Showing papers on "Thermal diffusivity published in 1969"

Self-diffusion of carbon and oxygen in calcite by isotope exchange with carbon dioxide

Abstract: Carbon self-diffusion coefficients were determined by exchange between calcite grains and a large reservior of C14-labeled CO2 at 250°–750°C. The progress of diffusion and the surface area of the grains were measured by the C14 uptake of the solid. Both carbon and oxygen self-diffusion coefficients were determined by exchange between calcite grains and a limited reservoir of C13- and O18-enriched CO2 at 650°–850°C. The progress of diffusion was measured by the change in isotope composition of the gas. On an Arrhenius plot of log D versus 103/T°K, the carbon self-diffusion coefficients are fitted to two straight lines. Carbon diffusivity determined by both methods in unannealed and annealed material at temperatures above 550°C are given by D = 1.3 × 103 exp(−88,000/RT)cm2/sec. Oxygen self-diffusion coefficients were higher than the corresponding carbon values by up to a factor of 4. Carbon diffusivity determined by the C14 method in unannealed calcite below 550°C are given by D = 4.6 × 10−16 exp(−17,000/RT)cm2/sec. Diffusivity in annealed material is lower by a factor of 50. The relative rates of oxygen and carbon self-diffusion indicate that there may be several mechanisms of oxygen migration in calcite in the high temperature region. A detailed examination of the C14 uptake results suggest that carbon self-diffusion is depth-dependent for the shallow penetration distances obtained in these experiments. Extrapolation of these results suggests that solid-state diffusion will not significantly alter the isotopic record of calcite under a wide range of geologic environments.

Variational Calculation of the Thermal Conductivity of Germanium

Abstract: The thermal conductivity of germanium has been calculated using an anharmonic isotropic continuum model. This model has been extended so as to incorporate umklapp processes by introducing a pseudo-reciprocal-lattice translation vector of magnitude equal to the diameter of the Debye sphere, in such a way that the wave vectors of the three phonons involved are coplanar. Boundary and mass-difference scattering are also included, and the calculations, besides giving good agreement with experimental values of thermal conductivity, provide information on the relative contributions of the different scattering processes to the thermal resistivity and of the different polarization branches to the conductivity. In particular, it is shown that almost all the heat transport is by transverse phonons.

Diffusion of Iron and Nickel in Magnesium Oxide Single Crystals

Abstract: Diffusion kinetics and mechanisms were studied in the FexO-MgO (vacuum), NiO-MgO (vacuum and air), and Fe203-MgO (air) systems. In the FexO-MgO system, Fe entered MgO by a redox reaction; the diffusivity and activation energy depended on concentration. In the NiO-MgO system in air the diffusivity depended on concentration and the activation energy did not; in vacuum both the diffusivity and activation energy were concentration-independent. In the Fe2O3-MgO system in the MgO phase the activation energy and diffusivity did not depend on concentration. Because of impurities, the diffusion results were for the extrinsic region. Formation of trivalent ions and consequent chemically created vacancies in the FexO-MgO and NiO-MgO (air) systems resulted in the concentration dependence of diffusivity. Concentration dependence of activation energy in the FexO-MgO system is associated with structural changes due to a change with concentration of the Fe3+ octahedraI/Fe3+ tetrahedral ratio. In the Fe2O3-MgO system structural changes do not occur during diffusion because this ratio remains constant.

Diffusivity of electrons and holes in silicon

Abstract: The variation of electron and hole diffusivity and drift velocity in the ``warm electron'' range of electric field (1–50 kV/cm) has been measured for 〈111〉‐oriented silicon at 300°K. These measurements were obtained from the transient currents produced from a thin layer of electrons or holes propagating through the high‐field region of a reverse‐biased p+−ν−n+ diode. Use of a sampling oscilloscope in conjunction with an analog‐to‐digital converter and averaging system resulted in significant improvement of the resolution and statistics of the measurement and allowed digital readout of the resulting data. Diffusivity of both electrons and holes was found to decrease slightly below the low‐field values in the electric field range of 6–50 kV/cm. Drift velocity versus electric field was measured and found to be reasonably consistent with the literature.

