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School of Materials Science and Engineering

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College of Engineering

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Particle cracking damage evolution in 7075 wrought aluminum alloy under monotonic and cyclic loading conditions

(Georgia Institute of Technology, 2005-11-22) Harris, James Joel

7xxx series Al-Zn-Mg-Cu-base wrought Al-alloy products are widely used for aerospace structural applications where monotonic and cyclic mechanical properties are of prime concern. Microstructure of these commercial alloys usually contains brittle coarse constituent particles or inclusions of Fe-rich intermetallic compounds and Mg2Si, typically in the size range of 1 to 50 micron. Plastic deformation and fracture of 7xxx series alloys (as well as of numerous other wrought Al-alloys) is associated with gradual microstructural damage accumulation that involves cracking of the coarse constituent particles, growth of voids around the cracked particles, and the void coalescence. To understand and model the microstructural damage evolution processes such particle cracking, quantitative microstructural data associated with the damage nucleation are required under monotonic as well as cyclic loading conditions. In the past quantitative characterization of particle cracking damage in these alloys has been problematic. However, with recent advances in digital image analysis and stereology based techniques, it is now possible to quantitatively characterize the damage nucleation in hot-rolled 7075(T6) Al-alloy (a typical alloy of 7xxx series) due to cracking of the Fe-rich coarse constituent particles. The objectives of this work are: * Quantitative characterization of the cracking of Fe-rich constituent particles as a function of strain under quasi-static loading. This involves measurements of number density of cracked particles, volume fraction of the cracked particles, their size, shape, and orientation distribution, as well as nearest neighbor distribution and two-point correlation functions to quantify spatial dispersion of the cracked particles in a series of interrupted uniaxial tensile test specimens at different strain levels. * Quantitative characterization of the cracking of Fe-rich constituent particles under cyclic loading to study the differences between the particle cracking damage due to monotonic and cyclic loading.
Piezoelectrically-Transduced Silicon Micromechanical Resonators

(Georgia Institute of Technology, 2005-08-26) Sivapurapu, Abhishek

This thesis reports on the design and fabrication of micro-electro-mechanical (MEM) resonators on silicon that are piezoelectrically-transduced for operation in the very high frequency (VHF) range. These devices have a block-type or beam-type design, and are designed to resonate in their in-plane and out-of-plane bulk extensional modes. Two piezoelectric materials were taken into consideration, zinc-oxide (ZnO) and lead-zirconate-titanate (PZT). The resonators are fabricated on silicon-on-insulator (SOI) wafers and the metal/piezo/metal stack of layers forming the device is built and patterned on the device layer silicon via photolithography techniques, RF sputtering (for the piezo-layer) and electron-beam evaporation (for the metal layers). The designing aspect involved ANSYS simulations of the mode-shapes and estimation of frequencies, and these have correlated well with experimental results. Devices with RF sputtered ZnO were successfully fabricated and tested to give high quality factors at reasonably high frequencies. A gold ground plane was implemented to reduce the feed-through level and increase the signal-to-noise ratio. Extensive characterization of PZT was also done as a replacement for ZnO, as the former material has a much higher piezoelectric coefficient (~20X that of ZnO) and can therefore extend the operation of these MEM resonators into the UHF range. Although the basic design of the device remains the same, incorporation of PZT complicates the process flow considerably with respect to the chemistry now involved with the patterning of different layers. The frequency response for ZnO-based resonators as well as all the characterization data for PZT has been reported.
A Nitride-Based Reaction for the Formation of a Three-Phase Molybdenum-Silicon-Boron Intermetallic Alloy

