<div>The rapid advance in modern electronics and photonics is pushing device design to the micro- and nano-scale, and the resulting high power density imposes immense challenges to thermal management. Promising materials like carbon nanotubes (CNTs) and graphene offer high thermal conductivity in the axial (or in-plane) directions, but their thermal transport in the radial (or cross-plane) directions are poor, limiting their applications. Hierarchical structures like pillared graphene, which is composed of many CNT-graphene junctions, have been proposed. However, thermal</div><div>interfacial resistance is a critical issue for thermal management of these systems. In this work, we have systematically explored thermal transport across interfaces,</div><div>particularly in pillared graphene and silicon/heavy-silicon.</div><div><br></div><div><div>First, by recognizing that thermal resistance of the 3D pillared graphene architecture primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics (NEMD), the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6 × 10−11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. Interestingly, when the CNT pillar length is small, the interfacial resistance</div><div>of the sp2 covalent junction is found to decrease as the CNT pillar length decreases, suggesting the presence of coherence effects. The junction resistance Rj is eventually</div><div>used in the network model to estimate the effective thermal conductivity, and the results agree well with direct MD simulation data, demonstrating the effectiveness of our model.</div></div><div><br></div><div><div>Then we identify three different mechanisms which can lead to thermal resistances across the pillared graphene junction: the material mismatch (phonon propagates from CNT to graphene), the non-planar junction (the phonon propagation direction must change), and defects (there are six heptagons at each junction). The NEMD results show that three mechanisms lead to similar resistance at the CNT-graphene junction, each at around 2.5 × 10−11 m2K/W.</div></div><div><br></div><div><div>Further, we have predicted the transmission function of individual phonon mode using the wave packet method at CNT-graphene junction. Intriguing phonon polarization conversion behavior is observed for most incident phonon modes. It is found that the polarization conversion dominates the transmission and is more significant at larger phonon wavelength. We attribute such unique phonon polarization conversion behavior to the dimensional mismatch across CNT-graphene interface. It is found that the transmission functions at the junction cannot be predicted by the conventional acoustic mismatch models due to the existence of dimensional mismatch. Further analysis shows that, the dimensionally mismatched interface, on one hand tends to reduce the transmission and conductance due to defects and the change of phonon propagation direction at the interface, while on the other hand tends to enhance the transmission and conductance due to the new phonon transport channel introduced by polarization conversion.</div></div><div><br></div><div><div>Finally, we address that many recent experiments have shown that the measured thermal boundary conductances (TBCs) significantly exceed those calculated using the Landauer approach. We identify that a key assumption that an interface is a local equilibrium system (different modes of phonons on each side of the interfaces are at the emitted phonon temperature Te), is generally invalid and can contribute to the discrepancy. We show that the measurable temperature for each individual mode is the ”modal equivalent equilibrium temperature” T rather than Te. Also,</div><div>due to the vast range of transmission functions, different phonon modes are out of local thermal equilibrium. Hence, the total conductance cannot be simply calculated as a summation of individual modal conductance. We modify the Landauer approach to include these effects and name it the ”Nonequilibrium Landauer approach”. Our approach has been used on the carbon nanotube (CNT)/graphene and Si/heavy-Si interfaces which are matched interfaces, and it gives 310% increases in TBC as compared to the conventional Landauer approach at CNT-graphene junction and even higher increase for Si/heavy-Si with small mass ratios. A convenient chart is created to estimate the conductance correction based on our approach, and it yields quite accurate results. Our work indicate that the measured high TBCs in experiments can be due to this nonequilibrium effect rather than the other proposed mechanisms, like inelastic phonon transmission and cross-interface electron-phonon coupling.</div></div><div><br></div><div><div>The results obtained in this study will provide a deeper understanding of nanoscale thermal transport across interfaces. This research also provides new perspectives of</div><div>atomic- and nano-scale engineering of materials and structures to enhance performance of thermal management.</div></div>

