It is well known [i] that a composite based on refractory oxides in which spherical granules are used as the filler is characterized by increased thermal-shock resistance and strength for a given porosity.We have studied batches consisting of a spherical filler and a finely milled component (binder) to ensure sintering.As the filler we used clayey microspheres and alumina spheres obtained in various ways (spheres A, B and K). The properties are given in Table I.The alumina microspheres are totally spherical particles.The strength of a single granule is 0. i N and the specific surface, 0.7 m2/g.According to the x-ray structural analysis, the microspheres are formed of y-AI203.The mass fraction of Al20s in them is 99.5%.The x-ray phase analysis with a DRON-I.5 diffractometer showed that the A, B, and K spheres were formed from a-Al2Os.It was established by electron microscopy studies* that the surface of the A spheres was smooth and consisted of individual fragments, and was penetrated by pores; the surface of the B spheres was not uniform and had a relief and the shell of the spheres made up of individual aggregates; the surface of the K spheres had a fairly well developed relief.As the finely milled component of the batch we used a mixture of corundum and y-Al2Os.The corundum for the binder was obtained by firing AI=O3, analytically pure grade, in an oil-fired furnace at 1750~ with a dwell of 5-6 h at the maximum temperature.The fired materSal was milled for I0 h in a rubber-lined vibromill with solid alloy spheres with a material:sphere ratio of 1:8. The specific surface of the material determined using the BET method was 2.1-2.3 m2/g. The x-ray phase analysis showed that the material was ~-A1203.The y-alumina, the reactive material obtained from the decomposition of hydroargillite, was characterized by a specific surface of 226 m~/g. Before the preparation of the batches, it was ground in a corundum mortar for 15-20 min.For the preparation of the specimens, we carefully mixed the filler and the finely ground component; the mixture obtained was wetted with polyvinyl alcohol to a moisture content of 8-10% and was then rubbed through a sieve.Specimens of diameter and height 15 mm were pressed from the mass obtained in this way at a pressure of 40-50 N/mm 2. The specimens were fired in an oil-fired furnace at 1750~ with a dwell of 6 h.The apparent density and open porosity of the refractories were determined in accordance with GOST 2409-80 by hydrostatic weighing of the specimens saturated with water.Five parallel specimens were used for the determination.The error evaluated from the magnitude of the dispersion of reproducibility was < 2%.Linear shrinkage relative to the diameter and height was calculated from measurements made using a micrometer to an accuracy of 0.01 mm.The compressive-strength tests were carried out on 15-mm-diameter specimens using an MR-5-1 breaking machine with an ultimate force of 500 N. The accuracy of the determination was 10%.

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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