Volume 1 I, number 1,2 MATERIALS LETTERS April 1991 Bi-Sr-Ca-Cu-0/Ca2Cu03 composites Judy M. Phillips, Shirley Krause, M.T. Lanagan, R.A. Olson, P.R. Sharpe, Michelle L. Ray, S.E. Dorris and KC. Goretta Materials and Components Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA Received 3 December 1990; in final form 21 December 1990 CaZCuOSand Sr,CuOj were examined for use as insulators with Bi-Sr-Ca-Cu-0 superconductors. Only CazCuO, could be successfully synthesized and sintered, the sintering required a slightly higher temperature than that required for the 85 K phase in the Bi-Sr-Ca-Cu-0 system. CazCuOa and the 85 K Bi-Sr-Ca-Cu-0 phase have nearly equal thermal expansion coefficients and can be coked into monolithic forms. Ca,CuOS is a highly conductive semiconductor at room temperature. CazCuOj is highly hygroscopic and exposure to moisture can reduce its conductivity by up to three orders of magnitude. 1. Introduction Fabrication of hid-temperature superconducting devices, such as solenoids, may require that the su- perconductors be coprocessed at high temperatures with insulating materials [ 1,2]. In addition, thin and thick superconductor films are grown on insulating substrates. Superconductors based on the Bi-Sr-Ca- 01-0 system are highly reactive, and no substrate has been shown to be completely inert to it [ 3-61. Even metals such as Ag and Au induce slight dele- terious changes in Bi-Sr-Ca-Cu-0 superconductors [7,8]. One approach to finding a compatible substrate fo? hip-temperature su~~onducto~ has been to use compounds within the superconductor system. For YBa2Cu30,, the YzBaCuO, phase, which is a poor semiconductor, has been used with some success [ 1,2,9- 111. Y,BaCuO, is inert to YBa.$&O, and is itself relatively stable [ 2,12,13 1. Examination of phase diagrams [ 14,15 ] suggests that, for supercon- ductors in the Bi-Sr-Ca-Cu-0 system, CazCuOs and Sr,CuO, may find similar use. Little information on these compounds has been published [ 16,171. The properties of CazCu03 are of special interest because it has been demonstrated that precipitates of CazCu03 in a Bi-Sr-Ca-Cu-0 matrix can pin flux lines and thus raise intragranular critical current densities substantially [ 18 1. The objectives of this investigation were to syn- thesize pure Ca,CuOs and Sr2Cu03 powders, deter- mine whether they could be sintered effectively, ex- amine the electrical properties of dense specimens, and fabricate composite structures with Bi-Sr-Ca- Cu-0 superconductors. CazCu03 and SrzCu03 are likely to be exposed to the atmosphere during pro- cessing; therefore, effects of moist air on phase sta- bility and electrical properties were also examined, 2. Experiments Ca,CuO, and Sr,Cu03 powders were synthesized by solid-state reaction of CuO and ~kaline-each carbonates. In order to minimize retention of CaO or SrO, which are highly hygroscopic, both powders contained 1% excess CuO. CuO is a stable com- pound that has exhibited little tendency to coat grain boundaries [ 19 1. Reagent-grade powders were mixed for 16 h in polyethylene jars containing methanol and ZrOz grinding media. The powders were dried, placed in shallow A1203 crucibles, and heated in air for 40 h at 900°C. One intermediate grinding in an agate mortar and pestle was performed on each powder. Final grinding in a tungsten carbide rotary mill yielded powders with a particle size of 2-5 pm. The resulting Ca,CuO, powder was phase pure by X-ray diffraction; the Sr2Cu03 contained traces of SrO. 10 0167-577x/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)