Attenuation ultrasound waves

It was observed that ultrasound stimulation (ultrasonication) also accelerated the crystallization of the more stable polymorphs of CB (111). A fundamental study of the effects of ultrasonication on the polymorphic crystallization of PPP and LLL showed that several factors, such as pressure effect, shear flow, cavitation, and thermal energy caused by absorption of attenuated ultrasound wave, may play concurrent effects of ultrasonication. As a result, there are optimal conditions for temperature and duration of ultrasonication to increase the rate of crystallization and the occurrence of the more stable polymorphs (20). This effect was also observed in CB (111). 

C. Ultrasound (200 kHz, 300 W) was stimulated to a 250-mL sample of CB at 32.3°C during cooling before crystallization. Form II occurred without ultrasonication, whereas Form V was observed when ultrasonication was done for 3 seconds. Further ultrasonication for 9 seconds formed a mixture of Form II and Form V, and only Form II was observed by the ultrasonication for 15 seconds. It is assumed that there are conflicting effects by ultrasonication promotion of nucleation by pressure effect and retardation of nucleation by thermal energy caused by absorption of attenuated ultrasound wave. The former effect may prevail at the ultrasonication for 3 seconds. The temperature rise, however, of the sample caused by absorption of attenuated ultrasound wave was 2°C for 9 seconds and 3.9°C for 15 seconds, and the latter effect may result in the case of cooling from above the melting point of CB. 

Ultrasound reflective spectroscopy application in the analysis of bubbly liquids has been known for many years. This technique is based on the transmission of ultrasound waves through a dispersion, and measuring the velocity and attenuation spectra. 

The attenuation of ultrasound (acoustic spectroscopy) or high frequency electrical current (dielectric spectroscopy) as it passes through a suspension is different for weU-dispersed individual particles than for floes of those particles because the floes adsorb energy by breakup and reformation as pressure or electrical waves josde them. The degree of attenuation varies with frequency in a manner related to floe breakup and reformation rate constants, which depend on the strength of the interparticle attraction, size, and density (inertia) of the particles, and viscosity of the Hquid. 

Ultrasound is used to obtain information about the properties of a material by measuring the interaction between a high frequency sound wave and the material through which it propagates. This interaction depends on the frequency and nature of the ultrasonic wave, as well as the composition and microstructure of the material. The parameters most commonly measured in an ultrasonic experiment are the velocity at which the wave travels and the extent by which it is attenuated. To understand how these parameters are related to the properties of foods it is useful to consider the propagation of ultrasonic waves in materials in general. 

In practice ultrasound is usually propagated through materials in the form of pulses rather than continuous sinusoidal waves. Pulses contain a spectrum of frequencies, and so if they are used to test materials that have frequency dependent properties the measured velocity and attenuation coefficient will be average values. This problem can be overcome by using Fourier Transform analysis of pulses to determine the frequency dependence of the ultrasonic properties. 

Side wall reflections. If the angle of diffraction of an ultrasonic wave leaving a transducer is large enough, reflections may occur from the side walls of the cell. This reflected ultrasound will interact with the ultrasound which has traveled directly through the sample and affects both velocity and attenuation measurements. It is therefore important to calculate the diffraction angle of the transducer and ensure that the side walls are far enough apart so that side-wall reflections do not interfere with the measurements [1]. 

Part II of the book deals with lesser known aspects of US for the analytical chemists such as its use as an energy source for detection purposes. Thus, ultrasound-based detection techniques viz. US spectrometry in its various modes including ultrasound attenuation, ultrasonic velocity, resonant ultrasound, laser-generated, ultrasound reflection and acoustic wave impedance spectroscopies) are dealt with in Chapter 9. Finally, Chapter 10 is devoted to seleoted applioations of US spectrometry — mostly non-analytical applications from whioh, however, analytical chemists can derive new, interesting analytical uses for ultrasound-based deteotion techniques. 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

Abstract Contribution of the Jahn-Teller system to the elastic moduli and ultrasonic wave attenuation of the diluted crystals is discussed in the frames of phenomenological approach and on the basis of quantum-mechanical theory. Both, resonant and relaxation processes are considered. The procedure of distinguishing the nature of the anomalies (either resonant or relaxation) in the elastic moduli and attenuation of ultrasound as well as generalized method for reconstruction of the relaxation time temperature dependence are described in detail. Particular attention is paid to the physical parameters of the Jahn-Teller complex that could be determined using the ultrasonic technique, namely, the potential barrier, the type of the vibronic modes and their frequency, the tunnelling splitting, the deformation potential and the energy of inevitable strain. The experimental results obtained in some zinc-blende crystals doped with 3d ions are presented. 

In most of the cases, an ultrasonic wave propagates adiabatically, so the (20) looks more naturally its right-hand side represents the adiabatic (non-relaxed) modulus and non-adiabatic contribution to the dynamic modulus. Recall that the relaxed (or isothermal) modulus should be regarded as quasi-static one. Figure 1 shows the frequency-dependent factor of non-adiabatic contribution as function of cox. One can see that transformation from isothermal-like to adiabatic-like propagation occurs in the vicinity cox = 1. The velocity of ultrasound is increased in this region, while the attenuation reaches its maximum value. 

Fig. 5 Temperature dependences of velocity [vi (T) - vi(4.2)/vt (4.2)] open circles) and attenuation of ultrasound (filled circles) with respect to the level at T = 4.2 K obtained in ZnSe Ct + at 54.4 MHz. Concentration of the impurity cr = 10 cm l Longitudinal wave, ultrasound passage I = 0.717 cm, propagation direction [110]. After Fig. 1 in [17]...

The presence of undissolved gas and of cavitation bubbles affects the transparency and refractive index of a liquid. Thus when a sonicated liquid is irradiated with light, X-rays, y-rays, or even high-frequency ultrasound, the attenuation and (or) refraction of the wave can be used to detect both the cavitation threshold and bubble density, and their variation with time. This is possible even within a very short period of the order of one ultrasonic cycle [138,139]. 

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