Heat capacity measurements of pulverized metal hydrides between 1.5 and 15 K by means of the Cu-tabletting method

Abstract

The heat capacity in the temperature range 1.5 to 15 K yields a number of important informations. The examination of transition metal hydrides, however, is difficult frequently, because the samples embrittle to a fine powder with increasing hydrogen content. Therefore expecially the hydrides of the lower half of the 3d transition group cannot be investigated by means of conventional techniques. In this case the copper-tabletting method is a helpful tool /1/. The finely grained hydride is mixed with high-purity copper powder and pressed to a tablet by applying a pressure of about 10 t/cm 2. The high ductility of Cu ensures mechanical stability of the tablet. The special advantage of Cu as a binder material is its high thermal conductivity which enables a fast distribution of heat within the sample. The heat capacity of Cu at low temperatures is small because of the almost vanishing electronic contribution, and its dependence on temperature can be described by a simple function. Therefore the contribution of the Cu-powder to the overall heat capacity of the specimen is small and can be substracted easily. The applicability of the method will be demonstrated by a number of examples. Informations about the low temperature phase boundaries of the V/H and the Nb/H system have been obtained, and the influence of H on the magnetic behaviour of Fe in V has been investigated. Further, a segregation of V/Ti alloys upon hydrogenation could be detected which cannot be observed neither by X-ray nor by neutron diffraction.