Defect Diffusion in Single Crystal Aluminum Oxide

Abstract: A defect diffusion coefficient was determined in single crystal aluminum oxide from 1400° to 1850°C by measuring the movement of the color boundary in a Ti3+− doped single crystal heated in air. In this temperature range the experimental data can be represented by The interpretation developed is that this equation represents the diffusivity for a defect which contributes to aluminum self-diffusion.

Effect of induced thermal stresses on the coefficients of thermal expansion and densities of filled polymers

Abstract: It is demonstrated that in a filled polymer the thermal stresses resulting from the difference in the thermal expansion coefficients of the filler and the polymer have significant effect on the apparent coefficient of thermal expansion of the composite. A model is constructed to aid the thermal stress analysis, and the results are found to agree well with the experimental data obtained from other sources. An expression for the apparent densities of filled polymers is also obtained but the agreement between the present prediction with an existing test result is found to be only qualitative.

Diffusion and permeation of deuterium in palladium-silver at high temperature and pressure

Abstract: The solubility, diffusion coefficient, and permeability of Pd-25 Ag alloy to deuterium have been measured between 0 and 1000 psi at 300 °, 400 °, and 500 °C. Substantial deviations from ideal behavior are observed, particularly for diffusion coefficients, so that permeability cannot be represented as a simple function of pressure. After the diffusion coefficients are corrected for chemical nonideality, an additional increase by a factor of two remains in the concentration dependence of diffusivity. This is attributed to the expanded lattice of the alloy when the deuterium-to-metal ratio is high. A strain-induced shift in solubility was also observed.

Thermal Properties of Polymers at Low Temperatures

Abstract: The existing measurements and theories of the low-temperature thermal properties, heat capacity, and thermal conductivity of polymers are reviewed with particular attention paid to the differences between partly crystalline and amorphous polymers. The most striking feature of the low-temperature heat capacity of polymers is that in the liquid helium temperature range the heat capacity does not depend upon the cube of the temperature as for other solids. Further, only well below 1°K does the heat capacity approach the value predicted on the basis of the sound velocity. This behavior indicates the presence of a small number of low-frequency modes of vibration in the frequency spectrum. The fact that such anomalous behavior seems linearly related to the crystallinity implies that this behavior is associated with the amorphous structure, perhaps with motions of pendent groups within cavities formed in the amorphous structure. The thermal conductivity of semicrystalline and amorphous polymers differs ...

The formation of carbon under well-stirred conditions

Abstract: The process of nonequilibrium formation of carbon in hydrocarbon flames has been studied in a well-stirred reactor, a device that allows combustion to proceed under conditions of intense back-mixing and without limitations of heat and mass transfer. The purpose of the study was to investigate the effect of back-mixing on carbon formation and to obtain information about the phenomenon under the well-defined conditions provided by such a combustor. The critical atom ratios, O/C, of the reactants at which carbon first appears measured under conditions of intense back-mixing were only 10%–20% lower than those determined in Bunsen-type flames, and still far removed from the equilibrium value of 1.0. These observations strongly support the view that nonequilibrium formation of carbon is a kinetic problem. Raising the flame temperature by pre-heating slightly decreases these critical ratios. The response of the point of incipient carbon formation to changes in the nature of the inert diluents (He, N2, Ar) and oxygen concentration is complex, suggesting that as well as reaction kinetics and temperature, the transport properties of the combustible mixture (thermal conductivity, diffusivity) are probably also important.

