engleski

Crystallization kinetics of sol-gel derived mullite-gahnite composite

Mullite (Al2 VI(Al2+2x IVSi2-2x)O10-x ; 0.18<x<0.88) is economically important material for high temperature and advanced ceramic applications. Incorporation of second phase particles, among which also gahnite (ZnAl2O4), is known to improve mechanical properties of mullite-based ceramics. The usual method for preparation of ceramic composites is based on solid state reactions of raw precursors constituents. Recently, sol–gel method was employed in the preparation of such composites. The advantages of such process are the achievement of excellent dispersion of the second phase particles as well as possibility of sintering at lower temperatures. In order to prepare mullite/gahnite composite, diphasic premullite gel with composition of 3:2 mullite (3Al2O3·2SiO2) doped with 9 mol % of zinc was prepared. Dried powder was calcined at 700 °C for 2 h to remove the precursor residuals. Upon annealing at higher temperatures, system yields with mullite and gahnite. The course of crystallization can significantly direct the microstructure in the mullitegahnite system. As microstructure of this system is crucial for presumed beneficial mechanical functionality of this composite, first a detailed insight in occurring crystallization processes was necessary. Thus a kinetic study of the crystallization processes was performed in nonisothermal conditions (heating rates 5-20 °C min−1) using differential thermal analysis (DTA). Several characteristic, but mutually overlapping exothermic effects could be observed on the DTA curves and they were assigned to crystal phases using X-ray diffraction (XRD). Those are ; at temperatures ~900 °C gahnite and Al-Si spinel (both with spinel structure), and at temperatures ~1200 °C two types of mullite. Gahnite crystallize at lowest temperature, followed by Al-Si spinel (γ-alumina with certain amount of Si in the structure), leaving some residual amorphous SiO2. At higher temperatures Al-Si spinel transforms to mullite rich in alumina (mullite I). This process yields with release of excess alumina which can readily react with Si-rich amorphous phase to from mullite with composition close to stoichiometric 3:2 ratio (mullite II). Crystallization effects on DTA curves were deconvoluted using profile fitting to the function derived from Johnson-Mehl- Avrami equation. A Kissinger equation was successfully applied in evaluation of the activation energies of obtained phases.

Mullite; gahnite; composite; crystallization kinetics

nije evidentirano