Showing papers on "Thermal diffusivity published in 1994"

Transport coefficients of argon, nitrogen, oxygen, argon-nitrogen, and argon-oxygen plasmas

Abstract: Calculated values of the viscosity, thermal conductivity, and electrical conductivity of argon, nitrogen, and oxygen plasmas, and mixtures of argon anti nitrogen and of argon anti oxygen, are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. These three combined diffusion coefficients fully describe di fusion of the two gases, irrespective of their degree of dissociation or ionizati on. The calculations, which assume local thermodynamic equilibrium, are performed! for atmospheric-pressure plasmas in the temperature range /torn 300 to 30,000 K. A number of the collision integrals used in calculating the transport coefficients are significantly more accurate than values used in previous theoretical studies, resulting in more reliable values of the transport coefficients. The results are compared with those of published theoretical and experimental studies.

Impedance Spectroscopy of Hydrating Cement‐Based Materials: Measurement, Interpretation, and Application

Abstract: This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement-based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel-based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.

Thermal conductivity, thermal diffusivity, and specific heat of thin samples from transient measurements with hot disk sensors

Abstract: Transient measurements of thermal conductivity, thermal diffusivity, and specific heat capacity have been performed with hot disk sensors in thin samples of metallic materials. With this new variation of the hot disk method the sample size can be reduced to a volume less than ten cubic centimeters for copper at room temperature. It is also shown that the specific heat capacity can be conveniently measured in transient recordings of slightly longer duration. On comparing with standard values the accuracy turns out to be better than 1% while the precision (standard deviation of the mean from six measurements) on the average is about 0.5% for all values recorded.

Diffusion in Lennard-Jones Fluids Using Dual Control Volume Grand Canonical Molecular Dynamics Simulation (DCV-GCMD)

Abstract: A new approach to calculating diffusivities, both transport as well as equilibrium, is presented. The dual control volume grand canonical molecular dynamics (or DCV‐GCMD) method employs two local control volumes for chemical potential control via particle creation/destruction as in grand canonical Monte Carlo (GCMC) simulations. The control volumes are inserted in a standard NVT molecular dynamics simulation yielding a simulation with stochastic chemical potential control that may be thought of as a hybrid GCMC‐MD approach. The geometrical control of the chemical potential enables a steady state chemical potential gradient to be established in the system. By measuring the density profile and flux, Fick’s law is used to determine the diffusivity. An example calculation is presented for a simple Lennard‐Jones system.

Convection enhanced diffusion for periodic flow

Abstract: This paper studies the influence of convection by periodic or cellular flows on the effective diffusivity of a passive scalar transported by the fluid when the molecular diffusivity is small. The flows are generated by two-dimensional, steady, divergence-free, periodic velocity fields.

The influence of thermal diffusion on laser ablation of metal films

Abstract: Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 μm have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.

Thermal diffusivity measurement of GaAs/AlGaAs thin-film structures

Abstract: This work develops a new measurement technique that determines the thermal diffusivity of thin films in both parallel and perpendicular directions, and presents experimental results on the thermal diffusivity of GaAs/AlGaAs-based thin-film structures. In the experiment, a modulated laser source heats up the sample and a fast-response temperature sensor patterned directly on the sample picks up the thermal response. From the phase delay between the heating source and the temperature sensor, the thermal diffusivity in either the parallel or perpendicular direction is obtained depending on the experimental configuration. The experiment is performed on a molecular-beam-epitaxy grown vertical-cavity surface-emitting laser (VCSEL) structure. The substrates of the samples are etched away to eliminate the effects of the interface between the film and the substrate. The results show that the thermal diffusivity of the VCSEL structure is 5-7 times smaller than that of its corresponding bulk media. The experiments also provide evidence on the anisotropy of thermal diffusivity caused solely by the effects of interfaces and boundaries of thin films. 23 refs., 9 figs., 1 tab.

