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The breast cancer susceptibility protein, BRCA1, is a tumor suppressor that helps maintain genomic integrity. Changes in BRCA1 that effect DNA repair processes can fuel cancer induction. The Kelly lab, at the Virginia Tech Carilion Research Institute, has recently developed a new methodology that employs silicon nitride (SiN) microchips to isolate BRCA1 assemblies from the nuclear material of breast cancer cells. These microchips are coated with adaptor proteins that include antibodies against target proteins of interest. The adaptor proteins are added in sequential steps to the coated microchips, followed by an aliquot of sample containing the protein of interest, such as BRCA1. The Kelly lab, partnered with Protochips Inc., developed these devices as a robust, tunable platform to monitor molecular processes, and refer to them as 'Cryo-SiN' in cryo-Electron Microscopy (EM) imaging. We are currently using Cryo-SiN to recruit BRCA1 protein assemblies to the microchip surface under mild conditions, while simultaneously preparing them for cryogenic preservation and EM imaging. This strategy presents a viable alternative to antibody affinity columns that require stringent elution steps to obtain protein complexes from the column. Another advantage of the microchip strategy is that it requires only a 30-minute nuclear extraction, a 60-minute enrichment procedure, and a 5-minute microchip capture step--a total of 95 minutes from initially lysing the cells to plunge-freezing the EM specimens. Therefore, these novel approaches represent a major departure from classical separation procedures that often require days to complete, during which time active protein assemblies can readily dissociate or become inactive. Overall, our use of BRCA1-specific microchips may reveal changes in the BRCA1 architecture during various stages of cancer progression--a major gap in knowledge that persists in cancer research. / M. S. / Modern advances in the imaging technology used for cryogenic electron microscopy (cryo-EM) have offered researchers an extraordinary view into the world of biology at the nanoscale. Supplemental to these technical innovations is the development of tunable substrates based on functional new materials that revolutionize the sequestering of biological components from human cells, such as protein complexes formed in breast cancer cells. New developments of novel viewing substrates, given traditional electron microscopy viewing grids have remained unchanged for decades, is the logical next step into the future of enhanced cryo-EM imaging. Tunable microchip substrates, made using recently enhanced micro-engineering techniques, are currently under development for use in cryo-EM imaging. In this work I have examined these microchip substrates for their capacity to streamline the isolation of biomolecules such as the protein most prominently cited in breast cancer, known as the breast cancer susceptibility protein (BRCA1). Utilizing these novel microchip substrates in the Kelly Lab, I have collected and analyzed data containing BRCA1 proteins, formed in human breast cancer cells, toward the development of 3-dimensional protein structures that allow us to peer into the structure-function relationships of these proteins. New and exciting Cryo-EM data, collected using these newly developed microchips, has the potential to reveal obscure disease mechanisms being propagated at the molecular level in modern clinical practice, such as breast cancer.

With the amalgamation of hard coating in cutting tools industries for three decades now, a stage with proven performance has been reached. Today, nearly 40% of all cutting tools used in machining applications are sheltered with coatings. Coatings have proven to dramatically improve wear resistance, increase tool life and enable use at higher speed. Over the years TiN, TiAlN and TiC have emerged as potential materials to coat machining tools. Chemical vapor deposition was the first technology to be used to deposit these coatings followed by physical vapor deposition. Currently, extensive use is being made of cathodic arc evaporation and sputtering for coatings components. The principal limiting factor in the performance of these cutting tools lies in their failure due to the brittleness of these coatings. These hard coatings, usually coated on soft steel substrates, are subjected to contact damage during service. This contact damage is driven by mismatch strain between the elastically deforming film on a plastically deforming substrate. Understanding of the contact damage is the key parameter for improvement in the coating design. Contact damage involves initiation of cracks and subsequent propagation within coating. Multiple cracking modes are seen in nitride coatings on soft substrate and mutual interaction of cracks may lead to spallation of the coating, exposing the substrate to extreme service conditions. Hence visualization of subsurface crack trajectories facilitates the classification of benign and catastrophic modes of failure, which consequently allows us to tailor the coating architecture to eliminate catastrophic failure. Multilayers have shown to perform better then monolayer coatings. In multilayer coatings, application specific particular properties can be engineered by alternately stack-ing suitable layers. The multilayer utilizes benefits of interfaces by crack deflection, crack blunting and desirable transition in residual stress across the interface. Hence, designing interfaces is the key parameter in the multilayer coating. However, very few studies exist that describe experimental visualization of deformation modes in multilayer coatings with different types of