DE69303901T2 - GETTER PUMP WITH HIGH PERFORMANCE - 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

The invention relates to an improved, high-performance getter pump suitable for creating and maintaining a vacuum, for example for use in an ultra-high vacuum chamber or in a high-energy particle accelerator.

Getter pumps used to create and maintain a vacuum are well known. The first commercially successful getter pump, described in U.S. Patent 3,780,501, used a folded metal strip in a housing with a getter metal embedded therein. Other examples of such getter pumps were described in U.S. Patents 3,609,064, 3,662,522, 3,961,879 and 4,137,012. Although these earlier getter pumps had great commercial success and widespread market acceptance, they still had the disadvantage of limited capacity within a given volume.

In order to increase this capacity, it was proposed to simply fill the pump housing with the getter material in the form of pellets having a similar size and shape to those (tablets) used in the pharmaceutical field, these pellets being typically cylindrical with a diameter of 5 to 10 mm and a height of 2 to 10 mm. However, when the housing is filled with these pellets, the access of the gas to the voluminous structure of the getter material is not satisfactory. Another disadvantage associated with the use of these pellets was their tendency to form undesirable loose particles; furthermore, the bulky structure may pose some safety problems because of the possibility of a highly exothermic reaction of the getter material during a possible ignition, especially when getter material with a low activation temperature is used.

It is therefore a first object of the invention to provide an improved getter pump substantially free from one or more of the above disadvantages.

A further object of the invention is to provide an improved getter pump with a higher uptake rate per unit volume with respect to known getter pumps.

Another object of the invention is to provide an improved getter pump with a higher absorption capacity per unit volume with respect to known getter pumps.

An additional object of the invention is to provide an improved getter pump which does not have to rely on folded and cut strips or pellets of the getter material.

Other objects of the invention will become apparent by reference to the following disclosure and drawings.

In its broadest aspect, the invention relates to an improved, high performance getter pump suitable for generating and maintaining a vacuum, for example in a high energy particle accelerator and in an ultra high vacuum chamber. This pump contains a plurality of porous, sintered sheets of a non-evaporable getter material, which have a

i) first main surface and

ii) have a second major surface which is substantially parallel to the first surface and separated from it by a thickness of 0.5 to 5.0 mm,

wherein said blades are arranged in a housing and separated from each other by a gas-conducting or empty space, the adjacent surfaces of adjacent blades being separated from each other by a distance of 0.5 to 10 mm.

These blades are arranged radially in the housing in a suitable manner so that their inner ends form a channel in the interior. The getter pump according to the invention is further equipped with a heater which heats the blades to the activation temperature and also to the desired working temperature and is attached to a vacuum housing with a flange. The porous, sintered blades of the pump according to the invention can have a shape which is selected from planar (in particular rectangular and optionally conical and/or chamfered), concave and combinations of these. Furthermore, the said blades have a density of 1 to 5 and preferably 1.5 to 3.5 g/cm³ and a surface area of 0.05 to 1 m²/g (preferably 0.1 to 1 m²/g).

The getter pump according to the invention can be used to maintain a vacuum in many areas of vacuum equipment and spaces, e.g. closed vacuum vessels (such as a Dewar or a vacuum vessel for liquid transfer), a particle accelerator (such as a synchrotron) and in an ultra-high vacuum chamber. The new getter pump can achieve a vacuum of 10⁻⁶ to even 10⁻¹² mbar (10⁻¹⁰ Pa).

A wide range of non-evaporable getter metals can be used to construct this pump. Examples include zirconium, titanium, hafnium, tantalum, thorium, uranium, niobium, mixtures and alloys of these metals with each other or with other metals, where these alloys may or may not be intermetallic compounds. These getter metals can be used alone or in mixtures with other materials, such as sintering materials. An exemplary, non-limiting range of non-evaporable getter metals used to make the porous sintered sheets mentioned include:

a) an alloy containing 84% Zr balanced with Al, as described for example 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 a Zr-Al alloy;

d) the intermetallic compound Zr2ni, described in US Patent 4,071,335;

e) the alloys Zr-M1-M2, according to US Patent 4,269,624, where M1 is V or Mb and M2 is Fe or Ni;

f) the Zr-Fe alloys according to US Patent 4,306,887;

g) some alloys of zirconium, vanadium and iron according to US Patent 4,312,669 as well as other alloys of zirconium and vanadium with smaller amounts of transition metals such as manganese;

h) some alloys of zirconium, titanium and iron, which are described in US Patent 4,907,948.

