Alireza Nojeh

Thermionic converters are promising candidates for static heat-to-electricity conversion due to their flexible form factor, low maintenance requirement, high power density, and potential for high efficiency. However, practical applications of thermionic converters have been hindered in the past by various engineering challenges such as the space charge effect and lack of materials with desirable properties. With the advent of microfabrication and nanotechnology, there has been renewed interest in thermionic conversion in the recent past. In particular, the micro-gap architecture has drawn significant attention as an elegant solution to mitigate the space charge effect. For example, micro-gap thermionic converters could enable chip-scale power generators. However, to make this a reality, apart from overcoming the engineering challenges, a thorough understanding of the devices' operation is necessary. The work presented in this thesis addresses this need by laying a foundation for multiphysics computational models for micro-gap thermionic converters both for single-stage devices and for various hybrid configurations including other mechanisms.For the single-stage thermionic converter, we develop self-consistent iterative models that consider the thermal and charge balance to accurately determine the electrode temperatures, space charge, thermal radiative coupling between the electrodes and its possible enhancement at small gaps due to the near-field effect, and the electrical and thermal losses in the lead resistance. This model shows how the micro-gap device performance and electrode temperatures are affected by the interelectrode gap size under the constraint of finite input power. Moreover, we reveal how the cathode material and its thermal coupling with the input energy source determine the nature of the device's response to light and its combined photo-thermionic behaviour.We also develop multiphysics models to investigate the prospects of hybrid thermionic-thermoelectric and thermionic-photovoltaic devices. We show that these different mechanisms can be operated in a complementary manner due to their different optimal temperature ranges of operation. Additionally, we show that, depending on the conversion mechanism of the second stage, such a hybrid device may or may not be more efficient than a single thermionic device. The above models represent a powerful approach to designing thermionic converters, developing new device concepts, and understanding their operation.

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Carbon nanotube forests are attracting significant attention due to their unique mechanical and electrical properties. There are research endeavours that explored these properties in which there was a need for patterning the material using conventional microfabrication techniques. These techniques, however, imposed some limitations on the patterning structure, and in turn, on the properties that can be investigated. This thesis work utilizes microplasma-based micro-electro discharge machining in novel patterning processes that enable exploring unique properties through MEMS and vacuum electronic devices.The first process investigates flat and rounded tip cylindrical electrodes in micropatterning of carbon nanotube forests. Both electrodes are used to pattern rectangular slots at different depths from which the discharge gap is measured. The rounded tip electrode shows a smaller discharge gap; however, the variation produced is larger, which affects the structural uniformity of the patterns. Consequently, the flat tip electrode is chosen in the fabrication of the first device in this work, which is a laterally suspended microcantilever made entirely of a carbon nanotube forest, to study the mechanical properties of the material. The microcantilever is electrostatically actuated to characterize its resonance. The measurement result fitted to a free vibrating microcantilever model is used to calculate the in-plane Young’s modulus of the material.The second process is a planarization process of carbon nanotube forests to produce macroscopically flat top surfaces using planar electrodes. This process allows for placing a carbon nanotube forest (emitter) electrode at close proximity down to a few tens of micrometers from a collector electrode in the second device presented in this work, which is a thermionic emission device. This device is used in an interelectrode-gap-variation platform to systematically study the mitigation of the space charge effect in-situ, which impedes the emitted electrons from reaching the collector. A conventional emitter, yttria, is also tested. The yttria emitter is found to follow the expected trend of increasing the output current in the space charge regime with decreasing the interelectrode gap. However, the carbon nanotube forest emitter exhibits an opposite behaviour in which the current decreases instead of increasing. Possible reasons are discussed to explain this unexpected behaviour.

