Powder signature

It is believed that this method of comparing two different powders most likely has an important future in industrial processing. Particle shape is very sensitive to the conditions under which the particles are made. Slight variations, which may not be measurable with normal means in an industrial operation, will significantly alter particle shape. Consequently, this characteristic can become a powerful tool not only for understanding the process but also for its control. In the long run, an equally important application of powder signatures may be their use in specifications. 

Plastic explosives contain one or more of the explosives listed above, moulded in an inert, flexible binder. Because powders do not readily hold a shape and TNT is the only common melt-castable explosive, most of the explosive powders (RDX, HMX, PETN, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)) are plasticized to make a mouldable material, for example, C-4, Semtex H, PE4, sheet explosive. A variety of plasticizers are added, but the maximum level is usually 10-15% because most plasticizers are inert and would degrade explosive output. Plastic explosives were originally developed for convenient use in military demolitions but have since been widely used in terrorist bombs. For detection techniques that rely on vapour signatures, such as canine olfaction, it is worth considering that the plasticizer is much more volatile than the explosive component. 

M. Williams, J.M. Johnston, P. Waggoner, J. Jackson, M. Jones, T. Boussom and S.F. HaUoweU, Determination of the canine detection odor signature for NG smokeless powder , Proceedings of the Second Explosives Detection Technology Symposium and Aviation Security Technology Conference, Atlantic City, N.J. Federal Aviation Administration, 1997. 

The wide variability of VOC signatures for smokeless powders and the variability in the smokeless powders used in the training of the canines tested may preclude the identification of odorants for low explosives with the most common VOC present, diphenylamine, not identified as an odorant for the detector dogs we have tested as discussed later. However, given the high occurrence of diphenylamine in smokeless powders, it may prove prudent to include a controlled source of diphenylamine during odor training, as it has the potential to be an explosive odorant. 

Figure 2 Partial Raman (top) and MAS NMR spectra (bottom) from hydrate samples recovered from Cascadia, taken at lOK and 173K, respectively. The spectral signatures indicate that methane is the principal guest for both large (major peak) and small cages(minor peaks). The line intensities show that the popidations are in an approximate 3-4 1 ratio (for large small cages), confirming the powder X-ray data assignment that it is a si hydrate. mbsf= meters below sea floor, cm bsf cm below sea floor. From ref. 41...

The above discussed the signature of individual particles. Powders, even from homogeneous sources, contain a variety of particle shapes. While a powder may have a variety of repeating particle shapes the composite of these different shapes cannot reproduce the typical particle shape of the powder. 

The surprising result of the evaluation of Fourier coefficient signatures of homogeneous powders is that there are usually five or more different shapes and the question arises as to the relation between these predominant shapes. Research is still continuing in this area. 

A number of methods have been proposed for particle shape analysis these include verbal description, various shape coefficients and shape factors, curvature signatures, moment invariants, solid shape descriptors, the octal chain code and mathematical functions like Fourier series expansion or fractal dimensions. As in particle size analysis, here one can also detect intense preoccupation with very detailed and accurate description of particle shape, and yet efforts to relate the shape-describing parameters to powder bulk behaviour are relatively scarce.10... 

In Figure 3, the Raman spectra of the spherulites in amorphous matrix and of the closed layer are shown. Although, the Raman spectrum of the amorphous phase lacks of several vibrational features like those observed in rubrene powder, there are clear signatures, which proves the presence of this phase in rubrene films. As it was already mentioned, the first one (but not exclusive) is the isotropy of the Raman spectrum upon light polarization. Second one is the broad band at 1373 cm-1 with a small shoulder at 1356 cm 1 (both labeled AM ). Namely, the both bands are clearly seen for amorphous phase, but completely missing for rubrene powder and closed rubrene layers. Isolated spherulites present intermediate... 

Fig. 3. Raman spectra of rubrene films spherulites in amorphous matrix (top), and from closed rubrene layer (bottom) 1,2 - the center and close to border of a spherulite 3 - amorphous matrix. Rubrene powder and mica substrate spectra are also shown as reference. AM - labels the signatures of the amorphous matrix.

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