Getter pump with high velocity of gas sorption

Abstract: A getter pump (10) is disclosed, wherein, inside a generally cylindrical chamber (12), there are three to eight getter structures (13, 13', ...), parallel to one another and to the chamber axis; each of these structures is formed of disks (14, 14', ...) of getter material supported by a central mounting (15, 15', ...). The getter elements are symmetrically arranged around the pump axis, and define a central cavity (17) having a heating member (16) housed therein, which is necessary for the activation and the working of the getter material. This pump has a higher sorption velocity with respect to getter pumps of similar dimensions.

Description: The present invention relates to a getter pump with high velocity of gas sorption. Getter pumps are long known in the vacuum field, and are appreciated in particular in that being static pumps, i.e. pumps without moving members. The working of these pumps is based on the chemisorption of reactive gas species such as oxygen, hydrogen, water and carbon oxides by elements made ofnon-evaporable getter materials (known in the field as NEG). The main NEG materials are titanium- or zirconium-based alloys. Getter pumps are known wherein the getter material is deposited onto generally flat metal supports, in form of layers less than a millimeter thick. Although widely used, these pumps have a small capacity of gas sorption, due to the small amount of getter material. In order to overcome such a drawback, the applicant has recently provided pumps having an increased capacity, wherein the getter elements consist of porous bodies formed of powders of sintered getter materials. Pumps of this type are disclosed, e.g. in patents US 5320496 and US 5324172. These pumps comprise a cylindrical metal chamber wherein a plurality of porous getter bodies are contained. In both cases, the set of getter bodies fills up the peripheral portion of the chamber, while leaving a cylindrical cavity in the center of the pump, which houses a heater for the activation and the efficient working of the getter material. The design of these pumps is intended to obtain a high capacity of gas sorption, but not to maximize the velocity of such a sorption, being in some uses the main feature required for the pump. In particular, these pumps have a reduced gas conductance in the portion between the inner wall of the pump body and the set of getter bodies; furthermore, the most peripheral portion of the getter bodies is only indirectly heated by conduction inside the getter body itself. These facts contribute to reduce the frequency of the effective collisions of the gas molecules on the getter bodies' portions being most peripheral with respect to the center of the pump, thus resulting in a reduced overall sorption velocity. Even slight modifications to the design of the two above pumps are not advisable, since, as the text of the aforementioned patents discloses, the good performances of these pumps are also due to the geometrical dimensions of the getter bodies (thicknesses, diameters, etc.) and to their exact arrangement inside the pump. Other pumps manufactured with porous getter bodies are known, but they are generally optimized for specific uses. A getter pump is known from patent application EP-A-753663, wherein the set of getter bodies, being disk-shaped, is supported by a central mounting having a heater housed therein. This pump is intended for use in portable instruments and its design aims to obtain a pump showing good performances but being small-sized, and requiring low powers for its heating; this pump hardly keeps its good characteristics when produced with larger dimensions. Patent application WO 96/17171 discloses a disk getter pump similar to the one of the aforementioned application. In this case the pump is integrated in a machinery for producing semiconductors. The set of getter disks with its central support is introduced into the work chamber without a housing, so that the sorption velocity is high, but this pump may be used only in this specific application, but not in any application of the getter pumps, such as e.g. being connected through suitable piping to scientific instruments. The object of the present invention is to provide a pump with high sorption velocity which can overcome the drawbacks shown by the pumps of the prior art. This object is achieved by a getter pump comprising: a metal housing defining a cylindrical chamber; a number, ranging from three to eight, of getter structures, each being formed of a plurality of porous disks obtained by sintering getter material powders and supported by a central mounting, said getter structures being arranged symmetrically around the center ofthe chamber and parallel with respect to its axis; a heater in the center of the chamber, being coaxial therewith; and an opening in the wall of said housing, connecting said getter structures with a space to be evacuated. The invention will be hereinafter described with reference to the drawings, wherein: Figure 1 shows a cross-sectional view, taken on a plan perpendicular to the pump axis, of a possible pump according to the invention; Figure 2 shows a partially cutaway view of the same pump, from an orthogonal direction with respect to the one of the previous view; Figures 3, 4 and 5 show different possible utilizations of the pump of the invention; Figures 6 and 7 show two cross-sectional views of prior art pumps; Figure 7a shows a detail of the pump of Figure 7; Figure 8 shows a comparison between the gas sorption curve of the pump of the invention and the one of a pump of the known art. By way of example of the pumps of the invention, Figures 1 and 2 show cross-sections of a pump provided with six getter structures formed of disks on a support, but, as previously stated, the number of such structures may range from three to eight, and preferably from four to six. In Figure 2, for the sake of clarity of the drawing, only the three structures farther from the viewpoint are shown, and only the farthest of them is shown in its whole vertical development, whereas the other are only partially shown. Referring to Figures 1 and 2, a