Browsing by Author "Suchicital, Carlos T. A."

Now showing 1 - 20 of 63
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    Additive Manufacturing of Copper via Binder Jetting of Copper Nanoparticle Inks
    Bai, Yun (Virginia Tech, 2018-06-01)
    This work created a manufacturing process and material system based on binder jetting Additive Manufacturing to process pure copper. In order to reduce the sintered part porosity and shape distortion during sintering, the powder bed voids were filled with smaller particles to improve the powder packing density. Through the investigation of a bimodal particle size powder bed and nanoparticle binders, this work aims to develop an understanding of (i) the relationship between printed part properties and powder bed particle size distribution, and (ii) the binder-powder interaction and printed primitive formation in binder jetting of metals. Bimodal powder mixtures created by mixing a coarse powder with a finer powder were investigated. Compared to the parts printed with the monosized fine powder constituent, the use of a bimodal powder mixture improved the powder flowability and packing density, and therefore increased the green part density (8.2%), reduced the sintering shrinkage (6.4%), and increased the sintered density (4.0%). The deposition of nanoparticles to the powder bed voids was achieved by three different metal binders: (i) a nanoparticles suspension in an existing organic binder, (ii) an inorganic nanosuspension, and (iii) a Metal-Organic-Decomposition ink. The use of nanoparticle binders improved the green part density and reduced the sintering shrinkage, which has led to an improved sintered density when high binder saturation ratios were used. A new binding mechanism based on sintering the jetted metal nanoparticles was demonstrated to be capable of (i) providing a permanent bonding for powders to improve the printed part structural integrity, and (ii) eliminating the need for organic adhesives to improve the printed part purity. Finally, the binder-powder interaction was studied by an experimental approach based on sessile drop goniometry on a powder bed. The dynamic contact angle of binder wetting capillary pores was calculated based on the binder penetration time, and used to describe the powder permeability and understand the binder penetration depth. This gained understanding was then used to study how the nanoparticle solid loading in a binder affect the binder-powder interactions and the printed primitive size, which provided an understanding for determining material compatibility and printing parameters in binder jetting.
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    The Adhesion Strength of a Plasma Sprayed Silicon Bond Coating on a Silicon Carbide Ceramic Matrix Composite
    Scherbarth, Austin Daniel (Virginia Tech, 2020-10-19)
    Silicon-based ceramics and ceramic matrix composites (CMCs), such as silicon carbide (SiC) fiber reinforced SiC, are promising candidates for hot section components in next generation turbine engines. Environmental barrier coatings (EBCs) are essential for implementing these components as they insulate and protect the substrate from reaction with water vapor in the engine environment. EBCs are typically deposited via atmospheric plasma spraying (APS) and preparing the component surfaces through cleaning and roughening prior to coating is a vital step to ensure sufficient coating adhesion. The adhesion of a plasma sprayed coating to the underlying component is one of the most important properties as the component will not be protected if the coating is not well adhered. Surface roughening of metallic components via grit blasting is well documented and understood, but much less is known about preparing ceramic and ceramic composite surfaces for thermal spray coating. Silicon coatings are often used as a bond coating between SiC-based components and EBC top layers, but the adhesion strength of plasma sprayed Si on these substrates, Si splat formation and the factors that affect coating formation and adhesion have not been well studied. The effects of automated grit blasting process parameters on surface roughness and material loss of a reaction bonded SiC (rb SiC) composite were evaluated. Surface roughness before and after grit blasting was evaluated with a confocal laser scanning microscope. The differences and advantages of automated grit blasting compared to manual grit blasting were observed. Most notably was the level of control at high nozzle traverse speeds resulting in reduction of material loss and consistency of roughening. At high nozzle traverse speeds, the amount of material loss decreased greatly with a small effect on induced surface roughness. The degree of grit blasting induced roughness and material loss was found to be largely dependent on the nature of the composite matrix and reinforcement, as well as blast nozzle traverse speed. A statistical model was developed to predict the substrate thickness loss and induced average roughness based on nozzle traverse speed and blast pressure for automated grit blasting. Additionally, laser ablation was used to create controlled, regularly patterned surface texture on rb SiC substrates to further investigate the role of texture parameters in Si coating adhesion. Si was plasma sprayed onto rb SiC substrates to deposit both thick coatings to evaluate adhesion strength and single splats to study splat formation. Surface roughness/texture, substrate preheat temperature and mean Si particle size were varied in plasma spray coating experiments to observe their role in coating adhesion strength. Si adhesion strength was found to be related to all three factors and a statistical model was developed to predict adhesion strength based on them. Substrate preheat temperature had a significant effect on both Si adhesion strength and Si splat formation on rb SiC. Single splat formation during plasma spraying of Si on SiC was simulated with software called SimDrop. Simulations of Si droplet impact, spreading and solidification during plasma spraying on smooth and textured SiC surfaces were used to investigate the effects of relevant process parameters on splat formation. Experimentally observed Si splats on smooth substrates at different temperatures during deposition were matched with simulated splats with the same spraying parameters. A change in thermal contact resistance with changing substrate preheat temperature was confirmed by the simulation results. The role of surface texture parameters for a regularly patterned surface texture in splat formation was demonstrated through simulation. This dissertation investigates methods of roughening and preparing a SiC composite substrate for plasma spray coating, as well as factors which affect the adhesion strength and splat formation of plasma sprayed Si through experiments and simulation. The observations made provide valuable insight for understanding and optimizing the manufacturing processes utilized to deposit strongly adhered coatings onto SiC-based composites. In addition, areas of interest in this field for future study and further investigation are introduced and suggested.
