Nontechnical Abstracts of PhD Theses: 2005

Pedro Andia

Advisor: F. Costanzo
Committee: G. Gray, L. Friedman, R. Messier, A. Belmonte, J. Todd

Mechanical Properties of Particle Systems Using a Molecular Dynamics Approach Inspired by Continuum Homogenization

In the last ten years the nanotechnology industry has developed promising devices that, in a near future, will have an impact in our everyday life. Nanotechnology relies on phenomena that occur at the nanoscale, that is, in length scales in which atoms and molecules are studied. The goal of nanotechnology is to use such phenomena to create systems with applications at human scales. The assessment of the mechanical behavior of nano-devices is crucial for estimating their reliability as well as for enhancing their designs. This mechanical behavior includes issues such as how stiff or flexible a structure is and/or how much force is required to break it. However, the conceptual understanding of these mechanical properties was originally developed in theories that apply at length scales that are at least tens of thousands of times larger than that of atoms. For this reason, a careful translation of ideas from human scales to the nanoscale is necessary.

Systems at the nanoscale are generally studied using discrete models, that is models that are based on an atomic description of the material. At larger scales, systems are usually modeled as continua, that is, the atomic or particle nature of the system is not considered. This dissertation presents an approach for reconciling the mechanical properties of solid material systems at the nanoscale with those same properties at larger scales. Thus, this reconciliation bridges the discrete and continuum description of material systems. In particular, the mechanical properties studied in this work are stress-strain curves, which are an indication of the how much the material deforms when subjected to a given set of forces. The behavior of materials at the nanoscale can be described by studying the interactions of individual atoms and molecules. Studying nanoscale systems in this manner is called molecular dynamics (MD) and MD can only be done using very large computer simulations. In order to obtain a description at a larger scale using information garnered from the nanoscale, this work determines appropriate averages of properties such as force, velocity, and deformation. As such, one important objective of this work is to properly link the understanding of the mechanical behavior of a material at human scales to the understanding of these same properties at atomic scales. In this way, a fundamental contribution to the understanding of mechanical behavior at the nanoscale is provided since a reconciliation is made between the notions of stress as found in the MD and physics communities and the notion of stress as is found in the classical mechanics community. Finally, important stress-strain curves as determined from deformation experiments performed on computers are presented.

Ravi Bollina

Advisor: R. M. German
Committee: J. Hellmann, R. Engel, I. Smid, B. Shaw, J. Todd

In situ Evaluation of Supersolidus Liquid Phase Sintering Phenomena of Stainless Steel 316L: Densification and Distortion

Powder Metallurgy (P/M) is a metal working technique to fabricate high quality, complex parts with close tolerances in an economical manner. The prime advantage of P/M is the ability to manufacture components with unique compositions and microstructures. P/M process comprises of shaping (compaction) and sintering. Shaping can be done by either die pressing, injection molding, extrusion, or slip casting. In practice, binders and lubricants are added to facilitate the shaping process. After shaping is accomplished these binders are removed by using thermal, solvent debinding or a combination of both. The final microstructure and properties are tailored during sintering. Sintering is a thermal treatment for bonding particles into a coherent structure via diffusion processes. In practice, majority of sintering happens by liquid phase sintering, a process in which liquid exits during some part of the sinter cycle. Supersolidus liquid phase sintering (SLPS) is a variant of liquid phase sintering. In SLPS, prealloyed powders are heated between the solidus and liquidus temperature of the alloy. Liquid forms along the grain boundaries within the particles, fragmenting them into individual grains. The semisolid compact densifies in response to the capillary force exerted by the liquid film along the grain boundaries. The rheological response of the compact during sintering determines the densification and distortion response. Above the solidus temperature the liquid formation decreases the apparent viscosity of the compact. The compact densifies when the sintering stress is higher than the inherent strength of the compact. If the viscosity drops too low, the inherent strength of compact is less than gravitational forces leading to distortion. Hence, there exists a critical viscosity window in which full density is achieved with minimal distortion.

SLPS is an important processing route for stainless and tool steels. Stainless steel 316L is an important class of steel which combines high strength to weight ratio, corrosion resistance, impact resistance along with its aesthetic appearance. This thesis focuses on processing of stainless steel 316L via SLPS by adding boron. The crux of this thesis is to understand the evolution of apparent viscosity and its relation to the microstructure. The densification behavior was modeled using Master Sintering Curve (MSC) concepts. The optimization of mechanical and corrosion properties by boron additions is researched.

Optimized sinter cycles can be designed from the knowledge gained to better control dimensions and final mechanical and corrosion properties. The numerical data obtained from experiments is used in finite element modeling of sintering.

Guodong Cai

Advisor: A. E. Segall
Committee: C. Lissenden, F. Costanzo, B. Shaw, J. Hellmann, J. Todd

Development of a Split-Beam Method for the Improved Laser Machining of Ceramics

All you can hear is a faint hiss. All of a sudden, a crack appears at the edge of a sheet of ceramics and travels relentlessly across the ceramics, finally, the sheet splits into two halves. However, this has nothing to do with magic; the operators at EES 108 rely instead on the creative laser technology.

During the past few decades, new materials like ceramics and composites have been introduced slowly but steadily into a number of manufacturing applications. Nonetheless, they do present problems due to their brittleness and the processing difficulty using conventional machining techniques. Although not visible to the naked eye, the edge of ceramics cut using these methods is full of tiny chips and splits, which are the starting points of microscopic cracks which spread across the cut edge. Any of these micro cracks can be the starting point of s fracture if the ceramics is subjected to mechanical stress. The use of lasers for processing ceramics is by no means new, but up to now, lasers are particularly advantageous for the precision machining of ceramics by virtue of their low force signature and cost effectiveness. However, the full potential of lasers has yet to be realized because of the fracture problems inherent to the brittle nature of ceramics and the associated higher manufacturing and environmental costs. One of the most vexing of these problems is the mixed-mode nature of early fractures and the separation burrs and chips that result.

