Patent Publication Number: US-2021166926-A1

Title: Electric Power Source Employing Field Emission

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Provisional Patent application for ELECTRIC POWER SOURCE WITH FIELD EMISSION, Filing Date Oct. 9, 2019 and 
     Provisional Patent application for THERMIONIC CONVERTER WITH FIELD EMISSION, Filing Date Dec. 3, 2018. 
     FEDERALLY SPONSORED RESEARCH 
     NONE 
     SEQUENCE LISTING 
     None 
     BACKGROUND OF THE INVENTION 
     This disclosure relates generally to an apparatus for generating electric power and more specifically to such an apparatus that produces electric power using heat from the environment as an energy source and employs electron field emission to complete an electric circuit. 
     This disclosure makes use of an attribute of conductors and semiconductors known as work function. Free electrons within a conductor or semiconductor material are held inside the material by an electric field barrier at the surface. Work function is most generally thought of as the amount of energy that a free electron within a material must be given so that it has enough kinetic energy to pass through the barrier and escape to the outside (see e.g. John D. McGervey in “Modern Physics”, 1971, page 72). Work function is usually expressed in electron volts. 
     The voltage that drives an electric current in this disclosure arises spontaneously when a low work function material and a higher work function material are connected by a wire. Electrons will flow from the low work function material to the higher work function material. 
     In this disclosure, electron field emission allows a current of electrons to cross a small gap between electrodes in order to complete a circuit. Electron field emission is quantum mechanical in nature and happens when a strong electric field is created at a material&#39;s surface and is oriented so as to pull on electrons near the surface, making the naturally occurring electric field barrier at the surface much thinner. Electrons can then tunnel through the thin surface barrier and move out of the material without being given kinetic energy equal to the work function in order to push through the barrier (see e.g. A. J. Dekker in “Solid State Physics”, 1959, pages 220 and 227). 
     DESCRIPTION OF THE RELATED ART 
     This disclosure is similar in many ways to a type of thermionic energy converter that operates with a high temperature source of heat. In both, potential energy brought about because of a difference in work functions between two materials drives an electric current. This potential energy is responsible for the contact potential difference or Volta potential that occurs when different conductor or semiconductor materials are brought into contact or are connected by a wire (see e.g. A. J. Dekker in “Solid State Physics” page 230). 
     The principle components of a high temperature thermionic energy converter are two electrodes that are made of different materials and are placed very close to each other. The materials that the two electrodes are made of have significantly different work functions. The two electrodes are connected peripherally by a wire. A source of heat is positioned near the higher work function electrode. The heat source should, in general, have a temperature above 1500 degrees K, (see e.g. David B. Go et al. in “Frontiers in Mechanical Engineering”, 8 Nov. 2017, page 3). There are lower temperature thermionic converters that develop a voltage through a different mechanism (see e.g. Khalid et al. In “IEEE Transactions on Electron Devices”, vol. 63, no. 6, June 2016, page 2232). 
     Without the heat source, there is an exchange of electrons through the wire in both directions between the lower work function electrode and the higher work function electrode in which the transfer of electrons is predominately from the lower to the higher work function electrode. As the electrons flow and a greater density of electrons gather in the higher work function electrode, an electric field arises that decreases the movement of electrons from the lower to the higher work function electrode and increases the electron flow from the higher to the lower work function electrode until an equilibrium is attained in which the electron flow in one direction equals the electron flow in the other direction. The difference in electric potential between the lower and the higher work function electrodes when the electron flow becomes equal is called the contact potential difference (see McGraw-Hill Dictionary of Scientific and Technical Terms, 6E, Copyright 2003). 
     With the heat source near the high work function electrode, even though the greater proportion of conduction electrons are moving through the wire from the lower work function electrode to higher work function electrode than in the other direction, an electric potential difference doesn&#39;t build up. Heat in the heated high work function electrode gives some electrons an amount of kinetic energy equal to or greater than the work function of the electrode material so that the electrons experience thermionic emission from the surface of the heated electrode into the space between the two electrodes. 
     The emitted electrons move across the small gap and enter the lower work function electrode completing an electric circuit. The electrons that have entered the lower work function electrode become part of the current of electrons that flows through the wire connecting the two electrodes and once again enter the higher work function electrode and once again are emitted. The higher work function electrode can be called the electron emitter and the lower work function electrode can be the electron collector. The current will flow as long as the heat source supplies energy to drive electrons out of the electron emitter. 
