Transient and steady state ionic conductivity in type-III fused silica

Abstract

Measurements of the transient and the steady state ionic conductivity in type-III fused silica were carried out in a non-blocking mode setup. Alkali (Li+, Na+ and K+) ions were electrolysed through the sample under a dc applied field in a 300 to 800 °C temperature range. The change in conductivity with time and the time required to attain steady state depended upon the type of alkali ion electrolysed through the sample as well as the electrolysing temperature. Except for the 350 °C transient conductivity profile, a maximum in the current-time behavior of K+ ion was observed at higher temperatures. In the dried as well as heat-treated samples, K+ ion transient conductivity at 600 °C showed a smooth approach to steady state. Li+ ion transient conductivity showed a minimum in the early stages of conduction for samples dried at 140 °C. This minimum shifted to lower times on increasing either the electrolysing temperature or the applied field. The minimum disappeared when the samples were heat treated at 600 to 650 °C for 1 to 2 d. Na+ ion electrolysis on the 140 °C dried sample showed instability in the early stages of transient conduction at 600 °C. The heat-treated samples always showed a higher instantaneous conductivity compared to the dried samples irrespective of the electrolysing ions but the steady state conductivities were similar. The protonic and the anionic (Cl-) species produced as a result of the interactions between the ≡SiCl groups and the dissolved H2 and H2O in the interstitial volume can account for the differences in the observed behavior. The steady state analysis showed that the Na+ ions were the most mobile ions followed by Li+ and K+ ions. The steady state conductivity was apparently independent of electric field strength and the different reactions which produce protons and Cl- ions.