Jedo Kim

Emission enhancement of sound emitters using an acoustic metamaterial cavity.

The emission enhancement of sound without electronic components has wide applications in a variety of remote systems, especially when highly miniaturized (smaller than wavelength) structures can be used. The recent advent of acoustic metamaterials has made it possible to realize this. In this study, we propose, design, and demonstrate a new class of acoustic cavity using a double-walled metamaterial structure operating at an extremely low frequency. Periodic zigzag elements which exhibit Fabry-Perot resonant behavior below the phononic band-gap are used to yield strong sound localization within the subwavelength gap, thus providing highly effective emission enhancement. We show, both theoretically and experimentally, 10 dB sound emission enhancement near 1060 Hz that corresponds to a wavelength approximately 30 times that of the periodicity. We also provide a general guideline for the independent tuning of the quality factor and effective volume of acoustic metamaterials. This approach shows the flexibility of our design in the efficient control of the enhancement rate.

Directional Reflective Surface Formed via Gradient-Impeding Acoustic Meta-Surfaces

Artificially designed acoustic meta-surfaces have the ability to manipulate sound energy to an extraordinary extent. Here, we report on a new type of directional reflective surface consisting of an array of sub-wavelength Helmholtz resonators with varying internal coiled path lengths, which induce a reflection phase gradient along a planar acoustic meta-surface. The acoustically reshaped reflective surface created by the gradient-impeding meta-surface yields a distinct focal line similar to a parabolic cylinder antenna, and is used for directive sound beamforming. Focused beam steering can be also obtained by repositioning the source (or receiver) off axis, i.e., displaced from the focal line. Besides flat reflective surfaces, complex surfaces such as convex or conformal shapes may be used for sound beamforming, thus facilitating easy application in sound reinforcement systems. Therefore, directional reflective surfaces have promising applications in fields such as acoustic imaging, sonic weaponry, and underwater communication.

Sound Pressure Level Gain in an Acoustic Metamaterial Cavity

The inherent attenuation of a homogeneous viscous medium limits radiation propagation, thereby restricting the use of many high-frequency acoustic devices to only short-range applications. Here, we design and experimentally demonstrate an acoustic metamaterial localization cavity which is used for sound pressure level (SPL) gain using double coiled up space like structures thereby increasing the range of detection. This unique behavior occurs within a subwavelength cavity that is 1/10th of the wavelength of the incident acoustic wave, which provides up to a 13 dB SPL gain. We show that the amplification results from the Fabry-Perot resonance of the cavity, which has a simultaneously high effective refractive index and effective impedance. We also experimentally verify the SPL amplification in an underwater environment at higher frequencies using a sample with an identical unit cell size. The versatile scalability of the design shows promising applications in many areas, especially in acoustic imaging and underwater communication.

Molecular Dynamics Simulation of Elastic Properties of Silicon Nanocantilevers

The molecular dynamics simulation of nanoscale cantilevers made of pure crystalline silicon with different lattice conditions is presented. Youngs moduli for various sized specimen is obtained by simulating clamped-free cantilever beam vibrations and static tensile responses. Youngs modulus decreases monotonically as the thickness of the specimen decreases. Although significant discrepancies exist between the simulated and experimentally determined Youngs modulus, incorporating a minute amount of voids in the specimen during simulation offers a partial account of this discrepancy. The dependence of the Youngs modulus on dimensional scaling is then applied to estimate thermal fluctuations of the cantilever under various temperatures, sizes, and lattice conditions and shows excellent agreement with the theoretical estimate based on the equipartition theorem. Finally, the applicability of the nanocantilevers as molecular mass sensors is demonstrated by simulating the change in the first flexural mode frequency as the number of silicon molecules placed at the tip of the cantilever is varied. The results show good agreement with the theoretical predictions of the Euler-Bernoulli beam vibration model.

