Acoustic properties dispersion relations

Perylene The two different crystalline phases, a perylene and perylene (see Chap. 2, Fig. 2.12) differ strongly also in their dynamic properties a perylene has four molecules or two dimers per unit ceU. From this, 24 internal modes result, 21 optical and three acoustic. They have also been observed and identified by inelastic neutron diffraction [17] and by Raman scattering [18]. Their spectrum has a width of about 4 THz, again similar to the cases of naphthalene and anthracene. For a perylene, the model treated in Sect. 5.6 again yields satisfactory theoretical dispersion relations. The low-energy internal modes are torsional (twisting) and butterfly modes. Their spectrum overlaps with that of the external modes. 

More recently, Yang and Thompson implemented this type of sensor in FI manifolds, which they consider ideal environments for relating the sensor s hydrodynamic response to the analyte s concentration-time profile produced by the dispersion behaviour of sample zones. Network analysis of the sensor generates multi-dimensional information on the bulk properties of the liquid sample and surface properties at the liquid/solid interface. The relationship between acoustic energy transmission and the interfacial structure, viscosity, density and dielectric constant of the analyte have been thoroughly studied by using this type of assembly [171]. 

Returning to acoustics, its lack of widespread aceeptanee may be related to the fact that it yields too mueh, sometimes overwhelming, information. Instead of dealing with interpretation of the acoustic spectra it is often easier to dilute the system of interest and apply light-based teehniques. It was often naively assumed that the dilution had not affected the dispersion characteristics. Lately, many researchers are coming to the realization that dispersed systems need to be analyzed in their natural concentrated form, and fliat dilution destroys a number of useful and important properties. 

The uncertainty related to the thermal expansion coefficient makes latex systems the most complicated systems for acoustics. This is important to keep in mind for testing a particular model of an acoustic instrument. Latex dispersions that are used as standards for light-based methods should be used with caution as in many cases the thermal-expansion properties of these standards are not well known. 

US velocity and attenuation measurements can also be used to determine solid-state material properties such as concentration and dispersion of fillers [908,909]. The active level of the acoustic emission signal of PP/talc composites was related to the degree of dispersion of the filler in the matrix [910]. LDPE/28-32 wt.% Mg(OH)2 and HDPE/0-10 wt.% Mg(OH)2 samples were examined over a wide range of temperatures (160-200°C) and pressures (up to 60 bar) to determine the effect of melt T, p and filler concentration on US velocity and attenuation in the melt [911]. Ultrasound velocity is affected by melt T, p and material density (filler content) US attenuation increases with increasing filler content. As US calculated filler concentrations deviated consistently from off-line TGA measured values (Fig. 1.43) it is obvious that further validation is required. In principle, extmsion processing data can be used to predict filler concentration. Accurate determination of filler concentration in real time is potentially useful to reduce excess and unnecessary usage of filler, to reduce scrap product and save production costs. 

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