Compound Mg2Ta2O7 and Its Interaction with Nd3TaO7

Compound Mg₂Ta₂O₇ crystallizing in the structure of cubic pyrochlore and stable in the temperature range from 1400°C to the melting point has been synthesized. The phase formation in the Mg₂Ta₂O₇ – Nd₃TaO₇ system in the temperature range from 1300°C to the melting point has been studied. The formation of Mg₂Ta₂O₇-based cubic solid solutions extending to 35 mol.% Nd₃TaO₇ has been established. A phase of varying composition with a fluorite-like structure stable to 1500 – 1550°C in the concentration range of 43 to 67 mol.% Nd₃TaO₇has been identified.     

Synthesis of Glass-Ceramic Materials in the BaO–PbO–B2O3–Al2O3–TiO2 System

The influence of PbO, B₂O₃, and Al₂O₃ additives on the glass formation and crystallization of glasses with a high total content of BaO and TiO₂ (65–75 wt % or 76–86 mol %) is investigated. It is shown that glasses of the compositions (wt %) 31–35 BaO, 12–17 PbO, 34–42 TiO₂, 10–13 Al₂O₃, and 2–3 B₂O₃ are promising materials for use in preparing glass-ceramic ferroelectrics based on the melting–molding–crystallization technology. These compounds are characterized by a relatively low melting temperature (1450°C), the absence of spontaneous crystallization during molding, and the possibility of controlling the phase composition of the material through the appropriate choice of the crystallization temperature.     

Manifestation of the mixed-cation effect in dilatometric properties of RO (R2O) · 2B2O3 borate glasses upon replacement of Na2O by BaO, Na2O by MgO, and BaO by MgO

The dilatometric properties, such as the temperature coefficient of linear expansion in the range 20–300°C, the structural temperature coefficient of linear expansion, and the glass transition temperature T_g of RO(R₂O) · 2B₂O₃ glasses are investigated upon replacement of Na₂O by BaO, Na₂O by MgO, and BaO by MgO. It is revealed that, upon these replacements, the composition dependence of the temperature coefficient of linear expansion exhibits an additive behavior, whereas the composition dependences of the structural temperature coefficient of linear expansion and the glass transition temperature T_g deviate from an additive behavior. These regularities are associated with the difference in the energy of cation-oxygen bonds and a different effect of cations on the coordination state of boron.Original Russian Text Copyright © 2004 by Fizika i Khimiya Stekla, Pevzner, Klyuev.     

Propriétés électrochimiques des solutions solides La2O3-SrO

Résumé Les matériaux à base d'oxyde de lanthane préparés renferment de 0 à 35% en mole d'oxyde de strontium. Seule la phase La₂O₃ a pu être mise en évidence lors de l'analyse radiocristallographique. La conductivité électrique a été mesurée en fonction de la température (600–1200°C) et de la pression partielle d'oxygène: entre 1 et 10^–7atm la conduction est mixte (ionique et électronique), au dessous elle est purement ionique. L'énergie d'activation de conduction ionique est pratiquement indépendante de la composition, elle est égale àE = 0.55 eV. La détermination du nombre de transport ionique et du domaine de stabilité redox a été effectuée par la méthode de l'électrode ponctuelle. Les résultats obtenus permettent de situer la limite inférieure de stabilité redox de l'électrolyte à 10^–27 atm à 1000°C.Les caractéristiques de l'électrolyte étudié sont comparées à celles de quelques électrolytes classiques.

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The La₂O₃ based materials which have been prepared contained 0–35 mol % SrO. Only the La₂O₃ phase could be seen by X-ray diffraction analysis.Electrical conductivity was measured as a function of temperature (600–1200°C) and partial oxygen pressure: from 1–10^–7atm, the conduction is mixed (ionic and electronic); below 10^–7 it is purely ionic.The activation energy for ionic conduction is nearly independent of composition, 0.55 eV. The determination of the ionic transport number and of redox stability domain was carried out by the point electrode method. The results allow the location of the lower limit of redox stability of the electrolyte at 10^–27atm at 1000°C.The characteristics of the studied electrolyte are compared with those of some classical electrolytes.

