Browsing by Subject "dispersion"

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    Accurate Recovery of H i Velocity Dispersion from Radio Interferometers
    (2017) Ianjamasimanana, R; Blok, W J G de; Heald, George H
    Gas velocity dispersion measures the amount of disordered motion of a rotating disk. Accurate estimates of this parameter are of the utmost importance because the parameter is directly linked to disk stability and star formation. A global measure of the gas velocity dispersion can be inferred from the width of the atomic hydrogen (H I) 21 cm line. We explore how several systematic effects involved in the production of H I cubes affect the estimate of H I velocity dispersion. We do so by comparing the H I velocity dispersion derived from different types of data cubes provided by The H I Nearby Galaxy Survey. We find that residual-scaled cubes best recover the H I velocity dispersion, independent of the weighting scheme used and for a large range of signal-to-noise ratio. For H I observations, where the dirty beam is substantially different from a Gaussian, the velocity dispersion values are overestimated unless the cubes are cleaned close to (e.g., ˜1.5 times) the noise level.
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    Influence of preparation variables on the dispersion of platinum on zeolite KL.
    (Elsevier, 1997) M'Kombe, C; Dry, M; O'Connor, C
    Various preparation conditions were investigated to evaluate their effect on the dispersion of Pt on zeolite KL. The highest dispersion was obtained by calcining in O2 at about 350 °C. Calcining in N2 or H2 resulted in lower dispersions. Irrespective of the gas atmosphere, heating at 600 °C resulted in sintering of the Pt. The duration of calcination, whether at 350 °C or 600 °C, had little effect on Pt dispersion. The rate at which the temperature was increased up to the calcination temperature and the rate of the O2 flow also had no effect. Subsequent to identical calcination procedures, reduction with H2 resulted in much higher Pt dispersions than reduction with CO. Below 350 °C the temperature of the H2 reduction did not have a marked effect on dispersion. At the 1.5% Pt level, different loading techniques, viz., liquid ion exchange, solid-state ion exchange, and incipient wetness impregnation, resulted in similar Pt dispersions. There was a good correlation between the percentage of Pt dispersion and n-hexane conversion in the aromatization reaction. Pt dispersions were determined by CO chemisorption, and the observed trends were confirmed by TEM. Hydrogen chemisorption was a less satisfactory method of measuring dispersion. © Elsevier Science Inc. 1997