Browsing by Subject "Material Science and Engineering"

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Atomic-scale investigations of multiwall carbon nanotube growth.
(2010-06) Behr, Michael John
The fundamental processes of carbon nanotube (CNT) growth by plasma-enhanced chemical vapor deposition (PECVD) were investigated using a suite of characterization techniques, including attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical emission spectroscopy (OES), Raman spectroscopy, convergent-beam electron diffraction (CBED), high-resolution transmission and scanning-transmission electron microscopy (TEM, STEM), energy dispersive x-ray spectroscopy, and electron energy-loss spectroscopy (EELS). It is found that hydrogen plays a critical role in determining the final CNT structure through controlling catalyst crystal phase and morphology. At low hydrogen concentrations in the plasma iron catalysts are converted to Fe3C, from which high-quality CNTs grow; however, catalyst particles remain as pure iron when hydrogen is in abundance, and produce highly defective CNTs with large diameters. The initially faceted and equiaxed catalyst nanocrystals are deformed by the surrounding CNT structure during growth. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (~1-3 degree) rotations, twists, and bends along their axial length between adjacent locations. Fe3C catalyst nanoparticles that are located inside the base of well-graphitized CNTs of similar structure and diameter do not exhibit a preferred orientation relative to the nanotube axis, indicating that the graphene nanotube walls are not necessarily produced in an epitaxial process directly from Fe3C faces. Chemical processes occurring at the catalyst-CNT interface during growth were inferred by measuring, ex situ, changes in atomic bonding at an atomic scale with EELS. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe3C, near where the CNT walls terminate at the catalyst base. An amorphous carbon-rich layer at the catalyst base provides the source for CNT growth. These results suggest that what is required for CNT growth is a graphene seed and a source of decomposed carbon. Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls are etched nonuniformly. Etch pits form at defective sites along the CNT, from which etching proceeds rapidly. It is determined that H etching occurs preferentially at graphene edges.
Catalytic partial oxidation of pyrolysis oils.
(2009-08) Rennard, David Carl
Pyrolysis oils, created from biomass by rapid heating in the absence of oxygen, are a promising intermediate for renewable fuels. Catalytic partial oxidation (CPO) can convert pyrolysis oils to synthesis gas, a mixture of CO and H2, which can be subsequently converted to synthetic renewable fuels: Fischer Tropsch alkanes, methanol, dimethyl ether, or H2 for fuel cells. CPO is rapid, with contact times of 10-30 ms, tunable to a select few types of products, and autothermal. The CPO of model compounds of pyrolysis oils, including acids, esters, and polyols is explored over Rh and Pt catalysts. Experiments over Rh achieve near equilibrium production of syngas. Over Pt, non-equilibrium olefins and aldehydes are observed, which give insight into the catalytic and homogeneous chemistry in CPO. Reactive Flash Volatilization (RFV), wherein liquid droplets are sprayed directly onto the catalyst surface, is also explored for both glycerol and three types of pyrolysis oil. A long-term study of RFV of glycerol explores the longevity of the noble metal catalyst in this technique.
Controlled electrochemical synthesis of giant magnetostrictive iron-gallium alloy thin films and nanowires.
(2012-04) Reddy, Kotha Sai Madhukar
Magnetostrictive Galfenol (Fe1-xGax, x = 10% - 40%) alloys have generated tremendous interest in recent times because of their potential as functional materials in various micro- and nano-electromechanical systems (MEMS/NEMS)-based transducers and sensors. Among the giant magnetostrictive alloys, Terfenol-D (Tb1-xDyxFe2) has the largest magnetostriction, but its brittle nature limits its applications. In contrast, the next best magnetostrictive alloy, Galfenol, is highly malleable and ductile while having the tensile strength of Iron. Electrochemistry is an economical route to fabricate 'very thick' films (upto several microns) or high-aspect ratio structures like nanowire arrays. However, the highly electropositive nature of gallium makes it very difficult to electrodeposit from aqueous solutions, similar in behavior to other non-ideal elements like molybdenum, phosphorus, tungsten etc. As a result, Fe1-xGax alloy plating has been severely plagued by non-repeatability in compositions from growth to growth, lack of uniformity in filling of pores when growing nanowires in nanoporous templates, undesired secondary hydrogen evolution reactions etc. In this study, a thorough understanding of the complex interplay between various deposition parameters (pH, overpotential, concentration, hydrodynamic conditions) was achieved, leading to an understanding of the deposition mechanism itself, thus allowing excellent control and ability to tune the alloy compositions. Arrays of nanowires were fabricated with alternating segments of the magnetostrictive alloy Fe1-xGax and Cu in nanoporous anodic aluminum oxide (AAO) templates. A novel rotating disk electrode-template (designed in-house) was used to optimize the nanowire length distributions by controlling the various aspects of electrodeposition like nucleation, kinetics and mass-transfer. Extensive structural characterization was done by X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), and magnetic characterization by vibrating sample magnetometry (VSM). Furthermore, of excellent promise in semiconductor spintronics, the feasibility of fabricating epitaxially nucleated Fe1-xGax thin films on GaAs having the desired (001) texture was demonstrated. Structural characterization using microdiffraction, high resolution ω - 2θ and rocking curve analysis revealed that the films grown on GaAs(001) are highly textured with <001> orientation along the substrate normal, and the texture improved further upon annealing at 300 °C for 2 hours in N2 environment. This was in contrast to films grown on polycrystalline brass substrates which exhibited undesired <011> texture out-of-plane. Rocking curve analysis on Fe1-xGax/GaAs structures further confirmed that the <001> texture in the Fe1-xGax thin film was indeed due to epitaxial nucleation and growth. A non-linear current-voltage plot was obtained for the Fe1-xGax/GaAs Schottky contacts, characteristic of tunneling injection, and showed improved behavior with annealing.
