Showing papers on "Thermal diffusivity published in 1987"

Thermal properties of AlAs/GaAs superlattices

Abstract: Thermal properties of semiconductor superlattices have been investigated for the first time. The thermal properties of AlAs/GaAs superlattices were measured with the ac calorimetric method. It is found that the thermal diffusivity and thermal conductivity of the AlAs/GaAs superlattices are larger than those of the AlGaAs alloy due to the suppression of alloy scattering in the superlattice. However, the thermal diffusivity and thermal conductivity decrease with a decrease in the superlattice period and seem to approach those of the Al0.5Ga0.5As alloy in the limit of short‐period superlattices.

Molecular dynamics of flow in micropores

Abstract: The method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in a micropore only a few molecular diameters wide. Fluid inhomogeneity is introduced by imposing an external potential that in one case simulates flat solid walls and in the other case causes density peaks in the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 1010–1011 s−1 of the simulation. The Green–Kubo relation is found to be valid for the diffusivity under the flow studied. We propose a local average density model (LADM) of viscosity and diffusivity, in which the local transport coefficients are those of homogeneous fluid at a mean density obtained by averaging the local density over a molecular volume. LADM predicts qualitatively correct velocity profiles, effective viscosities, and shear stresses using only equilibrium density profiles and molecular diameters. An analogous local equilibrium version of Enskog’s theory of diffusivity agrees well with the simulated pore diffusivities. Recently Vanderlick and Davis generalized Enskog’s theory of diffusivity to strongly inhomogeneous fluids. Their theoretical pore diffusion coefficient is also in good agreement with simulation results.

Minimal‐volume photoacoustic cell measurement of thermal diffusivity: Effect of the thermoelastic sample bending

Abstract: A minimal‐volume photoacoustic cell method is demonstrated for obtaining the thermal diffusivity of solids. It is based on the measurement of the acoustic signal as a function of the modulation frequency in the region where the sample is thermally thick. The method is tested by using metal, semiconductor, and polymer samples.

Thermobaricity, Cabbeling, and Water-Mass Conversion

Abstract: The efficient mixing of heat and salt along neutral surfaces (by mesoscale eddies) is shown to lead to vertical advection through these neutral surfaces. This is due to the nonlinearities of the equation of state of seawater through terms like ∂2ρ/∂θ∂p (thermobaric effect) and ∂2ρ/∂ θ2 (cabbeling). Cabbeling always causes a sinking or downwelling of fluid through neutral surfaces, whereas thermobaricity can lead to a vertical velocity (relative to neutral surfaces) of either sign. In this paper it is shown that for reasonable values of the lateral scalar diffusivity (especially below a depth of 1000 m), these two processes cause vertical velocities of the order of 10−7 m s−1 through neutral surfaces (usually downward!) and cause water-mass conversion of a magnitude equal to that caused by a vertical diffusivity of 10−4 m2 s−1 (often equivalent to a negative diffusivity). Both thermobaricity and cabbeling can occur in the presence of any nonzero amount of small-scale turbulence and so will not be detected by microstructure measurements. The conservation equations for tracers are considered in a nonorthogonal coordinate frame that moves with neutral surfaces in the ocean. Since only mixing processes cause advection across neutral surfaces, it is useful to regard this vertical advection as a symptom of various mixing processes rather than as a separate physical process. It is possible to derive conservative equations for scalars that do not contain the vertical advective term explicity. In these conservation equations, the terms that represent mixing processes are substantially altered. It is argued that this form of the conservation equations is the most appropriate when considering water-mass transformation, and some examples are given of its application in the North Atlantic. It is shown that the variation of the vertical diffusivity with height does not cause water-mass transformation. Also, salt fingering is often 3–4 times more effective at changing the potential temperature of a water mass than would be implied by simply calculating the vertical derivative of the fingering heat flux.

A fluid theory of ion collection by probes in strong magnetic fields with plasma flow

Abstract: A one‐dimensional fluid theory of Langmuir probe operation in strong magnetic fields is presented. Cross‐field diffusion of ions both into and out of the the collection region is consistently accounted for, in effect taking momentum and particle diffusivity to be equal. The results differ by significant factors from previous analyses, which did not account for outward diffusion but in effect set momentum diffusivity to zero. The differences are especially large when parallel flow of the external plasma is present. It is thus clear that the value assumed for the momentum diffusivity strongly affects the interpretation of recent probe measurements. It is argued that the present results offer a more reliable basis for this interpretation.

