Production of Powdered α-Fe2O3 with Multilevel Gradient Porosity

This article discusses methods for producing a material with gradient multilevel porosity by sintering layer-by-layer distributed α-Fe2O3 nanopowders and submicron powders. Nanopowders with an average particle size of 12 nm are obtained by coprecipitation and submicron powders in the form of hollow spheres are obtained by spray pyrolysis. The powders are consolidated by sintering in a muffle furnace, by hot pressing, and by spark plasma sintering (SPS) at various temperatures, loads, and holding times. It has been demonstrated that sintering in a muffle furnace and hot pressing do not allow one to obtain a compact sample of sufficient strength due to various activity of nano- and submicron powders. SPS produced powdered materials at holding temperatures of 700, 750, 800, and 900°C in 3 min. It has been established that a series of powders produced by SPS at 750°C is characterized by sufficient strength and an open porosity of 20% upon a total porosity of 37%. An increase in temperature in the frames of SPS leads to an increase in particle size in the bulk of nanopowders to the micron range and the partial destruction of hollow submicron spheres. While studying the phase composition of the produced samples, it has been revealed that it is identical to that of the initial powders. However, in the series of samples produced by hot pressing and SPS, the growth of crystals is observed in the bulk of nanopowders in the [001] direction of the highest electric and heat conductance along the punch axes. This is related with the temperature gradient between the bulk of nanopowders and punches and the lowest surface energy of plane (110), including the [001] direction.