Deformation mechanisms in nanoscale brittle materials.
(2011-05) Stauffer, Douglas Dean
In the past ten years nanotechnology has developed from a buzzword to an integral part of our modern life. The promise of bottom up devices has turned into better, faster, and stronger products utilizing nanoscale materials. Tires designed with carbon nanotubes, touchscreens, reformulated steel, self-cleaning fabrics, drug delivery systems, and semiconductor devices all rely on nanoscale materials. However, the mechanical property relationships are not fully understood, and the cross-roads of mechanical performance and electrical properties is still being explored. For example, the role of electrical contact in mechanical systems is important for reliability in systems that contain interconnect, switches, or relays. MEMS switches in particular can have reliability issues if the conducting area is decreased, or the switch fails due to plasticity. In this thesis, an attempt is made to characterize failure modes of several fundamental nanoscale materials using nanoindentation. In this thesis, ostensibly brittle materials such as alumina, chromia, and silicon are chosen as being archetypal examples of brittle materials. The use of conductive probe indentation is used here as a measure of plasticity under the indenter in constrained metal films with native oxide layers, as well as to determine the point of oxide fracture. In situ transmission electron microscope indentation is used to explore dislocation velocities and strain hardening in compressed silicon pillars. Dislocation velocities, in compression at room temperature, are found that approach that of those at 600°C in bulk tensile specimens. The dislocations, of unknown type, also contribute to strain hardening exponents of approximately 0.4 in pillars, and approach unity in silicon spheres.
Early stages of zeolite growth.
(2010-08) Kumar, Sandeep
Zeolites are crystalline nonporous aluminosilicates with important applications in separation, purification, and adsorption of liquid and gaseous molecules. However, an ability to tailor the zeolite microstructure, such as particle size/shape and pore-size, to make it benign for specific application requires control over nucleation and particle growth processes. But, the nucleation and crystallization mechanisms of zeolites are not fully understood. In this context, the synthesis of an all-silica zeolite with MFI-type framework has been studied extensively as a model system. Throughout chapters 2, 4 and 5, MFI growth process has been investigated by small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). Of fundamental importance is the role of nanoparticles (~5 nm), which are present in the precursor sol, in MFI nucleation and crystallization. Formation of amorphous aggregates and their internal restructuring are concluded as essential steps in MFI nucleation. Early stage zeolite particles have disordered and less crystalline regions within, which indicates the role of structurally distributed population of nanoparticles in growth. Faceting occurs after the depletion of nanoparticles. The chapter 6 presents growth studies in silica sols prepared by using a dimer of tertaprpylammonium (TPA) and reports that MFI nucleation and crystallization are delayed with a more pronounced delay in crystal growth.
Electromechanical characterization of quasi-one dimensional nanostructures of silicon, carbon, and molybdenum disulfide via symmetry-adapted tight-binding molecular dynamics.
