Showing papers on "Thermal diffusivity published in 1993"

Prediction of Polymer Properties

Abstract: Topological method for structure-property correlations volumetric properties thermodynamic properties cohesive energy and solubility parameter transition and relaxation temperatures surface tension and interfacial tension optical properties electrical properties magnetic properties mechanical properties properties of polymers in dilute solutions sheer viscosity thermal conductivity and thermal diffusivity transport of small penetrant molecules.

Thermal conductivity and localization in glasses: Numerical study of a model of amorphous silicon.

Abstract: Numerical calculations of thermal conductivity κ(T) are reported for realistic atomic structure models of amorphous silicon with 1000 atoms and periodic boundary conditions. Using Stillinger-Weber forces, the vibrational eigenstates are computed by exact diagonalization in harmonic approximation. Only the uppermost 3% of the states are localized. The finite size of the system prevents accurate information about low-energy vibrations, but the 98% of the modes with energies above 10 meV are densely enough represented to permit a lot of information to be extracted. Each harmonic mode has an intrinsic (harmonic) diffusivity defined by the Kubo formula, which we can accurately calculate for ω>10 meV

Ordinary and transition regime diffusion in random fiber structures

Abstract: The problem of bulk, transition and Knudsen regime diffusion in structures of freely overlapping fibers of various orientation distributions was numerically investigated, and the interrelation of the resulting effective diffusivities was examined. Fibers were randomly positioned and oriented in d = 1, 2, or 3 directions. A Monte Carlo simulation scheme was employed to determine the effective diffusivities from the mean-square displacement of random walkers traveling in the interior of the porous structure. The effective diffusivity was found to depend strongly on the orientational distribution of the fibers, porosity of the fibrous structures, and Knudsen number. The tortuosity factor decreased in general with increasing porosity, approaching at the limit of dilute beds the lower bound derived for each direction of diffusion from variational principles. The simulation results agreed well with experimental values of the bulk tortuosity of fibrous beds from the literature. It was also found that the reciprocal additivity or harmonic average effective diffusivity expression (Bosanquet formula), commonly used to estimate transition regime diffusivities from the values at the ordinary and Knudsen diffusion limits, provides an excellent approximation for the effective diffusivity of fibrous beds, except for that parallel to the fibers of a unidirectional structure.

On the molecular mechanism of thermal diffusion in liquids

Abstract: A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy. Simulations of isotope mixtures covering a range of densities and mass ratios show that the lighter component prefers the hot side of the system at stationary state. This implies a positive thermal diffusion factor in the definition we have adopted here. The molecular basis for the Soret effect is studied by analysing the energy flux through the system. In all cases we found that there is a difference in the relative contributions when we compare the hot and cold sides of the system. The contribution from the lighter component is predominantly flux of kinetic energy, and this contribution increases from the cold to the hot side. The contribution from the heavier component is predominantly energy transfer th...

Evaluation of vertical groundwater fluxes and thermal properties of aquifers based on transient temperature-depth profiles

Abstract: A series of type curves is presented for estimating vertical groundwater fluxes in relatively shallow aquifers by using seasonal changes in groundwater temperature-depth profiles. Vertical groundwater fluxes are estimated by fitting a dimensionless parameter to the type curves when the amplitude of annual variations of groundwater temperature at several depths and the thermal diffusivity of the aquifer are known or measured. Both upward and downward groundwater fluxes estimated by fitting temperatures observed in Nagaoka plain, Japan to the type curves agree well with the fluxes calculated from hydraulic conductivity and hydraulic gradient data. Measurements of seasonal changes in groundwater temperature and hydraulic gradients were analyzed to estimate the thermal diffusivity of the aquifer in situ that is close to the value reported in the literature.

Principles of particle selection for dispersion-strengthened copper

Abstract: A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rosler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.

Simulating the dynamic electrothermal behavior of power electronic circuits and systems

Abstract: The simulator solves for the temperature distribution within the semiconductor devices, packages, and heat sinks (thermal network) as well as the currents and voltages within the electrical network. The thermal network is coupled to the electrical network through the electrothermal models for the semiconductor devices. The electrothermal semiconductor device models calculate the electrical characteristics based on the instantaneous value of the device silicon chip surface temperature and calculate the instantaneous power dissipated as heat within the device. The thermal network describes the flow of heat from the chip surface through the package and heat sink and thus determines the evolution of the chip surface temperature used by the semiconductor device models. The thermal component models for the device silicon chip, packages, and heat sinks are developed by discretizing the nonlinear heat diffusion equation and are represented in component form so that the thermal component models for various packages and heat sinks can be readily connected to one another to form the thermal network. >

Low-temperature specific heat and thermal conductivity of glasses.

Abstract: The soft potential model (an extension of the tunneling model to include soft localised vibrations) is shown to describe the anomalous features of the specific heat C p and the thermal conductivity of glasses over the entire low-temperature range, up to and including the peak in C p /T 3 and the second rise of the thermal conductivity above the plateau

Eine neue Gleichung zur Berechnung von Diffusionskoeffizienten gelöster Stoffe

Abstract: To predict breakthrough curves for fixed-bed adsorbers, mass-transfer coefficients of the adsorbates are needed as input data. The film mass-transfer coefficients k F are often estimated from system parameters and liquid diffusivity D L using empirical correlations. A new equation for calculating D L from the molar mass of adsorbate, the dynamic viscosity of water, and the temperature is presented. Based on the results of test calculations, the validity of the new equation and the effects of possible errors in D L on the adsorber calculations are discussed in detail

Effect of surface diffusion on the creep of thin films and sintered arrays of particles

Abstract: An analysis is presented that illustrates the effect of surface diffusion on the creep of a uniform, sintered array of cylinders. The analysis is also appropriate for describing the creep of thin films bonded to substrates when there is no interfacial diffusion. The first part of the paper presents an analytical solution which can be obtained when it is assumed that a constant flux of matter out of the grain boundary is maintained. This solution illustrates the important physical phenomenon behind the problem in that a finite surface diffusivity causes a back stress to be developed owing to the enhanced surface curvatures in the region of the grain boundary. In the latter portion of the paper, this analytical solution is used to generate numerical solutions for more practical boundary conditions, and to illustrate the effects of finite boundary lengths.

