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SUBBARAO, E. C.; SUTTER, P. H.; HRIZO, J.
doi: 10.1111/j.1151-2916.1965.tb14794.xpmid: N/A
Compositions in the system ThO2‐YO1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite‐type structure were formed up to 20 to 25 mole % YO1.5 at 1400°C and up to 45 to 50 mole % YO1.5 at 2200°C. Density data showed that these solid solutions correspond to Th1—xYxO2—0.5x, having a complete cation sub‐lattice filled by Th4+ and Y3+ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO2 is soluble in YO1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y2O3 phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.
ROOKSBY, H. P.; WHITE, E. A. D.; LANGSTON, S. A.
doi: 10.1111/j.1151-2916.1965.tb14795.xpmid: N/A
A family of perovskite‐type compounds of composition AO.33BO3 was prepared by high‐temperature heat treatment of appropriate mixtures of trivalent rare‐earth oxide (A23+O3) and niobia or tantala (B25+O5). The A‐type cations investigated included La, Ce, Pr, Nd, Sm, Gd, Dy, Y, and Yb. The compounds which can be formed with niobia appear to be limited to those containing A‐type cations of radius greater than that of samarium, but with tantala the range extends down to ytterbium. The perovskitelike crystal structures are tetragonal or pseudotetragonal with an axial ratio ∼2. For the majority, the two‐cell pseudotetragonal unit shows either orthorhombic or monoclinic distortions, the lower monoclinic symmetry characterizing those containing the smallest rare‐earth cations, dysprosium, yttrium, and ytterbium. Niobia and tantala compounds of similar structure and composition were formed from ceria and praseodymia and the rare‐earth cations appeared to be substantially reduced to the trivalent state during the high‐temperature reactions.
doi: 10.1111/j.1151-2916.1965.tb14796.xpmid: N/A
The subsolidus phase equilibrium diagram for the pseudobinary join MgAl2O4‐Ga2O3 was determined. The shape of the exsolution boundary was obtained by heat‐treating samples pre‐ equilibrated at 1600°C. Crystalline solubility of Ga2O3 in MgAl2O4 decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg2+ions by Ga3+ ions to produce a cation defect spinel. The phase precipitated was the mono‐clinic δ‐Ga2O3 (=δ‐Al2O3 structure). Changes in the ratios of relative X‐ray diffraction intensities indicated that the crystalline solutions also disorder with temperature.
doi: 10.1111/j.1151-2916.1965.tb14797.xpmid: N/A
The rates of hydrolysis of polycrystalline magnesium fluoride slabs were investigated by a gravimetric method as functions of temperature from 745° to 835°C and water‐vapor pressure from 1 to 20 mm Hg. A linear rate law was found, and it was concluded that the rate‐determining process was chemical reaction at the MgF2‐MgO interface. A three‐step reaction mechanism is proposed: (1) the chemisorption of water vapor. (2) formation of a hydroxyfluoride complex. and (3) decomposition of the hydroxyfluoride complex to form products. Assuming that the chemisorption step was in equilibrium and that the HF pressure was negligible, an expression was developed showing that the rate was proportional to the concentration of chemisorbed water vapor. The rate dependence on water‐vapor pressure was expressed in terms of the Langmuir adsorption isotherm equation. Values of –16.9 kcal/mole and–4.2 eu were obtained for the enthalpy and entropy of chemisorption. The activation enthalpy and frequency factor for the decomposition of the chemisorbed intermediate, MgF2.H2O(chem), were 46.0 ± 0.5 kcal/mole and 1.75 × 1O10 sec−1.
MESSIER, DONALD R.; PASK, JOSEPH A.
doi: 10.1111/j.1151-2916.1965.tb14798.xpmid: N/A
The kinetics of the reaction between MgF2 and H20 vapor were gravimetrically determined on three specimen types: single crystal, sintered polycrystalline disk, and powder, in the range 950° to 1100°C at a water‐vapor partial pressure of 55.3 mm Hg. The rate‐influencing effects of changing specimen geometry and the formation of thick MgO product layers on specimens reacted to large extents complicated the evaluation of the rate measurements. Apparent activation enthalpies of 41.5 ± 1.6, 53.6 ± 2.7, and 23 kcal/ mole were obtained for the single‐crystal, polycrystalline, and powder specimens, respectively. The significance of these values is discussed with respect to possible rate‐influencing factors.
doi: 10.1111/j.1151-2916.1965.tb14799.xpmid: N/A
A technique for analysis of powdered or densified nonconductive materials by spark source mass spectrography is based on the use of a conducting probe against the compacted sample to initiate sparking. Results on MgO show significant quantities of anion impurities which are not generally reported. Analyses of CdS powder indicate low contamination from sample preparation.
doi: 10.1111/j.1151-2916.1965.tb14800.xpmid: N/A
The quenching method was used to determine phase relations at liquidus temperatures in the system cobalt oxide‐iron oxide‐silica in air. The crystalline phases coexisting in equilibrium with liquids in the system are silica (SO2), olivine (Co2SiO4), a spinel solid solution ((Co,Fe)3O4), and an oxide solid solution ((Co,Fe)O) with peri‐clase‐type structure. The lowest liquidus temperature is 1313°C. Composition relations among coexisting liquid and solid phases were established in the areas where solid solution crystals are involved in the equilibria.
doi: 10.1111/j.1151-2916.1965.tb14801.xpmid: N/A
Gallium orthoferrite (Ga2‐xFexO3) has a maximum thermal stability which coincides roughly with liquidus temperatures at oxygen pressures near atmospheric. As a result, changes in ambient oxygen pressure between 0.2 and 10 atm have a pronounced effect on equilibria. The compound exhibits a wide range in Ga:Fe ratio on both sides of the stoichiometric GaFeO3 but is essentially invariant in oxygen content to 1500°C in air. The orthoferrite bears many similarities to the corresponding aluminum compound Al2‐xFexO3.
doi: 10.1111/j.1151-2916.1965.tb14802.xpmid: N/A
The defect structure of tetragonal zirconia was investigated by measuring the temperature and oxygen partial pressure dependence of the electrical conductivity and the electronic transference number. Tetragonal zirconia was a mixed electronic and ionic conductor under all conditions studied. The n‐type electronic conductivity observed at high temperatures and low oxygen partial pressures was interpreted on the basis of a defect model involving fully ionized oxygen vacancies. The conductivity change associated with the monoclinic ⇒ tetragonal phase transformation was isothermal, but the rate of change was a function of the thermal history and the method of preparation of the sample.
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