Laminar deposits

Effect of C/H Ratio. The carbon-to-hydrogen (C/H) ratio of the gas mixture (CH4 and H2) entering the reaction chamber is an important factor in the control of the nature of the deposition. Higher C/H ratios (such as 1/4) favor laminar deposition and lower ratios (such as 1/14) favor isotropic deposition. 

Formation of Laminar Deposits by Pulsating Current Regimes... 

Despic AR, Jovic VD (1987) Electrochemical fonnatirai of laminar deposits of controlled structure and composition 1. Single current pulse galvanostatic technique. J Electrochem Soc 134 3004-3011... 

Smooth laminar deposition—occurring at a low deposition temperature and a high partial pressure of CH4, with no added H2, where [C2H2]/[C6H6] < 5... 

Graphitization of Columnar and Laminar Deposits. The columnar and laminar deposits described above have generally a turbostratic structure in the as-deposited condition, with a large interlayer spacing ( 0.344 nm) as revealed by x-ray diffraction. The material graphitizes readily when heat-treated at 2500°C for 90 minutes. 

Hardness. Being composed of minute crystallites with essentially random orientation, isotropic pyrolytic carbon lacks the easy interlayer slippage which is characteristic of the well-oriented laminar or columnar structures of pyrolytic graphite. As a result, it is considerably harder. This makes it easy to polish and the material can be given a high gloss. The wear resistance is usually superior to that of the columnar and laminar deposits of vitreous carbon. PS)... 

Additive levels below a certain threshold make the bath more snsceptible to imparity effects. For example, iron contamination of 100 mg/L without the recommended concentration of ductility-promoting agents was found to produce a columnar grain strnctnre at the hole corners. Similarly, organic contaminants such as photoresist can prodnce laminar deposits. 

The nodules of silicosis are well defined and located in the perivascular and peribronchiolar interstitium as well as in the paraseptal and sub-pleural interstitium (Fig. 5.1) and are preferentially distributed in the upper lobes. The silicotic nodule starts as a central zone of mononuclear cells surrounded by fibroblasts and collagen tissue. With time, the central zone becomes hypocellular, with concentric laminar deposition of reticulin, proteins, phospholipids and collagen in the periphery, giving a whorled appearance (Fig. 5.1). Adjacent vessels and bronchioles may become involved and destroyed by these nodules, with occlusion of their lumen. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Under higher waterside pressure conditions, consideration of bulk water turbulent flow, the thickness of the steam-water laminar flow sublayer film at the heat transfer surface, and the general waterside physicochemical operating conditions that exist are important issues in reviewing the potential risks of deposition, corrosion, and other problems that may occur within an operating boiler. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

In the case of laminar flow, the velocity of the gas at the deposition surface (the inner wall of the tube) is zero. The boundary is that region in which the flow velocity changes from zero at the wall to essentially that of the bulk gas away from the wall. This boundary layer starts at the inlet of the tube and increases in thickness until the flow becomes stabilized as shown in Fig. 2.4b. The reactant gases flowing above the boundary layer have to diffuse through this layer to reach the deposition surface as is shown in Fig. 2.3. 

CVD graphite can have several structural forms columnar, laminar, or isotropic. The columnar form is shown in Fig. 7.2. The crystallites are deposited with the basal planes (ab directions) essentially parallel to the deposition surface. Their structure tends to be columnar (cone-like) as a result of uninterrupted grain growth toward the reactant source. 

The laminar form consists of essentially parallel layers or concentric shells if deposited on a particle or fiber as shown in Fig. 

Effect of Temperature. Generally, isotropic deposits are obtained at higher temperatures (>1400°C) and laminar and columnar deposits at lower temperatures. 

In summary, isotropic deposits are obtained at high temperature, low pressures, and low C/H ratio. The opposite conditions favor the deposition of laminar and columnar deposits. 

Unsteady-state mass transfer caused by excessively fast current or potential ramps. This is especially likely to occur in measurements involving laminar flow past elongated surfaces and in free-convection studies, in which the establishment of secondary flow patterns may require long times. A compromise between the time sufficient to reach steady-state transport and the time necessary to avoid bulk depletion and surface roughening (in metal deposition) is required, and is found most reliably by preliminary experimentation. 

Diffusion is the dominant mechanism of lung deposition for radon daughter aerosols. It is generally assumed that airflow is laminar in the smaller airways and that deposition in each airway generation can be calculated adequately (Chamberlain and Dyson, 1936 Ingham, 1975). However, there is no such consensus on the treatment of deposition in the upper bronchi. Some authors (Jacobi and Eisfeld, 1980 NCRP, 1984) have considered deposition to be enhanced by secondary flow, on the basis of experimental results (Martin and Jacobi, 1972). It has been shown that this assumption reduces the calculated dose from unattached radon daughters by a factor of two (James, 1985). 

Pich, J., Theory of Gravitational Deposition of Particles from Laminar Flow in Channels, Aerosol Sci. 3 351-361 (1972). 

The deposition of ultrafine particles has been measured in replicate hollow casts of the human tracheobronchial tree. The deposition pattern and efficiency are critical determinants of the radiation dose from the short lived decay products of Rn-222. The experimental deposition efficiency for the six airway generations just beyond the trachea was about twice the value calculated if uniform deposition from laminar flow is assumed. The measured deposition was greater at bifurcations than along the airway lengths for 0.2 and 0.15 ym diameter particles ... 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

In comparison with the large amount of literature that is available on the deposition of particles from laminar fluid flows, literature on turbulent deposition is virtually non-existant [114]. It was mentioned that the trajectory and convective diffusion equations also apply when the fluid inertial effects are considered, including the case of turbulent flow conditions, provided one is able to express the fluid velocities explicitly as a function of position and time. 

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