Isotherms of the hydrogen–hafnium system

Abstract

The pressure–concentration isotherms for hydrogen–hafnium systems have been measured by the use of u.h.v. apparatus in 50-K steps from 1023 to 1273 K. Unlike other hydrogen–transition-metal systems, this system has isotherms characterized by a single sloped plateau which begins with a sharp shoulder and ends with a much rounded neck. When the temperature is lowered, the slope of the plateau becomes smaller until a horizontal plateau is attained having no critical or eutectic point. The sloped plateau may be interpreted as a collection of sequential plateaus. The multiplateau isotherm model constructed for the hydrogen–zirconium and hydrogen–titanium systems is applicable to the present system if the formalism is modified. The isotherms for the hydrogen–hafnium system may be explained using the higher-order multiplateau model, in contrast to the lower-order multiplateau model used in the case of zirconium or titanium. According to the former model, the occurrence of many plateaus is a consequence of progressive disorder of the interstitial hydrogen. A collective parameter, which involves the same number of degrees of freedom as the interstitial hydrogen, is introduced into the equation of state as a measure of the degree of disorder. The state parameters are specified by choosing T= 1223 K as the standard temperature. The estimated value of the collective parameter verifies the occurrence of such a disorder. The calculated curves are in good agreement with experimental results measured in the temperature range 1023–1273 K.