Electric field Induced phase transformation In ceramics

Abstract

It is known that: (1) an analogy exists between the electrical and mechanical properties of dielectric materials, (2) the polarization energy can be dissipated via phase transitions, and (3) phase transformation produced in ceramics under mechanical stress or at the time of crack propagation, increases fracture toughness. The influence of an electric field on the structure of an alumina (Al2O3) ceramic, with Zr02 (20%) is reported. It is shown that: (a) the percentage of Zr02 undergoing phase transformation from tetragonal to monolithic (t-->m) depends on the amplitude and duration of the applied electric field, (b) the phase transformation plays a dominant role on the conditionning of the insulator, (c) the sintering method influences the (t-->m) rate. The results presented will provide original insights into the conditionning mechanism, partial discharge mechanism, flashover, and on the role of internal stresses on voltage withstand of dielecrics.Electric field induced phase transformation have been reported in polymers /1/ in the frame of study of electrets formed in dielectrics submitted to poling. Electric field induced phase transformation is one possible mechanism for dielectric relaxation /U. Relaxation involves transfer of electrostatic energy to the phonon bath and can induce second order or first order phase transformation. However, the investigation of dielectric relaxation processes in solids have shown a clear analogy between electrical properties and mechanical properties./2/ The effect of electrets and related dielectric relaxation processes on electrical breakdown phenomena (flashover, punchthrough, bulk breakdown) has been recently shown /3,4/. The effect of electrets created by poling or UV irradiation on fracture toughness has been demonstrated on Yttria /5/. The effect of electrets on both toughness and breakdown voltage can be justified on the thermodynamical concept of first order phase transformation (see ref.1, p.18 and /6/). The aim of this work is therfore to investigate phase transformation of ceramics induced by electrets produced by poling. The target is also (i) to show that electrical conditioning of insulators can be related to such phase