Transcript Instrumental Analysis - Jagiellonian University

World famous
Surface chemists
Professor Gabor
A. Somorjai
Department of
Chemistry,
University of
California, Berkeley
 Developing low-energy electron diffraction (LEED)
for surface crystallography.
 Using LEED, high-resolution electron energy loss
spectroscopy (HREELS), and sum frequency
generation (SFG) to identify the bonding of
hydrocarbons as being similar to that in
organometallic clusters.
 the development of molecular surface science at
high pressures, pioneered the use of monolayer
sensitive techniques that could be used for
molecular studies at the solid-gas and solid-liquid
interface using high pressure-high temperature
STM and SFG.
Editor-in-chief of Catalysis Letters and serves on
the editorial board of eight other journals
 Published more than 775 papers in surface


sciences, heterogeneous catalysis, and solid state
chemistry;
Received several honorary degrees from several
international universities.
Educated a generation of leading scientists in
the field, including 93 Ph.D. students and 112
postdoctoral fellows.
Professor
Gerhard Ertl
Director, FritzHaber-Institut der
Max-PlanckGesellschaft,
Berlin
 Fundamental reactivity knowledge of
catalytic mechanisms gained from the
modern surface science approach.
 Correlate catalytic reactivity with the
structure and composition of the
heterogeneous catalytic surface as in
ammonia synthesis.
 Further applied this knowledge base to
the synthesis of specific microstructures
on the surface to carry out specific
reactions in high selectivity.
 Studies on carbon monoxide oxidation on
specific Pt crystallographic planes revealed the
dynamics of the oscillatory behavior of the
chemisorbed surface species.
 The development of the instrumentation that
makes these observations possible is regarded as
a breakthrough in surface science and a key step
in developing our understanding of the very
rapid and dynamic changes that many
heterogeneous catalyzed reactions experience.
UHV system
Vacuum Technology
Vacuum Technology
How to construct a simplified
Ultra-high Vacuum (UHV)
system?
 The need for Ultra-high
Vacuum
 Vacuum theory and
pumpimg laws
 Measurement of Pressure
 Vacuum pumps and their
characteristics
 Simplified Vacuum System
Design & Construction
Why is ultra-high vacuum
(UHV) necessary?
time” ~
the time it takes to contaminate a surface
with a single layer of molecular
adsorbates
 the monolayer time can be estimated as:
t = 4.2 ×10-6 / P
where t ~ seconds, P ~ Torr.
 “Monolayer
Want 1 hour to do an
experiment?
1 atmosphere
1 atmosphere
1 atmosphere
1 torr
1 micron Hg
1 millibar
1 torr
1 millibar
=
=
=
=
=
=
=
=
1.0133 bar
760 torr
1.0133×105 Pa
1 mm Hg
1 milliTorr
100 Pa
133.32 Pa
0.75 Torr
The pressure needed for for one hour to
“monolayer time” is equal to P < 1×10-9 Torr
Base pressure?
At least
-10
P < 2× 10 Torr
A common unit for “gas dose”
 Langmuir
(L) is defined as,
an exposure of gas at room temperature
at a pressure of P = 1× 10-6 Torr for 1
second (L = 10-6 Ts)
If one monolayer is created for 1 L exposure,
one should get 1 monolayer in one hour at a
pressure of P = 10-6 / 3600 = 3 ×10-10 Torr.”
Vacuum theory and
pumping laws
How the vacuum
is created?
Production of vacuum

to reduce gas density in given volume to below
atmospheric pressure with pump

enclosed vessel has continuous sources which
launch gas into volume and present pump with
continuous gas load

vacuum achievable at steady state is result of
dynamic balance between gas load and ability
of pump to remove gas form volume
Vacuum Theory using Ideal
Gas Properties

Mean velocity of a gas molecules of mass
M, at absolute temperature T, is given by
At T = 0 oC
He ~ 1200 m/s;
Ar ~ 380 m/s
N2 ~ 453 m/s;
H2O ~ 564 m/s

Mean free path, which is used to define the
various regions of gas flow, is given by
for air at R.T.,  (mm) = 6.6/P, P in Pa
 Particle flux, or the number of particle
striking a surface per unit area, or passing
through an imaginary plane of unit area, is
given by

The pressure, according to ideal gas law, is
given by
P = nkT

For a fixed volume containing a mixture of
different non-interacting gases,
The Three Regions of Gas Flow
When /d << 1, the flow is vicious, where
the vicious force is independent o the
pressure.
 When /d >> 1, it is in the free-molecular
flow regime, where the vicious drag is
linearly proportional to pressure.
 A third regions of gas-flow, Knudsen or
transition flow, is often used to describe the
region between these two limits.

