Synthesis behavior and grain morphology in mullite ceramics with precursor pH and sintering temperature
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Chang-Yong Jo High Temperature Materials Group, Korea Institute of Machinery and Materials, Changwon, Kyungnam 641-010, Korea
Jae-Ho Lee and Ungyu Paik Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Korea (Received 7 August 2002; accepted 10 October 2002)
The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite were studied using a stoichiometric mullite (3Al2O3 䡠 2SiO2) precursor sol. The mullite precursor sol was prepared by the dissolution of aluminum nitrate enneahydrate [Al(NO3)3 䡠 9H2O] into the mixture of silica sol. The precursor pH of the sols was controlled to the acidic (pH ≈ 1.5 to 2), intermediate (pH ≈ 4.5 to 5) and basic (pH ≈ 8.5 to 9) conditions. The gels dried from the synthesized aluminosilicate sols were formed into a disk shape under 20 MPa pressure; then the green bodies were sintered for 3 h in the temperature range of 1100–1600 °C. The synthesizing temperature of mullite phase was found to be above 1200 °C for pH ≈ 1.5 to 2, and above 1300 °C for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9. The grain morphology of the synthesized mullite was changed to a rod shape for pH ≈ 1.5 to 2, and granulate shape for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9 with increasing sintering temperature. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature. However, at higher pH, the precursor pH and sintering temperature did not affect the synthesis behavior and grain morphology.
I. INTRODUCTION
Mullite (3Al2O3 䡠 2SiO2) is attractive as a hightemperature structural material as well as semiconductor substrate due to its excellent high-temperature mechanical properties, good chemical and thermal stability, good dielectric properties, and low thermal expansion coefficient.1–7 Generally, mullite is produced by thermal decomposition of silimanite and kaoline. However, the product includes a certain amount of inclusions, which deteriorate mechanical and electrical properties. There have been many attempts to obtain high-purity mullite and/or to eliminate the amount of inclusions. During the last decade, precipitation and sol-gel methods, including fine sol and alkoxide ligands have been developed as homogeneous fine mullite powder manufacturing processes.5,8 Also, several results have been reported on the synthesis of monophasic or diphasic mullite from different precursors prepared by the precipitation a)
e-mail: [email protected] J. Mater. Res., Vol. 18, No. 1, Jan 2003
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and sol-gel methods.9–12 It is known that crystallization of the monophasic mullite occurs at about 980 °C,9,10 and that of diphasic mullite occurs above 1250 °C.9–12 The sol-gel method has advantages in mullite powder manufacturing, such as relatively low sintering temperature (980–1250 °C), high purity, and homogeneous products due to molecular-level mixing of starting materials, indicating that fine-scale mixing of alumina and silica reactants substa
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