Examinations on the Critical and Primary Crystallite Sizes during θ- to α-Phase Transformation of Ultrafine Alumina Powders

Abstract

Examination on the existence of the critical and primary crystallite size (d c r ∼ 17 nm and d p ) 50−55 nm) phenomena during θ-to r-phase transformation of the nanosized Al 2 O 3 powder was designed and conducted using two θ-Al 2 O 3 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 θ-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 r-size during the transformation can be predicted by means of the difference in the growth rate of the two θ-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.

Colloidal processing and sintering of nanosized transition aluminas

“…A typical example is the gamma alumina investigated in the present study [3]. Many attempts have been made to produce dense nanograined ceramics from transition aluminas, where both the phase transformations into the thermodynamically stable alpha alumina just above 1000°C and the thermal and processing history play important roles [4,5].…”

Effect of the aluminum source on the formation of yttrium aluminum garnet (YAG) powder via solid state reaction

“…For improving the diffusion, the nano grade aluminum sources such as ␣-Al 2 O 3 (100-150 nm, Forever Chemical Co., Taiwan) [18], -Al 2 O 3 (50 nm, Cerolox, USA), ␥-Al 2 O 3 (boehmite treated at 600 • C for 1 h) and boehmite (20 nm, AlOOH, Condea, USA) were used in this study. To form the mixing slurry, 2.28 g Y 2 O 3 and 1.72 g nano-grade alumina (2.26 g for boehmite) were added to 34 ml DI water.…”

θ-Crystallite growth restraint induced by the presence of α-crystallites in a nano-sized alumina powder system

“…Meanwhile, since y-to a-phase transformation lacks T c point, samples with coarser y-crystallite sizes will experience phase transformation at lower temperature. And it will eventually yield more a-formation at the same temperatures, compared with that yielded from samples with smaller y sizes [6].…”

“…The formation of one a-Al 2 O 3 nucleus occurs from one y-Al 2 O 3 crystallite. Continuous heating leads to the growth of acrystallites, exceeding the primary size (d p ¼ B50 nm, XRD-Scherrer formula on (0 1 2) a ) [3,5,6], and the phase transformation proceeds to accomplishment. Generally, the transformation starts at temperatures around 1000-1150 C (using a heating rate 10 C/min), or lower [7,8].…”

“…However, the transformation lacks a transformation temperature, T c in the G2T (Gibbs free energy and temperature) diagram [9], indicating that the transformation is triggered once the critical size of phase transformation, d cy is obtained and the rate of crystallite growth of y-phase can thus affect the temperature reduction of the phase transformation, if the kinetic requirement is satisfied. One interesting study examined the presence of residual y-crystallites in y-to a-phase transformation systems that underwent multiple thermal treatments [6]. Due to the critical size phenomena, it is inevitable that, even in a well thermal-treated system, there can be substantial small amounts of y-crystallites present, because these crystallites could not reach the critical size, d cy ; during the thermal treatment.…”