Ceramics-Silikáty 43, (2) 67 - 77 (1999)

Phase composition, microstructure, fast ionic conductivity, cracking process, and thermal residual stresses in LiF-RF₃ (R = rare earth element) eutectic composites, prepared by using horizontal directional solidification, are presented. A special emphasis is put on the relationships between the fast fluoride ion conduction, phase composition and microstructure of the composites, and between the mutual crystallographic orientation of phases, microstructure and thermal residual stresses. The temperature dependence of the ionic conductivity of the eutectic composites is compared with that of the corresponding single crystals. In the composite, which contains the tysonite-structured RF₃ phase, conductivity is significantly lower than that in the corresponding RF₃ single crystal. A pronounced conductivity enhancement occurs only in the LiF-LiGdF₄ eutectic composite. The influence of phase composition, microstructure, and mutual crystallographic orientation of both phases on the residual stresses distribution is modeled using the finite element method. The simulation of the thermal residual stresses, which appear due to the thermal expansion mismatch and the anisotropy of the thermomechanical properties of both phases, is performed for the fibrous LiF-TbF₃ (orthorhombic) directionally solidified eutectic composite.