MacDiarmid - Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

MacDiarmid Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

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Unformatted text preview: talline tetrahedral sites correspond to Al3+ substituting on the Zn2+ sites, whereas the amorphous geometries correspond to Al forming amorphous phases, presumably at grain boundaries.11 These may be precursors to Al-rich phases such as ZnAl2O4, which forms at the grain boundaries and inhibits growth of the ZnO grains.9,10,23 Previous studies indicate that the maximum amount of Al that can be incorporated in the ZnO lattice is 1%,9,10 although our previous 27Al NMR study indicated that under optimal synthesis conditions at 1% Al doping there was still a significant fraction of amorphous Al and in order to achieve maximum incorporation an excess of Al (up to 4 at%) needed to be added.11 A study of grain growth in bulk ZnO:Al with grain sizes of the order of micrometers and Al contents in the 0À1 mol % range also observed a factor of 2 increase in the grain growth activation energy, from 200 to 480 kJ/mol.23 This was attributed to formation of ZnAl2O4 crystallites at the grain boundaries that acted as pinning centers, which restricted further grain growth. Therefore, even for small amounts of added Al, some fraction of the dopant segregates to the interface as amorphous or crystalline Al-rich phases and interrupts the ZnO grain growth. This is evidenced both by the factor of 2 increase in the activation energy and noting that the overall grain size at a given temperature also tends toward smaller values as the Al concentration increases, which has been observed before.9,11,24 Consequently, this implies that the ZnO grain growth occurs through surface processes and addition of Al inhibits migration of atoms across the grain boundaries. There is no apparent Al-concentration dependence on the grain growth activation energy in the region of 1À4% Al. ’ CONCLUSION In situ, real-time synchrotron X-ray diffraction measurements yielded insights into the grain growth kinetics of ZnO:Al for 0À4 mol % Al during calcination at 400À800 °C. For all of the samples studied, the lattice parameters and volume of crystalline material did not change significantly with time nor was there a strong dependence of these on the Al concentration. The crystallite sizes grew with time and could be described using a relaxation model, which tends to the algebraic form of the generalized parabolic grain growth model at the highest temperatures studied. Activation energies for grain growth were extracted from an Arrhenius plot of the average grain sizes observed after 2 h. For the undoped ZnO experiments, the activation energy obtained was 24 ( 3 kJ/mol, while for all of the doped ZnO:Al experiments, the activation energy was constant within uncertainty at 43 ( 4 kJ/mol. This difference has been attributed to the segregation of Al to the crystallite interfaces, which then results in formation of Al-rich phases which interfere with the grain growth of ZnO. 21039 dx.doi.org/10.1021/jp207140g |J. Phys. Chem. C 2011, 115, 21034–21040 The Journal of Physical Chemistry C ’ ASSOCIATED CONTENT S b Supporting Information. Example of complete XRD pattern, and derivation of the equivalency o...
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This note was uploaded on 06/15/2013 for the course MSE 101 taught by Professor Sen during the Spring '12 term at Indian Institute of Technology, Kharagpur.

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