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Theoretical models are developed to predict the nature of the elevated temperature failure behavior in composites containing bridged cracks under small-scale creep conditions both for intermittently (or quasi-statically) and continuously growing matrix cracks that are fully bridged by continuous ceramic fibers. The time-dependence (or rate-dependence) in these models arises as a result of the presence of a viscous fiber/matrix interfacial layer. Under load this layer undergoes shear flow causing time-dependent pull-out of bridging fibers from the crack surfaces. The mechanics of time-dependent bridging is combined with a failure criterion based on secondary failure in a crack-tip creep process zone. The dependence of the matrix creep crack growth rates on flaw size and crack wake parameters as well as on composite microstructure is derived. It is shown that the crack wake plays a predominant role in influencing not only the magnitude of creep crack growth rates but also the relationship of growth rates to the crack sixes. The implications of the results for elevated temperature composite component design are discussed.

The material uniformity of cadmium zinc telluride (CZT) crystals in gamma-ray imaging detectors is examined using several existing techniques and a new technique called thermally stimulated current (TSC) imaging that has been developed for this dissertation. The TSC imaging model, simulations, and experimental demonstrations are presented here for the first time. CZT radiation detectors are used in nuclear medicine as well as other medical, industrial, national security, and scientific applications; however, the scarcity and cost of high-quality CZT materials have hindered the use of CZT in these applications. Understanding CZT's material properties and their effects on detector performance should be helpful in developing crystal growth methods that have improved yield of useful detector material. Data obtained from CZT samples using infrared transmission, electron microprobe, X-ray diffraction, and electron backscatter diffraction (EBSD) mapping methods are used to understand their crystal structure. Data obtained from these samples using both TSC imaging and conventional leakage current measurements while these samples were operated as pixelated detector arrays are used to understand their charge transport properties. Collimated gamma-ray mapping was used to understand the detector performance properties of these samples. Correlations among these spatially mapped data are investigated. Contrary to the suggestions of other researchers, it is found that leakage current is not inversely correlated with detector performance. Detector performance in these samples is well correlated with their crystal structure. High-angle grain boundaries are shown to trap charge carriers, and estimates of the locations of these boundaries are derived from the gamma-ray mapping data. EBSD distinguishes itself from X-ray diffraction methods in identifying the locations and types of grain boundaries intersecting the sample surface. Using the new TSC imaging method, evidence is obtained showing a higher density of a particular trap near incommensurate boundaries in a CZT sample. Other researchers have indicated that an electron trap associated with dislocations is present in CZT. Their observation is consistent with a conclusion drawn from these TSC imaging data that due to higher densities of dislocations near incommensurate grain boundaries these boundaries host electron traps while {111} twin boundaries do not.

Iridium silicides are of current interest as candidate detector materials for silicon based, Schottky-barrier infrared focal plane arrays. In this work, the growth and structure of codeposited IrSi₃ and Ir₃Si₄ films is discussed as well as the effect of annealing and deposition temperature on pure Ir film depositions. Nearly single-phase polycrystalline IrSi₃ films were formed by codeposition of Ir and Si in a 1:3 ratio at temperatures as low as 450°C. Localized epitaxial crystallite growth, identified by x-ray and electron diffraction, is found for IrSi₃ films formed at temperatures >600 °C, with a previously unreported c-axis epitaxial crystallite growth on Si(111) dominating at ∼700 °C. Single-phase polycrystalline Ir₃Si₄ films were formed by annealing room temperature 3:4 codeposited films, whereas localized epitaxial Ir₃Si₄ crystallite growth occurred for codeposition at temperatures of ∼500 °C. Annealed Ir films initially form IrSi crystallites at temperatures of ∼350 °C and further react with the substrate to form polycrystalline Ir₃Si5 at temperatures ≥ 550 °C. The Ir₃Si₄ phase, not found in annealed reactions, dominated the growth of silicide films formed by hot Ir depositions at 500 °C. A previously unreported Ir₃Si₄ epitaxial growth was identified for Ir depositions on Si(111)substrates. Resistivity measurements indicate that IrSi₃, IrSi, and Ir₃Si₄ films are metallic, where Ir₃Si₄ had the lowest resistivity of ∼60 μΩ-cm. Optical photoresponse and I-V measurements performed on diode structures indicate the barrier height of IrSi₃ on p-type Si(111) (∼0.33 eV) to be higher than that on p-type Si(100) (∼0.22-0.25 eV), limiting infrared imaging capability to the SWIR (1-3 μm) and MWIR (3-5 μm) atmospheric transmission windows, respectfully. Codeposited Ir₃Si₄ films display optical barrier heights between 0.125 to 0.175 eV on p-type Si(100), providing possible imaging capability in the LWIR (8-12 μm) spectral region. Ir₃Si₄ devices, displaying localized epitaxial crystallite growth, yield higher emission efficiency than polycrystalline Ir₃Si₄ films. Optical photoresponse measurements on a IrSi device also indicate a low optical barrier height (∼0.12 eV) providing access to the LWIR spectral region. Optical measurements on Ir₃Si5 films are also presented.

