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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 5
collisions between the air molecules and small (mass 2 or less) A theoretical interpretation of the results suggests that the loosely joified segments of the oil molecules account for the hydrogen atoms on the periphery of the oil molecules provide small average deflections which were observed. Simple elastic springiness to the oil molecule by first absorbing the momentum or inelastic collisions between the air molecules and the oil mole- . acquired at impact themselves and later sharing this momentum cules acting as units cannot account for the results obtained by with the balance of the oil molecule. The oil molecules do not the authors. act as hard, or as simply deformable spheres, on impact with air molecules.
Summary
Literature Cited
Experimental measurements were made on the deflections of oil molecules in air a t reduced pressures. These indicate that the approximate straight-line path for an oil molecule is much greater than its mean free path. This is in agreement with the general observation that many high vacuum devices, such as diffusion pumps, operate at pressures considerably higher than they would if the oil molecules ware appreciably deflected on a single encounter with an air molecule.
(1) Kapff, S. F., and Jacobs, R. B., Rev. Sei. Instruments, 18, 581 (1947). (2) Kennard, E. H., *“Kinetic Theory of Gases,” pp. 115-34, New York, McGraw-Hill Book Co., 1938. (3) Knauer, F.,2.Physik, 90,559 (1934). (4) Loeb, L. B., “Kinetic Theory of Gases,” p. 98, New York, McGraw-Hill Book Co., 1934. RECEIVED December18 1947
Surface Phenomena Useful in Vacuum Technique Le Roy Apker GENERAL ELECTRIC COMPANY, SCHENECTADY, N. Y.
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0D ER N v acu u m Studies of surface phenomena can give valuable inforsion pattern. Residual gases techniques can promation at pressures so low as to be unmeasurable by the are adsorbed selectively by duce pressures SO low that usual methods. Previous work o n thermionic emission the various vposed crystal they cannot be detected with from wires, and on field emission from single crystals, is faces, and the pattern a v a i l a b l e gages. Experimentioned briefly. The photoelectric emission from tungchanges radically as the surments in Nottingham’s labosten is very sensitive to residual gas in vacuum systems face becomes contaminated. ratory have shown that the and can be used to estimate partial pressures of active Because the gas condenses on common form of ionization gases. A simpler method involving a sudden burst a relatively cool substrate, of adsorbed gas into an ionization gage is also described. this technique gives informagage fails a t about 1.0-6 mition that cannot be obtained won. This effect is due to x-rays generated by electrons from thermionic emission measurements. The tube walls must be very clean, since the striking the grid (6). These rays eject photoelectrons from the collector and produce the same effect as positive ions flowing to this electrons are accelerated by several kilovolts. Thus, gas can be electrode. Thus the gage indicates micron, even though dislodged by impinging electrons. This field emission method the actual pressure is lower. Obviously, other methods are has been used in a valuable study of techniques for reducing needed for measuring the ultimate vacua attainable today. active gas pressures (4). At 10-5 micron, enough molecules strike a clean surface to Photoelectric Emission from Tungsten cover it with a monolayer in a few minutes. The adsorbed gas may produce radical changes in the behavior of the material. IB the laboratory at Schenectady, a procedure combining EvidentlY, experiments must be done quickly. For this reason, features of both thermionic and field emission methods has been little is known about Clean Surfaces, except for a few refractory used which involves measuring the photoelectric emission from a metals and several materials that are easily evaporated. For tungsten ribbon as a function of time ( 1 ) . The photocurrent this reason, also, adsorption phenornena may be used as sensitive from a metal increases approximately as the square of the quandetectors of certain gases. tity, hv ‘p. Here hv is the energy of the incident photons, and p is the work function of the surface. The primary effect of Thermionic Emission from Tungsten Wires contamination is to change the latter quantity. Therefore, the Langmuir’s experiments on adsorption by tungsten filaments photoelectric method is not so sensitive as the thermionic, which (3) showed that the thermionic emission from these wires Tvas involves currents varying as exp (-p/lcT). Both techniques extraordinarily sensitive to surface contamination-for example, measure an average effect for a polycrystalline surface and cannot cesium increased the current at 800” K. by a factor of 1020 detect highly selective adsorption by preferred crystal faces. At 1500” K., oxygen decreased it by a factor of 106. This deDespite these drawbacks, the photoelectric method is convenient crease is used in one modern form of leak detector (6). because it can be used at low temperatures and because little or no accelerating voltage is required. ~
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Field Emission from Single - Crystals
In Nottingham’s laboratory, a different method has proved successful ($1. A phosphor Screen collects the field emission from &verysmall single crystal of tungsten. After the metal is cleaned by flashing at a high temperature, it gives a characteristic emis-