Ce travail de thèse présente une nouvelle classe de microélectrodes de fibres de nanotubes de carbone (NT). Celles-ci sont réalisées par un filage en voie humide autorisant l’inclusion d’additifs au sein des fibres afin d’adapter leur formulation. Ainsi, le développement d’électrodes incluant la bilirubine oxydase (BOD) pour biopile enzymatique a permis d’obtenir un haut courant de réduction à l’aide d’un transfert d’électrons direct entre BOD et NT. Egalement, des actionneurs électromécaniques incluant une faible quantité de PVA réticulé sont proposés. De telles fibres génèrent une grande contrainte et présentent un temps de réponse court lorsqu’une faible tension leur est appliquée. La mobilité des NT les uns par rapport aux autres au sein de celles-ci a été réduite. Cette dernière est présente dans tout actionneur en NT et génère du fluage et une relaxation de contrainte de ces matériaux limitant ainsi leurs performances. Ces travaux ouvrent de nombreuses voies pour de nouvelles microtechnologies de conversion d’énergie, notamment appliquées au médical ou dans la micro-robotique. / This PhD work presents a new class of carbon nanotubes (NT) fibers microelectrodes. These fibers are produced by a wet spinning process which enables the inclusion of additives within the fibers in order to adapt their formulation. Thus, new microelectrodes for enzymatic biofuel cells that comprise bilirubin oxidase (BOD) have been realized in a one step process and enable a direct electron transfer process between the enzyme and NT at a high potential with a high reduction current. Furthermore, we also developed new NT microfibers including a small quantity of chemically crosslinked PVA for electrochemical actuators. They generate a large stress and a short response time when stimulated by a low voltage in an aqueous electrolyte. Moreover, the CNT mobility within these fibers is greatly reduced. The latter is present in any CNT actuator and induces creep and stress relaxation of these material prohibiting the possibility to obtain high actuating performances. The present results open routes towards the development of novel technologies for energy conversion potentially useful in micro-devices, biomedical applications and micro-robotics.

Robot-assisted minimally invasive surgery is used to perform intricate surgical tasks through small incisions using long, slender instruments. The miniaturization of these instruments is advantageous to both surgeon and patient because smaller instruments reduce trauma to surrounding tissue, decrease patient recovery times, and can be used in confined spaces otherwise inaccessible using larger instruments. However, miniaturization of existing designs is limited by friction between moving parts, the volume occupied by the end effector, and manufacturing and assembly constraints. The objective of this work is to develop and analyze concepts that can be used in robot-assisted needlescopic surgery. The concepts are intended for instrument shafts no larger than 3 mm in diameter. An ideal concept is one with large ranges of wrist and gripping motion. Concepts should also minimize friction and swept volume while maintaining a focus on manufacturability and ease of assembly. Multiple concepts were generated and evaluated using a tree classification scheme, proof-of-concept prototypes, and simplified mathematical models. Three unique concepts were further developed and tested—the Split CORE Grips, the Inverted Flexure Grips, and the Crossed Cylinders Wrist. The two grip concepts are instruments that incorporate one rotational degree of freedom and one gripping degree of freedom. The wrist concept incorporates two rotational degrees of freedom and could be coupled with a single DOF grip mechanism to form a functional instrument. In addition to concept development, a variety of fabrication techniques were investigated to better understand the challenges that arise when designing and fabricating devices at the 3 mm scale. To augment existing techniques, a novel fabrication technique was developed which uses layers of lithographically patterned carbon nanotube (CNT) composite material to form a 3D part. This method was used to prototype some of the designs at a 1:1 size scale.

