Showing papers on "Thermal diffusivity published in 2007"

Ultrafine nanoporous gold by low-temperature dealloying and kinetics of nanopore formation

Abstract: A low-temperature dealloying technique was developed to tailor the characteristic length scale of nanoporous gold for advanced functional applications. By systematically investigating the kinetics of nanopore formation during free corrosion, the authors experimentally demonstrated that the dealloying process is controlled by the diffusion of gold atoms at alloy/electrolyte interfaces, which strongly relies on the reaction temperatures. Low dealloying temperatures significantly reduce the interfacial diffusivity of gold atoms and result in an ultrafine nanoporous structure that has been proved to be useful with improved chemical and physical properties.

Heat transfer and fluid flow during keyhole mode laser welding of tantalum, Ti–6Al–4V, 304L stainless steel and vanadium

Abstract: Because of the complexity of several simultaneous physical processes, most heat transfer models of keyhole mode laser welding require some simplifications to make the calculations tractable. The simplifications often limit the applicability of each model to the specific materials systems for which the model is developed. In this work, a rigorous, yet computationally efficient, keyhole model is developed and tested on tantalum, Ti–6Al–4V, 304L stainless steel and vanadium. Unlike previous models, this one combines an existing model to calculate keyhole shape and size with numerical fluid flow and heat transfer calculations in the weld pool. The calculations of the keyhole profile involved a point-by-point heat balance at the keyhole walls considering multiple reflections of the laser beam in the vapour cavity. The equations of conservation of mass, momentum and energy are then solved in three dimensions assuming that the temperatures at the keyhole wall reach the boiling point of the different metals or alloys. A turbulence model based on Prandtl's mixing length hypothesis was used to estimate the effective viscosity and thermal conductivity in the liquid region. The calculated weld cross-sections agreed well with the experimental results for each metal and alloy system examined here. In each case, the weld pool geometry was affected by the thermal diffusivity, absorption coefficient, and the melting and boiling points, among the various physical properties of the alloy. The model was also used to better understand solidification phenomena and calculate the solidification parameters at the trailing edge of the weld pool. These calculations indicate that the solidification structure became less dendritic and coarser with decreasing weld velocities over the range of speeds investigated in this study. Overall, the keyhole weld model provides satisfactory simulations of the weld geometries and solidification sub-structures for diverse engineering metals and alloys.

Photoacoustic characterization of carbon nanotube array thermal interfaces

Abstract: This work describes an experimental study of thermal conductance across multiwalled carbon nanotube (CNT) array interfaces, one sided (Si-CNT-Ag) and two sided (Si-CNT-CNT-Cu), using a photoacoustic technique (PA). Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances (Si-CNT and CNT-Ag) and the two-sided CNT interface component resistances (Si-CNT, CNT-CNT, and CNT-Cu) and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one- and two-sided CNT interfaces measured using the PA technique are 15.8±0.9 and 4.0±0.4mm2K∕W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate (CNT-Ag), which is measured to be 14.0±0.9mm2K∕W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays (CNT-CNT), which is estimated to be 2.1±0.4mm2K∕W.

Impact of Pavement Thermophysical Properties on Surface Temperatures

Abstract: A one-dimensional mathematical model was developed, based on the fundamental energy balance, to calculate the pavement near-surface temperatures using hourly measured solar radiation, air temperature, dew-point temperature, and wind velocity data. An analysis was conducted to predict the diurnal temperature effects of pavement thermophysical properties with the aim of seeking an optimum composition of paving materials for future infrastructure projects. Appropriate paving materials not only ensure stability and safety for road users, but also the ability to mitigate heat absorption and high surface temperatures contributing to the Urban Heat Island Effect and human comfort. This paper evaluated the effects and sensitivities of the thermophysical properties on the pavement surface temperatures. The results indicated that both albedo and emissivity have the highest positive effects on pavement maximum and minimum temperatures, respectively, while increasing the thermal conductivity, diffusivity, and volumetric heat capacity help in mitigating the maximum but not the minimum pavement near-surface temperature.

Experimental Measurement of the Solubility and Diffusivity of CO2 in Room-Temperature Ionic Liquids Using a Transient Thin-Liquid-Film Method

Abstract: In this paper, results from an experimental investigation of carbon dioxide (CO2) solubility and diffusivity in the ionic liquids 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([b...

