Patent Publication Number: US-9852874-B2

Title: Electron exit window foil

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 13/885,555, filed Sep. 5, 2014, now allowed, which is a National Stage of PCT/SE2011/051340, filed Nov. 9, 2011 and claims priority to Swedish Patent Application No. 1051277-0, filed Dec. 2, 2010, the disclosures of all of which are incorporated herein by reference in their entirety. 
    
    
     THE FIELD OF THE INVENTION 
     The present invention refers to an electron exit window foil. More particularly, the present invention relates to an electron exit window foil for use in a corrosive environment and operating at a high performance. 
     PRIOR ART 
     Electron beam devices may be used to irradiate objects with electrons, e.g. for surface treatment. Such devices are commonly used within the food packaging industry, where electron beams are providing efficient sterilization of packages, e.g. plastic bottles or packaging material to be later converted into a package. 
     A main advantage with electron beam sterilization is that wet chemistry, using e.g. H 2 O 2 , may be avoided thus reducing the high number of components and equipment required for such wet environments. 
     An electron beam device typically comprises a filament connected to a power supply, wherein the filament is emitting electrons. The filament is preferably arranged in high vacuum for increasing the mean free path of the emitted electrons and an accelerator is directing the emitted electrons towards an exit window. The electron exit window is provided for allowing the electrons to escape from the electron beam generator so they may travel outside the electron beam generator and thus collide with the object to be sterilized and release its energy at the surface of the object. 
     The electron exit window typically consists of thin electron permeable foil that is sealed against the electron beam generator for maintaining the vacuum inside the electron beam generator. A cooled support plate in the form of a grid is further provided for preventing the foil to collapse due to the high vacuum. Ti is commonly used as the foil material due to its reasonable good match between high melting point and electron permeability, as well as the ability to provide thin films. 
     A problem with a Ti film is that it may oxidize, leading to reduced lifetime and operational stability. In order to achieve a long lifetime of the exit window, a maximum temperature of approximately 250 C should not be exceeded during the operation of the electron beam device. Typically, a high performance electron beam device is designed to provide 22 kGy at up to 100 m/min at 80 keV when used for sterilizing a running web of material. A plain Ti foil may thus not be used with such high performance electron beam devices, since the amount of emitted electrons transmitted through the window may cause temperatures well above this critical value. 
     In filling machines, i.e. machines designed to form, fill, and seal packages, sterilization is a crucial process not only for the packages, but for the machine itself. During such machine sterilization, which preferably is performed during start-up, the outside of the exit window will be exposed to the chemicals used for machine sterilization. A highly corrosive substance such as H 2 O 2 , which is commonly used for such applications, will affect the exit window by means of etching the Ti. 
     Different solutions for improving the properties of the exit window have been proposed to overcome the above-mentioned drawbacks. 
     EP0480732B describes a window exit foil consisting of a Ti foil, and a protective layer of Al that is forming an intermetallic compound by thermal diffusion treatment of the Ti/Al construction. This solution may be suitable for relatively thick exit windows, i.e. windows allowing a protective layer being thicker than 1 micron. However, an intermetallic compound is not acceptable on a thin Ti foil since it would reduce its physical strength. 
     EP0622979A discloses a window exit foil consisting of a Ti foil and a protective layer of silicon oxide on the side of the exit foil facing the object to be irradiated. Although the Ti foil may be protected by such layer, silicon oxide is very brittle and may easily crack in the areas where the foil is allowed to flex, i.e. the areas between the grids of the supportive plate when vacuum is provided. This drawback is making the foil of EP0622979A unsuitable for applications where the exit foil is exhibiting local curvatures, such as electron beam devices using a grid-like cooling plate arranged in contact with the exit foil. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce or eliminate the above-mentioned drawbacks. 
     A further object is to provide an electron exit foil that is able to decrease the heat load as well as the corrosion on the foil. 
     An idea of the present invention is thus to provide an electron beam generator having a prolonged operating lifetime, requiring a reduced service, and being more cost-effective than prior art systems due to inexpensive coating processes and the appliance of well-established X-ray manufacturing processes. 
     According to a first aspect of the invention, an electron exit window foil for use with a high performance electron beam generator operating in a corrosive environment is provided. The electron exit window foil comprises a sandwich structure having a film of Ti, a first layer of a material having a higher thermal conductivity than Ti, and a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment. 
