Crystallization kinetics and phase transformation characteristics in seeded monophasic cordierite gel
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Crystallization kinetics and phase transformation characteristics of (i- or a-cordierite seeded gels were compared with those of unseeded monophasic gels. The a-cordierite seeding modified the sequence of phase transformation and lowered the temperature of a-cordierite formation from amorphous gel by ^150 °C. The /x-seeded transformation was characterized by 3-dimensional isotropic growth with essentially zero activation energy for the nucleation step. Combining the nucleation theory with the transition state theory, we have separately estimated the activation free energy of nucleation and that of growth step in the crystallization of amorphous cordierite gel. The estimated activation free energies showed that the crystallization of unseeded cordierite gel is mainly growth controlled.
I. INTRODUCTION
II. EXPERIMENTAL
It is well known that many transformations in ceramics proceed by a nucleation and growth process. In a renowned work on the transformation of boehmite gel to a - A l 2 O 3 , Messing and his co-workers1'2 demonstrated that the epitaxial seeding significantly lowers the transformation temperature of boehmite gel and enhances the kinetics of the boehmite to a - A l 2 O 3 transformation by effectively eliminating the nucleation step. Recently, Roy and his co-workers3 applied the idea of epitaxial seeding to cordierite gel and found that the a-cordierite seeding lowers the crystallization temperature of a-cordierite by 125-150 °C. However, they focused mainly on the enhanced densification of cordierite through the nanocomposite (multiphasic) sol-gel processing4"6 and on the effect of a-cordierite seeding on the lowering of crystallization temperature of a-cordierite. In view of this, the main purpose of the present study was to examine systematically the effects of seeding (both [i- and a-cordierite) on the crystallization kinetics and phase transformation characteristics of amorphous cordierite prepared by the monophasic gel route. Combining the nucleation theory with the transition state theory for the rate process, we could separately estimate the activation free energy of nucleation for the critical size and that of the growth step in the crystallization of amorphous cordierite gel. This approach can be, in principle, applied to other systems that undergo a polymorphic phase transformation.
A monophasic cordierite gel was prepared from a stoichiometric mixture of TEOS (Si(OC2H5)4), A1(NO3)3 • 9H2O and Mg(NO 3 ) 2 • 6H 2 O in ethanol. In this procedure, aluminum nitrate and magnesium nitrate were separately dissolved in ethanol. The solutions were then combined, and to this mixture an appropriate amount of TEOS was added while stirring. The clear solution was allowed to gel in a 60 °C water bath, and then the gel was dried in a 100 °C oven. For the preparation of a seeded gel, either /i- or a-cordierite seed particles (2 wt. %) were added to a solution of aluminum nitrate and magnesium nitrate before TEOS was added. The /Li-cordierite seed was prepared by the monophasic sol-gel route described above
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