Thermal conductivity of composites with a filler of hollow spherical corundum granules

Abstract

One of the means of creation of thermal insulation materials with an increased porosity and heat resistance is the use of composites containing hollow spherical granules as a filler [i].Earlier the influence of the composition and quantity of a filler of alumina spherical granules on the density and strength of corundum refractories was investigated [2]. This article presents data on the thermal conductivity of such composites at temperatures from 20 to 400~Charges consisting of the filler and a fine-ground component providing sintering (binder) were investigated.Alumina microspheres and also corundum spherolites produced in the All-Union Institute for Abrasives and Grinding and in Kazakh Refractory Plant (spherolites A and K, respectively) were used as the filler [3]. The microspheres were produced by spraying of alumina in an argon plasma and the A and K spheres by inflation of a stream of electrofused corundum by compressed air on experimental (spherolites A) and production (spherolites K) units. The properties of the fillers are given in Table i. The finely ground component was corundum, 7-A1203, and mixtures of them, the properties and method of production of which are described in [2].To prepare the specimens the filler and the finely ground binder were mechanically mixed and the mixture was moistened with a 5% polyvinyl alcohol solution to a moisture content of 8-10% (depending upon composition) and sifted through a sieve with an opening size of 1.0 mm. Specimens with a diameter and a height of 15mm were pressed from the mixture obtained in this manner under a pressure of 40-50 N/mm 2. The specimens were fired in an oil furnace at 1750~ with a hold of 6 h.The apparent density was determined according to GOST 2409-80 by hydrostatic weighing of specimens saturated with water. The error determined from the amount of dispersion of reproducibility in five parallel determinations did not exceed 2%.The thermal conductivity X was measured on an IT-X-400 instrument, which uses the dynamic calorimeter method. The tests were made on specimens with a diameter of 15 and height of 3-5 mm, which were obtained by sawing from specimens prepared by the above method. The contact surfaces were ground and to improve the thermal contact of the specimens type PFMS-4 lubricant to TU 6-02-917 was used.As in [2], the simplex grid method of experiment planning was used [4].The mathematical relationships of the density and themal conductivity to composition were obtained in the form of fourth power polynomials:

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