CA2128416C - High capacity getter pump - Google Patents

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

P~'I'/IT93/O~D(440 --~dV~ 94/0295?
HIGH CAPACITY LETTER PUMP
The present invention relates to an improved high-ca-pacity Better pump, suitable for creating and maintain-ing the vacuum, for instance in an ultra-high vacuum chamber or in a high-energy particle accelerator..
Letter pumps are well known in the art and are suita-bte for creating and maintaining the vacuum. The first commercially successfull Better purrsp, described in VS
patent 3,?80,501, was employing, in a housing, a pleated metal strip having a ,Better metal embedded therein.
Additional examples of such Better pumps sere described in US patens 3;509,064; 3,662,52; 3,9b1,897 and 4,137,0124 Although these former Better pumps enjoyed a wide commercial success and market acceptance, they °15 sere still suffering from a drawback; residing in a limited sorption opacity inside a given volume.
In order to ~ir~crease said sprption capacity, it was sugge ted to simply fill the pump housing with a Better material in the f~rm of pellets; havjng size: and shape similar tb th~~e (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, Then the housing is filled with such pellets, the access of the gas to the bulky Setter, 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 ", 'CVO 94/02957 PCTIIT'9~/000~~
can show safety problems because ofi the possibility of a high exothermicity of the Better material during possi°
ble ignitions tin particular when the used Better mate- ' rial has a low activation temperature).
Accordingly, it is a ..fi~~st object ofi the present invention to provide an~~~t~i~mproved Better pump substantial-ly free from one or mare ofi the drawbacks hereinabove.
Another object of the invention is to provide an improved Better pump having a higher sorption rate per 1n unit volume, with respect to the Better pumps of the prior art.
A further object of the invention is to provide an improved Better pump having a higher sorption capacity per unit volume, with respect to the Better pumps of 1S the prior art.
An additional object of the invention is to provide an improved Better pump resorting neither to pleated coated strips nor to pellets of Better material.
Other objects of the invention will be apparent to 20 those of ordinary skill in the art, by reference to the following disclosure and drawings.
In its broadest aspect, the invention relates to an improved high-capacity Better pump, suitable for Great-,, ing and maintaining the vacuum, for instance ina high-25 energy particle accelerator and in an ultra°high vacuum chamber, said pump comprising a plurality of porous sintered blades made from a non-evaporable Better mate-rial having: .
i) a first main surface;
30 ii) a second main surface, essentially parallel to said .. , , r .. . . ' -. !'WO 9~1/OZ95'7 ~ ~ ~ ~ ~ ~ ~ P(.'I'/iT93/0~1040 - 3 °
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.~Cempty 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 malecules to enter the porous Better 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 exth~mities. The Better pump according to the invention are furthermore equipped with a'heater, for heating the blades at the activation tempera~~are and also at the desired operative temperature, and ~rith a flange fasten ing said housing to a vacuum.
The porous sintered blades of the pump according to the invention may have a shape seEected from planar Cin particular rectangular and optionally tapered and/or bevelled>, concave and combinations thereof.;P~oreover ~5 said blades have a density from 1 to 5 and prefers-bl~ from 1.5 to 3.5 g>cm3 and a surface area from 0.05 to 1 m'2/g Cpreferably 0.1 ° 1 m2/g).
The ge~ter pump according to the present invention may be employed for maintaining the vacuum in a wide range of vacuum deuices and apparatuses, for instance closed vacuum vessels dike e.g. a dewar or a vacuum -, PC,'f/ff93/OO~.U~
'6~~ 9/02957 jacket for a fluid transfer piping), particle accelera-tors (like for instance a synchrotron) and ultra-high vacuum chambers; the new Better pumps can maintain a vacuum level as high as 10-land even 10 12 mbar (10-10 r S Pa). ., A wide range of non-evaporable Better 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 intermetal-Lic compounds. These Better metals may be used alone or in admixture with other materials, like for instance antisintering agents. An exemplifying but not limiting series of non-evaporable Better metals for the manufactu-re of said porous sintered blades comprises:
a) an alloy containing ~4y 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!. alloy;
d) the inter,metallic compound Zr2Ni described e.g.
2S in US patent 4,071,335;
e) the ~ Zr-M1°M2 alloys according to US patent 4,269,62, where M1 is V or Nb and M2 is Fe or Ni; .
f) the Zr°Fe allays 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 ~~.~,~~'V~~ 9~4/~295? ~ ~ ~ ~ ~ PC'f'/IT'93/00040 -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 ofi the present 5 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 ofi hydrides. The combi-nations disclosed in GB patent application 2,077,487, in the name o~f the Applicant have proved to be particular-ly advantageous, being obtained firom:
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 t% bsw.):
a) 75! Zr - 206 V - 5~ Fe b) 45! Zr - 20~ V ° 35% Fe c) 45% Zr - 50% V - S% 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 combanations are traded by the Applicant as "SAES St 172"v 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 fiorm of a loose powder of Zr-V-Fe and/or Zr-1'i-Fe alloy particles, optionally in admixture with parti wvo 9~/ozg~~ ~c~r/r~~/oo~"