The widespread environmental occurrence of chemical pollutants presents an ongoing threat to human and ecosystem health. This challenge is compounded by the diversity of chemicals used in industry, commerce, agriculture and medicine, which results in a spectrum of potential fates and exposure profiles upon their inevitable release into the environment. This, in turn, confounds risk assessment, where challenges persist in accurate determination of concentrations levels, as well as spatial and temporal distributions, of pollutants in environmental media (e.g., water, air, soil and sediments). Passive sampling technologies continue to gain acceptance as a means for simplifying environmental occurrence studies and, ultimately, improving the quality of chemical risk assessment. Passive samplers rely on the accumulation of a target analyte into a matrix via molecular diffusion, which is driven by the difference in chemical potential between the analyte in the environment and the sampling media (e.g., sorbent phase). After deployment, the target analyte can be extracted from the sampling media and quantified, providing an integrated, time-weighted average pollutant concentration via a cost-effective platform that requires little energy or manpower when compared to active (e.g., grab) sampling approaches. While a promising, maturing technology, however, limitations exist in current commercially available passive samplers; they are typically limited in the types of chemicals that can be targeted effectively, can require long deployment times to accumulate sufficient chemical for analysis, and struggle with charged analytes. In this dissertation, we have designed a next-generation, nanofiber sorbent as a passive sampling device for routine monitoring of both legacy and emerging organic pollutant classes in water and sediment. The polymer nanofiber networks fabricated herein exhibit a high surface area to volume ratio (SA/V values) which shortens the deployment time. Uptake studies of these polymer nanofiber samplers suggest that field deployment could be shortened to less than one day for surface water analysis, effectively operating as an equilibrium passives sampling device, and twenty days for pore water analysis in soil and sediment studies. By comparison, most commercially available passive sampler models generally require at least a month of deployment before comparable analyses may be made. Another highlight of the nanofiber materials produced herein is their broad target application range. We demonstrate that both hydrophobic (e.g., persistent organic pollutants, or POPs, like PCBs and dioxin) and hydrophilic (e.g., emerging pollutant classes including pesticides, pharmaceuticals and personal care products) targets can be rapidly accumulated with our optimal nanofibers formulations. This suggests that one of our devices could potentially replace multiple commercial passive sampling devices, which often exhibit a more limited range of analyte targets. We also present several approaches for tailoring nanofiber physical and chemical properties to specifically target particular high priority pollutant classes (e.g., PFAS). Three promising modification approaches validated herein include: (i) fabricating carbon nanotube-polymer composites to capture polar compounds; (ii) introducing surface-segregating cationic surfactants to target anionic pollutants (e.g., the pesticide 2,4-D and perfluorooctanoic acid or PFOA); and (iii) use of leachable surfactants as porogens to increase nanofiber pore volume and surface area to increase material capacity. Collectively, outcomes of this work will guide the future development of next generation passive samplers by establishing broadly generalizable structure-activity relationships. All told, we present data related to the influence on the rate and extent of pollutant uptake in polymer nanofiber matrices as a function of both physical (specific surface area, pore volume, and diameter) and chemical (e.g., bulk and surface composition, nanofiber wettability, surface charge) nanofiber properties. We also present modeling results describing sampler operation that can be used to assess and predict passive sampler performance prior to field deployment. The electrospun nanofiber mats (ENMs) developed as passive sampling devices herein provide greater functionality and allow for customizable products for application to a wide range of chemical diverse organic pollutants. Combined with advances in and expansion of the nanotechnology sector, we envision this product could be made commercially available so as to expand the use and improve the performance of passive sampling technologies in environmental monitoring studies.

Nanomaterials (materials which have at least one dimensional feature with length less than 100 nanometers), and carbon nanotubes (CNTs) specifically, have exhibited great potential in water treatment. CNTs are cylindrical structures comprising single or multiple concentric graphene sheets and have diameters from less than 1 nanometer (nm) up to 50 nm (one nm is one millionth of a millimeter). Due to their unique and tunable structural, physical, and chemical properties, CNTs are used in environmental remediation as absorbents, catalysts or catalyst supports, membranes, and electrodes. However, a poorly understood determinant of the role of CNTs in water treatment is their interaction with chemical disinfectants (e.g., chlorine, chloramine, and ozone). To address these existing gaps in the environmental fate and reactivity of CNTs, this work establishes whether CNTs represent precursors for halogen and nitrogen containing disinfection byproducts (DBPs), which are products that form during a reaction of a disinfectant with organic matter in the water. In addition, we seek to understand how reaction with disinfectants alters CNT surface chemistry, and in turn impacts their environmental mobility and cytotoxicity. Finally, we determine how NOM and other aquatic variables known to impact DBP formation (e.g., Br−, NOM, and pH) influence the rate and products of CNT reaction with disinfectants. Outcomes of this work contribute to the current understanding of the role of carbon-based species as DBP precursors in disinfection and provide new context as to the environmental significance and implications of CNTs in natural and engineered aquatic systems.

