Tyler L. Westover

Photo- and thermionic emission from potassium-intercalated carbon nanotube arrays

Carbon nanotubes (CNTs) are promising candidates to create new thermionic- and photoemission materials. Intercalation of CNTs with alkali metals, such as potassium, greatly reduces their work functions, and the low electron scattering rates of small-diameter CNTs offer the possibility of efficient photoemission. This work uses a Nd:YAG (YAG denotes yttrium aluminum garnet) laser to irradiate single- and multiwalled CNTs intercalated with potassium, and the resultant energy distributions of photo- and thermionic emitted electrons are measured using a hemispherical electron energy analyzer over a wide range of temperatures. For both single- and multiwalled CNTs intercalated with potassium, the authors observe a temperature dependent work function that has a minimum of approximately 2.0 eV at approximately 600 K. At temperatures above 600 K, the measured work function values increase with temperature presumably due to deintercalation of potassium atoms. Laser illumination causes the magnitudes of collected e...

Thermionic emission energy distribution from nanocrystalline diamond films for direct thermal-electrical energy conversion applications

In the ongoing quest for energy production by nonconventional methods, energy conversion by vacuum and solid-state thermionic emission devices is one of the potentially efficient pathways for converting thermal energy directly into electrical power. The realization of practical of thermionic energy conversion devices strongly depends on achieving low work function materials, which is thus far a limiting factor. In an attempt to develop a new low work function thermionic material, this work reports thermionic emission energy distributions (TEEDs) from nanocrystalline diamond (NCD) films in the temperature range from 700 to 900u2009°C that reveal a consistent effective work function of 3.3 eV. The NCD films also exhibit emission peaks corresponding to higher work functions as indicated by shifts in their energy position and relative intensity as a function of temperature. These shifts thus appear to be related to instabilities in the NCD’s surface chemistry. The analysis of these data yields information on the ...

Experimental Study of Energy Exchange Attending Electron Emission from Carbon Nanotubes

Phenomena based on nanoscale transport processes offer new possibilities for direct refrigeration by electron emission between opposing electrodes across a vacuum region. The average energy of emitted electrons depends upon the magnitude and shape of the potential energy barrier in the vacuum region, which is affected by the emission gap, emitter work function (potential barrier height), and emitter tip geometry. Emitted electrons are replaced by other electrons to maintain charge continuity, and the difference in energy between the emitted and replacement electrons produces a heating or cooling effect, known as the Nottingham effect, at the emitter surface. Theoretical studies indicate the possibility of very large (> 100 W/cm2) cooling rates, but experimental confirmation is lacking due to challenging material and experimental requirements. To obtain the results discussed in this paper, the energy exchange attending electron emission from multi-walled carbon nanotube (MWNT) array samples is measured with an uncertainty of approximately 1 μ W. The results are found to depend strongly on the adhesive used to bind the MWNT arrays to the substrate, and this effect is explored by using both silver and carbon paints as the adhesive material. An attempt to determine the effect of the emitter work function by intercalating the MWNT arrays with potassium was unsuccessful. Heating curves as a function of the emission current are presented for various sample groups, and these curves provide insight into the mechanisms involved in the energy exchange associated with field emission from MWNT arrays, including the Nottingham effect and Joule heating.

Field emission from GaN and (Al,Ga)N∕GaN nanorod heterostructures

Vacuum field emission from GaN and (Al,Ga)N∕GaN nanorods with pyramidal tips has been measured. The turn-on fields, defined at a current density of 0.1μA∕cm2, were found to be 38.7 and 19.3V∕μm, for unintentionally doped GaN and (Al,Ga)N∕GaN nanorods, respectively. The 5nm (Al,Ga)N layer reduced the electron affinity at the surface, thereby lowering the turn-on field and increasing the current density. The nanostructures exhibit a field enhancement factor of approximately 65 and the work function of the (Al,Ga)N∕GaN nanorod heterostructure was estimated to be 2.1eV. The stability of the emission characteristics and the simple fabrication method suggest that intentionally doped and optimized (Al,Ga)N∕GaN nanorod heterostructures may prove suitable for field-emission device.

Photo- and Thermionic Emission From Potassium-Intercalated Single-Walled Carbon Nanotube Arrays

Vacuum thermionic electron emission has been considered for many years as a means to convert heat or solar energy directly into electrical power. However, an emitter material has not yet been identified that has a sufficiently low work function and that is stable at the elevated temperatures required for thermionic emission. Recent theoretical models predict that photonic and thermal excitation can combine to significantly increase overall efficiency and power generation capacity beyond that which is possible with thermionic emission alone. Carbon nanotubes (CNTs) intercalated with potassium have demonstrated work functions as low as 2.0 eV, and low electron scattering rates observed in small diameter CNTs offer the possibility of efficient photoemission. This study uses a Nd:YAG laser to irradiate potassium-intercalated single-walled CNTs (K/SWCNTs), and the resultant energy distributions of photo- and thermionic emitted electrons are measured using a hemispherical electron energy analyzer for a wide range of temperatures. We observe that the work function of K/SWCNTs is temperature dependent and has a minimum of approximately 2.0 eV at approximately 600 K. At temperatures above 600 K, the measured work function K/SWCNTs increases with temperature, presumably due to deintercalation of potassium atoms.Copyright

Thermionic Emission From Potassium-Intercalated Carbon Nanotube Arrays

Vacuum and solid-state thermionic emission have long been proposed as a means of converting heat or solar energy directly into electrical power. However, low work function materials must be developed before a reasonably efficient power generation device can be realized. In this work, thermionic emission energy distributions were measured for four samples including a single-crystal tungsten (100) sample, a pristine CNT mat, and two potassium-intercalated CNT mats. One of the potassium-intercalated CNT mats was composed largely of randomly oriented CNTs while the other CNT sample was grown in templated anodized alumina to align the growth pattern. Thermionic emission data obtained from the tungsten sample validated the experimental apparatus and method. The pristine CNT mat exhibited an emission distribution with a work function of 4.7 eV, while the potassium-intercalated samples exhibited work functions of approximately 3.1 and 3.4 eV for the randomly oriented and the templated meshes, respectively. The differences in the measured work function values for intercalated samples may be due to emitter tip differences. Both intercalated CNT samples showed some degradation after cooling from 510°C and reheating to the same temperature.Copyright

Thermionic Emission From Alkali Potassium-Intercalated Carbon Nanotube Arrays for Direct Energy Conversion

Vacuum and solid-state thermionic electron emission are potentially efficient means for converting heat or solar energy directly into electrical power. However, low work function materials must be developed before reasonable efficiency can be realized with a power generation device based on thermionic emission. In this work, carbon nanotube (CNT) arrays have been doped with potassium atoms using a two-zone vapor method to lower their work functions to 2–4 eV. We have previously shown that carbon nanotube emitters prepared in this way are stable in atmospheric air although undesirable oxide compounds can form on the carbon nanotube surface. Using a hemispherical electron energy analyzer to obtain thermionic emission energy distributions, we show that low work function emitters can be prepared from potassium-intercalated CNT mats at temperatures as low as 400°C and that emitters prepared in this way can be stable at temperatures up to 620°C.Copyright