(2010-11) Zhang, Dong-Bo
With a newly developed symmetry-adapted tight-binding molecular dynamics (MD) capability, we performed microscopic calculations on a variety of quasi-one dimensional silicon, carbon, and molybdenum disulfide nanostructures. In symmetry-adapted MD the helical symmetry instead of the standard translational symmetry is used. In the considered nanostructures, equivalent calculations can now be performed with a substantial smaller, in terms of the number of atoms, repeating domain. The symmetry-adapted method was utilized in the studied highlighted below. The stability of the most promising ground state candidate silicon nanowires with less than 10 nm in diameter was comparatively studied with with nonorthogonal tight-binding and classical potential models. The computationally expensive tight-binding treatment becomes tractable due to the substantial simplifications introduced by the presented symmetry-adapted scheme. It indicates that the achiral polycrystalline of fivefold symmetry and the hexagonal (wurtzite) wires of threefold symmetry are the most favorable quasi-one-dimensional silicon arrangements. Quantitative differences with the classical model description are noted over the whole diameter range. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces and strained morphologies. The elastic response for a large catalog of carbon nanotubes subjected to axial and torsional strain was next derived from tomistic calculations that rely on an accurate tight-binding description of the covalent binding. The application of the computationally expensive quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and ngular symmetries exhibited by the elastically deformed nanotubes. The elasticity of nanotubes larger than 1.25 nm in diameter can be represented with an isotropic elastic continuum. The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,−1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps. Symmetry-adaptedMD combined with density-functional-based tight-binding made possible to compute chiral nanotubes as axial-screw dislocations. This enabled the surprising revelation of a large catalog of MoS2 nanotubes that lack the prescribed translational symmetry and exhibit chirality-dependent electronic band-gaps and elastic constants. Helical symmetry emerges as the natural property to rely on when studying quasi-one dimensional nanomaterials formally derived or grown via screw dislocations. The nonlinear elastic response of carbon nanotubes in torsion was derived with the symmetry-adapted MD and a density-functional-based tight-binding model. The critical strain beyond which tubes behave nonlinearly, the most favorable rippling morphology, and the twist- and morphology-related changes in fundamental band gap were identified from a rigorous atomistic description. There is a sharply contrasting behavior in the electronic response: while in single-walled tubes the band-gap variations are dominated by rippling, multiwalled tubes with small cores exhibit an unexpected insensitivity. Results are assistive for experiments performed on nanotubes-pedal devices. Despite its importance, little is known about how complex deformation modes alter the intrinsic electronic states of carbon nanotubes. We considered the rippling deformation mode characterized by helicoidal furrows and ridges and elucidate that a new intralayer strain effect rather than the known bilayer coupling and &sigma-&pi orbital mixing effects dominates its gapping. When an effective shear strain is used, it is possible to link both the electrical and the mechanical response of the complex rippled morphology to the known behavior of cylindrical tubes. Moving on to graphene, to describe the strain stored in helical nanoribbons, we supplement the standard elasticity concepts with an effective tensional strain. Using &pi -orbital tight binding and objective molecular dynamics coupled with density functional theory, we show that twisting couples the frontier conduction and valence bands, resulting in band-gap modulations. In spite of the edges and ridges of the helical nanoribbons, from the effective strain perspective these band-gap modulations appear strikingly similar with those exhibited by the seamless carbon nanotubes.
Examination of transient carrier behaviors in organic field-effect devices via displacement current measurement.
(2011-01) Liang, Yan
Organic field-effect transistors (OFETs) are one of the key components of the ubiquitous flexible electronics in the near future. Since the first report in the mid-1980s, OFETs have been intensively studied for more than 20 years. However, most of these studies are based on steady state or quasi steady state DC measurements, which are not sensitive to transient processes including the formation and depletion of the conducting channel. These transient processes are essential parts of the device operation, and determine the response frequency of OFETs. For these reasons, the transient carrier behaviors during these processes warrant examination. This thesis develops displacement current measurement (DCM) as a technique to probe transient carrier behaviors. Instead of using OFETs for measuring displacement current, long-channel capacitors (LCCs) are used. A LCC can be viewed as a simplified OFET with only one channel contact to limit the carrier injection/extraction to one direction only. The channel of a LCC is elongated to millimeter range to increase the transient time and the displacement current associated with charging/discharging the channel. Displacement current has been measured from LCCs under cyclic gate voltage sweeps. The number of the injected, extracted, and trapped carriers can be calculated by integrating the displacement current with respect to time. A current peak has always been observed in the charging sweep, and it is attributed to the transient process of conducting channel formation. Analytical and numerical device models have been developed to understand the transient carrier behaviors. It is found that carrier mobility can be calculated from the slope of the displacement current peak. In addition, the evolution of the carrier distribution in the long channel during the conducting channel formation and depletion are presented in detail. Further more, the effects of carrier traps on the transient carrier behaviors are discussed. DCM has also been used to study the contact effects at metal-pentacene interfaces. It is found that the carrier trapping in the long channel of the LCCs with Au contacts is indirectly caused by the deep trap states at the pentacene-dielectric interface in the contact region generated by Au penetration. Low trapped carrier density is found in the LCCs with Cu contacts due to the shallow penetration of Cu atoms. In addition, ambipolar injection and transport are observed in a LCC with Al contact and a PMMA buffer layer between pentacene and SiO2. Thus DCM can be used to characterize the quality of metal-organic contacts. The conducting channel depletion dynamics under constant gate voltages has also been examined. It is found that the discharging displacement current can either follow an exponential decay or a power law decay, depending on the discharging gate voltage. A simple RC model has been given to explain these different decay behaviors. For the power law decay, the decay exponent measured from the experimental data is around 1.2 to 1.3, while the exponent predicted by the RC model is 2. The smaller exponent observed in experiment might be attributed to the effect of carrier traps.
Fabrication and characterization of organic single crystal and printed polymer transistors.
