Abstract:
A thermal battery including: a casing; a thermal battery cell disposed in the casing and operatively connected to electrical connections exposed from the casing; a first portion of a material capable of having an exothermic reaction positioned between the casing and the thermal battery cell; a second portion of a material capable of having an exothermic reaction positioned between the casing and the thermal battery cell; a first initiator for initiating the thermal battery cell; at least one second initiator for initiating the first portion; and a fuze in communication with the first and second portions for initiating the second portion resulting from the initiation of the first portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation-In-Part of U.S. application Ser. No. 13/306,959 filed on Nov. 29, 2011, which is a Continuation-In-Part of U.S. application Ser. No. 12/955,875 filed on Nov. 29, 2010, the entire contents of each of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure relates generally to components of thermal batteries, and more particularly to multi-functional insulating and heat generating materials for thermal batteries and the like. 
         [0004]    2. Prior Art 
         [0005]    Thermal batteries represent a class of reserve batteries that operate at high temperatures. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. The most common internal pyrotechnic is a blend of Fe and KClO 4 . Thermal batteries utilize a molten salt to serve as the electrolyte upon activation. The electrolytes are usually mixtures of alkali-halide salts and are used with the Li(Si)/FeS 2  or Li(Si)/CoS 2  couples. Some batteries also employ anodes of Li(Al) in place of the Li(Si) anodes. Reserve batteries are inactive and inert when manufactured and become active and begin to produce power only when they are activated. 
         [0006]    Thermal batteries have long been used in munitions and other similar applications to provide a relatively large amount of power during a relatively short period of time, mainly during the munitions flight. Thermal batteries have high power density and can provide a large amount of power as long as the electrolyte of the thermal battery stays liquid, thereby conductive. The process of manufacturing thermal batteries is highly labor intensive and requires relatively expensive facilities. Fabrication usually involves costly batch processes, including pressing electrodes and electrolytes into rigid wafers, and assembling batteries by hand. The batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. Thermal batteries, however, have the advantage of very long shelf life of up to 20 years that is required for munitions applications. 
         [0007]    Thermal batteries generally use some type of igniter to provide a controlled pyrotechnic reaction to produce output flame or hot particles to ignite the heating elements of the thermal battery. There are currently two distinct classes of igniters that are available for use in thermal batteries. The first class of igniters operate based on electrical energy. Such electrical igniters, however, require electrical energy, thereby requiring an onboard battery or other power sources with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. The second class of igniters, commonly called “inertial igniters,” operate based on the firing acceleration. The inertial igniters do not require onboard batteries for their operation and are thereby often used in high-G munitions applications such as in non-spinning gun-fired munitions and mortars. 
         [0008]    In general, the inertial igniters, particularly those that are designed to operate at relatively low impact levels, have to be provided with the means for distinguishing events such as accidental drops or explosions in their vicinity from the firing acceleration levels above which they are designed to be activated. 
         [0009]    Insulation and internal heat sinks are used to maintain the electrolyte in its molten and conductive condition during the time of use following their activation. The length of time that the electrolyte stays molten determines the active life of the battery. To increase the active life, the amount of available heat energy needs to be increased and/or more effective insulation material needs to be provided. For smaller size thermal batteries, the volume of the insulation material that can be provided becomes limited. In addition, since the ratio of the surface area to the enclosed molten material volume increases as the battery volume is decreased, the effectiveness of the insulation material decreases as the size of the thermal battery decreases. 
         [0010]    The following is a brief description of the thermal battery disclosed in U.S. Pat. No. 3,898,101 “Thermal Battery” by D. M. Bush, et al. However, it must be noted that this selection is for purposes of illustration only and is used for describing similar components with respect to the various embodiments disclosed herein. 
         [0011]    As it is shown in the schematic of FIG. 1, reproduced from U.S. Pat. No. 3,898,101, the thermal battery may include a plurality of electrochemical cells 10 stacked one upon the other in electrical series within a suitable casing 12 and thermal insulating barrier 14. Electrical connections may be made in an appropriate manner by suitable electrical leads and terminals 16, 17, and 18 to the respective positive and negative terminals of the upper and lower battery cells in the stack. The heat or thermal generating elements for the battery, which are generally positioned as a part of each battery cell with or without additional heat generating elements at each end of the battery, may be ignited to activate the battery by a suitable electrical match or detonator 20 and heat powder or fuse 22 which is coupled between the match 20 and the heating generating elements in each cell. The battery is normally formed by first stacking the individual cell elements to form separate cells and then the cells stacked together in the form shown in FIG. 1 and placed within the casing 12 and insulator 14 under suitable pressure, such as by a compression force applied by a bolt 23 passing through the center of the cells, or other suitable mechanisms. The so stacked battery cells may then be covered with an end insulator 24 and a casing cover 25 in an appropriate manner. The battery is operated by initiating the electrical match 20 and in turn the heat powder 22 and the individual heat generating elements of the cell stack and the electrical current drawn off through leads 16, 17, and 18. 
         [0012]    A need therefore exists for methods and materials that can be used to keep thermal batteries in general and small thermal batteries in particular operational longer following activation. For those applications in which the operational life of the thermal battery following activation is not an issue, such methods and material can be used to reduce the insulation volume requirement, thereby allowing the size of the thermal battery to be reduced. The material used for thermal insulation must also be electrically non-conducting. 
       SUMMARY 
       [0013]    Provided herein are methods to develop multi-functional heat insulation for thermal batteries and the like that can be used to provide heat to the battery to increase its operational time and performance as well as serving as heat insulation material. 
         [0014]    Further provided are methods to develop multi-functional heat insulation for thermal batteries and the like that can be used to provide heat insulation as well as provide heat to the battery on demand to prolong the battery operational time and performance. 
         [0015]    Still further provided are multi-functional insulation materials that can be used in thermal batteries to serve as thermal insulation as well as source of generating heat to the battery to prolong the battery operational time and performance. 