Molecular Transport and Pumping
Laws
Three parameters P, S, Q
 P:
pressure [Torr]
 S: volumetric flow [liter/sec]
 Q: throughput [Torr·liters/sec]
Q[Torr·liters/sec] = P[Torr]S[liter/sec]



Complete pumping equation is
Q = SP + VdP/dt
No pumping (S = 0), just a closed
chamber with a constant gas load from
outgassing and/or leaks.
P = (Q/V)t
Negligible outgassing or other leaking
sources, Q = 0, corresponding to Q << SP,
P = Poe-(S/V)t
Pumping law in the High and
Ultra-high vacuum regions



The ultimate pressure is the behavior of the
gas load over time.
In the HV and UHV region, the pressure
decrease with time (no leaks!),
P(t2) = P(t1)(t1/t2)
The final base pressure is related to some
ultimate values of Q and S,
Po = Qo/So
Sources of Gases in
Vacuum Systems

Leaks through vacuum vessel.

Virtual leaks from trapped gas
volumes.

Vaporization of volatile material.

Surface outgassing from adsorbed
gases on walls of vessel.

Volume outgassing from diffusion
of dissolved gases in bulk material
of vessel.

Permeation through porous
material or seals of vessel.

Backstreaming of volatile fluids
from pump.
Idealized initial pumpdown of a 100 L
system, size 50×50×40 cm, with a
roughing pump and UHV pump.
The UHV region can only be
achieved by bakeout.
So, it
drops!
Measurement of pressure




Mechanical phenomena gauges: measure
actual force exerted by gas (e.g. manometer).
Transport phenomena: measuring gaseous drag
on moving body (e.g. spinning rotor gauge) or
thermal conductivity of gas (e.g. thermocouple
gauge).
Ionization phenomena gauges: ionize gas and
measure total ion current (e.g. ion gauge).
Partial pressure residual gas analyzers:mass
spectrometers.
Vacuum gauges must
calibrated by



Comparison with absolute standard
calibrated from its own physical
properties.
Attachment to calibrated vacuum
system.
Comparison with calibrated
reference gauge.
Vacuum gauges used in vacuum
systems
Ultrahigh
High
Medium
Low
(< 10-7 Torr) (10-3 ~ 10-7 Torr) (1 ~ 10-3 Torr) (760 ~ 1 Torr)
Manometer
Manometer
Thermocouple Thermocouple
Ionization
Mass
spectrometer
Spinning rotor Spinning rotor
Ionization
Mass
spectrometer
Thermocouple gauge
For roughing vacuum (molecular flow regime)
measurements
Ionization gauges
Thermionic/hot cathode ionization gauges.
 Energetic beam of electrons (constant I-)
used to ionize gas molecules and produce ion
current.
I+ = p KI -, K: ion gauge sensitivity
 Upper pressure limit (10-3 Torr): secondary
ion ionization excitation, filament burn out.
 Lower pressure limit (10-10 Torr): secondary
electron current from X-ray emission.

Diagram of an ion gauge for
measuring UHV
阳极
阴极
收集极
Residual gas analyzers




More compact mass spectrometers with higher
sensitivity.
Gaseous ions formed in ion source box by
electron bombardment, extracted with suitable
fields, separated in analyzer and then collected
and measured.
Magnetic sector analyzer: masses separated by
static magnetic and electric fields.
Quadrupole mass analyzer: masses separated in
oscillating quadrupolar electric field.
The RGA 100 Residual Gas Analyzer
Quadrupole Mass Filter
Components
Principles of Filter Operation
Residual Gas Analysis
Vacuum pumps and their
characteristics

Gas transfer pumps:
(a) Positive displacement pumps that transfer
repeated volumes of gas from inlet to outlet
by compression ( e.g. rotary pump).
(b) Kinetic pumps that continuously transfer
gas from inlet to outlet by imparting
momentum to gas molecules (e.g. Diffusion
pump, turbomolecular pump).

