Cryogenic seal, cold head and vacuum pump

Abstract

Cryogenic seal having biasing member (e.g. spring 24) urging the seal (e.g. O-ring 21) towards a sealing surface 8, the seal 21 composes minimum 95% polymer and filler by weight, the polymer is minimu

Description

FIELD OF THE INVENTION The field of the invention relates to cryogenic seals and their use in cold heads and cryogenic vacuum pumps. BACKGROUND Cryogenic refrigerators used in cryocoolers or cryopumps, may comprise displacer elements that move within cylinders and that require an effective seal between the displacer and cylinder to inhibit gas leakage between the two. These seals need to be able to operate effectively at cryogenic temperatures and in very dry conditions with corresponding very high frictional forces. They therefore require properties of low wear and a low coefficient of friction. Conventionally fluorinated polymers such as Polytetrafluoroethylene PTFEs have been used. However, per and polyfluoroalkyl substances such as PTFEs are considered dangerous for human health and environmentally damaging because they collect in nature and cannot be easily degraded. It would therefore be desirable to eliminate or at least reduce their use. SUMMARY An aspect provides a cryogenic cold head according to claim 1. It was recognised that conventional cryogenic seals contain per and polyfluoroalkyl substances and that although these provide many of the properties required for these seals, the use of these materials is undesirable and may become restricted. Alternatives have therefore been sought which provide a low coefficient of friction at low temperatures and will not wear unduly. Although polymers may often be used in seals, cryogenic temperatures may cause these polymers to become brittle and this may make them not only subject to failure, but may also cause them to scratch and damage the surface they move against. It was found that the addition of filler materials alters the properties of the 13 01 26 polymers and can be used to maintain compliancy at low temperatures and provide a low coefficient of friction and low wear. In some embodiments, said sealing member further comprises up to 5% 5 Polytetrafluoroethylene PTFE, preferably up to 1%. In other embodiments, said sealing member consists of said polymer material and said at least one filler material. io It may be environmentally advantageous to have a seal with no per and polyfluoroalkyl substances within it. However, it may be that such a seal does not operate effectively enough for some applications at very low temperatures and thus, for some applications where temperatures are particularly low, perhaps below 10K it may be advantageous to have a seal with a small amount of PTFE 15 within it. In some embodiments, said cryogenic seal is configured to operate at temperatures between 80K and 4K, in some cases between 40K and 4K and in some below 10K. 20 In some embodiments, said polymer material is a ground material such as a powder. In some embodiments, said filler material comprises woven or knitted fibres 25 impregnated with said ground polymer material. In some embodiments, said elastic member comprises one of an O-ring or spring. In some embodiments, said seal comprises an intermediate member between 30 said elastic member and said sealing member. An intermediate member may be used to cushion the transfer of force between the elastic member and the sealing member. 13 01 26 ln some embodiments, said intermediate member is composed of plastic and is configured to retain said sealing member. The intermediate member may also be used to hold the sealing member in place. 5 In some embodiments, said seal is substantially cylindrical, said sealing member extending around an outer circumference of said elastic member. The cryogenic cold head comprises a displacer mounted to move within a io cylinder and at least one cryogenic seal, said at least one cryogenic seal being arranged to seal between an outer surface of said displacer and an inner surface of said cylinder. In some embodiments, said seal is mounted within a recess in said displacer, an 15 outer face of said sealing member abutting an inner surface of said cylinder. Said sealing member comprises a coefficient of thermal expansion that is equal to the coefficient of thermal expansion of said cylinder within a deviation of ± 20%, preferably within a deviation of ± 5%. 20 In order for the seal to operate effectively across a temperature range, then it may be advantageous if it has a similar coefficient of thermal expansion to the surface it seals against. Thus, in some embodiments, the polymer to filler ratio may be adjusted to achieve such a desired coefficient of thermal expansion. 25 In some embodiments, said cryogenic cold head comprising a two stage cold head comprising first and second stage displacers configured to move within respective cylinders and comprising at least two of said seals for sealing between said displacers and respective cylinders. 