Thermal rectification and negative differential thermal resistance in lattices with mass gradient

Abstract: We study thermal properties of one-dimensional (1D) harmonic and anharmonic lattices with a mass gradient. It is found that a temperature gradient can be built up in the 1D harmonic lattice with a mass gradient due to the existence of gradons. The heat flow is asymmetric in anharmonic lattices with a mass gradient. Moreover, in a certain temperature region, negative differential thermal resistance is observed. Possible applications in constructing thermal rectifiers and thermal transistors by using the graded material are discussed.

Artificial Fluid Properties for Large-Eddy Simulation of Compressible Turbulent Mixing

Abstract: An alternative methodology is described for Large-Eddy Simulation of flows involving shocks, turbulence and mixing. In lieu of filtering the governing equations, it is postulated that the large-scale behavior of an ''LES'' fluid, i.e., a fluid with artificial properties, will be similar to that of a real fluid, provided the artificial properties obey certain constraints. The artificial properties consist of modifications to the shear viscosity, bulk viscosity, thermal conductivity and species diffusivity of a fluid. The modified transport coefficients are designed to damp out high wavenumber modes, close to the resolution limit, without corrupting lower modes. Requisite behavior of the artificial properties is discussed and results are shown for a variety of test problems, each designed to exercise different aspects of the models. When combined with a 10th-order compact scheme, the overall method exhibits excellent resolution characteristics for turbulent mixing, while capturing shocks and material interfaces in crisp fashion.

Solubility, Diffusivity, and Permeability of Gases in Phosphonium-Based Room Temperature Ionic Liquids: Data and Correlations

Abstract: Reported are the solubility, diffusivity, and permeability data for various gases in five phosphonium-based ionic liquids at 30 °C, as determined with a lag-time technique. The ionic liquids have a viscosity range of 200−3000 cP. The gas solubilities and diffusivities of the phosphonium-based ionic liquids are of the same magnitude as the gas solubilities for the more familiar imidazolium-based liquids. The gas diffusivity appears to be inversely proportional to the viscosity with an average power of 0.35 for the phosphonium-based ionic liquids. This is in contrast to the power of 0.6 for the imidazolium-based ionic liquids, suggesting that the viscosity−diffusivity relationship varies for different classes of ionic liquids. Despite the generally higher viscosities of the phosphonium-based RTILs compared to the imidazolium-based RTILs, the similarity in thermodynamic and transport properties allows the consideration of the phosphonium-based RTILs as low-cost alternatives for reaction media or separation a...

Silicate melt properties and volcanic eruptions

Abstract: [1] Knowledge about the properties of silicate melts is needed by volcanologists and petrologists to evaluate the dynamics of volcanic eruptions and magmatic processes. These properties include the solubility and diffusivity of volatile components in silicate melts, silicate melt viscosity, and the fragmentation condition. Data and models of each property are reviewed and assessed. For rhyolitic melts many properties are sufficiently well known to allow realistic modeling of volcanic and magmatic processes. One interesting example is the role of speciation in the solubility and diffusivity of H2O and CO2. Even though both H2O and CO2 are present in silicate melts as at least two species, the complexity in the solubility and diffusion behavior of H2O and the simplicity of CO2 are due to differences in the speciation reaction: For the H2O component the stoichiometric coefficient is one for one hydrous species (molecular H2O) but is two for the other hydrous species (OH) in the species interconversion reaction, whereas for CO2 the stoichiometric coefficients for all carbon species are one. The investigation of the species reaction not only helps in understanding the solubility and diffusion behavior, but the reaction among the hydrous species also serves as a geospeedometer (cooling rate indicator) for hydrous rhyolitic pyroclasts and glass and provides a method to infer viscosity. For melts other than rhyolite, a preliminary description of their properties is also available, but much more experimental and modeling work is necessary to quantify these properties more accurately.

Simultaneous determination of thermal conductivity, thermal diffusivity and specific heat in sI methane hydrate

Abstract: SUMMARY Thermal conductivity, thermal diffusivity and specific heat of sI methane hydrate were measured as functions of temperature and pressure using a needle probe technique. The temperature dependence was measured between −20 ◦ C and 17 ◦ C at 31.5 MPa. The pressure dependence was measured between 31.5 and 102 MPa at 14.4 ◦ C. Only weak temperature and pressure dependencies were observed. Methane hydrate thermal conductivity differs from that of water by less than 10 per cent, too little to provide a sensitive measure of hydrate content in watersaturated systems. Thermal diffusivity of methane hydrate is more than twice that of water, however, and its specific heat is about half that of water. Thus, when drilling into or through hydrate-rich sediment, heat from the borehole can raise the formation temperature more than 20 per cent faster than if the formation’s pore space contains only water. Thermal properties of methane hydrate should be considered in safety and economic assessments of hydrate-bearing sediment.

