Characterization of HIFU transducers designed for sonochemistry application: Acoustic streaming.

Cavitation distribution in a High Intensity Focused Ultrasound sonoreactors (HIFU) has been extensively described in the recent literature, including quantification by an optical method (Sonochemiluminescence SCL). The present paper provides complementary measurements through the study of acoustic streaming generated by the same kind of HIFU transducers. To this end, results of mass transfer measurements (electrodiffusional method) were compared to optical method ones (Particle Image Velocimetry). This last one was used in various configurations: with or without an electrode in the acoustic field in order to have the same perturbation of the wave propagation. Results show that the maximum velocity is not located at the focal but shifted near the transducer, and that this shift is greater for high powers. The two cavitation modes (stationary and moving bubbles) are greatly affect the hydrodynamic behavior of our sonoreactors: acoustic streaming and the fluid generated by bubble motion. The results obtained by electrochemical measurements show the same low hydrodynamic activity in the transducer vicinity, the same shift of the active focal toward the transducer, and the same absence of activity in the post-focal axial zone. The comparison with theoretical Eckart's velocities (acoustic streaming in non-cavitating media) confirms a very high activity at the "sonochemical focal", accounted for by wave distortion, which induced greater absorption coefficients. Moreover, the equivalent liquid velocities are one order of magnitude larger than the ones measured by PIV, confirming the enhancement of mass transfer by bubbles oscillation and collapse close to the surface, rather than from a pure streaming effect.

Characterization of HIFU transducers designed for sonochemistry application: cavitation distribution and quantification.

Acoustic field distribution was determined in HIFU sonoreactors as well as localization of cavitation activity by crossing different techniques: modeling, hydrophone measurements, laser tomography and SCL measurements. Particular care was taken with quantification of this last technique by pixels or photon counting. Cavitation bubbles generated by HIFU are mainly located on the outer layer of the propagation cone in the post-focal zone. Greatest acoustic activity is not located at the geometrical focal, but corresponds to a high concentration of bubbles zone. On the contrary, the main sonochemical activity shifts slightly toward the transducer, whereas quenching of inertial cavitation is observed directly at the focal. Finally, SCL thresholds have been determined.

Energetic balance in an ultrasonic reactor using focused or flat high frequency transducers.

In order to undertake irradiation of polymer blocks or films by ultrasound, this paper deals with the measurements of ultrasonic power and its distribution within the cell by several methods. The electric power measured at the transducer input is compared to the ultrasonic power input to the cell evaluated by calorimetry and radiation force measurement for different generator settings. Results obtained in the specific case of new transducer types (composites and focused composites i.e., HIFU: high intensity focused ultrasound) provide an opportunity to conduct a discussion about measurement methods. It has thus been confirmed that these measurement techniques can be applied to HIFU transducers. For all cases, results underlined the fact that measurement of radiation pressure for power evaluation is more adapted to low powers (<15 W) and that measurement by calorimetry is a valid technique for global energy measurements. Composites and monocomponent transducers were compared and it appears that the presence of an adaptation glass plate reduces the efficiency of the monocomponent transducers. The distribution of ultrasonic intensity is qualitatively depicted by sono-chemiluminescence of luminol. Finally, the quantity of energy absorbed by samples placed in the sound field is determined and the temperature distribution monitored as a function of wall distance. This energetic balance allows us to understand the global behaviour of all experimental set-ups made up of a generator-transducer-liquid and sample.

Electrochemical study of silver thiosulphate reduction in the absence and presence of ultrasound.

The electrochemical reduction of silver thiosulphate was studied potentiostatically on platinum electrodes in the absence and presence of ultrasound (20 kHz). This system is irreversible and the reaction is both diffusion and kinetically controlled. The slowest step is the kinetic reaction especially the chemisorption of ions at the electrode surface. Ultrasound greatly improves the mass transport, which can be explained by changing from diffusion to mainly convection. This paper reports the effect of ultrasound upon electrode kinetic and mass-transport parameters at various RDE rotation speeds and ultrasonic intensities. It was found that the heterogeneous rate constant (kf) is improved in the presence of ultrasound due to the increase in the formal or standard heterogeneous rate constant (k0) (approximately by 10-fold under sonication).

