Patent Publication Number: US-2023141131-A1

Title: Laminated thermal barrier for anisotropic heat transfer

Description:
INTRODUCTION 
     The information provided in this section is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     The present disclosure relates to a laminated thermal barrier for anisotropic heat transfer. 
     Vehicles such as electric vehicles and hybrid vehicles propel the vehicles using electric motors and battery systems. The battery systems include one or more battery packs, each including one or more battery modules. Each of the battery modules includes multiple battery cells that are connected in series, parallel, and/or combinations thereof. 
     In these battery systems, thermal runaway occurs when heat generated by one or more battery cells exceeds heat dissipated to the surroundings. For example, thermal runaway occurs when a temperature of a battery cell increases due to a failure within the battery system. Elevated temperatures cause exothermic decomposition of materials in the battery cell. Eventually, a self-heating rate of the battery cell becomes greater than a rate at which heat can be dissipated to the surroundings. The temperature of the battery cell rises exponentially, and stability is ultimately lost, which results in all remaining thermal and electrochemical energy being released to the surroundings. 
     If not prevented, the thermal runaway worsens. The internal temperature of the battery system continues to rise, which causes the current of the battery system to increase, creating a domino effect. The increase in temperature in a single battery cell begins to adversely impact other battery cells in close proximity, and the pattern continues and spreads to neighboring battery cells, which is called thermal runaway propagation. The thermal runaway can propagate not only to the battery cells in a battery module but can also propagate across multiple battery modules in a battery pack, and further to neighboring battery packs in the battery system. 
     SUMMARY 
     A heat transfer system comprises first and second battery cells, a heat sink, and a plurality of sheets of a thermally conductive material arranged in a stack. The first and second battery cells are arranged adjacent to each other. The heat sink is arranged adjacent to the first and second battery cells. A first portion of the stack is disposed between the first and second battery cells. A second portion of the stack is disposed adjacent to and in thermal communication with the heat sink. The first portion of the stack conducts heat from the first battery cell while limiting heat conducted to the second battery cell. The second portion of the stack transfers the heat to the heat sink. 
     In another feature, the sheets are configured to provide a higher thermal contact resistance in the first portion of the stack than in the second portion of the stack. 
     In another feature, the first portion of the stack is configured to function as a thermal insulator across a thickness of the first portion of the stack and as a thermal conductor in a direction perpendicular to the thickness of the first portion of the stack. 
     In another feature, portions of the sheets in the first portion of the stack have a greater surface roughness than portions of the sheets in the second portion of the stack. 
     In another feature, portions of the sheets in the first portion of the stack are coated with a material that is thermally insulating or that increases contact resistance of the sheets in the first portion, and portions of the sheets in the second portion of the stack are not coated with the material or are coated with another material that decreases contact resistance of the sheets in the second portion. 
     In another feature, the heat transfer system further comprises one or more sheets of a thermally insulating material disposed between two or more of the sheets in the first portion of the stack. The second portion of the stack does not include any thermally insulating material between the sheets. 
     In another feature, the sheets in the second portion of the stack are arranged in a staggered manner with a portion of each of the sheets contacting the heat sink. 
     In another feature, the sheets in the second portion of the stack are joined together, and the joint is attached to the heat sink. 
     In another feature, the heat transfer system further comprises a layer of a thermally conductive paste disposed between the heat sink and at least a portion of at least one of the sheets in the second portion. 
     In another feature, the sheets are made of two or more different materials with relative positions of the sheets made of the two or more different materials in the stack being of any combination. 
     In another feature, the second portion of the stack is compressed with a greater force than the first portion of the stack. 
     In another feature, the material includes an anodized coating. 
     In another feature, the thermally insulating material includes a porous material. 
     In another feature, the two or more different materials include first and second metals. 
     In another feature, the two or more different materials include a metal and a non-metal. 
     In another feature, one or more of the sheets closer to the first battery cell are made of a material having a lower thermal conductivity than one or more of the sheets farther from the first battery cell. 
     In another feature, the first and second battery cells are arranged in a battery module along a first axis. The heat sink extends along the first axis. The first portion of the stack extends along a second axis that is perpendicular to the first axis. The second portion of the stack extends along the first axis. 
     In another feature, the first portion of the stack has a higher thermal contact resistance along the first axis than along the second axis. 
     In another feature, the first portion of the stack conducts more heat along the second axis than along the first axis. 
