The region of eastern and southern Africa is very diverse in environments and agronomic practices. The region has one of the highest per capita consumption of maize (Zea mays. L), which is predominantly produced by smallholder farmers. Some important constraints facing these farmers include drought and low fertility. For decades, the International Center for Wheat and Maize Improvement (CIMMYT) has been involved in developing maize genotypes that have high grain yields and are tolerant to drought, low fertility and other important constraints. This germplasm is developed for wide adaptation. However, the development of superior germplasm is significantly affected by interaction between genotypes and the environment (i.e., genotype by environment interaction, GEI). To estimate and understand GEI maize genotypes are evaluated in a range of environments representing as much variability of the target growing areas as possible. Because of dwindling resources needed to conduct testing in the region, it may not be possible to test in all potential target areas. Therefore, a careful process of site selection for testing is essential to improve efficiencies in cultivar testing and deployment. The objective of this research was to characterize the maize testing locations of the eastern and southern Africa region. Historical data from CIMMYT Regional Trials from 1999 to 2003 was used to characterize the environments and estimate genetic parameters. Environmnent and GEI showed consistently high contributions to the total variation observed among genotypes for grain yield. Environment contributed over 60% and sometimes up to 85% of total variation observed. Sequential retrospective pattern analysis (Seqret) was conducted on the adjusted standardized grain yield. A total of 7 groups of environments were identified. Repeatabilites, a measure of the proportion of phenotypic variation that is due to genetic differences, was reduced under stress conditions. The relationship among traits showed that anthesis-silking interval (ASI) is an important selective trait, which can improve selection efficiency for grain yield under stress conditions. Stability analysis provided an opportunity to observe the response and adaptation of genotypes to a wide range of environments. Variety ZM621 was a stable and high yielding genotype.

Genetic resource enhancement is the foundation of any good breeding program. Landraces from West Africa, interspecifics between Oryza sativa and Oryza glaberrima and improved lines from the West African Rice Development Association and other research centers were introduced to the Beaumont Rice Research center for in situ evaluation and characterization. Beside the introduction of seeds, milled samples were also introduced for grain chemistry analysis. Field evaluation combined with physicochemical and molecular characterization revealed unique characteristics among African germplasm. New rice for Africa (NERICA) lines performed well in the USA environment. Varieties like Nerica 2, Nerica 3, Nerica 4, and Nerica 5 need more attention because of their superior performance in yield and grain quality. Landraces did not perform well due to their height and late maturity and their resulting problems with lodging. The rapid visco analyzer RVA profiles showed that the cultivar Jaya has unusually strong paste viscosity features. Comparing West Africa samples grown in Cote d’Ivoire with those grown in Texas, parameters like AA, ASV, Hot, Cool, and CT were not stongly affected by the environment. According to the Stbk value, cultivars grown in Cote d’Ivoire will cook softer than when they were grown in Texas. The lack of the environmental effect is somewhat surprising considering the difference in latitude, soil types, weather patterns, and management practices between the two locations. Apparent amylose is a key element to characterize a rice cultivar; however certain varieties like Cocodrie and Dixiebelle have similar apparent amylose content but dramatically different functional qualities. A population derived from Cocodrie and Dixiebelle was developed for genotypic and phenotypic analysis of grain chemistry traits that affect functionality. It was concluded that the amount of soluble amylose in the grain had a significant effect on flour pasting properties, even when total apparent amylose content did not vary. Marker association studies revealed that the Waxy microsatellite and the Waxy exon 10 SNP markers were associated with soluble amylose content and RVA characteristics. These markers will speed up the development of new rice cultivars with desirable quality characteristics in West Africa and in the USA.

