Viscosity and Electrical Resistivity of Molten Alkali Borates

Abstract

The viscosity and electrical resistance of members of t h e systems Li20-B20a, Na20-Bz03, and K,0-B20s have been measured in the approximate temperature range 600' to 10QO°C. The log viscosity isotherms in the range 700" to 800°C.showed a minimum in the low-alkali region followed by a maximum as the alkali concentration was increased. This behavior could be explained by postulating a n equilibrium between two antithetical effects caused by the addition of alkali oxide to B2O3. The log electrical resistance isotherms decreased rapidly with increased alkali content up to about five equivalents of alkali ions per liter and decreased less rapidly in a linear fashion with further increase in alkali ion concentration. The equivalent conductance increased with alkali concentration, indicating behavior rather typical of liquids of low dielectric constant. The rate of increase of equivalent conductance was greater in the concentration region where the viscosity was increasing than where the viscosity was decreasing, thus illustrating that viscosity is not a controlling factor in electrical conduction. A plot of log R vs. log 7 gave straight lines whose slopes decreased with alkali content. The hypothesis that electrical resistance and viscosity of each glass were both related to the concentration of weak bonds in that glass was advanced to account for the constancy of the relation between the temperature coefficients of the two properties.

Borate Glass Structure

“…But even before the availability of NMR data, Krogh-Moe (44,105) had criticized the latter "anomalies" as not being acceptable proof of a saturation of NB04 in the 10-Z0 mo1% alkali oxide range. The viscosity of boron oxide at 500 0 C was found in increase with additions of sodium oxide up to 10 mol% where a peak in this property was noted (19). This, of course, invited the same kind of interpretation as the thermal expansion data, namely, an as sociation of increased cohe sion with increasing numbers of tetrahedral structures.…”

“…At this point the boron-oxide anomaly could no longer be explained in terms of boron coordination changes alone. Other example s of putative boron-oxide anomalie s have included the dependences of viscosity (10,19) and molar oxygen volume (13) on glass composition. But even before the availability of NMR data, Krogh-Moe (44,105) had criticized the latter "anomalies" as not being acceptable proof of a saturation of NB04 in the 10-Z0 mo1% alkali oxide range.…”

The surface viscosities and the foamines of molten oxides.

“…Fig. 15 showsthe bulk viscosity and the surface tension in the system Na20-B203reported by Shartsis et al,16) Kaiura and Toguri,17) and by Vogel,18) respectively, together with the surface and the bulk viscosities in the present work. According to the surface tension-composition curve in Fig.…”

Aerosol and Solution Modification of Particle-Polymer Interfaces

“…The exact structure of the borate melt does not concern us here, only that its physical properties make Na2B407 acceptable for generating continuous thin film coatings of BN on SiCw using the method described [33]. The viscosity of the sodium borate melt [34] is greatly reduced compared with the viscosity of boron oxide at elevated temperatures and penetration into the porous carbon layer on the whisker is, therefore, possible at lower temperatures (750-800 C) and before nitridation can occur. The unique effect of the Na20 on the properties of the borate melt may be to act as a fluxing agent to reduce the contact angle of the melt on the SiCw, thus producing a continuous BN coating [33].…”