     In another feature, the heat sink is arranged on a first side of the first and second battery cells. The heat transfer system further comprises a second heat sink. The second heat sink is arranged on a second side of the first and second battery cells that is opposite to the first side. The stack of the sheets further comprises a third portion that is disposed between the second heat sink and at least one of the first and second battery cells. The third portion of the stack transfers the heat conducted by the first portion of the stack from the first battery cell to the second heat sink. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIGS.  1 A and  1 B  show examples of a system for removing heat from one or more battery cells of a battery module using a stack of sheets of thermally conductive material disposed between the cells and a heat sink; 
         FIG.  2    shows the system of  FIG.  1 A  with an additional heat sink and with an additional portion of the stack of sheets disposed between the battery cells and the additional heat sink; 
         FIG.  3 A  shows an example of attaching portions of the sheets to the heat sink in a staggered manner; 
         FIG.  3 B  shows an example of joining portions of the sheets near the heat sink and attaching the joined portions of the sheets to the heat sink; 
         FIG.  4    shows an example of a surface of one of the sheets with different treatments performed on different portions of the surface; and 
         FIG.  5    shows an example of a layer of a thermally insulating material that can be disposed between two or more of the sheets shown in  FIGS.  1 A and  1 B . 
     
    
    
     In the drawings, reference numbers may be reused to identify similar and/or identical elements. 
     DETAILED DESCRIPTION 
     Typically, thermal runaway propagation is prevented by employing barriers formed using thick layers of aerogel insulation. The aerogel insulation is not only expensive but the thickness of the aerogel insulation layers precludes the use of the barriers between every battery cell. Accordingly, the barriers are placed between two battery cells after every N th  battery cell, where N is an integer typically greater than 3, which is insufficient to completely prevent thermal runaway propagation. A metallic heat sink/heat transfer plate is arranged adjacent to the barriers, which is relatively less effective for removing heat. 
     The present disclosure provides a heat removal system that prevents thermal runaway propagation using more compact and efficient material than aerogel insulation. The heat removal system provides both insulating and conducting properties. Specifically, the heat removal system prevents heat transfer between the battery cells by providing insulation between the battery cells. Additionally, the heat removal system conducts heat from the battery cells and transfers the conducted heat to a cooling plate attached to the heat removal system. 
     The heat removal system uses a stack of multiple thin conductive sheets placed between the battery cells. As explained below in detail, a thermal contact resistance provided by the stack between the battery cells provides a highly effective insulator in a through-plane direction (i.e., across the thickness of the stack and between the battery cells) while maintaining high in-plane conductivity (i.e., in a plane parallel to the length of the stack and parallel to the height of the battery cells). The thermal insulating properties are provided by the contact resistance across the thickness of the stack. 
     The contact resistance is created by stacking multiple sheets of a conducting material. When enough number of the conductive sheets are stacked, the sum of all the contact resistances becomes very high, simulating a highly effective insulator. However, because the sheets are highly conductive, the stack of the sheets still carries a significant amount of heat in the in-plane direction from the battery cell to the cooling plate. Furthermore, as explained below, the stack is constructed such that the stack has a relatively high contact resistance between the battery cells and a relatively low contact resistance where the stack is joined with the cooling plate/heat sink. 
     By using the stack of thin sheets, the heat removal system occupies a smaller area relative to the barriers formed using thick layers of aerogel insulation. The material used to form the heat removal system is less expensive than the aerogel insulation. Thus, the heat removal system provides better protection for neighboring battery cells against thermal runaway propagation relative to the barriers formed using thick layers of aerogel insulation. 
     More specifically, the heat removal system comprises a thermal barrier formed using a stack of thin sheets made of a highly conductive material. Non-limiting examples of the conductive material include metals, alloys, composites, and any combination thereof. First portions of the conductive sheets are stacked between the battery cells and have a relatively high surface roughness to increase a contact resistance between the conductive sheets. Second portions of the conductive sheets are stacked and connected to a heat sink and have a relatively low surface roughness to decrease the contact resistance between the conductive sheets. For example, the conductive sheets may be creased/wrinkled to reduce the contact area (i.e., to increase contact resistance) between the conductive sheets in the first portions when the conductive sheets are compressed between the battery cells. In the second portions, the contact area increases (i.e., contact resistance decreases) between the conductive sheets due to higher compression used between the heat sink and the cells than between the battery cells. 
