COnt..

1. 1
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:
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1. Vacuum Improves with Altitude (give three natural vacuum
examples)
Cont. Last Lecture

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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.
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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: Application
Large 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 monolayer
formation time
Clean
surfaces.
Fraction, adhesion, emission studies,
materials testing for space

6. 6
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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