<p> Aluminum is usually added to the zinc bath to form an Fe-Al interfacial layer which retards the formation of a series of Fe-Zn intermetallic compounds during the hot-dip galvanizing process. However, experimentally exploring the inhibition layer formation and obtaining useful experimental data to understand the mechanisms is quite challenging due to short times involved in this process. In this study, a galvanizing simulator was used to perform dipping times as short as O.ls and rapid spot cooling techniques have been applied to stop the reaction between the molten zinc coating and steel substrate as quickly as possible. In addition, the actual reaction time has been precisely calculated through the logged sample time and temperature during the hot-dipping process. The kinetics and formation mechanism of the inhibition layer was characterized using SEM, ICP and EBSD based on the total reaction time. For bath containing 0.2wt% dissolved AI, the results show that FeA13 nucleates and grows during the initial stage of the inhibition layer formation and then Fe2Als forms by a diffusive transformation. The evolution of the interfacial layer formed in a zinc bath with 0.13wt% dissolved AI, including Fe-Aland Fe-Zn intermetallic compounds, was a result of competing reactions. In the initial period, the Fe-Al reaction dominated due to high thermodynamic driving forces. After the zinc concentration reached a critical composition in the substrate grain boundaries, formation of Fe-Zn intermetallic compounds was kinetically favoured. Fe-Zn intermetallic compounds formed due to zinc diffusing to the substrate via short circuit paths and continuously grew by consuming Fe-Al interfacial layer after samples exited the zinc bath due to the limited Al supply. A mathematical model to describe the formation kinetics as a function of temperature for the 0.2wt% Al zinc bath was proposed. It indicated that the development of microstructure of the interfacial layer had significant influence on the effective diffusion coefficient and growth of this layer. However, the model underestimates the AI uptake by the interfacial layer, particularly at higher temperatures. This is thought to be due to the effect of the larger number of triple junctions in the inhibition layer leading to an underestimation of the effective diffusivity. </p> / Thesis / Master of Science (MSc)

Antimicrobial coatings in which the active agent (e.g. N-halamine) can regenerate activity represent a promising way to prevent microbial cross-contamination. A reported method for applying coatings containing antimicrobial N-halamines is layer-by-layer (LbL) application of polyelectrolytes, which form N-halamines upon cross-linking. Prior reports on dip layer-by-layer (LbL) fabrication have demonstrated the potential of this coating technology; however, spray LbL fabrication would enable more rapid coating and represents a more commercially translatable application technique. In this work, dip and spray LbL methods were used to coat polypropylene (PP) with N-halamine containing bilayers consisting of cross-linked polyethylenimine (PEI) and poly(acrylic acid) (PAA). Further experimentation with spray LbL fabrication used naturally occurring polyelectrolytes, chitosan and alginate. Materials were characterized using atomic force microscopy (AFM), ellipsometry, contact angle, fourier transform infrared spectroscopy, a chlorine content assay, and a dye assay for amine quantification. All methods of coating application exhibited a 99.999% (5-log) reduction against Listeria monocytogenes with application time for spray LbL taking less than 10% of the time required for dip LbL. Spray LbL fabrication of N-halamines is a rapid and inexpensive method to fabricate rechargeable antimicrobial surfaces.

Layer-by-layer (LBL) polyelectrolyte capsules can be modified to incorporate stimuli such as superparamagnetic nanoparticles which respond to a magnetic field only when it is turned on. Thus, they can act as a switch to load or unload their drug cargo on demand. Specifically, magnetite is incorporated into bilayer capsules made of alternating poly(allylamine hydrochloride) (PAH) and poly(sodium-p-styrenesulfonate) (PSS) which surrounds calcium carbonate core. The core is then dissolved using ethylenediaminetetraacetic acid (EDTA). These capsules are loaded with at FITC-BSA conjugate and examined with fluorescence to show the unloading of the FITC-BSA from capsules as it brightens the entire field of view of the microscope. The results suggest that we can next load and unload an anticancer drug such as doxorubicin using the combination of microcapsule and alternating magnetic field (AMF) to treat the cancer cells. Preliminary data interprets that the low frequency AMF we use has little to no adverse effect cells viability. This coincides with the general thought that low frequency AMF signals are not harmful to humans. Therefore, as an alternative to hyperthermia methods which use heat, it may be possible to deliver the anticancer drugs specifically to the cells when and where it is needed.