Transient plane source measurements of the thermal properties of hydrating cement pastes

Abstract: A transient plane source measurement technique is applied to assessing the heat capacity and thermal conductivity of hydrating cement pastes in their fresh state and during the course of 28 d of hydration at 20°C. Variables investigated include water-to-cement mass ratio (w/c – 0.3 or 0.4) and curing conditions (sealed or saturated curing). The heat capacity data for the fresh cement pastes are compared to a simple law of mixtures, and analytical expressions are developed to estimate the heat capacity as a function of degree of hydration for the two curing conditions. The measured thermal conductivities of the fresh pastes along with the known thermal conductivity of water are used to estimate the thermal conductivity of the original cement powder via application of the Hashin-Shtrikman (H-S) bounds. Hydration is seen to have only a minor influence on the measured thermal conductivity. Extension of the law of mixtures for heat capacity and the H-S bounds for thermal conductivity to predicting the corresponding properties of concretes are discussed.

Mesoscale Modeling of a Li-Ion Polymer Cell

Abstract: Finite element models of a three-dimensional, porous cathode were constructed and analyzed by the COMSOL multiphysics package (version 3.2). Four types of cathode active material particles, arranged in both regular and random arrays, were modeled. Experimental studies of Li/PEO-LiClO 4 /Li 1+x Mn 2 O 4 (where 0 < x < 1) were used to validate simulation results. Two parameters, Li ion diffusivity into Li 1+x Mn 2 O 4 particles, and contact resistance at the interface between cathode particles and the current collector, were obtained by curve-fitting discharge curves of simulation results of regular array models, with Li 1+x Mn 2 O 4 particles (3.6 μm) with experimental results. Diffusivities of Li ions were found to be 4 ×10 -13 , 6 ×10 -13 , 1×10 -12 , and 5 X 10 -12 cm 2 /s for Li 1+x Mn 2 O 4 particles sintered at 800, 600, 500, and 450°C, respectively. Contact resistances were found to be 3.5 Ω cm 2 for Li 1+x Mn 2 O 4 particles prepared at 600 and 800°C, and 10.5 Ω cm 2 for particles prepared at 450 and 500°C. Regular arrays were shown to increase achievable capacity from 5 to 50% of the theoretical capacity, compared with random arrays, at C/10 for samples sintered at 500°C. Smaller particle sizes of active material particles were also shown to be beneficial for high power density applications and for low diffusivity active materials.

Mean-field versus microconvection effects in nanofluid thermal conduction.

Abstract: Transient hot-wire data on thermal conductivity of suspensions of silica and perfluorinated particles show agreement with the mean-field theory of Maxwell but not with the recently postulated microconvection mechanism. The influence of interfacial thermal resistance, convective effects at microscales, and the possibility of thermal conductivity enhancements beyond the Maxwell limit are discussed.

Advances in pulsed-laser atom probe: instrument and specimen design for optimum performance.

Abstract: The performance of the pulsed-laser atom probe can be limited by both instrument and specimen factors. The experiments described in this article were designed to identify these factors so as to provide direction for further instrument and specimen development. Good agreement between voltage-pulsed and laser-pulsed data is found when the effective pulse fraction is less than 0.2 for pulsed-laser mode. Under the conditions reported in this article, the thermal tails of the peaks in the mass spectra did not show any significant change when produced with either a 10-ps or a 120-fs pulsed-laser source. Mass resolving power generally improves as the laser spot size and laser wavelength are decreased and as the specimen tip radius, specimen taper angle, and thermal diffusivity of the specimen material are increased. However, it is shown that two of the materials used in this study, aluminum and stainless steel, depend on these factors differently. A one-dimensional heat flow model is explored to explain these differences. The model correctly predicts the behavior of the aluminum samples, but breaks down for the stainless steel samples when the tip radius is large. A more accurate three-dimensional model is needed to overcome these discrepancies.

Thermal characterization of microscale conductive and nonconductive wires using transient electrothermal technique

Abstract: In this paper, a transient technique is developed to characterize the thermophysical properties of one-dimensional conductive and nonconductive microscale wires. In this technique, the to-be-measured thin wire is suspended between two electrodes. When feeding a step dc to the sample, its temperature will increase and take a certain time to reach the steady state. This temperature evolution is probed by measuring the variation of voltage over the wire, which is directly related to resistance/temperature change. The temperature evolution history of the sample can be used to determine its thermal diffusivity. A 25.4 m thick platinum wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying this transient electrothermal technique, the thermal diffusivities of single-wall carbon nanotube bundles and polyester fibers are measured. © 2007 American Institute of Physics. DOI: 10.1063/1.2714679

Mechanism of thermal transport in dilute nanocolloids.

