A Short History of Vacuum Terminology and Technology

1. Vapor Pressure of the Elements, #1 ~190Kb
2. Vapor Pressure of the Elements, #2 ~256Kb
3. Vapor Pressure of the Elements, #3 ~285Kb
4. Vapor Pressure of the Elements, #4 ~255Kb
5. Vapor Pressure of the Elements, #5 ~245Kb
6. Vapor Pressure of the Elements, #6 ~260Kb

• A collection of mathematical biographies taken from ‘A Short Account of the History of Mathematics’ by W. W. Rouse Ball (4th Edition, 1908).
• Vacuum Technology
• • Defining Parameters
  • Applications
  • Research
• Vacuum Equipment
• 1. Rotary Pump
  2. Mechanical Booster
  3. Molecular Pump
  4. Diffusion Pump
  5. Sputter Ion Pump
  6. “Vacuum Basics” from The Belljar via Roy Schmaus’ Page in Alberta

Amadeo Avogadro, conte di Quaregna e Ceretto was born August 9, 1776, in  Turin, Italy and died July 9,1856, also in Turin. He was a physicist who first set forth the hypothesis known as Avogadro’s Law, which states that equal volumes of gasses or ‘vapours’, at the same temperature and pressure contain the same number of molecules. This law explained why gasses chemically combine in simple proportions by volume and led Avogadro to believe that the elements of hydrogen, nitrogen, and oxygen exist as diatomic molecules in nature. A professor of higher physics at the University of Turin for many years, he stated by his hypothesis in 1811, but it was not generally accepted until after 1858, when the Italian chemist Stanislao Cannizzaro constructed a logical system of concepts of modern chemistry. Avogadro’s number is the number of molecules in one gram-molecular-weight of any substance.

AVOGADRO’S LAW, a statement that under the same conditions of temperature and pressure, equal volumes of different gasses contain an equal number of molecules. This empirical relation, proposed by the Italian physicist Amadeo Avogadro in 1811, can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas. The law is approximately valid for real gases at sufficiently low pressures and high temperatures.

The specific number of molecules is given by Avogadro’s constant. In the metre-kilogram-second (mks) system of units, Avogadro’s constant is the number of molecules in one kilogram-mole of a substance, defined as the molecular weight in kilograms. The value of Avogadro’s constant is 6.023 X 10²⁶ molecules per kilogram-mole in mks units or 6.023 X 10²³ molecules per gram-mole in the centimetre-gram-second (cgs) system of units. For example, the molecular weight of oxygen is 32, so that one kilogram-mole of oxygen has a mass of 32 kilograms and contains 6.023 X 10²⁶ molecules.

The volume occupied by one mole of gas is about 22.4 liters at standard temperature and pressure (STP), which are 0^oC and 1 atmosphere pressure, and is the same for all gases according to Avogadro’s law.

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Ludwig Eduard Boltzmann was born February 20, 1844, in Vienna  and died on September 5, 1906, in Duino, Italy. He was a physicist whose greatest achievement was in the development of statistical mechanics, which explains and predicts how the properties of atoms such as mass, charge, and structure determine the visible properties of matter such as viscosity, thermal conductivity, and diffusion.

After receiving his doctorate from the University of Vienna in 1866, he held professorships in mathematics and physics at Vienna, Graz, Munich, and Leipzig. He contributed substantially to the foundation and development of statistical mechanics, a branch of theoretical physics. The Boltzmann constant, k, is a fundamental constant of physics occuring in nearly every statistical formulation of both classical and quantum physics. Having dimensions of energy per degree of temperature, the Boltzmann constant has a value of 1.38062 x 10^-23 joules per degree Kelvin (^o K), or 1.38622 x 10^-16 ergs per ^oK

In the 1870s Boltzmann published a series of papers. He showed that the Second Law of Thermodynamics, which concerns energy exchange, could be explained by applying the laws of mechanics and the theory of probability to the motions of the atoms. In so doing, he made clear that the second law is essentially statistical and that a system approaches a state of thermodynamic equilibrium (equal energy distribution throughout) because equilibrium is overwhelmingly the most probable state in which matter occurs. During these investigations Boltzmann worked out the general law for the distribution of energy among the various parts of a system at a specific temperature.