Comparison of Single and Dual Probes for Measuring Soil Thermal Properties with Transient Heating

Abstract: Storage and transfer of heat in soils is governed by the soil thermal properties and these properties are therefore needed in many agricultural and engineering applications. In this paper we discuss solutions of the heat flow equation applicable to single and dual probe transient heating methods, and describe measurements made on air-dry sand to show how these methods can be used to obtain soil thermal properties. Measurements show that the two methods yield similar values of thermal conductivity. When determining thermal conductivity from the single probe data, it is best to use nonlinear curve fitting and to include a correction term in the model to account for the presence of the probe. Measurements of volumetric heat capacity made by using the dual probe heat-pulse method agreed well with independent estimates obtained using the de Vries method of summing the heat capacities of the soil constituents. The advantage of using the dual probe method together with the appropriate heat-pulse theory rather than the single probe is that all three soil thermal properties, the thermal diffusivity, volumetric heat capacity, and thermal conductivity, can be determined from a single heat-pulse measurement. Instantaneous heat-pulse theory can be used with the dual probe method to determine heat capacity from short duration heat-pulse data, but it should not be used to determine the thermal diffusivity and thermal conductivity.

Analysis of hydrogen atom transport in a two phase alloy.

Abstract: The theoretical basis for analysing hydrogen atom transport in a two-phase alloy is evaluated. Experimental measurements of hydrogen atom transport in a duplex stainless steel, thermally treated to give a varying volume fraction of austenite, are described and analysed to ascertain the relative effect on the effective diffusivity of interfacial trapping and of the reduced diffusivity but enhanced solubility of the austenite phase. The effective diffusivity of the as-received duplex stainless steel is a factor of 400 less than that for the fully ferritic steel. It is demonstrated that diffusion through the austenite has no effect on the hydrogen transport, despite the higher solubility of hydrogen in this phase. However, the presence of austenite creates a more tortuous path for the hydrogen transport. The enhanaced solubility of hydrogen atoms in the austenite phase, relative to that in the ferrite phase, can be considered to induce a trapping effect on the hydrogen atom transport; however, the effect is small relative to the trapping associated with the austenite-ferrite interface. The binding energy of the interfacial traps is estimated to be about 52 kJ mol −1 .

Laser-induced thermal acoustics: simple accurate gas measurements

Abstract: Laser-induced thermal acoustics (LITA), an optical four-wave mixing technique, has been used for sensitive measurement of the sound speed, thermal diffusivity, acoustic damping rate, and complex susceptibility of a gas. In LITA, laser-induced acoustic waves scatter laser light into a coherent, modulated signal beam. A simple expression accurately describes the signal. Atmospheric sound speeds accurate to 0.5% and transport properties accurate to 30% have been measured in a single shot without calibration. LITA spectra have been taken of weak spectral lines of NO2 in concentrations of less than 50 parts in 10^9. Signal reflectivities up to 10^4 are estimated.

Optical and thermal characterization of natural (Sepia officinalis) melanin

Abstract: The optical properties and the thermal diffusivity of natural cuttlefish (Sepia officinalis) melanin have been measured. The optical absorption and scattering properties of melanin particles were determined at 580 nm and 633 nm, using photometric and photothermal techniques. For the photometric studies, the absorption and the transport scattering coefficients were determined from the measurements of diffuse reflectance and transmittance. The scattering anisotropy was obtained from an additional measurement of the total attenuation coefficient and independently obtained by goniometry. For photothermal studies, pulsed photothermal radiometry was used to deduce the absorption and transport scattering coefficients via a model based on optical diffusion theory. Pulsed photothermal radiometry was also used to provide the thermal diffusivity of solid melanin pressed pellets.

Asymmetric Drainage in Foam Films

Abstract: The same basic mechanism is responsible for asymmetric drainage of thin circular films and marginal regeneration in vertical, rectangular films. A linear stability analysis showed that these phenomena are caused by a hydrodynamic instability that is produced by a surface-tension-driven flow and stabilized by surface viscosity, surface diffusivity, and the system length scale. Acriterion for the onset ofthis instability was derived. Experiments performed on small circular films of aqueous solutions of SDS and SDS-1dodecanol demonstrated the strong stabilizing effect of surface viscosity. Experimental results were found to be in good agreement with the predictions of the linear stability analysis.