Abstract: Thermal conduction modes in a nanocolloid (nanofluid) are quantitatively assessed by combining linear response theory with molecular dynamics simulations. The microscopic heat flux is decomposed into three additive fluctuation modes, namely, kinetic, potential, and collision. For low volume fractions (<1%) of nanosized platinum clusters which interact strongly with xenon host liquid, a significant thermal conductivity enhancement results from the self correlation in the potential flux. Our findings reveal a molecular-level mechanism for enhanced thermal conductivity in nanocolloids with short-ranged attraction and offer predictions that can be experimentally tested.

NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system.

Abstract: Lithium niobate has been chosen as a model system for spectroscopic studies of the influence of different structural forms and preparation routes of an ionic conductor on its ion transport properties. The Li diffusivity in nanocrystalline LiNbO3, prepared either mechanically by high energy ball milling or chemically by a sol-gel route, was studied by means of impedance and solid state 7Li NMR spectroscopy. The Li diffusivity turned out to be strongly correlated with the different grain boundary microstructures of the two nanocrystalline samples and with the degree of disorder introduced during preparation, as seen especially by HRTEM and EXAFS. Although in both samples nanostructuring yields an enhancement of the Li diffusivity compared to that in coarse grained LiNbO3, the Li diffusivity in ball milled LiNbO3 is much higher than in chemically prepared nanocrystalline LiNbO3. The former LiNbO3 sample has a large volume fraction of highly disordered interfacial regions which seem to be responsible for fast Li diffusion and to have a structure very similar to that of the amorphous form. This is in contrast to the chemically prepared sample where these regions have a smaller volume fraction.

Thermodiffusion of charged colloids: single-particle diffusion.

Abstract: An expression for the single-particle thermal diffusion coefficient of a charged colloidal sphere is derived on the basis of force balance on the Brownian time scale in combination with thermodynamics. It is shown that the single-particle thermal diffusion coefficient is related to the temperature dependence of the reversible work necessary to build the colloidal particle, including the core, the solvation layer, and the electrical double layer. From this general expression, an explicit expression for the contribution of the electrical double layer to the single-particle thermal diffusion coefficient is derived in terms of the surface charge density of the colloidal sphere, the electrostatic screening length, and its core radius, to within the Debye-Huckel approximation. This result is shown to explain experimental data, for both thin and thick double layers. In addition, a comparison with other theories is made.

Thermal radiation effects on MHD flow of a micropolar fluid over a stretching surface with variable thermal conductivity

Abstract: In this paper, the effects of variable thermal conductivity and radiation on the flow and heat transfer of an electrically conducting micropolar fluid over a continuously stretching surface with varying temperature in the presence of a magnetic field are considered. The surface temperature is assumed to vary as a power-law temperature. The governing conservation equations of mass, momentum, angular momentum and energy are converted into a system of non-linear ordinary differential equations by means of similarity transformation. The resulting system of coupled non-linear ordinary differential equations is solved numerically. The numerical results show that the thermal boundary thickness increases as the thermal conductivity parameter S increases, while it decreases as the radiation parameter F increases. Also, it was found that the Nusselt number increases as F increases and decreases as S increases.

Two-Phase Transients of Polymer Electrolyte Fuel Cells

Abstract: A three-dimensional transient model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes was developed to study the dynamics of gas-diffusion layer GDL dewetting and its impact on polymer electrolyte fuel cell performance. It was found that the dewetting of fuel cells by dry gas is characterized by several regimes of different time constants. These regimes can be classified by through-plane drying vs in-plane drying as well as by the differing water diffusivity in the anode and cathode. The water diffusivity in the anode GDL is several times larger than that in the cathode, therefore the anode side undergoes faster water loss to the dry gas stream. In addition, the land hampers the diffusive transport of water, therefore the liquid water tends to be trapped under the land and the water loss there starts only after through-plane drying of the GDL under the channel is completed. The different time constants of various dewetting regimes also affect the evolution of cell voltage due to the ohmic loss in the membrane. In addition, theoretical solutions are developed for the in-plane and through-plane drying regimes, and show good agreement with the numerically predicted time scales. © 2007 The Electrochemical Society. DOI: 10.1149/1.2734076 All rights reserved.