Aluminothermic reaction path in the synthesis of a TiB₂–Al₂O₃ composite

Abstract

Differential thermal analysis, in air and argon, in concert with x-ray diffraction, was performed on 2-and 3-component mixtures of Al, B 2 O 3 , and TiO 2 , to foster an understanding of the reaction path involved in the TiB 2 -forming thermite reaction. In argon, aluminum reacted with B 2 O 3 to form elemental boron, and reacted with TiO 2 to form AlTi 3 . These two products reacted just after boron was made available at ϳ1060 ± C to form TiB 2 . Formation of Al 18 B 4 O 33 by reaction between B 2 O 3 reactant and Al 2 O 3 product attenuated the yield of TiB 2 , but facilitated its formation by extraction of Al 2 O 3 reaction barriers.Thermite reactions utilize their released thermal energy to be self-sustaining. The reaction is initiated at one end of a powder compact and propagates through its length to completely convert the reactants to products. Use of thermite reactions in formation of advanced ceramics has shown a number of advantages. Low cost raw materials, e.g., ceramic oxides, can be used. High reaction temperatures permit rapid firing without the use of furnaces. Nonoxide ceramic compounds can be formed via rapid reaction and natural temperature quenching in air. 1 A variety of advanced ceramics and composites such as SiC, B 4 C, Al 2 O 3 -SiC, and MgO-B 4 C have been fabricated via thermite reactions. 2 Cutler et al. 3 investigated the feasibility of synthesizing various composite ceramics using aluminum-based thermite reactions. Al 2 O 3 -TiC, Al 2 O 3 -WC, Al 2 O 3 -TiC 0.5 N 0.5 , and Al 2 O 3 -HfB 2 powders were prepared by the aluminothermic reduction of the oxides (i.e., TiO 2 , SiO 2 , WO 3 , or HfO 2 ) in the presence of C, B, and/or N. The effect of TiB 2 additions to the reactant mixture was studied during the synthesis of a titanium diboridealumina composite by thermite reactions. 4 In previous work at Georgia Tech, thermite synthesis involving oxidation-reduction reactions has been used to produce pure TiB 2 and an Al 2 O 3 -TiB 2 composite. 5,6 The effect of aluminum particle size, powder compaction, and reactant preheating on the reaction behavior has been investigated for the Al -B 2 O 3 -TiO 2 system. 5 Logan et al. 6 studied particle-particle interactions during oxidation-reduction types of self-heating synthesis (SHS) reactions using laser exposure. The objective of this study was to determine the reaction path in the TiB 2 -Al 2 O 3 composite formation reaction using differential thermal analysis (DTA) and x-ray diffraction (XRD).The starting powders were aluminum (123-grade, Alcoa), amorphous boron oxide (Fisher Scientific), and titanium oxide (anatase, Fisher Scientific). Average particle sizes (Microtrac II laser particle size analyzer) were measured to be 15 mm for amorphous B 2 O 3 and 0.3 mm for TiO 2 . The particle size of Al was not measured, but the manufacturer's data showed that it was 15 -19 mm. The powders were weighed according to the stoichiometry of reactants to form anticipated products:and then mixed through extensive gentle stirring to ensure homogeneity. The re...

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