interfaces, e.g. nitride/nitride and nitride/metal. Thus the prime objective of the present study is to comprehend the influence of different interface structures as well as its architecture on the various contact damage modes in these coatings. TiAlN/TiN has shown better tribological properties compared to its constituent monolayers. There is an order of magnitude augmentation in loads for cracking without any hardness enhancement relative to monolayers of constituents, with the additional feature that both constituents exhibit similar hardness and modulus. The resistance to cracking is seen to increase with increase in number of interfaces. Hence this uniqueness in toughening without drastic reduction in mechanical properties provides the motivation for understanding the fundamental mechanisms of toughening provided by the interfaces in these hard/hard coatings. Another combination for the present study is with interfaces between hard-soft phases ZrN/Zr, a composite that seeks to compromise hardness in order to achieve greater toughness. The selected combination has potential of providing a model system without any substoichiometric nitrides influencing the interfacial structure. There is a great need to optimize the metal fraction/thickness for exploiting the benefits of toughening without much compromise on hardness and stiffness, since the principal applications of these coatings lies in preventing erosive and corrosive wear. As all the deformation modes in theses coatings are stress driven, the influence of different variables on stress field would dictate the emerging damage. To understand the role of stress fields on contact damage, finite element method and an analytical model was used to predict the stress field within the coating. The TiAlN/TiN coatings were deposited by cathodic arc evaporation, while sputtering was employed to procure the ZrN/Zr multilayer coatings with much finer layer spacing. Microstructural characterization of the as received coatings was done by XRD, scanning electron microscopy, focused ion beam cross section machining and transmission electron microscopy. Mechanical properties like hardness and modulus were evaluated by nanoindentation with restricted penetration depths to allow measurements that were not influenced by the substrate. Contact damage was induced by micro indentation at high loads. Indentations were examined from plan view as well as cross section for getting details of crack nucleation as well as propagation trajectories. Focused ion beam was used to examine cross sections of indents as well as to prepare electron transparent thin foils for transmission electron microscopy examination of subsurface damage induced by indentation. To emphasize specific issues in detail, the present work is divided into four sections: 1 Microstructure and mechanical characterization of the as deposited coatings of ZrN/Zr multilayer (while that of TiAlN/TiN has been reported elsewhere) 2 Details of contact damage in ZrN/Zr coating 3 Resolution of micro mechanistic issues in TiAlN/TiN coating utilizing detailed microscopy 4 The effect of change in architecture through heat-treatment of ZrN/Zr multilayer coatings on the mechanical behavior and contact damage Detailed microstructural, compositional and mechanical characterization was done on ZrN/Zr as received multilayer coatings. Thickness of metal layer was seen to influence the texture in the nitride, thick metal acquiring basal texture in turn inducing (111) texture in the nitride to reduce interfacial energy. Microstructure revealed that the nitride grows with interrupted columnar grains, renucleating at each metal/nitride interface. Presence of both phases was confirmed at even very low bilayer spacing, with slight changes in multilayers architecture, from planar interfaces to curved interfaces. The chosen system proved to be an ideal system for multilayer study without formation of secondary nitrides. Residual stress and hardness reduced with increase in metal layer thickness, whereas modulus was seen to follow the rule of mixture value. Detailed contact damage study of ZrN/Zr is reported in section two with influence of volume fraction and metal layer thickness. All the experimental results were corroborated with finite element methods. A comparative study of contact damage of multilayer with monolayer was carried out with cross section as well as plan view of indents. Metal plasticity was able to distribute damage laterally as well as vertically, hence reducing the stress concentration. There lies an optimum thickness of the metal providing maximum toughening by increasing the threshold load required for edge cracking. The sliding of columns is resisted by the metal. However, thick metal layers promote microcracking in individual nitride layers. Cracking is restricted to within individual nitride layers, eliminating through thickness cracking. The intermediate metal thickness was able to provide a mechanism of laterally distributing sliding and hence a higher tolerance level of the indentation strain that can be accommodated without cracking. Thin metal multilayers were seen to show delamination, strongly influenced by the multilayer architecture. We use the finite element method to understand the influence of stress fields in driving these various modes of damage for varying volume fraction and metal layer thicknesses. It is demonstrated how metal plasticity results in stress enhancement in the nitride layer compared to a monolayer and reduces the shear stress, which is the driving force for columnar sliding. The micro cracking to columnar shearing transition with metal thickness was explained with the help of average shear and normal stress across the multilayer which could explain the transition