A new high temperature design to determine electrical and thermal conductivities and thermoelectric power. Applications to the sintered composite AgNi (90/10) “pseudo-alloy”

Abstract

The aim of this paper is the description of a new high temperature design, tested up to 1100 °C, to measure electronic transport and to determine, for the first time to our knowledge at high temperatures, the electronic transport properties of a “pseudo alloys”, i.e. the silver nickel (90/10) one, which is used in contact materials for electrical engineering. Nickel and silver are mutually insoluble in liquid state. Thus, the alloy cannot be obtained by solidification. The solid is realized by sintering silver and nickel micrometric grains. This “pseudo alloy” is used in electrotechnics. To do these measurements, we had to develop a new experimental design that we describe, in order to measure very small resistance/resistivity up to 700 °C.We measure the electrical resistivity and the Absolute Thermoelectric Power (A.T.P. also called absolute thermopower or absolute Seebeck coefficient) of the Ag-Ni pseudo-alloy between room temperature and 700–800 °C. We determine the electronic thermal conductivity, using the Wiedemann–Franz law in the same temperature range. The electronic transport measurements do not only characterize the resistivity, the thermopower and the thermal conductivity for themselves, they also characterize change of phases happening in the solid. Indeed electronic transport is a probe of the state of matter and of its changes. For pseudo alloys, electronic transport properties allow also to determine ageing effects as function of temperature and of time through a drift of these properties.The variation of resistivity and thermopower are represented and discussed. Our results show that the thermal conductivity of the Ag-Ni “pseudo-alloy” is only 10% below that of the best thermal conductor (pure silver). We also observe, after two temperature cycles up to 700 °C that the thermal conductivity increases of about 1%. We compare the SEM micrographs of the samples before and after heating. Finally we showed that the behavior of a pseudo-alloy is different from that of an alloy and we interprete this behavior semi quantitatively.