J -'cles of Zr alone and/or Ti alone and with an expan-sion agent;
B) said loose powder tar the consequent mixture) is poured in a mould and sintered.
Said alloy particles have .y.referably a pre-sintering surface area equal to or higher than 0.15 and preferably 0.25 m /g and a pre-sintering particle sire up to 400 um, preferably from 1 to 128 um and even better from 1 to SO um.
1p Said Zr and/or Ti particles, in their turn, have preferably an average particle size 'from 1 to SS Nm and a surface area from 0.1 to 1.0 m2/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 ~ few hours, is generally considered as a sat~asfactory one, whereas a lo~rer temperature requires a long time; the sintering time should give rise to a dimensional stability.
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-carbonamide and/.or a carbamate like ammonium carbamate, in amounts from 0.1 to 15y b.~r.;, with respect to the non-evaporable Better material tpreferably 2 - 10%). The formula of azo-di-carbonamide is:

The heater may be arranged inside or outside the housing of the Better pump. An electrical current may ~~.2&41~
.._ ~~ r~v~ ~moz~s7 ~ ~~.-ria~~ioooao ~- 7 °
be allowed to flow directly through the getter material, as described e.g. in US patent 3,b09,Ob4 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 irradiated.
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 Better pump according to the present invention in operating conditionso Fig. 2 is an enlarged section view of a Better pump according to the present invention, taken along tine ZI-TI of Fig. 1;
Fig. 3 is a perspective view of a portion of the Better pump as shown in Fig. 2;
Fig. G is a section view of a Better pump according to the present, invention, taken along line IV°IV of Fig. 2;
Fig. 5 is a section view of a few blades according to the present invention, forming an angle o~ with the axial plane X-X of the pump;
Fig. 6 is a view similar to Fig. S 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 1'~
WO 94/02957 ~ ~ ~ ~ ~, ~ , PCy'/TT93/OO~QU :.
_ _ g _ 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 t~ypical pump according to the inv,eration, 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 gette~ 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 Better 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 ,~8, 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 cnnducta~ce.
The axis of each blade preferably forms with the axial plane %-% of the pump, as shown in Fig. 5, a small angle a. let us say from 1 to 15°, as to protect at least the inner wait of the housing Csee blade 18' on Fig> 5) and to consequently reduce the possible degassing . ~ .4 "Vtn .~ WU 94/02957 g _ from said wall. A proper choice of said a angle also ' makes it possible the full irradiation of the blades along the radial direction, thus avoiding an inhomoge neous heating of the porous Better material. Averall heating efficiency and power saving are further not-ne qlectable consepuences of such an arrangement. As to the profi le of the blade, it may be wa straight profi le or it can show a small concavity, like blade 18" on Fig. 6. In both cases of angle a deviation or concavity with respect to the axial direction not only heating of the blades is pramoted, but also gas~sorption.
The Better 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 (slats) 32, 33 are separated by a rib 34 radially extending from the annular plate 26.
Fins 3b, 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 ar~e holding one end of the blade 19. Getter pump 10 also has a second identical annular r~etent5on plate (not shown an the drawings) positioned at the bottom <not shown on the drawings) of the blades, 2S like blades 18, 190 20.
Getter pump 10 has a plurality of straps 40, 41, 42, each of which is spot r~elded to the periphery of both the fist annular retention plate 2b and the second annular retention plate 2b and the second annular reten-tion plate, not shown on the drawings. The same Better pump 10 has a thermocouple 47 and a lamp 44, providing . y_._ 1 dV() 9~1/i129~7 ~, ~ ~~ ~ '~'G°T/~'1C93/O~O~u~
~1 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 U (see Fig. 2) in communication with the gas conductances.
The get~ter pumps according to the present irmention have a sorption capacity several times greater, in a given volume, than the Better pumps of the prior art., Although the invention has been described in considerable detai l 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 inve-ration; 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.
EXAP9F'LE 1 ~?~~~ ~~p~° (~,anufact~re of the blades and assembl of the o y ______-~0 A porous sintered blade was manufactured starting from a loose powder of a Zr-V°Fe alloy showing the follow-ing feature:
° ~ composition (% b.w.):
Zr -~70; V = 2~.5: Fe - 5.5;
- average particle size - 1 - 128 um;
surfiace area - 0.25 m2/g.
The alloy powder was then thoraughly admixed, accord- .
ing to a weight ratio 1.5:1 with a Zr loose powder hav-ing the following features:
- average particle size - 1 - SS um;
surface area - 0.45 m2/g;