(2009-08) Xia, Yu
The key challenges in the development of organic electronics lie in the understanding of the charge transport physics and the realization of low cost device fabrication. Innovative studies on both aspects have been demonstrated in this thesis. On the fundamental side, first, charge transport and localization processes in various organic single crystal transistors have been investigated using a novel "air-gap"device geometry. Second, comparison of mobility - carrier density relation in polymer and single crystal transistors has been made by the utilization of different liquid gate dielectrics with extremely wide capacitance range, and fundamentally different charge transport mechanisms have been proposed. Third, direct measurement of the electrochemical potential at organic semiconductor/gate dielectric interfaces in electrolyte gated transistors has been achieved with the assistance of an embedded reference electrode. The correlation between the referenced turn-on voltages and the organic semiconductor ionization potentials has been discovered. Finally, an unusual negative differential transconductance behavior in electrolyte gated transistors upon inducing high gate carrier densities has been extensively investigated. On the application side, high performance polymer transistors and circuits were fabricated by a commercial aerosol jet printing technique. Printing not only saves the device manufacturing cost through its simple procedure, high throughput and low waste of materials, but also enables the fabrication of electronic devices over large area and on flexible substrates. All-printed transistors with exceptionally large transconductance of 10 mS/mm under 1 V of operating voltage have been realized with the application of specially designed printable high capacitance (>10 μF/cm2) ion gel as the gate dielectric material. Various device configurations and parameters have been investigated to further reduce the fabrication cost and improve the operating speed. Based on these transistors, high performance, low voltage operation logic and analog circuits such as inverters, NAND logic gates, D Flip-flop circuits and ring oscillators have been demonstrated.
Formation of salt crystal whiskers on nanoporous coatings and coating onto open celled foam.
(2012-02) Zhang, Heng
Salt crystal whiskers were grown from salt solution saturated nanoporous silica coatings. Coated substrates were partially immersed into an aqueous potassium chloride solution and then kept in a controlled relative humidity chamber for whisker growth. The salt solution was first wicked into the coating by capillary action, and then evaporation ensued and a supersaturated condition was reached. Crystals grew from the surface by a base growth mechanism in which salt ions were added to the surface of the crystal that was in contact with the nanoporous coating. Optical microscopy and SEM results demonstrated this mechanism. Crystals with whisker morphologies, typically 2 - 50 µm in lateral dimension and up to ~1 cm in length, emerged from the coating surface at a position above the original liquid level. Sheet-like crystals also formed from whiskers that had fallen flat onto the porous coating surface. Inspired by the sheet formation mechanism and liquid transportation phenomenon, a seeding technique was developed to reduce whisker width. Attritor ground salt particles were placed on the nanoporous coating surface to initiate simultaneous whiskers growth and salt nano-whiskers with lateral dimension as small as 50 nm were obtained on the surface of the coating. This crystal growth method can be applied to different materials, namely water soluble materials, and creates whisker crystals with controllable size and location on the nanoporous coating. Open celled foam is a three dimensional structure. In some applications, other materials are coated on internal surface of the foam to provide desired final product functionality. Because of their complicated 3D structures, coating onto foam is challenging. A new coating process that combines dip coating and spin coating was developed. Dip coating step was used to load the solution into the foam and a spin treatment step was added to remove the trapped liquid and redistribute the liquid to obtain uniform coating. The dip and spin process was also used to create -alumina and zeolite coatings, which are of interest for catalysis applications.
Hardening mechanisms of silicon nanospheres: a molecular dynamics study.
(2011-05) Hale, Lucas Michael
Much work has been done studying the compression of nanostructures of silicon as the measured properties can be related to structures present in MEMS and NEMS devices. In particular, spherical silicon nanoparticles are found to be much harder than bulk silicon during compression. Here, large scale molecular dynamics simulations are presented that investigate the yielding and hardening mechanisms of nanospheres. The resulting yield behavior is shown to vary with changes in temperature, sphere size, atomistic potential, and crystallographic orientation with respect to the loading direction. With the Tersoff potential, a strong temperature dependence is observed as hardness values near 0 K are much greater than 300 K values. beta-Sn forms during [100] crystallographic compressions which results in a slight hardening above 40 % strain. The Stillinger-Weber allowed for dislocation interactions to be studied in spheres comprised of up to one million atoms. Direct comparisons of the simulated results are made to experimental results indicating that the displacement excursions and low strain hardening behavior can be explained with dislocation activity. Further simulations investigated interactions affecting dislocations that might influence the properties of silicon nanostructures. The nature of dislocation-dislocation, dislocation-applied shear strain, and dislocation-free surface interactions are shown to be consistent with what is predicted by elementary dislocation theory. Presence of an oxide results in a more complex interaction as both the interface and the lattice strain associated with the oxide affect the dislocations. Depending on the geometry of the system, this oxide interaction may be repulsive resulting in dislocations becoming trapped in the system allowing for substantial hardening.
