Organizational Unit:

School of Materials Science and Engineering

Permanent Link

https://hdl.handle.net/1853/70762

Parent Organization

Organizational Unit

College of Engineering

ArchiveSpace Name Record

https://finding-aids.library.gatech.edu/agents/corporate_entities/1126

Full item page

Publication Search Results

Now showing 1 - 10 of 40

Multi-scale modeling of poly(3-hexylthiophene) and [6,6]-phenyl-c61-butyric acid methyl ester using coarse grained force field derived from DFT based atomistic force field

(Georgia Institute of Technology, 2015-12-02) Yoo, Hanjong

The power conversion efficiencies for the organic photovoltaic cells containing active layers of electron donors and acceptors are dependent of three morphological properties, namely the domain size of the electron donor phase, the interface-to-volume ratio of the blend and the percolation ratio. In this study, poly(3-hexylthiophene) (P3HT), poly(3-nonylthiophene) (P3NT), poly(3-dodecylthiophene) (P3DT), fullerene (C60) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends have been introduced as the active layers to understand the effect of the structural deformation of the active layer components on the morphological properties. The state-of-the-art coarse grained molecular dynamics simulations are employed to investigate the morphological properties of the active layer systems. We have developed Morse potential-based force field parameters to accurately describe potential energy surfaces between C60 and P3HT coarse grained models. Using the coarse-grained model, we can investigate much larger system during longer simulation time than using full atomistic model. We modified the electron donor and acceptor materials and analyzed how the modifications affect the morphological quantities of active layer in both microscopic macroscopic scales with weight ratio of 1:1.
Enhancing chiroptical signals from metamaterials via nonlinear excitation

(Georgia Institute of Technology, 2015-11-25) Rodrigues, Sean Phillip

As natural chiral materials demonstrate limited circularly dichroic contrasts, enhancement of these polarization dependent signals has long been the focus of chiral metamaterial research. By manipulating the geometric chirality of resonant plasmonic nanostructures, we are capable of enhancing light confinement to amplify chiral modified, nonlinear signals from quantum emitters. The metamaterial demonstrates a linear transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second harmonic responses from the two incident polarizations. Nonlinear and linear response images probed with circularly polarized lights show strongly defined contrast. As a second set of experimentation, the chiral center of the metamaterial is opened, providing direct access to place emitters to occupy the most light-confining and chirally sensitive regions. The resulting two-photon emission profiles from circularly polarized excitation displays mirrored symmetry for the two enantiomer structures. The efficiency of the nonlinear signal directly correlates to the chiral resonance of the linear regime. The nonlinear emission signal is enhanced by 40× that of the emitters not embedded in the metamaterial and displays a 3× contrast for the opposite circular polarization. Such manipulations of nonlinear signals with metamaterials open pathways for diverse applications where chiral selective signals are monitored, processed, and analyzed.
Auxetic behavior in some fiber network structures

(Georgia Institute of Technology, 2015-11-16) Verma, Prateek

Auxetic materials are a rare class of materials that exhibit negative Poisson’s ratio. While most substances (like a rubber band) become thinner in lateral direction when stretched, auxetic materials grow thicker. The broad objective of this research is to study the origins of auxetic behavior in fibrous networks and to develop predictive processing-structure-property relations for these materials systems. We start by examining out-of-plane Poisson's ratio in paper by investigating a range of carefully chosen commercial paper samples. Laboratory handsheets were also produced and examined for their out-of-plane auxetic response. Geometrical and finite element models were built to help understand the origin of and underlying mechanism responsible for this auxetic response. Additionally, we were able to create a similar auxetic response in needle-punched nonwoven fiber networks by a heat-compression treatment. A series of microscopic and tomographic characterization was performed. From results on paper and nonwovens, it is evident that the type of network stabilization (hydrogen bonding in paper and needle-punching in nonwovens) and the choice of subsequent processing conditions have a significant influence on the out-of-plane Poisson’s ratio in these materials. Ultimately, a fundamental understanding of the origins of deformation behavior in these fiber networks should lead to the prospect of rational design of new auxetics and, in turn, to new product development opportunities for fiber-network materials.
Perspectives on Degradation in Solid Oxide Fuel Cells Using X-ray Spectroscopies and Scattering

