Lectures Contents:

1. KINETIC MOLECULAR THEORY2. FLOW REGIMES: KNUDSEN NUMBER:3. TYPES OF SURFACE INTERACTIONS:4. OUT GASSING:5. PERMEATION:

*****************

1. KINETIC MOLECULAR THEORY

The Kinetic Molecular Theory is the theoretical background of vacuum physics. It is used to explain the physical principles of VST, with some basic assumptions that the molecules:

Are Small sphere Do elastic collision With no force of attraction Having negligible volume Travel randomly in straight line Remain in constant random motion

2. FLOW REGIMES (ray-zheem): Knudsen Number:

KNUDSEN NUMBER

It is used to find the nature of flow of the molecules inside the vacuum chamber.It is actually the ration/share/allowance between mean free path and the characteristic dimension of the molecules.

Kn= ʎ/Dʎ = Mean Free PathD = Characteristic Dimension There are three flow regime of the molecules

1. Viscous/Laminar/Continuum : Kn <0.012. Molecular: Kn >13. Transitional : 0.01 < Kn < 1

1

1. Viscous/Laminar/Continuum Flow

At the atmospheric pressure, the (many) molecules move in a near-compact manner thereby providing a flow and direction to the current. The predominant direction of all the gas molecules is the same as the macroscopic direction of the gas current, as noted in the figure below. The flow is governed by the viscous forces and is constant over time;

Where r indicates the radius of the tube, l indicates length, η indicates the dynamic viscosity of the gas, and p indicates the various pressuresDistance between molecules is small; collisions between molecules dominate; flow through momentum transfer; generally P greater than 0.1 mbar

When the motion of molecules is such that the molecules collide with themselves and the walls of container is called as Viscous flow i-e, S-M-M collision (Surface of container-Molecule-Molecule)

Knudsen number for this regime should be less than 0.01, this always occurs in low vacuum

2

2. MOLECULAR FLOW

Distance between molecules is large; collisions between molecules and wall dominate; flow through random motion; generally P smaller than 10 mbarThis flow obtains where the density of the molecules is lower; that is, in the presence of high vacuum level conditions, and, in terms of mean free path, the probability of collision between molecules is lower than the probability of molecule-wall collision.

In this regime the collision is only between molecules, because of very high vacuum the density of molecules decreases that causes less collision with the surface.

Where R indicates the constant of ideal gases, T indicates the absolute temperature, M indicates relative mass, d indicates the diameter of the piping, l indicates length, and p

indicates pressures.

3

3. TRANSITIONAL FLOW

Region between viscous and molecular flow

A formula, as sum of the contributions of the other two regimes, may be arrived at

When the motion of molecules is such that the molecules collide with themselves and the walls of container is called as Viscous flow i-e, M-M-S collision (Molecule-Molecule-Surface)

Knudsen number for this regime should be between 0.01 to 1.0, this occurs in medium vacuum

Table: table provides a breakdown of the regions of the vacuum with reference to pressure, indicating characteristics and typical effects.

P (mbar)[Pressure]

λ (cm) [mean free path]

t (s) Formation of Mono Layer

Flow Nature Typical Effects

Low vacuum 1 < P < 103 λ < 10-2 t < 10-5 Laminar/Continuum/Viscous

Convection dependent pressure

Intermediate Vacuum

10-3 < P <1 10-2 < λ < 10

10-5< t < 10-2 Transitional Significant changes in the thermal conductivity of gas

High Vacuum 10-7 < P <10-3 10 < λ < 105 10-2< t < 100 Molecular Significant reduction in the rate of impact

Ultra High Vacuum

P <10-7 λ >105 t >100 Molecular Dominate the effects of the surface

4

3. TYPES OF SURFACE INTERACTIONS:

When gas molecules interact with a solid surface, several phenomena can take place:ADSORPTION: Capture of molecules by the surface.BACKSCATTERING: Bouncing back of molecules impinging on the surface.DESORPTION: Emission of molecules by the surface.DIFFUSION: Penetration of adsorbed atoms from the surface into the solid bulk.DISPLACEMENT: Displacement of an adsorbed molecule by another impinging molecule.SURFACE REACTION: Formation of new molecules at the surface from the adsorbed molecules of different species.

4. OUT GASSING: The outgassing is defined as evolving of the gas from the solid surface. The outgassing rate can be taken as quantity of gas evolving from a unit area of a solid

surface in unit time. For a solid surface such as the chamber wall, the outgassing rate depends on the

surface's composition condition, vacuum and temperature. A well-pumped, well-baked vacuum-compatible material has a much lower outgassing

rate than the same material recently put under vacuum.

5

5. PERMEATION:

Permeation describes the flow of gases through a solid, Gases penetrate through some solid materials such as elastomers (O-rings). This happens in three steps.

- First gases dissolve into polymeric phase from high press side- In the second step these gases migrate towards the lower pressure (vac.) side.- Finally gases flow away from polymeric material.

6