Thermal conductivity of molding compounds for plastic packaging

Abstract: Alumina-loaded new molding compounds feature a thermal conductivity up to 1.5 W/mK, a water uptake smaller than 0.2 weight %, a coefficient of thermal expansion of 10 ppm/K, and an excellent popcorn resistance (0 cracks from 6). The thermal conductivity of such particulate-loaded polymers has been investigated as a function of the volume content (0-80%) of aluminum oxide, quartz, and fused quartz. The thermal expansion and the specific heat have also been recorded. A new model allows one to calculate the thermal conductivity and the thermal expansion within a couple of percent. >

Geochronology of the Larderello geothermal field: new data and the “closure temperature” issue

Abstract: The Larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4–9 km depth. Hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones, which implies that the field has always undergone a roughly monotonic cooling history (fluctuations < 40 K) since intrusion of the granite at 4 Ma. The heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of ≈ 32000 km3 rooted in the mantle. Borehole minerals from Larderello are thus a unique well-calibrated natural example of thermally induced Ar and Sr loss under geological conditions and time spans. The observations (biotites retain Ar above 450°C) agree well with other, albeit less precise, geological determinations, but contrast with laboratory determinations of diffusivity from the literature. We therefore performed a hydrothermal experiment on two Larderello biotites and derived a diffusivity D Lab(370°C)=5.3·10-18 cm2s-1, in agreement with published estimates of diffusivity in annite. From D Lab and the rejuvenation of the K/Ar ages we calculate maximum survival times at the present in-hole temperatures. They trend smoothly over almost two orders of magnitude from 352 ka to 5.3 ka, anticorrelating with depth: laboratory diffusivities are inconsistent not only with geological facts, but also among themselves. From the geologically constrained lifetime of the thermal anomaly we derive a diffusivity D G(370°C)=3.81·1021 cm2s-1, 3±1 orders of magnitude lower than D Lab. The cause of these discrepancies must be sought among various laboratory artefacts: overstepping of a critical temperature T *; enhanced diffusivities in “wet” experiments; presence of fast pathway (dislocation and pipe) diffusion, and of dissolution/reprecipitation reactions, which we imaged by scanning electron microscopy. These phenomena are minor in geological settings: in the absence of mineral transformation reactions, complete or near-complete resetting is achieved only by volume diffusion. Therefore, laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers.

Particle coarsening in binary and multicomponent alloys

Abstract: The theory of particle coarsening (Ostwald ripening) for multicomponent alloys was reviewed recently by Umantsev and Olson. In addition, they presented their own formulation for systems with a diagonal diffusivity matrix and they discussed how coarsening is affected by non-ideal behavior of the solid solution surrounding the precipitate particles. The present communication extends their work to include off diagonal terms in the diffusivity and shows that coarsening is independent of the solid solution thermodynamics followed by the continuous phase. This independence was recognized before, because changes in the diffusivity due to non-ideal behavior are balanced by changes in the concentration differences, due to surface curvature, that drive diffusion.

Micrometer scale visualization of thermal waves by photoreflectance microscopy

Abstract: We propose a novel approach of photoreflectance microscopy that provides a direct visualization of the phase contour lines of the thermal wave. The method is applicable to (possibly heterogeneous) samples of mediocre polish. In a locally homogeneous region it yields the local thermal diffusivity.

Comparison of six methods to determine unsaturated soil hydraulic conductivity.