The project developed a unique laser machining method, pre-scoring, that hold the promise of improving quality and throughout by stalling premature fractures and eliminating separation burrs and chips. The research was initially focused on understanding the groove methodology that ultimately lead to premature fractures, mixed-mode crack growth and the related separation burrs, and chips. To account for the statistical nature of fracture in ceramics, a Weibull type of analysis was integrated with the series of experiments to help quantify the time and location of fracture as the laser progresses along the cutting path. Finite element method (FEM) was used to modeling the relationships between the thermal transients caused by dual-laser machining. The resulting models were then combined with laser machining experiments to optimize a process of simultaneously scoring the surface to help control the path and surface of the final fracture. At last, two additional methods, offset alignments and angled pre-scoring, provided assistance in exploring to control/ delay premature fractures to extend the concept of pre-scoring.

Kyuhwan Chang

Advisors: J. Ruzyllo, O.O. Awadelkarim
Committee: M. Horn, J. Xu, P. Lenahan, J. Todd

Engineering of Semiconductor-Dielectric Interface in MOS Gate Structures

Complementary Metal-Oxide-Semiconductor (CMOS) structure built into silicon substrate is a key element of essentially any advanced electronic system. This means that performance of essentially any consumer electronics products such as personal computers, game consoles, DVD players, PDAs as well as industrial electronic instruments such as programmable logic controllers (PLCs), digital signal processing (DSP), and supercomputers depends on the performance of CMOS structure.. Consequently, in order to assure continued growth of broadly understood electronics it is imperative that no effort is spared to improve performance of a basic CMOS cell.

This project addresses one of the most important aspects of CMOS silicon technology which is a control of the physical and chemical characteristics of the interface between silicon and silicon dioxide because. Those characteristics have a defining impact on the electrical performance and reliability of CMOS devices. During the last half century, silicon/silicon dioxide interface has been improved significantly due to the tremendous amount of scientific studies and engineering efforts geared toward ensuring stable and predictable operation of MOS transistors based on silicon and silicon dioxide system. However, recent developments in semiconductor industry require further improvements of the quality of silicon/silicon dioxide interface. This cannot be accomplished without major several modifications of the CMOS devices and materials. Two of them were addressed in this research. First, a number of new dielectric materials is under investigation to overcome the limitation of silicon dioxide. Hafnium oxide and hafnium silicate are the most probable candidates of substitutes for silicon dioxide. Effects of process condition on film properties of hafnium silicates and on interface between silicon and hafnium silicates is investigated in this project. Second, instead of planar two-dimensional structure, three-dimensional structure has been considered to improve device performance. Later part of this project deals with the problems of this three-dimensional configuration of silicon and silicon dioxide interface.

Ravi Enneti

Advisor: R. M. German
Committee: J. Rose, D. Green, R. Engel, I. Smid, J. Todd

Thermal analysis and evolution of Shape Loss Phenomena During Polymer Burnout in Powder Metal Processing

Powder metallurgy technology deals with manufacturing of net shape or near net shape components starting from metal powders. The process consists of three primary steps i.e. shaping, polymer removal and sintering. In the shaping stage, the powders are formed to the required component shape either by use of external pressure or by utilizing the flowability property of the powder-polymer mix. Polymers are used as process aids during shaping of the components. The function of polymers involves providing lubrication to the powders and also handling strength to the shaped components. Following the shape-forming stage, polymers are removed from the shaped components by providing thermal energy. The removal of polymers with the aid of thermal energy is called as ''polymer burnout process''. After the polymers are removed, the components are heated to high temperatures to form bonds between the metal particles. This process is called 'Sintering'.

Polymer burnout is a very critical stage in powder metal technology. Defects like blistering, cracking, bloating usually occur during polymer burnout. Apart from the macro defects, any micro defects caused in this stage are exaggerated during the sintering. Improper removal of polymer can result in carbonaceous residues that can have a deleterious effect on mechanical, optical, thermal, magnetic, and electronic properties of the final sintered components.

The present research is aimed to understand the effect of metal powders on the polymer burnout process. The evolution of distortion or shape loss during polymer burnout is also studied in the current research. Experiments were carried out with 36L stainless steel metal powders and ethylene co-vinyl acetate (EVA) polymer. During the current research an innovative process was developed to die compact fine spherical powders. Components with high densities at low sintering temperatures were obtained from the powders prepared by this process. The addition of metal powders resulted in lowering of the polymer burnout temperatures. The polymer burnout temperatures were found to be sensitive to the surface area of the powders and also to the wettability of the powder surface. Powders with high surface area and exhibiting good wettability to EVA resulted in burnout of the polymer at lower temperatures.

The shape loss during polymer burnout was observed in situ and was found to be dependent on the degradation behavior of the polymer. The shape loss of the components occurred primarily during the softening of EVA. On onset of burnout of EVA, the distortion of the component was reversed. The recovery of shape loss was attributed to the degradation behavior of EVA.

Wei Luo

Advisor: J. L. Rose
Committee: B. Tittmann, C. Lissenden, E. Ventsel, Q. Zhang, J. Todd

Ultrasonic Guided Waves and Wave Scattering in Viscoelastic Coated Hollow Cylinders

Pipeline safety plays an important role in the transmission and distribution of energy, such as in fossil fuel and natural gas pipelines. To preserve the integrity and safety of these pipelines, a large percentage of them are coated with protective materials. However, environmental conditions, aging, and excavation accidents can compromise the effectiveness of these protective measures. Thus the inspection and monitoring become indispensable due to the high cost of replacement with new pipelines. Periodic or as necessary non-destructive evaluation (NDE) is required to tell the pipeline operator the current status of a pipe and whether remedial action is necessary. Methods exist for detecting cracks in pipelines, but they are either highly costly or with limited inspection abilities. Ultrasonic guided waves, because of their long range inspection ability, are being used more and more as a very efficient and economical pipeline inspection method.

