A spatially resolved diffraction method with a sub micrometer-diameter beam and 3D differential aperture technique together with MD simulations, SEM and OIM analysis are applied to understand the arrangements of voids, geometrically necessary dislocations and strain gradient distribution in samples of Al (123) single crystal shocked to incipient spallation fracture. We describe how geometrically necessary dislocations and effective strain gradient alter white beam Laue patterns of the shocked materials. We show how to quantitatively determine the orientation and density of geometrically necessary dislocations in the shock recovered Al samples being initially oriented for single slip.

Modified 9Cr-1Mo steels containing different Silicon (Si) content have been tested for evaluation of their metallurgical and corrosion properties. The results of tensile testing indicated reduced failure strain up to 400°C followed by its enhancement at a higher temperature. Stress corrosion cracking tests at constant load did not show any failure in any materials. However, reduced ductility and true failure stress were noted in slow strain rate (SSR) testing, indicating increased cracking susceptibility at elevated temperatures. The corrosion potential became more active with increasing temperature in polarization studies. A combination of intergranular and cleavage failures was observed in specimens tested by the SSR technique.

Film deposition methods have been the focus of renewed interest in the past decade due to calls for cheaper and more environment friendly deposition techniques as well as better quality of films. This paper describes a novel deposition technique: Microwave assisted chemical vapor deposition (MACVD). This technique utilizes inexpensive equipment and works at temperatures close to room temperature and ambient pressures. Deposition rates are very high (>1 micron a minute) and the resulting films are of high quality i.e. high density, stability. Conventional deposition techniques such as epitaxy, e-beam evaporation and LPCVD can achieve high quality films but the financial and environmental costs are high. This study considers the MACVD of a high k dielectric film (Ta2O5) on silicon for dielectric and insulation applications. Films generated are dense and stable with thicknesses varying from 60 nm to 62 microns. Depth profile studies of 575nm thick MACVD derived films show results similar to that of high quality films generated via MOCVD. Characterization of the resulting films was done using XRD, SEM, XPS, AES and profilometry techniques.

In this paper, we summarize recent progress in applying atomistic studies of cracking along interfaces of dissimilar materials under quasi-static crack growth conditions. We consider two linear-elastic material strips in which atoms interact with harmonic potentials, with a different spring constant in each layer leading to a soft and a stiff strip. The two strips are bound together with a tunable potential, which allows to independently control the interface and bulk fracture surface energy. An initial crack serves as initiation point for the failure. This provides a model system to investigate how elastic properties and interface strength interplay and determine the crack growth direction, leading to either interfacial cracking or branching into the film material. We observe a clear transition to interface failure when the interface fracture energy is less than 80% of the bulk fracture energy. We further find that branching in the film material is controlled by the elastic properties of the film material, suggesting interfacial cracking for extremely soft films and branching for stiffer films. Analysis of the virial stress field around the crack suggests that the circumferential hoop stress controls the branching behavior.

We made p-type nanoscale super lattice thermoelectric cooling devices which consist of multiple periodic layers of Si1−x Gex / Si, The thickness of each layer ranges between 10 and 50 nm. The super lattice was bombarded by 5 MeV Si ion with different fluencies aiming to form nano-cluster quantum dot structures. We estimated the thermo-electric efficiency of the so fabricated devices, measuring the thin film cross plane thermal conductivity by the 3rd harmonic method, measuring the cross plane Seebeck coefficient, and finally measuring the cross plane electric conductivity before and after ion bombardment. As predicted, the thermo-electric Figure of Merit of the films increases with increasing Si ion fluencies. In addition to the effect of quantum well confinement of the phonon transmission, the nano-scale crystal quantum dots produced by the incident Si beam further adversely affects the thermal conductivity by absorbing and dissipating phonon along the lattice, and therefore further reduces the cross plane thermal conductivity, This process increases the electron density of state therefore increasing Seebeck coefficient, and the electric conductivity.

Periodic cellular metals (PCMs) can offer higher specific strengths and stiffnesses than conventional (i.e. stochastic) metallic foams. This study examines the effects of PCM microstructure and loading conditions on the mechanical performance.

PCM cores with 95% open porosity were constructed from perforated 6061 aluminium alloy sheets using a perforation-stretching method. This method places planar, periodically-perforated sheet metal in an alternating-pin jig. The pins apply force out-of-plane, plastically deforming the sheet metal into a truss-like array of struts (i.e. metal supports) and nodal peaks (i.e. strut intersections). Micro-hardness profiles were taken in the PCM struts to investigate microstructural evolution during fabrication and after heat treatment.

