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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 275

Solution-based synthesis and processing of nanocrystalline ZrB₂-based composites

(Georgia Institute of Technology, 2008-11-24) Xie, Yanli

Zirconium- and tantalum-based diborides, and diboride/carbide composites are of interest for ultra-high temperature applications requiring improved thermomechanical and thermochemical stability. This thesis focuses on the synthesis, processing and sintering of nanocrystalline powders with Zr- and Ta-based diboride/carbide/silicide compositions. A solution-based processing method was developed to prepare reactive mixtures that were precursors for ZrB₂-based powders. The precursors reacted to form the ceramic powders after suitable pyrolysis and borothermal/carbothermal reduction heat treatments. Single-phase ZrB₂ powders were prepared with initial composition of C/Zr = 4.8 and B/Zr = 3.0. ZrB₂-based composite powders with ZrC, ZrO₂, TaB₂, TaC, SiC, TaSi₂ and B₄C were prepared with particle sizes of 10-500 nm for different phases based SEM micrographs. The composite powders were highly sinterable with proper processing methods developed to avoid and remove oxide impurities. The relative densities of ZrB₂/B₄C, ZrB₂/TaB₂, ZrB₂/TaB₂/B4C, ZrB₂/TaSi₂ were in the range of 91%-97% after pressureless sintering at 2020 ℃ for 1 h or 30 min.
Development of an anisotropic swelling hydrogel for tissue expansion: control over the degree, rate and direction of hydrogel swelling

(Georgia Institute of Technology, 2008-11-21) Lee, Jinhyun

Hydrogels are polymeric materials with chemically, physically or topologically crosslinked networks which have a capacity to absorb and retain water. They have been frequently used for many medical applications because of their useful physical properties such as oxygen permeability and excellent compatibility with living tissue and blood. The long term goal of this research is to develop a hydrogel system for potential use in reconstructive and plastic surgeries such as the closure of cleft palate defects and syndactyly (congenitally fused fingers or toes) repair. The medical requirements for such systems are not only a high degree of swelling, but also slow swelling rate, preferred direction of swelling (anisotropic swelling), appropriate mechanical strength, in addition to being biocompatible. A large degree of swelling would limit the number of surgical procedures required thereby reducing the cost and risk of surgery. A slow swelling rate can avoid tissue necrosis and help tissue growth during the tissue expansion process. Anisotropic swelling is required for specific surgical applications such as cleft palate repairs. Known to be biocompatible hydrogel systems, of a neutral gel system consisting of N-vinyl-2-pyrrolidinone (VP) and 2-hydroxyethyl methacrylate (HEMA) copolymers and an ionizable gel system of VP and acrylic acid (AA) copolymers were prepared using thermal and controlled UV-initiated polymerization. Using these VP/HEMA and VP/AA gel systems, various approaches to control their degree and rate of swelling were studied as a function of key controllable parameters. Their mechanical properties and structural characteristics determining their swelling behavior and mechanical properties also were investigated. Through these studies, how to control the key parameters that affect such swelling behavior was understood in addition to optimizing the gel systems for large degree of swelling, slow swelling rate, and mechanical integrity. Investigations into a number of methods to control the swelling rate were also undertaken for different VP/HEMA based gel systems. Multilayers of alternating gels and elastomer films (polybutadiene (PB) or polydimethylsiloxane (PDMS)) as well as gels encapsulated with the elastomer films were prepared. In addition, gels were prepared with inclusion of either silver nanoparticles or methacrylates with increasing the length of hydrophobic groups for the studies of swelling rate. In this work, two novel methods to control swelling direction (anisotropic swelling) of hydrogels were investigated. One method induces anisotropic swelling through structural gradients within the VP/HEMA gels synthesized by UV polymerization using gradient photomasks. A more promising method used stress induced anisotropic swelling for compressed VP/AA gels. The morphology-gradient VP/HEMA hydrogel system did not show large scale anisotropic swelling. However, the compressed VP/AA gels produced significant anisotropic swelling due to the controlled anisotropy of network morphology. A systematic study as a function of compression temperature, stain and strain rate was performed to derive an understanding of the anisotropic swelling behavior. These compressed gel systems produced not only a large degree of swelling and slow swelling rates but also high anisotropic swelling and proper mechanical stiffness of hydrogels. These materials are believed to be ideal candidates for tissue or skin expansion.
Stress corrosion cracking of duplex stainless steels in caustic solutions

