Separator in electrochemical heating element
BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an electrochemical heating element according to this invention. 
     FIG. 2 is a side sectional view of FIG. 1 taken at section 2--2 as indicated in FIG. 1. 
     FIG. 3 is a top sectional view taken at section 3--3 as indicated in FIG. 2. 
     FIG. 4 is a perspective view of a preferred alternate embodiment of the invention. 
     FIG. 5 is a sectional view of FIG. 4 taken at section 5--5 as indicated in FIG. 4. 
     FIG. 6 is a similar sectional view of a related embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the drawings, showing different preferred embodiments of this invention, like numerals designate like parts. 
     FIG. 1 is a perspective view showing an electrochemical heating element 10 according to this invention. Element 10 has three layers including a cathode layer 12, an adjacent separator layer 14, and an anode layer 16 adjacent to separator layer 14. Each layer is in intimate contact at its surface with the surface of the adjacent layers, this holding true for both surfaces of separator layer 14. 
     Cathode layer 12 includes an electrochemically active, nonmetallic, reducible substance which is conductive. Cathode layer 12 need not be formed of a reducible substance but may provide an electrochemically active surface upon which another material, for example, oxygen on an activated carbon-air electrode, is reduced. Cathode materials may be formed of a wide variety of substances such as manganese dioxide, metadinitrobenzene, silver chloride, silver oxide, copper fluoride, copper chloride and air depolarized cathode structures of the carbon and metal type. 
     The material for anode layer 16 can be selected from those metals and alloys which are known to be electrochemically active, for example, zinc, aluminum, magnesium, cadmium, lead, or alloys thereof. Anodes of aluminum and magnesium or their more common alloys are preferred because of their high inherent energy content and lack of concern for toxicity. The anode structure can take the form of thin metallic sheets or foils, powders, chips, granules or turnings pressed or rolled into a suitable conductive plate. 
     Separator layer 14 is formed of a non-conductive, porous, absobent material such as cotton, felt, or bibulous papers, which enable ions of an electrolyte to freely pass between the anode layer and the cathode layer. The separator material is sized to absorb and hold a sufficient amount of electrolyte solution between the electrodes to sustain the high rate electrochemical reaction to completion. 
     An electrolyte formed of an ionically conductive medium is placed within separator layer 14. The electrolyte may be an aqueous salt solution such as table salt (NaCl), or may be selected from a host of many other well known electrolyte materials. In those applications for which extremely high heat output is essential, highly acid or alkaline electrolytes can be used to great advantage. For example, water can be used in combination with a lithium metal anode, the electrolyte being lithium hydroxide which is produced spontaneously upon contact of the water with the lithium. This extremely high energy reaction could find use where high heat output per unit weight and area of heater is required. However, for the wide range of more common potential applications for the electrochemical heater, electrolytes consisting of an aqueous solution of sodium or magnesium chloride are preferred. 
     An electrolyte solution may be introduced into separator layer 14 in a number of ways. An electrolyte salt may be contained within the separator material in dry form, which when contacted with water dissolves to form the aqueous electrolyte solution. Alternatively, the dry salt can be intermixed or dispersed within the cathode or anode active materials. In both such cases, the activation of the heater element is by simple introduction of water and subsequent dissolving of the dry salt to form an electrolye within the separator material. Or, an aqueous electrolyte solution can be used directly for heater activation, that is, without any dry salt contained within the heater structure. Combinations of the above can also be used to good advantage. The placement of dry electrolyte salt within the heater, and activation with water or salt solution is governed by the speed at which it is desired for the reaction to initiate. For example, if a salt solution is used for heater activation the electrochemical reaction is initiated essentially instantaneously. On the other hand, if water is used for activation, the dry salt contained within the heater element must first dissolve before the electrochemical reaction can begin generating heat at the desired rate. 
     As illustrated best in FIGS. 2 and 3, electrically conductive connector means 18 extend through the three-layered element, electrically connecting anode layer 16 and cathode layer 12 through separator layer 14. Connectors 18 are sized to support the short-circuiting current produced when the electrochemical heating element is activated. Connectors 18, which are integrally contained as part of the element, serve a dual purpose: 1) holding the overall heater sandwich structure together -- keeping the individual layers in proper juxtaposition to one another, and 2) providing an internal short-circuiting means between the anode and cathode structures. Consequently, the fastening means must be mechanically strong while at the same time being electrically conductive. The fastening means may be selected from metal rivets, metal wire or staples, conductive carbon thread or similar materials. From the standpoint of heater performance, economics and ease of production, metal wire or staples are preferred. 
