Reoxygenation of vacuum-annealed YBa 2 Cu 3 O 6.9
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NTRODUCTION Recently high-temperature superconductors in the YBaCuO family have been intensively investigated.1^* The practical application of these materials for devices and sensors on a variety of substrates will be enhanced by the development of low-temperature processing techniques. The major requirements for low-temperature processing are that high quality grain interconnects be formed and that the proper oxygen content be established in the grains and grain boundaries while retaining the appropriate bulk stoichiometry and phase. To date, high processing temperatures (700-950 °C) have been employed to attain proper sintering of the grains and to establish the interconnects; a minimum of 500 °C has been required for the subsequent oxygen anneals to optimize the superconducting properties. Considerable work has been done studying the oxygen uptake in sintered material and its relationship to the tetragonal and orthorhombic phases. It appears that the single phase 94 K superconducting phase of YBa2 Cu3 O6 9 (123 material) belongs specifically to an orthorhombic structure variant of the perovskites.5~7 An orthorhombic-to-tetragonal phase transition occurs between 500 and 700 °C, and the transition temperature is a strong function of the oxygen partial pressure.8"10 The role of oxygen in the superconductor is complex, and the ease of oxygen in- and outdiffusion makes assessing the precise importance of small changes in the oxygen concentration difficult. In fact, considerable debate exists in the literature about the precise value of the subscript for the oxygen in the fully superconducting 123 material. Typical values range from 6.8-7.0, with some variability even in the neutron results.] '~13 We will not address the exact metal-oxygen ratio here, rather we will assume a value of 6.9, which seems currently the best consensus value. The data may need to be scaled when an exact value is determined. Oxygen diffusion appears to be quite rapid and reversible in the porous 732
J. Mater. Res. 2 (6), Nov/Dec 1987
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sintered ceramic material14 16 with indiffusion times between 15 and 400 s from 1000 to 400 °C and is a strong function of the grain size.17 This may reflect a much slower rate in the grains than through the porous network. A large difference is also observed between indiffusion and outdiffusion in the material with 1.1 eV reported for outdiffusion and 1.7 eV reported for indiffusion.18 The indiffusion and outdiffusion of oxygen are reversible and do not appear to affect the nature of the intergrain contacts in the superconducting phase. Although vacuum-annealed material has been reoxygenated successfully, the details of this process below 500 °C have not been examined. In this article we discuss the activation with oxygen anneals of nonsuperconducting tetragonal YBa2 Cu3 O 6A prepared by vacuum annealing YBa2 Cu3 O6 9. We demonstrate complete restoration of superconducting orthorhombic phase YBa2 Cu3 O6 9 at temperatures as low as 300 °C. II. EXPERIMENTAL A
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