Junichiro Shiomi

Stronger phonon scattering by larger differences in atomic mass and size in p-type half-Heuslers Hf1−xTixCoSb0.8Sn0.2

High lattice thermal conductivity has been the bottleneck for further improvement of the thermoelectric figure-of-merit (ZT) of half-Heuslers (HHs) Hf1−xZrxCoSb0.8Sn0.2. Theoretically, the lattice thermal conductivity can be reduced by exploring larger differences in the atomic mass and size in the crystal structure, leading to higher ZT. In this paper, we experimentally demonstrated that a lower thermal conductivity in p-type half-Heuslers can be achieved when Ti is used to replace Zr, i.e., Hf1−xTixCoSb0.8Sn0.2, due to larger differences in the atomic mass and size between Hf and Ti compared with Hf and Zr. The highest ZT peak, ∼1.0 at 800 °C, in the Hf1−xTixCoSb0.8Sn0.2 (x = 0.1, 0.2, 0.3, and 0.5) system was achieved using Hf0.8Ti0.2CoSb0.8Sn0.2, which makes this material useful in power generation applications.

Non-Fourier heat conduction in a single-walled carbon nanotube: Classical molecular dynamics simulations

The crystal structure and oxygen stoichiometry in two-layer Na{sub 0.74}CoO{sub 2} and Na{sub 0.38}CoO{sub 2} at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, {lambda}=1.1968 A and {lambda}=1.5403 Ang , are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na{sub 0.74}CoO{sub 2} and 0.99(2) for Na{sub 0.38}CoO{sub 2}, respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.We prepared a series of Mg{sub 1-x}(AlLi){sub x}B{sub 2} samples with 0{<=}x{<=}0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and x-ray diffraction confirms that Li enters the MgB{sub 2} structure even though in an amount less than nominal one. We performed susceptibility, resistivity, and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg{sub 1-x}Al{sub x}B{sub 2} samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties.High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals Cu{sub 2}O and Ag{sub 2}O as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to 240 K for Cu{sub 2}O, up to 470 K for Ag{sub 2}O) and anisotropic thermal displacements of M atoms (M=Cu,Ag). EXAFS measures a positive expansion of the nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 M-M next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid M-O rods.The crystal structure of the PbMg{sub 1/3}Ta{sub 2/3}O{sub 3} relaxor ferroelectric was studied under hydrostatic pressure up to {approx}7 GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion center, which correlates with an increase by {approx}50% of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and diffuse scattering in relaxor ferroelectrics.The ground state energy and pairing gap of the interacting Fermi gases calculated by the ab initio stochastic method are compared with those estimated from the Bardeen-Cooper-Schrieffer pairing Hamiltonian. We discuss the ingredients of this Hamiltonian in various regimes of interaction strength. In the weakly interacting (1/ak{sub F} or approx. 0, it becomes part of the bare Hamiltonian. However, the bare BCS Hamiltonian is not adequate for describing atomic gases in the regime of weak to moderate interaction strength -{infinity}<1/ak{sub F}<0 such as ak{sub F}{approx}-1.

Molecular Dynamics of Diffusive-Ballistic Heat Conduction in Single-Walled Carbon Nanotubes

The diffusive-ballistic heat conduction of finite-length single-walled carbon nanotubes has been studied by means of nonequilibrium molecular dynamics simulations. The length dependence of thermal conductivity is quantified for a range of nanotube lengths up to 1.6 µm at room temperature. A gradual transition from nearly pure ballistic to diffusive-ballistic heat conduction was identified from the thermal conductivity profile. In the diffusive-ballistic regime, the profile exhibits power-law length dependence and does not converge even for a tube length of 1.6 µm. Furthermore, the diameter dependence of thermal conductivity suggests considerable suppression of the diffusion effect as the diameter decreases.

On the importance of optical phonons to thermal conductivity in nanostructures

The contribution of optical phonons to thermal conductivity has typically been ignored. However, when the system size decreases to the nanoscale regime, optical phonons are no longer negligible. In this study, the contributions of different phonon polarizations to the thermal conductivity of silicon are discussed based on the phonon lifetimes extracted from a first principles approach. The results indicate that around room temperature, optical phonons can contribute over 20% to the thermal conductivity of nanostructures as compared to 5% in bulk materials. In addition, the temperature and size dependence of the contributions from acoustic and optical phonons are fully explored.

Microscopic mechanism of low thermal conductivity in lead telluride

Themicroscopic physics behind low-lattice thermal conductivity of single-crystal rock salt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carriers in bulk materials.

