Considerable increase in thermal conductivity of a polycrystalline CVD diamond upon isotope enrichment

We measured the temperature dependence of thermal conductivity of a polycrystalline CVD diamond with natural isotope composition and an isotope enriched (99.96% ¹²C) sample at temperatures from 5 to 420 K. The isotope enriched diamond demonstrates a considerable growth of thermal conductivity at temperatures above 80 K compared to the diamond with natural composition of isotopes. At room temperature the thermal conductivity reaches 24.3 W·cm^?1K^?1, and the isotope effect makes up not less than 34%.     

Possible isotope enrichment due to resonance trapping in a recombining plasma

The effect of resonance trapping is shown to produce a transient isotope enhancement of specific charged states of a low abundance isotope during the recombination and decay of a highly ionized plasma by reducing the recombination rate of the more abundant isotope. Such an effect might lead to an efficient isotope separation process, particularly for low vapor pressure elements.     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

Anomalous diffusion and thermal diffusion of light ions in a nonisothermal plasma

A turbulent state appearing upon the Cherenkov generation of slow ion acoustic waves by light ions and their induced scattering on heavy ions has been studied. The coefficients of the anomalous diffusion and thermal diffusion of light ions have been determined.     

Vacuum arcs for plasma centrifuge isotope enrichment

Requirements for economic plasma centrifuge isotope enrichment are reviewed. A distributed-discharge vacuum arc in chromium, as reported by A.I. Vasin et al. (1979), is described and a qualitative model for its operation is given. Experiments were performed on a modified distributed-discharge vacuum arc in which the parameters appear to be well suited for plasma centrifuge operation. A 2-m-long, 10-cm-diameter fully ionized DC chromium arc was produced with ion density and temperature in the range of 10¹³ cm^-3 and 0.16 eV. Chromium mass throughput rates in the range of 1 g/min are reported, with continuous running times of up to 40 min., limited only by chromium supply     

Influence of a magnetic field on diffusion and thermal diffusion in gaseous mixtures

The influence of a magnetic field on the transport properties of binary mixtures of a polyatomic and a noble gas is studied theoretically. Using an inverse operator technique, first thermal conductivity and viscosity are treated. Then diffusion and thermal diffusion are discussed in detail, with special emphasis on the composition dependence. A relation connecting the magnitudes of the field effects on thermal conductivity, diffusion, and thermal diffusion is derived. This relation is used to estimate the field effect on diffusion.     

Evaporative isotope enrichment as a constraint on reach water balance along a dryland river

Deuterium and oxygen-18 enrichment in river water during its transit across dryland region is found to occur systematically along evaporation lines with slopes of close to 4 in (2)H-(18)O space, largely consistent with trends predicted by the Craig-Gordon model for an open-water dominated evaporating system. This, in combination with reach balance assessments and derived runoff ratios, strongly suggests that the enrichment signal and its variability in the Barwon-Darling river, Southeastern Australia is acquired during the process of evaporation from the river channel itself, as enhanced by the presence of abundant weirs, dams and other storages, rather than reflecting inherited enrichment signals from soil water evaporation in the watershed. Using a steady-state isotope mass balance analysis based on monthly (18)O and (2)H, we use the isotopic evolution of river water to re-construct a perspective of net exchange between the river and its contributing area along eight reaches of the river during a drought period from July 2002 to December 2003, including the duration of a minor flow event. The resulting scenario, which uses a combination of climatological averages and available real-time meteorological data, should be viewed as a preliminary test of the application rather than as a definitive inventory of reach water balance. As expected for a flood-driven dryland system, considerable temporal variability in exchange is predicted. While requiring additional real-time isotopic data for operational use, the method demonstrates potential as a non-invasive tool for detecting and quantifying water diversions, one that can be easily incorporated within existing water quality monitoring activities.     

Ballistic diffusion induced by a thermal broadband noise

We present a thermal broadband noise from the difference between two Ornstein-Uhlenbeck noises, which can induce a ballistic diffusion, i.e., long-time mean square displacement of a free particle driven by this noise reads proportional to t(2). We apply this noise to a flashing ratchet and the mean velocity of the particle is calculated via Langevin simulation. The results show that a double peak of the mean velocity and flux reversal appears for the ratchet with large and small asymmetries, respectively; the inertia effect induces a large mean velocity and multireversal of flux. These rich and interesting phenomena are explained.     

Unusual thermal diffusion in polymer solutions

Thermal diffusion forced Rayleigh scattering results on thermal diffusion of poly(ethylene oxide) (PEO) in ethanol/water mixtures are presented. In water-rich solvent mixtures, PEO is found to migrate towards regions of lower temperature. This is typical for polymer solutions and corresponds to a positive Soret coefficient of PEO. In solvent mixtures with low water content, however, the polymer is found to migrate towards higher temperatures, corresponding to a negative Soret coefficient of PEO in ethanol-rich solutions. To our knowledge, this is the first observed sign change of the Soret coefficient of a polymer in solution. We also present a simple lattice model for the polymer solvent system and calculate Soret coefficients with statistical mechanics methods. The calculated values agree qualitatively with the experimental results.     