According to a preferred embodiment of the invention, the non-evaporable getter material is selected from alloys of Zr-V-Fe and of Zr-Ti-Fe, optionally in combination with zirconium alone and/or titanium alone, the latter two optionally in the form of hydrides. The combinations disclosed in patent application GB 2 077 487 in the name of the applicant have proved to be particularly advantageous and are obtained from:

1. A ternary particulate non-evaporable Zr-V-Fe getter alloy of a weight-based composition which, when plotted in a three-component diagram, lies within a polygon having the following points (wt.%) at its corners:

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 material selected from Zr and Ti, in which Zr and/or Ti particles have a smaller average size than the alloy particles. Such combinations are traded by the applicant under "SAES St 172".

An advantageous method for manufacturing the porous sintered blades of the pump of the invention begins with the above combinations and includes the following steps:

A) The vaporizable getter metal is prepared in the form of a loose powder of the Zr-V-Fe and/or Zr-Ti-Fe alloy particles, optionally mixed with particles of Zr alone and/or Ti alone and with an expansion agent;

B) The loose powders (or the resulting mixture) are poured into a mold and sintered.

The alloy particles preferably have a surface area after pre-sintering which is at least 0.15 and preferably 0.25 m²/g and a pre-sintering particle size of up to 400 µm, preferably from 1 to 128 µm or even better from 1 to 50 µm.

The Zr and/or Ti particles mentioned preferably have an average particle size of 1 to 55 µm, a surface area of 0.1 to 1.0 m²/g, whereby the weight ratio between the alloy particles and the Zr and/or Ti particles is in the range of 10:1 to 1:1.

A sintering temperature substantially between 700 and 1200 ºC, maintained for a period of between a few minutes and a few hours, is generally considered sufficient, whereas a lower temperature requires a long time and the sintering time should be responsible for the dimensional stability.

The expansion agent may suitably be a suitable inorganic and/or organic base containing nitrogen and/or phosphorus and decomposing completely below the sintering temperature, for example urea, azodicarbonamide and/or Carbamates, such as ammonium carbamate, in amounts of 0.1 to 15% by weight, based on the non-evaporable getter material (preferably 2 - 10%). The formula of azodicarbonamide is:

NH2-CO-N = N-CO-NH2

The heater can be located inside or outside the getter pump housing. An electric current can flow directly through the getter material, as described in US Patent 3,609,064, or heating can be achieved by conduction or radiation, e.g. by using a UHV quartz lamp.

In the latter case, the porous sintered blades of the pump should be slightly tilted towards each other (and with respect to the axial plane of the pump) in order to be able to irradiate them completely.

The following drawings (Fig. 1-10) serve to explain, but should in no way limit the scope of the invention:

Fig. 1 shows a schematic representation of a getter pump according to the invention under working conditions;

Fig. 2 shows an enlarged cross section of a getter pump according to the invention along the line II-II of Fig. 1;

Fig. 3 is a perspective view of a portion of the getter pump shown in Fig. 2;

Fig. 4 is a cross-sectional view of the getter pump along the axes IV-IV of Fig. 2;

Fig. 5 is a cross-sectional view of some blades forming a vane with an axial plane X-X of the pump;

Fig. 6 is a view similar to Figure 5 and shows a different shape of the leaves;

Fig. 7 shows a cross-section of the mold for sintering for flat, rectangular sheets;

Fig. 8 shows schematically the pumping system during the tests of the examples;

Fig. 9 expresses the results of some pumping tests in the form of a diagram and

Figure 10 shows a partial sectional view of a typical pump of this invention where the blades are arranged in different annular, superimposed rows (like crowns or cartridges).

Referring to the drawings in general and in particular, there is shown in Fig. 1 and Fig. 2 an improved non-evaporable getter pump. It has a gas-tight housing 10, is provided with a flange 12 which is intended to secure said housing 12 to the vacuum jacket 16. The getter pump 10 of Fig. 2 has many porous sintered leaves 18, 19, 20 inside its cylindrical housing 12 which are made of non-evaporable getter metal. A leaf 18 has a first flat surface 22 and a second flat surface 24, this second surface being parallel to the first surface 22. These two surfaces are separated by the distance "t" of about 0.5 - 5 mm thickness. Leaf 18 may, for example, have a rectangular shape. All of these leaves, such as leaves 18, 19, 20 etc., have a similar structure. The blades 18, 19, 20 etc. are arranged in a circle with a distance "c" from each other, which is substantially 0.5 and 10 mm. The empty space "c" between the adjacent sheets 18, 19, 20 etc. forms a gas line.

The axis of each blade forms a small angle a of about 1 to 15º with the axial plane X-X of the pump, as shown in Fig. 5, in order to protect the inner wall of the casing (as can be seen in blade 18 in Fig. 5) and to consequently reduce possible degassing from the wall. A good choice of the angle Q allows the blades to be completely irradiated along the radial direction, thus preventing inhomogeneous heating of the porous getter material. Above all, the effectiveness of heating and the energy saving are non-negligible consequences of this arrangement. As for the profile of the blades, this can be either straight or slightly concave, like blade 18" in Fig. 6. In both cases of angle a with respect to the axial direction, not only the heating of the blades but also the gas absorption is assisted.