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    Analysis of Strength Variation in Glass Due to Ion Exchange
    Kulp, Andrew Brooks (Virginia Tech, 2012-05-21)
    The main goal of this project was to compare the changes in statistical variation and Weibull characteristics of the strength of glass rods as modified by heat treatment with and without an ion exchange bath. Several sample groups of 30 sodium borosilicate glass rod specimens were heat treated at various temperatures in air and in a potassium nitrate salt bath to induce an ion exchange process. All samples were then tested to failure in 4-point bending to assess the resulting Modulus of Rupture (MOR). Statistical analysis techniques and Weibull analysis were used to study the variations which occur within and between strength distributions of each sample group. A smaller sampling of test groups was subjected to fractographic analysis to study the effect of ion exchange on fracture features. The data shows that the ion exchange process caused a statistically significant increase in the strength of the glass rods. Samples which were heat treated do not show any significant changes in average strength. The fractographic analysis suggests that no changes in fracture morphology occurred as a result of ion exchange process, and that the critical flaw size population was not significantly different.
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    Characterization of a Novel Biodegradable Material to Reduce Emission of Ammonia
    Adjei, Thomas (Virginia Tech, 2007-12-06)
    A novel biodegradable ammonia control material was developed from steam exploded corn cob and its adsorption capacity was studied by packed column and titration method. The packed column studies showed that the maximum absorption capacities of the raw corn cob (RCC) and the steam exploded corn cob (SECC) were 10.45 mg NHB3B/gRCC and 59.80 mg NHB3B/gSECC respectively. However, the titration of the water slurries with a NHB4BOH showed that the capacity of the SECC was 14.4 times that of RCC. The large difference between the packed column (SECC/RCC = 5.7) and the slurry titration (SECC/RCC = 14.4) was probably because: (1) the initial ammonia reaction products blocked the pores of the SECC and reduced diffusion into the pore structure; (2) the ammonia gas flow rates were too high and therefore the gas did not penetrate the pores; (3) the gas contact time was below the equilibrium value; and (4) since interior pore surface area is usually larger than the external surface area of a particle, it appears the low column SECC/RCC ratio is due to reactions on the SECC particle surface whereas the slurry result was a combination of both. Fourier Transform Infrared, FTIR spectroscopy was conducted on RCC, SECC, ammonia adsorbed on RCC and ammonia adsorbed on SECC in the range 4000–700 cmP-1P. The FTIR bands in the region between 1500 and 2000 cmPâ 1P showed a considerable difference between RCC and SECC. When SECC was treated with ammonia, the carboxylic functional group peak at 1700 cmP-1P was reduced and a new peak was observed at 1584 cmP-1P. The adsorption, desorption test and the heat of adsorption results suggested combined physisorption and chemisorption of ammonia on SECC but chemisorption was found to play an important role in ammonia removal. The BET specific surface area of RCC was 3.4 m2/g whilst that SECC was less than 1 m2/g. Although SECC had a low surface area compared with RCC its adsorption capacity was found to be greater than that of RCC meaning the adsorption process is chemically controlled. Also, the pore size distribution showed that RCC exhibited both macroporosity and mesoporosity whilst SECC showed only mesoporosity. It is interesting to note that upon steam exploding RCC, the macropores within RCC collapsed to form more mesopores in SECC. The high uptake of SECC was determined to be its small pore width compared to that RCC. Simultaneous Differential Scanning Calorimetry, DSC and Thermal Gravimetric Analyzer, TGA, was used to determine the heat of adsorption of ammonia on SECC. The heat of adsorption of ammonia on SECC was 33.00 kJ per mole of NHB3B. This study shows that SECC could be potentially used to remove NHB3B from various emission sources.
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    Characterization Study of Plasma Spray Attachment of Intrinsic Fabry-Perot Interferometric Sensors in Power Generation Applications
    Krause, Amanda Rochelle (Virginia Tech, 2012-06-11)
    The purpose of this study is to characterize the plasma spray deposits used for attaching intrinsic Fabry-Perot interferometric fiber optic strain sensors. The deposits must maintain adhesion at elevated temperatures without distorting the sensors' signals. Two different material systems were tested and modeled, a nickel based alloy and yttria-stabilized zirconia. The material properties of the deposits and the thermal stresses in the system were evaluated to determine attachment lifetime of the sensors. The encapsulated sensors' signals were collected before and after plasma spraying and at elevated temperatures. The material properties of the deposits were evaluated by electron microscopy, energy dispersive x-ray spectroscopy, scratch testing, thermal fatigue testing, and nanoindentation. The thermal stresses were evaluated by Raman spectroscopy and from finite element analysis in COMSOL® Multiphysics®. Several of the sensors broke during encapsulation due to the plasma spray processing conditions and the signals experienced distortion at elevated temperatures. The sensors can be treated to remove this interference to allow for this deposit attachment. The nickel based alloy's ductility and lamellar microstructure allowed for non catastrophic relaxation mechanisms to relieve induced thermal stresses. The yttria stabilized zirconia failed catastrophically at elevated temperatures due its lack of compliance to mismatches in thermal expansion. A high melting point metallic deposit, similar to the nickel based alloy, is desirable for fiber optic sensor attachment due to its ductility, thermal expansion, and dominant relaxation mechanisms. The processing conditions may need to be optimized to allow for the sensors' protection during encapsulation.
