Grain-Growth Phenomena in ZnO Ceramics in the Presence of Inversion Boundaries

ZnO grain-growth kinetics was studied in the ZnO+1 mol% Bi(2)O(3) system with the addition of 0.1 and 0.5 mol% SnO(2) between 950 degrees and 1300 degrees C. It was found that inversion boundaries (IBs) are the major factor influencing the ZnO grain growth in this system. After the nucleation of the IBs in the ZnO grains, these grains grow exaggeratedly and anisotropically in the direction of the inherent IB, causing a plate-like development of these grains. At 950 degrees C, the IB-induced grain-growth mechanism is characterized by a low grain-growth exponent (N) of 2, from 15 to 240 min of sintering. At higher temperatures (1100 degrees-1300 degrees C), the IB-induced grain growth is finished already after approximately 15 min and the N-value is even lower, ranging from 1.7 (1100 degrees C) to 1.4 (1300 degrees C), with the apparent energy for ZnO grain growth of similar to 148 kJ/mol. After the impingement of the plate-like ZnO grains with IBs, further growth follows the Ostwald-ripening mechanism, which is an incomparably slower growth process. In this growth stage, the N-value increases to similar to 3.5 and the apparent grain-growth energy is increased to similar to 353 kJ/mol (1100 degrees-1300 degrees C). After long sintering times, the samples reach an equilibrium microstructure comprised of only ZnO grains with IBs. During this stage, the grain growth is virtually stopped, which is reflected in very high N-values of 20 and more.