Solid-State and Vacuum Thermionic Energy Conversion

Abstract

A brief overview of the research activities at the Thermionic Energy Conversion (TEC) Center is given. The goal is to achieve direct thermal to electric energy conversion with >20% efficiency and >1W/cm 2 power density at a hot side temperature of 300-650C. Thermionic emission in both vacuum and solid-state devices is investigated. In the case of solid-state devices, hot electron filtering using heterostructure barriers is used to increase the thermoelectric power factor. In order to study electron transport above the barriers and lateral momentum conservation in thermionic emission process, the currentvoltage characteristic of ballistic transistor structures is investigated. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity. Cross-plane thermoelectric properties and the effective ZT of the thin film are analyzed using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n-and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. For vacuum devices, nitrogen-doped diamond and carbon nanotubes are studied for emitters. Sb-doped highly oriented diamond and low electron affinity AlGaN are investigated for collectors. Work functions below 1.6eV and vacuum thermionic power generation at temperatures below 700C have been demonstrated. Report Documentation Page INTRODUCTIONDirect thermal to electrical energy conversion systems that could operate at lower temperatures (300-650C) with high efficiencies (>15-20%) provide an attractive compact alternative to internal combustion engines for many applications in the W-MW range. They will also expand the possibilities for waste heat recovery applications. The Thermionic Energy Conversion Center's goal is to design, fabricate, and characterize direct energy conversion systems that meet the above requirements. The core of the solution is an integrated approach to engineer electrical and thermal properties of nanostructured materials and devices and fabricate more efficient solid-state and vacuumbased thermionic energy conversion systems. Solid-state material design is focused on increasing the efficiency of heterostructure thermionic power generators using embedded nanoparticles and metal/semiconductor superlattices. Vacuum effort focuses on the development of thermionic energy conversion based on thermionic-field emission from nanostructured carbon surfaces and low work function n-type wide band gap semiconductor collectors. Measurements of electrical, optical and thermal transport at both the device and nanostructure level are used to verify model predictions and thereby lay the foundation for improved device and materials design. Finally, various components will be integrated and packaged for systems demonstration. Fig. 1 displays a chart of the research groups participating in the Thermionic Energy Conversion Center.