MacDiarmid - Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

The capillaries were spinning throughout data

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Unformatted text preview: position below the sample and the collection run started. Thirty second exposures were collected in a continuous fashion for 2 h. The capillaries were spinning throughout data collection. Since the Mythen detector has small gaps between the various detector modules, data were collected from two alternating positions separated by 0.5° in 2θ. Each pair of scans was then merged (thus resulting in a time resolution of 1 min) and averaged to 0.005° step size. In addition, diffraction patterns were collected from a NIST LaB6 660b standard in order to refine the X-ray wavelength. Full pattern fitting was carried out using Topas 2.0 (Bruker), where each pattern was fitted using a minimal number of parameters (9-parameter Chebyshev background function, a and c lattice parameters, crystallite size, scale factor). This method provides much more robust results than, for example, simply applying the Scherrer equation to one or more peaks, particularly in cases where the peak intensities are low. ’ RESULTS In situ experiments were conducted for a variety of Al contents and temperatures as follows: 0% Al: 400, 500, 600, and 800 °C 1% Al: 500, 600, and 800 °C 2% Al: 600 and 800 °C 4% Al: 600 and 800 °C Experiments were attempted at 500 °C for the 2% and 4% Al samples, but a combination of the small amount of material and the poorer crystallinity expected of these materials, as encountered in earlier work,11 meant that the signal-to-noise ratio of the data obtained was too low to be usable. An example of a time series of XRD patterns (showing the ZnO 100, 002, and 101 peaks) is shown in Figure 1 for a sample containing 0% Al heated at 600 °C for 2 h (the complete final scan is also shown in the Supporting Information, Figure S1). As the hot air blower is moved into position beneath the sample in the X-ray beam, the sample can be visibly seen to turn white within a few seconds. Crystalline peaks corresponding to ZnO in the wurtzite structure are obtained from the very first scan. No other crystalline phases were observed in any of the experiments. Full pattern fitting was used to obtain the ZnO lattice parameters, crystallite size, and scale parameter (providing a relative measure of the volume of crystalline material) for each scan. These are shown as a function of time for the 0% Al, 600 °C example data set in Figure 2. The goodness-of-fit values for each time series varied little within each data set; the range of values is given in the Supporting Information, Table S1. The a and c lattice parameters do not change significantly throughout the measurement. This is the case for all of the samples studied at all temperatures. To deconvolute the effect of temperature on the lattice parameters (thermal expansion) and investigate the change in lattice parameter solely as a function of Al content, we performed an additional experiment where instead of heating up the hot air blower and then moving it into position beneath the sample the hot air b...
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