     As the current flows, electric power can be drawn by some device connected along the wire that links the two electrodes. The maximum voltage available depends on the difference in work functions between the electron collector and the electron emitter (see e.g. David B. Go et al. in “Frontiers in Mechanical Engineering”, 8 Nov. 2017, page 3). 
     It may appear that the source of energy that allows a high temperature thermionic converter to operate as an electric power source is heat added to the electron emitter. That is not the case. The difference in work functions creates a voltage to drive the electric current. The added heat serves a different function. 
     As in other electric power generation cycles, such as the rankine cycle, the working fluid (conduction electrons in this case of this disclosure) that has performed useful work must have its entropy returned to a low level that can be considered the starting point of a thermodynamic cycle. The heat added to the electron emitter in a high temperature thermionic converter fulfills the entropy reduction requirement by releasing electrons from the electron emitter so that they become lower entropy electrons in the space between the electron emitter and the electron collector. In the disclosure, unlike a high temperature thermionic energy converter, the necessary release of electrons as an entropy reduction step takes place through electron field emission rather than thermionic emission. 
     As will be more fully explained in the description of the disclosure, the actual energy source for a high temperature thermionic converter is not a high temperature heat source. The energy is found when electrons move from one material to another material that is in physical contact. Electrons must gain or lose kinetic energy so that they possess a kinetic energy equal to the Fermi energy of the material they are entering. The electrons that cross a boundary between materials exchange energy with lattice ions at the interface between the two materials so that the lattice become cooler or warmer as the electrons lose or gain kinetic energy. 
     DESCRIPTION OF THE INVENTION 
     The device of this disclosure can be built using the techniques of micro and nano manufacturing that are used to build such devices as microprocessors and nanotubes. As in a high temperature thermionic energy converter, voltage in the present invention is produced due to a difference in work function between two different materials. 
     In the present disclosure, a lower work function electrode and a higher work function electrode are placed very close to each other. The distance between them could be on the order of 100 nanometers or less. The higher work function electrode will be the electron emitter and the lower work function electrode will be the electron collector. The space between the electron collector and electron emitter should be in vacuum or at a very low pressure. 
     The electron emitter and the electron collector are connected peripherally by a wire. A load resistance is connected along the wire. The load resistance can be any device in need of electric power to function. As noted in the description of a thermionic energy converter, conduction electrons will spontaneously flow out of the lower work function electron collector through the wire, through the load resistance, and continue by wire into the higher work function electron emitter. 
     The driving mechanism for the spontaneous electron flow is what creates the voltage in this disclosure as well as in the thermionic energy converter. The mechanism can be seen when any two conducting or semiconducting materials with different work functions are brought together. At the interface between two metals, for example, conduction electrons move freely between the two materials. However, the number of electrons crossing the interface over time is not equal in both directions. Electrons will tend to stay in the higher work function material because lattice ions there provide a greater amount of electron screening than is provided by lattice ions in the material having the lower work function so that conduction electrons don&#39;t interact with each other as strongly and can gather more densely in the higher work function material. As in all spontaneous physical processes, the movement of electrons into the higher work function electron emitter is marked by an increase in entropy for the electrons that have traveled from the lower work function material of the electron collector. 
     The electrons that flowed from the electron collector, through wires and the load resistance arrive at the electron emitter. Electrons build up in the electron emitter, creating an electric potential difference between the electron emitter and the very nearby electron collector (see e.g. A. J. Dekker in “Solid State Physics” page 230) and therefore creating an electric field between the electron emitter and electron collector. 
     In order for a continuous current to flow in the present disclosure, free electrons in the electron emitter must move across the small gap to the electron collector, completing a circuit. The gap could be substantially in vacuum or contain a low pressure gas. The gap could also contain a material that allows ballistic transport of electrons. The release of electrons from the electron emitter will be accomplished through the impetus of the electric field described above, without adding heat at the emitter. 
