钇稳定二氧化锆(YSZ)纳米晶体的红外光谱分析

用微波法在乙醇介质中通过掺杂不同含量的钇合成二氧化锆单斜相(m-ZrO_2)、四方相(t-ZrO_2)、立方相(c-ZrO_2)或其混合相纳米晶体。t-ZrO_2和c-ZrO_2纳米晶体的XRD特征峰非常相似,难以区分。本文用红外图谱较好区分了t-ZrO_2和c-ZrO_2纳米晶体。结果表明:纯二氧化锆为60%左右的斜方相和40%左右的四方相组成,4%含量的钇稳定二氧化锆(4%YSZ)为85%四方相和15%斜方相组成,8%YSZ为一半四方相和一半立方相组成,15%YSZ及以上为立方相。     

(1-x)Ba4LiNb3O12-xBaTiO3（x=0～0.40）钙钛矿陶瓷的结构与微波介电性能

采用传统的固相法合成了(1-x)Ba4LiNb3O12-xBaTiO3(x=0～0.40)钙钛矿陶瓷。通过XRD,Raman,SEM以及电学测试对材料的结构和性能进行表征。XRD和Raman结果表明,Ti4+不利于六方相的稳定,一旦出现,部分或全部六方相即转变为立方相。随着x增大,陶瓷的介电常数(εr)逐渐增加,品质因数(Q×)逐渐降低,频率温度系数(τ)逐渐增加。立方相陶瓷与六方的相比,有较高的εr,较低的Q×和较大的τ。讨论了六方相的稳定性以及结构与性能之间的关系。     

先驱体合成法制备PMN-PT弛豫铁电体及其表征

采用先驱体合成法,以氧化物为原料制备了钙钛矿结构的(1-x)PMN—xPT弛豫铁电陶瓷粉末,研究了预烧温度以及PbTiO3固溶量等工艺参数对样品的相结构的影响．结果表明,随预烧温度升高,PMN—PT钙钛矿相含量增加,但温度过高,由于PbO的挥发钙钛矿相又减少;增加PbTiO3也可使钙钛矿相含量增加．     

准同型相界附近PMN-PT陶瓷的介电与压电性能

采用铌铁矿预产物合成法制备了组成在相界附近的(1-x)Pb(Mg1/3Nb2/3)O3-x PbTiO3弛豫铁电陶瓷.陶瓷样品X射线衍射相组成和相结构分析表明:所有陶瓷样品均为纯钙钛矿相,没有检测出其它杂相;而且组成在x=0.33 mol处存在一准同型相界(MPB),在该相界附近三方相和四方相共存,而远离该相界则分别为纯三方相和四方相.在该相界附近组成具有优异的介电和压电性能:εm～38000,Ec～5 kV/cm,P～31.5 μC/cm2,d33～500 pC/N,这主要是由于宏观上的三方和四方两相共存,或微观上的微畴和宏畴共存引起的.     

PMS-xPNW-(0.95-x)PZT压电陶瓷材料的温度稳定性

采用传统固相法制备了Pb(Mn1/3Sb2/3)0.05(Ni2/3W1/3)x(Zr0.495Ti0.505)0.95-xO3(PMS-xPNW-(0.95-x)PZT)四元系压电陶瓷,研究了NiW含量对PMS-xPNW-(0.95-x)PZT体系的相组成、微观结构、机电性能、温度稳定性的影响.结果表明:随着NiW量的增加,物相中四方相含量逐渐减少,三方相含量逐渐增大;适量的掺杂使PMSZT频移变小;机电耦合系数Kp,K31的温度稳定性得到改善;并且提高了体系的机电性能.     

PMS-xPNW-(0.95-x)PZT压电陶瓷材料的温度稳定性

采用传统固相法制备了Pb(Mn1/3Sb2/3)0.05(Ni2/3W1/3)x(Zr0.495Ti0.505)0.95-xO3(PMS-xPNW-(0.95-x)PZT)四元系压电陶瓷,研究了NiW含量对PMS-xPNW-(0.95-x)PZT体系的相组成、微观结构、机电性能、温度稳定性的影响.结果表明随着NiW量的增加,物相中四方相含量逐渐减少,三方相含量逐渐增大;适量的掺杂使PMSZT频移变小;机电耦合系数Kp,K31的温度稳定性得到改善;并且提高了体系的机电性能.     

