ELSEVIER Physica C 341-348 (2000) 2475-2476 www.elsevier.nl/Iocate/physc Melting and Solidification of Y1-xYbxBa2Cu3Oy" Influences of Yb doping and Oxygen Partial Pressure Yuxiang Zhou and Tong B. Tang Physics Department, H.K Baptist University, Waterloo Road, Kowloon, Hong Kong A comparative study was undertaken into the calcination conditions for the solid-state synthesis of pure Yl.xYbxBa2Cu307_ ~ (x=0, 0.25, 0.5, 0.75, 1.0), the starting materials being Y203, Yb203, BaCO3 and CuO. It was found that the calcination time should go up and the sintering temperature should lower, when the average rare earth ionic radius decreases. Moreover, the melting and the solidification temperatures of Yt.xYbxBa2Cu3Oy rise with decreasing Yb doping and increasing oxygen partial pressure, but depend only weakly on the particle size distribution. These deductions will be important to the fabrication of large-grain (Y,Yb)BaCuO 1. Introduction Recently, superconducting (RE)BCO materials have been fabricated in large grain form via peritectic solidification. Grains thus obtained can be several cm in diameter with Jc a few times 10SAcra 2 at 77K [1-4]. At the peritectic temperature Tp of 900 to 1080°C, RE-123 is formed from solid RE-211,a Ba-Cu-O based liquid phase and oxygen gas [3], (RE)2BaCuO5+Ba3CusOr.72+0.4202 - ) 2(RE)Ba2Cu3Or.2s Unfortunately there have been few reports on fabricating large grain (Yb, Y)BCO [5-7]. Hence this work, which aimed to clarify also the effects of particle size distribution and oxygen partial pressure on the underlying thermal events. 2. Experimental Solid-state reaction synthesis was used to produce Yl_xYbxBa2Cu3Oy (x=0, 0.25, 0.5, 0.75, or 1.0). Y203(99.9%), Yb203(99.9%), BACO3(99.9%) and CuO (99.5%), all from Fluka, were thoroughly mixed in the stoichiometric ratio, calcined in air at 880°C for 24 hours, then subjected to two consecutive sinterings at 900 and 920°C, both for 24 h with intermediate grinding. The product was pressed into pellets, and sintered at various temperatures from 860 to 920°C for different x for the same period of 24h, followed by annealing at 500°C in flowing 02 for 20h, before cooling down to room temperature in the furnace. The (Y,Yb)BaCuO powder thus obtained was milled in ethyl alcohol with agate balls in an agate jar for different periods (10, 20, 30 and 40h), dried at 130°C, then ground again in a mortar with a pestle. Powder X-ray diffraction patterns of both as- prepared and milled powder at room temperature, were taken in a Rigaku D/MAX-C diffractometer, equipped with a filter but no monochrometer. Phase identification was achieved through searches in the Powder Diffraction File database from JCPDS- International Center for Diffraction Data. For estimation of their particle size, ball milled samples were examined in a Jeol JSM-T330A Scanning Electron Microscope operated at 20 kV. For simultanious thermogravimetry (TG) and quantitative differential thermal analysis (qDTA), a SETARAM TGA 92-16 combined thermal analyser was used. Annealed alumina powder served as the reference in calorimetry. Measurements proceeded in different atmospheres as specified in the Results section. The program was always heating from 500 to 1100°C, isothermal for 30 minutes, then cooling, at the same rate of 5 K/min. 3. Results and discussion X-ray diffraction patterns showed that, for the series of samples fabricated under the same conditions, phase purity deteriorated as x increased. In YbBa2Cu3Oy, lower sintering temperature and longer calcination time reduced but could not eliminated the impurity phases if the sintering 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01196-5