@ Pergamon NONEQUILIBRIUM EFFECTS IN THE THERMAL CONDUCTIVITY OF B&BASED HIGH TEMPERATURE SUPERCONDUCTORS H. Bougrine, S. Sergeenkov**’ and M. Ausloost S.U.P.R.A.S. Institute of Physics, BS, University of LiZge. Sart Tihnan, B-4000 Liege, Belgium R. Cloots S.U.P.R.A.S. Institute of Chemistry, 86, University of LiGge, Sart Tilman, B-4000 Litge. Belgium and V.V. Gridin Department of Physics, University of Witwatersrand. Private Bag 3, P.O. Wits 2050. Johannesburg, RSA (Received 20 January 1994 by D. Van Dyck) Thermal conductivity of two Bi-based high temperature superconduct- ing ceramics. 2212 and 2223, with a markedly different microstructure have been examined. A pulse technique has been used. Depending on the heating pulse duration, the more weak-links-containing 2223 sample shows either equilibrium or nonequilibrium behavior. The temperature dependence of the thermal conductivity between 1IO K and 180K is found to obey the Arrhenius law. The experimental observations are interpreted as a thermally activated heat flux motion through the polycrystalline grain boundaries with an energy barrier of nearly IOmeV. It is likely due to a phase slip mechanism. An extra relaxation process due to the pore content/structure is thus emphasized in such regimes. Keywords: A: high T, superconductors, C: grain boundaries, D: electronic transport, heat conduction, flux pinning and creep. 1. INTRODUCTION In this communication we report the measure- ments of the thermal conductivity n(T) of Bi(2212) and Bi(2223) HTS ceramics. An a.c. heating pulse method has been used. Depending on the heating pulse duration (1 min or 3 min), the more weak-links- bearing 2223 sample is found to follow either non- equilibrium or equilibrium behavior of the TC. We unambiguously show that the dependence of n(T) obeys the Arrhenius law above the critical temperature in both samples. Furthermore. the appearance of a superconducting state is only loosely marked, and a very small peak only is discernible beIow T, under quasi-equilibrium conditions. However, in the presence of nonadiabatic effects, the peak is much enhanced, and some kind of hysteresis effect is observed. THERMAL conductivity (TC) of materials is a complicated property to measure and to interpret. In the case of high temperature superconducting (HTS) ceramics, review articles [I, 21 emphasize such difficulties. Among these ceramics, the case of Bi- based and Tl-based ones has attracted only moderate attention for the regime near the superconducting transition temperature T, [3-71. + Permanent address: Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia. ’ Present address: Department of Physics, University of Witwatersrand, Private Bag 3, P.O. Wits 2050. Johannesburg. RSA. ! E-mail: U215OMA @ BLIULGIl. Bitnet. We discuss the results in terms of sample microstructure and a thermally activated heat flux 571