Frequency Dependence of Electric Resistance and the Structure of Molten Oxide Systems, PbO-B2O3 and Na2O-B2O3

Abstract

The electric resistance of electrolytic solutions is usually measured using an A. C. bridge. Frequency dependence of resistance has been observed, and is considered as caused by electrode reactions at low frequencies up to several kHz. Electric resistance ρ is ususlly proportional to f-n(n; from 0.5 to 1).It is reported that resistance of molten alkali silicates, measured with Pt electrodes, depends upon frequency, and that Pt electrodes behave as oxygen electrode. It is also reported that the rate determining step of the electrode reaction is diffusion of O2- ion, and that the frequency dependence of ρ decreases with increasing activity a and diffusivity D of O2- ion. It is known that ao2- and Do2- depend upon the structure of molten oxides, therefore, it is able to discuss the structure of molten oxides on the basis of the relations between frequency dependence and the composition of the melts.In this study, frequency dependence in molten oxide systems, PbO-B2O3 and Na2O-B2O3, was measured, and the structure of molten borates was discussed. The results are the followings.(1) Electric conductivities above 5kHz showed poor frequency dependence. They increase sharply from 0 to 30mol% PbO or Na2O, and then slowly with further addition of PbO or Na2O. Such behavious of conductivity depend upon the structure of borate complex anion.(2) At about 0.2-5kHz, ρ is proportional to f-1, and it is considered that the frequency dependence of ρ in borate melts is mainly caused by the oxygen electrode reaction on the Pt electrodes as in silicates.(3) At the composition range from 0 to 30mol% PbO, or from 0 to 25mol% Na2O, frequency dependence d ρ/d(1/f), decreases with increasing PbO or Na2O concentration. It is caused by the dissociation of the borate network structure.(4) It is considered that the concentration of borate anion in cage structure, containing four-coordinated boron, increases at PbO content from 30 to 50mol%, or Na2O from 25 to 40mol%. And ao2- and/or Do2- decrease in this region, then d ρ/d(1/f) increases with increasing PbO or Na2O content.(5) For the melts containing more than 50mol% PbO or more than 40mol% Na2O, borate anion in the chain structure is dominant. The chain anions dissociate into smaller anions such as BO33-, O2- with increasing PbO or Na2O content, then frequency dependence of ρ decreases.