     The electron emitter and electron collector are placed so close together that the electric field between them can be very intense. If the work function difference between the electron emitter and the electron collector is one electron volt then the electric potential difference would be one volt. If the electron emitter and electron collector are shaped to present flat surfaces toward each other and are 0.1 microns apart, a one volt electric potential difference would cause an electric field between the emitter and the collector that would have a strength of 10 volts per micron (diminished by the Schottky effect). This electric field strength is enough to alter the electric field barrier at the surface of the electron emitter that holds free electrons within the electron emitter. The surface barrier becomes thinner (see e.g. A. J. Dekker in “Solid State Physics” page 230). 
     The thin surface barrier allows conduction electrons within the electron emitter to undergo quantum mechanical tunneling through the surface barrier. The tunneling through the barrier constitutes electron field emission. The emitted electrons are then drawn by the electric field to cross to the electron collector and complete the circuit, allowing a continuous electric current to flow through the load resistance, making this disclosure a continuous electric power source. 
     The number of electrons that are emitted and cross to the electron collector depends on the strength of the electric field between the electron emitter and the electron collector. The electric field strength can be increased by placing the emitter and collector closer together. Alternatively, the strength of the electric field at the emitting surface of the electron emitter can be increased by shaping the electron emitter so that the emitting surface facing the electron collector has a small radius of curvature. The electron emitter could be shaped so as to face the electron collector with a sharp point or a thin edge. The emitter or group of emitters could also be nanorods. 
     Materials with different work functions also have different Fermi energies. Electrons moving between two different materials in contact must gain or lose kinetic energy in order to cross the interface between materials. Electrons gain or lose kinetic energy by taking heat from the lattice or giving heat to the lattice. Considering materials that have significantly different work functions and could be used as the electron emitter and the electron collector in the present disclosure, the electron emitter with its higher work function would have a Fermi energy that would be higher than the Fermi energy of the electron collector. As electrons move from the electron collector, through wires, through a load resistance and into the electron emitter, the electrons must gain or lose kinetic energy as they enter each new material at the Fermi energy of that material. The net result is that each electron that starts a journey in the electron collector and ends in the electron emitter will have, on average, drawn heat energy from the materials it passed through. The heat is the energy source that allows a difference in work functions to drive a current in the disclosure. Heat drawn from the interfaces between materials in a thermionic energy converter is also the energy source for that electric power source. 
     The heat that the device of this disclosure gathers at the junctions between different materials can come from any source. While this disclosure is intended to operate by taking heat from the local environment, which ultimately comes from the sun, the heat could come from such sources as fossil fuels, nuclear or geothermal. The heat could also come from an engineered gathering of solar energy as with lenses or mirrors or dark colored absorbers. 
     At extremely low temperatures there would be very little heat available for conduction electrons to gain enough kinetic energy to move from a low Fermi energy electron collector to a higher Fermi energy electron emitter. At such low temperatures, zero point energy would be the energy source that would allow electrons to enter the electron emitter. 
     A material for the electron emitter would be chosen from a group with relatively high work functions. Some examples of materials that the electron emitter could be made of or coated with are: carbon, carbon nitride, tungsten, tantalum, molybdenum, rhenium, osmium, platinum, nickel, silicon, doped silicon, or a mixture thereof. 
     There are also many choices of material for the electron collector. Some examples of materials with relatively low work functions that the electron collector could be made of or coated with are: lanthanum, lanthanum hexaboride, cerium, cerium hexaboride, barium, barium carbonate, barium oxide, cesium, silicon, doped silicon, or a mixture thereof. 
     As with any other direct current power source, such as a chemical battery, individual examples of the present disclosure can be connected in parallel to provide a higher electric current in a circuit. 
     Additionally, while the heat energy taken in at the junctions between higher and lower work function materials allows this disclosure to function, it also cools the junctions between materials and anything near the junctions, allowing this disclosure to function as a cooling device. 
     While this disclosure is intended to describe a power source that doesn&#39;t require a heated electron emitter, a heat source near the electron emitter would raise the kinetic energy of some free electrons in the electron emitter and would increase the rate of electron field emission. 
     The foregoing description is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  illustrates a perspective drawing of an embodiment of the disclosure with flat, close spaced electron emitter and electron collector. 
         FIG. 2  illustrates a schematic drawing of an embodiment of the disclosure in which the electron emitter has a sharp point. 
         FIG. 3  illustrates a schematic drawing of an embodiment of the disclosure in which the electron emitter is a nanorod. 
         FIG. 4  illustrates a perspective drawing of an embodiment of the disclosure in which the electron emitter and the electron collector are in the shape of rectangular solids on a substrate. 