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Heat conduction phenomena of carbon nanotubes (CNTs) have attractedgreat interest both from the viewpoint of engineering applications andfundamental science, but the thermal conductivity of individualsingle-walled CNTs remains a rather controversial topic. Starting froman empirical, realistic atomic interaction potential, we study thelattice thermal conductivity (LTC) of single-walled CNTs by employingtwo approaches: quantum mechanical calculations of three-phononscattering rates in the framework of the Peierls-Boltzmann transporttheory (PBTT) and classical molecular dynamics (MD) simulations.First, we compare the system-size and temperature dependence of theLTC determined from an iterative solution of the linearized phonontransport equation in the framework of the PBTT and from anonequilibrium MD (NEMD) approach. At room temperature, qualitativelysimilar trends for the tube-length dependence are found in the limitof short tubes, where an extensive regime of ballistic heat transportprevailing in CNTs of lengths L ≲ 1 µm is independently confirmed. In the limit of long tubes, the PBTT-derivedLTC diverges. Using PBTT and equilibrium MD approaches, we performnumerical calculations of acoustic phonon lifetimes to clarify thesource of divergence. NEMD-derived temperature dependencies obtainedfor micrometer-long CNTs and temperatures T ≤ 800 K confirmthe 1/T behavior of the LTC at moderately high temperatures.Next, we revisit the tube-length dependence of the LTC by use of therelaxation time approximation in the PBTT. Through a combination ofnumerics and analytical considerations, we derive exact asymptoticscaling laws of the LTC. In particular, we demonstrate the importanceof tensile lattice strain, previously overlooked in the long-standingdispute over tube-length convergence vs divergence of the LTC. Namely,it is proved that, in the long-tube limit, the relaxation timeapproximation yields a finite value for stress-free but an infinitevalue of the LTC in any stretched tube configuration.Lastly, we pursue a matrix inversion approach to solve the linearizedphonon transport equation in the framework of the PBTT. Here, it isshown that violations of acoustic sum rules cause spurious convergentbehaviors of a length-dependent LTC.

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Thermal energy conversion promises readily available sources of clean, renewable energy through technologies such as thermionic or thermoelectric devices, which can enhance existing sources such as solar or co-generation. Such devices require sustained high temperature gradients between an electron emitter material and an electron collector, which presents challenges due to factors such as heat dissipation. Carbon nanotube (CNT) forests are a promising candidate for efficient thermal energy conversion due to the highly localized heating (“Heat Trap”) that occurs when they are spot heated by a focused light beam. Because of this, temperatures over 1700 °C and temperature gradients of 10 °C per micrometre are achievable with powers less than 50 W/cm², making CNT forests a promising material for low-cost, miniaturized thermal energy conversion. However, there is much mystery regarding the mechanisms underlying the Heat Trap, particularly anomalous effects such as temperature decay and recovery over a period of hours, and how they are connected to the complex, multiscale structure of the CNT forest. In order to maximize the potential of the Heat Trap, it is important to understand how these various internal interactions come together in assemblies of one-dimensional (1-D) nanoscale objects to produce and influence such phenomena.This thesis outlines the development of a semi-empirical computational model for simulating these 1-D nanomaterial systems, incorporating various models for near-field radiation and analogues to structural features in CNT forests such as defects and density variations. This model showed how the forest structure was a major contributor to the Heat Trap due to nanoscale effects such as near-field radiative energy exchange, which also showed that nanoengineering the structure through methods such as increased CNT density could improve heat retention and thermal conversion efficiency. In addition, simulations were able to explain the observed temperature decay and recovery, attributing it to slow diffusion of adsorbates such as oxygen. This process was non-destructive, a promising sign for long-term device stability. These results demonstrate the potential of the model as a framework for improving the effectiveness of thermal energy conversion and related applications, and for further study into thermal applications of general multiscale nanomaterial systems.

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Thermionic conversion involves the direct conversion of heat, including light-induced heat, from a source such as solar energy, to electricity. The progress of thermionic converters has been limited by issues such as the space charge effect and lack of materials with desirable mechanical, electrical and thermal properties. Nanotechnology could help address some of the main challenges that thermionic converters encounter. However, existing models, which were developed for macroscopic converters, are not adequate for many aspects of nanostructured devices. The work presented in this thesis primarily advances a new model to partially address this void and study emergent thermionic devices. We demonstrate a self-consistent and iterative approach to the Vlasov-Poisson system that overcomes the inherent limitations of the traditional methods. This approach serves as the foundation for more advanced and yet crucial cases of the operation of thermionic converters in the presence of back-emission, grids in the inter-electrode region and low-pressure plasmas. We develop the physics of the device in the presence of grids and demonstrate that momentum gaps could arise in the phase space of the electrons; taking into account these gaps, which had not been noticed in the past, is key to designing efficient thermionic converters and we predict improvements of 3 orders of magnitude in current density using a properly designed grid. We also develop the physics of the device in the presence of low-pressure plasmas, which are prime candidates for reducing space charge. We show that the output power density of a thermionic converter can improve by a factor of ~ 10 using a modest plasma pressure of 500 Pa. On a different front, we have also improved the traditional analytical model and developed an approach to extract the internal device parameters such as emission area and workfunction based on a limited set of experimental output characteristics. These parameters are highly dependent on the operating conditions and ex-situ measurements are not applicable. Therefore, our approach allows for a more systematic study of the device and material properties, which is key to further the development of thermionic converters, in particular based on novel materials and nanostructures.

