Akira Nagakubo

Elastic constants of cubic and wurtzite boron nitrides

We synthesized pure polycrystalline cubic boron nitride (cBN) and wurtzite boron nitride (wBN) by the direct conversion method from hexagonal boron nitride, and measured their longitudinal-wave elastic constants CL between 20 and 300 K using picosecond ultrasound spectroscopy. Their room-temperature values are 945 ± 3 GPa and 930 ± 18 GPa for cBN and wBN, respectively. The shear modulus G of cBN was also determined by combining resonance ultrasound spectroscopy and micromechanics calculation as G = 410 GPa. We performed ab-initio calculations and confirmed that the generalized gradient approximation potential fails to yield correct elastic constants, which indicated the necessity of a hybrid-functional method.

Elasticity and hardness of nano-polycrystalline boron nitrides: The apparent Hall-Petch effect

Nano-polycrystalline boron nitride (BN) is expected to replace diamond as a superhard and superstiff material. Although its hardening was reported, its elasticity remains unclear and the as-measured hardness could be significantly different from the true value due to the elastic recovery. In this study, we measured the longitudinal-wave elastic constant of nano-polycrystalline BNs using picosecond ultrasound spectroscopy and confirmed the elastic softening for small-grain BNs. We also measured Vickers and Knoop hardness for the same specimens and clarified the relationship between hardness and stiffness. The Vickers hardness significantly increased as the grain size decreased, while the Knoop hardness remained nearly unchanged. We attribute the apparent increase in Vickers hardness to the elastic recovery and propose a model to support this insight.

Erratum: “Monitoring of Longitudinal-Wave Velocity and Attenuation of SrTiO3 at Low Temperatures Using Picosecond Ultrasound Spectroscopy”

We measured the longitudinal-wave velocity and its attenuation in SrTiO3 between 20 and 300 K using picosecond ultrasound spectroscopy. From the temperature dependence of the velocity and attenuation, we monitored the cubic–tetragonal phase transition of SrTiO3 near 100 K, whereas no more transitions were indicated below 100 K. From the measured attenuation coefficients, we estimate the relaxation time τ. Because of the ultrahigh frequency measurements, the product ωτ is larger than unity, where the traditional theory for phonon–phonon interaction fails to explain the relaxation time. We then derived the relationship between the relaxation time and attenuation for ωτ>1.

Elastic constants of GaN between 10 and 305 K

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.

Temperature behavior of sound velocity of fluorine-doped vitreous silica thin films studied by picosecond ultrasonics

Vitreous silica (v-SiO2) shows anomalous temperature dependence of velocity, including positive temperature coefficient of velocity (TCV) and velocity minimum around 70 K. The former characteristic allows its application in acoustic-resonator devices as a temperature compensating material. In this paper, we study the temperature dependence of velocity of fluorine-doped v-SiO2 (v-SiO2–xFx) thin films using picosecond ultrasonic spectroscopy. To correct the temperature increase caused by irradiation with light pulses, we calculated the steady temperature increase in the measuring volume with a finite volume method, considering the temperature dependence of thermal conductivity, and find that temperature in the measurement region remains high even when the back surface is cryogenically cooled. Using the corrected temperature, we determine TCV of v-SiO2–xFx thin films for 0<x<0.264, which increases as x increases and is smaller than reported bulk values by a factor of 0.5−0.7. The velocity minimum is absent f...

Hopping conduction and piezoelectricity in Fe-doped GaN studied by non-contacting resonant ultrasound spectroscopy

Using the antenna-transmission acoustic-resonance technique, we measured temperature dependencies of mechanical resonance frequencies and attenuation of an Fe-doped GaN. A strong internal-friction peak appears during temperature change, at which reduction in frequency occurs. The peak temperature rises as frequency increases, indicating the phonon-assisted hopping conduction of carriers between Fe centers. The Arrhenius plot yields the activation energy of the hopping conduction to be 0.23 ± 0.05 eV. The frequency reduction of a quasi-plane-shear resonance mode yields the piezoelectric coefficient e15 = 0.332 ± 0.03 C/m2.

Piezoelectric coefficients of GaN determined by hopping conduction of carriers

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.

Refractive index and extinction coefficient of Si at 400 nm between 10 and 300 K

Optical properties of Si are important and widely studied. However, temperature dependence of the refractive index for visible light at low temperatures has not been measured. In this study, we measured the complex refractive index of Si at 400 nm between 10 and 300 K by picosecond ultrasound spectroscopy. The measured refractive index at room temperature agrees well with reported values, confirming the accuracy of our measurement, and we found that the refractive index at 10 K is smaller than reported values.

Acoustic properties of co-doped AlN thin films at low temperatures studied by picosecond ultrasonics

(Mg0.5Zr0.5)xAl1?xN and (Mg0.5Hf0.5)xAl1?xN thin films are AlN-base piezoelectric materials, and their piezoelectric coefficients are higher than those of pure AlN, being promising materials for acoustic devices. However, their acoustic properties remain unknown because of measurement difficulty for deposited thin films. In this study, we measure their longitudinal-wave elastic constants C33 and their temperature coefficients using picosecond ultrasound spectroscopy for 0 < x < 0.13; we obtain C33 = 398.2 ? 0.7 GPa for pure AlN, and it largely decreases by doping Mg, Zr, and Hf, leading to a minimum values of 316.8 ? 1.6 GPa for (Mg0.5Zr0.5)0.126Al0.874N.

Temperature behavior of sound velocity of SiON thin films studied by picosecond ultrasound

Amorphous thin films are used as components of acoustic resonators because they could control temperature coefficient of resonator. Because elastic constants of thin films are often different from those of bulk materials, the direct measurement of sound velocity and temperature coefficient of velocity (TCV) for individual thin films becomes important in designing resonators. Because The TCV of amorphous silicon oxide is opposite in sign to that of silicon nitride, we expect that a specific-composition SiON thin film show the zero TCV. We here measure the longitudinal-wave velocity of SiON thin films with various nitride concentrations using picosecond ultrasound method.