         [0016]    Still further yet provided are multi-functional insulation materials that can be used in thermal batteries to serve as thermal insulation as well as source of heat to the battery on demand to prolong the battery operational time and performance. 
         [0017]    Accordingly, a thermal battery is provided. The thermal battery comprising: a casing; a thermal battery cell disposed in the casing and operatively connected to electrical connections exposed from the casing; a fuel and oxidizer mixture disposed at least partially between the casing and the battery cell; and one or more initiators for initiating one or more of the thermal battery cell and the fuel and oxidizer mixture; wherein the fuel and oxidizer mixture produces an exothermic reaction upon initiation and forms a reaction product being a thermal insulator. 
         [0018]    The casing can include a casing cover. 
         [0019]    The thermal battery cell can be selected from a list consisting of perchlorates, nitrates, permanganates, fluorinated polymers and metal oxides. 
         [0020]    The fuel and oxidizer mixture can comprise silicon nanosponge particles and porous silicon particles. The silicon nanosponge particles can be prepared from metallurgical grade silicon powder having an initial particle size ranging from about 1 micron to about 4 microns, the silicon nanosponge particles can have a plurality of nanocrystals having pores. The porous silicon particles can be prepared from a metallurgical grade silicon powder having a solid core surrounded by a porous silicon layer having a thickness greater than about 0.5 microns. The reaction product of the fuel and oxidizer mixture can be silica. 
         [0021]    The thermal battery can further comprise an insulator disposed between the fuel and oxidizer mixture and the casing. 
         [0022]    The thermal battery can further comprise an additional insulator disposed between the fuel and oxidizer mixture and the battery cell. 
         [0023]    The thermal battery can further comprise an insulator disposed between the fuel and oxidizer mixture and the battery cell. 
         [0024]    The fuel and oxidizer mixture can comprise at least first and second fuel and oxidizer mixtures separated by an insulator. 
         [0025]    Also provided is a method of initiating a thermal battery. The method comprising; disposing a thermal battery cell in a casing; disposing a fuel and oxidizer mixture at least partially between the casing and the battery cell; initiating the fuel and oxidizer mixture; wherein the initiating includes producing an exothermic reaction and forming a reaction product being a thermal insulator. 
         [0026]    The method can further comprise insulating the exothermic reaction on a side of the fuel and oxidizer mixture between the fuel and oxidizer mixture and the casing. 
         [0027]    The method can further comprise insulating the exothermic reaction on a side of the fuel and oxidizer mixture between fuel and oxidizer mixture and the battery cell. 
         [0028]    The disposing of the fuel and oxidizer mixture can comprise disposing first and second fuel and oxidizer mixtures between an insulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other features, aspects, and advantages of the apparatus of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0030]      FIG. 1  illustrates a schematic of a cross-section of a thermal battery and igniter assembly of the prior art. 
           [0031]      FIG. 2  illustrates a schematic of a first embodiment of a thermal battery. 
           [0032]      FIG. 3  illustrates a close-up view of the casing and insulation section of the thermal battery shown in  FIG. 2 . 
           [0033]      FIG. 4  illustrates a close-up view of the casing and insulation sections of a second embodiment of a thermal battery. 
           [0034]      FIG. 5  illustrates a close-up view of the casing and insulation sections of a third embodiment of a thermal battery. 
           [0035]      FIG. 5  illustrates a close-up view of the casing and insulation sections of a fourth embodiment of a thermal battery. 
           [0036]      FIG. 6  illustrates a close-up view of the casing and insulation sections of a fourth embodiment of a thermal battery. 
           [0037]      FIG. 7  illustrates a schematic of a fifth embodiment of a thermal battery. 
           [0038]      FIG. 8  illustrates a variation of the thermal battery of  FIG. 7 . 
           [0039]      FIG. 9  illustrates a schematic of a close-up view of a section of the thermal battery embodiment of  FIG. 7 . 
           [0040]      FIG. 10  illustrates the schematic of a cross-sectional view A-A of the section shown in  FIG. 9  showing a possible method of integrating a delay fusing for delayed initiation of a pocket of active insulation material. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    An embodiment of a thermal battery includes a mixture of fuel(s) and oxidizer(s) which exhibits an exothermic reaction upon initiation, generating heat to prolong the battery operation and where the reaction product (including any residual fuel) is one that can provide thermal insulation. Preferred fuels for the aforementioned multi-functional insulation material are silicon nanosponge particles and porous silicon particles as described in U.S. Pat. Nos. 7,560,085 and 7,569,20, the contents of which are incorporated herein by reference. Silicon nanosponge particles are prepared from a metallurgical grade silicon powder having an initial particle size ranging from about 1 micron to about 4 microns. Each silicon nanosponge particle has a structure comprising a plurality of nanocrystals with pores disposed between the nanocrystals and throughout the entire nanosponge particle. Porous silicon particles having a particle size &gt;0.5 micron are also prepared from a metallurgical grade silicon powder but comprise a solid core surrounded by a porous silicon layer having a thickness greater than about 0.5 microns. The silicon nanosponge and porous silicon particles together with appropriate oxidizers can be formulated to burn at a desired rate and to form the chemical compound silicon dioxide, SiO 2 , also known as silica. Silica has very high thermal insulation and electrical insulation characteristics. By using the proper type and amount of oxidizers, the amount of gasses that can be generated during the process of burning of the silicon nanosponge material is minimized. The Table shows the expected reaction of silicon with various oxidizers and the estimated heat of reaction. Oxidizers including but not limited to perchlorates, nitrates, permanganates, fluorinated polymers and metal oxides can be used. The oxidizer may be chosen based on the desired burn rate and ignition characteristics. The Brunauer.Emmet.Teller (B.E.T.) surface area of the silicon nanosponge and porous silicon particles can also be changed as described in U.S. Pat. No. 7,560,085, the contents of which are also incorporated herein by reference. The burn rate and heat output can also be controlled by varying the particle size, surface area and porosity of the porous silicon particles. Hereinafter, the silicon nanosponge materials and the porous silicon particles together (“treated”) with the appropriate oxidizers are referred to as the “porous silicon-based pyrotechnic” material. 
         [0042]    It will be appreciated by those of ordinary skill in the art that the relative amount of oxidizer used may be selected to oxidize (burn) a desired portion of the silicon nanosponge or porous silicon particle to generate the desired amount of heat per unit volume of the aforementioned “porous silicon-based pyrotechnic” material used in the thermal battery and/or to control (minimize) the amount of gasses that the oxidization process could generate. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Summary of reactions of Silicon with various oxidizers and the 
               
