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35. Measurements of nonlinearity parameter B/A of water-saturated glass beads under uniaxial compressive loading.

Author

Lee, Hunki, Noh, Eunghwy, Ohm, Won-Suk, and Kwon, Oh-Cho

Subjects
GLASS beads, COMPRESSION loads, GREEN'S functions, PLASMA beam injection heating, OCEAN bottom
Abstract

This Letter reports measurements of nonlinearity parameter B/A of water-saturated glass beads under uniaxial compression representative of the gravitational loading in seabed. The finite-amplitude method in conjunction with the multi-Gaussian beam model is used to determine B/A, in which the use of an accurate source function is found to be important in achieving the overall measurement accuracy. The estimated B/A is on the order of a hundred and decreases with increasing uniaxial compression up to 50 kPa, so that B/A could change as much as 20% over a depth of 5 m for a typical coarse sandy sediment. [ABSTRACT FROM AUTHOR]

Published
2019
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48. Control of electromechanical properties of multilayer ceramic capacitors for vibration reduction.

Author

Ohm, Won‐Suk, Kim, Dongjoon, Ko, Byung‐Han, and Park, No‐Cheol

Subjects
*ELECTROMECHANICAL devices, *CERAMIC capacitors, *IMAGE stabilization, *FERROELECTRIC ceramics, *VIBRATION (Mechanics)
Abstract

Abstract: A multilayer ceramic capacitor (MLCC) contains layers of ceramics as the dielectric materials. It has been known that Class 2 MLCCs, made of ferroelectric ceramics such as barium titanate, tend to suffer from electromechanical coupling hence vibration, which leads to the generation of acoustic humming noise, a source of annoyance in many modern electronic devices. In this article, a repoling method to control the electromechanical properties and the resulting vibration of MLCCs is presented. The repoling protocol hinges on the understanding that two independent mechanisms are responsible for the electromechanical coupling in MLCCs: piezoelectricity and electrostriction of the ceramic layers. The vibration due to piezoelectricity is linearly proportional to the input voltage, whereas the vibration due to electrostriction shows a quadratic dependence. Given the DC bias and the AC input voltage under normal operating conditions, the vibration is composed of the fundamental component at the frequency of the AC input and the second harmonic component spawned by the quadratic nonlinearity of electrostriction. It is demonstrated that by engineering the coefficients of piezoelectricity and electrostriction of the ceramic layers through a carefully designed repoling treatment, vibration reduction can be achieved for both the fundamental and second harmonic components. Especially, the fundamental component of vibration can be reduced significantly, as the piezoelectric effect is made to offset the electrostrictive effect. [ABSTRACT FROM AUTHOR]

Published
2018
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49. Acoustically sticky topographic metasurfaces for underwater sound absorption.

Author

Lee, Hunki, Jung, Myungki, Kim, Minsoo, Shin, Ryung, Kang, Shinill, Ohm, Won-Suk, and Kim, Yong Tae

Subjects
ACOUSTICS, ABSORPTION of sound, UNDERWATER acoustics, METAMATERIALS, SOUND
Abstract

A class of metasurfaces for underwater sound absorption, based on a design principle that maximizes thermoviscous loss, is presented. When a sound meets a solid surface, it leaves a footprint in the form of thermoviscous boundary layers in which energy loss takes place. Considered to be a nuisance, this acoustic to vorticity/entropy mode conversion and the subsequent loss are often ignored in the existing designs of acoustic metamaterials and metasurfaces. The metasurface created is made of a series of topographic meta-atoms, i.e., intaglios and reliefs engraved directly on the solid object to be concealed. The metasurface is acoustically sticky in that it rather facilitates the conversion of the incident sound to vorticity and entropy modes, hence the thermoviscous loss, leading to the desired anechoic property. A prototype metasurface machined on a brass object is tested for its anechoicity, and shows a multitude of absorption peaks as large as unity in the 2–5 MHz range. Computations also indicate that a topographic metasurface is robust to hydrostatic pressure variation, a quality much sought-after in underwater applications. [ABSTRACT FROM AUTHOR]

Published
2018
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