from cracking and sliding to interfacial delamination in thin metal layer multilayers with enhancement in interfacial shear stress. TiAlN/TiN multilayer allowed to exploit a form of compositional contrast to measure the strain with respect to depth. Layers acting as strain markers quantify the amount of sliding in terms of the offset in layers with respect to depth within the coating. We illustrate with transmission electron micrographs, the flaw generation that occurs as a result of sliding of misaligned column boundaries. These boundary kinks,upon further loading, may lead to cracks running at an angle to the indentation axis in an otherwise dense, defect free, as deposited coating. A previous study illustrates the increase in resistance of multilayers to multiple modes of cracking that are seen in the monolayer nitride coatings on steel substrates. We provide evidence of the enhanced plasticity, seen as macroscopic bending, which in reality is column sliding in a series of distributed small steps. We discuss the role of misfit dislocations in spreading the material laterally to accommodate the constraints during indentation and lattice bending. Interfacial sliding is seen to reduce the stress concentration by distributing the vertical column sliding and accommodating the flaws generated by the sliding of misaligned column boundaries. Some preferred boundaries with special orientation relations do slide, while near the substrate, the sliding is facilitated by the relaxation in intrinsic residual stresses. An analytical model which was formulated earlier is used to support our experimental findings. Investigations of the plausible reasons for the naturally occurring multilayer mollusc sea shells to reach stiffnesses equal to the upper bound of the rule of mixture value have concluded that its brick and mortar organization is responsible for its exceptional mechanical properties. Inspired by the same model, heat treatment was used to change the architecture of the soft-hard metal/nitride combination from that of the planar interface of the as deposited multilayer to a brick and mortar arrangement. Such an interconnected ZrN microstructure was successfully achieved and the stiffness and hardness were both seen to increase relative to the as received coatings. The possible reasons for this enhancement are discussed in term of this newly emerged architecture ,change in residual stress as well as changes in stoichiometry after heat treatment. The contact damage, though, was found to be more catastrophic relative to the as deposited coating with increased propensities for edge and lateral cracking. This was attributed to the interconnected nitrides formed in the brick and mortar architecture as well as residual stress changes due to the dissolution of Zr in ZrN to form off-stoichiometric nitrides. The cracks feel the presence of the metal and deviate from the otherwise smooth trajectory and take a path along the interface of the metal packet and the interconnected nitride. Summarizing, the present study clearly illustrates the fact that interfaces play an important role in damage control under contact loading. Fracture and deformation are either controlled by metal plasticity, distributing the column sliding in metal/nitride multilayers or by interfacial sliding mediated by interfacial misfit dislocations in case of the nitride/nitride multilayer coatings. The effective role of interfaces is to distribute damage laterally as well as horizontally to relieve stresses and hence enhance the damage tolerance under indentation. Optimum metal layer thickness has been proposed for maximum toughening in the metal/nitride multilayer coating and the role of interfaces in providing modes of plasticity is presented for the nitride/nitride multilayer coatings by use of extensive transmission electron microscopic investigations. A new interconnected architecture coatings provides a unique way of combining stiffness and toughness along with scope for further developing such configurations with improved mechanical properties.

[Truncated abstract] Over the past decade, the group III-nitride semiconducting compounds (GaN, AlN, InN, and their alloys) have attracted tremendous research efforts due to their unique electronic and optical properties. Their low thermal carrier generation rates and large breakdown fields make them attractive for the development of robust electronic devices capable of reliable operation in extreme conditions, i.e. at high power/voltage levels, high temperatures and in radiation environments. For device applications in radiation environments, such as space electronics, GaN-based devices are expected to manifest superior radiation hardness and reliability without the need for cumber- some and expensive cooling systems and/or radiation shielding. The principle aim of this Thesis is to ascertain the level of susceptibility of current GaN-based elec- tron devices to radiation-induced degradation, by undertaking a detailed study of 60Co gamma-irradiation-induced defects and defect-related effects on the electrical characteristics of n-type GaN-based materials and devices . . . While the irradiation-induced effects on device threshold voltage could be regarded as relatively benign (taking into account that the irradiation levels employed in this study are equivalent to more than 60 years exposure at the average ionising dose rate levels present in space missions), the observed device instabilities and the degradation of gate current characteristics are deleterious effects which will have a significant impact on the performance of AlGaN/GaN HEMTs operating in radiation environments at low temperatures, a combination of conditions which are found in spaceborne electronic systems.