     The first layer may be arranged between the film and the second layer, or the film may be arranged between the first layer and the second layer. 
     The second layer may comprise at least two layers of different materials, which is advantageous in that different mechanical and/or physical properties of the foil, such as erosion resistance and strength, may be tailor made for the particular application. 
     The first layer may be selected from a group consisting of materials having a ratio between thermal conductivity and density being higher than of Ti. 
     The first layer may be selected from the group consisting of Al, Cu, Ag, Au, or Mo, and the second layer may be selected from the group consisting of Al 2 O 3 , Zr, Ta, or Nb. 
     The corrosive environment may comprise H 2 O 2 . Hence, the foil may be implemented in electron beam devices operating in machines being subject to corrosive sterilizing agents, such as for example filling machines within the food packaging industry. 
     The electron exit window foil may further comprise at least one adhesive coating between the Ti film and first layer or the second layer. Said adhesive coating may be a layer of Al 2 O 3  or ZrO 2  having a thickness between 1 and 150 nm. This is advantageous in that any reaction or material diffusion is prevented at the film/layer interface or the adhesion between the Ti film and a layer or between two layers is improved. 
     According to a second aspect, an electron beam generator configured to operate in a corrosive environment is provided. The electron beam generator comprises a body housing and protecting an assembly generating and shaping the electron beam, and a support carrying components relating to the output of the electron beam, said support comprising an electron exit window foil according to the first aspect of the invention. 
     The advantages of the first aspect are also applicable for the second aspect of the invention. 
     According to a third aspect of the invention, a method for providing an electron exit window foil for use with a high performance electron beam generator operating in a corrosive environment is provided. The method comprises the steps of providing a film of Ti, providing a first layer of a material having a higher thermal conductivity than Ti onto a first side of said film, and providing a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment. 
     The step of providing a flexible second layer may comprise arranging said flexible second layer onto a second side of said film. 
     The step of providing a flexible second layer may comprise arranging said flexible second layer onto said first layer. 
     At least one of steps of providing a first layer or providing a flexible second layer may be preceded by a step of providing an adhesive coating onto said film. 
     According to a fourth aspect of the invention, a method for providing a high performance electron beam device is provided. The method comprises the steps of attaching a film of Ti onto a frame, processing said film by providing a first layer of a material having a higher thermal conductivity than Ti onto a first side of said film, and providing a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment, and attaching said foil-frame subassembly to a tube housing of an electron beam device for sealing said electron beam device. 
     The advantages of the first aspect of the invention are also applicable for the third and the fourth aspects of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, exemplary embodiments of the invention will be described in greater detail, with reference to the enclosed drawings, in which: 
         FIG. 1  is a schematic cross sectional isometric view of an electron beam device according to prior art, 
         FIG. 2  is a schematic cross sectional perspective view of an electron exit window foil and a foil support plate, and 
         FIG. 3 a - f    are schematic cross sections of electron exit window foils according to different embodiments. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1  an electron beam device is shown. The electron beam device  100  comprises two parts; a tube body  102  housing and protecting an assembly  103  generating and shaping the electron beam, and a supportive flange  104  carrying components relating to the output of the electron beam, such as a window foil  106  and a foil support plate  108  preventing the window foil  106  from collapsing as vacuum is established inside the device  100 . Further, during operation of the electron beam device the foil  106  is subject to excessive heat. Thereby, the foil support plate  108  also serves the important purpose of conducting heat generated in the foil  106  during use away from the foil  106  of the device. By keeping the foil temperature moderate a sufficiently long lifetime of the foil  106  may be obtained. 
     With reference to  FIG. 2 , an electron exit window is shown comprising the foil  106  and the foil support  108 . The support  108  is arranged inside the electron beam device such that vacuum is maintained on the inside of the exit window. This is indicated by P 1  and P 2  in  FIG. 2 , where P 1  denotes atmospheric pressure outside the exit window and P 2  represents vacuum on the inside. 
     During manufacturing, the foil support plate  108 , being of copper, is preferably attached to the flange  104  forming a part of the tube body  102 . The flange  104  is generally made of stainless steel. The window foil  106  is then bonded onto a separate frame thus forming a foil-frame sub assembly. The foil  106  is subsequently coated, in order to improve its properties regarding for instance heat transfer. The foil-frame subassembly is subsequently attached to the tube body  102  to form a sealed housing. 