Al替代Ni对A2B7型贮氢电极合金性能的影响

用冷坩埚磁悬浮熔炼方法制备La0.7Mg0.3（Ni0.85-xCo0.15Alx）3.4贮氢电极合金，采用XRD、三电极体系及SEM研究相结构、电化学性能及电极的表面状态。Rietveld法全谱拟合分析表明，该体系合金为多相结构，主相为ce2Ni7型六方相，还包括CaCu5型六方相、PuNi3型菱方相、MgCu2型立方相及BCr型正交相。Al元素为ce2Ni7型主相的有利形成元素，且Al替代Ni后，各组成相的晶胞体积均增加。P-C-T曲线显示随着Al替代量X的增加，合金放氢平台的平台区域变窄，平台压力降低，平台特性变差。电化学性能测试表明，随着X增加合金电极的最大放电容量降低，高倍率放电性能降低，循环稳定性明显提高。     

Nd掺杂K0.53Na0.47NbO3基无铅压电陶瓷的结构和性能

采用固相反应合成法制备稀土Nd掺杂0.942(Na0.53K0.47)NbO3-0.058LiNbO3(KNNLN)无铅压电陶瓷。采用XRD、红外和拉曼等方法对其结构进行表征,并测试其介电性能。结果表明:掺杂稀土Nd能够有效地固溶到KNNLN基体中;Nd掺杂通过Nb位替代而扰乱了Nb—O6键并使得其强度变弱,进而导致KNNLN样品的相结构变化。Nd掺杂量在1%~2%时,样品中出现了四方相和立方相共存区;随着Nd掺杂量的进一步增加,KNNLN的晶体结构发生从四方和斜方相共存向四方和立方相共存的连续变化;稀土Nd掺杂能够明显地提高样品的室温介电常数并在一定程度上降低KNNLN基陶瓷的室温介电损耗。2%Nd掺杂KNNLN压电陶瓷样品的压电系数d33和介电常数εr值分别约为128 pC/N和694。相比之下,未掺杂KNNLN压电陶瓷的d33和εr值分别约为87 pC/N和545。     

铸钢中(Mn,Fe)S单晶生长形态的研究

本文用SEM 505扫描电镜观察到ZG 25钢铸件皮下显微气孔壁上(Mn,Fe)S单晶生长台阶花样和平衡状态的晶体形态.平衡状态面心立方(Mn,Fe)S单晶是由{111}晶形的八个晶面和{100}晶形的六个晶面组成的十四面体.是O_h—m3m类立方晶系对称图形的典型实例.     

六方Al14.2(Fe,V)3.0Si1.3准晶近似晶体相的电子衍射分析

利用选区电子衍射和会聚束衍射研究了喷射沉积Ａｌ－Ｆｅ－Ｖ－Ｓｉ合金中一种六方相的晶体结构,其点阵常数为：ａ＝２．５２ｎｍ,ｃ＝１．２６ｎｍ,空间群为Ｐ６／ｍｍｍ。该六方相与立方相具有固定取向关系,且ＥＤＳ分析表明,两相成分相近。系统对比分析ＳＡＥＤ花样发现,六方相的某些带轴衍射谱具有立方相的二十面本结构基元表现出的衍射特征。表明六方相也是一种准晶近似晶体相,其结构构架可能由（Ｆｅ＋Ｖ）和（Ａｌ＋Ｓ     

Size effects on the crystal structure of nanoscale BaTiO<Subscript>3</Subscript> powders prepared by hydro-thermal synthesis

The size effect on crystal structure has been investigated for nanoscale BaTiO₃ powders prepared by hydrothermal methods. The powders were composed of both the cubic and tetragonal phases at room temperature. The relative volume fraction of the tetragonal phase was found to be approximately 30% by X-ray diffraction and Raman spectroscopy. High resolution transmission electron microscopy (HRTEM) revealed that the surface region of the powders was of the tetragonal phase, while the core was of the cubic phase.     

Synthesis of tetragonal BaTiO3 film on Ti substrate by micro-arc oxidation

BaTiO3 films on Ti substrate were fabricated by alternative current（AC） and direct current（DC） micro arc oxidation （MAO）. Microstructures of films were investigated by means of SEM, XRD and TEM. The results show that the amorphous phase and primitive cubic phase are the main phases in the films prepared by AC MAO. Even after being annealed at 1 200 ℃ for 8 h, only a few tetragonal phases can be observed in films prepared by AC MAO. However, tetragonal BaTiO3 phase can be produced by DC MAO directly. In the films prepared by DC MAO, a mixture of cubic phase and tetragonal phase is formed. After sparking spacious distribution, sparking duration and temperature gradient near sparking sites were taken into account, and a mechanism of synthesis of tetragonal BaTiO3 phase by DC MAO was proposed.     

Mechanism and conditions of the chessboard structure formation

The observations of the pseudo-periodical chessboard (CB) microstructure in metal and ceramic solid solutions indicate that this is a general phenomenon. We propose a theory and three-dimensional (3-D) computational modeling explaining the origin of the CB microstructure in the cubic → tetragonal decomposition. The 3-D modeling demonstrates that the formation of two-phase CB structures is contingent on the formation of a compositionally stabilized precursor state with the tweed structure that is spontaneously formed at the initial stage of the transformation. The modeling has shown that this tweed structure is a distribution of spatially correlated tetragonal nanodomains whose spatial arrangement has the CB topological features. This precursor tweed structure serves as a template for the precipitation of the equilibrium cubic phase. The CB-like tweed template channels the microstructure evolution towards the two-phase CB structure whose complex and detailed 3-D geometry is in excellent agreement with electron microscopic observations. The thermodynamic analysis and obtained evolution sequences allow us to formulate the necessary thermodynamic, structural and kinetic conditions for the CB structure formation. Reasons for its relative stability are discussed. It is also shown that the coherency between the cubic and tetragonal phases comprising the CB structure produces the stress-induced tetragonality of the cubic phase, orthorhombicity of the tetragonal phase, and rotations of cubic phase rods. These effects should diminish and disappear upon lifting of coherency.     

