弛豫铁电体的微畴-宏畴转变与生长动力学

制备了铅基弛豫铁电体0.9 Pb(Mg1/3Nb2/3)O3-0.1 PbTiO3和Pb(Zn1/3Nb2/3)O3基陶瓷.利用修正的局域冻结玻璃模型研究了弛豫铁电体微畴-宏畴转变动力学和微畴生长动力学，结果表明,微畴-宏畴转变过程是量子化的过程，微畴-宏畴转变的驱动力主要是热驱动力和能量驱动力，微畴-宏畴转变稳定性由活化能和状态数来判断，而微畴生长动力学的稳定性可以用稳定性判据值大小确定.     

温度控制的原位脱羧3-氟邻苯二甲酸与铅(Ⅱ)配位聚合物的合成、晶体结构及荧光性质

Pb（NO₃）₂与3-氟邻苯二甲酸（H2Fpht）和2,2′-联吡啶（bpy）在不同水热反应温度下获得了2个新的配位聚合物:[Pb（Fba）（bpy）（NO₃）]_n（1）和[Pb₄O₂（Fba）₄]_n（2）。3-氟邻苯二甲酸配体经由原位脱羧生成了3-氟苯甲酸根（Fba）。配合物1具有Pb-NO₃-Pb双螺旋链结构,是由硝酸根桥联四方反棱柱[PbO₆N₂]结构单元而形成,而Fba与bpy配体螯合配位于中心铅离子。配合物2为PbOH-Pb链状结构,包含四面体结构单元[Pb₄（μ4-OH）₄]⁴⁺,且具有4个晶体学独立的铅离子中心:Pb（1）O₆、Pb（2）O₆、Pb（3）O₅和Pb（4）O₅。这两个配合物链分别经氢键作用力自组装为三维超分子网络结构。室温下,测定了配合物的固体荧光光谱,均显示为基于配体的荧光发射。有意思的是,通过荧光淬灭机理配合物2可以选择性地检测硝基苯。     

共沉淀法合成Pb3Nb2O8纳米粉

在PbO-Nb₂O₅-KOH-H₂O体系中,于90℃下得到纳米级Pb₃Nb₂O₈陶瓷粉.原料来源及配比、溶液碱度及合成温度对产物物相的形成有较大影响.在Pb₃Nb₂O₈的合成中,必须以可溶性铌酸盐和醋酸铅作为反应原料,同时以KOH调节体系中铌酸盐的聚集状态.当n(Pb)/n(Nb)接近1/1,KOH浓度在1～3mol/L时,得到Pb₃Nb₂O₈纯相,当KOH浓度大于3mol/L时容易生成反应活性较低的PbO,产生杂相.     

激光拉曼光谱研究NiMoP浸渍液活性组分的结构 Ⅱ.不同有机添加剂的影响

采用激光拉曼光谱（LRS）技术对添加有乙二醇和柠檬酸的NiMoP浸渍液和用该浸渍液制备的NiMoP/Al₂O₃催化剂进行了表征,研究了乙二醇和柠檬酸含量对NiMoP浸渍液及浸渍过程中活性相组成、结构的影响。结果表明,乙二醇使NiMoP(0.063)浸渍液中H_{x\[PMo11O39\]⁽7-x)- 或Hx\[PMo9O31\]⁽3-x)-及Hx\[PMo12O40\]⁽3-x)- 杂多阴离子结构转化成Hx\[P2Mo5O23\]⁽6-x)- 结构,而柠檬酸能保持NiMoP(0.063)浸渍液中Hx\[P2Mo5O23\]⁽6-x)-、Hx\[PMo11O39\]⁽7-x)- 或Hx\[PMo9O31\]⁽3-x)-及Hx\[PMo12O40\]⁽3-x)-杂多阴离子结构共存,但导致Hx\[P2Mo5O23\]⁽6-x)-含量降低,Hx\[PMo12O40\]⁽3-x)-含量升高。相对乙二醇而言,柠檬酸有机添加剂能够更有效地阻止浸渍过程中各杂多阴离子结构在载体氧化铝孔道中的分解。}     

{(Me2NH2)2[Fe2(ox)2Cl4]•H2O} n (ox=oxalate)中客体水分子诱导的介电弛豫行为

以双齿配位的草酸根为配体和FeCl₃反应得到了化合物{(Me₂NH₂)₂[Fe₂(ox)₂Cl₄]•H₂O} _n (ox=oxalate) ( 1•H₂O). 在真空和加热的条件下, 1•H₂O脱水发生单晶到单晶的转变, 得到了化合物{(Me₂NH₂)₂[Fe₂(ox)₂Cl₄]} _n ( 1). 磁性研究表明客体水分子存在与否对化合物磁相互作用没有明显影响. 介电性质研究表明客体水分子的存在可影响化合物中客体胺和氢原子的动态弛豫过程. 化合物 1•H₂O在250 K, 80 K和50 K呈现三个介电弛豫过程, 而化合物 1仅在80 K表现出单一弛豫过程.     

