Electrical conductance through adsorbed water molecules on freshly fractured glass surfaces

Plate glass sheets of low emissivity glass were cut into small rectangles and scratched with a diamond tipped scribe from one edge to another. The scratch was deep enough to locally remove the conducting tin oxide surface and to fracture the glass beneath, leaving behind conducting electrodes on either side. They were connected by gold wires to a standard meter to measure the complex admittance at different relative humidities (14-79%) and frequencies (20 Hz to 1 MHz). The raw data was converted to the complex susceptibility after subtracting the signal due to the bulk glass substrate and analyzed using the universal dielectric response model developed by Jonscher. The change in the conductance and capacitance, as measured in a parallel mode, gave a measure of the humidity sensing ability of the scratch and roughly followed an exponential dependence. A control plain glass sample showed a very small effect. The Jonscher index “n” for the susceptibility (χ′) in the high frequency range was seen to be about +0.5, which is characteristic of ionic or hopping conductance and which we expect to be the Grotthuss process. The somewhat lower values of the index seen for the dielectric loss (χ″) data are typical of a lossy material. A combined plot of χ′ versus χ″ of all the data for all humidities and frequencies showed that the Jonscher model was well obeyed in the present work, with a log-log slope close to one as required by the model. Although one scratch sample is highlighted in this work, measurements are presented that show reproducibility between a number of samples as well as a minor amount of aging in repeat measurements on the same sample.