Mullite crystallization in zirconia incorporated aluminosilicate ceramics prepared from novel monophasic liquid precursor

This paper deals with the synthesis of a series of zirconoaluminosiloxane (ZAS) as monophasic liquid precursors in toluene for zirconia/mullite ceramics. Precursors are synthesized by the zirconium modification of aluminosiloxane. The effect of zirconium incorporation in the aluminosilicate network is studied by FTIR, NMR, and TG/DTA analysis. 29Si and 27Al clearly indicates that zirconium is chemically bonded to the aluminosilicate network and the network connectivity increases with increase in zirconium content. 27Al NMR also reveals the progressive distortion of octahedral symmetry of AlO6 due to zirconium incorporation, validating the insertion of Zr through Al–O–Zr bonds where Al is four coordinated. These observations attest the monophasic nature and the chemical homogeneity of the synthesized precursors, which is the main factor contributing to the low temperature crystallization of ceramic phases. Ceramic conversion studies of the precursors confirmed the formation of crystalline zirconia/mullite ceramic phases at a low temperature of 900 °C. The XRD and microstructure analysis by SEM/EDX indicated that the mullite crystallization can be controlled by zirconia incorporation. This effect is discussed in terms of the occupancy of zirconia either as intra-granular or inter-granular particles. The zirconia concentration for maximum crystallization of mullite is optimized.