(Georgia Institute of Technology, 2015-11-16) Lai, Samson Yuxiu

Solid oxide fuel cells (SOFCs) represent a major piece of a next-generation, renewable, clean energy economy and contribute to combating anthropogenic climate change by efficiently converting chemical energy into electrical energy through electrochemical reactions. However, despite adding significant chemical, mechanical, and microstructural complexity to push SOFC performance ever higher, cost and durability remain significant barriers to SOFC commercialization. Two of these issues are cathode stability in atmospheres containing carbon dioxide and water vapor and anode stability in fuel containing hydrogen sulfide. With regards to those aspects, state-of-the-art SOFC cathodes (La1-xSrxMnO3-δ and La1-xSrxCo1-yFeyO3-δ) and anodes (NiO and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ) are studied to understand the interactions between contaminant and electrode. In this work, powerful in situ and operando x-ray spectroscopy and scattering experiments provide deep insight into the physiochemical phenomena that define the behavior of SOFC electrode materials. These studies demonstrate that proper combination of in situ and operando experiments, due partially to the powerful intensity and capabilities of synchrotron x-rays, can provide unique information that has never before been possible and is critical to gaining new perspectives and to better understand data where a single perspective may only lead to ambiguous conclusions. Such a multi-pronged characterization approach is vital to gaining a better understanding of complex SOFC materials and providing critical insights for rational design of next-generation SOFC electrode materials.
The mechanochemistry in heterogeneous reactive powder mixtures under high-strain-rate loading and shock compression

(Georgia Institute of Technology, 2015-11-16) Gonzales, Manny

This work presents a systematic study of the mechanochemical processes leading to chemical reactions occurring due to effects of high-strain-rate deformation associated with uniaxial strain and uniaxial stress impact loading in highly heterogeneous metal powder-based reactive materials, specifically compacted mixtures of Ti/Al/B powders. This system was selected because of the large exothermic heat of reaction in the Ti+2B reaction, which can support the subsequent Al-combustion reaction. The unique deformation state achievable by such high-pressure loading methods can drive chemical reactions, mediated by microstructure-dependent meso-scale phenomena. Design of the next generation of multifunctional energetic structural materials (MESMs) consisting of metal-metal mixtures requires an understanding of the mechanochemical processes leading to chemical reactions under dynamic loading to properly engineer the materials. The highly heterogeneous and hierarchical microstructures inherent in compacted powder mixtures further complicate understanding of the mechanochemical origins of shock-induced reaction events due to the disparate length and time scales involved. A two-pronged approach is taken where impact experiments in both the uniaxial stress (rod-on-anvil Taylor impact experiments) and uniaxial strain (instrumented parallel-plate gas-gun experiments) load configurations are performed in conjunction with highly-resolved microstructure-based simulations replicating the experimental setup. The simulations capture the bulk response of the powder to the loading, and provide a look at the meso-scale deformation features observed under conditions of uniaxial stress or strain. Experiments under uniaxial stress loading reveal an optimal stoichiometry for Ti+2B mixtures containing up to 50% Al by volume, based on a reduced impact velocity threshold required for impact-induced reaction initiation as evidenced by observation of light emission. Uniaxial strain experiments on the Ti+2B binary mixture show possible expanded states in the powder at pressures greater than 6 GPa, consistent with the Ballotechnic hypothesis for shock-induced chemical reactions. Rise-time dispersive signatures are consistently observed under uniaxial strain loading, indicating complex compaction phenomena, which are reproducible by the meso-scale simulations. The simulations show the prevalence of shear banding and particle agglomeration in the uniaxial stress case, providing a possible rationale for the lower observed reaction threshold. Bulk shock response is captured by the uniaxial strain meso-scale simulations and is compared with PVDF stress gauge and VISAR traces to validate the simulation scheme. The simulations also reveal the meso-mechanical origins of the wave dispersion experimentally recorded by PVDF stress gauges.
Structure Property Relationships in Polyethylene Blown FIlms

