Synthesis and properties of polycrystalline (Tl2)(Ba2)(CaCu2)O(x) and (Tl2)Ba2Ca2Cu3O(x).

摘要

Polycrystalline samples of Tl(sub 2)Ba(sub 2)CaCu(sub 2)O(sub x) and Tl(sub 2)Ba(sub 2)Ca(sub 2)Cu(sub 3)O(sub x) superconductors with high (>95 wt %) phase purity were prepared by a solid state reaction of Tl(sub 2)O(sub 3) with a mixture of BaCuO(sub 2), Ca(sub 2)CuO(sub 3), and CuO. Starting compositions of Tl(sub 1+(delta))BaCaCu(sub 2)O(sub x) and Tl(sub 2+(delta))Ba(sub 2)Ca(sub 2)Cu(sub 3)O(sub x) were fired in semi-sealed reaction vessels for durations of 12 to 48 hours in flowing oxygen with low, reproducible thallium losses. Samples fired at temperatures below 870(degrees)C produced predominately the 2212 phase, while samples above 870(degrees)C yielded the 2223 phase. While pressed mixed oxides reacted in a single firing step produced low-density ((le)50% theoretical) material, samples prepared by a two-step sequence of reactive calcination, intermediate grinding, and sintering yielded higher density material (>70% theoretical). The sintered polycrystalline superconductors were characterized by x-ray diffraction, reflected light and electron microscopy, magnetic susceptibility, and dc transport resistivity. Critical current density measurements by direct transport and magnetic moment hysteresis methods are presented for the denser samples. Transport J(sub c) values up to 300 A/cm(sup 2) at 75 K were observed for moderately dense material (5.2 g/cm(sup 3), 74% theoretical) in transverse H fields up to 10 kOe. 10 refs., 16 figs.