Browsing by Subject "Interface"

Now showing 1 - 10 of 10

Compatibilization of polyolefin blends
(2015-08) Thurber, Christopher
Polymer blends are used to access unique combinations of properties beyond those of neat homopolymers. Blends confer flexibility in tailoring a specific material to a given application, and in some cases, they can lend improved properties compared to their constituent materials. Some examples of blend-synergistic properties in the literature include toughness enhancement, increased chemical resistance, increased modulus, and improved processability. Given the breadth of properties that can be improved by blends, they are employed extensively in commercial products, with more than a third of all polymer resins used in blends (Utracki, 2003). Most polymer pairs are immiscible, thus their blends require compatibilization to aid dispersion in the melt state and to transfer stress across interfaces in the solid state. Block copolymers have proven to be successful compatibilizers, in both premade and reactively formed systems. This thesis focuses mainly on reactive systems. The reaction at immiscible polymer interfaces is kinetically limited and most reactions are too slow for applications, so general methods of increasing interfacial reaction rate have been investigated. This work also seeks to find new tools for measuring localization and conversion in polymer blends, with the ultimate goal of making useful, economical materials, and understanding the resulting structures. This thesis attempts to further our knowledge of compatibilization of polyolefin blends in particular. Chapter 2 attempts to create facile reactive compatibilization schemes for polyolefins with poly(methyl methacrylate). Chapters 3 and 4 examine the use of catalysts to increase interfacial reaction rate between functional polyethylene and polylactide. Chapter 3 demonstrates stannous octoate catalyst is localized at the interface, and blends show better compatibilization than those with a more active but non-localized tin chloride dihydrate catalyst. Chapter 4 uses cobalt octoate catalyst to increase interfacial reaction rate by ~90-fold and the extension at break of polylactide majority blends to ~200%. Structural dependence of copolymers on compatibilization efficiency in polypropylene/polyethylene blends is investigated in Chapter 5. Finally, a small scale coextruder is created using a dual-bore capillary rheometer, with the potential to examine the effect of flow on copolymer localization, catalyst localization, and interfacial reaction rate (Chapter 6).
Complexity at cobaltite interfaces: the interplay between strain, stoichiometry, magnetism and transport
(2014-12) Bose, Shameek
Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells and gas membranes. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial "dead layers". Working with the prototypical SrTiO3 (001)/La1-xSrxCoO3 (LSCO) system, our group recently discovered that this degradation in the magnetism and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the interplay between the strain state and the ordering of oxygen vacancies. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure. We first demonstrate a synthesis technique that utilizes a unique high pressure oxygen plasma to sputter LSCO thin films over a wide doping range 0.05  x  0.80. Then, using reciprocal space mapping and transmission electron microscopy, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). We then provide cross sectional electron energy loss spectroscopy data to show that this strain and orientation control preserves both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface. SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we use scanning tunneling microscopy to provide direct real space images of the magneto-electronic phase separation in ultrathin LSCO on SrTiO3(001). Our work thus demonstrates the ability to utilize oxygen vacancy ordering as a tunable control parameter to tailor interfacial electronic and magnetic properties, with profound implications for the myriad other systems that exhibit unique properties due to such ordering.
Flow characterization on a thin film spinning apparatus
(2014-09) Alvarado, Alonso Antonio
In industrial milling operations that use comminution and wet-comminution techniques, the reduction of the particle size is usually achieved through crushing the sample with a material harder than the product. These methods are convenient when the required median particle size is above 400 um. However, to obtain post-milling particle distributions with 85% sub-micron particles (in number) is both energy intensive, and time consuming. For conventional milling machines, to have the required output in several ton/hr of a product, having a large number of particles in the micron or sub-micron sizes at an affordable rate is cumbersome.Here, a wet-comminution machine that has shown to achieve the aforementioned milestones in the laboratory scale is studied. However, when the machine is scaled to industrial processes, it was recorded that some of the product variables are difficult to scale. In these studies, we attempt to understand the mechanisms by which this machine operates in order to achieve successful scaling. The apparatus operates completely on fluid mechanics principles, it consists of two concentric cylinders, the inner cylinder that has a smaller radius than the outer, rotates while the larger is held stationary. The inner cylinder is also shorter in length than outer, hollow in the inside and has transversal holes where the shaft attaches to the apparatus. The apparatus can operate in batch condition, where the liquid volume is much less than the volume of the apparatus, typically 0.3Vt, 0.42Vt and 0.54Vt. In addition, the apparatus can operate with throughflow, which the upper plate covering the apparatus is reduced in radius.Two component Laser Doppler Velocimetry (LDV) was used to obtain even-time averaged statistics of the azimuthal and axial velocities, in the gap and underneath the impeller. Also, Flow Visualization using Kalliroscopic particles was performed as means of observing large scale structures in the gap. Moreover, single plane Particle Image Velocimetry (PIV) was used to acquire statistics of the axial and radial velocities in the gap, and both underneath as well as above the inner cylinder.It was found that at both throughflow conditions, the topology of the apparatus creates a free spinning boundary both at the bottom and above the inner cylinder. Near the bottom, the thickness of the boundary was found to decrease with Reynolds number to a limiting value, where Re; is based on gap thickness and inner cylinder tip speed. For Re > 2546, the liquid/air interface thickness is constant for a given holding volume. In the regions above and underneath the inner cylinder, corner vortices were detected; if viewing the left-hand-side, the lower one rotating counter clockwise, while the upper rotates clockwise. The thickness of these vortices was found to be constant for various axial flows at Re = 1110 and 2230. The radial length scale of the stationary vortices was found to be ~2.5d;.The flow generated inside of the gap was characterized to have Taylor vortex signatures. It was found that the length scale of the Taylor vortices in the gap is rather insensitive to Reynolds number or holding volume ratio. The average vortex pair wavelength; was found to be 3.6d. Average flow statistics in batch condition indicate that in the gap, at Re = 1110 and 2230, the azimuthal velocity is 0.5U over much of the length. Similarly, it was found that the net axial flow through the gap is close to zero.
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.
Mixed convection in horizontal fluid-superposed porous layers
(2013-08) Dixon, John Mark
Mixed convection in horizontal fluid-superposed porous layers is studied in the following work. Much research has been done in the field of natural, mixed, and forced convection in a porous layer. Several studies have investigated natural and forced convection in a two-domain system that includes a porous and a fluid layer, but mixed convection has not been addressed. This problem can be found in many natural and engineering applications. Some examples include beach sand, human lungs, bread, gravel, soil, rock, packed bed reactors, fiberglass insulation, thermal energy storage systems, electronic cooling, crude oil extraction, nuclear reactors, and the list goes on. The present study is motivated by the wide range of applications and seeks to fill the gap in the literature regarding mixed convection. The problem considers a long, narrow channel that is partially filled with a porous layer and has a fluid layer above the porous layer. The channel is partially heated on the bottom and cross flow along the length of the channel is added in varying degrees. The problem is studied at a fundamental level, with the governing equations being derived, non-dimensionalized, discretized, and solved numerically. The two layers are treated as a single domain and the porosity is used as a switching parameter, causing the governing equations to transition from an extended form of the Darcy-Brinkman-Forchheimer equation in the porous layer to the Navier-Stokes equations in the fluid layer. This method avoids the need for interfacial boundary conditions to be explicitly defined at the interface between the two domains. Several dimensionless numbers are varied and their effects on the overall Nusselt number of the system are documented. The parameters varied include the Peclet number, the Rayleigh number, the porous layer height ratio, the Darcy number, the Prandtl number, and the conductivity ratio between the solid and fluid phases. In addition, the impact of the various additional terms in the extended form of Darcy's law is investigated and documented as well. The conductivity ratio, Darcy number, porous layer height ratio, Rayleigh number, and Peclet number all have a strong effect on the overall Nusselt number of the system, while the Prandtl number, the Brinkman term, the Forchheimer term, and the convective terms have a negligible effect. A critical Peclet number was observed, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the porous layer height ratio. A critical porous layer height ratio was also found, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the Peclet number. The streamlines capture the transition from the natural convection regime to the forced convection regime. In the transition region the flow patterns have characteristics of both domains. The isotherms capture the plume flow and show the influence of the cross flow on the shape and character of the plume. An experimental apparatus is designed in order to collect data over a similar range of parameters as explored numerically. The average error between the numerical and experimental results is 30%, with a peak of 67%. The numerical results show good agreement with the experimental data within the bounds of uncertainty. The experimental results confirm the presence of a critical Peclet number. However, they do not show the same trends at intermediate porous layer height ratios. The effect of the porous layer height ratio, η=h_p⁄H, on the Nusselt number is shown to be small in the range of η = 0.5 to η = 1 and large in the range of η = 0 to η = 0.5. Also, the transition to the forced convection regime occurs earlier for the numerical results than it does for the experimental results. This points towards future research opportunities that focus on the lower range of porous layer height ratio values.
Moving Pictures, Empty Words: Cinema as Developmental Interface in the Chinese Reconstruction, 1932-1952
(2017-06) Chen, Hongwei
This dissertation is a genealogical study of the relationship between instructional technologies and uneven development. It focuses on the work of the Chinese educational film movement, which unfolded as a mélange of governmental and non-governmental initiatives over the course of the 1930s and 1940s. As I argue, educational cinema presented Chinese interlocutors with a "developmental interface," that is, an equivocal material and metaphorical framework for negotiating the technical, economic, and cultural asymmetries produced by modern imperialism and capital accumulation. Challenging unidirectional conceptions of media instrumentality, which are often based on flattening notions of the state and medium specificity, the project approaches the educational film as an interface, defined as a surface connecting heterogeneously structured realities, defined by distributions of workability and unworkability. Inserted at the rough edges between Confucian traditions of popular uplift, modern models of pedagogical discipline, and the international circulation of communication technologies, jiaoyu dianying/"educational cinema" comprised a particularly unworkable interface, caught between the dispersive temporalities of acute developmental unevenness, on the one hand, and the path-determining technological and institutional forms that defined international modes of media governance, on the other. As an interface for developmental desires, educational cinema united teachers, politicians, filmmakers, and engineers under a common framework, promising them a direct line to masses otherwise dispersed by social fragmentation, illiteracy, poor roads, dialect differences, and an intensifying rural-urban divide. As a global aesthetic and technical reality, it subjected its users to a new, and no less unequal, milieu of international technology exchanges, expert knowledges, and mass-mediated visibility. Drawing on interdisciplinary methods of institutional history alongside the close reading of films, reports, diagrams, and teaching guides generated by Chinese instructional bodies, I show how cinema participated in the metamorphoses of institutional power, literary authority, temporality, and affective texture that defined Chinese Republican-era cultural crisis.
Noise detection and transport measurements of spin valve systems.
(2011-08) Guo, Feng
Electronic noise not only limits the performance of magnetic devices in practical applications but also provides valuable physical insights into these devices. The first part of this thesis discusses how the low frequency noise in magnetic tunnel junctions and giant magnetoresistance devices can be used to understand the fundamental noise sources. Previously, the low frequency noise in these systems has been reported to have an enormously large magnitude when the magnetization switches. This was attributed to magnetic fluctuations. An alternative mechanism of a slow drift in the device resistance is discussed, and we show how it produces noise spectra that are similar to those in previous reports. We conclude that this resistance drift causes a measurement artifact and the low frequency magnetic noise is not present in the measured samples within measurement error. As a second part of the thesis, we discuss a pronounced voltage dependent conductance feature present at nonzero bias in some magnetic tunnel junctions. The presence of this feature depends upon the oxidation condition for creating the barrier, and this effect is found to be interfacial in nature. We describe how the electronic structures and density of states at the barrier interfaces could be responsible for this effect, and possibility of utilizing the conductance measurement to probe the interfacial states.
