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EXPERIMENTAL INVESTIGATION OF THERMOPHYSICAL PROPERTIES OF NIOBIUM AT HIGH TEMPERATURES

Using the experimental setup for investigation of the thermophysical properties of refractory materials under high pressure and temperature a few experiments with the different metals were carried out. The experimental method is based on fast uniform heating of the small sample by current pulse. The novelty of our technique consists in the temperature feedback heating that provides currant interruption at the desired temperature measured by pyrometer with delay less than 3 μs. The aim of the work is to improve the accuracy of temperature measurements near the phase transition points, where the optical properties of materials change dramatically. The technique allows reaching the heating rate of 10 to 10 K s−1 under high static gas pressure up to several kbars. An idea of the method consists in the rapid heating of the specimen up to the melting point in a time of 25 μs to 1 ms. Thus, through measuring the surface temperature of the sample T (t), electrical current I(t) and voltage drop e(t) across the central part of the specimen during the experiment, one can determine temperature dependence of enthalpy HP (T ), specific heat capacity CP (T ) and other properties of the material. One of advantages of our setup is a possibility of precise temperature measurement by special pyrometer [1]. It carries out the measurements in a narrow spectral band around 25 nm at the wavelengths λ1 = 0.650 μm and λ2 = 0.900 μm. The pyrometer has mirror field stop, which forms at the specimen surface a small area of view with the diameter of 0.3 mm. It allows using foil samples with various emitting cavities to provide thermal radiation of the sample being close to the greybody spectrum. A high temperature investigation of such parts of the heat capacity as lattice anharmonism, electronic part and influence of equilibrium vacancies is of profound importance [2]. These factors come out mostly at the temperature region of Tm − 100 K in refractory metals, in particular in niobium, that is why this material has been chosen for our investigations.