Nanowires are high aspect ratio nanostructures with structural diameters on the order of nanometers to hundreds of nanometers. In this work, the optical properties of highly crystalline silicon nanowires grown by the Vapor-Liquid-Solid (VLS) method surrounded by a thin silicon shell are investigated for thin film solar cell applications. Crystalline core nanowires were surrounded by a conformal amorphous silicon shell and exhibited extremely high absorption of 95% at short wavelengths (??<550nm) and very low absorption of <2% at long wavelengths (??>780nm). Nanowires were disordered with average lengths ranging from 1.3 to 2.3 ??m. The absorption increased at longer wavelengths as a function of amorphous shell radial thickness, significantly higher than the absorption of a reference planar a-Si thin film. In addition, a new method to grow epitaxial silicon at low growth temperatures on glass substrates is demonstrated. Highly crystalline silicon nanowires with an average length of 800 nm were used as the seed crystal to grow an epitaxial silicon shell around, using a low temperature process. The nanowire core was grown at 400??C, and the shell was grown at about 150??C. Such epitaxial grown nanowire shells could be used as a building block for nanotechnology applications in which epitaxial silicon is required over large-area substrates such as glass. Furthermore, the epitaxial silicon shell nanowires exhibited absorption > 90% up to a wavelength of 600 nm, which was significantly higher than that of a planar 1 ??m nanocrystalline silicon film. The high absorption exhibited by nanowires with both amorphous and crystalline silicon shells makes them promising for use in photovoltaic and photodetector applications. Silicon nanowires were incorporated into thin film silicon n-i-p solar cells in two configurations: as a nanostructured back reflector, and in core-shell nanowire solar cells. First, domed-shaped nanostructures were fabricated by coating an array of silicon nanowires with a thick layer of amorphous silicon. After the nanostructures were coated with Ag and ZnO:Al, they were used as the backreflector in an n-i-p amorphous silicon solar cell. The nanostructured backreflector improved light scattering within the solar cell, leading to a short circuit current of 14.8mA/cm2, a 13% improvement over that of the planar device, which had a Jsc=13.1 mA/cm2. The overall conversion efficiency of nanostructured backreflector device was ?? = 8.87%, a strong improvement over that of the planar device (?? = 7.47%). Silicon nanowires were also incorporated into core-shell nanowire solar cells. The first device architecture investigated consisted of nanowires incorporated as the intrinsic absorption layer between a planar n+ layer and conformal p+ layer. However, the fabricated devices exhibited very low collection efficiencies of < 2% due to the presence of impurities incorporated by the catalyst used during nanowire growth. As a result, the device architecture was modified such that the nanowires provided high aspect ratio structure to enhance absorption in a shell material, but the nanowires themselves were not used as an active device component. Nanowire core-amorphous silicon shell solar cells, on average 525 nm long and about 350nm in total diameter, exhibited an impressive low total reflectance of <3% in the wavelength interval of 410 nm < ?? < 640nm and exceeded 10% only for ??>700 nm. As a result, the core-shell nanowire devices exhibited enhancement in quantum efficiency at low wavelengths, ?? < 500nm and high wavelengths, ?? > 600nm as compared to a planar device. The resulting short circuit current was 14.1 mA/cm2 compared to 12.3 mA/cm2 for the planar device, an improvement of ~15%. Nanowire core- nanocrystalline silicon shell solar cells were also fabricated using the same device architecture. Core-shell nanowires with an average length of 800 nm showed significant enhancement in quantum efficiency over all wavelengths as compared to a 1 ??m thick planar solar cell. The core-shell nanowire device had a short-circuit current of 16.2 mA/cm2 , a ~25% improvement over that of the planar thin film solar cell (Jsc=13.0 mA/cm2). Core-shell nanowire devices did, however, have lower open circuit voltage compared to the planar device. Non-conformal coverage was found to be a limiting factor in device performance, but further improvements can be expected with optimization of the n-i-p deposition conditions and nanowire density.

