Particles ultrasonic wave

Ultrasonic methods can also be applied to velocity measurements based on measurement of the Doppler shift in the frequency of an ultrasonic wave scattered from a moving particle. The angle between the velocity vector and the direction of ultrasound propagation must be known, which practically limits the appHcation of the technique to the measurement of unidirectional flows. However, this Hmitation may be overcome again by the use of an array of transducers [11]. 

Ghasemi et al. [24] have obtained nanostructured Pb02 (50-100 nm) using [5-PbO precursor and in the presence of ammonium peroxydisulfate as an oxidant. Here, the ultrasonication dispersed and then cracked the [5-PbO particles, thereby increasing the contribution of their surface area. Such an ultrasonic treatment resulted in an enhancement in the oxidation of PbO to Pb02 has been observed. Ultrasonic waves also have been found to inhibit the formation of Pb02 particles larger than 150 nm. 

Ultrasonic relaxation loss, of vitreous silica, 22 429-430 Ultrasonics, for MOCVD, 22 155 Ultrasonic spectroscopy, in particle size measurement, 13 152-153 Ultrasonic techniques, in nondestructive evaluation, 17 421—425 Ultrasonic testing (UT) piping system, 19 486 of plastics, 19 588 Ultrasonic waves, 17 421 Ultrasonic welding, of ethylene— tetrafluoroethylene copolymers,... 

Sonication using ultrasonic cleaner baths remains a popular extraction approach particularly for controlled-release products. In sonication, an ultrasonic wave of 20-40 kHz generated by a piezoelectric transducer is used to produce the formation and collapse of thousands of microscopic bubbles (cavitations) in the water bath to facilitate the break up of the solid particles and the subsequent dissolution of the API. Note that parameters such as the wattage power of the sonicator, presence of the perforated tray, depth of the water level, bath temperature and the number of sample flasks sonicated might all affect the extraction rate. For... 

Agglomerated sol particles (10 nm use of ultrasonic waves, laser scattering)... 

Other Methods. Other reductants like hydrazine, sodium metal, etc. can be used for the reduction of metal ions. Decomposition of metal salts or complexes by heat treatment is sometimes used for synthesis of fine particles as well. In this case the valence of metals in the fine particles should be carefully examined. Recently, irradiation of ultrasonic wave was applied to the synthesis of colloidal dispersions of metal fine particles. 

Ultrasonic waves are a mechanical disturbance which passes thru the medium by the progressive displacement of particles. The particles do not travel in the direction of the source but vibrate about their mean fixed position. The amplitude of the wave is the distance from peak to peak and therefore is the maximum displacement of a particle in the medium. The period (T) is the time required to complete one cycle and the frequency (f) refers to the number of cycles per unit time. The unit of frequency is the Hertz (Hz, one cycle per second) and it is the reciprocal of the period. The rate at which sound travels thru the medium is the velocity (c, meters per second). The wavelength (X, meters), is the distance between adjacent cycles. Therefore, the relation,between wavelength, velocity and frequency is given by... 

The action of ultrasonic waves seems likely to be the result of mechanical rupture of the long chain molecules of nitrocellulose on being rubbed by the violently oscillating particles of solvent. Thus it is one of rare examples of depolymerization brought about by mechanical forces. Further evidence in favour of the supposition that mechanical forces may cause the substance to depolymerize is provided by the... 

The sonophotocatalytic system is effective for overall water splitting as shown in Fig. 12.2 and Table 12.1. This system requires, properly, a photocatalyst such as particulate Ti02. As ultrasonic waves pass through the solution, the properties of the solution influence a sonochemical reaction. In particular, negative effects are considered in the presence of powdered photocatalysts. The effects of fine particles in the solution on the sonochemical reaction have been noted so far. For example, Yasuda et al.19) reported the effects of insoluble particles, such as silicon oxide (Si02) or aluminum oxide (Al203), in the reactant solution on the sonochemical reaction and demonstrated that the reaction rate constant depended on particle properties, particle size and number of particles. It is assumed that a powdered photocatalyst suspended in the solution obstructs the transmission of ultrasonic waves. In this section, the influence of the photocatalyst powder suspended in solution on the sonochemical reaction is examined. 

A mathematical description of an ultrasonic wave must describe the dependence of the particle displacement on distance and time, and the reduction of its amplitude with distance traveled through the material. For plane sinusoidal waves the following equation is appropriate ... 

