Influence of Ethylene Glycol on the Mullite Crystallization Processes Analyzed by Rietveld Refinement

• Autor(a) principal: Fernandes,Flaviano Willians
• Data de Publicação: 2013
• Outros Autores: Campos,Tiago Moreira Bastos, Cividanes,Luciana de Simone, Machado,João Paulo Barros, Simonetti,Evelyn Alves Nunes, Thim,Gilmar Patrocínio
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Journal of Aerospace Technology and Management (Online)
• Texto Completo: http://old.scielo.br/scielo.php?script=sci_arttext&pid=S2175-91462013000400431
• topic: Mullite; Sol-gel; Ethylene glycol; Rietveld refinement

ABSTRACT: Mullite is an excellent structural material due to its high temperature stability, high electrical insulation capabilities and creep resistance. This material has a number of technological applications, such as rocket nozzles used in the aerospace industry. In this work, mullite was obtained by sol-gel process, using silicic sol, aluminum nitrate and ethylene glycol, besides the following volume ratios of silica sol dispersion to ethylene glycol: 1/0; 1/1; 1/2; and 1/3. After drying, the samples were thermal treated at temperatures of 1,000; 1,100; 1,200 and 1,250°C. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and specific surface area (Bruner-Emmett-Teller - BET). SEM showed that mullite particles are fine and nearly equiaxed. The sample without ethylene glycol showed 3/2 mullite after heat treatment at 1,250°C. The sample with intermediate ethylene glycol concentration presented two crystallization processes: the first at 1,000°C forming mullite and spinel phases, and the second at 1,250°C forming only 3/2 mullite. However, the sample with the highest ethylene glycol concentration crystallized directly to mullite at 1,000°C with the highest yield. There is a strong dependence on the specific surface area with temperature. The Rietveld refinement showed that the a cell lattice of mullite and the Al/Si molar ratio in the mullite formula depend on the ethylene glycol presence and on the calcination temperature. The lattice parameters b and c are not dependent on the alumina content, but the parameter a increases with the increase in the alumina content. Samples prepared with higher ethylene glycol concentrations reached higher mullite yields at lower temperatures.