Recognition of Belousov-Zhabotinsky-type oscillations in autosynthetic micropyretic reactions

Abstract

Micropyretic synthesis is an autosynthesis route which requires minimal external energy. During synthesis, a high-temperature reaction front propagates in a uniform or unstable manner depending on the conditions chosen, thereby converting a reactant mix to the desired products accompanied by a large amount of entropy production. Sometimes banded microstructural features are noted to form (frozen into the final solid), often with a fixed periodicity. This banded structure is thought to be the signature of an oscillating wave front. In previous articles by us and others, banded structures and other residual instabilities have been thought to arise from variations in the combustion front velocity caused by a mismatch between the heat diffusion rate and the heat production rate from the product synthesis (i.e. from exceeding the critical Lewis number and related bifurcations). Such oscillations have been modeled in the past by a single overall reaction formulation and corresponding heat flow solution which we refer to as the extended Merzhanov model. Spiral fronts are also recognized in the literature. We believe now that banded structures could also arise from entropy generating, dissipative, Belousov-Zhabotinsky (BZ) type reactions. Such reactions are discussed in this article. The overall BZ reaction progresses in the form of a nonlinear oscillator with several intermediary steady-state sub-reactionscontained in a reaction volume. We propose sub-reaction formulations by which a BZ reaction may occur during micropyretic synthesis. Micropyretic synthesis can display both types of oscillations, i.e. the Lewis type and the BZ type. We discuss materials systems where the BZ oscillations are possible, namely, the Ti-B and Ni-Al alloy systems. A comparison with experiments shows that the micropyretic product-chemistry is adequately predicted for the first time by a BZ formulation in both the Ti-B and several Ni-Al alloy systems. Wherever feasible compositional data reported in the literature are compared with the new predictions. It appears invoking the BZ yields a better picture of the final products.