Highly heat-resistant mullite-silicon carbide materials

Abstract

Cross-linking additions of silicon carbide and waste material from ferroalloy production make it possible to increase the heat-resistance index for mullite-containing refractories. For briquette specimens based on a charge containing aluminum powder it reaches 45 thermal cycles (1300°C -water).The possibility of describing the thermal conductivity of refractories, proposed by non-equilibrium thermodynamics together with methods of statistical physics, makes it possible to break into the mechanism of the heat transfer phenomenon as a non-equilibrium process, and to establish the analytical dependences for thermal conductivity on ceramic matrix parameters.In the thermodynamic model developed in this article by the authors for thermal conductivity it shown that the heat resistance of refractories is governed on one hand by heat transfer conditions in the ceramic matrix, and on the other hand by the capacity of its structure to generate non-destructive thermal stresses with presence of a temperature gradient. Elements are developed on the basis of this model for producing thermally stable forsterite-containing refractories [1 -3].The theoretical model of heat resistance provides a systematic nature to the particular results obtained and it is possible to transfer it to mullite-containing material elements already developed. Creation of new forms of highly heat-resistant aluminosilicate ceramics is possible due to development of composites based on high-alumina raw material with non-traditional additions of technogenic waste materials.The aim of this work is a study of the effect of non-equilibrium processes on formation of the structures of mullite-silicon carbide refractories, and the role of modifying additions and charge composition on heat-resistance index. High alumina raw material, i.e. kyanite is used in order to prepare refractories. In the North-West part of the Kola Peninsula there are large deposits of kyanite shales, i.e. Keivy. Here 27 deposits have been discovered and eight explored, and 23 of them are placed on the state balance. The largest are the Novaya Shuururta, Tyapsh Manyuk and Chervurta deposits. The predicted reserve of kyanite ore is two billion tons [4].The starting components of the charge are: briquettes made of kyanite (without additions, with addition of aluminum powder or with aluminum hydroxide calcined at 800°C), silicon carbide, ferrosilicon production waste material. A charge was mixed with a temporary binder (polyvinyl alcohol), and compacted specimens were placed in a carburizing atmosphere.For mullite there is typical anisotropy of thermal expansion (a a = 5.2 × 10 -6 K -1 , a b = 7.1 × 10 -6K-1 , a c = 2.4 × 10 -6 K -1 at 298 -1098 K), and thermal conductivity l = 3.0 -3.5 W(m × K). The different linear thermal expansion coefficients (LTEC) over crystallographic axes lead to the occurrence within the volume of the material of thermal stresses with unidirectional heating or cooling. An increase in heat resistance index is obtained as a result of improving heat transfer and reduci...

Modified mullite-cordierite materials

“…4), we show the dependence of the thermal stability on the form and amount of structure additives in the compositions considered in paper [6] and the present study. Refractories in the charges of which there is magnesium oxide in active form have the highest thermal stability.…”

“…There was a difference from compositions previously examined, in which there was active aluminum oxide made from hydroxide and aluminum powder [6], in that high thermal stability was attained even when we added only the waste from ferrosilicon production in the MgO-Al 2 O 3 -SiO 2 system ( During annealing, the products from these reactions (Al 2 O 3 and SiO 2 ) at the instant of segregation in the presence of MgO produce cordierite.…”

“…Figure 1 shows the dependence of the thermal resistance of these refractories on the charge composition. The mullite-cordierite-graphite specimens have thermal resistance greater than that of mullite-graphite ones, namely up to 30 heat cycles (Table 1, composition 3) [5]. Figure 2 shows the dependence of the thermal stability for mullite-cordierite-graphite refractories on the form of the antioxidant and the amount of carbonaceous component in the charge.…”

“…We have examined the effects of charge composition and structuring additives on the structure formation for mullite-bearing refractories based on kyanite, including the thermal stability of these [5,6]. It has been found that the thermal stability of mullite-bearing refractories can be raised by the following: 1) adding carbon to the charge, which increases the thermal conductivity and produces a plastic matrix around the grains of mullite, which allows them to deform under varying temperatures without disrupting the surrounding material.…”

Mullitization of aluminosilicate diphasic gel in the presence of nickel oxide additive

“…The average crystallite size after calculation using equation (5) for mullite sample with nickel oxide additive has been given in Table 5. With the increase in temperature, the size of the mullite crystallites became shorter, suggesting the accumulation of significant amount of nickel oxide-bearing phases at the grain boundary of mullite samples.…”