He also derived the theorem of equipartition of energy known as the Maxwell-Boltzmann law. This law states that the average amount of energy used for each different direction of motion of an atom is the same. He derived the integro-differential equation for the change of the distribution of atoms due to collisions and laid the foundations and much of the structure of statistical mechanics. Boltzmann was also one of the first Europeans to recognize the importance of the electromagnetic theory proposed by James Clerk Maxwell of England. His work on statistical mechanics was strongly attacked by many who did not believe in the atomic theory and wanted to base all of physical science on energy considerations only. There was much misunderstanding of his ideas by those who did not fully grasp the statistical nature of his reasoning. His conclusions were finally supported by the series of discoveries in atomic physics that began shortly before 1900 and by the explanation of fluctuation phenomena, such as Brownian motion which is the random movement of microscopic particles suspended in a fluid. These phenomena in the physical world could be explained only by statistical mechanics. Tragically ill and depressed, Boltzmann took his own life in 1906.

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Robert Boyle was born January 25, 1627, in Listmore, County Cavan in Ireland and died December 30, 1691, in London. He was a chemist and a natural philosopher. He was noted for his pioneer experiments on the properties of gasses and espousal of a corpuscular view of matter. It was a forerunner of the modern theory of chemical elements.

Sent to Eton College in 1635, Boyle read Galileo’s works during a European tour (1639-44). At Oxford he collaborated with Robert Hooke in constructing an air pump with which he conducted the experiments leading to his publication (1662) of the relationship, now known as Boyle’s law, that at constant temperature the volume of a gas is inversely proportional to its pressure.

In The Sceptical Chymist (1661) Boyle attacked the Aristotelian and alchemical views of the composition of matter, proposing that it consists of a single, primary substance that forms clusters, or corpuscles, of varying complexity. Beginning in 1668, he lived in London and continued experimental work on the calcination of metals and the distinction between acids and alkalies. Celebrated during his lifetime, Boyle promoted Christianity abroad and advocated the scientific study of nature as a religious duty. He endowed a series of lectures on the consistency of Christian religion and scientific investigation.

Boyle’s law, called Mariotte’s Law in Europe, is a relation concerning the compression and expansion of a gas at constant temperature. This empirical relation, formulated by Boyle in 1662, states that the pressure (p) of a gas varies inversely as its volume (v) at constant temperature; in equation form, pv=k, and is a constant. The relationship was also discovered by the French physicist Edme Mariotte in 1676.

The law can be derived from the kinetic theory of gasses assuming a perfect (ideal) gas. Real gasses obey Boyle’s law at sufficiently low pressures, although the product pv generally decreases slightly as the gas pressure is raised to appreciably higher values.

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Jacques-Alexandre Charles was born November 12, 1746, in Beaugency, France and died April 7, 1823, in Paris. He was a mathematician, physicist, and inventor. With the Robert brothers, Nicolas and Anne-Jean, he built one of the first hydrogen balloons. Along with Nicolas Robert, he was the first to ascend in a hydrogen balloon in 1783. In the course of several flights he rose to an altitude of more than a mile.

From clerking in the finance ministry, he turned to science and experimented with electricity. He produced several inventions, including a hydrometer and reflecting goniometer, and improved the Gravesande’s heliostat and Fahrenheit’s aerometer. He was elected in 1795 to the Academie des Sciences, Paris, and subsequently became a professor of physics. His published papers mainly concerned mathematics.

About 1787 he developed Charles’ law concerning the expansion of gasses. Charles’ law is a statement that the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature, under conditions of constant pressure. This empirical relation was formulated first by Charles about 1787, and later by Joseph Gay-Lussac. It is a special case of the general gas law, which states that the product of the absolute temperature (t) and a constant equals the product of the pressure (p) and the volume (v) or pv=kt, and can be derived from the kinetic theory of gasses under the assumption of a perfect (ideal) gas. Measurements show that, at constant pressure, the thermal expansion of real gasses is nearly the same at sufficiently low pressure and high temperature, showing that Charles’s law is approximately valid.

