MECHANISMS OF COMBUSTION AND STRUCTURE FORMATION IN SHS- SYSTEMS WITH PARTICIPATION OF TWO AND MORE CHEMICAL REACTIONS
- Prof. Evgeny LEVASHOV
- Prof. Evgeny LEVASHOV (National University of Science and Technology "MISiS")
The mechanisms of combustion and structure formation in perspective systems Ta-Ti-C, Mo-Si-B, Ta-Zr-C, Zr-Si-B, Ti-C-Ca3(PO4)2, Cr-Al-Si-B, Si-C-B were well characterized using a combination of various techniques including dynamic XRD, stop combustion front, SEM, TEM, Raman spectroscopy, etc.
It was established that gas transport reactions is to control the combustion in some cases. In system (100%–X)(Ti+0,5C) + X(Ta+C) with X= 10 and 30% an abrupt increase of Uc and Tc occurs as a result of the transfer from the splitting to merging mode, which is accompanied by an increase in heat release as a result of two parallel chemical reactions. In the case of X= 50%, dependences Tc(T0) and Uc(T0) are linear over an a wide range of T0.
The following processes are defined the SHS for Si-rich Mo-Si-B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo3Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi2 grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO3 gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Kinetics of the SHS process, stages of chemical transformations and structure formation of ceramic materials in the Cr-Al-Si-B system were investigated. The effect of green mixture composition and initial temperature on the combustion rate Uc and combustion temperature Tc, which reduce with increasing Al content, was studied. An increase in the initial temperature of the SHS process causes a linear increase of Uc and Tc in the range of T0 = 290–750 K. This is a fact that each composition is characterized by the similar combustion mechanism, when the stages of chemical reactions of product formation remain unchanged. However, an increase in T0 above 750 K, probably, may lead to exponential character of Uc growth. Furthermore, an increase in Al content increases the proportion of the Al–Si eutectic melt. The stages of chemical transformations and the mechanism of structure formation in the combustion wave were studied. Dependences Tc(T0) and Uc(T0) of mechanically activated (MA) Ta-Zr-C exothermic mixtures were determined. The self-heating phenomenon is observed in argon atmosphere at T0 > 380 K due to zirconium particles oxidation by adsorbed oxygen. ZrO2 was formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into ZrC. TaC was formed in the combustion zone, while the single-phase (Ta, Zr)C with the lattice parameter of 0.4479 nm was formed closer to the post-combustion zone. Kinetics and mechanism of SHS process, stages of chemical transformation and structure formation in systems Zr-Si-B, Ti-C-Ca3(PO4)2, Si-C-B were also discussed.