Efeito da adição de MgO na síntese de mulita “in situ”

Abstract

The mullite is the only crystalline phase formed in the SiO2-Al2O3 system, being a refractory material with high melting point, excelente resistance to termal shock, electrical resistivity and low termal conductivity. It can be used traditional and advanced applications. High temperatures and long sintering times are required due to the high activation energy required to diffuse the ions in the mullite network. To optimize the process, power precursors with submicron grain size or using sintering additives. In this study, MgO was used to obtain the reactive sintering mullite from the mixture of Al(OH)3 and coloidal SiO2, and the mixture of Al2O3 and coloidal SiO2. To verify its effect on mulitization and sintering of mullite in these compositions, 0,8; 1,6; 2,4; 3,1 and 3,9 wt% MgO were added. The compositions were sintered at 1350, 1450 and 1550 ºC for 4 hours. Initially the compostions were analyzed by dilatometry and differential termal and thermogravimetric analysis. After sintering, they were analyzed by X ray diffraction, X ray fluorescence, apparent density and porosity measurements, scanning eléctron microscopy, three-point flexural strength and impedance spectroscopy. From the obtained results, it was observed that the addition of MgO reduced the mulitization temperature and the sintering temperature of the compositions studied, from 1400 ° C to around 1350 ° C. In addition, it has been observed that the higher the MgO contente and the sintering temperature, the larger and more anisotropic the mullite grains are formed and the greater their flexural strength. The composition M5 sintered at 1550 ºC obtained the highest resistance value of 28 MPa. There was a drastic reduction of porosity at 1450 and 1550 ºC, contrary to what happened at 1350 ºC, which can be observed in the SEM images and the values of apparent porosity. In which the composition M5 sintered at 1550 ºC obtained the lowest porosity value of 2%, and the composition M0 sintered at the same temperature obtained 13,1%. The composition MA1 sintered at 1450 ºC, obtained the highest value of resistance in the value of 65,2 MPa, due to the appearance of the glassy phase, which reinforced its microstructure. With the increase of the amount of MgO, there was formation of the spinel phase, together with the mullite, remaining residual α-alumina in some compositions. In the impedance spectroscopy test, only a semicircle was obtained in the Nyquist representation, associated with the contribution of only the interior of the grain, with a capacitance around 10-11-10-12 F. It was noted that the electrical conductivity of the material is dependent on the test temperature, that the higher the temperature the higher the conductivity. In addition, the appearance of phases such as spinel, alumina and vitreous phase favored the electrical resistivity of the material, considerably.