An ultrasonic wave is a mechanical wave at frequencies (usually larger than 20 kHz) higher than the human's audible frequency range. For civilian applications, ultrasonic techniques have been used widely in medical diagnostics and NDE of material and structures for many years. These applications basically utilize ultrasonic waves in frequency ranges higher than several MHz and so the wave propagation distance is very limited due to the high attenuation at those frequency ranges. These waves are usually called bulk waves due to the small wavelength compared to the size of bulk wave propagation media. When the wavelength is equivalent to media geometry size at low frequency (kHz) range, waves are called guided waves which can propagate along a wave guide for as long as hundreds of feet. Therefore, guided wave inspection is much more efficient than the tedious point-by-point bulk wave inspection.

Guided waves in pipes are quite complicated in terms of dispersion and mode diversity. They can be categorized into axisymmetric including longitudinal and torisonal waves, and non-axisymmetric waves including flexural longitudinal and flexural torsion waves. Axisymmetric waves and phased array focusing are the two main techniques for long range pipeline inspection. Focusing techniques can increase energy impingement, locate defects, and enhance greatly inspection sensitivity and propagation distance of guided waves. A typical scenario of long range guided wave inspection is to generate guided waves from one single transducer position, which will propagate with long distance and then impinge onto any possible defects with the occurrence of wave scattering. The inspection strategy is to acquire the possible reflected waves from defects and to analyze the waves for defect detection, locating, sizing and characterization. Therefore, the inspection distance is am important parameter evaluating the guided wave inspection ability.

However, the viscoelastic nature of coating materials leads to significant attenuation consequently reducing guided wave inspection distance. Because of the variation of coating materials and the complexity of the wave mechanics in viscoelastic multilayered structure, many aspects and questions on guided wave inspection in coated pipe still remain unknown and very challenging. In this work, guided wave propagation, scattering and phased array focusing in viscoelastic coated pipes were studied for the first time via numerical method, analytical method as well as some experimental measurements. A powerful 3-dimensional finite element tool was developed first for the modeling of any guided wave propagation and focusing in a coated pipe. Wave scattering studies were then followed on three-dimensional defects with respect to inspection and sizing potentials. Some exciting results were acquired in which phased array focusing potentials in coated pipes were proved. Some criteria on wave attenuation reduction and consequently inspection distance increment were established. A process from experimental measures to theoretical models has been established as a tool to evaluate the guided wave inspection potential of in-field coated pipes. Most of the work has never been studied before and therefore the accomplishments achieved have a high impact for the future long range guided wave inspection of coated pipe.

Chih-Yi Peng

Advisor: S. J. Fonash
Committee: S. Ashok, O. Awadelkarim, J. Xu, S. Joshi, J. Todd

The Use of Arrayed Nano-Dimensional Template Structures for Controlled Growth

Nanotechnology is an emerging field of the scientific and engineering research that has roots that have been developing for decades. It offers the promise of letting us fabricate an entire new generation of products that are cleaner, stronger, lighter, and more precise by using nanoscale fundamental building blocks. A promising approach where nanotechnology can lead the way is in the fabrication of nanowires and their assembly into nanoelectronic devices by nanotechnology. Devices based on the nanowires, may find use in applications ranging from molecular electronics to chemical sensors. Over the past few years, nanowires of different materials have attracted much attention, and considerable progress has already been made in their synthesis and their application in devices. However, a significant obstacle in the application of these nanostructures has been the difficulty in handling, maneuvering, and integrating them to form a complete system. The challenge still has to be faced due to the lack of post-synthesis process suitable for the hierarchical organization of these nanoscale building-blocks into functional assemblies and, ultimately, useful systems. If nanowires could be easily aligned, arranged into patterns, and contacts and interconnects set-up, the impact would be tremendous in many areas. Among the most frequently used nanowire synthesis methods used to gain some control of creating nanostructures is the approach to forming nanoscale structures in commercially available porous membranes as the growth templates which define the diameter and length of the nanowires. Once the membrane is removed and the nanowires liberated from the membrane, one must employ some pick-and-then-place techniques to capture the nanostructures and to position them at the point of use. Such manipulation can be very time-consuming and arduous. There is an urgent need for an engineered approach for better control of manufacturing and assembling of oriented nanomaterials in order to advance to the next stage of nanoscience and nanotechnology.

In this research we developed a fabrication procedure for producing nanowires in arrays and circuits without the need for any pick-and-then-place processing. The approach developed uses nanodimensional channels as permanent templates for the formation of nanomaterial arrays with precise dimensional, positional, and orientational control as well as with built-in electrical access, when appropriate. These nanochannel templates were fabricated by the combined use of top-down process and the sacrificial material approaches. The dimensions of the nanochannels can be as small as 20 nm high, 20 nm wide and several hundred microns long. Two versions of these nanochannel templates for the nanomaterial growth were fabricated in this research, and the growth of different nanomaterials in these nanochannels was demonstrated.

In the first version of nanochannel templates, we synthesized Poly(methyl methacrylate) (PMMA) by radical polymerization, and polythiophene (PT) by photopolymerization. We also showed the carbon nanofibers grew inside and grow out of this nanochannel template with nickel catalyst in the nanochannels in chemical vapor deposition (CVD) system. Then our second version of nanochannel templates, this version with built-in electrodes and channel access region, was developed. The use of permanent, horizontal, encapsulated, nanochannel growth templates to electrochemically produce arrays of individual polyaniline nanoribbons with fully controlled dimensions, position, alignment, encapsulation, and electrical contacting was demonstrated. The structure was used for ''grow-in-place'' electrochemical synthesis and direct characterization of the produced conducting polyaniline. We used the built-in contacts of our templates to independently assess the electrical properties of the polyaniline nanoribbons and of the polyaniline/platinum contacts. The resulting electrical characterizations were done by both two-point and four-point configurations.

The approach we developed in this thesis gives us precise control over nanowire geometry, orientation, and location, and the nanowires can be organic, conducting, insulating, or semiconducting materials, and even the combinations. The procedure is general, allowing the synthesis of different materials inside the nanochannels, and opens the door to ''grow-in-place'' manufacturing.