This thesis presents work done to improve the switching speed and contrast performance of electrochromic devices. Layer-by-Layer (LbL) assembly was used to deposit thin electrochromic films of materials ranging from organic, inorganic, conducting polymers, etc. The focus was on developing new materials with high contrast and long lifecycles. A detailed switching-speed study of solid-state EC devices of already-developed (PEDOT (Poly(3,4-ethylenedioxythiophene)), polyviologen, inorganic) materials and some new materials (Prodot-Sultone) was performed. Work was done to achieve the optimum thickness and number of bilayers in LbL films resulting in high-contrast and fast switching. Device sizes were varied for comparison of the performance of the lab-made prototype device with the commercially available "small pixel" size displays. Symmetrical EC devices were fabricated and tested whenever conducting polymers are used as an EC material. This symmetrical configuration utilizes conducting polymers as an electroactive layer on each of two ITO-coated substrates; potential is applied to the two layers of similar conducting polymers and the device changes color from one redox state to another. This method, along with LbL film assembly, are the main factors in the improvement of switching speed results over already-published work in the literature. PEDOT results show that EC devices fabricated by LbL assembly with a switching speed of less than 30 ms make EC flat-panel displays possible by adjusting film thickness, device size, and type of material. The high contrast value (84%) for RuP suggests that its LbL films can be used for low-power consumption displays where contrast, not fastest switching, is the prime importance. In addition to the electrochromic work, this thesis also includes a section on the application of LbL assembly in fabricating electromechanical bending actuators. For bending actuators based on ionic polymer metal composites (IPMCs), a new class of conductive composite network (CNC) electrode was investigated, based on LbL self-assembled multilayers of conductive gold (Au) nanoparticles. The CNC of an electromechanical actuator fabricated with 100 bilayers of polyallylamine hydrochloride (PAH)/Au NPs exhibits high strain value of 6.8% with an actuation speed of 0.18 seconds for a 26 µm thick IPMC with 0.4 µm thick LbL CNCs under 4 volts. / Ph. D.

According to a 1999 Federal Highway Administration statistic, the U.S. has around 8.2 million lane-miles of roadways that need to be maintained and rehabilitated periodically. Therefore, in order to reduce rehabilitation costs, pavement engineers need to optimize the rehabilitation procedure, which is achieved by accurately knowing the existing pavement layer thicknesses and localization of subsurface defects. Currently, the majority of departments of transportation (DOTs) rely on coring as a means to estimate pavement thicknesses, instead of using other nondestructive techniques, such as Ground Penetrating Radar (GPR). The use of GPR as a nondestructive pavement assessment tool is limited mainly due to the difficulty of GPR data interpretation, which requires experienced operators. Therefore, GPR results are usually subjective and inaccurate. Moreover, GPR data interpretation is very time-consuming because of the huge amount of data collected during a survey and the lack of reliable GPR data-interpretation software. This research effort attempts to overcome these problems by developing new GPR data analysis techniques that allow thickness estimation and subsurface defect detection from GPR data without operator intervention. The data analysis techniques are based on an accurate modeling of the propagation of the GPR electromagnetic waves through the pavement dielectric materials while traveling from the GPR transmitter to the receiver. Image-processing techniques are also applied to detect layer boundaries and subsurface defects. The developed data analysis techniques were validated utilizing data collected from an experimental pavement system: the Virginia Smart Road. The layer thickness error achieved by the developed system was around 3%. The conditions needed to achieve reliable and accurate results from GPR testing were also established. / Ph. D.

The formation of layered nanoparticle films on the surface of wood fibers is reported in this study. The layer-by-layer (LbL) assembly technique was comprehensively investigated as a non-covalent surface modification method for lignocellulosic fiber. Nanocomposite-based lignocellulosic fibers were successfully fabricated by sequential adsorption of oppositely charged poly(diallydimethylammonium) chloride (PDDA) and clay nanoparticles in a number of repeated deposition cycles. Nanocomposite fibers displayed layered structure as indicated by the electrokinetic potential studies and scanning electron microscopy (SEM) analysis. Layer-by-layer films of PDDA and clay impacted the thermal stability of wood fibers. Average degradation temperature at 5 and 10% weight loss for modified fibers with 4 bi-layers increased by up to ~24 and ~15°C, respectively. Significant char residue formed for the LbL modified fibers after heating to 800°C, indicating that the clay-based coating may serve as a barrier, creating an insulating layer to prevent further decomposition of the material. Layer-by-layer film formation on wood fibers was investigated as a function of parameters related to fiber composition and solution conditions (ie. presence of lignin, salt concentration and pH). Elemental analysis of modified fibers revealed that PDDA adsorption to the fibers was reduced for all solution conditions for the samples with the highest content of lignin. Upon extracting the non-covalently attached lignin, the samples showed the greatest amount of PDDA adsorption, reaching to 1.5% of total mass, under neutral solution conditions without the presence of added electrolyte. Furthermore, the influence of both the amount of PDDA adsorbed onto the fiber surface and electrokinetic potential of modified fibers on subsequent multilayer formation was quantified. Under select fiber treatments, great amount of PDDA/clay (up to ~75% total mass for only 4 bi-layers) was adsorbed onto wood fibers through the LbL process, giving these high surface area fibers nanocomposite coatings. LbL modified fibers were melt compounded with isotactic polypropylene (PP) and compression molded into test specimens. The effect of LbL modification as a function of the number of bi-layers on composite performance was tested using the tensile, flexural, dynamic mechanical and thermal properties of fiber reinforced thermoplastic composites. LbL modified fiber composites had similar modulus values but significantly lower strength values than those of unmodified fiber composites. However, composites composed of LbL modified fibers displayed increased elongation at break, increasing by more than 50%, to those of unmodified samples. DSC results indicated that crystallization behavior of PP is promoted in the presence of wood fibers. Both unmodified and LbL modified fibers are able to acts as nucleating agents, which cause an increase of the crystallinity of PP. Moreover, results from tensile and flexural strength, dynamic mechanical analysis and water absorption tests revealed that the material (PDDA or clay) at the terminal (outer) layer of LbL modified fiber influences the performance of the composites. These findings demonstrate control over the deposition of nanoparticles onto lignocellulosic fibers influencing terminal surface chemistry of the fiber. Further investigation into using renewable fibers as carriers of nanoparticle films to improve fiber durability, compounding with thermoplastics that have higher melt processing temperatures, and tailoring terminal surface chemistry to enhance adhesion is justified by this research. / Ph. D.

The typical architecture of the proton exchange membrane fuel cell (PEMFC) contains a layer called the microporous sublayer (MSL). The MSL is a mixture of carbon black and polytetrafluoroethylene (PTFE), which is typically applied to the gas diffusion layer (GDL). The composition (wt.% PTFE) and loading (mg/cm2) can be varied to optimize the electrochemical performance of the PEMFC and the overall adhesion of the layers within the PEMFC. This research establishes correlations that characterize the performance and adhesion of the layers within the PEMFC based on composition, loading, fabrication pressure, and fabrication time. MSL loading was varied from 1.5-4 mg/cm2, composition was varied from 10-50 wt.% PTFE, fabrication pressure was varied from 3.45-10.34 MPa, and fabrication time was varied from 2-8 minutes. Using these four factors, correlations were created, and optimal solutions for each response were identified. The adhesion correlation identifies a low MSL loading, mid-range MSL composition, high fabrication pressure, and high fabrication time as desirable factors. The performance correlation suggests that the PEMFC performance is enhanced with low MSL loadings, low MSL PTFE content, and a low fabrication pressure and does not find fabrication time to be a significant factor in the correlation. / Master of Science

