Microwave sintering of nanometer and micrometer ferrite powders

Two ferrite powders, one nanometre and one micrometre-sized, have been processed into small pellets using two different green forming routes, viz. die pressing and slip casting. Prior to forming using slip casting the powders were dispersed into water using a range of surfactants and then wet ground in an attempt to reduce the presence of agglomerates in the as-received powder. The pellets were subsequently microwave sintered using a TE₁₀₂ single mode applicator operating at 2.45 GHz. The die pressed samples produced from the nanometre-sized powder displayed microstructures that were increasingly duplex in nature as the sintering temperature increased. This was believed to arise from the highly agglomerated nature of the as-received powder. When a wet grinding route was introduced followed by slip casting to prevent reagglomeration of the powder, the number of agglomerates was very significantly reduced and a largely submicron microstructure was achieved. Unfortunately, most of the slip cast pellets suffered from cracking during the sintering process indicating the presence of stresses in the green bodies produced. Nevertheless, the results achieved suggest that it is possible to microwave sinter nanometre powder compacts without suffering major grain growth provided the initial powder is not agglomerated.