Showing papers on "Thermal diffusivity published in 1986"

Hydraulic Conductivity and Diffusivity: Laboratory Methods

Abstract: This chapter describes several laboratory methods of determining the hydraulic conductivity and hydraulic diffusivity. Water moves through soil in response to various forces acting upon it. The chemical species water may be transported due to bulk movement of the liquid phase or soil solution, or it may be transported by diffusion relative to the mean motion of the liquid phase. The chapter deals with bulk movement, under isothermal conditions, of the liquid phase in response to mechanical driving forces. However, the transport of water in the gas phase by vapor diffusion will be included in the measured hydraulic conductivity and diffusivity, especially at low water contents. The concept of parameter identification has been applied to the determination of the parameters in the hydraulic conductivity and water retention functions. The method involves the measurement of some aspect of the response of a soil water flow system to a set of applied boundary conditions.

Transient thermoreflectance from thin metal films

Abstract: This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one‐dimensional heat flow equation using a two‐parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

A concentration polarization model for the filtrate flux in cross-flow microfiltration of particulate suspensions

Abstract: Cross-flow filtration with microporous membranes is increasingly used in the separation and concentration of particulate suspensions. Existing models for the filtrate flux are inadequate for correlating experimental observations and are based on contradictory physical mechanisms. We propose that the flux is limited by the formation of a dynamic concentration polarization boundary layer consisting of a high concentration of retained particles. A simple model is developed incorporating a shear-enhanced diffusivity of the large particles which arises from mutually induced velocity fields in the shear flow of the concentrated suspension. Predictions of the model agree well with experimental data for a variety of particulate suspensions. The model provides both a fundamental understanding of the physical phenomena governing flux and a rational basis for design of improved cross-flow filters.

A Mathematical Model of Silicon Chemical Vapor Deposition Further Refinements and the Effects of Thermal Diffusion

Abstract: We describe a mathematical model of the coupled gas‐phase chemical kinetics and fluid mechanics in a chemical vapor deposition (CVD) reactor. This paper presents refinements to our earlier model of the CVD of silicon from silane. The model predicts gas‐phase temperature and velocity fields, concentration fields for seventeen chemical species, and deposition rates. The major new features are a multicomponent transport model including thermal diffusion and a new formulation of the boundary conditions that describe deposition. A significant result is that thermal diffusion is predicted to make an important contribution to species density profiles and generally to reduce deposition rates.

Thermal conductivity of amorphous solids

Abstract: The thermal conductivities of glassy solids, when scaled by material parameters, have a similar magnitude and temperature dependence, from the lowest temperatures yet measured, to the melt. Both empirical results and theoretical models are reviewed briefly. It is emphasized that the thermal conductivity of an amorphous solid is not understood in any temperature range.

Porosities and Diffusivities of Some Nonsorbing Species in Crystalline Rocks

Abstract: The diffusion of nonsorbing species in different rock materials and fissure coating materials has been studied on a laboratory scale. The nonsorbing species were iodide, Uranine, and Cr-EDTA. The results show that the effective diffusivity of iodide in rock materials with fissure coating material is of the same magnitude as or higher than the effective diffusivity of iodide in rock materials without fissure coating material. The results also show that the variations in the rock material are too large to give one value of the diffusivity in a rock material from a certain area. The estimated effective diffusivity of iodide in rock materials without fissure coating material was found to be in the range 1 x 10/sup -14/ m/sup 2//s to 70 x 10/sup -14/ m/sup 2//s.

