Diffusion deposition

Martin, D. and W. Jacobi, Diffusion Deposition of Small-sized Particles in the Bronchial Tree, Health Phys. 23 23-29 (1972). 

Different. Ingham (1975) diffusion deposition but corrected by turbulent diffusion factors in airways 0 to 6 as measured by Cohen (1986). 

It is well known that enhanced deposition in the first few airways occurs due to the turbulence produced. Turbulent diffusion is accounted for by using factors (ratio of observed deposition to calculated diffusion deposition) to correct the diffusion deposition. These had formerly been measured by Martin and Jacobi (1972) in a dichotomous plastic model of the upper airways. The data used here are from measurements performed by Cohen (1986) using hollow casts of the upper bronchial tree which included a larynx. This cast was tested using cyclic flow with deposition measured for 0.03, 0.15 and 0.20 urn diameter particles. Her turbulent diffusion factors are used in the calculation here (14 for generation 0, and 2 for generations 1 to 6). 

Konstandopoulos, A. G. Convective-diffusive deposition of fractal-like aggregates and the microstructure and properties of the resulting nanostructured deposits (to be submitted) (2007). 

A critical problem with adhesion layers arises from grain boundary diffusion. Deposited films tend to be polycrystalline and granular. The electrochemistry of the adhesion film is frequently much less desirable than the electrochemistry of the primary film. Moreover, minute contamination of the primary metal film surface by adhesion components can dramatically degrade the electron transfer properties (e.g., electrochemical reversibility, as evidenced by cyclic voltammetric peak potential separation) of the film [58], Thus it is essential that the adhesion layer is not exposed to solution. While the rate of diffusion of adhesion metals through the bulk of the primary layer is quite slow, grain boundary diffusion along the surfaces of grains is much faster. In many cases, the adhesion layer can seriously compromise the performance of the electrode. This is particularly a problem for chromium underlayers. Recently a codeposited Ti/W adhesion layer has been recommended as an alternative to chromium, with reportedly better adhesion and fewer interferences than Cr. A procedure was also described to recondition these electrodes to minimize interference by adhesion layer metals [58]. 

Diffusive deposition of gases. Although we have not formally investigated the deposition of condensible gases, the overall concentrations of Br and Se in impactor samples collected downstream of a hot-side ESP at a large western coal-fired power plant far exceeded their concentrations as determined by filter sampling (16). In addition, the concentrations of Br and Se were nearly uniform on all stages... 

But concentration change, and hence diffusive deposition, is observed. Thus, there must be an additional mechanism operating which tends to enhance deposition of small particles by diffusion. ... 

Today, a typical process flow for advanced ICs consists of 300 to 500 steps, 30% of which are wafer cleaning steps." Many process steps during IC fabrication may introduce contamination, which must be cleaned before the next process step. For example, in processes such as steam oxidation, resist etching, and ion implantation, metallic contamination typically introduces a surface concentration of 10 to lO Vcm. The need for wafer cleaning can be separated into three areas (1) preparation of the wafer surfaces for oxidation, diffusion, deposition, and metallization (2) preparation for the application of photoresist and (3) removal of photoresist after the etching process." ... 

Dimensionless diffusion deposition parameter or mass flux away from reaction zone... 

Several sampling methods in eommon use rely upon air filtration to remove bioaerosols from a sampled air volume. Generally the aerosols are removed by a combination of impaction on the filter surface as well as interception, electrostatic attraction, and diffusion deposition. After sampling, filters are agitated in a solution to re-entrain the microbes and this solution is then serially diluted and plated onto appropriate culture media. 

Air(l) Soil (3) Diffusion Rain Dissolution Wet Deposition Dry Deposition Water (2) —Sediment (4) Sediment (4) —Water (2) Diffusion Deposition Diffusion Resuspension... 

Martin D, Jacobi W. 1972. Diffusion deposition of small-sized particles in the bronchial tree. Health Phys 23 23-29. 

Based on observed differences in the distribution of smooth muscle cells in patients at autopsy, some asthmatics may profit from a more diffuse deposition pattern including both large and smaller bronchi, while the optimal deposition pattern for others may be the larger bronchi. 

Coating (immersion, electrolytic metal deposition, diffusion deposition, hot-dip metal coating, evaporation coating, and metal spray coating)... 