非绝热分子动力学模拟A位阳离子对钙钛矿热载流子弛豫的影响

钙钛矿具有优异的光学和电学性质, 近年来成为太阳能电池领域的研究热点. 大量实验报道钙钛矿热载流子弛豫时间变化顺序为CsPbBr₃>MAPbBr₃(MA=CH₃NH₃)>FAPbBr₃[FA=HC(NH₂)₂], 但A位阳离子(Cs ⁺, MA ⁺, FA ⁺)对弛豫快慢的影响机制仍不明确. 采用基于含时密度泛函理论的非绝热动力学方法研究了上述3种钙钛矿热电子和热空穴的能量弛豫动力学, 计算得到的热载流子弛豫时间与实验结果吻合. 结果表明, A位阳离子通过静电和氢键作用影响其与无机Pb—Br骨架的电子-振动耦合, 使非绝热耦合强度遵从FAPbBr₃>MAPbBr₃>CsPbBr₃的变化趋势, 进而使热载流子弛豫时间尺度变化趋势与之相同, 表明合理选择A位阳离子可以优化钙钛矿太阳能电池的性能.     

氨基苯甲酸修饰的DTPA螯合物及其弛豫性能

用邻-、对-氨基苯甲酸修饰二乙三胺五乙酸(DTPA),分别得到两种分子中携带芳香基团的双酰胺型DTPA衍生物,合成了它们的顺磁性金属离子螯合物,并研究了其中部分螯合物的inuitroNMRT₁弛豫率(R₁).结果表明,Gd(Ⅲ)DTPA-B4ABA^2-和GdDTPA-B2ABA^2-对水质子的弛豫率R₁分别为5.5和5.4L·mmol^-1·s^-1,接近相应的母体螯合物GdDTPA^2-的R₁值5.2L·mmol^-1·s^-1.     

单核Gd化合物的场依赖磁弛豫和磁热效应研究

合成了两例具有局域D_4d对称性的单核钆配合物: Gd(THD)₃L (THD=2,2,6,6-四甲基庚烷-3,5-二酮, L=Phen (1,10-菲罗琳, 1), Bpy (2,2'-联吡啶, 2). 磁性研究表明其在低温下具有较大的磁热效应, 在2.5 K时, 0到5 T外磁场下的磁熵变(-ΔS_m)分别高达17.0和18.4 J·kg^-1·K^-1. 研究发现, 化合物1, 2还同时具有明显的场依赖磁弛豫性质. 通过分析弛豫时间随外加磁场和温度变化的关系, 发现其低场下的弛豫行为可以用共振声子俘获机制来解释, 高场下的弛豫行为则可能来源于直接过程.     

溶胶-凝胶法制备(Ba0.85Ca0.15)(Ti0.9Zr0.1)O3无铅压电陶瓷研究

锆钛酸钡钙基无铅压电陶瓷由于具有较好的压铁电性能且环境友好而备受研究者关注,但其存在烧结温度高(>1450℃)、居里温度低等缺点。为降低锆钛酸钡钙基压电陶瓷的预烧及烧结温度,并优化其电学性能,本研究采用溶胶-凝胶法成功合成了(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃无铅压电陶瓷的前驱粉体,并在预烧温度为1000℃、烧结温度为1420℃时成功制备了(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃陶瓷,相较于传统固相法,陶瓷的预烧温度降低了250℃,烧结温度降低了30℃,成功改善了陶瓷的烧结行为,得到了铁电性能、介电性能和压电性能优良且致密性良好的低温烧结陶瓷。     

利用ESR技术研究α-蒎烯,β-蒎烯光敏氧化反应机理

本文主要利用电子顺磁共振(ESR)自旋捕获技术研究9,10 二氰基蒽(DCA)敏化α-蒎烯(αP),β-蒎烯(βP)光氧化反应.提供了在乙腈中α-蒎烯和β-蒎烯的光氧化反应过程中存在超氧负离子基(O₂^-)和单重态氧(¹O₂)的直接证据;在四氯化碳溶剂中只捕获到¹O₂;在正己烷中没有捕获到O₂^-或¹O₂.ESR实验结果进一步证明在乙腈中光敏氧化反应的¹O₂可能来自O₂^-和反应底物α、β-蒎烯正离子自由基之间的电荷复合(CR).     