The popularity of III-nitride materials has taken up the semiconductor industry to newer applications because of their remarkable properties. In addition to having a direct and wide band gap of 3.4 eV, a very fascinating property of GaN is the band gap tuning from 0.7 to 6.2 eV by alloying with Al or In. The most common orientation to grow optoelectronic devices out of these materials are the polar c-plane which are strongly affected by the intrinsic spontaneous and piezoelectric polarization fields. Devices grown in no polar orientation such as (1 0 –1 0) m-plane or (1 1 –2 0) a-plane have no polarization in the growth direction and are receiving a lot of focus due to enhanced behaviour. The first part of this thesis deals with the development of non-polar epimGaN films of usable quality, on an m-plane sapphire by plasma assisted molecular beam epitaxy. Growth conditions such as growth temperature and Ga/N flux ratio were tuned to obtain a reasonably good crystalline quality film. MSM photodetectors were fabricated from (1 0 -1 0) m-GaN, (1 1 -2 0) a-GaN and semipolar (1 1 -2 2) GaN films and were compared with the polar (0 0 0 2) c-GaN epilayer. Later part of the thesis investigated (1 0 -1 0) InN/ (1 0 -1 0) GaN heterostructures. Further, we could successfully grow single composition nonpolar a-plane InxGa1-xN epilayers on (1 1 -2 0) GaN / (1 -1 0 2) sapphire substrate. This thesis focuses on the growth and characterisation of nonpolar GaN, InxGa1-xN and InN by plasma assisted molecular beam epitaxy and on their photodetection potential. Chapter 1 explains the motivation of this thesis work with an introduction to the III-nitride material and the choice of the substrate made. Polarization effect in the polar, nonpolar and semipolar oriented growth is discussed. Fabrication of semiconductor photodetectors and its principle is explained in details. Chapter 2 discusses the various experimental tools used for the growth and characterisation of the film. Molecular beam epitaxy technique is elaborately explained along with details of the calibration for the BEP of various effusion cells along with growth temperature at the substrate. Chapter 3 discusses the consequence of nitridation on bare m-sapphire substrate. Impact of nitridation step prior to the growth of GaN film over (1 0 -1 0) m-sapphire substrate was also studied. The films grown on the nitridated surface resulted in a nonpolar (1 0 -1 0) orientation while without nitridation caused a semipolar (1 1 -2 2) orientation. Room temperature photoluminescence study showed that nonpolar GaN films have higher value of compressive strain as compared to semipolar GaN films, which was further confirmed by room temperature Raman spectroscopy. The room temperature UV photodetection of both films was investigated by measuring the I-V characteristics under UV light illumination. UV photodetectors fabricated on nonpolar GaN showed better characteristics, including higher external quantum efficiency, compared to photodetectors fabricated on semipolar GaN. Chapter 4 focuses on the optimization and characterisation of nonpolar (1 0 -1 0) m-GaN on m-sapphire by molecular beam epitaxy. A brief introduction to the challenges in growing a pure single phase nonpolar (1 0 -1 0) GaN on (1 0 -1 0) sapphire without any other semipolar GaN growth is followed by our results achieving the same. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of m-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. Au/ nonpolar GaN schottky diode was fabricated and temperature dependent I-V characteristics showed rectifying nature. Chapter 5 demonstrates the growth of (1 0 -1 0) m-InN / (1 0 -1 0) m-GaN / (1 0 -1 0) m-sapphire substrate. Nonpolar InN layer was grown at growth temperature ranging from 3900C to 440C to obtain a good quality film at 4000C. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. RSM images on the asymmetric plane revealed highly strained layers. InN band gap was found to be around 0.8 eV from absorption spectra. The valance band offset value is calculated to be 0.93 eV for nonpolar m-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.82 eV. Chapter 6 focuses on the optimization of nonpolar (1 1 -2 0) a-GaN on (1 -1 0 2) r-sapphire by molecular beam epitaxy. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of a-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrate. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. UV photo response of a-GaN film was measured after fabricating an MSM structure over the film with Au. EQE of the photodetectors fabricated in the (0 0 0 2) polar and (1 1 -2 0) nonpolar growth directions were compared in terms of responsively, nonpolar a-GaN showed the best sensitivity at the cost of comparatively slow response time. Chapter 7 demonstrates the growth of non-polar (1 1 -2 0) a-plane InGaN epilayers on a-plane (1 1 -2 0) GaN/ (1 -1 0 2) r-plane sapphire substrate using PAMBE. The high resolution X-ray diffraction (HRXRD) studies confirmed the orientation of the films and the compositions to be In0.19Ga0.81N, In0.21Ga0.79N and In0.23Ga0.77N. The compositions of the films were controlled by the growth parameters such as growth temperature and indium flux. Effect of variation of Indium composition on the strain of the epilayers was analyzed from the asymmetric RSM images. Further, we report the growth of self-assembled non-polar high indium clusters of In0.55Ga0.45N over non-polar (1 1 -2 0) a-plane In0.17Ga0.83N epilayer grown on a-plane (1 1 -2 0) GaN / (1 -1 0 2) r-plane sapphire substrate. The structure hence grown when investigated for photo-detecting properties, showed sensitivity to both infrared and ultraviolet radiations due to the different composition of InGaN region. Chapter 8 concludes with the summary of present investigations and the scope for future work.

