Patent Number: 
Section: claims

1. A thermionic (TI) power cell, comprising:a layer of radioactive material that generates heat due to radioactive decay;a layer of electron emitting material disposed on the layer of radioactive material; anda layer of electron collecting material, wherein the layer of electron emitting material is physically separated from the layer of electron collecting material to define a chamber between the layer of electron collecting material and the layer of electron emitting material, wherein the chamber is substantially evacuated to permit electrons to traverse the chamber from the layer of electron emitting material to the layer of electron collecting material, and further wherein heat generated over time by the layer of radioactive material causes a substantially constant flow of electrons to be emitted by the layer of electron emitting material to induce an electric current to flow through the layer of electron collecting material when the layer of electron collecting material is connected to an electrical load. 2. The TI power cell of claim 1, wherein the layer of electron emitting material includes an array of spikes to concentrate the electrons emitted by the layer of electron emitting material at tips of the spikes, wherein the tips of the spikes point toward the layer of electron collecting material. 3. The TI power cell of claim 2, wherein the tips of the spikes are separated from one another by a lateral spacing of approximately 100 micrometers to approximately 1 millimeter. 4. The TI power cell of claim 2, wherein the chamber is defined by a gap of approximately 100 micrometers to approximately 1 millimeter between the tips of the spikes and the layer of electron collecting material. 5. The TI power cell of claim 2, further comprising a layer of spacer material disposed between the layer of electron emitting material and the layer of electron collecting material,wherein the layer of spacer material exposes the tips of the spikes so that the exposed tips are disposed in the chamber. 6. The TI power cell of claim 1, further comprising:a first terminal coupled to the layer of electron emitting material; anda second terminal coupled to the layer of electron collecting material to induce the electric current from the first terminal to the electrical load and from the electrical load to the second terminal. 7. The TI power cell of claim 1, further comprising a layer of insulating material disposed between the layer of radioactive material and the layer of electron emitting material. 8. The TI power cell of claim 1, wherein:the layer of electron emitting material encapsulates the layer of radioactive material; andthe layer of electron collecting material surrounds the layer of electron emitting material. 9. The TI power cell of claim 1, wherein the layer of radioactive material includes a thin plate of plutonium-238. 10. The TI power cell of claim 1, wherein the layer of radioactive material, the layer of electron emitting material, and the layer of electron collecting material are enclosed by an outer shell. 11. A method for generating an electric current, comprising:heating a layer of electron emitting material disposed on a layer of radioactive material by radioactive decay of the layer of radioactive material;emitting electrons from the layer of electron emitting material to a layer of electron collecting material by thermionic emission, wherein the layer of electron emitting material is physically separated from the layer of electron collecting material to define a chamber between the layer of electron collecting material and the layer of electron emitting material, wherein the chamber is substantially evacuated to permit electrons to traverse the chamber from the layer of electron emitting material to the layer of electron collecting material; andinducing an electric current to flow through the layer of electron collecting material connected to an electrical load, wherein heat generated over time by the layer of radioactive material causes a substantially constant flow of electrons to be emitted by the layer of electron emitting material. 12. The method of claim 11, wherein emitting the electrons includes emitting electrons from an array of spikes disposed on the layer of electron emitting material to concentrate the electrons emitted by the layer of electron emitting material at tips of the spikes, wherein the tips of the spikes point toward the layer of electron collecting material. 13. The method of claim 12, wherein emitting the electrons includes emitting electrons from the tips of the spikes that are separated from one another by a lateral separation of approximately 100 micrometers to approximately 1 millimeter. 14. The method of claim 12, wherein emitting the electrons includes emitting electrons from the tips of the spikes to the layer of electron collecting material that are separated by a gap of approximately 100 micrometers to approximately 1 millimeter. 15. The method of claim 12, wherein emitting the electrons includes emitting electrons from the tips of the spikes that are exposed by a layer of spacer material disposed between the layer of electron emitting material and the layer of electron collecting material to the layer of electron collecting material. 16. The method of claim 11, wherein inducing the electric current includes inducing the electric current to flow from a first terminal coupled to the layer of electron emitting material to the electrical load and from the electrical load to a second terminal coupled to the layer of electron collecting material. 17. The method of claim 11, wherein a layer of insulating material is disposed between the layer of radioactive material and the layer of electron emitting material. 18. The method of claim 11, wherein:the layer of electron emitting material encapsulates the layer of radioactive material; andthe layer of electron collecting material surrounds the layer of electron emitting material. 19. A thermionic (TI) power cell, comprising:a heat source, wherein the heat source includes an exhaust manifold;a heat conductive layer that provides heat from the heat source;a layer of electron emitting material disposed on the heat conductive layer; anda layer of electron collecting material, wherein the layer of electron emitting material is physically separated from the layer of electron collecting material to define a chamber between the layer of electron collecting material and the layer of electron emitting material, wherein the chamber is substantially evacuated to permit electrons to traverse the chamber from the layer of electron emitting material to the layer of electron collecting material, and further wherein heat generated over time by the heat source causes a substantially constant flow of electrons to be emitted by the layer of electron emitting material to induce an electric current to flow through the layer of electron collecting material when the layer of electron collecting material is connected to an electrical load.