This thesis concerns the fabrication and modification of novel oxygen and glucose biosensors as well as the application of these biosensors in oxygen and glucose transport research in cell constructs. In Chapter 1, the principle and development of biosensors has been reviewed. Particular attention is paid to oxygen and glucose mass transport research in cell constructs which are crucial for bio-scaffold design in tissue engineering. Chapter 2 details the materials and methods in oxygen and glucose sensor fabrication, modification and characterization. Chapter 3 presents research into practical challenges in oxygen and glucose sensors. For oxygen sensor, membrane biofouling and sensitivity to stirring effect have been detected and successful progresses have been made to reduce their effects. For glucose sensor, membrane biofouling and oxygen tension reliance affect their performance. Remarkable contributions have been made to improve glucose sensors’ stability and reliability. In particular, micro-biosensors have been introduced in the interests of better sensor adaptability for further biomedical applications. Chapter 4 is the experimental section for biosensor applications, and thus provides a detailed description of the cell culture models used in the thesis. Chapter 5 describes the oxygen partial pressure and glucose concentration measurements using biosensors. 2D and 3D cell culture constructs are investigated and results are discussed in this section. It deserves to be mentioned that the modified oxygen and glucose sensor in this thesis are excellent for in vitro biomedical applications, the simultaneously investigation of PO2 and glucose concentration gradient in 3D cell constructs is also a pioneering work in this research field. Chapter 6 illustrates the overall conclusions resulting from the experiments described in the thesis and points out possible future research directions.

The increasing availability of nanopowders and nanotubes combined with new processing techniques is enabling the development of new multifunctional materials. Carbon Nanotubes (CNTs) are one of the recently discovered allotropic forms of carbon. They have exceptional mechanical, electrical and thermal properties. The application of CNTs in the reinforcement of ceramic nanocomposites has not yet been fully investigated and is the subject of this study. Alumina is the main matrix used in this study. CNTs need to be de-agglomerated and homogeneously distributed in ceramic nanocomposites. Dimethylformamide (DMF) produces fine and stable CNT and alumina dispersions. All nanocomposites were sintered by Spark Plasma Sintering (SPS). Nanocomposites prepared using DMF dispersions showed better dispersions, higher electrical conductivity and mechanical properties as compared to those prepared using ethanol dispersions. The addition of CNTs or Carbon Black (CB) to alumina significantly aids its densification. The CNTs produce significant grain growth retardation. CNTs were found to be well preserved in alumina after being SPSed up to 1900 oC. Structural preservation of CNTs in ceramic nanocomposites depends on the nature of ceramic and SPS processing conditions. The electrical conductivity of alumina – CNT nanocomposites is four times higher as compared to alumina – CB nanocomposites due to the fibrous nature and high aspect ratio of CNTs. Alumina coated CNTs were used for better interfacial adhesion with the matrix. Oxidative resistance of CNTs was increased by coating them with alumina and by decreasing the grain boundary area in alumina – CNT nanocomposites. Coated and uncoated CNTs showed higher mechanical reinforcement in alumina nanocomposite as compared to CB. The future for ceramic – CNT nanocomposites is very bright, especially for applications associated with the electrical and thermal properties. Apart from a good understanding of nanocomposites, the commercial development of CNT based technologies heavily relies on the availability and price of CNTs.

<p> Three dimensional fiber reinforced foam cores (3DFRFC) can have improved mechanical properties under specific strain rates and fiber volumes. This study explored different manufacturing techniques for the 3DFRFC and tested the specimens at dynamic loading rates of 69&ndash;10³ s^&ndash;1. Flexural bend test showed that glass fibers made the samples stronger yet more brittle while quasi-static compression tests showed a decrease in performance with 3DFRFC. High strain impact tests validated previously published studies by showing an 18&ndash;20% reduction in the maximum force experienced by the fiber reinforced core and its ability to dissipate the impact force in the foam core sandwich panel. The results show potential for the cost-effective manufacturing method used in this study to produce an improved composite foam core sandwich panel for armored applications where high strain rates are present and reduce the overall weight of vehicles while maintaining the desired strength performance.</p><p>

<p>Abundant experimental analyses and theoretical computational analyses that had been performed on metals to understand anisotropy and its evolution and its dependence on initial orientation of grains have failed to provide theories that can be used in macro-scale plasticity. Ductile metals fracture after going through a large amount of plastic deformation, during which the anisotropy of the material changes significantly. Processed metal sheets or slabs possess anisotropy due to textures produced by metal forming processes (such as drawing, bending and press braking). Metals that were initially isotropic possess anisotropy after undergoing forming processes, <i>i.e</i>., through texture formation due to large amount of plastic deformation before fracture. It is therefore essential to consider the effect of anisotropy to predict the characteristics of fracture and plastic flow performances in the simulation of ductile fracture and plastic flow of materials. Crystal plasticity simulations carried out on grains at the meso-scale level with different initial orientations (ensembles) help to derive the evolution of anisotropy at the macro-scale level and its dependence on initial orientation of grains. This paper investigates the evolution of anisotropy in BCC and FCC metals and its dependence on grain orientation using crystal plasticity simulations and texture analysis to reveal the mechanics behind the evolution of anisotropy. A comparison of anisotropy evolution between BCC and FCC metals is made through the simulation, which can be used to propose the theory of anisotropy evolution in macro-scale plasticity. </p><p> <i>Keywords</i>: ensembles; grains; initial orientation; anisotropy; evolution of anisotropy; crystal plasticity; textures; homogeneity; isotropy; inelastic; equivalent strain </p>

