US2894679A - Pump - Google Patents

Description

July 14, 1959 Filed Nov. 23, 1953 TO SYSTEM TO BE R. G. HERB PUMP 5 Sheets-Sheet 1 GUIDE TUBE Ti WIRE 7 COPPER ELECTRODES INVENTOR RAYMOND G .HERB.

'5 BY M WQ/hcM ATTORNEY COGJNG & CURRENT'LEAD- July 14, 1959 R. GHERB 2,894,679

PUMP Filed Nov. '25, 1953 I 5 Sheets-Sheet 2 5 COMPRESSION HEADER 3 FILAMENT 2' INNERGRIDV 1 OUTER GRID E 23 AcnvE suRIAcE GUIDE TUBE I GRAPHITE CRUCIBLE COPPER TUBING ,FOR

F EEDER To MECHANICAL PUMP INVENTOR RAYMOND swans.

ATTORNEY 'TO'SYSTEM 10 BE 'EVACUAT'ED July 14, 959 R. G. HERB 2,894,679

} PUMP Filed Nov. 23, 1953 5 Sheets-Sheet 3 F E-1 I I TO SYSTEM TO BE EVACUATED SPOOL FOR 'n WIRE Ti WIRE FEEDER GUIDE TUBE FOR Ti WIRE I OUTER GRID :I- IOOO V ACTIVE SURFACE TITANIUM WIRE ....I.G. JOB n a l X TUNGSTON FILAMENT +|OO V APHI POST 1000 3 INVENTOR RAYMOND G. HERB ATTORNEY R. G. HERB v PUMP 5 Sheets-Sheet 4 Filed NOV. 23, 1953 v .m W MR Mm D 1 o 3 M mm m lm 0 w M M r v 2 mm K wm F k A r 6: q V? Y xxfix B s mm mm vm al Q A W :k A///// //A// mm m? July 14,1959 R. G. HERB 2,894,679

PUMP

Filed Nov. 23, 1955 5 Sheets-Sheet 5 INVENTOR RAYMOND G. HERB.

BY 644 .4. file- 99 ATTORNEY United States Patent PUMP Madison, Wis., assignor to Wisconsin Madison, Wis., a cor- Raymond G. Herb,

Alumni Research Foundation, poration of Wisconsin The present invention relates to vacuum pumps which are capable of producing and maintaining a high vacuum free of organic vapors. Excluding noble gases, pumps embodying this invention have proven in practice to have pumping speeds about ten times greater than those of ordinary oil-diffusion pumps of the same physical size. Systems evacuated by the usual combination of an oil or mercury diffusion pump in series with a mechanical fore pump are contaminated by organic vapors from oils always present in mechanical pumps. Where the oil type diffusion pump is used, the diffusion pump is also a source of organic vapors. Other organic components, such as rubber 0 rings and plastic parts, also contribute to the presence of organic vapors. The use of watercooled bafl'les and liquid air-cooled traps reduces, but does not eliminate the contamination by organic vapors. In addition to organic vapor considerations, mercury type diffusion pumps require elaborate liquid air-cooled traps to prevent mercury vapor from entering the system to be evacuated.

In many systems, the presence of organic vapors is undesirable since the vapors may condense on the interior surfaces forming a chemical impurity and changing the properties of those surfaces. The improved pumps of the present invention can pump at exceptional speeds while maintaining a high vacuum entirely free of organic or mcrcural vapors without the use of traps or bafiies of any kind.

In the present invention, a suitable apparatus for the continuous or intermittent evaporation of a gettering sub stance, called a getter, is located in a container or chamber so that the vaporized getter condenses on the interior surfaces of the container and forms an active surface.

'The getter to be evaporated is chosen so that the gases in 'the chamber combine with the condensed getter and are :removed from the gaseous state. With continuous evaporation and condensation of new gettering substance, an

:active surface is constantly provided and a pumping ac- :tion results.

