Energy transport and conversion in electron emission processes

Tyler L Westover, Purdue University

Abstract

Vacuum electron emission has been proposed as a means of achieving direct refrigeration and also for converting heat directly into electrical power; however, a suitable material with a sufficiently low and stable work function has not been found. Numerous approaches based on nanomaterials and quantum phenomena are being explored to overcome this obstacle. This thesis contributes to this effort by investigating energy transport associated with electron emission from nanoscale-size structures using both theoretical models and experimental data. Specifically, a non-equilibrium Green’s function (NEGF) method is employed to simulate the energy exchange associated with field emission for a simple one-dimensional configuration with flat-plate electrodes. The effects of electrode work function, applied electric field, quantum confinement in the emitter structure, and vacuum gap distance in the nanometer range are reported. The results yield important insights into the thermodynamics associated with electron emission across nanometer-scale vacuum gaps. ^ To complement the theoretical models developed in this thesis, experiments were also conducted to measure the energy exchange associated with field emission from carbon nanotubes. An apparatus capable of measuring a heating or cooling effect with an uncertainty of approximately 1 μW was fabricated and this apparatus was utilized to measure the energy exchange associated with field emission from several CNT samples. Significant heating due to field emission has been registered for all cases, and results exhibit a nearly linear relationship between emitter heating and emission current. The magnitude of the measured emitter heating is much greater than that expected from the Nottingham effect alone, indicating that Joule heating is dominant. The heating effect is also found to depend strongly on the adhesive used to bind the CNT arrays to the substrate, and this effect has been explored using silver and carbon paints as the adhesive material. ^ Because of their low turn-on voltage and high current density, carbon nanotubes (CNTs) are excellent field emitters, and a considerable amount of research is currently in progress to extend their use to new applications. A potential challenge to this effort is the heat that is transferred to the anode by high-energy emitting electrons. This thesis investigates the heating of a thin, disc-shaped steel anode by electrons field-emitted from an individual multiwalled carbon nanotube. The steady-state temperature distribution in the anode during electron field emission has been measured by an infrared camera, and this distribution is compared to that predicted by a numerical model. By assuming that the electron distribution in the beam follows a Gaussian distribution, a good fit to the anode temperature profile is obtained which provides an estimate of the beam spreading radius. ^ Devices based upon electron emission may also be used to convert heat and solar energy to electrical power. Electrons absorb energy from a heat source and thermionically emit across a vacuum gap to create an electrical current. Although the use of vacuum gaps reduces the losses caused by heat conduction and convection, electron emission-based power generation devices are limited to high temperatures due to a lack of suitable emitter materials with sufficiently low work functions. Carbon nanotubes (CNTs), by virtue of their unique mechanical, electrical, and optical properties, offer a potential pathway to a practical solar energy conversion device. CNTs are robust emitters even at high temperatures, and their work functions can be reduced by intercalating alkali metals into the carbon lattice. Furthermore, CNT arrays have very high optical absorption coefficients in the dominant solar wavelengths, making them attractive for solar energy conversion applications. ^ Single-walled and multi-walled CNT arrays have been intercalated with potassium to reduce their work functions to 2-3 eV, and the energy distributions of emitted electrons are measured with a hemispherical energy analyzer. Results show that emitters prepared in this way are highly sensitive to light in the dominant solar wavelengths. The effective work function of both single-walled and multi-walled carbon nanotubes intercalated with potassium (K/SWCNT and K/MWCNT, respectively) is temperature-dependent and has a minimum of approximately 2 eV in the neighborhood of 300°C for K/SWCNTs while the minimum for K/MWCNTs is approximately 2.4 eV in the neighborhood of 500°C. Results from control samples without CNTs are also presented and display substantially inferior emission.^

Degree

Ph.D.

Advisors

Timothy Fisher, Purdue University.

Subject Area

Engineering, Mechanical

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