getter pump 10 according to the invention comprises an essentially cylindrical housing 11, possibly consisting of a length of tube being closed at least at one end by a blind flange 21, or of a length of tube having at one end a base welded thereon (a solution not shown in the drawings); in both cases the open end carries a flange which may be used for closing the pump with another blind flange, or for connecting it to pipings or chambers to be evacuated, as hereinafter described. Housing 11 defines a chamber 12; inside chamber 12 getter structures 13, 13', ... are arranged, each being formed of disks 14, 14', 14", ..., supported by a central mounting 15, 15', .... The disks are kept at a desired mutual distance along the central mounting by spacing members (not shown in the drawing), which may be metal ringlets integral with the disks or loose, or the disks may be provided with such a cross-section as to have a portion thicker than the rest of the disk, so as to form a spacing integral with the disk. Getter structures 13, 13', ... are symmetrically arranged in chamber 12 around its center, wherein heater 16 is housed. The drawing shows a resistance heater, but different kinds of heaters may be used, as hereinafter described. By this arrangement, there is a void volume in the pump central region 17, and a series of void volumes in regions 18, 18', ..., between two adjacent getter structures in the peripheral portion of the chamber. These void volumes, extending for the whole height of the pump, are extremely important for obtaining high sorption velocities, since they provide conductances for conveying the gas onto the surfaces of the getter material. Housing 11 has an open end 19 with a flange 20 for connecting the pump to the space to be evacuated or to suitable counterflanges for the junction with pipings. The flange generally carries one or more gaskets (not shown in the drawing) made of polymeric material or metal depending upon the desired vacuum level, according to modalities known to those skilled in the art. Referring to Figures from 3 to 5, various possible configurations are shown for using the pumps of the invention. Figure 3 shows the use of pump 10 connected to a generic chamber C to be evacuated through a piping T, generally made of metal and possibly jointed, according to modalities known in the vacuum field. Figure 4 shows a configuration wherein pump 10 is directly connected, through flange 20, to the chamber C to be evacuated. Finally, Figure 5 shows a configuration for using pump 10 wherein this latter is directly introduced into chamber C; in this case the pump upper opening 19 is simply left open and flange 20 is not used, while housing 11 acts as a shield for preventing possible particles of getter materials from moving through the chamber, and for making the heating of getter structures more homogeneous. As previously said, the number of the getter structures in the chamber 12 according to the invention ranges from three to eight, and preferably from four to six. A number of getter structures smaller than three or greater than eight results in drawbacks due to the ineffective filling of the space inside the chamber. In particular, with two getter structures, the void volume in regions 18, 18' is exceeding, while the space for housing the heater is scarce; on the contrary, with a number of getter structures greater than eight, the void volume in region 17 in the center of the pump is exceeding, resulting in a reduction of the getter material, the overall pump size being the same, and in a reduction of the heating efficiency of member 16. A number of getter structures ranging from four to six allows to obtain the best compromise between the mutual distances of the getter structures and their distance from the heater, as well as between volume of getter material and volume of the conductances in regions 17 and 18, 18'. Getter structures 13, 13', ... may be kept in the desired geometry e.g. by mounting them onto suitable metal sections, so as to obtain a "stack" of such structures then to be introduced in chamber 12; otherwise, inner walls 22 of the circular bases of housing 11 may be provided with suitable seats (not shown), which hold, e.g., the ends of mountings 15, 15', ... ; the metal section, as well as other details relevant to the method for assembling the getter structures in the pump, are not shown in the drawings. The assembling methods herein mentioned, as well as other possible alternative methods, will be evident to those skilled in the field of mechanical constructions. Housing 11 is generally made of metal, preferably AISI 304L or 316L steel, as well as mountings 15, 15', ... of the getter structures. It may be a one-piece housing, obtained by welding the various metal component members once having introduced getter structures 13, 13', ... in space 12. However, the housing is preferably formed of at least two parts, which may be vacuum-tight tightened to one another; for example, circular base 21 may be a blind flange, which may be screwed on a counterflange integral with the lower edge of the same housing; the tightness is ensured by one or more gaskets placed between the flanges. This structure is preferred in that it allows operations of pump maintenance, e.g. recurring replacements of the heater or of the getter structures. Disks 14, 14', 14", ... , are obtained by the known methods for sintering powders of getter materials. A wide variety of getter materials may be used, generally comprising titanium and zirconium, their alloys with one or more elements selected among the transition metals and aluminium, and mixtures of one or more of these alloys with titanium and/or zirconium. Among the materials more commonly used for producing getter pumps, there are the alloy having weight percent composition Zr 70% - V 24.6% - Fe 5.4%, manufactured and sold by the applicant under the tradename St 707™, and a mixture comprising 60% by weight of St 707™ alloy and 40% by weight of zirconium, manufactured and sold by the applicant under the tradename St 172. The use is preferred of the materials disclosed in patent application EP-A-719609, in the applicant's name, which is