Through the history of mankind, novel materials have played a key role in techno- logical progress. As we approach the limits of scaling it becomes difficult to squeeze out any more extensions to Moore’s law by just reducing device feature sizes. It is important to look for an alternate semiconductor to silicon in order to continue making the progress predicted by Moore’s law. Among the various semiconductor options being explored world-wide, the III-nitride semiconductor material system has certain unique characteristics that make it one of the leading contenders. We explore the III-nitride semiconductor material system for the unique advantages that it offers over the other alternatives available to us. This thesis studies the device applications of epitaxial III-nitride films and nanos- tructures grown using plasma assisted molecular beam epitaxy (PAMBE) The material characterisation of the PAMBE grown epitaxial III-nitrides was car- ried out using techniques like high resolution X-ray diffraction (HR-XRD), field emis- sion scanning electron microscopy (FESEM), room temperature photoluminescence (PL) and transmission electron microscopy (TEM). The epitaxial III-nitrides were then further processed to fabricate devices like Schottky diodes, photodetectors and surface acoustic wave (SAW) devices. The electrical charcterisation of the fabricated devices was carried out using techniques like Hall measurement, IV and CV measure- ments on a DC probe station and S-parameter measurements on a vector network analyser connected to an RF probe station. We begin our work on Schottky diodes by explaining the motivation for adding an interfacial layer in a metal-semiconductor Schottky contact and how high-k di- electrics like HfO2 have been relatively unexplored in this application. We report the work carried out on the Pt/n-GaN metal-semiconductor (MS) Schottky and the Pt/HfO2/n-GaN metal-insulator-semiconductor (MIS) Schottky diode. We report an improvement in the diode parameters like barrier height (0.52 eV to 0.63 eV), ideality factor (2.1 to 1.3) and rectification ratio (35.9 to 98.9 @2V bias) after the introduction of 5 nm of HfO2 as the interfacial layer. Temperature dependent I-V measurements were done to gain a further understanding of the interface. We observe that the barrier height and ideality factor exhibit a temperature dependence. This was attributed to inhomogeneities at the interface and by assuming a Gaussian distribution of barrier heights. UV and IR photodetectors using III-nitrides are then studied. Our work on UV photodetectors describes the growth of epitaxial GaN films. Au nanoparticles were fabricated on these films using thermal evaporation and annealing. Al nanostruc- tures were fabricated using nanosphere lithography. Plasmonic enhancement using these metallic nanostructures was explored by fabricating metal-semiconductor-metal (MSM) photodetectors. We observed plasmonic enhancement of photocurrent in both cases. To obtain greater improvement, we etched down on the GaN film using reac tive ion etching (RIE). This resulted in further increase in photocurrent along with a reduction in dark current which was attributed to creation of new trap states. IR photodetectors studied in this thesis are InN quantum dots whose density can be controlled by varying the indium flux during growth. We observe that increase in InN quantum dot density results in increase in photocurrent and decrease in dark current in the fabricated IR photodetectors. We then explore the advantages that InGaN offers as a material that supports surface acoustic waves and fabricate InGaN based surface acoustic wave devices. We describe the growth of epitaxial In0.23 Ga0.77 N films on GaN template using molecular beam epitaxy. Material characterisation was carried out using HR-XRD, FESEM, PL and TEM. The composition was determined from HR-XRD and PL measurements and both results matched each other. This was followed by the fabrication of interdigited electrodes with finger spacing of 10 µm. S-parameter results showed a transmission peak at 104 MHz with an insertion loss of 19 dB. To the best of our knowledge, this is the first demonstration of an InGaN based SAW device. In summary, this thesis demonstrates the practical advantages of epitaxially grown film and nanostructured III-nitride materials such as GaN, InN and InGaN using plasma assisted molecular beam epitaxy for Schottky diodes, UV and IR photodetec- tors and surface acoustic wave devices.

