Crystallization of Aluminum Hydroxide in a Sodium Aluminate Solution on a Heterogeneous Surface

Aluminum hydroxide was crystallized in a sodium aluminate solution as bayerite at room temperature. Primary particles appeared as conical pyramids. They were born on particles of alpha-Al2O3 which had been preformed on a surface of FeCrAl alloy mesh as a result of its annealing. The primary nucleation was discussed using a two-step model that consists of a formation of prenucleation clusters and their transformation into nuclei. The induction period is an inversely proportional function to a square of the substrate surface and depends on its nature. Stirring the solution affected the primary nucleation preventing the formation of the prenucleation clusters. At an early stage, in a supersaturated state, the lateral surface of the primary pyramids grew via filling kink sites and forming the secondary pyramids. The secondary pyramids appeared from the prenucleation clusters. The pyramid base grew by a screw mechanism with an unwinding spiral. When a supersaturate degree of the solution decreased, the lateral surface grew by filling kink sites only, whereas the screw mechanism transformed into a usual modification of the screw mechanism with the shrinking spiral. The change in the growth mechanism transformed the primary cone-shaped particles into the final needle-shape particles.