Dispersion, attenuation, and crosstalk are several major challenges that both a high-speed digital and a microwave serial link must overcome to achieve their desirable performance. These phenomena are directly related to the frequency dependency of the dielectric property of the material used in package and interconnect. The dielectric property of a material is commonly measured by its manufacturer in a particular direction at a few discrete frequencies using resonator and waveguide methodology. Since the dielectric property may vary during manufacturing processing, the measurements taken by the manufacturer might be not adequate. Moreover, the dielectric property of a material in a bandwidth that covers at least the second harmonics of the fundamental operational frequency is required to accurately predict the link performance. One of the efforts in this research is to investigate the methodology of realizing broadband characteristics of the dielectric property of a material in its "as packaged" configuration using various transmission line topologies, such as microstrip line and Co-Planar Waveguide (CPW). Transitions from CPW to other transmission line topologies are mandatory if CPW probes are used to achieve broadband and repeatable measurements. Since microstrip line is one of the transmission line topologies involved in this research, a research effort is dedicated to develop a broadband CPW-to-microstrip line transition. An effort is also expended to creating casual material models that can be used in electromagnetic simulators to appropriately model the link based on the polarization mechanism of the materials. In addition to focusing on the measurement method in frequency domain, Short Pulse Propagation (SPP), a time domain method, is investigated as well. A virtual test bench is created to investigate the correlation between impedance variations in stripline structures due to fabricated tolerance and the attenuation predicted by SPP.

AM60B magnesium alloy is being increasingly used in auto industry in applications that usually involve various formats of cyclic loading scenarios. Therefore, the fatigue response of this alloy is investigated in this thesis. Our investigation is focused on characterization of the influence of compressive stress cycles within a given cyclic loading scenario on alloy’s crack propagation response. In the first part of this dissertation, fatigue crack growth rate (FCGR) of AM60B alloy subject to cyclic loadings with various stress ratios (both positive and negative) is investigated and a modified model is proposed to predict the FCGR under a wide range of stress ratios. Subsequently, using the modified model, the experimental results of the crack propagation tests are condensed into a single line in a logarithmic scale and the integrity of a proposed FCGR model is investigated. The investigation is continued by studying the influence of compressive stress cycle (CSC) on FCGR. Constant and random amplitude loadings with several magnitudes of CSCs are applied, leading to considerable acceleration in FCGR. The stress distribution ahead of the crack tip is also studied using the finite element method. The tensile residual stress and plastic zone are characterized upon the removal of the CSCs. The acceleration in the crack propagation is shown to be governed by the tensile zone ahead of the crack tip. Furthermore, application of an overload within an otherwise constant amplitude loading (CAL) has been known to retard the crack propagation, thus increase the fatigue life. This retardation would be a function of the affected zone and retardation magnitude. It is shown in this thesis that the affected zone would be influenced by the “sensitivity” of the material to overload. Moreover, it is also demonstrated that the nature of baseline CAL loading would also affect the retardation response and dimension of the affected zone. Therefore, modification to the Wheeler model is proposed, thereby enabling the model to account for material’s sensitivity and nature of the baseline loading. The integrity of the proposed model is verified by the experimental results obtained in this project, as well as those reported by other investigators for other alloys.

Semiconductor nanowires (NWs) are anticipated to play a crucial role in future electronic and optoelectronic devices. Their practical applications remain hindered by an urging need for feasible strategies to tailor their optical and electronic properties. Strategies based on strain and alloying are limited by issues such as defects, interface broadening and alloy scattering. In this thesis, a novel method to engineer the optoelectronic properties based on strain-free periodic structural modulations in chemically homogeneous Nanowire Twinning Superlattices (NTSLs) is experimentally demonstrated. NTSLs are an emerging new class of nanoscale material, composed of periodically arranged rotation twin-planes along the length of NWs. The main objective of this thesis is to establish the relationship between the electronic energy band gap (Eg) and the twin-plane spacing (d) in NTSLs, quantified using a periodicity parameter, based on ZnSe. ZnSe was chosen because of its excellent luminescence properties, and potential in fabrication of optoelectronic devices in the near-UV and blue region of the spectrum. A prerequisite to establishing this correspondence is a prior knowledge of the photoluminescence (PL) response and the nature of fundamental optical transitions in defect-free single crystal ZnSe NWs with zinc-blende (ZB) and wurtzite (WZ) crystal structures. There has been no systematic work done yet on understanding these fundamental optical processes, particularly on single NWs and in relation to their crystalline microstructure. Therefore, the secondary objective of this thesis is to study the influence of native point defects on the optical response of single ZnSe NWs in direct relation to their crystalline microstructure. The PL response from single ZB and WZ NWs was determined unambiguously, and excitonic emission linewidths close to 1 meV were observed, which are the narrowest reported linewidths thus far on ZnSe NWs. Based on this and extensive optical and structural characterization on individual NTSLs, a linear variation in Eg is shown through a monotonic shift in PL peak position from ZnSe NTSLs as a function of d, with Eg's that lie between those of ZB and WZ crystal structures. This linear variation in Eg was also validated by ab Initio electronic structure calculations. This establishes NTSLs as new nanoscale polytypes advantageous for applications requiring tunable band gaps.

