The Electrical Conductivity of Sodium Metasilicate–Silica Glasses

JSGT 1932 V16 T450-T477

Determinations have been made in an evacuated silica vessel (p<0·001 mm Hg) of the electrical resistance of a series of fourteen pure, well-annealed (schedules given) sodium metasilicate-silica glasses ranging in composition from SiO₂50·21%, Na₂O 49·79% to SiO₂91·6, Na₂O 8·40%, the temperature range being from below 100° up to the softening temperature (400–560°, according to the glass). Electrodes suitable for these glasses, which have a very wide range of thermal expansion, were devised, and involved platinising the ends of the cylindrical rod specimens and using stainless-steel screw cups over the ends with a platinum foil packing. The standard rate of heating applied was 1° per minute throughout the range of measurements, and the effects of heating rate, presence of strain, and of incipient devitrification were studied. The specific electrical resistance ranged, for the end glasses of composition quoted, from 2·09×10⁵ to 1·31×10³ at 100° and from 71·8 to 7·5×10⁴at 400°. The curves for all the glasses obtained by plotting log. sp. resistance against the inverse of absolute temperature are approximately straight and parallel up to the transformation point, but also contain, what are now shown for the first time by electrical resistance methods, points of inflection or transition points. The transformation points (T_g) range from 396° to 465°, agreeing closely with those determined from thermal expansion by Turner & Winks, but the transition points by the two methods are different. Rasch & Hinrichsen's law is found to be valid provided that new constants are employed in each temperature range marked off by transition and transformation temperatures. The curves obtained for different temperatures from 50° to 400° by plotting log. sp. resistance against composition consist of two straight lines intersecting at a point corresponding with the disilicate composition Na₂O.2SiO₂.

E. Seddon, E. J. Tippett &W. E. S. Turner