The present dissertation compiles three aspects of my Ph.D. work on OLED device design, fabrication and characterization. The first chapter is a review of the concepts and theories describing the mechanisms of organic electroluminescence. The second chapter makes use of these concepts to articulate some basic principles for the design of efficient and stable OLEDs. The third chapter describes the main characterization and sample preparation techniques used along this dissertation. Chapter IV describes the processing of efficient organic electroluminescent EL devices with ITO\TPD\AIQ₃\Mg:Ag structures. The screen printing technique of a hole transport polymeric blend was used in an unusual mode to render thin films in the order of 60-80 nm. EL devices were then fabricated on top of these sp films to provide ∼0.9% quantum efficiencies, comparable to spin coating with the same structures. Various polymer:TPD and solvent combinations were studied to find the paste with the best rheological properties. The same technique was also used to deposit a patterned MEH-PPV film. Chapter V describes my research work on the wetting of TPD on ITO substrates. The wetting was monitored by following its surface morphology evolution as a function of temperature. The effect of these surface changes was then correlated to the I-V-L characteristics of devices made with these TPD films. The surface roughness was measured with tapping AFM showed island formation at temperatures as low as 50-60°C. I Also investigated the effect of the purity of materials like AlQ3 on the device EL performance, as described in Chapter VI. In order to improve the purity of these environmentally degradable complexes a new in situ purification technique was developed with excellent enhancement of the EL cell properties. The in situ purification process was then used to purify/deposit organic dyes with improved film formation and EL characteristics.

Plans for development of human colonies on the Moon, Mars or other planets will require the investigation of new structural materials. In order to foster self-sufficiency and to make the colonies economically feasible, materials must be developed from locally available resources when possible. In this investigation a material made from a lunar soil simulant has been developed and tested for its mechanical properties. The simulant was mixed with varying percentages of aluminum, stainless steel and carbon steel fibers and heated to 1100°C to form a solid material. Beam shaped samples were cut from these specimens for bending tests. From the intact portions of the tested beams, samples for compression testing were cut and tested. Analysis of the results includes bending strength, compressive strength, and investigation of elastic moduli. The material was found to have significant strength in bending and compression. Results indicate the presence of fibers significantly changes the behavior of the material.

This research describes the preparation of electrically and thermally conductive polymer composites. The filler used is short carbon fibers. These were dispersed in methyl methacrylate (MMA) and settled under different vibrational and gravitational forces, resulting in well packed sediments. To improve further the dispersability of the fiber/MMA system, steric stabilization was attempted by using organic dispersants of increasing chain length. Subsequent polymerization of the dense sediments produced composites with high fiber volume fractions. The electrical and thermal conductivities of these composites were studied. Fiber size, distribution, orientation and volume fraction are shown to have a profound influence on these properties. A general effective media equation, which relates percolation and effective media theories, is shown to describe the electrical conductivity of the composites. The specific thermal conductivity of the high fiber fraction composites is greater than that of stainless steel. Applications include electronic packaging and electromagnetic interference shielding.

An analytical model for the specific gas detection of oxygen, carbon dioxide, and water vapor using zirconia amperometric oxygen sensors has been developed. Sensors of this type have been designed, fabricated, and tested using planar ceramic technology. Furthermore, an experimental setup has been designed and constructed for sensor characterization. This testbed can accurately control gas partial pressures as well as the total system pressure over a wide range of flow rates. Extensive effort has been put into design and construction of this testbed to ensure accurate scientific measurements. Special attention has been paid to ensuring that the apparatus is leak-tight from air to ensure accurate measurements at low oxygen partial pressures. Results of the experimentation for oxygen detection as well as the detection of carbon dioxide and water vapor are presented. The effects of electronic conduction in the zirconia electrolyte at low oxygen partial pressures are examined. Possible applications of the sensor, as well as suggestions for further research are discussed.

<p> This study develops a novel sandwich composite material using plant based materials for potential use in nonstructural building applications. The face sheets comprise woven hemp fabric and a sap based epoxy, while the core comprises castor oil based foam with waste rice hulls as reinforcement. Mechanical properties of the individual materials are tested in uniaxial compression and tension for the foam and hemp, respectively. The sandwich composite is tested in 3 point bending. Flexural results are compared to a finite element model developed in the commercial software Abaqus, and the validated model is then used to investigate alternate sandwich geometries. Sandwich model responses are compared to existing standards for nonstructural building panels, showing that the novel material is roughly half the strength of equally thick drywall. When space limitations are not an issue, a double thickness sandwich biocomposite is found to be a structurally acceptable replacement for standard gypsum drywall.</p>