     In an alternative embodiment, the exit window foil  106  is attached directly to the flange, being attached to the support plate, before the flange is welded to the tube body. 
     In this embodiment, the exit window foil is consequently coated prior to being attached to the tube body  102 . 
     With reference to  FIG. 3 a - f   , different embodiments of an electron exit window foil  106   a - f  are shown. 
     Starting with  FIG. 3 a   , the foil  106   a  comprises a thin film of Ti  202 . The Ti film  202  has a thickness of approximately 5 to 15 microns. On a first side of the Ti film  202 , a thermally conductive layer  204  is arranged. The thermally conductive layer  204  is provided in order to transfer heat along the exit foil such that a reduced temperature is achieved across the entire foil  106 . The thermally conductive layer  204  is provided by means of any suitable process, such as sputtering, thermal evaporation, etc, and should allow for a sufficient improvement in thermal conductivity for lowering the temperature of the electron exit window foil  106   a  while still allowing the foil to bend into the apertures of the support plate  108  when vacuum is applied. Preferably, the material of the thermally conductive layer  204  is chosen from the group consisting of Al, Cu, Ag, Au, and Mo. Although other materials, such as Be, may have a higher ratio between thermal conductivity and density they are considered as poisonous and hence not preferred, especially in applications in which the electron beam device is arranged to process consumer goods. 
     On the other side of the Ti film a protective layer  206  is arranged. The protective layer  206  is provided by means of any suitable coating process, such as sputtering, thermal evaporation, etc. Preferably, the material of the protective layer is chosen from the group consisting of Al2O3, Zr, Ta, and Nb due to their resistance against hydrogen peroxide containing environments. It should thus be understood that the protective layer  206  is facing the atmospheric environment, i.e. the objects to be sterilized. 
     The thickness of the thermally conductive layer  204  is preferably between 1 and 5 microns and the thickness of the protective layer  206  is substantially less than 1 micron. Preferably, the thickness of the protective layer  206  is approximately 200 nm. By keeping the window foil  106  as thin as possible, the electron output is maximized. The thickness of the protective layer  206  should thus be designed such that it is capable of protecting the Ti film from a) corrosion by hydrogen peroxide or other aggressive chemical agents which may be provided in the particular application, and b) corrosion caused by the plasma created by the electrons in the air. Further, the thickness of the protective layer  206  should ensure tightness and physical strength, such that the second layer  206  is flexible in order to allow the entire foil to bend and conform to the apertures of the support plate  108  when vacuum is applied. A yet further parameter may be the density, for allowing electron transmittance through the protective layer  206 . 
     By arranging the thermally conductive layer  204  and the protective layer  206  on opposite sides of the Ti foil, stress in the layers may be reduced. For example, if using Al as the thermally conductive layer and Zr as the protective layer, the Ti foil arranged in between those layers will reduce some of the stress induced upon heating. This is due to the fact that the coefficient of thermal expansion of Ti lies between the corresponding value of Al and Zr. 
       FIG. 3 b    shows another embodiment of a foil  106   b . Here, the thermally conductive layer  204  and the protective layer  206  are provided on the same side of the Ti film  202  such that the protective layer  206  is coated directly on the thermally conductive layer  204 . This structure may be advantageous for electron beam devices, for which the electron exit window foil must be mounted to the tube housing before coating, i.e. not allowing coating of the side of the Ti foil facing the interior of the electron beam device. 
       FIGS. 3 c  and 3 d    show two different embodiments similar to what has been previously described with reference to  FIGS. 3 a  and  b   . However, in  FIGS. 3 c  and 3 d    the protective layer  206  comprises at least two layers of different materials; a first layer  208  and a second layer  209 . The first layer  208  and the second layer  209  of the protective layer  206  are both selected from the group consisting of Al 2 O 3 , Zr, Ta, and Nb, or alloys thereof. It should however be understood that each one of the layers  208 ,  209  could per se be a sandwich of two or more protective layers. 