     

Mechanism and Kinetics of Formation of Solid Solutions in the Nd2SrAl2O7–Ho2SrAl2O7 System

The processes of phase formation in the Nd₂O₃–Ho₂O₃–SrO–Al₂O₃ system are investigated in the temperature range 1100–1530°C. It is revealed that the structural–chemical mechanism of formation of (Nd _x Ho_{1 – x} )₂SrAl₂O₇ solid solutions depends on the temperature and composition. An increase in the holmium content and in the temperature leads to a crossover from the mechanism including the formation of LnAlO₃ and LnSrAlO₄ as intermediate products to the mechanism in which the interaction of Ln ₂O₃ and SrAl₂O₄ is a limiting stage.     

Thermomechanical properties of ceramics in the systems Al2O3-TiO2 and Al2O3-TiO2-mullite

Conclusions We studied certain properties of ceramics in the systems Al₂O₃-TiO₂ and Al₂O₃-TiO₂-mullite, obtained by the use of the double-stage synthesis of aluminum titanate.We established the nature of the change in the high-temperature strength in relation to the ratio of Al₂O₃ and aluminum titanate. The maximum high-temperature strength (bending) at 1200°C is possessed by ceramic with a corundum matrix and a volume proportion of aluminum titanate equal to 40–45%.It is established that the addition of CaO + SiO₂ made in amounts of up to 1.0–1.5% contributes to the partial breakdown of the aluminum titanate in the compositions Al₂O₃-TiO₂ and the production of a ceramic with a bending strength of 160–190 N/mm² at 20–200°C, thermal-shock resistance 650–800°C, and thermal conductivity of 1.9–2.1 W/(m·K).We studied the effect of the mullite concentration on the properties of the ceramic in the system Al₂O₃-TiO₂-mullite. The introduction of mullite in amounts of not more than 50%, containing up to 3% of impurities, contributes to an increase in the ceramic's strength in the range 20–1300°C and in the thermal shock resistance.Translated from Ogneupory No. 2, pp. 22–26, February, 1988.     

The System BaO – Al2O3 – Fe2O3

Results of a study of the subsolidus structure of the BaO – Al₂O₃ – Fe₂O₃ system at 1300°C are presented. Based on thermodynamic and experimental data, the triangulation of the system, previously unreported, is carried out with regard for all phases stable at this temperature. Geometrical and topological characterization of the system is given.     

The long-term high-temperature behavior of refractories produced from ZrO2 stabilized with Nd2O3

Conclusions A technology was developed for the production of zirconia refractories from ZrO₂ stabilized with Nd₂O₃. The thermal strength of the product is adequate for long-term service at a large (200–300°C/mm) temperature gradient. Products based on a zirconia — neodymium solid solution can be used several times.It was established that no appreciable Nd₂O₃ vaporization and, consequently, no appreciable destabilization of the ZrO₂ develops in a neutral medium at 2100–2500°C.The solid-phase processes developing at 2100–2500°C in products from a mixture of 70% cubic solid solution (88 mole % ZrO₂+12 mole % Nd₂O₃) and 30% unstabilized ZrO₂ fired at 1750°C consist of the redistribution of the Nd₂O₃ between the cubic solid solution Nd₂Zr₂O₇-ZrO₂ and the unstabilized ZrO₂, and the diffusion of some of the Nd₂O₃ from the cooler to the working zone.Translated from Ogneupory, No. 3, pp. 52–55, March, 1976.     

Structural features of Sc3TaO7

Conclusions The compound Sc₃TaO₇ synthesized at 1300°C has a monoclinic modification which is stable in vacuum in the 25–1700°C range. We detected a phase inversion of the monoclinic modification into the tetragonal at 1800°C, accompanied by a change in the chemical composition with respect to oxygen. We determined the temperature coefficients of linear expansion of the monoclinic modification of Sc₃TaO₇ in the 25–1700°C range, and detected a marked anisotropy of temperature expansion in the crystal lattice.The author wishes to thank O. N. Vasil'eva for assistance with the experiments.Translated from Ogneupory, No. 7/8, pp. 14–16, July–August, 1992.     

Strain and Fracture of a Ceramic Based on Lanthanum Chromite

The bending behavior of a ceramic — a lanthanum chromite-based solid solution La_{1 – x} Ca_xCrO₃ (x = 0.025) doped with 5 – 40 wt.% Y₂O₃ — is studied in the temperature range of 20 – 1400°C. At 20°C the composition with 5 – 10 wt.% Y₂O₃ is strained inelastically and exhibits relatively superior mechanical properties in comparison to compositions containing 20 – 40 wt.% Y₂O₃. These compositions are prone to brittle fracture within the linear elastic strain. Based on the strain and fracture characteristics of lanthanum chromite at 20°C, its mechanical behavior at high temperatures is predicted.     