In situ characterization of dynamic structures of coatings
(2012-04) Song, Jin-Oh
We have developed new methods and apparatus to characterize the structure changes of coatings in situ. The techniques enabled the study of critical factors to control during drying or curing process to avoid excess materials use and coating defects. The instruments have been used to investigate the effect of process conditions on the structure development and final coating properties. A magnetic microrheometer for in situ measurement of local viscosity of coatings was designed since conventional bulk rheometry cannot be used to follow the temporal and spatial gradients of viscosity in drying or curing coatings. Micron-sized magnetic probe particles under a magnetic field gradient act as probes for local rheological responses in coatings. Viscosity-time profiles were measured in drying aqueous PVA coatings, and the results revealed the correlation between sagging defects and viscosity build-up. The development of viscosity gradient through the thickness in UV curing epoxy coatings was also characterized to study wrinkling defects, skin formation, and structure or composition gradients through the coating thickness. The microstructure of drug-polymer coatings was also characterized using confocal Raman microscopy. A drying apparatus was built to control the coating method, drying temperature, and air flow during the characterization since the size and distribution of drug phase and polymer structure in the coating strongly depend on the process conditions. The dependence of environmental inputs to the drug and polymer coating morphology during drying was investigated in order to elucidate and optimize either the processing conditions or coating formulation.
Interfacial coupling between immiscible polymers: flow accelerates reaction and improves adhesion.
(2011-10) Song, Jie
As the workhorses of the plastics industry, polyolefins are consumed in the largest volume of all types of polymers. Despite their wide use, polyolefins suffer from poor adhesion and compatibility with other polar polymers due to their intrinsic low polarity and lack of functional groups. The first goal of this study is to enhance interfacial adhesion between polyolefins with other polymers through coupling reaction of functional polymers. We have used functional polyethylenes with maleic anhydride, hydroxyl, primary and secondary amino groups grafted through reactive extrusion. Functional polyolefins dramatically improved the performance of polyolefins, including adhesion, compatibility, hardness and scratch resistance, and greatly expand their applications. The second goal is to understand the factors affecting adhesion. We systematically investigated two categories of parameters. One is molecular: the type and incorporation level of functional groups. The other is processing condition: die design in extruders, reaction time and temperature. The interfacial adhesion was measured with the asymmetric dual cantilever beam test and T-peel test. The extent of reaction was quantified through measuring anchored copolymers via X-ray photoelectron spectroscopy. A quantitative correlation between adhesion and coupling reaction was developed. A coextruded bilayer system with coupling reaction at interfaces was created to clarify processing effects on the kinetics of coupling reactions. For the reaction between maleic anhydride modified polyethylene and nylon 6, the reaction rate during coextrusion through a fishtail die with compressive/extensional flow was strikingly almost two orders of magnitude larger than that through a constant thickness die without compressive flow. The latter reaction rate was close to that of quiescent lamination. We attribute the reaction acceleration through the fishtail die to the large deformation rate under the compressive/extensional flow condition. The deformation generated stretched chains leading to complimentary functional groups exposed to each other and forcing reactive species to overcome the interfacial diffusion barrier. We also found reaction acceleration through a fishtail die for the coupling of functional PE with thermoplastic polyurethane. This work illustrates that enhancing the compressive/extensional flow during polymer processing may create opportunities for increasing adhesion and designing new reactions and products.
The mechanical response of common nanoscale contact geometries
(2008-03) Mook, William Moyer
Characterizing the mechanical response of common nanoscale contact geometries is vitally important to fields such as microelectromechanical systems (MEMS) where the behavior of nanoscale contacts can in large part determine system reliability and lifetime. Therefore a research program was undertaken that focused on the development of innovative nanoindentation-based techniques capable of quantifying the mechanical response of freestanding nanostructures. Nanoindentation was used since it is a non-destructive, high resolution technique that has been proven to be very useful in characterizing materials at the nanoscale. Examples of tested structures include single crystalline nanoparticles and polycrystalline nanoposts. From these experiments methods to characterize the structures' effective elastic modulus, flow stress, fracture toughness and activation volume required for plasticity have been developed. It was noted that both modulus and toughness in nanoparticles scale with average contact stress. This result has lead to the development of an experimental analysis technique that accounts for the hydrostatic component of pressure which develops in a material under contact. The effect of hydrostatic pressure on indentation modulus is currently not accounted for in nanoindentation even though it is shown to be important at length scales below 100 nm.
Modeling of diblock copolymers in selective solvents.
