Heat transport in a four-perpendicularity-bend quantum waveguide

Using the scattering matrix method, we investigate acoustic phonon transmission and thermal conductance in a four-perpendicularity-bend quantum waveguide at low temperatures. The transmission spectrum of the quantum waveguide displays a series of resonant peaks and dips; and when one of the bend heights is larger than or equal to the minimum of the dimensions of the phonon channel in the quantum waveguide, a stop-frequency gap will appear; and some single four-perpendicularity-bend quantum waveguides with larger bend heights exhibit narrower width or smaller number of the stop-frequency gaps than that with smaller bend heights. The thermal conductivity is much sensitive to the change of the smaller heights and longitudinal lengths of the bend section; and the thermal conductivity decreases with the increasing of the temperature first, then increases after it reaches a minimum. The investigations of multiple four-perpendicularity-bend waveguides connected in series indicate that the first additional waveguide suppresses the transmission coefficient and forms the stop-frequency gap; and two additional resonance peaks will be formed when each four-perpendicularity-bend waveguide is added in the series. The results could be useful for controlling thermal conductance artificially and the design of phonon devices.     

Effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films

By using the continuum elastic approximation model and the transfer matrix method, we investigate the effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films. Our work show that most acoustic phonons can easily pass the structure, but some only have much less transmission probabilities and form corresponding dips in the transmission spectrum. With the change of the structure parameters such as the width of diffusion layers and defect layer, the number of unit cell and the density of containing Al in diffusion layers and defect layer, the magnitude of the frequencies of acoustic phonons corresponding to the dips almost remain unchanged, but the transmission coefficients corresponding to the dips change at different degree, and the transmission probabilities of some frequencies are very sensitive to the variation of the above-mentioned structure parameters. These results can provide some references in controlling the transmission coefficients of acoustic phonons, devising parts of acoustic apparatus and theoretical investigation related.     

Phonon Transmission and Thermal Conductance in Fibonacci Wire at Low Temperature

We investigate the phonon transmission and thermal conductance in a general Fibonacci quasicrystal by the model of lattice dynamics and the technique of transfer matrix. It is found that quasiperiodic distribution of masses may greatly destroy the phonon transport at both low and high frequencies and thus may affect the thermal conductance. The thermal conductance increases with temperature at low temperatures and displays saturation with further increase of the temperature. Such saturation behaviour is preserved even when the mass ratio of atoms in the Fibonacci chain is changed.     

Hole-interface optical phonon relaxation rates with valence band-mixing effects

We theoretically investigate the hole-interface optical phonon scattering rates for a InGaAs-AlGaAs quantum well structure, taking into account the valence-band mixing. The dispersion relation and the electrostatic potentials for interface optical phonon modes are obtained based on the macroscopic dielectric continuum model. For the hole dispersion relation, the Luttinger-Kohn Hamiltonian is used. The hole-interface optical phonon interaction is evaluated by the Fermi's golden rule taking into account the Bloch overlap factor.Our results show that the hole-interface phonon scattering rates within the parabolic band approximation are different from those including valence band mixing effects. Especially, in the low energy region, the hole-interface phonon scattering rates within the parabolic band approximation are overestimated very significantly.     

Conductance of a Quantum Dot in the Presence of a Phonon Field

We theoretically investigate the electrical transport property of a quantum dot with longitudinal optical phonons. The conductance through the dot connected to two leads is calculated by the nonequilibrium Green function within the Landauer-Büttiker framework. The numerical examples of the conductance with different electron-phonon coupling strengths show that the presence of a phonon field typically results in the suppression of the main peak accompanied by some phonon side peaks. Both the main peak and the side peaks axe sensitive to the electron-phonon coupling strength, which is related to temperature. Our results for this system are consistent with some related previous works but the calculation is comparatively simple.     

Thermal Conductivity Measurement of Submicron-Thick Aluminium Oxide Thin Films by a Transient Thermo-Reflectance Technique

Thermal conductivity of submicron-thick aluminium oxide thin films prepared by middle frequency magnetron sputtering is measured using a transient thermo-reflectance technique. A three-layer model based on transmission line theory and the genetic algorithm optimization method are employed to obtain the thermal conductivity of thin films and the interracial thermal resistance. The results show that the average thermal conductivity of 330- 1000nm aluminium oxide thin films is 3.3 Wm^-1K^-1 at room temperature. No significant thickness dependence is found. The uncertainty of the measurement is less than 10%.     