               
                 theoretical heat of reaction per unit mass and unit volume 
               
             
          
           
               
                   
                 □Hr (kJ/g) 
                 □Hr (kJ/cc) 
               
               
                 Reaction 
                 (Si + oxidizer) 
                 (Si + oxidizer) 
               
               
                   
               
             
          
           
               
                 Si + O 2  → SiO 2   
                 −15.2 
                 −4.5 
               
               
                 2Si + NaClO 4  → 2SiO 2  + NaCl 
                 −10.4 
                 −16 
               
               
                 2Si + KClO 4  → 2SiO 2  + KCl 
                 −9.4 
                 −18 
               
               
                 5Si + 4KNO 3  → 5SiO 2  + 2N 2  + 2K 2 O 
                 −6.1 
                 −10 
               
               
                 Si + (C 2 F 4 ) n  → SiF 4  + 2C 
                 −6.2 
                 −12 
               
               
                 Si + 2CuO → SiO 2  + 2Cu 
                 −3.0 
                 −12 
               
               
                 3Si + 2Bi 2 O 3  → 4Bi + 3SiO 2   
                 −1.4 
                 −8.5 
               
               
                   
               
             
          
         
       
     
         [0043]    It is noted that the silicon nanosponge materials and porous silicon particles as well as silica have very high thermal insulation (very low thermal conductivity) characteristics and are therefore good candidates for use as thermal barriers in thermal batteries. In addition, when necessary, particularly for the ease of manufacturing, the silicon particles may be used with appropriate binders to allow them to be formed or molded into the desired shape for use in thermal batteries. However, the molding method should preserve the porosity and surface area of the materials in order to maintain the oxidation characteristics. In general, binders that generate minimal amount of gas when heated to the thermal battery activation temperatures are highly desirable since such gasses can degrade the performance of the thermal battery. 
         [0044]    As discussed above, currently available thermal batteries have various electrochemical cell and other internal component and initiation designs. Almost all thermal batteries, however, generally use the insulation materials to enclose the hot interior of the thermal batteries (items  14  and  24  in  FIG. 1 ) and provide a thermal insulating barrier to keep the battery operational for the required length of time. Hereinafter and for the sake of describing the various embodiments disclosed below, the hot interior elements of thermal batteries and the initiation device  20  (excluding the insulating thermal barriers  14  and  24  and the outside shell  12  and the cap  25 — FIG. 1 ) are represented as a single interior element  51  as shown in the schematic of the first embodiment  50  illustrated in  FIG. 2 . 
         [0045]    In the schematic of the first embodiment  50  illustrated in  FIG. 2 , the aforementioned interior element  51  is enclosed within an appropriate casing  52  and cover  53 , usually stainless steel and hermetically sealed. The space between the interior element  51  and the casing  52  and cover  53  is filled with the aforementioned “porous silicon-based pyrotechnic” material  54  and  55 , respectively. The thermal battery leads are indicated by numerals  56  and  57 . 
         [0046]    It will be appreciated by those skilled in the art that any portion of the volume  54  and  55  that is filled with the aforementioned “porous silicon-based pyrotechnic” may instead be filled with any other commonly used (usually organic) insulation material. This might be particularly elected to be done for the cover region  55  where the battery leads  56  and  57  are located. 
         [0047]    In operation, once the thermal battery is activated by igniting the heat generating elements of the thermal battery inside the element  51 ,  FIG. 2 , the “porous silicon-based pyrotechnic” material  54  and  55  are also ignited as the consequence of the thermal battery activation via the heat generating elements of the battery or via separately provided pyrotechnic elements (not shown). Once the “porous silicon-based pyrotechnic” material  54  and  55  are ignited, as a result of at least partial silicon sponge material burning (oxidation), at least a portion of the silicon sponge material is converted to silica. As a result, firstly, heat is generated, which would have the beneficial effect of keeping the thermal battery operational longer or at least require lesser amounts of heat generating elements, thereby allowing the construction of relatively smaller thermal batteries that would stay operational the same length of time. Secondly, the conversion of the already substantially thermally insulating silicon sponge material to silica would generally increase its thermally insulating characteristics. As a result, the burning of the “porous silicon-based pyrotechnic” material  54  and  55  has the substantial effect of turning it into an effective thermal barrier while initially providing heat to the thermal battery core  51 . 