..., ~~ 94/02957 PC~'/IT93/00040 and with 5% b.w. of ammonium carbamate tNH2 - CO - 0 -NH4).
The resulting mixture was loaded into the rectangular graphite mould of Fig. 7, and sintered at 1000°C for 10 S minutes; the resulting blade was 75 mm long, 20 mm wide and 1.4 mm thick. The surface area of the porous sintered blade was 0.14 - 0.15 m2/g and the geometrical tvisible) surface of the blade was approximately 33 cm2. 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 t56 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 ratib, namely the ratio between the geometrical surface of the blades and the volume of the housing was 3.1 cm2/cm~ and the diameter of the inner channel; defined by the internal extremities of the radially arranged blades was S8 mm. The volume ratio, namely the ratio betr~een the overaCl volume of the blades and ,the empty volume of the housing wa's 0.21 cm3/.cm3 and he' mass ratio, was approximately 0.64, g/cm3.
PART '~~~~-t~um~in~-te~~) Its sch~matiCally shown in figure 8, the Better pump GP was fastened to a vacuum chamber tVC), cannected to a high vacuum pumping system tVP) by means of a piping having a known conductance tC) tcalibrated conductance).
The experimental vacuum chamber was evacuated by the main pumping group down to a pressure in the range of ~f~ 94/02957 ~ ~ 1P(,°I'/Tlf~3/OOO~a~
~~.~~y ~
n - 1z -$ tort.
Heating of the Better pump (activation) was achieved , by using an internal quartz limp, coaxial with the_.hous-ing of the pump and not sho.w~n in the figure.
5 The quartz lamp was swlv~ched on and the Better blades were irradiated until reaching the temperature of 500°C.
Such temperature was maintained for 1 hour. .i'he lamp was subsequently switched off and the getterJmaterial was brought back to room temperature (z5°C). At this , 10 point, a known test gas fCQ) 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-°Alpert3 BAG 1 and GAG 2 were used °~o continuously measu°
re the pressure values bQfore arid after the known conduct-ance (C).
By property operating the valve CV>, the pressure (P~) upstream of the calibrated conductance was kept at a constant level (1.5 x 10 4 tort), and the pressure (Pg) downstream thereof i.e. in the proximity of the Better was.monitpred fgr a few hours; said pressure (Pg) was lower than the pressure tPm) upstream of the gas conductance, . because ~ the Better pump .was adsorbing z5 Part of the gas entering the volume (VC). The increase of the amount of gad adsorbed by the Better material was corresponding to a reduction of the pumping rate and therefore to ah iincrease of pressure (Pg).
From pressure Pm (tort), from gas conductance C Cl/s) and from the change of pressure P (tort) along the time, it was possible to calculate the pumping rate G