(2012-06) Thiagarajan, Raghuram
Phase behavior and micellization kinetics of diblock copolymer surfactants in selective solvents influence many processes. We study the driving forces behind the self-assembly of a diblock copolymer AB, consisting of a solvent-philic block (B) and a solvent-phobic block (A), in selective solvents (S). We investigate this system using self-consistent field theory (SCFT), which is a coarse-grained, approximate theory with a proven track record for polymer mixtures. It discards the effects of fluctuations. Micellar transformations between spherical, cylindrical, and bilayer curvatures are tracked in the dilute regime. We determine thermodynamic and structural properties of these isolated aggregates such as the critical micelle concentration (CMC), the critical micelle temperature (CMT), the solvent penetration of the core, and the core radius of micellar morphologies within the context of SCFT. We also investigate the morphological variation from ordered phases, found in the concentrated regime, to isolated aggregates upon copolymer depletion. Depleting this blend of surfactant causes these stable structures to swell and ultimately unbind. The unbinding transition of the ordered phases is compared with the morphology transformations observed in the dilute regime. We also delineate two phase coexistence regions between ordered phases, and ordered phases and a solvent rich macrophase. Furthermore, we quantify the effective interactions between the aggregates themselves. Intriguingly, for spherical micelles, the free energy of BCC, and FCC phases can be described in terms of a single effective pair potential that depends on micellar aggregation number, however, this aggregation number changes significantly with the concentration and temperature. The kinetic barriers to association and dissociation of diblock copolymers in various selective solvents are calculated. We study the variation of these kinetic barriers for both block copolymers in small molecule solvents and block copolymers in a homopolymer matrix. The kinetic barriers are found to be very sensitive to temperature and surfactant concentration. They also become prohibitive except in a modest range of temperature near the CMT, or in sufficiently highly supersaturated or subsaturated solutions near the equilibrium CMC. The dependence of kinetic barriers upon the chain lengths and solvent quality is also studied.
Recognition and assembly at multiple length-scales.
(2010-05) Olmsted, Brian Keith
Many molecular materials capable of crystallizing into an ordered solid state may assume multiple packing arrangements. This behavior is called polymorphism and is common among organic molecules such as pharmaceuticals and dyes. Controlling the nucleation of specific polymorphic crystals is not well understood, but is tantamount to the development and manufacture of new industrial products. One phenomena that has been observed to influence crystal orientation, growth rate, and morphology is epitaxy. Epitaxy refers to a condition by which a crystalline substrate presents a similar two-dimensional lattice to a crystalline plane of a nucleating species, resulting in a condition that lowers the energy barrier to nucleation and results in a preferential orientation of crystal growth on the substrate. Therefore, epitaxial nucleation may provide routes to selectively nucleate polymorphs and attain control over otherwise unpredictable crystallization events. The literature provides several examples of epitaxial relationships between a substrate and a crystal overlayer in fields involving inorganic crystals as well as organic crystals, and because epitaxy relies on geometric comparisons between lattice parameters, computational prediction of epitaxy is an active area of research. Our laboratories have developed software; named GRACE, to attempt to predict epitaxial relationships and this software has been used to verify epitaxy reported in the literature. One particularly useful feature of GRACE is its ability to handle a library of substrates and screen them against a corresponding database of crystal structures available as candidate crystal overlayers. In this capacity GRACE allows large libraries of substrates and crystals to be reduced to an experimentally manageable size, whereby combinatorial crystallizations can be tested for selective nucleation arising from epitaxial interfaces. This research also focuses on other aspects of nucleation that are not yet fully understood. Epitaxial interfaces are by definition, abrupt. However, a specialized class of crystals involving a domain that completely overgrows a core crystal by epitaxial mechanisms has revealed a zone of intermixing spanning close to a micron. In situ Atomic Force Microscopy (AFM) reveals the mechanisms for these observations and provides insight into how epitaxial interfaces behave mechanistically. Notably, it was revealed that process conditions between phases of growth in the formation of core-shroud heterocrystals may yield controllable interfacial thicknesses between crystalline domains, It was also discovered that the propensity for abrupt, epitaxial interfaces may be limited by the thermodynamic behavior of specific crystal interfaces under conditions of near-equilibrium. Although the use of in situ AFM is excellent for the study of crystal growth, the mass-transfer limitations at crystallizing interfaces inside an (AFM) fluid cell are not directly discernable and the assumption is typically made that conditions in the bulk solution are the same inside the cell. By implementing computational fluid dynamic (CFD) simulations for flow and mass transport, in situ AFM was studied to determine how the different conditions at the crystal surface are in comparison to the bulk solution outside the cell. The geometry of the internal volume of the AFM fluid cell imparts specific fluid flow and mass transport limitations on the environment directly at the area of investigation for crystal growth and in some cases may have significant ramifications for the appropriate correlation of bulk solution variables to crystal growth variables. The results of the CFD calculations indicate that differences are significant, though usually minor and these results may prove useful for future fluid cell design. Finally, photolithographic techniques were employed to produce millions of micron-sized particles with shapes mimicking molecular contours and other crystallographically significant contours to study how symmetry and packing originates at the micron length-scale. Although much is known about assembly at the molecular level for symmetry and packing, the assembly of anisotropic particles at longer length scales, which involve different interactive forces, has not been studied. This work concludes by performing preliminary work in elucidating the general behavior towards symmetry and packing in two-dimensions of micron-sized particles by using gravitational gradients and dielectrophoresis.