referred to as for the details of the chemical composition and the disk production. Heater 16 may consist of a lamp, e.g. a quartz lamp, or a resistance, e.g. obtained by helicoidally winding a metal wire around a ceramic support, as known in the field. The high pumping speed of the pumps according to the invention is due to their specific geometry. In fact, when comparing a pump according to the invention with the pump of the aforementioned patent US 5320496, shown in Figure 6, this latter is seen to have a small gas conductance in the peripheral region of getter bodies 60, 60', ... , the outer surface of which is thus little accessible to the gases, resulting in a reduced overall sorption velocity. On the contrary, as previously stated, the pump of the present invention has a high gas conductance, and an easy access of gases to all the surfaces of the getter elements, by virtue of the void volumes in regions 17 and 18, 18', ... . As for the pump of the aforementioned patent US 5324172, the configuration of the getter elements allows an easy conveyance of the gas onto all the surfaces of the getter elements, but the efficiency of their irradiation by the central heater is not optimal; in fact, as shown in Figures 7 and 7a, in this pump the only portion of the getter elements being directly heated is a small rectangular surface 70, having as its sides the height and thickness size of the element, while the remaining portion is only heated by heat conduction inside the body itself. On the contrary, in the pump according to the invention, the surface of each getter element being directly exposed to the irradiation of heater 16 is greater, and equal to half the disk circumference multiplied for its thickness. The following Examples show how the sorption velocity tests are carried out on a getter pump according to the invention, compared to the sorption velocity of a pump of the known art. EXAMPLE 1 A getter pump according to the invention was assembled, formed of a housing which defined a cylindrical chamber, 135 mm high and having an inner diameter of 92 mm, open at its upper portion and containing six getter structures, each comprising 50 disks having a diameter of 2.54 cm. When observing the pump from its upper opening, the six getter structures appeared to be inscribed in an annulus such that a void volume having a diameter of 31 mm was at the center of the pump, while the structures were about 3 mm apart from the inner wall of the housing. At the center of the pump a quartz lamp was housed for heating the getter material. The disks were made of the aforementioned St 172 alloy. The sorption velocity test was carried out according to the ASTM F 798-82 standard, at a temperature of 250 °C, by using CO as the test gas. The test results are reported in double logarithmic scale in Figure 8 as curve 1; the trend of gas sorption velocity (V) is shown, measured in liters per second (1/s) as a function of the amount of sorbed gas (Q), measured in mbar per liter (mbar•1). EXAMPLE 2 (COMPARATIVE) The test of Example 1 was repeated by using a pump having the same dimensions and materials as the pump in Example 1, except for being provided with sheets of getter material arranged as disclosed in the aforementioned patent US 5324172 instead of stacked getter disks. When observing the pump from its upper opening, the sheets of getter material appeared to be arranged in an annulus having the same dimensions as the pump of Example 1. The results of this test are reported in Figure 8 as curve 2. By comparing the curves in Figure 8, the dimensions of the pump and the volume of getter material therein being the same, the pump of the invention has at the start a sorption velocity about five times greater than the pump of the prior art.

Claims: A getter pump with high sorption velocity comprising: a metal housing (11) defining a cylindrical chamber (12); a number, ranging from three to eight, of getter structures (13, 13',..), each being formed of a plurality of porous disks (14, 14', 14", ...), obtained by sintering powders of getter material and supported by a central mounting (15, 15', ..), said getter structures being arranged symmetrically around the center of the chamber and parallel to its axis; a heater (16) at the center of the chamber, being coaxial therewith; and an opening (19) in the wall of said housing, connecting said getter structures with a space to be evacuated. A getter pump according to claim 1, comprising four to six getter structures. A getter pump according to claim 1, wherein the housing is formed of at least two parts which can be vacuum-tight tightened to one another. A getter pump according to claim 3, with a base (21) consisting of a blind flange which can be tightened to the lower edge of the cylindrical wall of the housing (11), at a portion thereof molded as a counterflange. A getter pump according to claim 1, wherein the disks of getter material are made of sintered powders of a metal selected between titanium and zirconium. A getter pump according to claim 1, wherein the disks of getter material are made of sintered powders of metal alloys of titanium and/or zirconium with one or more elements selected among the transition metals and aluminium. A getter pump according to claim 1, wherein the disks of getter material are produced by sintering a mixture of powders of titanium and/or zirconium and of an alloy comprising titanium and/or zirconium and one or more elements selected among the transition metals and aluminium. A getter pump according to claim 6, wherein the disks of getter material are produced by using an alloy having weight percent composition Zr 70% - V 24.6% - Fe 5.4%. A getter pump according to claim 7, wherein the disks of getter material are produced by using a mixture of powders of an alloy having weight percent composition Zr 70% - V 24.6% - Fe 5.4% and powders of metallic zirconium. A getter pump according to claim 9, wherein the mixture of powders comprises 60% by weight of alloy and 40% by weight of metallic zirconium.

Inventors: CONTE ANDREA, CONTE, ANDREA
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1998
Main classification: H01J7
IPC codes: H01J7, F04B37