Diffusive transport in a Rayleigh-Bénard convection cell.

Abstract: The effective diffusivity for a passive impurity in a steady convecting flow is calculated in the experimentally relevant limit of large P\'eclet number (p=vl/D). The expulsion of the concentration gradients from the region of closed stream lines leads to the enhancement of the diffusivity D by a factor of ${p}^{1/2}$.

Effective diffusion in laminar convective flows

Abstract: The effective diffusion coefficient D* of a passive component, such as test particles, dye, temperature, magnetic flux, etc, is derived for motion in periodic two‐dimensional incompressible convective flow with characteristic velocity v and size d in the presence of an intrinsic local diffusivity D Asymptotic solutions for effective diffusivity D*(P) in the large P limit, with P∼ vd/D, is shown to be of the form D*=cDP1/2 with c being a coefficient that is determined analytically The constant c depends on the geometry of the convective cell and on an average of the flow speed along the separatrix The asymptotic method of evaluation applies to both free boundary and rough boundary flow patterns and it is shown that the method can be extended to more complicated patterns such as the flows generated by rotating cylinders, as in the problem considered by Nadim, Cox, and Brenner [J Fluid Mech 164, 185 (1986)] The diffusivity D* is readily calculated for small P, but the evaluation for arbitrary P requires numerical methods Monte Carlo particle simulation codes are used to evaluate D* at arbitrary P, and thereby describe the transition for D* between the large and small P limits

Theory of resistive pressure-gradient-driven turbulence

Abstract: Saturated resistive pressure‐gradient‐driven turbulence is studied analytically and with numerical calculations Fluid viscosity and thermal diffusivity are retained in the analysis and calculations Such dissipation guarantees the existence of a stable, high‐m dissipation range, which serves as an energy sink An accurate saturation criterion is proposed The resulting predicted pressure diffusivity scales similarly to the mixing length estimate but is significantly larger in magnitude The predictions of the analytic theory are in good quantitative agreement with the numerical results for fluctuation levels

Thermal conductivity of Inconel 718 and 304 stainless steel

Abstract: The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known

Thermal conductivity of the amorphous alloy Fe40Ni40P14B6 between 80 and 300 K

Abstract: The thermal conductivity of the amorphous alloy Fe40Ni40P14B6 is investigated with the aim to separate out the lattice contribution and to compare it with that of amorphous dielectrics. The problems of the influence of the radiative transfer of heat between the sample and its environment, which causes errors in the determination of thermal conductivity, are discussed. The authors found that the temperature dependence of the lattice thermal conductivity of the alloys is qualitatively the same as that of amorphous dielectrics.

Effect of Cracking on Drying Permeability and Diffusivity of Concrete

Abstract: The increase of overall drying permeability and diffusivity of concrete due to cracking is determined experimentally and formulated mathematically. The test specimens are C-shaped beams deformed by a tie rod and reinforced on the tensile face so that uniformly spaced cracks are produced. The difference in the loss of weight for various drying periods between cracked and uncracked specimens is measured and used to quantify the effect on permeability and diffusivity. The overall drying diffusivity and permeability in the cracking direction, which is theoretically proportional to the crack width cubed and inversely proportional to the crack spacing, is found to increase about 2.25 times for crack width 0.1 mm and crack spacing 70 mm. Although appreciable, this value is two orders of magnitude less than the theoretical upper bound predicted on the basis of viscous flow calulation if it is assumed that the cracks are of constant thickness, have planar walls, and are continuous. It is concluded that even though the major cracks are seen to be continuous on the specimen surface, they must be discontinuous in the specimen interior, perhaps being interconnected by much narrower necks with a width about 10 times smaller. This fact is of interest for deducing fracture process zone models from visual observations of cracks on the specimen surface. Although approximate, the presently derived formula for the increase of diffusivity and permeability is directly usable in finite element programs for drying or wetting of concrete.

Experimental and Numerical Study of the Thermal Degradation of PMMA

Abstract: Thermal degradation of PMMA subjected to a radiant heat flux was studied both experimentally and numerically. Very accurate measurements of mass loss rate and temperature profiles were performed for three constant heat fluxes: 1.5,2.3 and 3.0 W·cm-'. The results show that the mass loss rate cannot be directly related to the surface temperature and a contribution of the sub-surface region of the sample to the rate of gasification must be taken into account in a thermal degradation odel. A heat source term was introduced in the conduction equation and a kinetic equation was used to locally calculate the mass loss rate in each slab of the solid. The density and the thermal conductivity of the material were assumed temperature independent while for the specific heat the values measured by differential scanning calorimetry in the range 20-400·C were retained. The predictions of the model are in very close agreement with the experimental results for the evolution with time of both the temperature profi...

