Patent Publication Number: US-11640999-B2

Title: Thermoelectric cooler including a single, solid, and electrically insulative support/plate having a planar side directly affixed to upper electrical connections and non-planar side to a raised structure

Description:
PRIORITY CLAIM(S) 
     This application is a continuation of U.S. patent application Ser. No. 16/562,785, filed on Sep. 6, 2019; which claims priority to U.S. Provisional Patent Application No. 62/739,622, filed Oct. 1, 2018; 62/743,136, filed Oct. 9, 2018; 62/744,398, filed Oct. 11, 2018; and 62/805,653, filed Feb. 14, 2019; which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present application is related generally to x-ray detectors. 
     BACKGROUND 
     X-ray detectors sometimes need to be inserted into small locations. Small size is useful. 
     For high resolution, x-ray detectors are cooled to low temperatures, such as around −20° C. for example. For more efficient cooling, it is helpful to minimize thermal resistance. 
     Electromagnetic interference can disrupt optimal operation of x-ray detection devices, such as for example PIN photodiodes or silicon drift detectors (SDD). 
     For material analysis, x-rays are directed towards to a sample. The sample then reemits x-rays which are characteristic of the chemical composition of the sample. Surrounding materials, including materials of the x-ray detector, can emit x-rays which can interfere with x-ray signal from the sample. It can be beneficial to block x-rays from these surrounding materials from hitting the x-ray detection device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS (DRAWINGS MIGHT NOT BE DRAWN TO SCALE) 
         FIG.  1    is a schematic perspective-view of a cooler  10  for an x-ray detector, comprising a thermoelectric cooler (TEC)  20  with pillars  12  electrically coupled to upper electrical connections  23  and lower electrical connections  24 , a support  15  including a base  16  with a planar side  16   P  and a non-planar side  16   N  opposite of the planar side  16   P , the planar side  16   P  directly affixed to the upper electrical connections  23 , and a raised structure  17  on the non-planar side  16   N  extending away from the base  16 , in accordance with an embodiment of the present invention. 
         FIG.  2    is a schematic perspective-view of a TEC  20 , comprising pillars  12  electrically coupled in series and extending between upper electrical connections  23  in a top plane  13  and lower electrical connections  24  in a bottom plane  14 , in accordance with an embodiment of the present invention. 
         FIG.  3    is a schematic perspective-view of a cooler  30  for an x-ray detector, similar to cooler  10 , further comprising the raised structure  17  including a rib  17   r  extending around the perimeter of the support  15 , in accordance with an embodiment of the present invention. 
         FIG.  4    is a schematic perspective-view of a cooler  40  for an x-ray detector, similar to cooler  10 , further comprising the raised structure  17  includes four separate posts  17   P  with a post  17   P  at each of four corners of the support  15 , in accordance with an embodiment of the present invention. 
         FIG.  5    is a schematic side-view of an x-ray detector  50 , including cooler  10 ,  30 , or  40 , further comprising a cap  55  affixed to the raised structure  17  of the support  15 , forming a cavity  57  between the cap  55  and the non-planar side  16   N  of the support  15 , and a silicon drift detector (SDD)  56  affixed to the cap  55 , in accordance with an embodiment of the present invention. 
         FIG.  6    is a schematic view of a bottom face  55   b  of the cap  55 , electronic components  51  carried by the bottom face  55   b  of the cap  55 , wire bonds  62  electrically connecting the electronic components  51  to traces  61 , and wire bonds  62  electrically connecting the electronic components  51  to the SDD  56  through holes extending through the cap  55 , in accordance with an embodiment of the present invention. 
         FIG.  7    is a schematic side-view of an x-ray detector  70 , comprising a TEC  20  sandwiched between a pair of electrically insulating materials  11 , a support  15 , which can be metallic, shaped as described above and carried by the TEC  20 , a cap  55  as described above carried by the support  15 , and an SDD affixed to cap  55 , in accordance with an embodiment of the present invention. 
         FIG.  8    is a schematic side-view of an x-ray detector  80 , comprising a TEC  20 , a plate  81 , a support  15 , and an SDD  56 ; a bottom side  81   b  of the plate  81  affixed to upper electrical connections  23  of the TEC  20 ; a distal end  17   d  of a raised structure  17  of the support  15  affixed to the plate  81 , forming a cavity  57  between the plate  81  and the support  15 ; and a planar side  16   P  of the support  15  affixed to the SDD  56 ; in accordance with an embodiment of the present invention. 
