Combustion synthesis of porous oxynitride materials under conditions of forced filtration of reacting gas

• Combustion synthesis of sialon in forced filtration mode was investigated.
• The procedure for synthesis of sialon with macroporous structure was suggested.
• Coflowing filtration leads to the creation of super-adiabatic synthesis conditions.
• Removing the adsorbed gases from the reaction zone plays significant role.

Combustion synthesis is a promising technique for producing the oxynitride ceramics, which represent a new class of materials with high resistance to high temperatures and to intensive wear. The different experimental ways of control of combustion parameters were considered, such as sample porosity, porosity structure of samples, different experimental configurations (coflowing and opposite flow systems), and pressure. Kinetic features of nitrogenation of reacting samples (aluminium, silicon oxide and sialon powders) were analyzed at different conditions. Forced nitrogen filtration through porous sample substantially improved the process of combustion synthesis (yield with higher nitrogen content, porous structure). The application of coflowing configuration intensifies the combustion synthesis and provides the re-heating process by convective heat transfer from combustion products to the initial mixture. It was demonstrated that this process leads to super-adiabatic temperature rise in the combustion zone. Coflowing configuration allow one to synthesize porous sialon with the conversion ratio up to 0.995 with combustion rate 0.8 mm/s and maximum combustion temperature about 1900 °C. The parameters of porous samples were varied in a wide range: sample porosity: 40–75%, the average sizes of the skeleton elements – from 240 μm to 3.5 mm; average pore-channel size 10–390 µm; percentage of closed porosity 16–30%.