Entrapment/capture pumps,
retain molecules by sorption or
condensation on internal surfaces
(e.g. sorption pump, sublimation
pump, sputter ion pump, cryogenic
pump).
The different vacuum pumps
Ultrahigh
(< 10-7 Torr)
High
Medium
Low
(10-3 ~ 10-7 Torr) (1 ~ 10-3 Torr) (760 ~ 1 Torr)
Rotary
Sorption
Diffusion
Diffusion
Turbomolecular Turbomolecular
Sublimation
Sputter ion
Cryogenic
Sublimation
Sputter ion
Cryogenic
Rotary
Sorption
1. Roughing pumps (1 atmosphere
to 1-10 micron)
Rotary vane (oil) mechanical pumps
low cost, durable, long life
high pumping speed
oil-backstreaming must be controlled
 Cryosorption pumps (sorption pumps)
very clean
inexpensive and simple
limited capacity, frequent reconditioning

Rotary vane mechanical pumps
Sorption pumps
The sorption pump has no moving parts and therefore
no oils or other lubricants. (5 liters of liquid nitrogen)
2. Diffusion pumps (high
vacuum and UHV)
Low cost per unit pumping speed, very
high pumping speeds
 Very well understood
 Hard to destroy
 Continuous operating expense (LN2)
 Potential for serious vacuum accidents
 “Open system”:Forbidden in certain
applications

3. Turbomolecular pumps (high
vacuum and UHV)





Medium to high cost per unit pumping speed
Very clean, pumps rare gases
Requires periodic maintenance which can be
expensive
Difficult to reach very low UHV base
pressures
“Open system”:Forbidden in certain
applications
A typical turbomolecular pump
 High vacuum port
 Three-phase motor
 Water-cooling
 Stator package
 Rotor
 Motor shaft
 Ball bearing
 Lubrication duct
 Fore-vacuum port
4. Titanium sublimation
pumps (HV and UHV)




Very inexpensive and simple
Requires periodic maintenance, which
is cheap
Often misused, which limits their
performance
Selective in what it pumps (good for
oxygen, N2, air, not for rare gases)
A typical titanium sublimation pump
5. Cryopumps

Expensive per unit pumping speed

Very high pumping speeds are possible

Pumping hydrogen (pumps everything)

Requires periodic recharging

Vibration can be a serious problem
6. Ion pumps (also called
sputter-ion pumps)

Expensive per unit pumping speed

Low pumping speed

Generates hydrocarbons

Has a memory effect

Very low maintenance

Moderately difficult to destroy

Excellent base pressures



Does not pump rare gases well
Does not pump hydrogen
Closed system: very safe against vacuum
accidents
A typical
ion-pump
An ‘ideal’ UHV work-station
consists of several types of pump
used in different applications



a cryo-sorption pump or trapped rotary
pump for initial pumpdown from
atmosphere
a turbo-molecular pump to pump rare
gases, assist in initial pump-down, and to
pump load-locks
an ultra-high vacuum pump. Depending on
the application, this can be an ion-pump/Ti
sublimation pump, or a diffusion pump.
The baking of an UHV system
Simplified vacuum system
design

Materials for ultra-high vacuum

Construction materials for UHV

Common vacuum problems
Materials for Ultra-high
vacuum
Properties required:
(a) Low vapor pressure.
(b) Bakeable to > 200 oC without
losing mechanical strength.
(c) Impermeable to gases.
(d) Inert towards reaction with other
materials in system or vacuum
process.
(e) Inert towards irradiation by
electromagnetic or particle beams.
(f) Easy machining and fabrication into
suitable components.
Metals
(a) Stainless steel:
 Excellent all round material.
 Distortion during welding.
(b) Aluminum and aluminum alloys:
 Good corrosion resistance, easily machined
and jointed.
 Poor strength at high temperatures, high
distortion when welding .
(c) Nickel alloys:

High strength at high temperatures, excellent
corrosion resistance.

High cost, machining problems.
(d) Copper:

Easily machined, good corrosion resistance,
especially oxygen free, high conductivity
grade (OFHC) material.

Difficult to braze in hydrogen atmosphere.
(e) Brass:
 Good
corrosion resistance.
 Zinc
evaporates out at temperatures
above 100 oC.
(f) Mild steel:
 Not
generally used as it is liable to rust.
Plastics
(a) PTFE: low outgassing rate, good electrical
insulator, heat resistant, self lubricating.
(b) Polycarbonate: moderate outgassing rate
and water absorption, good electrical
insulator.
(c) Nylon and acrylic: high outgassing rate and
water absorption rates, self lubricating.
(d) PVC: high outgassing rate and water
absorption rates.
(e) Polyethyene: only suitable if well outgassed.
(f) Nitrile rubber: easily jointed, sealing rings.
(g) Viton: low outgassing, heat resistant, sealing
rings.
Common vacuum problems

Improper cleaning techniques

Incompatible materials

Leaks

Virtual leaks