The isostatic pressure processing of high Tc superconductors and their properties

Abstract: The high Tc superconductors are highly anisotropic, ceramic materials which makes it difficult to sinter them at atmospheric pressure to full density. High porosity is detrimental to their mechanical and electrical properties, so the use of isostatic pressures was used to give higher density materials with improved properties. Two general types of isostatic pressing investigated were (1) hot isostatic pressing or HIPing and (2) cold isostatic pressing or CIPing. The HIPing of the YBCO superconductor 123 in a glass encapsulation was shown to retain the oxygen leading to a material which is superconducting without an additional heat treatment, but the 123 phase is unstable in a high oxygen partial pressure, so a partial decomposition occurred at the temperatures required to give a low porosity. A low or high oxygen pressure can be obtained in the HIP depending on the type of encapsulation material used. Maintaining a high oxygen pressure by HIPing in glass makes it possible to quickly form dense samples of the 124 phase from 123 plus CuO by a reaction HIP sintering technique developed. Advantages of this method to form the 124 phase include the high density and short reaction time relative to low pressure synthesis of 124. Disadvantages of this method include the risk of cracking due to thermal expansion mismatch with the glass encapsulation, the presence of unreacted CuO and the lack of grain alignment to improve Jc. The BSCCO 2223 superconductor can be HIPed to nearly full density with good current transport properties without additional heat treatments. The thermal expansion of the 124 phase was measured by low temperature X-ray diffraction, and the 2223 phase by high temperature X-ray diffraction. The c-axis expansion was higher than for the a-axis or b-axis for both materials. A buffer layer of silver plus MgO powder could prevent thermal expansion cracking for HIPing of the 2223 phase superconductor. The mechanical properties are of little interest if the electromagnetic properties are poor, so the final portion of the research is directed toward critical transport current (Ic) in the BSCCO 2223 phase superconductor. The Ic of bars of the 2223 phase superconductor could be raised to the limits of the magnetic self-field (Hs) of the transport current by CIPing without resorting to the more complex HIP process. The Hs limit for the 2223 phase was observed to be about 7000 A/m for our bars with a low oxygen partial pressure final heat treatment. This critical value of Hs appears to be a fundamental limit to the current transport in bulk 2223 materials using the current solid state sintering technique, since it is of the same order of magnitude as for the best bulk 2223 specimens reported in the literature. Determination of the selffield limit provides a very useful method to compare the quality of specimens with different geometries. Finally, it was discovered that an AC transport current can be used to examine flux vortex motion from the DC voltage drop near the normal DC Ic transition in bars of the 2223 superconductor. Two methods of inducing a DC voltage drop for an AC transport current were observed (1) when a DC offset voltage is present in the nominally AC current or (2) when an asymmetric magnetic field is applied to the specimen. The sign of the longitudinal electric field parallel to the current transport can be changed by reversing the direction, or by reversing the asymmetry of the applied magnetic field. The AC Ic transition phenomena provides evidence for the penetration of flux vortices when Hs equals Hc1 for the grains at the surface of the specimen.