Effects of ultrasonic irradiation on the properties of coatings obtained by electroless plating and electro plating.

This paper deals with the effects of ultrasonic irradiation on electroless copper coating i.e. metallic deposition on non-conductive substrates and on electroplating on metallic substrates. Ultrasonic irradiation was both applied during activation (surface preparation for the electroless coating) and during plating steps in both cases. Several parameters were monitored, such as plating rates, practical adhesion, hardness, internal stress versus varying acoustic powers and frequencies. Optimum conditions for irradiation time, frequency and power were determined for each step. It appears clearly that ultrasound use affects deposit properties. Then, changes in the coating mechanisms can be discussed, and several parameters will be explored in this paper, to explain enhancement of deposit properties: increase in catalyst specific area, stirring dependence, surface energy evolution, dihydrogen desorption, structure of coating.

Influence of ultrasound power and frequency upon corrosion kinetics of zinc in saline media.

This paper is devoted to zinc corrosion and oxidation mechanism in an ultrasonically stirred aerated sodium sulfate electrolyte. It follows a previous study devoted to the influence of 20 kHz ultrasound upon zinc corrosion in NaOH electrolytes [Ultrason. Sonochemis. 8 (2001) 291]. In the present work, various ultrasound regimes were applied by changing the transmitted power and the wave frequency (20 and 40 kHz). Unlike NaOH electrolyte which turns the zinc electrode into a passive state, Na2SO4 saline media induces soft corrosion conditions. This allows a study of the combined effects of ultrasonically modified hydrodynamic and mechanical damage (cavitation) upon the zinc corrosion process. A series of initial experiments were carried out so as to determine the transmitted power and to characterize mass transfer distribution in the electrochemical cell. Zinc corrosion and oxidation process were subsequently studied with respect to the vibrating parameters. When exposed to a 20 kHz ultrasonic field, and provided that the electrode is situated at a maximum mass transfer point, the corrosion rate reaches values six to eight times greater than in silent conditions. The zinc oxidation reaction, in the absence of competitive reduction reactions, is also activated by ultrasound (20 and 40 kHz) but probably through a different process of surface activation.

Ultrasound influence on the activation step before electroless coating.

This paper is devoted to the electroless plating of non-conductive substrates under ultrasound at 530 kHz. The ultrasonic irradiation is applied to the activation and to the plating steps. Effects are measured by following the final copper thickness obtained in 1 h of plating time, easily correlated to the average plating rate. It appears that ultrasound has a strong influence on the plating rates enhancement, and assumptions can be made that this increase could be linked to the catalyst cleaning. This is confirmed by XPS measurements.

Electroless copper coating of epoxide plates in an ultrasonic field.

This paper reports the study of ultrasonic irradiation effects on electroless copper coating on an epoxide resin. Several parameters were monitored, such as plating rates, practical adhesion and internal stress, versus varying acoustic powers at a constant frequency of 530 kHz. Exposure conditions were characterised by both transmitted power and interfacial mass transfer coefficients. Optimum conditions expressed in irradiation time and power were determined. The use of ultrasound during electroless copper plating affects the plating rates and the deposits properties, particularly the practical adhesion which increases whereas the internal stress decreases. Then, the changes in the coating mechanisms are discussed.

Electrochemical behaviour of zinc in 20 kHz sonicated NaOH electrolytes.

This paper describes the influence of 20 kHz ultrasound upon the corrosion behaviour of zinc in NaOH electrolytes. A systematic study of the effects exerted by ultrasound on the electrochemical interface was first carried out, so as to determine the transmitted power and to characterize mass transfer at the electrode. Then attention was focused on the corrosion passivation mechanism of zinc in sonicated NaOH solutions (0.1 and 1 M). Investigations were carried out using electrochemical techniques to determine corrosion and passivation kinetics parameters. SEM analysis of the sonicated zinc surfaces provides complementary information on the oxide layer composition and structure.