(Georgia Institute of Technology, 2015-11-16) Kannan, Abhiram

The fabrication of blown films is a complex industrial process that has received some attention in the past from both industry and academia with the intention to establish detailed property-structure-processing linkages between polymers and their resulting blown films. Unfortunately, a clear understanding of the molecular level architectural variables, which control important properties of blown films such as resistance to tear and the resistance to puncture, are not fully developed. The current work uses powerful synchrotron based in situ X-ray scattering techniques to explore the morphologies of blown films as well as morphological evolution under uniaxial strain, for a series of polyethylenes whose architectures are well understood. Firstly, a number of protocols based on traditional analyses of X-ray scattering are developed which aid in the quantification of both the crystalline and non crystalline phases of the overall semicrystalline morphology of films. The analysis protocols developed allow the parametrization of dimensionality and orientation for both these phases at atomistic and mesoscopic length scales. Secondly, empirical relationships are established between the pertinent extracted parameters and molecular architectures of the polyethylenes under investigation. This enables the elucidation of those aspects of the molecular architecture of polyethylene the targeted manipulation of which is likely to result in the attainment of desired tear and puncture properties in blown films. Thirdly, quantitative relationships relating the dimensional and orientational parameters to the tear and puncture resistance properties are developed to determine the origin of these properties. Complementing the central theme of this dissertation are studies on the modeling of semicrystalline microstructures and the theoretical simulations of X-ray scattering from the same. Supplementing the analyses protocols developed from the traditional understanding of scattering phenomena are statistical analyses of scattering based on concepts borrowed from the realm of big-data analytics.
Understanding the process-structure-property relationship in biodegradable polymer nanocomposite films

(Georgia Institute of Technology, 2015-11-13) Sullivan, Erin M.

The focus of this study was to explore process-structure-property relationships in biodegradable polymer nanocomposite films in order to eliminate the commonly used trial and error approach to materials design and to enable manufacturing of composites with tailored properties for targeted applications. The nanofiller type and concentration, manufacturing method and compounding technique, as well as processing conditions were systematically altered in order to study the process-structure-property relationships. Polylactic acid (PLA) was used as the polymer and exfoliated graphite nanoplatelets (GNP), carbon nanotubes (CNT), and cellulose nanocrystals (CNC) were used as reinforcement. The nanocomposite films were fabricated using three different methods: 1) melt compounding and melt fiber spinning followed by compression molding, 2) solution mixing and solvent casting, and 3) solution mixing and electrospinning followed by compression molding. Furthermore, the physical properties of the polymer, namely the crystallization characteristics were altered by using two different cooling rates during compression molding. The electrical response of the composite films was examined using impedance spectroscopy and it was shown that by altering the physical properties of the insulating polymer matrix, increasing degree of crystallinity, the percolation threshold of the GNP/PLA films is significantly reduced. Additionally, design of experiments was used to examine the influence of nanofiller type (CNT versus GNP), nanofiller content, and processing conditions (cooling rate during compression molding) on the elastic modulus of the composite films and it was concluded that the cooling rate is the primary factor influencing the elastic modulus of both melt compounded CNT/PLA and GNP/PLA films. Furthermore, the effect of nanofiller geometry and compounding method was examined and it was shown that the high nanofiller aspect ratio in the CNT/PLA films led to decreased percolation threshold compared to the GNP/PLA films. The melt compounded GNP/PLA films displayed a lower percolation threshold than the solution cast GNP/PLA films most likely due to the more homogeneous distribution and dispersion of GNP in the solution cast films. Fully biodegradable and biorenewable nanocomposite films were fabricated and examined through the incorporation of CNC in PLA. Through the addition of CNC, the degree of crystallinity of the matrix was significantly increased. Focusing the design space through investigation of process-structure-property relationships in PLA nanocomposites, can help facilitate nanocomposites with tailored properties for targeted applications.
Impact-initiated combustion of aluminum

(Georgia Institute of Technology, 2015-11-11) Breidenich, Jennifer L.

