JOURNAL OF MATERIALS SCIENCE 30 (1995) 3995-4002 Processing of bulk YBa2Cua07_ ceramics prior to peritectic solidification WAI LO, D. A. CARDWELL, S.-L. DUNG, R. G. BARTER IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK The influence of the physical and chemical properties of precursor green bodies on the properties of fully melt-processed YBa2Cu3074 ceramic (YBCO) have been investigated. Cold isostatic pressing has been found to allow better control of the size and distribution of Y2BaCuOs phase inclusions in the melt=processed ceramic than a combination of die pressing and sintering. A study of the loss of liquid from YBCO during partial melting has revealed that the total percentage weight loss is sensitive to both the heating rate and proportion of excess 211 phase and is maximum at a temperature corresponding to the peak of the differential thermal analysis partial melting endotherm. Densification and expansion processes in the sample, which compete during melting, have been investigated in detail. The temperature where the expansion rate is maximum has been found to coincide with the maximum rate of oxygen desorption by the sample. The expansion process terminates on completion of partial melting whereas the densification process, which is dominated by the volume proportion of liquid, continues but at a reduced rate. The use of dense green bodies and an optimum heat-treatment process has been found to be essential for the fabrication of large-grain melt-processed YBCO if a fine distribution of 211 phase inclusions and the homogeneity and shape of the sample are to be retained. 1. Introduction A variety of melt-processing techniques have been used to fabricate large-grain YBa2Cu3OT_~ ceramics (YBCO) which exhibit critical current densities of up to 50000A cm -2 at 77 K and 1 T [1-4]. This repres- ents a significant improvement over the critical cur- rent densities of sintered YBCO, which are around 1 000 A cm-2 [5], and highlights the potential of melt- processed materials for a number of high-field engin- eering applications such as magnetic bearings, fault current limiters and current leads [6-8]. Significantly, the best melt-processed YBa2Cu3OT-~ (the so-called 123 phase) typically contains a fine, unconnected dispersion of Y2BaCuO5 phase material (the so-called 211 phase) embedded in the 123 matrix [1,2,9-11] which is not present in sintered material. The presence of this second phase in melt-processed samples is considered to enhance their ability to pin magnetic flux and hence account for the observed increase in critical current density [12]. Control of the formation of the 211 phase during melt processing is considered essential, therefore, if material with the required electrical and magnetic properties is to be fabricated. All melt-processing techniques used to fabricate large-grain YBCO ceramics exploit a peritectic reac- tion at 1015 ~ from which the 123 phase is formed from the 211 phase and a 3BaCuO2 + 2CuO-based Iiquid phase (L), i.e. Y2BaCuO5 +3BaCuOa +2CuO --* 2YBa2Cu306.5 (211) (L) (123) (1) 0022-2461 9 1995 Chapman & Hall The 211 phase and the liquid in this reaction can be produced by rapidly heating a presintered green body of the desired composition to a temperature well above the peritectic temperature, Tp. Formation of the required 123 phase is then achieved by cooling the partially molten YBCO material slowly through T;. It is desirable to add up to 30 mol % of the 211 phase to the 123 phase prior to melt processing, both to gener- ate more flux pinning sites and to prevent loss of liquid during melting. Semisolid solidification processes [13], such as that described by Equation 1, place certain requirements on the precursor YBCO green body if melt processing is to be performed effectively, Firstly the 211 phase particles in Y2BaCuOs-enriched YBazCuuOT_~ should remain fine in the green body state if they are to form a fine dispersion in the fully melt-processed material. Secondly, the material must be able to retain the liquid phase resulting from the peritectic reaction within its bulk at temperatures sig- nificantly above Tp to enable formation of the 123 phase on cooling. This depends on the homogeneity and density of the precursor green body and on the size distribution of the 211 phase particles it contains. Finally, the green body should be free from composi- tional and surface contaminants which form hetero- geneous grain nucleation sites and hence limit the grain size that can be achieved during the growth process. The purpose of this study was to investigate the influence of the physical and chemical properties of precursor green bodies, prepared using a variety of 3995