Self-propagating combustion reactions

As a further approximation, it may be assumed drat dre elements conform to Neumann and Kopp s rule that dre heat capacities of compounds and elements are about 25Jgatom Consequently the htral temperature, 7j, which is achieved at the reaction front is given by 

Apart from simple compounds, composite materials may also be prepared by combustion synthesis. Thus a composite of TiB2 and MgO can be formed 

TiOa + B2O3 + 5Mg = TiBa + 5MgO and a composite of MgO and B4C can be prepared tlrrough the reaction 

As an example of the production of oxide systems, the ceramic superconductor YBa2Cu307 j has been prepared tlrrough the reaction 

The reaction mechanisms are quite complicated in these syntheses, the kinetics depending on inter-diffusion rates in neighbouring particles, the formation of transient liquid phases, and in some cases, the vapour transport of a reactant. The presence of the latter can be detected by dre retarding effect of increased pressure in an inert surrounding atmosphere. 

---

This method, based on a self-propagating combustion reaction [4], requires a low ignition temperature to induce a combustion reaction between the catalyst precursor salts and an organic matrix as fiael (e.g. urea). The final... 

Munir, Z. A., and Anselmi-Tamburini, U., Self-propagating exothermic reactions The synthesis of high-temperature materials by combustion. Mater. Sci Reports, 3,277 (1989). 

Z. A. Munir, U. Ansekni-Tamburini, Self-Propagating Exothermic Reactions The Synthesis of High-Temperature Materials by Combustion, Material Science Reports, A Review Journal 15 (1989), 277. 

Figure 1. Photographic sequence of self-propagating combustion synthesis reaction between Ti powder compact and gaseous nitrogen.

Another method of densification makes use of reaction hot-pressing or self-propagating combustion sintering of MC-B powder mixtures under pressure ... 

Self-propagating exothermic reactions the synthesis of high-temperature materials by combustion. Mater. Sci. 

For the low-temperature synthesis of LaA103, Taspinar and Tas (1997) proposed a self-propagating combustion synthesis from aqueous solution containing urea and the respective nitrate salts. A pure perovskite phase was obtained at 1023 K, which is one of the lowest temperatures reported for the powder s)mthesis of LaA103 so far. Han et al. (2006) used a combustion method for the s)mthesis of Eu " -activated YAIO3 and GdA103 by redox reactions between the respective metal nitrates and glycine in a preheated furnace at 773 K. 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

Combustion Synthesis. Besides solid-state reactions requiring heating even for several days at high temperatures to be completed, fast reactions are also known which give their products in minutes or seconds. Several nearly equivalent names are used for these reactions such as solid-state combustion, combustion synthesis, self-propagating (self-sustained) high-temperature synthesis. Solid state direct combinations and solid-state metathesis have been described. 

---