The discovery of carbon nanotubes (CNTs) in 1991 by Iijima revealed the possibility of ultra-strong materials exploiting the properties of materials at smaller length scales. The superior strength, stiffness, and ability to perform under extreme conditions motivated researchers to investigate further on CNTs and similar materials at nanoscale. This resulted in discovery of various nanostructures such boron nitride nanotubes (BNNTs), graphene, hexagonal boron nitride sheets etc. Many of such nanostructures exhibited superior strength and stiffness comparable to that of CNTs. Out of these nanotubes, BNNTs have recently attracted attention from researchers due to their excellent mechanical properties similar to that of CNTs along with better chemical and thermal stability. Thus, BNNTs can be used for varieties of applications such as protective shield for nanomaterials, optoelectronics, bio-medical, nano spintronics, field-emission tips in scanning tunneling and atomic force microscope, and as reinforcement in composites. BNNTs are also used in other applications such as water cleansing, hydrogen storage, and gas accumulators. To exploit these ultra-strong materials, the mechanics of materials under different conditions of loading and failure need to be studied and understood. Also, to make use of the material in a nanocomposite or other applications, the material properties should be evaluated. The present work is focused on the computational study of the mechanics of nanotubes with special reference to BNNTs and CNTs. Note that the attention is not given to the material but to the nanostructure and mechanics. Hence depending on the state-of-the-art, BNNTs and CNTs are used wherever it is appropriate along with justifications. The chapter-wise outline of the present work is given below. The first chapter is an introduction along with a state-of-the-art literature review. The second chapter introduces the molecular simulation methodology in brief. The chapters from the third to the seventh present the work in detail and describe the major contributions. The final chapter summarizes the work along with a few possible directions to extend the present work. Chapter 1 In this chapter, the importance of computational techniques to study the mechanics at the nanoscale is outlined. A brief introduction to various nanostructures and nanotubes are also given. A detailed literature review on the mechanics of nanotubes with special attention to elastic properties, buckling, tensile failure, and as reinforcement in nanocomposites is presented. Chapter 2 In this chapter, the molecular simulation technique is outlined. The molecular dynamics (MD) simulation is one of the most common simulation techniques used to study materials at the nanoscale. A few interatomic potentials that are used in an MD simulation are explained. Theories linking continuum mechanics with the molecular dynamics are also explained here. Chapter 3 In this chapter, the elastic behavior of single-walled BNNTs under axial and torsional loading is studied. Molecular dynamics (MD) simulation is carried out with a tersoff potential for modeling the interatomic interactions. Different chiral configurations with similar diameter are considered to study the effect of chirality on the elastic and shear moduli. Furthermore, the effects of tube length on elastic modulus are also studied by considering different aspects ratios. It is observed that both elastic and shear moduli depend on the chirality of a nanotube. For aspect ratios less than 15, the elastic modulus reduces monotonically with an increase in the chiral angle. For chiral nanotubes the torsional response shows a dependence on the direction of loading. The difference between the shear moduli against and along the chiral twist directions is maximum for a chiral angle of 15◦, and zero for zigzag (0◦) and armchair (30◦) configurations. Chapter 4 Buckling of nanotubes have been studied using many methods such as MD, molecular mechanics, and continuum based shell theories. In MD, motion of the individual atoms are tracked under an applied temperature and pressure, ensuring a reliable estimate of the material response. The response thus simulated varies for individual nanotubes and is only as accurate as the force field used to model the atomic interactions. On the other hand, there exists a rich literature on the understanding of continuum mechanics based shell theories. Based on the observations on the behavior of nanotubes, there have been a number of shell-theory-based approaches to study the buckling of nanotubes. Although some of these methods yield a reasonable estimate of the buckling stress, investigation and comparison of buckled mode shapes obtained from continuum analysis and MD are