Sol-gel processing of cordierite: Effect of seeding and optimization of heat treatment
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Three series of cordierite powders were prepared by the sol-gel route: a single phase (monophasic) gel prepared from alkoxides, a nominally triphasic nanocomposite gel made with two nanosized powders and one solution phase, and a truly compositionally triphasic nanocomposite gel prepared from three nanosized powders. Crystalline a-cordierite seeds were also incorporated with the gels and their effectiveness as nucleating agents was investigated and found to lower the crystallization temperature of a-cordierite by 125-150 °C. The densification behavior of powder compacts was examined and alterations made to the heat treatment until optimum conditions were found. The truly triphasic compact sintered at 1300 °C for 2 h resulted in 100% of theoretical density whereas the nominally triphasic and monophasic pellets densified to 96% and 80%, respectively. The enhanced densification achieved with powder compacts prepared from triphasic nanocomposite gels is due in part to the excess free energy of the three components.
I. INTRODUCTION
Cordierite as a ceramic material has a wide range of uses and applications stemming from its important properties of low thermal expansion and dielectric constant coupled with high chemical and thermal stability. For example, extruded cordierite honeycomb substrates coated with catalysts have become a vital part of the catalytic converter system in automobiles withstanding severe temperature and chemical environments.1"3 Other severe conditions where cordierite ceramics are employed include interior walls of high temperature furnaces and industrial heat exchangers for gas turbine engines.3"5 Research is currently underway to develop cordierite as an alternative substrate to alumina for electronic applications.6"12 Preparing dense cordierite glass-ceramics has long been a problem because of the narrow sintering range near cordierite's incongruent melting point12"14 and because of water molecules in the cavities of the cordierite structure which vaporize near the melting point, leaving cracks and pores within the sintered body.612 Low densities lead to inferior mechanical properties, and consequently extensive research has been conducted to find ways of circumventing the problem. One answer was found with the use of sintering aids such as K2O615 or TiO2,16"20 which increased the density from less than 90% to 98% of theoretical. The drawback, however, was that these nucleating agents not only lowered the crystallization temperature and increased density but also increased the coefficient of thermal expansion.1-2'5'21 Other researchers focused on altering the J. Mater. Res., Vol. 5, No. 5, May 1990
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synthesis techniques—specifically, utilizing a solution sol-gel (SSG) process5'9"12'22"25 which was developed by Roy and co-workers in the early fifties26"29 and has proven to be a breakthrough in ceramic processing. This technique allowed the preparation of fine reactive powders which can be crystallized and sintered at lower temperatures with
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