This work focuses on understanding the impact-initiated combustion of aluminum powder compacts. Aluminum is typically one of the components of intermetallic-forming structural energetic materials (SEMs), which have the desirable combination of rapid release of thermal energy and high yield strength. Aluminum powders of various sizes and different levels of mechanical pre-activation are investigated to determine their reactivity under uniaxial stress rod-on-anvil impact conditions, using a 7.62 mm gas gun. The compacts reveal light emission due to combustion upon impact at velocities greater than 170 m/s. Particle size and mechanical pre-activation influence the initiation of aluminum combustion reaction through particle-level processes such as localized friction, strain, and heating, as well as continuum-scale effects controlling the amount of energy required for compaction and deformation of the powder compact during uniaxial stress loading. Compacts composed of larger diameter aluminum particles (~70µm) are more sensitive to impact initiated combustion than those composed of smaller diameter particles. Additionally, mechanical pre-activation by high energy ball milling (HEBM) increases the propensity for reaction initiation. Direct imaging using high-speed framing and IR cameras reveals light emission and temperature rise during the compaction and deformation processes. Correlations of these images to meso-scale CTH simulations reveal that initiation of combustion reactions in aluminum powder compacts is closely tied to mesoscale processes, such as particle-particle interactions, pore collapse, and particle-level deformation. These particle level processes cannot be measured directly because traditional pressure and velocity sensors provide spatially averaged responses. In order to address this issue, quantum dots (QDs) are investigated as possible meso-scale pressure sensors for probing the shock response of heterogeneous materials directly. Impact experiments were conducted on a QD-polymer film using a laser driven flyer setup at the University of Illinois Urbana-Champaign (UIUC). Time-resolved spectroscopy was used to monitor the energy shift and intensity loss as a function of pressure over nanosecond time scales. Shock compression of a QD-PVA film results in an upward shift in energy (or a blueshift in the emission spectra) and a decrease in emission intensity. The magnitude of the shift in energy and the drop in intensity are a function of the shock pressure and can be used to track the particle scale differences in the shock pressure. The encouraging results illustrate the possible use of quantum dots as mesoscale diagnostics to probe the mechanisms involved in the impact initiation of combustion or intermetallic reactions.
Development of oxidation resistant molybdenum-silicon-boron composites