Theory of ion‐temperature‐gradient‐driven turbulence in tokamaks

Abstract: An analytic theory of ion‐temperature‐gradient‐driven turbulence in tokamaks is presented. Energy‐conserving, renormalized spectrum equations are derived and solved in order to obtain the spectra of stationary ion‐temperature‐gradient‐driven turbulence. Corrections to mixing‐length estimates are calculated explicitly. The resulting anomalous ion thermal diffusivity χi=0.4[(π/2)ln(1+ηi)]2[(1+ηi)/τ]2 ρ2pcs/Ls is derived and is found to be consistent with experimentally deduced thermal diffusivities. The associated electron thermal and particle diffusivity, and particle and heat‐pinch velocities are also calculated. The effect of impurity gradients on saturated ion‐temperature‐gradient‐driven turbulence is discussed and a related explanation of density profile steepening during Z‐mode operation is proposed.

Hydrogen trapping phenomena in metals with B.C.C. and F.C.C. crystals structures by the desorption thermal analysis technique

Abstract: The types and amounts of lattice defects acting as trapping sites of hydrogen in metals with b.c.c. and f.c.c. structures and those induced by hydrogen during cathodic charging have been determined by the thermal analysis technique. Trapping parameters such as the trap activation energy, trap binding energy and trap density for each trap have been examined by using the mathematical models derived from the existing trap theory. From the results of thermal analysis of b.c.c.-structured metals, it is observed that the interfaces of non-metallic inclusions such as iron oxides, Al2O3, MnS and TiC are strong trapping sites for hydrogen with a high trap activation energy while the lattice imperfections, grain boundaries, dislocations and microvoids are shallow traps. The energy levels of hydrogen around each trapping site are as follows: for a strong trapping site, the saddle point energy is higher than the activation energy of lattice diffusion, which means that trapping and detrapping of hydrogen are difficult but, in a shallow trapping site, the saddle point energy is lower so that trapping and detrapping are easy. In pure nickel with an f.c.c. crystal structure, dislocation and grain boundaries have low trap activation energies compared with the activation energy for lattice diffusion. Equations for lattice solubility and diffusivity under atmospheric hydrogen pressure in pure nickel have been obtained by the thermal analysis technique and are as follows: C((H atom) atom-1)= 1.57 × 10-3exp−11.80 kJRT D(cm2s-1)=7.5 × 10-3exp−39.23 kJRT It is found that internal microcracks or microvoids are generated and are the major trapping sites of hydrogen when pure iron is cathodically charged.

Photothermal investigation of transport in semiconductors: Theory and experiment

Abstract: The photothermal deflection technique has been extended as a contactless, in situ method to investigate transport in solids with an emphasis on semiconductors. A theoretical model is developed which quantitatively describes the transport behavior, and is shown to be in excellent agreement with experimental results. For semiconductors, this approach yields the thermal diffusivity, the electronic diffusivity, the minority‐carrier lifetime and the surface recombination velocity.

Two‐beam photoacoustic phase measurement of the thermal diffusivity of solids

Abstract: A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring the phase lag between a front and rear illumination, at a single chopping frequency. The method is tested using some semiconductor and glass samples.

Thermal transport studies of electrically conducting materials using the transient hot-strip technique

Abstract: The transient hot-strip (THS) method has been used for thermal conductivity and thermal diffusivity studies of electrically conducting materials by introducing a thin electrically insulating layer between the hot strip and the metallic material under study. The insulating layer introduces a certain thermal contact resistance between the hot strip (heat source cum temperature sensor) and the surface of the sample to be studied. To account for this thermal resistance a theory has been developed which indicates how measurements on these kind of materials should be performed and how the reduction of data from transient recordings should be carried out to give reliable results. The new experimental approach, which should be applied whenever a thermal contact resistance is suspected, has been demonstrated by two series of measurements on a stainless steel at room temperature.

