Influences of oxygen partial pressure on YBCO grain growth by a zone melting method
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Influences of oxygen partial pressure on YBCO grain growth by a zone melting method X. Yao, M. Sumida, and Y. Shiohara Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome Koto-ku, Tokyo 135, Japan (Received 5 October 1995; accepted 22 April 1996)
This study investigates the influences of Y–Ba –Cu–O compounds solidification under different oxygen partial pressures by a zone melting method. In the Y–Ba –Cu–O system, P(O2 ) ranged from 0.02 to 1 atm, and the following reaction occurred during heating: YBa2 Cu3 O61d (Y123) ! Y2 BaCuO5 (Y211) + L. With an increase of oxygen partial pressure or decrease of pulling rate, the morphology of the solidified interface changed from mushy to equiaxed, cellular, and planar. The continuous Y123 grains were readily obtained under high oxygen partial pressure. Based on the constitutional supercooling theory and combining the result of the yttrium solubility limit for different oxygen partial pressures, the influences of oxygen partial pressure on Y123 morphological evolution were clarified.
I. INTRODUCTION
Oxygen partial pressure as a control parameter has a significant effect in the Ln –Ba–Cu –O (Ln: Y and lanthanide) system. For physical properties, oxygen partial pressure has been reported to be able to control the substitution of Ln for Ba in crystal, and further control the critical temperature Tc and critical current Jc . By applying oxygen partial pressure of 0.01 atm, the OCMG process1,2 has been developed to achieve a high Jc Nd–Ba –Cu–O superconductor under a high magnetic field. For fabrication of single crystals, low oxygen pressure of 0.02 atm has been used in fabricating YBCO and SmBCO single crystals under a lower growth temperature than those grown in air atmosphere.3,4 In addition, a directional Y210 phase has been reported to form under oxygen partial pressure below 0.01 atm by using the laser-heated floating zone (LHFZ) technique.5 In brief, the role of oxygen partial pressure is very important to the Ln–B–Cu –O system. By changing oxygen partial pressure, it may affect liquid properties, melting and solidification conditions, growth morphologies, phase formation, and further physical properties. Therefore, it may be an efficient way to optimize Ln–Ba –Cu–O compound morphologies and superconductor properties by controlling oxygen partial pressure. The aim of this work is to study the correlation between oxygen partial pressure and growth morphologies of Y–Ba –Cu– O compounds, especially for the Y123 solidified interface. For this, oxygen partial pressures were selected from 1 atm down to 0.02 atm since lower oxygen partial pressure (less than 0.01 atm) may lead to a Y210 phase.5 In theoretical investigations, we tried to apply the constitutional supercooling theory6,7 because it is believed to be a key to explain growth morpholoJ. Mater. Res., Vol. 11, No. 11, Nov 1996
gies by using some thermol-physical properties data. Fortunately, a recent publication8 has provided yttrium solubility at a temperature
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