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 C L between 20 and 300 K using picosecond ultrasound spectroscopy. Their room-temperature values are 945 6 3 GPa and 930 6 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 ¼ 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. V

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Hydrogen-gas sensing at low concentrations using extremely narrow gap palladium nanoclusters prepared by resistive spectroscopy

A highly sensitive hydrogen-gas sensor fabricated using MEMS technology is presented. The sensor chip consists of glass substrates, silicon substrate, and an AT-cut quartz crystal resonator, which is embedded in the microchannel constructed on the substrates. The quartz resonator has a fundamental resonant frequency of 165 MHz and a 200 nm palladium film deposited on its single surface as the hydrogen-gas sensing material. The MEMS hydrogen-gas sensor operates in a wireless manner by exciting and detecting the resonator vibration using the non-contacting antennas. The curvature induced resonant frequency change of the resonator plate caused by the expansion of the palladium film is used for the detection of the hydrogen gas. We succeeded in improving the hydrogen absorption rate and then the sensitivity for the hydrogen-gas detection by applying the air-plasma treatment method, and clarified the role of palladium oxide in lowering the energy barrier for the hydrogen-atom migration from surface to subsurface with the X-ray photoelectron spectroscopy. Thus sensitivity enhanced MEMS hydrogen-gas sensor exhibits a detection limit of 10 ppm or less at room temperature.

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 ∼1  mm dimensions.

(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.

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 Gr€ uneisen 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. Published by AIP Publishing.

Sensitivity of hydrogen-gas sensor based on lattice expansion of palladium highly depends on the surface morphology of palladium. We find that the sensitivity can be significantly improved by exposing the as-deposited palladium film to low-power plasma. The hydrogen-gas detection was performed by a wireless-electrodeless AT-cut quartz-crystal resonator with 125-MHz fundamental resonance frequency. It detects hydrogen gas through bending deformation of the resonator caused by the volume expansion of palladium. The surface morphology of palladium film before and after the plasma treatment were analyzed using AFM. The plasma treated palladium film exhibits rougher surface, finer grains, voids, and grain boundary extension. Such morphology and structure changes along with defects induced by the ion bombardment during the plasma treatment strongly contribute to increase in hydrogen absorption rate and then the sensor sensitivity. We further investigate the thickness dependence of the sensitivity, revealing an optimum palladium film thickness of 300 nm.