Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO₄, Hf_1.26Ti_0.74O₄, and ZrTiO₄ (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO₄ is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α _a =+(8.7±0.5)×10⁻⁶°C⁻¹, α _b =−(5.2±0.5)×10⁻⁶°C⁻¹, and α _c =+ (5.3±0.5)×10⁻⁶°C⁻¹. The thermal expansion of Hf_1.26Ti_0.74O₄ was similar to that of HfTiO₄ but that of ZrTiO₄ was markedly less anisotropic. The compressibilities of HfTiO₄ and ZrTiO₄ also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO₂) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.     

Thermal Expansion Anisotropy of Oxides and Oxide Solid Solutions

The thermal expansion properties of the oxides stishovite (the rutile form of SiO₂), tetragonal vanadium dioxide, and zincite (ZnO) were measured. Solid solutions of rutile-, corundum-, and zincite-structure oxides were prepared, and the thermal expansion properties were measured in an effort to find compositions with more isotropic thermal expansion coefficients. The changes in the observed lattice constants and the changes expected on the basis of the cation radii were compared for both pure oxides and the oxide solid solutions. The difference in these changes indicates the variations to be expected in the thermal expansion anisotropy. A composition range containing the thermally isotropic composition in the system A1₂O₂,-Cr₂O₃ was determined, and the isotropic composition was estimated to be 75% Al₂O₃-25% Cr₂O₃.     

锆离子印迹聚合物分离锆和铪的研究

以锆(Ⅳ)离子为印迹离子,磷酸三丁酯(TBP)为络合剂,4-乙烯基吡啶和苯乙烯为功能单体,二乙烯基苯为交联剂,采用本体聚合法制备了锆离子印迹聚合物(Zr-IIPs)和控制聚合物(CPs).用1∶1盐酸将锆离子印迹聚合物中的锆离子洗脱,释放出印迹空腔.对Zr-IIPs和CPs进行了FT-IR和XRD分析表征.研究了pH及...     

Crystal Structures and Mechanism of Thermal Expansion of High Cordierite and Its Solid Solutions

The crystal structure of high cordierite and that of high cordierite solid solutions containing either Mn or Ga or Ge were refined. The structural changes caused by substitutions and the characteristic features of deformations of each T1O₄, T2O₄ tetrahedron and MO₆ octahedron were clarified. Thorough crystal chemical considerations led to a conclusion that the structure of Mn-bearing solid solution at room temperature may be regarded as a model structure of high cordierite at an elevated temperature. On the basis of the thermal expansion mechanism derived from this consideration, the thermal expansion behavior of the solid solutions is presented.     

Refractory Diborides of Zirconium and Hafnium

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB₂) and hafnium diboride (HfB₂) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB₂ and HfB₂ have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B₂O₃ volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi₂ improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB₂ and HfB₂ ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.     

Solvent Extractive Separation of Zirconium and Hafnium from Hydrochloric Acid Solutions by Organophosphorous Extractants and Their Mixtures with Other Types of Extractants

From 1 to 4 M HCl medium, zirconium was selectively extracted over hafnium by organophosphorous extractants (D2EHPA, PC88A, Cyanex 272). In order to increase the separation factor Zr/Hf, mixtures of organophosphorous extractants with amines (Alamine 336, Aliquat 336) or cationic extractants (Versatic acid 10, LIX 63) were employed in 1–4 M HCl medium. Mixtures with Versatic acid 10 and LIX 63 led to depression in the extraction percentages. But the mixture with LIX 63 was found to be the most effective in separating the two metals among the extractant systems tested in this study. The highest separation factor of around 9.5 was obtained with a mixture of 0.01 M LIX 63 + 0.05 M Cyanex 272 at HCl concentration of 2–4 M. The Zr and Hf were effectively stripped from the loaded mixture of LIX 63 + Cyanex 272 by sulfuric acid solution.     

Tunable Linear Thermal Expansion Coefficient of Amorphous Zirconium Tungstate

Zirconium tungstate (α-ZrW₂O₈) exhibits isotropic negative thermal expansion over a wide range of temperatures. Under high pressure it transforms to a metastable amorphous phase with positive thermal expansion coefficient. α-ZrW₂O₈ can be fully recovered by annealing the amorphous phase at temperatures slightly above 600°C, at room pressure. In this work, we describe the dependency of the linear thermal expansion coefficient of amorphous ZrW₂O₈ with the annealing temperature, showing that it can be continuously tuned, in two stages, from about 7.5° to −9°C⁻¹. These two stages are related to the relaxation of the amorphous phase and the onset of recrystallization, respectively.     

