Abstract:
A device for the removal of hydrogen from a vacuum enclosure at cryogenic temperatures, especially high energy particle accelerators which comprises a metal support preferably in the form of a strip of aluminium and a composition able to sorb hydrogen adherent to the support in particular on at least one surface of the strip. The composition comprises a porous absorber of H 2  O, preferably powdered aluminium oxide and in contact with palladium oxide which preferably covers, at least partially, the water absorber.

Description:
This is a continuation of Ser. No. 800,434, filed Nov. 29, 1991. 
    
    
     BACKGROUND TO THE INVENTION 
     In traditional high energy particle accelerators there has been used a distributed non-evaporable getter (NEG) system for reaching the ultimate vacuum in the system. This consists of coating a metallic non-evaporable getter on one or both surfaces of a supporting strip and then disposing the strip along substantially the whole length of the vacuum chamber constituting the beam tube of the accelerator. See U.S. Pat. No. 3,620,645 and C. Benvenuti and J-C. Decroux, Proceedings of the 7th International Vacuum Congress (Dobrozemsky, Vienna, 1977) p. 85. 
     Unfortunately there are disadvantages related to the use of non-evaporable getters in that they have to undergo a heating process to make them active and sorb unwanted gases, including hydrogen. Such heating can place an unacceptably high thermal load on the cryogenic cooling systems associated with the accelerator. Furthermore when the NEG is cooled down to the cryogenic temperature its sorption capacity becomes limited to the surface area only of the sorbing material with a consequent reduction of its ability to sorb hydrogen. 
     In addition as the accelerator reaches higher and higher circulating beam energies synchrotron radiation becomes more important as it tends to stimulate desorption of gas from the inner wall of the beam tube. In accelerators where superconducting magnets are used this gas is essentially hydrogen, with a very small amount of CO. 
     BRIEF OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a device and process for the removal of hydrogen from a vacuum at cryogenic temperatures which is free from one or more of the disadvantages of prior art hydrogen removal systems. 
     It is another object of the present invention to provide a device and process for the removal of hydrogen from a vacuum at cryogenic temperatures which does not place a thermal load on the cryogenic system of the vacuum chamber. 
     It is yet another object of the present invention to provide a device and process for the removal of hydrogen from a vacuum at cryogenic temperatures which is not limited by the surface area in its sorption of hydrogen. 
     It is a further object of the present invention to provide a device and process for the removal of hydrogen from a vacuum at cryogenic temperatures which is capable of sorbing hydrogen desorbed from the walls of a particle accelerator or storage device. 
     These and other advantages and objects of the present invention will become evident with reference to the following drawings thereof and description wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional representation of a strip useful in the present invention. 
     FIG. 1A is an enlarged view of that portion so indicated on FIG. 1. 
     FIG. 2 is a representation of a vacuum enclosure of a high energy particle accelerator incorporating a strip of the present invention. 
     FIG. 3 is a graph showing the sorption properties, for hydrogen, of a powder prepared according to the present invention. 
     FIG. 4 is a graph showing the sorption properties, for hydrogen, of a strip prepared according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a device 10 for the removal of hydrogen from a vacuum at cryogenic temperatures. By cryogenic temperatures is meant those temperatures equal to, or below the temperature of boiling oxygen. It comprises a metal support in the form of a metal strip 12 which can be any metal to which aluminum oxide can adhere, but preferably is a metal having a high thermal conductivity such as copper, silver, molybdenum and Nichrome. Aluminium is the preferred metal. 
     The aluminium strip has a length much greater than its width forming an upper surface 14 and a lower surface 16. The thickness of strip 12 is preferably between 25 μm and 1000 μm and more preferably between 100 μm and 800 μm. At lower thicknesses it becomes too thin to be handled without breaking. At greater thicknesses it becomes excessively bulky and rigid. 
     A hydrogen sorbing composition of matter 18 adheres the upper surface 14 of strip 12 but could just as well adhere also to lower surface 16. Composition of matter 18 comprises aluminium oxide, or, more in general, porous physical sorbents of moisture in contact with palladium oxide. The aluminium oxide is in the form of a powder 20 and preferably has a particle size of between 5 μm and 80 μm. At lower particle sizes the aluminium oxide becomes dangerous to handle (health hazard) while at larger particle sizes it has a lower surface area per unit mass and is less efficient as a sorber of H 2  O. 
     The palladium oxide is preferably in the form of a thin layer 22 covering the aluminium oxide powder 20. The weight ratio of aluminium oxide to palladium oxide is from 99.9:0.1 to 50:50 and preferably is from 99.5:0.5 to 90:10. A higher ratios there is too little palladium oxide to efficiently perform its hydrogen conversion function for a sufficiently long time. At lower ratios the palladium oxide blocks the sorption of H 2  O by the aluminium oxide and the additional cost is not offset by proportionally increased sorption. The palladium oxide is present therefore as a multiplicity of clusters or islands on the surface of the aluminium oxide. 
     Hence, on contacting the composition with hydrogen the palladium oxide, at the cryogenic temperature, is transformed into palladium and H 2  O, and the H 2  O is sorbed directly by the aluminium oxide without going through the vapour phase. 
     If the H 2  O were to be released as vapour it would be able to condense upon the walls of the beam tube only to he released as vapor once again by the synchrotron radiation. This increase in the partial pressure of the H 2  O vapour would severely degrade the quality of the circulating particles in the particle accelerator. 
     It is known from UK patent N° 921,273 to use palladium oxide in combination with an absorber of H 2  O such as zeolite, but physically separated therefrom. 
     FIG. 2 shows a vacuum enclosure 40 comprising an outer wall 42 and a beam tube 44 held at cryogenic temperatures, of a high energy particle accelerator. There is a device 46 comprising a metal strip 48 of aluminium having a length much greater than its width. It forms an upper surface 50 and a lower surface 52. The thickness is 40 μm. 
     A hydrogen sorbing composition of matter 54 is adherent to both surfaces. The composition 54 was produced following Example 3 (below) with the particles of aluminium oxide having an average particle size of between 3 μm and 7 μm. There was 3 mg of aluminium oxide per cm 2 . Co-deposited as clusters, on the surface of the aluminium oxide was palladium oxide, Its concentration was 0.3 mg/cm 2 . 
     Device 46 is held by a rod 56 on to the outside surface 58 of a beam tube 60. Beam tube 60 contains a slit 62 approximately 2 mm wide connecting the beam area 65 with annular outer side chamber 66. 
     It will be realized that as used in the instant specification and claims the term aluminium oxide embraces hydrated aluminium oxide and all known forms which generally are known as γ-alumina. Other porous physical adsorbents efficient for H 2  O sorption are also included. Any technique of applying the composition of the present invention to the metal support can be used. Non-limiting examples are given in the following Table I. 
     
