Vacuum Technology

the aggregate of methods and devices for obtaining, maintaining, and controlling a vacuum.

The historical development of physics and chemistry, and also of several branches of industry, has been inseparable from the development of vacuum technology. Heron of Alexandria (probably in the first century A.D.) described devices that may be considered prototypes of the pneumatic mechanisms later used for the creation of rarefaction. The first experiments with a vacuum were conducted in the 1540’s. In 1654 the German scientist O. von Guericke conducted his experiment with Magdeburg hemispheres, vividly demonstrating the existence of atmospheric pressure. The pump he used was the first pump for obtaining a vacuum.

Figure 1. Gaede molecular pump.

The production of the incandescent light bulb (in 1879) brought about further development in vacuum technology. A significant contribution was made by the German scientist W. Gaede. In 1905 he first used the rotary mercury pump, in 1913 he created the first molecular pump (Figure 1), and in 1915 he published an account of the diffusion pump (Figure 2). In 1916 the American scientist Langmuir created the condensate removal mercury-vapor pump (Figure 3).

Figure 2. The first diffusion pump.

The rapid development of vacuum technology is associated with the development of electronics, nuclear power engineering, and accelerator technology. Modern achievements in the field of vacuum distillation, the widespread use of vacuum-metallurgical and vacuum-chemical processes, work in the field of controlled thermonuclear reactions, the technology of obtaining thin films and ultrapure materials for spacecraft, and the testing of spacecraft under conditions close to those of outer space all became possible because of the high developmental level of modern vacuum technology. The First International Congress on Vacuum Technology took place in June 1958 in Belgium, resulting in the creation of the International Union for Vacuum Science, Technique, and Applications.

Figure 3. Langmuir’s first condensate removal mercury-vapor pump.

A vacuum system or vacuum unit consists of a vessel connected to vacuum pumps, a vacuometer, vacuum accessories, leak detectors, and other devices. The type of vacuum pump for maintaining a vacuum during the performance of a given process is selected on the basis of its operating pressure range, ultimate pressure, and evacuation rate in a given range (see Figure 4). The order for obtaining a high vacuum is as follows: mechanical pre-evacuation pumps are

Figure 4. Regions of operation of various vacuum pumps (in N/m²): (1) water-ring pumps, (2) reciprocating pumps, (3) oil-diffusion booster pumps, (4) mechanical booster pumps, (5) diffusion pumps, (6) getter ion pumps.

used from atmospheric pressure to 10^-1 newtons per sq m (N/m²), or 10^-3 mm of mercury (mm Hg); diffusion pumps are used to reduce pressure to 10^-5 N/m² (10^-7 mm Hg); and getter ion pumps are used to reduce pressure to 10^-9 N/m²(10^-11 mm Hg). It is impossible to obtain pressures on the order of 10^-6-10^-7 N/m² (10^-8-10^-9 mm Hg) and less without the preliminary removal of gas from the walls of the vessel being evacuated.

In series connection of pumps, the amount of gas is Q = p₁s₁ = p₂s₂ = … = p_is_i, where p_i is the inlet pressure and s_i is the evacuation rate. The pumps are selected in such a way that the inlet pressure in each successive pump decreases and never reaches the maximum outlet pressure of the preceding pump. The completeness of utilization of the pumps in a vacuum is determined by each pump’s evacuation rate (s_p) and by the resistance of the channel connecting the pump to the vacuum system element being evacuated. Thus, the effective evacuation rate is

where u = 1/w is the transmissive capacity of the vacuum channel, a quantity that is the inverse of the channel’s resistance (measured in units of evacuation rate, l/sec). Consequently it is always the case that s_e < s_p; s_e < u.

The following dependence relationship exists between the amount of gas flowing through the vacuum duct Q = p_is_i, the transmissive capacity u of the vacuum duct, and the difference between pressures at its ends: Q = u(p₂ - p₁). The value of u is usually determined by the nature of the gas, its state, the geometry of the vacuum duct, and the conditions of gas flow.