Ultrasonics can be used to determine the size of particles in microheterogeneous materials in a manner analogous to light scattering. An ultrasonic wave incident upon an ensemble of particles is scattered by an amount which depends on the size of the particles and the ultrasonic wavelength. The scattered waves, interact with the incident wave, which modifies its phase and amplitude. Thus velocity and attenuation measurements can be used to determine particle size. 

Reducing the course material in size in a colloid mill or by means of ultrasonic waves generally does not lead to a higher distribution ratio, i.e. to smaller particles. As it is smaller particles tend to form larger ones under the influence of mechanical forces and due to the attraction between the particles. These problems are solved to a large extent when a surface active substance and possibly a solvent are added during the reduction process. 

Some of the simplest ultrasonic measurements involve the detection of the presence/absence of an object or its size from ultrasonic spectrum (Coupland and McClements, 2001). An ultrasonic wave incident on an ensemble of particles is scattered in an amount depending on size and concentration of the particles. As the ultrasonic parameters depend on the degree of the scattering, it can therefore be used to provide information about particle size. 

Absorption is an unavoidable consequence of passing ultrasonic waves through matter. In particular, to reach the cavitation zone in a solution, large-amplitude ultrasound must pass through the solution. Generally, the higher is the viscosity of a solution, the higher is the ultrasonic absorption. There is an additional loss associated with finely divided particles, known as... 

O Neill, T.J., Tebbutt, J.S., Challis, R.E. 2001. Convergence criteria for scattering models of ultrasonic wave propagation in suspensions of particles. IEEE Trans. Ultrasonics, Ferro-electrics and Frequency Control 48, 419-424. 

Although the effect of the concentration of suspension on the results of the measurement of pigments has never been proven, the development of techniques able to cope with concentrations closer to the applications if of interest These would make it possible, for example, to determine the particle size distribution in a dispersion paint or in a reaction vessel where a pigment is produced by the precipitation process. A measurement technique having no problems, in principle, with high concentration dispersions is the scattering of ultrasonic waves. Nevertheless the instruments on the market have up to now failed to realize the great expectations of this technique. 

A plane ultrasonic wave can be described in terms of the displacement of a particle from its equilibrium position as a function of the distance the wave has travelled as follows ... 

Electrokinetic sonic amplitude a.c. electric field particles liquid ultrasonic waves pressure amplitude per unit field strength ESA NV- m-i... 

A mathematical model, Allegra-Hawley [227,228], prediets the attenuation of ultrasonic waves as a funetion of frequency for each particle size distribution and concentration. Some mechanical, thermodynamic and transport properties of both phases are needed. The relationship between the size, concentration and frequency is obtained from the solution of the... 

For liquids, the velocity of ultrasound depends on the compressibility and density of the liquid. For suspensions, the velocity depends also on the drag of particles in the liquid under the influence of the ultrasonic wave. At low frequencies, small particles tend to move in phase with the liquid and the ultrasonic velocity may differ widely from that in the pure liquid. As particle size and ultrasonic frequency increases, the particles tend to lag more and more behind the movement of the liquid and the ultrasonic velocity approaches that of the suspension acting as a uniform fluid. There is a transition frequency range between complete entrainment and no entrainment of the particles that can be used to obtain particle size information. The hydrodynamic model of Marker and Temple [267 ] can be used to calculate ultrasonic velocity. This model takes into account the effects of fluid viscosity, of concentration, density and elastic modulus of both particles and fluid and can predict ultrasonic velocities accurately for volume fractions between 5% and 20%. Ultrasonic velocity measurements in the 50 kHz to 50 MHz can be used to determine particle size distributions in the range of about 0.1 to 30 pm. 

CSIRO Minerals has developed a particle size analyzer (UltraPS) based on ultrasonic attenuation and velocity spectrometry for particle size determination [269]. A gamma-ray transmission gauge corrects for variations in the density of the slurry. UltraPS is applicable to the measurement of particles in the size range 0.1 to 1000 pm in highly concentrated slurries without dilution. The method involves making measurements of the transit time (and hence velocity) and amplitude (attenuation) of pulsed multiple frequency ultrasonic waves that have passed through a concentrated slurry. From the measured ultrasonic velocity and attenuation particle size can be inferred either by using mathematical inversion techniques to provide a full size distribution or by correlation of the data with particle size cut points determined by laboratory analyses to provide a calibration equation. 

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