The mechanism of combination of the gas and the gettering substance depends on the type of getter evaporated and the kinds of gases present in the pumping system. Chemical combination, adsorption, absorption, gas-metal solutions, and solutions of compounds of the gas in the gettering metal are possible means of initial trapping of a gas which is subsequently buried under new condensing metal.

Where the attraction between the gas molecules and the freshly condensed active surface is insufiicient to trap the gas molecules at the surface until they are securely buried by new getter, means may be provided to increase the afiinity of the gas molecules for the active surface by ionization dissociation or a combination of these such that the products are more elfectively trapped. To further improve the trapping action, means may be provided to drive the gaseous ions into the active surface sufficiently deep to prevent escape before burial by new getter con- 2,894,679 Patented July 14, 1959 iCQ densing on the surface. The features of ionization or dissociation of the gas molecules and driving the gaseous ions to the active surface of gettering material are claimed in my continuation-impart application S.N. 546,025 which was filed November 10, 1955, now Patent No. 2,850,225.

Any one of a number of metals or alloys may serve as the gettering substance or material in practicing the invention. Titanium, zirconium, uranium and other metals particularly in groups IV and V of the periodic table, for example, can be employed. Titanium (Ti) is a preferred metal as it provides the desired active surface and forms stable solids with very low vapor pressures. In an em bcdiment where titanium was used as the getter in the pump illustrated in Figure 3, pumping speeds were meas ured and are given in the following table.

Table Temperature of active surfacezapproximately 20 C.

Rate of evaporation of Ti=7 mgn1./m1n. Lowest pressure obtainedzapproximately 1 X 10- mm. Hg.

Ion Gauge Pressure in mm. of Hg Pumping Gas Speeds (liters/ second) Hydrogen Except for argon and helium, the pumping speeds are about ten times those for an oil diffusion pump of the same physical dimensions. These pumping speeds were measured by bleeding in gas at a known rate through a small tube ending near the geometrical center of the pump container in order that the gas meets little impedance.

To improve the pumping speed for argon and helium, which must be ionized and driven into the surface, a magnetic field can be imposed to increase the path length of the ionizing electrons. An oscillating electric field can be used to make the electrons execute oscillatory paths of increased length. Higher acceleration voltages will in crease the penetration of the ions into the surface and may improve pumping speeds and the ultimate vacuum. Outgassing of the gettering substance before it is used also may increase its ability to take up gas and thus improve the pumping speeds and ultimate vacuum.

Embodiments illustrating my invention are shown in the drawings.

Figure 1 a partial section elevational view of a pump using a resistance heated crucible for evaporation of the getter, with the vacuum producing apparatus connected in series between the system to be evacuated and a cooperating vacuum producing device, such as a mechanical pump.

Figure 2 is a partial section elevational view of a mod ification of the pump shown in Figure 1 with additional apparatus for the ionization or dissociation, or combination of the same, of gas molecules and the acceleration of gaseous ions into the wall. The gettering apparatus is similar to that shown in Figure 1 except that the pump has been rotated about a vertical axis.

Figure 3 is a partial section elevational view of a preferred modification of the invention where the evaporation is from a post heated by electron bombardment and one filament provides electrons for both evaporation and ionization. The vacuum producing apparatus of Figs. 2 and 3 is connected in parallel with a cooperating vacuum producing device, such as a mechanical pump.

Figure 4 is a horizontal cross section of the wire feeding device shown in Figure 3.

Figure 5 is a fragmentary side elevational view of the 23 wire feeding device as seen from the right hand side in Figure 3.

Figure 6 is a sectional view showing the outer end of the feeding device in cross section illustrating the engagement thereof with an external rotary actuator.

Figure 7 is an enlarged detailed sectional view of the wire feeder as seen in Figure 3.

Figure 8 is a view of the wire feeder similar to that shown in Figure 7 illustrating the position of the cooperating parts of the device in one phase of its operation.

Figure 9 is a view similar to Figure 8 illustrating a different operating position of the device.