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Carbon nanotubes (CNTs) are promising materials for many applications due to their attractive electronic, optical, mechanical and thermal properties. Currently, the main challenge facing their widespread usage is the inability to fabricate nanotube devices reproducibly. Dielectrophoresis (DEP) is a versatile method for the fabrication of nanostructures from a solution. However, while this method offers advantages such as overall control over the positioning of the nanotubes and the possibility of pre-selecting the types of CNTs, there are many unknowns about how the process works, leading to unreliability and irreproducibility. Although there have been reports on different parameters affecting DEP results, some important factors such as the movement of the solution and the interactions among nanotubes during the process have often been neglected.This thesis presents a combination of experimental and modeling efforts to investigate the mechanisms at work during DEP. Experiments were performed to evaluate the influence of the conductivity of the solution. A framework based on finite-element method simulations was developed to unveil the mechanisms involved. The results showed that variations in the conductivity of the solution, leading to changes in electrothermal movements, could lead to substantial differences in the outcome. Higher levels of repeatability were achieved by using low-conductivity solutions.The mutual interactions of nanotubes during DEP were also investigated using both experiments and simulations; it was shown that these could lead to the formation of periodic patterns in the deposited nanotubes.Finally, a particle tracing simulation formalism was developed, allowing one to follow the CNTs in their journey from the solution to the substrate, taking into account several influential factors: the DEP force, the movement of the solution itself, and the Brownian movement of the CNTs. Metallic and semiconducting nanotubes were traced in various scenarios and the effective forces were explored every step of the way.The work reported in this thesis thus leads to a better understanding of the DEP process and the mechanisms involved in the deposition of nanotubes, and potentially that of other nano-objects, taking us a step closer to engineering reproducible processes for nano-device fabrication.

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Carbon nanotubes (CNTs) are relatively new materials with exceptional properties which have attracted significant interest in the past two decades. The ability to grow arrays of vertically aligned carbon nanotubes, so called CNT forest, opened up opportunities to develop different types of novel devices enabled by the material. A key in facilitating micro-electro-mechanical systems (MEMS) applications of the material is the ability to pattern the material in a batch mode with high precision and high reproducibility. Patterning CNT forests prior to, during or after the growth is reported. The mentioned techniques are, however, primarily for the formation of two-dimensional types of patterns (with uniform heights). Laser micromachining is reported to shape CNT forests for different applications while exhibiting its inherent limitations including tapered sidewalls, lack of high-precision depth control, and thermal damages. Hence, there is a need to develop machining techniques to fabricate CNT forests in any shape for MEMS and other applications. This thesis is based on the idea that a powerful micromachining technique is a path that should be taken to reach a successful integration of smart materials such as nanotubes and MEMS (and other) devices to achieve more complex and improved devices. This work develops an effective micromachining technique based on dry micro-electro-discharge machining (µEDM) to produce free-form, three-dimensional (3D) patterns out of CNT forests with high precision (~2-µm machining tolerance), high-aspect-ratios (of about 20), high reproducibility, and at very small machining voltages (~10 V) which corresponds to several orders of magnitude smaller discharge energy (0.5 nJ compared to 15 µJ). The machining mechanism has been found to be different from the one in typical µEDM. Furthermore, techniques to achieve high removal precision with tighter tolerance are investigated. Also, elemental and molecular analysis of the machined structures is carried out to observe the level of cross-contamination of the process. To demonstrate an application of the processed nanotubes, high-power MEMS switches that integrate micropatterned CNT forests as electrical contact have been developed. Micropatterned CNT forests as field emitters and atomic force microscopy (AFM) probe tips are also demonstrated.