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Made available in DSpace on 2014-06-11T19:27:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-06-30Bitstream added on 2014-06-13T18:55:39Z : No. of bitstreams: 1 souza_ga_me_guara.pdf: 3512611 bytes, checksum: e67ec93171ad6e8015c80fa5a52bf34e (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho investiga os constituintes de um aço médio carbono e baixa liga com estrutura bainítica/martensítica, obtida através de tratamento isotérmico em baixas temperaturas, a 336 ºC e 286 ºC (próximas à temperatura de transformação martensítica), utilizando-se da microscopia óptica e de processamento metalográfico com os reagentes nital, LePera, LePera Modificado e solução aquosa à base de metabissulfito de sódio, para análise qualitativa e quantitativa das fases presentes. O ataque químico com nital foi eficiente na identificação da fase bainita, revelada com coloração marrom escuro ou preta, porém tornou difícil a distinção entre as fases martensita e austenita retida. Da mesma forma, através do ataque químico LePera foi possível a observação da fase bainita na coloração marrom escuro ou preta e o constituinte martensita-austenita retida com tonalidade clara. O reagente LePera Modificado mostrou-se mais eficiente do que o reagente LePera, principalmente para estruturas constituídas quase que totalmente pela estrutura bainítica, apresentando maior nitidez na identificação da fase bainita, revelada na coloração marrom, e do constituinte formado pelas fases martensita e austenita retida (em tonalidade clara). O ataque químico com reagente à base de metabissulfito de sódio permitiu a caracterização da austenita retida (observada em tonalidade branca). Também utilizouse o reagente Contorno de Grão para revelar o contorno de grão da estrutura austenita, formada previamente na zona de austenitização. Uma combinação do reagente Contorno de Grão e reagente nital permitiu a visualização da nucleação e crescimento das ripas da bainita a partir dos contornos de grãos austeníticos. PALAVRAS-CHAVE: caracterização microestrutural, bainita, martensita, AISI 4340. / This work investigated the microstructures of the medium carbon and low alloy steel with mixed structure bainite/martensite, obtained through isothermal treatment in low temperatures (close to the temperature of martensitic transformation) to 336 ºC and 286 oC, by optical microscopy and metallographic methods with the etchings nital, LePera, LePera Modified and sodium metabisulfite reagent for qualitative and quantitative analysis of the present phases. The etching with nital was efficient in the identification of the bainite, revealed with brown darkness or black coloration; however it turned difficult the distinction between the martensite and retained austenite. In the same way, through the color tint etching with LePera reagent was possible the observation of the bainite in the brown darkness or black coloration and the martensite-austenite in the clear or straw-colored. Etching technique LePera Modified was more efficient than the LePera, mainly for microstructure constituted almost that totally for the bainitic structure, producing better color contrast in the identification of the bainite, revealed in the brown coloration, and the martensiteretained austenite (in white). The etching with sodium metabisulfite reagent allowed the characterization of the retained austenite (appears as small bright white particles). The Austenite Grain Boundary reagent was used to reveal the grain boundaries prioraustenite. A combination of the Austenite Grain Boundary reagent and nital reagent allowed the observation of the nucleation and growth of the laths of the bainite from the grain boundaries prior-austenite.