(Georgia Institute of Technology, 2005-07-18) Middlemas, Michael Robert

The alloy Mo-3Si-1B (wt%) may have the fracture toughness and oxidation resistance required for use as jet turbine engine blades. Mo-3Si-1B (wt%) forms a three-phase mixture of and #945;-Moss, A15 (Mo3Si) and T2 (Mo5SiB2). It has been observed that at high-temperatures, the A15 and T2 intermetallics form a oxidation resistant borosilicate glass coating. To achieve the proper combination of mechanical and thermal properties, the material must have a molybdenum matrix with a fine dispersion of intermetallics to produce a continuous protective layer. In this project, reactive sintering of molybdenum, Si3N4 and BN powders was used to create a semi-continuous molybdenum matrix with a fine dispersion of the A15 and T2 intermetallics. Sintering of the materials was further enhanced by the use of submicron-sized reactants. X-ray diffraction analysis was used verify the desired phases were formed. It was determined that formation of the A15 intermetallic phases begins as low as 1200?nd formation of T2 begins at 1300? The reactions are complete by 1400? Samples with bulk densities as high as 95% of theoretical were produced. Scanning electron microscopy images reveal a microstructure with dispersed intermetallics in a semi-continuous molybdenum matrix with grain sizes on the order of 1-4 and #956;m. It was found that by varying parameters such as mixing method and heating rates, it is possible to engineer the final microstructure, changing the level of dispersion of the intermetallics and continuity of the matrix.
Laser Processing of Biological Materials

(Georgia Institute of Technology, 2005-07-14) Patz, Timothy Matthew

I have explored the use of the matrix assisted pulsed laser evaporation (MAPLE) and MAPLE direct write (MDW) to create thin films of biological materials. MAPLE is a novel physical vapor deposition technique used to deposit thin films of organic materials. The MAPLE process involves the laser desorption of a frozen dilute solution (1-5%) containing the material to be deposited. A focused laser pulse (~200 mJ/cm2) impacts the frozen target, which causes the solvent to preferentially absorb the laser energy and evaporate. The collective action of the evaporated solvent desorbs the polymeric solute material towards the receiving substrate placed parallel and opposite to the target. The bioresorbable polymer PDLLA and the anti-inflammatory pharmaceutical dexamethasone were processed using MAPLE, and characterized using Fourier transform infrared spectroscopy, atomic force microscopy and x-ray photoelectron spectroscopy. MDW is a CAD/CAM controlled direct writing process. The material to be transferred is immersed in a laser-absorbing matrix or solution and coated onto a target or support positioned microns to millimeters away from a receiving substrate. Using a UV microscope objective, a focused laser pulse is directed at the backside of the ribbon, so that the laser energy first interacts with the matrix at the ribbon/matrix interface. This energy is used to gently desorb the depositing material and matrix onto the receiving substrate. I have deposited neuroblasts within a three-dimensional extracellular matrix. These two laser processing techniques have enormous potential for functional medical device and tissue engineering applications.
New Carbon-Silicon Carbide Composite Board Material for High Density and High Reliability Packaging

(Georgia Institute of Technology, 2005-06-23) Kumbhat, Nitesh

Current board technologies are inherently performance-limited (FR-4) or cost-prohibitive (Al2O3/AlN). Next-generation high-density packaging applications would necessitate a new base substrate material to achieve ultra-fine pitch solder-joint reliability and multiple layers of fine-line wiring at low cost. The NEMI 2000 roadmap defines the need for 4-8 layers of 5-10 m wiring for future system boards. The 2003 ITRS roadmap calls for organic substrates with less than 100-m area-array pitch in the package or board by year 2010. Solder-joint reliability at such fine-pitch is a matter of concern for the industry. Use of underfills reduces thermal stresses but increases cost and, in addition, their dispensing becomes increasingly more complicated with the shorter gaps required for future interconnects. Therefore, there is a pronounced need to evaluate board materials with CTE close to that of Si for reliable flip-chip on board without underfill. Recently, a novel manufacturing process (using polymeric precursor) has been demonstrated to yield boards that have the advantages of organic boards in terms of large-area processability and machinability at potentially low-cost while retaining the high stiffness (~250 GPa) and Si-matched CTE (~2.5 ppm/㩠of ceramics. This work reports the evaluation of novel SiC-based ceramic composite board material for ultra-fine pitch solder-joint reliability without underfill and multilayer support. FE models were generated to model the behavior of flip-chips assembled without underfill and subjected to accelerated thermal cycling. These models were used to calculate solder-joint strains which have a strong direct influence on fatigue life of the solder. Multilayer structures were also simulated for thermal shock testing so as to assess via strains for microvia reliability. Via-pad misregistration was derived from the models and compared for different boards. Experiments were done to assemble flip-chips on boards without underfill followed by thermal shock testing so as to get the number of cycles to failure. To assess microvia reliability, 2 layer structures containing vias of different diameters were fabricated and subjected to thermal cycling. Via-pad misalignment was also studied experimentally. Modeling and experimental results were corroborated so as to evaluate thermomechanical suitability of C-SiC for high-density packaging requirements.
Nanocarving of Titania Surfaces Using Hydrogen Bearing Gases