30 Although the seals are cryogenic seals in that they can operate effectively at cryogenic temperatures many of the selections of polymer and filler mixtures will 13 01 26 also function well at non-cryogenic temperatures and thus, in some embodiments, the same design of seal can be used throughout the cold head. In some embodiments, said cryogenic cold head further comprises a mechanical 5 drive for moving said displacer, and a plurality of said seals, one of said seals being mounted to seal between a shaft of said mechanical drive driving said displacer and a housing of said mechanical drive. In some embodiments, said cryogenic cold head further comprises a pneumatic io drive for moving said displacer, and a plurality of said seals, one of said seals being mounted to seal between a shaft of the control piston of the displacer and the wall of the control cylinder of said pneumatic drive driving. In some cases, the seal may be used in the displacer where it may operate close 15 to room temperature and be used to reduce or prevent the gas flow between the low-pressure region of the drive used to move the displacer, and the displacer working volume. This seal may be located between the shaft of the pushing rod and the housing of the mechanical drive. 20 Another aspect provides a cryogenic refrigerator comprising a cryogenic cold head according to one aspect. A yet further aspect provides a cryogenic vacuum pump comprising a cryogenic cold head according to one aspect. 25 Other aspects may provide a helium or hydrogen liquefaction system comprising a cryogenic cold head according to one aspect, an MRI system comprising a cryogenic cold head according to one aspect; or a high temperature super conductor cooler comprising a cryogenic cold head according to one aspect or a 30 quantum computer system comprising a cryogenic cold head according to one aspect. 13 01 26 Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. 5 Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. io BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 shows a cross section through a seal according to one embodiment; Figure 2 shows a cross section through a seal according to a further 15 embodiment; and Figure 3 shows a cryopump comprising a cold head comprising seals according to an embodiment. DESCRIPTION OF THE EMBODIMENTS 20 Before discussing the embodiments in any more detail, first an overview will be provided. Many cryogenic seals contain Per- and polyfluoroalkyl substances such as PTFE, which help to provide the desired low wear and low coefficient of friction 25 proprieties at low temperatures. These substances are environmentally damaging and their use may well be banned or restricted in the future. Embodiments therefore seek to provide an alternative cryogenic seal with no or reduced Per- and polyfluoroalkyl substances and that will still operate effectively at the required low temperatures. 30 Embodiments provide a cryogenic seal for sealing between cooperating inner and outer cylindrical surfaces at cryogenic temperatures. The seal comprising: an 13 01 26 elastic member for biasing the seal towards one of the cylindrical surfaces; and a sealing member, wherein at least 95% by weight of the sealing member comprises: a polymer material that is not a per- or polyfluoroalkyl substance and at least one filler material, the polymer material making up at least 50% of the 5 composition of the sealing member. The disclosure relates to such seal systems, as well as a displacer and refrigerator or cryocooler using such a seal system. Cryogenic refrigerators for example the GM type comprise a cylinder, a displacer, a mechanism for moving io the displacer within the cylinder, and a valve. For correct and energy efficient operation, there are a number of seals used between the components, so that the gas flow is directed as desired. Many of the seals are made of materials which contain polymers. Conventionally a high fraction of such polymer seals consists of fluorinated compounds. As noted above embodiments seek to provide 15 effective seals without, or at least with reduced, fluorinated polymers. Embodiments provide a seal system comprising a sealing member that is either free of per- and polyfluoroalkyl substances PFAS, or has less than 5% PFAS, preferably less than 1%, in some cases less than 0.5%, and is suitable for use at 20 cryogenic temperatures in the region of 2 to 80 K, preferably in the region of 4 to 30 K. Such use may be in the second stage of a cold head comprising a second stage cylinder with an inner diameter preferably between 15 and 40 mm, made of 25 metallic material, preferably stainless steel, and a second stage displacer with a cylindrical shape, which has an outer diameter suitable for fitting within the second stage cylinder and which can be moved linearly and / or rotationally within the cylinder in order to meet the requirements of the corresponding cooling cycle, for example the GM cryogenic process. 30 The displacer body may be made of stainless steel, phenolic resin composite material or PEEK. The seal is in contact with the cylinder and displacer, and 13 01 26 provides a sealing function between them, and the sealing member of the seal is composed of at least one ground polymeric material and further supporting materials, where the supporting material (for example fillers) are in concentrations each higher than 0.2% by weight and together less than 50% by 5 weight, and where the ground material may be one of the following polymer materials or a mixture of more than one of the following polymer materials: i. PAI: Polyamide-imide ii. PI: Polyimide iii. PEEK: Polyether ether ketone io iv. PE: Polyethylene v. PP: Polypropylene vi. PPS: Polyphenylene sulfide vii. PVC: Polyvinylchloride viii. PVDC: Polyvinylidene chloride 15 ix. Polyphosphazene x. Silicone: Poly(organo)siloxane, for example MQ (Methyl-Silicone), VMQ (Vinyl-Methyl-Silicone), PVMQ (Phenyl-Vinyl-Methyl-Silicone), PMQ (Phenyl-modified Silicone) xi. Synthetic resins: epoxy resins 20 xii. NBR: nitrile butadiene rubber xiii. PET: Polyethylene terephthalate xiv. Polychloroprene xv. PBI: Polybenzimidazole. 