         [0048]    A close-up view  58  of the casing and insulation section  52  and  54 , respectively, is shown in  FIG. 3 . A similar close-up view may also be considered for the cover  53  and its underlying the insulation section  55  and the following embodiments may also be employed in their construction. In the following embodiments, novel methods to construct different configurations of the insulation layer using the aforementioned silicon sponge and “porous silicon-based pyrotechnic” material  54 ,  FIGS. 2 and 3 , are disclosed. The advantages and possible shortcomings of each embodiment when used in different types and sizes of thermal batteries and/or their applications are also discussed. 
         [0049]    A second embodiment is shown schematically in the close-up view  60  (as replacing the wall section close-up view  58  of the embodiment  50  shown in  FIGS. 2 and 3 ) of  FIG. 4 . In the embodiment of  FIG. 4 , an insulation layer  61  (e.g., using any one of the currently available materials known in the prior art) is used between the casing  52  and the aforementioned “porous silicon-based pyrotechnic” material  54 . 
         [0050]    In operation, once the thermal battery is activated by igniting the heat generating elements of the thermal battery inside the element  51 ,  FIGS. 2 and 4 , the “porous silicon-based pyrotechnic” material  54  is also ignited as the consequence of the thermal battery activation via the heat generating elements of the battery or via separately provided pyrotechnic elements (not shown). Once the “porous silicon-based pyrotechnic” material  54  is ignited, as a result of at least partial silicon sponge material burning (oxidation), at least a portion of the silicon sponge material is converted to silica. As a result, firstly, heat is generated, which would have the beneficial effect of keeping the thermal battery operational longer or at least require lesser amounts of heat generating elements, thereby allowing the construction of relatively smaller thermal batteries that would stay operational the same length of time. Secondly, the conversion of the already substantially thermally insulating silicon sponge material to silica would generally increase its thermally insulating characteristics. As a result, the burning of the “porous silicon-based pyrotechnic” material  54  has the substantial effect of turning it into an effective thermal barrier while initially providing heat to the thermal battery core  51 . The addition of the insulation layer  61  will ensure that the generated heat is not conducted out of the thermal battery casing  52 . 
         [0051]    It is noted that similar two-layer design (layers  61  and  54  in  FIG. 4 ) may be used under the cover  53  ( FIG. 2 ) to achieve the aforementioned effect. 
         [0052]    In a third embodiment  70 , at least one insulation layer (e.g., using any one of the currently available materials known in the art) and at least one layer of aforementioned “porous silicon-based pyrotechnic” material is used between the aforementioned casing  52  (and possibly the cover  53 ) and the interior element  50  of the thermal battery ( FIGS. 2 and 3 ). As an example, an additional layer of insulation  71  (using any one of the currently available materials known in the art) may be added to the embodiment of  FIG. 4  between the “porous silicon-based pyrotechnic” material  54  and the interior element  51  as shown in the schematic of  FIG. 5 . The insulation layer  71  may be added to facilitate the packaging of the “porous silicon-based pyrotechnic” material  54 , which may be in the form of “loose powder” without the use of added binders that could otherwise generate unwanted gasses. 
         [0053]    In operation, once the thermal battery is activated by igniting the heat generating elements of the thermal battery inside the element  51 ,  FIGS. 2 and 4 , the “porous silicon-based pyrotechnic” material  54  may also be packaged to be ignited (e.g., by providing an opening in the insulation layer  71 —not shown) as a consequence of the thermal battery activation via the heat generating elements of the battery. However, the “porous silicon-based pyrotechnic” material  54  is preferably ignited via separately provided pyrotechnic elements (not shown), possibly a certain period of time before or after the aforementioned thermal battery initiation depending on the design of the thermal battery and its operational requirements and the temperature of the environment to achieve optimal performance of the thermal battery. Once the “porous silicon-based pyrotechnic” material  54  is ignited, as a result of at least partial silicon sponge material burning (oxidation), at least a portion of the silicon sponge material is converted to silica. As a result, firstly, heat is generated, which would have the beneficial effect of keeping the thermal battery operational longer or at least require lesser amounts of heat generating elements, thereby allowing the construction of relatively smaller thermal batteries that would stay operational the same length of time. Secondly, the conversion of the already substantially thermally insulating silicon sponge material to silica would generally increase its thermally insulating characteristics. As a result, the burning of the “porous silicon-based pyrotechnic” material  54  has the substantial effect of turning it into an effective thermal barrier while initially providing heat to the thermal battery core  51 . The addition of the insulation layer  61  will ensure that the generated heat is not conducted out of the thermal battery casing  52 . 
