Sticking


In the presence of oxygen even at ambient temperature, hydrocarbons can undergo a process of deterioration by oxidation that can form viscous substances commonly called gums. This can cause a variety of undesirable incidents blockage of engine fuel pump membranes, plugging of needle valves or injectors, sticking of the carburetor level control mechanism or even sticking (or gumming up) of piston rings in their grooves. Anti-oxidant additives of the alkyl-p-phenylenediamine or alkyl p-amino-phenol families are added to the gasoline in concentrations of 10 to 20 ppm as soon as it is produced in the refinery to avoid these problems. However, several methods exist to verify the quality of the finished products.  [c.242]

To avoid these problems, refiners commonly use additives called detergents" (Hall et al., 1976), (Bert et al., 1983). These are in reality surfactants made from molecules having hydrocarbon chains long enough to ensure their solubility in the fuel and a polar group that enables them to be absorbed on the walls and prevent deposits from sticking. The most effective chemical structures are succinimides, imides, and fatty acid amines. The required dosages are between 500 and 1000 ppm of active material.  [c.243]

A phenomenon that may develop, especially at small sliding speeds, is known as stick-slip friction in which the slider moves in jumps that may be of quite high frequency. This effect is partly a consequence of having some play and lack of rigidity in the mechanism holding the slider. Once momentary sticking occurs, the slider is pushed back against the elastic restoring force of the holding mechanism as a result of the continued motion of the latter. When the restoring force exceeds that corresponding to Us, the slider moves forward rapidly, overshoots if there is sufficient play, and sticks again. The greater the difference between static and sliding frictional coefficients, the more prone is the system to stick-slip friction with lubricated surfaces, there may be a fairly well defined temperature above which the phenomenon is observed. Friction force microscopy on smooth graphite surfaces illustrates molecular stick-slip friction [11]. Slip occurs when the derivative of the periodic force equals the spring constant of the AFM. Stick-slip friction is encouraged if uk decreases with sliding speed (why ) and with increasing W [12]. A very common, classroom illustration of stick-slip friction is, incidentally, the squeaking of chalk on a chalkboard. An important and interesting feature of current models of earthquakes is stick-slip motion of the tectonic plates.  [c.436]

The rate of physical adsorption may be determined by the gas kinetic surface collision frequency as modified by the variation of sticking probability with surface coverage—as in the kinetic derivation of the Langmuir equation (Section XVII-3A)—and should then be very large unless the gas pressure is small. Alternatively, the rate may be governed by boundary layer diffusion, a slower process in general. Such aspects are mentioned in Ref. 146.  [c.661]

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes  [c.705]

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.  [c.294]

In order to calibrate the sticking coefficient, one needs to detemiine the exposure, i.e. how many molecules  [c.294]

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.  [c.294]

Figure Al.7.8. Sticking probability as a fimction of surface coverage for tliree different adsorption models. Figure Al.7.8. Sticking probability as a fimction of surface coverage for tliree different adsorption models.
Figure Bl.19.24. Friction loop and topography on a heterogeneous stepped surface. Terraces (2) and (3) are composed of different materials. In regions (1) and (4), the cantilever sticks to the sample surface because of static friction The sliding friction is tj on part (2) and on part 3. In a torsional force image, the contrast difference is caused by the relative sliding friction, Morphological effects may be Figure Bl.19.24. Friction loop and topography on a heterogeneous stepped surface. Terraces (2) and (3) are composed of different materials. In regions (1) and (4), the cantilever sticks to the sample surface because of static friction The sliding friction is tj on part (2) and on part 3. In a torsional force image, the contrast difference is caused by the relative sliding friction, Morphological effects may be
Sticking, for simplicity, with a simple atomic system, the kinetic energy may be written  [c.2243]

Jackson B 1994 Quantum and semiclassical calculations of gas surface energy transfer and sticking Comput. Rhys. Commun. 80 119  [c.2323]

At first sight, one might think that any treatment of the properties of a polymer chain has to emanate from its microscopic chemical stmcture since it detennines the populations of the different rotational isomeric states of a given torsional angle. However, in polymers without well defined confonnations, the correlation between the torsional angles along the chain decays rapidly. Beyond that length scale, typically a few nanometres and referred to as persistence length [Kl], one can think of the chain as a succession of jointed sticks, with unrestricted angles at the junctions. Such a chain with a large number of segments is called Gaussian. It can be easily shown [H] that for a chain consisting of segments of length the root mean square of the distance between the two chain ends averaged over all possible confonnations is  [c.2517]

