MacDiarmid - Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

Figure 9 fits of the relaxation model to 4 al

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Unformatted text preview: of this equation, it is common to fit the data as t(D) rather than D(t). Model 3: Grain growth with size-dependent impediment (see refs 13 and 14 and references therein). This satisfies the differential equation dDðt Þ a3 ¼ À b 3 Dð t Þ dt Dð t Þ 21037 |J. Phys. Chem. C 2011, 115, 21034–21040 The Journal of Physical Chemistry C ARTICLE Figure 8. Fits of the relaxation model to 2% Al crystallite size versus time data for different temperatures. Figure 10. Fitted parameters m and D∞ as a function of temperature for different Al contents for the fits shown in Figures 6À9. Figure 9. Fits of the relaxation model to 4% Al crystallite size versus time data for different temperatures. temperatures using the relaxation model (Model 4) are shown in Figures 6À9. The fitted parameters m and D∞ are shown for all samples as a function of temperature in Figure 10. For the experiments with 0% Al, m decreases with increasing temperature while D∞ increases with increasing temperature. These trends are less pronounced for experiments with nonzero Al content. At 800 °C m is approximately 0.1, which for the generalized parabolic grain growth (recalling that the two models have equivalent forms in these conditions) corresponds to a value of n of 10. Similar values have been observed before in nanocrystalline grain growth studies.15,16 Since D∞ tended toward excessively large numbers at 800 °C it was not possible to use the D∞ values in an Arrhenius plot. Instead, the last 10 min of D(t) values were averaged (termed Df), and a plot of ln Df versus 1/T is given in Figure 11 for samples of various compositions. These obey Arrhenius relationships (as indicated by the linear fits) from which the activation energies can be extracted. The activation energies are shown as a function of Al content in Figure 12. A value of 24 ( 3 kJ/mol was obtained for undoped ZnO, while for all of the Al-doped experiments the values were around 43 ( 4 kJ/mol. Figure 11. Arrhenius plot of Df (average last 10 min) vs 1/T. Linear fits are shown. ’ DISCUSSION For all compositions and temperatures studied, the initial nucleation and grain growth is very fast, with peaks corresponding to crystalline ZnO being observable by the end of the first scan after insertion into the hot air stream, i.e., nucleation occurs in less than 30 s, with the average crystallite size after 1 min being larger than 10 nm for most of the temperatures studied. This implies a low activation energy, which is borne out in Figure 12. The activation energy for grain growth in undoped ZnO has been quoted in the literature as being 275 kJ/mol.17 The value we 21038 |J. Phys. Chem. C 2011, 115, 21034–21040 The Journal of Physical Chemistry C Figure 12. Activation energy extracted from the slopes of the fitted lines in Figure 11 as a function of Al content. obtained, of 24 ( 3 kJ/mol, is less than one-tenth of the bulk value. Grain growth activation energies in nanocrystalline oxides have been o...
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