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John Dalton was born September 6, 1766, in Eaglesfield, Cumberland, England and died July 27, 1844, in Manchester. A chemist and physicist, he developed the atomic theory of matter and hence is known as one of the fathers of modern physical science.

Dalton, a teacher at the age of 12, spent most of his life in Manchester. He began his studies of meteorology in 1787. He made valuable observations concerning the Aurora Borealis, the trade winds, and the cause of rain. He determined the point of the maximum density of water and published the results of work on color blindness.

In chemistry, his major work began around 1800. Dalton developed the law known as Dalton’s Law of partial pressures. Dalton’s Law states that the total pressure of gas is equal to the sum of the partial pressures of the individual component gasses. The partial pressure is the pressure that each gas would exert if it, alone, occupied the volume of the mixture at the same temperature. This empirical relation was stated in 1801. It follows from the kinetic theory of gasses under the assumption of a perfect (ideal) gas, and assumes no chemical interaction between the component gasses. It is approximately valid for real gasses at sufficiently low pressures and high temperatures.

He also found that gasses expand as their temperature is raised. Further experiments showed the solubility of gasses in water, the rate of diffusion of gases, and the constancy of composition of the atmosphere. Dalton determined the relative weights of atoms, developed the laws of definite and multiple proportions, and finally formulated the atomic theory which states that all elements are composed of tiny, identical, and indestructible particles. Many of these findings were included in his New System of Chemical Philosophy.

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Evangelista Torricelli was born October 15, 1608, in Faenza, Italy and died October 22, 1647 in  Florence. He was a hysicist and mathematician who invented the barometer. Inspired by Galileo’s writing, he wrote a treatise on mechanics, De Motu (“Concerning Movement”), which impressed Galileo. In 1641 Torricelli was invited to Florence, where he served the elderly astronomer as secretary and assistant during the last three months of Galileo’s life. Torricelli was then appointed to succeed him as professor of mathematics at the Florentine Academy.

Two years later, pursuing a suggestion by Galileo, he filled a four-foot-long glass tube with mercury and inverted the tube into a dish. He observed that some of the mercury did not flow out and that above the mercury in the tube was a vacuum. Torricelli became the first man to create a sustained vacuum. After much observation, he concluded that the variation of the height of the mercury from day to day was caused by changes in the atmospheric pressure. He never published his findings, however, because he was too deeply involved in the study of pure mathematics. He pursued calculations of the cycloid, which is a geometric curve described by a point on the rim of a turning wheel. In his Opera Geometrica (“Geometric Work”), published in 1644, Torricelli included his findings on fluid motion and projectile motion. His developments in geometry aided in the eventual development of integral calculus.

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Joseph Fourier was born March 21, 1768, in Auxerre,
France and died on May 16, 1830. A brilliant mathematician, he developed the partial differential equations governing the steady-state and time-dependant propagations of heat in solid bodies. He solved these equations with an infinite series of trigonometric equations, now known as a Fourier Series. His analytical methods for solving heat transfer problems departed from others of the time by distinguishing between interior and surface conditions, treating each separately. He was awarded the Grand Prize in Mathematics from the Institut de France in 1810 for his work.

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Gustave de Coriolis was born May 21, 1792 in Paris, France and died September 19, 1843. 
He is best remembered for his discovery of the Coriolis force relating to centrifugal motion. In a rotating frame of reference, he found a force that was perpendicular both to the flow direction and to the axis of rotation. He revealed how the laws of motion could be used in this rotating frame of reference if an extra force, called the Coriolis acceleration, was added to the equations of motion. Coriolis also developed relationships for the work and kinetic energy involved in the movements of solid bodies.

