3.11 Conduction in Glasses

• Glasses have a random 3-D network and contain glass forming agents such as SiO₂ , B₂O₃ , and Al₂O₃

• Interstitial positions are occupied by modifier ions such as Na⁺ , K⁺, and Li⁺ or blocking ions such as Ca²⁺ and Mg²⁺.

• Conductivity increases as the temperature increased as barrier for migration for mobile modifier ions is easily overcome by thermal activation.

• Usually as the size of the modifier ion decreases, the conductivity increases.

• As the concentration of blocking ions increases, the conductivity decreases.

       3.11.1 Molten Silicates

• Molten silicates are of commercial importance as glass melting is typically done in electrical furnaces using resistive heating of glass.

• Electrical conductivity is dependent upon the concentration of alkali ions (R: Na⁺, Li⁺, K⁺).

• For R₂O greater than 22%, highest conductivity is achieved for Li₂O followed by Na₂O and then K₂O while for R₂O < 22%, Na₂O yields the highest conductivity followed by K₂O and Li₂O.

• Typical electrical resistivity for soda-silicate glasses varies between10^-2 to 50 Ω-cm and depends strongly upon the alkali content.

• Silicates also contain alkali earth ions such as Ca²⁺, Sr²⁺ etc but the conductivity of alkali earth containing silicate glasses is about an order of magnitude lower than alkali containing silicates.

• In alkali earth compounds, highest conductivity is obtained by incorporating CaO followed by MgO, SrO and BaO, however the differences are very small.