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Recently, the rare earth nitrides have received a considerable attention from theorists and experimentalists due to their potential applications in spintronic, piezoelectric, and thermoelectric devices. In this work, erbium nitride (ErN) and scandium nitride (ScN) crystals were grown and characterized experimentally, and the growth process was modeled and simulated. Erbium nitride (ErN) is a rare earth nitride notable for its magnetic and optical properties. Here we report on its growth on a non-native substrate, tungsten foil, via physical vapor transport, and its characterization. The source material was erbrium metal that was converted to ErN by heating in nitrogen. Subsequently, it was sublimed to form the ErN crystals. The operating conditions were 1620-1770 ⁰C and 150-330 Torr in pure nitrogen. The growth rate increased exponentially with temperature with an activation energy of 508 kJ/mol, and inversely with pressure. X-ray diffraction revealed the ErN preferentially adopted a (100) orientation, the same as the dominant orientation of the tungsten sheet. The lattice constant was 4.853 Å. The crystal shapes and sizes were dependent on the temperature, as revealed by SEM and optical microscopy. The ErN crystals were highly faceted, bound by (100) and (111) crystal planes. The ErN compound deviated from stoichiometry: the Er:N atomic ratio ranged from 1:1.15 to 1:1.2 according to EDX and XPS elemental analysis. Raman spectra was in good agreement with theoretical predictions. Scandium nitride single crystals (14–90 µm thick) were grown on tungsten (100) single crystal substrate by physical vapor transport in the temperature range of 1850-2000 ⁰C and pressure of 15-35 Torr. Epitaxial growth was confirmed using in-plane ɸ scan and out-of-plane x-ray diffraction techniques which revealed that ScN exhibited cube-on-cube growth with a plane relationship ScN (001) || W (001) and normal direction ScN [100] || W [110]. Atomic force microscopy revealed the surface roughness decreased from 83 nm to 18 nm as the growth temperature was increased. X-ray diffraction (XRD) rocking curves widths decreased indicating the crystal quality improved with increasing growth temperature. The lowest XRC FWHM was 821 arcsec, which is so far the lowest value reported for ScN. Scanning electron microscopy (SEM) exhibited the formation of macrosteps and cracks on the crystal surface with latter due to the mismatch of ScN’s and tungsten’s coefficients of thermal expansion . In general for crystal growth, material should deposit on the seed crystal and not on any adjacent supporting structures. This efficiently uses the source material and avoids the possibility of spurious polycrystals encroaching on, and interfering with the single crystal growth. To achieve this goal, a new crucible design with a cooling fin in contact with the seed was simulated and experimentally demonstrated on the physical vapor transport (PVT) crystal growth of scandium nitride. The heat transfer of the growth cavity for a conventional crucible and a modified crucible with the cooling fin were modeled theoretically via computational fluid dynamics (CFD) with FLUENT. The CFD results showed that the seed in the modified crucible was approximately 10 °C cooler than the crucible lid, while in the conventional crucible the temperature of the seed and lid were uniform. The experimental results showed that increasing the temperature gradient between the source and the seed by employing the cooling fin led to a dramatic increase in the growth rate of ScN on the seed and reduced growth on the lid. The relative growth rates were 80 % and 20 % on the seed and lid respectively, in the modified crucible, compared to 25% and 75% with the conventional crucible. Thus, the modified crucible improved the process by increasing the species transporting to the seed by sublimation.

abstract: Composite materials are finally providing uses hitherto reserved for metals in structural systems applications – airframes and engine containment systems, wraps for repair and rehabilitation, and ballistic/blast mitigation systems. They have high strength-to-weight ratios, are durable and resistant to environmental effects, have high impact strength, and can be manufactured in a variety of shapes. Generalized constitutive models are being developed to accurately model composite systems so they can be used in implicit and explicit finite element analysis. These models require extensive characterization of the composite material as input. The particular constitutive model of interest for this research is a three-dimensional orthotropic elasto-plastic composite material model that requires a total of 12 experimental stress-strain curves, yield stresses, and Young’s Modulus and Poisson’s ratio in the material directions as input. Sometimes it is not possible to carry out reliable experimental tests needed to characterize the composite material. One solution is using virtual testing to fill the gaps in available experimental data. A Virtual Testing Software System (VTSS) has been developed to address the need for a less restrictive method to characterize a three-dimensional orthotropic composite material. The system takes in the material properties of the constituents and completes all 12 of the necessary characterization tests using finite element (FE) models. Verification and validation test cases demonstrate the capabilities of the VTSS. / Dissertation/Thesis / Masters Thesis Engineering 2015