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Hendrik A. Lorentz was born July 18, 1853 in Arnheim, Netherlands and died February 4, 1928. He refined Maxwell’s electromagnetic theory in his doctoral thesis and was appointed professor of mathematical physics at Leiden University in 1878. He is best known for the Fitzgerald-Lorentz Contraction, a hypothesis that played a role in discrediting the notion that the vacuum of space is filled with a material medium called ether. Lorentz also proposed that light waves were due to oscillations of an electric charge in the atom. His mathematical theory of the electron brought him a Nobel Prize in 1902. More information .

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Blaise Pascal was born June 19, 1623 in Clermont-Ferrand, France and died August 19, 1662. He invented the first digital calculator which roughly resembled the mechanical calculators of the 1940s. His studies in geometry, hydrodynamics, hydrostatic pressure and atmospheric pressure led him to invent the syringe and the hydraulic press. His studies also led to the discovery of Pascal’s Law of Pressure, wherein external forces to a confined fluid are transmitted uniformly in all directions. Pascal’s vacuum-in-a-vacuum experiment and his theory of pressure equilibria resulted in the determination that the pressure exerted by a vacuum is zero. More information is available in an excerpt from `A Short Account of the History of Mathematics (4th edition, 1908) by W. W. Rouse Ball.

Quotations:
“We are usually convinced more easily by reasons we have found ourselves than by those which have occurred to others.” Pensees, 1670.

It is the heart which perceives God and not the reason.” Pensees, 1670.

“Man is equally incapable of seeing the nothingness from which he emerges and the infinity in which he is engulfed.” Pensees, 1670.

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Christaan Huygens was born April 14, 1629 at The Hague, Netherlands and died July 8, 1695. His father “discovered Rembrandt”. He was a technical physicist and a contemporary of Isaac Newton. He patented the first pendulum clock and derived the law of centrifugal force for uniform circular motion. An accomplished vacuum research specialist, he constructed some of the first practical vacuum pumps. His theory of light propagation through air and vacuum departed from conventional wisdom, which held that a vacuum had material properties. His view was that a vacuum was without physical properties and is, of course, the foundation of all vacuum technology today.

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Joseph-Louis Gay-Lussac was born December 6, 1778 in Limoges, France and died May 9, 1850. A brilliant experimentalist, he is known for developing the Law of Combining Volumes (more information) and his hypotheses on the interactions of gasses. He also developed the lead chamber process for the production of sulphuric acid The tall absorption towers are referred to as Gay-Lussac Towers even today. His work on iodine research is considered by many to be a model of chemical research. While Gay-Lussac was the first to isolate the element boron, he lost out on its discovery to Humphry Davy, who isolated it nine days later but was the first to publish the results.

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Otto von Guericke was born November 20, 1602 in Magdeburg, Germany and died May 11, 1686. A physicist, engineer and natural philosopher, von Guericke invented the first air pump and used it to study vacuums and the role of air in combustion and respiration. His first pumps were inverted pumps (for fire fighting) that could evacuate larger volumes of air than could be done using static methods. His pumps were used to demonstrate the well-known Magdeburg experiment where two teams of eight horses were unable to pull apart two evacuated brass hemispheres. His latter pump designs used lever pumping systems. In 1663, von Guericke also built the first machine to create an electric spark.

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The above data were compiled from a number of sources including some 65 year old encyclopedias. If you found this data interesting or useful, please let us know. If you have a useful tract or note, send it to us and we will consider posting it.

Vacuum Technology is the term applied to all processes and physical measurement carried out under conditions of below-normal atmospheric pressure.
A process or physical measurement is generally performed under vacuum for one of the following reasons:

1. to remove the constituents of the atmosphere that could cause a physical or chemical reaction during the process (e.g., vacuum melting of reactive metals such as titanium)
2. to disturb an equilibrium condition that exists at normal room conditions, such as the removal of occluded or dissolved gas or volatile liquid from the bulk of material (e.g., degassing of oils, freeze-drying) or desorption of gas from surfaces (e.g., the cleanup of microwave tubes and linear accelerators during manufacture)
3. to extend the distance that a particle must travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes
4. to reduce the number of molecular impacts per second, thus reducing chances of contamination of surfaces prepared in vacuum (e.g., in clean-surface studies and preparation of pure, thin films).

