IA-1271 VACUUM TECHNOLOGY PART I. A. ROTH •/ ' I *^*1 v, I , << / l\« 1 lktfsi»?AiV8ffi/ri IA-127I Israel Atomic Energy Commission A. ROTH Vacuum Technology October 1972 582 p. This is the text of a Postgraduate Course given by the author at the Faculty of Engineer­ ing of the Tel-Aviv University, After an introduction dealing with the main applications and history of vacuum technology, the course discusses relevant aspects of rarefied gas theory, and treats in detail molecular, viscous and intermediate flow through pipes of simple and complex geometry. Further chapters deal with relevant physico- chemical phenomena (evaporation-condensation, sorptlon-desorption, permeation), pumping and measuring techniques, and special techniques used for obtaining and maintaining high vacuum (sealing techniques, leak detection). (Parts I & II). VACUUM TECHNOLOGY PART I A. Roth Israel Atomic Energy Commission October 1972 Head Vacuum Technology Dept. Soreq Nuclear Research Centre I CONTENTS Page 1. Introduction 1 1.1 The vacuum 1 1.11 Artificial vacuum 1 - Vacuum ranges 4 - Composition of the gas 4 1.12 Natural vacuum , 6 Vacuum on earth 6 Vacuum in space 6 1.2 Fields of application and importance 7 1.21 Applications of vacuum techniques 7 1.22 Importance of vacuum technology 13 1.3 Main stages in the history of vacuum techniques .... 14 1.4 Li terature sources 18 2. Rarefied gas theory for vacuum technology • 25 Commonly used symbols 25 2.1 Physical states of matter 27 2.2 Perfect and real gas laws 34 2. 21 Boyle' s law 34 - McLeod's gauge 35 2.22 Chales1 law 37 2.23 The general gas law 38 2.24 Molecular density 42 2.25 Equation of state of real gases 44 2.3 Motion of molecules in rarefied gases 46 2.31 Kinetic energy of molecules 46 2.32 Molecular velocities 49 2.33 Molecular incidence rate 51 2.4 Pressure and mean free path 53 2.41 Mean free path 53 2.42 Pressure units 57 II Page 2.5 Transport phenomena in viscous state 61 2.51 Viscosity of a gas 61 2.52 Diffusion of gases 65 - Diffusion pump (principle) 66 2.6 Transport phenomena in molecular state 68 2.61 The viscous and molecular states 68 2.62 Molecular drag 70 - Time to form a monolayer 71 - Molecular pump (principle) 71 - Molecular gauge (principle) 72 2.7 Thermal diffusion and energy transport 73 2.71 Thermal transpiration 73 2.72' Thermal diffusion 74 2.73 Heat conductivity of rarefied gases 75 -. Heat conductivity in viscous state 75 - Heat conductivity in molecular state 77 - Thermal conductivity gauge (principle) 82 Appendix • • 83 3. Ga» flow at low pressures , 87 Connonly used symbols •. • < 67 3.1 Flow regimes, conductance and throughput B9 3.11 Flow regimes 89 - The Reynold number 90 -. The Knudsen number 91 3.12 Conductance 92 - Parallel and series connection 94 3.13 Throughput and pumping speed 95 3.2 Viscous and turbulent flow 99 3.21 Viscous flow-conductance of an aperture 99 3.22 ViBcoua flow-conductance of a cylindrical plpe-Polseuille'a law 103 3.23 Viscous flow-surface slip 107 3.24 Viscous flow-rectangular cross section 108 3.25 Viscous flow-annular cross section 110 3.26 Turbulent flow Ill Ill Page 3.3 Molecular flow 112 3.31 Molecular flow-conductance of an aperture .... 112 3.32 Molecular flow-conductance of a diaphragm .... 113 3.33 Molecular flow-long tube of constant cross section , 115 - Circular cross section ,.....,,.,,.,.......... 117 - Rectangular cross section 117 - Triangular cross section ,,.......,.,,.,...... 118 - Annular cross section 118 3.34 Molecular flow-short tube of constant cross section 119 - Circular cross section 120 - Rectangular cross section 121 - Annular cross section 121 3.4 Conductance of combined shapes .*...-...„........... 122 3.41 Molecular flo^-tapered tubes 122 - Circular cross section 124 - Rectangular cross section 125 3*42 Molecular flow-elbows 125 3.43 Molecular flow-traps 126 3.44 Molecular flow-optical baffles 133 - Conductance of baffles with straight plates .. 134 - Conductance of baffles with concentric plates 135 3.45 Molecular flow-seal interface ..* 138 3.5 Analydico-Btatistlcal calculation of conductances... 142 - Transmission probability for elbows .......... 147 - Transmission probabiliry for annular pipes — 148 - Transmission probability for baffles 149 3.6 Intermediate flow — 154 3.61 Knudsen * s equation , 154 3.62 The minimus conductance — ...... 155 3.63 The transition pressure 157 3.64 Limits of the intermediate range 158 3.65 General equation of flow 159 3.66 The viscous-molecular intersection point ..... 