Nanocrystalline (NC) metals and alloys are known to possess superior mechanical properties, e.g., strength, hardness, and wear-resistance, as compared to conventional microcrystalline materials. NC metals are characterized by a mean grain size <100 nm; in this grain size regime, inelastic deformation can occur via a combination of interface-mediated mechanisms viz., grain boundary sliding/migration, and dislocation nucleation from grain boundary sources. Recent studies have suggested that these interface-mediated inelastic deformation mechanisms in fcc metals are influenced by non-glide stresses and interfacial free volume, unlike dislocation glide mechanisms that operate in microcrystalline fcc metals. Further, observations of tension-compression strength asymmetry in NC metals raise the possibility that yield and inelastic flow in these materials may not be adequately described by solely the deviatoric stress. Unfortunately, most literature concerning the mechanical testing of NC metals is limited to uniaxial deformation or nanoindentation techniques, and the multiaxial deformation behavior is often predicted assuming initially isotropic yield and subsequent flow normal to the yield surface. The primary objective of this thesis is to obtain a better understanding of the nature of inelasticity in NC metals by simulating multiaxial deformation at the atomistic resolution, and developing methods to interpret the results in ways that would be useful from a continuum constitutive modeling viewpoint. First, we have presented a novel, statistical mechanics-based approach to unambiguously resolve the elastic-plastic transition as an avalanche in the proliferation of mobile defects. This approach is applied to nanocrystalline Cu to explore the influence of pressure and multiaxial stress states on the inelastic deformation behavior. The results suggest that initial yield in nanocrystalline Cu under biaxial loading is only weakly anisotropic in the 5 nm grain size regime, and that plastic flow evolves in a direction normal to the von Mises yield surface. However, triaxial deformation simulations reveal a significant effect of the superimposed hydrostatic stress on yielding under shear. These results are analyzed in detail in order to assess the influence of pre-existing internal stresses and interfacial excess volume on the inelastic deformation behavior. Further, we have studied the effects of imposed hydrostatic pressure on the shear deformation behavior of Cu bicrystals containing symmetric tilt interfaces, as well as Cu nanocrystals of different grain sizes. Most interfaces exhibit an increase in shear strength with imposed compressive hydrostatic pressure. However, for some interfaces, this trend is reversed. Neither the sign nor the magnitude of the pressure-induced elevation in shear strength appears to correlate with interface structure or particular deformation mechanism(s). In Cu nanocrystals, we observe that imposed compressive pressure leads to strengthening under shear deformation, and the effect of imposed pressure on the shear strength becomes stronger with increase in grain size or temperature. Activation parameters for shear deformation have been computed for these nanocrystals, and computed values seem to agree with existing experimental and theoretical estimates. Finally, we have proposed some modifications to conventional isothermal molecular dynamics algorithms, in order to isolate dislocation nucleation events from interfacial sources, and thereby permit explicit computation of the activation parameters for such events.

This thesis investigates the effect of microstructural strengthening mechanisms on the overall mechanical performance of micro-truss periodic cellular metals (PCMs). Prior to the author’s work, the primary design considerations of micro-truss PCMs had been topological issues, i.e. the architectural arrangement of the load-supporting ligaments. Very little attention had been given to investigate the influence of microstructural effects within the cellular ligaments. Of the four broad categories of strengthening mechanisms in metals, only solute and second phase strengthening had previously been used in micro-trusses; the potential for strengthening micro-truss materials by work-hardening or grain size reduction had not been addressed. In order to utilize these strengthening mechanisms in micro-truss PCMs, two issues needed to be addressed. First, the deformation-forming method used to produce the micro-trusses was analyzed in order to map the fabrication-induced (in-situ) strain as well as the range of architectures that could be reached. Second, a new compression testing method was developed to simulate the properties of the micro-truss as part of a common functional form, i.e. as the core of a light-weight sandwich panel, and test the effectiveness of microstructural strengthening mechanisms without the influence of typical high-temperature sandwich panel joining processes, such as brazing. The first strengthening mechanism was achieved by controlling the distribution of plastic strain imparted to the micro-truss struts during fabrication. It was shown that this strain energy can lead to a factor of three increase in compressive strength without an associated weight penalty. An analytical model for the critical inelastic buckling stress of the micro-truss struts during uniaxial compression was developed in terms of the axial flow stress during stretch forming fabrication. The second mechanism was achieved by electrodeposition of a high-strength nanocrystalline metal sleeve around the cellular ligaments, producing new types of hybrid nanocrystalline cellular metals. It was shown that despite the added mass, the nanocrystalline sleeves could increase the weight-specific strength of micro-truss hybrids. An isostrain model was developed based on the theoretical behaviour of a nanocrystalline metal tube network in order to predict the compressive strength of the hybrid materials.

