The use of internal standards for measurement of temperature in BigDiff
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Date
2002-01-29
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Australian Institute of Physics
Abstract
A simple furnace has been constructed to enable X-ray diffractograms to be obtained at elevated temperatures in BigDiff, on the Australian beamline at the Photon Factory, Tsukuba, Japan. Measurement of the specimen temperature presents a problem, and this poster describes an attempt to measure the temperature using the thermal expansion of an internal standard. α-alumina was chosen as the internal standard, but because of uncertainty regarding its coefficients of thermal expansion it was decided to compare temperatures based on α-alumina with those based on tungsten, using a composite specimen. Data in BigDiff are recorded on image plates. It has long been known that the direction of reading of an image plate is not strictly parallel to its edge, with the result that the starting angles of strips across the width of a plate are not exactly the same. To overcome this problem exposures at the same temperature were recorded on opposite sides of the plate. Comparison of these diffractograms revealed that the pixel spacing also varied across the width of the plate, and down the length of a strip. As a result it was not possible to calculate cell parameters with sufficient accuracy using Rietveld refinement of the data. Instead, temperatures have been calculated from the shifts of individual lines from their supposed positions at the reference temperature. Comparing temperatures calculated from different lines showed that at each temperature the entire pattern had been shifted parallel to its length by an amount on the order of 50 μm, but not the same for all patterns, presumably because movement of the cassette in BigDiff was not strictly parallel to the guide rails. After making allowance for the above effects temperatures can be measured to a precision of ±2K, but the biggest uncertainty, almost an order of magnitude greater, remains lack of knowledge of the coefficients of thermal expansion.
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Keywords
Aluminium oxides, X-ray diffraction, Furnaces, Accuracy, Image processing, KEK Photon Factory, Standards, Temperature measurement, Thermal expansion, Tungsten
Citation
Ball, C. J., & Thorogood, G. J. (2002). The use of internal standards for measurement of temperature in BigDiff. Paper presented at the 26th Annual Condensed Matter Physics Meeting, Wagga Wagga, NSW (Australia), 29 January - 1 February 2002, (pp. 64).