Development of High Vacuum Technique Contribution to Purely Scient ijic Inves t iga t io ns Saul Dushman GENERAL ELECTRIC CO., SCHENECTADY, X. Y .
T h e article reviews the origins of high vacuum technique and its contributions in solving problems of a fundamental nature, especially- in physics, and discusses the significance from a purely scientific point of view of investigations at extremely low pressures.
A
LTHOUGH the French have preceded us by initiating the publication of a journal (Le V i d e ) which is to be devoted exclusively to developments in the science and technological aprlications of vacuum technique, papers in this symposium come from the first occasion in this country on which a group of scientists and engineers have gathered to discuss topics in this field. Such an event signifies that a new field in t,echnology has come of age. It has emerged from the purely scientific research laboratory and has become a n important industrial development, the extent and importance of which are measured in terms of millions of dollars of annual business. A4sMorse ( 7 )points out, the recent war catalyzed extensive and very important developments in the industrial applications of vacuum technique, and the fact that, a meeting such as this symposium attracted so large an &endance is to be regarded as evidence of t,he general interest in this field. Furthermore, t,he papers and discussions will serve to demonstrate thk growing importance, in a number of industrial fields, of the applications of high vacuum technique. This discussion is limited largely to a brief review of the origins of high vacuum technique, its contributions in the solution of problems of a fundamental nature, especially in physics, and a discussion of the significance from a purely scient,ific point of view of investigations at extremely low pressures.
Development of Vacuum Technique The first industrial application of vacuum technique was made by Edison in his invention of the carbon filament lamp. The pressures obtained 1vit.h the rotary oil pumps available up to about 1910 were of t,he order of 10-1 mm. To improve t8hevacuum in the sealed-off lamps, a red phosphorus getter was introduced. About, 1911 Langmuir began his investigations on the causes of blackening in vacuum tungsten filament lamps. This work led him not only to the invention of the gas-filled lamp but also to an investigation of the phenomenon of electron emission from incandescent met,als, during the course of which he discovered t,he electron space charge effect. I n cont,radiction to the views expressed a t t,hat, t.ime by many of the foremost scientists, he concluded that in order t o obtain pure electron emission that is characteristic of a metal, it is absolutely essential to work with extremely clean surfaces. Since such surfaces could be obtained only in an extremely high vacuum, it became necessary to dcvelop a technique for producing and maintaining such high vacua. This work led not only to the applicat,ion of these observations to the development of electronic tubes, but also to an immense number of other applications of vacuum technique as an important contributing factor in investigations of a fundamental nature. The first and most important of these applications was t,he invention by Coolidge of the hot-cathode x-ray tube. This device not only proved a boon t,o the medical profession but also made it possible, as a result of the investigations by the Braggs, Debye, Scherer, and Hull, to use x-ray diffraction methods for the determination of the arrangement of atoms in cryshl lattice struct,ures.
M a y 1948
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The use of vacuum technique made possible the discovery by Davisson and Germer of the wave nature of electrons and the development of the electron diffraction method for the study of the structures of surface layers and the arrangement of atoms in the molecules of a gas or vapor. The early work on the evacuation of electronic devices was aided materially by the development by Gaede of his molecular pump (1913). While this was not a high-speed pump, as compared with present-day pumps, i t was so much better than rotary oil or mercury pumps available a t that time with respect to both speed and degree of vacuum obtained, that it immediately found wide application i n the commercial production of Coolidge x-ray and the other types of hot-cathode high vacuum devices. With the invention by Gaede (1915) of his mercury vapor diffusion pump and by Langmuir (1916) of the mercury condensation pump, a new era opened up in the extensive application of vacuum technique. I n 1928-29 Burch, in England, demonstrated that mercury in these pumps could be replaced by high boiling point hydrocarbons and very soon afterward Hickman and his associates began their pioneering investigations on the applications of synthetic esters of phthalic and sebacic acids. As a result of this work and of investigations on high boiling point petroleum distillates, it has been possible to construct vapor pumps with speeds of exhaust ranging up t o and above 7000 liters per second (15,000 cubic feet per minute). The manner in which these high speed pumps contributed towards the success of the Manhattan Project and other developments during the recent war is too well known to require more than mere mention. The diverse and extensive technological applications of these developments are discussed in the subsequent papers of this symposium.
Scientific Developments Based on High Vacuum Technique M7hUethe early form of hot-cathode x-ray tube was designed for a maximum of 100 kv., subsequent developments in design of both the tube and the high voltage generator made i t possible to increase the range of operating voltage, and a t present a 2,000,000-volt equipment is available for use in both therapeutic work and the inspection of very large metal objects. Other devices, the development of which has been made possible by means of high vacuum technique, are the electron microscope and high energy particle accelerators such as the cyclotron, betatron, and synchrotron. The contributions which have been made by means of the cyclotron to our knowledge of the nucleus have been publicized so widely as to make further elaboration of the statement unnecessary. The more recently developed devices, by which it will probably be possible to accelerate electrons to 500,000,000 volts or even higher, will undoubtedly enable us t o gain even more profound knowledge of nuclear structures and of the origin of cosmic rays. The accurate determination of nuclear masses by the mass spectrometer, another vacuum device, has given us fundamental data for the verification of the Einstein mass-energy relation, The more recent applications of this device in gas analysis and as a leak-detector in vacuum technique have. proved, and wiIl prove, even more valuable in both purely scientific and technological investigations. It is extremely stimulating to one who has been interested in this field for over 30 years to view the widespread application which vacuum technique has received in so many different fields of physics, chemistry, and metallurgy. Gages for the measurement of low pressures have also received considerable attention and there are available to the research worker, as well as t o the production engineer, a variety of such gages. I n the research laboratory a high vacuum system consisting of a rotary oil pump, a vapor pump, and gages, is essential equipment for investigations ranging over a wide range of problems such as thermionic and photoelectric emission, electri-
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cal discharges in gases at low pressure, solubilities of gases in metals, glass, and other solids, determinations of amount and compositions of gases adsorbed and absorbed by’ materials used in vacuum devices, and chemical reactions a t low pressures, including the characteristics of getters and rates of evaporation. It is absolutely essential in investigating the electron emission characteristics of surfaces to have such surfaces as free as possible from adsorbed gases. Furthermore, it is necessary in the case of electronic devices to avoid effects due to collisions between electrons and gas molecules. This signifies that the pressures used must be so low that the mean free path for electrons or molecules is large compared with the distance from any one surface to any other. In fact, this is the criterion by means of which we many di-tinguish the range of pressures designated as high vacua from the range of higher pressures. Through the investigations of Gaede, Knudsen, Langmuir, Clausing, and others we have learned a great deal about the laws which govern ,the behavior of gases a t these low pressures. Some of these conclusions which have been deduced from the kinetic theory of gases are reviewed briefly below.
