DTA/TGA Study of Eutectic Melting in the System BaF₂-BaO-Y₂O₃-CuO_x−H₂O

Abstract

Differential thermal analysis and thermogravimetric analysis (DTA/TGA) experiments have been completed in the BaF₂-BaO-Y₂O₃-CuO_x-H₂O system at various oxygen pressures to determine the effect of adding solid BaF₂and gaseous H₂O (at p_H2O = 2.5 kPa) on the eutectic melting in the system BaO-Y₂O₃-CuO_x. We have investigated the eutectic melting temperature over the range of p_O2 = 20 Pa to p_O2 = 0.1 MPa under the following conditions: 1) without BaF₂ or H₂O; 2) with H₂O only; 3) with BaF₂ only; 4) with both BaF₂ and H₂O. Results indicate that without BaF₂ or H₂O, eutectic melting is depressed from 917°C at p_O2 = 0.1 MPa to 838°C at p_O2 = 1 kPa. At oxygen pressures below 1 kPa, down to p_O2 = 20 Pa, no further lowering of eutectic melting temperature was observed. The topologies of the temperature vs. log p_O2 curves for various combinations of BaF₂ and H₂O were similar to that for the experiments without BaF₂ or H₂O, but all curves were shifted to lower temperatures. For the experiments with addition of BaF₂ only, H₂O only, and BaF₂ with H₂O, the eutectic melting temperatures as a function of p_O2 were lowered by, respectively, 5°C - 15°C, 15°C - 25°C and 20°C - 30°C. Application of results to the “BaF₂ex-situ” method of processing Ba₂YCu₃O_x coated conductors is discussed.