Effect of Phase Transformation on the Sintering of Lead Barium Metaniobate Solid Solutions

The densiflcation behavior and microstructure development were examined for Pb_1-xBa_xNb₂O₆ (x = 0.0, 0.15, and 0.30) with the low-temperature rhombohedral form. The rhombohedral form transformed to the high-temperature tetragonal form during sintering. The phase transformation increased the rate of grain growth, which influenced the microstructure development and densiflcation. The densiflcation proceeded partly before the phase transformation occurred, and the microstructure changed by the sequential formation of necks, chainlike grains, and equiaxed grains. The microstructure just after the phase transformation was determined by the microstructure just before the phase transformation. The chainlike and equiaxed grains changed to elongated and large equiaxed grains, respectively. Pore shape also changed.     

Thermal Expansion of Synthetic β-Spodumene and β-Spodumene—Silica Solid Solutions

Synthetic β-Spodumene and several β-Spodumene-silica solid solutions were prepared, and their thermal expansion behavior was studied using high-temperature X-ray diffraction techniques. Data indicate that all β-Spodumenes are characterized by a pronounced anisotropy of thermal expansion much the same as that of the silica polymorph keatite. Anisotropy increases slightly and the volume expansion decreases as the compositions become richer in SiO₂. Specimens with more than 80.8 wt% SiO₂ at 130O°C and 1 atm contain P-cristobalite along with β-Spodumene. The lattice parameters of β-Spodumene compositions throughout the solid solution range were determined. The structural aspects of the anisotropy of thermal expansion are discussed.     

Thermal Expansion of Binary Borate Glasses and Their Structure

The possible structural transformations in binary borate glasses are analyzed by comparing the phase diagrams and the data available in the literature on the thermal expansion of glasses.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

Solid Solutions of Rare-Earth Cuprates and Their Electrical Properties

Mixed cuprates based on La–Sm, La–Gd, Pr–Y, Pr–Nd, and Sm–Gd oxides are synthesized, and their physicochemical properties are investigated. The electrical properties of the studied cuprates are interpreted within the model of oxidation–reduction reactions.     

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO₂ and HfO₂ lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO₂ is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO₂ at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 Å is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.     

Formation and Transformation of Cubic Lead Niobate Pyrochlore Solid Solutions

Formation of lead niobate pyrochlores by the reaction of PbO with Nb₂O₅ in various molar ratios and at various temperatures has been investigated. It has been shown that for a PbO/Nb₂O₅ molar ratio between 1.5 and 3.0, a pyrochlore solid solution having cubic symmetry is formed at low temperatures (∼500-600°C). X-ray diffraction data show evidence for the existence of compositional inhomogeneity for this phase which contains part of the Pb in the Nb site leading to the formula Pb_{1.5+ x} (Nb_{2- y} Pb _y )O_7-delta (0.0 < x < 0.5; 0.0 < y < 0.5). A phase-pure Pb-deficient pyrochlore (Pb_1.5Nb₂O_6.5) and Pb-excess pyrochlores (Pb_2.5Nb₂-O₈, Pb₃Nb₂O₈) are formed by the decomposition of this cubic solid solution and subsequent reaction at high temperatures. These results indicate that the mechanism of the structural transformation from the low-temperature (LT) cubic solid solution to the corresponding high-temperature (HT) phases is reconstructive.     

晶化时间对MgO-Al2O3-SiO2微晶玻璃相转变及热膨胀性能的影响

采用XRD、DTA、SEM等测试方法,对MgO-Al2O3-SiO2（MAS）系微晶玻璃的析晶和微观结构进行了研究,讨论了不同的晶化时间对MAS微晶玻璃析晶行为及其热膨胀性能的影响。结果表明：在1050℃保温,堇青石能快速地晶化析出。随着晶化时间的增长,堇青石相逐渐增多,当晶化2h时几乎完全析出,析出晶粒大小约为2～5um。MAS系微晶玻璃的热膨胀系数与相组成有着密切的关系,随着晶化时间的延长,热膨胀系数逐渐减小。