     Interposed in the plane between cathode layer 12 and separator layer 14, and separator 14 and anode layer 16 are structural spacing means in the form of discrete mechanical spacers 20, which are placed immediately adjacent to connectors 18. Spacers 20 are in the form of small, electrically inert, square pads which having been placed in the appropriate location and pierced by connectors 18 as connectors 18 interconnect layers 12, 14 and 16. Spacers 20 may be made of plastics, paper, cardboard, rubber, and a wide variety of other materials. Spacers 20 need not be electrically inert. A wide variety of metallic and nonmetallic materials would be suitable. Electrically inert materials are preferred since such could not interfere in any way with the short-circuiting function provided by connectors 18 in electrochemical heating element 10. 
     As is best illustrated in FIG. 3, spacers 20 cover an essentially negligible portion of the surface area of element 10. Spacers 20 serve to concentrate and focus the compression, caused by the interconnection of the layers, on specific points eliminating the more even distribution of compression across the element which can substantially interfere with the absorptive capacity of separator layer 14. 
     FIGS. 4 and 5 illustrate a highly preferred embodiment of this invention. The electrochemical heating element shown in these figures has cathode layer 12, separator layer 14, anode layer 16 and connectors 18 as in the previously described embodiment. However, the structural spacing means interposed between the electrode layers 12 and 16 includes cathode layer 12 being formed to define a recessed surface portion 22 as shown best in FIG. 5. Recessed surface portion 22 is in the form of a square recess centrally located on cathode layer 12 as illustrated in FIG. 4. Separator layer 14 includes a mating or wick portion 24 on its upper surface, as shown in FIG. 5, which is shaped to be approximately complementary to recessed surface portion 22 of cathode layer 12. Mating or wick portion 24 is received into recessed surface portion 22. 
     Connectors 18 are placed through the heating element in areas adjacent recessed surface portion 22 and mating or wick portion 24. Wick 24, by virtue of the recessing of a portion of cathode layer 12 and the positioning of connector means 18, will remain substantially uncompressed. Therefore, wick 12 is able to absorb and hold an adequate amount of electrolyte solution during activation of the element. In effect, the wick structure stands &#34;free&#34; within the heater element assembly, although in contact with the recessed surface of cathode layer 12. It should be noted that for large sized heater elements the wick could be located or held in place with a staple or fastener, provided the bulk of the wick structure remains uncompressed. 
     FIG. 6 illustrates a complex element 26 having five layers including two cathode layers 12 and two separator layers 14. Each adjacent cathode layer and separator layer are formed and operate in the same manner as the cathode and separator layer of the element illustrated in FIGS. 4 and 5. Complex element 26, however, has additional connector means 28 which connect one pair of adjacent cathode and separator layers with anode layer 16 in a subassembly which is later joined to the other pair of adjacent cathode and separator layers. 
     Experiments have shown that the wicking structure need not take the rectangular shape illustrated in FIGS. 4 through 6. Numerous other shapes and sizes have been evaluated and all have proven to be effective. Circular, trapezoidal, triangular and irregular shapes are examples. The rectangular wick illustrated in the drawings is preferred because of its ease of handling in production and its cost effectiveness in utilization of raw materials. A preferred variation of the rectangular wick illustrated in drawing FIGS. 4 through 6 is a recessed surface portion and mating portion which are rectangular but extend completely across the element in one direction, forming, in effect, a stripe thereacross. 
     In especially preferred embodiments, the recessed surface portion and mating portion are substantially centrally located on the heating element as shown in FIGS. 4 through 6. It is preferred that the width of such surface portion 22 and mating portion 24 be within the range of about one-eighth to one-half of the width of the heat-generating element and most preferred that it be on the order of about one-third of the width of the element, at least in one direction. The recessed surface portion and mating portion need not be centrally located. Other locations or a multiplicity of locations would be acceptable. However, the preferred centrally located wicking structure in the preferred size range has shown itself to be most advantageous in achieving efficient heat output and reproducibility thereof. 
     One practical application of this invention is the heating of pre-packaged food such as the type referred to as &#34;retort&#34; packaged food. The following data, determined by the actual heating of 5 ounce (142 g) portions of retort packaged food, will show the advantages and benefits of the most preferred embodiment of this invention, the electrochemical heating element including a wicking structure. In all cases, the food samples were temperature stabilized at 4° C (39.2° F), and the heater elements were activated using a 23.3% aqueous solution of sodium chloride. In developing these data, simple elements of the type illustrated in FIGS. 4 and 5 were utilized as well as simple elements having no wicking structure. In each case the elements had 9 sq in of surface area and 3.5 g magnesium foil anode layers. Total BTU output was measured for 15 minutes and 20 minutes. 
     The BTU outputs for 15 minutes ranged from 12-21 BTU without a wick and from 21-23 BTU with a wick. Over 20 minutes the improvement provided by the invention raised the BTU output range from 15-24 to 26-28. 
     These and other data show that the spread in experimental results is substantially reduced by use of the wick structure. It also illustrates that the total heat output may be substantially increased by use of the wick structure. Such reproducibility and increased heat output are extremely important in a wide variety of applications for electrochemical heat generating elements. 
     While in the foregoing specification, this invention has been described in relation to certain preferred embodiments, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.