Truss cores were tested in two limiting uniaxial compression conditions. In the first, the PCM cores are placed between smooth compression platens where the nodes are laterally free and compressive forces are resisted through PCM node-bending (i.e. free compression). In the second, the PCM cores were placed between plates where the nodes are laterally confined and compressive forces are resisted through PCM beam-buckling (i.e. confined compression). Compression response was analyzed in terms of peak compressive strength, elastic modulus, and energy density absorbed upon densification; response values were used to illustrate the effect of compression test conditions. In addition, PCM cores were tested in the age-hardened state and annealed state to determine microstructural effects on compressive response.

Analysis of PCM response in free- and confined-compression conditions indicates a greater force resistance in beam-buckling over node-bending resistance mechanisms. The compressive strength, elastic modulus, and energy density of heat-treatable AA6061 PCMs are be found to respond: 1) over a wide range of value, dependent on the microstructure; 2) over a wide range of value, dependent on the PCM compression conditions; and 3) equally, if not more repeatable and with higher compressive strength-to-weight ratio than conventional metal foams.

Advances in microelectromechanical systems has generated an ever growing demand for novel insulating material applicable to high temperature systems. Photonic bandgap materials are appealing for such applications, specifically Ta2O5 due to its high index of refraction, refractory nature and negligible absorbance in the infrared region. The challenge faced in the realization of such materials is the synthesis of crack free Ta2O5 films whose thickness is in the order of a quarter wavelength of the incident infrared radiation.

This work seeks to investigate the effect of addition of polyvinyl pyrollidone (PVP) as a binder material in the sol gel synthesis of thick, uniform and crack free Ta2O5 films. Incorporation of PVP into the sol precursor has enabled uniform and crack free films with thicknesses of up to 2.4 microns to be realized. Chemical probing of the precursor was conducted via TGA, FTIR, and NMR analysis of the sol to elucidate the processes behind this film formation. The calcined oxide films were characterized via SEM, XRD and XPS.

Microstructural defects are introduced in materials upon irradiation with energetic particles. These defects can cause degradation of mechanical properties and contribute to material failure. Transmuted helium in irradiated stainless steels exerts deleterious effects on material properties. We have performed kinetic Monte Carlo (kMC) simulations of point defect diffusion and clustering in bcc alpha iron. The model includes helium and vacancy diffusion and spontaneous clustering and dissociation of the point defects from the clusters. We employ the kMC simulations to investigate the time evolution of the point defect configuration leading to defect clustering and bubble formation. The concentration of embryonic point defect clusters is determined as a function of the simulation time.

Femtosecond (fs = 10−15 sec.) laser ablation of Si(100) with thermally grown oxide films was studied with pump/probe imaging techniques in order to determine the role of film thickness on ablation dynamics. Two different imaging geometries were used in this study. Front view images were formed with the reflection of a delayed probe pulse from the area of a sample irradiated with a pump pulse. By changing the delay between the pump and probe pulses, images were obtained showing the evolution of the surface as a function of time (0 – 12 ns after the arrival of the pump pulse). The side view imaging technique, also known as shadowographic imaging, an image was formed of a delayed probe pulse which passed through the ablation plume produced by a pump pulse parallel to the sample surface. Both laser induced shock wave propagation and material removal were observed to change with increased thermal oxide thickness.

We have introduced various nanopowders in the precursor of glassy polymeric carbon (GPC) and studied its electrical, thermal, and mechanical properties as well as its chemical structure. In general the GPC ware produced at AAMU is used for making crucibles, heat exchangers, and for prosthetic devices because of its biocompatibility. GPC ware at AAMU is synthesized from a phenolic resin solution from Georgia Pacific in a pyrolyser system at temperatures between 100 °C all the way to 2800 °C. The heat treatment includes several stages: gelling, curing, postcuring, precarbonization and carbonization. The fabrication of GPC is complicated because of the high production rate of gaseous products in critical temperature ranges where out-diffusion is relatively slow. Special care should be taken in temperature programming to avoid kilning faults and misshapen or porous GPC end results. In this work we have introduced SiC, CNT or Al2O3 to the precursor and studied the properties of the final product treated at 1000oC pyrolysis temperature.

