Order-Disorder Phenomenon in Lead Scandium Tantalate

Lead scandium tantalate (PST) is a ferroelectric relaxor with the perovskite structure of A(B'B")O₃. By suitable heat treatment, the B-site cations can be brought from a stucturally disordered state into various degree of ordering. The degree of ordering is strongly affected by the amouth of vacancies present in the materials. To suppress PbO loss during the sintering or annealing process, a PbO-rich atmosphere is supplied by materials having high PbO vapor pressure, such as PbZrO₃. For PST ceramics with nearly zero weight loss, very long annealing times and higher annealing temperatures are required for ordering. The higher Pbo-loss materials are found to be easily ordered. The introduction of a reducing atmosphere during annealing enhances the ordering process. The ordering process is characterized quantitatively by X-ray diffraction and qualitatively by Raman spectroscopy.     

Iron Diffusion in Iron-Aluminate Spinels

The diffusion coefficient of ⁵⁹Fe in iron-aluminate spinels (Fe₃O₄-FeAl₂O₄) was measured by serial sectioning. The results were analyzed with a defect model, assuming that the predominant defects were cation vacancies and interstitials. At 1380°C, diffusion occurs in Fe₃O₄ and Fe(Fe_1.51Al_0.49))O₄ by an interstitial mechanism at low P_O2 and a vacancy mechanism at high P_O2* Diffusion of iron is by the vacancy mechanism for higher hercynite concentrations, except that the results for the end-member, FeAl₂O₄, suggest an interstitial mechanism. At 10⁻⁶ atm O₂, an Arrhenius plot of D _Fe exhibits a minimum for Fe₃O₄ and Fe(Fe_1.51 A1_0.49)O₄. This effect is explained in terms of a mechanism change. The D _Fe is essentially independent of temperature in Fe(Fe_1.03Al_0.97)O₄ and Fe(Fe_0.54Al_1.46)O₄.     

Elastic Moduli of Refractory Spinels

Elastic moduli of spinel phases in the system Mg(Al, Cr, Fe)₂O₁ were determined from sonic analyses of porous, polycrystalline specimens prepared by hot-pressing. A special iterative least-squares technique (ILS) and standard curve-fitting techniques were used to obtain moduli at zero porosity by extrapolation. A minimal standard error of estimate was achieved in all cases by using an exponential form for the porosity dependence and the ILS technique. Moduli were checked for self-consistency. Extrapolated moduli for MgAl₂O₁ agreed well with respective moduli calculated from single-crystal elastic constants and Birch's law. Longitudinal and shear sound velocities decreased with increasing phase density in the solid-solution systems and systematics indicate that elastic moduli of arbitrary solid solutions can be estimated to lt; ± 10% from the phase density.     

Study of Order-Disorder in Rock-Salt-Related Structures by Infrared Spectroscopy

Infrared spectra were determined for a series of oxides related by ordering and distortion to the rock-salt structure. The expected spectrum of the tetragonal 1:1 ordering (space group D _4h¹⁹) contains 5 ir-active and 8 Raman-active bands. For the trigonally distorted structure (space group D _3d⁵) factor group calculations yield 4 ir and 2 Raman modes. The expected additional structure appears in the spectra of the tetragonal materials but is broadened and washed out by the long-range polarization field. The spectra of the trigonal materials contain twice the predicted number of modes. The broadened bands of the 6-coordinated structures are compared with the sharp spectra of 4-coordinated γ-LiAlO₂.     

Mixed Order-Disorder and Displacive Character of Ferroelectric Transitions

The coexistence of order-disorder and displacive features in Barium Titanate has been noted, among others by Zhong, Vanderbilt and Rabe [1], and has been illustrated recently by NMR work [2]. In this work we make basically two points. One is mainly of historical significance [3]. The other takes note of the previously reported coexistence of order-disorder and displacive features in partially deuterated triglycine sulfate and selenate crystals [4], which are at the opposite end of BaTiO₃ with regard to their order-disorder vs. displacive character.     

Thermal Segregation of Cations in Iron Aluminate Spinels

Thermal segregation experiments were performed on open and encapsulated wafers of Fe(Fe_1.51Al_0.49)O₄ spinels using CO₂ laser radiation as a heat source. The correlation of "apparent" marker movement and cation segregation led to the conclusion that the free exchange of gaseous oxygen at the external spinel surfaces is necessary to cause significant redistribution of aluminum and iron in a temperature gradient. This was explained by the different mobilities of different cations in a net vacancy (low. Effectively no segregation was observed for the zero vacancy flux condition.     

Structural Mechanism of Cluster Formation in Ordered Chalcogenide Spinels

The structural mechanism of 1 : 1 ordering in tetrahedral positions of the spinel structure is investigated within the framework of the group-theoretical methods as applied to the theory of second-order phase transitions. The results of the calculations performed demonstrate that the mechanism of formation of a low-symmetry ordered phase appears to be more complex than traditionally considered earlier. It is found that the formation of the low-symmetry ordered phase is accompanied by the appearance of clusters consisting of cations and anions.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Talanov.     

Reactivity of Aluminum Spinels in the Ethanol Steam Reforming Reaction

Aluminum spinels, MAl₂O₄ with M²⁺ = Cu, Zn or Ni, were prepared by citrate method and its reactivity in the ethanol steam reforming reaction was studied at 500 °C. They were characterized by TG, BET surface area, XRD, FTIR and TPR techniques. The catalytic behavior depended strongly on the nature of divalent metal which determines the physico-chemical properties of samples. In all cases, very active catalysts with a complete ethanol conversion were reached. Hydrogen yield follows the order of ZnAl > NiAl > CuAl.     