         FIG. 5  illustrates a perspective drawing of an embodiment of the disclosure in which the electron emitter and the electron collector are in the shape of rectangular solids that are held above a substrate by spacers. 
         FIG. 6  illustrates a perspective drawing of an embodiment of the disclosure with a heat source. 
     
    
    
     REFERENCE NUMERALS IN DRAWINGS 
     
         
           11  electron emitter 
           12  electron collector 
           13  load resistance 
           14  emitter wire 
           15  collector wire 
           16  pointed electron emitter 
           17  pointed electron emitter base 
           18  nanotube emitter 
           19  nanotube emitter base 
           20  long electron emitter 
           21  long electron collector 
           22  substrate 
           23  raised electron emitter 
           24  raised electron collector 
           25  emitter spacer 
           26  collector spacer 
           27  heat source 
       
    
     DETAILED DESCRIPTIONS OF FIGURES 
       FIG. 1  shows an electron emitter  11  facing an electron collector  12 . The surface material of electron emitter  11  has a higher work function than the surface material of electron collector  12 . Electron emitter  11  is connected by an emitter wire  14  to a load resistance  13 . Load resistance  13  can be any device that requires electric power. Electron collector  12  is connected by a collector wire  15  to load resistance  13  resulting in an electrical connection between electron emitter  11  and electron collector  12 . The electrical connection between electron emitter  11  and electron collector  12  allows electrons to spontaneously flow, due to the difference in work functions, from electron collector  12  to electron emitter  11  creating an electric field between electron emitter  11  and electron collector  12 . The electric field created between electron emitter  11  and electron collector  12  causes field emission of electrons from electron emitter  11 . The emitted electrons cross the small vacuum gap to electron collector  12 , completing the electric circuit. The complete circuit allows a continuous current to flow and provide electric power to load resistance  13 . 
       FIG. 2  is a cutaway view that shows a pointed electron emitter  16  facing electron collector  12 . Pointed electron emitter  16  is composed of a material that, at the surface of the sharp tip, has a higher work function than the surface material of electron collector  12 . Pointed electron emitter  16  is held in place by a pointed electron emitter base  17 . Pointed electron emitter base  17  is made of a material that will conduct electricity and is connected by emitter wire  14  to load resistance  13 . Load resistance  13  can be any device that requires electric power. Electron collector  12  is connected by collector wire  15  to load resistance  13  resulting in an electrical connection between electron collector  12  and pointed electron emitter  16 . The electrical connection between electron collector  12  and pointed electron emitter  16  allows electrons to spontaneously flow, due to the difference in work functions, from electron collector  12  to pointed electron emitter  16  creating an electric field between electron collector  12  and pointed electron emitter  16 . The tip of pointed electron emitter  16  has a small radius of curvature which causes an enhancement of the electric field strength. The electric field strength at the tip of pointed electron emitter  16  would be greater than that at the flat emitting surface of electron emitter  11  of  FIG. 1 . The electric field created between pointed electron emitter  16  and electron collector  12  causes field emission of electrons from pointed electron emitter  16 . The emitted electrons cross the small vacuum gap to electron collector  12 , completing the electric circuit. The complete circuit allows a continuous current to flow and provides electric power to load resistance  13 . 
       FIG. 3  is a cutaway view that shows a nanotube emitter  18  facing electron collector  12 . Nanotube emitter  18  is composed of a material that has a higher work function than the material of electron collector  12 . Nanotube emitter  18  is held in place by a nanotube emitter base  19 . Nanotube emitter base  19  is composed of a material that will conduct electricity and is connected by emitter wire  14  to load resistance  13 . Load resistance  13  can be any device that requires electric power. Electron collector  12  is connected by collector wire  15  to load resistance  13  resulting in an electrical connection between electron collector  12  and nanotube emitter  18 . The electrical connection between electron collector  12  and nanotube  18  allows electrons to spontaneously flow, due to the difference in work functions, from electron collector  12  to nanotube emitter  18  creating an electric field between electron collector  12  and nanotube emitter  18 . The tip of nanotube emitter  18  facing electron collector  12  has a small radius of curvature which causes an enhancement of the electric field strength. The electric field strength at the tip of nanotube emitter  18  would be greater than that at the flat emitting surface of electron emitter  11  of  FIG. 1 . The electric field created between nanotube emitter  18  and electron collector  12  causes field emission of electrons from nanotube emitter  18 . The emitted electrons cross the small vacuum gap to electron collector  12 , completing the electric circuit. The complete circuit allows a continuous current to flow and provides electric power to load resistance  13 . 
