Mechanism of electromagnetic emission in plastically deformed ionic crystals

Experiments on the plastic deformation of LiF ionic monocrystals under uniaxial compression are performed with simultaneous recording of acoustic (AE) and electromagnetic (EME) emissions. A strong correlation between AE and EME events has been found, which clearly demonstrates that the observed EME is caused by a dynamical interaction between moving dislocations and charged vacancies in the ionic lattice during work hardening. The mechanism proposed to explain EME is based on the assumption that gliding edge dislocations sweep up the vacancies of a preferable sign. As a result, when a dislocation pileup is formed, a certain nonequilibrium charge density is accumulated at its head, resulting in electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pileup bursts through the stoppers and quickly loses its bounded charge. The relaxation of this charge produces an intrinsic polarization current generating an electric pulse. It is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force $\sim Bv$ ($B$ is the friction coefficient and $v$ is the vacancy velocity) in a self-consistent electric field determined by the distribution of the total charge density. A nonlinear integro-differential equation of motion for the nonequilibrium charge density is derived. For a special form of the initial charge density distribution, an automodel solution of this equation describing the polarization current has been built. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies (the analog of an edge dislocation extra plane) in LiF has been estimated to be $B\simeq 0.9\times {10}^{-5}\mathrm{g}{\mathrm{cm}}^{-1}{\mathrm{s}}^{-1}$. This value is in accordance with the corresponding coefficient for dislocations in ionic lattices. DOI: http://dx.doi.org/10.1103/PhysRevB.76.024106 © 2007 The American Physical Society