Temperature-dependent evolution of grain growth in mullite fibres

Mullite grain size characteristics of four different alumino silicate fibres heat-treated between 1400 and 1700 °C in a time frame of 0.5-100 h have been determined. Mullite grain sizes show little change up to 1500 °C though the grain microstructure develops from mosaic-type to facetted. Above 1500 °C grain growth kinetics follow the empirical law D1/n − D01/n = kt (D0 = initial grain size, D = grain size after annealing, 1/n = grain growth exponent, k = reaction constant, and t = firing time). Between 1500 and 1600 °C the grain growth exponent is remarkably low (1/n = 1/12) but above 1600 °C grain growth exponents reach normal values of ≈1/3. The vacancy drag model has been used to explain the complex grain growth behaviour of mullite.