In units in which the required evacuation rate is so great that it cannot be achieved by pumps mounted outside the vessel being evacuated, the absorptive properties of an atomized metal such as titanium are utilized in a process analogous to that occurring in getter ion pumps. One or more evaporators are mounted inside the vessel being evacuated; these evaporators deposit titanium on the inner walls of the chamber. To remove a gas that is not absorbed by titanium, a diffusion pump is connected to the vessel being evacuated.

One of the tasks of vacuum technology is the measurement of low pressures (down to 10^-12 N/m² [10^-14 mm Hg] and lower) and the attainment of a hermetic state in the vacuum system, particularly at the locations where its individual elements are joined. The measurement of extremely low pressures requires special apparatus. Leakage is monitored by special leak detectors.

Vacuum technology is widely used both in industry and in laboratory practice. For example, the mass production of various types of electronic vacuum equipment is inseparable from the improvement of methods of achieving and maintaining a high vacuum. The production of this equipment requires the removal of gases (outgassing) and the use of getters for the preservation of the vacuum. The vacuum treatment of such equipment is performed by automatic multiposition rotary exhaust pumps. The electronic devices pass through positions for mounting, evacuation, warm-up, outgassing (in order to remove adsorbed gases from the inner surfaces), atomization of the getting substances, unsoldering, and removal. The purification and separation of high-molecular-weight organosilicon compounds, polymerization products, oily petroleum fractions, complex esters, alcohol, concentrates of vitamins, and other products are all carried out in a vacuum of 10^-1 N/m² (10^-3 mm Hg). Outgassing, the treatment of insulating materials and cables, the sealing of transformers and condensers, and the drying of materials (such as plastics) that cannot be dried at atmospheric pressure are carried out in a vacuum. Heat-sensitive substances such as egg albumen, enzymes, mother’s milk, antibiotics, bacterial cultures, and vaccines are dried (at room temperature or higher) and freeze-dried under vacuum conditions. Vacuum pumps are used to remove solvents from substances that cannot be heated (for example, explosives) and to increase the concentration of solutions.

The vacuum has found application in the thermal and cathodic atomization of metal for plating and metallizing various materials—for example, in the production of optical and household mirrors, Christmas toys, reflectors for automobile and airplane lights, and metal and plastic decorations. The processing of cloth during dyeing, the metallization of paper, ceramics, and the matrices of phonograph records and semiconductor materials, and the deposition of protective and decorative films are carried out under vacuum in the operating pressure range between 10^-2 and 10^-4 N/m²(10^-4-10^-6 mm Hg).

In metallurgy the recovery of metals from ores and the reduction of their chemical compounds and the smelting, refining, and degassing of metals are carried out in a vacuum. The processes of smelting, evaporation, and distillation of metals in a vacuum form the basis for obtaining materials of high purity. Highly efficient multiplate steam-ejector pumps and booster pumps (steam-jet and mechanical) are used in metallurgy to create vacuums with an operating pressure as low as 10^-2 N/m² (10^-4 mm Hg).

In modern experimental physics, the techniques of vacuum technology permit the operation of electrophysical instruments and devices in which the movement of beams of charged particles is produced. The study of the physical properties of solid-body surfaces and also certain research requiring gases of high purity are possible only under ultrahigh vacuum conditions.

In devices in which the volumes to be evacuated are in the hundreds of cubic meters, continuous evacuation is accomplished by means of a great number (up to several dozen) of highly efficient pumps in parallel operation, with an evacuation rate of tens to hundreds of cubic meters per second. In addition to diffusion pumps, getter ion pumps, which have a high evacuation rate and a residual pressure of less than 10^-8N/m² (10^-10 mm Hg), are widely used.

The solution of many complex scientific and technological problems requires the attainment of pressures of 10^-14 N/m²(10^-16 mm Hg) and lower, as well as the measurement of such pressures. This in turn requires advanced measuring instruments, highly sensitive methods of checking hermeticity, the creation of adequate seals in the ultrahigh vacuum apparatus, and the preparation and purification of the surfaces of the vessels being evacuated so that these surfaces will not emit contaminating gases.