Patent Publication Number: US-8989354-B2

Title: Carbon composite support structure

Description:
CLAIM OF PRIORITY 
     Priority is claimed to U.S. Provisional Patent Application Nos. 61/486,547, filed on May 16, 2011; 61/495,616, filed on Jun. 10, 2011; and 61/511,793, filed on Jul. 26, 2011; which are herein incorporated by reference. 
    
    
     BACKGROUND 
     It is important for support members in support structures, such as x-ray window support structures, to be strong but also small in size. Support structures in x-ray windows can support a film. X-ray windows can be used for enclosing an x-ray source or detection device. X-ray windows can be used to separate a pressure differential, such as ambient air pressure on one side of the window and a vacuum on an opposing side, while allowing passage of x-rays through the window. 
     X-ray windows can include a thin film supported by the support structure, typically comprised of ribs supported by a frame. The support structure can be used to minimize sagging or breaking of the thin film. The support structure can interfere with the passage of x-rays and thus it can be desirable for ribs to be as thin or narrow as possible while still maintaining sufficient strength to support the thin film. The support structure and film are normally expected to be strong enough to withstand a differential pressure of around 1 atmosphere without sagging or breaking. 
     Materials comprising Silicon have been use as support structures. A wafer of such material can be etched to form the support structure. 
     Information relevant to x-ray windows can be found in U.S. Pat. Nos. 4,933,557, 7,737,424, 7,709,820, 7,756,251, 8,498,381; U.S. Patent Publication Numbers 2008/0296479, 2011/0121179, 2012/0025110; and U.S. Patent Application Nos. 61/408,472 61/445,878, 61/408,472 all incorporated herein by reference. Information relevant to x-ray windows can also be found in “Trial use of carbon-fiber-reinforced plastic as a non-Bragg window material of x-ray transmission” by Nakajima et al., Rev. Sci. Instrum 60(7), pp. 2432-2435, July 1989. 
     SUMMARY 
     It has been recognized that it would be advantageous to provide a support structure that is strong. For x-ray windows, it has been recognized that it would be advantageous to provide a support structure that minimizes attenuation of x-rays. The present invention is directed to support structures, and methods of making support structures, that satisfy these needs. 
     In one embodiment, the apparatus comprises a support frame defining a perimeter and an aperture and a plurality of ribs comprising a carbon composite material extending across the aperture of the support frame and carried by the support frame. Openings exist between the plurality of ribs. A film can be disposed over, carried by, and span the plurality of ribs and can be disposed over and span the openings. The film can be configured to pass radiation therethrough. 
     In another embodiment, a method of making a carbon composite support structure comprises pressing at least one sheet of carbon composite between non-stick surfaces of pressure plates and heating the sheet(s) to at least 50° C. to cure the sheet(s) into a carbon composite wafer. Each sheet can have a thickness of between 20 to 350 micrometers (μm). The wafer can then be removed and a plurality of openings can be laser cut in the wafer, forming ribs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional side view of a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional side view of a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic top view of a carbon composite wafer in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic top view of a carbon composite support structure, wherein carbon fibers in a carbon composite material are directionally aligned with a longitudinal axis of a plurality of ribs across an aperture of a support frame, in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic top view of a carbon composite support structure comprising a carbon composite material that includes carbon fibers directionally aligned in two different directions; in accordance with an embodiment of the present invention; 
         FIG. 6  is a schematic top view of a carbon composite support structure with ribs that have at least two different cross-sectional sizes, in accordance with an embodiment of the present invention; 
         FIG. 7  is a schematic top view of a carbon composite support structure with intersecting ribs, in accordance with an embodiment of the present invention; 
         FIG. 8  is a schematic top view of a carbon composite support structure with hexagonal shaped openings and hexagonal shaped ribs, in accordance with an embodiment of the present invention; 
         FIG. 9  is a schematic top view of a section of a carbon composite support structure with a hexagonal shaped opening, hexagonal shaped ribs, and carbon fibers directionally aligned with longitudinal axes of the ribs, in accordance with an embodiment of the present invention; 
         FIG. 10  is a schematic top view of a carbon composite support structure with triangular shaped openings, triangular shaped ribs, and carbon fibers directionally aligned with longitudinal axes of the ribs, in accordance with an embodiment of the present invention; 
         FIG. 11  is a schematic top view of a carbon composite support structure with two ribs extending in one direction and two ribs extending in a different direction and carbon fibers that are directionally aligned with longitudinal axes of the ribs, in accordance with an embodiment of the present invention; 
         FIG. 12  is a schematic cross-sectional side view of multiple stacked support structures, including a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 13  is a schematic top view of a stacked support structure including a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 14  is a schematic top view of a stacked support structure including a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 15  is a schematic cross-sectional side view of a multi-layer support structure including a carbon composite support structure, in accordance with an embodiment of the present invention; 
         FIG. 16  is a schematic top view of an irregular-shaped support frame, in accordance with an embodiment of the present invention; 
         FIG. 17  is a schematic top view of a support structure with an irregular-shaped support frame, in accordance with an embodiment of the present invention; 
         FIG. 18  is a schematic top view of a support structure with a support frame that does not completely surround or enclose the ribs, in accordance with an embodiment of the present invention; 
         FIG. 19  is a schematic cross-sectional side view of an x-ray detector, in accordance with an embodiment of the present invention; 
         FIG. 20  is a schematic cross-sectional side view of an x-ray window attached to a mount, in accordance with an embodiment of the present invention; 
         FIG. 21  is a schematic cross-sectional side view showing pressing and heating at least one sheet of carbon composite to form a carbon composite wafer, in accordance with an embodiment of the present invention; 
         FIG. 22  is a schematic top view of ribs disposed over and supported by a support frame, in accordance with an embodiment of the present invention; 
         FIG. 23  is a schematic cross-sectional side view of an x-ray window attached to a mount, with the support frame facing the interior of the mount; in accordance with an embodiment of the present invention; 
         FIG. 24  is a schematic cross-sectional side view of an x-ray window attached to a mount, with the support frame facing the exterior of the mount; in accordance with an embodiment of the present invention; 
         FIG. 25  is a schematic top view of a carbon composite support structure, including a plurality of cross-braces disposed between a plurality of ribs, in accordance with an embodiment of the present invention; 
         FIG. 26  is a schematic top view of a carbon composite support structure, including a plurality of cross-braces disposed between a plurality of ribs, in accordance with an embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     
         