Nanomechanics emerged as chemists and physicists began fabricating nanoscale objects. However, there are some materials that have neither been fabricated nor mechanical investigated at the nanoscale, such as rhodium. Rhodium is used in many applications, especially in coatings and catalysis. To contribute to the understanding the nano-properties of this important material, rhodium was fabricated and mechanically investigated at the nanoscale. The nanopillars approach was employed to study size effects on mechanical properties. Nanopillars with different diameters were fabricated using electroplating followed by uniaxial compression tests. SEM was used as a quality control technique by imaging the pillars before and after compression to assure the absence of buckling, barrelling, or any other problems. Transmission electron microscopy (TEM) and SEM were used as microstructural characterization techniques, and the energy-dispersive X-ray spectroscopy (EDX) was used as the chemical characterization technique. Due to substrate induced effects, only the plastic region of the stress-strain curves were investigated, and it was revealed that rhodium softens with decreased nanopillar diameter. This softening/weakening effect was due to the nanocrystallinity of the fabricated pillars. This effect is consistent with the literature that demonstrates the reversed size effect of nanocrystalline metals, i.e., smaller is weaker. Further studies should focus on eliminating the substrate effect that was due to the adhesion layers between Rh and the silicon substrate being softer than Rh, consequently, causing Rh to sink into the adhesion layer when compressed and thus perturbing the stress-strain curve. Moreover, further investigation of other properties of Rh is required to achieve a comprehensive understanding of Rh at the nanoscale, and to render it suitable for specific, multivariable applications.

We have designed and engineered bi-catalyst magnesium hydride composites with superior sorption performance to that of ball milled magnesium hydride catalyzed with the individual baseline catalysts. We have examined the effect of single-walled carbon nanotube (SWCNT)-metallic nanoparticle additions on the hydrogen desorption behavior of MgH2 after high-energy co-milling. We showed the synergy between SWCNT's and metallic nanoparticles in catalyzing the sorption of magnesium hydride. The optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This microstructure is optimum, presumably because of the dense and uniform coverage of the defective SWCNTs on the MgH2 surface. Cryo-stage transmission electron microscopy (TEM) analysis of the hydride powders revealed that they are nanocrystalline and in some cases multiply twinned. Since defects are an integral component of hydride-to-metal phase transformations, such analysis sheds new insight regarding the fundamental microstructural origins of the sorption enhancement due to mechanical milling. The nanocomposite shows markedly improved cycling as well. Activation energy analysis demonstrates that any catalytic effect due to the metallic nanoparticles is lost during cycling. Improved cycling performance is instead achieved as a result of the carbon allotropes preventing MgH2 particle agglomeration and sintering. The nanocomposite received over 100 sorption cycles with fairly minor kinetic degradation. We investigated the catalytic effect of Fe + Ti bi-metallic catalyst on the desorption kinetics of magnesium hydride. Sub-micron dimensions for MgH2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy milling. The composites containing Fe shows DSC desorption temperature of 170 °C lower than as-received MgH2 powder, which makes it suitable to be cycled at relatively low temperature of 250 °C. The low cycling temperature also prevents the formation of Mg2FeH6. The ternary Mg-Fe-Ti composite shows best performance when compared to baseline ball milled magnesium hydride with only one catalytic addition. With a very high BET surface area it also shows much less degradation during cycling. The synergy between Fe and Ti is demonstrated through use of TEM and by carefully measuring the activation energies of the baseline and the ternary composites. / Materials Engineering

Bulk nanocrystalline 304L and 316L austenitic stainless steels (SS) were produced by equal channel angular pressing(ECAP) at elevated temperature. The average grain size achieved in 316L and 304 L SS is ~ 100 nm, and grain refinement occurs more rapid in 316 L SS than that in 304L. Also the structures are shown to retain a predominant austenite phase. Hardness increases by a factor of about 2.5 in both steels due largely to grain refinement and an introduction of a high density of dislocations. Tensile strength of nanocrystalline steels exceeds 1 GPa with good ductility in both systems. Mechanical properties of ECAPed 316L are also shown to have less dependence on strain rate than ECAPed 304L. ECAPed steels were shown to exhibit thermal stability up to 600oC as indicated by retention of high hardness in annealed specimens. Furthermore, there is an increased tolerance to radiation-induced hardening in the nanocrystalline equiaxed materials subjected to 100 keV He ions at an average dose of 3-4 displacement-per-atom level at room temperature. The large volume fraction of high angle grain boundaries may be vital for enhanced radiation tolerance. These nanocrystalline SSs show promise for further research in radiation resistant structural materials for next-generation nuclear reactor systems.

Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-02-02Bitstream added on 2014-06-13T18:50:38Z : No. of bitstreams: 1 leite_dmg_me_bauru.pdf: 1278982 bytes, checksum: f2b696ae90b2738246ee3c7750b37751 (MD5) / A recente descoberta de propriedades ferromagnéticas em alguns semicondutores magnéticos diluídos (DMS) trouxe a esta classe de materiais um grande potencial para aplicações em dispositivos de controle de spin. Um DMS é basicamente formado por um semicondutor dopado por íons magnéticos, os quais têm o papel de criar um momento magnético local e também, em algumas situações, de introduzir portadores livres no material. Entre os DMSs conhecidos, o 'GA IND.1-x'MN IND.XN' surge como o mais forte candidato a aplicações práticas por apresentar até o momento a mais alta temperatura de transição ferromagnética ('T IND.C' 'DA ORDEM DE' 400 k). Até o presente, os filmes de 'GA IND.1-x'MN IND.XN' com propriedades ferromagnéticas relatados na literatura foram preparados por epitaxia por feixe molecular (MBE). Neste trabalho, descrevemos a preparação de filmes nanocristalinos de 'GA IND.1-x'MN IND.XN' com diferentes conteúdos de Mn (0,00 'MENOR' x 'MENOR' 0,18) pela técnica de RF-magnetron sputtering reativo. Analisamos os efeitos da incorporação de Mn na estrutura e nas propriedades ópticas destes filmes através de medidas de difração de raios-X e de absorção óptica entre o ultravioleta (6,5 eV) e infravermelho próximo (1,4 eV). Os resultados apontam um aumento do parâmetro de rede e do índice de refração, uma diminuição do gap ótico e um aumento da densidade de estados de defeitos no interior do gap conforme se aumenta o conteúdo de Mn nos filmes de 'GA IND.1-x'MN IND.XN' preparados por sputtering. Estes resultados são semelhantes aos reportados para a incorporação de Mn em filmes monocristalinos de 'GA IND.1-x'MN IND.XN' com propriedades ferromagnéticas preparados por MBE. / The recent discoveries related to the ferromagnetic properties in some diluted magnetic semiconductors (DMS) have attracted considerable attention on this class of material due to their potential application on spin control devices. A DMS is basically formed by a semiconductor doped with magnetic ions with the purpose of creating local magnetic moments and, in some situations, to introduce free carriers in the material. Among the known DMSs, 'GA IND.1-x'MN IND.XN' is the one with the highest ferromagnetic transition temperature ('T IND.C' 'DA ORDEM DE' 400 k), and it is consequently on of the stronger candidates for practical applications. Until now, the 'GA IND.1-x'MN IND.XN' films with ferromagnetic properties described in the literature were prepared by molecular beam epitaxy (MBE). In this work, we report the preparation of nanocrystalline 'GA IND.1-x'MN IND.XN' films (0,00 'MENOR' x 'MENOR' 0,18) by reactive RF-magnetron sputtering technique. We analyzed the Mn incorporation effects on structure and optical properties of the films by X-ray diffraction measurements and optical absorption between UV (6,5 eV) and near infrared (1,4 eV). The results show the increase of the lattice parameters and of the refractive index, a decrease of the optical gap and a increase of defect states in the gap when Mn concentration is increased. These results are similar to those reported for Mn incorporation in monocrystalline 'GA IND.1-x'MN IND.XN' films prepared by MBE with ferromagnetic properties.

