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Miniaturization of mechanical milling for powder X-ray diffraction

To enable mechanical milling of small (0.1–1.0 g) samples, a cylindrical grinding vessel machined from polypropylene and furnished with tungsten carbide rods has been designed and produced for use inside the conventional jar of a McCrone Micronizing Mill. The vessel is about one-seventh the volume of the conventional jar supplied by the manufacturer. The conditions of milling for both the conventional and the miniaturized-grinding assemblies were tested using quartz sand as a limiting case. The median grain sizes of the resultant powders were measured by an X-ray gravitational-sedimentation method, with contamination from the grinding media measured by Rietveld refinement and by instrumental neutron activation analysis. The use of tungsten carbide grinding elements permits rapid wet milling of a small sample to the same median grain size in about one-third of the time required by a regular sample ground in corundum. The relative contamination (by tungsten carbide on a weight basis) using the miniaturized-grinding assembly is about 6(1)% of the proportion of corundum contamination yielded by the conventional grinding assembly.