Dependence of heat transport on the strength and shear rate of prescribed circulating flows

We study numerically the dependence of heat transport on the maximum velocity and shear rate of physical circulating flows, which are prescribed to have the key characteristics of the large-scale mean flow observed in turbulent convection. When the side-boundary thermal layer is thinner than the viscous boundary layer, the Nusselt number (Nu), which measures the heat transport, scales with the normalized shear rate to an exponent 1/3. On the other hand, when the side-boundary thermal layer is thicker, the dependence of Nu on the Peclet number, which measures the maximum velocity, or the normalized shear rate when the viscous boundary layer thickness is fixed, is generally not a power law. Scaling behavior is obtained only in an asymptotic regime. The relevance of our results to the problem of heat transport in turbulent convection is also discussed. Received 28 November 2001 Published online 25 June 2002     

Basic aspects of microindentation in multilayered poly(ethylene terephthalate)/polycarbonate films

The microhardness H of multilayered poly(ethylene terephthalate) (PET)/polycarbonate (PC) films, produced by continuous layer multiplying coextrusion has been determined. These materials present rather uniform laminates up to thousands of layers in the micrometre and submicrometre range. The micromechanical properties have been investigated as a function of layer thickness of the single polymer components, the total number of layers, the film thickness and the influence of heat treatment. The microhardness of the microlayered structure has also been determined across the profile in the parallel direction to the packing of the layers. The hardness in the vicinity of the PET/PC phases has been examined. Results reveal that the influence of the interphase on the H values for the samples with a large number of layers is rather small. The most important parameter in determining the final hardness of the multilayered films is the ratio of the penetration depth to the thickness of the layer. Upon heating, a microhardness increase is observed as a consequence of a double contribution: the crystallization of the PET layers, on the one hand, and the physical ageing of the PC zones on the other.     

Ferrimagnetic resonance spectra of magnetic bubble films at low microwave frequencies

Magnetic bubble films exhibit a number of ferrimagnetic resonance modes due to the spatial variation of the anisotropy. The resonance frequencies have been measured as a function of the applied bias fieldH ₀. In the lower field range the magnetization of the transient layer, which has negative anisotropy, is not yet parallel toH ₀. In this range the resonance frequencies are shifted to higher values due to pinning effects. In films grown by the vertical dipping method an additional layer on top of the transient layer is observed within which the magnetization rotates from the direction in the transient layer to that of the bulk of the film. In films grown by horizontal dipping no such layer could be detected. Each ferrimagnetic resonance mode excites transverse elastic waves in the film due to the magnetoelastic interaction and thus gives rise to elastic resonances of the whole crystal, film and substrate. These elastic resonances lead to a fine-structure of the ferrimagnetic resonances. The observed fine-structure vanishes periodically with frequency and from this behaviour the thickness of the magnetic film and of the transient layer has been determined.     

Rapid communication: Permeability of hydrogen in two-dimensional graphene and hexagonal boron nitride sheets

This paper concerns an application to optimal energy by incorporating thermal equilibrium on MHD-generalised non-Newtonian fluid model with melting heat effect. Highly nonlinear system of partial differential equations is simplified to a nonlinear system using boundary layer approach and similarity transformations. Numerical solutions of velocity and temperature profile are obtained by using shooting method. The contribution of entropy generation is appraised on thermal and fluid velocities. Physical features of relevant parameters have been discussed by plotting graphs and tables. Some noteworthy findings are: Prandtl number, power law index and Weissenberg number contribute in lowering mass boundary layer thickness and entropy effect and enlarging thermal boundary layer thickness. However, an increasing mass boundary layer effect is only due to melting heat parameter. Moreover, thermal boundary layers have same trend for all parameters, i.e., temperature enhances with increase in values of significant parameters. Similarly, Hartman and Weissenberg numbers enhance Bejan number.     