<p> Since its discovery, the carbon nanotube (CNT) has been proposed as one of the ultimate materials for its electrical, thermal and mechanical properties due to its incredibly strong <i>sp</i>² bonds, low defect density, and large aspect ratio. Many experimental results on individual CNTs have confirmed these outstanding theoretically predicted properties. However, scaling these properties to the macroscopic regime has proved to be challenging. This work focused on the synthesis and measurement of highly conducting, macroscopic, CNT assemblies. Scaling up the synthesis of vertically aligned multiwalled CNT (MWNT) forests was investigated through the development of a large, 100mm, wafer scale, cold wall chemical vapor deposition chamber. In addition to the synthesis, two distinct CNT assemblies have been investigated. A linear morphology where CNTs are strung in series for electrical transport (CNT wires) and a massively parallel 2D array of vertically aligned CNTs for Thermal Interface Material (TIM) applications. </p><p> Poymer-CNT wire composites have been fabricated by developing a coaxial CNT core-polymer shell electrospinning technique. The core-shell interactions in this system have been studied by way of Hansen's solubility parameters. The most well defined CNT core was achieved using a core solvent that is semi-immiscible with the shell solution, yet still a solvent of the shell polymer. Electrical characterization of the resulting CNT core has shown a two orders of magnitude increase in conductivity over traditional, homogeneously mixed, electrospun CNT wires. </p><p> A number of vertically aligned MWNT assemblies were studied for their thermal interface properties. Double-sided Silicon substrate (MWNT-Si-MWNT) TIM assemblies were characterized using a DC, 1D reference bar, thermal measurement technique. While attempts to control MWNT density via a micelle template technique produced only 'spaghetti like' CNTs, sputter deposited catalyst provided stark variations in array density. Relevant array morphologies such as density, height, and crystallinity were studied in conjunction with their thermal performance. A Euler buckling model was used to identify the transition between increasing and decreasing resistance with density over array height, these two regimes are explained by way of contact analysis. </p><p> Self catalyzing Fecralloy substrate MWNT TIMs were studied in a similar vein to the Silicon based assemblies. This substrate was investigated because of its malleability, ease of CNT synthesis and increased CNT adhesion. The growth behavior was studied with respect to the array morphologies, i.e. array height, density, crystallinity, and diameter, while the contact resistance was evaluated using a DC, 1D reference bar technique. The best performing samples were found to have a factor of two increase over their Si counterparts. Temperature dependent thermal measurements offer insight into the interfacial phonon conduction physics and are found to agree with other temperature dependent studies, suggesting inelastic scattering at the MWNT-Cu interface. Due to the challenges associated with deliberately controlling a single array morphology, a statistical approach was used for identifying the influences of the multivariate array morphology on contact resistance. Showing the strongest correlation with array height, following a <i>R ~ L</i>^&minus;0.5. Several models were investigated to help explain this behavior, although little insight is gained over the empirical relations. </p><p> To better characterize these MWNT TIM assemblies two experimental techniques were developed. A transient 3&omega; thermal measurement technique was adapted to characterize the thermal performance of CNT TIMs, offering insight into the limiting resistance in a mulilayer material stack. The MWNT-growth substrate interface was found to dominate in the Si samples while the MWNT-opposing substrate interface dominated in the Fecralloy samples. These measurements strongly supported the DC thermal measurements and the qualitative observations of substrate adhesion. Additionally, a new technique for observing nano sized contacts was established by viewing contact loading through an electron transparent membrane, imaged under an SEM. The contrast mechanism is explained by a voltage contrast phenomenon developed by trapped charges at the interface. The resolution limits have been studied by way of electron beam interactions and the use of Monte Carlo simulations, showing nanometer resolution with appropriate experimental conditions. The real MWNT contact area was found to be less than 1/100<i>^th</i> the apparent contact area even at moderate pressures and the number of contacting CNTs is approximately 1/10<i>^th</i> the total number of CNTs. These results confirm experimental measurement values for van der Waals adhesion strengths and thermal interface resistance.</p>

In this work, we present a new computational method based on a nonconforming domain decomposition technique for modeling of phase transitions. Phase transitions are the result of thermal or mechanical loading in ferromagnetic materials or shape memory materials. Modeling of phase transitions is important because it can help to predict or control the behavior of these materials. This thesis will focus on phase transitions characterized by two directions of magnetization in the case of ferromagnets and two variants of Martensite in the case of shape memory materials. In both types of materials, branching occurs near an internal surface which is characterized by complex microstructures. These microstructures occur at a minimum energy state. The new computational method simulates the branching behavior of these microstructures near an internal surface. We approximate the microstructures via energy minimization. We minimize the total stored energy stored in vicinity of internal surface with the minimizing function representing the microstructures. We compare the numerical results obtained by the new technique with those obtained by a more standard technique, one not incorporating nonconforming domain decomposition. Furthermore, we verify the various energy scaling laws used to predict the total stored energy near an internal surface. Among these laws, we verify the local-in-y scaling property which has been conjectured but not proven.