     Optionally, the first portions of the conductive sheets may be coated with a thin thermally insulating layer. For example, the insulating layer may include an anodized coating. Alternatively, insulating sheets of a thermally insulating material may be placed in between the first portions of the conductive sheets at any interval. The insulating sheets may include a solid pattern or a pattern with holes. 
     The conductive sheets are made to conduct heat away from a first mass (e.g., a first battery cell) to a second mass (e.g., a heat sink or a cooling plate), while protecting a third mass (e.g., a second battery cell adjacent to the first battery cell) from heat generated by the first mass. The conductive sheets are compressed tightly together where the conductive sheets contact the second mass to promote heat transfer. Alternatively, the conductive sheets are welded together where the conductive sheets join the second mass to promote heat transfer. The conductive sheets may use a conductive paste where the conductive sheets are joined together with the second mass to promote heat transfer through the thickness of the conductive sheets at the location of the joint. The conductive sheets may be staggered where the conductive sheets join the second mass to promote heat transfer so that a portion of each conductive sheet contacts the second mass. The conductive sheets may be made up of multiple types of materials. Surface treatments can also be applied to the various portions of each individual conductive sheet. These and other features of the heat removal system of the present disclosure are described below in detail. 
     Throughout the present disclosure, a battery system is used only as an example to illustrate an application of the heat removal system. The heat removal system of the present disclosure can be used in any application where heat generated by one mass can adversely affect a neighboring mass, which in turn can cause a domino effect in other neighboring masses. The heat removal system of the present disclosure can be used to prevent similar heat propagation in any application. 
       FIGS.  1 A and  1 B  show examples of a heat removal system  100  according to the present disclosure. In  FIG.  1 A , a side cross-sectional view of the heat removal system  100  is shown. For example, the heat removal system  100  is installed in a battery module of which only two battery cells, a first battery cell  102  and a second battery cell  104 , are shown for illustrative purposes. Hereinafter, for convenience, the first and second battery cells  102 ,  104  are simply called the cells  102 ,  104 ; and it is understood that the cells  102 ,  104  are located in a battery module of a battery pack. 
     Suppose that the cell  102  is experiencing thermal runaway, and the cell  104  that is neighboring or adjacent to the cell  102  is functioning normally (i.e., the cell  104  is not generating an abnormal amount of heat). The heat removal system  100  conducts heat from the cell  102  and transfers the conducted heat to a heating sink (or a cooling plate)  106 . The heat removal system  100  prevents the heat from the cell  102  from flowing to the neighboring cell  104  and causing thermal runaway. 
     The heat removal system  100  comprises a plurality of sheets  108 - 1 ,  108 - 2 ,  108 - 3 , and  108 - 4  (collectively the sheets  108 ) made of a thermally conductive material. Only four sheets  108  are shown for example only. Fewer or more than four sheets  108  can be used instead. The sheets  108  are made of a highly thermally conductive material. For example, the sheets  108  can be made of a metal or an alloy. Alternatively, the sheets can be made of a composite material. In some examples, one set of the sheets  108  can be made of a first conductive material (e.g., a metal), and another set of the sheets  108  can be made of a second conductive material (e.g., a composite material). 
     In some examples, at least one of the sheets  108  closest to the cell  102  (e.g., sheet  108 - 1 , or sheets  108 - 1  and  108 - 2 ) can be made of a first conductive material, and the remaining sheets  108  can be made of a second conductive material having a higher thermal conductivity than the first conductive material. Many other permutations and combinations of conductive materials can be used to make the sheets  108 . 
     The sheets  108  are stacked together and are disposed between the two adjacent cells  102 ,  106 . The sheets  108  include two portions: a first portion  110  of the sheets  108  is compressed between the cells  102 ,  104 ; and a second portion  112  of the sheets  108  is disposed and compressed between the heat sink  106  and the top of the cell  102  as shown. The first portion  110  of the sheets  108  extends from the bottom of the cells  102 ,  104  to the top of the cells  102 ,  104 . The first portion  110  of the sheets  108  is compressed between the cells  102 ,  104 . The second portion  112  of the sheets  108  extends beyond the first portion  110  and is folded at about right angle over the top of the cell  102  as shown at  111 . 