Diffusion of gold in dislocation-free or highly dislocated silicon measured by the spreading-resistance technique

Abstract: The diffusion of Au in dislocation-free or plastically deformed Si (1011 to 1013 dislocations/m2) was measured with the aid of the spreading-resistance technique. The Au profiles produced indislocation-free Si slices by in-diffusion from both surfaces possess nonerfc-type U shapes as predicted by the so-called kick-out diffusion model. This model is used to calculate the contribution of self-interstitials to the (uncorrelated) Si self-diffusion coefficient,D =0.064×exp(−4.80 eV/kT)m2 s−1, from the present and previous data on the diffusivity and solubility of Au in Si in the temperature range 1073–1473 K. Inhighly dislocated Si the diffusion of Au is considerably faster than in dislocation-free Si. From the erfc-type penetration profiles found in this case, effective Au diffusion coefficients were deduced and combined with data on the solubility of Au in Si. ThusC D i=0.0064 ×exp(−3.93 eV/kT)m2 s−1 was obtained in the temperature range 1180–1427 K, whereC andD i are the solubility and diffusivity of interstitial Au in Si.

Heat pulse propagation studies in TFTR

Abstract: The time-scales for sawtooth repetition and heat pulse propagation are much longer (tens of milliseconds) in the large tokamak TFTR than in previous, smaller tokamaks. This extended time-scale, coupled with more detailed diagnostics, has led us to revisit the analysis of heat pulse propagation as a method to determine the electron heat diffusivity χe in the plasma. A combination of analytic and computer solutions of the electron heat diffusion equation is used to clarify previous work and to develop new methods for determining χe. Direct comparison of the predicted heat pulses with soft-X-ray and ECE data indicates that the space-time evolution is diffusive. However, the χe determined from heat pulse propagation usually exceeds that determined from background plasma power balance considerations by a factor ranging from two to ten. Some hypotheses for resolving this discrepancy are discussed.

Measurements of thermal properties for human femora

Abstract: In this study, density, specific heat, thermal conductivity, and thermal diffusivity were measured experimentally along the lengths of human cadaveric femora. Fresh and dry bone samples were selected from both male and female specimens, and for different age groups varying between 44 and 73 years old. Measured values for specific heat vary between 1.14 and 2.37 J/gm degrees C; for thermal conductivities the range is from 0.16 to 0.34 W/m degrees C; and for thermal diffusivities the range is from 0.10 to 0.23 cm2/sec, depending on whether the bone samples were fresh or dry, cancellous or cortical. The experimental results are presented in non-dimensional coordinates and are compared with the few other data available in the literature.

Mirage-effect measurement of thermal diffusivity. Part II: theory

Abstract: A three-dimensional theory of a mirage-effect technique for measuring thermal diffusivity of solids is presented. A formula that incorporates sizes and separations of the heating and probe beams; t...

Experimentally inferred ion thermal diffusivity profiles in the Doublet III tokamak: Comparison with neoclassical theory

Abstract: Measurements of ion temperature profiles, obtained on a single-shot basis, have been coupled with transport analysis to obtain spatial profiles of the ion thermal diffusivity ?i for a series of 18 neutral-beam-heated discharges in the Doublet III tokamak. The expanded boundary divertor discharges examined in this study had the following range of parameters: Bt = 2.53 T, Ip = 0.35?0.89 MA, vertical elongation = 1.4?1.9, n?e = (3.7?7.9) ? 1013 cm?3, PT = 3.5?6.6 MW, and ?i* = 0.03?0.22 , for r/a = 0.2?0.7. The experimentally inferred values of ?i have been compared with the values of the ion thermal diffusivity predicted by neoclassical theory. On the basis of a detailed error analysis, the results for all discharges examined are the same: ?i is approximately equal to near the plasma centre, but for r/a between about 0.2 and 0.8, ?i is both larger than and almost certainly has a spatial variation different from that of . The differences between ?i and are larger than can be explained by errors in the experimental measurements and indicate that there are processes operative for the loss of ion heat which are not included in the usual transport models for ion confinement.

