Preparation and Properties of Hot-Pressed YBa2Cu3O7-x

Autor: Frank P. Gerstle, Eugene L. Venturini, Ronald E. Loehman, Roger Howard Moore, W.F. Hammetter
Rok vydání: 1989
Předmět:
Zdroj: Journal of the American Ceramic Society. 72:669-674
ISSN: 1551-2916
0002-7820
DOI: 10.1111/j.1151-2916.1989.tb06193.x
Popis: We have hot-pressed dense, bulk specimens of the YBa2Cu3O7-x composition to near-theoretical density. Powders made by the mixed-oxide method were pressed at 6.9 MPa (1000 psi) in Al2O3-lined SiC dies at 950°C in air or oxygen for 30 min. Relative densities of 94% to 97% of theoretical were obtained for speciments as large as 200 g. In the aspressed condition the ceramic has an oxygen stoichiometry near YBa2Cu3O6.28, as determined by thermogravimetric reduction in hydrogen and by wet chemical analysis. Scanning electron microscoy shows a microstructure of elongated grains that are partially oriented by the hotpressing. Oxidation of bulk specimens of dense, as-hot-pressed material produced intergranular porosity, presumably from anisotropic contraction as individual grains are converted from the tetragonal to the orthorhombic structure. We studied the reoxidation kinetics of hot-pressed specimens to the orthorhombic, superconducting state by thermogravimetry in flowing oxygen. Isothermal weight gains can be fitted to an Avrami model with an activation energy of 96 kJ / mol. Superconducting quantum interference detector (SQUID) measurements of magnetic susceptibility reveal a small anisotropy that is related to the preferred orientation seen in the microstructure. The average thermal expansion coefficient (CTE) of the ashot-pressed, tetragonal material is 135 × 10−7°C−1 from room temperature to 800°C, which increases to 155 × 10−7°C−1 for reoxidized specimens. For reoxidized material the CTE is not linear with temperature, but shows a sharp break at approximately 450°C. Measurements of Young's modulus show there is a 36% decrease for reoxidized specimens as compared with ceramics in the as-hot-pressed condition. The decrease in modulus appears to be related to the development of microstructural porosity and cracking as a result of the reoxidation.
Databáze: OpenAIRE
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