         [0054]    It is noted that similar multi-layer design (layers  61 ,  54  and  71  in  FIG. 5 ) may be used under the cover  53  ( FIG. 2 ) to achieve the aforementioned effect. 
         [0055]    It will be appreciated by those skilled in the art that the embodiment  70  may be constructed with multi-insulation (e.g., using any one of the currently available materials known in the art) and the aforementioned “porous silicon-based pyrotechnic.” For example, one may use more than one sandwiched layers of insulation (e.g., using any one of the currently available materials known in the art) and “porous silicon-based pyrotechnic” materials to provide the means of generating heat by igniting the different “porous silicon-based pyrotechnic” layers sequentially to achieve optimal operational performance of the thermal battery by keeping the battery electrolyte at the desired temperature for a longer period of time. 
         [0056]    It is also appreciated by those skilled in the art that neither the insulation material such as layers  61  and  71  in  FIG. 5  (e.g., using any one of the currently available materials known in the art) nor the “porous silicon-based pyrotechnic” material layers such as  54  in  FIG. 5 , have to completely cover the entire side, bottom and/or the top surfaces of the thermal battery core  51 . For example, “pockets” or “rings” of “porous silicon-based pyrotechnic” material can be provided within the insulation material layers  61  and/or  71  to localize their generated heat in those areas. 
         [0057]    It will also be appreciated by those skilled in the art that any insulation material could be used for layers  61  and/or  71  in  FIG. 5 . For example, the layer  71  may be formed using the flexible fuel comprising at least one polymeric binding material and porous silicon particles dispersed throughout the polymeric binding material as disclosed in the U.S. Patent application 2009/0101251 of Subramanian, et al. filed on Apr. 23, 2009, the entire contents of which is incorporated herein by reference. 
         [0058]    As shown in  FIG. 6 , two or more of the “porous silicon-based pyrotechnic” material layers  54 ,  54   a  can be provided with insulating layers  61 ,  71  disposed therebetween. In the configuration of  FIG. 6 , an additional insulting layer can be provided between the thermal battery casing  52  and the “porous silicon-based pyrotechnic” material layer  54   a  (as shown in  FIG. 5 ). 
         [0059]    In another embodiment, the aforementioned two or more “porous silicon-based pyrotechnic” material layers (hereinafter also referred to as “active insulation” material), for example, the “porous silicon-based pyrotechnic” material layer  54  shown in the schematic of  FIG. 5  or the “porous silicon-based pyrotechnic” material layers  54  and  54   a  shown in the schematic of  FIG. 6  or when more “porous silicon-based pyrotechnic” material layers are employed, the “porous silicon-based pyrotechnic” material layers that are separated from the thermal battery core  51  by at least one insulation layer (such as the insulation layer  71  in the schematics of  FIGS. 5 and 6 ) may be divided into separate compartments. Each one of these compartments are then separated from the other “porous silicon-based pyrotechnic” material filled compartments and layers and the thermal battery core by thermal insulation materials such as those used in the insulation layers  71 , to prevent the initiation (burn) of the “porous silicon-based pyrotechnic” materials in one compartment or layer from initiating the burn of the “porous silicon-based pyrotechnic” materials in another compartment or layer. An example of such an embodiment  100  is shown schematically in  FIG. 7 . 
         [0060]    In the schematic of  FIG. 7  and for the sake of simplicity, the embodiment  100  is illustrated with only four individual “rings” of “porous silicon-based pyrotechnic” material filled compartments  101 ,  102 ,  103  and  104  and a top and bottom compartments  105  and  106 , respectively. The porous silicon-based pyrotechnic” material filled compartments  101 ,  102 ,  103  and  104  are referred to as “rings” since  FIG. 7  illustrates the thermal battery  100  in cross-section and the thermal battery is considered to be cylindrical (but can be of any shape). For the same reason, the thermal battery is considered to have only one “layer” of “porous silicon-based pyrotechnic” material, which is divided into the aforementioned six compartments ( 101 ,  102 ,  103 ,  104 ,  105  and  106 ). The “porous silicon-based pyrotechnic” material filled compartments  101 ,  102 ,  103 ,  104 ,  105  and  106  are separated by “bands” of thermal insulations made out materials such as material used for the insulation layer  71  ( FIGS. 5 and 6 ), indicated by numerals  107 ,  108 ,  109 ,  110  and  111  in the schematic diagram of  FIG. 7 . The “porous silicon-based pyrotechnic” material filled compartments  101 ,  102 ,  103 ,  104 ,  105  and  106  are also each provided by initiation devices  112 ,  113 ,  114 ,  115 ,  116  and  117  (such as electrical initiation devices), respectively, so that they can be individually and independently initiated at any desired and appropriate time as described below to achieve optimal thermal battery performance. 
