Showing papers by "Hirotsugu Ogi published in 2016"

Nucleus factory on cavitation bubble for amyloid β fibril.

Abstract: Structural evolution from monomer to fibril of amyloid β peptide is related to pathogenic mechanism of Alzheimer disease, and its acceleration is a long-running problem in drug development This study reveals that ultrasonic cavitation bubbles behave as catalysts for nucleation of the peptide: The nucleation reaction is highly dependent on frequency and pressure of acoustic wave, and we discover an optimum acoustical condition, at which the reaction-rate constant for nucleation is increased by three-orders-of magnitudes A theoretical model is proposed for explaining highly frequency and pressure dependent nucleation reaction, where monomers are captured on the bubble surface during its growth and highly condensed by subsequent bubble collapse, so that they are transiently exposed to high temperatures Thus, the dual effects of local condensation and local heating contribute to dramatically enhance the nucleation reaction Our model consistently reproduces the frequency and pressure dependences, supporting its essential applicability

Inspection of stress corrosion cracking in welded stainless steel pipe using point-focusing electromagnetic-acoustic transducer

Abstract: Point-focusing electromagnetic-acoustic transducers (PF-EMATs) for shear-vertical (SV) waves were developed for crack inspection of stainless-steel pipes. The transducer has improved defect detectability by accumulating SV waves generated by concentric line sources at a focal point in phase. An optimum frequency for defect detection was found to be 2 MHz, with which a crack of 0.5 mm depth near a weld was clearly detected. The EMAT exhibited defect detectability comparable to that of a conventional phased-array piezoelectric transducer, indicating that this new EMAT is highly practical for the non-contacting evaluation of stress-corrosion cracking in stainless steels.

Elastic constants of GaN between 10 and 305 K

Abstract: Using the antenna-transmission resonant ultrasound spectroscopy, we measured the elastic constants of GaN between 10 and 305 K using 72 resonance frequencies. The mode Gruneisen parameter is determined from temperature dependence of each elastic constant, which is larger along the c axis than along the a axis, showing anisotropy in lattice anharmonicity. The zero-temperature elastic constants, determined using the Einstein-oscillator model, yield the Debye characteristic temperature of 636 K. The ab-initio calculation is carried out for deducing the elastic constants, and comparison between calculations and measurements at 0 K reveals that the local-density-approximation potential is preferable for theoretically evaluating characteristics of GaN. The theoretical calculation also supports the anisotropy in lattice anharmonicity.

Thermal Mode Spectroscopy for Thermal Diffusivity of Millimeter-Size Solids

Abstract: Heat conduction possesses (thermal) modes in analogy with acoustics even without oscillation. Here, we establish thermal mode spectroscopy to measure the thermal diffusivity of small specimens. Local heating with a light pulse excites such modes that show antinodes at the heating point, and photothermal detection at another antinode spot allows measuring relaxation behavior of the desired mode selectively: The relaxation time yields thermal diffusivity. The Ritz method is proposed for arbitrary geometry specimens. This method is applicable even to a diamond crystal with $\ensuremath{\sim}1\text{ }\text{ }\mathrm{mm}$ dimensions.

Elastic constant C11 of 12C diamond between 10 and 613 K

Abstract: We measured the temperature dependence of the elastic constant C11 of a 12C diamond monocrystal using picosecond ultrasonics between 10 and 613 K. We found that C11 is almost temperature independent below room temperature; the temperature coefficient around 300 K is −6.6 MPa/K. Our results show a significantly higher Einstein temperature than reported values by ∼30%, indicating that diamond has a larger zero-point energy, which remains dominant around ambient temperature. We also calculated the temperature dependence of the elastic constants using ab-initio methods, resulting in good agreement with measurements. Our study shows that below-ambient-temperature measurements are not sufficient to extract the Debye temperature and the Gruneisen parameter of high-Debye-temperature materials.

Impact of interface stiffness in surface-wave resonances on nanostrip-attached substrates

Abstract: Surface waves are often excited by interdigitated transducers consisting of many nanostrips attached on a substrate, and it has been recognized that the mass and stiffness of the attached nanostrips affect surface-wave resonances to some extent. Here, we reveal the more noticeable influence of the interfacial stiffness between strips and substrate at high frequencies. This influence is confirmed by exciting and detecting surface-wave resonances up to $\ensuremath{\sim}6$ GHz by picosecond ultrasound spectroscopy. The resonance frequency significantly decreases and attenuation increases as the interfacial stiffness decreases for silicon substrate. However, low-attenuation branches appear along the Rayleigh-wave-resonance dispersion curve for silica substrate, and the resonance frequencies remain nearly identical to those of the Rayleigh waves. Previous models fail to reproduce these surface-wave resonance behaviors. The proposed theoretical model, involving the interfacial stiffness, consistently explained them, indicating the importance of the interface bond strength in designing surface-wave resonators.

Piezoelectric coefficients of GaN determined by hopping conduction of carriers

Abstract: Using resonant ultrasound spectroscopy, we monitored the resonance-frequency and internal-friction behaviors of a GaN monocrystal at elevated temperatures. An internal-friction peak appears with increasing temperature, at which reduction of frequency occurs. The frequency shift reflects the disappearance of the apparent piezoelectricity due to hopping conduction of carriers, allowing us to accurately determine the piezoelectric coefficients eij. We measured the frequency decrements of eight vibrational modes to inversely determine three independent eij: Our values are e15=−0.22±0.02 C/m2, e31=−0.14±0.02 C/m2, e33=1.15±0.05 C/m2.

Point-Focusing Electromagnetic-Acoustic Transducer for Crack Inspection

Abstract: Stress corrosion cracking in stainless-steel pipes is a critical failure in atomic power plants and chemical plants. In ultrasonic testing, piezoelectric transducers are generally used, in which reproducibility of amplitude measurements is not necessarily high because of effects of coupling materials and contacting conditions between the transducers and specimens. Comparing to the transducers, an electromagnetic acoustic transducer (EMAT) requires no coupling materials and is less sensitive to the contacting conditions, making the reproducibility higher. However, lower signal to noise (S/N) ratio has been a disadvantage. For increasing the S/N ratio, we developed a point-focusing EMAT (PF-EMAT) [1]. It generated shear-vertical (SV) waves from concentric line sources on a top surface of a specimen. Coil configuration of the EMAT was designed so that the SV waves were accumulated in phase at a focal point on the bottom surface, which increased the S/N ratio and improved the spatial resolution. We have designed PF-EMATs operated at different frequencies, and applied to artificially fabricated defects on stainless steel specimens. In this presentation, we show the results, and discuss availability of the PF-EMAT to crack inspection.