(Georgia Institute of Technology, 2008-11-19) Bhattacharya, Ananya

Duplex stainless steels (DSS) with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical and pulp and paper mills. However, many DSS grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfide-containing caustic solutions. Although stress corrosion cracking of duplex stainless steels in chloride solution has been investigated and well documented in the literature, SCC mechanisms for DSS in caustic solutions were unknown. Microstructural changes and environmental factors, such as pH of the solution, temperature, and resulting electrochemical potential also influence the SCC susceptibility of duplex stainless steels. In this study, the role of material and environmental parameters on corrosion and stress corrosion cracking of duplex stainless steels in caustic solutions were investigated. Results showed that the austenite phase in the DSS is more susceptible to crack initiation and propagation in caustic environment, which is different from that in the low pH chloride environment where the ferrite phase is the more susceptible phase. This study also showed that alloy composition and microstructural changes in duplex stainless steels due to different heat treatments could affect their SCC susceptibility. Moreover, corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS indicated that the metal sulfide compounds formed along with oxides at the metal surface in the presence of sulfide containing caustic environments made the steel susceptible to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.
Reactive molding and self-assembly techniques for controlling the interface and dispersion of the particulate phase in nanocomposites.

(Georgia Institute of Technology, 2008-11-07) Pranger, Lawrence A.

This research explored the processing and properties of PNCs using a polyfurfural alcohol (PFA) matrix. The precursor for PFA, furfuryl alcohol (FA) is sourced from feedstocks rich in hemicellulose, such as corn cobs, oat hulls and wood. To exploit FA as a polymerizable solvent, cellulose whiskers (CW) and montmorillonite clay (MMT) were used as the nanoparticle phase. Results from PNC processing show that CW and MMT can be dispersed in the PFA matrix by means of insitu polymerization, without the use of surfactants or dilution in solvents. Both CW and MMT nanoparticles catalyze the polymerization of furfuryl alcohol (FA). Moreover, the insitu intercalative polymerization of FA in the interlayer galleries of MMT leads to the complete exfoliation of the MMT in the PFA matrix. CW and MMT both function as effective matrix modifiers, increasing the thermal stability of PFA nanocomposites compared to pure PFA polymer. The increased thermal stability is seen as significant increases in the onset of degradation and in residual weight at high temperature. This research also explored the surface functionalization of Cu, Ni and Pt substrates by self-assembly of a range of difunctional linker molecules. Characterization by XPS and PM-IRRAS indicate that diisocyanides and dicarboxylic acids both form chemically "sticky" surfaces after self-assembly on Cu and Ni. Sticky surfaces may provide a means of increasing nanoparticle dispersion in metal nanocluster filled PNCs, by increasing their interaction with the matrix polymer. Another potential application for sticky surfaces on Cu is in the ongoing miniaturization of circuit boards. The functionalization of Cu bond pad substrates with linker molecules may provide an alternate means of bonding components to their bond pads, with higher placement accuracy compared to solder bumps.
Shape preserving conversion reaction of siliceous structures using metal halides: properties, kinetics, and potential applications

(Georgia Institute of Technology, 2008-11-07) Shian, Samuel

BaSIC, which stands for Bioclastic and Shape-preserving Inorganic Conversion, is shape-preserving chemical conversion process of biological (or man-made) silica structures for producing complex 3-D microscale structures. This dissertation reports the BaSIC reaction of halide gases (i.e., TiF4, ZrF4, and ZrCl4) with 3-D silica structures, (i.e., diatom frustules, silicified direct-write assembly scaffolds, and Stöber silica spheres) to produce titania and zirconia replicas of the original 3-D structures. The kinetics of reaction of silica with titanium tetrafluoride gas is analyzed by using a novel HTXRD reaction chamber, nitrogen adsorption, and transmission electron microscope (TEM). The crystal structure and the temperature-induced phase transformation (from the room temperature hexagonal R-3c structure to the higher temperature cubic Pm3m structure) of polycrystalline TiOF2 that was synthesized through metathetic reaction of silica with TiF4(g) is reported. Additionally, potential applications of the converted titania diatom frustules (i.e., as a fast micron-sized ethanol sensor, and as a pesticide hydrolyzing agent) are also demonstrated in this work.
Colloidal gold nanorods, iridescent beetles and breath figure templated assembly of ordered array of pores in polymer films

