Microanalysis of Cuprate Superconductors Using the Color of Polarization
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MICROANALYSIS OF CUPRATE SUPERCONDUCTORS USING THE COLOR OF POLARIZATION H.A. HOFF*, W.L. LECHTER**, M.S. OSOFSKY*, AND G.A. CANDELA* *Naval Research Laboratory,Washington, DC 20375-5000 "**Sachs/Freeman Associates, Inc.,Landover, MD 20875 ABSTRACT A technique for the identification of individual anisotropic grains in a heterogeneous and opaque material involves the observation of grain color in a reflecting light optical microscope. Such colors are generally characteristic of particular phases. We have studied the presence of color in many superconducting cuprates and found a strong correlation between color and the existence of superconductivity. The change in color from insulator to superconductor facilitates the selection of grains for microanalysis. Color variations in a Tl-Sr-Ca-Cu-O material are investigated using quantitative energy dispersive x-ray spectroscopy. Cation concentrations are found not to correlate with the changes in color. This apparent lack of correlation of color with composition is discussed. INTRODUCTION The discovery of superconductivity with high onset temperatures (Tc) in cuprates necessitates producing homogeneous quantities of these materials. The processing of 'single' crystal, polycrystalline bulk, or thin film, can be assisted by the microscopic examination of texture and the ready differentiation at room temperature of the superconducting phase from insulating phases on a grain-by-grain basis. Distinctive colors may be seen while viewing with crossed polarizers (polarization colors) [1]. The cuprate superconductors (both hole carrier and electron) have their own characteristic polarization color independent of phase and dependent only on being superconductors [2,3]. As the insulating precursor is often colorless, there is a characteristic change of polarization color as the material is doped from insulator to superconductor [2]. This change is correlated with doping and consistent with the presence and increase in free carriers [3]. The changes in color and the presence of identifying colors for secondary phases allows identification for microanalysis. We used the presence of the characteristic golden color to isolate superconducting grains for subsequent microprobe analysis of the first Tl-Sr-Ca-Cu-O system superconductor [4]. Changes in color have been investigated with quantitative energy dispersive x-ray spectroscopy (EDXS) in an effort to learn more about the Tl-Sr-Ca-Cu-O cuprates. EXPERIMENTAL To produce a sample of the same nominal composition Tl.7Sr3Ca0.7Cu4.30x [4], finely ground powders of SrC0 3 , CaCO3 and CuO in stoichiometric proportions are mixed and calcined at 915 0 C for 3.5 hrs. The resulting compound is reground and mixed with the appropriate amount of T1 2 0 3 , pressed into a rod and encapsulated in Pyrex glass (evacuated and backfilled with Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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argon). For the hot isostatic press (HIP), the capsule is heated to 750 0 C and held for -1 hr, to soften the glass, ramped up to 900*C and pr
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