Charles L. Randow

Advisors: G. Gray, F. Costanzo
Committee: A. Belmonte, J. Cusumano, E. Mockensturm, J. Todd

A Directed Continuum Model of a Columnar Thin Film

One of the main tasks of engineers and scientists is to develop accurate models of materials or structures observed in reality. Such models are essential for researchers studying how best to use a particular material or structure in a given application. Ideally, such models can even give insights into how something behaves before such behavior is even observed in an experiment. Different approaches used in modeling have different advantages and disadvantages. For example, some models take into account highly detailed information about the nature of matter at an atomic scale. The trade-off for such detailed knowledge is that we are only able to model relatively small amounts of matter for small periods of time. On the other extreme, classical continuum models are capable of describing the behavior of large objects over long periods of time. The trade-off for this type of knowledge is that we must greatly simplify our understanding of reality. For example, in classical continuum theories we consider material to be completely uniform, effectively ignoring the fact that atoms, crystal grain boundaries, or any small cracks exist. Even so, classical continuum theories are used in many engineering applications every day throughout the world.

One area where classical continuum theories may not adequately predict a material's behavior occurs when our material or structure is very small and atomic interactions must be explicitly accounted for, e.g., the nanoscale. There is no doubt that the field of nanotechnology will prove the source of many of the technological developments in the 21st century. It is therefore critical that scientists and engineers be engaged in fundamental research, which includes understanding and modeling a wide variety of nanoscale phenomena.

The goal of this research effort is to propose a continuum model that is capable of accounting for long-range atomic interactions. This proposed continuum model cannot be classical since we must account for long-range interactions. On the other hand, we want to take advantage of the benefits of a continuum approach, namely the ability to model large systems over long periods of time. The model being proposed is actually based on work nearly 100 years old that was published in 1909 by brothers Eugéne and François Cosserat. In a classical continuum model, each point of a material in the model is thought of in a straightforward manner as a point of the material in the real world. That is, when an object stretches in the real world, the points of the model comprising the body likewise stretch. In a Cosserat-type of model, each point contains additional information; in principle, each point may contain any information the modeler desires. Since the Cosserat-type of model contains more information than a classical continuum model, it is capable of modeling much more complicated properties. In this work, a particular nano-scale structure serves as the motivation for a nano-scale Cosserat-type continuum model.

The primary result of this research is the development of a model based on the structure of a nano-scale thin film. Included in this development is a scheme that characterizes the behavior of the film and accounts for variations in the behavior. A number of different possible uses for such a theory are suggested (including thin film buckling and thin films with varying curvature) and results from the model are compared with results published by experimentalists. It is hoped that this model will serve as a benchmark for future models incorporating nonlinear interactions, nonlinear motions, and time.

Yanan Sha

Advisor: V. K. Varadan
Committee: J. Abraham, P. Lenahan, O. O. Awadelkarim, J. Ruzyllo

Design and Development of Integrated Polymer-Based Acoustics

The steadily growing needs for clinic diagnosis, imaging, structure evaluation, and underwater detection have promoted the development of low-cost, highly sensitive piezoelectric polymer sensors. Since the discovery of PVDF and PVDF-TrFE in 60's, piezoelectric polymer materials have been regarded as alternative transducer materials to PZT and single crystals due to their wide angular response and a broadband, flat frequency response. Additionally, their better acoustic impedance match with water and tissue offer them wide biomedical and underwater application. Today, piezoelectric polymer sensors are among the fastest growing of the technologies within the $18 billion worldwide sensor market.

On the other hand, the tremendous growth of IC industry has also propelled the research and development of integrated polymer sensors in diverse applications. In the last two decades, many research work were carried out to integrate piezoelectric polymers with silicon ICs to fabricated various device, such as acoustic transducers, pyroelectric detectors, infrared image sensors and robotic tactile sensors. The integrated sensor technology not only reduces the interference between sensing elements and electronics, but also offers better performance and design flexibility for sensor arrays. In the early research on integrated sensors, single MOSFETs are employed to interfere with piezoelectric polymer due to the limitation of the design and fabrication technology. Many special techniques, such as micromachining and inserting dielectric layers, were employed to improve the device sensitivity. In recent year, with the fast growth of IC industry, IC chips with more complex circuit structures are employed as the signal readout circuits for integrated sensors in order to ensure good performance of the overall sensors.

In this thesis, two types of integrated PVDF-TrFE based acoustic sensors, integrated PVDF-TrFE based acoustic sensor and integrated on-chip PVDF-TrFE based CMOS acoustic sensor are developed. For the first type of device, the PVDF-TrFE film are prepared on silicon wafer and then integrated with interfacial circuit on the PCB board level. Charge amplifier is employed in the signal conditioning circuit in order to amplify the PVDF-TrFE electrical signal and minimize the parasitic capacitance of PVDF-TrFE film and connecting wires. As the basic structure for charge and voltage amplifier, differential amplifier is introduced and explained in detail. When connected with piezoelectric polymer sensor, it effectively amplifies the electrical signal corresponding to the physical input while rejects the noise due to the environmental changes. The final device is encapsulated by Rho-C rubber for underwater measurement.

For on-chip integrated PVDF-TrFE based CMOS acoustic sensor, the PVDF-TrFE film and interfacial circuits are integrated on silicon chip level. The signal conditioning circuits are designed to distribute along the edge of a 2mm by 2.5 mm silicon die. All of their inputs are connected to a large metal pad located in the middle of the silicon die, which is the interface to PVDF-TrFE film. The CMOS interfacial circuit is designed based on MOSIS AMIS 0.5 technology and a standard mix-signal circuit design procedure is followed. The layout design and circuit simulation are provided in detail. Additionally, the sample preparation for PVDF-TrFE film and its integration techniques with silicon wafer are presented.

Paul Sunal

Advisor: M. Horn
Committee: R. Messier, S. Ashok, S. Tadigadapa, R. Collins, J. Todd

Evaluation of Momentum Effects on Ti-Si-N Nanocomposite Material Properties Prepared by Pulsed dc Reactive Sputtering

This thesis informs individuals of an emerging technology called ''superhard nanocrystalline composite thin films''. These thin films are coatings no thicker than a fraction of the width of a hair, but can protect underlying surfaces from various types of wear that occur in harsh working environments, such as on a jet engine rotor operating in a sandy, desert environment.