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    Chemically modified Ta₂O₅ thin films for dynamic random access memory (DRAM) applications
    Desu, Chandra S. (Virginia Tech, 1998-07-17)
    Increasing demand for high-density memories has necessitated the search for new materials with higher dielectric constants to satisfy the minimum charge storage density requirements. Several materials such as Ta₂O₅, BST¹, BBT² are being investigated to replace the currently used Si based oxide/nitride dielectrics. Among the materials under investigation, Ta₂O₅ is one of the most promising, especially from the fab compatibility point of view. Ta₂O₅ thin films offer a six-fold increase in dielectric constant compared to conventional dielectrics. However, the significant improvement in dielectric constant is offset by higher leakage currents compared to conventional dielectrics. Improvement in both, dielectric and insulating properties is required for the successful integration of Ta₂O₅ thin films into devices. In the current research work, it was demonstrated that by chemically modifying the tantalum pentoxide matrix, significant improvements in its electrical properties can be achieved which would enable the fabrication of a reliable high-density memory device. In the present work, the effects of Al addition on Ta₂O₅ thin films were systematically studied. The structural and electrical properties of these chemically modified thin films were investigated in detail to establish their potential for device applications. The effects on dielectric and insulating characteristics due to incorporation of Al in Ta₂O₅ matrix were studied in capacitor configuration. A metallorganic solution decomposition (MOSD) technique was used to deposit thin films onto Pt coated Si(100) substrates. The capacitors were fabricated by sputter depositing Pt electrodes on the top surface of the films. The dielectric and insulating properties of pure and modified Ta₂O₅ thin films and their dependence on film composition, processing temperature, and the thickness were discussed and an attempt was made to provide theoretical understanding for the experimental observations. The dielectric and insulating properties of Ta₂O₅ were found to be significantly modified by addition of Al. It was observed that Al addition has decreased the leakage currents approximately by an order of magnitude and improved thermal and bias stability characteristics of Ta₂O₅ capacitors. For example, the leakage currents in crystalline pure Ta₂O₅ thin films were found to be 4.5 x 10⁷ A/cm² in a 1MV/cm dc field which decreased to 3.4 x 10⁸ A/cm² for 10% Al modified Ta₂O₅ thin films. A typical dielectric constant of 42.5 was obtained for 10% Al modified Ta₂O₅ thin films. This is significantly higher compared to the commonly reported dielectric constant of 25 to 35 for Ta₂O₅ thin films. This enhancement was attributed to strong (100) orientation exhibited by both pure and modified Ta₂O₅ thin films. The high dielectric constant, low dielectric loss, low leakage currents and low temperature coefficient of capacitance suggest the suitability of Al modified Ta₂O₅ as a capacitor dielectric for future generation DRAM applications. ¹Barium strontium titanate, ²Barium bismuth tantalate
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    Computer Aided Design and Fabrication of Magnetic Composite Multilayer Inductors
    Fielder, Robert Stanley (Virginia Tech, 2000-12-04)
    Computer modeling using finite element analysis (FEA) was performed to examine the effects of constructing multilayered thick film inductors using an artificially modulated magnetic composite structure. It was found that selectively introducing regions of low permeability material increased both the inductance and the current carrying capacity compared to thick film inductors made with single material magnetic cores. Permeabilities of the composite cores ranged from 1 to 220. The frequency for the models ranged from 0 to 5.0 MHz. Experimental devices were constructed using thick film screen printing techniques and characterized to validate the models and to determine the effectiveness of the design modifications. Quantitative comparisons were made between inductors of single permeability cores with inductors produced with magnetic composite cores. It was found that significant (> 130%) increases could be gained in saturation current with only a 12% decrease in inductance. It was found that the key parameters affecting performance were 1) the placement of low permeability regions, 2) the extent of non-uniform flux distribution within the structure, and 3) the volume fraction of low permeability material.
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    Crush Strength Analysis of Hollow Glass Microspheres
    Dillinger, Benjamin Eugene (Virginia Tech, 2016-09-21)
    Porous Wall Hollow Glass Microspheres (PWHGMs) were developed by the Savannah River National Laboratory. What makes these microspheres unique is the interconnected porosity spread throughout their wall allowing various materials to travel from the surface to the hollow interior. With their characteristic porosity, the PWHGMs are a great tool for encapsulating or filtrating different materials. Unfortunately, there is little information available on the mechanical properties of PWHGMs. The main goal of this research was to develop a method to crush individual microspheres and statistically analyze the results. One objective towards completing this goal was to measure the microsphere diameter distribution. Microsphere diameter is a major factor affecting strength as well as the Weibull parameters. Two different methods, microscopy counting and laser light scattering, used in the research yielded similar distributions. The main objective of this research was to analyze the crush strength of individual microspheres. Using nanoindentation, data were collected to analyze the crush strength of PWHGMs in uniaxial compression. Nanoindentation data were used to analyze how the strength of the PWHGMs changes through the different stages of production and at different diameter ranges. Data for 3M commercial microspheres were compared to ARC microspheres. Most data were analyzed using a statistical technique known as the two parameter Weibull analysis. The data indicated that the strength generally decreased as the microsphere diameter increased. Scattering in the data was nearly the same across all sample sets tested. Results indicated that the PWHGMs were weaker than the ARC hollow glass microspheres (HGMs). This is primarily due to the addition of wall porosity in the PWHGM.
