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Why Is There Vacuum? (The sequel to Bill Cosby’s “Why Is There Air?”)

Why Is There Vacuum? (The sequel to Bill Cosby’s “Why Is There Air?”). Matthew C. DeLong University of Utah OptoElectronic Materials Laboratory 7 January 2008. Low: 1 atm to 1 Torr Medium: 10 -3 ( 1 m m) to 1 Torr High: 10 -8 to 10 -3 Torr Ultra High: 10 -12 to 10 -8 Torr

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Why Is There Vacuum? (The sequel to Bill Cosby’s “Why Is There Air?”)

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  1. Why Is There Vacuum?(The sequel to Bill Cosby’s “Why Is There Air?”) Matthew C. DeLong University of Utah OptoElectronic Materials Laboratory 7 January 2008

  2. Low: 1 atm to 1 Torr Medium: 10-3 ( 1 mm) to 1 Torr High: 10-8 to 10-3 Torr Ultra High: 10-12 to 10-8 Torr Extreme: < 10-12 Torr Note: low vacuum ↔ high pressure Drying, drinking straws Sputtering Thermal evaporation, e-gun, SEM STEM, FIM, AES, SIMS Anti-particle accumulators, space simulation Ranges of Vacuum

  3. Pressure: Units of Measure • Pressure exerted by a column of fluid: • P ≡ F/A = mg/A = ghA/A = gh  h • 1 Atm (mean sea level) = 760 Torr = 1013 mBar = 1.01x105 Pa = 101.3 kPa = 14.7 psi = 34 ft. water • Average atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…

  4. Gauge Pressure: measured with respect to ambient. Absolute pressure: measured with respect to vacuum Car tires, basketballs, boilers, LN2 tanks, JFB compressed air supply… Vacuum systems, cathode ray tubes, light bulbs, barometers “Kinds of Pressure”

  5. Low Medium High Ultra High Extreme Mechanical (Bourdon), Hg column, capacitance Thermocouple, Pirani Ionization [hot and cold (Penning) cathode] Ionization (hot cathode: Bayard-Alpert) Modulator Bayard-Alpert Measurement Techniques

  6. Bourdon Gauge (Mechanical)

  7. Capacitance Manometer • A = Annular electrode • D = Disk electrode • S = Substrate • G = Getter (in vacuum space) • Differential capacitance between annulus and disk depends on pressure difference between Test Chamber and “Getter”.

  8. Heat Transfer of Gases • Conductivity is linear in pressure over about 2 orders of magnitude. • Molecular flow regime • Pirani and thermocouple gauges

  9. Ionization gauges • Hot cathode: more sensitive; less forgiving • Cold cathode: less sensitive; more forgiving

  10. Chambers et al. P.84

  11. Mean Free Path in Gases With sufficient accuracy for approximate calculations we may take: λ = 7 x 10-3/p mbar-cm λ = 5 x 10-3/p Torr-cm λ = 5/p μmHg-cm

  12. Roughing pump comparisons:Oil Sealed Pumps

  13. Roughing pump comparisons:Dry Roughing Pumps

  14. Rotary Vane Mechanical Pump • Robust • Inexpensive • Operates to ambient pressure • Single stage and two stage

  15. Sorption Pump • Clean: no oil • Very inexpensive: 170,000 Torr-liters for $1000 + 8.5 l LN2 • Requires LN2 • Air adsorbs onto zeolite at 77K • 10-3 Torr capability

  16. Oil Vapor Diffusion Pump • Robust (silicone oil!) • Low maintenance: no moving parts • Requires backing • 10-3 – 10-7 Torr Vacuum system

  17. Turbomolecular Pump • Requires backing: Operates only <1 Torr • Clean: no oil • Expensive: Approximately triple the cost of a rotary vane mechanical pump and oil diffusion pump • Limited lifespan

  18. Getter pump • Low maintenance: no moving parts • 10-4 – 10-10 Torr • Requires backing • Clean: no oil • Based on chemical reaction of “air” with very reactive metals

  19. Vac-Ion Pump (Sputter/Getter) • Clean: no oil • 10-4 – 10-10 Torr • Not cheap! • Require backing

  20. References • A. Chalmers, B.K. Fitch, and B. S. Halliday, Basic Vacuum Technology, IOP Publishing, Bristol (1998). TJ/940/C45/1998. • D. Hucknall, Vacuum Technology and Applications, Butterworth-Heinemann, Oxford (1991). TJ/940/H83 (1991). • Vacuum Equipment, Granville-Phillips Co., Boulder CO. TJ/940/G7. • R. R. LaPelle, Practical Vacuum Systems, McGraw-Hill, New York (1972). • David Joy, “New Lecture 3” on course website.

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