原子层沉积技术沉积的FeOx包裹层显著提高Pd/C催化剂在甲酸分解放氢反应中的催化活性

Hydrogen generation from formic acid (FA) has received significant attention.The challenge is to obtain a highly active catalyst under mild conditions for practical applications.Here atomic layer deposition (ALD) of FeO_x was performed to deposit an ultrathin oxide coating layer to a Pd/C catalyst,therein the FeO_x coverage was precisely controlled by ALD cycles.Transmission electron microscopy and powder X-ray diffraction measurements suggest that the FeO_x coating layer improved the thermal stability of Pd nanoparticles (NPs).X-ray photoelectron spectroscopy measurement showed that deposition of FeO_x on the Pd NPs caused a positive shift of Pd3d binding energy.In the FA dehydrogenation reaction,the ultrathin FeO_x layer on the Pd/C could considerably improve the catalytic activity,and Pd/C coated with 8 cycles of FeO_x showed an optimized activity with turnover frequency being about 2 times higher than the uncoated one.The improved activities were in a volcanoshape as a function of the number of FeO_x ALD cycles,indicating the coverage of FeO_x is critical for the optimized activity.In summary,simultaneous improvements of activity and thermal stability of Pd/C catalyst by ultra-thin FeO_x overlayer suggest to be an effective way to design active catalysts for the FA dehydrogenation reaction.     

Number-Phase Quantization Scheme and the Quantum Effects of a Mesoscopic Electric Circuit at Finite Temperature

For L-C circuit, a new quantized scheme has been proposed in the context of number-phase quantization. In this quantization scheme, the number n of the electric charge q(q=en) is quantized as the charge number operator and the phase difference θ across the capacity is quantized as phase operator. Based on the scheme of number-phase quantization and the thermo field dynamics (TFD), the quantum fluctuations of the charge number and phase difference of a mesoscopic L-C circuit in the thermal vacuum state, the thermal coherent state and the thermal squeezed state have been studied. It is shown that these quantum fluctuations of the charge number and phase difference are related to not only the parameters of circuit, the squeezing parameter, but also the temperature in these quantum states. It is proven that the number-phase quantization scheme is very useful to tackle with quantization of some mesoscopic electric circuits and the quantum effects.     

Thermal transport in twisted few-layer graphene

Twisted graphene possesses unique electronic properties and applications, which have been studied extensively. Recently, the phonon properties of twisted graphene have received a great deal of attention. To the best of our knowledge,thermal transports in twisted graphene have been investigated little to date. Here, we study perpendicular and parallel transports in twisted few-layer graphene(T-FLG). It is found that perpendicular and parallel transports are both sensitive to the rotation angle θ between layers. When θ increases from 0° to 60°, perpendicular thermal conductivity κ_(||) first decreases and then increases, and the transition angle is θ = 30°. For the parallel transport, the relation between thermal conductivity κand θ is complicated, because intra-layer thermal transport is more sensitive to the edge of layer than their stacking forms. However, the dependence of interlayer scattering on θ is similar to that of κ⊥. In addition, the effect of layer number on the thermal transport is discussed. Our results may provide references for designing the devices of thermal insulation and thermal management based on graphene.     

A thermal conductivity model for nanofluids including effect of the temperature-dependent interfacial layer

The interfacial layer of nanoparticles has been recently shown to have an effect on the thermal conductivity of nanofluids. There is, however, still no thermal conductivity model that includes the effects of temperature and nanoparticle size variations on the thickness and consequently on the thermal conductivity of the interfacial layer. In the present work, the stationary model developed by Leong et al. (J Nanopart Res 8:245–254, 2006) is initially modified to include the thermal dispersion effect due to the Brownian motion of nanoparticles. This model is called the ‘Leong et al.’s dynamic model’. However, the Leong et al.’s dynamic model over-predicts the thermal conductivity of nanofluids in the case of the flowing fluid. This suggests that the enhancement in the thermal conductivity of the flowing nanofluids due to the increase in temperature does not come from the thermal dispersion effect. It is more likely that the enhancement in heat transfer of the flowing nanofluids comes from the temperature-dependent interfacial layer effect. Therefore, the Leong et al.’s stationary model is again modified to include the effect of temperature variation on the thermal conductivity of the interfacial layer for different sizes of nanoparticles. This present model is then evaluated and compared with the other thermal conductivity models for the turbulent convective heat transfer in nanofluids along a uniformly heated tube. The results show that the present model is more general than the other models in the sense that it can predict both the temperature and the volume fraction dependence of the thermal conductivity of nanofluids for both non-flowing and flowing fluids. Also, it is found to be more accurate than the other models due to the inclusion of the effect of the temperature-dependent interfacial layer. In conclusion, the present model can accurately predict the changes in thermal conductivity of nanofluids due to the changes in volume fraction and temperature for various nanoparticle sizes.     

