Selection of appropriate systems for flux growth of single-crystal YBa2Cu3O7 –y

Abstract

In order to grow large single crystals of high quality, a physico-chemical investigation has been carried out on the growth system of YBa₂Cu₃O_{7 –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 (Y_0.99Pr_0.01)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.