(Georgia Institute of Technology, 2005-05-18) Rick, Helene Sylvia

An investigation of surface structures formed on polycrystalline and single crystal TiO2 (titania) samples having under gone various heat treatments in a controlled hydrogen bearing atmosphere was conducted. The study included the recreation and examination of the process discovered by Sehoon Yoo at Ohio State University to form nanofibers on the surface of polycrystalline TiO2 disks. Fibers were formed by heating samples to 700??in a 5%H2 95%N2 gas stream. The nanofibers formed during this processes are approximately 5-20 nanometers in diameter and can be 100??f nanometers long. The fibers do not actually grow on the surface, but are what remain of the surface as the material around them is removed by the gas stream V i.e., nanocarving. The mechanism of fiber formation and the effect of varying experimental parameters remained unknown and were explored within this study. This included changing gas composition, flow rate, and changes in sample preparation. The effect of isovalent doping and impurities within the starting powder were examined. Sintering temperature and time was investigated to determine the effect of grain size and surface morphologies prior to nanocarving. The effect of elevated temperature and 5%H2 95%N gas on the surface of TiO2 single-crystal wafers was also investigated. Test methods include Thermogravimetric Analysis (TGA), Mass Spectrometry (MS), Scanning Electron Microscopy (SEM), and X-ray diffraction (XRD) analysis.
Reactive replacement and addition of cations in bioclastic silica and calcite

(Georgia Institute of Technology, 2005-05-05) Allan, Shawn Michael

Numerous organisms produce ornately detailed inorganic structures (often known as shells) with features on length scales from 50 nm to several centimeters. One class of such organisms are the diatoms; microscopic algae that form silica frustules. Another group of algae, the coccolithophorids, produce similar calcium carbonate structures. Over 100,000 species comprise these two classes of algae, every one of which is endowed with a unique cytoskeleton structure. Using various types of displacement reactions, the chemistry of the original structure can be modified to produce a new material. Magnesium vapor has been found to displace the silicon in diatom frustules to yield an MgO structure. The conversion has been reported at temperatures from 650°C to 900°C. In the current work, the conversion and processing of silica frustules to MgO was examined in depth. The effect of reaction temperature on grain size and extent of conversion was evaluated. With the goal of obtaining high purity MgO structures, various methods for removing the silicon products of reaction were investigated. Wet chemistry and high temperature vapor etches were evaluated. The MgO reaction served as an intermediate step in the production of magnesium tungstate diatoms, which were imbued with photoluminescent properties. Reactions were identified to allow the conversion of calcium carbonate (calcite) structures to alternative chemistries. Calcite sand-dollars were converted to calcium tungstate or calcium molybdate by aqueous solution chemistry. In this process, sand dollar tests (shells) and coccolithophore frustules were reacted with ammonium para-molybdate or ammonium para-tungstate. The reactions were evaluated for shape preservation, phase purity, and photoluminescence of the structures.
Processing and Characterization of Energetic and Structural Behavior of Nickel Aluminum with Polymer Binders

(Georgia Institute of Technology, 2005-04-21) Martin, Morgana

A polymer-based composite reinforced with a mixture of Ni and Al powders was investigated as an example of a multifunctional structural energetic material. Micron-sized Ni powder, nano/micron-sized Al powders, and Teflon or epoxy were fabricated as bulk materials by pressing or casting. The thermally initiated reaction response of these materials was evaluated using differential thermal analysis coupled with x-ray diffraction. The analyses showed evidence of thermally initiated reactions between Ni and Al powders, as well as between Ni+Al and Teflon. Nano-sized Al powder showed a preference for reaction with Teflon over Ni, while micron-sized Al reacted strongly with Ni regardless of the presence of a binder. Teflon was shown to be very reactive with the Ni+Al/nano Al mixture, whereas epoxy was not reactive with the metallic powders, and also inhibited reaction between Ni and nano Al. The structural/mechanical behavior of these materials was evaluated using elastic and plastic property measurements via static and dynamic compression tests. Dynamic mechanical testing using reverse Taylor anvil-on-rod impact tests combined with velocity interferometry gave qualitative and quantitative information about the transient deformation and failure response of the composites. The material containing 20wt% epoxy and nano-sized Al powder showed the most superior mechanical properties in terms of elastic modulus and static and dynamic compressive strength. The addition of Ni and Al powders to the epoxy matrix increased the strength of the composites, and their tendency toward brittle fracture, as evidenced by Ni particle pullout in SEM analysis. The results illustrate that nano-sized Al particles provide significant enhancement to strength of epoxy composites, but at the expense of reactivity. The nano-Al particles get dissociated from the Ni and Al mixture and swept into the epoxy, generating a nano-Al containing epoxy matrix with embedded Ni particles. The chemical reactivity of the system is thus sacrificed as contacts between Ni and Al powders are minimized. A mixture of nano-sized Ni and Al particles may however provide the best combination of high strength and reactivity.
Evaluation of Metakaolins for Use as Supplementary Cementitious Materials