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Due to their unique optical properties, carbon nanotubes have been widely investigated for use in photonic and optoelectronic devices and optical absorption and emission with nanotubes have been achieved in experiments. On the other hand, the structural characteristics of nanotubes, e.g. the chirality, diameter, and length, as well as other factors such as the polarization of the incident light, presence of a magnetic field and mechanical deformation can significantly affect the optical properties of these structures. Some of these effects have been theoretically studied at the tight-binding approximation level. However, a systematic first-principles-based study of nanotubes that addresses these effects did not exist in the literature prior to the present work. This thesis aims at performing such a fundamental study. We first describe a method for calculating the dipole moments and transition rates in nanotubes. This also enables the study of selection rules, based on which a modified set of rules is defined. The probability of absorption is studied in the full range of infrared-visible-ultraviolet. We show that π-σ*, σ-π*, and σ-σ* transitions that are neglected in previous works are allowed and can lead to high probabilities of transition. We then investigate several effects caused by the curvature of the nanotube sidewall and their impacts on the optical properties. The overall effect is shown to not only depend on the diameter, but also on the chirality of the nanotube. Through the study of the light polarization effect, we show that the overall transition rate spectrum of the perpendicularly polarized light is suppressed for smaller-diameter nanotubes in the IR/VIS range. In the UV region, however, perpendicular polarization is generally absorbed at a higher rate compared to parallel polarization. Finally, we show how the absorption spectra of short nanotube segments can be different from those of long nanotubes. We examine the effect of length on individual absorption peaks and also investigate the effect of spin on the optical properties of nanotube segments. The calculation method described in this thesis and the results can be used to estimate the effects of structural and environmental factors on the optical absorption properties of nanotubes.

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Recently there has been growing interest in the interaction of light and nanomaterials,especially carbon nanotubes. Although there exist a large numberof studies on the physical and chemical properties of nanotubes with variousspectroscopic techniques, only a limited number of works have looked at thisinteraction for electron source applications. The work presented in this thesisdemonstrates light-induced electron emission from arrays of nanotubeswith a broad range of wavelengths and light intensities. I demonstrate thatarrays of nanotubes have a quantum efficiency of > 10-⁵ in the photoelectricregime, which is comparable to that of metal photocathodes such as copper.Nanotubes are also expected to have better operational lifetime than metalsbecause of their complete chemical structure. I also demonstrate that,based on an effect called "Heat Trap", a spot on the surface of a nanotubearray can be heated to above 2,000 K using a low-power beam of light with abroad range of wavelengths from ultraviolet to infrared. Light-induced heatingof a typical bulk conductor to electron emission temperatures requireshigh-power lasers. This is because of the efficient dissipation of heat generatedat the illuminated spot to the surroundings, since electrical conductorsare also typically excellent thermal conductors. I show that the situationcan be drastically different in an array of nanotubes. This behaviour hasfar-reaching implications for electron sources. For example, the fabricationcost of light-induced electron sources can be signicantly reduced since thenanotube-based cathode can be heated to thermionic emission temperatureswith inexpensive, low-power, battery-operated handheld lasers as apposedto high-power or pulsed laser sources, which are currently required for metalcathodes. Arrays of nanotubes can also be shape engineered because of theirsparse nature. I have demonstrated that the emission current density can be increased by a factor of 4 by densifying the array with a liquid-inducedshrinkage that works by pulling the nanotubes closer together. The Implications of the findings reported in this thesis go beyond conventional electronbeamtechnologies. For instance, they could lead to novel devices such asthermionic solar cells, solar displays and new types of optical modulatorsand thermoelectrics.

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Carbon nanotubes have great potential for nanoscale devices. Previous studies have shown the prospects of carbon nanotubes as stable, low-voltage electron emitters for vacuum electronic applications. Yet, their electron emission mechanisms are far from being fully understood. For example, it is not completely clear how nanotubes interact with an external electron beam and generate secondary electrons. In addition to its fundamental scientific importance, understanding these mechanisms and properties will facilitate the engineering of nanotube-based devices for applications such as vacuum transistors, electron multipliers, X-ray devices for medical imaging, etc.This thesis presents an experimental and theoretical investigation into the interaction of electron beams with carbon nanotubes. First-principles simulations are carried out to qualitatively analyze the possible direct interaction mechanisms of electron beams with nanotubes. An experimental study of electron yield (total, backscattered and secondary electron yields) from individual nanotubes and collections of nanotubes is reported. The experiments reveal low secondary electron yield from individual nanotubes. A different backscattered electron emission behaviour compared to that in bulk materials is observed in nanotube forests due to unusually high electron penetration range in them.A semi-empirical Monte Carlo model for the interaction of electron beams with collections of nanotubes is presented. Physically-based empirical parameters are derived from the experimental data. The secondary electron yield from individual nanotubes is first investigated in the light of the commonly used energy loss model for solids. Finally, the problems of using the traditional models for individual nanotubes are identified and an approach to modeling secondary and backscattered electron emission from such nanostructures is presented.The experiments and analysis presented in this thesis provide a platform for investigating backscattered and secondary electron emission also from nanostructures other than nanotubes.

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