MÖSSBAUER STUDY ON RECOVERY OF COLD-WORKED Fe-Al ALLOYS

Le processus de la restauration isotherme de l'ordre atomique dans les alliages Fe-Al écrouis a été étudié par spectrométrie Mössbauer. On a analysé les spectres observés pour obtenir à chaque étape du recuit la distribution du champ interne dans les alliages. La probabilité d'existence des configurations atomiques a pu être obtenue aussi, en supposant une fonction de distribution Gaussienne du champ hyperfin. Les alliages ordonnés de type DO3, magnétiques ou non-magnétiques selon leur composition, ont été désordonnés et rendus magnétiques par laminage. Pour ces alliages, on assiste à une restauration très lente,au cours de recuit, vers l'état initial de type DO3 en passant par un ordre de type B2. Les alliages ordonnés de type B2, non-magnétiques, ont été partiellement désordonnés par laminage et ont donné un spectre Mössbauer comportant des caractéristiques magnétiques et non-magnétiques. Les alliages laminés se sont rapidement réordonnés pour donner les états initiaux par le recuit.The process of atomic reordering of cold-worked Fe-Al alloys on isothermal annealing has been investigated by means of Mössbauer spectroscopy. Observed spectra were analyzed to obtain the distribution function of the internal magnetic field in the alloys at each stage of annealing. The probabilities of the nearest neighbor configurations of constituent atoms could also be obtained by assuming the Gaussian distribution function of hyperfine field acting on iron atoms with various number of iron neighbors. The alloys ordered with DO3 symmetry, which are either magnetic or nonmagnetic depending on their composition, were changed by cold working into disordered and strongly magnetic states. These alloys recovered very slowly in the course of annealing to their initial state by way of B2 type of order. The alloys with B2 type of order, which are nonmagnetic, were partially disordered by cold working, and exhibited the Mössbauer spectra of both magnetic and nonmagnetic characters. The alloys recovered promptly to their initial state by annealing

Influence of the bubble-bubble interaction on destruction of encapsulated microbubbles under ultrasound

Influence of the bubble-bubble interaction on the pulsation of encapsulated microbubbles has been studied by numerical simulations under the condition of the experiment reported by Chang et al. [IEEE Trans. Ultrason Ferroelectr. Freq. Control48, 161 (2001)]. It has been shown that the natural (resonance) frequency of a microbubble decreases considerably as the microbubble concentration increases to relatively high concentrations. At some concentration, the natural frequency may coincide with the driving frequency. Microbubble pulsation becomes milder as the microbubble concentration increases except at around the resonance condition due to the stronger bubble-bubble interaction. This may be one of the reasons why the threshold of acoustic pressure for destruction of an encapsulated microbubble increases as the microbubble concentration increases. A theoretical model for destruction has been proposed

Mechanism of enhancement of sonochemical-reaction efficiency by pulsed ultrasound

The enhancement of sonochemical-reaction efficiency by pulsed ultrasound at 152 kHz has been studied experimentally through absorbance measurements of triiodide ions from sonochemical oxidation of potassium iodide at different liquid volumes to determine sonochemical efficiency defined by reacted molecules per input ultrasonic energy. The mechanism for enhancement of the reaction efficiency by pulsed ultrasound is discussed using captured images of sonochemiluminescence (SCL), and measured time-resolved signals of the SCL pulses and pressure amplitudes. The high sonochemical-reaction efficiency by pulsed ultrasound, compared with that by continuous-wave ultrasound, is attributed both to the residual pressure amplitude during the pulse-off time and to the spatial enlargement of active reaction sites

Spatial distribution enhancement of sonoluminescence activity by altering sonication and solution conditions

An intensified charge-couped device (CCD) camera was used to capture raw images of multibubble sonoluminescence, generated by 168 and 448 kHz ultrasound. The effect of various air and surfactant concentrations, and pulse conditions on the acoustic pressure distribution, percentage of standing wave component, the structure of the sonoluminescence activity, and speed of streaming was investigated. It was observed that the enhancement in the sonoluminescence intensity by appropriate degassing, pulsing, and addition of sodium dodecylsulfate were closely related to an expansion in the spatial distribution of sonoluminescence activity. This broadening in the spatial distribution is correlated with a high percentage of standing wave component. This effect stems from the reduction in the attenuation of the acoustic field by inhibiting the formation of large coalesced bubbles

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. "Transient" cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, "stable" cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble-bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both "stable" and "transient" cavitation bubbles which pulsate nonlinearly with the period of ultrasound

Optical cavitation probe using light scattering from bubble clouds

To understand the behaviour of systems containing clouds of bubbles (multibubble system) in real sonochemical reactors, a new diagnosis method, i.e., optical cavitation probe (OCP), has been proposed. When a laser beam is introduced into the cavitation bubble cloud, the scattered light intensity changes by the collective oscillation of cavitation bubbles. The frequency domain spectrum of the scattered light contains rich information on the cavitation bubble clouds, comparable with the acoustic emission spectra detected by a hydrophone. The significant merits of OCP, such as capability for spatially resolved, non-invasive measurement of the cavitation bubble clouds, robustness even in a violent cavitation field have been experimentally demonstrated. © 2008 Elsevier B.V. All rights reserved

Fabrication of silver nanoparticles deposited on boehmite sol for surface enhanced Raman scattering

The composite consisting of silver nanoparticles deposited on boehmite hybrid was synthesized by NaBH4 reduction technique. The morphology of the composite was studied by TEM, UV/Vis spectrophotometer and particle sizer. The size of the silver nanoparticles deposited on the surface of the boehmite ranged from 10 nm to 100 nm. The contact of silver nanoparticles increased by means of deposition of silver nanoparticles on the boehmite sol and the aggregation of the composites. This leads to the appearance of a shoulder at 450 nm in the UV–Vis absorption spectra with the addition of 0.15 mg and 1.5 mg boehmite. It was found that the intensity of the SERS in the case of the composite was higher than for silver colloids consisting of a concentration of silver greater than 3.2 mM

Study of an acoustic field in a microchannel

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed