US3501617A

US3501617A - Apparatus for sublimating and evaporating metals

Info

Publication number: US3501617A
Application number: US685674A
Authority: US
Inventors: Helmut Vedder
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1967-11-24
Filing date: 1967-11-24
Publication date: 1970-03-17
Anticipated expiration: 1987-03-17

Description

March 17, 1970 APPARATUS FOR Filed NOV. 24, 1967 H. VEDDER 3,501,617

SUBLIM ATING AND EVAPORATING METALS 3 Sheets-Sheet 1 l N V E NTO 12 I He/mz/f Meobe/f APPARATUS FOR SUBLIMATING AND EVAPORATING METALS Filed m 24, 1967 H. VEDDER March 17, 1970 3 Sheets-Sheet 2 )NVENTOIZ HeMz/f Mew 48V BYWEMWH, flm

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APPARATUS FOR SUBLIMATING AND EVAPORATING METALS March 17, 1970 5 Sheets-Sheet 5 Filed Nov. 24, 1967 INVENT'OIZ 3O He/ma/ Meade? av Wmwflmkdl, 3W

Elm ATTORNEYS United States Patent 3,501,617 APPARATUS FOR SUBLIMATING AND EVAPORATING METALS Helmut Vedder, Downers Grove, Ill., assignor to Chicago Bridge & Iron Company, Oak Brook, Ill., a corporation of Illinois Filed Nov. 24, 1967, Ser. No. 685,674 Int. Cl. F22b 1/28 US. Cl. 219-271 15 Claims ABSTRACT OF THE DISCLOSURE Disclosed is a device, hereinafter referred to as a filament, for subliming or evaporating materials. The filament comprises a carrier rod or carrier tubing having a series of noneonsumable elements intermittent consumable elements made of materials to be sublimed or evaporated by electrical power either directly energized as an electrical resistor orindirectly by resistant heating, induction or electron bombardment.

This invention relates to apparatus and methods for subliming or evaporating materials.

It is well known that relatively small changes in the temperature of certain materials will significantly alter their evaporation or sublimation rate. The rate increases as the temperature increases. Generally, it is desirable to produce high evaporation or sublimation rates. One state-of-the-art method for evaporating or subliming materials is to place the material to be sublimed (consumable material) in a container, such as a shallow dish, which may be heated by passing an electrical current through the container. The material for the container must have a melting point which is significantly higher than that of the material to be sublimed or evaporated and both materials should not combine and form an alloy having a melting point unsuitable to the operation temperature. A drawback of the container method is the fact thatit cannot be used in all positions since the sublimed material would' fall out of the container, and the fact that the evaporation or sublimation is necessarily narrowly directional. Also, heat transfer to the consumable material is generally rather poor because of poor thermal contact.

Another prior art technique is to hang small pieces of the material to be sublimed or evaporated from a straight or spirally wound refractory metal Wire or stranded wire filament which is electrically heated. This prior art method involves a waste of consumable material because much of it will melt instead of evaporating or subliming and large molten pieces of it will drop from the wire filament. Also, in order to obtain high evaporation rates, the consumable material must be heated to near its melting point. This can cause local melting. The molten material will then combine with the wire filament material and form an alloy with a lower melting point, thus causing failure of the wire filament. There is also poor thermal contact between the wire filament and the consumable material, which makes it necessary to use a very high filament temperature. Control of the temperature in this operation is very diflicult. As a result, there are produced local hot spots on the consumable material which causes it to melt and to combine with the wire filament material to form alloys which melt at lower temperatures than does the original filament. This causes premature failure of the wire filament.

Evaporation-sublimation devices employing a consumable material in the form of a wire wound around a heating rod have also been used but have failed be- "ice cause the wires only touched the rod and adjacent wires along a line contact. Only through this line contact could electricity and heat be conducted from the heating rod to the outside and through adjacent wires from oneend of the device to the other. In areas where there is poor contact between the wires, the consumable material melts, drops are formed and the molten material licks away adjacent areas of the device. This local decreasing of the cross-section of the filament results in higher current densities and even higher local temperaturse, thus leading to the final complete destruction of the device.

Various other forms of evaporation and sublimation devices have been used but they all have deficiencies, including at least some of those just described. There is thus a clear need for improved sublimation and evaporation units which permit controlled sublimation or evaporation at temperatures which are high enough to provide for desirable rates of evaporation or sublimation and yet sufficiently low enough to avoid, in most cases, actual melting of the consumable material.

This invention relates to apparatus and methods for subliming or evaporating materials without the disadvantages listed above. While the filament of this invention will be described hereinafter with regard to titanium as the consumable material, it should be understood that other consumable materials besides or in addition to titanium can be used. By consumable material is meant a material which evaporates or sublimes from the device or filament, as distinguished from other parts of the device which do not evaporate or sublimate under conditions of use to an extent considered as spoiling to the process to be performed.

The filament provided by this invention comprises an elongated carrier rod having a series of consumable plates thereon with spacer plates of nonconsumable material intermittent the consumable plates. The rod and spacer plates are advisably of a material which has a melting point higher than titanium or whatever consumable material is used. Thus, the rod and spacer plates can be suitably made of tungsten, columbium, tantalum, molybdenum, or alloys of these materials, carbon or other materials with a high melting point.

The invention will now be described in conjunction with the attached drawings in which:

FIGURE 1 is a partial sectional view on the line 1-1 of FIGURE 2, and shows a carrier rod with plates of titanium or other consumable material intermittently separated by spacer plates;

FIGURE 2 is a sectional view along the line 2-2 of FIGURE 1;

FIGURE 3 shows a pair of rods like those of FIGURE 1 joined at their ends;

FIGURE 4 is similar to FIGURE 2but shows a milled surface on the carrier rod;

FIGURE 5 is similar to FIGURE 2 but shows optional striated or knurled surfaces which can be employed on the spacer and/or titanium plates;

FIGURE 6 is an enlarged view of the striations or knurls on the plate of FIGURE 5 such as can be formed by a milling operation;

FIGURE 7 is similar to FIGURE 1 but shows, in crosssection, some of the plates with striated or knurled surfaces to facilitate tight continuous contact between adjacent surfaces;

FIGURE 8 is a partial view in cross-section of a filament having a hollow carrier rod;

FIGURE 9 is a cross-sectional view of a filament using a carrier tube containing a separate heating element therein;

FIGURE 10 is a sectional view along the line 1010 of FIGURE 9;

FIGURE 11 is a partial plan view of a filament made of strips of alternating consumable and nonconsumable material wound in layers on a carrier rod;

FIGURE 12 is a sectional view of the filament of FIG- URE 11 taken on the line 1212;

FIGURE 13 is a sectional view of another embodiment of the filament of this invention; and

FIGURE 14 is a sectional view of another embodiment of the filament of this invention have the nonconsumable plates integral with the carrier rod.

So far as is practical, the various parts and elements which appear in the different views of the drawings will be identified by the same number.

As shown in FIGURE 1, the filament of this embodiment of the invention broadly comprises an elongated carrier rod on which a series of consumable plates 11, such as of titanium, are threaded on nonconsumable spacer plates 12 placed intermittently between pairs of plates 11. The nonconsumable spacer plates 12 are made of a material which has a higher melting point than the consumable plates 11 and which does not appreciably sublime or evaporate at the filament operating temperatures. Generally, the spacer plates are made of a metal, such as tungsten, columbium, tantalum, molybdenum or allows of these materials. Similarly, such materials, as well as carbon, can be used for the carrier rod 10 since they provide the necessary electrical resistance for heating the filament with high amperage and low voltage.

Although FIGURES 1 and 2 show a carrier rod with consumable titanium plates 11 and nonconsumable spacer plates 12 which are circular, it is obvious that square, triangular or multi-sided shaped plates can also be used satisfactorily in producing the filament provided herewith. It is generally advisable, because of convenience and ease of fabrication, to employ a rod of circular cross-section and circular fiat washers for the titanium plates and the spacer plates. Metal rods of the desired composition are available commercially and the washers can be produced conveniently by stamping them from commercially available sheets of metal.

The sublimation or evaporation filaments of FIGURES 1 and 2 can be readily used by attaching an electrical terminal at the end of each carrier rod in order to heat the filament by electrical energy.

With reference to FIGURE 3, there is shown a particularly suitable form of the filament comprising two carrier rods 10 placed substantially side-by-side with two ends placed together and joined by an electrically conductive tab 13 which can be made of any material which will not significantly melt or sublime at the operating conditions of the filament. Titanium plates 11 and spacer plates 12 are then positioned on the rods 10 in a threaded arrangement and the other ends of the rods threaded into electrical terminal elements 15 of such a size and shape as to be readily connected to an arrangement for supplying a source of electrical energy to the filament. The

plates

11 and 12 are threaded tightly together so that they are in intimate contact with adjacent plates so that electrical and heat energy can be transmitted readily through the entire filament.

An evaporation and sublimation filament having the washers arranged around a carrier rod provides an excellent way for transfer of heat and electricity from the center rod to the outside surface. As a result, local hot spots are avoided.'

The nonconsumable spacer material intermittently placed between consumable material performs an extremely important function. When local melting of the consumable material occurs, it is retained in place between the non-consumable spacer material. This prevents large globules of molten material from forming and also prevents localized thinning of the filament. As a result, when electrical energy is used for heating, current densities through the unit are kept uniform, thereby providing uniform temperatures. The amount of alloying between the carrier rod and the consumable metal is also limited by the amount of consumable metal in contact with the carrier rod. That is, consumable metal washers or groups of these melted in the direct neighborhood of the carrier rod cannot melt together with the next consumable washers or groups of these in the area close to the carrier rod because they are separated by the spacer washers.

The filament of FIGURES 1 to 3 has proven able to sublime large amounts of the consumable metal at high rates, while providing good reliability, until almost all of the consumable metal has been sublimed, as long as the proper power level is maintained. The proper power level is that which sublimes the consumable material at the maximum rate without melting the outside surface. The nonconsumable spacer washer also serves the function of conducting heat away from hot spots which might be locally generated, thus again reducing the failure rate. The nonconsumable spacer plates also act as heat transfer fins to more uniformly heat the consumable material, thus decreasing any temperature difference between the inside surface of the consumable washer and its outer surface.

FIGURES 4 to 14 of the drawings illustrate further embodiments of the filament of this invention. However, all the embodiments of the invention as shown in the drawings have in common the provision of a carrier rod with nonconsumable spacer material placed thereon intermittently between consumable material, also supported by the rod, which can be evaporated or sublimed. The lateral walls of the nonconsumable material prevent running of any consumable material which may become molten during heating. In this way, all molten consumable material is localized and prevented from licking other molten consumable material into a pool or large drops. The nonconsumable material thus functions as intermittent barriers or dams which restrict or bar flow of molten consumable material, produced by the fact that the cohesive force in molten metal is greater than its adhesion. Of course, to suppress forming large drops a nonconsumable material should be used which is easily wetted by the consumable material.

In order for the consumable plates 11 and nonconsumable spacer plates or washers 12 of the filament of FIG- URES 1 to 3 to be placed in improved electrical and heat transfer contact with the carrier rod, the surface of the rod 10 can be milled to provide a grooved surface 16 as shown in FIGURE 4. The hole in the

washers

11 and 12 can be slightly smaller than the outer diameter of the grooved rod 10 so that a slight pressure is needed to force the washer onto the rod during the threading operation.

With reference to FIGURES 5-7, at least one surface of the consumable titanium or spacer plates can be grooved, striated or knurled 17 in order to obtain improved electrical and heat transfer contact between adjacent plates or washers. The raised peaks 18 as shown in FIGURE 6, formed by cross-milling of the surface of the plates, are squeezed into the adjacent surface when the plates are in threaded position on the carrier rod 10 and pressure is applied thereto.

FIGURE 8 shows another embodiment of the filament having a hollow carrier rod 19 which supports titanium plates 11 and spacer plates 12 in a manner similar to that shown in FIGURE 1.

FIGURES 9 and 10 show still another embodiment of the invention. Tubular carrier rod 20 has titanium plates or washers 22 and spacer plates 21 tightly threaded over the outer-surface thereof. The end of tube 20 is covered by plate 23 to provide a heat shield. A separate or indirect electrical resistance or other type of heat source 24 is positioned inside of tube 20 adjacent to its inner surface to provide means to raise the temperature of the filament sufficiently high to cause sublimation of the titanium. This filament can be heated by the use of either indirect resistance, or direct resistance, heating.

FIGURES 11 and 12 illustrate still another embodiment of the invention. Rod 30, which can be a metal such as tantalum, has three layers 31 of a consumable material, such as titanium, in the form of strips tightly wound thereon. The consumable material strips 31 are placed one on top of the other with sufficient space provided between them to accommodate strips 32 wound, one on top of the other, onto rod 30. Strips 32 are made of a nonconsumable material, which may be metallic or nonmetallic, such as of a suitable ceramic material.

Although FIGURE 12 shows strips 31 and 32 having the same width, either one can be wider than the other although to minimize consumption of metal it is advisable to use nonconsumable strips 32 more narrow than consumable strips 31. Also, two or more strips 31 and/or 32 can be wound side-by-side if desired. All of

strips

31 and 32 are tightly wound onto rod 30 so that good face-toface contact is obtained between

adjacent strips

31 and 32 at abutting edges, and between top and bottom contacting surfaces of strips in the same stacks. The ends of rod 30 can be provided with suitable means to support it in posi tion and/or to provide electrical current thereto. When desired, rod 30 can be made of a suitable material which is nonconductive to electrical energy such as when heating 'is to be effected by indirect means or when electrical energy is supplied directly to the

strips

31 and 32, both of which must be metal in such case.

FIGURE 13 illustrates a further embodiment of the invention. In this embodiment, rod 40 is metallic and has plates of nonconsumable material 41 placed intermittent to consumable plates 42. Plates 41 can be metallic or nonmetallic. Specifically, rod '40 and plates 41 can be tantalum and plates 42 can be titanium. This embodiment is shown to have plates 42 about twice as thick as plates 41. However, the thickness of the plates can be varied according to the characteristics of the material being evaporated or sublimated, i.e., the temperature employed and the rate of evaporation or sublimation desired.

The embodiment of FIGURE 14 has a rod 50 which plates 51, integrally formed therein. The space between adjacent plates 51 is filled with a consumable material 52, such as titanium. Rod 50 and plates 51 can be made of a nonconsumable material, such as of a metallic or ceramic material which is not substantially sublimed or evaporated during use of the filament. Specifically, rod 50 and plates 51 can be tantalum and consumable material 52 can be titanium.

In the filaments provided herewith, it is advisable to have the nonconsumable spacer washer at least as big in diameter as the consumable titanium washers to provide adequate barriers against melted titanium running together and forming large molten drops. In other words, the dimensions of the nonconsumable washer should usually be no less than the dimensions of the consumable material, except in thickness and as to thickness the Washer can be thicker or thinner than the consumable material. The barrier plates do not excessively melt or evaporate or sublime under the conditions of use. The thickness of the spacer plates can usually be considerably less than the distance between consecutive spacer plates, said distance being filled with a single consumable plate, such as titanium plate or plurality of titanium plates.

It is also advisable at times to braze the metal elements of the filament together. This can be readily done by placing very thin nickel washers between the other washers or by plating the carrier rod, or plating one side of the titanium washer, with nickel or another suitable metal or alloy, and heating the filament under vacuum until the brazing material melts and is drawn by capillary attraction to flow between the plates and rod. Other conventional brazing techniques can also be employed.