Abstract: Knowledge of soil hydraulic properties is required for soil-water flow models. Although many studies of individual methods exist, comparisons of methods are uncommon. Therefore, we compared application ranges and results for six laboratory methods for determining hydraulic conductivity or diffusivity on eolian sand, eolian silt loam, marine sandy loam, and fluviatile silt loam. The methods, hot air, sorptivity, crust, drip infiltrometer, Wind's evaporation, and one-step outflow, fall into three groups: (i) those that only yield a conductivity curve; (ii) those that yield a simultaneous estimate of conductivity, diffusivity, water content, and pressure head; and (iii) those that yield a diffusivity curve. Diffusivities were converted to conductivities with a water retention curve. One main difference between the methods was the pressure head-water content range. Despite the large differences between the methods, the results for the first two groups tended to be similar. The results of the third group did not match well with those of the first two. It proved difficult to compare these methods correctly due to hysteresis. View complete article To view this complete article, insert Disc 5 then click button8

Diffusion of Spheres in Entangled Polymer Solutions: A Return to Stokes-Einstein\ Behavior

Abstract: Dynamic light scattering has been used to follow the tracer diffusion of polystyrene spheres (R200 nm) in dilute, semidilute, and entangled solutions of poly(vinyl methyl ether) (M w = 1.3×10 6 ). Over this range of matrix concentrations, 0 ≤ c[η] ≤ 36, the diffusivity drops by almost 5 orders of magnitude. Near c * (∼ [η] -1 ) for the matrix, the diffusivity exceeds that estimated from the bulk solution viscosity via the Stokes-Einstein relation by a factor of about 3. Such expositive deviations« from Stokes-Einstein behavior have been reported previously in several systems. However, once the matrix concentration is sufficiently high for entanglements to be effective, Stokes-Einstein behavior is recovered

Compositional Gradients in Petroleum Reservoirs

Abstract: This paper describes methods for calculating the one-dimensional, vertical variation in composition with depth caused by gravity and thermal gradients. The Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) cubic equations of state (EOS) are used as thermodynamic models. Examples of calculated compositional gradients are given for reservoir fluid systems ranging from black oil to near-critical oils. A solution algorithm is suggested for solving the isothermal gravity/chemical equilibrium (GCE) problem. The algorithm is simply an adaptation of a method proposed by Michelsen for calculating saturation pressure. The problem of false (unstable) solutions is discussed, and the subsequent need for applying phase stability analysis to identify such false solutions. Finally, an algorithm is given for determining the location of a gas-oil contact (GOC). A model for treating both gravity and thermal gradient has been used to quantify the potential effect of thermal diffusion on compositional grading. The model used was proposed by Belery and da Silva. Unfortunately, the physics and thermodynamics of thermal diffusion are not well understood. This model is only one of several approaches which have been suggested for treating thermal diffusion. Examples given in the authors paper show that thermal diffusion can have a marked effect on compositional grading, with themore » possibility of enhancing, reducing, or completely eliminating gradients caused by gravity alone. The authors illustrate the potential danger of using gradient calculations for defining original hydrocarbon distributions (oil and gas in place) when limited fluid samples and PVT data are available. Furthermore, guidelines are given for when to use gradient calculations, and how to develop an EOS fluid characterization for reservoirs exhibiting compositional variation.« less

Anisotropic thermal diffusivity and conductivity of YBCO(123) and YBCO(211) mixed crystals. I

Abstract: The anisotropic thermal diffusivity α of the highly c-axis-oriented Y–Ba–Cu–O bulk superconducting crystals has been measured quasi-simultaneously with the thermal conductivity κ. The estimated values of the specific heat C by use of α and κ values parallel and perpendicular to the c-direction agreed with each other. In these crystals prepared by the modified melt texture growth (MMTG) method, fine Y2BaCuO5 particles are dispersed in the YBa2Cu3O7-x superconducting matrix phase. Based on a simple model, the thermal diffusivity and the specific heat of the YBa2Cu3O7-x matrix phase are separated using independently measured data of a Y2BaCuO5 polycrystal. The influence of Y2BaCuO5 particles on the thermal properties of the mixed crystals is discussed.

A high-temperature laser-pulse thermal diffusivity apparatus

Abstract: A high-temperature laser-pulse apparatus for the measurement of thermal diffusivity in the temperature range from 1500 to 2500 K has been designed. constructed, and tested at the National Institute of Standards and Technology. A curve-fitting method is introduced by which the entire experimental temperature history curve is fitted with the theoretical curve under the boundary condition of radiative heat losses. The new apparatus and the curve-fitting method permit thermal diffusivity measurements with an uncertainty of not more than 3%.