         
           
             As used herein, the terms “about” or “approximately” are used to provide flexibility to a numerical value or range by providing that a given value may be “a little above” or “a little below” the endpoint. 
             As used herein, the term “carbon fiber” or “carbon fibers” means solid, substantially cylindrically shaped structures having a mass fraction of at least 85% carbon, a length of at least 5 micrometers and a diameter of at least 1 micrometer. 
             As used herein, the term “directionally aligned,” in referring to alignment of carbon fibers with ribs, means that the carbon fibers are substantially aligned with a longitudinal axis of the ribs and does not require the carbon fibers to be exactly aligned with a longitudinal axis of the ribs. 
             As used herein, the term “rib” means a support member and can extend, linearly or with bends or curves, by itself or coupled with other ribs, across an aperture of a support frame. 
             As used herein, the term “substantially” refers to the complete or nearly complete extent or degree of an action, characteristic, property, state, structure, item, or result. For example, an object that is “substantially” enclosed would mean that the object is either completely enclosed or nearly completely enclosed. The exact allowable degree of deviation from absolute completeness may in some cases depend on the specific context. However, generally speaking the nearness of completion will be so as to have the same overall result as if absolute and total completion were obtained. The use of “substantially” is equally applicable when used in a negative connotation to refer to the complete or near complete lack of an action, characteristic, property, state, structure, item, or result. 
           
         
       
    
     DETAILED DESCRIPTION 
     Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
     As illustrated in  FIG. 1 , a support structure  10  is shown comprising a support frame  12  and a plurality of ribs  11 . The support frame  12  can include a perimeter P and an aperture  15 . The plurality of ribs  11  can comprise a carbon composite material and can extend across the aperture  15  of the support frame  12  and can be carried by the support frame  12 . Openings  14  can exist between the plurality of ribs  11 . Tops of the ribs  11  can terminate substantially in a common plane  16 . 
     The carbon composite material can comprise carbon fibers embedded in a matrix. The carbon fibers can comprise a carbon mass fraction of at least 85% in one embodiment, at least 88% in another embodiment, at least 92% in another embodiment, or 100% in another embodiment. The carbon fibers can comprise carbon atoms connected to other carbon atoms by sp 2  bonding. The carbon fibers can have a diameter of at least 1 micrometer in one embodiment, at least 3 micrometers in another embodiment, or at least 5 micrometers in another embodiment. Most, substantially all, or all of the carbon fibers can have a length of at least 1 micrometer in one embodiment, at least 10 micrometers in another embodiment, at least 100 micrometers in another embodiment, at least 1 millimeter in another embodiment, or at least 5 millimeters in another embodiment. Most, at least 80%, substantially all, or all of the carbon fibers can be aligned with a rib. Most, at least 80%, substantially all, or all of the carbon fibers can have a length that is at least half the length of the rib with which it is aligned in one embodiment, or at least as long as the rib with which it is aligned in another embodiment. The carbon fibers can be substantially straight. 
     In one embodiment, such as if the support structure  10  is used as an x-ray window, a film  13  can be disposed over, carried by, and span the plurality of ribs  11  and can be disposed over and span the openings  14 . The film  13  can be configured to pass radiation therethrough. For example, the film  13  can be made of a material that has a low atomic number and can be thin, such as for example about 5 to 500 micrometers (μm). The film  13  can have sufficient strength to allow differential pressure of at least one atmosphere without breaking. The film  13  can be hermetic or air-tight. The film  13  can combine with one of the support structures described herein and a shell to form a hermetic enclosure. 
     The film  13  can comprise highly ordered pyrolytic graphite, silicon nitride, polymer, polyimide, beryllium, carbon nanotubes, carbon nanotubes embedded in a polymer, diamond, diamond-like carbon, graphene, graphene embedded in a polymer, boron hydride, aluminum, or combinations of these various materials. The film  13  can include a stack of layers, and different layers in the stack can comprise different materials. 
     In one embodiment, the film  13  comprises a plurality of layers stacked together, including an aluminum layer disposed over a thin film layer comprising a material selected from the group consisting of highly ordered pyrolytic graphite, silicon nitride, polymer, polyimide, beryllium, carbon nanotubes, carbon nanotubes embedded in a polymer, diamond, diamond-like carbon, graphene, graphene embedded in a polymer, boron hydride, and combinations thereof. Aluminum can be a gas barrier in order to provide a hermetic film. Aluminum can be used to prevent visible light from passing through the window. In one embodiment, the aluminum layer can have a thickness of between 10 to 60 nanometers. 
     The film  13  can include a protective layer over the aluminum layer. The protective layer can provide corrosion protection for the aluminum. The protective layer can comprise amino phosphonate, silicon nitride, silicon dioxide, borophosphosilicate glass, fluorinated hydrocarbon, polymer, bismaleimide, silane, fluorine, or combinations thereof. The protective layer can be applied by chemical vapor deposition, atomic layer deposition, sputter, immersion, or spray. A polymer protective layer can comprise polyimide. Use of amino phosphonate as a protective layer is described in U.S. Pat. No. 6,785,050, incorporated herein by reference. 
     In some applications, such as analysis of x-ray fluorescence, it can be desirable for the film  13  to comprise elements having low atomic numbers such as hydrogen (1), beryllium (4), boron (5), and carbon (6). The following materials consist of, or include a large percent of, the low atomic number elements hydrogen, beryllium, boron, and carbon: highly ordered pyrolytic graphite, polymer, beryllium, carbon nanotubes, carbon nanotubes embedded in a polymer, diamond, diamond-like carbon, graphene, graphene embedded in a polymer, and boron hydride. 
     In one embodiment, the support frame  12  comprises a carbon composite material. The support frame  12  and the plurality of ribs  11  can be integrally formed together from at least one layer of carbon composite material. As shown in  FIG. 1 , the support frame  12  and the plurality of ribs  11  can have substantially the same thickness t 1 , 
     As shown in  FIG. 2 , the plurality of ribs  11  and support frame  12  of support structure  20  can be separately formed, can be formed of separate materials and/or can have different thicknesses (t 2 ≠t 3 ). In one embodiment, a thickness t 3  of the support frame  12  can be at least 10% thicker than a thickness t 2  of the ribs 
             11   ⁢       (           t   ⁢           ⁢   3     -     t   ⁢           ⁢   2         t   ⁢           ⁢   2       &gt;   0.1     )     .           
In another embodiment, a thickness t 3  of the support frame  12  can be at least 20% thicker than a thickness t 2  of the ribs
 