         FIG.  9    is a schematic view of one embodiment of the non-planar side  16   N  of the support  15  of  FIG.  8   , electronic components  51  carried by the non-planar side  16   N  of the support  15 , wire bonds  62  electrically connecting the electronic components  51  to traces  61  and to the SDD  56  through holes extending through the support  15 , and the raised structure  17  of the support  15  including four separate posts  17   P  with a post  17   P  at each of four corners of the support  15 , in accordance with an embodiment of the present invention. 
         FIG.  10    is a schematic side-view of an x-ray detector  100 , comprising a TEC  20 , a plate  81 , and a PIN photodiode  106 , with a sequence of the TEC  20 , the plate  81 , then the PIN photodiode  106 ; the plate  81  having a bottom side  81   b  directly affixed to upper electrical connections  23  of the TEC  20 ; and the PIN photodiode  106  directly affixed to a top side  81   t  of the plate  81 ; in accordance with an embodiment of the present invention. 
         FIG.  11    is a schematic top-view of x-ray detector  100 , in accordance with an embodiment of the present invention. 
         FIG.  12    is a schematic side-view of an x-ray detector  120 , comprising a blocking ceramic  121  sandwiched between an x-ray detection device  122  and a cooling mechanism  123 , the blocking ceramic  121  including a metal blocking layer  124  sandwiched between a pair of ceramic layers  125 , in accordance with an embodiment of the present invention. 
         FIG.  13    is a schematic side-view of an x-ray detector  130 , similar to x-ray detector  120 , further comprising the metal blocking layer  124  including three layers of different metals C, M, and F, in accordance with an embodiment of the present invention. 
         FIG.  14    is a schematic perspective view of an x-ray detector  140 , similar to x-ray detectors  120  and  130 , further comprising the pair of ceramic layers  125  adjoin each other at four outer corners, in accordance with an embodiment of the present invention. 
         FIG.  15    is a schematic perspective view of an x-ray detector  150 , similar to x-ray detectors  120 ,  130 , and  140 , further comprising one of or both of the pair of ceramic layers  125  including a cup facing the metal blocking layer  124 , the metal blocking layer  124  is located in the cup, the pair of ceramic layers adjoin each other in a co-fired bond at an entire outer perimeter of each of the pair of ceramic layers, and a ground connection G to the metal blocking layer  124  extends through a hole  151  in the blocking ceramic  121  from a side of the blocking ceramic  121  on which the x-ray detection device  122  is located, in accordance with an embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     As used herein, the term “adjoin” means direct and immediate contact. 
     As used herein, the phrase “directly affixed” means the objects affixed have, at most, an adhesive, solder, or both between them, but no other structural components. 
     As used herein, the term “μm” means micrometer(s). 
     As used herein, the terms “trace” and “traces” mean electrically conductive layers, typically metals such as copper or gold, such as on a circuit board for conducting electricity between electronic components. 
     The terms “top” and “bottom” are used herein as relative terms relative to the orientation of the Figures. 
     DETAILED DESCRIPTION 
     First SDD Embodiment 
     As illustrated in  FIG.  1   , a cooler  10  for an x-ray detector is shown comprising a thermoelectric cooler (TEC)  20  comprising electrical components for thermoelectric cooling, including pillars  12  extending between upper electrical connections  23  in a top region and lower electrical connections  24  in a bottom region, and interconnected by the electrical connections  23  and  24 . The pillars  12 , the upper electrical connections  23 , the lower electrical connections  24 , or combinations thereof can be electrically conductive. The upper electrical connections  23  in the top region can be located in a top plane  13 . The lower electrical connections  24  in the bottom region can be located in a bottom plane  14 . 
     The pillars  12  can be electrically coupled to the upper electrical connections  23  and to the lower electrical connections  24 . Example maximum resistance between each pillar  12  and each electrical connection (each upper electrical connection  23  and each lower electrical connection  24 ) include ≤10 −2  ohms, ≤10 −3  ohms, ≤ 10   −4  ohms, ≤10 −5  ohms, or ≤10 −6  ohms. The pillars  12  can be electrically coupled in series, and electrically coupled to each other through the upper electrical connections  23  and the lower electrical connections  24 . Electrical connections between adjacent pillars  12  can alternate between the top region and the bottom region. Alternating pillars  12  can be made of a different material with respect to each other. For example, the pillars  12  can alternate between p-doped and n-doped semiconductors. 
