New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al and HTSC oxides

A new differential thermal analysis (DTA) device was designed and installed in a hot isostatic pressure (HIP) furnace in order to perform high-pressure thermodynamic investigations up to 2 k-bar and 1200 C. Thermal analysis can be carried out in inert or oxidizing atmosphere up to p(O2) 400 bar. The calibration of the DTA apparatus under pressure was successfully performed using the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard calibration references. The thermal properties of these metals have been studied under pressure. The values of δV (volume variation between liquid and solid at Tm), ρsm (density of the solid at Tm) and αm (linear thermal expansion coefficient at Tm) have been extracted. A very good agreement was found with the existing literature and new data were added. This HIP-DTA apparatus is very useful for studying the thermodynamics of those systems where one or more volatile elements are present, such as high TC superconducting oxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 k-bar under reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading to the formation of the 2223 phase was found to occur at higher temperatures when applying pressure: the reaction DTA peak shifted by 49 C at 2 k-bar compared to the reaction at 1 bar. This temperature shift is due to the higher stability of the Pb-rich precursor phases under pressure, as the high isostatic pressure prevents Pb from evaporating