Reconstructing oxide surfaces.
(2009-06) Riesterer, Jessica Lori
The work presented here is concentrated on surfaces and interfaces in alumina (Al2O3), anorthite (CaAl2Si2O8), silica (SiO2) and rutile (TiO2). While each of these materials have different crystal structures and measurable properties, they all exhibit similar mechanisms for fundamental behavior. The topics researched and discussed lead into each other. Faceting describes the movement of atoms to a lower energy configuration. While faceting of the surface is only considered, grain boundaries can be faceted. In cross-section, facets resemble grain boundary grooves. Grooves and ridges form where a grain boundary intersects the surface of a material. The grooves facilitate grain boundary migration and diffusion. The surface tension at the groove is governed by Young's equation, which balances the interfacial forces between the solid and vapor. Glass films can wet or dewet the surface a grain boundaries. Whether the film wets or dewets depends on the surface energy of the surface and the liquid. Capillary forces determine the type of dewet patterns formed on the surface. Again, the surface-vapor-liquid interfaces are governed by Young's equation. Liquid films at grain boundaries facilitate densification and grain boundary migration. Liquid phase sintering (LPS) uses capillary forces and the dissolution/reprecipitation process to sinter green compacts to a high density at lower temperatures. Capillary forces and surface tension can also cause the liquid film to penetrate or exude from the grain boundary. Various forms of microscopy have been used to characterize and relate these phenomena.
Stress development in particulate, nano-composite and polymeric coatings.
(2009-09) Jindal, Karan
The main goal of this research is to study the stress, structural and mechanical property development during the drying of particulate coatings, nano-composite coatings and VOC compliant refinish clearcoats. The results obtained during this research establish the mechanism for the stress development during drying in various coating systems. Coating stress was measured using a controlled environment stress apparatus based on cantilever deflection principle. The stress evolution in alumina coatings made of 0.4 micron size alumina particles was studied and the effect of a lateral drying was investigated. The stress does not develop until the later stages of drying. A peak stress was observed during drying and the peak stress originates due to the formation of pendular rings between the particles. Silica nanocomposite coatings were fabricated from suspension of nano sized silicon dioxide particles (20 nm) and polyvinyl alcohol (PVA) polymer. The stress in silica nano-composite goes through maximum as the amount of polymer in the coating increases. The highest final stress was found to be ~ 110MPa at a PVA content of 60 wt%. Observations from SEM, nitrogen gas adsorption, camera imaging, and nano-indentation were also studied to correlate the coatings properties during drying to measured stress. A model VOC compliant two component (2K) acrylic-polyol refinish clearcoat was prepared to study the effects of a new additive on drying, curing, rheology and stress development at room temperature. Most of the drying of the low VOC coatings occurred before appreciable (20%) crosslinking. Tensile stress developed in the same timeframe as drying and then relaxed over a longer time scale. Model low VOC coatings prepared with the additive had higher peak stresses than those without the additive. In addition, rheological data showed that the additive resulted in greater viscosity buildup during drying.
Structure and mechanical properties of elastomeric block copolymers.
(2010-12) Alfonzo, Carlos Guillermo
This research presents the synthesis (by anionic polymerization and catalytic hydrogenation) and characterization of two types of block copolymers: CMC and XPX. In CMC, C is glassy poly(cyclohexylethylene) and M, the matrix, can be semicrystalline poly(ethylene) E, rubbery poly(ethylene-alt-propylene) P, or rubbery poly(ethylethylene) EE, or a combination to yield: CPC, CEEC, CEC, CPEEC and CEPC, with fC ≈ 0.18 – 0.30. XPX materials have X = CEC, fC ≈ fE, and fP ≈ 0.40 – 0.60. Block copolymer phase behavior and morphology were examined through a combination of DSC, rheology, SAXS, WAXS and TEM. CMC materials are meltordered due to block thermodynamic incompatibility with TODT > Tg (C) ≈ 147 °C and show lamellar or C cylinder morphologies. The design of XPX yields melt disordered materials up to high Mn with microphase segregation induced by E crystallization. Two high Mn XPX polymers are melt ordered above Tm(E) and show two correlation lengths in SAXS assigned to the C – E and X – P length scales. TEM images indicate that all XPX materials, irrespective of melt segregation, are characterized by composite glassy and crystalline hard domains dispersed in rubbery P at room temperature. Tensile and recovery testing at room temperature show that CMC and XPX materials, with the exception of plastic CEC, behave as thermoplastic elastomers with tunable properties. Interestingly, melt disordered XPX materials have competitive mechanical properties comparable to the strongest CMC polymers, but with advantageous processing. For melt ordered CMC, Tprocess > TODT, which is dependent on Mn, while for melt disordered XPX, Tprocess > Tm(E) ≈ 100 °C independent of Mn. The deformation of melt disordered XPX materials, probed by recovery studies and WAXS, suggests that deformation is first taken by P, then E and finally C, which causes ultimate failure, as agreed in the literature for conventional SBS and SIS thermoplastic elastomers. This implies that strain recovery in XPX materials can be comparable to that of CPC if materials contain low hard block content or are stretched to strains below the onset of E deformation. Finally, a collection of data of mechanical properties, namely modulus E, strain at break εb, tensile strength σTS and tension set εs, obtained from CMC, XPX and previously reported materials were examined. Most notably, E and εs were found to be strongly correlated with the volume fractions of C and E, as properties increase with (fC + fE)δ, where δ = 1 – 2.4. Ultimate properties such as σTS and εs are unaffected by changes in composition as failure is dictated by that of the hard domains and values are similar above a minimum amount of hard block. In addition, E, σTS, and εb are inversely correlated to rubber entanglement molecular weight Me, which implies that modulus and ultimate properties are affected by the ability of the rubber network to redistribute stress by entanglement slippage. However, εs is unresponsive to Me variations, which indicates that irrecoverable deformation in these materials results from deformation of the hard domains.
