Journal of the American Ceramic Socieny-Wu er al. Vol. 85. No. 10 (A) (B) e505q己 40 nm 40 nm 505H4q 3b052b8b 40 nm 28 nm (E) 40 nm Fig. 4. TEM photographs of the used precursor (A)and the hydrothermal powders synthesized at 245 C in the neutral medium(B) and the basic medium (C), and at 200(D) and 245 C(E) in the acid medium. The reaction time was fixed at I8 h of sample weight, and it is expressed by negative Am. When the IV. Discussion two parts, the total Am is equal to the algebraic sum of the two Formation o Ceo crystallites Synthesized under arts Am. Similarly, when the DTA curves are out of range and are divided into two parts, the whole curve may be obtained by Because the solubility products of some precipitated sub- joining the two parts after shifting vertically the divided part. stances are bigger (for example, K p(AgCI) The dotted curves indicate a temperature change from room K p(Baso4)=I I X 10,etc). when Ostwald ripening occurs temperature to 1000 C, an ascent of the curve denotes a rise in the smaller grains dissolve more quickly, the solute diffuses more temperature, and a descent of the curve denotes a fall in slowly, and the larger grains grow more slowly: therefore, it is temperature. The whole curve may be obtained by joining the assumed that the growth kinetics are controlled by diffusion or divided parts after shifting vertically the divided parts. The surface reaction. However, the solubility product of Ce(OH)a endothermic peaks with the temperature below 150C are (i.e. CeO, 2H- O)is 2 x 10 and is far smaller than for one of elated to dehydrating of the examined samples those precipitated substances. When Ostwald ripening occurs, the2464 Jtnirnal of the American Ceramic Societ\—Wii et al. Vol. 85. No. 10 Fig. 4. TEM phoiographs of lhe used precursor (A) and the hydrolhermal powders synthesized al 245''C in the neutral medium (B) and the basic medium (C). and ai 200'' (D) and 245X (E) in the acid medium. The reaction time was fixed at 18 h. of satiiple weight, and it i.s expressed by negative Am. When the TG curves are out of range of the plotter and are divided into two parts, the total A;» is equal to the algebraic sum of the two parts Am. Similarly, when the DTA curves are out of range and arc divided into two parts, the whole ctirve may be obtained by joininj! the two parts after shifting vertically the divided part. The dotted curves indicate a temperature change from room tetiiperauirc to 1000°C. an aseent of the curve denotes a rise in temperature, and a descent of the curve denotes a fall in temperature. The whole curve may be obtained by joining the di\ided parts after shifting vertically the divided parts. The endotherinic peaks with the temperature below ISO^C are related to dehydrating of the examined samples. IV. Discussion (1) Formation of CeOi Crystallites Synthesized under Hydrothermal Conditions Because the solubility products"* of some precipitated sub￾stances are bigger (for example. A',,,(AgCI) ^ I.S X 10 '" and A',p(BaSO4) - 1.1 X 10 '". etc), when Ostwald ripening occurs the smaller grains dissolve more quickly, the solute diffuses move slowly, and lhe larger grains grow more slowly; therefore, it is assumed that the growth kinetics are controlled by diffusion or surface reaction. However, the solubility product"* of Ce(0H)4 (i.e., CeO;-2H,O) is 2 X 10 •"* and is far smaller than for one of those precipitated substances. When Ostwald ripening occurs, the