Moojoon Kim

Electromechanical coupling coefficient of an ultrasonic array element

One of the most important parameters for characterizing piezoelectric materials is the so-called electromechanical coupling coefficient, k, which describes the electromechanical coupling strength. Although this parameter should be an intrinsic material parameter, it appears to depend on the aspect ratio of the resonator. There are three different values defined for three extreme geometries, k33, k33′, and kt, and they differ by more than 50%. Unfortunately, these three values cannot describe resonators of general geometries and also create conceptual confusion. Here, we provide a unified formula that will accurately describe the coupling coefficient of rectangular slender bar transducer array element with any aspect ratio.

Aspect ratio dependence of electromechanical coupling coefficient of piezoelectric resonators

The most important parameter characterizing a piezoelectric material is the electromechanical coupling coefficient, which specifies the conversion efficiency between electrical and mechanical energies for a given vibration mode. For the resonance along the poling direction, there are two coupling coefficients defined, i.e., kt and k33, which are for resonators of the same mode but two extreme aspect ratios and they differ substantially. We have derived a unified formula for this coupling coefficient as a function of the vibrator aspect ratio. The unified formula can provide more accurate description to the effective coupling coefficient of resonators not satisfying the extreme aspect ratio requirements.

Experimental technique for characterizing arbitrary aspect ratio piezoelectric resonators

The electromechanical coupling coefficient k is the most important parameter for the characterization of a piezoelectric material and for the design of electromechanical devices. Standard resonance technique for measuring the k value is based on one-dimensional approximation, which needs to use samples having extreme geometries. For small size crystals and/or due to geometrical constraints in many devices, piezoelectric resonators may not be made into such extreme geometries. An averaging scheme has been developed to tackle this challenging experimental task, and the so obtained k values agree well with theoretical predictions of k values for arbitrary aspect ratio resonators.

Ultrasonic streaming and cavitation for nano particle dispersion

Recently, non-contact type ultrasonic dispersion technique is attracting attention in various industrial fields. To disperse the nano particle suspension effectively, new dispersion system was suggested using arrayed piezoelectric vibrators with high resonant frequency. The suggested system has high intensity ultrasonic field and stirring effect caused by eddying current because the piezoelectric vibrators, which are arrayed on inner surface of a metal cylinder and tilled by the regular degree, generate focused ultrasound and make streaming effect on nano particle suspension. The system has the erosion dispersion caused by the ultrasonic shock wave due to the ultrasonic cavitation as well as the division dispersion caused by shear stress of the fluid from the eddy current due to the ultrasonic streaming in the suspension, simultaneously. To confirm these effects, TiO2 suspension was dispersed with the suggested system and the effectiveness of the systems was confirmed.

Separation of single nano particles from suspension by using ultrasonic atomization

It is very important to obtain non-agglomerated nano particle state for practical application of nano technology. In order to improve the functionality of products using nano particles, more precise control of particle size distribution is required in their synthesis process. However, synthesized nano particles are agglomerated due to physical and chemical reasons, and it veils the unique properties of nano particles and has disturbed their practical use. In this study, we propose a separation method for nano particles from suspension by using the droplets as the separation space. The particle size distribution of nano particles in the recollected suspension was measured. From the results, it was confirmed that it is possible to separate and to recollect monodispersed nano particles from original suspension with 0.002 wt.% TiO2 powder by using suggested method.

Sound speed change with concentration in nano particle suspension

Based on the multi-scattering theory for the micro particles, sound speed of the TiO2 nano particle suspension with ultra-low concentration was estimated. Good agreement was found between the estimated results and the experimental ones. The sound speed of 5 MHz ultrasound was measured with high accuracy, decreasing by about 0.15 m/s as concentration of suspension was changed in the range from 0.002 wt.% to 0.02 wt.%. From the experimental result, it is expected that the concentration of nano particle suspension is possibly estimated from the sound speed change.

Single nano particle collecting by using ultrasonic humidifier

The single nano particle state was obtained by using a ultrasonic humidifier. To control the surface tension, the concentration of methanol changed in the suspension of TiO2 nano particle. The collecting effect was investigated by measuring the particle size distribution. As the surface tension was decreased by increasing the concentration of methanol, the particle size at the peak of distribution was decreased and the number of particles per unit volume was increased. The nano particles with intended size could be collected by controlling the surface tension of the suspension.

Trapping of microorganism using cylindrical standing ultrasound waves and its application to water purification

In biological fields, it is known that the ultrasound is useful for trapping of biological cells or microorganism. Recently, several experimental results of micro-particle trapping by acoustic standing wave fields which are formed by plane waves have been reported. In this study, we confirm that the standing waves by a cylindrical ultrasonic transducer can trap and aggregate the microorganism such as green algae, chlorella, etc. Those microorganism were trapped at specific positions determined by acoustic pressure distribution, and the density of aggregation is increased according to the lapse time after the transducer operating. The dense colony of microorganism is easily filtered out by a sieve. A water purification system using this phenomenon was designed and the efficiency was verified by considering the density change of the microorganism after the ultrasonic treatment. Consequently, it is shown that the standing wave in a cylindrical transducer can be applied to water purification.