Efeito do tamanho e distribuição de tamanho inicial das partículas da hematita, na cinética de sinterização assistida por micro-ondas

Abstract

This thesis aimed to study the effect of the initial particle size and its distribution of hematite on microwave-assisted sintering kinetics. Two synthesis methods were used: sol-gel and seeded-mediated growth. By sol-gel the average particle size ranged from 30 to 1500 nm, and possessed large distribution, and, by seeded growth, from 70 to 200 nm and had a narrower distribution of particles. Thus, the shrinkage of the samples was analyzed using conventional, hybrid microwave at 2.45 GHz (to 30 nm nanopowders) and microwave assisted at 30 GHz dilatometry trials with heating rates of 5 to 20 oC/min. For the samples at 30 GHz, the in situ electrical resistance was measured using the four-wire method.

For kinetics study, the non-isothermal methods were adopted, from Woolfrey- Bannister and from Dorn to the initial stage and from Wang-Raj to the

intermediate stage of sintering. The effect of the initial particle size was observed, as well its distribution, showing striking differences in the microwave at 30 GHz sintering kinetics, compared with the conventional sintering. The smaller the average particle size, the higher the densification rate, accelerating the sintering of hematite. Lower activation energy values are needed for both stages, besides the temperatures of onset of shrinkage, ranges of initial and intermediate stages and activation energies were lower, when compared to conventional heating as well as with the 30 nm nanopowders, sintered at 2.45 GHz. A narrower distribution promoted a lower densification rate. As the heating rate increased, smaller grain size and relative density values were obtained using microwave energy at 30 GHz. The sintered samples exhibited weak ferromagnetic behavior.