The Debye-Falkenhagen theory of electrical relaxation in glass

The Debye-Falkenhagen-Tomozawa theory (DFT theory) of electrical relaxation in glass is reviewed. This theory interprets the principal electrical relaxation in ionically conducting glasses in terms of relaxation of Debye-Huckel ion atmospheres. Electrical relaxation data is presented to demonstrate that in typical alkali silicate glasses the concentration of free ions is too great for the DFT theory to hold. Further data in a series of borosilicate glasses containing Na₂O at the 0.01 mol% level offers evidence for the validity of the theory in the dilute alkali regime. However, a quantitative fit to the theory can only be made under the assumption that less than 8% of the sodium in the glass is present as free ions at 400°C. This result is discussed in terms of the Bjerrum theory of ionic association.