Concentric artificial impedance surface for directional sound beamforming

Utilizing acoustic metasurfaces consisting of subwavelength resonant textures, we design an artificial impedance surface by creating a new boundary condition. We demonstrate a circular artificial impedance surface with surface impedance modulation for directional sound beamforming in three-dimensional space. This artificial impedance surface is implemented by revolving two-dimensional Helmholtz resonators with varying internal coiled path. Physically, the textured surface has inductive surface impedance on its inner circular patterns and capacitive surface impedance on its outer circular patterns. Directional receive beamforming can be achieved using an omnidirectional microphone located at the focal point formed by the gradient-impeding surface. In addition, the uniaxial surface impedance patterning inside the circular aperture can be used for steering the direction of the main lobe of the radiation pattern.

Conical microspike morphology formation and control on various metal surfaces using femtosecond laser pulse

Formation of conical microspikes on various metal surfaces (316L stainless steel, Ti-6Al-4V, and Al5754) under femtosecond irradiation at high repetition rate is reported. Two types of microcone morphologies formed at these high repetition rates under high and low-fluence conditions were clearly distinguished. At low fluence (near the ablation thresholds), conical spikes with high aspect ratio and nonuniform distribution forms through random evolution. At high fluence, semiuniform conical spikes are formed through a simultaneous progressive evolution procedure with increasing the number of scans. Experimental results are presented showing the progression of random microspike formation to uniform microspikes as fluence increases and show how scan-speed affects the size of the spikes. Also, extraordinary absorption coefficient is measured for nonuniform conical spike covered 316L stainless steel formed under near threshold condition.

Enhanced carbon tolerance of Ir alloyed Ni-Based metal for methane partial oxidation

Carbon plugging of active catalytic sites significantly degrades the performance of the hydrocarbon reformers. In this study, we show that small amount of Ir alloyed to Ni/CeO2 nanoparticle exhibit promising improvements to the carbon tolerance properties. XRD analysis indicates that the synthesized nanoparticles are comprised of independent NiO and CeO2 particulates and that the added Ir atoms tend to stay on the surface consistent with the theoretical calculation results of the proposed Ir-Ni alloy. The Ir rich samples show higher methane cracking rate and better carbon removal characteristics. Also, the CO selectivity result shows that adding Ir can prolong the lifetime of the Ni active sites despite a slight drop in the initial partial oxidization reforming rate. Our findings highlight the enhancement effects of Ir on the Ni-based metal carbon tolerance properties and bring us one step closer to finding a solution for the carbon plugging problem.

Formation mechanism of micro-spikes on various metals with femtosecond laser pulses

Formation of conical micro-spikes on various metal surfaces (316L stainless steel, Ti-6Al-4V, and Al5754) under femtosecond irradiation at high repetition rate is reported. Two types of micro-cone morphologies formed at these high repetition rates under high and low-fluence conditions were clearly distinguished. At low fluence (near the ablation thresholds), conical spikes with low aspect ratio and non-uniform distribution forms through random evolution. At high fluence, semi-uniform conical spikes with higher aspect ratios are formed through simultaneous progressive evolution procedure with increasing the number of scans. Experimental results are presented showing the progression of random micro-spike formation to uniform micro-spikes as fluence increases and how scan-speed affects the size of the spikes. For both formation processes, the scan speed is found to be an effective parameter to control the micro-cone size (or micro-cone number density).Formation of conical micro-spikes on various metal surfaces (316L stainless steel, Ti-6Al-4V, and Al5754) under femtosecond irradiation at high repetition rate is reported. Two types of micro-cone morphologies formed at these high repetition rates under high and low-fluence conditions were clearly distinguished. At low fluence (near the ablation thresholds), conical spikes with low aspect ratio and non-uniform distribution forms through random evolution. At high fluence, semi-uniform conical spikes with higher aspect ratios are formed through simultaneous progressive evolution procedure with increasing the number of scans. Experimental results are presented showing the progression of random micro-spike formation to uniform micro-spikes as fluence increases and how scan-speed affects the size of the spikes. For both formation processes, the scan speed is found to be an effective parameter to control the micro-cone size (or micro-cone number density).