The getter pump 10 has a first annular retention plate 26 made of a metal sheet with a plurality of radially arranged gas paths, such as paths 28, 29, 30, 31, 32 and 33. Adjacent gas paths (columns) 32, 33 are separated by a strut 34 extending radially from the annular plate 26.

The ribs 36/38 of the radial strut 34 may be axially parallel to each other and spatially separated from each other by a distance substantially equal to the width of the blade 19, the ribs 36/38 supporting one end of the blade 19. The getter pump 10 also has a second identical annular retention plate (not shown in the drawings). is arranged at the bottom (not shown in the drawings) of the blades such as blades 18, 19, 20.

The getter pump 10 has a plurality of retaining rings 40, 41, 42 each of which is welded to the periphery of both the first annular retention plate 26 and the second annular retention plate 26' and the second annular retention plate is not shown in the drawings. The same getter pump has a thermocouple 47 and a lamp 44 which provide heat to the blades at the activation temperature and also at the operating temperature (Fig. 10). The energy required for the lamp 44 is supplied by the energy source 46 (Fig. 1). The inner ends of the blades define an inner channel of diameter D (Fig. 2) in communication with the gas lines.

The getter pumps according to the invention have a much greater sorption capacity in a given volume than the known getter pumps. Although the invention has been described in detail with reference to some preferred embodiments, it is to be understood that many changes and modifications can be made within the scope of the patent claims. The following examples, which are intended in particular for illustrative purposes, are in no way intended to limit the scope and spirit of the invention.

EXAMPLE 1 Part "A" Manufacturing the blades and assembling the pump)

A porous sintered sheet was prepared, starting from a loose powder of a Zr-V-Fe alloy with the following properties:

- Composition (wt%):

Zr = 70; V = 24.5; Fe = 5.5;

- average particle size = 1 - 128 µm;

- Surface area = 0.25 m²/g

The alloy powder was then carefully mixed with a loose Zr powder with the following properties in a weight ratio of 1.5:1:

- average particle size = 1 - 55 µm;

- Surface area = 0.45 m²/g;

and

5 wt.% ammonium carbamate (NH₂-CO-O-NH₄).

The resulting mixture was molded into a rectangular graphite mold as shown in Fig. 7 and sintered at 1000 ºC for 10 minutes. The resulting sheet was 75 mm long, 20 mm wide and 1.4 mm thick. The surface area of the porous sintered sheet was 0.14-0.15 m²/g and the geometric (visible surface area of the sheet) was about 33 cm². The density of the sheet was 3 g/cm³.

A total of 112 blades were manufactured in the same way and these blades were arranged radially in two identical superimposed rows (56 blades in each cartridge) and with an equal angular distance in a stainless steel cylinder housing, which has an inner diameter of 100 mm, with the outer major dimension of the blades being almost in contact with the inner wall of the casing (clearance = 1 mm). The surface area ratio, namely the ratio between the geometric surface of the blades and the volume of the casing, was 3.1 cm²/cm³ and the diameter of the inner channel defined by the inner extremities of the radially arranged blades, was 58 mm. The volume ratio, namely the ratio between the total volume of the blades and the void volume of the casing, was 0.21 cm²/cm³ and the mass ratio was about 0.64 g/cm³.

Part "B" (Pump test)

As shown schematically in Figure 8, the getter pump GP was attached to a vacuum chamber (VC) and connected to the high vacuum pumping system (VP) by means of a tube of known conductivity (C) (calibrated conductivity). The experimental vacuum chamber was evacuated by means of the main pumping system to a pressure in the range of 10-8 torr. Heating of the getter pump (activation) was achieved using an internal quartz lamp, coaxial to the pump housing and not shown in the figure.

The quartz lamp was turned on and the getter sheets were irradiated until a temperature of 500 ºC was reached. Such a temperature was maintained for 1 hour. The lamp was then turned off and the getter material was brought back to room temperature (25 ºC). At this point, a known test gas (CO) was flowed from a high purity container (R) through the tube connecting the pump system and the calibrated line. The gas flow was regulated by means of a UHV sapphire valve.

Two pressure control indicators (Bayard-Alpert) BAG 1 and BAG 2 were used to continuously measure the pressure values before and after the known line (C).

By properly operating the valve (V), the pressure upstream (Pm) of the calibrated line is maintained at a constant level (1.5 x 10-4 torr) and the pressure downstream (Pg) of it, i.e. near the getter, is monitored for several hours, this pressure (Pg) being lower than the pressure upstream (Pm) of the gas line because the getter pump adsorbed part of the gas constituting the volume (VC). The increase in the amount of gas adsorbed by the getter material corresponded to a reduction in the pump speed and therefore to an increase in the pressure (Pg).