Water transport inside a single-walled carbon nanotube driven by a temperature gradient

In this work, by means of molecular dynamics simulations, we consider the mass transport of a water cluster inside a single-walled carbon nanotube (SWNT) with a diameter of about 1.4 nm. The influence of the non-equilibrium thermal environment on the confined water cluster has been investigated by imposing a longitudinal temperature gradient on the SWNT. It is demonstrated that the water cluster is transported with an average acceleration proportional to the temperature gradient. Additional equilibrium simulations suggest that the temperature dependence of the potential energy of the confined water is sufficient to realize the transport. In particular, for a system with a hydrophobic interface, the water-water intrinsic potential energy appears to play a dominant role. The transport simulations were also performed for a system with a junction between two different SWNTs. The results suggest that an angstrom difference in diameter may result in a large barrier for water being transported through a small diameter SWNT.

Gallium arsenide thermal conductivity and optical phonon relaxation times from first-principles calculations

In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density functional theory calculations. Phonon dispersion is calculated from a dynamical matrix constructed using the harmonic force constants and phonon relaxation times are calculated using Fermis Golden rule. The calculated GaAs thermal conductivity agrees well with experimental data. Thermal conductivity accumulations as a function of the phonon mean free path and as a function of the wavelength are obtained. Our results predict a significant size effect on the GaAs thermal conductivity in the nanoscale. Relaxation times of optical phonons and their contributions from different scattering channels are also studied. Such information will help the understanding of hot phonon effects in GaAs-based devices.

Tunable electrical and thermal transport in ice-templated multilayer graphene nanocomposites through freezing rate control.

We demonstrate tunable electrical and thermal conductivities through freezing rate control in solution-based nanocomposites. For a prototypical suspension of 1 vol % multilayer graphene suspended in hexadecane, the solid-liquid electrical conductivity contrast ratio can be tuned from 1 to 4.5 orders of magnitude for freezing rates between 10(2) and 10(-3) °C/min. We hypothesize that this dramatic variation stems from ice-templating, whereby crystal growth drives nanoparticles into concentrated intercrystal regions, increasing the percolation pathways and reducing the internanoparticle electrical resistance. Optical microscopy supports the ice-templating hypothesis, as these dramatic property changes coincide with changing crystal size. Under the same range of freezing rates, the nanocomposite solid-liquid thermal conductivity contrast ratio varies between 2.3 and 3.0, while pure hexadecanes varies between 2.1 and 2.6. The nanocomposites thermal conductivity contrast ratios and solid phase enhancements are greater than effective medium theory predictions. We suggest this is due to ice-templating, consistent with our electrical measurements, as well as nanoparticle-induced molecular alignment of alkanes.

Reduction of phonon lifetimes and thermal conductivity of a carbon nanotube on amorphous silica

We use molecular dynamics simulations to examine the phonon lifetimes in (10,10) carbon nano-tubes (CNTs), both when isolated and when supported on amorphous SiO2 substrates. We deter-mine the Umklapp, normal, boundary and CNT-substrate phonon scattering rates from the com-puted inverse lifetimes. Suspended CNTs have in-plane optical phonon lifetimes between 0.7-2 ps, consistent with recent experiments, but contact with the substrate leads to a lifetime reduction to the 0.6-1.3 ps range. The thermal conductivity of the supported CNT is also computed to be ~30 percent lower than that of the isolated CNT. The thermal boundary conductance estimated from the CNT-substrate phonon scattering rates is in good agreement with that computed from the Green-Kubo relation and with previous experimental results. The results highlight that solid substrates can strongly affect and could be even used to tune the thermal properties of CNTs.

Temperature-Dependent Phonon Conduction and Nanotube Engagement in Metalized Single Wall Carbon Nanotube Films

Interfaces dominate the thermal resistances in aligned carbon nanotube arrays. This work uses nanosecond thermoreflectance thermometry to separate interface and volume resistances for 10 microm thick aligned SWNT films coated with Al, Ti, Pd, Pt, and Ni. We interpret the data by defining the nanotube-metal engagement factor, which governs the interface resistance and is extracted using the measured film heat capacity. The metal-SWNT and SWNT-substrate resistances range between 3.8 and 9.2 mm(2)K/W and 33-46 mm(2)K/W, respectively. The temperature dependency of the heat capacity data, measured between 125 and 300 K, is in good agreement with theoretical predictions. The temperature dependence demonstrated by the metal-SWNT interface resistance data suggests inelastic phonon transmission.