Lecture Contents:

1. Vacuum improves with altitude

2. Why Vacuum is necessary?

3. Why Perfect vacuum cannot be created?

4. Relationship between several concepts that defines Degree of

Vacuum:

---*****---1. Vacuum Improves with Altitude (give three natural vacuum examples)

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Cont. Last Lecture

---*****---

2. Why Vacuum is necessary? …… (P R O D C)

Because.

To Provide the clean environment

To Reduce the atmospheric load

To Offer the moisture less condition

To Decrease atmospheric particle flux/density.

To Control process accordingly

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3. Why Perfect vacuum cannot be created?

Perfect vacuum with a gaseous pressure of absolute zero value is only a philosophical concept that is never observed in practice due to the following reasons:

1. The walls of a vacuum chamber emit light in the form of black-

body radiation (photon at particular temperature & pressure).

2. Out gassing of the container material.

3. There is no such a vacuum pump, that can produce and maintain

vacuum of the range 10 - ∞ mbar.

---*****---

4. USEFUL CONCEPTS and Properties That Characterize the Degree of Vacuum: Molecular Density, Mean Free Path &Time for Monolayer.

1) Molecular density: Average number of molecules per unit volume.

Physical

Situation

Objective: Application

Low

molecular

density

Remove active

atmospheric

constituents.

Remove occluded or

dissolved gas.

Decrease energy

transfer.

Lamps (incandescent, fluorescent,

electric discharge tubes), melting,

sintering, packaging etc

Drying, dehydration, concentration,

freeze-drying, degassing, etc

Thermal insulation, electrical

insulation, vacuum microbalance,

space simulation

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2) Mean free path (λ, Lamda): or Molecular Mean Free Path

Average distance travelled by a molecule before hitting another one (ternary and higher-order collisions are negligible) OR

The average distance between successive/consecutive collisions is the MEAN FREE PATH (mfp). OR

Physical Situation Objective: ApplicationLarge mean free

path

Avoid collision. Electron tubes, cathode ray tubes,

television tubes, photocells,

photomultiplier. X-ray tubes,

accelerators,

storage rings, mass spectrometers,

isotope separators, electron

microscopes, electron beam welding,

heating, coating (evaporation,

sputtering) etc.

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For Nitrogen at around 20oC, the mean free path is as shown.

3) Time to form a Monolayer:

The time for one atomic layer of gas to adsorb on the surface. OR Time required for freshly cleaved surface to be covered by a layer of gas of

one molecule thickness. OR The time to saturate a surface with one layer of molecules is called the

monolayer formation time. It depends on molecular size and arrival rate, the temperature and pressure

of a gas. This time is given by the ratio between the number of molecules required to

form a compact monolayer (about 8 x 1014 molecules/ cm2) and the molecular incidence rate.

Physical Situation

Objective: Application

Long monolayerformation time

Clean surfaces.

Fraction, adhesion, emission studies, materials testing for space

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Summary:

Sticking Coefficient: S = number of adsorbed / number of incident At 3 x 10-5 Torr, it takes about one second for a monolayer of gas to

adsorb on a surface assuming a sticking coefficient, S = 1. At 10-9 Torr, it takes 1 hour to form a monolayer for S = 1. For most gases at room temperature S<<1, so the monolayer time is

much longer.

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