The transformation sequences in the cubic → tetragonal decomposition

The decomposition of a generic supersaturated binary cubic solid solution into a mixture of cubic and tetragonal phases is investigated by phase field microelasticity modeling and simulations. It is shown that the decomposition in such a system is not necessarily developed by conventional nucleation and growth of the tetragonal phase. There are three temperature and composition ranges where the sequences of transient structures formed are different. The transformation pathways are predicted and the corresponding thermodynamic parameters are identified. In particular, the simulations reveal unusual transformation sequences occurring in the process of isostructural decomposition followed by cubic → tetragonal MT confined within one of the decomposed cubic phases. Mechanisms for the formation of the stress-accommodating multi-domain aggregates of the tetragonal phase and the checkerboard-like structures comprised of parallel rods of cubic and tetragonal phases are discussed.     

Novel thermal expansion of lead titanate

Lattice parameters of lead titanate were precisely re-determined in the temperature range of -150-950℃ by high precision XRPD measurements. It was clarified that there was no any evidence for a new phase transition at low tempera-tures. Tetragonal distortion strain decreases with temperature increasing. A novel thermal expansion was observed, positive thermal expansion from -150℃ to room temperature (RT) and above 490℃, and the negative thermal expansion in the temperature range of RT-490℃. A big jump of thermal expansion coefficient is attributed to the tetragonal-cubic phase transition. A rationalization for the negative thermal expansion of PbTiO3 is due to the decrease of anion-anion repulsion as polyhedra become more regular at heating. The mechanisms of positive and negative thermal expansions were elucidated as the same nature in the homogenous tetragonal phase at present case.     

Tetragonal-cubic phase boundary in nanocrystalline ZrO2-Y2O3 solid solutions synthesized by gel-combustion

By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO₂-Y₂O₃ solid solutions, the presence at room temperature of three different phases depending on Y₂O₃ content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7-10 nm, were synthesized by a nitrate-citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO₂-Y₂O₃ solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined.     

A stabilization mechanism of zirconia based on oxygen vacancies only

The microscopic mechanism leading to stabilization of cubic and tetragonal forms of zirconia (ZrO₂) is analyzed by means of a self-consistent tight-binding model. Using this model, energies and structures of zirconia containing different vacancy concentrations are calculated, equivalent in concentration to the charge compensating vacancies associated with dissolved yttria (Y₂O₃) in the tetragonal and cubic phase fields (3.2 and 14.4% mol, respectively). The model is shown to predict the large relaxations around an oxygen vacancy, and the clustering of vacancies along the 111 directions, in good agreement with experiments and first principles calculations. The vacancies alone are shown to explain the stabilization of cubic zirconia, and the mechanism is analyzed.     

Influence of heat treatment on nanocrystalline zirconia powder and plasma-sprayed thermal barrier coatings

Nanostructured zirconia top coat was deposited by air plasma spray and NiCoCrAlTaY bond coat was deposited on Ni substrate by low pressure plasma spray.Nanostructured and conventional thermal barrier coatings were heat-treated at temperature varying from 1050 to 1 250oC for 2-20 h.The results show that obvious grain growth was found in both nanostructured and conventional thermal barrier coatings(TBCs)after high temperature heat treatment.Monoclinic/tetragonal phases were transformed into cubic phase in the agglomerated nano-powder after calcination.The cubic phase content increased with increasing calcination temperature.Calcination of the powder made the yttria distributed on the surface of the nanocrystalline particles dissolve in zirconia when grains grew.Different from the phase constituent of the as-sprayed conventional TBC which consisted of diffusionlesstransformed tetragonal,the as-sprayed nanostructured TBC consisted of cubic phase.     

Mechanical instabilities and structural phase transitions: The cubic to tetragonal transformation

A variety of technologically important crystalline phases are predicted by first-principles electronic structure methods to be mechanically unstable at 0 K, raising fundamental questions about the finite temperature excitations that stabilize these phases at high-temperature. Here, we show that anharmonic vibrational degrees of freedom can stabilize a cubic phase that is mechanically unstable at 0 K with respect to a tetragonal distortion. We develop an effective anharmonic strain Hamiltonian for a cubic lattice and parameterize its coefficients to first-principles calculations of the energy surface of TiH₂, a compound that undergoes a cubic to tetragonal transformation around 300 K and for which the cubic phase is predicted to be mechanically unstable. Monte Carlo simulations applied to the effective Hamiltonian predict a cubic to tetragonal phase transition and provide insight about the true nature of the high-temperature cubic phase.