             11   ⁢       (           t   ⁢           ⁢   3     -     t   ⁢           ⁢   2         t   ⁢           ⁢   2       &gt;   0.2     )     .           
In another embodiment, a thickness t 3  of the support frame  12  can be at least 50% thicker than a thickness t 2  of the ribs
 
     
       
         
           
             11 
             ⁢ 
             
               
                 ( 
                 
                   
                     
                       
                         t 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         3 
                       
                       - 
                       
                         t 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                     
                       t 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   &gt; 
                   0.5 
                 
                 ) 
               
               . 
             
           
         
       
     
     For simplicity of manufacture, it can be desirable to form the plurality of ribs  11  and the support frame  12  in a single step from a single wafer of carbon composite, as shown in  FIG. 1 . In one embodiment, the support frame  12  and the plurality of ribs  11  were integrally formed together from at least one layer of carbon composite material. Having the support frame  12  and the plurality of ribs  11  integrally formed together from at least one layer of carbon composite material can be beneficial for simplicity of manufacturing. For a stronger support frame  12  compared to the plurality of ribs  11 , it can be desirable to form the plurality of ribs  11  and support frame  12  separately and have a thicker support frame  12 , as shown in  FIG. 2 . 
     In one embodiment, the plurality of ribs  11  and/or support frame  12  can have a thickness t of between 20 to 350 micrometers (μm) and/or a width of between 20 to 100 micrometers (μm). In another embodiment, the plurality of ribs  11  and/or support frame  12  can have a thickness t of between 10 to 300 micrometers (μm) and/or a width w of between 10-200 micrometers (μm). In one embodiment, a spacing S between adjacent ribs  11  can be between 100 to 700 micrometers (μm). In another embodiment, a spacing S between adjacent ribs can be between 700 micrometers (μm) and 1 millimeter (mm). In another embodiment, a spacing S between adjacent ribs can be between 1 millimeter and 10 millimeters. A larger spacing S allows x-rays to more easily pass through the window but also provides less support for the film  13 . A smaller spacing S may result in increased, undesirable attenuation of x-rays but also provides greater support for the film  13 . 
     Use of carbon composite material, which can have high strength, in a support structure, can allow a high percentage of open area within the support frame  12  and/or reduce the overall height of the plurality of ribs  11 , both of which are desirable characteristics because both increase the ability of the window to pass radiation. The openings  14  can occupy more area within the perimeter P of the support frame  12  than the plurality of ribs  11  in one embodiment. In various embodiments, the openings  14  can occupy greater than 70%, greater than 90%, between 70% to 90%, between 85% to 95%, between 90% to 99%, or between 99% to 99.9% of the area within the perimeter P of the support frame  12  than the plurality of ribs  11 . 
     Embodiments with openings  14  occupying a very large percent of the area within the perimeter P of the support frame  12  may be used in an application in which a strong film is used and only needs minimal support. Such embodiments may also be used in an application in which at least one additional support structure, such as an additional polymer support structure, is disposed between the carbon composite support structure and the film  13 . The additional support structure can be the secondary support structure  128  shown in  FIG. 12  or the secondary support structure  158  shown in  FIG. 15 . 
     As shown in  FIG. 3 , a carbon composite sheet  30  can have carbon fibers  31  aligned substantially in a single direction, such as along longitudinal axis A 1 . As shown in support structure  40  in  FIG. 4 , carbon fibers  31  can be aligned such that the carbon fibers  31  in the carbon composite material are directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11  across the aperture. 
     In various figures and embodiments, the carbon fibers  31  in the carbon composite material can be directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11 . In one embodiment, all of the carbon fibers  31  can be directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11 . In another embodiment, substantially all of the carbon fibers  31  can be directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11 . In another embodiment, at least 80% of the carbon fibers  31  can be directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11 . In another embodiment, at least 60% of the carbon fibers  31  can be directionally aligned with a longitudinal axis A 1  of the plurality of ribs  11 . 
     The carbon fibers  31  can comprise solid structures having a length that is at least 5 times greater than a diameter of the carbon fibers  31  in one embodiment, a length that is at least 10 times greater than a diameter of the carbon fibers  31  in another embodiment, a length that is at least 100 times greater than a diameter of the carbon fibers  31  in another embodiment, or a length that is at least 1000 times greater than a diameter of the carbon fibers  31  in another embodiment. 
