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Vacuum Technology-2 Concepts and Key Points

Vacuum Technology-2 Concepts and Key Points. Jim Jr-Min Lin 林志民 Institute of Atomic and Molecular Sciences Academia Sinica, Taipei, Taiwan 106 原分所 Department of Applied Chemistry National Chiao Tung University, Hsinchu, Taiwan 300 交大應化系. Outlines:. 1

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Vacuum Technology-2 Concepts and Key Points

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  1. Vacuum Technology-2 Concepts and Key Points Jim Jr-Min Lin林志民 Institute of Atomic and Molecular Sciences Academia Sinica, Taipei, Taiwan 106 原分所 Department of Applied Chemistry National Chiao Tung University, Hsinchu, Taiwan 300 交大應化系

  2. Outlines: 1 Mean Free Path of gas molecules: Viscose flow Vs. Molecular flowGas flow: Throughput, Conductance, Pumping speed Pumps: Mechanical, Roots, Turbo, Diffusion, Dry, Ion, Cryo, Gauges: Mechanical, Thermal conductance, Ionization, Chambers: Joints (metal, elastomer), parts 2 Practical concerns: Surface Material: SUS, Al alloy, ceramic, plastic, Baking Virtual leak Leak test UHV

  3. Surface Outgas Concern example: 10-3 mol H2O, 18 mg, liquid 1.8x10-2 cm3; gas 2.4x10-2ℓ at 1 atm If 10-3 mol H2O in chamber surface and if the outgas pressure is 10-7 torr, how long it takes to pump down to 10-8 torr when the pumping speed is 1000 ℓ/s? Initial pumping throughput is 10-7 torr * 1000 ℓ/s = 10-4 torr ℓ/s Total amount of water is 1 atm * 2.4x10-2ℓ = 760 torr * 2.4x10-2ℓ = 1.8 x 101 torr ℓ Assume: outgas rate ∝ water amount ∝ P, i.e.∝ pump out throughput Q=PS i.e. first order rate equation

  4. example of exponential decay same area under both curves

  5. Pump out time constant = t 2.3 t = 115 hr to reduce 10x pressure Oftenfor a chamber with big surface area Big chamber ≠ big surface area

  6. How to minimize surface area Oxide could be porous Remove thick surface oxide: electro polish SUS chamber and parts basic wash (NaOH solution) Al alloy acid wash copper/brass parts sand blast Dirty surface is thicker Cleaning Strong detergent is much more efficient than solvent

  7. Estimate the effect of Baking When temperature rises by 100 oC, outgas rate rises by roughly two orders of magnitude, i.e., 10-5 torr instead of 10-7 torr Initial pumping throughput is 10-5 torr * 1000 ℓ/s = 10-2 torr ℓ/s to P = 10-10 torr, P0/P = 105 Practically, it takes a little bit longer (≲100 hr) ∵ Single exponential delay is only an approximation Deeper water has smaller outgas rate, thus longer t

  8. bake uniformly is important to avoid distortion Aluminum foil on SUS chamber, heating tape on the aluminum foil, another layer of aluminum foil to reduce heat loss Degas ion gauge during baking Clean ion gauge and its surrounding by excess heating Don’t bake oily surface. oil  tar

  9. Estimate the effect of using plastic parts plastic may absorb H2O to 1~2 % w/w Assume 100 g plastic can absorb ~1.8 g H2O = 0.1 mol If the initial outgas pressure is 10-7 torr, t = 5000 hr If the initial outgas pressure is 10-6 torr, t = 500 hr more troublesome is that most plastics cannot be baked Use only Inert material: Teflon, PE, PP, Kel-F, Viton, Teflon insulated wire High temperature material : polyimide (Vespel, Kapton), Kalrez (O-ring) Less absorption Bakable to 200oC less inert than Teflon

  10. Material outgas (volume outgas) It lasts forever! More than your life! SUS: H2 & CO. SUS316L can be vacuum firing at 1000 oC to remove deeper contaminants Al alloy: less H2 & CO. Bakable to 120 oC Zn & Cd alloy have high vapor pressure High temperature increase outgas @ bake out Cooling can reduce outgas @ use

  11. Sealing Concern: 100% seal low outgas bakable Metal seal: copper gasket & ConFlat flange are preferred two surfaces may fuse together use silver plated screws in SUS taps convenient non-consuming O-ring seal: Viton O-ring bakable to 100 oC 15 ~ 18 % compression to seal volume compression is not allowed sealing surface polish is important small leak is possible (Hard to find small leaks) Careful to use viton gasket on conflat flanges very easy to leak for size larger than 4.5” not cheap

  12. Dynamic Seal without differential pumping 760 torr * L = 10-7 torr * 1000 ℓ/s L = 1.3 x10-7ℓ/s with differential pumping 760 torr * L = P2 * 0.1 ℓ/s P2 = 1 x10-3torr P2*L=P3*1000 ℓ/s P3= 1.3x10-13 torr a virtual leak 760 torr 760 torr rotatable or translational 0.1 ℓ/s 760 torr O-ring may trap gas & water ⇒ Virtual leak P2 P3 10-7 torr O-ring: 15  2 % compression + Grease polished surfaces to have the above leak rate careless work makes 1~2 orders worse 1000 ℓ/s

  13. Virtual Leak: Gas trapped in a vacuum system, can’t be found from outside 1 cc at 1 atm = 760 x 107 cc at 10-7 torr = 7.6 x 106ℓ at 10-7 torr 1x10-7 torr 1 cc at 1 atm pumped at 1000 ℓ/s, leak: 1.32x10-7ℓ/s Estimate pump down time constant if leak out rate is 1.32x10-7ℓ/s and the chamber is pumped at 1000 ℓ/s, that is, 760 torr x 1.32x10-7ℓ/s = 10-7 torr x 1000 ℓ/s 10-1 pressure drop takes 2.3t O-ring can trap small bubbles and release them when being moved.

