Tracer diffusion of Ba and Y in YBa 2 Cu 3 O x
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K. C. Goretta Materials and Components Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 4 March 1992; accepted 18 May 1992)
Tracer self-diffusion of Ba and Y and the diffusion of Dy, Ho, and Gd, which substitute for Y, have been measured in polycrystalline YBa 2 Cu 3 0 x over temperature and oxygen partial pressure ranges of 850 to 980 °C and 103 to 105 Pa, respectively. The diffusion of Ba is slower than that of oxygen or copper, with a high activation energy of about 890 ± 80 kJ/mole. Large anisotropy has also been observed, with diffusion along the c-axis being more than three orders of magnitude slower than diffusion in randomly oriented polycrystals. Diffusion coefficients of Ba were, within experimental uncertainty, independent of oxygen partial pressure over the range measured. The diffusion coefficients of the Y-site species were nearly identical and an activation energy of about 1.0 MJ/mole was estimated, in agreement with that for high-temperature deformation. Attempts to speed up the kinetics through creation of point defects on the Y site by doping proved to be unsuccessful. These results are compared to cation diffusion in cubic perovskites and simple oxides.
I. INTRODUCTION Diffusion studies of A-site cations have been carried out as a part of an investigation of anion and cation tracer diffusion in the Y - B a - C u - 0 (YBCO) system. YBCO is highly nonstoichiometric, with a distorted, layered perovskite structure. Study of diffusional motion may yield information on atomic defects and their relations to structure. For example, it has been found that oxygen ordering in YBCO plays a very important role in superconductivity. Therefore, the transport properties associated with the motion of oxygen have been a major focus in the literature.1 9 An activation energy of about 100 kJ/mole for oxygen diffusion has been generally obtained. However, different oxygen partial pressure (Po 2 ) dependences have been observed for chemical diffusion of oxygen. In an extensive study of tracer diffusion, Rothman et al.1 found that PQ2 has little influence on oxygen diffusion rates. Surprisingly few measurements of cation diffusion10"15 have been carried out, however, notwithstanding its importance to scientific and technical interests. The kinetics of high-temperature processes, such as sintering, sinter-forging, creep, and grain growth, are governed by the motion of cations, which have much slower diffusion rates than does oxygen.14'15 Control of these processes is crucial for achieving satisfactory properties in bulk materials. A complete study, including all of the cations and oxygen as functions of composition, temperature, oxygen pressure, and crystal orientation, is required for the better understanding of diffusion mechanisms, their relations to high-temperature pro2308
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cesses, and to the associated defect structures in this material. In a previously published paper,13 we reported tracer diffusion results of Cu and the elements that substitute
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