Through a mathematical and computational model of the physical behavior of shape memory alloy wires, this study shows that localized heating and cooling of such materials provides an effective means of damping vibrational energy. The thermally induced pseudo-elastic behavior of a shape memory wire is modeled using a continuum thermodynamic description based on an improved Landau-Devonshire potential. Our construction of the potential function allows the model to account for particular alloys as well as the general solid-state phase transformation, improving over traditional potentials that idealize many of the material properties or focus only on individual processes. The material's thermodynamic response is modeled using a nonlinear conservation of momentum and a nonlinear heat equation. The heat equation introduces an inhomogeneous version of the Fourier heat flux, thereby describing the discontinuous heat flow associated with shape memory materials more thoroughly than standard, continuous heat dissipation mechanisms do. This continuum thermodynamic model is then solved computationally to determine the resulting state of the wire in time. Continuous time Galerkin methods and affine finite elements treat the temporal and spatial discretizations of the model, respectively. Traditional methods for solution of the resulting finite-dimensional, nonlinear, nonconvex system of equations must introduce a significant artificial dissipation to achieve existence of solutions. The solution of the discrete system here uses a novel combination of the damped Newton method and a homotopy method for minimizing the artificial dissipation. This combination, inspired by the well-known Method of Vanishing Viscosity for the solution of scalar hyperbolic conservation laws, reduces the artificial dissipation errors introduced by traditional approaches for such nonlinear, nonconvex thermomechanical models. Computational tests show that the proposed model successfully describes the relevant physical processes inherent in shape memory alloy behavior. Further computational experiments then confirm that up to 80% of an initial shock of vibrational energy can be eliminated at the onset of a thermally-induced phase transformation.

No description available.

The principles governing pattern formation in discotic nematic liquid crystalline fibers subjected to uniaxial extensional flows are established. Computational and analytical methods are used in conjunction with bifurcational techniques to simulate the structural characteristics of the orientational patterns that arise by stretching discotic nematic liquid crystalline materials. The analytical and numerical results are in excellent agreement with actual cross-sectional fiber textures obtained by melt spinning carbonaceous mesophases. This work reproduces the main structural features of the oscillatory zig-zag pattern commonly observed in mesophase carbon fibers, and identifies the process conditions that lead to this peculiar fiber texture. In addition, the temperature driven texture transitions and the emergence of random pattern also observed during the industrial manufacturing of mesophase carbon fibers are captured by the simulations and thoroughly explained using classical viscoelastic theories of liquid crystalline materials.

<p> Vanadium oxide has emerged as a potential electrochemical capacitor material due to its attractive pseudocapacitive performance; however, it is known to suffer from capacitive degradation upon sustained cycling. In this work, the electrochemical cycling behavior of anodically electrodeposited vanadium oxide films with various surface treatments in aqueous solutions is investigated at different pH. Quantitative compositional analysis and morphological studies provide additional insight into the mechanism responsible for capacitive degradation. Furthermore, the capacitance and impedance behavior of vanadium oxide electrochemical capacitor electrodes is compared for both aqueous and nonaqueous electrolyte-solvent systems. Alkali metal chloride and bromide electrolytes were studied in aqueous systems, and nonaqueous systems containing alkali metal bromides were studied in polar aprotic propylene carbonate (PC) or dimethyl sulfoxide (DMSO) solvents. The preferred aqueous and nonaqueous systems identified in the half-cell studies were utilized in symmetric vanadium oxide whole-cells. An aqueous system utilizing a 3.0 M NaCl electrolyte at pH 3.0 exhibited an excellent 96% capacitance retention over 3000 cycles at 10 mV s^-1. An equivalent system tested at 500 mV s^-1 displayed an increase in capacitance over the first several thousands of cycles, and eventually stabilized over 50,000 cycles. Electrodes cycled in nonaqueous 1.0 M LiBr in PC exhibited mostly non-capacitive charge-storage, and electrodes cycled in LiBr-DMSO exhibited a gradual capacitive decay over 10,000 cycles at 500 mV s^-1. Morphological and compositional analyses, as well as electrochemical impedance modeling, provide additional insight into the cause of the cycing behavior. Lastly, reduced graphene oxide and vanadium oxide nanowire composites have been successfully synthesized using electrophoretic deposition for electrochemical capacitor electrodes. The composite material was found to perform with a higher capacitance than electrodes containing only vanadium oxide nanowires by a factor of 4.0 at 10 mV s^-1 and 7.5 at 500 mV s^-1. The thermally reduced composite material was examined in both symmetric and asymmetric whole cell electrochemical capacitor devices, and although the asymmetric cell achieved both higher energy and power density, the symmetric cell retained a higher capacitance over 50,000 cycles at 200 mV s^-1.</p>