  14. Leak Check Spread CH3OH or C2H5OH on a possible leak to see if pressure rises Acetone is OK for metal, bad for O-ring, bad for health Response rise time ~ few seconds, Don’t move too fast. It takes very long to dry out the solvent. Very long fall time From lower spots to higher spots. Helium leak check: Spread He to see if PHe rises MASS is required. RGA or He leak detector fast ≲ 1 sec ∵ light mass ∴ fast speed He is fast to escape, fast to pump down low background inert from higher spots to lower spots He is easy to reach a nearby spot. ⇒ isolation

  15. RGA provide very important information high m28/m32 (4:1) indicate air leak daughter ion is useful. CO+ C+ O+ Vs. N2+ N+ H2O is very common Don’t make vacuum chamber wet, especially at a rainy day

  16. Good vacuum practices No leak Clean: traps for oil pumps: molecular sieve, LN2 Metal & non-porous ceramic is excellent Plastic and grease: as less as possible Confident sealing. Finding a leak is labor consuming. Bakable for 10-10 torr or better Good local conductance for pumping speed Gas composition (partial pressure) is often more important than the total pressure, as most vacuum parameters are species dependent.e.g. surface laser burn, background masses RGA is very nice to have

  17. Example of utilizing a sorption pump to recycle isotope gases, see §3.4.3 O2 (18O, 97%, > NT$10000/ℓ) ℓ: std liter (1 ℓ at standard condition, i.e. 298K, 760 torr) gas inlet If pump 2 = mechanical pump, O2 will be mixed with air and oil. If pump 2 = molecular sieve sorption pump, O2 can be absorbed at liquid nitrogen temperature and retrieved at room temperature. P1= 1x10-5 torr f 25mm Question: How much O2 (in atm ℓ) is used per hour? pump 2 turbo pump 1 2000 ℓ/s f 6mm molecular sieve

  18. Quiz: Write answer on a page of A4 paper A mixture of 10% C4H10 in He flows into a vacuum chamber pumped by a turbo pump. The Ion Gauge reading is 5x10-6 torr. (a) What is the true pressure? (b) What is the pressure reading for a 20% mixture at the same flow rate? Note: Given that the relative electron impact cross section: s(N2)=1, s(He)=0.15, s(C4H10)=5

  19. Practically clean chamber, turbo pump, not baked, 10-9 torr clean chamber, 2 serial turbo pumps, baked, 10-11 torr (compression ratio for H2) Untra High Vacuum < 1x10-10 torr Example of 1 x 10-12 torr

  20. He cold head (remove displacer to bake) 50K <10K room temperature 5x10-11 torr LN2 3x10-11 torr (much cleaner) cold head 1x10-12 torr LN2 78K

  21. How to make Ultra High Vacuum (UHV): Outgas rate = pumping speed x pressure 100 ℓ/s x 10-11 torr 2 serial Turbo Pumps: 10-11 torr, H2 compression ratio Ion Pumps : 10-11 torr, small pumping speed, memory effect Getter Pumps: Titanium Sublimation pump, Non-evaporative getter Cryopump: 10-10 torr, not-bakable, memory effect Ultimate pressure of pumps Outgas rate Materials: Stainless Steel, Al alloy, OFHC copper, ceramic, teflon, kapton Surfaces: mechanical polish (glue on sand paper!), electro polish, acid, base Bake & Contamination: Firing, porous oxides, oil→tar→charcoal Low temperature: LN2, cryohead Effective pumping speed S-1 = S1-1 + S2-1 Virtual Leaks

  22. Turbo pumps: 400 ℓ/s, 600 ℓ/s compression ratio: 104 for H2, 106 for He,109 for N2 150oC Oil, grease, or magnetic bearing and insulated wires Fore line back stream when Electric shutdown Getter pumps: Ion, Ti, NEG No foreline needed, no continuous electricity needed 250oC, 400oC Not for every gas, memory effect Low maintenances for low load systems

  23. Cryopumps: >1500 ℓ/s, bake to 70oC, Not for every gas, memory effect Outgas due to activated carbon absorber Electric shutdown Cryopump + turbo pump: Very high pumping speed even for H2 at 10-9 torr Bakable Cryohead without absorber: high pumping speed for H2 at <10-11 torr Low outgas  10-12 torr

  24. SUS304, SUS304L: Cr 18%, Ni 10%, Fe, C<0.2 % or Low carbon < 0.08% SUS316, SUS316L: Cr 18%, Ni 10%, Mo 2-3%, std and Low carbon Sand ballasting, basic detergent, Acid dip, Electro polish, DI water Easy to be welded, Bake to 250oC, SUS316L: Fire at 1000oC at 10-8 torr Major outgas: H2, CO Al alloy 6061-T6, and others Low H2, CO outgas Welding at outside, must clean before welding NaOH(aq), HNO3(aq), Al2O3 is porous. Mirror finish parts is available (Japan) 120oC, high temperature will change tempering condition

  25. Oxygen Free High Conductance copper, Beryllium copper Brass and bronze could be dirty (zinc, phosphor) Special (strong) acid brightening Plastic: gas/water permittivity is high Teflon absorb water 10 times less than usual plastics, but still too much for UHV Teflon, PE, PP, might be OK for 10-8 torr, others are only good to 10-6 torr. polyimide (Kapton) is bakable, 10-10 torr Ceramic could be porous Al2O3 (alumina) (thermal conductance better than SUS) Vacuum firing

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