Studies of block copolymer melts by field theory and molecular simulation.
(2009-11) Qin, Jian
The thesis covers theoretical and simulation studies of various phase behavior related to block copolymers and homopolymer blends. The non-frustrated triblock copolymer phase behavior is examined using the numerical SCFT (self-consistent field theory). The long ranged composition fluctuations in binary homopolymer blends and diblock copolymer melts are studied using both a renormalized fluctuating field theory and (for diblock copolymer) Monte Carlo simulations.
Synthesis and characterization of copper zinc tin sulfide nanoparticles and thin films.
(2012-07) Khare, Ankur
Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. CZT(S,Se) solar cells have already reached power conversion efficiencies >10%. One of the ways to improve upon the CZTS power conversion efficiency is by using CZTS quantum dots as the photoactive material, which can potentially achieve efficiencies greater than the present thin film technologies at a fraction of the cost. However, two requirements for quantum-dot solar cells have yet to be demonstrated. First, no report has shown quantum confinement in CZTS nanocrystals. Second, the syntheses to date have not provided a range of nanocrystal sizes, which is necessary not only for fundamental studies but also for multijunction photovoltaic architectures. We resolved these two issues by demonstrating a simple synthesis of CZTS, Cu2SnS3, and alloyed (Cu2SnS3)x(ZnS)y nanocrystals with diameters ranging from 2 to 7 nm from diethyldithiocarbamate complexes. As-synthesized nanocrystals were characterized using high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive spectroscopy to confirm their phase purity. Nanocrystals of diameter less than 5 nm were found to exhibit a shift in their optical absorption spectra towards higher energy consistent with quantum confinement and previous theoretical predictions. Thin films from CZTS nanocrystals deposited on Mo-coated quartz substrates using drop casting were found to be continuous but highly porous. Annealing CZTS nanocrystal films at temperatures as low as 400°C led to an intense grain growth; however, thin films from CZTS nanocrystals cracked on annealing due to their high porosity. Although quantum confinement in CZTS is only accessible in nanocrystals of diameters less than 5 nm, the high volume of the ligands as compared to the volume of the nanocrystals makes it a challenge to deposit continuous compacted thin films from small nanocrystals. Films deposited from thermal decomposition of a stoichiometric mix of metal dithiocarbamate complexes were found to be predominantly CZTS. These films from complexes were found to be continuous but microporous. The diameter of the spheres making up the microporous structure could be changed by changing the anneal temperature. The structural composition of the final film could be altered by changing the heating rate of the complexes. CZTS exists in three different crystal structures: kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures. Due to the similarity in the crystal structures, it is extremely difficult to distinguish them based on X-ray diffraction. We computed the phonon dispersion curves for the three structures using ab-initio calculations, and found characteristic discontinuities at the Γ-point which can potentially be used to distinguish the three. In addition, the Γ-point phonon frequencies, which correspond to the Raman peak positions, for the three structures were found to be shifted from each other by a few wavenumbers. By deconvoluting the experimental Raman spectra for both CZTS and Cu2ZnSnSe4 (CZTSe) using Gaussian peaks, we observed that the most intense Raman scattering peak in both CZTS and CZTSe is a sum of two different peaks which correspond to scattering from their respective kesterite and stannite phases. The electronic, structural, and vibrational properties of a series of CZTS-CZTSe alloys (CZTSSe) were studied using ab-initio calculations. The S-to-Se ratio and the spatial distribution of the anions in the unit cell were found to determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. X-ray diffraction patterns and phonon distribution curves were found to be sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.

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