Ion-Exchange in Sheet Glass by Electrolysis

Abstract

The ion-exchange process was investigated by electrolyzing potassium ions into commercial sheet glass below its strain point. The concentration of potassium ions diffused into the glass sheet under the electric field gave a rectangular profile. The analytical measurements showed that no more than 93% of sodium ions were replaced by the potassium ions.
The apparent resistivity ρ, increased linearly with the increase of thickness ε of the ion-exchanged layer, as expressed in the equation, ρ=ρ₀+ρ₁ε/L, where L is the thickness of the glass sheet, ρ₀ and ρ₁ are resistivities related to the diffusion of sodium ions and potassium ions in the sheet glass respectively. The activation energy for conductivity due to those ions was obtained from the temperature dependence of ρ₀ and ρ₁. The potassium diffusion coefficient in the sheet glass was estimated from the Einstein relation by use of an assumed correlation factor.
The stress profile in the layer was measured by means of a polarization microscope with the aid of a Eerek compensator. The compressive stress increased appreciably with the depth of the ion-exchanged layer and abruptly changed into the tensile stress at the boundary of the layer. The characteristics of the stress profile were explained by the different degree of stress relaxation which is greater at the layer nearer to the surface.
When potassium ions were diffused into one side of a glass sheet, the sheet was bent in order to balance stresses due to the ion-exchange. The stresses in the layer calculated from the curvature of the bend were not changed with the temperature of the treatment and became fairly larger than the observed values when the sheet was treated at higher temperatures, because the curvature was not affected by stress relaxation.