Sticking coefficients

Here, if Z is expressed in moles of collisions per square centimeter per second, r is in moles per square centimeter. We assume the condensation coefficient to be unity, that is, that all molecules that hit the surface stick to it. At very low Q values, F as given by Eq. XVII-3 is of the order expected just on the basis that the gas phase continues uniformly up to the surface so that the net surface concentration (e.g., F2 in Eq. XI-24) is essentially zero. This is the situation... 

The probability for sticking is known as the sticking coefficient, S. Usually,. S decreases with coverage. Thus, the sticking coefficient at zero coverage, the so-called initial sticking coefficient,. S q, reflects the interaction of a molecule with the bare surface. 

In order to calibrate the sticking coefficient, one needs to detemiine the exposure, i.e. how many molecules... 

L exposure would produce 1 ML of adsorbates if the sticking coefficient were unity. Note that a quantitative calculation of the exposure per surface atom depends on the molecular weight of the gas molecules and on the actual density of surface atoms, but the approximations inlierent in the definition of tire Langmuir are often inconsequential. 

Another limitation of tire Langmuir model is that it does not account for multilayer adsorption. The Braunauer, Ennnett and Teller (BET) model is a refinement of Langmuir adsorption in which multiple layers of adsorbates are allowed [29, 31]. In the BET model, the particles in each layer act as the adsorption sites for the subsequent layers. There are many refinements to this approach, in which parameters such as sticking coefficient, activation energy, etc, are considered to be different for each layer. 

Figure A2.3.18 The excess energy in units of NkT as a fiinction of the concentration for the RPM and SEM 2-2 electrolyte. The curves and points are results of the EfNC/MS and HNC approximations, respectively, for the binding and the electrical interactions. The ion parameters are a = 4.2 A, and E = 73.4. The sticking coefficients = 1.6x10 and 2.44x 10 for L = all and a/3, respectively.

The addition of potassium to Fe single crystals also enliances the activity for ammonia synthesis. Figure A3.10.19 shows the effect of surface potassium concentration on the N2 sticking coefficient. There is nearly a 300-fold increase in the sticking coefficient as the potassium concentration reaches -1.5 x 10 K atoms cm ... 

G is a multiplier which is zero at locations where slip condition does not apply and is a sufficiently large number at the nodes where slip may occur. It is important to note that, when the shear stress at a wall exceeds the threshold of slip and the fluid slides over the solid surface, this may reduce the shearing to below the critical value resulting in a renewed stick. Therefore imposition of wall slip introduces a form of non-linearity into the flow model which should be handled via an iterative loop. The slip coefficient (i.e. /I in the Navier s slip condition given as Equation (3.59) is defined as... 

Methods for determining fiber-to-fiber friction have been developed (29—31). The friction coefficient can also be measured in terms of the force required to pull entwined fibers apart (32—34) or the force necessary to remove a single fiber from a mass of fibers under pressure (35). Another test involves an apparatus wherein one or a series of parallel fibers are mounted across a small bridge similar to a violin bridge. This is then pressed against a surface that may be another fiber or some other material, and the fibers alternately sHp and stick as they sHde across each other (36,37). 

We get a quite different answer if we include the friction between the die and the forging. The extreme case is one of sticking friction the coefficient of friction is so high that a shear stress k is needed to cause sliding between die and forging. The total area between the dies and piece c is given by... 

Because of their light weight, ability to be moulded into intricate shapes in one piece, low coefficients of friction and absence of slip-stick behaviour, acetal resins find use as bearings. 

C2 molecules. Thus the respective rate of arrival and subsequent sticking coefficient are Fa k e, and Yq, which are normalized so that Fa + Tb + Fc = 1, and consequently the model has two parameters, namely Fa and Yq-... 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

This is our principal result for the rate of desorption from an adsorbate that remains in quasi-equihbrium throughout desorption. Noteworthy is the clear separation into a dynamic factor, the sticking coefficient S 6, T), and a thermodynamic factor involving single-particle partition functions and the chemical potential of the adsorbate. The sticking coefficient is a measure of the efficiency of energy transfer in adsorption. Since energy supply from the... 

Note that if sticking is controled by site-exclusion only, i.e., if S 6,T) = 5 o(P)(l — 0), this rate is that of a first-order reaction at low coverage. This simple picture breaks down when either the sticking coefficient depends dilferently on the coverage, as it does for instance for precursor-mediated adsorption, or when lateral interactions become important. It then does not make much physical sense to talk about the order of the desorption process. 

The simplest choice is to set all Ai = 0 in (44). Physically this means that an adsorbing particle will not experience any interactions with its prospective neighbors, and sticking is solely controled by the availability of sites. Thus the sticking coefficient becomes (for A,- = 0)... 

The sticking coefficient at zero coverage, Sq T), contains the dynamic information about the energy transfer from the adsorbing particle to the sohd which gives rise to its temperature dependence, for instance, an exponential Boltzmann factor for activated adsorption. 

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