Electrophysical properties of the potassium aluminate-natural clay system

Abstract

Refractory clays and articles based on them are widely used in the national economy [1][2][3].They are mainly used as heat-resistant coatings in the ferrous-metals industries [i], in the production of building products [2], and for electrovacuum and radioengineering ceramics [3]. Usually, refractory goods are made from various sorts of clays with the use of lime or other bonds [3, 4].In the development of new formulas for refractory bodies, an important role is played by improving the methods of checking the quality of the refractories. A large part of them in terms of electrophysical properties relate to the group of dielectrics or semiconductors. The measurements of the specific electric conductivity of refractories during research is used as a sensitive parameter of the solid-phase reactions, processes of sintering, crystallization, etc. [4].This article presents data, absent from the literature, on time-related changes in the specific electric conductivity of refractories on the basis of aqueous solutions of potassium aluminate with natural clays. We present the results of conductivity measurements on hardened specimens from the above systems during heating at 20-800~The compositions studied are used as refractory coatings for metals, and also as high-temperature adhesives.The study used Prosyanovsk kaolin, Idzhevansk bentonite, chamotte produced by the Novomoskovsk refractories factory, and a water solution of potassium aluminate with a density of 1.54 g/cm s, prepared by the method described in [5]. The mixtures were prepared by slaking from i0 to 50%* of clay with a water solution of potassium aluminate.The specific electrical conducitivity of the suspension was measured with an AC bridge (P5010) and a cell with plane-parallel electrodes (GOST 6581--75), In this case the electrical conductivity of the suspension was studied during storage for 1-30 days.The electroconductivity of hardened mixtures during heating was measured with the method described in [4]. The specimens were prepared as follows:37% clay was slaked in 63% water solution of potassium aluminate, after which the mixture was hardened at 300~ in i h in a drying chamber.Then the mixture was ground in an agate mortar to a particle size of 80-i00 ~m. The resulting mixture was manually pressed into tablets 16 mm in diameter and 1.5-2.0 mm thick. The tablets were placed between the two steel, platinized electrodes of a metering cell which was placed in the resistance furnace.The furnace was heated to the prescribed temperature and then switched off, and the volume resistance of the specimen was measured.The value of the specific electric conductivity of these systems was calculated from the well-known formula [4].The data obtained show (Fig. i) that the addition of even small amounts of clay (up to 10%) to the water solutions potassium aluminate reduces by a factor of 2-4 their specific electric conductivity; the maximum reduction in the values of u is noted in the supensions containing bentonite (by a factor of four).Further increase in the concent...