Vol 26, No 2 (2017)

Stability of combustion wave during its passage through a gaseous gap between two reactive layers was studied by mathematical modeling. The character of transient processes in the gap was explored as a function of thermophysical parameters of the layers, kinetic parameters, and gap width at a predominant role of radiative heat transfer. The passage of combustion wave through the gap was found to be accompanied by the attainment of superadiabatic temperatures within the transition zone. A critical temperature for burning failure at the interface was determined.

Influence of ignition conditions on the modes of unstable combustion of planar symmetrical green samples was numerically explored in terms of 2D theoretical model. Within the range of instability, ignition conditions were found to define the number of self-propagating hot spots and their trajectories. In case of uniform distribution of ignition temperature over the inner boundary of annular sample, the combustion front propagates in the form of expanding ring.

Aluminum oxynitride (γ-AlON) powders were prepared from low-caloric Al–Al₂O₃ mixtures by SHS under high pressure of nitrogen gas in the presence of heat-generating agent, Mg(ClO₄)₂, as a booster. The preparation procedure was characterized and optimized with respect to Al/Al₂O₃ ratio, amount of heatgenerating additive, and nitrogen gas pressure. The morphology of synthesized γ-AlON powders was found to depend on Al powder brand. Compared to γ-AlON powders obtained in chemical furnace, we managed to prepare less sintered arenaceous cakes that could be easily disintegrated into target γ-AlON powder. SHSproduced powders may find their application in production of optical ceramics.

Explored was the influence of compacting pressure (P) and green density (ρ) on the properties of Zr-doped mineral-like pyrochlore ceramics Y₂(Ti_{1 – x}Zr_x)₂O₇ (x ≤ 0.3) prepared by SHS method. The optimal ρ values that provide minimal porosity and maximal mechanical strength of synthesized ceramics were found. An increase in ρ was found to decrease combustion temperature and increase pyrochlore lattice parameter a. Green density was also found to affect phase composition of the SHS-produced ceramics under study.

Overviewed are existing synthetic procedures for preparation of powders used as raw materials in fabricating optical polycrystalline ceramics such as aluminum oxynitride 9Al₂O₃ · 5AlN (AlON), yttrium aluminum garnet 5Al₂O₃ · 3Y₂O₃ (YAG), and magnesium aluminate spinel Al₂O₃ · MgO (MAS). Most promising for practical implementation seems to be solution-combustion synthesis from dehydrated gels containing appropriate inorganic salts and organic fuels.