25 The supporting or filler materials for the seal system may be one or more of the following: a) Carbon fibres b) Para-aramid (Kevlar) c) Graphite 30 d) Carbon black e) Glass fibres f) Stainless steel fibres 13 01 26 g) Wool fibres h) Cotton fibres i) Carbon Nanotubes and / or Fullerenes j) Mica particles 5 k) Boron Nitride powder I) Molybdenum disulfide m) Metallic powders (for example stainless steel, bronze, messing, copperberyllium bronze, copper, indium, tin, or other metal or alloy) n) Rotaxanes and / or Catenanes with a molecular mass higher than 1000 io amu. Where the supporting material are fibre materials they may be in the form of woven or knitted material, impregnated with the ground polymer material. 15 In some cases the ground polymeric material and filler are selected, and / or the ground material-to-filler ratio is adjusted in a way, that the seal material has nearly the same CTE (coefficient of thermal expansion) as the metallic material of the cylinder. 20 In some embodiments the seal further comprises supporting and energising elements, in some cases O-rings, which serve to adjust the force, which acts between the seal, displacer and cylinder. In some embodiments the energising element comprises a ring, in some cases 25 an O-ring and in others the energising element comprises a metallic spring. In some embodiments, the energising element further comprises an intermediate holding ring made of plastic material, which serves to support the seal in its place, and a metallic spring ring, which is applying force on the holding ring. 30 Figure 1 shows a seal system 20 according to an embodiment. Seal system 20 comprises a sealing member 21 made of a polymeric material and filler mix, an intermediate member 22 and a spring member 24. The sealing system 20 is 13 01 26 mounted within a recess in a displacer 9 which moves up and down as shown by the arrow within cylinder 8. This displacer may be a displacer within a cryogenic cold head and may contain a regenerator material. The seal system 20 may have a cylindrical shape and the outer circumference of the seal system 20 may 5 abut the inner circumference of cylinder 8 and seal to it. The intermediate member 22 may be formed of a plastic material and may be configured to hold sealing member 21 in position. The spring 24 provides a biasing force for biasing the seal against cylinder 8. The composition of sealing io member 21 is selected to make it suitable for use at cryogenic temperatures. The material is also selected to provide a low coefficient of friction and a resistance to wear that is relatively high. The coefficient of thermal expansion may be selected to be similar to that of the material of the cylinder 8 and this may be done by choosing the composition of the filler and polymer materials 15 accordingly. The filler and the polymer materials are each selected from the lists given earlier in the disclosure. Figure 2 shows an alternative embodiment. As in the embodiment of Figure 1 the sealing system is mounted within a recess within displacer 9. However, in this 20 embodiment the elastic member 24 is in the form of an O-ring. The sealing member is a polymer and filler mix as in the Figure 1 embodiment and the materials may be selected with a coefficient of thermal expansion similar to that of cylinder 8 and with the desired low friction and resistance to wear properties at cryogenic temperatures. In this embodiment there is no intermediate member. 25 Figure 3 shows a cryogenic vacuum pump 1 according to an embodiment. Cryogenic vacuum pump 1 comprises a vacuum vessel 2, an inlet 3 for receiving gas and a refrigeration unit 4 including a cold head 5 extending into the vacuum vessel 2 for cooling a cryopanel structure 6 for capturing gas in the vacuum 30 vessel 2. The refrigeration unit 4 includes a two stage cold head 5 comprising a first stage 7 and a second stage 18. Each stage comprises a displacer 9 configured to move backwards and forwards to cool a refrigerant. A motor 10 is 13 01 26 configured to drive the motion of the displacers 9. Seals 20 according to an embodiment can be seen between the displacer 9 and the cylinder 8 of the first stage and the displacer 9 and the cylinder 8 of the second stage. The seals in the first stage will operate at temperatures above cryogenic temperatures and 5 although the seals are suitable for use at cryogenic temperatures, they may also operate well at these slightly higher temperatures. The seals in the second stage will operate at cryogenic temperatures. In some embodiments there may be further seals within the motor unit 10 and these may operate close to room temperature and be used to reduce or prevent the gas flow between the low io pressure region of the mechanical drive and the displacer working volume. The refrigerator system 4 that is used in figure 3 to cool the cryopanels of a cryopump may be used in other embodiments for helium or hydrogen liquefaction or in a MRI system. They may also be used in the cooling of high temperature 15 superconductors or for cooling quantum computer systems. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and 20 modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. 10 15 REFERENCE SIGNS 1 cryogenic vacuum pump 2 vacuum vessel 3 inlet 4 refrigerator 5 cold head 6 cryopanels 7 first stage 8 cylinder 9 displacer 10 mechanical drive 18 second stage 20 seal 21 sealing member 22 intermediate member 24 elastic member 13 01 26