The promise of a material that is as hard as diamond appears to finally be closer to reality. The particular material is a mixture of two binary compounds: titanium nitride (TiN) and silicon nitride (Si3N4). When TiN forms crystals of a sufficiently small size (5-10 billionths of a meter, called nanocrystalline in this size regime) and Si3N4 coats the interfaces of these TiN nanocrystals, the mechanisms responsible for softening a material are arrested. This new potentially superhard material is yet to be fully understood and the as hard as diamond properties must still be replicated on a large scale by researchers across the world.

To understand why this material is so hard, the fundamental principles required to fabricate superhard materials must be understood. The necessary laboratory setup consists of a vacuum system capable of removing 99.9999% of the air from the chamber and two material sources, known as sputter guns, inside the chamber. The nanocrystalline material is deposited onto another material called a substrate. For most applications today, this substrate is steel. Using a technique called sputter deposition, the nanocrystalline material is created slowly in a very thin sheet that covers the surface of the substrate. This is why the material is termed a nanocrystalline composite thin film.

Using an instrument called a nanoindenter, the fabricated thin films are tested for their hardness. Using a diamond pyramidal tip, the instrument presses into the film and the resistive force is measured. Published results show a maximum hardness that is comparable to that of diamond (60-100 GPa). The impression left by the nanoindentation also shows these thin films to be quite tough, which means they will not shatter like a ceramic when a large force is applied. The maximum hardness achieved by the films fabricated here is 33 GPa; to put this value in perspective, it is 5 times harder than hard alloy tool steel. An issue of oxygen contamination within these films has been confirmed as the limiting constraint with respect to hardness, since it reacts with the Si3N4 portion of the nanocomposite and weakens the overall film.

Not only is the oxygen contributing to the lower hardness, but also the process of measuring the hardness. The instrument uses very shallow indents to measure the hardness and is on the order of 5 to 20% of the total film thickness. When the surface of the film is rough, it throws off the measuring process and gives erroneous data. Using an indent depth that is deeper than the depth of the surface roughness, yet not deeper than 15% of the film thickness is necessary if good data is desired.

An understanding of the complex processes involved with the film deposition that takes place inside the vacuum chamber was completed as well. This includes analysis of the plasma (ionized gas) and its role in determining the physical and chemical properties of the resulting nanocomposite thin film.

It appears that the plasma properties can be used to engineer the physical and chemical properties of the thin films. By using one variable, the momentum-per-atom arriving at the growing film surface during deposition, one can engineer properties such as the nanocrystal size and the structure of the nanocrystals at atomic, nano- (10×), and micro- (10,000×) scales---all of which determine the hardness of the thin film. This momentum-per-atom is simply the mass times the velocity of the arriving atoms at the film surface (similar to the pieces of puzzle falling and stacking in a game of Tetris). In general, the higher the momentum-per-atom, the higher is the hardness. This is attributed to the densification of the thin film, which stabilizes the nanostructural movement of the crystals under high mechanical loads and limits the amount of oxygen that can get into the film.

The main driving force for investigation into nanocomposite thin films, besides advancing scientific understanding, is the loss of over $300 billion annually (around 7% of the Gross National Product) to corrosion and wear failures. Use of these nanocomposite thin films could reduce this figure to some extent, which is continuing to grow since that 1990 estimate. All in all, these superhard thin films will give various industries a definite advantage over the current materials used in a host of applications. Benefits should be shared among the automotive, metal forming, and microelectronics sectors of the economy.

Fei Wang

Advisor: A. Lakhtakia
Committee: R. Messier, M. Horn, B. Tittmann, V. Gopalan, J. Todd

Optics of Slanted Chiral Sculptured Thin Films

Sculptured thin films (STFs) are a class of nanomaterials that emerged during the 1990s from the widely used columnar thin films. The microstructure of an STF is an assembly of virtually identical, curved, and parallel nanowires with diameter 10-100 nm. A wide variety of two-dimensional and three-dimensional nanowire morphologies can nowadays be realized on large area substrates. Being porous, STFs contain voids of characteristic shapes and sizes. As these voids can be filled with different materials, STFs can function in many different ways and for many different applications in optics and biotechnology. For example, STFs have been designed, fabricated, and tested to filter out selected frequencies of light, change the handedness of light, and optically sense infiltration by moisture. Research on using STFs as biochemical sensors, frequency-tunable lasers, optical pulse-shapers, and transmission-inhibition materials is also occurring.

When the nanowires of a STF are shaped as helixes, the film possesses the property of structural handedness, also known as chirality. Light can also be handed. The attraction of chiral STFs is attributed to the circular Bragg phenomenon displayed by them. Briefly, normally incident light is mostly reflected within a certain wavelength-regime when it has the same handedness as that of the chiral STF; while reflection is little when the handednesses of light and the chiral STF are opposite to each other.

The helical nanowires of a chiral STF stand upright on a substrate. Therefore, the optical periodicity of a chiral STF is unidirectional--along the normal to the substrate plane. It appears possible to fabricate a chiral STF with its helical nanowires slanted at an angle to the normal to the substrate plane. In this way, a slanted chiral STF is formed with optical periodicity both perpendicular and parallel to the substrate plane. The optical importance of slanted chiral STFs lies in the coupling of the dual periodicities: whereas the periodicity perpendicular to the substrate plane gives rise to the circular Bragg phenomenon, the periodicity parallel to the substrate plane leads to the optical response being spatially multiplexed and discrete.

The objective of this thesis is to theoretically establish an optical framework for slanted chiral STFs. This objective is achieved by investigating the optical responses of slanted chiral STFs to several types of excitation sources, such as plane waves, optical beams, and dipoles. These studies would help integrate slanted chiral STFs with semiconductor chips used in the electronics industry.

