Ultra-alto vuoto - Dipartimento di Fisica

Fisica e Tecnologia del vuoto
Vuoto: definizione e condizioni sperimentali
Cinetica molecolare (gas perfetto,
distribuzione di Maxwell)
Regimi di flusso, capacità e impedenza
(Fluidodinamica)
Tecniche da vuoto: misura di pressione e
pompe da vuoto
Vuoto: definizione e condizioni sperimentali
Basso vuoto 105 — 102 Pa
1atm=1.01315 x 105 Pa
Vuoto medio 102 — 10-2 Pa
Alto vuoto 10-2 —10-6 Pa
Ultra-alto vuoto < 10-6 Pa
Vuoto estremo < 10-10 Pa
L’atmosfera lunare: H2, He, Ne, Ar pressione totale
10-6 Pa
Teoria cinetica
●
Consideriamo un gas come N particelle indipendenti
e non interagenti contenute in un volume V, in moto
casuale con velocità media v
Volume del recipiente = V
Numero di molecole = N
Densità = N/V
PV=NKBT
p=NKBT
PV=NKBT
VM = 22.4 l
at 273 K
p= n KBT
and atmospheric pressure
N= 6.02 x 1023
Maxwell-Boltzmann Distribution
Some results from Kinetic Theory
1
mν
2
Average kinetic energy
Average velocity
Pressure
ν =
8kT
=
πm
2
3
= kT
2
8 RT
T
= 145
πM
M
p = nkT
Pressure
(mbar)
Mean free
path (m)
103
6 x 10-8
1
6 x 10-5
10-3
6 x 10-2
10-6
6
10-10
6 x 105
Definitions of Vacuum Regimes:
1.) Rough Vacuum: ~0.1-760 torr (atmospheric pressure is 760 torr)
Pa
2.) Medium Vacuum: ~ 0.1 to 10-4 torr
3.) High Vacuum: ~ 10 –4 torr 10 -8 torr 10-6 Pa
4.) Ultrahigh Vacuum: < 10-8 torr
1atm=1.01325x105
Gas flow:
1.) Viscous Flow regime: gas density (pressure) is high enough, many molecule-molecule
collisions occur and dominate the flow process (one molecule “pushes” another). Collisions with
walls play a secondary role in limiting the gas flow.
2.) Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions
occur and molecule-chamber wall collisions dominate the flow process (molecules are held back
by wall)
Fisica e Tecnologia del vuoto
Vuoto: definizione e condizioni sperimentali
Cinetica molecolare (gas perfetto,
distribuzione di Maxwell)
Regimi di flusso, capacità e impedenza
(Fluidodinamica)
Tecniche da vuoto: misura di pressione e
pompe da vuoto
Vuoto: definizione e condizioni sperimentali
L’atmosfera lunare: H2, He, Ne, Ar pressione totale
10-6 Pa
Plots of relevant vacuum features vs. pressure
Residual Gas
CO
H2O
O2
CO2
CH4
N2
Solid Surface
Bulk Solid
Adsorbed Atoms & Molecules
Why to Work in Ultra High Vacuum?
Why to Work in Ultra High Vacuum?
If the sticking probability of an atom or a molecule hitting a solid surface is approximately
equal to 1, one monolayer (ML) of atoms or molecules from the residual gas is adsorbed at the
surface in:
1 sec
@ p = 1 x 10-6 mbar
100,000 sec
@ p = 1 x 10-11 mbar
Utra High Vacuum (UHV):
p = 10-10-10-11 mbar
Teoria cinetica
●
Consideriamo un gas come N particelle indipendenti
e non interagenti contenute in un volume V, in moto
casuale con velocità media v
Volume del recipiente = V
Numero di molecole = N
Densità = N/V
PV=NKBT
p=NKBT
Kinetic Theory
●
●
Mean free path λ
λ
Pressure
(mbar)
Mean free
path (m)
103
6 x 10-8
1
6 x 10-5
10-3
6 x 10-2
10-6
6
10-10
6 x 105
Kinetic Theory
The pressure, p, exerted
on the walls of the vessel
depends on the
molecular impingement
rate or flux, J
Maxwell-Boltzmann
Distribution
PV=NKBT
VM = 22.4 l
at 273 K
p= n KBT
and atmospheric pressure
N= 6.02 x 1023
Maxwell-Boltzmann Distribution
Some results from Kinetic Theory
1
mν
2
Average kinetic energy
Average velocity
Pressure
ν =
8kT
=
πm
2
3
= kT
2
8 RT
T
= 145
πM
M
p = nkT
Pressure
(mbar)
Mean free
path (m)
103
6 x 10-8
1
6 x 10-5
10-3
6 x 10-2
10-6
6
10-10
6 x 105
Definitions of Vacuum Regimes:
1.) Rough Vacuum: ~0.1-760 torr (atmospheric pressure is 760 torr)
Pa
2.) Medium Vacuum: ~ 0.1 to 10-4 torr
3.) High Vacuum: ~ 10 –4 torr 10 -8 torr 10-6 Pa
4.) Ultrahigh Vacuum: < 10-8 torr
1atm=1.01325x105
Gas flow:
1.) Viscous Flow regime: gas density (pressure) is high enough, many molecule-molecule
collisions occur and dominate the flow process (one molecule “pushes” another). Collisions with
walls play a secondary role in limiting the gas flow.
2.) Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions
occur and molecule-chamber wall collisions dominate the flow process (molecules are held back
by wall)
Perchè ultra-alto-vuoto?
Roughing pumps
• Rotary vane (a.k.a. mechanical, roughing) – positive
displacement
• Sorption (e.g., contains zeolite cooled with LN2)
• Diaphragm dry and membrane pumps – zero oil
contamination
• Scroll pumps (the highest speed dry pump)
Turbomolecular Pumps
Gas molecules are accelerated from the vacuum
side to the exhaust side
Depends on impact processes between the
pumped molecules:
Molecular mass of the gas
rotor velocity ( not as good for He and H2)
• Ti
sublimation getter pump
• Ion getter pump
+ Effective for H2, H2O and CO
- need HV to start
J=
p
=
2π mkT
pN A
2π MRT