A measurement of hydrogen ion transport parameters in Tokamak discharges

Abstract: By modulating the small gas feed required to maintain a steady-state Tokamak discharge a small density perturbation may be induced from whose propagation particle transport parameters may be adduced. Experiments in TEXT show that the phenomena may be adequately described by a simple model with constant diffusion plus inward convection except near the density limit. Diffusion coefficients and convective velocities has been determined for a broad range of hydrogen discharges and found to scale approximately as (nq)-1. The results are plausibly related to impurity transport parameters, thermal diffusivity, and particle confinement times. Diffusion coefficients for deuterium are systematically lower than for hydrogen.

Photoacoustic measurement of thermal diffusivity of polymer foils

Abstract: The photoacoustic measurement of polymer foils, typically 170–200 μm thick, is discussed. It is shown that the measurement based upon the phase lag between the front and rear illuminations is applicable only in a limited range of frequencies from 6 to 12 Hz. The dominant mechanism responsible for the photoacoustic signal, in almost the entire frequency range 10–100 Hz, is proven to be the thermoelastic bending of the foil samples. The thermal diffusivity is then obtained from the frequency dependence of the front‐phase illumination data.

Tokamak heat transport – a study of heat pulse propagation in JET

Abstract: The propagation of heat pulses originating from sawtooth activity in JET has been investigated in a series of limiter discharges with the following parameters: plasma current, Ip 3 MA, toroidal magnetic field, BT 3 T and elongation, κ = 1.45. The auxiliary power was varied such that the total power ranged from 2 to 13.5 MW. Electron temperature perturbations in a 20 cm region around a minor radius of r = (2/3)a were recorded with high time resolution, using a 12 channel electron cyclotron emission polychromator. From these measurements the electron heat diffusivity was derived. Over the whole range of powers considered, was found to be independent of power and to lie in the range of 2.5 ± 0.5 m2s−1. The quantity is compared to as derived from global power balance analysis. For Ohmic heating, the latter is lower than by a factor of 2.5. For increasing auxiliary power, approaches . A model for the dependence of the local χe n the temperature gradient is presented; it permits a unified description of the heat pulse behaviour, the deterioration of confinement and a certain degree of profile consistency. The model does not invoke non-local parameters such as the total power input. It is shown that the present heat pulse data, subject to this interpretation, contradict the τE scaling laws of the typical form τE ∝ P−0.5.

A self-heated thermistor technique to measure effective thermal properties from the tissue surface

Abstract: A microcomputer based instrument to measure effective thermal conductivity and diffusivity at the surface of a tissue has been developed. Self-heated spherical thermistors, partially embedded in an insulator, are used to simultaneously heat tissue and measure the resulting temperature rise. The temperature increase of the thermistor for a given applied power is a function of the combined thermal properties of the insulator, the thermistor, and the tissue. Once the probe is calibrated, the instrument accurately measures the thermal properties of tissue. Conductivity measurements are accurate to 2 percent and diffusivity measurements are accurate to 4 percent. A simplified bioheat equation is used which assumes the effective tissue thermal conductivity is a linear function of perfusion. Since tissue blood flow strongly affects heat transfer, the surface thermistor probe is quite sensitive to perfusion.

Thermal diffusivity of igneous rocks at elevated pressure and temperature

Abstract: Thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 /sup 0/C and pressures of 200 MPa. The measuring method was based on the concept of cylindrical symmetry and periodic heat pulses. The seven rocks measured were Westerly (Rhode Island) granite, Climax Stock (Nevada) quartz monzonite, Pomona (Washington) basalt, Atikokan (Ontario, Canada) granite, Creighton (Ontario, Canada) gabbro, East Bull Lake (Ontario, Canada) gabbro, and Stripa (Sweden) granite. The diffusivity of all the rocks showed a positive linear dependence on inverse temperature and, excluding the East Bull Lake gabbro, showed a linear dependence on quartz content. (Quartz content varied from 0 to 31% by volume.) Diffusivity in all cases rose or remained steady with increasing confining pressure. The pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 MPa, depending on rock type. The pressure effect (measured as a percentage change in diffusivity) is stronger in the four rocks of granite composition than in the three of basaltic composition. Our results agree well with existing thermal diffusivity measurements at atmospheric pressure.

