Materials Letters 15 ( 1992) 13- 18 North-Holland The effects of excess CuO on the grain growth kinetics, sintering and microstructure of the YBa2Cu307 _6 superconductor M.W. Shin, A.I. Kingon, T.M. Hare and C.C. Koch Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Received 29 June 1992 The influence of CuO on the grain growth kinetics, densification and microstructure of YBazCu,O,_s was studied. The YBa2Cuj0,_6 material with sufficient liquid phase revealed a grain growth exponent of about 0.25 after isothermal annealing at 990°C. The addition of CuO greatly enhanced the densification during sintering through the presence of the liquid phase. A microstructure containing highly aligned grains (d> 200 pm) was produced by a compositional gradient of CuO powder. 1. Introduction An inherent phase-equilibria characteristic of YBazCu30,_a superconductor is that it is a “line compound” with a very limited range of cation solid solubility. When a bulk compact is produced by rou- tine ceramic powder processing, various second phases are easily induced by the local deviation of the mixed powder from the ideal ratio of cations. Grain growth of the YBazCu30,_a (YBCO) at typ- ical processing temperatures (above 890°C) might be regarded as a liquid-phase-assisted process wherein the operating mass transport mechanisms are quite different from those of the classical solid-state sin- tering process. In view of this consideration, it is worth investigating the grain growth phenomenon for this material as a function of excess second phase, i.e. CuO, concentration. in the low-temperature region where a lower expo- nent (0.2) was observed. Recently, Shin et al. [ 51 reported that the grain growth exponent of YBazCu30,_d was about 0.21 in the temperature range from 950 to 990°C. Thermal analysis (DTA) indicated that there existed some liquid phase dur- ing the grain growth, but the liquid phase was not found to be fully spread along the grain boundaries from the microstructural observation. It is well known that a highly aligned microstruc- ture is required to overcome the weak-link problem in bulk YBa2Cu30,_+ It has been found that the tex- tured microstructure can be obtained by partially or completely melting this material [ 6,7]. Tiefel et al. zyxwvuts 181 reported grain growth enhancement in In many experiments of liquid-phase sintering, under diffusion controlled conditions, the grain growth exponent was found to be about 0.33 [ l-31 consistent with the theoretical derivation of grain growth kinetics for liquid-phase processing [2]. In a previous study, Chu and Dunn [4] studied the grain growth of YBa2Cu30,_6 and found an expo- nent of 0.33 at 975°C. The result was interpreted as grain growth in the presence of a liquid phase. They found that at this sintering temperature, the grain morphology was more isotropic than what was found YBa2Cu307_-6 by the addition of Ag,O. They con- cluded that Ag,O doping enhances the decomposi- tion and induces melting of YBazCu307_J at lower temperatures. They attributed the grain growth en- hancement to the melting which occurred at the sin- tering temperature ( 980’ C). In this paper we report densification behavior, grain growth kinetics and microstructural development of YBaZCu307_-6 when it is coupled with CuO during sintering. 2. Experimental procedures The YBazCu30,_J powder was made by a typical 0167-577x/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved. 13