Engineering Design and Scale-Up of Combustion Synthesis Reactors
Vladimir Hlavacek Principal Investigator
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Many exothermic noncatalytic solid-solid or solid-gas reactions liberate enough heat so that they can, after being ignited, self-propagate. These reactions, being ignited locally by an external energy source with short term service, may propagate throughout the sample at a certain rate and occur in a narrow zone which separates the fresh substances and reaction products. This process has many names: in the USSR it is referred to as self-propagating high temperature synthesis (SHS) and in the US as combustion synthesis of inorganic materials (CSIM). The objective in development of this process is to learn how to effectively utilize the heat release from the reaction. In ideal fulfillment of this objective, the synthesis reaction is thermally self-sustaining and the product is a very fine powder (which can be easily hot pressed or sintered) or a fully dense shape that does not need follow-on processing. The purpose of the PI's research is to apply chemical reaction engineering principles to the scale-up of reactors used for CSIM (the ultimate goal is the scale-up of CSIM technology to industrial scale). He has been working in this area for eight years and has already obtained qualitative and quantitative information on the dynamics of CSIM. In this project the plan is to follow the microscopic progress of CSIM reactions by observing them under a specially designed microscope. He plans to use these results to estimate kinetics factors such as activation energy, frequency factors, etc. Because heat transfer characteristics become more significant as the reactor size increases, he plans to measure heat transfer parameters, taking into account radiation also. The parameters evaluated from experimental investigations of kinetics and transport mechanisms will be utilized in numerical modelling of CSIM systems. New generation non-tradition models will be developed which will take into account the effects of film cracking, melting, evaporation and sintering. In order to understand the mechanism for simultaneous synthesis and densification, a thermoplastic stress analysis in monolithic bodies will be performed. The numerical simulation will help design an optimal experimental course by predicting the conditions under which the desired products can be synthesized. They will also help in visualizing the conditions which lead to loss of stability and the associated non-linear phenomena occurring in the CSIM system. The following systems will be investigated: 1. Synthesis of titanium carbide from titanium and carbon (solid-solid reaction with moderate expulsion absorbed gases). 2. Magnesiothermic reduction of titanium and boric oxides for synthesis of titanium diboride (solid-solid with violet gas expulsion). 3. Aluminothermic preparation of niobium from niobium oxide (solid-solid with melting). 4. Synthesis of titanium nitride from titanium and nitrogen (gas-solid at low pressures). 5. Synthesis of silicon nitride from silicon and nitrogen (gas-solid at high pressures). 6. Manufacture of high density tiles of titanium carbide (simultaneous synthesis and densification).