Analysis of thermal expansivity of iron (Fe) metal at ultra high temperature and pressure

In the present investigation we have explained the thermal and compression properties of HCP iron (Fe) at high pressure with variable temperature (isobars) and at high temperature with variable pressure (isotherm). The usual Tait equation of state is modified by incorporating the effect of thermal pressure. The calculated values of pressure for different isotherms and isochors and thermal expansivity (α) as a function of both temperature and pressure have been compared with those values obtained by Isaak et al and Wasserman et al.      

Influence of the particle size distribution on the thermal conductivity of nanofluids

In a previous study, we have obtained an equation to predict the thermal conductivity of nanofluids containing nanoparticles with conductive interface. The model is maximal particle packing dependent. In this study, the maximal packing is obtained as a function of the particle size distribution, which is the Gamma distribution. The thermal conductivity enhancement depends on the averaged particle size. Discussion concerning the influence of the suspension pH on the particle packing is made. The proposed model is evaluated using number of sets from the published experimental data to the thermal conductivity enhancement for different nanofluids.     

Study of the interface Si-nc/SiO2 by infrared spectroscopic ellipsometry and X-ray photoelectron spectroscopy

SiO_x films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO₂ matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm⁻¹ range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO_x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO₂ layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si_4−nO_n) (n=0–4) has also revealed the presence of a SiO_x phase. Consistency between infra-red and XPS results is discussed.     

Simulating the thermal stability and phase changes of small carbon clusters and fullerenes

The thermal stability, phases and phase changes of small carbon clusters and fullerenes are investigated by constant energy Molecular Dynamics simulations performed over a wide range of temperatures, i.e., from to above the melting point of graphitic carbon. The covalent bonds between the carbon atoms in the clusters are represented by the many-body Tersoff potential. The zero temperature structural characteristics of the clusters, i.e., the minimum energy structures as well as the isomer hierarchy can be rationalized in terms of the interplay between the strain energy (due to the surface curvature) and the number of dangling bonds in the cluster. Minimization of the strain energy opposes the formation of cage structures whereas minimization of the number of dangling bonds favors it. To obtain a reliable picture of the processes experienced by carbon clusters as a function of temperature, both thermal and dynamical characteristics of the clusters are carefully analyzed. We find that higher excitation temperatures are required for producing structural transformations in the minimum energy structures than in higher lying isomers. We have also been able to unambiguously identify some structural changes of the clusters occurring at temperatures well below the melting-like transition. On the other hand, the melting-like transition is interrupted before completion, i.e., the thermal decomposition of the clusters (evaporation or ejection of or units) occurs, from highly excited configurations, before the clusters have fully developed a liquid-like phase. Comparison with experiments on the thermal decomposition of and a discussion of the possible implications of our results on the growth mechanisms leading to the formation of different carbon structures are included. Received: 25 March 1998 / Received in final form: 30 October 1998     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

General Solutions for Hydromagnetic Free Convection Flow over an Infinite Plate with Newtonian Heating,Mass Diffusion and Chemical Reaction

