Abstract
In order to grow large single crystals of high quality, a physico-chemical investigation has been carried out on the growth system of YBa2Cu3O7 –y(YBCO). The phase diagram of the flux system BaO–CuO was established by improved thermogravimetric analysis (ITG), which confirmed the eutectic composition of 0.28 BaO + 0.72 CuO. The phase diagram of (Y0.99Pr0.01)BCO–(0.28 BaO + 0.72 CuO) by differential thermal analysis (DTA) showed a narrow liquidus range and is applied here to the crystal growth of YBCO. The effects of various additives on the crystal growth and superconductivity have been examined; the best additive was found to be a small amount of BaF2, which improved the size and the quality of the superconducting crystals. The functions of the fluoride ion are discussed on the basis of the mixed-cluster model. The preferential adsorption of F– on the crystal surface encourages more effective crystal growth by a screw dislocation mechanism. As a result, single crystals up to 20 × 10 × 0.5 or 12 × 10 × 1 mm3 in size have been obtained from the flux system and a superconducting transition temperature of 92.5 K with transition width < 1 K has been reached in the as-grown state.
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