Powders, test methods particle size characteristics

Maintenance of the physical characteristics of samples under test was to be maximized. The methods should be chosen to prevent this and should "...provide the minimal interference with the irradiation of samples under test." The particle size of powders was to be "...with a particle size distribution representative of material as released for use..." and "...a thickness of not more than (NMT) 5mm."... 

Methods for evaluating the performance of powder coatings are the same as those used for conventional coatings. Test methods for coating powders include particle size distribution, powder free flow, sintering, fluidization characteristics, and others. They have been reviewed in detail and reported in the literature (120,121). In addition, the American Society for Testing and Materials has issued a comprehensive standard that covers the most important test methods for the evaluation and characterization of powder coatings (122). 

Madon and Boudart propose a simple experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions [R. J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438]. The experiment involves making rate measurements on catalysts in which the concentration of active material has been purposely changed. In the absence of artifacts from transport limitations, the reaction rate is directly proportional to the concentration of active material. In other words, the intrinsic turnover frequency should be independent of the concentration of active material in a catalyst. One way of varying the concentration of active material in a catalyst pellet is to mix inert particles together with active catalyst particles and then pelletize the mixture. Of course, the diffusional characteristics of the inert particles must be the same as the catalyst particles, and the initial particles in the mixture must be much smaller than the final pellet size. If the diluted catalyst pellets contain 50 percent inert powder, then the observed reaction rate should be 50 percent of the rate observed over the undiluted pellets. An intriguing aspect of this experiment is that measurement of the number of active catalytic sites is not involved with this test. However, care should be exercised when the dilution method is used with catalysts having a bimodal pore size distribution. Internal diffusion in the micropores may be important for both the diluted and undiluted catalysts. 

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