Messung thermophysikalischer Parameter unterkühlter Kupfer-Zirkonium-Schmelzen

Abstract

An electrostatic levitator for containerless processing has been constructed and used for the measurement of the specific heats of glass-forming, undercooled metal melts. The heating power is independent of the levitation force, so that the levitation of liquid specimens with low melting points is possible. To avoid gas discharging at this field strength, a vacuum of 10:sup:-4:/sup: Pa was necessary.:br:The levitator provides a positioning accuracy of better than 100 µm for specimen masses up to 1 g. A xenon lamp with a radiation power of 300 W allowed temperatures of up to 1650 K. For the measurements of specific heats, techniques involving the measurement of cooling rates and solidification holding times were devised, which worked with the heating switched off and could therefore be used for high undercoolings.:br:For a systematic test of the application to the measurement of the specific heats of undercooled melts of glass-forming alloys, the Cu-Zr system, which has a wide glass-forming composition range, was selected. Seven congruently solidifying alloys in this system were investigated. This selection provided for testing in a wide composition range (38-72 at. % Zr) and made it possible to use the holding time method, which is not applicable for non-congruent solidification.:br:Maximum undercoolings of 80 to 260 K, depending on the alloy composition, were attained. No correlation was observed between the maximum undercooling and the specimen size, indicating that heterogeneous nucleation predominated. The specific heat of the melt as the melting temperature was found to be in the range 36 to 47 J mol:sup:-1:/sup:K:sup:-1:/sup: and increased with falling temperature in all alloys, as has been observed in other glass-forming systems. On the basis of the temperature dependence of the specific heat, the alloys investigated could be classified in three groups, which differed significantly in melting temperature.