Diffusion in κ-carrageenan gel beads

Abstract: Using a well-mixed and temperature-led vessel, the diffusion characteristics of various solutes into spherical κ-carrageenan gel beads were experimentally investigated. The diffusion coefficient of glucose was markedly affected by the glucose concentration and the operating temperature. In all cases the diffusivity obtained was noticeably smaller than that of glucose in pure water. The experimental data also indicated an inverse relationship between the diffusivity and the polymer concentration used in the gel preparation. As well, the glucose diffusivity was affected by the presence of other solutes in the glucose solution. Electrolytes such as ammonium sulfate, KCl, and CaCl2 were observed to enhance the diffusion coefficient. On the other hand, the addition of arginine or bovine serum albumin had an adverse effect on the diffusivity. No diffusion of albumin into the gel beads was observed, and such a solute created a significant mass transfer resistance during the diffusion process.

The effective diffusivity of fibrous media

Abstract: A procedure based on averaging the conservation equations in a homogeneous, disordered fibrous medium is used to demonstrate that in the limit of long times, macroscopic versions of Fick's and Fourier's laws may be used to relate the average flux to the average gradient in driving force. The asymptotic behavior in the limit of low volume fraction of the effective diffusivity (or conductivity) in such a medium is determined for all values of the Peclet number, P = Ua/Df, where U is the average velocity through the bed, a is the fiber radius, and Df is the molecular diffusivity of the solute in the fluid. The convective disturbance caused by the fibers is found to have a large influence on the rate of mass transfer even at moderate Peclet numbers and low volume fraction.

Mirage-effect measurement of thermal diffusivity. part i: experiment.

Abstract: A mirage-effect thermal-wave method for the measurement of thermal diffusivities of solids is described. Data from two different laboratories are provided for various pure elements and compound sem...

Thermal conductivity of two porous media as a function of water content, temperature, and density

Abstract: Appropriate values for the thermal conductivity, which include the contribution of vapor transfer, are required to describe heat transport in a porous medium. We measured the apparent thermal diffusivity of a glass bead medium and a Norfolk sandy loam (Typic Paleudult) as a function of water content

Defect distribution in ion‐implanted silicon. A Monte Carlo simulation

Abstract: A Monte Carlo simulation is given of the defect distribution within thecascade launched by medium-heavy ions in ion bombarded silicon. The alteration of the ion and recoil distributions, due to dose effects, are accounted for and described. The results favourably compare with the results,reported in literature, on anomalous diffusivity phenomena occurring duringrapid thermal annealing. Es wird eine Monte-Carlo-Simulation der Defektverteilung in der durch Ionenbombardement mit mittelschweren Ionen in Silizium hervorgerufenen Kaskade durchgefuhrt. Die Anderungen der Ionen-und Ruckstosverteilungen infolge der Dosiseffekte werden zugeordnet und beschrieben. Die Ergebnisse stimmen ausgezeichnet mit den Ergebnissen der Literatur uber eine anomale Diffusion wahrend der schnellen thermischen Ausheilung uberein.

Temperature fluctuations and heat transport in the edge regions of a tokamak

Abstract: Electron temperature fluctuations have been investigated in the edge region of the Caltech research tokamak [S. J. Zweben and R. W. Gould, Nucl. Fusion 25, 171 (1985)], and an upper limit to this fluctuation level was found at Te/Te <~ 15%. This measurement, together with previous measurements of density and electric and magnetic field fluctuations, allows a unique comparison of the heat transport resulting from three basic turbulent mechanisms: (1) heat flux from the particle flux resulting from microscopic density and electric field fluctuations; (2) thermal conduction resulting from microscopic temperature and electric field fluctuations; and (3) thermal conduction resulting from microscopic magnetic field fluctuations. The measurements indicate that, in the edge regions, the electron heat transport caused by the measured turbulence-induced particle flux is comparable to or greater than that caused by the thermal conduction associated with the electron temperature and electric field fluctuations, and is significantly greater than that resulting from the measured magnetic fluctuations. This electron heat loss caused by the plasma turbulence is found to be an important electron energy loss mechanism in the edge regions.