Because surface chemical reactions occur at a localized geometrical interface between phases, transport processes are strongly coupled with these reactions. The overall reaction can be decomposed into a series of steps— attachment or sticking of reactants to the surface diffusion of reactants on the surface fonnation of reaction product disposition of reaction product—any one of which can be rate-limiting for the overall reaction. Fundamental progress in understanding etching and deposition reactions relies upon isolating one of these steps for investigation in simplified model circumstances, either theoretical or experimental. A comprehensive review published in 1994 summarizes such fimdamental results from reactive molecular beam studies of both etching and deposition reactions. That review is highly recommended as an introduction to the field, since it also describes the fundamental concepts in adsorjDtion dynamics and chemisoriDtion, as well as the relevant experimental teclmiques  [c.2926]

In view of the complex kinetics and dynamics of the overall reaction, attention was directed to dynamical studies of individual steps, particularly the attachment of etchant molecules to surfaces. The emerging theme is that the sticking probability and the stmcture of the adsorbate layer depend strongly on the energy of the incoming molecules and on the stmcture of the surface, and that chemisoriDtion can be highly site-selective. For example, at low incident kinetic energy where precursor-mediated chemisoriDtion dominates, CI2 fonns large islands of SiCl on Si(l 11)-(7 X 7) while at high incident energy direct activated chemisoriDtion dominates, and Cl is adsorbed only at isolated sites [19].  [c.2930]

Excessive pressure differentials between the borehole and the formation. For instance, if the pressure of the mud column is much higher than the formation pressure the drill pipe may become sucked against the borehole wall (differential sticking). This often happens when the pipe is stationary for some time, e.g. while taking a deviation survey. Prevention methods include reduced mud weights, adding of friction reducing components to the mud, continuous rotation / moving of string, adding of centralisers or use of spiral drill collars to minimise contact area between string and formation, low fluid loss mud system.  [c.57]

It is a probe whose the coil support is a small circular sticks with a straiglit section. The aim of our study is to assimilate the resulting magnetic field to a material point, hi order to minimize the lateral field, we have chosen the construction of conical coil where the lateral field at a contact point in respect to a straight configuration is decreased with an exponential factor. The results obtained from the curves are as follow  [c.292]

The third region is one for which the Q values are of the order of chemical bond energies the r values become quite large, indicating that desorption may be slow, and F as computed by Eq. XVII-3 becomes preposterously large. Such values are evidently meaningless, and the difficulty lies in the assumption embodied in Eq. XVII-3 that the collision frequency gives the number of molecules hitting and sticking to the surface. As monolayer coverage is approached, it is to be expected that more and more impinging molecules will hit occupied areas and rebound without experiencing the full Q value. One way of correcting for this effect is taken up in the next section, which deals with the Langmuir adsorption equation.  [c.603]

As with any system, there are complications in the details. The CO sticking probability is high and constant until a 0 of about 0.5, but then drops rapidly [306a]. Practical catalysts often consist of nanometer size particles supported on an oxide such as alumina or silica. Different crystal facets behave differently and RAIRS spectroscopy reveals that CO may adsorb with various kinds of bonding and on various kinds of sites (three-fold hollow, bridging, linear) [307]. See Ref 309 for a discussion of some debates on the matter. In the case of Pd crystallites on a-Al203, it is proposed that CO impinging on the support  [c.736]

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials.  [c.283]

Atom abstraction occurs when a dissociation reaction occurs on a surface in which one of the dissociation products sticks to the surface, while another is emitted. If the chemisorption reaction is particularly exothennic, the excess energy generated by chemical bond fomiation can be chaimelled into the kinetic energy of the desorbed dissociation fragment. An example of atom abstraction involves the reaction of molecular halogens with Si surfaces [27, 28]. In this case, one halogen atom chemisorbs while the other atom is ejected from the surface.  [c.295]

The most basic model for chemisorption is that developed by Langmuir. In the Langmuir model, it is assumed that there is a finite iiumber of adsorption sites available on a surface, and each has an equal probability for reaction. Once a particular site is occupied, however, the adsorption probability at that site goes to zero. Furthemiore, it is assumed that the adsorbates do not difhise, so that once a site is occupied it remains iimeactive until the adsorbate desorbs from the surface. Thus, the sticking probability S goes to zero when the coverage, 0, reaches the saturation coverage, 0q. These assumptions lead to the following relationship between the sticking coefficient and the surface coverage.  [c.296]

