ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15139.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Vapor pressure of gadolinium metal and carbon‐rich gadolinium dicarbide was measured by the Knudsen effusion technique using an automatic recording balance. Knudsen pressures calculated on the basis that Gd(g) is the major gaseous species did not vary significantly as a function of orifice diameter or sample surface area. Corrections were made for a minor gaseous species, GdC2(g). The entropy and heat content of GdC2were estimated. For the reaction Gd(l) = Gd(g) the third‐law ΔH°298of 95.2 ± 0.3 kcal/g‐atom was in agreement with the second‐law value of 97.3 ± 0.8 kcal/g‐atom. Third‐ and second‐law ΔH°298were combined to give a value of 125 ± 9 kcal/mole for the reaction GdC2(s) = Gd(g) + 2C(s). From these values the enthalpy of formation of carbon‐rich GdC2was calculated to be — 30 ± 9 kcal/mole. Studies of rare earth dicarbide vaporization behavior are briefly reviewed and discussed, and their possible application to a self‐purifying, high‐temperature

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15140.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

SrTiO3solid solutions containing Sr0.5NbO3were studied by X‐ray diffraction and dielectric measurements. Single‐phase perovskite solid solutions were formed up to the composition containing 30 mole % Sr0.5NbO3. The addition of niobium resulted in a symmetry change from cubic (SrTiO3) to tetragonal (solid solution) and thec/aratio increased with increasing niobium content. The dielectric constant for the solid solutions was lower than that for SrTiO3. Pronounced dielectric relaxation peaks and dispersion in permittivity were observed for the composition 7SrTiO3:3Sr0.5NbO3. The activation energy for the relaxation process is about 0.42 ev. It is suggested that the existence of the A‐site vacancy in the perovskite ABO3lattice introduced by the niobium substitution distorts the oxygen octahedra, producing more than one possible “noncentral” site for the Ti4+ion. The relaxation arises from the thermal motion over potential barriers separating these alternat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15141.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

A partial phase diagram for the system Na3AlF6‐Li3AlF6was constructed from DTA and X‐ray diffraction measurements. A region of solid solution extends from Na3AlF6to the limiting composition Na2LiAlF6. At compositions between Na1.5Li1.5AlF6and NaLi2AlF6a cubic phase resembling the mineral cryolithionite is stable over a narrow range of temperatures, which shifts progressively to higher temperatures with increasing lithium content. A single phase can be quenched from samples annealed within this range except near the ideal cryolithionite composition (Na1.5Li1.5AlF6) where the upper temperature limit is too low to permit recombination of two solid phases to cryolithionite even with prolonged annealing. Cryolithionite precipitated from aqueous solution, a sample of the mineral, and the solid solutions containing from 53 to 65 mole % Li3AlF6have identical powder patterns except for differences of lattice constants. Solid solubility of Na3AlF6in Li3AlF6reaches 30 mole % at the eutectic temperat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15142.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two‐phase brittle‐matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass‐alumina system was negligible. The glass‐porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi‐component systems under uniaxial and biaxial stress states can be attributed in part to micro‐structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle cera

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15143.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Thermal expansion measurements, dielectric measurements, and X‐ray diffraction studies were made of the PbTiO3‐BaZrO3system. A dielectric constant maximum of 11,300 was found at the Curie temperature for the Pb0.80‐Ba0.20Ti0.80Zr0.20O3composition. The transition from a ferroelectric tetragonal phase to a para‐electric cubic phase for the 10, 20, and 30 mole % BaZrO3in PbTiO3compositions was of first order. X‐ray diffraction determination of the lattice parameters (25°C) showed that this tetragonal‐to‐cubic phase transition occurred at a composition of 44

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15144.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Results of a new experimental approach to static fatigue of glass are presented. Using the double‐cantilever cleavage technique, it was possible to observe crack motion and to accurately measure crack velocities in glass. The measured crack velocity is a complicated function of stress and of water vapor concentration in the environment. Experimental results are discussed with reference to current theories of static fatigu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15145.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The isothermal transformation rates for quartz to tridymite were studied in the presence of binary silicate liquids between the eutectic temperature and 1470°C. Oxide pairs included Na2O‐SiO2, PbO‐SiO2, FeO‐SiO2, and Cu2O‐SiO2. Micrographic and X‐ray analytical procedures were used. Transformation was most rapid at intermediate temperatures, thus providing typical TTT curves. The rates increased as the liquid contents of the binary pairs were increased. Transformation was more rapid for the pairs which had higher SiO2contents in their equilibrated liquids. Transformation proceeded as a solution‐reprecipitation reaction. Initially, metastable cristobalite precipitated more rapidly than the tridymite; however, it redissolved and eventually disappeared, leaving only tridymite as the solid phase. An empirical equation was adapted to the tr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15146.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The oxidation rate of silicon carbide fibers of submicroscopic dimensions in static air was investigated by a gravimetric technique at 800°, 900°, and 1000°C. The fibers can be held near 800°C for several hours without significant oxidation, but they rapidly oxidize at 1000°C. A theoretical model for diffusion‐controlled oxidation of the fibers, taking into account a changing reaction interfacial area, was obeyed to more than 60% conversion of the silicon carbide to silica. For the diffusion‐controlled oxidation an enthalpy of activation of 55.8 or 39.8 kcal/mole was calculated depending on whether an amorphous silica sheath was initially

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15147.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The radiation transmittance of specimens of nine types of brick having a wide range of densities was measured with a nuclear density gage. A plot of radiation transmittance versus bulk density yielded the exponential curve typical for the fundamental radiation law, showing that the method is sensitive to wide density variations between bricks of different types. Further tests on larger groups of specimens of specific types showed that the method is less sensitive to smaller differences but nevertheless is useful for determining variations in density within and between refractories of various compositions and thicknesses.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15148.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY