US3517161A - Sublimation unit and system - Google Patents

Description

June 23, 1970 T. $.GRACZYK SUBLIMATION UNIT AND SYSTEM Filed Sept. 13. 1967 INVENTOR. 7 #1406 0-5 S.- GRACZYK BY n 747 A TTOR/V' Y.

United States Patent O 3,517,161 SUBLIMATION UNIT AND SYSTEM Thadeus S. Graczyk, Rochester, N.Y., assignor, by mesne assignments, to The Bendix Corporation, Detroit, Mich., a corporation of Delaware Filed Sept. 13, 1967, Ser. No. 667,551 Int. Cl. C23c 13/00 U.S. Cl. 219275 1 Claim ABSTRACT OF THE DISCLOSURE A sublimation unit for getter vacuum pumps or evap oration deposit apparatus, comprising several resistance heated sublimation elements which are spaced from each other and which are interconnected in electrical parallel within the unit.

A sublimation system including such a unit is also disclosed.

BACKGROUND OF THE INVENTION Field of the invention The subject invention relates to the sublimation of material and, more particularly, tosublimation units and systems useful in getter vacuum pumps and evaporation deposit apparatus.

Description of the prior art The use of sublimation techniques to provide getter material in vacuum systems or deposits in evaporation deposit apparatus is well known. In general, these techniques reside in the use of a source of sublimation material in conjunction with a condensing surface or substrate to be coated or covered by the deposition of material Sublimated from the source which is suitably oriented with respect to and appropriately spaced from the con densing surface or substrate in accordance with wellknown principles.

A familiar species of these techniques includes the use of electrical resistance heating for the sublimation of suitable getter or vacuum deposit materials. In most instances this is accomplished by providing one or more wires of the material to be sublimated in heat-transfer relationship with one or more filaments of electrically resistive refractory material. conventionally, the filaments and wires are combined into one elongated sublimation element.

This type of sublimation element is widely used in the art because of its relatively low cost and of the ease with which it can be handled and energized. These elements are, however, vulnerable to local resistance increases at one or more points along their lengths.

Local resistance increases may, for instance, be caused by imperfections in wire thickness, the presence of impurities, variations in oxide layers, or filament and wire separations. The result is local overheatnig which leads to a melting of sublimation material. The locally melted material is drawn by surface tension forces to adjacent regions of the element where it impairs the sublimation process. After a relatively short time, the local overheating reaches a point where the refractory filament burns out while substantial portions of the sublimation material are still on the element.

Another drawback of the sublimation elements under consideration is their general sensitivity to overheating.

3,517,161 Patented June 23, 1970 This sensitivity is due to the fact that the temperature range of effective sublimation is close to the melting point for most suitable sublimation materials. While this sensitivity affects in its basic aspect the entire sublimation element, the problem raised thereby is compounded by the above mentioned proneness to localized resistance increases. In practice, this problem manifests itself not only by an accelerated destruction of the sublimation element, but also by the fact that even the use of a regulated power source for the element cannot continuously assure an operation within the temperature range where sublimation proceeds most effectively for the particular materials.

SUMMARY OF THE INVENTION The subject invention provides sublimation units and systems with self-regulating features which materially prolong the useful life of sublimation elements.

More specifically, the invention provides a sublimation unit comprising a plurality of elongated sublimation elements each including sublimation material and each defining at least one electrically conductive path between a first region and a second region of the particular element. The sublimation unit according to the invention further includes means mounting the sublimation elements in a mutually spaced relationship, means electrically interconnecting the mentioned first regions of the sublimation elements and providing a first terminal for these elements, and means electrically interconnecting the mentioned second regions of the sublimation elements and provid ing a second terminal for the sublimation elements.

The invention also provides a sublimation system comprising a plurality of sublimation elements each including sublimation material and electrically conducting means for heating the sublimation material, and means mounting the sublimation elements in a mutually spaced relationship. This sublimation system further includes a source of electric currrent having output terminals and providing at these output terminals a voltage which increases as a function of the resistance of a load connected to the output terminals, and means for connecting each sublimation element across the mentioned output terminals and in electrical parallel to the other sublimation elements.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing illustrates a preferred embodiment of a sublimation unit and a sublimation system according to the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The sublimation unit 10 shown in the drawing includes four elongated sublimation elements 11, 12, 13, 14. As shown by way of example and on a magnified scale in the outline 15, each sublimation element is composed of a filament 16 and of a wire 17 of sublimation material wound around the filament 16. In accordance with conventional practice, the filament may be composed of a solid wire or of a rope of stranded wires of an electrically conducting refractory material, such as tungsten. The sublimation material may be a material, such as titanium, which has a lower melting point than tungsten. While titanium is an excellent getter material for vacuum pumps, other sublimation materials may be chosen as material for the wire 17, particularly if the unit 10 is not employed as a getter source, but as a source of material to be vacuum deposited. In the latter case, the wire 17 is made of the material which is desired to be deposited.

In the subslimator unit each sublimation element 11, 12, 13 and 14 has a first end portion 20, 21, 22 or 23 connected to a first terminal and mounting member 25 in which these first end portions are retained in bores 26, 27, 28 and 29 by set screws, two of which are visible at 30 and 31. Similarly, each sublimation element has a second end portion 33, 34, and 36 connected to a second terminal and mounting member 38, which also has bores (not shown) and set screws, two of which are shown at 40 and 41, for retaining the end portions 33 to 36.

The first member 25 is connected to a rod 43 which provides a first terminal 44 for the sublimation unit 10. The second member 38 is connected to a rod 45 which provides a second terminal 46 for the sublimation unit 10. It will be appreciated from a consideration of the drawing that the members 25 and 38 mount the sublimation ele ments 11, 12, 13 and 14 in a mutually spaced relationship and connect these elements in an electrically parallel relationship between the unit terminals 44 and 46. This electrical interconnection is thus effected inside the unit 10, rather than by external wires.

The sublimation unit 10 is mounted between mounting members 48 and 49 which are located inside a vacuum vessel (not shown) in which the sublimation is to take place. Wires 51 and 52, which may extend through conventional vacuum lead-throughs (not shown), connect the unit terminals 44 and 46 to the output terminals 53 and 54 of an electric power supply 55. Set screws 56 and 57 removably retain the terminal rods 43 and 45 in electrical contact with the wires 51 and 52.

The power supply is of a type which provides an output voltage V across the output terminals 53 and 54 which increases as a function of the resistance of a load connected to these output terminals. In the illustrated embodiment, the load just referred to is the sublimation unit 10, or more exactly the parallel-connected sublimation elements 11 to 14. I

Power sources which provide an output voltage that increases as a function of the load resistance include constant current and constant power sources, both of which are conventional, and both of which are suitable as power sources in the sublimation system of the subject invention. Other suitable power sources for the subject sublimation systems are those which have a substantial internal resistance, such as an internal resistance which corresponds substantially to the total electrical resistance of the parallel-connected elements 11 to 14.

Accordingly, the power supply 55 (which can be any of the aforementioned power supplies suitable for sublimation systems) is symbolically illustrated as a battery 59 being connected between the output terminals 53 and 54 in series with a resistor 60. The resistor 60 is intended to represent the internal resistance of the power supply 55, although it will be understood that an external impedance or resistor may be connected in series with the terminal 53 or the terminal 54 in order to supplement the impedance of the internal resistance of the power supply 55.

An electric current i flowing from the power supply 55 to the sublimation unit 10 will be subdivided into the current components i i i and L; in the parallel branches constituted by the sublimation elements 11, 12, 13 and 14. If the electrical resistances of these elements are of equal value, the current components just referred to will also be of equal value, and each element will perform substantially the same sublimation operation as the other sublimation elements of the unit 10.

For the purpose of illustration it is now assumed that during the operation of the unit 10 one of the elements, element 11, of elements 11, 12, 13 and 14 increases its resistance element 11. As has been described above, this would result in overheating and accelerated destruction of the sublimation element 11 if it were the only element connected to the power supply. The same detrimental result would follow if the sublimation elements 11 to 14 were connected to a constant-voltage power supply.

A self-regulating action takes place, however, if the parallel-connected sublimation elements 11 to 14 are energized by the power supply 55 which, as mentioned above, is of a constant-current or constant-power type, or is of another type which provides an output voltage that increases as a function of the output load resistance. This self-regulating action mainifests itself by the fact that the sublimation elements 12, 13 and 14 serve as shunts for the element 11 when a resistance increase occurs in the latter element. Accordingly, each of the elements 12, 13 and 14 absorbs a larger share of the total current i than the increased-resistance element 11, which materially decreases the probability of an accelerated destruction of the ele ment 11 by local overheating.

A further self-regulating action is due to the fact that each of the elements 12, 13 and 14 will sublimate more material than the element 11, as each of these elements 11 13 and 14 conducts more current than the element 11 after the resistance of the latter element has increased as mentioned above. Accordingly, the resistance of the elements 12, 13 and 14 increases until it substantiall equals the resistance of the elemnt 11, whereupon the currents i i i and L, are again balanced.

The self-regulating action described here with respect to the element 11 does, of course, also take place with respect to any of the sublimation elements 12, 13 and 14. Moreover, should the element 11, for instance, burn out, the described self-regulating action will be carried out by the remaining sublimation elements 12, 13 and 14, thereby prolonging the life of the sublimation unit 10.

The theoretical minimum of the number of sublimation elements present and initially active in the sublimation unit 10 is two, and sublimation units with only two parallelconnected sublimation elements may be used to take advantage of at least the fundamental aspects of the selfregulatory action according to the subject invention. However, I have found that this self-regulatory action is improved as the number of sublimation elements is increased, since current increments are then distributed over more shunt paths when one or more elements are affected by local resistance increases. Through practical tests I have determined that improved results are obtained with a minimum of three parallel-connected sublimation elements, these results being further improved in a typical case by the use of four elements as shown in the drawing. As an upper limit, I prefer six parallel-connected sublimation elements, since the use of more than six elements generally either requires an extremely high current input or the provision of rather thin sublimation elements which would be difficult to manufacture.

While specific embodiments of my invention are described and illustrated herein, modifications thereof within the spirit and scope of the invention will be apparent to those skilled in the art.

I claim:

1. A sublimation unit comprising:

a first electrically conductive mounting means having a plurality of holes arranged in mutually spaced relationship, said first mounting means having a terminal for receiving an electric potential;

a second electrically conductive mounting means having a plurality of holes arranged in the same mutually spaced relationship as the holes in said first mounting means, said second mounting means having a terminal for receiving an electric potential;

a plurality of straight filaments mounted in the holes of said first and second mounting means so that said mounting means are spaced from each other with said filaments extending therebetween in mutually spaced relationship, said filaments in electrical contact with said first and second mounting means; and

a plurality of wires of sublimation material separately wound around each of said filaments, said sublimation wires contacting said filaments so that when a potental is applied to the terminals of said first and second mounting means said filaments conduct electrical currents that uniformly raise the temperature of said filaments to sublimate said wires.

References Cited UNITED STATES PATENTS 2,969,448 1/1961 Alexander 219-271 3,068,337 12/1962 Kuebric et al. 219275X 6 3,231,715 1/1966 Hall 118-49.1 X 3,231,846 1/1966 Radke 219271 X 3,274,372 9/1966 Frank 219-271 5 JOSEPH V. TRUHE, Primary Examiner P. W. GOWDEY, Assistant Examiner US. Cl. X.R.

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