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

Spatial Distribution of Acoustic Cavitation Bubbles at Different Ultrasound Frequencies

Images of sonoluminescence, sonophotoluminescence and sonochemiluminescence are recorded in order to semi-quantitatively compare the spatial distribution of the cavitation activity at three different ultrasound frequencies (170 kHz, 440 kHz and 700 kHz) and at various acoustic amplitudes. At all ultrasound frequencies investigated, the sonochemically active cavitation zones are much larger than the cavitation zones where sonoluminescence is observed. Also, the sonochemically active bubbles are observed at relatively lower acoustic amplitudes than that required for sonoluminescence bubbles to appear. The acoustic power required for the observation of the initial cavitation bubbles increases with an increase in the ultrasound frequency. The cavitation bubbles are observed relatively uniformly throughout the reactor at 170 kHz whereas they are located away from the transducer at the higher frequencies used in this study. While these observations highlight the complexities involved in acoustic cavitation, possible reasons for the observed results are discussed

Bubble population phenomena in sonochemical reactor: II. Estimation of bubble size distribution and its number density by simple coalescence model calculation

A simple bubble population model, with emphasis on the bubble–bubble coalescence, is proposed. In this model, the bubble size distribution is simulated through the iteration of fundamental bubble population process: generation, dissolution, and coalescence. With this simple modelling, the bubble size distribution experimentally observed by the pulsed laser diffraction method and the void rate obtained by the capillary system at 443 kHz were successfully simulated. The experimental results on the bubble population growth by the repetitive pulsed sonication and the effect of pulse width on the bubble population were recreated by the numerical simulation in a semi-quantitative manner. The importance of coalescence of bubbles especially for the effect of addition of surfactant is demonstrated. By decreasing the coalescence frequency by one order of magnitude in the simulation, both the drastic decrease in the total bubble volume as well as the depression of bubble size distribution centring from a few tens of microns in water to a few microns in a dilute surfactant solution can be simultaneously derived

Development and optimization of acoustic bubble structures at high frequencies

At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios

The detection and control of stable and transient acoustic cavitation bubbles

Acoustic cavitation plays a significant role in many applications that include therapeutic medicine, sonochemsitry and materials processing. The type of cavitation bubbles generated, namely transient or stable (repetitive transient), is important for optimizing the efficiency of these applications. We report on a multibubble sonoluminescence (MBSL)-based experimental technique for the detection and the control of type of cavitation at low and high ultrasound frequencies. It is observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles, as reported earlier. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generates a significant amount of stable cavitation, as evidenced from the quenching of sonoluminescence by volatile solutes. Evidence for the existence of transient cavitation at a higher ultrasound frequency (440 kHz) is also provided

Bubble population phenomena in sonochemical reactor: I Estimation of bubble size distribution and its number density with pulsed sonication - Laser diffraction method

To characterize the bubble populations (size and its number distribution) in a sonochemical reactor, a simple but powerful technique based on the Fraunhofer laser diffraction (LD) has been proposed. In this method, the acoustic wave disturbance to the laser probe in the sonochemical reaction field was eliminated by the temporal separation using pulsed sonication (pulsed LD). With this relatively simple strategy, the temporal development of the bubble size distribution could be evaluated by pulsed LD. A number density of bubbles was estimated by using a calibration data obtained with monosized standard particles. In addition, the effect of pulse length and a surfactant on the bubble population phenomena in a multibubble system are discussed

The detection and control of stable and transient acoustic cavitation bubbles

Acoustic cavitation plays a significant role in many applications that include therapeutic medicine, sonochemsitry and materials processing. The type of cavitation bubbles generated, namely transient or stable (repetitive transient), is important for optimizing the efficiency of these applications. We report on a multibubble sonoluminescence (MBSL)-based experimental technique for the detection and the control of type of cavitation at low and high ultrasound frequencies. It is observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles, as reported earlier. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generates a significant amount of stable cavitation, as evidenced from the quenching of sonoluminescence by volatile solutes. Evidence for the existence of transient cavitation at a higher ultrasound frequency (440 kHz) is also provided

Bubble population phenomena in sonochemical reactor: I Estimation of bubble size distribution and its number density with pulsed sonication - Laser diffraction method

To characterize the bubble populations (size and its number distribution) in a sonochemical reactor, a simple but powerful technique based on the Fraunhofer laser diffraction (LD) has been proposed. In this method, the acoustic wave disturbance to the laser probe in the sonochemical reaction field was eliminated by the temporal separation using pulsed sonication (pulsed LD). With this relatively simple strategy, the temporal development of the bubble size distribution could be evaluated by pulsed LD. A number density of bubbles was estimated by using a calibration data obtained with monosized standard particles. In addition, the effect of pulse length and a surfactant on the bubble population phenomena in a multibubble system are discussed

Development and optimization of acoustic bubble structures at high frequencies

At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios