MELTING AND VAPORIZATION OF THE 1223-PHASE IN THE SYSTEM (TL-PB-BA-SR-CA-CU-O)

The melting and vaporization of the 1223 [(Tl,Pb):(Ba,Sr):Ca:Cu] oxide phase in the system (Tl-Pb-Ba-Sr-Ca-Cu-O) have been investigated usin g a combination of dynamic methods (differential thermal analysis, the rmogravimetry, effusion) and post-quenching characterization technique s (powder x-ray diffraction, scanning electron microscopy, energy disp ersive x-ray spectrometry). Vaporization rates, thermal events, and me lt compositions were followed as a function of thallia loss from a 122 3 stoichiometry. Melting and vaporization equilibria of the 1223 phase are complex, with as many as seven phases participating simultaneousl y. At a total pressure of 0.1 MPa the 1223 phase was found to melt com pletely at (980 +/- 5) degrees C in oxygen, at a thallia partial press ure (p(Tl2o)) of (4.6 +/- 0.5) kPa, where the quoted uncertainties are standard uncertainties, i.e., 1 estimated standard deviation. The mel ting reaction involves five other solids and a liquid, nominally as fo llows: 1223 --> 1212 + (Ca,Sr)(2)CuO3 + (Sr,Ca)CuO2 + BaPbO3 +(Ca,Sr) O + Liquid Stoichiometries of the participating phases have been deter mined from microchemical analysis, and substantial elemental substitut ion on the 1212 and 1223 crystallographic sites is indicated. The 1223 phase occurs in equilibrium with liquids from its melting point down to at least 935 degrees C. The composition of the lowest melting liqui d detected for the bulk compositions of this study has been measured u sing microchemical analysis. Applications to the processing of superco nducting wires and tapes are discussed.