     The first and second portions  110 ,  112  are continuous; that is, the sheets  108  in the first and second portions  110 ,  112  are simply folded to form the first and second portions  110 ,  112  but are otherwise continuous. Alternatively, the first and second portions  110 ,  112  can be two separate portions of the sheets  108  that are joined together at  111 . The first and second portions  110 ,  112  of the sheets  108  are designated as such so that the different properties and structural details of the sheets  108  in the first and second portions  110 ,  112  can be described by conveniently referencing the two portions. 
     The heat sink  106 , which includes a cooling plate, compresses the second portion  112  of the sheets  108  between the heat sink  106  and the top of the cell  102  (and may be another cell neighboring the cell  102  on the opposite side of the cell  104 , not shown). The heat sink  106  compresses the second portion  112  of the sheets  108  with a much greater force than the force with which the first portion  110  of the sheets  108  is compressed between the cells  102 ,  104 . Alternatively, as shown in  FIG.  1 B , the heat sink  106  may include two layers  106 - 1  and  106 - 2 , and the second portion  112  of the sheets  108  is compressed between the two layers  106 - 1  and  106 - 2  of the heat sink  106 . Note that the heat sink  106  can include a cooling or heating plate in a battery pack. Alternatively, a portion of the battery pack can serve as the heat sink  106 . Further, neighboring cells can also serve as the heat sink  106 . 
     In  FIG.  1 A , in the first portion  110 , the sheets  108  provide a thermal contact resistance in a through-plane direction  120  (i.e., along a plane extending across the thickness of the sheets  108  in the first portion  110  between the cells  102 ,  104 ). The contact resistances of the sheets  108  connect in series. Depending on the number of the sheets  108  stacked and compressed together, a sum of all of the contact resistances of the sheets  108  in the first portion  110  becomes very high in proportion to the number of sheets  108  used. As a result, the first portion  110  of the sheets  108  functions like a highly effective thermal insulator that prevents heat from the cell  102  from transferring to the cell  104 . The insulating effect of the first portion  110  of the sheets  108  can be enhanced further by coating the sheets  108  in the first portion  110  with an insulating material or by using layers of an insulating material between the sheets  108  in the first portion  110  of the sheets  108  as described below in detail. 
     While the first portion  110  of the sheets  108  functions as a highly effective thermal insulator between the cells  102 ,  104 , the sheets  108  themselves are made of a highly conductive material. Therefore, the sheets  108  maintain high conductivity in an in-plane direction  122  (i.e., in a plane parallel to the length of the stack of the sheets  108  and parallel to the height of the cells  102 ,  104 ). In general, the through-plane direction  120  can be called a horizontal direction, and the in-plane direction  122 , which is perpendicular to the through-plane direction  120 , can be called a vertical direction. The first portion  110  of the stack functions as a thermal insulator across a thickness of the first portion  110  of the stack (i.e., in the horizontal direction) and as a thermal conductor in a direction perpendicular to the thickness of the first portion  110  of the stack (i.e., in the vertical direction). 
     Further, the second portion  112  of the sheets  108  does not include any insulating coating or insulating material between the sheets  108 . Instead, the sheets  108  in the second portion  112  are connected to the heat sink  106  in one or more ways described below to reduce contact resistance between the sheets  108  in the second portion  112  and to enhance heat transfer between the sheets  108  in the second portion  112  and the heat sink  106 . Accordingly, the sheets  108  provide a lower thermal contact resistance in the second portion  112  of the stack than in the first portion  110  of the stack. The heat conducted from the cell  102  by the sheets  108  in the first portion  110  is readily transferred from the sheets  108  in the second portion  112  to the heat sink  106 . 
     Optionally, the sheets  108  may be coated with an insulating material. For example, the sheets  108  may be coated with the insulating material or a material that can increase the contact resistance of the sheets  108  only in the first portion  110  of the stack. The sheets  108  in the second portion  112  of the stack are not coated with the insulating material. Alternatively, a plurality of layers  114 - 1 ,  114 - 2 , and  114 - 3  (collectively the layers  114 ) of an insulating material can be disposed between the sheets  108  in the first portion  110 . The layers  114  are not disposed between the sheets  108  in the second portion  112 . The sheets  108  in the second portion  112  of the stack may be coated with materials that reduce contact resistance of the sheets  108  in the second portion  112  of the stack. 