sparse. Previous studies show that a direct application of shell theories to study nanotube buckling often leads to erroneous results. In this chapter, the nonlocal effect on the mechanics of nanostructures is studied using Eringen’s nonlocal elasticity. The buckling of carbon nanotubes is considered as an example to demonstrate and understand the nonlocal effect in the nanotubes. Single-walled armchair nanotubes with the radius varying from 3.4nm to 17.7nm are considered and their critical buckling stresses are predicted based on multiscale modeling techniques including classical and nonlocal continuum mechanics theories and MD simulation. Fitting nonlocal mechanics models to MD simulation yields a radius-dependent length-scale parameter, which increases approximately linearly with the radius of carbon nanotube. In addition, the nonlocal shell model is found to be a better continuum model than the nonlocal beam model due to its ability to include the circumferential nonlocal effect. Chapter 5 In this chapter, the effects of geometrical imperfections on the buckling of nanotubes are studied. The present study reveals that a major source of the error in continuum shell theories in calculating the buckling stress can be attributed to the geometrical imperfections. Here, geometrical imperfections refer to the departure of the shape of the nanotube from a perfect cylindrical shell. Analogous to the shell buckling in the macro-scale, in this work the nanotube is modeled as a thin-shell with initial imperfection. Then a nonlinear buckling analysis is carried out using the Riks method. It is observed that this proposed approach yields significantly improved estimate of the buckling stress and mode shapes. It is also shown that the present method can account for the variation of buckling stress as a function of the temperature considered. Hence, this turn out to be a robust method for a continuum analysis of nanotubes taking in the effect of variation of temperature as well. Chapter 6 In this chapter, the effects of Stone-Wales (SW) and vacancy defects on the failure behavior of BNNTs under tension are investigated using MD simulations. The Tersoff-Brenner potential is used to model the atomic interaction and the temperature is maintained close to 300 K. The effect of a SW defect is studied by determining the failure strength and failure mechanism of nanotubes with different radii. In the case of a vacancy defect, the effect of an N-vacancy and a B-vacancy is studied separately. Nanotubes with different chirality but similar diameter are considered first to evaluate the chirality dependence. The variation of failure strength with the radius is then studied by considering nanotubes of different diameter but same chirality. It is observed that the armchair BNNTs are extremely sensitive to defects, whereas the zigzag configurations are the least sensitive. In the case of pristine BNNTs, both armchair and zigzag nanotubes undergo brittle failure, whereas in the case of defective BNNTs only the zigzag ones undergo brittle failure. An interesting defect-induced plastic behavior is observed in defective armchair BNNTs. For this nanotube, the presence of a defect triggers mechanical relaxation by bond breaking along the closest zigzag helical path, with the defect as the nucleus. This mechanism results in a plastic failure. Chapter 7 In this chapter, the utility of BNNTs as reinforcement for nanocomposites with metal matrix is studied using MD simulation. Due to the light weight, aluminium is used as the matrix. The influence of number of walls on the strength and stiffness of the nanocomposite is studied using single-and double-walled BNNTs. The three body tersoff potential is used to model the atomic interactions in BNNTs, while the embedded atom method (EAM) potential is used to model the aluminium matrix. The van der Waals interaction between different groups — the aluminium matrix with the nanotube or the between the concentric tubes in double walled BNNT — is modeled using a Lennard Jones potential. A representative volume element approach is used to model the nanocomposite. The constitutive relations for the nanocomposite is also proposed wherein the elastic constants are obtained using the MD simulation. The nanocomposite with reinforcement shows improved axial stiffness and strength. The double-walled BNNT provides more strength to the nanocomposite than the single-walled BNNT. The BNNT reinforcement can be used to design nanocomposites with varying strength depending on the direction of the applied stress. Chapter 8 The summary of the work with a broad outlook is presented in this chapter. The major conclusions of the work are reiterated and possible directions for taking the work further ahead are mentioned.