From the pressure Pm (torr), from the gas conductivity C(L/s) and from the change of the pressure P�9 (torr) over time, it was possible to calculate the pumping speed G(L/s) of the getter pump as a function of the amount of adsorbed gas (torr xL). It is known that the amount of gas (Qi) flowing through the gas line (C) at a given point is given by

Q i = C (Pm - Pg) (torr x L/s).

This amount of gas per unit time corresponds to the amount of gas (per unit time) adsorbed by the getter pump and can be expressed as G x Pg (torr x L/s), namely as the product of the pumping speed of the getter time of the pressure (Pg) in the vicinity of the same getter. By equating the two amounts it is possible to obtain

GxPg = (Pm - Pg);

From which follows

G(t) = C[(Pm - Pg(t)]/Pg(t).

The total amount of gas Q adsorbed by the getter pump in time t can be obtained, as is known, by integrating this amount of gas Qi adsorbed per unit time over time

Q = Qi dt = G (t) x Pg (t) dt.

The results of this measurement, namely the course of the pumping speed of the getter as a function of the amount of gas adsorbed by it, are shown in Figure 9, in which G (pumping speed) is plotted against Q (sorption capacity) and in which these data (line 1) are compared with the results (line 2) obtained using a known getter pump (SAES Getters GP 200) described in US Patent 3,662,522 with an equal housing volume.

From this comparison it is clear that the pumping speed of the improved getter pump GB according to the invention is more than twice the speed of conventional GP 200 pumps, based on the coated strips. It is also clear that the sorption capacity measured when the pumping speed of the two pumps drops below 100 L/s was more than an order of magnitude higher than that of the known pumps. The improved getter pump according to the invention therefore provides significantly higher sorption and capacity characteristics than a traditional NEG (non-evaporable getter) pump for a given casing volume.

EXAMPLE 2

Example 1 was repeated a second time by replacing carbon monoxide with nitrogen. Also in this case, the pumping speed and sorption capacity were significantly higher with respect to the standard GP 200 pumps.

EXAMPLE 3

Example 1 was repeated once again replacing carbon monoxide (CO) with hydrogen (H2). Again, the pumping speed of the improved getter pump was greater than twice that of GP 200. Since the hydrogen capacity of the NEG material used to make the pump is much greater for CO and N2, the test was stopped after the pump had sorbed 1333 Pa x L H2 and well before the point at which the pumping speed begins to drop.

Claims (9)

1. A high performance getter pump suitable for generating and maintaining a vacuum, comprising a plurality of porous sintered sheets of a non-evaporable getter material, which i) first main surface and ii) have a second major surface which is substantially parallel to the first surface and separated from it by a thickness of 0.5 to 5.0 mm, wherein said blades are arranged in a housing and separated from each other by a gas-conducting or empty space, the adjacent surfaces of adjacent blades being separated from each other by a distance of 0.5 to 10 mm. 2. A pump according to claim 1, wherein said blades are arranged in a substantially radial manner and define with their inner ends an inner channel about a symmetrical longitudinal axis of the pump, and also a heating means connected to the housing and a mounting flange are provided. 3. Pump according to claim 2, wherein the porous sintered blades have a shape selected from planar, in particular rectangular and optionally conical and/or chamfered, concave and Combinations thereof, wherein the axes of these blades form an angle with the axial planes and pass for each blade through the longitudinal axis of the pump, and wherein this angle is preferably between 10 and 150. 4. A pump according to claim 1, wherein the porous sintered sheets have a density of 1 to 5 and preferably 1.5 to 3.5 g/cm³ and a surface area of 0.5 to 1 m²/g. 5. Pump according to claim 1, wherein the 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 this metal is optionally mixed with a non-sintering material. 6. Pump according to claim 5, wherein the 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. Pump according to claim 6, wherein the non- evaporable getter material is a combination obtained from I) a ternary particulate Zr-V-Fe non- evaporable getter alloy of a composition by weight which, when in a Three-component diagram is plotted, within a polygon with the following points (wt.%) at its corners: 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 material selected from Zr and Ti, wherein Zr and/or Ti particles have a smaller average size than the alloy particles. 8. Method for generating and/or maintaining a vacuum in devices and/or apparatus which are to be kept under vacuum and in particular under a vacuum of a magnitude equal to or greater than 10⁻⁴4 and also higher than 10⁻¹⁰ Pa, characterized in that such devices and/or apparatus are connected to or attached to the improved high-performance getter pump according to claim 1. 9. Method according to claim 8, wherein these devices and/or apparatus are selected from - Vacuum vessels, such as a Dewar or vacuum jacket a line for transferring liquids, - Ultra-high vacuum chambers, - a particle accelerator and in particular a syncrotron.