Objectives: 1 To correlate carbon nanotube (CNT) concentrations measured by Method 5040 relative to particle count concentrations; 2 to correlate CNT concentrations measured by Method 5040 relative to black carbon concentrations measured with an aethalometer; 3 to compare elemental carbon (EC) concentrations measured by Method 5040 among various CNT types and purities. Methods: CNT samples were collected using 25 mm quartz fiber filters and analyzed for EC by Method 5040. An aethalometer was simultaneously used to measure black carbon concentrations. Samples sent for EC analysis included various CNT types (multi-walled, single-walled) and purities (high, low). Levels of EC concentration were subjected to a two-way analysis of variance having two levels of CNT type and two levels of purity. Results: No correlation was established between CNT count and EC concentration, but a correlation was found between CNT volumetric and total carbon (TC) concentration. A significant correlation between black carbon and TC concentration was found. Method 5040 was found to have a positive bias for TC, and the aethalometer was found to have a positive bias for black carbon. Lastly, this study found that CNT type had no effect on EC concentration, but purity did have a significant effect on EC concentration. Conclusions: Samples analyzed by Method 5040 were found to have 6 - 19% EC content, and thus surprisingly high amounts of organic carbon. It is reasoned that significant amounts of impurities were introduced to CNT samples while travelling through the experimental apparatus. When TC concentrations were plotted against black carbon concentrations a significant relationship was found and the bias of Method 5040 and the aethalometer cancelled out. Future research is needed to investigate the aethalometer as a surrogate for Method 5040. Until then, those conducting CNT exposure assessments should use a 25 mm cassette and increase the volume sampled to achieve a reporting limit lower than the NIOSH recommended CNT REL of 7 µg/m3.

Nous nous sommes intéressés à l'optimisation de la dispersion des nanotubes de carbone (CNTs) dans une matrice semi-cristalline : le polyamide 6 (PA-6), et une matrice vitreuse : le poly(méthacrylate de méthyle) (PMMA). Nous avons réalisé des dispersions dans le PA-6 et le PMMA par voie fondu. Nous avons montré que la conductivité et le seuil de conductivité étaient nettement améliorés après un post traitement thermique, quelle que soit la matrice (PMMA ou PA-6). Dans le cas du PMMA, l'étude cinétique de la conductivité en fonction de la température nous a permis de préciser le mécanisme de formation de contacts entre CNTs observé dans le fondu. Une étude structurale et morphologique des composites de PA-6 a été réalisée. Nous montrons la disparition des sphérolites de PA et avons mis en évidence la croissance de lamelles trans-cristallines de PA-6 perpendiculairement à la surface des CNTs. Les CNTs ont aussi été dispersés à partir d'un pré-composite copolymère à blocs/nanotube. Le copolymère à blocs utilisé est un polystyrène-b-polybutadiène-b-polyméthacrylate de méthyle (SBM), les différences de compatibilité de chaque bloc avec les CNTs, la matrice ou le solvant sont utilisées pour stabiliser et disperser les CNTs. L'utilisation du SBM a permis d'améliorer l'état de dispersion dans les deux matrices pour deux méthodes de dispersion : par voie solvant et par voie fondu. Dans le PA-6, l'utilisation du SBM a également permis d'abaisser le seuil de conductivité, grâce à une localisation spécifique des CNTs. Ils sont localisés à l'interface PA-6/SBM pour les pré-composites réalisés par voie fondu et dans la matrice PA-6 pour les pré-composites réalisés par voie solvant.