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    Crystallization of Lithium Disilicate Glass Using Variable Frequency Microwave Processing
    Mahmoud, Morsi Mohamed (Virginia Tech, 2007-04-24)
    The lithium disilicate (LS2) glass system provides the basis for a large number of useful glass-ceramic products. Microwave processing of materials such as glass-ceramics offers unique benefits over conventional processing techniques. Variable frequency microwave (VFM) processing is an advanced processing technique developed to overcome the hot spot and the arcing problems in microwave processing. In general, two main questions are addressed in this dissertation: 1. How does microwave energy couple with a ceramic material to create heat? and, 2. Is there a "microwave effect" and if so what are the possible explanations for the existence of that effect? The results of the present study show that VFM processing was successfully used to crystallize LS2 glass at a frequency other than 2.45 GHz and without the aid of other forms of energy (hybrid heating). Crystallization of LS2 glass using VFM heating occurred in a significantly shorter time and at a lower temperature as compared to conventional heating. Furthermore, the crystallization mechanism of LS2 glass in VFM heating was not exactly the same as in conventional heating. Although LS2 crystal phase (Orthorhombic Ccc2) was developed in the VFM crystallized samples as well as in the conventionally crystallized samples as x-ray diffraction (XRD) confirmed, the structural units of SiO4 tetrahedra (Q species) in the VFM crystallized samples were slightly different than the ones in conventionally crystallized samples as the Raman spectroscopy revealed. Moreover, the observed reduction in the crystallization time and apparent temperature in addition to the different crystallization mechanism observed in the VFM process both provided experimental evidence to support the presence of the microwave effect in the LS2 crystallization process. Also, the molecular orbital model was successfully used to predict the microwave absorption in LS2 glass and glass-ceramic. This model was consistent with experiments and indicated that microwave-material interactions were highly dependent on the structure of the material. Finally, a correlation between the Fourier transform infrared reflectance spectroscopy (FTIRRS) peak intensities and the volume fraction of crystals in partially crystallized LS2 glass samples was established.
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    Current-Mode Control: Modeling and its Digital Application
    Li, Jian (Virginia Tech, 2009-04-14)
    Due to unique characteristics, current-mode control architectures with different implementation approaches have been widely used in power converter design to achieve current sharing, AVP control, and light-load efficiency improvement. Therefore, an accurate model for current-mode control is indispensable to system design due to the existence of subharmonic oscillations. The fundamental difference between current-mode control and voltage-mode control is the PWM modulation. The inductor current, one of state variables, is used in the modulator in current-mode control while an external ramp is used in voltage-mode control. The dynamic nonlinearity of current-mode control results in the difficulty of obtaining the small-signal model for current-mode control in the frequency domain. There has been a long history of the current-mode control modeling. Many previous attempts have been made especially for constant-frequency peak current-mode control. However, few models are available for variable-frequency constant on-time control and V2 current-mode control. It's hard to directly extend the model of peak current-mode control to those controls. Furthermore, there is no simple way of modeling the effects of the capacitor ripple which may result in subharmonic oscillations in V2 current-mode control. In this dissertation, the primary objective to investigate a new and general modeling approach for current-mode control with different implementation methods. First, the fundamental limitation of average models for current-mode control is identified. The sideband components are generated and coupled with the fundamental component through the PWM modulator in the current loop. Moreover, the switching frequency harmonics cannot be ignored in the current loop since the current ripple is used for the PWM modulation. Available average models failed to consider the sideband effects and high frequency harmonics. Due to the complexity of the current loop, it is difficult to analyze current loop in the frequency domain. A new modeling approach for current-mode control is proposed based on the time-domain analysis. The inductor, the switches and the PWM modulator are treated as a single entity to model instead of breaking them into parts to do it. Describing function method is used. Proposed approach can be applied not only to constant-frequency modulation but also to variable-frequency modulation. The fundamental difference between different current-mode controls is elaborated based on the models obtained from the new modeling approach. Then, an equivalent circuit representation of current-mode control is presented for the sake of easy understanding. The effect of the current loop is equivalent to controlling the inductor current as a current source with certain impedance. The circuit representation provides both the simplicity of the circuit model and the accuracy of the proposed model. Next, the new modeling approach is extended to V2 current-mode control based on similar concept. The model for V2 current-mode control can accurately predict subharmonic oscillations due to the influence of the capacitor ripple. Two solutions are discussed to solve the instability issue. After that, a digital application of current-mode control is introduced. High-resolution digital pulse-width modulator (DPWM) is considered to be indispensable for minimizing the possibility of unpredicted limit-cycle oscillations, but results in high cost, especially in the application of voltage regulators for microprocessors. In order to solve this issue, a fully digital current-mode control architecture which can effectively limit the oscillation amplitude is presented, thereby greatly reducing the design challenge for digital controllers by eliminating the need for the high-resolution DPWM. The new modeling strategy is also used to model the proposed digital current-mode control to help system design. As a conclusion, a new modeling approach for current-mode control is fully investigated. Describing function method is utilized as a tool in this dissertation. Proposed approach is quite general and not limit by implementation methods. All the modeling results are verified through simulation and experiments.