160 3.67 Integrated equation of flow 164 IV Page 3.7 Calculation of vacuum systems 16B 3.71 Sources of gas in vacuum systems 168 3.72 Pumpdown In the viscous range 170 3.73 Pumpdown in the molecular range 174 3.74 Steady state with distributed gas load 178 3.75 Nomographic calculations 181 4» Physico-chemical phenomena in vacuum techniques 187 4.1 Evaporation-condensation 187 4.11 Vapours in vacuum systemB 187 4.12 Vapour pressure and rate of evaporation 188 4.13 . Vapour pressure data 190 4.14 Cryopumping and vacuum coating 195 - Cryopumping 195 - Vacuum coating 200 4.2 Solubility and permeation 203 A.21 The permeation process 203 4.22 Permeation through vacuum envelopes 208 4.23 Consequences of permeation 211 4.3 Sorption 215 4.31 Sorption phenomena 215 4.32 Ad sorption energies 215 4.33 .Monolayer and sticking coefficients .......... 221 4.34 Adsorption isotherms 224 4.35 True surface 226 4.36 Sorption of gases by ahsorbants 229 - Sorption by activated charcoal 229 - Sorption by zeolites 231 - Sorption by silica gel 232 4.4 DeBorptlon-outgassing <•• 233 4.41 Desorptlon phenomena 233 4.42 Outgaseing 234 4.43 Outgaesing rates 238 V Page 5. Production of low pressures 243 5.1 Vacuum pumpB 243 5.11 Principles of pumping 243 5.12 Parameters and classifications 244 5.2 Mechanical pumps 248 5.21 Liquid pumps 248 5.22 Piston pumps 250 5.23 Hater ring pumps . 252 5. 24 Rotating-vane pumps 253 - Gas ballast , 257 5.25 Sliding-vane pumps 261 5.26 Rotating-plunger pumps 264 5.27 Roots pumps 265 5. 28 Molecular pumps 267 5.3 Vapour pumps 269 5.31 Classification 269 5.32 Vapour ejector pumps 271 5.33 Diffusion pumps 274 - Pumping speed 274 - Ultimate pressure 276 - Roughing and backing 277 - Pump fluids 279 - Fractionating pumps , 282 - Back streaming and back-migration 283 - Characteristic curves 284 5.4 Ion pumps 286 5.41 Classification 286 5.42 Ion pumping 287 5.43 Kvapor-±on pumps ?PS - Small evapor-ion pumps 289 - Large evapor-ion pumps 290 - The Orbitron pump - 292 5.44 Sputter-ion pumps 294 VI 5.5 Sorption pumps 298 5. SI Mature of sorption pumping 296 5.52 The sorption pump 302 5.53 Multistage sorption pumping 303 5.6 Cryopumping 308 5.61 Cryopumping mechanism 308 5.62 Cryopumping arrays 316 5.63 Cryotrapping 320 5.64 Cryopumps 323 5.65 Liquid nitrogen traps 324 5.7 Gettering 328 5.71 Gettering principles 328 5-72 Flash getters , 331 5.73 Bulk and coating getters 334 5.74 Gettering capacity 336 5.8 Pumping by dilution 337 5.9 Measurement of pumping speed 333 5.91 Methods of measurement 338 5.92 Constant pressure methods 338 5.93 Constant volume methods 343 5.94 Measurement of the pumping speed of mechanical and diffusion pumps 344 6. Measurement of low pressures 347 6.1 Classification and selection of vacuum gauges 347 6.2 Mechanical gauges 349 6.21 Bourdon gauge 349 6.22 Diaphragm gauges 349 6.3 Gauges using liquids 354 6.31 U-tube manometers 354 6.32 Inclined manometers 355 6.33 Differential manometers 356 6.34 The Dubrovln gauge , 356 VII Page 6.35 The McLeod gauge 359 - Sensitivity and limitations 359 - Raising systems 365 Forma of McLeod gauges 367 6.4 Viscosity (molecular) gauges 371 6.41 The decrement gauge 371 6.42 The rotating molecular gauge 373 6.43 The resonance type viscosity gauge 374 6.5 Radiometer (Knudsen) gauge 374 6.6 Thermal conductivity gauges 377 6.61 Thermal conductivity and heat losses 377 6.62 Pirani gauge 379 6 ,63 The thermocouple gauge 382 6.64 The thermistor gauge , 384 6.65 Combined McLeod-Pirani gauge 385 6.7 Ionization gauges 385 6.71 The discharge tube 385 6.72 Hot-cathode ionization gauges 386 - Principles 386 " Common ionization gauge 389 - Bayard-Alpert gauge 392 - Lafferty gauge 392 - Klopfer gauge , 395 6.73 Cold-cathode ionization gauges 396 - Penning gauge 396 - The inverted magnetron gauge 397 - Redhead magnetron gauge 398 6.74 Gauges with radioactive sources 399 6.8 Calibration of vacuum gauges 401 6.81 General 401 6.82 McLeod gauge method 401 6.83 Expansion method ,... 401 6.84 Plow method 402 6.85 Dynamical method - 403 VIII 6.9 Partial pressure measurement: ....................... 404 6.91 General 404 6.92 Magnetic Reflection mass spectrometers 405 6.93 the trochoidal mass spectrometer 408 6.94 The omegatron 409 6.95 The Farvifcron 430 6.96 The quadrupola 412 6.97 Time-of-flight mass spectrometers ............ 413 7. Hifth, vacuum technoloj&y 415 7.1 Criteria for selection o£ materials ................ 415 7.11 General 415 7.12 Mechanical strength , 415 7.13 Permeability to gases 417 7.14 Vapour pressure and gas evolution ............ 417 7.15 Working conditions 417 7.16 Metal vessels and pipes 418 7.17 Glass vessels and pipes •. • 419 7.18 Elastomer and plastic pipes ....-..........•«• 420 7.2 Cleaning techniques 422 7.21 Cleaning of metals 422 7.22 Cleaning of glass 428 7.23 Cleaning of ceramics 429 7.24 Cleaning of rubber 430 7.25 Baking 430 7.3 Sealing techniques 430 7.31 General* classification » 430 7.32 Permanent sealB 4 31 - Welded seals 431 - Brazed seals 438 - Glass-glass seals ...........................