Effect of Particle Size and Sintering Temperature on the Formation of Mullite from Kyanite and Aluminum Mixtures

Abstract

The effect of particle size and sintering temperature of the mixtures of kyanite and metallic aluminum related to the thermal transformation of kyanite into primary mullite and free silica was studied. In addition, the reaction between α-Al2O3 (in situ produced by aluminum oxidation) and the silica was obtained in cristobalite structure from kyanite to obtain secondary mullite. The kyanite powders were milled by 0.5, 3, 6, and 12 hours and then were mixed with aluminum powder, which were previously milled by 3 hours. After that, the powders were characterized by X-ray diffraction technique (XRD), scanning electronic microscopy (SEM), differential thermal analysis (DTA), and thermogravimetric analysis (TGA), and the particle size was determined in a centrifugal analyzer particle size Shimadzu model SA-CP4. The mixed powders were pressed uniaxially into cylindrical samples (compacts), and then sintering was conducted at 1100, 1200, 1300, 1400, 1500, and 1600°C; these samples were characterized by XRD, SEM, and thermodilatometry analysis (TD); density and open porosity measurements were performed by the Archimedes method. The samples were thermally etched to observe the microstructure, which consisted of mullite equiaxial grains contained in a glassy phase. It was observed that the nonmilled kyanite mineral becomes into mullite plus silica at temperatures between 1400 and 1500°C. When the particle size was reduced at sizes less than 1 µm, the transformation temperature was low until 200°C; the X-ray patterns of the sintered samples at 1400°C, ground for 6 hours, showed mullite peaks with small reflections of cristobalite and α-Al2O3, and these samples exhibited high density and low open porosity.

Electrical resistivity and flexural strength of mullite‐bonded porous SiC: Effect of transition metal additives

“…In kaolin‐added porous SiC, the mechanical strength of the sample increased due to the formation of cristobalite and mullite, which are the transformation products of kaolin 15,41 . Kaolin starts to change into mullite above 1000°C, 20 and this transformation intensifies with increasing temperature (1100–1600°C) 15,42 . Das et al 41 .…”

“…15,41 Kaolin starts to change into mullite above 1000 • C, 20 and this transformation intensifies with increasing temperature (1100-1600 • C). 15,42 Das et al 41 also reported an enhanced amount of mullite formation by increasing the kaolin content in the samples. The XRD results of the present study clearly indicate the formation of mullite and cristobalite at 1300 • C, whereas no such phase was detected at 1100 • C. This finding reveals that a temperature of 1100 • C was insufficient for the formation of cristobalite and mullite, which was also obvious from the mechanical strength results (Figure 14A).…”

Effect of particle size of associated rare earth kaolin powder on the sintering and properties of synthetic mullite

“…Size reduction in the feedstock is very effective in reducing the firing temperature for powder synthesis, and reducing the powder size increases the surface activity and the number of nucleation sites, shortening the diffusion distance and improving the degree of homogeneity. 11 Hernández et al 12 studied the effect of kyanite powder particle size and sintering temperature on the synthesis of mullite. The results show that the transformation and growth rate of mullite depend on the particle size and specific surface area, and the reduction of particle size affects the thermal decomposition of kyanite, thus causing expansion as well as secondary mullite formation reactions and increasing shrinkage, while grinding significantly increases the reactivity of kyanite.…”

“…Size reduction in the feedstock is very effective in reducing the firing temperature for powder synthesis, and reducing the powder size increases the surface activity and the number of nucleation sites, shortening the diffusion distance and improving the degree of homogeneity 11 . Hernández et al 12 . studied the effect of kyanite powder particle size and sintering temperature on the synthesis of mullite.…”