Figure 10 is a view similar to that shown in Figures 8 and 9 illustrating a third operating position of the device.

Considering Figure l in detail, the pumping vessel is a metal (steel) housing or cylinder 10 capped off by flanges 11 as shown in the drawing utilizing copper gasket vacuum seals 12. The connection to the system to be evacuated is through inlet port or conduit 13. A mechanical roughing pump is connected to the pumping vessel by outlet port or conduit 14 through vacuum valve 15. Titanium wire 16 is continuously or intermittently fed through the guide tube 17 by a feeder mechanism (shown in more detail in Figures 4l0) into a crucible 18 machined out of graphite rod. The crucible is heated to a temperature of about 2000 C. by passing a current through it via the copper electrodes 19. Titanium is evaporated from the crucible and condenses on the cylindrical interior of the pumping vessel, forming the active surface at 20. Gas entering the pumping vessel contacts the active surface and combines with the titanium or other gettering substance. Since the active surface is constantly being formed, a pumping action results. To start this pump, the roughing pump is connected by opening valve 15. The crucible can be heated to outgas the graphite while the roughing pump is connected. When a vacuum of about 10 mm. Hg is obtained, the roughing pump is cut off from the pumping vessel by closing valve 15. The crucible is raised to the proper temperature and wire feeding is started. Where certain components of the gases do not combine with the gettering substance, this embodiment of the invention can be "used in series or in parallel with other vacutun producing devices. This embodiment can also be used in series or parallel with other vacuum-producing devices to pump components including organic vapors and thus eliminate the necessity of cold traps. In an embodiment of this type connected in parallel to an oil diffusion pump, the estimated pumping speed for hydrogen is about ten times that for an oil difiusion pump of the same physical dimensions.

In the embodiment shown in Figure 2, the evaporation mechanism is the same as that of the embodiment in Figure l but is shown at right angles. The embodiment of Figure 2 contains apparatus for the dissociation and ionization of the gases, and the acceleration of the ions into the active surface. Thermionic electrons are emitted from a cathode consisting of a hair pin filament 21 and are accelerated outward by placing the inner grid or cylindrical anode 22 at a suitable positive potential with respect to the cathode. The anode 22 presents a relatively small cross-sectional area to the incident electrons so that most of the electrons pass through into the region between anode 22 and the outer grid or anode 23. Anode 23, also of small crosssectional area, is electrically connected to anode 22 so that the electrons coast across the field-free region between the anodes at full ionizing energy. The cathode 21 is at a sufficiently positive potential with respect to the wall of the pumping vessel so that the electrons which pass out of anode 23 are repelled from the wall and return to the above mentioned field-free region without loss of energy. After traversing the field-free region, the reversed electrons enter the volume enclosed by anode 22 where they are again reversed in direction unless they strike the filament. This is not probable because of the small area subtended by the filament. Thus the electrons take up an oscillatory motion through both anodes resulting in a long path length before capture by the anodes or the filament. The energized electrons dissociate the gas into components which can be taken up by the titanium or other gettering substance. The electrons also ionize the gas. Since the volume enclosed by anode 22 is small compared to that of anode 23, thermal motion in the field-free region will most likely bring the ions to the region between anode 23 and the wall rather than to the interior of anode 22. The high positive potential of anode 23 with respect to the wall drives the ions into the active layer 20 where they are trapped long enough to be buried by new condensing metal or gettering substance. A small amount of additional gas may .be removed by combination of the gas with the hot filament. Hydrogen, nitrogen, oXygen, carbon dioxide, freon, sulfur-hexafiuoride, ethane, methane, ammonia, argon, helium, and air were pumped by this embodiment without the assistance of any other vacuum-producing device.

To put this embodiment in operation, a rough vacuum is established by connecting a mechanical pump to the pumping vessel through valve 15. The crucible is heated to outgas the graphite. When a pressure of about 10* mm. of Hg is reached, the filament 21 is heated and outgassed. Feeding of titanium wire 16 is started from spool 24, through guide tube 17 and at the same time the anodes 22 and 23 are raised to voltages of about 1000 volts. Valve 15 is closed and the pressure steadily decreases to about 3 X 10- mm. Hg. Outgassing of the pumping vessel while the roughing pump is connected can be employed to reduce the virtual leak of gases coming out of the old titanium layer and other metal parts.

The embodiment of Figure 3 is an improvement on those of Figure l and Figure 2 in several respects. Titanium or other gettering substance is evaporated from a graphite post 25 which is heated by electron bombardment. This method of evaporation improved the lifetime of pump components and reduced the power requirements. The geometry of the embodiment is such that the getter is evaporated onto a larger area and the active layer is more uniform. The same cathode, in the form of a spiral filament 21, is used as an electron source for electron bombardment, ionization, and dis sociation. The electrons used for ionization and dissociation are drawn outward from. the cathode 21 by placing inner grid or anode 22' at a positive potential of 1000 volts with respect to the cathode. The anode 22 is constructed to present a minimum cross-sectional area to the electrons so that most of the electrons pass into the region between anode 22 and outer grid or anode 23. Anode 23', which is also constructed to capture a minimum number of incident electrons, is electrically connected to anode 22' and thus the electrons maintain full ionizing energy until they pass through anode 23'. Outside anode 23', the electrons are reversed in direction due to a potential drop of 1100 volts from anode 23' to the wall. The electrons travel back through the volume between the anodes and are reversed again, unless they suffer a collision with the filament. This is not probable because of the small cross-sectional area presented by the filament. Thus the electrons oscillate over long path lengths before being captured by the anodes or the filament. Because of the small volume enclosed by anode 22 compared to that of anode 23', most ions formed between the anodes will not intercept anode 22', but escape with thermal velocities into the region between anode 23' and the wall. Outside anode 23, the ions are accelerated to high velocities and driven into the active layer 20 where they are trapped and buried.

Illustrative pumping speeds and pressures obtained with this embodiment are given in the table referred to above. Except for argon, helium and air (since air contains 1% argon) pumping speeds are high compared to those for oil diffusion pumps of about the same physical dimensions. With the pump shown in Figure 3, this system is started most easily by placing a furnace around the pumping vessel and outgassing the pumping vessel at about 300 C. while the roughing pump is connected through valve When a pressure of about 10* mm. of Hg is obtained, the furnace is removed and the roughing pump is disconnected by closing valve 15. The post and anodes are raised to proper voltages such as those shown in the drawing. The filament heating current is turned up until a suitable electron current is drawn to the post and to the anodes. When the post is hot enough to evaporate the gettering substance, the feeder is turned on and evaporation commences. For this embodiment it was desirable to cool the active surface once the system is started. The values in the table above were measured with water-cooled walls to provide an active surface at about C.

In the embodiments of Figures 1, 2 and 3, the getter to be evaporated is in wire form. The choice of feeding mechanism used to move the wire 16 from the spool 24 to the evaporation device is somewhat arbitrary providing the mechanism has a positive feeding action capable of overcoming the frictional resistance met by the wire in moving through the guide tube. In addition, the feeding mechanism must transmit motion into the vacuum system without the use of organic gaskets or grease-packed seals. Since frictional forces increase in a vacuum, a feeding mechanism free of sliding surfaces in the vacuum is desirable.

The preferred wire feeding device used in the embodiments of Figures 13 is shown in Figures 410. The action of this mechanism is positive, and it contains no organic components or sliding surfaces in the vacuum.

As shown in Figures 4, 5 and 6, a flexible thin-walled tube 30 is mounted on a shoulder in a hole through a steel block 31 by a rigid, vacuum-tight silver solder joint 32. The top of the tube is capped off by a driving plug 33 which is silver soldered into the tube. Under normal pumping conditions, atmospheric pressure exists inside the flexible tube while a vacuum is maintained outside. Aluminum or copper gaskets 34 on each end of the ceramic cylinder 35 provide the vacuum seal between the block 31 and the pump wall of the pumping vessel 10. The ceramic cylinder 35 and ceramic cylinder 36 electrically insulate the feeding mechanism from the wall of the pumping vessel 10 and prevent grounding of the evaporation device through the feeding mechanism when the wire contacts the evaporation device. Such insulation is necessary in the embodiment of Figure 3 where the evaporation device is not at ground potential.