Microbial electrochemical systems (MESs) have attracted much research attention in recent years due to their promising applications in renewable energy generation, bioremediation, and wastewater treatment. In a MES, microorganisms interact with electrodes via electrons, catalyzing oxidation and reduction reactions at the anode and the cathode. The bacterial community of a high power mixed consortium MESs (maximum power density is 6.5W/m��) was analyzed by using denature gradient gel electrophoresis (DGGE) and 16S DNA clone library methods. The bacterial DGGE profiles were relatively complex (more than 10 bands) but only three brightly dominant bands in DGGE results. These results indicated there are three dominant bacterial species in mixed consortium MFCs. The 16S DNA clone library method results revealed that the predominant bacterial species in mixed culture is Geobacter sp (66%), Arcobacter sp and Citrobacter sp. These three bacterial species reached to 88% of total bacterial species. This result is consistent with the DGGE result which showed that three bright bands represented three dominant bacterial species. Exoelectrogenic bacterial strain SX-1 was isolated from a mediator-less microbial fuel cell by conventional plating techniques with ferric citrate as electron acceptor under anaerobic conditions. Phylogenetic analysis of the 16S rDNA sequence revealed that it was related to the members of Citrobacter genus with Citrobacter sp. sdy-48 being the most closely related species. The bacterial strain SX-1 produced electricity from citrate, acetate, glucose, sucrose, glycerol, and lactose in MFCs with the highest current density of 205 mA/m�� generated from citrate. Cyclic voltammetry analysis indicated that membrane associated proteins may play an important role in facilitating electron transfer from the bacteria to the electrode. This is the first study that demonstrates that Citrobacter species can transfer electrons to extracellular electron acceptors. Citrobacter strain SX-1 is capable of generating electricity from a wide range of substrates in MFCs. This finding increases the known diversity of power generating exoelectrogens and provids a new strain to explore the mechanisms of extracellular electron transfer from bacteria to electrode. The wide range of substrate utilization by SX-1 increases the application potential of MFCs in renewable energy generation and waste treatment. Anode properties are critical for the performance of microbial electrolysis cells (MECs). Inexpensive Fe nanoparticle modified graphite disks were used as anodes to preliminarily investigate the effects of nanoparticles on the performance of Shewanella oneidensis MR-1 in MECs. Results demonstrated that average current densities produced with Fe nanoparticle decorated anodes were up to 5.9-fold higher than plain graphite anodes. Whole genome microarray analysis of the gene expression showed that genes encoding biofilm formation were significantly up-regulated as a response to nanoparticle decorated anodes. Increased expression of genes related to nanowires, flavins and c-type cytochromes indicate that enhanced mechanisms of electron transfer to the anode may also have contributed to the observed increases in current density. The majority of the remaining differentially expressed genes were associated with electron transport and anaerobic metabolism demonstrating a systemic response to increased power loads. The carbon nanotube (CNT) is another form of nano materials. Carbon nanotube (CNT) modified graphite disks were used as anodes to investigate the effects of nanostructures on the performance S. oneidensis MR-1 in microbial electrolysis cells (MECs). The current densities produced with CNT decorated anodes were up to 5.6-fold higher than plain graphite anodes. Global transcriptome analysis showed that cytochrome c genes associated with extracellular electron transfer are up-expressed by CNT decorated anodes, which is the leading factor to contribute current increase in CNT decorated anode MECs. The up regulated genes encoded to flavin also contribute to current enhancement in CNT decorated anode MECs. / Graduation date: 2013

