The present invention relates to devices that generate electrical current and, in particular, to a composite structure and methods of making and utilizing the composite structure to convert ambient heat to electrical current. 
Over the years, many attempts have been made to harness energy from our environment in order to generate electricity.  As a result, numerous electrochemical and thermoelectrical devices have been developed to convert solar energy to electricity.  For example, attempts to convert solar energy to electricity have spawned some major technologies such as photovoltaic conversion devices.  The heat content of solar radiation emitting electromagnetic radiation and particles is used to provide heat for generating electricity. 
One attempt to achieve the conversion of solar energy to electricity is found in U.S. Pat.  No.  5,421,909 issued to Ishikawa et al.  which is directed to a photovoltaic device having a semiconductor layer, front and back electrodes, and a surface protection layer.  The photovoltaic device of Ishikawa et al.  converts electromagnetic radiation directly into electricity.  Photovoltaic devices, however, require the use of semiconducting materials to absorb electromagnetic radiation.  Semiconductor materials require a degree of care and technical expertise to produce and can be expensive. 
Another method of generating electricity is through the use of an electrochemical system, such as the electrode process which is the principle process in electrochemical batteries.  Important aspects of the electrode process include oxidation and reduction occurring as a result of electron transfer in coupled chemical reactions.  Coupled reactions are initiated by production or depletion of the primary products which are reactants at the electrode surface.  The chemical reaction utilized to produce electrical energy requires supplying electrons to an electrode forming a negative terminal and removing the electrons from the positive terminal.  In a lead storage battery, for example, electrons are supplied to a negative terminal by the oxidation of metallic lead.  At the positive terminal, lead is reduced.  The electrons flowing in an external circuit from the negative to the positive terminal constitute the desired electric current.  However, electrochemical systems utilize a redox reaction which ultimately deteriorates the source of chemical components in the systems. 
Examples of efforts to generate electricity through the use of an electrochemical system such as electrode processes in batteries have been described in U.S. Pat.  Nos.  3,837,920, 4,188,464, 4,892,797, 5,279,910, and 5,419,977.  More specifically, U.S. Pat.  No.  3,837,920 to Liang et al.  is directed to a battery containing a solid electrolyte, an alkali metal anode, and a heavy metal cathode.  The battery of the Liang et al. ' 920 references utilizes a redox reaction. 
U.S. Pat.  No.  4,188,464 to Adams et al.  is directed to a composite electrode in bipolar electrolytic cells.  The electrode includes an intermediate graphite layer interposed between two polymeric layers.  Each side of the polymeric layers is in contact with an anode layer and a cathode layer.  The electrode, however, functions as a battery and involves electrolysis. 
U.S. Pat.  No.  4,892,797 to Ran et al.  involves a bipolar electrode for electrochemical cells and process for manufacturing the same.  The bipolar electrode contains an electrically conductive intermediate layer interposed between an electronegative layer and an electropositive layer.  The intermediate layer is a plastic substrate which includes electrically conductive particles.  The electronegative layer can be silver coated nickel particles or aluminum.  The electropositive layer can be a metal such as silver, copper, nickel, and lead.  The bipolar electrode of the Ran et al. ' 797 reference, however, functions in electrochemical cells requiring a redox reaction. 
U.S. Pat.  No.  5,279,910 to Cysteic et al.  is directed to an improved battery for reversible operation at ambient temperature.  The battery includes a negative electrode, a composite positive electrode, an electrochemically active material, an electrolyte, and optionally an electron conductive material.  The Cysteic et al. ' 977 battery requires a redox reaction. 
U.S. Pat.  No.  5,419,977 to Weiss et al.  is directed to an electrochemical device for production of electrical energy.  The electrochemical device involves operatively combined capacitors which can increase capacitance density and energy storage capability.  The electrochemical device of the Weiss et al. ' 977 reference requires a redox reaction. 
Another method for direct conversion of heat into electrical energy is via thermoelectrical devices based on the Seebeck effect, Peltier effect, and Thomson effect.  The Seebeck effect concerns electromotive force (EMF) generated in a circuit composed of two different conductors whose junctions are maintained at two different temperatures (e.g., hot and cold junctions). 
Peltier effect generates temperature differences from electrical energy.  Peltier effect refers to the reversible heat generated at the junction between two different conductors when current passes through the junction.  One of the conductors is connected to a cold junction and the other conductor is connected to a hot junction. 
Thomson effect involves the reversible generation of heat in a single current-carrying conductor along which a temperature gradient is maintained.  Thomson heat is proportional to the product of the current and the temperature gradient.  Thomson heat is referred to as reversible in the sense that the conductor changes from a generator of Thomson heat to an absorber of Thomson heat when the direction of either the current of the temperature gradient is reversed. 
Some examples of thermoelectrical devices are thermocouples (e.g., P-type thermoelectric conversion materials) and thermoelectric materials consisting of an oxide with perovskite structure.  Thermoelectrical devices, however, are known to have disadvantages of relatively low efficiencies and high cost per unit of output.  Examples of efforts to generate electricity via thermoelectrical devices are disclosed in U.S. Pat.  Nos.  4,969,956 and 5,057,161.  U.S. Pat.  No.  4,969,956 to Kreider et al.  is directed to a transparent thin film thermocouple and a method of manufacturing the same.  The thermocouple of Kreider et al.  includes a positive element of indium tin oxide and a negative element of indium oxide formed on a surface by reactive sputtering with the elements being electrically joined to form a hot junction for conversion of heat into electricity.  The reactive sputtering is accomplished with a magnetron source in an argon and oxygen atmosphere. 
U.S. Pat.  No.  5,057,161 to Komabayashi et al.  is directed to a p-type iron silicide thermoelectric conversion material.  The thermoelectric conversion material patent involves Seebeck effect which requires two different temperatures. 
Each of the technologies set forth above require conditions which, to a certain extent, constrain their use for a simple current-producing device which operates as a mere function of ambient temperature.  It is therefore an object of the present invention to provide a composite structure and a method for generating electrical current which overcomes the disadvantages generally associated with the prior art.