The electrical resistivity of pure hexagonal boron nitride in the c direction was measured from 700 degrees C to 1400 degrees C. Special attention was paid to allowance for electrical leakage of the measurement rig. The logarithm of the resistivity increased linearly with the reciprocal of absolute temperature, suggesting that conduction was intrinsic in the temperature range considered. A gap width of 7.1+or-0.1 eV between valence and conduction bands was deduced.
Experiments are described in which the momentum flux of gas atoms, remitted normal to the surface of a hot clean tungsten ribbon immersed in a low pressure of helium or argon, is measured with a torsion balance and the thermal accommodation coefficient deduced. Data are presented in which the tungsten temperature range was 700-1900 K for helium and 1100-1700 K for argon.If it is assumed that the normal remitted momentum flux is that expected on assumption of the cosine emission relation, accommodation coefficients much larger (and for argon physically impossible) than those found previously by other workers are implied. A model is proposed which assumes that atoms impinging on and remitted from the hot tungsten ribbon conserve momentum in directions parallel to the surface. This results in a remitted flux, in the direction of the normal, greater than the cosine relation would predict. The resulting accommodation coefficients are then of the same order as those found using the total-heat-loss method. The method here reported is believed to be novel: its accuracy increases with the temperature of the hot solid; and it permits the measurement of translational thermal accommodation without dependence on the temperature coefficient of resistance of the solid and hence is applicable to alloys and to non-metals. For metals which have a normal temperature coefficient of resistance, the method allows translational accommodation to be measured and internal energy accommodation to be-deduced.
The behaviour of hot gold filaments, in vacuum and in oxygen at pressures of a few microns of mercury, has been studied. Two main effects were observed-the disappearance of gaseous oxygen and a rate of evaporation of gold considerably greater in oxygen than in vacuum.The rate of disappearance of oxygen ( E ) depended on the purity of the gold, but the amount which disappeared was too large to be explained as oxidation of impurities in the metal. The disappearance is attributed to an impurity-catalyzed conversion of oxygen molecules from the ground to the metastable lAg state, the 0; attacking the walls of the reaction vessel. E decreased as the walls became protected by a film of evaporated gold. The enhanced rate of evaporation of gold in 0 2 is attributed to the removal of an inhibiting surface film of impurity. In N2, E was zero and there was no enhanced evaporation of gold.The activation of molecular oxygen is discussed in appendix 1 by D. M. Gilbey. Appendix 2 by D. Clark reports on the structure of the evaporated gold deposits.
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