(Georgia Institute of Technology, 2008-11-05) Sharma, Vivek

Water drops that nucleate and grow over an evaporating polymer solution exposed to a current of moist air remain noncoalescent and self-assemble into close packed arrays. The hexagonally close packed, nearly monodisperse drops, eventually evaporate away, leaving a polymer film, with ordered array of pores. Meanwhile, typical breath figures or dew that form when moist air contacts cold surfaces involve coalescence-assisted growth of highly polydisperse, disordered array of water drops. This dissertation provides the first quantitative attempt aimed at the elucidation of the mechanism of the breath figure templated assembly of the ordered arrays of pores in polymer films. The creation and evolution of a population of close packed drops occur in response to the heat and mass fluxes involved in water droplet condensation and solvent evaporation. The dynamics of drop nucleation, growth, noncoalescence and self-assembly are modeled by accounting for various transport and thermodynamic processes. The theoretical results for the rate and extent of evaporative cooling and growth are compared with experiments. Further, the dissertation describes a rich array of experimental observations about water droplet growth, noncoalescence, assembly and drying that have not been reported in the published literature so far. The theoretical framework developed in this study allows one to rationalize and predict the structure and size of pores formed in different polymer-solvent systems under given air flow conditions. While the ordered arrays of water drops present an example of dynamics, growth and assembly of spherical particles, the study on colloidal gold nanorods focuses on the behavior of rodlike particles. A comprehensive set of theoretical arguments based on the shape dependent hydrodynamics of rods were developed and used for centrifugation-assisted separation of rodlike particles from nanospheres that are typical byproducts of seed mediated growth of nanorods. Since the efficiency of shape separation is assessed using UV-Vis-NIR spectroscopy and transmission electron microscopy (TEM), the present dissertation elucidates the shape dependent parameters that affect the optical response and phase behavior of colloidal gold nanorods. The drying of a drop of colloidal gold nanorods on glass slides creates coffee ring like deposits near the contact line, which is preceded by the formation of a liquid crystalline phase. The assemblies of rods on TEM grids are shown to be the result of equilibrium and non-equilibrium processes, and the ordered phases are compared with two dimensional liquid crystals. The methodology of pattern characterization developed in this dissertation is then used to analyze the structure of the exocuticle of iridescent beetle Chrysina gloriosa. The patterns were characterized using Voronoi analysis and the effect of curvature on the fractions on hexagonal order of tiles was determined. Further, these patterns were found to be analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. In summary, the dissertation provides the crucial understanding required for the widespread use of breath figure templated assembly as a method for manufacturing porous films, that requires only a drop of polymer solution (dilute) and a whiff of breath! Further, the dissertation establishes the physical basis and methodology for separating and characterizing colloidal gold nanorods. The dissertation also suggests the basis for the formation and structure of tiles that decorate the exoskeleton of an iridescent beetle Chrysina gloriosa.
High dielectric constant polymer nanocomposites for embedded capacitor applications

(Georgia Institute of Technology, 2008-09-17) Lu, Jiongxin

Driven by ever growing demands of miniaturization, increased functionality, high performance and low cost for microelectronic products and packaging, embedded passives will be one of the key emerging techniques for realizing the system integration which offer various advantages over traditional discrete components. Novel materials for embedded capacitor applications are in great demand, for which a high dielectric constant (k), low dielectric loss and process compatibility with printed circuit boards are the most important prerequisites. To date, no available material satisfies all these prerequisites and research is needed to develop materials for embedded capacitor applications. Conductive filler/polymer composites are likely candidate material because they show a dramatic increase in their dielectric constant close to the percolation threshold. One of the major hurdles for this type of high-k composites is the high dielectric loss inherent in these systems. In this research, material and process innovations were explored to design and develop conductive filler/polymer nanocomposites based on nanoparticles with controlled parameters to fulfill the balance between sufficiently high-k and low dielectric loss, which satisfied the requirements for embedded decoupling capacitor applications. This work involved the synthesis of the metal nanoparticles with different parameters including size, size distribution, aggregation and surface properties, and an investigation on how these varied parameters impact the dielectric properties of the high-k nanocomposites incorporated with these metal nanoparticles. The dielectric behaviors of the nanocomposites were studied systematically over a range of frequencies to determine the dependence of dielectric constant, dielectric loss tangent and dielectric strength on these parameters.
Rapid production of polymer microstructures