         [0061]    In the schematic of the embodiment  100  illustrated in  FIG. 7 , the initiation devices  112 ,  113 ,  114 ,  115 ,  116  and  117  that are provided for the initiation of the “porous silicon-based pyrotechnic” material filled compartments  101 ,  102 ,  103 ,  104 ,  105  and  106 , respectively, are shown to be inserted at various locations on the top cap  53  and side (casing)  52  of the thermal battery assembly. This positioning of the initiation devices is mainly for ease of illustration. In practice, however, the initiation devices  112 ,  113 ,  114 ,  115 ,  116  and  117  and their wiring can all be located inside of the thermal battery casing and are bundled and brought out for connection to the appropriate circuitry through the top cap  53  (or wherever the thermal battery leads  56  and  57  are routed out). 
         [0062]    In the schematic of the embodiment  100  illustrated in  FIG. 7 , the aforementioned thermal battery core  51  is also enclosed in the appropriate casing  52  and cover  53 , usually stainless steel and hermetically sealed. The thermal battery leads are indicated by numerals  56  and  57 . The thermal battery core (activation) initiation device (such as the electrical initiation element  20  shown in the schematic of  FIG. 1 ) is not shown in the schematic of  FIG. 7  for clarity. 
         [0063]    In operation, the thermal battery is activated by igniting the heat generating elements of the thermal battery core  51 ,  FIG. 2 , using the thermal battery activation initiator (not shown). As a result, the temperature of the thermal battery core  51  is increased and the solid electrolyte is melted, thereby enabling the thermal battery to provide electrical energy. In thermal batteries without the present “active insulation” material layer(s), the temperature of the thermal battery core will then begin to slowly decrease until the battery electrolyte begins to solidify, thereby rendering the thermal battery inactive. When the thermal battery core temperature nears its inactivation state, the provided “active insulation” material of the thermal battery can be initiated to provide heat to elevate the thermal battery core, thereby allowing the battery to stay active longer. When the aforementioned “porous silicon-based pyrotechnic” (active insulation) is used as the fuel and oxidizer mixture, following ignition of at least partial silicon sponge material, at least a portion of the silicon sponge material is converted to silica. As a result, firstly, heat is generated, which would have the beneficial effect of keeping the thermal battery operational longer or at least require lesser amounts of heat generating elements, thereby allowing the construction of relatively smaller thermal batteries that would stay operational the same length of time. Secondly, the conversion of the already substantially thermally insulating silicon sponge material to silica would generally increase its thermally insulating characteristics. As a result, the burning of the “porous silicon-based pyrotechnic” material also has the substantial effect of turning it into an effective thermal barrier while initially providing heat to the thermal battery core  51 . 
         [0064]    It is appreciated by those skilled in the art that at higher temperature levels, the thermal battery core would lose heat at higher rates. Thus, by allowing the added heat generated by the aforementioned “active insulation” material to be provided after the thermal battery core has cooled down to temperatures close to the inactivation temperature, the total amount of time that the thermal battery is going to stay active is increased. The embodiment  100  shown in the schematic of  FIG. 7  provides the means for such heat generation as the thermal battery is cooled, while the aforementioned division of the provided fuel and oxidizer mixtures into individual pockets that can be independently initiated would provide the following added significant benefits:
       It would not allow the aforementioned individual fuel and oxidizer mixtures to be initiated (burned) at any desired time following thermal battery activation. This provides the means of maximizing the thermal battery run time, i.e., the time duration during which the thermal battery is active and provides electrical energy to the system; and   It provides the opportunity to provide heat as needed to the regions of the activated thermal battery that has cooled to close to the electrolyte solidification temperature (i.e., thermal battery deactivation). In such embodiments, the thermal battery core temperature at its difference surface areas are measured to identify those regions at/around which the fuel and oxidizer mixture pocket should be initiated. In general, the thermal battery voltage and current (power) levels that can be provided can be used as an indication of the thermal battery core temperature. However, for the aforementioned heating by local fuel and oxidizer mixture pockets, the thermal battery core surface temperature needs to be measured at as many areas as practical to determine which fuel and oxidizer mixture pocket to be initiated to achieve optimal thermal battery performance, i.e., the maximum thermal battery run (activation) time.       
 