     In one embodiment, carbon composite material in a support structure can comprise a stack of at least two carbon composite sheets. Carbon fibers  31  in at least one sheet in the stack can be directionally aligned in a different direction from carbon fibers  31  in at least one other sheet in the stack. For example, support structure  50  shown in  FIG. 5  includes a carbon composite sheet with carbon fibers  31   a  aligned in one direction A 1  and at least one carbon composite sheet with carbon fibers  31   b  aligned in another direction A 2 . In the various embodiments described herein, the support frame  12  can be made from the same carbon composite sheet(s) as the plurality of ribs  11 , or the support frame  12  can be made separately from the plurality of ribs  11  and can be made from a different material. 
     In one embodiment, an angle between sheets having carbon fibers  31  aligned in different directions is at least ten degrees (|A 2 −A 1 |&gt;10 degrees). In another embodiment, an angle between sheets having carbon fibers  31  aligned in different directions is at least thirty degrees (|A 2 −A 1 |&gt;30 degrees). In another embodiment, an angle between sheets having carbon fibers  31  aligned in different directions is at least forty five degrees (|A 2 −A 1 |&gt;45 degrees). In another embodiment, an angle between sheets having carbon fibers  31  aligned in different directions is at least sixty degrees (|A 2 −A 1 |&gt;60 degrees). 
     In another embodiment, carbon fibers  31  in the carbon composite material can be randomly aligned. For example, an initial sheet with randomly aligned carbon fibers may be used. Alternatively, many sheets can be stacked and randomly aligned. The sheets can be pressed together and cut to form the desired support structure. 
     As shown in  FIG. 6 , a support structure  60  can include multiple sized ribs  11   a - e . For example, different ribs can have different cross-sectional sizes. This may be accomplished by cutting some ribs with larger widths w and other ribs with smaller widths w. Five different rib cross-sectional sizes are shown in  FIG. 6  ( 11   e &gt; 11   d &gt; 11   c &gt; 11   b &gt; 11   a ). 
     In one embodiment, the plurality of ribs  11  have at least two different cross-sectional sizes including at least one larger sized rib with a cross-sectional area that is at least 5% larger than a cross-sectional area of at least one smaller sized rib. In another embodiment, a difference in cross-sectional area between different ribs can be at least 10%. In another embodiment, a difference in cross-sectional area between different ribs can be at least 20%. In another embodiment, a difference in cross-sectional area between different ribs can be at least 50%. Different rib cross-sectional sizes is described in U.S. Patent Application Publication Number 2012/0213336 which claims priority to provisional U.S. Patent Application No. 61/445,878, filed on Feb. 23, 2011, both incorporated herein by reference. 
     As shown in  FIG. 7 , a support structure  70  can include a plurality of ribs  11  extending in different directions A 3  and A 4 . For example, one rib or group of ribs  11   f  can extend in one direction A 3  and another rib or group of ribs  11   g  can extend in another direction A 4 . Ribs extending in different directions can cross perpendicularly or non-perpendicularly. Carbon fibers can be aligned with a longitudinal direction of the ribs. For example, in  FIG. 7 , some of the carbon fibers can be directionally aligned with a longitudinal axis A 3  of one rib or group of ribs  11   f  and other carbon fibers can be directionally aligned with a longitudinal axis A 4  of another rib or group of ribs  11   g . In one embodiment, carbon fibers can be substantially aligned in one of two different directions A 3  or A 4 . 
     As shown in  FIG. 8 , a support structure  80  can include a plurality of ribs  11  that extend nonlinearly across the aperture  15  of the support frame  12 . The plurality of ribs  11  can be arranged to form a single hexagonal shaped opening or multiple hexagonal shaped openings  14   a  as shown in  FIG. 8 . 
     Shown in  FIG. 9  is an expanded section of the plurality of ribs  11  of a support structure  90  with carbon fibers aligned in three different directions A 5 -A 7  and directionally aligned with a longitudinal axis A 5 -A 7  of at least one rib  11 . One group of carbon fibers  31   h  can be directionally aligned A 5  with at least one rib  11   h , another group of carbon fibers  31   i  can be directionally aligned A 6  with at least one other rib  11   i , and another group of carbon fibers  31   j  can be directionally aligned A 7  with at least one other rib  11   j . Hexagonal-shaped carbon composite support members, especially with carbon fibers aligned with the plurality of ribs  11 , can provide a strong support structure. 
     Shown in  FIG. 10  is a support structure  100  with carbon fibers aligned in three different directions A 8 -A 10  and directionally aligned with a longitudinal axis A 8 -A 10  of at least one rib  11 . One group of carbon fibers  31   k  can be directionally aligned A 8  with at least one rib  11   k , another group of carbon fibers  31   m  can be directionally aligned A 9  with at least one other rib  11   m , and another group of carbon fibers  31   n  can be directionally aligned A 10  with at least one other rib  11   n . Triangular-shaped carbon composite support members, especially with carbon fibers aligned with the ribs  11 , can provide a strong support structure. 
     Choice of arrangement of ribs, whether all in parallel, in hexagonal shape, in triangular shape, or other shape, can be made depending on needed strength, distance the ribs must span, type of film supported by the ribs, and manufacturability. 
     As shown in  FIG. 11 , a support structure  110  can include a small number of ribs  11 , such as for example two ribs  11  in each of two different directions A 11 -A 12 . Alternatively, the support structure  110  could include only a single rib, a single rib in each of two different directions, or a single rib in each of at least three different directions. This may be desirable for supporting a film  13  that is very strong, and only needs minimal support. Carbon fibers  31   p  &amp;  31   o  can be directionally aligned with longitudinal axes of ribs  11 . For example, as shown in  FIG. 11 , carbon fibers  31   o  can be directionally aligned with a longitudinal axis A 11  of ribs  11   o  and carbon fibers  31   p  can be directionally aligned with a longitudinal axis A 12  of ribs  11   p.    
