Advisor: J. L. Rose
Committee: R. German, B. Tittmann, R. Engel, C. Ruud, J. Todd
An Ultrasonic Tomographic Signature Approach for Material Property Gradient Analysis in Powder Metals and Natural Wave Guide Structural Health Monitoring
Computerized tomography (CT) is a cross-sectional image reconstruction technique via collecting energy data (x-ray, radioisotopes, ultrasound, etc.) along the perimeter of an object in some pattern. First introduced to the field of diagnostic medicine with an x-ray source, CT technology has been successfully exploited in many areas, including x-ray and ultrasonic CT in diagnostic medicine, bore-hole ultrasonic CT in the geophysical field and x-ray and ultrasound CT in the nondestructive evaluation (NDE) field. In this thesis work two engineering applications of the CT technology were studied: ultrasonic tomography for powder metals and ultrasonic tomography for natural wave guide monitoring.
Powder Metallurgy (P/M) is an attractive metalworking technology, which shapes metal particles directly into near net-shaped parts by compaction and sintering. Key steps of P/M include preparing of metal power, compaction of metal powder into solids with close shape and reasonable strength (green parts), and sintering of the green parts into high density, high strength, near-shape parts. Due to its capability to produce complex parts with low-cost and high productivity, P/M finds extensive applications in industries such as automobile, medical, nuclear and manufacturing. Despite its success, one technological limitation of P/M fabrication process is the shape distortion of the final compacts, which, if successfully overcome, will lead much broader application of P/M technology.
In the need of an effective nondestructive way to facilitate the study of powder metallurgy manufacturing process, the first part of this thesis focuses on design and realization of a Laser based ultrasonic bulk wave tomography system for density distribution testing. Laser beams are applied to excite ultrasonic waves in cylindrical powder metal samples, and air transducers are exploited to receive the transmitted ultrasonic waveforms. Due to the similarity of image area shape in this study with those in medical X-ray CT applications and the high resolution requirement on the resulting images, a filtered back projection algorithm is selected for image reconstruction, which is famous in X-ray CT discipline. Results show that this tomography technique is a good candidate of nondestructive study of powder compacts with the potential of being applied for in-situ process monitoring.
The second part of thesis focuses on ultrasonic guided wave tomography for nature wave guide monitoring. Ultrasonic guided waves are special ultrasonic waves which propagate along natural wave guided such as plates and pipes. Ultrasonic guided wave tomography is a new and promising method that combines the advantages of both tomography and guided waves. It exploits the CT principle while taking ultrasonic guided waves as an energy source. As a method applying guided waves, it has unique applications for natural wave guide structural health monitoring.
Sparse array guided wave tomography is investigated as a low resolution imaging methodology for health monitoring which provides many advantages. First, the data collection procedure is not as tedious as other tomography systems because of its low data amount requirement. In this study, one system has only eight transducers and another sixteen transducers. Even though no scanning of the structure is needed, the resulting precision is sufficient for most industrial purposes. Second, in a sparse array case, direct matrix inversion techniques are feasible for tomographic reconstruction which leads to a fast reconstruction procedure. In addition, this system is robust in a sense that it is less sensitive to signal noise contamination.
Advisor: R. M. German
Committee: R. Engel, D. Green, I. Smid, C. Binet, J. Todd
The Micromechanical Influences on the Constitutive Laws of Sintering for a Continuum Model
Particulate and powder materials are used extensively to manufacture net-shape products, parts that resemble the final desired shape and therefore require minimal or no secondary operations. The processes involved in producing these net-shaped parts involve consolidation of the powder into a slightly larger net-shape preform, and then sintering of the preform into a densified part with the desired final dimensions. Sintering is a thermally activated process that bonds the powder particles together to form a densified material. Intermediate thermal processes are often used to remove binder that sticks the powder particles together during the consolidation step, especially in the case of powder injection molding. The green or unsintered preform typically experiences some shrinkage during sintering as the porous material densities. In order to produce dimensionally precise final parts, it is necessary to be able to predict the deformation that takes place. Generally tooling is designed by an expensive trial and error process, to determine the deformation that will take place during sintering. Accurate computer simulations of sintering deformation can significantly reduce time and cost considerations for tooling design. To this end, constitutive equations governing the rheological response of the porous material during sintering, are used to simulate part shrinkage and distortion, therefore providing a useful and cost-effective solution for tooling design.