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    Decision-Making Framework for the Selection and Design of Shading Devices
    Olbina, Svetlana (Virginia Tech, 2005-02-11)
    Most shading device systems installed in windows or glass walls are used only for protection from overheating and glare, neglecting other possible functions, such as increasing the daylight level in the space or collecting solar energy. The blinds are usually made of opaque or translucent materials, and if they are partially open/closed or completely closed, a direct view to the outside is blocked. A balance between a sufficient amount of daylight and protection from overheating of the space in summer is not often achieved due to inappropriate control of the blinds’ tilt angle. There is also a need for specific guidance for the selection and design of shading device systems in the windows. This research develops a general decision-making framework (DMF) that can be used by architects and manufacturers of shading devices. The general DMF is a guide for the user in analyzing shading device performance in the process of selection/design of the shading device. This research also develops a specific DMF to better understand and validate the general DMF. The specific DMF, based on illuminance and luminance, is used for an analysis of daylighting performance of shading devices to select the best possible existing system or new system among several alternatives. Architects or manufacturers of shading devices, as the users of the DMF, analyze various systems of blinds applied on a particular building and at a given location. The users of the DMF can apply either an experimental procedure or computer simulation that provides information about illuminance and luminance levels in the space. Based on the analysis of the results of the experiments or simulations, the user of the DMF decides which blinds to select. The specific DMF proposes a methodology for both the analysis of the daylighting performance and for the process for making a decision based on the results of the analysis. A case study is conducted in order to validate the DMF. Three types of shading devices are tested: an existing system, a patented system, and a new system, proposed by this research. The shading devices are installed in an office space located in Roanoke, Virginia. The software Autodesk VIZ 4 is used to simulate daylighting performance. The output information, such as illuminance and luminance levels in the space, is used as a basis for making the decision about which type of blinds to apply. A new system of shading device, which has a triangular cross section and is made of clear plastic with a silver coating on one side, shows better performance than the existing shading device and the patented shading device, given the research limitations. By using the specific decision-making framework, a shading device manufacturer/designer is able to understand the shading device daylighting performance from his design-imposed criteria. Selection of the shading device, given the designer's daylighting objectives, is better achieved. Existing shading devices are also able to be analyzed from a building designer's perspective. This analysis is based on the designer-imposed daylighting criteria. The specific decision-making framework helps the designers of the buildings, together with the clients, select the most appropriate shading device based on daylighting performance. The decision-making framework is a model for development of decision-making software that will help designers of buildings, facades, and shading device systems in the selection/design of shading device systems in all phases of the design.
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    Densification Behavior of Ceramic and Crystallizable Glass Materials Constrained on a Rigid Substrate
    Calata, Jesus Noel (Virginia Tech, 2005-05-04)
    Constrained sintering is an important process for many applications. The sintering process almost always involves some form of constraint, both internal and external, such as rigid particles, reinforcing fibers and substrates to which the porous body adheres. The densification behavior of zinc oxide and cordierite-base crystallizable glass constrained on a rigid substrate was studied to add to the understanding of the behavior of various materials undergoing sintering when subjected to external substrate constraint. Porous ZnO films were isothermally sintered at temperatures between 900°C and 1050°C. The results showed that the densification of films constrained on substrates is severely reduced. This was evident in the sintered microstructures where the particles are joined together by narrower necks forming a more open structure, instead of the equiaxed grains with wide grain boundaries observed in the freestanding films. The calculated activation energies of densification were also different. For the density range of 60 to 64%, the constrained film had an activation energy of 391 ± 34 kJ/mole compared to 242 ± 21 kJ/mole for the freestanding film, indicating a change in the densification mechanism. In-plane stresses were observed during the sintering of the constrained films. Yielding of the films, in which the stresses dropped slight or remained unchanged, occurred at relative densities below 60% before the stresses climbed linearly with increasing density followed by a gradual relaxation. A substantial amount of the stresses remained after cooling. Free and constrained films of the cordierite-base crystallizable glass (glass-ceramic) were sintered between 900°C and 1000°C. The substrate constraint did not have a significant effect on the densification rate but the constrained films eventually underwent expansion. Calculations of the densification activation energy showed that, on average, it was close to 1077 kJ/mole, the activation energy of the glass, indicating that the prevailing mechanism was still viscous flow. The films expanded earlier and faster with increasing sintering temperature. The expansion was traced to the formation of pores at the interface with the silicon substrate and to a lesser extent on aluminum nitride. It was significantly reduced when the silicon substrate was pre-oxidized at 900°C, leading to the conclusion that the pore formation at the interface was due to poor wetting, which in turn was caused by the loss of the thin oxide layer through a reaction with the glass.
