Mullite (3Al2O3-2SiO2) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. Thejexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phuse were observed along the grain boundaiy. ULLlTE (3A120,-2Si02) is one of the tensivel y studied crystalline phases in the AI2On-SiO2 binary system. Because of the growing interest in using mullite as a high-temperature structural material due to its high creep resistance,' low thermal expansion, and good thermal and chemical stability, different processes of preparationx-'] of high-purity mullite have been tried recently. Except for the mullite prepared by the process given in Ref. 10, others showed low mechanical strength. The main difficulty in the preparation of mullite is to get a homogeneous mixing of Al2O3 and Si02. In the present process the sol-gel technique was employed to get a homogeneous mixing of A120n and S O z . The mullite powder prepared was sintered to >98% of its theoretical density at a tem-CONTRIBUTING EDITOR-E. R. F u L w

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Na2Fe2Ti6Ol6 , monoclinic, C2/m, a = 12.267(6), b = 3.823 (5), c = 6.483 (3) A, fl = 107.16 (5) °, Z = 4, D x = 3.97 g cm -3. The crystalswere prepared by heating a mixture of Fe and TiO 2 in 10 M NaOH solution, sealed in a gold tube, at 650°C and 1000 atm for 21 days. Black platy crystals were obtained.The substance is isostructural with Na0.2TiO2, with mean M(1)-O, M(2)--O and Na-O distances of 1.993, 1.973 and 2.715 A respectively.

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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. [