High Density Bulk Shape Rapid Consolidation of the Yba₂Cu₃0_7-X Superconductor

Abstract

A novel rapid densification technique for fabrication of bulk shape YBa 2 Cu 3 07_xSuperconductor is presented. The Ceracon process is a one-step, quasi-isostatic consolidation route utilizing conventional P/M equipment and set-up.The Ceracon technology has enabled successful fabrication of bulk shapes such as discs, cylinders, hollow cylinders and spheres along with significant increases in the density up to 95-98% of the theorertical. The superconducting volume fraction is preserved due to short hold times at the operating temperatures and avoidance of high processing temperatures.Results based on densities, microstructure, susceptibility measurements, X-ray diffraction patterns and TGA measurements are discussed.Bulk high temperature superconductors have been limited in their applications by non-optimal values of mechanical and superconducting properties. The poor properties exhibited by the ceramic superconductors can be attributed to unfavorably modified properties of the bulk material resulting from processing conditions used in conventional consolidation techniques. A novel processing route, namely the Ceracon Process, for the fabrication of high density bulk YBCO superconductor is discussed.The Ceracon process (shown in Fig.l) is a unique, one-step, quasi-isostatic, hot consolidation technique [1] akin to Hot Isostatic Pressing (HIPing) but utilizes a ceramic particulate material as a pressure transmitting medium (PTM), instead of the gas used in HIPing. Cold iso-pressed green preforms are preheated and subsequently immersed in hot ceramic PTM. Pressures up to 900 MPa are applied to the PTM for hold times up to 200 seconds. The uniaxial pressure applied to the grain bed is transmitted to the superconductor as quasi-isostatic pressure and consolidation of the part occurs in a time span of minutes. The Ceracon processed samples are maintained at 930*C for 24 h in flowing oxygen and slowly cooled to room temperature to allow for full recovery of oxygen content, critical to the formation of the superconducting phase. The control parameters for this process include the part temperature, the PTM temperature, the pressure, part preheat time and hold time at the high temperature and pressure.The Ceracon-consolidated and annealed YBCO specimens were characterized for physical density, microstructure (optical and SEM), magnetization measurements, thermogravimetric analysis and X-ray diffraction. Magnetic measurements were performed in a Quantum Design magnetometer with an applied field of 20 G. Fig. 2 (a) and (b) show a polarized light optical micrograph and a scanning electron micrograph in the secondary imaging mode of a Ceracon processed and annealed YBCO superconductor. A virtually full-dense microstructure is observed with no observable intergranular porosity.It should be noted that such a microstructure is typical throughout the bulk of the consolidated YBCO and is not restricted to a single area or zone and is consistently reproducible. An important point to note is that typical sample weights for disc shape...