SURFACE-RESISTANCE OF BULK AND THICK-FILM YBA2CU3OX

The surface resistance, R(s), of high temperature oxide superconductors (HTS) varies as f(n), where f is frequency and the exponent n varies generally between 1 and 2. The R(s) of copper varies approximately as f1/2. The crossover frequency, fc, between copper and HTS is a convenient indicator of sample quality in radio-frequency and microwave applications, the goal being as high a crossover frequency as possible. The R(s) of YBa2Cu3O7 (YBCO) has been measured by a coaxial method i.e. a copper outer shield and a HTS central conductor and also by using an all-YBCO TE011 cavity. The microstructure of the ceramic has been deliberately altered so that variables such as starting powder size and phase purity, ceramic density, phase composition, grain size and number of grain boundaries may be evaluated. A number of experiments have been conducted so that the variables chosen might be studied independently. The results show that R(s) varies in a complex manner according to the microstructure and that as sintered grain size is reduced to a certain size the R(s) is reduced. Further decreases in grain size are associated with an increase in the number of grain boundaries and an increase in the R(s) suggesting that grain boundary detritus and grain boundaries themselves are sources of loss. The R(s) is also highly dependent on crystallographic orientation and examples are given to show that R(s) diminishes in c-axis oriented YBCO thick films.