THE ROLE OF DISLOCATIONS AND GRAIN BOUNDARIES IN TRANSPORT PROCESSES IN IONIC CRYSTALS

Abstract

After a brief survey of the defects introduced by plastic deformation and of their removal by annealing, tlie influence of plastic deformation upon ionic conductivity is considered. Both enhancement and s~~ppression of the extrinsic conduction has been found experimentally in addition to an enhancement at very high temperatures in the intrinsic region 1'. These effects can be explained in terms of dislocations and their interactions during plastic flow. In polycrystalline material of near theoretical density, the grain boundaries appear to enhance only the extrinsic conductivity and to increase the observed activation energy in tliis region. 1 . Introduction. There is a growing volume of evidence that tlie presence of dislocations in ionic crystals may modify their transport properties in a number of different ways. The purpose of this paper is to review tlie experimental findings and to indicate what precautions lllust be taken to tnininiise the influence of dislocations upon measurements of ionic conductivity. Since most dislocations are generated by plastic defor~iiation, the paper starts with a brief survey of the defects introduced by plastic deformation and of the way in which these defects are removed by annealing. Subsecluently the influence of dislocations and grain boundaries on ionic conductivity is considered. 2. Defects introduced by plastic deformation. At room temperature plastic deformation is not homogeneous in ionic crystals, and i t is often difficult to compare the results of dilferent workers who strain their specimens plastically by tlie same amount but produce dilyerent defect densities and distributiolls by using dilferent specimen geometries. The shape of the stress-strain curve gives a good idea of the nature of the dcformation process, and where possible this should be measured in all deformation experiments. F o r specimens which arc tall compared with their cross-section, conipresscd along tlie [OOI] tall axis, three stages of work-hardening are fbu~ id with changes of slope after about 4 ;; and about 8 % plastic co~iipression [I]. Thesc three stages represent differelit modes of dcform:~tion, a n d diffcrent rates of accumulation o f dislocations ill thc crystal, with different effects upon the transport properties, and thus it is important to know whether a crystal has been deformed into stage I, stage I1 or stage 1II. By contrast, specimens which are squat compared with their cross-section, for example niost ionic conductivity specimens compressed along the squat axis, enter stage 111 deformation at very small plastic strains with a high rate of accumulation of dislocations and of generation of point defects. Most of the experiments on plastic deformation of NaCl have been carried out on tall crystals with dimensions of the order of 5 x 12 x 25 mm, having initial dislocation densities, inside the sub-grains, of between 10' and IOcm? The rate of increase of dislocation density is low in stage 1 and increases more rapidly in stage 11, to a total screw dislocation density of some 8 x 1O7/crn\af~er 8 O/;: compression. The moving screw dislocatiolls leave trails of debris, in the form of vacancy clusters and edge dislocation dipoles, oriented in [I001 directions in the slip plane. The density of edge dislocations and debris is about 10 tilnes that of screw dislocations during stage 11. By measuring the change in bulk density of the crystals during deformation the rate of accuniulation of point defects can bc estimated, although it is difficult lo dislinguisli between vavancies and interstitials in tliis way. Both species should be formed as a consequence of thc geometry of the deformation process, although in the alkali halides tlie life-time of' the intcrstitinls is likely to be very short at room temperature. Interpr-cling tlic long-term change of density produced by p l~~s t i c deformation in tcrms Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973939