Behavior of Gases at Low Pressure I n the range of normal pressures (760 to about 100 mm. of mercury) the coefficients of heat conductivity and viscosity of gapes are independent of the pressure, because both energy and momentum are transferred from one surface to another, adjacent to it, by means of collisions between the molecules. According t o the kinetic theory of gases, the coefficient of viscosity, ?, and that of thermal conductivity, A, are given by the relations =
0.5pvL
(1)
and A = scVpvL
where p = density, ZJ = average velocity of molecules, E = constant, c, = specific heat per unit mass a t constant volume, and L = mean free path. Furthermore, we have the relations ZJ = 14,551
v’/T/M
cm. see.-’
(3) where T = absolute temperature in degrees Kelvin, and M = molecular weight in grams, and [4)
where n = number of molecules per cc. and 6 = molecular diameter in om. Since n varies linearly with p , it follows that, at constant temperature, L varies inversely as p , and therefore both 17 and X should be independent of the pressure, a t constant temperature. The experimental verification of these conclusions served t o give the kinetic theory of gases wide acceptance. Equation 1 can be written in the form (5)
where p is the pressure in the microns (1 micron = 10-3 mm. of mercury). For air at 25’ C. (room temperature), q = 1.845 X lo-hpoise, and consequently L = 5.09/p cm. Equations 1 and 2 can be applied only at higher pressuresthat is, a t pressures for which d / L is greater than approximately 100, where d denotes the distance over which heat or momentum transfer occurs. At low pressures, molecules travel from one surface to another one adjacent to it, without suffering collisions. This range of pressures is therefore designated that of “free molecule flow.” It may be characterized by the criterion that the value of the ratio d / L is approximately equal to or less than 1. We shall designate this as the region of high vacua or extremely low pressures.
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7 80
In this range of pressures the rates of transfer of both momentum and energy vary linearly with the pressure and this fact has therefore been applied t o the development of a number of designs of low pressure gages. We may now consider briefly a few relations which have been deduced from the kinetic theory of gases, which are of importance in high vacuum investigations. At low pressures, the rate a t which molecules strike a surface is given by the relation Y
=
l/4
nv, molecules cm.-2 set.-'
(6)
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For organic vapors, as pointed out by Hickman, the value of M / T is approximately 1. Hence, the value of G is about 0.6 p (grams per square meter per second)-that is, at p = 1 micron, the maximum theoretical rate of evaporation is 0.6 gram per square meter per second, or about 4.7 pounds per square meter per hour. Finally, there exists a difference between the rate of flow of gases a t normal pressures and the rate of flow at low pressures.
For the whole range of pressures the rate of flow of gas through a cylindrical tube can be expressed by the relation
which can be expressed in the form Y
= 3.513 X 1019p/d/MT
where p = pressure in microns. For nitrogen a t 25 O C., Y = 3.84 X 1017p , molecules see.-’ Since the number of molecules, Ns, required to form a layer one molecule thick (monolayer) is about 18 X 1014 it follows that the time, to, required to form a monolayer, assuming that each molecule condenses on the surface, is 1YS
to = - = 2 X lO-S/p seconds Y
that is, at p = ,
micron, to is about 2 seconds.
These relations are of importance, as shown by L. Apker, in determining the rate of contamination of a clean surface in a high vacuum. Actually, not every molecule incident on a surface “sticks.” There is a constant re-evaporation which increases with the temperature, and as shown by Langmuir, the fraction of the surface covered at equilibrium, e, at any pressure p , is given by a relation of the form
e=-
bp
1
+ bp
(7)
where b is a constant which is proportional to the “life” of the adsorbed molecules on the surface. Equation 6 also gives the rate at which molecules evaporate from a surface at very low pressures of residual gases. From this equation it follows that the rate of evaporation is given by the equation,
G
= 5.833 X
-
10-5 p d M / T gram cm.-2 sec.-l
(8)
where Q = micron liters per second p 2 - pl = difference in pressures (in microns) a t the two ends of the tube a = radius and I = length of tube (in em.) L, = mean free path a t the average pressure, pa= 0.5 (PZ PI) = ap,/L1 L1 = mean free path a t 1micron, in em. 2 = function of a/L. which increases from the value 0.81 for a/L. > 100 to 1.0 for a/L, 0.1 If we plot log Q versus log p a , the slope has the value 1 a t low pressures and the value 2 at high pressures, with a transition from 1 t o 2 in the range a/La = 100 to a/L, = 1 approximateiy. At the lower range of pressures, Q varies linearly with p z - pl; at the higher range, Q varies with the product, (pz pl)p,.
+