     Shown in  FIG. 12 , a support structure  120  can include multiple stacked support structures  127 - 128 . A primary support structure  127  can comprise a primary support frame  12  defining a perimeter P and an aperture  15 ; a plurality of primary ribs  11  extending across the aperture  15 . The primary ribs  11  can be carried by the primary support frame  12 . Openings  14  can exist between the primary ribs  11 . The ribs can comprise a carbon composite material. The primary support structure  127  can be made according to one of the various carbon composite support structures described herein. Tops of the primary ribs  11  can terminate substantially in a single plane  16 . 
     A secondary support structure  128  can be stacked on top of the primary support structure  127 , and thus between the primary support structure  127  and the film  13 , as shown in  FIG. 12 . Alternatively, the primary support structure  127  can be stacked on top of the secondary support structure  128 , and thus the primary support structure  127  can be disposed between the secondary support structure  128  and the film  13 . The secondary support structure  128  can attach to the primary support structure  127  at a plane  16  at which primary ribs  11  terminate. 
     The secondary support structure  128  can comprise a secondary support frame  122  defining a perimeter P and an aperture  125  and a plurality of secondary ribs  121  extending across the aperture  125 . The secondary ribs  121  can be carried by the secondary support frame  122 . Openings  124  can exist between the secondary ribs  121 . The secondary support structure  128  can be disposed at least partly between the primary support structure  127  and a film  13  or the secondary support structure  128  can be disposed completely between the primary support structure  127  and the film  13 . Tops of the secondary ribs  121  can terminate substantially in a single plane  126 . 
     In one embodiment, the secondary support frame  122  and secondary support ribs  121  are integrally formed and can be made of the same material. In another embodiment, the secondary support frame  122  and secondary ribs  121  are not integrally formed, are separately made then attached together, and can be made of different materials. 
     In another embodiment, the primary support frame  12  and the secondary support frame  122  are a single support frame and support both the primary ribs  11  and the secondary ribs  121 . The primary support frame  12  and the secondary support frame  122  can be integrally formed and can be made of the same material. The primary support frame  12 , the primary ribs  11 , and the secondary support frame  122  can be integrally formed and can be made of the same material. The secondary ribs  121  can thus be supported by the primary ribs  11 , the primary support frame  12 , and/or the secondary support frame  122 . 
     In one embodiment, primary ribs  11  provide support for the secondary ribs  121 , and thus may be called a secondary support frame  122  for the secondary ribs  121 . For example, a primary support structure  127  can be formed, secondary ribs  121  can be formed, then the secondary ribs  121  can be placed on top of or attached to the primary support structure  127 . An adhesive can be sprayed onto the primary or secondary support structure or both and the two support structures can be pressed and adhered together by the adhesive. 
     In one embodiment, the secondary support structure  128  comprises a polymer. In another embodiment, the secondary support structure  128  comprises photosensitive polyimide. Use of photosensitive polymers for support structures is described in U.S. Pat. No. 5,578,360, incorporated herein by reference. 
       FIGS. 13-14  show a top view of support structures  130  &amp;  140 , each with a primary and secondary support structure. In  FIG. 13 , secondary ribs  121   a  are supported by primary ribs  11  and by secondary support frame  132 . In  FIG. 14 , secondary ribs  121   b  are supported by primary ribs  11  and by primary support frame  142 . Thus, support frame  142  can serve as both primary and secondary support frame. 
     Shown in  FIG. 15 , support structure  150  can include multiple stacked support structures  157 - 158 . A primary support structure  157  can comprise a primary support frame  12  defining a perimeter P and an aperture  15 ; a plurality of primary ribs  11  extending across the aperture  15 . The primary ribs  11  can be carried by the primary support frame  12 . Openings  14  can exist between the primary ribs  11 . The ribs  11  can comprise a carbon composite material. The primary support structure  157  can be made according to one of the various carbon composite support structures described herein. 
     A secondary support structure  158  can be disposed at least partly on top of the primary support structure  157 . The secondary support structure  158  can comprise a secondary support frame  152  defining a perimeter P and an aperture  155  and a plurality of secondary ribs  151  extending across the aperture  155 . The secondary ribs  151  can be carried by the secondary support frame  158  and/or the primary ribs  11 . Openings  154  can exist between the secondary ribs  151 . The secondary support structure  158  can be disposed at least partly between the first support structure  157  and a film  13 . Tops of the secondary ribs  151  can terminate substantially in a single plane  156 . 