Thermal aging of ceramics based on Al2O3-TiO2, Al2O3-SiO2 compositions

Conclusions Following an investigation of the thermal aging at 800°C in air of ceramics based on Al₂O₃ TiO₂ (Nos. 1–3) and Al₂O₃-TiO₂-SiO₂ (No. 4), it was established that the ceramics based on Al₂O₃-TiO₂-SiO₂ are the most heat-stable at 800°C over prolonged periods.Translated from Ogneupory, No. 1, pp. 21–23, January, 1990.     

Technical properties of compositions in the system,MgO-Cr2O3-ZrO2

Conclusions In the region of the pseudotrinary system ZrO₂-MgCr₂O₄-MgO, it is possible to obtain high-density, chemically resistant and strong products with refractoriness-under-load values (2 kg/cm²) above 1720°C. With this, it is necessary to make combined use of unfired ZrO₂, presynthesized magnesio-chromite, and high-fired magnesia. The final temperature of firing for the products is 1700–1750°C.We evolved three groups of compositions, of which the best factors are possessed by compositions with a predominance of ZrO₂. They are characterized by simplicity of preparation and high sinterability.An addition of ZrO₂ to the spinel-periclase composition increases their density and chemical resistance in relation to calcium oxide, and magnesio-chromite improves the resistance to iron oxides.Translated from Ogneupory, No.1, pp.55–60, January, 1967.     

Conduction in Bi2O3-based oxide ion conductors under low oxygen pressure. I. Current blackening of the Bi2O3-Y2O3 electrolyte

The cathodic current blackening of Bi₂O₃-based oxide ion conductors was examined for the Bi₂O₃-Y₂O₃ electrolyte at low oxygen pressure. In air, more than 500 mA cm^–2 d.c. could be passed at 600° C without causing changes in the electrolyte itself. However, in argon gas, a limiting current of 3 mA cm^–2 was observed and the electrolyte was blackened at the cathode side. The limiting current was ascribed to control by the diffusion of oxygen gas at the cathode. The blackened oxide was found to consist of a mixture of Bi metal and Bi₂O₃-Y₂O₃ solid solution and to exhibit the equilibrium oxygen partial pressure almost corresponding to that of the Bi, Bi₂O₃ mixture.     

High oxygen ion conduction in sintered oxides of the Bi2O3-Er2O3 system

The phase diagram of the Bi₂O₃-Er₂O₃ system was investigated. A monophasic f c c structure was stabilized for samples containing 17.5–45.5 mol% Er₂O₃. Above and below this concentration range polyphasic regions appear. The f c c phase showed high oxygen ion conduction. The ionic transference number is equal to one for specimens containing 30 mol% Er₂O₃ or less, while an electronic component is introduced at low temperatures for specimens containing 40–60 mol% Er₂O₃. Between 673 K and 873 K a maximum in the conductivity was found at 20 mol% Er₂O₃. (Bi₂O₃)_0.8.(Er₂O₃)_0.20 is found to be the best oxygen ion conductor so far known. The conductivity at 773 K and 973 K is 2.3 ^–1m^–1 and 37 ^–1 m^–1 respectively. These values are 2–3 times higher than the best oxygen ion conductor reported for substituted Bi₂O₃ systems and 50–100 times higher than those of stabilized zirconia (ZrO₂)_0.915(Y₂O₃)_0.085 at corresponding temperatures.     

Density measurements of molten Y3Al5O12 and Y2O3-Al2O3 compounds using an electrostatic levitation technique

The densities of molten Y₂O₃-Al₂O₃ compounds, including yttrium aluminum garnet (Y₃Al₅O₁₂), were determined over a wide temperature range that included an undercooled region, using an electrostatic levitation furnace. The density of the molten Y₃Al₅O₁₂ varied with temperature according to the relationship 3750 − 0.25(T − T_m) (kg/m³) with T_m = 2240 K and for the range of 1300 K ≤ T ≤ T_m, yielding the thermal expansion coefficient ɑ = 6.7×10⁻⁵ K⁻¹. The molar volumes of molten (100-x)Y₂O₃-xAl₂O₃ (x = 0, 33.3, 50, 55, 62.5, 76.5, 81.5, or 100 mol%) were found to vary with the value of x in a linear manner within the superheated temperature range. However, the molar volumes in the undercooled region deviated from those calculated using an ideal solution model owing to attractive interactions between Y₂O₃ and Al₂O₃.     