For any vacuum process, the limiting parameter for the maximum permissible pressure can be defined by:

• the number of molecules per unit volume (reasons 1 and 2),
• the mean free path (reason 3),
• or the time required to form a monolayer (reason 4).

At room temperature and normal atmospheric pressure, one cubic foot (0.03 cubic metre) of air contains approximately 7×1023 molecules moving in random directions and at speeds of around 1,000 miles per hour (1,600 kilometers per hour). The momentum exchange imparted to the walls is equal to a force of 14.7 pounds for every square inch of wall area. This atmospheric pressure can be expressed in a number of different units (see Table), but until recently it was commonly expressed in terms of the weight of a column of mercury of unit cross section and 760 milimetres (mm) high. Thus, one standard atmosphere equals 760 mm Hg, but to avoid the anomaly of equating apparently different units, a term, torr, has been postulated. One standard atmosphere = 760 torr (1 torr =1 mm Hg). This term was replaced in 1971 by SI unit defined as the newton per square metre (N/m2), and called the pascal (one pascal=7.5×10-3 torr).

The first major use of vacuum technology in industry occurred about 1900 in the manufacture of electric light bulbs. Other devices requiring a vacuum for their operation followed, such as various types of electron tube. Furthermore, it was discovered that certain processes carried out in a vacuum achieved either superior results or ends actually unattainable under normal atmospheric conditions. Such developments included the “blooming” of lens surfaces to increase the light transmission, the preparation of blood plasma for blood banks, and the production of reactive metals such as titanium. The advent of nuclear energy in the 1950s provided impetus for development of vacuum equipment on a large scale. Increasing applications for vacuum processes were steadily discovered, as in space simulation and microelectronics.

Applications of Vacuum

Industrial vacuum applications range from mechanical handling (such as the manipulation of heavy and light items by suction pads) to the deposition of integrated electronic circuits on silicon chips. Obviously, vacuum requirements are as widely varied as the particular processes using vacuums. In the rough vacuum range from about one torr to near atmosphere, typical applications are mechanical handling, vacuum packing and forming, gas sampling, filtration, degassing of oils, concentration of aqueous solutions, impregnation of electrical components, distillation, and steel stream degassing.

At lower pressures down to about 10-4 torr, many metallurgical processes such as melting, casting, sintering, heat treatment, and brazing can derive benefit. Chemical processes such as vacuum distillation and freeze-drying also need this range of vacuum. Freeze-drying is used extensively in the pharmaceutical industry to prepare vaccines and antibiotics and to store skin and blood plasma. The food industry freeze-dries coffee mainly, although most foods can be stored without refrigeration after freeze-drying, and the technique is receiving widespread acceptance.

The pressure range down to about 10-6 torr is used for cryogenic (low-temperature) and electrical insulation. It is used in the production of lamps; television picture tubes, X-ray tubes; decorative, optical, and electrical thin-film coatings; and mass spectrometer leak detectors.

In thin-film coating, a metal or compound is evaporated under high vacuum from a source onto a base material or substrate. The base material is generally plastic for decorative coatings; glass for optical coatings; and glass ceramic, or silica for electrical coatings. Thickness of the film can vary from about 1/4 wavelength of visible light to 0.001 inches or more. In the optical field, antireflection coatings are deposited on lenses for cameras, telescopes, eyeglasses, and other optical devices, considerably reducing the amount of light reflected by the lenses and thus giving a brighter transmitted image.

To achieve vacuum high enough for thin-film coating and for other industrial uses requiring pressures down to 10-6 torr, a pumping system consisting of an oil-sealed rotary pump and a diffusion pump is used. The oil-sealed rotary pump (sometimes referred to as forepump) “roughs” the chamber down to a pressure of about 0.1 torr, after which the roughing valve is closed. The fore valve and high-vacuum baffle valve are then opened so that the chamber is evacuated by the diffusion pump and rotary pump in series.

