Aluminothermic Synthesis of Al 2 O 3 /ZrO 2 /SiC Composite Powders
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ALUMINOTHERMIC SYNTHESIS OF A12 0 3 / ZrO2 / SiC COMPOSITE POWDERS
J.P. ERAUW
Materials Research Unit - Flemish Institute for Technological Research SCK/VITO, Boeretang 200, B-2400 Mol (Belgium)
ABSTRACT The synthesis of A120 3 / ZrO 2 / SiC composite powders by aluminothermic reduction of zircon in the presence of carbon is reported. The effect of the temperature on the progress of the reaction, as assessed by XRD analysis, is outlined. The morphological characteristics of the reaction products are described. Based on the experimental results, a reaction scheme is proposed. The possible propagation of the reaction is discussed in terms of the adiabatic temperature.
INTRODUCTION A significant attention has recently been focused on reaction-based ceramic-ceramic composites owing to the difficulty to obtain dense, flaw-free components by conventional processing routes. In particular the use of exothermic reactions, often referred to in the literature as combustion synthesis or SHS (Self-propagating Hfigh-temperature Synthesis), has received renewed interest in recent years. The attractiveness of this method comes on the one hand from the low-energy costs implied by the ability of those exothermic reactions to propagate in a self-sustaining mode, and on the other hand, from the expected higher purity and reactivity (sinterability) of the reaction products. Its feasibility has already been demonstrated in a number of ceramic / ceramic systems including A120 3 -TiC [1,2], A12 0 3-SiC [2,3], A12 0 3 -TiB2 [4]. The present study aims at extending this method to the synthesis of A120 3 / ZrO2 / SiC composite powders by means of an aluminothermic reduction of zircon (ZrSiO4) in presence of carbon. In view of the toughening potential of zirconium oxide dispersions, this extension deserves attention. The overall investigated reaction may be formulated as 3 ZrSiO 4 + 4 Al + 3 C ->
2 A12 0 3 + 3 ZrO2 + 3 SiC.
(1)
Calculations from relevant thermodynamic data show that this reaction is exothermic and brings about a negative free energy change (AG0 298 = -745 kJ/mole ; AH- 298 = -769 kJ/mole). By the use of zircon, the present study stands out against a previous research [2] on this system where a dispersion of zirconia within an A12 0 3 / SiC composite powder was obtained by adding a commercial ZrO 2 powder. THEORETICAL CONSIDERATIONS Only highly exothermic reactions can lead to a potential self-sustaining propagation of a combustion front. In this respect, the adiabatic temperature (Tad) appears a useful parameter. The adiabatic temperature is by definition the temperature reached by the products under Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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adiabatic conditions as a result of the heat released by the reaction. Though this temperature constitutes an upper limit seldom reached by the products because of the heat losses, it is generally accepted that it should be above 2000 K if the propagation has to be self-sustaining. The adiabatic temperature (Tad) of the reaction (1) has been calcu
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