         [0067]    It will be appreciated by those skilled in the art that as the thermal battery core is cooled, the coldest regions of the thermal battery core will always be on its exterior surfaces. Thus, to determine the location (regions) of lowest thermal battery core temperatures, one would only need to monitor the thermal battery core surfaces. 
         [0068]    In a variation of the embodiment of  FIG. 7 , temperature sensors such as  118  shown in the schematic of  FIG. 7  are distributed over the surface of the thermal battery core  51 , preferably behind the insulation layer  71  that covers the surface of the thermal battery core (in the schematic of  FIG. 7  only one such temperature sensor  118  is shown for the sake of clarity, however, a plurality of the temperature sensors  118  can be provided at various positions). Such temperature sensors are generally required to be capable of measuring temperatures of up to around 500 degrees C., and are well known in the art, such as those based on thermocouples or changes in the resistance of electrical resistor elements. It will be appreciated by those skilled in the art that in general, only a few such temperature sensors are generally required for the purpose of determining at which location additional heat is to be provided to keep the thermal battery core activated, i.e., to keep the thermal battery electrolyte molten and above certain temperature. This is generally the case since by performing thermal modeling of the thermal battery, one can predict the cooling pattern of the thermal battery core and thereby ensure that the aforementioned temperature sensors are strategically positioned to detect temperature of these regions of the thermal battery core. When a cooler region (a region in which additional heat is determined necessary in order to keep the thermal battery electrolyte molten and above a certain predetermined temperature) is detected, a corresponding portion of porous silicon-based pyrotechnic material  101 ,  102 ,  103 ,  104 ,  105 ,  106  and  107  can be initiated with a corresponding initiation device  112 ,  113 ,  114 ,  115 ,  116  and  117 . 
         [0069]    In addition and/or in place of one or more of the temperature sensors  118 , the electrical initiators  112 - 117  are used to serve as temperature sensors before their activation to ignite the corresponding fuel and oxidizer mixture pockets. Most current electrical initiators are constructed as low resistance filaments (wires) that are heated by supplied current to initiate pyrotechnic materials. The actual resistance of such filaments is dependent on their temperature, and can therefore be used as temperature sensor by monitoring their electrical resistance. The advantages of using the electrical initiators as temperature sensors are that firstly the need for at least some of the temperature sensors is eliminated, and secondly, when a drop in the power produced by the thermal battery is detected and/or when thermal battery core temperature is detected to be closing to its inactivation temperature, then the fuel and oxidizer pocket(s) at which their electrical initiators show lowest temperatures, need only be initiated, noting that the actual temperature values are not required to determine which fuel and oxidizer mixture needs to be initiated. 
         [0070]    In another embodiment, particularly for smaller thermal batteries, one may only need to formulate a thermal model for the thermal battery core cooling following the battery activation and determine the sequence with which the fuel and oxidizer mixture pockets need to be initiated. 
         [0071]    In the embodiment  100  of  FIG. 7 , only one layer of fuel and oxidizer mixture is used for the purpose of illustration. It will be, however, appreciated by those skilled in the art that more than one such layer, divided in a number of individual pockets of various shapes (not all “ring type” as shown in the schematic of  FIG. 7 ) may also be used and provided with initiation devices to ignite individually or as groups of two or more. In fact, in certain applications where the thermal battery is not cylindrically shaped or has varying cross-sectional shapes, fuel and oxidizer mixture pockets of various shapes and sizes may be provided at various positions around the thermal battery core so that through their ignition at proper times the thermal battery core temperature could be kept above the battery inactivation temperature, thereby maximizing the thermal battery runtime. As a result, thermal batteries with relatively long runtime having a non-cylindrical shape or varying cross-section to match the available space in munitions can be effectively developed. 
         [0072]    As an example, consider the embodiment  150 , a thermal battery with a “D-shaped” cross-section as shown in the schematic of  FIG. 8 . The thermal battery  150  is constructed with the housing  151 , preferably made out of stainless steel, which houses the thermal battery core  152 . The space between the housing  151  and the thermal battery core is considered to be filled with an insulation material  153 . Pockets  154 ,  155 ,  156  and  157  of fuel and oxidizer mixture are distributed around the thermal battery core as shown in the schematic of  FIG. 8 , where the thermal battery core would tend to cool faster due to being further away from the center of the thermal battery core. The fuel and oxidizer pockets  154 ,  155 ,  156  and  157  are provided with individual (preferably electrical) initiators as shown in the schematic of  FIG. 7  or two or more are grouped together and each group is provided with an initiator (not shown in the schematic of  FIG. 8 ). Then when the thermal battery is activated and the thermal battery core begins to cool and its certain regions begin to cool down to close to the inactivation temperature of the thermal battery core, one or more of the fuel and oxidizer pockets  154 ,  155 ,  156  and  157  can be initiated to heat said region of the thermal battery and keep the battery activated. As a result, the runtime of such irregularly shaped thermal batteries can be significantly increased. 
         [0073]    In general, the cooling pattern of thermal batteries of various shapes and those with thermal battery cores of irregular shapes can be readily and closely determined through widely available computer modeling software and simulation of their transient thermal response following activation. Such cooling patterns can then be used to determine the optimal shape of the thermal battery core, insulation material and optimal size, shape and distribution of the fuel and oxidizer mixture filled pockets to achieve maximum battery runtime. 
