定性;三氯化铁能促进钛酸铝的低温合成,但其稳定钛酸铝的效果也不明显;乙醇铁虽不利于低温合成钛酸铝,但却能够大大提高钛酸铝的抗热分解能力.乙醇铁能与前驱体混合液中形成的铝、钛醇盐形成异质缩聚键合,致使铁能进入钛酸铝晶格形成同溶体,从而提高其热稳定性能.     

PSN-PMN-PT∶Er3+晶体Judd-Ofelt理论分析

为了探索Er3+在铁电晶体场中的发光特性,采用高温溶液法生长了Er3+掺杂Pb(Sc1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3弛豫铁电单晶.该晶体在紫外-可见-近红外吸收光谱380～1800 nm波段出现了7个吸收峰;在980 nm泵浦光源照射下,该晶体的发射光谱出现了强绿光和强红光上转换发射带,分别对应于Er3+的4S3/2→4I15/2和4F9/2→4I15/2跃迁;利用Judd-Ofelt理论计算了晶体的振子强度三参数(Ω2＝2.54×10-20 cm2,Ω4＝1.36×10-20 cm2,Ω6 ＝2.38× 10-20 cm2)以及Er3+在PSN-PMN-PT晶体场中的跃迁几率、能级寿命和荧光分支比.研究结果表明 PSN-PMN-PT∶Er3+弛豫铁电晶体是一种新型发光晶体.     

福建物构所分子基铁电晶体材料研究获新进展

铁电晶体是在居里温度以下电偶极子自发排列形成电畴,并可以随外加电场而使自发极化反向的一种材料。有极轴且无对称中心是铁电体的必要条件,因此,居里温度以下的铁电体必然也具有压电性。铁电材料对电信号表现出高介电常数,对温度改变表现出大的热释电响应,在应力或声波作用下具有强的压电效应     

近零膨胀Zr0.5Hf0.5V1.4P0.6O7材料的制备及其性能研究

采用传统的固相法合成了近零膨胀氧化物功能陶瓷材料Zr_0.5Hf_0.5V_1.4P_0.6O₇,用X射线衍射(XRD)、Raman光谱和热膨胀法对Zr_0.5Hf_0.5V_1.4P_0.6O₇的热膨胀系数、各向同性和相变进行了测试,通过Hf⁴⁺/P⁵⁺共掺杂使得材料具有较低的热膨胀系数,研究发现合成的Zr_0.5Hf_0.5V_1.4P_0.6O₇具有Pa3立方相结构,从334 K附近到673 K较宽的温度范围内的线性热膨胀系数为-1.56×10^-6 K^-1,表现出稳定的近零热膨胀特性。由于固溶体内部微结构的影响造成膨胀仪实验结果与变温X射线衍射结果存在一定的差距。Zr_0.5Hf_0.5V_1.4P_0.6O₇具有的近零膨胀特性为通过负热膨胀材料合成膨胀系数可控的材料提供了基础。     

Properties of porous glasses with embedded ferroelectric materials

Methods of introducing two ferroelectric materials, viz. NaNO₂ and (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB), into porous glass matrices and the results of dielectric, pyroelectric, thermal expansion and SEM investigations are presented. The best filling degree was achieved for NaNO₂ introduced from the melt, whereas a significantly worse filling effect was achieved for a water solution as the introducing agent. A splitting was observed in the temperature dependence of dielectric permittivity related to the incommensurate phase of NaNO₂ embedded in porous glass from a water solution, whereas no incommensurate phase was observed in the case of glass filled with NaNO₂ from the melt. The confined materials turned out to possess ferroelectric properties and exhibited phase transitions. The negative size effect was observed for porous glasses filled with NaNO₂ and PMACB. The above conclusions are based on the anomaly of dielectric, pyroelectric and dilatometric properties observed in the phase transition regions.     

Epitaxial growth of ferroelectric oxide films

This review paper begins with a brief overview of the most common ferroelectric materials, the perovskites and the Aurivillius families. The epitaxial growth of ferroelectric epitaxial films can be a viable approach to improve the ferroelectric properties, in particular for the layered perovskites. Defects related to a polycrystalline structure, which lead to a degradation of ferroelectric properties like remanent polarisation, piezoelectric coefficient, charge retention, and may cause time-dependent fatigue problems, can be prevented. However, it is also to be considered that effects connected with thin films like substrate clamping, strain or finite thickness may limit the film properties. Substitution of elements allows the adjustment of the film characteristics to the device function. Additionally, the orientation of the films can be controlled by the appropriate choice of the substrate, which is important due to the anisotropy of the ferroelectrics.The deposition methods commonly used for ferroelectric oxide layers are reviewed, particularly with regard to epitaxial growth. The conditions under which stoichiometric, crystalline growth can be obtained are described. The paper primarily focuses on the MO-CVD technique.Furthermore whether epitaxial growth of ferroelectric films occurs or not depends on several conditions like lattice mismatch between film and substrate, surface orientation and crystal symmetry of the substrate, thermal expansion of film and substrate. The influence of these parameters on epitaxial growth is discussed. Local epitaxial growth of ferroelectric layers on metallic electrodes is also mentioned due to its importance in device fabrication. The site-engineering concept is shortly reviewed as the substitution of elements constitutes a simple way to modify film properties in thin film technology.     

