The effect of separating layer between reacting media and molding template on the porous structure of combustion synthesized Ni-Al intermetallics

•Porous intermetallics comprised of mm-sized elements were combustion synthesized.•The influence of boundary conditions on the porous structure was investigated.•Effect of separating layers covering the surface of molding template was considered.•The average size of spheroidal elements increases when using cellulose-based layers.•Carbon released by cellulose pyrolysis intensify the coalescence of Ni-Al melt. This research focuses on identifying conditions under which porous intermetallics with a welded granular structure comprised of mm-sized spheroidal alloy elements can be directly obtained by combustion synthesis using μm-sized metal powders as starting reagents. The heat loss of reacting medium to a molding template used to shape a powder mixture of reagents significantly reduce the size of the alloy elements in manufactured material, especially near the mixture/template interface. One possible solution to this problem is to employ a separating layer between the powder mixture and the molding template. The effect of separating layers, both chemically inert (ceramic-based) and active (cellulose-based), on the size of Ni-Al elements has been experimentally studied. It was found that notable structure modification can be achieved by using cellulose-based separating layers. The carbon-containing products released during cellulose pyrolysis interact with the reacting medium and promote the coalescence of metal melts. Changing the thickness of the cellulose-based separating layer is an easy way to control the porous structure and permeation properties of combustion synthesized Ni-Al-based intermetallics, which makes them relevant for practical application.