Dielectric relaxation study in electron‐irradiated ferroelectric poly(vinylidene fluoride‐trifluoroethylene) (80/20 mol%) copolymer

The relaxor ferroelectric (RFE) behavior in high‐energy electron‐irradiated poly(vinylidene fluoride‐trifluoroethylene) [P(VDF‐TrFE)] 80/20 mol % copolymer system is characterized over a broad frequency and temperature range. The dielectric properties remarkably vary with the irradiated dose in terms of the change from normal ferroelectric (FE) to RFE phase. During the RFE–paraelectric (PE) transition, the dielectric constants, as a function of temperature, can be described by the Vogel–Fulcher (V–F) relation. It has been found that the relationship between the real and imaginary part of dielectric constant in irradiated copolymer can be well fitted with modified Cole–Cole equation and Debye relaxation equation, exhibiting similar features as inorganic RFEs. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2972–2980, 2005     

Supramolecular bola-like ferroelectric: 4-methoxyanilinium tetrafluoroborate-18-crown-6

Molecular motion is one of the structural foundations for the development of functional molecular materials such as artificial motors and molecular ferroelectrics. Herein, we show that pendulum-like motion of the terminal group of a molecule causes a ferroelectric phase transition. Complex 4-methoxyanilinium tetrafluoroborate-18-crown-6 ([C(7)H(10)NO(18-crown-6)](+)[BF(4)](-), 1) shows a second-order ferroelectric phase transition at 127 K, together with an abrupt dielectric anomaly, Debye-type relaxation behavior, and the symmetry breaking confirmed by temperature dependence of second harmonic generation effect. The origin of the polarization is due to the order-disorder transition of the pendulum-like motions of the terminal para-methyl group of the 4-methoxyanilinium guest cation; that is, the freezing of pendulum motion at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase. The supramolecular bola-like ferroelectric is distinct from the precedent ferroelectrics and will open a new avenue for the design of polar functional materials.     

Study of the relaxor behavior in BaTi1−xHfxO3 (0.20 ≤ x ≤ 0.30) ceramics

The influence of Hf doping on the structure and dielectric properties of BaTiO₃ has been studied. For this purpose Ba(Ti_1−xHf_x)O₃ ceramics were prepared through solid-state reaction route at close compositions, having x = 0.20, 0.22, 0.23 and 0.30. The study was aimed to locate the exact hafnium concentration for normal to relaxor crossover in these ceramics. X-ray diffraction followed by Rietveld refinement, reveals the formation of single phase with Pm3m cubic structure. Temperature and frequency dependence of real (ɛ′) and imaginary (ɛ″) parts of the dielectric permittivity have been studied in the temperature range of 90–350 K, at frequencies between 0.1 kHz and 100 kHz. The dielectric permittivity variations with temperature show deviation from Curie–Weiss behavior and strong frequency dispersion. The deviation from Curie–Weiss behavior, discontinuous jump along with the change in the slope of T_m vs Hf concentration plot, and the degree of relaxation (γ) approaching ∼2, indicate a crossover from normal to relaxor ferroelectrics. Substitution of Hf⁴⁺ for Ti⁴⁺ in BaTiO₃ introduces structural disorder, causing perturbations like local electric and strain fields. These perturbations reduce the long-range polar order resulting in relaxor behavior.     

Time autocorrelation function analysis of master equation and its application to atomic clusters

We derive the energy fluctuation Delta(2)E, and the time autocorrelation kappa(tau) and its Fourier transformation--the fluctuation spectra S(omega)--of the master-equation transition matrix. The contribution from each eigenmode of the transition matrix to these fluctuation quantities reveals the relevant importance of the individual mode in the relaxation processes. The time scales associated with these relaxation processes are determined by the corresponding eigenvalues. Unlike traditional time evolution analysis, the autocorrelation function and fluctuation spectra analysis does not involve an arbitrary initial population. It is also more suitable for analyzing the underlying dynamic, kinetic behavior near the equilibrium and the behavior of the long-time-scale rare events. We utilize our technique to analyze the solid-liquid phase coexistence of the 13-atom Morse cluster and the fcc-to-icosahedral structure transition of the 38-atom Lennard-Jones cluster. For the processes studied, the fluctuation spectra from the master equation simplify the analysis of the transition matrix, and the important relaxation modes are easily extracted.     