First, the response of slanted chiral STFs to plane waves is investigated by developing a robust numerical procedure for reflection and transmission calculations. The prominent feature of the planewave response is exhibited by the circular Bragg phenomenon in a non-specular mode--that is, a normally incident co-handed plane wave is obliquely reflected for the most part, when the circular Bragg phenomenon occurs.

Next, the optical response of a slanted chiral STF with a central twist defect is explored. The central twist defect is introduced into the slanted chiral STF by purposely rotating its upper half by an angle about the helical axis of nanowires in relation to its lower half. Due to the twist defect, wave resonance occurs in a very narrow regime, i.e., it is localized within the wavelength-regime wherein the circular Bragg phenomenon occurs. Additionally, there is a crossover phenomenon associated with the localization of wave resonance by chiral STFs: The localization is seen as a hole in the reflection spectrum when the slanted chiral STF is relatively thin, but as a hole in the transmission spectrum when the thickness is large. This remarkable crossover phenomenon is mathematically elucidated, and can be harnessed for the design of ultra-narrowband optical filters.

Finally, the optical responses of slanted chiral STFs to both optical beams and dipolar radiations are examined, which are a step closer to practical applications. In general, an optical beam is laterally shifted on reflection. Particular interest arises in the so-called Goos-Hänchen shift which is the lateral shift of optical beam when it is totally reected. Lateral shifts of optical beam on reflection by slanted chiral STFs are computed, with emphasis on the Goos-Hänchen shift. Being comparable with the dimensions of nano-materials, these lateral shifts are too important to be neglected in nanotechnology. The radiation pattern of dipolar source in the presence of the slanted chiral STF is presented, which expresses the circular Bragg phenomenon in the coordinate space.

Potential applications of slanted chiral STFs are suggested in this thesis as optical beamsplitters and couplers, spectral-hole filters, and biochemical sensors.

Ryan Wolfe

Advisors: B. A. Shaw
Committee: R. Messier, M. Horn, H. Pickering, E. Sikora, J. Todd

Imparting Passivity to Vapor Deposited Magnesium Alloys

Most metals tend to react with oxygen when exposed to the environment. Because of this, we find that most metals are found in nature in the form of oxide ore. Even though we convert these ores to metals in order to make them useful, the natural tendency to revert back to the oxide still exists. When metals slip back into their previous state, expensive and dangerous situations can arise. Bridges collapse, airplanes crash, and cars require new paint jobs. The oxidation of metals is termed corrosion.

Corrosion is a natural although often preventable consequence of making things from metal. The metallic form is a high energy state, which is analogous to putting a ball on the top of a hill. Corrosion can be thought of as the tendency for a metal to revert to its natural state, much like a ball will tend to roll down a hill. This goal of this research is to prevent the metaphorical magnesium ball from rolling down the corrosion hill. This research is important because magnesium is very light, so its utilization would enhance the performance of vehicles and aircraft, for example. However, current magnesium alloys can not be used because they corrode too quickly.

In order to prevent magnesium from corroding, alloying elements must be added. Although the addition of dissimilar metals may often be detrimental to corrosion properties, special processing techniques can be used to attain beneficial corrosion attributes. In particular, it has been shown to be beneficial to produce materials by condensing metal vapor to form a solid metal alloy.

The properties of these solidified alloys can be optimized by controlling the energy of the condensing atoms, in addition to varying the alloying elements and their concentrations. Controlled atmospheric exposure and various electrochemical testing are used to assess the corrosion properties of the alloys, which are affected by many factors. Annealing, or heating of the alloys, alters the stress state. The crystallinity of the alloys is examined using x-ray diffraction (XRD). Scanning electron microscopy (SEM) reveals the structure of the alloys. X-ray photoelectron spectroscopy (XPS) shows that the alloys attain passivity by an enrichment of passivating alloying elements (Ti, e.g.) in the outer (passive) film.

This research has broad societal implications. The magnesium alloys developed in this work are more corrosion resistant than any commercial magnesium alloy. These new materials can be used to produce engineering components and vehicles with unprecedented combinations of lightness and durability. When utilized in aeronautical equipment, for instance, these low density, corrosion resistant magnesium alloys will allow for higher payloads, greater acceleration, and longer range, among other benefits such as longer inspection intervals.

When materials fail, they need to be replaced. The ensuing primary metal production and subsequent fabrication releases pollutants into the environment. Blast furnaces contribute to acid rain, aluminum smelters release greenhouse gases, and magnesium refinement produces noxious fumes. If corrosion can be stopped, then less material would need replaced, thus preserving the environment. Moreover, the ores and the fossil fuels needed to produce metals are nonrenewable resources which ought not be squandered.

Thomas Yurick, Jr.

Advisors: G. Gray, F. Costanzo
Committee: R. Messier, S. Mohney, M. Horn, J. Todd

An Investigation of Clustering During the Early Stages of the Sculptured Thin Film Growth via Molecular Dynamics

As nano-technology continues to revolutionize our daily lives, nano-engineered materials take on a more prominent role. One example of a nano-engineered material is that of sculptured thin films. Sculptured thin films or STFs are a special class of thin films that have a characteristic shape imparted to them on the nano-scale, during the deposition process. This characteristic shape can be that of a zig-zag, chevron, or helices. Applications for these STFs vary from micro-electronics to medical applications, however, it is most likely that the best use of them is yet to be discovered.

As with any engineering problem, simulation can play a key role in gaining understanding and insight. This is certainly true with the deposition of STFs as well. However, the simulation of an STF entails the capability of simulating each and every atom that makes up the STF. It is the manipulation of the impinging film atoms, during the deposition, that produce the characteristic shape. Luckily, today's fast computer processors coupled with an atomistic simulation method called Molecular Dynamics allows for such a simulation.

This work focused on the use of a custom parallel Molecular Dynamics, or MD, program for the simulation of cluster formation during very early stages of STF growth. Once the simulated thin film morphology was obtained, a qualitative analysis of the simulated thin film morphology was performed by visualizing the thin film surface. A qualitative analysis of the thin film morphology was also performed by estimating the fractal dimension of the simulated surface via the Slit-Island Method.

