Mass deposition rate

Crystal growth rate may be expressed either as a rate of linear inerease of eharaeteristie dimension (i.e. veloeity) or as a mass deposition rate (i.e. mass flux). Expressed as a veloeity, the overall linear erystal growth rate, G (=dL/dt where L is the eharaeteristie dimension that is inereasing). The rate of ehange of... 

The mass deposition rate is also equal to the total flux of solute adding to the erystal surfaee, i.e. 

FK Force of repulsion, N g Standard gravitational acceleration, 9.8067 meters/sec2 G Mass velocity, (kg)/(meter2)(sec) Gde Mass deposition rate due to electrostatic forces, (kg)/(meter2)(sec) Gdg Mass deposition rate due to gravity, (kg)/(meters 2)(sec) h Planck s constant =... 

As with nucleation, classical theories of crystal growth 3 20 2135 40-421 have not led to working relationships, and rates of crystallisation are usually expressed in terms of the supersaturation by empirical relationships. In essence, overall mass deposition rates, which can be measured in laboratory fluidised beds or agitated vessels, are needed for crystalliser design, and growth rates of individual crystal faces under different conditions are required for the specification of operating conditions. 

Methods used for the measurement of crystal growth rates are either a) direct measurement of the linear growth rate of a chosen crystal face or b) indirect estimation of an overall linear growth rate from mass deposition rates measured on individual crystals or on groups of freely suspended crystals 35,41,47,48). 

Because the rate of growth depends, in a complex way, on temperature, supersaturation, size, habit, system turbulence and so on, there is no simple was of expressing the rate of crystal growth, although, under carefully defined conditions, growth may be expressed as an overall mass deposition rate, RG (kg/m2 s), an overall linear growth rate, Gd(= Ad./At) (m/s) or as a mean linear velocity, // (= Ar/At) (m/s). Here d is some characteristic size of the crystal such as the equivalent aperture size, and r is the radius corresponding to the... 

Table 16.5 Total Mass Deposition Rates and Percent Conversion of Methane to Plasma Polymer at Various Axial Positions...

Table 5 Total mass deposition rates and percent conversion of methane to plasma polymer at various axial positions. Conditions are 8.00 A, 2000 seem argon, 10.0 seem methane, and 560mtorr...

Garside et al. (1982) developed an elegant technique to evaluate crystal growth kinetics from an integral mode of batch experiments. For size-independent growth, the crystal mass deposition rate (Rq) can be given by... 

In order to coat substrates with diameters in excess of 4 inches, reactors working at lower frequency have been developed in recent years. Using a 75 kW source operating at 915 MHz, an apparatus was built by ASTeX permitting a coatable area of 8 inches diameter and a total mass deposition rate which is a factor of 10-100 larger than for the 5 kW HPMS source [25]. 

The negatively self-biased substrate surface is bombarded with ions which are generated in the discharge (C (H and noble gas ions if noble gases are added). However, not only the C H ions introduce carbon into the film. Comparing the calculated mass deposition rates of major ions (CHj for CH4 gas and C H as well as C6H5 for CsH precursor) with the experimentally determined deposition rates Catherine and Couderc [6] found clear discrepancies in the correlations between rate and RF power and between rate and pressure. In order to overcome these, the adsorption of activated species and their incorporation into the growing film must be taken into account additionally to explain the experimental data. 

The chemical-vapor deposition of diamond films and their applications are reviewed by C.-P. Klages and by R. S. Sussmann et al. in Parts II and III, respectively. To date the most effective CVD method (with the greatest mass deposition rate) is based on the hydrogen/hydrocarbon gas mixtures. In this method diamond is formed for kinetic reasons according to the simplified reaction ... 

There is no simple or generally accepted method of expressing the rate of growth of a crystal, since it has a complex dependence on temperature, supersaturation, size, habit, system turbulence, and so on. However, for carefully defined conditions crystal growth rates may be expressed as a mass deposition rate Rq (kgm s ), a mean linear velocity v(ms ) or an overall linear growth rate G (ms ). The relationships between these quantities are... 

Alternatively, expressing the overall mass deposition rate Rq (kgm s ) as in equations 6.14 and 6.61 the overall linear growth rate G may be expressed as... 

Overall growth rates for potash alum measured in the fluidized bed crystallizer coincide very well with those predicted from face growth rates measured in the single crystal cell Figure 6.22). The alums grow as almost perfect octa-hedra, i.e. eight (111) faces, so it is a simple matter, using the crystal density, pc, to convert linear face velocities to overall mass deposition rates Rq = pcV u ))-... 

The mass and energy conservation equations have been listed in Table 2.1. The constitutive equations for the transport parameters and the mass deposition rate have been summarized in Tables 2.2 and 2.3, respectively. The gas phase (mixture) properties have been computed according to Reid et al. [14], using the pure component data supplied by Daubert and Danner [15]. Uniform initial temperature profiles are taken without any mass deposited... 

Crystal growth rates can be expressed in a number of different ways. For the purpose of characterizing the growth kinetics in a maimer that is relevant to the design of a crystallization process, it is convenient to express growth rates in terms of mass deposition rates or overall growth rates ... 

For the purpose of simplification, the assumption has been made in Figure 11.3 that the time for a recirculation process is sufficient to reduce the supersaturation down to negligible levels. This reduction in supersaturation is, however, a function of the mean mass deposition rate and the growing crystal surface area present [2] ... 

The smaller the active crystal surface available, the slower the mass deposition rate d7w/df, and the larger the supersaturation remaining after each recirculation cycle. The point 0, in Figures 11.3 and 11.4 moves up, if crystal growth does not desupersaturate to Ac —> 0. As this residual supersaturation is added to the newly created supersaturation, it is certainly possible that the metastable zone width will be... 

The mass deposition rate dmidt (Eq. (9.2)) can be described as exponential function with supersaturation AC as exponent basis and the crystal surface A as linear factor. The proportionaUty factor /Cg considers the influence of the temperature. 

For large-scale application of electrospinning technology, it is important to know how to maximise and control the deposition rate of spun fibre. Driving polarity, substrate material and current flow were examined to quantify their effect on fibre deposition rate. Conductivity of the substrate and polarity of driving electric charge were found to affect the mass deposition rate. Higher deposition rates were the result of the production of thicker fibres and an increase in deposition speed. 

However, mass deposition rates are low, limiting the possible range of applications to certain niche markets. There is currently only a small number of companies in commercial production of electrospun fibre. Then-products are specialist filtration elements and electrospinning equipment. In order to increase the commercial potential, a coherent model of the electrospinning process based on fundamental mechanisms for a single jet and multiple jets is required. This model can then be used to devise manufacturing processes. 

It was observed that when the operating polarity was reversed, that the mass deposition rate of fibre increased significantly. The work that follows investigates what fundamental mechanisms could explain this change in fibre deposition. 

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