Oxygen mean free path

In Figure 2 we presented the permeability coefficient K of oxygen as a function of the mean gas pressure experimentally obtained for a sample of porous material from acetylene black modified with 35% PTFE. The experimental linear dependence is obtained. The intercept with the abscissa corresponds to the Knudsen term DiK. The value obtained is 2,89.1 O 2 cm2/s. The slope of the straight line is small, so that the ratio K,/ Dik at mean gas pressure 1 atm. is small ( 0.1) which means that the gas flow is predominantly achieved by Knudsen diffusion and the viscous flow is quite negligible. At normal conditions (1 atm, 25°C) the mean free path of the air molecules (X a 100 nm) is greater than the mean pore radii in the hydrophobic material (r 20 nm), so that the condition (X r) for the Knudsen-diffusion mechanism of gas transport is fulfilled. 

The mean free path X, of a molecule in air can be calculated from the sizes of the molecules involved. The most probable collision partners for a trace molecule (such as CFC-12) in air are molecular nitrogen (N2) and oxygen (02). The trace molecule i is hit whenever its center gets closer to the center of an air molecule than the critical distance, rcrit = r, + rair (Fig. 18.8). Picturing the molecules as spheres, the molecular radius r, can be estimated from the collision cross-section A listed in chemical handbooks such as the Tables of Physical and Chemical Constants (Longman, London, 1973) ... 

Bodenstein makes the calculation in the following way, which is probably sufficiently accurate. The chance that an oxygen molecule hits a nitric oxide molecule actually in collision with one of its own kind bears the same ratio to the chance that it hits a single nitric oxide molecule as the molecular diameter bears to the mean free path. This gives the ratio of ternary to binary collisions as 10-8/10-5 or 1 to 1,000. The number of collisions between oxygen and nitric oxide molecules under the experimental condi-... 

For the hole of the diameter larger than the mean free path of O2 diffusion, the flux of oxygen, Jo, is given as... 

In the device of Fig.6a (sensor with distributed cavity), depending on the dimensions of the pores of the diffusion barrier, the oxygen diffusion can be bulk diffusion or Knudsen diffusion(ll). In the former case, the sensor output (limiting current Ig) is given by Eq. (9). Knudsen diffusion occurs when the average pore diameter is much smaller than the mean free path of the gas molecules, in which case collisions between molecules and pore walls are the dominant events. In this case(121. Dq -k T /2,and... 

Example For oxygen gas at 25 C, calculate, (a) the mean free path at 1 atm pressure, (b) the mean free path at I Ch3 mm Hg pressure, (c) the number of collisions per second per molecule,... 

The analytical depth profiling for these systems (e.g. the polystyrene data is shown in Figure 5) revealed that the reaction is essentially confined to the topmost monolayer of material ( ). This is entirely reasonable in terms of the plasma chemistry since the most prominent reactive species is atomic oxygen f ich is expected to have an extremely short mean free path in hydrocarbon polymers. This serves as a very good example of the powerful nature of XPS when applied to the study of the surface modification of polymers. 

Calculate the mean free path in a sample of oxygen gas at 27°C and 1.0 atm. Solution... 

This equation shows that the tunnel effect is possible. In addition, the emitted carrier has the energy of the order of e E(IX, where X is the mean free path. This energy is a little less than 1 eV, and the positive hole with this energy can react with water to yield H+, and evolve oxygen, after Eq. 6. Two protons react with the two trapped electrons at the micro cracks to evolve hydrogen. 

A CVD process is generally performed in a vacuum condition. The two main factors that require a vacuum condition are (1) to remove or minimise the active atmospheric constituents that could cause undesirable physical or chemical reactions (especially oxygen and water vapour) and (2) to improve the coating uniformity by increasing the mean free path of precursor gases. As the pressure is decreased from its atmospheric value of almost 760 Torr to 0.5 to 1 Torr, the free mean path increases by a factor of 1000, as shown by Equation (2.10). 

Assuming that the molecules are rigid elastic spheres a typical value for the mean free path for a gas, say oxygen, can be calculated from (2.516). Consider a typical room temperature at 300 (K) and a pressure of 101325 (Pa). The collision diameter of molecular oxygen can be set to 3.57 x 10 ° (m) in accordance with the data given by [51], example 1.4. We can then calculate the mean free path / for oxygen ... 

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