     Some secondary ribs  151   b  can be disposed between primary ribs  11  or the primary support structure  12  and the film  13 . Other ribs  151   a  can extend down and be disposed partly between primary ribs  11 . This embodiment can be made by first creating a primary support structure  157 , then pouring a liquid photosensitive polymer on top of the primary support structure  157 . The photosensitive polymer can be patterned and developed to form ribs  151  and to harden the polymer. 
     Stacked support structures may be useful for spanning large distances. For example, it can be impractical to use a polymer support structure to span large distances. Use of an underlying carbon composite support structure can allow the polymer support structure to span the needed large distance. 
     Most of the figures herein show circular support frames. Although it may be more convenient to use circular support frames, other support frame shapes may be used with the various embodiments described herein. Shown in  FIG. 16  is an irregular shaped support frame  162  with a perimeter P and aperture  15 . Shown in  FIG. 17  is support structure  170  with ribs  11  attached to irregular shaped support frame  162 . Outer ribs may form the support frame. 
     Most of the figures herein show support frames which totally surround and enclose ribs. A support frame with an enclosed perimeter can provide greater strength and support for ribs and thus is a preferred embodiment, however, the various embodiments described herein are not limited to fully enclosed support frames. Shown in  FIG. 18  is a support structure  180  that has an opening  182  in the support frame  12 . Thus the support frame  12  need not totally surround and enclose ribs  11 . The embodiments shown in  FIGS. 16-18  are applicable to the various embodiments of support structures described herein. 
     As shown in  FIG. 19 , an x-ray detection unit  190  can include a support structure  195  according to one of the embodiments described herein. A film  13  can be disposed over the support structure  195 . The support structure  195  and the film  13  can comprise an x-ray window  196 . The x-ray window  196  can be hermetically sealed to a mount  192 . An x-ray detector  191  can also be attached to the mount  192 . The mount  192  and window  196  can comprise a hermetically sealed enclosure. The window  196  can be configured to allow x-rays  194  to impinge upon the detector  191 , such as by selecting a window  196  that will allow x-rays  194  to pass therethrough and by aligning the detector  191  with the window  196 . In one embodiment, the support frame  12  and the mount  192  are the same and the plurality of ribs  11  are attached to this support frame  12  and mount  192 . The film  13  can be hermetically sealed to the mount  192  and an x-ray detector  191  can be attached to the mount  192 . The x-ray window  196  and mount  192  can also be used with proportional counters, gas ionization chambers, and x-ray tubes. 
     As shown in  FIG. 20 , a mounted window  200  can include a film  13  disposed over a support structure  201  attached to a mount  202 . The support structure  201  can be one of the embodiments described herein including carbon composite ribs  11 . The film  13  can comprise a plurality of layers stacked together, including a thin film layer  203  and an outer layer  205 . The outer layer  205  can include at least one layer of polymer, at least one layer of boron hydride, at least one layer of aluminum, or combinations of these layers. The thin film  203  can be comprised of a material selected from the group consisting of highly ordered pyrolytic graphite, silicon nitride, polymer, polyimide, beryllium, carbon nanotubes, carbon nanotubes embedded in a polymer, diamond, diamond-like carbon, graphene, graphene embedded in a polymer, or combinations of these various materials. 
     The thin film layer  203 , the support structure  201 , or both can be hermetically sealed to a mount  202 , defining a sealed joint  204 . The outer layer  205  can extend beyond a perimeter of the thin film layer  203  and can cover the sealed joint  204 . The outer layer  205  can provide corrosion protection to the sealed joint. 
     Shown in  FIGS. 23-24 , an x-ray window  230  can be attached to a mount  231 . The window  230  can be hermetically sealed to the mount  231 . The x-ray window  230  can be one of the various embodiments described herein. The window  230  and mount  231  can enclose an interior space  232 . The interior space  232  can be a vacuum. 
     As shown in  FIG. 23 , the plurality of ribs  11  can be disposed between the film  13  and the interior space  232 . As shown in  FIG. 24 , the film  13  can be disposed between the plurality of ribs  11  and the interior space  232 , thus the plurality of ribs  11  can be separated from the interior space  232  by the film  13 . 
     Having the plurality of ribs  11  between the film  13  and the interior space  232 , as shown in  FIG. 23 , can allow for easier support of the film  13 , but this embodiment may have a disadvantage of certain carbon composite material components outgassing into the vacuum of the interior space  232 , thus decreasing the vacuum. Whether this problem occurs is dependent on the level of vacuum and the type of carbon composite material used. 
     One way of solving the problem of carbon composite material components outgassing into the interior space  232  is to dispose the film  13  between the plurality of ribs  11  and the interior space  232 . A difficulty of this design is that gas pressure  233  outside of the window  230  and mount  231  can press the film  13  away from the support frame  12  and/or plurality of ribs  11 . Thus, a stronger bond between the film  13  and the plurality of ribs  11  and/or support frame  12  may be needed for the embodiment of  FIG. 24 . 