2~2~4~.6 '-~~~ ~N4 94/02957 - PC'Tl1Ta93/00040 (L/s) of the Better pump as a function of the amount of adsorbed gas (tort x L). As it is known, the amount of gas (Qi) flowing at a certain point through gas c-onduct-ance lC> is supplied by:
Q i - C (P - P ) (tort 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 Better pump, which can be expressed as G x Pg (taJrr x l/s) namely as the product of the pumping rate of the Better times the pressure (Pg> in the proximity of the same Better. By equalizing the two amounts it is possible to obtain:
G x Pg - C (Pm - Pg);
wherefrom:
G (t) - C L(Pm - Pg (t).] /Pg (t) ~'he overall amount of gas Q adsorbed by the Better 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 = ~.f Qi dt - J' G (t) x Pg (t) dt 'the results of this measuresnent, namely the progress of the pumping rate of the Better as a function of the gas amount adsorbed by the same, are reported in figure 9, plotting G'(pumping rate) versus 4 tso'rption capacity), where these data tlie~e 1) are compared with the results (line 2) obtained using a Better pump according to the prior art (SAES Letters GP 200) described in US patent 3,b62,~22 and having ~n equal housing volume.
From the comparison it is clear that the pumping rate of the improved Better pump GP according to the invention is more than twice the rate of the traditional VVC9 94f 02957 ~ ~ ~ ~ P~'~"/I'x'9~f OOOa" o' GP 200 pumps based on coated strips. xt is also clear that the sorption capacity, as measured when the pumping rate of the two pumps drops below 100 l/s, is mor_~ than one order of magnitude higher with respect to the former -S pump. The improved Better pump according to the invention therefore provides far significantly higher sorption and capacity features than a traditional NEG (non--evapo-rable Better) pump for a given housing volume.

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 1 was repeated a further time by replacing carbon monoxide (CO) by hydrogen (H2). Also in this case the pumping rate of the improved Better 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 N2, the test was stopped after the pump had sorbed 10 torr x t of H2 and much before the point where the pumping rate starts to slow down. .

Claims (14)

1. An improved high-capacity getter pump, suitable for creating and maintaining a vacuum, comprising a plurality of porous sintered separate and distinct blades made from a non-evaporable getter material and having:
i) a first main surface;
ii) a second main surface, essentially parallel to said first main surface and spaced therefrom by a thickness of 0.5-5.0 mm;
wherein said separate and distinct blades are arranged in a housing and are separated from each other by a gas conductance in the form of an empty intermediate space, with the adjacent surfaces of adjacent 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 arranged in a substantially radial way, defining with their inner extremities 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 axes forming an angle with the axial planes, passing for each blade through said longitudinal axis of the pump, said angle being between 1° and 15°

4. The pump of claim 1, wherein said porous sintered blades have a density from 1 to 5 g/cm³ and a surface area of from 0.05 to 1 m² /g.

5. The pump of claim 1, wherein said non-evaporable getter material is a metal selected from the group consisting of zirconium, titanium, hafnium, tantalum, thorium, uranium, niobium, mixtures and alloys thereof with other metals.

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.

7. The pump of claim 6 wherein said non-evaporable getter metal is selected from the group consisting of:
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 having as its corners the following points:
a) 75% Zr--20% V--5% Fe b) 45% Zr--20% V--35% Fe c) 45% Zr--50% V--5% Fe; and II) a particulate non-evaporable getter metal, selected from Zr and Ti, wherein these particles have a smaller average size than the alloy particles.

8. The pump of claim 4, wherein said porous sintered blades have a density from 1.5 to 3.5 g/cm³.

9. The pump of claim 5, wherein said non-evaporable getter material is admixed with an anti-sintering agent.

10. The pump of claim 6, wherein said non-evaporable getter metal is admixed with one or more members selected from the group consisting of: Zr, Ti, Zr hydride, and Ti hydride.

11. 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 temperature and is preferably selected from urea, azo-di-carbonamide 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-sintering surface area equal to or higher than 0.15 and preferably 0.25 m2/g and a pre-sintering particle size up to 400 mµ, preferably from 1 to 128 mµ and even better from 1 to 50 mµ; said Zr and/or Ti particles having an average particle size from 1 to 55 mµ, as well as a surface area from 0.1 to 1.0 m2/g and wherein the weight ratio between the alloy particles and said Zr and/or Ti particles is from 10:1 to 1:1.

12. The method of claim 11, 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.

13. 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 than 10-6 and even 10-12 mbar, characterized in that said devices and/or apparatuses are connected or fastened to the improved high-capacity getter pump of claim 1.

14. The process of claim 13, 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 synchrotron.