Methods for Large Scale Synthesis

In general, methods for the large-scale production of advanced materials by combustion synthesis consist of three main steps: (a) preparation of the green mixture, (b) high-temperature synthesis, and (c) post-synthesis treatment. A schematic diagram of these steps is presented in Fig. 4. The first step is similar to the procedures commonly used in powder metallurgy, where the reactant powders are dried (e.g., under vacuum at 80 to 100 °C), weighed into the appropriate amounts, and mixed (e.g., by ball mixing). For some applications, cold pressing of the green mixture is necessary, especially for the production of low-porosity or poreless materials. Typically, no plasticizer is used, and the density of the cold-pressed compacts varies from 40 to 80% of the theoretical density. The final procedure in sample preparation determines the type of product to be synthesized: a powder product results from loose powder reactants, while sintered products are yielded from cold-pressed compacts. Pressing the green mixture into special molds or machining pressed initial compacts yields complex-shaped articles.

Fig. 4 Generalized schematic diagram of combustion synthesis technologies

The main production technologies of combustion synthesis by the SHS mode are presented in the second block of Fig. 4. They may be classified into several major types: powder production and sintering, densification, casting, and coating.

The third main step of combustion synthesis technologies is post-synthesis treatment. Not all products require this step. Powder milling and sieving are used to yield powders with a desired particle size distribution. Annealing at elevated temperatures (800 to 1200 °C) removes residual thermal stress in brittle products. The synthesized materials and articles may also be machined into specified shapes and surface finish.

The VCS mode is used primarily for the synthesis of weakly exothermic systems. Various types of heaters, mostly commercially available furnaces but also spiral coil and foil heaters, are used to preheat the sample to the ignition point. To date, VCS materials have been produced only in laboratories.

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