Studies on the Synthesis and Utilization of Fine Alumina Powder for the Fabrication of Sintered Mullite Ceramic

Abstract

The high demand and various applications of mullite in structural ceramic instigate the ceramic researchers to study in-depth for the development of mullite ceramic with improved quality and performance. Mullite is an excellent material for advanced electrical, optical, and structural ceramics applications. An extensive literature study shows that solid-state reaction and the chemical route can synthesize the dense mullite ceramic. In solid-state, kaolinite is used for mullite synthesis due to its occurrence in nature, lower cost and good availability. It is an excellent natural source of silica and alumina but needs extra alumina from outside to prepare stoichiometric mullite. The sintering of mullite is difficult due to the slow aluminium and silicon ion interdiffusion in the mullite lattice structure. Different sintering additives are tried to solve the issue. In the clay alumina system, most additives are added in a higher amount, and the densification is enhanced by the extensive liquid formation, resulting in poor strength. Incorporating fine alumina powder can also improve densification and mechanical strength by reducing the porosity with inter-particle voids fillings and faster sintering. The reactivity and particle size of the added α-Al2O3 can be modified by applying different synthesis routes. The approach in this research work is first to synthesize fine alumina powder using various chemical routes. Then the use of these synthesized alumina powder and calcined kaolinite for further processing of mullite ceramic. Some oxide additives are also tried here in nominal amounts to get a higher sintered density. The strengthening of bulk mullite ceramic using a rare earth oxide additive is attempted to obtain columnar interlocking mullite grains. Fine α-alumina powder is prepared following two different methods named sol-gel and combustion. The detailed characterization of synthesized α-alumina powder is carried out in each case and is added to kaolinite for further processing of mullite. A batch comprising kaolinite and available commercial boehmite is also prepared for comparison. A detailed study on adding these three different α-Al2O3 in kaolinite showed the effectiveness of the combustion synthesized alumina powder in producing sintered mullite ceramic with a theoretical density of 87%. Three different oxide additives, TiO2, MgO and La2O3, are chosen, and the effects on densification and strength development are studied. The maximum densification achieved for the clay- α-Al2O3 system in the presence of 3% MgO and 2% TiO2 reported earlier are 92% and 80% at 1600°C. However, in the present work, 1% MgO and 1% TiO2 individually is sufficient to improve the densification up to 94% at 1600°C. The use of reactive alumina here helps reduce the dose of additive, which removes the possibility of the formation of secondary phases like aluminium titanate and magnesium aluminate spinel. 3% La2O3 additive further enhances the densification up to 96% with improvement in flexural strength. The elongated mullite grains with interlocked structures are found in the micrographs. Therefore, using a combustion synthesized α-Al2O3 in kaolinite reduces the sintering additive's dose and assists in developing a sintered, low-cost, high-strength mullite ceramic.