Nobutomo Nakamura

Elastic, anelastic, and piezoelectric coefficients of langasite: resonance ultrasound spectroscopy with laser-Doppler interferometry

This paper presents advanced techniques to determine all independent elastic-stiffness coefficients C/sub ij/, the associated internal friction Q/sub ij//sup -1/, and piezoelectric coefficients e/sub ij/ of monocrystal langasite (La/sub 3/Ga/sub 5/SiO/sub 14/) using a single rectangular parallelepiped specimen. Langasites crystal structure belongs to the trigonal system with point group 32, and thus possesses six independent C/sub ij/, two e/sub ij/, and two dielectric coefficients /spl epsiv//sub ij/. All of the elastic and piezoelectric coefficients affect the mechanical resonance frequencies of the solid specimen, and measuring them very accurately permits one to determine the C/sub ij/ and e/sub ij/ with known density, dimensions, and e/sub ij/. We developed a piezoelectric tripod to support the specimen upward and measured the free-vibration resonance frequencies with minimum load from its own weight. This weak and stable acoustic coupling ensures accuracy of the frequency measurement better than 10/sup -5/, enough to determine the coefficients reliably. Our C/sub ij/ fall in the range of results measured with previous (conventional) methods. Our e/sub 11/ is smaller than the reported values by 1.2-13%, and e/sub 14/ is larger by 44-97%. For the internal friction measurement, we used a solenoid coil to vibrate the specimen without any contact. The longitudinal-wave internal friction considerably exceeds the shear-wave internal friction, which can be explained by phonon-phonon interactions.

Elastic, anelastic, and piezoelectric coefficients of α-quartz determined by resonance ultrasound spectroscopy

All independent components of the elastic constants, internal friction, and piezoelectric coefficients of synthetic α-quartz have been simultaneously determined by resonance ultrasound spectroscopy coupled with laser-Doppler interferometry. Seventeen crystals obtained from Z and X regions with various infrared-absorption values were used; for each crystal, a complete set of the coefficients was determined using 72 resonance frequencies, which were measured by a needle-transducer tripod in a vacuum at a constant temperature with frequency-error limit of 0.001%. The infrared-absorption value and grown region did not strongly affect the material coefficients. Among the eight (elastic plus piezoelectric) coefficients, piezoelectric coefficients were significantly different from previously reported values. The six independent internal-friction components showed a positive correlation with the temperature derivatives of the corresponding elastic constants, indicating that the dominant damping mechanism was the ...

Elastic, anelastic, and piezoelectric coefficients of GaN

We report elastic, anelastic, and piezoelectric coefficients of wurtzite GaN measured by resonant-ultrasound spectroscopy coupled with laser-Doppler interferometry. Five rectangular parallelepiped specimens, measuring 6.5 × 2.0 × 4.0 mm3, cut from two single crystals were used. Our values of elastic and piezoelectric coefficients were C11 = 359.4 GPa, C12 = 129.2 GPa, C13 = 92.0 GPa, C33 = 389.9 GPa, C44 = 98.0 GPa, e15 = 0.10 C/m2, e31 = 0.17 C/m2, and e33 = 0.29 C/m2. In anelastic coefficients, anisotropy was observed between Q11−1 and Q33−1.

Observation of higher stiffness in nanopolycrystal diamond than monocrystal diamond

Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grain-boundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials.

Off-diagonal elastic constant and sp2-bonded graphitic grain boundary in nanocrystalline-diamond thin films

This letter studies the relationship between the off-diagonal elastic constant C12 and bond configuration in nanocrystalline-diamond (NCD) thin films deposited by the nitrogen-doped chemical vapor deposition method. The film thickness was varied between 2.4 and 11.3μm. The elastic constants were measured by resonant-ultrasound spectroscopy coupled with laser-Doppler interferometry. The diagonal elastic constants C11 and C44, and Young’s modulus in NCD films are smaller than those of the bulk polycrystalline diamond and microcrystalline-diamond (MCD) thin films, and they decrease as the film thickness decreases. However, the off-diagonal elastic constant of the NCD films is significantly larger than that of the bulk diamond, while that of the MCD films is smaller. Micromechanics calculations revealed that this exceptional enhancement of C12 occurs when the material includes randomly distributed thin graphitic plates in the isotropic diamond matrix. Thus, this result indicates that the NCD films consist of ...

