Study of the mechanism of andalusite-Al°-N2 reaction using the combination of DTA-TG-DTG techniques

Abstract

Alumina based ceramics have important engineering applications due to their good physicochemical properties but their performance may be enhanced by addition of a second phase. Aluminosilicates as andalusite submitted to aluminium reduction and the simultaneous nitriding (N2 atmosphere) leads to the obtention of alumina and phases of the Si-Al-O-N system. These phases are of great interest for structural ceramics. Mixes prepared with andalusite and Al° (different content) were treated with N2 up to 1550 °C., using a heating rate of 5 °/min. Thermal effects (DTA) and weight changes (TG, DTG) of nitriding reactions during heating were evaluated. The reaction mechanism was determined using DTA-TG-DTG techniques. Results indicate that Si° and AlN are important intermediate phases during reactions. The final products were a-Al2O3 with sialons, mainly b’ phase and polytype sialons. The final formed phases depend on the Al° content and can be predicted using the Si-Al-O-N phase diagram.

Characterization of a chiastolite-type andalusite: structure and physicochemical properties related to mullite transformation

“…Wala et al [6] identified graphite in the mineral composition of andalusite from South Africa and France, thus supporting the above mass spectrometric results. The followed insignificant weight loss (1200 °C-1600 °C) with no gas released was ascribed to the structural reorganization process of andalusite, leading to the very gradual crystallization of mullite (equation ( 2)) [30].…”

“…This behavior can be explained by the existence of melting elements (impurities), particularly because of the collapse of the mica-muscovite (KAl 2 Si 3 AlO 10 (OH) 2 ) network (figure 9(a)) [31,32]. However, the curve stabilizes between 1300 and 1500 °C (D 4 ) owing to the mullitization process, where the Al/Si site reorganization reactions yield 3:2-mullite (3SiO 2 •2Al 2 O 3 ) and amorphous silica (SiO 2 ) according to equation (2) [30]. The significant shrinkage (D 5 ) occurring between 1500 °C and 1600 °C can be related to the sufficiently low viscosity of a probably slight amount of amorphous phase (silica rich owing to the decomposition of andalusite and melting with initial impurities) [27,28].…”

Combination of Mechanical Activation and Silicothermal Reduction and Nitridation Process to Form X-Sialon by Using Andalusite Precursor

“…[387]/113 [2] and aluminothermic reduction and nitridation [18,19] processes are two mechanisms which apply to synthesize siolons.…”