Synthesis of Hg-Re-Ba-Ca-Cu-O superconductors by a two-step method
A new two-step method has been developed to synthesize (Hg,Re)Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ with lower annealing temperatures as compared to conventional methods using a precursor which contains the (Hg,Re)Ba/sub 2/CaCu/sub 2/O/sub y/ phase. In the first step, high phase purity (Hg,Re)Ba/sub 2/CaCu/sub 2/O/sub y/ superconductors are prepared from commercially available Ba/sub 2/CaCu/sub 2/O/sub y/ precursors, HgO, and Re/sub 2/O/sub 7/. In the second step, the finely ground (Hg,Re)Ba/sub 2/CaCu/sub 2/O/sub y/ is mixed with Ca-Cu-O and HgO. This precursor mix is encapsulated in evacuated quartz tubes and annealed for 8 to 48 h between 750/spl deg/C and 850/spl deg/C. The resulting samples have (Hg,Re)Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ as a majority phase and exhibit a critical temperature of 133 K as prepared. The superconducting grains are platelike and randomly oriented. The grain sizes depend on the annealing time and temperature and reaches up to 30 /spl mu/m diameter and 5 /spl mu/m thickness. Second phases are mainly unreacted Ba/sub 2/CaCu/sub 2/O/sub y/ and Ca-Hg-O.