Vacuum Materials
Vacuum Materials
materials used in vacuum equipment and instruments. The basic requirements of vacuum materials are a low vapor pressure at operating temperatures and the possibility of easy outgassing. In addition, vacuum materials for vacuum-instrument casings must have low gas permeability. Vapor pressure, gassing, and gas permeability are the primary features of vacuum materials that determine the upper limit of the attainable vacuum and the possibility of maintaining it over an extended period of time. Other requirements of vacuum materials are specified according to the type of application. For example, materials for vacuum tubes must have sufficient strength at high temperatures and be good insulators or, conversely, good conductors.
| Table 1. Inorganic construction materials | |||
|---|---|---|---|
| Temperature at vapor pressure of 1.3 MN/m2(10-5 mm Hg) (°C) | Temperature coefficient of linear expansion in internal from ) 0° to 100°C (α × 107) (°C-1) | Melting point (°C) | |
| Metals | |||
| Aluminum............... | 841 | 238 | 658 |
| Copper............... | 934 | 165 | 1083 |
| Iron............... | 1083 | 119 | 1535 |
| Molybdenum............... | 1955 | 55 | 2625 |
| Nickel............... | 856 | 133 | 1452 |
| Palladium............... | 1157 | 116 | 1554 |
| Platinum............... | 1585 | 90 | 1773.5 |
| Silver............... | 751 | 189 | 960.5 |
| Tantalum............... | 2402 | 652 | 996 |
| Titanium (iodide)............... | 1336 | 81 (20°-200°C) | 1725 |
| Tungsten............... | 2564 | 44 | 3410 |
| Zirconium (iodide)............... | 1836 | 54 (20°-200°C) | 1845 |
| Alloys | |||
| L-68 brass............... | — | 184 | 938 |
| Fernico (Covar)............... | — | 45-55 (20°-300°C) | 1450 |
| Monel............... | — | 137 | 1250 |
| Nichrome............... | — | 125 | 1400 |
| Stainless steel | |||
| 1Kh18N9(Ela-1)............... | — | 160 | 1400 |
| 1Kh18N9T (Ela-1T)............... | — | 160 | 1450 |
| Miscellaneous materials | |||
| Electrographite............... | 2129 | 8-18 (linear) | 3800-3900 |
| Fused quartz............... | — | 5 | 1700 |
| Glass............... | — | 30-120 | — |
| Mica (muscovite)............... | — | 30 | 1300 |
| Ceramics | |||
| Alumina............... | — | 46-70 (20°-100°C) | 2000 |
| Magnesia............... | — | 70-80 (20°-100°C) | 1600 |
Vacuum materials are divided into the following basic groups: construction materials, getters (gas absorbers), vacuum oils, and materials used as pressure fluids for pumps and vacuum gauges (for example, mercury), pastes, lubricants, lacquers, and cements. Some properties of the more important vacuum construction materials are presented in Tables 1, 2, and 3.
| Table 2. Organic construction materials | ||||||
|---|---|---|---|---|---|---|
| Gas separation rate at 20°C | Coefficient of gas permeability at 20° C | |||||
 |  |  | ||||
| Helium | Nitrogen | Helium | Nitrogen | |||
| NK natural-base rubber............... | (5-8) × 10-6 | (4-6) × 10-6 | 1.3 × 10-16 | 2.3 × 10-17 | 1.3 × 10-7 | 2.3 × 10-8 |
| SKN-26 synthetic nitrile-base rubber............... | (3-4) × 10-5 | (2-3) × 10-5 | 5.2 × 10-17 | 2.5 × 10-18 | 5.2 × 10-8 | 2.5 × 10-9 |
| SKN-40 synthetic nitrile-base rubber............... | (3-4) × 10-5 | (2-3) × 10-5 | 3.6 × 10-12 | 3.9 × 10-18 | 3.6 × 10-8 | 3.9 × 10-9 |
| SKTV-1 polyvinylsiloxane-base rubber............... | (1-3) × 10-5 | (1-2) × 10-5 (250°C) | — | 2.0 × 10-15 (25°C) | — | 2.0 × 10-6 (25°C) |
| Fluoroplast-4 (Teflon)............... | (4-7) × 10-7 | (3-5) × 10-7 (150°C-250°C) | 2.3 × 10-16 | 8.4 × 10-18 | 2.3 × 10-7 | 8.4 × 10-9 |
| Polyethylene............... | (7-13) × 10-7 | (5-10) × 10-7 | 2.5 × 10-17 | 2.5 × 10-18 | 2.5 × 10-8 | 2.5 × 10-9 |
| Polyethyleneterephthalate............... | (3-7) × 10-8 | (2-5) × 10-8 | 7.2 × 10-18 | 2.7 × 10-20 | 7.2 × 10-9 | 2.7 × 10-11 |
| Polyethylenepolyamine-strengthened ED-5 epoxide resin............... | (3-7) × 10-4 | (2-5) × 10-4 (60°C) | 1.0 × 10’-17 | — | 1.0 × 10-8 | — |
Metals are used in the manufacturing of housings, pumps, valves, casings, electrodes, and gas absorbers. Glass is the basic material for flasks, pipes, tubes, and so on. Tubes and wires are made from synthetic materials (polyethylene, polyterafluorethylene, polyamide, and others) and cured rubber. Vacuum lubricants and pastes serve to seal detachable and fixed connections. Lacquers are used for sealing scratches and coating surfaces, and cements are used for tube bases.
| Table 3. Vacuum lubricants, pastes, lacquers, and cements | |||||
|---|---|---|---|---|---|
| Vapor pressure at 20°C | Fusing temperature (°C) | Maximum operating temperature (°C) | |||
| (N/m2) | (mm Hg) | Purpose (use) | |||
| Lubricants | |||||
| High-vacuum............... | 10-4 | 10-6 | — | -40 to +200 | Sealing valves and ground joints |
| Lucricil............... | 10-3 | 10-5 | 40 | — | ” |
| Ramsay............... | 10-2 | 10-4 | — | 30 | ” |
| Apiezone............... | 10-1 | 10-3 (at 200°C) | 43 | 30 | ” |
| Apiezone............... | 10-5 | 10-10 | 47 | — | Seating of force-fit joints |
| Pastes | |||||
| Picein............... | 10-5 | 10-7 | — | 40 | Sealing ground-glass and polished metal joints |
| Denison............... | 10-3 | 10-5 | — | 60 | |
| Rubber-beeswax compound | 10-1 to 10-2 | 10-3 to 10-4 | 60 | — | |
| Apiezone............... | 10-1 | 10-3 | 45; 85 | — | Sealing fixed connections |
| Mendeleev............... | — | — | — | 50 | |
| Khotinskii cement............... | 10-1 | 10-3 | — | 40 | For tube bases |
| Glyptal varnish............... | 3 × 10-2 | 2 × 10-4 | — | 200 | Sealing abrasions and coating surfaces |
REFERENCES
Bälitskii, A. V. Tekhnologiia izgotovleniia vakuumnoi apparatury, 2nd ed. Moscow-Leningrad, 1966.Lebedinskii, M. A. Elektrovakuumnye materialy, 2nd ed. Moscow-Leningrad, 1966.
E. N. MARTINSON and E. G. PLESHCHENKO