(Georgia Institute of Technology, 2005-04-20) Justice, Joy Melissa

Two metakaolins were evaluated for use as supplementary cementitious materials in cement-based systems. The metakaolins varied in their surface area (11.1 v. 25.4 m2/g), but were quite similar in mineralogical composition. Performance of metakaolin mixtures was compared to control mixtures and to mixtures incorporating silica fume as partial replacement for cement at water-to-cementitious materials ratios of 0.40, 0.50, and 0.60. In this study, the early age properties of fresh concrete and the mechanical and durability properties of hardened concrete were examined. Early age evaluations aimed to determine the reactivity of metakaolin (heat of hydration) and its effect on mixture workability (slump, setting time, unit weight). In addition, three types of shrinkage were monitored in metakaolin-cement systems: chemical, autogenous, and free. Compressive, tensile, and flexural strength and elastic modulus were measured at various concrete ages. The influence of metakaolin addition on durability was assessed through accelerated testing for sulfate resistance, expansion due to alkali-silica reaction, and through rapid chloride permeability measurements. To further quantify the underlying mechanisms of metakaolin's action, the microstructure of pastes was examined. Calcium hydroxide (CH) content was determined using thermogravimetric analysis and verified using differential thermal analysis. Surface area and pore size distribution were evaluated via nitrogen adsorption. These analyses yielded information about the pozzolanic reactivity of metakaolin, associated CH consumption and pore structure refinement, and resulting improvements in mechanical performance and durability of metakaolin-concretes.
Investigation of Reactions between Barium Compounds and Tungsten in a Simulated Reservoir Hollow Cathode Environment

(Georgia Institute of Technology, 2005-03-24) Schoenbeck, Laura

Reservoir-type dispenser hollow cathodes are currently being developed for use on NASAs Prometheus 1 mission. In these cathodes, the reaction between a barium source material and tungsten powder contained in a cavity surrounding a porous tungsten emitter produces barium vapor which is crucial to operation of the cathode. The primary objective of this research was to investigate the reactions between tungsten and a commercial barium source material in a simulated reservoir hollow cath-ode environment. Mixtures of tungsten and a barium calcium aluminate material were sealed inside molybdenum capsules with porous tungsten closures and heated to 1000?1200?and 1300?or 100, 200, and 400 hours. Based on the reaction products, which were identified to be BaAl2O4 and Ba2CaWO6, a reaction was proposed for the barium calcium aluminate material with tungsten. The bottom pellets in the capsules were found to have reacted to a much further extent than the top pellets in all of the samples, possibly due to a temperature gradient or excessive moisture in the base of the capsules. Quantita-tive and semi-quantitative x-ray analysis results did not show a clear trend as to how the concentrations of BaAl2O4 and Ba2CaWO6 vary with time. Most of the barium source materials are hygroscopic, and hydration of the materi-als would substantially reduce the performance of the cathode. Therefore, the environ-mental stability of several barium compounds, 3BaO??2O3 (B3A), 6BaO????2O3 (612), 4BaO????O3 (411), Ba2.9Ca1.1Al2O7 (B4ASSL), and Ba3Sc4O9, were investi-gated in order to evaluate their suitability for use as barium source materials. A micro-balance was used to measure weight gain of the materials as they were exposed to dew points of ??C and 11?t room temperature. The results showed that B3A hydrated more extensively than any of the other materials tested in the low- and intermediate-humidity environments, while the 612, 411, and B4ASSL materials were all reasonably stable in the low-humidity environment. The Ba3Sc4O9 was extremely stable compared to the barium aluminates in the intermediate-humidity conditions.