(Georgia Institute of Technology, 2015-11-06) Marshall, Peter

The development of molybdenum - silicon - boron (Mo-Si-B) composites having a combination of high temperature strength, creep, and oxidation residence has the potential to substantially increase the efficiency of gas turbines. The refractory nature of the αMo, Mo3Si (A15), and Mo5SiB2 (T2) phases results in good strength and creep resistance up to 1300°C. At this temperature, the formation of a borosilicate surface scale from the two intermetallic phases is able to provide oxidation resistance. However, realization of these advantages has been prevented by both a high brittle to ductile transition temperature and difficulty in forming the initial surface borosilicate to provide bulk oxidation resistance. This dissertation addresses two factors pertaining to this material system: 1) improvements to powder processing techniques, and 2) development of compositions for oxidation resistance at 1300°C. The processing of Mo-Si-B composites is strongly tied to their mechanical properties by establishing the αMo matrix, limiting impurity content, and reducing silicon supersaturation. These microstructural aspects control the brittle to ductile transition temperature which has traditionally been too high for implementation of Mo-Si-B composites. The processing here built upon the previously developed powder processing with silicon and boron nitrides which allowed for a low oxygen content and sintering of fine starting powders. Adjustments were made to the firing cycle based upon dew point measurements made during the hydrogen de-oxidation stage. Under a relatively high gas flow rate, 90% of the total water generated occurred during a ramp of 2°C /min between 450 and 800°C followed by a hold of 30 minutes. The oxidation resistance of Mo-Si-B composites was studied for a wide range of compositions. Silicon to boron atomic ratios were varied from 1 to 5 and iron, nickel, cobalt, yttria, and manganese were included as minor additions. In all these compositions, the αMo volume fraction was kept over 50% to ensure the potential toughness of the composite. For the oxidized surface glass, a silica fraction of 80 to 85% was found to be necessary for the borosilicate to have a sufficiently high viscosity and low oxygen permeability for oxidation resistance at 1300°C. For the Mo-Si-B bulk composition this corresponds to a Si/B atomic ration of 2 to 2.5. Higher viscosity compositions failed due to spallation of poorly attached, high silica scales. Lower viscosity compositions failed from continuous oxidation, either through open channels or repetitive MoO3 bubble growth and popping. Additionally, around 1% manganese was necessary for initial spreading of the borosilicate at 1300°C. In conjunction with flowing air to prevent MoO3 accumulation, oxidation weight loss rates below 0.05 mg/cm2-hr were measured. Finally, a theory is proposed here to describe the mechanisms responsible for the development of oxidation resistance. This theory involves three stages associated with: 1) generation of an initial surface borosilicate, 2) thickening of the borosilicate layer, and 3) slow parabolic oxidation controlled by the high silica surface scale.
Responsive micro- and nano-structures through interfacial assembly of star polymers

(Georgia Institute of Technology, 2015-11-05) Xu, Weinan

Responsive polymeric nanostructures have attracted much attention in recent years due to their abilities to adapt and respond to external stimuli, and potential applications in bio-sensing, self-healing coatings, drug delivery, tunable catalysis, and bio-imaging. Star polymers have emerged as novel building blocks for such assembled structures due to their unique architectures and multiple responsive properties. A challenging task in this filed is how to precisely control the interactions between star polymers and with other components, and maintain the responsive properties of the functional stars in the assembled nanostructures. Therefore, the goal of the proposed work is to understand the responsive properties and interactions of star polymers in different conditions, including solution and interfaces, and utilize them as building blocks for polymeric micro- and nano-structures such as polymersomes, ultrathin films and microcapsules, which have intriguing properties in terms of stability, responsiveness and functionalities compared with conventional linear polymers based structures. Specifically, in the first place, we studied the solution phase behavior of responsive star polymers by using in situ (small angle neutron scattering) SANS, and showed that in semidilute solution, the temperature induced phase separation for thermo-responsive star polymers are significantly different from that of their linear counterparts. The star polymers show limited microphase separation with aggregates composed of several molecules, while the corresponding linear polymers have LSCT (low critical solution temperature) type phase separation. Secondly, we studied the responsive properties and assembly of amphiphilic star polymers at the air/water interface and in Langmuir-Blodgett monolayer. We found that the confined interface environment leads to different conformational changes and assembly behaviors of the star polymers compared with those in solution state. For instance, when there is a hydrophilic to hydrophobic transition, the polymers tend to go from water subphase to the air/water interface, rather than showing coil to globule transition in aqueous solution. Thirdly, we utilized the star polymers as major component to fabricate 3D responsive microstructures such as thin shell microcapsules, by using layer-by-layer (LbL) assembly technique, which has rarely been explored before, especially for complex star block copolymers. The assembly microcapsules have hierarchical multicompartmental structure, which enables the encapsulation and release of multiple molecules simultaneously. The shell of the multilayer microcapsules has porous 3D network structure, with fine controlled permeability. Lastly, for star polymers with multiple responsive properties, we found that their responsiveness is well maintained after being assembled into microstructures, so that the microcapsules have multiple responsive properties. The multiple responses in structure and permeability to external stimuli enable the controlled and programmable delivery of multiple cargo molecules, such as those we demonstrated in this study: microcapsules with pH and temperature dual responsiveness, as well as ionic conditions and UV dual responsive properties.