Diffusion, tracer studies

Johnson, W. B. et al., 1975, Gas Tracer Study of Roof-Vent Effluent Diffusion at Millstone Nuclear Power Station, Atomic Industrial Forum AIF/NESP 0076. 

Unidirectional, first-order transfer rates (day1) between compartments were developed for 6 age groups, and intermediate age-specific values are obtained by linear interpolation. The range of age-specific transfer rate values are given in Table 2-8. The total transfer rate from diffusible plasma to all destinations combined is assumed to be 2,000 day"1, based on isotope tracer studies in humans receiving lead via injection or inhalation. Values for transfer rates in various tissues and tissue compartments are based on measured deposition fractions, or instantaneous fractional outflows of lead between tissue compartments (Leggett 1993). 

Surface path at low overpotential. Qualitative and quantitative analysis of impedance data, tracer studies, as well as various studies of thin-film electrodes suggest that under low-overpotential LSM operates primarily via a surface-mediated mechanism (like Pt). This conclusion appears to be consistent with the properties of LSM, which is fully oxygen stoichiometric under ambient Pq. However, little is known about how far the active region of reduction extends beyond the solid/solid interface (via surface diffusion) or the relative importance of chemical steps (on the LSM surface) vs electrochemical kinetics at the solid/solid interface. 

The results of the multiple nonreactive tracer study showed that secondary contaminant sources may form within the bedrock matrix, and that the importance of this source increases with continued contaminant discharge through the bedrock fracture network. This may be particularly important for reactive contaminants such as radionuclides, where matrix diffusion can enhance solute retardation by many orders of magnitude. 

Transverse dispersivity has been less studied, but it is smaller than the longitudinal dispersivity. Measurements from tracer studies indicate that the transverse horizontal dispersivity is about 10% of the longitudinal dispersivity in the bedding plane, and the transverse vertical dispersivity is about 1% (Gelhar, 1997). Klenk and Grathwohl (2002) found that the transverse vertical dispersivity was determined mostly by diffusion. 

Rao, V. K., and Brzustowski, T. A. "Tracer Studies of Jets and Diffusion Flames in Cross-Flow." Combustion Science and Technology T1 (1982) 229-39. 

Of course, there are more bulk properties of interest than the above parameters related to transport of the fast ions and electrons. Metal cation transport is minor, but still a most crucial parameter, because it eventually leads to membrane walkout, demixing, or decomposition in chemical gradients. Methods used for investigating metal cation diffusion comprise reactivity studies, interdiffusion couples, and tracer studies, using analytical SEM, EPMA, SIMS or radioactivity for the diffusion profile analyses. 

In the case of micellar solutions, it is not possible to separate the diffusivity of the surfactant as a whole from singly dispersed amphiphiles and micelles. Lindman and Brun (1973), using radiolabeled tracer studies, and Lindman et al. (1984), using magnetic resonance with C labeling and heavy water as a solvent, showed that... 

The success of silica potential evolution is demonstrated by Litton and Garofalini,29 who investigated the kinetics of Si and O self-diffusion in the bulk of vitreous silica. These authors report computed activation energies for self-diffusion in bulk vitreous silica in good agreement with experimental values obtained from diffusion-coupled tracer studies. 

Arsenic radiotracer diffusion was studied in intrinsic and in homogeneously doped extrinsic single crystals. Under intrinsic conditions, the tracer diffusion coefficient was described by ... 

Tracer studies indicated that the diffusion of Ge at 1100 to 1265C, for Ge concentrations which were below the level at which new dislocations were introduced, was given by ... 

The diffusivity was studied at 900 to 1200C by using deep level transient spectroscopy and tracer methods. The surface concentrations which were deduced from the tracer method agreed with solubility data, and the concentration of electrically active interstitial Mn was found to represent 60 to 70% of the total Mn content. Both methods yielded identical diffusion coefficients. These could be described by ... 

Tracer diffusion was studied in natural and synthetic single crystals of diopside, using stable tracers enriched in 54pe, at 950 to llOOC, under latm. The diffusion profiles were determined by using an ion microprobe. For directions parallel to the c-axis and... 

The use of isotopes or tracers is a common means of studying diffusion. Tracer methods permit measurements of self-diffusion, that is, the diffusion of the crystal components in a crystal. Furthermore, they allow measurements of diffusion in homogenous materials, that is, without imposing chemical gradients (when one disregards the difference in atomic weight between atoms in the crystal and of the tracer). 

Electron microprobe analysis and secondary ion mass spectroscopy (SIMS) combined with sputtering techniques also provide excellent tools for studying penetration and diffusion of foreign ions. As oxygen does not have a radioactive isotope suitable for tracer studies, SIMS is particularly useful for studying oxygen diffusion employing the stable isotope. 

Other reactors and uses of tracers. Tracers are used extensively in all other two-phase (gas-liquid, gas-solid, liquid-solid) and three-phase reactor types (gas-liquid-solid, liquid-liquid-solid). Tracers confined to a single phase are used to determine the RTD of that phase and evaluate its flow pattern. Tracers that can be transported from one phase to another are frequently used for evaluation of various rate parameters and transport coefficients such as mass transfer coefficients, particle effective diffusivity, adsorption rate constants, kinetic rate constants, etc. The interpretation of tracer studies in evaluation of the above parameters is always dependent on the selected model for the system. We do not attempt to review this vast literature but will just cite a few examples as good starting points for the interested reader. 

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