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    Design and Analyses of a Dimple Array Interconnect Technique for Power Electronics Packaging
    Wen, Sihua (Virginia Tech, 2002-08-02)
    This research developed a novel, non-wire bond semiconductor interconnect technology, termed the Dimple Array interconnect (DAI), with significantly improved electrical, thermal and mechanical characteristics for power electronics applications. In the DAI structure, electrical connections onto the devices are achieved by solder bumps formed between the silicon device and arrays of dimples stamped on a metal sheet flex. This research first presents the design of the materials, electrical and thermal performance, reliability, and the fabrication process of the DAI. It was found that due to the use of solder material, the current handling capability and thermal management of Dimple Array interconnected devices are significantly better than those using wire bonds. In addition, the shorter and wider solder joints reduce parasitics, which is a serious problem in wire bond interconnects. The proposed fabrication process of the DAI is simpler than other developing integrated power packaging technologies, such as flip chip and deposited metallization integration. DAI was successfully demonstrated in a half-bridge power electronics module with much improved electrical characteristics. The study then focuses on the thermomechanical reliability of Dimple Array packages as compared to conventional controlled collapse bonding (CCB) flip chip packages. Experimental approaches, such as power cycling and temperature cycling tests, and numerical simulation with the help of finite element analysis (FEA) were used. The thermal cycling test shows that dimple solder joints display an eightfold reliability improvement over the conventional CCB solder joints. The power cycling test showed that the measured forward voltage can not reliably reflect the integrity of the solder joint interconnect. However, from metallographic cross-section images of these samples, it was concluded that the DAI solder joints are more reliable than the CCB solder joints under power cycling conditions. FEA results showed excellent correlation with experiments in predicting that the Dimple Array solder joints are more fatigue-resistant due to a reduced stress/strain concentration. Furthermore, failure mechanisms were explored using the mapped stress/strain distribution within the models. It was found that the CCB solder joint has a highly localized strain concentration at the device/solder interface, while strains are more uniformly distributed over the whole Dimple Array solder joint.
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    Design, Fabrication and Testing of Fiber-Reinforced Cellular Structures with Tensegrity Behavior using 3D Printed Sand Molds
    Jorapur, Nikhil Sudhindrarao (Virginia Tech, 2017-02-15)
    The overall goal of this work is to improve the structural performance of cellular structures in bending applications by incorporating tensegrity behavior using long continuous fibers. The designs are inspired by the hierarchical cellular structure composition present in pomelo fruit and the structural behavior of tensegrity structures. A design method for analyzing and predicting the behavior of the structures is presented. A novel manufacturing method is developed to produce the cellular structures with tensegrity behavior through the combination additive manufacturing and metal casting techniques. Tensegrity structures provide high stiffness to mass ratio with all the comprising elements experiencing either tension or compression. This research investigates the possibility of integrating tensegrity behavior with cellular structure mechanics and provides a design procedure in this process. The placement of fibers in an octet cellular structure was determined such that tensegrity behavior was achieved. Furthermore, using finite element analysis the bending performance was evaluated and the influence of fibers was measured using the models. The overall decrease in bending stress was 66.6 %. Extending this analysis, a design strategy was established to help designers in selecting fiber diameter based on the dimensions and material properties such that the deflection of the overall structure can be controlled. This research looks to Additive Manufacturing (AM) as a means to introduce tensegrity behavior in cellular structures. By combining Binder Jetting and metal casting a controlled reliable process is shown to produce aluminum octet-cellular structures with embedded fibers. 3D-printed sand molds embedded with long continuous fibers were used for metal casting. The fabricated structures were then subjected to 4 point bending tests to evaluate the effects of tensegrity behavior on the cellular mechanics. Through this fabrication and testing process, this work addresses the gap of evaluating the performance of tensegrity behavior. The overall strength increase by 30%. The simulation and experimental results were then compared to show the predictability of this process with errors of 2% for octet structures without fibers and 6% for octet structures with fibers.
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    Design, Synthesis and Characterization of Porous Silica Nanoparticles and Application in Intracellular Drug Delivery
    Munusamy, Prabhakaran (Virginia Tech, 2010-06-29)
    Nanoparticle mediated drug delivery approaches provide potential opportunities for targeting and killing of intracellular bacteria. Among them, the porous silica nanoparticles deserve special attention due to their multifunctional properties such as high drug loading, controlled drug release and targeting of organs/cells. A review of the functional requirements of an ideal drug delivery system is provided. A general comparison between different drug delivery carriers and key issues to be addressed for intracellular drug delivery is discussed. Acid catalyzed and acid-base catalyzed, sol-gel derived, silica xerogel systems were investigated for sustained release of an aminoglycosides antimicrobial against salmonella infection in a mouse model. The release of gentamicin from the inner hollow part of the carrier is delayed. Further, the higher porosity of the acid–base catalyzed silica xerogel allows for high drug loading compared to the acid catalyzed silica xerogel system. Efficacy of these particles in killing intracellular bacteria (salmonella) was determined by administering three doses of porous silica loaded gentamicin. This proved to be useful in reducing the salmonella in the liver and spleen of infected mice. Furthermore, the presence of silanol groups provides the ability to functionalize the silica xerogel system with organic groups, poly (ethylene glycol) (PEG), to further increase the hydrophilicity of the silica xerogel matrix and to modify the drug release properties. Increase in the hydrophilicity of the matrix allows for faster drug release rate. In order to facilitate controlled drug release, magnetic porous silica xerogels were fabricated by incorporating iron particles within the porous silica. The particles were fabricated using an acid-base catalyzed sol-gel technique. The in-vitro drug release studies confirm that the release rate can be changed by the magnetic field "ON-OFF" mechanism. This novel drug release methodology combined with the property of high drug loading capacity proves to be influential in treating salmonella intracellular bacteria. The potential application of any drug delivery carrier relies on the ability to deliver the requisite drug without adversely affecting the cells over the long term. We have developed silica/calcium nanocomposites and evaluated their solubility behavior. The solubility of particles was characterized by particle size measurements for different periods of time. It was found that the solubility behaviour of the silica/calcium particles was dependent on their calcium content. The results obtained demonstrate the potential to use mesoporous silica/calcium nano-composites for drug delivery applications. The significant contribution of this research to drug delivery technology is on design and development of the novel porous core-shell silica nano-structures. This new core-shell nano-structure combines all the above mentioned properties (high drug loading, magnetic field controlled drug release, and solubility). The main aim of preparing these porous core-shell particles is to have a control over the solubility and drug release property, which is a significant phenomenon, which has not been achieved in any other drug delivery systems. The shell layer acts as a capping agent which dissolves at a controllable rate. The rate at which the shell layer dissolves depends on the composition of the particles. This shell prevents the drug "leakage" from the particles before reaching the target site. The core layer drug loading and release rate was modified by application of a magnetic field. Additionally, inclusion of the calcium ions in the core layer destabilizes the silica network and allows the particles to dissolve at an appropriate rate (which can be controlled by the concentration of the calcium ions).