Various researchers are interested in the structure of the surface pressure fluctuations for the development and use of noise prediction techniques for helicopter and turbomachinery rotors. This study, conducted in the Virginia Tech low speed boundary layer wind tunnel, covered the effects of zero and favorable streamwise pressure gradient flows on the surface pressure fluctuation spectra, coherence and convective wave speeds in turbulent boundary layers for momentum Reynolds numbers from 3000 to 18,800. The acceleration parameter, pressure gradient flow. K is near 2x10⁻⁷ for the favorable Small pinhole condenser microphones were used to obtain the surface pressure fluctuation data for all test cases. The longitudinal and lateral coherence functions and the convective wave speeds were obtained for both streamwise pressure gradient flows. The results presented are for the surface pressure fluctuation spectra nondimensionalized by different groupings of the outer and inner boundary layer variables. The grouping using the outer variables, U_e, π_w and δ₁ collapse the spectra for the low to middle range of frequencies for most test cases. The grouping using the inner variables, U_π and ν, collapse the spectra for the middle to high range of frequencies for all test cases. The value of p¹/r_w was near 3.8 and 2.8 for the smallest values of d⁺ in the zero and favorable pressure gradient flows, respectively. The spectral data was corrected using the correction developed by G.M. Corcos, but the pinhole correction developed by Bull and Thomas was not used in the data reduction process. However, some discussion is included on the effects of the pinhole correction for the results of this study. The coherence exhibits a decay that is not exponential in some cases, but the Corcos similarity parameters ωΔx/U_c and ωΔz/U_c collapse the data for all test cases. C The ratio of U_c/U_e shows an increase with increasing ωδ₁/U_e up to a certain value of ωδ₁/U_e where U_c/U_e becomes constant. This was observed in the present results for both streamwise pressure gradient flows. The experimental results presented show good agreement with previous research. / M.S.

Layer-by-layer deposition techniques were used to fabricate films with second order nonlinear optical (NLO) properties. These materials are key to the development of electro-optic modulators used in fiber optic communication systems. Performance benefits and lower manufacturing costs are driving the development of organic NLO materials as replacements for inorganic crystalline materials such as lithium niobate. The layer-by-layer deposition technique in which polyelectrolytes are deposited on a surface by electrostatic effects is called the Ionically Self-Assembled Monolayer or ISAM method. The role of the optically inactive polycation's structure on deposition and chromophore orientation was studied by fabricating films with several different polycations. While the specific interactions responsible for chromophore orientation in ISAM films remains unclear, hydrogen bonding and electrostatic effects are ruled out as the sole sources of orientation. The highest values of χ(2) were observed under pH conditions that resulted in flat and thin layers. The relationship between pH and the optical homogeneity of the film was also explored. Deposition of polymers under pH conditions in which the polymer chains were aggregated in solution results in films that are not suitable for use in devices. In this work, a new layer-by-layer deposition technique was developed. Coined hybrid deposition, it relies on covalent bonds and electrostatic interactions for film fabrication. Optically inactive polyamines were used as sources of positive charges and as binding sites with optically active low molecular weight chromophores functionalized with a reactive triazine ring and negative charged sulfonate groups. Polar ordering of the chromophores was obtained when the deposition was done under conditions in which covalent bonding was the preferred attachment mechanism for the chromophore molecules. pH conditions in which electrostatic attachment dominated resulted in poorer orientation. The effect of adding ionic salts to the dye solutions was studied, with hopes of increasing the chromophore density in the film by shielding inter-dye electrostatic repulsions. A linear relationship in deposited amount, as characterized by absorbance/bilayer, was observed as the salt concentration was increased. Little effect on χ;(2) was observed for films made with the as-received Procion Red MX-5B chromophore. However, films fabricated from purified Procion Brown MX-GRN showed a definite dependence on added salt. Exceptional χ(2) values were obtained for Procion Brown films deposited using 0.5 M NaCl and PAH. The importance of depositing from non-aggregated solutions was again highlighted, as films made with the less soluble Procion Orange were significantly less homogeneous than those made from Procion Red and Procion Brown which were highly soluble. The role of polycation structure on the deposition and orientation of Procion Brown and Red was examined. / Ph. D.

Better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation. Instead of spatial averaging, this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels. Velocity data were collected with a one dimensional laser Doppler velocimeter (LDV) under single layer emergent and submerged flow conditions, and through two layers of vegetation. The effects of dowel arrangement, density, and roughness are also examined under the single layer experiments. The results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it. The region immediately behind a dowel, where the vorticity and turbulence intensity are highest, is characterized by a velocity spike near the bed and an inflection point near the top of the dowel arrays. With two dowel layers, the velocity profile in the region behind a tall dowel exhibits multiple inflection points and the highest turbulence intensities are found there. / Master of Science