La thèse résulte d'une collaboration entre le laboratoire académique G2Elab et les entreprises Aperam Amilly et Aperam Imphy.Les matériaux magnétiques nanocristallins de type Finemet sont constitués d'une phase nanocristalline et d'une phase amorphe. Cette structure singulière leur confère des anisotropies magnéto-cristalline et magnéto-élastique évanescentes. On peut alors induire, par le biais de recuits adaptés, une anisotropie contrôlée conditionnant la forme du cycle d’Hystérésis et la perméabilité. D’un point de vue applicatif, il s’agit d’une aptitude capitale puisque les caractéristiques du circuit magnétique peuvent être adaptées pour répondre à des cahiers des charges spécifiques. La mise au point des protocoles de recuit mis en œuvre industriellement est cependant empirique.Le sujet de la thèse porte donc sur la mise au point d'un modèle capable de prédire l'amplitude K_u de l'anisotropie induite sur ces matériaux en fonction des paramètres du recuit sous champ (température T_re, champ appliqué H_re) et des caractéristiques structurales (fraction cristalline f_c, taille moyenne D des nanograins, composition y de la phase cristalline Fe1-ySiy). / This thesis is the result of a collaboration between Grenoble Electrical Engineering laboratory, Aperam Alloys Amilly and Aperam Alloys Imphy manufactories.The magnetic materials nanocrystalline Finemet are made of Fe-Si nanocrystallites embedded in a residual amorphous phase. This unconventional crystallographic structure features vanishing magnetocristalline and magnetoelastic anisotropies. As a result, it is possible to induce a cohenrent magnetic anisotropy in such material by suitable annealing treatments, allowing to control the shape of the hysteretic loop and permeability. In view of applications in electronic devices, this attract a great interest, the magnetic circuit characteristics being could be easily adapted by this way to satisfy the requirement of the regarded sensor or actuator.However, the optimization of annealing parameters (temperature, duration, amplitude of applied field…) to fit the desired properties is focused on time and resources consuming, which are based on empirical experience at present.As a result, the aim of this work is to build a model which will be able to predict the magnitude of induced anisotropy according to the field annealing parameters and the structural ones (crystalline fraction f_c, size of nanograins D, and composition of FeSi phase).

xxi, 134 p. : ill. (some col.) / Hydrogenated nanocrystalline silicon (nc-Si:H) is a semiconducting material that is very useful as a thin film photovoltaic. A mixture of amorphous and crystalline silicon components, nc-Si:H shows good carrier mobilities, enhanced infrared response, and high resilience to light-induced degradation of its electronic properties, a thermally reversible degenerative phenomenon known as the Staebler-Wronski Effect (SWE). However, production of nc-Si:H is difficult in part because the structural and electronic properties of this material are not well understood. For example, its electronic properties have even been observed by some authors to improve upon prolonged light exposure, in direct opposition to the SWE observed in purely amorphous thin film silicon. We used several junction capacitance based measurements together with characterization methods such as Raman spectroscopy and secondary ion mass spectroscopy to better understand the structure/function relationships present in nc-Si:H. Drive level capacitance profiling (DLCP) was used to determine densities, spatial distributions, and energies of deep-gap defects. Transient photocapacitance (TPC) and transient photocurrent (TPI) were used to characterize optical transitions and the degree of minority carrier collection. Materials had crystallite volume fractions between 20% and 80% and were deposited using RF and modified VHF glow discharge (PECVD) processes at United Solar Ovonic, LLC. Measurements were made as a function of metastable state: annealed states were produced by exposing the material to temperatures above 370K for 0.5h and the lightsoaked state was produced by exposure to 200mW/cm 2 610nm long-pass filtered light from an ELH halogen source for 100h. We identified two deep defects in nc-Si:H. A primary defect appearing throughout the material at an electronic transition energy of roughly 0.7eV below the conduction band, and a second defect 0.4eV below the conduction band which was localized near the p/i junction interface. Results suggested that the deeper defect is related to the presence of oxygen and is located in grain boundary regions. The energy depth of this defect appears also to be somewhat dependent on metastable state. This phenomenon, and the universal decrease in minority carrier collection upon lightsoaking are accounted for in a model of electronic behavior we have developed over the course of this study. / Committee in charge: Dr. Miriam Deutsch, Chairperson; Dr. J. David Cohen, Advisor; Dr. Roger Haydock, Member; Dr. Heiner Linke, Member Dr. Mark Lonergan Outside Member

abstract: Mechanical behavior of metallic thin films at room temperature (RT) is relatively well characterized. However, measuring the high temperature mechanical properties of thin films poses several challenges. These include ensuring uniformity in sample temperature and minimizing temporal fluctuations due to ambient heat loss, in addition to difficulties involved in mechanical testing of microscale samples. To address these issues, we designed and analyzed a MEMS-based high temperature tensile testing stage made from single crystal silicon. The freestanding thin film specimens were co-fabricated with the stage to ensure uniaxial loading. Multi-physics simulations of Joule heating, incorporating both radiation and convection heat transfer, were carried out using COMSOL to map the temperature distribution across the stage and the specimen. The simulations were validated using temperature measurements from a thermoreflectance microscope. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016