In this work a manufacturing method for UN, ZrN and (U,Zr)N pellets was established at the nuclear fuel laboratory at KTH Stockholm/Sweden, which consists of the production of nitride powders and their sintering into pellets by spark plasma sintering. The nitride powders were produced by the hydriding-nitriding route using pure metal as starting material. This synthesis was performed in a stream of the particular reaction gas. A synthesis control and monitoring system was developed, which can follow the reactions in real time by measuring the gas flow difference before and after the reaction chamber. With the help of this system the hydriding and nitriding reactions of uranium and zirconium were studied in detail. Fine nitride powders were obtained; however, the production of zirconium nitride involved one milling step of the brittle zirconium hydride. Additionally uranium and zirconium alloys with different zirconium contents were produced and synthesized to nitride powders. It was found that also the alloys could be reduced to fine powder, but only by cyclic hydriding-dehydriding. Pellets were sintered out of uranium nitrides, zirconium nitrides, mixed nitrides and alloy nitrides. These experiments showed that relative densities of more than 90% can easily be achieved for all those powders. Pellets sintered from mechanically mixed nitride powders were found to still consist of two separate nitride phases, while nitride produced from alloy was demonstrated to be a monophasic solid solution both as powder and as sintered pellets.

This thesis discusses the growth of thin-film silicon layers suitable for solar cells using liquid phase epitaxy and the behaviour of oxide LPCVD silicon nitride stacks on silicon in a high temperature ambient.¶ The work on thin film cells is focussed on the characteristics of layers grown using liquid phase epitaxy. The morphology resulting from different seeding patterns, the transfer of dislocations to the epitaxial layer and the lifetime of layers grown using oxide compared with carbonised photoresist barrier layers are discussed. The second half of this work discusses boron doping of epitaxial layers. Simultaneous layer growth and boron doping is demonstrated, and shown to produce a 35um thick layer with a back surface field approximately 3.5um thick.¶ If an oxide/nitride stack is formed in the early stages of cell processing, then characteristics of the nitride may enable increased processing flexibility and hence the realisation of novel cell structures. An oxide/nitride stack on silicon also behaves as a good anti- reflection coating. The effects of a nitride deposited using low pressure chemical vapour deposition on the underlying wafer are discussed. With a thin oxide layer between the silicon and the silicon nitride, deposition is shown not to significantly alter effective life-times.¶ Heating an oxide/nitride stack on silicon is shown to result in a large drop in effective Lifetimes. As long as at least a thin oxide is present, it is shown that a high temperature nitrogen anneal results in a reduction in surface passivation, but does not significantly affect bulk lifetime. The reduction in surface passivation is shown to be due to a loss of hydrogen from the silicon/silicon oxide interface and is characterised by an increase in Joe. Higher temperatures, thinner oxides, thinner nitrides and longer anneal times are all shown to result in high Joe values. A hydrogen loss model is introduced to explain the observations.¶ Various methods of hydrogen re-introduction and hence Joe recovery are then discussed with an emphasis on high temperature forming gas anneals. The time necessary for successful Joe recovery is shown to be primarily dependent on the nitride thickness and on the temperature of the nitrogen anneal. With a high temperature forming gas anneal, Joe recovery after nitrogen anneals at both 900 and 1000oC and with an optimised anti-reflection coating is demonstrated for chemically polished wafers.¶ Finally the effects of oxide/nitride stacks and high temperature anneals in both nitrogen and forming gas are discussed for a variety of wafers. The optimal emitter sheet resistance is shown to be independent of nitrogen anneal temperature. With textured wafers, recovery of Joe values after a high temperature nitrogen anneal is demonstrated for wafers with a thick oxide, but not for wafers with a thin oxide. This is shown to be due to a lack of surface passivation at the silicon/oxide interface.

A new material system was developed for fabricating the combustion engine of an unmanned aerial vehicle. The material system consisted of a mixture of nanoscale and microscale particles of silicon nitride. Magnesia and yttria were used as sintering additives. The powders were mixed with a paraffin binder system. The binder-powder was analyzed for its properties and molding attributes. The study involved several steps of the development and processing. These steps include torque rheometery analysis, mixing scale-up, property measurements of binder-powder, injection molding, binder removal, sintering, scanning electron microscopy analysis and mechanical properties measurements. Simulations of the injection molding process were conducted to assess the feasibility of manufacturing a ceramic engine and to determine its optimal process parameters. The model building required for the simulation was based on flow and solidification behavior data compiled for the binder-powder mixture. The simulations were performed using the Moldfow software package. A design of experiments approach was set up in order to gain an understanding of critical process parameters as well as identifying a feasible process window. Quality criteria were then analyzed in order to determine the optimal production parameters. The study resulted in the successful development of design parameters that will enable fabrication of silicon nitride engine components by powder injection molding. / Graduation date: 2012