     This stronger bond between the film  13  and the plurality of ribs  11  and/or support frame  12  can be achieved by use of polyimide or other high strength adhesive. The adhesive may need to be selected to achieve desired temperatures to which the window will be subjected. An adhesive which will not outgas may also need to be selected. The bond between the film  13  and the plurality of ribs  11  and/or support frame  12  may be improved by treating the surface of the plurality of ribs  11 , support frame  12 , and/or film  13  prior to joining the surfaces. The surface treatment can include use of a potassium hydroxide solution or an oxygen plasma. 
     Another method of solving the problem of carbon composite material outgassing into the interior space  232  is to select carbon composite materials that will not outgas, or will have minimal outgassing. A carbon composite material including carbon fibers embedded in a matrix comprising polyimide and/or bismaleimide may be preferable due to low outgassing. Polyimide and bismaleimide are also suitable due to their ability to withstand high temperatures and their structural strength. 
     As shown on x-ray windows  250  and  260  in  FIGS. 25-26 , the plurality of ribs  11   r  can be substantially straight and parallel with respect to one another and arrayed across the aperture  15  of the support frame. The x-ray windows  250  and  260  can further comprise a plurality of intermediate support cross-braces  251  extending between adjacent ribs of the plurality of ribs  11   r . The cross-braces  251  can span an opening between adjacent ribs without spanning the aperture  15  of the support frame. The cross-braces  251  can comprise a carbon composite material. The plurality of cross-braces  251  can be substantially perpendicular to the plurality of ribs  11   r.    
     The cross-braces  251  can be laterally off-set with respect to adjacent cross-braces  251  of adjacent openings so that the cross-braces  251  are segmented and discontinuous with respect to one another. For example, in  FIG. 25 , central cross braces  251   a  are disposed between alternating pairs of ribs  11   r  and disposed at approximately a midpoint across the aperture  15 ; outer cross braces  251   b  are disposed between alternating pairs of ribs  11   r  and offset from the midpoint across the aperture  15 . Thus, central cross braces  251   a  and outer cross braces  251   b  are both disposed between alternating pairs of ribs  11   r , but the central cross braces  251   a  are disposed between different alternating pairs of ribs  11   r  than the outer cross braces  251   b.    
     The cross-braces  251  can be disposed at approximately one third of a distance in a straight line parallel with the ribs from the support frame across the aperture. The cross-braces  251  can be laterally off-set with respect to adjacent cross-braces  251  of adjacent openings so that the cross-braces  251  can be segmented and discontinuous with respect to one another. For example, in  FIG. 26 , upper cross braces  251   c  (called upper due to their position in the upper part of the figure) can be disposed between alternating pairs of ribs  11   r  and disposed at approximately one third of the distance across the aperture  15 . Lower cross braces  251   d  (called lower due to their position in the lower part of the figure) can be disposed between alternating pairs of ribs  11   r , different from the alternating pairs of ribs  11   r  between which upper cross braces  251   c  are disposed. Lower cross braces  251   d  can be disposed at a one third distance across the aperture  15 , but this one third distance is from an opposing side of the aperture  15  from the upper cross braces  251   c.    
     How to Make: 
     Carbon composite sheets (or a single sheet) can be used to make a carbon composite wafer. Due to the toughness of carbon composite material, it can be difficult to cut the small ribs required for an x-ray window. Ribs can be cut into the wafer, in a desired pattern, by laser mill (also called laser ablation or laser cutting). 
     The optimal matrix material can be selected based on the application. A carbon composite material including carbon fibers embedded in a matrix comprising polyimide and/or bismaleimide may be preferable due to low outgassing, ability to withstand high temperatures, and high structural strength. 
     A composite with carbon fibers with sufficient length can be selected to improve structural strength. Carbon fibers that extend across the entire aperture of the window may be preferred for some applications. 
     Carbon composite sheet(s) can comprise carbon fibers embedded in a matrix. The matrix can comprise a polymer, such as polyimide. The matrix can comprise bismaleimide. The matrix can comprise amorphous carbon or hydrogenated amorphous carbon. The matrix can comprise a ceramic. The ceramic can comprise silicon nitride, boron nitride, boron carbide, or aluminum nitride. 
     In one embodiment, carbon fibers can comprise 10-40 volumetric percent of the total volume of the carbon composite material and the matrix can comprise the remaining volumetric percent. In another embodiment, carbon fibers can comprise 40-60 volumetric percent of the total volume of the carbon composite material and the matrix can comprise the remaining volumetric percent. In another embodiment, carbon fibers can comprise 60-80 volumetric percent of the total volume of the carbon composite material and the matrix can comprise the remaining volumetric percent. Carbon fibers in the carbon composite can be substantially straight. 
     A carbon wafer can be formed by pressing, at an elevated temperature, such as in an oven for example, at least one carbon composite sheet between pressure plates. Alternatively, rollers can be used to press the sheets. The pressure plates or rollers can be heated in order to heat the sheets. The sheets can be heated to at least 50° C. A single sheet or multiple sheets may be used. Carbon fibers in the carbon composite sheet(s) can be randomly aligned, can be aligned in a single direction, can be aligned in two different directions, can be aligned in three different directions, or can be aligned in more than three different directions. 