Thermoelastic and Piezoelastic Properties of Hexagonal RbMnCl3

Temperature and pressure derivatives of the elastic constants, indices of refraction, dielectric constants, Faraday effect and thermal expansion of RbMnCl₃ near T_c = 278 K confirm the existence of a second-order phase transition. A strongly anomalous behaviour is observed with the elastic shear stiffness c₄₄ which possesses a positive temperature derivative and a negative pressure derivative. These properties and also the pressure derivative of the transition temperature, dT_c/dp = 6.2 K kbar⁻¹ closely resemble the anomalous behaviour of two other ferroelectric species, betaine borate and betaine hydrogen maleinate.     

Thermal characterization of a vinylidene fluoride-trifluorethylene (75–25) (%mol) copolymer film

Vinylidene fluoride-trifluorethylene (VDF-TrFe) copolymers – exhibits ferroelectric properties due to the special arrangement of the chain units in the crystalline phase. The co-polymers have attracted intense scientific and technological interest due to the possibility of studying the ferroelectric phase transition, as it occurs before the melting of the material. Being that the piezo- and pyroelectric properties of the material are the most interesting from a practical point of view, they have been intensively studied in the recent years. Nonetheless, there is a lack of information about the thermal behavior of other relevant characteristics of the samples. In this work, P(VDF-TrFE) (75/25) has been characterized by mechanical tests and a series of thermal analysis techniques, including thermal mechanical analysis (TMA) and differential scanning calorimetry (DSC). The films exhibit slight mechanical anisotropy, and in the longitudinal direction larger mechanical properties, due to the higher structural organization. DSC results show two well-defined peaks corresponding to the ferroelectric-paraelectric transition and the melting of the paraelectric phase at higher temperatures. Two relaxation processes were found (β and α_c) by dynamic mechanical analysis (DMA). The β process, attributed to segmental motions in the amorphous phase, appears at the same temperature in both directions, but its intensity was found to be higher for the films tested in the longitudinal direction. At higher temperature, the α_c relaxation was related to the thermal variation of the dimensions of the films in the longitudinal direction during heating, recorded by TMA.     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

General concept of increasing crystal symmetry with an increase in temperature

A general concept of the symmetry transformation of a material during its thermal expansion and final polymorphic transition is proposed based on numerous experimental data. Within this concept, the atomic nature of the transformation of the crystal structure with a change in temperature is considered. Particular attention is paid to the thermal expansion of materials, along with the well-known tendency of a thermally induced increase in the crystal symmetry, which is generally considered to be the directionality of material transformation during polymorphic transitions.     

Thermal expansion in the AgGa(S1−xSex)2 solid solutions from X-ray diffraction data

The lattice parameters a and c as well as the axial thermal expansion coefficients in the AgGa(S_1-xSe_x)₂ solid solutions with chalcopyrite-type structure were determined as a function of temperature in the range from 80 to 700 K and composition x using an X-ray powder diffractometry technique. It is found that the thermal expansion coefficients were anisotropic and for all the solid solutions the thermal expansion coefficients along the tetragonal c-axis were negative whereas those along the a-axis and the volume coefficients were positive. The directions in which the crystal thickness does not change as temperature varies, were found. The composition dependences of these coefficients were non-linear.     

Czochralski growth and constitutional studies on single crystals of potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2−xNb_5+xO₁₅ (KLN) is a very promising material for the conversion of infrared light to light in the visible region. However, growing of single crystals is known to be complicated due to the considerable anisotropy of the growth rate and the thermal expansion behaviour. The single crystals of KLN, Mg²⁺‐doped KLN, as well as the mixed crystals of potassium lithium tantalate niobate K₃Li₂(Nb_1−xTa_x)₅O₁₅ (KLTN) were grown by the Czochralski technique. The chemical analyses of the samples were performed by atomic absorption spectroscopy (AAS) and X‐ray fluorescence analysis (XRF). The element concentrations along the single crystals were measured by the electron microprobe analysis (EMPA) to clarify the segregation phenomena in the grown crystals. The elements distribution coefficients were also calculated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

(Sm0.5Gd0.2Nd0.3)2(Hf0.3Ce0.7)2O7复合氧化物的热物理性能

为寻求新型热障涂层用陶瓷材料,本文采用高温固相烧结法制备了(Sm_0.5Gd_0.2Nd_0.3)₂(Hf_0.3Ce_0.7)₂O₇复合氧化物。利用XRD分析了其晶体结构,SEM分析其显微组织,膨胀仪测试其热膨胀性能,激光热导仪测试其热扩散系数。结果表明,成功制备了具有单一萤石晶体结构的(Sm_0.5Gd_0.2Nd_0.3)₂(Hf_0.3Ce_0.7)₂O₇复合氧化物。其显微组织结构致密,晶界清晰无其他相存在。由于复杂的元素组成和较大的原子量,其热导率明显低于7YSZ和Sm₂Ce₂O₇。其较低的热膨胀系数则归因于B位离子较小的离子半径,但其热膨胀系数依然满足热障涂层的要求。