         [0074]    In the embodiment  150  of  FIG. 8 , only one layer of thermal insulation material and only four pockets of fuel and oxidizer mixture are shown for the purpose of illustration. It will be appreciated by those skilled in the art that in practice, more than one layer of insulation material and more or less pockets of fuel and oxidizer mixture of various appropriate shapes and sizes may be used to achieve optimal thermal battery performance, including runtime and minimal total size to fit the available space in munitions. 
         [0075]    In another variation, that part of the surfaces of the cell core can be covered with the exothermic material as discussed above and another part of the surfaces of the cell core can be covered with commonly used thermal insulation material. This variation is particularly suitable when the shape of the thermal battery is such that certain regions cools faster than the others, for example, the sides of the “D-shaped” design in  FIG. 8 . 
         [0076]    In yet another embodiment, particularly for smaller thermal batteries, the “porous silicon-based pyrotechnic” material filled compartments (e.g.,  101 - 104  in the embodiment of FIGS.  7  and  154 - 157  in the embodiment of  FIG. 8 ) can be initiated sequentially a very short time after the thermal battery ( 100  and  150  in  FIGS. 7 and 8 , respectively). In such applications, it is generally not necessary to provide individual (e.g., electrically activated) initiation devices (e.g.,  112 - 117  in the embodiment  100  of  FIG. 7 ). Rather the initiation of the “porous silicon-based pyrotechnic” material filled compartments ( 101 - 104  in the embodiment of  FIG. 7 ) may still be required to be sequential (in any desired order and even more than one simultaneously), but only need to be delayed a certain amount of time. For such applications, the required delay time may be provided by means of delay fuses. As an example, such delay fusing may be provided to the embodiment  100  of  FIG. 7  as follows. 
         [0077]    The close-up view of a section of the thermal battery embodiment  100  indicated by the numeral  160  in  FIG. 7  is shown in the schematic  FIG. 9 . The thermal battery core  51  and casing  52  are shown in  FIG. 9  as well as the “active insulation” material layer  161  and  162  ( 102  and  103  in  FIG. 7 ) with the aforementioned “band”  163  ( 109  in  FIG. 7 ) of thermal insulations made out materials such as material used for the insulation layer  71  ( FIGS. 5 and 6 ). In the present embodiment, however, the insulation “band”  163  can be made from a more rigid insulation material such as those made from ceramics or the like. 
         [0078]    The cross-sectional view A-A of the section of the thermal battery shown in  FIG. 9  is shown in  FIG. 10 . In this embodiment, a delay, such as a fuse cord  164 , which can be pyrotechnic base, is embedded in the “band”  163  of thermal insulations, which can be formed of relatively rigid material such as ceramic or the like, preferably such that it is as far as possible from the batter cell  51 . Then once the “active insulation” material layer  161  has been ignited and begins to burn, then at some point one end  165  of the delay fuse cord  164  is ignited. The delay fuse cord  164  will then begin to burn, and depending on its length and burn rate, after a predetermined (delay) time, the flame reaches the other end  166  of the delay fuse cord  164  and the “active insulation” material layer  162  is ignited. It is appreciated by those skilled in the art that by selecting the proper fuse cord material and length, the user can design the aforementioned “active insulation” layers (pockets) to be sequentially (or more than one at the same time) be ignited after a desired time delay to maximize the thermal battery run time. 
         [0079]    It is appreciated by those skilled in the art that the first end of the above delay fuses  164  may be similarly ignited by a thermal battery initiator or by the thermal battery heat generating pyrotechnic material (heat pallets or the like) as the thermal battery itself is initiated. Similarly, delay fuse cords may be provided between any two or more “active insulation” pockets in a thermal battery, for example, between pockets  154 - 157  inside the insulation material  153  in the embodiment  150  of  FIG. 8 . 
         [0080]    It is also appreciated by those skilled in the art that the materials of the aforementioned “active insulation” may be formulated to make them burn at relatively slow rates. Such methods and mixtures for slowing the rate of burn in pyrotechnic material are well known in the art. Then when used in a thermal battery, each “active insulation” layer or pocket can generate heat over a relatively longer period of time than it would if it would burn very rapidly. In addition, local hot spots are also minimized. Such an embodiment is particularly useful in very small (miniature) thermal batteries since it would also eliminate the need for initiation and/or delay fuse cords and the “active insulation” material in such batteries may be ignited at the same time as the thermal battery or a very short time thereafter by employing one or more of the aforementioned delay fuse cords that are initiated at the time of thermal battery initiation as previously described by the thermal battery initiator or its heat generating pyrotechnic elements (heat pallets). 
         [0081]    It is also appreciated by those skilled in the art that other pyrotechnic materials may also be used in place of the aforementioned “porous silicon-based pyrotechnic” materials as “active insulation” materials in the different embodiments. In general, it is highly preferred that the pyrotechnic materials used provide good thermal insulation characteristics following ignition. The user must however consider that the amount of heat loss due to degradation of the thermal insulation properties that are encountered by replacing high grade insulation materials with pyrotechnic materials and its ignited by products is significantly less than the gain in the generated heat input due to the burning of the pyrotechnic material. 
         [0082]    While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.