(Georgia Institute of Technology, 2008-08-25) Nagarajan, Pratapkumar

The goal of this research is to develop an integrated polymer embossing module, with which difficult-to-emboss polymer microstructures and microparts can be fabricated in a cost-effective manner. In particular, the research addresses three major limitations of the hot embossing process, namely, long cycle time, difficulty in producing shell patterns, and difficulty in building up a high embossing pressure on thick substrates. To overcome these limitations, three new technical approaches two-station embossing, rubber-assisted embossing, and through-thickness embossing were developed and investigated. Fundamental understanding of these new embossing techniques were achieved through extensive experimental and theoretical studies involving parametric experiments, rheological characterization, surface investigation, mathematical modeling, and computer simulation.
Development of commercial, sustainable processes for dyeing generic, unmodified polypropylene fiber

(Georgia Institute of Technology, 2008-08-25) Gupta, Murari Lal

Identification of viable vat dye candidates of a trichromatic series (compatible red, yellow and blue colorants) plus an orange based on the developed single-stage acid leuco vat dyeing process for unmodified polypropylene (PP) flat woven fabrics has been achieved with adequate fastness properties to washing, crocking and dry-cleaning: C. I. Vats Orange 1, Yellow 2 and Red 1 have been certified, whereas Vat Blue 6 is a marginal candidate. Vat Blue 1 has been demonstrated to be a viable colorant for dyeing of PP fiber as a single colorant. Molecular dynamics simulation and solubility parameter (SP) approaches have been utilized to screen the potential vat dye candidates for generic PP coloration. Experimental K/S results have exhibited good correlation with the predicted mixing energy of acid leuco vat dyes-PP fiber and the calculated dyes' SP's. The low SP/mixing energy acid leuco vat dyes (e.g., C. I. Vat Red 1) have shown better color-yield/fastness properties than the high SP/mixing energy vat dyes (e.g., C. I. Vat Brown 1), exhibiting that increasing difference of SP between the vat dye and the PP fiber, coupled with a higher mixing energy of dye-PP blend, resulted in decreased interactions between the two. For example, C.I. Vat Brown 1 with its high SP and calculated mixing energy with PP gave least color yield than the certified vat dyes, all with lower SP's and mixing energies. Cross-section micrographs of the dyed fibers revealed the absence of "ring-dyeing". Experimentally determined kinetic parameters such as affinity of dyeing and heat of dyeing quantified the presence of interaction between acid leuco vat dyes and PP fiber. Tensile test and X-ray crystallinity results have confirmed that dyeing process did not alter the tensile strength and modulus of the dyed PP textiles significantly. PP Fabrics dyed with simulated continuous dyeing processes (pad-steam and pad-dry heat) demonstrated good color yields and levelness with adequate fastness to crocking, washing and dry cleaning.
Cellulose fiber reinforced nylon 6 or nylon 66 composites

(Georgia Institute of Technology, 2008-08-20) Xu, Xiaolin

Cellulose fiber was used to reinforce higher melting temperature engineering thermoplastics, such as nylon 6 and nylon 66. The continuous extrusion - direct compression molding processing and extrusion-injection molding were chosen to make cellulose fiber/nylon 6 or 66 composites. Tensile, flexural and Izod impact tests were used to demonstrate the mechanical properties of the composites. The continuous extrusion-compression molding processing can decrease the thermal degradation of cellulose fiber, but fiber doesn't disperse well with this procedure. Injection molding gave samples with better fiber dispersion and less void content, and thus gave better mechanical properties than compression molding. Low temperature compounding was used to extrude cellulose fiber/nylon composites. Plasticizer and a ceramic powder were used to decrease the processing temperature. Low temperature extrusion gave better mechanical properties than high temperature extrusion. The tensile modulus of nylon 6 composite with 30 % fiber can reach 5GPa; with a tensile strength of 68MPa; a flexural modulus of 4GPa, and a flexural strength of 100MPa. The tensile modulus of nylon 66 composites with 30 %fiber can reach 5GPa;with a flexural modulus of 5GPa; a tensile strength of 70MPa; and a flexural strength of 147MPa. The effect of thermal degradation on fiber properties was estimated. The Halpin-Tsai model and the Cox model were used to estimate the composite modulus. The Kelly-Tyson model was used to estimate the composite strength. The result indicates that the change of fiber properties determines the final properties of composites. Fiber length has a minor affect on both modulus and strength as long as the fiber length is above the critical length.