     For example, the corrosion protection layer  206  itself could be a multilayer structure comprising an oxide, a metal, an oxide, a metal, etc. According to a specific embodiment such multilayer structure may be formed by a first layer of ZrO 2 , a second layer of Zr, a third layer of ZrO 2 , and a fourth layer of Zr. This is advantageous in that a potential disruption in one of the sub layers does not induce a significant reduction of the overall corrosion protection of the protective layer  206 . 
     In order to achieve good adherence between the different layers/films of the electron exit window foil, adhesive barrier coatings may be provided at the interface. Such coatings may be a thin layer of Al 2 O 3  or ZrO 2 , having a thickness between 1 to 150 nm, preferably between 50 and 100 nm. The use of such coatings is advantageous in that they prevent any reaction or diffusion of material at the interface between Ti and the thermally conductive layer and/or the protective layer. Reaction or diffusion may result in the formation of an intermetallic compound which negatively changes the characteristics of the materials involved. In the case of a thin Ti foil it may get reduced physical strength. Further, the presence of intermetallic compounds may reduce the thermal conductivity and the corrosion protective ability of the thermally conductive layer  204  and the protective layer  206  respectively. 
       FIG. 3 e    describes a further embodiment similar to that of  FIG. 3 a    but provided with barrier coatings of the kind described above. The electron exit window foil  106   e  comprises a sandwich structure having a film of Ti, a first layer  204  of Al having a higher thermal conductivity than Ti, and a flexible second layer  206  of Zr being able to protect said film  202  from a corrosive environment, wherein the second layer  206  is facing the corrosive environment. 
     The thermally conductive layer  204  of aluminium (Al) is arranged on a first side of the titanium (Ti) foil. A first barrier coating  210   a  of zirconium oxide (ZrO 2 ) is provided in between said Ti film  202  and said Al layer  204 . On the other side of the Ti film  202  the protective layer  206  of zirconium (Zr) is arranged. A second barrier coating  210   b  of zirconium oxide (ZrO 2 ) is provided in between the Ti film and the Zr layer  206 . This embodiment is advantageous in that the Ti foil  202  is surrounded on one side by Al as the thermally conductive layer and on the other side by Zr as the protective layer. Since the coefficient of thermal expansion of Ti lies between the corresponding values of Al and Zr, some of the stress induced during heating of the foil will be reduced. As an alternative one or both of the barrier coatings  210   a ,  210   b  may instead be made of aluminium oxide (Al 2 O 3 ). It is an advantage if the barrier coatings are based on a material provided in either the thermally conductive layer or in the protective layer. For example, if the protective layer is zirconium and the thermally conductive layer is aluminium, it is preferred that either aluminium oxide or zirconium oxide are used for the barrier coatings. This is due to the fact that the layers are applied by a sputtering machine. In a such machine sputter targets are used, one for each material that should be deposited. One and the same target can be used for both e.g. zirconium and zirconium oxide. The same applies for aluminium and aluminium oxide. Hence, it is preferred if the barrier coating is an oxide of a material used in either the corrosion protection or the thermal conductivity layer. 
       FIG. 3 f    shows an embodiment similar to that of  FIG. 3 b    but provided with barrier coatings. The electron exit window foil  106   f  comprises a sandwich structure having a film of Ti, a first layer  204  of Al having a higher thermal conductivity than Ti, and a flexible second layer  206  of Zr being able to protect said film  202  from a corrosive environment, wherein the second layer  206  is facing the corrosive environment. The thermally conductive layer  204  and the protective layer  206  are provided on the same side of the Ti film  202 . On top of the Ti film  202  a first barrier coating  210   a  is coated. The barrier coating  210   a  is made of aluminium oxide (Al 2 O 3 ). The thermally conductive layer  204  of aluminium (Al) is coated on said first barrier coating  210   a . On the thermally conductive layer  204  there is in turn coated a second barrier coating  210   b . Said barrier coating  210   b  is also made of aluminium oxide (Al 2 O 3 ). Finally, the protective layer  206 , being made of zirconium (Zr), is coated on said second barrier coating  210   b . As an alternative one or both of the barrier coatings  210   a ,  210   b  may instead be made of zirconium oxide (ZrO 2 ). 
     In both the embodiments of  FIGS. 3 e  and 3 f    the protective layer  206  may be a multilayer structure as described in relation to  FIGS. 3 c    and  3   d.    
     The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.