Surface creep

Molecules arrive at the surfaces of traps and baffles by volume flow and surface creep. Molecules are trapped in vacuum systems by binding with energies much greater than kT of the surface, where k is Boltzmann s constant and Tthe absolute temperature, or by lowering the temperature of the surface in such a way that kT is less than the heat of physisorption of a molecular species on a surface. 

We observe that for the bonds depicted in Figs, lb and le, an atom M or an atom R, to which the chemisorbed particle C is attached, are more weakly bound to the lattice than the normal ions M+ or, respectively, Rr. As a result, in some cases we can expect that the molecule CM or CR may evaporate that is, the particle C upon desorption may carry off with it an atom of the lattice, thereby violating the stoichiometric composition of the crystal. In all cases such adsorption should facilitate surface creep which plays such an important role in the sintering, recrystallization, and disintegration of solids in reaction. This may also explain the well-known influence of adsorption on the surface mobility of the adsorbent atoms. 

Figure 3.40. Principle of a surface creep measurement for a viscoelastic monolayer. At t = 0 a constant shear stress is (instantaneously) applied and maintained till t = The shear strain Ax/Ay is followed eis a function of time. At t = t this stress is instantaneously removed. Idealized behaviour.

Surface creep must be very low, both to restrict loss of lubricant from bearings, etc. and to avoid contamination of other components. Creep is a surface tension effect where oil surface tensions are typically in the range 18-30 mN/m. To mitigate surface creep, thermal gradients should be avoided or minimised and the surface finish of components should be optimised. Capillary creep occurs if Ra> 0.1 mm, thermal creep occurs if Ra < 0.6 mm. 

In the lubrication of ball bearings, oil is transported by surface creep from the grease supply to the raceway this creep is favored by the molecular heterogeneity of ordinary petroleum oils, whose more volatile components have lower surface tensions than the total oil mixture. If one imdertakes to lubricate with a radiation-resistant alkyl aromatic oil, however, the situation may be reversed, and the deliberate inclusion of a minor amount of an oil of lower surface tension is indicated. The recession phenomena reported for silicone liquids on a 400° surface remind us that at the high temperatures and temperature gradients existing in some military equipment oil films maybe expected... 

FIGURE 11. Permanent tensile surface creep strain in mullite-SiC whisker composites versus time. Star indicates that the specimen failed. Published with permission of the J. Am. Ceram. Soc. (31)... 

Soil and dust particle mobilization by winds entails physical parameters such as saltation, surface creep, and suspension (Nicholson, 1988a Sehmel, 1980). Saltation, relevant to particles in the size range of 1(X)—500 pm, can move 50—75% of surface particles. Larger particles, 500—1000 pm in size, move by surface creep, up to 25% of all particles. Suspension, the process by which particles enter the ambient air, applies to a size <100 pm and involves up to 40% of soil by weight. 

After extensive investigations, we consistently observed that once the temperature has reached Tg, the sample begins to soften and e tip sinks into the surface (creep). At a constant temperature of 384 K (10 degrees above Tg), after thermal equilibrium was obtained (30 minutes), a load was applied to a film of PS (Mw 6.5 M). A hole is formed, which is 0.65 pm in diameter, figure 7a. The time the cantilever was in contact with the sanple at constant load and tenq)erature will be referred to as the... 

The radiant energy E from a hot surface is given by E = dT A, where d is the emittance of the surface, T is the absolute temperature (Kelvin), and A is the area of the emitting surface. Radiant energy from the hot vaporization source heats all of the surfaces in the deposition chamber, leading to a rise in the substrate temperature, desorption of gases from surfaces, and surface creep of contaminants. Radiant heating of the substrate and interior surfaces can be minimized by ... 

The surface conductivity of glass makes long creeping distances necessary. 

The properties of a single crystal creeping wave probe The a single crystal creeping wave probe is suitable for testing various artificial defects such as surface cracks, FBH, columned hole and SDH etc. and its distance amplitude cruve is shown in Fig.6... 

Flexural stress SiC mpture curves are shown in Figure 3 (27). AU. the forms tend to be fairly resistant to time-dependent failure by elevated temperature creep. In addition, SiC shows outstanding resistance to oxidation even at 1200°C as a result of formation of a protective high purity siUca surface layer (28). 

Skin Patch-Tested Repellents. SmaU areas of human forearms are marked and treated with smaU amounts of repeUent on a unit area basis to ensure that the treatment rate is always the same between subjects (7). The patches are tested at 0 and 4 hours against smaU numbers (ca 15) of mosquitoes. This method does not consider creep, movement of repeUent across the skin surface, or the iateraction between two chemicals owiag to such lateral movement of chemical. 

The role, design, and maintenance of creepproof barriers in traps, especially those in oil DPs, remain to be fully explored. In general, uncracked oil from a DP is completely inhibited from creeping by a surface temperature of <223 K. On the other hand, a cold trap, to perform effectively in an ordinary vacuum system, must be <173 K because of the vapor pressure of water, and <78 K because of the vapor pressure of CO2. For ultracontroUed vacuum environments, LN temperature or lower is required. CO2 accumulation on the trap surface must be less than one monolayer. The effectiveness of a LN trap can be observed by the absence of pressure pips on an ionization gauge when LN is replenished in the reservoir. 

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