Physicochemical research is conducted to examine the phase formation under dynamic and static annealing of MgO-Al 2 O 3 -SiO 2 tempered glass (composition range adjacent to stoichiometric cordierite). Thirty glass compositions are examined in this system and the range is determined over which single-phase ceramic materials with the lowest possible thermal expansion coefficient are synthesized under specific crystallization conditions.Melt crystallization provides materials based on alumina compounds, including mullite (3Al 2 O 3 ⋅ 2SiO 2 ), forsterite (2MgO ⋅ SiO 2 ), and cordierite (2MgO ⋅ 2Al 2 O 3 ⋅ 4SiO 2 ) with uniform chemical and phase compositions and controlled content of impurity phases. Two processing options may be used: (i) forming of parts from molten glass and their subsequent annealing (T ≥ 1000°C) for crystallization and (ii) production of granulated glass, its grinding, forming, and annealing using conventional ceramic processing.Single-phase α-cordierite with low thermal linear expansion coefficient (TLEC) and high thermal resistance was produced by cooling a cordierite melt at a rate of 1.5 °C/sec to 900-1100°C with subsequent annealing at these temperatures for four to five days [1]. To promote the crystallization of cordierite and decrease the volume content of the glass phase, oxide additions are used to form crystallization nuclei in glass: 3-5 wt.% ZrO 2 [2], 0.1-15 wt.% Nb 2 O 5 , Ta 2 O 5 [3], ≤10 wt.% TiO 2 [4]. This leads to a 25% decrease in annealing time, and the content of crystalline cordierite reaches 98-99 vol.%.The thermal linear expansion coefficients of samples representing stoichiometric cordierite ceramics and cordierite-based solid solutions are 2.0 ⋅ 10 -6 °C -1 over a range between 25 and 1000°C [5]. A substantial amount of the glass phase that had no admixtures of alkaline or alkaline-earth metals (ratios 1 : 1 : 4, 1 : 1 : 5, and 1 : 1 : 6 containing to 81% Si 2 ) increased the thermal expansion coefficient from 2.0 to 3 ⋅ 10 -6 °C -1 . The high content of crystalline cordierite (95-97%) resulted from arc melting of a cordierite charge consisting of serpentinite, quartz sand, and alumina [6]. When glass was heated at a rate of 10 °C/min, differential thermal analysis (DTA) revealed an exothermic effect of cordierite crystallization with a peak at 1000°C. There are no data on the thermal expansion of crystallized cordierite samples. In contrast to conventional negative attitude to alkaline oxide admixtures, it was established in [7] that about 7.56 wt.% K 2 O introduced into cordierite glass in accordance with the formula 4MgO ×

Refractory packing masses are being used more and more in the manufacture of linings for heat assemblies. One of the important properties of packing masses is the constancy of their volume or the very slight increase in volume during the preparatory heat treatment and later service. A slight increase in the volume of the packing lining on heating produces compressive stresses which have a favorable effect on the strength of the lining and prevent cracking.The present article reports some results of studies of refractory packing masses based on chamotte containing kaolin or milled pyrophillite as the finely dispersed component.In the preparation of the packing masses we used the following: chamotte (TU 14-8-58-72) containing 42%* fractions of 2.5-1.0 mm; 22%, 1.0-0.4 ram; 27%, 0.4-0.063 mm; and 9?0, < 0.063 mm; finely milled Glukhovetsk kaolin; and finely milled pyrophillite from the Mozyr'-Ovrunsk Deposits. As the bonding agent, we used water glass (WG) of density 1.4 g/cm 3 (GOST 13078-67) and an aluminochromophosphate bonding (ACPB) of density 1.6 g/cm a (TU 6-18-166-73).We thought it would be interesting to replace the kaolin by the pyrophillite since heat treatment even at comparatively low temperatures (1150-1300~ causes the pyrophillite to decompose into mullite and an active silicon oxide; this improves the sintering qualities of the refractory lining and increases its strength.We prepared four compositions of a chamotte packing mass (Nos. 1-4). All the masses contained 75% chamotte and 25% pyrophillite (Nos. 1 and 3) or of kaolin (Nos. 2, 4). To mass Nos. 1 and 2 we added 15% water glass and to masses 3 and 4, 15% ACPB.In the preparation of masses we added to the preliminarily mixed dry powders the appropriate bonding agent and the mixture was incorporated to a uniform state. All the specimens were pressed under a pressure of 25 MPa. For the compressive strength tests we used specimens in the form of cylinders, diameter and height 20 ram, previously dried at 100~ and fired for 2 h at various temperatures. The open porosity of the specimens was 23-25%. The change in the length of the specimens after heat treatment was determined from the change in length of beams measuring 8 x 8 x 50 mm. The thermal-shock resistance was determined using a heat-cycle method (800~ --water) for specimens of diameter and height 30 mm and also using a method which involved heating hollow cylinders in rings of external diameter 50 and internal 25 mm and depth 20 mm [1].

Ceramics made from aluminum titanate constitute one of the most interesting oxide materials as a result of their unusually low temperature coefficient of linear expansion ~. Together will the prolonged operational charactristics of these ceramics, several investigators have noted the low mechanical strength which hinders the application of the material; attempts to increase the mechanical strength by adding various substances lead as a rule to a simultaneous increase in the linear expansion coefficient (TKLR) [I].The thermal expansion of aluminum titanate was studied for the first time in [2]. This showed the presence in this compound of hysteresis of thermal expansion during heating and cooling of the specimen, which was mainly explained by the microcracking of the structure of the ceramic made from titanate with crack sizes of about 7.5 nm.Such cracks possess the capacity for reverse closing (collapse) and opening during heating and cooling of the specimens respectively.This phenomenon was recently confirmed indirectly [3] by investigating the acoustic emission of ceramics made from aluminum titanate in relation to the test temperature of the specimens.Unfortunately, direct observations in the electron microscope of the microcracked structure of this type are difficult because of the coincidence of the microcracks formed with the grain boundaries of the material.The microcraoked structure of the ceramics made of aluminum titanate is a consequence of the clearly expressed anisotropy of the thermal expansion of AIzTi05, which causes the development of internal stresses, leading to the formation of internal ruptures.The internal stresses arising during cooling of the ceramic from its sintering temperature to room temperature were detected also in AI203 Furthermore, microcracking is noted only in certain of the above-mentioned materials, and depends on the magnitude of the anisotropy of thermal expansion [9]; in the fine-grained materials microcracking is not observed [5,7]. The critical grain size, below which microcracking does not occur, depends on the magnitude of the anistropy of the thermal expansion and equals for Fe2TiO 3 3 #m, and for MgTi205 5 ~m [9]. For the aluminum titanate this dimension is 1-2 ~m.Microcracking of ceramics occurs at rather high temperatures, which according to the data of several authors are located within the range 500-700~ [2, 3, 5, i0, ii]. For material with low anisotropy of thermal expansion such as, for example, A1203, relaxation of the stresses at these temperatures may be sufficient to prevent the formation of microcracks. With a high anisotropy, as, for instance, in materials of the pseudobrookite structure (AI2TiOs, FezTiO5, MgTi2Os, etc.), the high-temperature relaxation of the intergrain stresses proves to be inadequate, and with further cooling of the ceramic mierocracks are formed.During the transition through the critical size of the grain in materials of this group there is a sharp change in most of the physical properties.For fine-grained structures there is typically...

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