     The TEC  20  can be sandwiched between an electrically insulating material  11  and a support  15 . The support  15  can be electrically insulative. The electrically insulating material  11 , the support  15 , or both can be ceramic. The TEC  20  is illustrated in  FIG.  2    without the electrically insulating material  11  and the support  15  in order to show more clearly components of the TEC  20 . 
     The support  15 , can be a single, solid, electrically insulating material. Thus, the support  15  can be free of adhesive-filled interface joints. This is in contrast to multiple ceramics bonded together which can have increased resistance to heat transfer at interfaces between the ceramics. The support  15  can include a base  16  with a planar side  16   P  and a non-planar side  16   N  opposite of the planar side  16   P . 
     The upper electrical connections  23  can be formed on the planar side  16   P  of the support  15  by depositing a thin metal film then patterning traces needed for attachment to the pillars  12 , and for electrical current flow between adjacent pillars  12 . The upper electrical connections  23  and the support  15  can be attached to the pillars  12  by solder or other suitable means. 
     A raised structure  17  on the non-planar side  16   N  can extend away from the base  16 . A purpose of the raised structure  17  is to provide a cavity  57  as described below. The raised structure  17  can have various shapes. It can be beneficial for the raised structure  17  to be long with a large surface area for contact with the cap  55  described below, to allow increased heat transfer between the raised structure  17  and the cap  55 . It can also be useful, however, for the raised structure  17  to have openings or gaps to allow improved evacuation of the cavity  57  during sealing of the detector. The design of the raised structure  17  for each application can be based on a balance of these competing interests. 
     For example, the raised structure  17  can be a rib  17   r  extending around some or all of a perimeter of the support  15 . The raised structure  17  of cooler  10  in  FIG.  1    extends partially around the perimeter of the support  15 . The raised structure  17  of cooler  30  of  FIG.  3    extends all of the way around the perimeter of the support  15 . Thus, cooler  10  likely will have inferior heat transfer than cooler  30  between the raised structure  17  and the cap  55 , but it can be easier to evacuate a detector with cooler  10 . Holes can be drilled into cooler  30  in order to allow evacuation. Examples of the extension of the rib  17   r  around the perimeter of the support  15  include: ≥25%, ≥50%, ≥75%, or ≥90%; and ≤75%, ≤90%, ≤95%, or ≤99% of a perimeter of the support  15 . 
     As illustrated on cooler  40  in  FIG.  4   , the raised structure  17  can comprise four separate posts  17   P , with a post  17   P  at each of four corners of the support  15 . It can be relatively easy to evacuate a detector made with cooler  40 , but cooling can be reduced due to a smaller surface area of contact of the raised structure  17 . 
     As illustrated in  FIG.  5   , x-ray detector  50  can include a cap  55  carried by the support  15 . The cap  55  can be electrically insulative, can have a top face  55   t , and can have a bottom face  55   b  opposite the top face  55   t . The bottom face  55   b  of the cap  55 , the top face  55   t  of the cap  55 , or both, can be flat, planar surfaces. The bottom face  55   b  of the cap  55  can be affixed to the raised structure  17  of the support  15 , forming a cavity  57  between the cap  55  and the base  16  of the support  15  on the non-planar side  16   N  of the support  15 . The support  15  and underlying TEC  20  can be as described for cooler  10 ,  30 , or  40 . A silicon drift detector (SDD) can be affixed to the top face  55   t  of the cap  55 . The cap  55  can be affixed to the support  15  by epoxy. The SDD  56  can be affixed to the cap  55  by epoxy. 
     Also illustrated in  FIG.  5   , electronic component(s)  51  can be carried by the bottom face  55   b  of the cap  55 . The electronic component(s)  51  can extend into the cavity  57 . The support  15  can be a single, solid, electrically insulative structure (i.e. the only solid, electrically insulative structure) between the upper electrical connections  23  in the top region of the TEC  20  and the electronic component(s)  51 . 
     The bottom face  55   b  of the cap  55  is illustrated in  FIG.  6   , showing the electronic components  51 , traces  61  extending in a single plane along the bottom face  55   b  of the cap  55  from the electronic components  51  to or near an outer perimeter of the cap  55 , and wire bonds  62  electrically connecting the electronic components  51 , traces  61 , and the SDD  56  through holes extending through the cap  55 . For example, the electronic component(s)  51  can include an amplifier, a transistor, a thermistor, a capacitor, or combinations thereof. 
     The support  15  can be free of electronic circuit traces. Thus, the function of the support  15  can be to electrically isolate the upper electrical connections  23  from the electronic components  51  and to provide the cavity  57  for containing the electronic components  51  and the wire bonds  62 . 
     Second SDD Embodiment 
     As illustrated on x-ray detector  70  in  FIG.  7   , a TEC  20  as described above can be sandwiched between a pair of electrically insulating materials  11 . A support  15  can be carried by the TEC  20 . The support  15  can be directly affixed to one of the pair of electrically insulating materials  11 . 
     The support  15  can include a base  16  with a planar side  16   P . The planar side  16   P  can face the TEC  20 . The support  15  can include a non-planar side  16   N  opposite of the planar side  16   P . A raised structure  17  on the non-planar side  16   N  can extend away from the base  16 , as described above. 
     A cap  55 , as described above, can be carried by the support  15 . The cap  55  can be electrically insulative. The cap  55  can include a top face  55   t  and a bottom face  55   b  opposite the top face  55   t . The bottom face  55   b  of the cap  55 , the top face  55   t  of the cap  55 , or both, can be flat, planar surfaces. The bottom face  55   b  of the cap  55  can be affixed to the raised structure  17  of the support  15 , forming a cavity  57  between the cap  55  and the base  16  of the support  15  on the non-planar side  16   N  of the support  15 . A silicon drift detector (SDD) can be affixed to the top face  55   t  of the cap  55 . Electronic component(s)  51  can be carried by the bottom face  55   b  of the cap  55 . The electronic component(s)  51  can extend into the cavity  57 . 
     The support  15  can be metallic. One benefit of using a metallic support  15  is increased thermal conductivity, which can assist heat transfer away from the SDD  56 . Another benefit is lower cost (metal can be formed easily into the shapes of the support  15  described above). Another benefit is that an electrically conductive metal can shield the SDD  56  and the electronic component(s)  51  from electromagnetic interference from the TEC  20 . Another benefit is that a metallic support  15  can block fluoresced x-rays from the TEC  20  from interfering with the SDD  56 . 
     It can be helpful for the support  15  to be made of low atomic number elements, to minimize interference in the SDD  56  by x-rays fluoresced from the support  15 . Example materials for the support include aluminum, nickel, or both. For example, a material composition of the support  15  can be ≥20 mass percent, ≥50 mass percent, ≥75 mass percent, ≥90 mass percent, or ≥95 mass percent aluminum. 
     Nickel can be useful by blocking higher energy x-rays. X-rays fluoresced by nickel can also cause more interference with the SDD  56  than x-rays fluoresced by lower atomic number elements. The blocking ability of nickel can be achieved with reduced interference by embedding the nickel within or between lower atomic number material(s), such as aluminum. For example, a layer of aluminum can be deposited on each of two opposite faces of the nickel, or the nickel support  15  can be plated (electroplated or electroless) with aluminum or other low atomic number metal or metalloid. 
     Third SDD Embodiment 
     As illustrated on x-ray detector  80  in  FIG.  8   , a TEC  20  as described above can be sandwiched between electrically insulating materials  11  and  81 . One of these electrically insulating materials, a plate  81 , can have a top side  81   t  and a bottom side  81   b  opposite of the top side  81   t . The top side  81   t  and the bottom side  81   b  of the plate  81  can be flat, planar surfaces. 
     The plate  81  can be a single, solid, electrically insulative structure extending across the upper electrical connections  23 . The plate  81  can be free of adhesive-filled interface joints. 
     X-ray detector  80  can also comprise a support  15 , like the support  15  described above. The non-planar side  16   N  of the support  15  can face the top side  81   t  of the plate  81 . The raised structure  17  can extend towards the plate  81 . The raised structure  17 , such as a distal end  17   d  farthest from the base  16 , can be directly affixed (e.g. by epoxy) to the plate  81 , forming a cavity  57  between the plate  81  and the base  16  of the support  15  on the non-planar side  16   N  of the support  15 . The planar side  16   P  of the support  15  can be affixed (e.g. by epoxy) to the SDD  56 . 
     Electronic component(s)  51  can be carried by the non-planar side  16   N  of the support  15  and can extend into the cavity  57 . The non-planar side  16   N  of the support  15  is illustrated in  FIG.  9   , with traces  61 , exposed to air, extending in a single plane along the non-planar side  16   N  of the support  15  from the electronic component(s)  51 , between the posts  17   P  or through a gap in the rib  17   r , to an outer perimeter of the support  15 . There can be multiple traces  61 . Each trace  61  can extend in a single plane along the non-planar side  16   N  of the support  15  from an electronic component  51 , between the posts  17   P  or through a gap in the rib  17   r , to an outer perimeter of the support  15 . Each trace  61  can extend through a different gap in the rib  17   r . The electronic components  51  and the wire bonds  62  can have other characteristics as described above. 
     The plate  81  can be a single, solid, electrically insulative structure (i.e. the only solid, electrically insulative structure) between the upper electrical connections  23  in the top region of the TEC  20  and the electronic component  51 . The plate  81  and the support  15  can be free of adhesive-filled interface joints. 
     Pin Photodiode 
     As illustrated on x-ray detector  100  in  FIG.  10   , a TEC  20  as described above can be sandwiched between electrically insulating materials  11  and  81 , for electrical isolation of the electrical connections  23  and  24 . One of these electrically insulating materials, a plate  81 , can have a top side  81   t  and a bottom side  81   b  opposite of the top side  81   t . The top side  81   t  and the bottom side  81   b  of the plate  81  can be flat, planar surfaces. 
     The plate  81  can be a single, solid, electrically insulative structure extending across the upper electrical connections  23 . The plate  81  can be free of adhesive-filled interface joints. 
     As illustrated in  FIGS.  10 - 11   , a PIN photodiode  106  can be directly affixed (e.g. by epoxy) to the top side  81   t  of the plate  81 . As illustrated in  FIG.  10   , trace(s)  61  can be directly affixed to the top side  81   t  of the plate  81 . The plate  81  can be a single, solid, electrically insulative structure (i.e. the only solid, electrically insulative structure) between the upper electrical connections  23  in the top region of the TEC  20  and the PIN photodiode  106 . In addition to the PIN photodiode  106 , other electronic components  51  (e.g. an amplifier, a transistor, a thermistor, a capacitor, or combinations thereof), and traces  61  adjacent to the electronic components  51 , can be affixed to the top side  81   t  of the plate  81 . The traces  61  can be exposed to air along their entire length. Wire bonds  62  can interconnect the PIN photodiode  106 , the electronic components  51 , the traces  61 , and additional circuitry not shown. 
     All Embodiments 
     The various embodiments described herein can have reduced layers between the TEC  20  and the SDD  56  or the PIN photodiode  106 . Although this can be beneficial for improved heat transfer between these components, it can also reduce x-ray shielding between these components. Thus, the various embodiments described herein can be particularly helpful for lead free detectors. In the various x-ray detector embodiments described herein, all solder bonds can be lead-free. 
     Bonding the components of the TEC  20  or other x-ray detector components can result in solder or adhesive at least partially covering bonding pads, such as bonding pads used for wire bonds to connect the electronic components  51  and traces  61  to external circuitry. As illustrated in  FIGS.  3 ,  5 ,  8   , and  9 , a channel  35  at an outer perimeter of the support  15  or the plate  81  can be a reservoir for excess solder or adhesive. This channel  35  can fill with excess solder or adhesive instead of the solder or adhesive covering the bonding pads. 
     The channel  35  can be located in the raised structure  17  of the support  15 . The raised structure  17  and the channel  35  can extend for an equivalent distance around the outer perimeter of the support  15 . 
     The channel  35  can extend all or part of the way around the perimeter of the support  15  or the plate  81 . For example, the channel  35  can extend ≥25%, ≥50%, ≥75%, or ≥95% and ≤100%, ≤90%, ≤80%, or s 60% around the outer perimeter of the support  15  or the plate  81 . 