Plasticizer diffusion, deposition, and accumulation on a material surface are controlled by the degree of the compatibihty between the plasticizer and the matrix, the surface energy of the liquid/solid interface, and the volatility of plasticizer. Volatihty and migration are controlled by different properties of the plasticizers therefore they are not related (Figure 7.8). Plasticizers have high boiling points so evaporation is typically slow. The rate can be calculated from the Hertz equation ... 

Particulate matter is collected (sampled) either from suspension in the air (usually by filtration) or by collection of depositing particles as they fall out of the atmosphere under gravity or by turbulent or diffusive deposition. In general, airborne particles collected by air filtration are 10 pm or less in diameter, while particles greater than this size predominate in deposit gauge collectors. In this section, the analysis of several important pollutants... 

Overcamp, T. J. 1976. A General Gaussian Diffusion-Deposition Model for Elevated Point Sources, Journal of Applied Meteorology, vol. 15, pp. 1167-1171. 

Particles of >10 nm diameter are subject to significant sedimentation rates as a result of gravitational forces (the velocity may be estimated crudely by use of Stoke s Law). Consequently their atmospheric lifetime is severely limited by the gravitational settling process, and by impaction upon surfaces. Particles of <10 jum diameter are removed only relatively slowly from the atmosphere, those greater than ca. 0.3 /im by impaction processes and the smaller particles by diffusive deposition. The deposition flux may be estimated from the deposition velocity and some data are presented in Section 4.2.2. 

Papastefanou and Bondietti (1991a, 1991b) performed experiments on the diffusive deposition of aerosol particles on wire screens and, in particular, used Pb deposition as a measure of the collection efficiency of the screens for aerosol-associated attached radionuclides in outdoor air, at Oak Ridge National Laboratory, Oak Ridge, Tennessee (35 58 N, 84 17 W) during the summer period. Stainless steel wire screens (60, 200, as well as 40 and 100 mesh/inch) were used in the experiments to collect the unattached species of radon decay products in ambient aerosols. Glass fibre filters were used as back-up to collect the radon decay products which passed the wire screens. The screens were separated from the back-up filter by a spacer screen (4 mesh/inch) to prevent contamination by the filter deposit (e.g., " Pb atoms) via a-recoil. 

Pe Peclet number it characterizes the intensity of diffusion deposition, and an increase in the Peclet number will decrease the single-fibre diffusion efficiency, Pe fMt... 

Kirsch AA, Chechuev PV. Diffusion deposition of aerosol in fibrous filters at intermediate Peclet numbers. Aerosol Sci Technol 1985 4(1) 11—6. 

In the case of significant accidental exposures, it will often be necessary to use parameter values in the calculation of tissue or organ equivalent doses and effective dose that ate specific to the conditions of exposure and to the individual. Similarly, in routine situations it may be necessary to take account of the particular circumstances of exposure rather than using default parameters. The new model for the respiratory tract [34] adopts an AMAD of 5 pm as a default particle size when no specific information is available. Regional deposition of airborne particles is subject to the mechanisms of sedimentation, impaction and diffusion. Deposition throughout the respiratory system and hence inhalation dose coefficients depend upon aerosol parameters, such as the AMAD. Similarly, ingestion dose coefficients depend upon the choice of an appropriate/j value. 

The EXWAT-model represents an open system with two compartments the fluid and the sediment. As in the fugacity approach of Mackay, equilibria are assumed both for the chemicals dissolved in the fluid and sorbed on the suspended matter and for chemicals dissolved in the pore-water and sorbed on the particulates of the sediment. The volatility was calculated with the formulas developed by Southworth, Liss and Slater (Lyman et al. 1982 Southworth 1979 Liss and Slater 1974). No differentiation was made for the various degradation processes of chemicals in the dissolved or sorbed state, although different degradation behaviour of molecules due to their molecular state can be expected. Similar to Mackay s approach, diffusion, deposition and resuspension is used to describe the exchange processes between sediment and fluid. The main results are called descriptors as seen in SCHEME 2. Several of these are defined in TABLE 2. 

Frohlich s syndrome, dystrophia adiposo-genitalis, occurs in children, and is due to infantile hypopituitarism and hypothyroidism. The victims are stunted, stupid, and often distorted by diffuse deposition of fat. 

FIGURE 2.2 Particle capture mechanism A - particle captured by interception B - particle captured by inertial impaction C - particle captured by diffusive deposition. 

---