Phase transformations in stabilized ZrO2-Al2O3 compositions and properties of zirconium-corundum refractories

Solid-phase reactions in mixtures of cubic ZrO₂ with different compositions synthesized by induction melting and Al₂O₃ with different thermal prehistories are investigated at a temperature of 1750°C. Data on the change in the principal engineering properties of zirconium-corundum refractories as a function of their chemical composition are obtained.Translated from Ogneupory, No. 12, pp. 5–8, December, 1995.     

Conditions for Preparation of Oxide Components and Their Effect on Properties of Al2O3 – ZrO2 – SiC Composite

Results of a study of phase formation in the Al₂O₃ – ZrO₂ – SiC system sintered under vacuum and in a reducing medium at 1350 – 1550°C are reported. Conditions for preparation of Al₂O₃ and ZrO₂ powders from hydroxides are specified and the effect of specific surface, temperature, and holding time on the reaction between oxide and carbide components is considered. Results of microstructural, x-ray phase, thermogravimetric, and chemical analyses of precursor materials and end products are discussed. Optimum composition (20% SiC, 15% ZrO₂, 65% Al₂O₃ ) and dispersity of the mixture for obtaining a composite with a strength of about 200 MPa by a method other than high-temperature toughening are determined.     

On the Phase Separation and Crystallization of Glasses in the MgO–Al2O3–SiO2–TiO2 System

The changes in the structure and phase composition of glasses in the MgO–Al₂O₃–SiO₂–TiO₂ system (at different TiO₂ contents and ratios MgO : Al₂O₃) upon their heat treatment in the temperature range 700–960°C are investigated by small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. The influence of gallium oxide additives on the phase separation and crystallization is analyzed. It is demonstrated that the heat treatment results in the phase separation, which occurs through the spinodal decomposition mechanism. A regular structure formed upon phase separation is retained after the completion of crystallization in inhomogeneity regions. The interference effects due to the regularity in the distribution of nanocrystals in the vitreous matrix bring about a decrease in the light scattering intensity and provide transparency of glass-ceramic materials.     

Comparison of 18O Isotopic Exchange After and During the Interaction of N2O, NO, and NO2 with Fe Ions in Ferrierites

Nitrous oxide deposits its oxygens on Fe-ferrierites at 200–250 °C in contrast with nitric oxide and nitrogen dioxide. This oxygen is readily exchangeable for ¹⁸O₂ at room temperature and the reaction proceeds via a single-step exchange mechanism. All three nitrogen oxides in a mixture with ¹⁸O labeled dioxygen undergo isotopic exchange (IE) at 200–250 °C, N₂O via the same single-step mechanism, while NO and NO₂ react via a multiple-step mechanism. Zeolitic oxygens participate in IE above 250 °C during temperature-programmed desorption of surface species formed in the reaction of nitrogen oxides with ¹⁸O₂.     

Properties of compositions based on the system Al2O3-Mg(Al1−x,Crx)2O4

Conclusions Magnesia-alumina spinel and highly chromic spinel MgAl_0.4Cr_1.6O₄ and MgCr₂O₄ retard the grain growth of corundum during firing of the product. Low-chromic spinel MgAl_1.6Cr_0.4O₄ and MgAlCrO₄ added in small quantities intensify the corundum recrystallization.Small (5–10% by weight) additions of spinel Mg(Al_1–x, Cr_x)₂O₄ increase the strength of the corundum specimens, but only the high-alumina spinel improves their sintering.The sintering of mixtures of Al₂O₃ and Mg(Al_1–x Cr_x)₂O₄ is impaired during the substitution of magnesia-alumina spinel by magnesia chromite, and with an increase in the quantity of spinels from 5 to 30%.A small addition (5–10%) of high-alumina spinel of the composition Mg(Al_1–x, Cr_x)₂O₄ where x 0.5 to the alumina precalcined at 1450°C enables us to obtain dense, strong, and thermally shock-resistant corundum products.Translated from Ogneupory, No. 3, pp. 53–56, March, 1972.