In Research

Almost every research laboratory uses vacuum directly in its experiments or employs equipment that depends on vacuum for its operation. The lowest pressures are obtained in the research laboratories, where equipment is generally similar to, but smaller than that used by industry.

Typical of the research equipment using vacuum down to about 10-6 torr are the electron microscope, analytical mass spectrometer, particle accelerator, and large space simulation equipment. Particle accelerators range from small van de Graaff machines to large proton synchrotrons.

In space simulation, large units that simulate space around a complete vehicle require a vacuum of 10-6 torr or below. Such vessels incorporate a complete shroud at liquid nitrogen temperature and a port through which high-intensity light can be beamed to simulate the sun’s radiation.

In the pressure region down to and below 10-9 torr, research applications include electrical insulation, thermonuclear energy conversion experiments, microwave tubes, field ion microscopes, field emission microscopes, storage rings for particle accelerators, specialized space simulator experiments, and clean-surface studies. In many experiments it is not only necessary to reach such pressures of 10-9 torr but to reduce the hydrocarbons in the residual gases to an absolute minimum. Even small traces of hydrocarbons can render the results unreliable. To achieve a vacuum of this order the vacuum vessel and the equipment inside must be cleared of residual gas (degassed) to the greatest extent possible. A common solution is to bake the whole apparatus for a number of hours at about 350oC while maintaining a vacuum in the 10-5 torr region. Baking at this temperature requires the use of all-metal sealing rings. To eliminate hydrocarbons, the unit is pumped down to about 10-3 torr using sorption pumps; and from there, sputter ion pumps and titanium sublimation pumps complete the task down to 10-9 torr or below.

Vacuum Equipment

Oil-sealed Rotary Pump

Capacities are available from 1/2 to 1,000 cubic feet per minute, operating from atmospheric pressure down to as low as 2 x 10-2 torr for single-stage pumps and less than 5 x 10-3 torr for two-stage pumps. The pumps develop their full speed in the range from atmosphere to about one torr. The speed then decreases to zero at their ultimate pressures. Two of the most common designs are useful for pumping both liquids and gases. One is a two-bladed pump in which the rotor is eccentric to the stator, forming a crescent-shaped volume swept by the blades through the outlet valve. The second, a rotary piston pump, similar to a single blade, is part of the sleeve fitting around the rotor. The blade is hollow and acts as an inlet valve, closing off the pump from the system when the rotor is at top center.

Ultimate pressures attainable are limited by leakage between the high and low-pressure sides of the pump (due mainly to carry over of gases and vapors dissolved in the sealing oil that flash off when exposed to the low inlet pressure) and decomposition of the oil exposed to high temperature spots generated by friction.

Gas ballasting helps to prolong pump life because it removes the chief source of pump contamination, condensable vapors. The gas ballast is a vented exhaust that admits a small amount of air at atmospheric pressure to the compression side of the pump, thus permitting most condensable vapors to pass through the pump without condensing.

Typical applications of this pump are in food packaging, high-speed centrifuges, and ultraviolet spectrometers. It is also widely used as a forepump or a roughing pump, or both, for most of the other pumps described.

Mechanical Booster

Capacities are available from 100 to 70,000 cubic feet per minute, operating usually in the pressure range from 10 to 10-3torr. The peak speed of the pump is developed in the pressure range from 1 to 10-2 torr. The speed at the lower end of the pressure range depends on the type of forepump used. A typical mechanical booster uses two figure-eight-shaped impellers, synchronized by external gears, rotating in opposite directions inside the stator. The gas is trapped from between the impellers and the stator wall and transferred from the high vacuum to the fore vacuum side of the pump. The gears are oil-lubricated but are external to the pump, so that the impellers run dry. Clearance between the impellers and the stator wall is generally about .002 to .010-inch. As a consequence, back leaking of gas occurs at a rate governed by the pressure difference between input and output and the type of gas being pumped. Under normal running conditions, a pressure difference of about ten to one is obtained. The mechanical booster must be backed by another pump in series when working in its normal pressure range. The most frequently used type of forepump is the oil-sealed rotary pump. Typically, the mechanical booster is employed for pumping vacuum-melting furnaces, in an impregnation plant for electrical equipment, and in low density wind tunnels.