Approximate Theory of Emitter‐Push Effect

Abstract: An approximate theory of the emitter‐push effect is presented. The rate of vacancy generation during emitter diffusion is formulated using Prussin's model of dislocation distribution. A time and space average rate 〈R〉 is defined, and a quasi‐steady state approximation of vacancy distribution is obtained. It is shown that the ratio of the effective to the intrinsic base diffusivities, which is also equal to the local supersaturation of vacancies in the vicinity of the base—collector junction, is given by Db/Db*=(Cv/Cv*)xjb≈16π−1/2a−3β{1−√π ierfc[xr/2(Dt)1/2]} C0D(CsDs*)−1+1, where Db is the base diffusivity, Cv is the vacancy concentration, the asterisk denotes the condition of an intrinsic semiconductor with vacancy at thermal equilibrium, a is the lattice unit cell dimension, β is the coefficient of lattice contraction, xr is the dislocation penetration depth, D is the emitter diffusivity, C0 the emitter surface concentration, Cs the concentration of lattice sites, and Ds the self‐diffusivity. It is also...

The interfacial stresses in particulate composite systems

Abstract: It is postulated that property gradients exist at the interface forming an “interphase” between the insert and the matrix. Such gradients might be caused by partial solubility and diffusivity of the two phases comprising a composite. For simplicity, the “interphase” is replaced by a shell exhibiting averaged properties (Eshell = 1/2 Einsert + 1/2 Ematrix, etc.). An exact linear elastic solution was employed to evaluate stress fields throughout all regions. The possible effect of the “interphase” and its thickness on several physical properties of the composite are evaluated in terms of the particular stress fields.

Method of Thermal Diffusivity Measurement

Abstract: Measurement techniques of thermal diffusivity of rocks and minerals at high pressures and temperatures have been developed. The technique utilizes the Angstrom method. Samples as small as several millimeters in linear dimension can be measured. The methods were tested on quartz, fused silica, basalt, and serpentinized peridotite. Linear and cylindrical sample geometries are tested. It is found that, for relatively opaque samples, the cylindrical sample geometry is most suitable for measurements at high pressures. The linear sample geometry is adequate for measurements of small samples at moderately high temperatures.

Vertical diffusivity from radon profiles

Abstract: Measurements of the vertical profile of atmospheric radon (Rn222) concentration at a tower site near Washington, D.C., are interpreted in terms of vertical diffusivity (Kz). A mean uniform radon flux with height is derived for near-adiabatic conditions from expressions normally used to compute moisture flux; the radon flux term is assumed to apply to all meteorological conditions and is used in the conventional flux-gradient equation to yield an average vertical diffusivity for a 90-meter height interval. Average derived vertical diffusivities range from about 3.1 × 105 cm2 sec−1 during unstable convective periods to about 8.3 × 103 cm2 sec−1 during stable inversion periods, based on radon concentrations measured over 1- to 4-hour periods. In a gross sense the derived Kz values are consistent with measured conventional meteorological and turbulence parameters.

Thermal Conductivity of ThO2-UO2 System

Abstract: The thermal diffusivity of the ThO2-UO2 system in solid solution was measured by laser pulse method at temperatures ranging from 20° to 500°C. Reproducibility of the data was confirmed to be within 3%. The compositions of the samples were ThO2, ThO2-1%UO2, ThO2-5%UO2 and ThO2-10%UO2. The thermal conductivity was calculated from measured thermal diffusivity data and the specific heat data available in literature and corrected to zero porosity by using Loeb's equation, in which the shape factor is unity. The values on ThO2 thus obtained agreed very well with the data found in literature, throughout the range of temperature of the experiments. The thermal conductivity of ThO2, ThO2-1%UO2, ThO2-5%UO2 and ThO2-10%UO2, at 20°C, were 0.0312, 0.0288, 0.247 and 0.0184 cal·cm/sec· °C·cm2, respectively.

Diffusivity measurements of the argon–mercury vapor system

Abstract: An experimental method for the determination of the diffusivities of metal vapor – gas mixtures has been examined. An investigation of the mercury vapor – argon system in the temperature range 180–340 °C indicated that the experimental diffusivities agreed well with the diffusivities calculated from the kinetic theory of gases. The temperature dependence of the diffusivity was found to vary with the absolute temperature to the 1.7 power.