Thermodiffusion in polymer solutions as observed by forced Rayleigh scattering

Abstract: Thermodiffusion, the cross coupling between temperature and concentration, is studied in dilute polymer solutions by forced Rayleigh light scattering. First a spatial temperature grating is created within the solution by absorption of an optical interference grating and subsequently a concentration grating builds up. The time‐dependent diffraction efficiency of both gratings is described by a phenomenological model from which the thermal diffusivity, the ordinary diffusion coefficient, and the thermodiffusion coefficient are obtained without the need for absolute intensity measurements or calibration with a known standard. No labeling of the polymer chains with photochromic dyes is required. Experimental details and potential pitfalls are discussed. Measurements are carried out on polystyrene in ethyl acetate and the obtained values compare well with data taken from the literature.

Capillary water migration in rock: process and material properties examined by NMR imaging

Abstract: We report the application of proton nuclear magnetic resonance (NMR) imaging to the measurement of water content distributions in Lepine limestone, a typical constructional stone. The method is used to observe the kinetics of the absorption of water into this material by capillarity. The water content distributions are consistent with the predictions of unsaturated flow theory. The hydraulic diffusivity of Lepine stone is found to be an approximately exponential function of the water content, in agreement with experimental data on other porous materials. The best estimate of the diffusivity function is D (m2s−1) = 6.3 × 10−9 exp (4.90θr), whereθr is the normalized volumetric water content. The sorptivity estimated from NMR data is in close agreement with the directly measured value (1.00 mm min−1/2). NMR imaging methods appear promising as a non-destructive and rapid laboratory means of determining moisture distributions, especially for the purpose of accurate measurement of the capillary transport properties of porous materials.

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

Abstract: From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm−1 K−1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.

Oxygen Tracer Diffusion in La2‐xSrxCuO4‐y Single Crystals

Abstract: The tracer diffusion of O-18 in La(2-x)Sr(x)CuO(4-y) single crystals (x = 0 to 0.12) has been measured from 400 to 700 C in 1 atm of oxygen using SIMS analysis. Evidence for diffusion by a vacancy mechanism was found at low strontium contents. Oxygen diffusivities for x greater than or = 0.07 were depressed by several orders of magnitude below the diffusivity for undoped La2CuO(4+/-y). The observed effects of strontium doping on oxygen diffusivity are discussed in terms of defect chemical models. The decreasing oxygen diffusivity with increasing strontium was attributed to the ordering of oxygen vacancies at large defect concentrations. A diffusion anisotropy D(sub ab)/D(sub c) of nearly 600 was also found at 500 C.

On the photodeflection method applied to low thermal diffusivity measurements

Abstract: The photodeflection method when applied to measure the low thermal diffusivity of some materials gives inconsistent results. In this article a way to extend the thermal diffusivity range of measurements using the phase of the photodeflection signal is presented. A comparison with computer simulations and experimental results shows good agreement.

Kinetic Theory Analysis of Flow-Induced Particle Diffusion and Thermal Conduction in Granular Material Flows

Abstract: The kinetic theory model assumes that the particles are smooth, identical, and nearly elastic spheres, and that the binary collisions between the particles are isotropically distributed throughout the flow. The particle diffusivity and effective thermal conductivity are found to increase with the square root of the granular temperature, a term that quantifies the kinetic energy of the flow. The theoretical particle diffusivity is used to predict diffusion in a granular-flow mixing layer, and to compare qualitatively with recent experimental measurements

A mathematical model of turbulent heat and mass transfer in stably stratified shear flow

Abstract: It is commonly assumed that heat flux and temperature diffusivity coefficients obtained in steady-state measurements can be used in the derivation of the heat conduction equation for fluid flows. Meanwhile it is also known that the steady-state heat flux as a function of temperature gradient in stably stratified turbulent shear flow is not monotone: at small values of temperature gradient the flux is increasing, whereas it is decreasing after a certain critical value of the temperature gradient. Therefore the problem of heat conduction for large values of temperature gradient becomes mathematically ill-posed, so that its solution (if it exists) is unstable.In the present paper it is shown that a well-posed mathematical model is obtained if the finiteness of the adjustment time of the turbulence field to the variations of temperature gradient is taken into account. An evolution-type equation is obtained for the temperature distribution (a similar equation can be derived for the concentration if the stratification is due to salinity or suspended particles). The characteristic property which is obtained from a rigorous mathematical investigation is the formation of stepwise distributions of temperature and/or concentration from continuous initial distributions.

Numerical analysis of the deformation and solidification of a single droplet impinging onto a flat substrate

Abstract: An existing model has been modified to explore the deformation and solidification of a single droplet impinging on a substrate. The modification accounts for possible solid fraction of material at impact. Numerical results predict that the kinetic energy dominates the process at impinging velocities greater than about 100 m s−1. In addition, the thermal diffusivity of the solidifying material controls the process, but the temperature of the substrate relative to the melting temperature of the material must be considered when comparing materials. It is believed that droplets solidifying into thinner, wider discs would reduce porosity; therefore, dense materials accelerated to high speed would solidify into masses with the highest bulk density.