Jungsoon 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.

Horizontal molecular orientation in solution-processed organic light-emitting diodes

Horizontal orientation of the emission transition dipole moments achieved in glassy vapor-deposited organic thin films leads to an enhancement of the light out-coupling efficiency in organic light-emitting diodes (OLEDs). Here, our combined study of variable angle spectroscopic ellipsometry and angle dependent photoluminescence demonstrates that such a horizontal orientation can be achieved in glassy spin-coated organic films based on a composite blend of a heptafluorene derivative as a dopant and a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl as a host. Solution-processed fluorescent OLEDs with horizontally oriented heptafluorene emitters were then fabricated and emitted deep blue electroluminescence with an external quantum efficiency as high as 5.3%.

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.

Visualization of Thermal Distribution Caused by Focused Ultrasound Field in an Agar Phantom

Ultrasound waves have been widely used not only in medical diagnostic systems but also in medical treatment. There has been increasing interest in the thermal distribution caused by ultrasound waves. In this study, using thermochromic particles, a visualization method was suggested in a mimic phantom for organic materials. By blending various thermochromic particles with different critical temperatures, the thermal field distribution in the phantom could be observed. The effect of a concave-type ultrasound transducer on the temperature distribution in an agar phantom mixed with thermochromic particles was observed. The temperature distribution corresponded to changes in the brightness distribution of gray. It was confirmed that the thermal distribution pattern in the vicinity of the focal area varies with the time exposure to ultrasound waves.

Effect of Focused Ultrasound on Residual Particle Size Distribution in Water

The residual particle distribution in pure water could be a problem in the nanoscience fields because nanoparticles are usually suspended in pure water such as deionized or distilled water. The effect of ultrasound on the residual particle distribution in pure water should be analyzed because the nano particles used in nanoscience are usually dispersed using ultrasound power. In this study, using a cylindrical piezoelectric vibrator, a noncontact-type focusing ultrasound system was fabricated to keep the purity of the water. The residual particle distribution in pure water was investigated. The number of residual particles increased as the ultrasound exposure time increased.

Dispersion Method Using Focused Ultrasonic Field

The dispersion of powders into liquids has become one of the most important techniques in high-tech industries and it is a common process in the formulation of various products, such as paint, ink, shampoo, beverages, and polishing media. In this study, an ultrasonic system with a cylindrical transducer is newly introduced for pure nanoparticle dispersion. The acoustics pressure field and the characteristics of the shock pulse caused by cavitation are investigated. The frequency spectrum of the pulse from the collapse of air bubbles in the cavitation is analyzed theoretically. It was confirmed that a TiO2 water suspension can be dispersed effectively using the suggested system.

Aspect Ratio Dependence of Electromechanical Coupling Coefficient k31 of Lateral-Excitation Piezoelectric Vibrator

Using the theory of coupled vibrations, the lateral-excitation mode is theoreticaliy analyszed. As a result, a formula for the electromechanical coupling coefficient k31 was derived as a function of the aspect ratio between the width and length of a rectangular thin-plate piezoelectric vibrator. This formula can be used in the design for device fabrication using a lateral-excitation-mode piezoelectric vibrator with an arbitrary aspect ratio.

Arrayed Ultrasonic Transducers on Arc Surface for Plane Wave Synthesis

In ultrasonic computed tomography (UCT), it is necessary to synthesize a plane wave using waves emitted from sound sources arranged in the interior surface of a cylinder. In order to transmit a plane wave into a cylindrical surface, an ultrasonic transducer which has many vibrating elements with piezoelectric transverse effect arrayed on an arc surface is proposed. To achieve a wide beam width, the elements should have a small radiation area with a much narrow width. The measured electroacoustic efficiency for the elements was approximately 40% and the beam width defined by -3 dB level from the maximum was as wide as 120 deg. It was confirmed that plane wave synthesis is possible using the proposed transducer array.

Acoustic Characteristics of a Tissue Mimicking Phantom for Visualization of Thermal Distribution

An ultrasonic tissue mimicking phantom is a useful tool for the performance testing and calibration of ultrasonic medical devices. However, it is hard to estimate the thermal distribution caused by the ultrasound in the human body using a conventional phantom. In this study, a tissue mimicking phantom to visualize the thermal distribution was developed by using thermochromic particles. The acoustic characteristics of the phantom were investigated according to the ingredient ratio of the phantom. Changing the ingredient ratio, the acoustic characteristics of the phantom can be adjusted to those of human tissue.