                       TABLE I______________________________________TECHNIQUE            SUPPORT______________________________________Electroless plating, Alco-deposition ofPd + Al O.OH withoxidationBonding of Al.sub.2 O.sub.3 /PdO                any metalparticles onto stripsPlasma spray coating any metalwith Al.sub.2 O.sub.3  and thenPd deposition withoxidationElectrophoretic deposition                Mo, Taof Al.sub.2 O.sub.3  + sintering at                refractory metals1550° C. or above,                (m.p. &gt;1550° C.)then Pd deposition andoxidationAnodic Oxydation with                Alsuccessive Pd depositionand oxidation______________________________________ 
    
     The invention may be better understood by reference to the following examples wherein all parts and percentages are by weight unless otherwise indicated. These examples are designed to teach those skilled in the art how to practice the present invention and represent the best mode presently known for practicing the invention. 
     EXAMPLE 1 
     This example illustrates the preparation of a powder suitable for use in the present invention. 
     50 g of Al 2  O 3  (γ-alumina) of maximum particle size 80 μm and having a surface area of 300 m 2  g, where placed in a glass vessel and degassed, under vacuum, at 120° C. for 40 minutes. After cooling there was added a solution of 2.5 g of Pd in the form of PdCl 2  in 40 cm 3  water. The solution was again evaporated under vacuum at 45° C. thus depositing PdCl 2  over the surface of the Al 2  O 3 . 
     A quantity of solution of NaHCO 3  was added sufficient to turn all the PdCl 2  into Pd(OH) 2  by the reaction: 
     
         PdCl.sub.2 +2NaHCO.sub.3 →Pd(OH).sub.2 +2NaCl+2CO.sub.2. 
    
     Formaldehyde was then added in sufficient quantity to reduce the Pd(OH) 2  to Pd metal. The powder was then rinsed to remove reactants and then dried in an oven at 80° C. for 6 hours and then oxidized in a flow of pure O 2  at 350° C., for 3 hours. 
     EXAMPLE 2 
     A sample of powder prepared exactly as in Example 1 was placed in a test apparatus designed to measure the sorption characteristics according to ASTM (American Society for Testing and Materials) standard procedure N° F798-82. The test gas used was hydrogen at a pressure of 3×10 -6  torr (×10 -6  mbar). The sample was held at a temperature of -196° C. The test results are shown on FIG. 3 as curve 1. 
     EXAMPLE 3 
     A strip of aluminium 30 mm ×0.2 mm ×10 cm was immersed in a solution of PdCl 2  where the following reaction took place 
     
         2Al+3PdCl.sub.2 →3Pd+2AlCl.sub.3, 
    
     The solution was made slightly acid whereupon the following reaction took place 
     
         2AlCl.sub.3 +4H.sub.2 O→2Al O(OH)+6 HCl 
    
     resulting in a co-deposition of Pd and hydrated aluminium oxide. 
     The strip of coated aluminium was washed and rinsed and dried at 80° C. in air and then heated in a flow of pure O 2  at 350° C. 
     EXAMPLE 4 
     The test of Example 2 was repeated except that a piece of the coated strip prepared as in Example 3 was used. The test results are shown in FIG. 4 as curve 2. 
     Although the invention has been described in detail with reference to high energy particle accelerators or storage rings it will be realized that it may be applied to any device held at cryogenic temperatures where the pumping of hydrogen is a problem. for instance in Dewars, or anywhere where thermal insulation is required and insulation by liquid nitrogen is provided. Cryogenic fluid transport lines are another example. The hydrogen sorbing composition can be also held directly on structural parts of the device such as its walls. 
     It could be placed on the baffles of a cryopump for instance.