Molecular Pump

Capacities are available up to 20,000 cubic feet per minute, with an operating range of 10-1 to 10-10 torr, when backed by an oil-sealed rotary pump. The full speed of the pump is developed in the very wide pressure range from 10-2 to 10-9 torr. In the molecular pump, a high rotational speed rotor (up to 32,000 revolutions per minute) imparts momentum to the gas molecules, moving them along the small clearance between the rotor and stator. The molecular drag pump also employs this principle of operation. For ultimate base pressures it has been almost entirely displaced by the faster and simpler turbo molecular pump, in which radial slots in both rotor and stator fins actuate the pump. A number of compression stages are employed but because of its design, larger clearances can be tolerated between rotor and stator than were possible in the molecular drag pump. The molecular drag pump is sometimes built integral with a turbo molecular pump to allow the use of vert clean “dry” backing pumps. These hybrids are often used in semiconductor processing where oil vapor backstreaming would contaminate processes but high pumping speeds are needed.

Vapor Diffusion Pump

This pump is mainly used on equipment for the study of clean surfaces and in radio frequency sputtering. Pumping speeds are available up to 190,000 cubic feet per minute with an operating pressure range of 10-2 to less than 10-9 torr when water-cooled baffles are used and less than 10-11 torr when refrigerated baffles are employed. The pumping speed for a vapor pump remains constant from about 10-3 torr to well below the ultimate pressure limitations of the pump fluid. The best fluids allow pressures of better than 10-9 torr. The diffusion pump is initially evacuated by an oil-sealed rotary pump to a pressure of about 0.1 torr or less. When the pump fluid in the boiler is heated, it generates a boiler pressure of a few torr within the jet assembly. High-velocity vapor streams emerge from the jet assembly, impinge and condense on the water or air-cooled pump walls, and return to the boiler. In normal operation part of any gas arriving at the inlet jet is entrained, compressed, and transferred to the next stage. This process is repeated until the gas is removed by the mechanical forepump.

The oil-vapor booster pump works on the same principles as the diffusion pump, but it employs a higher boiler pressure. Normal operating pressure range is 1 to 10-4 torr. When backed by an oil-sealed rotary pump, this pump is widely used for achieving high vacuum in thin-film evaporation units, accelerators, and in TV tube pumping.

Sputter Ion Pump

Capacities are available up to 14,000 cubic feet per minute, with an operating pressure range of 10-11 torr. The full speed of the pump is developed in the pressure range from about 10-6 to 10-8 torr, although the characteristic at the lower pressure is dependent on the pump design. This pump uses a cathode material such as titanium vaporized or sputtered by bombardment with high velocity ions. The active gasses are pumped by chemical combination with the sputtered titanium, the inert gasses by ionization and burial in the cathode, and the light gasses by diffusion into the cathode.

A typical pump consists of two flat rectangular cathodes with a stainless steel anode between them made up of many open-ended boxes. This assembly, mounted inside a narrow box attached to the vacuum system, is surrounded by a permanent magnet. The anode is operated at a potential of about seven kilovolts (kV), whereas the cathodes are at ground potential.

The sputter ion pump has low speeds and sometimes instability when pumping inert gases. To improve its characteristics other types of sputter ion pumps have been developed: the slotted cathode, triode, differential, and magnetron pumps.

To start up a sputter ion pump it is necessary to reduce the pressure to at least 2 x 10-2 torr, and preferably much lower, by means of a roughing pump. Sputter ion pumps can operate in any position and do not need water or liquid nitrogen supplies. They have a long life and can provide very clean, ultrahigh vacuum, free of organic contamination and vibration. They are employed mainly for the clean-surface studies and in those applications where any organic contamination will give unsatisfactory results.

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