Evaluation of thermal diffusion factor and diffusion coefficient from measurements on a trennschaukel

Measurements have been taken on a glass trennschaukel of eight tubes for the Ar-He system as a function of composition and with its hot and cold bulbs at 100°c and 0°c. For each mixture, several speeds for the mechanical push-pull motion of the gas which cover a wide enough range were tried. The results have been interpreted to determine the thermal diffusion factor α _T, in conventional fashion as well as on the basis of a theory characterizing the low speed operation, developed here. Four such different possibilities of α _t evaluation are discussed and values compared with the traditional ones obtained from the convection-free two-bulb apparatus. Lastly we discuss also in brief the prospect of estimating the diffusion coefficient as an important byproduct from measurements on trennschaukel runs.     

The isotope effect of the impurity diffusion in electrotransport

Determining the isotope effect of impurity diffusion in an electrotransport experiment under steady state conditions permits the evaluation of the effective charge of the impurity. In addition, the intrinsic diffusion coefficient of the solvent can be determined, which allows us to find the vacancy flow factor. Making use of other diffusion data it is then possible to calculate atomic jump frequency ratios in the neighborhood of a vacancy.     

Correlation and isotope effects for cation diffusion in magnetite

The simultaneous diffusion of ⁵²Fe and ⁵⁹Fe has been measured in Fe₃O₄ as a function of equilibrium oxygen partial pressure (10^?9 <p_o2 < 10^?4 atm) at 1200°C. The p_o2 dependence of D goes through a minimum near 10^?6 atm in agreement with earlier data of Dieckmann and Schmalzried. Comparison of the isotope effect results with correlation-factor calculations suggests that at p_o2 γ 10^?6, diffusion occurs predominantly by vacancy jumps between the normally occupied octahedral sites on the spinel lattice; jumps between tetrahedral sites probably play a lesser role. At p_o2< 10^?6 atm, diffusion occurs by an interstitialtype mechanism involving the simultaneous migration of two atoms. Five of the seven interstitialcy jumps considered in our correlation-factor calculations are consistent with the experimental results.     

Statistical mechanics of thermal diffusion

Expressions of the flows of atoms A and B of a binary system in a crystal are derived as the response to the imposed gradients of temperature and chemical potentials. The formulation is done using the pair approximation of the Path Probability Method of irreversible statistical mechanics and atomic migration is assumed to be via the vacancy mechanism. The energy carried by photons (and electrons) under the temperature gradient is assumed to be independent of the atomic flux. For the case near equilibrium, linear relations are derived among the atomic fluxes, the energy flux (associated with atomic flux) and the gradients. The Onsager reciprocal relations are proved to hold among the coefficients, including those related to energy flows. The heat of transport (energy carried by a diffusing atom) and the heat conduction due to atomic flux are thus unambigously derived.     

On thermal diffusion of nanoparticles in gases

Thermal diffusion of nanoparticles in gases is studied with the help of the kinetic theory and using the nanoparticle-molecule interaction potential developed earlier by the authors. The dependence of the thermal diffusion coefficient of nanoparticles on their radius, volume concentration, and the temperature of carrier gas is analyzed. The results are compared with the data for gas mixtures.     

Diffusion and thermal diffusion coefficients of a relativistic gas mixture

Values up to order c^-2 are given for the diffusion and thermal diffusion coefficients of binary relativistic gas mixtures of hard spheres and Israel particles.     

Effect of hydrogen enrichment of laminar ethylene diffusion flames on thermal structure and soot yields at pressures up to 10 bar

A series of high-pressure experiments were conducted to assess the influence of hydrogen enrichment of laminar diffusion flames of nitrogen-diluted ethylene on the thermal flame structure and soot yields at pressures above atmospheric. In parallel experiments, added hydrogen is replaced by helium, either in equal mole fractions or in mass fractions, to evaluate the thermal, dilution, and direct chemical interaction effects of hydrogen in soot formation. Experiments covered pressures from atmospheric to 10 bar. In the first set of experiments, conducted at 3, 6, and 10 bar pressure, base fuel was an ethylene-nitrogen mixture with 33.3% ethylene and 66.7% nitrogen (by mole as well as by mass). This base fuel was doped with either hydrogen or helium such that hydrogen and helium mass fractions and mole fractions in the fuel stream are matched in two cases. In the second set of experiments, which were conducted at 1.2 bar pressure with ethylene as the base fuel, hydrogen or helium is added such that additive mole fraction in the fuel stream was 44%. Temperature measurements in the first set of experiments indicate that, when hydrogen is added to nitrogen-diluted ethylene, the changes in the temperature field of the co-flow diffusion flames are negligible, except at lower in the flame where hydrogen added flames display slightly higher temperatures. When helium is added instead of hydrogen, however, the temperatures were measurably lower than those of the base fuel. Results show that, once the dilution effects are accounted for, the hydrogen addition to ethylene does not suppress soot formation by direct chemical interaction at elevated pressures. These findings, which are not in agreement with the previous experimental results obtained at atmospheric pressure, are discussed in terms of the higher molecular diffusivity of hydrogen and shorter residence times of high-pressure flames.     

Nonlinear thermal diffusion of electrons in ionized gases

The authors have obtained a solution of Boltzmann's transfer equation for electrons in a slightly ionized gas and a simple model semiconductor, in the presence of a high temperature gradient. An explicit expression for f₀(the isotropic part of the electron velocity distribution function) correct to terms involving the square of the temperature gradient has been obtained. Using this solution, explicit expressions for non-linear electric and thermal current densities due to high temperature gradient have been obtained.