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|Type:||Artigo de periódico|
|Title:||Kinetics Of The γ → α-alumina Phase Transformation By Quantitative X-ray Diffraction|
|Abstract:||This work reports the kinetics of α-alumina transformation from γ-alumina, prepared by the sol-gel method from a solution of saturated aluminum nitrate and urea. The γ-alumina phase transformed directly into α-alumina at 750, 800, 850 and 900 oC, without any intermediate phases, such as θ- or δ-alumina. The kinetics of γ → α-alumina phase transformation was monitored by determination of α-alumina fraction formed through quantitative X-ray diffraction (XRD), with calcium fluoride added as an internal standard. The crystallised fractions of α-alumina increased sigmoidally with time, indicating that the γ → α-alumina phase transformation had a nucleation and growth mechanism. The kinetic parameters for this transformation were determined through the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and the Arrhenius' law. The apparent activation energy, the Avrami exponent, n, and the t 0.75/t 0.25 ratio determined for the transformation were, respectively, of (201 ± 4) kJ mol -1 (2.1 ± 0.2) and (2.1 ± 0.1). The apparent activation energy is lower than the values previously reported for the γ → α-alumina transformation, as a consequence of the high surface area (425 m 2/g) of γ-alumina. The t 0.75/t 0.25 ratio of (2.1 ± 0.1) suggested that the α-alumina growth was plate-like, which was confirmed by the SEM micrograph of α-alumina. © Springer Science+Business Media, LLC 2007.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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