Effect of the Heating Rate and Processing Time on Grain Growth of Hematite Nanopowders in Conventional and Microwave-Assisted Sintering

Microwave-assisted sintering of ceramic materials has proven to be a very favorable processing technique that can promote lower grain growth and densification with shorter dwell times. Hematite is considered a good microwave absorber due to its high loss tangent value, calculated in the range of 0.001 to 0.2 between room temperature and 750 ºC for 2.45 GHz¹, thus showing good interaction with this electromagnetic radiation. However, there are few studies on grain growth kinetics of hematite during microwave sintering, as well as the relationship with grain growth parameters. Therefore, the aim of this work was to evaluate the effect of the heating rate and dwell time on grain growth kinetics, during microwave-assisted sintering (2.45 GHz) of hematite nanopowders. In an initial characterization, dilatometry tests were performed by conventional heating and microwave-assisted heating using the same heating rate (20 °C/min). From these results, the temperature ranges of the initial and intermediate stages of sintering and the onset of linear shrinkage were determined. Considering these results, the samples were sintered in a conventional oven from 750 ºC to 1200 ºC with increments of 50 ºC, varying the dwell time in 6, 12 and 36 minutes. Thus, the diffusional mechanism (N) could be calculated, a value used for the approximate calculation of microwave sintering kinetics. Additionally, sintering was performed in a microwave oven using three heating rates (20, 30 and 50 °C/min) to evaluate the effect of the heating rate on grain growth. The estimated activation energies for the grain growth process during microwave sintering were approximately 237.5 to 272.3kJ/mol, which were higher compared to conventional sintering, in the range of 206.0 to 242.0 kJ /mol. It was found that with increasing microwave heating rate, the activation energy for grain growth tends to be higher.

Keywords:
Microwave sintering kinetics; Hematite nanopowders; Grain growth kinetics