Production of α-Fe ₂O ₃ powder material with multilevel gradient porosity

Abstract

The paper studies methods for obtaining a multilevel gradient porous material by the layer-by-layer sintering of distributed α-Fe2O3 nanopowders and submicron powders. Nanopowders with an average particle size of 12 nm were obtained by the coprecipitation method, and submicron powders, which are hollow spheres, were obtained using the spray pyrolysis method. Powders were consolidated by sintering in a muffle furnace, hot pressing, and spark plasma sintering (SPS) at various temperatures, loads, and holding times. It was shown that muffle furnace sintering and hot pressing methods cannot provide a compact of enough strength due to the different activity of nanopowders and submicron powders. Powder materials were obtained by spark plasma sintering when holding at 700, 750, 800, and 900 °С for 3 min. It was found that a series of samples obtained by SPS at 750 °С has sufficient strength and open porosity of 20 % with a total porosity of 37 %. Rising temperature in this method leads to an increase in the particle size in the nanopowder volume to a micron size and partial destruction of hollow submicron spheres. It was found during the study that the phase composition of samples obtained is identical to the phase composition of initial powders. However, for a series of samples obtained by hot pressing and SPS in the nanopowder volume, there is a directed growth of crystals towards the highest electrical and thermal conductivity [001] along the punch axis. This is due to the temperature gradient between the powder volume and punches and the lowest value of the plane surface energy (110), which includes direction [001].