Effect of Porosity on the Thermal Conductivity of ThO2

Abstract: The eflect of porosity on the thermal conductivity of sintered ThO2 of densities ranging from 90 to 95% of theoretical density was investigated at 20°C. The thermal conductivity was determined by means of laser pulse. Discrepancies were observed between the experimental results and the relation derived by Loeb. This discrepancy was explained by applying a modified Maxwell formula derived by Brailsford et al. The samples were also examined metallographically. It is concluded that the existence of fluid continuous regions contribute significantly to the thermal resistance of such materials as ThO2.

Radiation Diffusion in Antarctic Ice Media

Abstract: THE thermal diffusion equation for solid, homogeneous conductors can usually be solved conveniently using standard Fourier methods1. These solutions cannot, however, be applied when the medium does not behave as a genuine conductor; that is, when convective and radiative processes create energy sources and sinks within the medium. The effect of absorbed solar radiation on the thermal diffusion in clear ice has been described elsewhere2. In the coastal ablation zone of Antarctica, it causes startling changes in the heat storage in the upper 8 m of the ice, strongly modified overall heat transfer patterns in the ice and increases of the effective thermal conductivity of the medium by factors of up to two. Similar effects on the effective thermal parameters have been obtained in the low density permeable snow medium on the Antarctic Plateau3, although, there, convective transfer of heat in the snow must be taken into account also.

THERMAL PROPERTIES OF β-SILICON CARBIDE FROM 20 TO 2000°C

Abstract: Thermal and mechanical properties have been measured on chemical vapor deposited, polycrystal β-SiC. Density was measured from 20°C (3.166 gm/cm3) to 2000°C (3.075). Linear thermal expansion, measured between 20 and 2000°C, showed a total differential expansion of less than 1%. Enthalpy and specific heat were characterized between 0 and 2000°C and thermal diffusivity and thermal conductivity from 200 to 2000°C. Hardness and flexural strength were measured to 1500°C. Maximum strength is at 1000°C, dropping to 2/3 of its room temperature value at 1500°C.

Radial temperature measurements of alternating current arcs

Abstract: Using the shock-wave technique, measurements of the radial distribution of gas temperature of free-burning alternating current arc columns have been determined. Temperature distributions have been determined at different times in the current cycle for arcs of 8 to 25 A r.m.s. The results show that at any point in the arc column the temperature has a mean value on which is superimposed an oscillatory component having a phase and amplitude dependent upon the spatial position. The alternating current arc exhibits, therefore, a cyclic heat-wave phenomenon. From the variations of phase and amplitude of temperature oscillation, thermal diffusivity values have been deduced. The results show that the a.c. arc has a lower temperature, lower conductance and greater power dissipation than the comparable d.c. arc. Consideration indicates that these effects result from the cyclic radial gas flow generated by the oscillatory nature of the discharge.

Application of Newton's law to body cooling

Abstract: Newton's law of cooling was used to analyze the fall in rectal temperature post mortem in 55 cases during refrigeration in a mortuary. As with solids of low thermal conductivity, there was an initial curvilinearity to the semilog plot lasting 1 to 11 hours (related to pelvic circumference) while the proper internal temperature distribution was becoming established. Thereafter there was a linear trend representing a constant percent cooling rater 1 which lasted up to 24 hours for adults. This was followed by a less steep trend of cooling rater 2, which was 37% less thanr 1. This diminution in cooling rate occurred when the rectal temperature fell below 10°C and was ascribed to a reduction in the thermal diffusivity of fat.r 1 had the highest correlation with pelvic circumference. For the nude body in still airr 1=15.6% to 18.4% (T r −T a )/hour for children, 3.8% to 9.4% for adults. Wind increasedr 1; the decrease due to clothing was questionable because of the paucity of cases.