HIGH CAPACITY GETTER PUMP

Abstract: An improved high-capacity getter pump, suitable for creating and maintaining the vacuum, comprising a plurality of porous sintered blades made from a non-evaporable getter material and having a first main surface; a second main surface, parallel to said first surface and spaced therefrom by a thickness of 0.5-5.0 mm; wherein said blades are arranged in a housing and are separated from each other by a gas conductance, with the adjacent surfaces of adjacent blades being spaced from each other by a distance of 0.5-10 mm.

Description: HIGH CAPACITY GETTER PUMP The present invention relates to an improved high-ca¬ pacity getter pump, suitable for creating and maintain¬ ing the vacuum, for instance in an ultra-high vacuum chamber or in a high-energy particle accelerator. Getter pumps are well known in the art and are suita¬ ble for creating and maintaining the vacuum. The first commercially successfull getter pump, described in US patent 3,780,501, was employing, in a housing, a pleated metal strip having a getter metal embedded therein. Additional examples of such getter pumps were described in US patents 3,609,064; 3,662,522; 3,961,897 and 4,137,012. Although these former getter pumps enjoyed a wide commercial success and market acceptance, they were still suffering from a drawback, residing in a limited sorption capacity inside a given volume. In order to increase said sorption capacity, it was suggested to simply fill the pump housing with a getter material in the form of pellets, having size and shape si ilar to those (tablets) used in the field of drugs; such pellets were typically showing a cylindrical shape, with a diameter of 5-10 mm and a height of 2-10 mm. However, when the housing is filled with such pellets, the access of the gas to the bulky getter structure is far from being satisfactory. Another drawback, bound to the use of said pellets, was their tendency to produce undesired loose particles; moreover the bulky structure can show safety problems because of the possibility of a high exothermi ci ty of the getter material during possi¬ ble ignitions (in particular when the used getter mate¬ rial has a low activation temperature) . Accordingly, it is a first object of the present invention to provide an improved getter pump substantial¬ ly free from one or more of the drawbacks hereinabove. Another object of the invention is to provide an improved getter pump having a higher sorption rate per unit volume, with respect to the getter pumps of the prior art . A further object of the invention is to provide an improved getter pump having a higher sorption capacity per unit volume, with respect to the getter pumps of the prior art. An additional object of the invention is to provide an improved getter pump resorting neither to pleated coated strips nor to pellets of getter material. Other objects of the invention wi ll be apparent to those of ordinary ski ll in the art, by reference to the following disclosure and drawings. In its broadest aspect, the invention relates to an improved high-capaci y getter pump, suitable for creat¬ ing and maintaining the vacuum, for instance in a high- energy particle accelerator and in an ultra-high vacuum chamber, said pump comprising a plurality of porous sintered blades made from a non-e aporable getter mate¬ rial having: i) a first main surface; i ) a second main surface, essentially parallel to said first surface and spaced therefrom by a thickness of 0.5-5.0 mm; wherein said blades are arranged in a housing and are separated from each other by a gas conductance (empty intermediate space), with the adjacent surfaces of adja¬ cent blades being spaced from each other by a distance of essentially 0.5-10 mm. The gas conductances between adjacent blades allow the gas molecules to enter the porous getter structures at a fast rate and the higher porosity of the porous sintered blades better promotes the efficiency of the gas sorption with respect to the pleated strips and to the pellets (or tablets) of the prior art. Said blades are suitably arranged in a radial way in said housing, defining an inner channel with their inner extremities. The getter