World famous Surface chemists

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World famous Surface chemists
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Professor Gabor A. Somorjai Department of
Chemistry, University of California, Berkeley
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• 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.

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Editor-in-chief of Catalysis Letters and serves
on the editorial board of eight other journals
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Professor Gerhard Ertl Director,
Fritz-Haber-Institut der Max-Planck-Gesellschaft,
Berlin
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• 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.

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• 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.

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Vacuum Technology

• How to construct a simplified Ultra-high Vacuum
  (UHV) system?

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Why is ultra-high vacuum (UHV) necessary?

• Monolayer 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.

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Want 1 hour to do an experiment?
1 atmosphere 1.0133
bar 1 atmosphere
760 torr 1 atmosphere
1.0133105 Pa 1 torr
1 mm Hg 1 micron Hg
1 milliTorr 1 millibar
100 Pa 1
torr 133.32 Pa
1 millibar
0.75 Torr
The pressure needed for for one hour to
monolayer time is equal to P lt 110-9 Torr
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Base pressure?

• At least
• P lt 2 10-10 Torr

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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.
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Vacuum theory and pumping laws
How the vacuum is created?
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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

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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
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• 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

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• The pressure, according to ideal gas law, is
  given by
• P nkT
• For a fixed volume containing a mixture of
  different non-interacting gases,

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The Three Regions of Gas Flow

• When ?/d ltlt 1, the flow is vicious, where the
  vicious force is independent o the pressure.
• When ?/d gtgt 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.

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Molecular Transport and Pumping Laws

• Three parameters P, S, Q
• P pressure Torr
• S volumetric flow liter/sec
• Q throughput Torrliters/sec
• QTorrliters/sec PTorrSliter/sec

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• 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 ltlt SP,
• P Poe-(S/V)t

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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

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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.

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• 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.

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Idealized initial pumpdown of a 100 L system,
size 505040 cm, with a roughing pump and UHV
pump.
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The UHV region can only be achieved by bakeout.
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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 analyzersmass
  spectrometers.

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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.

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Vacuum gauges used in vacuum systems
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Thermocouple gauge
For roughing vacuum (molecular flow regime)
measurements
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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.

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Diagram of an ion gauge for measuring UHV
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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.

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The RGA 100 Residual Gas Analyzer
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Quadrupole Mass Filter Components
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Principles of Filter Operation
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Residual Gas Analysis
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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).

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• Entrapment/capture pumps,
• retain molecules by sorption or condensation
  on internal surfaces (e.g. sorption pump,
  sublimation pump, sputter ion pump, cryogenic
  pump).

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The different vacuum pumps
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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

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Rotary vane mechanical pumps
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Sorption pumps
The sorption pump has no moving parts and
therefore no oils or other lubricants. (5 liters
of liquid nitrogen)
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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 systemForbidden in certain applications

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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 systemForbidden in certain applications

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A typical turbomolecular pump
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? High vacuum port ? Three-phase motor ?
Water-cooling ? Stator package ? Rotor ? Motor
shaft ? Ball bearing ? Lubrication duct ?
Fore-vacuum port
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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)

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A typical titanium sublimation pump
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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

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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

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• Does not pump rare gases well
• Does not pump hydrogen
• Closed system very safe against vacuum accidents

A typical ion-pump
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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.

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The baking of an UHV system
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Simplified vacuum system design

• Materials for ultra-high vacuum
• Construction materials for UHV
• Common vacuum problems

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Materials for Ultra-high vacuum
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Properties required

• (a) Low vapor pressure.
• (b) Bakeable to gt 200 oC without losing
  mechanical strength.
• (c) Impermeable to gases.

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(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.
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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 .

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• (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.

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• (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.

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Plastics

• (a) PTFE low outgassing rate, good electrical
  insulator, heat resistant, self lubricating.
• (b) Polycarbonate moderate outgassing rate and
  water absorption, good electrical insulator.

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• (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.

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Common vacuum problems

• Improper cleaning techniques
• Incompatible materials
• Leaks
• Virtual leaks