Resonance acoustic-phonon spectroscopy for studying elasticity of ultrathin films

Ultrahigh-frequency phonon resonances were excited in ultrathin films (∼5nm) by femtosecond light pulses, and their resonance frequencies were measured to determine the through-thickness elastic constants. The studied materials were Pt and Fe. The elastic stiffness increases with decreasing film thickness for both materials, whereas the normal strain showed opposite thickness behavior. Analysis of the wave propagation using the third-order elastic constants explained these trends.

Study of Elastic Anisotropy of Cu Thin Films by Resonant-Ultrasound Spectroscopy Coupled with Laser-Doppler Interferometry and Pump-Probe Photoacoustics

Understating elastic properties of thin films is a matter of deep interest both in scientific and industrial fields. In this paper, we propose a combination of resonant-ultrasound spectroscopy coupled with the laser-Doppler interferometry and pump-probe photoacoustics for measuring anisotropic elastic constants of deposited thin films. Among the five independent elastic constants, the resonant-ultrasound spectroscopy is sensitive to the in-plane elastic constants, C11, C13, and C66, and pump-probe photoacoustics to the out-of-plane elastic constant, C33. We apply this to Cu thin films deposited on monocrystalline Si substrates by the magnetron-sputtering method. Cu thin films show elastic anisotropy, C33>C11. The cause of this elastic anisotropy is attributed to the textured structure and the columnar structure. These effects are estimated by X-ray diffraction measurements and micromechanics calculations.

Mode Conversion and Total Reflection of Torsional Waves for Pipe Inspection

The group velocity of each higher torsional mode in a pipe depends on the wall thickness and it will convert to a lower mode when the thickness is smaller than a critical value, the so-called cut-off thickness. The fundamental torsional mode, T(0,1), and the first higher mode, T(0,2), are generated in pipes by an electromagnetic acoustic transducer (EMAT) and their mode conversion behaviors are investigated by changing the shape of a defect on the pipe. It is confirmed that the conversion behavior is sensitive to the shape of thickness transition and the total reflection of the T(0,2) mode occurs in a pipe with a tapered defect. On the basis of the mode conversion, defects with a cross-sectional loss larger than 35% are detectable.

Measurement of Elastic Constant and Refraction Index of Thin Films at Low Temperatures Using Picosecond Ultrasound

In this paper, a picosecond ultrasound measurement is conducted to evaluate the low-temperature elastic and optical properties of thin films and semiconductors. Specimens are cooled with liquid He through a heat exchanger in a cryostat, and an ultrahigh-frequency acoustic pulse is generated using a femtosecond light pulse, which propagates in the film-thickness direction. Pulse echoes of the longitudinal wave and Brillouin oscillation are observed by the changes in reflectivity of the time-delayed probe light, which depend on the material, and give the longitudinal-wave out-of-plane elastic constant. When the stiffness is known, the Brillouin oscillation provides the refractive index. We determined the stiffness of a Pt thin film and the refractive index of Si at 5 K. The methodology developed in this paper is useful for studing the elastic and optical properties of metallic thin films and transparent materials at cryogenic temperatures.

Stacking-fault structure explains unusual elasticity of nanocrystalline diamonds

This letter reveals that unusual elasticity of nanocrystalline diamond is consistently explained by stacking fault inside the diamond grains instead of the graphitic plate inclusion, which was only possible mechanism. Ab initio calculation shows that stacking fault introduced in the diamond structure behaves as graphitic sp2 bonds, and the elastic constants calculated from the strain-energy relationship agree with the acoustic measurements.