Claims

1. A cryogenic cold head comprising a displacer mounted to move within a cylinder and at least one cryogenic seal arranged to seal between an outer5 surface of said displacer and an inner surface of said cylinder for sealing between cooperating inner and outer cylindrical surfaces at cryogenic temperatures said seal comprising:a sealing member;an elastic member for biasing said sealing member towards one of saidio cylindrical surfaces; whereinat least 95% by weight of said sealing member comprises: a polymer material and at least one filler material; whereinsaid polymer material consists of at least one of the following: PAI: Polyamide-imide, PI: Polyimide; PEEK: Polyether ether ketone; PE: Polyethylene;15 PP: Polypropylene; PPS: Polyphenylene sulfide; PVC: Polyvinylchloride; PVDC: Polyvinylidene chloride; Polyphosphazene; Silicone; a synthetic or epoxy resin; NBR: nitrile butadiene rubber; PET: Polyethylene terephthalate; Polychloroprene; PBI: Polybenzimidazole; andsaid filler material comprises at least one of the following: Carbon fibres;20 Para-aramid; Graphite; Carbon black; Glass fibres; Stainless steel fibres; Wool fibres; Cotton fibres; Carbon Nanotubes; Fullerenes; Mica particles; Boron Nitride powder; Molybdenum disulfide; Metallic powder; Rotaxanes and / or Catenanes with a molecular mass higher than 1000 amu.wherein said polymer material comprises at least 50% by weight of said25 sealing member; andsaid at least one filler material each comprise at least 0.2% by weight of said sealing member; andwherein said sealing member comprises a coefficient of thermal expansion that is equal to the coefficient of thermal expansion of said cylinder within a30 deviation of ± 20%.13 01 262. A cryogenic cold head according to claim 1, wherein said sealing member further comprises up to 5% Polytetrafluoroethylene PTFE.

3. A cryogenic cold head according to claim 1, wherein said sealing member 5 consists of said polymer material and said at least one filler material.

4. A cryogenic cold head according to any preceding claim, wherein said cryogenic temperatures are temperatures between 80 and 4K.io 5. A cryogenic cold head according to any preceding claim, wherein said polymer material comprises a ground material and said filler material comprises woven or knitted fibres impregnated with said ground polymer material.

6. A cryogenic cold head according to any preceding claim, wherein said15 elastic member comprises one of an O-ring or spring.

7. A cryogenic cold head according to any preceding claim, further comprising an intermediate member between said elastic member and said sealing member.

208. A cryogenic cold head according to claim 7, wherein said intermediate member is composed of plastic and is configured to retain said sealing member.

9. A cryogenic cold head according to any preceding claim, wherein said seal 25 is substantially cylindrical, said sealing member extending around an outer circumference of said elastic member.

10. A cryogenic cold head according to any preceding claim, wherein said seal is mounted within a recess in said displacer, an outer face of said sealing30 member abutting an inner surface of said cylinder.

11. A cryogenic cold head according to claim any preceding claim, wherein said sealing member comprises a coefficient of thermal expansion that is equal to the coefficient of thermal expansion of said cylinder within a deviation of ± 5%.5 12. A cryogenic cold head according to any preceding claim, said cryogeniccold head comprising a two stage cold head comprising first and second stage displacers configured to move within respective cylinders and comprising at least two of said seals for sealing between said displacers and respective cylinders.io 13. A cryogenic refrigerator comprising a cryogenic cold head according to any preceding claim, further comprising a mechanical drive for moving said displacer, and a plurality of said seals, one of said seals being mounted to seal between a shaft of said mechanical drive driving said displacer and a housing of said mechanical drive.CM1514. A cryogenic vacuum pump comprising a cryogenic cold head according to any one of claims 1 to 12.

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