Aggregation of defects and thermal-electric breakdown in MgO

Transmission electron microscopy has been used to study defects in as-grown MgO single crystals and in crystals which had undergone high-temperature electrical conduction for long periods of time. Besides dislocation loops of (a/2〈110〉) Burgers vectors and precipitates, a new type of defect, dislocation loops of α〈100〉 Burgers vectors lying in {100} planes, has been observed in electric-field-treated specimens. The nature of both (a/2) 〈110〉 and a〈100〉 loops was determined to be vacancy type. The relation of this microstructure to the observed increase in electrical conductivity, which ultimately results in thermal-electric breakdown of the samples, is discussed.