Thermal Diffusivity of Film/Substrate Structures Characterized by Transient Thermal Grating Method

Transient thermal grating method is used to measure the thermal diffusivity of absorbing films deposited on transparent substrates. According to periodically modulated dielectric constant variations and thermoelastic deformations of the thin films caused by the transient thermal gratings, an improved optical diffraction theory is presented. In the experiment, the probing laser beam reflectively diffracted by the thermal grating is measured by a photomultiplier at different grating fringe spaces. The thermal diffusivity of the film can be evaluated by fitting the theoretical calculations of diffraction signals to the experimental measured data. The validity of the method is tested by measuring the thermal diffusivities of absorbing ZnO films deposited on glass substrates.     

Temperature Dependence of Thermal Conductivity of Nanofluids

Mechanism of thermal conductivity of nanofluids is analysed and calculated, including Brownian motion effects, particle agglomeration and viscosity, together influenced by temperature. The results show that only Brown- Jan motion as reported is not enough to describe the temperature dependence of the thermal conductivity of nanofluids. The change of particle agglomeration and viscosity with temperature are also important factors. As temperature increases, the reduction of the particle surface energy would decrease the agglomeration of nanopartieles, and the reduction of viscosity would improve the Brownish motion. The results egree well with the experimental data reported.     

Length Dependence of Thermal Conductivity of Single-Walled Carbon Nanotubes

Dependence of the thermal conductivity on the length of two armchair single-walled carbon nanotubes （SWNTs） is studied by the nonequilibrium molecular dynamics （MD） method with Brenner Ⅱ potential. The thermal conductivities are calculated for （5, 5） and （7, 7） SWNTs with lengths ranging from 22 to 155nm. The results show that the thermal conductivity of SWNTs is sensitive to the length and it does not converge to a finite value when the tube length increases up to 155nm, however it obeys a power law relation.     

Phonon Transport and Thermal Conductivity in an Acoustic Filter

We investigate the phonon ballistic transmission and the thermal conductivity in a dielectric quantum structure. It is found that these observable quantities sensitively depend on geometric parameters, and are of quantum character. The total transmission coetfficient as a function of the reduced waveguide-length exhibits periodical behaviour and the reduced thermal conductance decreases below the ideal universal value for the low temperature. Our results show that one can control the thermal conductivity of the structure and make all kinds of acoustic filters to match practical requirements in devices by adjusting the geometric parameters.     

Thermal diffusivity and Biot number: A new experimental method

A new simple method is presented for measuring thermal diffusivity and Biot number in cylindrical samples made of relatively highly conducting materials, subjected to laminar air flow. The basic idea is a heat source in the middle section of the sample, acting also as a thermocouple; only one additional temperature sensor at the cylinder basis is required to give all information, without requiring any hypothesis about the effective time dependence of the heat source.     

Thermal and carrier transport originating from photon recycling and non-radiative recombination in laser micromachining of GaAs thin films

Coupled thermal and carrier transports (electron/hole generation, recombination, diffusion and drifting) in laser photoetching of GaAs thin film is investigated. A new volumetric heating mechanism originating from SRH (Shockley–Read–Hall) non-radiative recombination and photon recycling is proposed and modeled based on recent experimental findings. Both volumetric SRH heating and Joule heating are found to be important in the carrier transport, as well as the etching process. SRH heating and Joule heating are primarily confined within the space-charge region, which is about 20 nm from the GaAs surface. The surface temperature rises rapidly as the laser intensity exceeds 10⁵ W/m². Below a laser intensity of 10⁵ W/m², the thermal effect is negligible. The etch rate is found to be dependent on the competition between photovoltaic and photothermal effects on surface potential. At high laser intensity, the etch rate is increased by more than 100%, due to SRH and Joule heating. Received: 24 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-310/206-2302, E-mail: xiang@seas.ucla.edu     

Determination of Thermal Conductivity of Liquids by a Multi-Point Laser Pump Method

As an advanced optical method, a multi-point pump method is presented for measurements of thermo-physical properties of liquids. Meanwhile, based on the laser-induced thermal grating method, a new theory model is presented and used to analyse the thermal effects caused by the multi-point laser pump, by which the thermal conductivity of liquids can be obtained. The results of some typical liquids, such as water, ethanol and acetone, are presented and are consistent with those of acknowledged values, demonstrating that the multi-point method is simple and useful for characterizing thermal properties of liquids.     