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    Designing Microstructure through Reverse Peritectoid Phase Transformation in Ni₃Mo Alloy
    Khalfallah, Ibrahim (Virginia Tech, 2016-12-07)
    High-energy ball milling and powder metallurgy methods were used to produce a partially alloyed nickel and molybdenum of γ-Ni₃Mo composition (Ni-25at.%Mo). Milled powders were cold-compacted, sintered/solutionized at 1300°C for 100h sintering followed by quenching. Three transformation studies were performed. First, the intermetallic γ-Ni₃Mo was formed from the supersaturated solution at temperatures ranging between 600°C and 900°C for up to 100h. The 100% stable γ-Ni₃Mo phase was formed at 600°C after 100h, while aging at temperatures ranging between 650°C and 850°C for 25h was not sufficient to complete the transformation. The δ-NiMo phase was observed only at 900°C as cellular and basket strands precipitates. Second, the reversed peritectoid transformation from γ-Ni₃Mo to α-Ni and δ-NiMo was performed. Supersaturated solid solution samples were first aged at 600C for 100h followed by quenching to form the equilibrium γ-Ni₃Mo phase. After that, the samples were heat treated between 910°C and 1050°C for up to 10h followed by quenching. Regardless of heat-treatment temperature, samples heat-treated for shorter times exhibited small precipitates of δ-NiMo along and within grain boundaries of α-Ni phase, and it coarsened with time. Third, the transformation from the supersaturated solution α-Ni to the peritectoid two-phase region was performed. The samples were aged between 910°C and 1050°C for up to 10h followed by quenching. Precipitates of δ-NiMo were observed in the α-Ni matrix as small particles and then coarsened with aging time. In all three cases, hardness values increased and peaked in a way similar to that of traditional aging, except that the peak occurred much rapidly in the second and third cases. In the first case, hardness increased by about 113.6% due to the development of the new phases, while the hardness increased by 90.5% and 77.2% in the second and third cases, respectively.
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    Determination of Surface Free Energies and Aspect Ratio of Talc
    Lobato, Emilio Marcus de Castro (Virginia Tech, 2004-11-12)
    Microcalorimetric measurements and contact angle measurements were conducted to assess the surface chemistry of the mineral talc. The contact angles were performed on both flat and powdered samples and the results were used to determine the surface free energy components and parameters (SFEC) using the acid-base theory for solids, according to the van Oss-Chaudhury-Good approach. It was found that the surface hydrophobicity of talc increases with decreasing particle size up to a limit after which hydrophilicity (polarity) increases. The increase in hydrophobicity was attributed to the increase of the delamination of the lamellar talc particles. Delamination is a comminution mechanism that preferentially exposes talc's hydrophobic basal planes, while fracture is another mechanism that breaks the lamellae, rupturing covalent bonds thus exposing more hydrophilic edge surfaces. The decrease in hydrophobicity, beyond a given particle size, could be related to the prevail of fracture over delamination during grinding which generated more hydrophilic edge surfaces. The flow microcalorymetry combined with thin layer wicking allowed the separate estimation of the SFEC at the basal plane and edge surfaces of talc. The results suggested that the basal surface of talc is monopolar basic, while the edge surface is monopolar acidic, which are in agreement with the crystal structure of the mineral. The combination of two particle size distribution techniques, which are based on different physical principles, permitted the quantitative determination of the aspect ratio of highly anisometric particles, such as talc. The same trend obtained using flow microcalorimetry was observed for the evolution of the aspect ratio as a function of particle fineness, i.e. the fracture prevails over delamination after achieving a maximum aspect ratio value of about 35. The agreement between two distinct methods was considered rather encouraging.
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    Echogenic Biomaterials for Medical Ultrasound Tracking
    Contreras, Jerry (Virginia Tech, 2020-06-29)
    As the world population ages, hospital discharges of geriatric patients to nursing homes have increased. Patients with peripherally inserted central catheters (PICCs) are routinely discharged with the catheters in place. PICCs, only capable of being tracked through x-ray imaging, will routinely experience complications due to thrombosis or accidental dislodgement from poor at-home care. Routinely, elderly patients will be forced to revisit the hospital to have the catheter replaced using x-ray imaging, exposing them to hospital borne illness. Catheters with the capability to be tracked without the need of x-ray imaging would greatly benefit the ill and elderly, providing decreased stress to the patients and increase nursing home capabilities. This project seeks to develop the field of real-time ultrasound tracking of polymeric medical devices, through fabrication of ultrasound responsive polymer-glass composites. Optimal composition will be researched through three complimentary approaches. The first approach seeks to develop a polyurethane-glass microparticle composite to understand the relationship between microparticle loading and ultrasound imaging. In the second approach, manufacturing and end-use complications will be simulated to evaluate the effects on mechanical and ultrasonic properties. Furthermore, impacts from in-vitro long term catheterization to the sample mechanical and ultrasound morphologies would be analyzed. In the third approach, optimization from the previous approaches would assist in the replacement of medical grade polyurethane with medical grade thermoset silicone in hopes to prove the ability for the research to be transferable to other medical polymeric devices. The stated approaches will be useful for setting a path towards the development of ultrasonic imaging as the standard for medical device tracking.
