Evaluating the Chemical Compatibility of Potential Substrate Materials for Bi-Sr-Ca-Cu-O Films
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EVALUATING THE CHEMICAL COMPATIBILITY OF POTENTIAL SUBSTRATE MATERIALS FOR Bi-Sr-Ca-Cu-O FILMS K.E. Williams, M.M. Matthiesen, D.A. Rudman Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT Potential substrate and buffer layer materials for Bi 2Sr 2 CaCu 2 O, (BSCCO) were surveyed using bulk ceramic processing techniques. Of the materials tested, only Ag was inert with respect to BSCCO. MgO slightly degrades the superconducting onset temperature and volume fraction. LaGaO 3 may be a feasible ceramic substrate for BSCCO, although some reaction appears to occur.
INTRODUCTION Electronics applications of the recently discovered oxide superconductors will require high quality thin films. In order to grow such films, it is necessary to find chemically compatible materials for use as substrates or as buffer layers between the superconductor and chemically incompatible substrates. In studies of the YBa 2 Cu3 O., (YBCO) system, Cimna et al. developed a survey method using bulk superconductor material [1]. Powders of high temperature superconductor and candidate material are mixed, then annealed to drive any chemical reactions to completion. The resulting samples are tested for degradation of superconductivity. The present study adapts this method to search for substrate materials that will be chemically compatible with Bi 2 Sr2 CaCu 2 0 . (BSCCO). Our intent was to find truly inert substrates, although the method can be extended to determine reaction rates and products.
EXPERIMENTAL PROCEDURE Composite BSCCO-substrate samples [2] were fabricated to contain 50 vol % BSCCO with 50 vol % candidate substrate. These compositions were chosen to maximize the interfacial area between the reactants and, therefore, the extent to which reaction can occur at a givemn temperature. The powders were mixed dry, pressed in a 5/8 inch die, and sintered in air at 8500 C. As the goal of the study was to find inert substrates, high sintering temperatures and long times were used. For each candidate substrate material, we prepared a set of five samples, which were sintered for times ranging from 1 hour to 4 days. Each set of samples was accompanied through the process by a BSCCO pellet as a control. Additionally, a set of BSCCO samples was sintered separately. After sintering, each sample was reground to powder form and characterized. SQUID magnetometer measurements were used to determine superconducting onset temperature and magnetic susceptibility. Volume susceptibility was determined from the sample mass and the densities of the two materials (BSCCO and substrate) originally mixed. Comparison of these results with the control samples allowed us to determine whether degradation of either the superconducting onset temperature or volume fraction had occurred. X-ray diffraction was used to confirm the magnetometer results and to discern the nature of any reaction.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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Table I: Summary of materials tested
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