Coherent Dynamics of Direct-Current-Driven Quantum-Dot-Array with Two Time-Dependent Embedded Impurities

The coherent dynamics of dc-driven quantum-dot array with two embedded time-dependent impurities is investigated, the exact crossings and avoided crossings in the quasienergies are associated with the evolution dynamics of the electron by the introduction of the envelope lines of the site＇s maximum returning probability. Through a perturbative scheme, the time-dependent Hamiltonian is replaced by an equivalent static one, whose structure reveals the new quantum tunnelling mechanism in the time-periodic driving system and sheds light on the alternative interpretation of dynamic localization and delocalization.     

Measurement of rotational temperature in CO2 waveguide laser medium

A small-signal gain in CO₂ waveguide laser medium has been measured on rotational-vibrational transitions in the P-branch of the (0, 0, 1)-(0, 2⁰, 0) band. It has been found that the rotational temperature is well defined in the waveguide laser system where high excitation power is injected and a large amount of energy is flowing through vibrational, rotational, and translational degrees of freedom. The rotational temperature is slightly higher than the translational temperature.     

A constrained variational principle for heat conduction

The heat equation is re-studied in this Letter in view of variational theory. By the semi-inverse method, a variational principle for the heat conduction is obtained, which is first appeared in the literature. The physical understanding of the obtained variational principle still needs further explanation.     

Conductance fluctuation and degeneracy in nanocontact between a conductive AFM tip and a granular surface under small-load conditions

Conductive-tip atomic force microscope (c-AFM) has been extensively used in measuring electrical properties of surface nanostructures, but the electrical conduction in c-AFM tip-sample contacts in nanometer scale is not well understood. In the present work, we experimentally investigated the electrical properties of the nanocontact between a W₂C-coated c-AFM tip and granular gold film under small-load (∼5 nN) at ambient air conditions. We found that under a constant bias voltage (10 V), the electrical current passing through the tip-sample junction at fixed location of sample surface dramatically fluctuated and degenerated. By quantitatively estimating the mechanical and electrical aspects of the nanocontact, we explained the observed phenomena as mechanical instabilities, electron tunneling transport and atomic rearrangements at the contact junction. We think that our results are important for the realistic application of c-AFM in nanoelectronic measurement.     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Local Heating Effect of Flow Past a Circular Cylinder

Heating effects of air flows past a two-dimensional circular cylinder at low Reynolds numbers and low Mach numbers are investigated by numerical simulation. The cylinder wall is heated partially rather than heated on the whole surface as with previous researches. The heating effects are completely different for various heating locations on the cylinder surface. Heating either windward or leeward side stabilizes the flow and reduces or completely suppresses vortex shedding from the cylinder at supercritical Reynolds numbers, which is consistent with previous results of heating on the whole surface of the cylinder. However, as the lateral sides of the cylinder （perpendicular to the stream-wise direction） are heated, an adverse effect is found for the first time in that the flow is destabilized and vortex shedding can be excited at subcritical Reynolds numbers. As the lateral sides of the cylinder are cooled, the flow is stabilized.     

Charge and Heat Transport in Polycrystalline Metallic Nanostructures

Metals are typically good conductors in which the abilities to transport charge and to transport heat can be related through the Wiedemann-Franz law. Here we report on an abnormal charge and heat transport in polycrystalline metallic nanostructures in which the ability to transport charge is weakened more obviously than that to transport heat. We attribute it to the influence of the internal grain boundaries and have formulated a novel relation to predict the thermal conductivity. The Wiedemann-Franz law is then modified to account for the influence of the grain boundaries on the charge and heat transport with the predictions now agreeing well with the measured results.