Motion is imparted to the wire 16 by the proper manipulation of the wobble stick 37 which pivots on bearing surface 38 at the base of the flexible tube 30 with one end of the stick in a loose fitting hole in the driving plug 33 and the other end extending a suitable distance out of the clearance hole 49. The shape of the wobble stick in the flexible tube maximizes the rigidity of the stick but permits flexure of the tube and stick without binding. When the outer end of the wobble stick is displaced from the relaxed position on the axis of the clearance hole (Figure 6), the stick pivots on bearing 38 and the upper end of the stick displaces the driving plug in the opposite direction causing the tube to bend. Since the wobble stick exerts a deflecting force at the end of the tube, the flexure of the tube is that of a simple cantilever beam loaded at the end. By exerting the deflecting force at the end of the tube rather than at some point between the end" and the base a maximum displacement of the driving plug is obtained. For a given displacement of the driv-f. ing plug, exertion of the deflecting force at the end of the tube minimizes the stresses in the tube wall and the resulting strains are well below the elastic limit of the metal used in the tube.

In operation, the outer end of the wobble stick is driven in a circular path by a solid steel rod 39 which has been cut flat on one side 40 to accommodate therather pivots on bearing 38 in such a way that both ends. trace a circular path, neglecting constraining leaves 46,.

4'7 and 48.

The circular motion impressed on the driving plug;

imparts linear motion to the wire in a cycle of events. as shown in Figures 8-10. Figure 7 shows the position. of the driving plug 33 when in inoperative position prior to engagement of wobble stick 37 with flat 40 of rotary rod 39. The driving plug presses the wire against the driving leaf 42, which is hinged on spring 43, with sufficient force to bend spring 43 away from the block 31. The locking arm 44 mounted on spring 43 by holder 45 is thus lifted from the wire and the wire is free to move as shown in Figure 8. The driving plug thrusts the wire forward, carrying the hinged driving leaf along under enough force to keep the locking arm withdrawn. Figure 9 shows the end of the thrust. As the driving plug disengages the wire, spring 43 forces the locking arm against the wire preventing the wire from slipping back as shown in Figure 10. The driving plug presses against the hinged constraining leaf 46 as it moves away from the wire. Constraining leaves 46 and 47 determine the length of the thrust by limiting the travel of the driving plug parallel to the wire. Upon disengaging constraining leaf 46, the driving plug engages hinged constraining leaf 48 which prevents the driving plug from swinging too far out to engage leaf 47 properly. After engaging leaf 47, the driving plug binds the wire against driving leaf 42 and the cycle is repeated. Because of the constraining and driving leaves, the driving plug moves in a rectangular rather than circular path. The Wire moves forward in increments of about of an inch, and the number of increments or rod rotations per minute varies from A to 20. Where the metal to be evaporated in practicing this invention is in Wire form, other feeding mechanisms can be used, but the simplicity, reliability, positive action, and lack of organic components or grease in the feeding device described above make the choice of other mechanisms limited. I

When the getter is in wire form and is fed from a spool 24 as shown above, it was found that the wire had a tendency to resume its prior curvature on leaving the end of the guide tube. This characteristic of the wire made it diflicult to accurately bring the wire to the hot crucible or post for evaporation. With continued investigation it was discovered that the wire lost all tendency to resume its prior curvature when the guide tube was heated. With a heated guide tube, for example, it was found that the wire proceeded unsupported from the end of the tube in a straight line striking the evaporation device accurately at the desired spot even though the distance between the evaporation device and the tube end was sufliciently great to minimize the amount of metal condensing on the tube end. The guide tube or a portion thereof can be heated by any means including electrical means such as a resistance coil, high he quency induction coil, etc. In practice, however, it has been found preferred to heat the lower section of the guide tube by heat radiating from the evaporation device. The optimum positioning of the guide tube to prevent plugging by condensing getter and the optimum temperature at Which the guide tube should be heated can be readily ascertained by preliminary test. In all cases, the temperature of the guide tube should be below the point at which the getter loses its wire form or melts.

