MacDiarmid - Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

Crystalline zno is rst observed above 400 c due to

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Unformatted text preview: lower was heated up in place at 5 °C/min (i.e., the sample temperature was slowly ramped up to 800 °C). The lattice parameters for this experiment are shown as gray lines in Figure 3. Crystalline ZnO is first observed above 400 °C. Due to the very weak peak intensities at this stage, the uncertainties are quite large. Once the sample temperature reached 800 °C, it was then cooled at 15 °C/min. Over the range from 400 to 800 °C the a lattice parameter increases linearly with temperature. The c lattice parameter does not appear to be completely reversible; between 400 and 600 °C the c parameter is roughly constant and then starts to increase as the temperature rises above 600 °C. 21035 dx.doi.org/10.1021/jp207140g |J. Phys. Chem. C 2011, 115, 21034–21040 The Journal of Physical Chemistry C ARTICLE Figure 2. Results from fitting the data shown in Figure 1: lattice parameters a and c, crystallite size, and scale parameter (providing a relative measure of the volume of crystalline ZnO). Figure 4. Crystallite size versus time for different temperatures (as indicated in a) for (a) 0% Al, (b) 1% Al, (c) 2% Al, and (d) 4% Al. Figure 3. Final a and c lattice parameters for different Al contents as a function of temperature (symbols) compared with a temperature ramp experiment with 0% Al (gray line). On the cooling leg, the c parameter decreases linearly with decreasing temperature. Superimposed on the heating ramp experiment in Figure 3 are the a and c lattice parameters averaged from the last 20 min of data collection for all samples and temperatures studied (where the hot air blower was first heated to temperature and then moved into place beneath the sample). Within the uncertainties, the lattice parameters of the doped samples show no significant departure from those found for the 0% Al temperature ramp experiment. This is consistent with earlier findings for ZnO:Al calcined powders measured at room temperature using synchrotron X-ray diffraction.11 The trend of the scale parameter in Figure 2 is also constant with time for all samples and temperatures studied. This indicates that the volume of crystalline ZnO is not changing over time. The crystallite size in Figure 2, however, increases over time in an approximately logarithmic or sublinear power law fashion. These two pieces of information indicate that nucleation of ZnO is rapid and then grain growth occurs between crystalline regions; no new ZnO crystallization is occurring at later times. Comparative plots of crystallite size versus time are shown for all samples in Figure 4. In general, it was observed that the higher the temperature, the larger the crystallite size at any given time. This is consistent with earlier ex situ synchrotron X-ray diffraction results.11 Apart from the 1% Al sample calcined at 800 °C, the crystallite size at a given size and temperature decreases as the Al content increases, which is consistent with earlier ex situ X-ray diffraction results.11,12 Given the excellent time r...
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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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