A typical photocell described in (1) was sealed off and was gettered with tungsten vapor. After the tube had been standing for a week at 3000K,, no photocurrent was ejec$ed from the tungsten cathode by radiation of wave length 2537 A. (hv = 4.89 electron volts). The ribbon was then flashed for 10-second periods a t
May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
temperatures that were increased successive1 in 100O steps. Nothing happened until 1000° K. was reached: At this point, the ionization gage indicated a burst of gas. The peak pressure was 5 X micron and the reading quickly returned to 2 X as the gas was taken up by the walls of the tube. After this flash, photocurrent was observed. The work function had therefore decreased to a value below 4.89 electron v o h . Flashing at temperatures up t o 2000" K. did not affect the emission, but at 2100" K. i t increased by about 6%. The work function was then 4.5 volts. It was assumed that this treatment produced a clean surface, as the current was not changed by flashing the tungsten even at temperatures just below its melting point. When this clean surface remained at 300" K., the emission slowly decreased. It fell exponentially at a rate of about 0.1 % per minute and reached immeasurably small values after several days. !Flashing another filament or heatin the glass wall of the tube contaminated the ribbon very qui&ly. Contamination rates in other tubes varied by several factors of 10, although the ionization gages indicated identical pressures. The exhaust schedule, the getter, and the collector material all affected these rates, but they were uninfluenced by the presence of the radiation. Changing the magnitude of the photocurrent (normally about 10-11 ampere) had no effect. However, with an accelerating potential of 50 volts, a few milliamperes of thermionic current from the tungsten dislodged enough gas from the collector to be detected even by the ionization gage, Although these observations tire not understood in detail, they are valuable for detecting active gases. The sensitivity of such adsorption methods increases with the time that one is willing to wait for contaminants to condense. Although the results are not quantitative on a n absolute scale, relative contamination rates can be determined with considerable precision. Let us assume for the moment that the work function, p, increases linearly with the amount of gas adsorbed. Then the rate at which Y, changes can be used to determine the rate a t which molecules condense on the surface. T h k in turn gives the partial pressure of active gmes if one knows the reflection coefficient for the incident molecules. In the absence of other information, it may be assumed that reflection is negligible. For the photocell mentioned above, the partial pressure thus computed was about 10-8 micron.
Sudden Evaporation of Adsorbed Gas Tungsten is useful also for detecting active gas by a simpler method.
847
contamination rates it is a simple matter to determine the direction and relative rate of flow.
Nature of Adsorbed Gas Well known researches on the composition of the active gases mentioned have been carried out at higher pressures. From Langmuir's data: the author has concluded that the component which evaporated in his experiments i n 10 seconds a t 2100' K. was probably oxygen. The gas that came off at 1000' K. has .not been definitely identified. Work on this subject and related ones is in progress in several laboratories, with the mass spectrometer playing an invaluable role. Although adsorption phenomena may be complex in mixtures such as the residual gases in vacuum systems, one may anticipate that methods of the type discussed here will be applied to analysis as well as detection. Even a t present, they are extremely valuable for improving vacuum techniques.
Acknowledgment It is a pleasure to acknowledge indebtedness to Miss J. E. Dickey, E, A. Taft, and R. L. Watters for. their generous help and to S. Dushman and A. W. Hull for valuable discussions.
Literature Cited Apker, L., Taft, E., and Dickey, J., Phys. Rev:, 73,46(1948). Daniel, J. H., Ibld., 61,668 (1942); see also earlier papers cited in this reference. Langmuir, I., IND.ENG.CHEM.,22,390 (1930). Moore, N. H.,thesis, Massachusetts Institute of Technology, 1941.
Nelson, R. B., Rev. Sci. Imtruments, 16,65 (1946). Nottingham, W. B., paper presented at M.I.T. Eleotronica Conferenae, 1947. RECIIVEDNovember 12, 1947.
"Production of Orange Juice Concentrate and Powder" by H. W . Schwars and F. E. Penn, also presented at the Symposium on High Vacuum, appears in the Pilot Plant section on page 938 of this issue.
When the ressure in a system has fallen below t f e usable range of an ionization gage, flashin a small tungsten filament produces a %urst of gas that is easily measurable (say, 10-8 micron). If the filament is allowed to stand for 2 or 3 minutes it will adsorb enough gas to give another readable burst (10-4 micron). The peak pressure increases at first linearly with the time during which the filament remains cool. It finally saturates at the value first mentioned. This reading agrees, in order of magnitude, with that computed for the evaporation of a monolayer from the wire.
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Tungsten is highly sensitive to the gases from glass; hence this technique is convenient in determining when a tube or constriction is sufficiently degassed to permit a clean seal-off. For a system suitably prepared, the contamination rate decreases by a factor of several thousand, a short time after the constriction is closed. A similar * technique is useful for measuring flow of active gas during a n exhaust. Two detectors are connected to opposite en& of a constriction-the seal-off constriction, for instance. From the
Dryers and Concentrators in Commercial Scale Plant f o r Dehydration of Orange Juice
