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 –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

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 = 1.01x10 5 Pa = kPa = 14.7 psi = 34 ft. water Average atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…

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!

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

“Kinds of Pressure” 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

Measurement Techniques 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

Bourdon Gauge (Mechanical)

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”.

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

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)

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

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

Ionization gauges Hot cathode: more sensitive; less forgiving Cold cathode: less sensitive; more forgiving

Chambers et al. P.84

Roughing pump comparisons: Oil Sealed Pumps TypeAdvantagesDisadvantages Rotary vaneLow ultimate pressure. Low cost Long pump life. Backstreams oil. Produces hazardous waste. Rootes LobeVery high pumping speed Frequent maintenance. Requires a purge gas. Requires a backing pump. Must be absolutely horizontal. Rotary pistonHigh volume Low cost Noise. Vibration Safety Valve.

Roughing pump comparisons: Dry Roughing Pumps ScrollClean. Low "dry" ultimate pressure. Easily serviceable Quiet. Technology is well known. Limited bearing life. Limited scroll life. Permeable to small gases. Not hermetically sealed. Clean applications only. DiaphragmLow cost. Quiet. Easily serviced. Low pumping speed. High ultimate pressure. Frequent service required. Hook and Claw No backstreaming. Low ultimate pressure Expensive Screw rotorLow ultimate vacuum. Less maintenance than hook & claw Expensive Dry pistonLow ultimate pressureExpensive SorptionCleanRequires LN2.

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

Sorption Pump Clean: no oil Very inexpensive: 170,000 Torr-liters for $ l LN2 Requires LN2 Air adsorbs onto zeolite at 77K Torr capability

Oil Vapor Diffusion Pump Vacuum system Robust (silicone oil!) Low maintenance: no moving parts Requires backing – Torr

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

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

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

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). gauges.php