This research is aimed at synthesizing millimeter-scale carbon nanotube arrays (CNTA) by conventional chemical vapor deposition (CCVD) and water-assisted chemical vapor deposition (WACVD) methods, and exploring their application as catalyst supports for electrochemical supercapacitors. The growth mechanism and growth kinetics of CNTA under different conditions were systematically investigated to understand the relationship among physical characteristics of catalyst particles, growth parameters, and carbon nanotube (CNT) structures within CNTAs. Multiwalled CNT (MWCNT) array growth demonstrates lengthening and thickening stages in CCVD and WACVD. In CCVD, the lengthening and thickening were found to be competitive. By investigating catalyst particles after different pretreatment conditions, it has been found that inter-particle spacing plays a significant role in influencing CNTA height, CNT diameter and wall number. In WACVD, a long linear lengthening stage has been found. CNT wall number remains constant and catalysts preserve the activity in this stage, while MWCNTs thicken substantially and catalysts deactivate following the previously proposed radioactive decay model in the thickening stage of WACVD. Water was also shown to preserve the catalyst activity by significantly inhibiting catalyst-induced and gas phase-induced thickening processes in WACVD. Mn3O4 nanoparticles were successfully deposited and uniformly distributed within millimeter-long CNTAs by dip-casting method from non-aqueous solutions. After modification with Mn3O4 nanoparticles, CNTAs have been changed from hydrophobic to hydrophilic without their alignment and integrity being destroyed. The hydrophilic Mn3O4/CNTA composite electrodes present ideal capacitive behavior with high reversibility. This opens up a new route of utilizing ultra-long CNTAs, based on which a scalable and cost-effective method was developed to fabricate composite electrodes using millimeter-long CNTAs. To improve the performance of the composites, -MnO2 nanorods were anodically pulse-electrodeposited within hydrophilic 0.5 mm-thick Mn3O4 decorated CNTAs. The maximum gravimetric capacitance for the MnO2 nanorods/CNTA composite electrode was found to be 185 F/g, and that for -MnO2 nanorods was determined to be 221 F/g. After electrodeposition, the area-normalized capacitance and volumetric capacitance values were increased by a factor of 3, and an extremely high area-normalized capacitance of 1.80 F/cm2 was also achieved for the MnO2 nanorods/CNTA composite. / Materials Engineering

This Ph.D. thesis focuses on the high-frequency electrical capabilities of the carbon-nanotube, field-effect transistor (CNFET). The thesis can be categorized into three stages, leading up to an assessment of the RF capabilities of realistic array-based CNFETs. In the first stage, the high-frequency and time-dependent behavior of ballistic CNFETs is examined by numerically solving the time-dependent Boltzmann transport equation (BTE) self-consistently with the Poisson equation. The RF admittance matrix, which contains the transistor’s y-parameters, is extracted. At frequencies below the transistor’s unity-current-gain frequency fT, the y-parameters are shown to agree with those predicted from a quasi-static equivalent circuit, provided that the partitioning factor for the device charge is properly extracted. It is also shown that a resonance behavior exists in the transistor’s y-parameters. In the second stage, non-quasi-static effects in ballistic CNFETs are examined by analytically developing a transmission-line model from the BTE and Poisson equation. This model includes nonclassical transistor elements, such as the "quantum capacitance" and "kinetic inductance," and it is shown to represent the intrinsic (contact-independent) transistor’s behavior at high frequencies, including a correct prediction of the resonances in the y-parameters. Moreover, it is shown that the kinetic inductance can be represented using lumped elements in the transistor’s small-signal equivalent circuit, and it is demonstrated that the resulting circuit is capable of modeling intrinsic CNFET behavior to frequencies beyond fT. In the last stage, by building upon the first two stages, a comprehensive study is performed to assess the RF performance potential of array-based CNFETs. First, phonon scattering is incorporated into the time-dependent BTE to study the impacts of collisions on different aspects of intrinsic CNFET operation, including the intrinsic fT and the small-signal equivalent circuit. These results are then further extended by adding the effects of extrinsic (contact-dependent) parasitics and then examining the behavior of key RF figures of merit, such as the extrinsic fT, the attainable power gain, and the unity-power-gain frequency. The results are compared to those of state-of-the-art high-frequency transistors and to the next generation of RF CMOS, and they provide an indication of the potential advantages of array-based CNFETs for RF applications. / Micro-Electro-Mechanical Systems (MEMS) and Nanosystems