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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 5 March 2009. Applications. In each case: why is vacuum relevant and how is it used? The drinking straw! Food preservation Thin film deposition

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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 5 March 2009

  2. Applications • In each case: why is vacuum relevant and how is it used? • The drinking straw! • Food preservation • Thin film deposition • Thermal (including e-gun) • Sputtering • Purity enhancement • Vacuum sublimation • Protection from ambient atmosphere (Including cryogenic applications) • Thermal insulation • The vacuum thermos bottle! • Cryogenic applications • High temperature applications • Importance of emissivity to both

  3. Pressure: Units of Measure(How is pressure related to vacuum?) • 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. Physics of Using Vacuum to Pump Fluids • How does a drinking straw actually work? • Do you actually “suck” liquid up the straw? • How does a vacuum-based water pump work? • Why is the maximum depth from which you can pump water with a “vacuum pump” about 28 feet in Salt Lake? • This is a physics class: start with a free body diagram!

  5. 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

  6. 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”

  7. 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

  8. Bourdon Gauge (Mechanical)

  9. 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”.

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

  11. Molecular vs. Viscous Flow • Molecular flow • Mean free path length is larger than apparatus dimensions • Molecular collisions are primarily with apparatus walls • Viscous flow • Laminar flow • Parabolic velocity profile • Molecular collisions are primarily with other molecules (in fluid)

  12. 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

  13. Homework 1 • Derive one of the above expressions. • Due Monday 9 March.

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

  15. Chambers et al. P.84

  16. Roughing pump comparisons:Oil Sealed Pumps

  17. Roughing pump comparisons:Dry Roughing Pumps

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

  19. 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

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

  21. 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

  22. 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

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

  24. 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). • http://www.wallaceandtiernan.net/absolute_gauges.php

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