     Blocking Metal Layer 
     As illustrated in  FIGS.  12 - 15   , x-ray detectors  120 ,  130 ,  140 , and  150  can comprise a blocking ceramic  121  sandwiched between an x-ray detection device  122  (e.g. PIN photodiode, silicon drift detector, etc.) and a cooling mechanism  123  (e.g. thermoelectric cooler). The blocking ceramic  121  can include a metal blocking layer  124  sandwiched between a pair of ceramic layers  125 . 
     Electromagnetic interference from electronics in the cooling mechanism  123  can disrupt optimal operation of the x-ray detection device  122 . The metal blocking layer  124  can be grounded to allow it to block this electromagnetic interference. As illustrated in  FIG.  14   , there can be a ground connection G to the metal blocking layer  124  in an opening in a perimeter of the metal blocking layer  124 , between the pair of ceramic layers  125 . Alternatively, as illustrated in  FIG.  15   , the metal blocking layer  124  can be encapsulated within the pair of ceramic layers  125 , except for a hole  151  in the blocking ceramic  121  through which the ground connection G can pass to the metal blocking layer  124 . The hole  151  can be located at a side of the blocking ceramic  121  on which the x-ray detection device  122  is located. 
     The metal blocking layer  124  can block electromagnetic interference arising out of the cooling mechanism  123  from interfering with the x-ray detection device  122 . The metal blocking layer  124  can block fluoresced x-rays from the cooling mechanism  123 , to prevent such x-rays from interfering with the x-ray detection device  122 . Materials for the x-ray detection device  122  can be selected for optimal protection of the x-ray detection device  122 . For example, the metal blocking layer  124  can include one or more of the following: nickel, aluminum, tantalum, molybdenum, titanium, tungsten, cobalt, titanium, chromium, a metal with an atomic number ≥13, a metal with an atomic number ≤79, or combinations thereof. 
     As illustrated in  FIG.  13   , the metal blocking layer  124  can include three layers, a layer C closest to the x-ray detection device  122 , a middle layer M, and a layer F farthest from the x-ray detection device  122 . For improved blocking of x-rays and to minimize stray x-ray interference with the x-ray detection device  122 , the layer C closest to the x-ray detection device  122  can have a lowest atomic number of the three layers, the middle layer M can have an intermediate atomic number, and the layer F farthest from the x-ray detection device  122  can have a largest atomic number of the three layers. Although not shown in the figures, there can be two layers, four layers, or more than four layers. Cost can be weighed against increased efficiency due to more layers. Each of these layers can have a different material composition than the other layers. 
     The metal blocking layer  124  can be affixed to the pair of ceramic layers  125  by various methods, including solder or epoxy. Each added layer to the stack (e.g. solder or epoxy) can result in undesirable impurities or reduced cooling of the x-ray detection device  122  due to increased resistance to heat transfer at junctions. As illustrated in  FIG.  15   , one or both of the pair of ceramic layers  125  can include a cup or a cavity for holding metal blocking layer  124 , which can avoid the use of the solder or epoxy. 
     The pair of ceramic layers  125  can adjoin or touch  141 . The metal blocking layer  124  and the pair of ceramic layers  125  can be formed in a single blocking ceramic by co-firing, to bond the pair of ceramic layers  125  together at locations where they touch  141 . The pair of ceramic layers  125  can adjoin each other at four outer corners as illustrated in  FIG.  14   . The pair of ceramic layers  125  adjoin each other 75%, but less than all, of a perimeter of the pair of ceramic layers  125 . As shown in  FIG.  15   , the x-ray detector  150  can comprise one of or both of the pair of ceramic layers  125  including a cup facing the metal blocking layer  124 , and the metal blocking layer  124  can be located in the cup. The pair of ceramic layers  125  can adjoin each other at an entire outer perimeter of each of the pair of ceramic layers  125  as illustrated in  FIG.  15   . Thus, co-firing the pair of ceramic layers  125  can capture the metal blocking layer  124  between the pair of ceramic layers  125  without added epoxy or solder layer. 
     There can be tight contact between the metal blocking layer  124  and the pair of ceramic layers  125  for improved heat transfer. Thus, the metal blocking layer  124  can include a top face  114   T  adjoining one of the pair of ceramic layers  125  across all of the top face  114   T  and a bottom face  114   B  adjoining the other of the pair of ceramic layers  125  across all of the bottom face  114   B .