Cation Distribution and Defect Chemistry of Iron-Aluminate Spinels

A model describing cation distribution and defect concentrations in iron-aluminate spinels is presented. Cation distribution equilibrium constants are derived from phase-boundary data at 1500° and 1280°C. Estimations of the defect concentrations for the entire solid solution are possible using defect equilibrium constants for Fe₃O₄.     

Synthesis of Magnesium Aluminate Spinels from Bauxites and Magnesias

The synthesis of refractory Mg-Al spinel aggregates from bauxites and magnesias was addressed by applying the alumina–magnesia–silica ternary equilibrium phase diagram as the fundamental basis. Four different alumina sources (bauxites) and four different magnesia sources were investigated. These were fired in air through 1700°C and their reactions were monitored using X-ray diffraction. Evolution of their microstructure was observed by scanning electron microscopy. Initially, the periclase reacts with the free corundum of the bauxite to produce Mg-Al spinel. The periclase then reacts with the mullite in the bauxites to yield additional spinel and also some forsterite. Spinels were produced for all sixteen different raw-material combinations.     

Electronic Conduction and Thermopower of Magnetite and Iron-Aluminate Spinels

Electrical-conductivity and thermoelectric-power measurements of magnetite and iron-aluminate spinels at high temperatures confirm a small-polaron model of electronic conduction. Electrical properties corroborate the previously established cation distribution and defect model. Iron cations become randomly distributed between sublattices at the highest temperature. An n-to-p transition therefore occurs at the 50%Fe₃O₄-50%FeAl₂O₄ composition. Cation distribution and defect equilibria appear to be independent of composition.     

Magnetic Pulsed Compaction of Nanopowders Based on Lithium-Containing Spinels

This paper reports on the results of investigations into the compactibility and structure of powder materials based on lithium-containing spinels that have been considered promising candidates for electrodes of lithium-ion batteries. Dynamic compaction of the powders is accomplished using the uniaxial magnetic pulsed compaction technique, which seems to be an efficient tool for assembling flat and cylindrical lithium-ion batteries. Experiments are performed with sol-gel-derived powders of the electrode materials belonging to the class of lithium-containing spinels (LiMn₂O₄, Li₄Ti₅O₁₂), as well as with powder coatings applied in the form of a thin uniform film to an aluminum foil. The influence of the additive type [graphite, poly(vinyl difluoride), carbon black] and temperature on the compactibility of the powder materials under the action of pressure pulses with an amplitude up to 1.6 GPa is analyzed.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Kaigorodov, Kelder, Schoonman, Ivanov, Nikonov, Medvedev, Rempel’, Nozdrin, Ivanova.     

Electrical Conductivity and Thermal Expansion of Spinels at Elevated Temperatures

Binary oxide spinels composed of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, and Zn were synthesized. Electrical conductivity was measured in air at 500°–800°C. Thermal expansion was measured from room temperature to 1000°C. Ferrite spinels have thermal expansion coefficients of 11–12 ppm/K, compared with 7–9 ppm/K for other spinels except Cu–Mn and Co–Mn which show anomalous behavior. The highest electrical conductivity among transition metal spinels was found for MnCo₂O₄ (60 S/cm at 800°C) and Cu_1.3Mn_1.7O₄ (225 S/cm at 750°C).     

铁水中钒氧化及钒铁尖晶石形成的研究

以热力学计算结果为依据对铁水中钒的氧化行为及钒渣中钒、铁存在的价态进行了理论分析,模拟铁水中钒的氧化反应制取钒渣,运用X衍射分析了钒铁尖晶石的分子组成,研究了FeO对钒铁尖晶石形成的影响.结果表明,铁水中的钒主要被氧化成V3+,也有少部分被氧化成V2+,钒渣中的铁有Fe2+和Fe3+两种形式;当初渣中氧化剂(FeO)的摩尔量与铁水中V的摩尔量比X<2时,钒渣中的钒铁尖晶石以FeV2O4的形式存在;当23时,钒铁尖晶石以Fe2VO4的形式存在.     

Particle Size Comparison of Hydrothermally Synthesized Cobalt and Zinc Aluminate Spinels

Nanosized CoAl₂O₄ and ZnAl₂O₄ powders with particle sizes of 67 and 6.5 nm are synthesized under hydrothermal conditions at 245°C for 20 h. The precursors are reacted at different temperatures to provide intermediate phase transformations using X-ray diffraction (XRD), infrared (IR) spectra, and thermal gravity and differential thermal analysis (TG-DTA). XRD patterns and IR spectra demonstrate that the CoAl layered double hydroxide structure (CoAl-LDHs) is more stable than ZnAl layered double hydroxide structure (ZnAl-LDHs) when they are hydrothermally treated. The different thermal stability of the CoAl- and ZnAl-LDHs results in the different aluminum source, e.g., β-Al(OH)₃ for ZnAl₂O₄ vs γ-AlO(OH) for CoAl₂O₄, when the aluminate spinels are formed. The different aluminum sources lead to the particle size difference. The phenomenon is reasonably expounded based on the nucleation theory from the microscopic scale.     

谈谈化学平衡中的标准平衡常数

可逆反应达到平衡时,根据物质在反应中的存在形式,其标准平衡常数可表示成cKθ、pKθ或Kθ。不同形式的标准平衡常数之间既有区别又有联系。结合例题与推导,详细讨论了标准平衡常数的表示形式、换算关系和实际应用。