Phase-transformation kinetics in triphasic cordierite gel
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S. Zec The Vincˇa Institute of Nuclear Sciences, Belgrade, Yugoslavia
S. Drmanic´ and Lj. Kostic´-Gvozdenovic´ Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade, Yugoslavia (Received 4 July 2000; accepted 2 November 2000)
A triphasic cordierite-type gel was prepared from silica sol, boehmite sol, and an aqueous solution of Mg(NO3)2 ⭈ 6H2O. The silica sol was obtained from water glass by the ion exchange method, while boehmite sol was obtained by peptization of freshly prepared Al(OH)3. Phase transformations occurring in the gel were studied by differential scanning calorimetry, x-ray diffractometry, and Fourier transform infrared spectrometry. Spinel was observed to crystallize from the gel prior to cristobalite; their reaction subsequently yielded ␣-cordierite. At higher temperatures, ␣-cordierite transformed into modulated -cordierite. Kinetic parameters of ␣-cordierite formation and ␣-cordierite to modulated -cordierite transformation were determined by differential scanning calorimetry under nonisothermal conditions. Formation of ␣-cordierite was found to be a diffusion-controlled process, with an overall activation energy of Ea ⳱ 1242 ± 66 kJ/mol. During the ␣ → -cordierite transformation, a modulated phase was formed by surface transformation of ␣-cordierite. The overall activation energy for the formation of the modulated structure is Ea ⳱ 583 ± 77 kJ/mol. I. INTRODUCTION
Cordierite ceramics have been a subject of numerous investigations carried out in recent decades due to their good properties such as low thermal expansion coefficient and dielectric constant and high chemical and thermal stability. 1,2 Traditional methods of cordierite synthesis by the solid-state reaction are applied to obtain electroporcelain and cordierite ceramics for the refractory industry.3 However, in this way, it is not possible to produce dense cordierite ceramics without sintering aids4 because of the narrow sintering range near the incongruent melting point (1460 °C) of cordierite. These aids, on the other hand, increase the thermal expansion coefficients and dielectric constant. For that reason, most of the studies on cordierite ceramics made recently have focused on the synthesis of fine, homogeneous, and reactive powders,5–7 which crystallize and can be sintered at lower temperatures without sintering aids. It is well known that alkoxy-derived gel powders are very fine and reactive.8–14 However, in multicomponent systems, such as cordierite, it is very difficult to determine the optimum conditions for alkoxide hydrolysis and condensation, the reason why inhomogeneity in the gels and the resulting powders occurs. Gel-derived compacts often suffer from the commencement of crystallization before densification is nearly completed, leaving large J. Mater. Res., Vol. 16, No. 2, Feb 2001
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pores unremoved. This problem is most frequently encountered in the densification of multicomponent silicacontaining gels that undergo viscous sinterin
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