Langmuir adsorption adequately describes the behaviour of many systems in which strong chemisorption takes place, but it has limitations. For one, the sticking at surface sites actually does depend on the occupancy of neighbouring sites. Thus, sticking probability usually changes with coverage. A connnon observation, for example, is that the sticking probability is reduced exponentially with coverage, i.e.  [c.297]

If adsorption occurs via a physisorbed precursor, then the sticking probability at low coverages will be enhanced due to the ability of the precursor to diflfiise and find a lattice site [30]. The details depend on parameters such as strength of the lateral interactions between the adsorbates and the relative rates of desorption and reaction of the precursor. In figure Al.7,8 an example of a plot of S versus 0 for precursor mediated adsorption is presented.  [c.298]

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.  [c.298]

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. 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.
Clusters can undergo a variety of chemical reactions, some relevant only to clusters. The simplest reaction involving clusters is association, namely the sticking of ligands to an ionic core. For association to occur, the ion-neutral complex must release energy either by radiating or by collision with an inert third body. The latter is an important process in the earth s atmosphere [128. 129, 130 and 131] and the fonner in interstellar clouds [101]. Cluster ions can be fonned by photon or electron interaction with a neutral cluster produced in a supersonic expansion [132]. Another process restricted to clusters is ligand-switching or the replacement of one ligand for another. Often exotiiennic ligand-switching reactions take place at rates near the gas kinetic limit, especially for small values of n [72, 133]. Chemical-reactivity studies as a function of cluster size show a variety of trends [93, 127. 133]. Proton-transfer reactions are often unaffected by solvation, while nucleophilic-displacement reactions are often shut down by as few as one or two solvent molecules.  [c.816]

The first two of these we can readily approach with the knowledge gained from the studies of trappmg and sticking of rare-gas atoms, but the long timescales involved in the third process may perhaps more usefiilly be addressed by kinetics and transition state theory [35].  [c.906]

Andersson S, Wiizen L, Persson M and Harris J 1989 Sticking in the quantum regime H2 and D2 on Cu(IOO) Phys. Rev. B40 8146  [c.916]

U ] Guo X-C, Bradley J M, Hopkinson A and King D A 1994 O2 interaction with Pt 100 -hexR0.7°—scattering, sticking and saturating Surf. Sc/. 310 163  [c.919]

It is widely accepted that the rate-detemrining step in NH synthesis is the dissociative adsorption of N2, depicted in a Lennard-Jones potential energy diagram in figure A3,10,17 [46], This result is clearly illustrated by examining the sticking coefficient (the adsorption rate divided by the collision rate) of N2 on different Fe crystal faces (figure A3,10,181 [48], The concentration of surface nitrogen on the Fe single crystals at elevated temperatures in UHV was monitored with AES as a fiinction of N2 exposure. The sticking coefficient is proportional to the slope of the curves in figure A3.10.18. The initial sticking coefficients increase in the order (110) < (100) <(111), which is the same trend observed for the ammonia synthesis catalytic activity at high-pressure (20 atm). This result indicates that the pressure gap for annnonia synthesis can be overcome the kinetics results obtained in UHV conditions can be readily extended to the kinetics results obtained under high-pressure reaction conditions.  [c.945]

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  [c.946]

Not only does the sticking coefficient increase, but with the addition of potassium as a promoter, N2 molecules are bound more tightly to the surface, with the adsorption energy increasing from 30 to 45 kJ moN A consequence of the lowering of tlie N2 potential well is that the activation energy for dissociation (E in Figure A3.10.17 ) also decreases. Thus, the promotion of Fe annnonia synthesis catalysts by potassium appears to be primarily an electronic effect.  [c.946]

Figure A3.10.19 Variation of the initial sticking codFicient of N2 with increasing potassium surface concentration on Fe(lOO) at 430 K [50], Figure A3.10.19 Variation of the initial sticking codFicient of N2 with increasing potassium surface concentration on Fe(lOO) at 430 K [50],
Tribological phenomena have been known to mankind since prehistorical times when friction between wooden sticks was used to produce fire. The first historical record of tribology described the use of lubrication in the  [c.2740]


See pages that mention the term Sticking : [c.180]    [c.60]    [c.159]    [c.310]    [c.686]    [c.706]    [c.706]    [c.706]    [c.296]    [c.303]    [c.500]    [c.903]    [c.2722]    [c.2810]   
Computational methods in surface and colloid science (2000) -- [ c.388 , c.389 ]