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Mullitekordierite composite powders containing different proportions of cordierite were prepared by the sol-gel method using boehmite, colloidal silica, and Mg(NO& -6Hz0. Mullite and cordierite sols were prepared separately and mixed to form the composite sol. Mullitization temperature depends on the cordierite content in the composite. Also, ecordierite crystallizes at a lower temperature in a mullite-rich (MC20) composite. The XRD patterns of the powders calcined at 1450°C for 12 h showed that mullite and cordierite exist as two different phases, and no additional phases were observed. The IR absorbance spectra of composites showed characteristic peaks corresponding to both mullite and cordierite. The sintered density of the powders increases with temperature up to 1450°C and decreases beyond the melting point of cordierite (1455°C). The microstructure of MC30 sintered at 1440°C for 3 h consisted of acicular grains, whereas in MC40 and MC50 equiaxed grain morphology was observed under similar sintering conditions. The flexural strength and Vickers hardness decreases with the increase of cordierite content in the composite. Dielectric constant and thermal expansion showed a similar behavior. [

002ChemInform Abstract Mullite (3 Al2O3•2 SiO2)/cordierite (2 MgO•2 Al2O3•5 SiO2) composite powders containing up to 60 wt.% cordierite (M/C 60) are prepared by the sol-gel process using boehmite, colloidal silica, and Mg(NO3)2 as starting materials. For M/C 10, crystallization of mullite and cordierite is not observed (formation of a MgO-mullite solid solution), while these phases are detected as the only phases present in M/C 40 composites sintered at 1450 rc C. The sintered density of all composites increases with increasing temp. up to 1450 rc C and then decreases. The flexural strength decreases with increasing cordierite content. The room temp. flexural strength of M/C 40 is 300 MPa and is maintained up to 1200 rc C. A thermal shock resistance of 375 rc C is observed for M/C 40 and, therefore, this material may be useful for engineering applications that require high thermal shock resistance and low thermal expansion.

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