Relaxation dynamics in tert-butylpyridine/tristyrene mixture investigated by broadband dielectric spectroscopy

We investigated, by means of dielectric spectroscopy, the relaxation dynamics of glass forming binary mixtures composed by the quite rigid polar molecules tert-butylpyridine dissolved in the apolar solvent tristyrene. By changing the relative concentration of the components we observed a transition from a relaxation scenario with a structural process and an excess wing to that with a structural process and a well resolved secondary process. Another relaxation process, slower than the latter, was observed, well below Tg. Our detailed analysis evidenced that the secondary relaxation with shorter relaxation time can be identified as the Johari-Goldstein relaxation for all the mixtures, whereas the new relaxation process was attributed to a different type of motion of tert-butylpyridine needing a larger amount of free volume for the molecular rotation.     

Application of calorimetry and thermodynamics to critical problems in materials science

Calorimetry and thermodynamic studies have long been playing a very important role in the research fields of fundamental science and technology. Some topics and examples of thermodynamics studies are given, and the details are explained on the basis of the present author’s experience, focusing attention to application of adiabatic calorimetry and thermodynamics to solve critical problems in materials science: (1) condensed gas calorimetry and third law entropy, (2) phase transition and polymorphism in simple molecular crystals, (3) incommensurate phase transitions, (4) particle size effects on the phase transitions in ferroelectric/ferroelastic crystals, (5) relaxor ferroelectrics and multi-ferroics, and some other topics in materials science and technology.     

Studies of structural,dielectric relaxor and electrical characteristics of lead-free double Perovskite:Gd2NiMnO6

In the current communication, structural, microstructural, dielectric, relaxor, impedance, AC conductivity, and electrical modulus characteristics of double perovskite Gd₂NiMnO₆ (synthesized by a solid state reaction route) as a function of temperature (25–125 °C) and frequency (1 kHz–1MHz) have mainly been reported. From preliminary X-ray structural analysis, it is found that the crystal structure of the material is monoclinic. In temperature dependence of dielectric constant analysis, relaxor behaviour of the material was observed. Such type of behaviour is described by modified Curie–Weiss law and a Vogel–Fulcher law. From Nyquist plots, the existence of grain and grain boundary effect in the material is observed. The non–Debye type of relaxation is confirmed from the complex impedance spectroscopy. From the impedance data, the determined grain resistance reduces with increment of temperature showing negative temperature co-efficient of resistance (NTCR)-type nature of the material which also confirmed from conductivity analysis. Again, non-Debye type of relaxation phenomena is observed from the analysis of modulus spectroscopy which is also proved by complex impedance plot. From these result it may be concluded that this material may be used for different high temperature applications.     

Thermal analysis of cellulose acetate modified with caprolactone

Cellulose acetate (CA) was modified with caprolactone (CL) under various reaction conditions in an internal mixer. The thermal behavior and relaxation transitions of the samples were determined by dynamic mechanical analysis and differential scanning calorimetry. Various relaxation transitions were detected in externally and internally modified cellulose acetate by DMTA. These were assigned to the glass transition of the main chain, to the movement of single glucose units and to hydroxymethyl groups. The β′ transition must belong to structural units larger than a single glucose ring and their formation must depend on sample preparation conditions. No transition could be assigned to grafted polycaprolactone (PCL) chains by DMTA. Contrary to other groups, we could not detect even the transitions of modified CA by DSC. Only the crystallization of oligomeric PCL homopolymer was observed mostly when it diffused to the surface of the sample.     

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

We performed dynamic Monte Carlo simulations of stress relaxation in parallel-aligned and uniaxially stretched bulk amorphous polymers at low temperatures.We observed an extra-slowing down in the early stage of stress relaxation,which causes nonlinear viscoelasticity as deviated from Debye relaxation and Arrhenius-fluid behaviors observed previously at high temperatures.Meanwhile,fluctuation analysis of stress relaxation revealed a substantial increase in the stretch fractions of polymers at the transient periods of high-temperature Debye relaxation.Structural analysis of free volume further revealed the scenario that,at low temperatures,the modulus of polymer entropy elasticity decreases with temperature and eventually loses its competition to the imposed modulus (Deborah number becomes larger than one),and hence upon stress relaxation under constant strains,monomers are firstly accumulated nearby two stretching ends of polymers,resulting in tentative global jamming like physical cross-linking there,and thus retarding the coming transient state of stress relaxation.We concluded that intermolecular cooperation raises physical crosslinking for nonlinear viscoelasticity of polymer stress relaxation as well as the rubbery states unique to bulk amorphous polymers.The new microscopic mechanism of the fluid-rubbery transition of polymers may bring insights into the intermolecular cooperation mechanism of glass transition of small molecules,if the fluid-rubbery transition is regarded as an extrapolation of glass transition from low to high molecular weights.