A novel technique for measuring solute diffusivities in entrapment matrices used in immobilization.

Abstract: A novel technique has been developed for measuring effective solute diffusivities in entrapment matrices used for cell immobilization. In this technique radiotracers were used to measure effective diffusivities and equilibrium partition coefficients of the solute between the liquid and solid matrix. Ca-alginate was used in this study, because it is one of the most commonly employed matrices for the immobilization of microbial, plant and mammalian cells. The experimental apparatus consisted of a single spherical Ca-alginate bead which was attached to a rotating rod and immersed in water containing C14-glucose. The rotational speed of the spherical bead was controlled and resulted in excellent mixing, and negligible external film mass transfer resistance, which allowed the measurement of true effective solute diffusivity within the solid matrix. The rates of C14-glucose diffusion within the Ca-alginate sphere were measured using a scintillation spectrometer. A mathematical model of unsteady-state diffusion in a sphere was used with appropriate boundary conditions, and the effective diffusivity of glucose was found from the best fit of the experimental data using a computer regression analysis method. Using 2% (w/v) Ca-alginate beads in this new radiotracer technique the effective diffusivity and partition coefficient of glucose were found to be 6.62 × 10−10 m2/s and 0.98, respectively. The accuracy, advantages, and simplicity of this new method for diffusivity measurements are also compared to other existing methods.

Solubility, diffusion and thermodynamic properties of silver in silicon

Abstract: Concentration-depth profiles of Ag in Si have been measured with the aid of neutron activation analysis combined with serial removal of sections. The Ag diffusion appears to be very fast. In the bulk of dislocation-free Si wafers saturation is achieved after short periods of annealing. From this the authors conclude that interstitial Agi is the predominant configuration in Si without dislocations. Equilibrium concentrations of Agi are determined for temperatures between 1287 and 1598 K. The results are thermodynamically analysed, taking into account Ag-Si liquidus data. In dislocated Si much higher Ag concentrations are observed, which vary irregularly with the penetration depth. A comparison of the diffusion and solubility of Ag and Au in Si suggests that in Si with dislocations substitutional Ags may arise from Agi-Ags transitions. Finally an estimate of the Agi diffusivity is obtained.

Thermal stability of InGaAs/InP quantum well structures grown by gas source molecular beam epitaxy

Abstract: Single InGaAs/InP quantum wells and superlattices grown by gas source molecular beam epitaxy were subjected to brief anneals at temperatures in the 600–850 °C range. The resulting increases in the well thickness and changes in composition were monitored by low‐temperature photoluminescence and transmission electron microscopy. Very sharp well‐barrier interfaces are found to be present even after annealing at the highest anneal temperature. These results can be modeled assuming diffusivity proportional to the square of concentration with D0=7×1010 cm2/s and an activation energy of Q=5.8 eV.

Thermophysical Properties of Meat Products: General Bibliography and Experimental Values

Abstract: Experimental values for thermophysical properties of different kinds of meats are here presented such as: thermal conductivity, enthalpy, heat capacity, thermal diffusivity and density. All these values were obtained from an extensive selection of the existent bibliography. They correspond to measurements made at the normal temperature range during cooling and freezing processes. In the present work a complete and detailed table of bibliography is presented in order to facilitate the attainment of information about experimental data, measurement techniques and mathematical equations.

Self‐diffusion in fluids in microporous solids

Abstract: Enskog’s kinetic theory of tracer diffusion has been generalized recently to strongly inhomogeneous fluids and is here applied to self‐diffusion in fluids in slit pores, i.e., fluids confined between parallel, flat, solid walls. In pores narrower than ten molecular diameters, the diffusivity deviates significantly from its value in bulk phase at the same temperature and chemical potential. Because of the confining pore walls, the fluid tends to form layers. The pore diffusivity oscillates as a function of pore width, a local minimum occurring when the packing of a given number of fluid layers is favored and a local maximum in the region of transition between these favored packing widths. The theory predicts fluid density distributions in good agreement with computer simulations of a similar fluid. The predicted diffusivities are in good qualitative agreement with computer simulations, although quantitatively the predicted oscillations are sharper than those observed in the computer simulations. This work represents the first application of Enskog’s kinetic theory of diffusion in strongly inhomogeneous fluids.