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The microstructures of gas-pressure-sintered materials from &Si3N4 powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850" to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The ratedetermining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from mSi3N4 powder despite the smaller aspect ratio of the grains. [

Properties of Dense Polycrystalline Oxyhydroxyapatite 455 VI. ConclusionsIt has been shou n by Raman spectroscopy that in the first stage of melting in a reacting 30K,C0,,-70SiOZ-1 As,O,, glass-forming batch, all the arsenic is in the pentavalent state. dissolved as AsO,3-in a potassium-rich liquid phase. which contains metasilicate and carbonate. The high oxidation state of arsenic is then stabilized by the high potassium oxide concentration. When the reactions proceed and the product glass composition is reached. the melt becnmes mnre acidic and part of the arsenic goes into the trivalent state. causing an oxygen evolution at the right place and time.Solution-grow n crjstals of hydrovjapatite were sintered into polycrj stalline ox3 hydroxyapatite bodies. using the range 1050 to 1450 C. The heat capacity, thermal diffusility. and thermal conducti\itj of the sintered bodies were measured by the laser flash method at 130-1000 K. The sintered bodies \\ere 94.4 to 99.4'~ of theoretical density and 0.8 to 12 p m in grain size. Sintering is accompanied by grain grabs th and by kacancj formation and cell contraction due to thermal dehydration. Tjpical values of the heat capacity, thermal diffusil ity, and thermal conductivity at room temperature are 0.73 J/g K, 0.0057 cm2/s and 0.013 Jis cm K, respectkelj. Lou-temperature thermal conductivitj increased with increasing temperature. similarly to that of amorphous solids. This odd behavior is discussed in terms of phonon mean free path.

ABSTRACT:High strength polyethylene fiber (Toyobo, Dyneema® fiber: hereinafter abbreviated to DF) has a negative thermal expansion coefficient. Relation between fiber structure and thermal strain of DF used as reinforcement of DF reinforced plastic (DFRP) for cryogenic use was investigated. The crystallinities and orientation angles of several kinds of polyethylene fibers having different modulus from 15 to 134Gpa (herein after abbreviated to DFs) were measured by NMR and X-ray. We obtained the parameters of the mechanical series-parallel model composed of crystal and amorphous by crystallinity and modulus. Thermal expansion coefficients of DFs were estimated by mechanical seriesparallel model. All DFs having different modulus showed negative thermal expansion coefficients in the temperature range from 180 to 300K, and absolute values of those markedly increased by increasing tensile modulus of DF. The estimated thermal expansion coefficients showed negative values, and thermal strains showed a similar curve to observed ones mostly. Average thermal expansion coefficients in the temperature range from 180 to 300K estimated by mechanical model agreed with the observed ones.

An oxynitride glass containing 18 at.% N was prepared under high N2 pressure. The glass was transparent and very hard, with a Vickers hardness of 12.0 GPa.