pump according to the invention are furthermore equipped with a heater, for heating the blades at the activation temperature and also at the desired operative temperature, and with a flange fasten- ing said housing to a vacuum. The porous sintered blades of the pump according to the invention may have a shape selected from planar (in particular rectangular and optionally tapered and/or bevelled), concave and combinations thereof. Moreover said blades have a density from 1 to 5 and prefera¬ bly from 1.5 to 3.5 g/cm and a surface area from 0.05 2 2 to 1 m /g (preferably 0.1 - 1 m /g) . The getter pump according to the present invention may be employed for maintaining the vacuum in a wide range of vacuum devices and apparatuses, for instance closed vacuum vessels (like e.g. a dewar or a vacuum jacket for a fluid transfer piping), particle accelera¬ tors (like for instance a synchrotron) and ultra-high vacuum chambers; the new getter pumps can maintain a -6 -12 -10 vacuum level as high as 10 and even 10 mbar ( 10 Pa). A wide range of non-evaporable getter metals may be employed for the manufacture of the pumps according to the invention, for instance zirconium, titanium, hafnium, tantalum, thorium, uranium, niobium, mixtures thereof and alloys of these metals with each other and with other metals, such alloys being or being not inter etal- lic compounds. These getter metals may be used alone or in admixture with other materials, like for instance anti sintering agents. An exemplifying but not limiting series of non-evaporable getter metals for the manufactu¬ re of said porous sintered blades comprises: a) an alloy containing 84% Zr, balance Al, as describ¬ ed e.g. in US patent 3,203,901; b) a metal composition according to US patent 3,584,253, based on Zr, Ta, Hf, Nb, Ti or U; c ) a metal composition according to example 3 of US patent 3,926,832, based on a combination of Zr with Zr-A L a I loy; d ) the intermeta L Ii c compound Zr Ni described e.g. in US patent 4,071,335; e ) the Zr-M1-M2 alloys according to US patent 4,269,624, where M1 is V or Nb and M2 is Fe or Ni; f ) the Zr-Fe alloys according to US patent 4,306,887; g ) certain alloys of zirconium, vanadium and iron, as described in US patent 4,312,669, as well as other alloys of zirconium and vanadium and minor amounts of transition metals such as manganese; h) certain alloys of zirconium, titanium and iron, as described in US patent 4,907,948. According to a preferred embodiment of the present invention, said non-evaporable getter metal is selected from the Zr-V-Fe alloys and the Zr-Ti-Fe alloys, optional¬ ly in combination with Zr alone and/or Ti alone, these last being optionally in the form of hydrides. The combi¬ nations disclosed in GB patent application 2,077,487, in the name of the Applicant have proved to be particular¬ ly advantageous, being obtained from: I) a ternary particulate Zr-V-Fe non evaporable getter alloy having a composition (by weight) lying, when plotted on a ternary diagram, within a polygon hav- ing as its corners the following points (% b.w„): a) 75% Zr - 20% V - 5% Fe b) 45% Zr - 20% V - 35% Fe c) 45% Zr - 50% V - 5% Fe II) a particulate non-evaporable getter metal, selected from Zr and Ti, wherein the Zr and/or Ti particles have a smaller average size than the alloy particles. Such combinations are traded by the Applicant as "SAES St 172". One advantageous method for manufacturing the porous sintered blades of the pump according to the invention, starting from the combinations hereinabove, comprises the following steps: A ) said non-evaporable getter metal is prepared in the form of a loose powder of Zr-V-Fe and/or Zr-Ti-Fe alloy particles, optionally in admixture with parti- cles of Zr alone and/or Ti alone and with an expan¬ sion agent; B) said loose powder (or the consequent mixture) is poured in a mould and sintered. Said alloy particles have preferably a pre-sintering surface area equal to or higher than 0.15 and preferably 2 0.25 m /g and a pre-sintering particle size up to 400 μm, preferably from 1 to 128 μm and even better from 1 to 50 μm. Said Zr and/or Ti particles, in their turn, have preferably an average particle size from 1 to 55 μm and 2 a surface area from 0.1 to 1.0 m /g, wherein the weight ratio between the alloy particles and said Zr and/or Ti particles is suitably from 10 : 1 to 1 : 1. A sintering temperature substantially between 700 and 1200°C, maintained for a time comprised between a few minutes and a few hours, is generally considered as a satisfactory one, whereas a lower temperature requires a long time; the sintering time should give rise to a dimensional stab lity. The expansion agent may suitably be an inorganic and/or organic base containing nitrogen and/or phospho¬ rus, which completely decomposes below the sintering temperature, for instance urea, azo-di-carbona ide and/or a carbamate like ammonium carbamate, in amounts from 0.1 to 15% b.w., with respect to the non-evaporable getter material ( preferably 2 - 10%). The formula of azo-di-carbona ide is: NH - C0 - N = N - C0 - NH The heater may be arranged inside or outside the housing of the getter pump. An electrical current may be allowed to flow directly through the getter material, as described e.g. in US patent 3,609,064 or heating may be carried out by conduction or by radiation, for instan¬ ce by means of a UHV quartz lamp. In this latter case, the porous sintered blades should be slightly tilted with respect to each other ( and with respect to the axial plane of the pump), in order to be fully i rradiated. The following drawings (Figs. 1-10) are supplied for illustrative purposes but do not limit in any way the scope of the invention; in particular: Fig. 1 is a schematic representation of a getter pump according to the present invention in operating conditions; Fig. 2 is an enlarged section view of a getter pump according to the present invention, taken along line II-II of Fig. 1; Fig. 3 is a perspective view of a portion of the getter pump as shown in Fig. 2; Fig. 4 is a section view of a getter pump according to the present invention, taken along line IV-IV of Fig. 2; Fig. 5 s a section view of a few blades according to the present invention, forming an angle α with the axial plane X-X of the pump; Fig. 6 is a view similar to Fig. 5 showing a different shape of the bLades; Fig. 7 shows a section view of a mould for sintering planar rectangular blades; Fig. 8 schematically shows the pumping system employed during the tests of the examples; Fig. 9 reports the results of a few pumping tests in the form of a diagram; and Fig. 10 shows a partially cut-away view of a typical pump according to the invention, where the blades are arranged in different superimposed annular rows (crowns or cartridges). Referring now to the drawings in general and in parti¬ cular Figs. 1 and 2, there is shown an improved non-eva- porable getter pump 10, having a gas-tight housing 12 provided with a flange 14, which constitutes means for fastening said housing 12 to a vacuum vessel 16. The getter pump 10 of Fig. 2 has a plurality of porous sintered blades 18, 19, 20, inside a cylindrical housing 12, consisting of a non-evaporable getter metal. Blade 18 has a first planar surface 22 and a second planar surface 24, substantially parallel to said first surface 22, spaced from the first surface by a distance "t" (thickness) of about 0.5-5 mm. Blade 18 can be for instan- ce rectangular in shape. All the blades, like blades 18, 19, 20 and so on, have a simi lar structure. Blades 18, 19, 20 and so on are radially arranged, with adjacent blades spaced from each other by a distance "c" substan¬ tially between 0.5 and 10 mm. The empty space "c" between adjacent blades 18, 19, 20 and so on constitutes a gas conductance . The axis of each blade preferably forms with the axial plane X-X of the pump, as shown in Fig. 5, a small angle , let us say from 1 to 15°, as to protect at least the inner wall of the hous-ing (see blade 18' on Fig. 5) and to consequently reduce the possible degassing from said wall. A proper choice of said angle also makes it possible the full irradiation of the blades along the radial direction, thus avoiding an inhomoge- neous heating of the porous getter material. Overall heating efficiency and power saving are further not-ne- glectable consequences of such an arrangement. As to the profile of the blade, it may be a straight profile or it can show a small concavity, like blade 18" on Fig. 6. In both cases of angle α deviation or concavity with respect to the axial direction not only heating of the blades is promoted, but also gas sorption. The getter pump 10 has a first annular retention plate 26, made from a metal sheet, having a plurality of radially arranged gas passages like passages 28, 29, 30, 31, 32 and 33. Adjacent gas passages (slots) 32, 33 are separated by a rib 34 radially extending from the annular plate 26. Fins 36, 38 of the radial rib 34 can be axial, parall¬ el to each other and spaced apart from each other by a distance substantially equal to the width of the blade 19; said fins 36, 38 are holding one end of the blade 19. Getter pump 10 also has a second identical annular retention plate (not shown on the drawings) positioned at the bottom (not shown on the drawings) of the blades, Like blades 18, 19, 20. Getter pump 10 has a plurality of straps 40, 41, 42, each of which is spot welded to the periphery of both the first annular retention plate 26 and the second annular retention plate 26 and the second annular reten- tion plate, not shown on the drawings. The same getter pump 10 has a thermocouple 47 and a lamp 44, pro iding for the heating of the blades at the activation tempera¬ ture and also at the operative temperature (see Fig. 10). The power required by the lamp 44 is supplied by a power source 46 (Fig. 1). The inner ends of the blades define an inner channel, having diameter D (see Fig. 2) in communication with the gas conductances. The getter pumps according to the present invention have a sorption capacity several times greater, in a given volume, than the getter pumps of the prior art. Although the invention has been described in considerable detail with reference to certain preferred embodiments, it will be understood that many changes and modifications can be carried out without departing from the scope of the invention; the following examples, in particular are supplied for illustrative purposes but do not limit in any case the scope and the spirit of the invention. EXAMPLE_1 Part_"A"_ manufacture_of_ he_ ) . ^ __aιJ__^^^e_^l ^_of _the A porous sintered blade was manufactured starting from a loose powder of a Zr-V-Fe alloy showing the follow¬ ing features: composition (% b.w.): Zr = 70; V = 24.5: Fe = 5.5; - average particle size = 1 - 128 μm; 2 surface area = 0.25 m /g. The alloy powder was then thoroughly admixed, accord¬ ing to a weight ratio 1.5:1 with a Zr loose powder hav¬ ing the following features: - average particle size = 1 - 55 μ ; 2 surface area = 0.45 m /g; and with 5% b.w. of ammonium carbamate (NH - CO - 0 - NH,). 4 The resulting mixture was loaded into the rectangular graphite mould of Fig. 7, and sintered at 1000°C for 10 minutes; the resulting blade was 75 mm long, 20 mm wide and 1.4 mm thick. The surface area of the porous sintered 2 blade was 0.14 - 0.15 m /g and the geometrical (visible) 2 surface of the blade was approximately 33 cm . The density of the blade was 3 g/cm . An overall number of 112 blades were prepared in the same way; said blades were radially arranged in two identical superimposed rows (56 blades in each cartridge) and at equal angular distance, in a stainless steel cylindrical housing having an inner diameter of 100 mm, with the external main size of the blades being nearly in contact with the inner wall of the housing (clearance = 1 mm). The surface ratio, namely the ratio between the geometrical surface of the blades and the volume of 2 3 the housing was 3.1 cm /cm and the diameter of the inner channel, defined by the internal extremities of the radially arranged blades was 58 mm. The volume ratio, namely the ratio between the overall volume of the blades and the empty volume of the housing was 0.21 cm /cm 3 and the mass ratio was approximately 0.64 g/cm . As schematically shown in figure 8, the getter pump GP was fastened to a vacuum chamber (VO, connected to a high vacuum pumping system (VP) by means of a piping having a known conductance (C) (calibrated conductance). The experimental vacuum chamber was evacuated by the main pumping group down to a pressure , in the range of -8 1 0 t o r r . Heating of the getter pump (activation) was achieved by using an internal quartz lamp, coaxial with the hous¬ ing of the pump and not shown in the figure. The quartz lamp was switched on and the getter blades were irradiated until reaching the temperature of 500°C. Such temperature was maintained for 1 hour. The lamp was subsequently switched off and the getter material was brought back to room temperature (25°C ) . At this point, a known test gas (CO) coming from a high purity reservoir (R) was allowed to flow through the piping connecting the pumping system and the calibrated conduct¬ ance. The gas flow was controlled by means of a UHV sapphire valve. Two pressure control gauges (Bayard- -Alpert) BAG 1 and BAG 2 were used to continuously measu¬ re the pressure values before and after the known conduct¬ ance (C) . By properly operating the valve (V), the pressure (P ) upstream of the calibrated conductance was kept at m -4 a constant level (1.5 x 10 torr), and the pressure ( P ) downstream thereof i.e. in the proximity of the 9 getter was monitored for a few hours; said pressure ( P ) was lower than the pressure (P ) upstream of the g m gas conductance, because the getter pump was adsorbing part of the gas entering the volume (VC). The increase of the amount of gas adsorbed by the getter material was corresponding to a reduction of the pumping rate and therefore to an increase of pressure (P ). 9 From pressure P (torr), from gas conductance C (l/s) m and from the change of pressure P (torr) along the time, it was possible to calculate the pumping rate G (l/s) of the getter pump as a function of the amount of adsorbed gas (torr x I) . As it is known, the amount of gas (Qi) flowing at a certain point through gas conduct¬ ance (C) is supplied by: Q i = C (P - P ) (torr x L/s) m g Such amount of gas per time unit was coincident with the amount of gas (per time unit) adsorbed by the getter pump, which can be expressed as G x P (torr x l/s) namely as the product of the pumping rate of the getter times the pressure (P ) in the proximity of the same 9 getter. By equalizing the two amounts it is possible to obtain: G x Pg = C (Pm - Pg); wheref rom: G (t) = C [(Pm - Pg (t)J /Pg (t) The overall amount of gas Q adsorbed by the getter pump at the time t can be obtained, as is known, by integrating along the time the amount of gas Qi adsorbed per time unit: Q = / Q i dt = / G (t) x Pg (t) dt The results of this measurement, namely the progress of the pumping rate of the getter as a function of the gas amount adsorbed by the same, are reported in figure 9, plotting G (pumping rate) versus Q (sorption capacity), where these data (line 1) are compared with the results (line 2) obtained using a getter pump according to the prior art (SAES Getters GP 200) described in US patent 3,662,522 and having an equal housing volume. From the comparison it is clear that the pumping rate of the improved getter pump GP according to the invention is more than twice the rate of the traditional GP 200 pumps based on coated strips. It is also clear that the sorption capacity, as measured when the pumping rate of the two pumps drops below 100 l/s, is more than one order of magnitude higher with respect to the former pump. The improved getter pump according to the invention therefore provides for significantly higher sorption and capacity features than a traditional NEG (non-evapo¬ rable getter) pump for a given housing volume. EXAMPLE_2 Example 1 was repeated a second time, by replacing carbon monoxide by nitrogen. ALso in this case the pump¬ ing rate and the sorption capacity were significantly higher with respect to the standard GP 200 pumps. EXAMPLE_3 Example 1 was repeated a further time by replacing carbon monoxide (CO) by hydrogen (H ). Also in this case the pumping rate of the improved getter pump was more than twice the value of GP 200. Since capacity of hydrogen of the NEG material used for pump manufacturing is much higher than that for CO and N , the test was stopped after the pump had sorbed 10 torr x I of H and much before the point where the pumping rate starts to slow down.

Claims: C L A I M S 1. An improved high-capacity getter pump, suitable for creating and maintaining the vacuum, comprising a plurality of porous sintered blades made from a non-evapo¬ rable getter material and having: i) a first main surface; ii) a second main surface, essentially parallel to said first surface and spaced therefrom by a thickness of 0.5-5.0 mm; wherein said blades are arranged in a housing and are separated from each other by a gas conductance ( empty intermediate space), with the adjacent surfaces of adja¬ cent blades being spaced from each other by a distance of essentially 0.5-10 mm. 2. The pump of claim 1, wherein said blades are arrang- ed in a substantially radial way, defining with their inner extre ities an inner channel around a longitudinal axis of symmetry of the pump, there being also provided a heater and a fastening flange connected to said housing. 3. The pump of claim 2, wherein said porous sintered blades have a shape selected from planar (in particular rectangular and optionally tapered and/or bevelled), concave and combinations thereof, with the axes of said blades forming an angle with the axial planes, passing for each blade through said longitudinal axis of the pump, said angle being preferably comprised between 1° and 15°. 4. The pump of claim 1, wherein said porous sintered blades have a density from 1 to 5 and preferably 3 from 1.5 to 3.5 g/cm and a surface area from 0.05 to 1 2 m /jg ) . 5. The pump of claim 1, wherein said non-evaporable getter material is a metal selected from zirconium, titanium, hafnium, tantalum, thorium, uranium, niobium, mixtures and/or alloys thereof and/or with other metals and wherein said metal is optionally admixed with anti- sintering agent . 6. The pump of claim 5, wherein said non-evaporable getter metal is selected from: a) Zr - V - Fe alloys; and b) Zr - Ti - Fe alloys; optionally in combination with Zr alone and/or Ti alone, optionally in the form of Zr hydride and Ti hydride. 7. The pump of claim 6, wherein said non-evaporable getter metal is a combination obtained from: I) a ternary particulate Zi—V-Fe non-evaporable getter alloy having a composition (by weight) lying, when plotted on a ternary diagram, within a polygon having as its corners the following points (% b.w.): a) 75% Zr - 20% V - 5% Fe b) 45% Zr - 20% V - 35% Fe c) 45% Zr - 50% V - 5% Fe II ) a particulate non-evaporable getter metal, selected from Zr and Ti, wherein the Zr and/or Ti particles have a smaller average size than the alloy particles. 8. A method for the manufacture of a porous sintered blade of a pump according to claim 6, wherein: said getter metal is prepared in the form of a loose powder of alloy particles, optionally in admixture with particles of Zr alone and/or Ti alone and with an expansion agent; said expansion agent is an inorganic or organic base containing nitrogen and/or phosphorus, which completely decomposes below the sintering temperatu¬ re and is preferably selected from urea, azo-di-car¬ bonamide and carbamates; - said loose powder (or the consequent mixture) is loaded into a mould and sintered at a temperature from 700 to 1200°C; and wherein said aLloy particles have a pre-sinteri ng surface area equal to or higher than 0.15 and preferably 2 0.25 m /g and a pre-si ntering particle size up to 400 mμ, preferably from 1 to 128 mμ and even better from 1 to 50 μ; said Zr and/or Ti particles having an average particle size from 1 to 55 mμ, as well 2 as a surface area from 0.1 to 1.0 m /g and wherein the weight ratio between the alloy particles and said Zr and/or Ti particles is from 10 : 1 to 1 : 1. 9. The method of claim 8, wherein said expansion agent is ammonium carbamate and wherein the carbamate amount is between 0.1 and 15% b.w. (preferably 2-10%) with respect to said non-evaporable getter metal. 10. A process for creating and/or maintaining vacuum in devices and/or apparatuses to be kept under vacuum and especially a vacuum at a level equal to or higher —6 —1 ? than 10 and even 10 mbar, characterized in that said devices and/or apparatuses are connected or fastened to the improved high-capacity getter pump of claim 1. 11. The process of claim 10, wherein said devices and/or apparatuses are selected from: - vacuum vessels, like for instance a dewar or the vacuum jacket of a piping for the transfer of fluids; ultra-high vacuum chambers; a particle accelerator and in particular a synchro¬ tron.

Inventors: FERRARIO BRUNO, MANINI PAOLO, FERRARIO, BRUNO, MANINI, PAOLO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1993
Main classification: H01J7
IPC codes: F04B27, F04B37, H01J7