In an effort to decrease the computational expense, an improved simulation method was introduced which allows for, at least from a computational point of view, the number of particles in the MD TF simulation to remain constant. This was accomplished by creating specialized layers of particles within the growing TF, with each layer performing a specific function, i.e. fixed, thermostated, and free. What is new, is that an additional layer was added, in this work, that contains particles which have been removed from the bottom of the simulation as the film height increases. These removed particles do not interact with other particles in the simulation, and as such, decrease the computational workload which would normally be associated with their presence in the simulation. The new method was shown to be 2.3 times faster than the pre-existing methods for the same simulation size and deposition conditions.

The new MD simulation method was then used to produce a series of simulated TFs, representing the very early stages of the deposition, which were de-posited under low adatom deposition conditions favorable to STF growth. The most evident characteristic effect on the morphology was produced by changing the deposition angle from 0° through 75°, and depositing the TF at 0.5 eV and 300 K. The morphology changed from a nearly complete covering of the substrate by a network of clusters of particles at 0°, to isolated individual clusters of particles at 75°. The change in morphology occurred gradually with the increasing deposition angle, accompanied by more and more of the substrate visible with each increase in deposition angle.

An examination of the effect of deposition energy and temperature on the TF morphology was also performed When comparing the results of an increase in deposition energy from 0.1 eV to 2.0 eV, the higher energy was accompanied by a greater coverage of the substrate than the low energy case, and in general, larger and flatter clusters of particles. The 0.1 eV case was characterized by much smaller clusters and much more of the substrate visible. It was clear that the additional kinetic energy provided for increased adatom mobility of the de-posited particle. Finally, at least visually, the change in deposition temperature provided the least visual evidence of a change in morphology.

Accompanying the qualitative analysis of the simulated TF morphology, a quantitative analysis was also performed using the Slit-Island method. The TF surface was analyzed as a function of time during the deposition. Starting with the surface of the substrate, the fractal dimension began at 2.86, and in general, decreased as the TF grew. The amount that the fractal dimension decreased, and the time during the simulation that it decreased, varied from simulation to simulation. When considering the change of fractal dimension, as a function of time, for increasing deposition angle, for 0°, 35°, 45°, and 55°, there is a distinct transition in the morphology, from a relatively high fractal dimension to a lower fractal dimension, which occurred around the midpoint of the deposition. This transition was not observed for deposition angles 65°, and 75°, for which the decrease was more gradual. The change in fractal dimension of the final TF morphology for each deposition angle is also interesting. Essentially, the fractal dimension of the final morphology begins at 2.55 for 0°, increases to a maximum of 2.76 for 65° and then decreases to 2.55 for 75°.

Lastly, the variation of fractal dimension with deposition energy and de-position temperature was also examined. The morphology resulting from a deposition energy of 0.1eV was compared to the morphology resulting from a deposition energy of 2.0 eV. In general the fractal dimension was lower for the higher deposition energy, and was true throughout the simulation. Like-wise, when the morphology of a simulation deposited at 100K was compared to that of a simulation deposited at 700 K, the higher temperature produced a lower fractal dimension throughout the simulation. Something that was not particularly evident when comparing the morphology visually.

These results, in themselves, are not unexpected. It is well known that in-creasing the deposition energy or temperature produces a smoother film, or that changes in the deposition angle produce self-shadowing. However, the ability to simulate a TF, detect and visualize the surface, and then quantify changes in the morphology is what is important. Perhaps even more important, is the ability to experimentally deposit a TF and directly compare it to a simulation, both qualitatively and quantitatively.

Li Zhang

Advisor: J. Rose
Committee: B. Tittmann, C. Lissenden, N. Salamon, E. Smith, J. Todd

Guided Wave Focusing Potential in Hollow Cylinders

Acoustical waves at a frequency above 20 kHz are called ultrasonic waves, which cannot to be heard by the human ear's. There are two different types of ultrasonic waves: bulk waves and guided waves. A bulk wave is the wave propagating in an infinite medium and is not affected by the boundaries of the medium. A bulk wave in air can be heard as a train whistle as reflected from a far away mountain. Guided waves include the ultrasonic waves in bounded waveguides, such as plates, pipes, and rods. Due to their excellent penetration possibility and great inspection sensitivity, ultrasonic waves are widely used in the non-destructive evaluations (NDE) of various structures.

Bulk waves in elastic isotropic materials involve longitudinal waves and transverse waves. The particle motion direction of a longitudinal wave is parallel to the wave propagation direction (a gas explosion underground); the particle motion direction of a transverse wave (also known as a shear wave) is perpendicular to the wave propagation direction (a shear force from an earthquake). The dominant particle motions of the so-called longitudinal guided waves in a hollow cylinder are either in the axial direction or in the radial direction; the torsional guided waves involve dominant circumferential-direction particle motions. Based on the particle behavior, the longitudinal or torsional guided waves can be separated into individual mode groups. Each group contains one axisymmetric wave mode with axisymmetric energy distribution and infinite flexural modes with non-axisymmetric energy distributions. All the wave modes in one mode group have pretty similar particle motions. By applying an axisymmetric excitation source, one can only generate axisymmetric guided waves in an elastic isotropic tubular structure.

Prior research shows that a group of guided waves in cylindrical structures can be focused at a predetermined spot by aligning the phases of all the major wave modes. There are usually two ways to focus at a particular circumferential position in a pipe: 1) using a partially loaded transducer to make the ultrasonic energy be naturally focused; 2) applying input time delays and amplitude controls of a multi-channel phased transducer array to focus at a preconcerted point.

However, the natural focusing and the phased array focusing are not always applicable. First, the transducers themselves have many limitations to achieve focusing. The excited wave types and groups are decided by the incident waves generated by the transmitters. In addition, appropriate excitation frequencies and reasonable transducer sizes are important for controlling energy distribution in pipes. Second, the properties of the hollow cylinder itself also strongly affect the focusing potential. The energy distributions in a pipe change with the pipe size and material. Some geometry and material inhomogeneities, such as elbows, branches, and anisotropic welds, may also influence the guided wave behavior in a tubular structure.

