Examinations on the critical and primary crystallite sizes during θ- to α-phase transformation of ultrafine alumina powders

Examination on the existence of the critical and primary crystallite size (d(c)(a) similar to 17 nm and d(p) = 50-55 nm) phenomena during theta- to alpha-phase transformation of the nanosized Al2O3 powder was designed and conducted using two theta-Al2O3 powders of different crystallite sizes. Due to the critical size, the size dependence of the transformation temperature, the rate of transformation, and the inevitable presence of residual theta-phase in the thermal-treated and re-treated 100%-transformed sample can be expected. Due to the two characteristic sizes and the formation of primary crystallites by the coalescence growth of the critical crystallites, the variation in mean alpha-size during the transformation can be predicted by means of the difference in the growth rate of the two theta-powder (different crystallite sizes) systems associated with proper thermal treatment techniques. Further, it is important to note that the presence of primary crystallites at various thermal-treated samples becomes possible.