Studies of weight changes and oxygen diffusion in oxide superconductors
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. INTRODUCTION
II. EXPERIMENTAL SECTION
The role of thermal treatment in the fabrication of oxide superconductors is critical.1"3 In the widely studied YBa2Cu3OA: (YBCO) superconductor, the calcination step usually requires 850-900 °C treatment and a (900-930 °C) sintering step. Above all, the samples must go through an annealing step, ca. 400-500 °C, in an oxygen-containing environment to allow the oxygen to diffuse into the sample. Studies of the thermal behavior and oxygen diffusion are therefore of great interest. A number of oxygen diffusion studies have previously been reported.4"17 The reported isotropic diffusion constants and Arrhenius activation energies are summarized in Table I. There is wide divergence of values reported. More recent reports10"1216 on diffusion constants tended to hover near 10"11 to 10"13 cm2/s at 500 °C. For activation energy, the medium value appears to be 20 kcal/mole. Tu et al.9 and Ottaviani et al.17 used electrical resistivity measurements and actually found two separate processes occurring at 200-370 °C in oxygen with distinctly different activation energies. Rothman et al}2 have reviewed the available data on oxygen diffusion. Previously, several studies7141617'20 have been made of YBCO, using thermogravimetric analysis (TGA). Most of these studies14'16'17 focused on weight gain as a function of oxygen uptake to study the oxygen diffusion phenomenon. Weight loss measurement was de-emphasized because of a perceived lack of experimental accuracy. In this work, we have carried out careful weight loss measurements and found good diffusion rates and activation energies. The information we obtained turned out to be complementary to the previous studies on the subject.
A. Materials
a)
Presented at the Materials Research Society Meeting, Boston, Massachusetts, in November, 1988. b) Address correspondence to this author. J. Mater. Res., Vol. 6, No. 2, Feb 1991 http://journals.cambridge.org
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The YBCO used in this study was purchased from W. R. Grace, Davison Division. It was found to be more than 95% YBazCuaO, (1-2-3) phase by x-ray diffraction. The grain size was found to range from 2 ju,m to 20 /im in diameter. The median was 9.2 /im and the mean was 11.0 [iva. by optical microscopy (for 400 particles counted). Before use, the YBCO powders were charged with oxygen at 500 °C for 12 h. B. TGA
Thermogravimetric analysis was performed with DuPont 951 TGA module interfaced to a DuPont 9900 Thermal Analyzer Controller. For each sample approximately 27-35 mg were loaded into the platinum boat at room temperature and either held isothermally (at 400, 500, 600, 650, 700, or 750 °C) or heated dynamically (5 °C/min). The weight loss profile of each sample was recorded with respect to time. The time required for the TGA module to equilibrate at the isothermal test temperature was about 3 min. Dried nitrogen, oxygen, or air was purged through the TGA chamber at the rate of 100 ml/min during each study. C. X-ray diffraction
X-ray studies were performed with a Philips A
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