Glassy Polymeric Carbon (GPC) is obtained by a molding technique, in various shapes, from a phenolic resin precursor. The heat treatment of the precursor is achieved in three stages up to 1000 °C. Similar GPC materials produced in our laboratory displayed large strain to failure ratio, small thermal expansion coefficient and low density. Like all carbon forms, is attacked by oxygen, especially atomic oxygen. Nevertheless the kinetics for reaction with atmospheric oxygen is very slow. We investigated the composition and structural changes of the phenolic precursor as a function of temperature and evaluated materials stability when exposed to high temperatures in presence of hydrogen or oxygen.

We have used synchrotron X-ray powder diffraction (SXRPD) to investigate the structural behavior of cesium dihydrogen phosphate upon heating. Temperature-resolved data collected at ambient-pressure demonstrate that a transition from the room-temperature monoclinic phase (P21/m; a=7.90Å, b=6.39Å, c=4.87Å, and β=107.64°) to a high-temperature cubic phase (Pm3m; a=4.96Å) occurs at T=237°C. The high-temperature phase is not stable under ambient-pressure conditions, even in the absence of further heating. On the other hand, SXRPD measurements carried out under high-pressure (∼1GPa) evidence a transition from monoclinic to a stable cubic phase (Pm3m, a=4.88Å) at a temperature within the 255°C-275°C range. A 1000-fold increase in the proton conductivity (indicating the transition to the superprotonic phase) was previously observed under the same non-ambient conditions. Therefore, our results represent strong evidence that the superprotonic behavior in cesium dihydrogen phosphate is associated with a monoclinic-to-cubic polymorphic structural transition and not with chemical modifications.

During the fourth servicing mission of the Hubble Space Telescope (HST), the second set of solar arrays (SA-II) was replaced with a third set and the SA-II was brought back to Earth. A section of the retrieved SA-II solar array drive arm (SADA) multilayer insulation (MLI), which experienced 8.25 years of space exposure, was provided to NASA Glenn Research Center for environmental durability analyses of the top layer of silver-Teflon fluorinated ethylene propylene (Ag-FEP). Because the SADA MLI had solar and anti-solar facing surfaces and was exposed to the space environment for a long duration, it provided a unique opportunity to study solar effects on environmental degradation of Ag-FEP, a commonly used spacecraft thermal control material. Therefore, the objective of this research was to characterize the degradation of retrieved HST SADA Ag-FEP with particular emphasis on solar radiation effects. Data obtained included tensile properties, solar absorptance, surface morphology and chemistry. The solar facing surface of the SADA was found to be extremely embrittled and contained numerous through-thickness cracks. Tensile testing indicated that the solar facing surface lost 60% of its mechanical strength and 90% of its elasticity while the anti-solar facing surface had ductility similar to pristine FEP. The solar absorptance of both the solar facing surface (0.155 ± 0.032) and the anti-solar facing surface (0.208 ± 0.012) were found to be greater than pristine Ag-FEP (0.074). Solar facing and anti-solar facing surfaces were microscopically textured, and locations of isolated contamination were present on the anti-solar surface resulting in increased localized texturing. Yet, the overall texture was significantly more pronounced on the solar facing surface indicating a synergistic effect of combined solar exposure and increased heating with atomic oxygen erosion. The results indicate a very strong dependence of degradation, particularly embrittlement, upon solar exposure with orbital thermal cycling having a significant effect.

The purpose of this paper is to report the experimental results of boron doping on 60-80 micron size diamond particles using Field Enhanced Diffusion with Optical activation (FEDOA) [1-5]. Diamond is a wide band gap material with unique combinations of optical, thermal, mechanical and electronic properties that can be useful for a number of applications including optoelectronic applications and micro sensor technology. The incorporation of boron into diamond has been proven to change its electrical properties and convert the diamond from insulator to a p-type semiconductor [3]. A promising technique for incorporation of impurities into diamond is FEDOA. FEDOA drives impurities into single crystalline diamond material and we have used this method in this study [5-7]. FEDOA uses a combination of thermal diffusion with bias plus thermal ionization and optical ionization I simultaneously. A modified version of FEDOA was implemented for the diffusion of boron in natural diamond type II/a powder of size 60-80 microns (Figure 1). The diamond powder was obtained from Microdiamant and has 99.9% purity. The boron powder used in the experiment was amorphous, 325 mesh 90 %(Assay), Mg (5%) nominal obtained from AESAR. A ratio of 3:1 Boron-Diamond mixture was used for the doping process. A heating element and a diamond-boron powder mixture holder were designed and incorporated in the FEDOA system. Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) were used to study the diamond-doped morphology and identify impurities. Boron and hydrogen concentration results in the doped samples were studied using Prompt Gamma Neutron Activation Analysis (PGNAA) at the University of Missouri Research Reactor (MURR). Raman analysis of the treated samples was done as part of this work. The experimental results and analysis show that the samples were doped with boron. It was also found that samples with high boron concentration exhibited a high electrical conductivity. This work presents additional evidence that boron can be diffused into natural diamond powders. It also demonstrates that the FEDOA diffusion process is not only a powerful technique for the diffusion of impurities into wide band-gap materials in the form of single crystal plates, polycrystalline plate but also in a powder form with the modified FEDOA process.

Three nickel-base austenitic alloys, namely Alloy C-22, Alloy C-276 and Waspaloy have been tested for evaluation of their corrosion resistance in an acidic solution at ambient and elevated temperatures. The results of stress corrosion cracking studies indicate that none of these materials did exhibit any failure at constant load. The cracking susceptibility determined by the slow strain rate technique was gradually enhanced at higher temperatures showing reduced ductility and true failure stress. The critical potentials determined by the polarization technique, became more active (negative) with increasing temperature. The fractograpic evaluations by scanning electron microscopy (SEM) revealed ductile failure in Alloys C-22 and C-276. However, Waspaloy showed brittle failure at elevated temperature.

Earlier particle experiments in the 1970s on Pioneer-10 and -11 and Voyager-1 and -2 provided Jupiter flyby particle data, which were used by Divine and Garrett to develop the first Jupiter trapped radiation environment model. This model was used to establish a baseline radiation effects design limit for the Galileo onboard electronics. Recently, Garrett et al. have developed an updated Galileo Interim Radiation Environment (GIRE) model based on Galileo electron data. In this paper, the GIRE model was utilized to generate trapped proton and electron spectra as a function of Rj (Rj = radius of Jupiter = ∼71,400 km). Using these spectra and a high-energy particle transport codes (MCNPX and HZETRN), radiation exposures and dose effects for a variety of shielding materials (Al, polyethylene [PE], and Ta plus several other elemental materials for “Graded-Z” portion of the paper) and thicknesses are presented for the Icy Moon, Europa, Ganymede, and Callisto for several orbital inclinations. In addition, an in-depth discussion and absorbed dose calculations are presented for “Graded-Z” materials and several computer codes were utilized for comparison purposes. We find overall there is generally quite good agreement between the various computer codes utilized in the study: MCNPX (Monte Carlo) vs. HZETRN (deterministic) for slab shielding and the comparison of “Graded-Z” shielding using the CEPXS, NOVICE, and NASA JPL codes. Finally, we conclude that the merits of using “Graded-Z” materials that include PE, due to cost and weight, should aid future Jupiter mission planners and spacecraft designers.

We studied the effects of the low energy ions to induce nucleation of nanoscale crystals on and near surface of silica nano-layer containing low concentrations of Au. Suprasil substrates were coated with thin layer of gold followed by low-energy ion irradiation in a glow discharge plasma. The formation of nanoscale crystals due to low energy ion irradiation were then studied using RBS and optical absorption spectrometry.

Silicon carbide is a promising wide-bandgap semiconductor intended for use in fabrication of high temperature, high power, and fast switching microelectronics components running without cooling. For hydrogen sensing applications, silicon carbide is generally used in conjunction with either palladium or platinum, both of them being good catalysts for hydrogen. Here we are reporting on the temperature-dependent depth profile modifications of tungsten electrical contacts deposited on silicon carbide substrates.

A new experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (on the order of 4 GPa). This testing procedure experimentally determines the pressure-dependent shear strength of thin foil specimens. The experiments provide calibration data for models of materials subjected to extreme pressures such as the Steinberg-Guinan hardening model and can assist in model validation for discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 1 to 4 GPa. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure.

The Gas Turbine-Modular Helium Reactor (GT-MHR) and the Very-High-Temperature Reactor (VHTR) are next-generation high-temperature reactor types that are being designed to operate under normal conditions with primary coolant outlet temperatures in the range of 850 °C and 1000 °C, respectively. A new type of silicon carbide based diode neutron detector is currently under development in order to monitor the neutron flux in this environment. An important problem, in this context, is the long-time reliability of the diodes under continuous irradiation at high temperatures. In this paper, we discuss a computational methodology to study the accumulation of radiation damage in the detectors as a function of temperature and its influence on the electrical properties.