The purpose of this research work was to investigate the focusing possibility in hollow cylinders. The frequency and angle tuning (FAT) technique was presented to enhance the natural focusing inspection and to make sure that a pipe is thoroughly scanned for defects. Influence of the excitation source for natural focusing potential and phased array focusing potential was also studied. Contour charts of focusing potential at particular distances in a pipe were employed as a directory to find a right transducer size and frequency to carry out focusing. When guided waves are focused beyond small defects and transversely isotropic welds, the focal location is not affected by these geometry or material inhomogeneities. Nevertheless, a seam weld leads to anomalous variations of energy distributions and inaccuracy of phased array focusing. Also an elbow in a pipe extremely changes the guided wave behaviors. The so-called time reversal technique is used to achieve phased array focusing beyond an elbow. The time reversal technique utilizes a phased array as a time-reversal mirror (TRM), which records the signals from either a transmitter or a reflector at the preselected focal point and then re-emits the time-reversed signals. This time reversal technique can be implemented to focus beyond an inhomogeneous area in an isotropic hollow cylinder.

Nanyan Zhang

Advisor: V. K. Varadan
Committee: J. Abraham, M. Urquidi-Macdonald, B. Tittmann, Q. Wang, J. Todd

Carbon Nanotubes Based Functional Materials for MSL and Biosensor Applications

Carbon nanotubes (CNTs) have stirred intensive investigations during the last decade due to their tiny size (nanomete in diameter and micrometer in length) and unique properties. The exceptional properties already observed in CNTs include, but not limited to, their remarkable mechanical strength, superb electrical and thermal conductivity, high chemical stability, and high accessible surface area for adsorption. Consequently, carbon nanotubes are one of the most promising candidates as materials in several diverse high-end applications, which can be roughly classified into nano-scale and macro-scale based on the size of the final products. In the nano-scale range, carbon nanotubes can be used in nanofabrication, nano-electronics and devices, biotechnology, and chemistry, etc. In the macro-scale range, carbon nanotubes have applications as the reinforcement filler for composites with high mechanical, electrical, and thermal properties, electrode for rechargeable Lithium ion battery, and support for catalysts and so on.

Since carbon nanotubes consist of pure carbon element with highly conjugated stable structure, they are insoluble in any known organic solvents. The insolubility makes it extremely difficult to explore their properties and applications. Even worse, carbon nanotubes have a propensity to aggregate to bundle or wrap together due to high surface energy and surface area. Therefore, manipulating single CNT or dispersing bundle CNTs is very difficult, and this is the bottleneck for their potential commercial applications.

This thesis is proposed to synthesize functional materials based on carbon nanotubes and to explore possible applications of the produced materials. Carbon nanotubes used in this thesis were synthesized by microwave chemical vapor deposition (MWCVD) method and carefully purified to remove impurities. The purified carbon nanotubes were further oxidized by oxidant named permanganate in organic solvent using phase transfer catalyst. Several functional groups, namely, hydroxyl, carboxyl, carbonyl, or quinonic groups have been successfully attached to the surface of the CNTs. As compared with other reported methods, our novel functionalization procedure gives significantly higher yield and high functional group density. These functional groups make the CNTs soluble in common solvents such as water, DMF, and DMAc, so that wet chemistry becomes possible in future applications. More importantly, these abundant functional groups are highly reactive and have been used as the grafting sites to chemically connect the CNTs with other organic polymers or bio-molecules (proteins). The reactions of the hydroxyl and carboxyl groups with isocyanide and epoxide have been utilized to successfully bind UV-curable monomers with carbon nanotubes to form UV-curable solution, which can be cured by MicroStereoLithography (MSL) UV light laser. The MSL system can solidify UV-curable solution layer by layer to the form patterned 3D micro-structure. This chemically bonded nanotube solution has potential applications in UV coating and MicroElectroMechanical system (MEMS) devices. The functional groups can also facilitate the fabrication of well-dispersed uniform CNTs/polymer nanocomposites. As an example, hydroxyl and carboxylic acid groups at the CNTs surface can react with water soluble polymers such as polyvinyl alcohol (PVA) and polyacrylic acid (PAA). The carbon nanotubes were wrapped into the long polymer chains and the resulting nanocomposites have good solubility in water and polar solvents. Furthermore, since the carbon nanotube-reinforced composites have excellent electrical conductivity, they were used as the electrode of electrochemical biosensors. With glucose oxidase (GOx) enzyme as the model biomolecule, the performances of enzyme/CNTs electrode were fabricated to detect the trace of GOx in diluted solutions.

Rui Zhang

Advisor: R. Engel
Committee: N. Salamon, C. Lissenden, R. German, P. Michaleris, J. Todd

Numerical Simulation of Solid-State Sintering of Metal Powder Compact Dominated by Grain Boundary Diffusion

This research focuses on the study of the dimensional change during the manufacturing processes of metal powder parts. Industrial products such as gears and filters can be made from loose powders that are pressed tightly in molds. After compaction, the part is weak because the particles are loosely joined. An additional step to bond the particles together occurs when the part is subjected to a sintering cycle at high temperature in a controlled environment. In addition to the part being stronger, it is also denser.

The problem in the sintering process is that while the part becomes denser, it also changes shape and size. These changes may be undesirable if they require extra cost to machine the parts after sintering. The goal of this research is to develop a numerical model that can predict the shape change and thereby reduce the cost. In order to achieve this task, three questions are to be answered. First, how can we link the material characteristics of the problem with the mechanical response. Second, how can we prove that the model is valid and appropriate for this problem. Third, how can we implement the model with a computational approach that can be practiced and verified. All of the three questions have been investigated in this research and the numerical model has been applied to the sintering simulations of cylindrical stainless steel 316L and bronze parts. The experimental and numerical results that predict axial shrinkage consistently agree for a range of particle size distribution. The result refines the understanding of sintering and provides an approach that can be expanded to address complex shapes that are encountered in industrial applications.

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This page was last updated on January 18, 2006.