B. Sh. Braverman

Saffman–Taylor problem in filtration combustion

In the hydrodynamic approximation, postulating the combustion zone thickness to be much smaller than the channel width, the problem of filtration combustion (FC) has the same spectrum of solutions as the well-known Saffaman-Taylor (ST) problem of the displacement of a high-viscosity liquid by a low-viscosity one. In both cases, the spectrum of solutions includes families of symmetrical and asymmetrical fingers. In the ST problem, asymmetrical fingers are considered unrealizable because of the effect of surface tension at the interface between the liquids, which suppresses the formation of asymmetrical fingers. For FC, there is no exclusion of asymmetrical solutions, since the interphase boundary is a reaction front with zero surface tension. In the present work, it is experimentally demonstrated that an increase in the channel width above a critical value, a planar FC wave loses its stability and the combustion front propagates along one of the channel walls in the form of an asymmetrical finger.

Possibility of nitriding industrial ferroalloys in a nitrogen-containing gas flow

Combustion of ferroalloy powders in a nitrogen-containing gas flow produces nitrogencontaining ligatures in the form of separate unbonded granules at a pressure close to atmospheric pressure.

Combustion of Porous Specimens Under Non-One-Dimensional Filtration

An experimental method for studying the combustion of porous specimens under non-one-dimensional filtration of a gaseous reagent is proposed. Propagation of a surface-combustion moving normally to the gas-permeable lateral surface of a specimen was observed in situ for the first time. When the surface fronts meet at the center of the specimen, the temperature increases, which explains previous experimental results. Key words: surface combustion, non-one-dimensional filtration, metal powders, superadiabatic heating, chromium nitrides.

Nonthermal Nature of Unsteady Combustion of Chromium in Nitrogen

When specimens pressed from a chromium powder react with nitrogen at pressures of 1–8 MPa and relative densities of 0.47–0.55, unsteady combustion of a nonthermal nature, which was not known previously, is observed. It is shown that the unsteadiness is due to cracks formed ahead of the combustion front. The specimens fail as a result of an increase in the condensed-phase volume upon formation of nitrides.

Chromium combustion in nitrogen

Results of investigation of chromium combustion in nitrogen are presented. It is shown that the combustion proceeds in the solid state. In the combustion wave, chromium reacts with nitrogen in steps. The first step—the formation of Cr2N—proceeds in the separation regime. The maximum combustion temperature is limited by the temperature of dissociation of CrN. Afterburning is observed behind the combustion front.

Nitride-based materials SHS-produced from ferroalloys: II. Silicon nitride as a support for Ag-Containing catalysts for partial oxidation of ethylene glycol

Ag-containing catalytic systems immobilized on SHS-produced and acid-leached Si3N4 supports showed good service parameters in partial oxidation of ethylene glycol to glyoxal. Compared to similar catalytic systems on aluminosilicate supports, our catalysts exhibit more uniform distribution of active centers over support surface, narrow size distribution of Ag particles, higher chemical resistance, and lower carbon deposition.

Combustion of TiAl alloy in nitrogen

It has been shown that Ti–Al–N ternary compounds, belonging to MAX phases (ceramic materials that can be processed as metals) can be produced by combustion of a granular powder of T65Yu35 (TiAl) alloy in nitrogen flow at a pressure close to atmospheric pressure. Combustion is accompanied by transfer of part of the aluminum through the gas phase. The propagation velocity of the combustion zone and the maximum temperature increase with increasing flow rate.

On the possibility of obtaining refractory metal nitrides in a calciothermic process

The possibility of obtaining refractory titanium nitride TiN in a calciothermic process is shown. The dependence of the combustion rate and temperature on the nitrogen pressure is studied. The phenomenon of super-adiabatic heating during combustion of the Ca-TiO2-N2 system is found.

Superadiabatic heating in combustion chromium in nitrogen

Combustion of chromium powder in SHS reactors is examined. It is shown that the maximum temperature in the central part of the powder samples is higher than the adiabatic temperature. The observed phenomenon is due to the non-one-dimensionality of the combustion front. Overheating of the central part leads to melting of the product and a decrease in the degree of conversion, which should be taken into account in nitration of samples of large diameter.

Calciothermic reduction of titanium dioxide and molybdenum trioxide under pressure of nitrogen gas

Explored was the phenomenology of calciothermic reduction in model green mixtures containing metal oxides with strong (TiO2) and weak (MoO3) Me–O bonds. In case of high affinity metal (Ti), the process occurs in two stages: (1) combustion reaction of Ca with N2 yielding calcium nitride and (2) calciothermic reduction of TiO2 to give TiN. In case of low affinity metal (Mo), the above reactions proceed in parallel, without noticeable time delay.