Although the nonconsumable spacer plates and consumable plates can be varied according to any desired arrangement, it is usually advisable to arrange the spacer plates in an arithmetic series betwen the titanium plates.

evaporated from the filament. However, since some metals evaporate faster than others, the ratio of metals on the filament must be adjusted to obtain the desired alloy composition. For example, to deposit an alloy containing 50% nickel and 50% iron, the filament would advisably contain two iron washers to one nickel washer since nickel sublimes twice as fast as iron.

The filament to which this invention pertains may be heated in a variety of ways. In addition to heating by either direct resistance or indirect resistance, or combinationss, thereof, as described above, the filament may be heated by induction, radio frequency energy, or by an electron bombardment either alone or in combination with other methods. Non-electrical methods of heating may be used. Any of these methods of heating or combinations thereof which achieve higher filament surface temperatures, relative to interior temperatures, are most advantageously employed since such techniques limit the migration of impurities to the filament surface. Migration of the impurities to the filament surface, especially when the consumable metal melts, diminishes the effective surface area from which sublimation or evaporation can occur, thus reducing the effectiveness of the filament. This problem will be overcome, to some extent, by operating the filament below the melting point of the consumable material. It has been found that temperatures below, but within 10% of, the melting point of the consumable metal will give suflicient sublimation and at the same time min imize melting from localized hot spots.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A filament comprising:

a pair of elongated electrically conductive carrier rods having an electrical conductor means joining an end of each rod together in substantially side-by-side position,

each rod having consumable material thereon with nonconsumable spacer material carried on the rod intermittent to the consumable material,

said rod, spacer material and electrical conductor means being of material which has a higher melting point than the consumable material, and,

an electrical terminal at the other end of each of the rods for connecting to a source of electrical energy.

2. A filament comprising:

each rod having a series of consumable plates threaded thereon with nonconsumable spacer plates threaded on the rod intermittent to the consumable plates,

said rod, spacer plates and electrical conductor means being of metal which has a higher melting point than the consumable plates, and

3. A filament according to claim 2 in which the consumable plates are of titanium and in which the rods, spacer plates and electrical conductor means are tungsten, molybdenum, tantalum, columbium or alloys thereof.

4. A filament according to claim 2 in which the plates are substantially flat circular washers with a central hole through which the rod is threaded.

5. A filament comprising:

each rod having consumable sublimatable material thereon with nonconsumable spacer material carried on the rod intermittent to the consumable material,

said electrical conductor means being of rigid material for supporting and holding said rods,

said rod, spacer material, and electrical conductor means being of material which has a higher melting point than the consumable material, and,

6. A filament comprising:

a pair of elongated electrically conductive carrier rods having an electrical conductor means joining an end of each rod together in substantially sideby-side position,

each rod having a. series of consumable sublimatable plates threaded thereon with nonconsumable spacer plates threaded on the rod intermittent to the consumable plates, said electrical conductor means being of rigid material for supporting and holding said rods,

said rod, spacer plates and electrical conductor means? are substantially fiat circular washers with a central hole 11. A filament according to claim 6 in which the plates are brazed together and to the rod.

12. A filament comprising:

each rod having a series of titanium plates threaded thereon With nonconsumable spacer plates threaded on the rod intermittent to the consumable plates,

said rods, spacer plates and electrical conductor means are tungsten, molybdenum, tantalum, columbium or alloys thereof,

said consumable and nonconsumable spacer plates are substantially flat circular washers with a central hole through which the rod is threaded, and

13. A filament according to claim 12 in which the washers are threaded or strung tightly on the rod and into tight contact with adjacent washers.

14. A filament according to claim 12 in which the diameter of the spacer washers is at least as large as the diameter of the consumable washers.

I 15. A filament according to claim 12 in which the washers are brazed together and to the rod.

References Cited UNITED STATES PATENTS 1,891,074 12/1932 Winter 313-339 2,056,621 10/ 1936 Roberts 313-338 X 2,112,969 4/1938 Maurogenis 313-338 X 2,477,279 7/ 1949 Anderson 313-346 FOREIGN PATENTS 1,023,598 3/1966 Great Britain.

JOSEPH V. TRUHE, Primary Examiner PETER W. GOWDEY, Assistant Examiner US. Cl. X.R. 118-495