     In  FIG. 4 , a long electron emitter  20  sits on a substrate  22 . Substrate  22  is composed of an insulating material. A long electron collector  21  sits on substrate  22  and faces long electron emitter  20 . Long electron emitter  20  is composed of a material that at has a higher work function at its surface than the material at the surface of long electron collector  21 . Long electron emitter  20  is connected by emitter wire  14  to load resistance  13 . Load resistance  13  can be any device that requires electric power. Long electron collector  21  is connected by collector wire  15  to load resistance  13  resulting in an electrical connection between long electron collector  21  and long electron emitter  20 . The electrical connection between long electron collector  21  and long electron emitter  20  allows electrons to spontaneously flow, due to the difference in work functions, from long electron collector  21  to long electron emitter  20 , creating an electric field between long electron collector  21  and long electron emitter  20 . The edges of long electron emitter  20  have a small radius of curvature, which causes an enhancement of the electric field strength along the length of each edge of long electron emitter  20  with respect to the flat surfaces of long electron emitter  20 . The electric field created between long electron emitter  20  and long electron collector  21  causes field emission, principally from the edges of long electron emitter  20 . The emitted electrons cross the small vacuum gap to long electron collector  21 , completing the electric circuit. The complete circuit allows a continuous current to flow and provides electric power to load resistance  13 . 
       FIG. 5  shows an electric power source in which a raised electron emitter  23  faces a raised electron collector  24 . Raised electron emitter  23  sits on an emitter spacer  25 . Raised electron collector  24  sits on a collector spacer  26 . Emitter spacer  25  and collector spacer  26  sit on substrate  22 . Emitter spacer  25  and collector spacer  26  are composed of an insulating material. Substrate  22  is composed of an insulating material. 
     The surface of raised electron emitter  23  is composed of a material that has a higher work function than the material that composes the surface of raised electron collector  24 . Raised electron emitter  23  is connected by emitter wire  14  to load resistance  13 . Load resistance  13  can be any device that requires electric power. Raised electron collector  24  is connected by collector wire  15  to load resistance  13  resulting in an electrical connection between raised electron collector  24  and raised electron emitter  23 . The electrical connection between raised electron collector  24  and raised electron emitter  23  allows electrons to spontaneously flow, due to the difference in work functions, from raised electron collector  24  to raised electron emitter  23 , creating an electric field between raised electron collector  24  and raised electron emitter  23 . The edges of raised electron emitter  23  have a small radius of curvature, which causes an enhancement of the electric field strength along the length of each edge of raised electron emitter  23  with respect to the flat surfaces of raised electron collector  23 . The electric field created between raised electron emitter  23  and raised electron collector  24  causes field emission, principally from the edges of raised electron emitter  23 . The emitted electrons cross the small vacuum gap to raised electron collector  24 , completing the electric circuit. The complete circuit allows a continuous current to flow and provides electric power to load resistance  13 . 
       FIG. 6  shows electron emitter  11  facing electron collector  12 . The surface material of electron emitter  11  has a higher work function than the surface material of electron collector  12 . A heat source  27  is positioned near electron emitter  11 , raising the temperature of electron emitter  11 . Electron emitter  11  is connected by emitter wire  14  to load resistance  13 . Load resistance  13  can be any device that requires electric power. Electron collector  12  is connected by collector wire  15  to load resistance  13  resulting in an electrical connection between electron emitter  11  and electron collector  12 . The electrical connection between electron emitter  11  and electron collector  12  allows electrons to spontaneously flow, due to the difference in work functions, from electron collector  12  to electron emitter  11  creating an electric field between electron emitter  11  and electron collector  12 . The electric field created between electron emitter  11  and electron collector  12 , aided by heat added to electron emitter  11  by heat source  27 , causes field emission of electrons from electron emitter  11 . The emitted electrons cross the small vacuum gap to electron collector  12 , completing the electric circuit. The complete circuit allows a continuous current to flow and provide electric power to load resistance  13 .