Dust formation

The state-of-the-art i -Si H films (Table 3) are deposited at the rate of 1—3 A/s with the gas utilization rate on the order of 15%. Larger gas utilization rates, hence larger deposition rates, usually result in inferior properties than those indicated in Table 3. Increasing the deposition rate by merely increasing the power leads to dust formation. The use of higher excitation frequency can lead to deposition rates in excess of 15 A/s and still give relatively good film properties (7). 

Various methods are used for evaluatiag the quaflty, ie, physical strength and ensyme dust formation, of the granulate. In the elutriation process, a sample of product is fluidised ia a glass tube with a perforated bottom plate for 40 miautes. Dust from the sample is collected oa a filter and the ensyme activity measured. An acceptable dust level is when less than 5—10 ppm of the activity of the sample has been collected. In the so-called Heubach method, 20 g of granulate is elutriated. During the elutriation, four steel balls are rotated ia the bed ia order to evaluate the impact of attritioa oa the dust release of the ensyme. The dust is collected oa a filter and measured. The acceptable dust level is very low. 

Method of Adding Liquids When the addition of hquids may be desirable (see Dust Formation and Elec trostatic Charge ), this should be considered when designing the mixing system rather than hastily improvised. The purpose of the liquid shomd be considered, whether for (I) dust suppression, (2) product, or (3) heating and cooling. If a viscous hquid must be well distributed, this requirement should be considered when choosing the mixer. 

Handle solids as prills or pellets rather than powders to minimize the possibility of dust formation... 

Use enclosed conveyors and sieves for coal and coke handling. Use sprinklers and plastic emulsions to suppress dust formation. Provide windbreaks where feasible. Store materials in bunkers or warehouses. Reduce drop distances. 

Crystallization is an important separation proeess that purifies fluids by forming solids. Crystallization is also a partiele formation proeess by whieh moleeules in solution or vapour are transformed into a solid phase of regular lattiee strueture, whieh is refleeted on the external faees. Crystallization may be further deseribed as a self-assembly moleeular building proeess. Crystallo-graphie and moleeular faetors are thus very important in affeeting the shape (habit), purity and strueture of erystals, as eonsidered in detail by, for example, Mullin (2001) and Myerson (1999). In this ehapter the internal erystal strueture and external partiele eharaeteristies of size and shape are eonsidered, whieh are important indieators of produet quality and ean affeet downstream proeessing, sueh as solid-liquid separation markedly. Larger partieles separate out from fluids more quiekly than fines and are less prone to dust formation whilst smaller partieles dissolve more rapidly. 

Often, it is essential that crystals are not damaged unduly by the drying operation. And in the case of food and pharmaceutical grade materials, care should be taken to avoid contamination and powder leakage via dust formation. 

Modulation of the RF excitation has been used in an attempt to increase the deposition rate. Increasing the gas pressure or raising the power generally leads to dust formation and deterioration of material properties. To overcome this problem, one can pulse the plasma by modulating the RF signal in amplitude with a square wave (SQWM). Depending on the regime (a or y ), different effects are observed. 

This paper reports on the aspects of dust formation in livestock buildings, the material composition of the dust, the emission of dust-bome odourants like volatile fatty acids (VFA) and simple phenols and indoles from piggeries, the importance of particle-borne... 

Calcium carbide for acetylene is mainly packed in returnable steel bulk containers ranging in capacity from 2.5—4.5 t, suitable for lift trucks and unloading conveyors. The granular carbide is lightly oiled with alubricating oil (see Lubrication and lubricants), which decreases the rate of reaction when exposed to moist air and also reduces dust formation during handling. 

Also, one classical question is whether dust formation initiates mass-loss or whether dust is formed as a result of mass-loss. It is to be noted that the latter process may be rather easy, once mass-loss occurs by another mechanism. This problem can be examined on the basis of recent observations of CO radio emission lines, by which stellar mass-loss rate has been determined with better accuracy than by any other method for a large sample of red giant stars, and terminal flow velocities have also been determined with high accuracy( e.g.,Knapp,Morris,1985). The result revealed that the momentum in the stellar wind and that in the stellar radiation do not necessarily show good correlation(e.g.,Zuckerman,Dyck,1986). Also, a necessary condition for the winds to be accelerated by radiation pressure on dust( Mv [Pg.160]

Despite the observational identification of a possible site of dust formation, however, the dust-driven wind could not be applied to stars without dust envelope, as noted in Sect.II. Then, a more interesting possibility is a turbulence-driven wind,in which the high turbulent pressure of the transition layer(or cool corona) pushes the gas out of star,just as the high thermal pressure in corona does in solar-type stars. In fact, if the turbulent zone is extended to about 10 stellar radii, the local escape velocity there may already be small enough to be comparable with the observed flow velocities. Thus, the Maxwellian tail of the turbulent motion in the quasi-static molecular formation zone can directly lead to stellar mass-loss in all... 

The cool extended shells of giants and supergiants are well known to be places of copious dust formation as indicated by the occurrence of pronounced extinction, reddening, and polarization of the continuous star light and by the appearance of particular absorption features, both manifesting the interaction of photons with particles considerably larger than atoms or molecules. 

An adequate answer to these questions must be based on the detailed study of the processes of formation and growth of dust particles in these environments. However, dust formation cannot be considered as an isolated problem because due to their huge absorption cross sections even a small contamination of the atmospheres by circumstellar dust may have a significant influence on the radiative transfer and (via energy- and momentum-coupling) on the thermodynamic and hydrodynamic structure of the dust forming shell. 

For describing dust formation and growth we adopt a frame of reference which is moving with the flow. Then the system of moment equations which describes these processes in a sufficient approximation reduces to... 

All calculations presented in the following chapters are restricted to dust formation in C-stars and are performed under the assumption of chemical equilibrium for the molecular reactions among the various chemical elements. The element abundances have to be specified as additional external parameters. For dust driven winds the amount of condensed material determines the velocity field and in particular the terminal velocity of the wind. For this reason, the abundances of the dust forming elements and the terminal velocity of the wind are coupled very closely. For the case of M-stars similar calculations have been performed by Kozasa et al. (1984) considering formation of MgSi03 grains. 

Fig. 2 shows an HR-diagram with contour lines of constant mass loss rates. The lines have been calculated for M, = 1 M, and ec/e0 = 2- The dust driven winds are confined to a well defined area in the upper right corner of the HRD. Since effective dust formation requires sufficient high densities, there exists a minimum mass loss rate of about 10-7 M yr-1 (Gail, Sedlmayr, 1987b). Because of the obvious correlation between luminosity and mass loss, this defines a limit line (the broken line in Fig. 2). Below this line no dust driven winds are possible. This fact is in keeping with the observation that C-stars with extended envelopes always have mass loss rates above 10 7 t yr (Knapp and Morris, 1985).

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