     The presence of the insulating material (coating or the layers  114 ) in the first portion  110  of the sheets  108  further increases the contact resistance of the sheets  108  in the first portion  110 , which in turn prevents heat transfer from the cell  102  to the cell  104 . The absence of the insulating material (coating or the layers  114 ) in the second portion  112  of the sheets  108  and optional use of a coating that reduces contact resistance of the sheets  108  in the second portion  112  allow heat conducted by the sheets  108  from the cell  102  to be readily transferred to the heat sink  106 . Note that while the layers  114  are shown between every pair of the sheets  108  in the first portion  110 , the layers  114  can be disposed in any other manner. The coating and the layers  114  are shown and described in further detail with reference to  FIGS.  4  and  5   . 
       FIG.  2    is similar to  FIG.  1 A  except that  FIG.  2    shows an additional heat sink  130  at the bottom of the cells  102 ,  104 . The heat sink  130  is parallel to the heat sink  106 , both of which are parallel to the through-plane direction  120 . In  FIG.  2   , the sheets  108  include a third portion  132  that is structurally similar to the second portion  112  in all respects. Therefore, the third portion  132  of the sheets  108  is not described in further detail for brevity. The third portion  132  transfers heat conducted by the first portion  110  of the sheets  108  from the cell  102  to the heat sink  130  in the same manner as the second portion  112  of the sheets  108  transfers the heat conducted by the first portion  110  of the sheets  108  from the cell  102  to the heat sink  106  as described above. Note that as shown in  FIG.  1 B , the heat sink  130  may also include two layers, and the third portion  132  can be compressed between the two layers of the heat sink  130 . Further, the heat sink  130  can also include a cooling or heating plate in a battery pack. Alternatively, a portion of the battery pack can serve as the heat sink  130 . Further, neighboring cells can also serve as the heat sink  130 . 
       FIGS.  3 A and  3 B  show different ways in which the sheets  108  in the second portion  112  can be connected to each other and to the heat sink  106 . In  FIGS.  3 A and  3 B , partial side cross-sectional views of the heat removal system  100  are shown without the cells  102 ,  104 . The cells  102 ,  104  are omitted to focus on the different ways in which the sheets  108  in the second portion  112  can be connected to each other and to the heat sink  106 . 
     Note that the sheets  108  in the third portion  132  (shown in  FIG.  2   ) can be connected to each other and to the heat sink  130  (shown in  FIG.  2   ) in the same manner. In  FIGS.  3 A and  3 B , the layers  114  are shown for completeness. The layers  114  can be arranged differently or omitted altogether as already described above. Further, the description of other features such as surface treatment and surface roughness of the sheets  108  in the first and second portions  110 ,  112  provided elsewhere applies equally to the elements shown and described below with reference to  FIGS.  3 A and  3 B . 
     In  FIG.  3 A , in the second portion  112 , the sheets  108  are arranged in a staggered manner as shown. In the staggered or staircase-like arrangement shown, at least a portion of each of the sheets  108  in the second portion  112  is connected to the heat sink  106 . In addition, all of the sheets  108  in the second portion  112  contact each other. As already described above, the sheets  108  in the second portion  112  are neither coated with nor separated by any insulating material. Instead, as described below, the sheets  108  in the second portion  112  may have a surface roughness less than that of the sheets  108  in the first portion  110 . As a result, the contact resistance of the sheets  108  in the second portion  112  is much less than the contact resistance of the sheets  108  in the first portion  110 . The layered manner of arranging the sheets  108  shortens the path for heat transfer in the through-plane direction  120  for more of the sheets  108  in the second portion  112 . Therefore, the sheets  108  in the second portion  112  readily transfer the heat, which is conducted from the cell  102  by the sheets  108  in the first portion  110 , to the heat sink  106 . 
     Further, a thermally conductive paste may be used between the heat sink  106  and the portions of the sheets  108  in the second portion  112  that contact the heat sink  106 . The thermally conductive paste further enhances the heat transfer from the sheets  108  in the second portion  112  to the heat sink  106 . 
     In  FIG.  3 B , the sheets  108  in the second portion  112  are joined (e.g., welded) together as shown at  134 . The joint  134 , that is the location where the sheets  108  in the second portion  112  are joined or welded together, is connected to the heat sink  106 . The joint  134  provides a zone of very high thermal conductivity along the through-plane direction  120  in which to transfer heat to the heat sink  106 . Further, a conductive paste may be used between the joint  134  and the heat sink  106  to further enhance the heat transfer from the sheets  108  in the second portion  112  to the heat sink  106 . 