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    The Effect of Carbon Concentration on the Amorphization and Properties of Mechanically Alloyed Cobalt-Carbon Alloys
    Elmkharram, Hesham Moh A. (Virginia Tech, 2021-04-27)
    Magnetic alloys that are amorphous exhibit soft magnetic properties; hence they play an essential role in electronic and electrical systems and devices. They are used in applications that include electrical power generation and transmission, electronic motors, solenoids, relays, magnetic shielding, and electromagnets. This work was an attempt to investigate the solid-state formation of Co-C amorphous alloys, their thermal stability and magnetic properties. Amorphous Co-C alloys with compositions of 2 to 40 at.% C were successfully synthesized from elemental Co and C (graphite) using mechanical alloying, a solid-state powder processing technique. All alloy compositions were milled for up 40 hours. After 20h of milling some of the alloys (≤ 20 at.% C) had partially amorphized, while the higher concentrations had completely amorphized. After 40h of milling, complete amorphization was observed in all alloys, except for the 2 and 5 at.% C alloys. The thermal analyses of the milled powders showed very interesting results. DSC results indicated that alloys with compositions through 20 at.% C crystalized in two steps; the lower temperature event precipitated metastable cobalt carbide from the amorphous phase, followed by the eventual transformation to fcc cobalt and graphite from both the remaining amorphous and the metastable carbide at the higher temperature. Two types of carbides were observed - Co3C in the 2 and 5 at.% C alloys, and Co2C in the higher carbon alloys through 20 at.% C. For compositions above 20 at.% C, only one step crystallization was observed, that of the decomposition of the amorphous phase to amorphous carbon and cobalt – primarily fcc phase. Activation energy calculations show that the low temperature carbide precipitation was controlled by carbon diffusion, while the high temperature decomposition reaction forming cobalt and amorphous carbon was controlled by cobalt diffusion. Room temperature magnetic measurements of the milled powders were made using vibrating sample magnetometer (VSM). High saturation magnetization (Ms) and very low coercivity (Hc) are desired for efficient performance of soft magnets. But in this study, Ms decreased with both carbon composition and milling time. It decreased from 195 Am2/kg for the un-milled pure Co to between 178 and 44 Am2/kg for the alloys, with the worst being the 40 at.% C sample milled for 40h. The Ms drop as function of composition made sense, as its related to the volume fraction of cobalt in the alloy. However, the Ms drop as a function of milling time is unclear. In the case of Hc, its value did drop from 12.7 kA/m for the un-milled pure Co to between 7.5 and 1.3 kA/m when the C content is less than 15 at.%. These gains are not significant enough to favor the use of these alloys as soft magnets. Amorphous metal alloys tend to have strengths that are much higher than their crystalline counterparts, and they have hardness values comparable to those of particulate ceramic materials used to reinforce metal matrices. The Co-C amorphous alloy with 40 at.% C that had been milled for 40h (the most stable of all the samples) was used to reinforce cobalt matrix by powder processing methods that included spark plasma sintering (SPS) at temperatures below those of crystallization. Volume fraction ranged from 1 to 20 % reinforcement. The densities of these composites were between 81 and 85 % of theoretical values, hence there were substantial porosities. Despite this the matrix strengthening of the cobalt matrix, as assessed by Vickers microhardness tests, was significant. Hardness increased from 210 HV for unreinforced matrix to 537 HV for the 20 vol.% amorphous. The primary contributor to the strengthening appears to be boundary strengthening by the particles whose average size of about 4 microns is comparable to the grain size of the matrices of the composites. The hardness data fits the Hall Petch-like relationship based on particle spacing. Having a reinforcement particle with a chemistry similar to that of the matrix as is the case in this study, has the potential to improve interfacial bonding and also minimize the difference between the components' coefficient of thermal expansions, which are major issues with the use of ceramics to reinforce metal matrices. The microstructures of the composites indicated good bonding at their interfaces.
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    The Effect of Microwaves on Aqueous Corrosion of Glass
    Lynch, Matthew (Virginia Tech, 2006-07-24)
    Glass corrodes in aqueous environments. The corrosion process is well-understood for many circumstances involving long periods of time at room temperature as well as processes that involve conventional heating, but the effect of microwave energy on glass corrosion has never been fully investigated. It was suspected that microwaves may alter or accelerate the aqueous corrosion processes that occur in glass which contribute to migration into foods or other materials. Lithium disilicate (Li2O-2SiO2) and commercial soda-lime glass were corroded using both conventional and microwave heating in this study. The results did not clearly show substantial differences in corrosion under the test conditions, but leave open the possibility of an altered mechanism in some circumstances. These findings suggest the need for testing at a lower microwave frequency, specifically 2.45 GHz.