The addition of interstitial solute atoms to niobium, in concentrations below the solubility limit, lowers the transition temperature. Interstitial oxygen has the largest effect, decreasing T, by 0.93'K per at. %; while increasing the resistivity in the normal state by 5.2 pQ cm per at.%. Magnetization curves obtained on niobium (R99s Q/Rgo K 500) and on similar specimens containing interstitial oxygen or nitrogen are substantially reversible and are similar to the shape predicted by Abrikosov for superconductors of the second kind. The field first penetrating the sample, Bzs, less than the thermodynamic critical field, II" decreases with increasing concentration of the interstitital atom, while HN, the field at which the normal state is restored (as determined from magnetization measurements), increases. The ratio, HN/H" is a linear function of p". When the solubility limit is exceeded, hysteresis effects become more pronounced similar to those predicted by Bean s model. Resistive measurements at low current density in longitudinal magnetic fields indicate that both niobium and its interstitial solid solutions exhibit some superconducting properties above IIN.
The specific heat of high-purity Acheson graphite prepared by the National Carbon Company has been measured from 13° to 300 0 K. In the region 13° to 54°K the Cp data follows a]'2 dependence quite accurately in agreement with previous experimental work and recent theoretical investigations of specific heat in strongly anisotropic solids.On the basis of some recent studies for other highly anisotropic solids, it is suggested that the specific heat of graphite will eventually follow a ]'2 dependence at still lower temperatures.The derived thermodynamic functions, entropy, enthalpy, and free energy, have been determined by graphical integration and tabulated at integral values of temperature up to 300 o K. The entropy of graphite at 298.16°K is 1.372±0.OO5 cal/g-atom deg, of which 0.004 is extrapolated from 13° to OOK assuming the third law and the 1'2 dependence.
INTRODUCTIONM EASUREMENTS on the specific heat of graphite commenced with those of Weber l in 1875, and Zakrzewiski 2 in 1891. The room temperature values deviated considerably from the predictions of the classical Dulong and Petit law. Nernst's3 investigation represents the first real attempt to measure the specific heat of graphite below ordinary temperatures (25.8° to 92.6°K). These measurements were followed by those of Magnus,4 who measured the specific heat of graphite over a large temperature range (44.1° to llOOOK), and the work of Jacobs and Parks,6 whose measurements 2.2 2.0
Specific heat data on diamond at temperatures between 20° and 300°K are reported. Comparisons of the data have been made with the predictions of the Debye theory. The deviation of the specific heat from the value given by the three-dimensional continuum theory in the low temperature region can be qualitatively described in terms of a superposition of a simple Einstein frequency. It has been found that variations that occur in expressing reduced characteristic temperatures as a function of reduced temperatures for diamond are qualitatively similar to those of the face-centered cubic metals Al, Cu, and Ag.
The values of entropy, enthalpy, and free energy have been determined and tabulated at integral values of temperature from 25° to 300°K. The entropy of diamond at 298.16°K is 0.568±0.005 cal/g-atom/deg.
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