The problem of hydromagnetic free convection flow over a moving infinite vertical plate with Newtonian heating, mass diffusion and chemical reaction in the presence of a heat source is completely solved. Radiative and porous effects are not taken into consideration but they can be immediately included by a simple rescaling of Prandtl number and magnetic parameter. Exact general solutions for the dimensionless velocity and concentration fields and the corresponding Sherwood number and skin friction coefficient are determined under integral form in terms of error function or complementary error function of Gauss. They satisfy all imposed initial and boundary conditions and can generate exact solutions for any problem with technical relevance of this type. As an interesting completion, uncommon in the literature, the differential equations which describe the thermal, concentration and momentum boundary layer, as well as the exact expressions for the thicknesses of thermal, concentration or velocity boundary layers were determined.Numerical results have shown that the thermal boundary layer thickness decreases for increasing values of Prandtl number and the concentration boundary layer thickness is decreasing with Schmidt number. Finally, for illustration,three special cases are considered and the influence of physical parameters on some fundamental motions is graphically underlined and discussed. The required time to reach the flow according with post-transient solution(the steady-state),for cosine/sine oscillating concentrations on the boundary is graphically determined. It is found that, the presence of destructive chemical reaction improves this time for increasing values of chemical reaction parameter.     

Phonon heat transport in gallium arsenide

The lifetimes of quantum excitations are directly related to the electron and phonon energy linewidths of a particular scattering event. Using the versatile double time thermodynamic Green’s function approach based on many-body theory, an ab-initio formulation of relaxation times of various contributing processes has been investigated with newer understanding in terms of the linewidths of electrons and phonons. The energy linewidth is found to be an extremely sensitive quantity in the transport phenomena of crystalline solids as a collection of large number of scattering processes, namely, boundary scattering, impurity scattering, multiphonon scattering, interference scattering, electron–phonon processes and resonance scattering. The lattice thermal conductivities of three samples of GaAs have been analysed on the basis of modified Callaway model and a fairly good agreement between theory and experimental observations has been reported.     

Remote sensing of thermal profile in the lower atmosphere by a radio-acoustic system

Summary A new radio-acoustic methodology, for the remote sensing of temperature profile in the lower troposphere, has been developed in the last few years. Since 1972 different types of radio-acoustic sounders have been designed and tested in the USA, Europe and Japan. The technique used for measurements in a Doppler tracking of a short acoustic pulse with a radar. Measurement of the sound speed as a function of delay from the start of the acoustic beam leads to the acquisition of the temperature vertical profile. After a brief review of the experimental radio-acoustic systems, the metric radio-acoustic sounder developed and tested at Istituto di Cosmogeofisica of the National Research Council (Italy) is analysed. Results of this sounder and its performance in fog-capping thermal inversion measurements are discussed. A comparison of the data measured with the radio-acoustic system and with traditional methods shows:a) sounder ability to produce vertical thermal profile with temperature accuracy and height discrimination comparable with conventional soundings;b) advantages typical of remote-sensing technique;c) applicability of the system both in assessing thermodynamic conditions in the planetary boundary layer (PBL) and in sensing conditions conductive to high concentrations of air pollutants at ground level. Preliminary results of the completely automatized system and application to air quality management are also presented. This work was partially supported by the Commission of the European Communities (CEC) under contract No. ENV·348 I(S) and by a research contract by ENEL-CRTN Milano.     

Asymptotic theory of boundary layers of weakly ionized thermalplasmas on emitting electrodes

The paper deals with collision-dominated boundary layers of weakly ionized thermal plasmas on emitting electrodes. The case considered is when the dominating ionization mechanism in the plasma is ionization by electron impact, and the dominating recombination mechanism is recombination with an electron as a third body. The ratio of the Debye length to the recombination length is treated as a small parameter, and the method of matched asymptotic expansions is employed. Analytical formulas have been obtained for the distributions of the number densities of ions and electrons and of the electrostatic potential in each asymptotic zone. Formulas have been obtained describing the voltage drop in the boundary layer as a function of the density of the electric current coming from the plasma to the electrode