| AVS 58th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN+NS-TuA |
| Session: | Nanostructured Materials for Thermophotovoltaics, Thermoelectrics & Plasmonics |
| Presenter: | William Paxton, Vanderbilt University |
| Authors: | W.F. Paxton, Vanderbilt University J.L. Davidson, Vanderbilt University W.P. Kang, Vanderbilt University |
| Correspondent: | Click to Email |
Introduction
Thermionic energy conversion is a candidate technology for the efficient conversion of thermal energy directly to electrical energy. In a thermionic converter, thermally excited electrons are emitted from the surface of a heated cathode into a vacuum gap. These electrons are then collected by a cooler anode and driven through an external load back to the cathode. The thermionic emission current density of a heated cathode can be described by the Richardson Equation (Eq. 1).
J = AT2e-Φ⁄kT (1)
where: J: Thermionic emission current density (A/cm2); A: Richardson constant (A/cm2 T2); T: Temperature (K); Φ: Material’s work function (eV); and k: Boltzmann constant (eV/T)
It can be seen from Eq. 1 that a material with a lower work function, Φ, can achieve higher current densities at lower temperatures than a material with a higher work function, which implies lower work function values translate into better energy converters. Prior attempts to construct efficient thermionic converters were limited by available materials with work function values ranging from 3.5eV to 5eV requiring extreme cathode temperatures in order to achieve useable output power values. In this study, the thermionic emission properties of nitrogen-incorporated diamond films are examined as a potential electrode material to enhance the efficiency of such a device.
Experimental
Polycrystalline diamond films were synthesized on molybdenum substrate via Microwave Plasma-Enhanced Chemical Vapor Deposition (MPCVD). The source gasses were H2, CH4, and N2, microwave power was 1.5KW, and the nominal pressure was 50Torr. Scanning electron micrographs of the samples demonstrated uniform film coverage and a thickness of ~100µm.
Thermionic emission characterization was performed in a vacuum environment with a base pressure of 1 x 10-7 Torr. The diamond films were resistively heated and the temperature was constantly observed by a dual color pyrometer. Electron emission current was collected with an electrically isolated anode biased at a constant voltage of 100V positioned 0.5cm above the heated cathode.
Results and Discussion
Observation of the electron emission current above the noise level began at 600oC and increased exponentially with temperature up to 800oC. Analysis of this data demonstrated agreement with the Richardson equation with a correlation coefficient of 0.99. From this data, the nitrogen-incorporated diamond samples were determined to have a work function value less than 2eV which is considerably lower than previously mentioned materials. These results exhibit diamond’s potential as an interesting cathode material for a thermionic energy converter.