Production of Ceramic Composite Materials of Aluminum‐Silicon Dioxide‐Dolomite System Using SHS Process

Abstract

The self-propagating high-temperature synthesis (SHS) of industrial refractories from low-cost domestic raw materials (dolomite and silica) using aluminum powder as a reducing agent is investigated. The phase composition, microstructure and combustion wave velocity are studied for different compositions of a powder charge. Differential thermal analysis has revealed that at low heating rates, about 10 K/min, which are typical of traditional furnace synthesis, the self-ignition is impossible because of oxidation of aluminum in air. Thermodynamic modeling has been used for studying the interaction mechanism in the SHS wave. The effect of preliminary mechanical activation of a charge mixture on the SHS wave velocity is investigated. The feasibility of the cost end energy efficient SHS method for producing refractories for hightemperature applications, e.g., furnace lining, in this system is demonstrated. INTRODUCTION Industrial refractory ceramic materials (RCM) and articles made from them are used for a wide range of high temperature applications such as lining of furnaces, casting ladles, etc. They are supposed to possess high heat resistance, thermal stability, and mechanical strength. Such a combination of properties are can be attained in refractories containing aluminum magnesium spinel, MgAl2O4, which has a high melting temperature, Tm=2135 ° , and is chemically stable towards many liquid metals and slags, and silicon carbide, which imparts high heat resistance, electrical and thermal conductivity to a material. In recent years, a number of attempts was made to produce high performance RCM from mineral substances using reduction-oxidation reactions in the regime of self-propagation high-temperature synthesis (SHS). The latter is often termed as combustion synthesis (CS) or solid flame. In this process, a heterogeneous reaction front, being initiated by heating a mixture of reactive powders by a local heat source, e.g., resistance coil or electric arc, propagates progressively through the charge as a combustion wave with temperature TCS leaving behind hot interaction product. The basic feature of SHS is that the heat released due to exothermal reactions in the wave front initiates the thermal reactions in the adjacent layer thus sustaining displacement of the combustion wave. Alternatively, the powder charge is preheated as a whole to a certain temperature at which the reaction starts either throughout the volume providing fast self-heating of the specimen to the final temperature TCS (the so-called thermal explosion mode of CS) or initiates spontaneously at an edge of the sample and then propagates through the preheated material as an SHS wave. SHS is characterized by a high value of TCS reaching 3500 C in highly exothermic systems such as Ti-B, relatively fast velocity of the SHS wave, ~0.1 to 10 cm/s in different systems, a high rate of self-heating, up to 10 K/s, steep temperature gradient, up to 10 K/cm, rapid cooling after synthesis, up to 100 K/s, and fast accomplishment of conversion. It should be noted that traditional furnace synthesis of refractory compounds requires a much longer time, ~1-10 h, for the same initial composition, particle size and close final temperature, and necessitates the use of costly and energy-consuming high-temperature facilities. It has been demonstrated both experimentally and theoretically that in many systems phase and structure formation during SHS proceeds via uncommon interaction mechanisms from the point of view of the classical Materials Science. To improve the degree of conversion in difficult-to-react systems and exert a closer control over CS, in some cases mechanical activation (MA) of starting powder mixtures is used. In this connection, an urgent problem is the development RCM and efficient technologies for their production from low-cost domestic raw materials basing on the cost and energy saving concept of SHS. Thus, the objective of this research is investigating experimentally a possibility of obtaining RCM for furnace lining applications in the regime of SHS using dolomite, CaMg(CO3)2, and silica sand, which occur in Belarus, as reducible compounds and aluminum powder as a reducing agent. RESEARCH METHODOLOGY In this work, a variety of experimental methods was used together with a theoretical study, namely thermodynamic modeling of SHS. Experimental procedure For producing porous RCM, fine-dispersed powders of silica, dolomite and aluminum were used in a different mass ratio (see Table I). In a series of experiments, the green powder mixture was subjected to mechanical activation in a rotary ball mill with a rotation speed of 1 revolution per second for several hours using 5-10 mm diameter wear-resistant steel balls as milling bodies with the ball-to-powder mass ratio of 2 to 4. Table I. Green powder compositions for SHS, wt.% No. Al SiO2 dolomite, CaMg(CO3)2 1 20 30 50