The present invention may be practiced according to the embodiments of the figures described above, but it is to be distinctly understood that the invention is not limited to these illustrative embodiments. In particular, the geometries and proportions of the pumping vessel and inner components are not limited to those of the above embodiments. The optimum area for the active surface will vary from one application to another and can be increased by the use of fins in the pumping vessel.

The gettering substance to be evaporated is not restricted to titanium, but may be any gettering substance.

The embodiments discussed are not to be construed as excluding other means of evaporation in practicing this invention. The evaporation process may be intermittent rather than continuous. It may be desirable in some embodiments to outgas the getter to be evaporated before it is used and thus increase the getters capacity to take up gas. Suitable magnetic fields, both constant and oscillating, can be used to increase the electron path length and consequently improve the ionization. The electron accelerating voltage applied to the anodes can also be constant or oscillatory. The voltages applied to the filament, the anodes, and the Wall may differ from values mentioned above, the optimum conditions for a particular pump being readily ascertainable by preliminary tests. The optimum temperature of the active surface may vary in different systems and may be of different values for different parts of the active surface. This can be readily accomplished by the use of standard water jackets on the pump using water at different temperatures. Starting procedures will vary in different embodiments of the invention and can involve the rapid evaporation or flash gettering of a small amount of getter in an apparatus distinct from that for the continued evaporation. A primer charge of getter can also be evaporated from the crucible or post at the start of the pumping operation. Localized bake outs can also be employed in place of the bake out described above with reference to the operation of the pump illustrated in Figure 3. Various other modifications coming within the spirit and scope of my invention will be obvious to those skilled in the art.

I claim:

1. Vacuum producing apparatus for use with a cooperating vacuum producing device, comprising a housing defining a chamber, means. for coupling one end of the chamber to a system. to be evacuated, means for coupling the other end of the chamber to said cooperating vacuum producing device, and means located inside the chamber for the continued evaporation of a getter material and located to cause the evaporated getter to condense on a surface inside the chamber to trap gas molecules, so that the gettering action assists said cooperating vacuum producing device in the evacuation of the system, and so that the gettering action in the chamber serves as a trap between the system to be evacuated and said cooperating vacuum producing device to prevent any vapors which may be emitted from said co- 0 erating vacuum producing device from entering the system which is to be evacuated.

2. Vacuum producing apparatus for use with a cooperating vacuum producing device, comprising a housing defining a chamber, means for coupling the chamher to a system to be evacuated, means for coupling the chamber to said cooperating vacuum producing device, means located inside the chamber for evaporating a portion of getter material and located to cause the evaporated getter material to condense on a surface inside the chamber to trap gas molecules, and means located inside the chamber for evaporating additional portions of getter material to trap additional gas molecules and to bury the gas previously trapped by the condensed getter, so that both the gettering action in the chamber and said cooperating vacuum producing device serve to evacuate the system.

3. The apparatus of claim 2 wherein said chamber is coupled in parallel with said cooperating vacuum producing device.

4. The apparatus of claim 2 wherein said chamber is coupled in series with said cooperating vacuum producing device.

References Cited in the file of this patent UNITED STATES PATENTS 2,100,045 Alexander Nov. 23, 1937 2,153,786 Alexander Apr. 11, 1939 2,521,345 Cortright Sept. 5, 1950 2,636,664 Hertzler Apr. 28, 1953 2,727,167 Alpert Dec. 13, 1955 2,755,014 Westendorp July 17, 1956

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