     A layer of polyimide can be bonded (such as with pressure) to one surface of the carbon composite sheet(s) prior to pressing the sheets. The polyimide layer can be placed between carbon composite sheets, or on an outer face of a stack of carbon composite sheets. The polyimide layer can be cut along with the carbon composite sheet(s) into ribs and can remain as a permanent part of the final support structure. The layer of polyimide film can be between 5 and 20 micrometers thick in one embodiment. One purpose of the polyimide layer is to make one side of the carbon composite sheet(s) smooth and flat, allowing for easier bonding of the x-ray window film. Another purpose is to improve final rib strength. The layer of polyimide can be replaced by another suitable polymer. High temperature resistance and high strength are two desirable characteristics of the polymer. 
     In one embodiment, carbon fibers of a single sheet, or carbon fibers of all sheets in a stack, are aligned in a single direction. A first group of ribs, or a single rib, can be cut such that a longitudinal axis of the rib(s) is aligned in the direction of the carbon fibers. 
     In another embodiment, at least two carbon composite sheets are stacked and pressed into the wafer. Carbon fibers of at least one sheet are aligned in a first direction and carbon fibers of at least one other sheet are aligned in a second direction. A first group of ribs, or a single rib, can be cut having a longitudinal axis in the first direction to align with the carbon fibers aligned in the first direction and a second group of ribs, or a single rib, can be cut having a longitudinal axis in the second direction to align with the carbon fibers aligned in the second direction. In one embodiment, an angle between the two different directions is least 10 degrees. In another embodiment, an angle between the two different directions is least 60 degrees. In another embodiment, an angle between the two different directions is about 90 degrees. 
     In another embodiment, at least three carbon composite sheets are stacked and pressed into the wafer. Carbon fibers of at least one sheet are aligned in a first direction, carbon fibers of at least one sheet are aligned in a second direction, and carbon fibers of at least one sheet are aligned in a third direction. A first group of ribs, or a single rib, can be cut having a longitudinal axis in the first direction to align with the carbon fibers aligned in the first direction, a second group of ribs, or a single rib, can be cut having a longitudinal axis in the second direction to align with the carbon fibers aligned in the second direction, and a third group of ribs, or a single rib, can be cut having a longitudinal axis in the third direction to align with the carbon fibers aligned in the third direction. An angle between any two directions can be about 120 degrees. The structure can form hexagonal-shaped or triangular-shaped openings. 
     In one embodiment, each carbon composite sheet in a stack can have a thickness of between 20 to 350 micrometers (μm). 
     The plates used for pressing the carbon composite sheets into a wafer can have non-stick surfaces facing the sheet(s) of carbon composite. The plates can have fluorinated flat silicon surfaces facing the sheets. For example,  FIG. 21  shows a press  210  including two plates  211  and at least one carbon composite sheet  212  between the two plates  211 . The carbon composite sheet(s)  212  can include a layer of polyimide or other polymer. 
     Pressure P can be applied to the carbon composite sheet(s)  212  and the carbon composite sheet(s) (and optionally a layer of polymer, such as polyimide) can be heated to a temperature of at least 50° C. to cure the sheet(s) of carbon composite into a carbon composite wafer. Temperature, pressure, and time can be adjusted based on thicknesses of the sheets, the number of sheets, matrix material, and desired final characteristics of the wafer. For example, carbon composite sheets comprising carbon fibers in a polyimide matrix have been made into wafers at pressures of 200-3000 psi, temperatures of 120-200° C., and initial sheet thickness of 180 micrometer (μm). 
     The wafer can be removed from the press and the wafer can be cut to form ribs and/or support frame. The wafer may be cut by laser milling or laser ablation. A high power laser can use short pulses of laser to ablate the material to form the openings by ultrafast laser ablation. A femtosecond laser may be used. Ablating wafer material in short pulses of high power laser can be used in order to avoid overheating the polymer material in the carbon composite. Alternatively, a non-pulsing laser can be used and the wafer can be cooled by other methods, such as conductive or convective heat removal. The wafer can be cooled by water flow or air across the wafer. The above mentioned cooling methods can also be used with laser pulses, such as a femtosecond laser, if additional cooling is needed. 
     The ribs, formed by the laser, can be formed of a single original layer of carbon composite material or multiple layers of carbon composite material and can include at least one layer of polyimide. If a polyimide layer is used in the stack, then the ribs can comprise carbon composite and polyimide and thus polyimide ribs will be attached to and aligned with the carbon composite ribs. 
     As shown in support structure  220  in  FIG. 22 , ribs  11  can be formed separately from the support frame  12 . Ribs  11  can then be laid on top of the support frame  12 . An adhesive may be used to hold the ribs in place. The support frame  12  can be a ring a material or a mount, such as mount  192  shown in  FIG. 19  or mount  202  shown in  FIG. 20 . 
     It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.