       FIG.  4    shows an example of one of the sheets  108  (hereinafter the sheet  108 ). Specifically,  FIG.  4    shows a front view of the sheet  108  and also shows a cross-sectional view of the sheet  108  taken along the line A-A, which is seen in  FIGS.  1 A- 3 B . In the front view, the dimensions d and h respectively denote the depth and height of the cells  102 ,  104 , of which the height his shown in  FIGS.  1 A- 2   . 
     The sheet  108  has a first portion  110  that is disposed between the cells  102 ,  104 . The sheet  108  has a second portion  112  that is disposed between the heat sink  106  and the cell  102 . The sheet  108  is folded at  111  at about right angle to form the first and second portions  110 ,  112 . These portions are already described above as respective portions of the sheets  108  stacked together. Various surface treatments of these portions are now described in further detail. The description provided with reference to  FIG.  1 A  applies to all of the sheets  108  shown and described above with reference to  FIGS.  1 A- 3 B . Further, while a third portion  132  of the sheet  108  is not shown, it is to be understood that the third portion  132  of the sheet  108  is identical to the first portion  110  of the sheet  108 . 
     The first portion  110  of the sheet  108  may be treated in a manner to increase the surface roughness and to increase the thermal contact resistance. The second portion  112  of the sheet  108  may be treated in a manner to decrease the surface roughness and to decrease the thermal contact resistance. Surface roughness can be increased in many ways. For example, to increase surface roughness, the first portion  110  of the sheet  108  can be brushed, wrinkled, or creased. To decrease surface roughness, the second portion  112  of the sheet  108  may be smooth (or less rough than the first portion  110 ). 
     To further increase the thermal contact resistance, instead of or in addition to wrinkling, the first portion  110  of the sheet  108  may be treated using a surface treatment. For example, the first portion  110  of the sheet  108  may be coated with a thin layer of an insulating material. For example, the thin layer may include an anodized coating. The second portion  112  of the sheet  108  is not coated using such a surface treatment. Instead, a conductive paste may be used to join the second portion  112  of the sheet  108  to the heat sink  106  to enhance heat transfer from the second portion  112  of the sheet  108  to the heat sink  106 . 
     In  FIG.  4   , the difference in roughness between the first and second portions  110 ,  112  of the sheet  108  and the presence and absence of the coating on the first and second portions  110 ,  112  of the sheet  108  are shown using two different types of shading. A line denoted by reference numeral  111  shown between the two types of shading (i.e., between the first and second portions  110 ,  112  of the sheet  108 ) represents the fold  111  where the sheet  108  is folded at about right angle to form the first and second portions  110 ,  112  of the sheet  108 . Further, while only one side of the sheet  108  is shown, it is understood that the other side of the sheet  108 , which is not shown, may be identical or similar to the side shown in  FIG.  4   . 
       FIG.  5    shows an example of the layer  114  of insulating material shown in  FIGS.  1 A- 3 B . Specifically,  FIG.  5    shows a front view of the layer  114  and also shows a cross-sectional view of the layer  114  taken along the line B-B, which is seen in  FIGS.  1 A- 3 B . In the front view, the dimensions d and h respectively denote the depth and height of the cells  102 ,  104 , of which the height h is shown in  FIGS.  1 A- 2   . 
     For example, the layer  114  can comprise a mesh of an insulating material. For example, the layer  114  comprise fiber glass. For example, the layer  114  can comprise a thin sheet of any thermally insulating material. The thin sheet may be solid or porous (i.e., can include air pockets). The thin sheet can include any regular or irregular pattern of holes. 
     Note that the layer  114  is optional. The layer  114  can be used between every pair of the sheets  108  in the first portion  110 . The layer  114  can be between any of the sheets  108  in the first portion  110  used using any regular or irregular pattern. The layer  114  can be used instead of or in addition to the thin coating of an insulating material applied to the sheets  108  in the first portion  110 . The layer  114  is not used in the second portion  112 . 
     Throughout the present disclosure, references have been made to insulating and conductive materials. The insulating and conductive properties of these materials are to be understood in the context of the ability or inability of these materials to conduct and transfer heat. An insulating material is to be understood as being thermally insulating, and a thermally conductive material is to be understood as being thermally conductive. 
     The foregoing description is merely illustrative in nature and is not intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. 
     It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure. 
