Using Carbon-Based Nanomaterials and Microscale Geometry for Enhanced Thermionic Energy Conversion in Space Applications

To improve the capabilities of extraterrestrial scientific instruments, including satellites and rovers, and to make manned missions possible, more efficient methods of energy conversion need to be developed. Over the past several decades, the standard energy sources have been solar collection or radioactive decay. These sources must be converted to electricity and different methods have been used, most commonly photovoltaic cells for solar-to-electric and thermoelectric generators for heat-to-electric. The problem with both of these methods is the decrease in conversion efficiency at higher temperatures. A possible alternative power source is thermionic energy converters (TECs). Thermionic energy conversion is the process in which thermal energy, from sources such as waste, solar or nuclear heat, is converted to electrical energy by the thermal emission of electrons across an electrode gap, generating current. TECs, unlike other methods of power generation, become more effective at higher temperatures, but this requires that the cathode must be elevated to very high temperatures (>1500 K). As reviewed by Rasor[1] and Adams[2], TEC can be coupled with solar concentrators for solar TEC or used “in-” or “outcore” for nuclear TEC. And because TEC is particularly suited for high temperature heat rejection, it is ideal for those applications where other methods, such as photovoltaics and thermoelectrics, become inefficient. TECs have been limited in the past due to the high temperatures required to gain significant emission current. This directly impacts the possible cathode materials, because they must have both a melting temperature greater than the operating regime and a low work function for effective emission, with the most commonly used being tungsten, which has a work function of 4.6 eV. However, carbon-based nanomaterials have shown significant promise for enhanced TEC. The material this research is investigating is nanodiamond due to its extremely high operating temperature, negative electron affinity[3], and, when doped with nitrogen, a work function as low as 1.5 eV[3]–[5]. An additional problem that limits the current in thermionic energy converters is the accumulation of negative space charge. There are many methods that have been used to mitigate the effect of negative space charge: reducing the spacing between the cathode and anode (the interelectrode gap) to allow for less build-up of the electron concentration, introducing a third electrode to draw electrons away from the cathode, and introducing positive ions into the interelectrode gap to neutralize the effect of negative space charge. Historically, the most common method employed was introducing cesium ions to neutralize the effect of negative space charge[6], [7]. However, cesium has several drawbacks including needing to be kept at elevated temperatures to maintain its gaseous state, and its extreme reactivity which further limits the materials that can be used in TECs. To try and keep the benefit of positive ions without the drawbacks of cesium, there has been research interest in using an inert gases as the ion source in TECs[8]. In inert gas TECs a plasma is created by quickly pulsing a voltage between the two electrodes, causing a rapid build-up of ions in the interelectrode gap. Once the voltage is turned off, the ions slowly diffuse towards the cathode. These ions have two effects: canceling out the screening effect of negative space charge in the gap, and increasing the current of electrons, called the ion-enhanced Schottky effect[9]. My research has focused on combining two space charge mitigation strategies outlined previously: inert gas plasmas and micron scale interelectrode gap spacing. Using a theoretical approach, I showed that the ion-enhanced Schottky effect can be quite substantial[10], especially in cases where the gas density and electric field are chosen the maximize the efficiency of ionization. The final portion of the NSTRF was focused on building on this to design operating conditions for which the power produced through the ion-enhanced Schottky effect is great enough to overcome the power deficit caused by igniting the plasma. In concert with this, I have synthesized nitrogendoped diamond that can act as electrodes for TECs and worked to perform thermionic testing on the resulting materials.