Solving the two-interface problem in vibrational sum frequency generation spectroscopy applied to multilayer thin film systems
(2014-03) O'Brien, Daniel Bruce
This dissertation describes advances made in applying sum frequency generation spectroscopy (SFG, in particular vibrational SFG or VSFG) to multilayer thin film systems. Application of VSFG to thin film systems is motivated by the challenge of characterizing molecular structure at the active boundary in organic field-effect transistors, these are inherently buried interfaces. VSFG is a surface-selective probe of molecular structure; however, when VSFG is applied to an organic thin film, the detected signal has contributions from two potential sources - the two interfaces of the organic - which must be separated. The problem is further confounded by optical interferences inherent in multilayer thin film systems. An intuitive mathematical model is developed; postulating a solution to the two-interface problem of SFG applied to planar and stratified multilayer structures. The model system for this dissertation is thin films of the small molecule N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) vapor deposited on silica thin film substrates, consistent with an oFET thin film geometry. The interference model is used for an extensive simulation analysis that reveals intricacies contained in the intensity data of VSFG applied to that system. VSFG experiments performed on samples with PTCDI-C8 deposited as gradient thicknesses provide compelling evidence that the model gives an accurate description of optical interference effects and that it can be used to separate contributions to the total VSFG signal intensity. The supplementary materials contain a collection of Mathematica notebooks that can be used to investigate optical interference effects on SFG data collected from systems composed of an arbitrary number of thin film layers.
A theoretical study of stress induced slip at polymer-polymer interfaces
(2013-12) Gustafson, Andrew Aaron
The phenomena of stress-induced tangential slip at interfaces between immiscible polymer melts is studied by analytic theory and simulation. Characteristic interfacial slip velocities are predicted, at which the interfacial stress behavior qualitatively changes. Interfacial stress is found to be primarily governed by the slip direction alignment of interfacially entangled strands, and the convective release of interfacial entanglements. Simulations combine a slip-link description of polymer entanglement with a self-consistent field description of a polymer-polymer interface.
Valence electronic structure of semiconductor quantum dot and wide band gap oxide interfaces by ultraviolet photoelectron spectroscopy.
(2009-08) Timp, Brooke Andrea
Energy level alignment is an important factor in efficient charge transfer at an interface between two semiconductors. This topic is explored in model systems that are relevant to quantum dot-sensitized solar cells, inorganic semiconductor nanoparticles adsorbed on single crystal wide band gap oxide substrates, using ultraviolet photoelectron spectroscopy. Cadmium selenide quantum dots are assembled on a ZnO (10-10) surface using 3-mercaptopropionic acid linkers. The valence band maximum of the CdSe quantum dots is found to be located at 1.1 ± 0.1 eV above the valence band maximum of ZnO, nearly independent of the size of the quantum dots (2.1-4.2 nm). This finding suggests that, upon adsorption, there is strong electronic interaction between CdSe quantum dots and the ZnO surface. As a result, varying the quantum dot size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of the ZnO surface. Sub-monolayer films of PbSe quantum dots are prepared on single crystal substrates, ZnO (10-10) and TiO2 (110), and exposed to ligand solutions, either hydrazine or 1,2-ethanedithiol (EDT) in acetonitrile. Interfacial energy alignment is measured as a function of quantum dot size, substrate and ligand treatment. The affect of the ligand treatments on the energy alignment is substrate-dependent. The valence band maximum of the dots is size-independent on ZnO due to strong electronic interactions with the substrate; in particular, EDT-treated films show significant enhancement of quantum dot valence band intensity due to electronic coupling with the ZnO surface. In contrast, the quantum dot valence band maximum is size-dependent and shows a smaller shift between ligand treatments for films on TiO2, suggesting weaker quantum dot-substrate interactions. In most cases the measured alignment predicts that electron injection from a photoexcited PbSe quantum dot to either ZnO or TiO2 will necessitate the involvement of higher-lying levels above the first excitonic transition.

University Digital Conservancy

Browse by Subject

Browsing by Subject "Interface"