Synthesis of Mullite Powders Through a Suspension Combustion Process

Abstract

This work reports on the synthesis of mullite powders by a suspension combustion process. Aluminum nitrate, as a source of Al, and a colloidal silica suspension, as a source of Si, are used as reagents, and urea serves as the fuel. This colloidal suspension allows a complete mixing of the reactants, thus promoting a core-shell reaction to produce the mullite. The powders obtained need to be treated at a high temperature to form the mullite. However, when ammonium nitrate is also added as a combustion aid, the mullite phase is directly obtained without any further treatment. A chemical reaction involving all these compounds is proposed, and the thermodynamic parameters are calculated in order to predict the temperature reached during the combustion process. The addition of ammonium nitrate reduces the temperature of the reaction by ca. 1501C. Once the conditions for the combustion reaction are optimized, the Al/Si ratio is modified in an attempt to obtain monophasic mullite. J ournal

Influência do Y2O3 na sinterização rápida de mulita comercial

“…na rede e contornos de grão do material [4][5][6], o que faz com que pesquisas que visem aumentar a densificação, intensificar a cinética de sinterização e/ou reduzir a temperatura de queima sejam de grande importância para se produzir corpos com elevada resistência e a um custo relativamente menor [7]. Nesse sentido, muito interesse tem sido devotado à análise da influência de aditivos na melhora da sinterabilidade de corpos mulíticos [8][9][10][11].…”

Solution combustion synthesis of α-Al2O3 using urea

“…This method has also been successfully applied to the production of alumina [9][10][11][12][13][14][15] and other Al-based compounds [16][17][18][19][20][21][22][23][24][25].…”

“…The SCS process is based on redox reactions between metal salts and reducing agents such as glycine [12,14,16,21] or urea [9][10][11][12]17,18,24,25]. Generally, an organic reducing agent should also be a complexing agent.…”

“…As a result, during the evaporation of the reaction solution, the gelation prevents the divided saltingout of the solution components and facilitates the formation of an easily flammable xerogel and the initiation of combustion or the solution combustion synthesis reactions. The combustion temperature can be calculated a priori from the available thermodynamic data for various reactants and products [17,18,25,26] according to the following equation:…”

“…However, a-Al 2 O 3 is formed after the combustion reaction of aluminium nitrate with urea [10][11][12]. But the presence in the reaction mixture combustion aids increasing gas production or ballast of a solid phase, reducing the actual temperature of the process and leads to the synthesis of y-Al 2 O 3 [17,18].…”