Sintering of Industrial Mullites in the Presence of Magnesia as a Sintering Aid

Abstract

The sinterability of two industrial mullite powders, in the presence of MgO as a sintering aid, was investigated. A glassy phase, which was generated during preparation, was present in both powders; this glassy phase had a strong influence on sintering, depending on its content, composition, and spatial distribution. MgO promoted sintering in the presence of a liquid phase, both in the as-received materials and in samples washed with HF, in which most of the pre-existing glassy phase was eliminated. Investigations using transmission electron microscopy, coupled with energy-dispersive spectroscopy, as well as dilatometric measurements and X-ray diffraction data, on washed and unwashed materials and on quenched and slow-cooled samples allowed a better understanding of the influence of MgO and the glassy phase on the sintering behavior and the formation of new phases. Most of the phases, in fact, can be explained by using the MgO-Al 2 O 3 -SiO 2 phase diagram, even in such complex systems.

Sinterização ultrarrápida por micro-ondas do compósito mulita-cordierita

“…Com esta desestabilização, o MgO, originário desta fase, começa a interagir com o SiO 2 , principalmente o originário desta mesma fase, formando uma fase líquida rica em SiO 2 contendo MgO a partir da faixa de temperatura de 1410 ºC a 1425 ºC [3,8,17]. A elevação da temperatura gera maior desestabilização, originando maior quantidade de MgO que, por sua vez, interage mais com o SiO 2 .…”

“…Ao exceder este ponto, tem início o mecanismo de sinterização com fase líquida, fator que proporciona uma maior densifi cação, devido à alteração da cinética do processo de sinterização, e uma melhora das propriedades mecânicas [7]. O MgO, presente na fase cordierita, aumenta a densifi cação da fase mulita [8] através da desestabilização da composição química da zona próxima ao contorno do grão de mulita, induzindo à mudança da composição 2Al 2 O 3 .SiO 2 para 3Al 2 O 3 .2SiO 2 ; o SiO 2 excedente forma uma fase líquida com o MgO na região do contorno do grão. Este processo de desestabilização seguido da formação de uma fase líquida de composição MgO-SiO 2 ocorre até o momento em que todo o MgO forme fase líquida, podendo chegar a uma confi guração na qual existe uma camada de alumina em volta do grão de mulita [7].…”

Evaluation of the Influence of MgO and La2O3 on the Fast Sintering of Mullite

“…The role of sintering additives has been attributed to the formation of liquid phase and to a reduction in viscosity of the glassy (or liquid) phase or to a reduction in mullite formation temperature in gel-derived powders, thereby leading to higher mobility of diffusing species [7][8][9][10][11] . Reports in the literature on studies involving mullite have focused on the nucleation of gel-derived mullites and doped mullites 5,7,[12][13][14][15][16] and on the conventional sintering of mullite compacts [17][18][19][20] . Few researches have investigated the fast sintering behavior of mullite bodies [21][22][23][24][25] , and even fewer studies 26,27 have sought to ascertain the influence of additives on the rapid sintering of mullite.…”

“…Few researches have investigated the fast sintering behavior of mullite bodies [21][22][23][24][25] , and even fewer studies 26,27 have sought to ascertain the influence of additives on the rapid sintering of mullite. In this context, some studies have observed benefits in the use of MgO 9,18,28 and L 2 O 3 10,29-31 as sintering aids in the densification, microstructure evolution and strength of sintered mullite bodies. Studies 17,18 on the sintering of industrial mullites in the presence of magnesia as a sintering aid attributed the enhanced densification of doped mullite bodies to the formation of liquid phase.…”

“…In this context, some studies have observed benefits in the use of MgO 9,18,28 and L 2 O 3 10,29-31 as sintering aids in the densification, microstructure evolution and strength of sintered mullite bodies. Studies 17,18 on the sintering of industrial mullites in the presence of magnesia as a sintering aid attributed the enhanced densification of doped mullite bodies to the formation of liquid phase. The amount of glassy phase significantly influenced the sinterability of mullite, which decreased when the amount of glassy phase was reduced.…”