Electric conduction of glasses at intermediate field strengths

Abstract

This study concerns four commercial glasses containing from 4 to 13 mol% alkali, a mixed alkali glass and three semi-conducting glasses containing from 5 to 10 mol% Fe 2 O 3 . Deviations from Ohm's law are detected somewhat beyond 10 4 V cm −1 and reach 10% at 10 5 V cm −1 . They are well described by a sinh function with activation lengths from 17 to 22 Å. These lengths are several Å greater than, and therefore consistent with, the saddle-point distances previously determined at the onset of Poole-Frenkel (P-F) conduction at 2 × 10 5 V cm −1 . The mechanism with a constant activation length through the ohmic and intermediate field range is thus succeeded by the P-F mechanism with the activation length decreasing as E − 1 2 . The simple electrostatic interactions required by the P-F effect imply a low density of free charges. It is suggested that the initial activation length is the radius of isolated Coulomb traps cut off at about the 1.5 kT energy level and that the effect of the field is to enhance the thermal dissociation rather than the mobility. This hypothesis invalidates the use of a sinh function to evaluate activation lengths. It is shown however that a cosh function (to describe field-assisted dissociation) can also represent the intermediate field data.