     Spatial and functional relationships between elements (for example, between modules, circuit elements, semiconductor layers, etc.) are described using various terms, including “connected,” “engaged,” “coupled,” “adjacent,” “next to,” “on top of,” “above,” “below,” and “disposed.” Unless explicitly described as being “direct,” when a relationship between first and second elements is described in the above disclosure, that relationship can be a direct relationship where no other intervening elements are present between the first and second elements, but can also be an indirect relationship where one or more intervening elements are present (either spatially or functionally) between the first and second elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” 
     In the figures, the direction of an arrow, as indicated by the arrowhead, generally demonstrates the flow of information (such as data or instructions) that is of interest to the illustration. For example, when element A and element B exchange a variety of information but information transmitted from element A to element B is relevant to the illustration, the arrow may point from element A to element B. This unidirectional arrow does not imply that no other information is transmitted from element B to element A. Further, for information sent from element A to element B, element B may send requests for, or receipt acknowledgements of, the information to element A. 
     In this application, including the definitions below, the term “module” or the term “controller” may be replaced with the term “circuit.” The term “module” may refer to, be part of, or include: an Application Specific Integrated Circuit (ASIC); a digital, analog, or mixed analog/digital discrete circuit; a digital, analog, or mixed analog/digital integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor circuit (shared, dedicated, or group) that executes code; a memory circuit (shared, dedicated, or group) that stores code executed by the processor circuit; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. 
     The module may include one or more interface circuits. In some examples, the interface circuits may include wired or wireless interfaces that are connected to a local area network (LAN), the Internet, a wide area network (WAN), or combinations thereof. The functionality of any given module of the present disclosure may be distributed among multiple modules that are connected via interface circuits. For example, multiple modules may allow load balancing. 
     The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, data structures, and/or objects. The term shared processor circuit encompasses a single processor circuit that executes some or all code from multiple modules. The term group processor circuit encompasses a processor circuit that, in combination with additional processor circuits, executes some or all code from one or more modules. References to multiple processor circuits encompass multiple processor circuits on discrete dies, multiple processor circuits on a single die, multiple cores of a single processor circuit, multiple threads of a single processor circuit, or a combination of the above. The term shared memory circuit encompasses a single memory circuit that stores some or all code from multiple modules. The term group memory circuit encompasses a memory circuit that, in combination with additional memories, stores some or all code from one or more modules. 
     The term memory circuit is a subset of the term computer-readable medium. The term computer-readable medium, as used herein, does not encompass transitory electrical or electromagnetic signals propagating through a medium (such as on a carrier wave); the term computer-readable medium may therefore be considered tangible and non-transitory. Non-limiting examples of a non-transitory, tangible computer-readable medium are nonvolatile memory circuits (such as a flash memory circuit, an erasable programmable read-only memory circuit, or a mask read-only memory circuit), volatile memory circuits (such as a static random access memory circuit or a dynamic random access memory circuit), magnetic storage media (such as an analog or digital magnetic tape or a hard disk drive), and optical storage media (such as a CD, a DVD, or a Blu-ray Disc). 
     The apparatuses and methods described in this application may be partially or fully implemented by a special purpose computer created by configuring a general purpose computer to execute one or more particular functions embodied in computer programs. The functional blocks, flowchart components, and other elements described above serve as software specifications, which can be translated into the computer programs by the routine work of a skilled technician or programmer. 
     The computer programs include processor-executable instructions that are stored on at least one non-transitory, tangible computer-readable medium. The computer programs may also include or rely on stored data. The computer programs may encompass a basic input/output system (BIOS) that interacts with hardware of the special purpose computer, device drivers that interact with particular devices of the special purpose computer, one or more operating systems, user applications, background services, background applications, etc. 
     The computer programs may include: (i) descriptive text to be parsed, such as HTML (hypertext markup language), XML (extensible markup language), or JSON (JavaScript Object Notation) (ii) assembly code, (iii) object code generated from source code by a compiler, (iv) source code for execution by an interpreter, (v) source code for compilation and execution by a just-in-time compiler, etc. As examples only, source code may be written using syntax from languages including C, C++, C#, Objective-C, Swift, Haskell, Go, SQL, R, Lisp, Java®, Fortran, Perl, Pascal, Curl, OCaml, Javascript®, HTML5 (Hypertext Markup Language 5th revision), Ada, ASP (Active Server Pages), PHP (PHP: Hypertext Preprocessor), Scala, Eiffel, Smalltalk, Erlang, Ruby, Flash®, Visual Basic®, Lua, MATLAB, SIMULINK, and Python®.