The constitutive equations of sintering are applied to a continuum model by viewing the consolidated powder compact as a continuous, porous medium rather than a collection of discrete particles and pores. Experimental research suggests that a linear viscous flow model, similar to creep models, provides the most accurate description of the material response of the porous continuum during sintering. Therefore, the constitutive equations of sintering describe the linear viscous strain rate response to stress, and are characterized by key material and system dependent parameters. In this thesis, these key parameters are identified as apparent viscosity, effective shear and bulk viscosity, and sinter stress. The viscosities describe the resistance against flow and how it changes with density and temperature, while the sinter stress describes the potential for flow and densification. These parameters define the Newtonian form of the linear viscous response model. A new parameter, the critical stress for flow, is introduced and incorporated into the constitutive equations for sintering, altering the equations to a Bingham form of the linear viscous response model. The critical stress for flow indicates a threshold stress value that must be attained before material flow occurs. It can be directly linked to the strength of the porous body during sintering.
This study gives experimental evidence in support of the Bingham modification of the constitutive equations of sintering to include a critical flow stress parameter. Both the Newtonian and Bingham forms of the constitutive equations are programmed into a one-dimensional analytical solution using Visual Fortran, and into a three-dimensional finite element analysis using ABAQUS 6.2-6 with the user-subroutine CREEP. Using these computer-aided analysis tools, critical sintering experiments that were conducted for this study are simulated and the results from both the Newtonian and Bingham formulations are compared. From these results it is clear that the Bingham modification to the constitutive laws offers a more accurate prediction of sintering behaviour, especially when considering prior thermal processing history such as thermal debinding. The shortcomings of both the model formulations arc critiqued and suggestions for improvements and further work are given. These results help in developing more accurate predictions of sintering shrinkage and distortion, therefore making part and tooling design more efficient. They also provide academic insight, into the mechanisms of sintering and the influence of in situ strength evolution on densification and distortion.
Advisor: S. J. Fonash
Committee: O. O. Awadelkarim, A. Sen, M. Urquidi-Macdonald, J. Todd
Electrical Detection of Bio-molecular interactions using Nanometer-scale Gap Electrode Structures
Detecting biological molecules faster and with greater sensitivity has immediate impact on medicine and healthcare. Disease identification and drug development rely on the ability to detect and identify biological chemicals and their interactions. Currently, most detection techniques rely on optical methods such as a color change to determine the presence of a biological compound. The ability to perform these tests electrically has the potential for faster and more sensitive results and the ability to couple easily with computer analysis.
A unique sensor device and technique for detecting biological molecules has been developed. Here, an electrical sensor with nanometer-scale dimensions is made using standard process and equipment found in the semiconductor industry. This sensor has the ability to be chemically modified to detect specific biological compounds. This is the first time a device combining these properties effectively for commercial use has been presented. Processes used in the fabrication of the device are shown to be critical in preserving the ability for chemical modification as a sensor.
A unique chemical linking method has also been developed which allows effective biological modification of the device. This modification of the device allows for specific biological molecules to interact with surfaces of the device. The modification can be used to tailor the device to interact with a range of biological molecules including DNA, proteins and other small bio-molecules. The ability of the chemistry to interact with a variety of different species is important for commercialization, because a versatile platform is more economically viable to develop.
Using an example system, this device electrically detected the interaction of two biological molecules. The detection scheme makes the use of biologically modified gold nano-particles, which change the electrical characteristics of the device. The sensitivity of the device is among the highest reported for electrical techniques, and the technique itself appears robust enough for commercial development.
Overall, this unique and cost-effective sensor offers sensitive detection of biological molecules.
Advisor: J. L. Rose
Committee: O. O. Awadelkarim, V. K. Varadan, B. Tittmann, S. Tirupatikumara, J. Todd
Aspects of Guided Waves in Structural Health Monitoring
Ultrasonic guided waves have been made popular by their capability to inspect larges areas of aircrafts and long lengths of pipe from a single sensor position. This is in comparison to conventional nondestructive testing (NDT) techniques that require mechanical movement of sensors to inspect large areas. These techniques include conventional ultrasonic and eddy current testing.
Structural health monitoring (SHM) is a more modern version of NDT. In this thesis, SHM is defined as a monitoring technique in which sensors are embedded in or mounted on a structure permanently. These sensors are used to provide feedback on the mechanical integrity of the structure through analysis of a measurable feature. Some structural characteristics commonly used to provide feedback on mechanical integrity include vibration, temperature, material interaction with ultrasound, conductivity, and material interaction with radiation. The monitoring frequency can be real-time, based on flight hours for aircraft, or service life for pipelines. Since SHM applications require technology that is compatible with permanent installation and long term monitoring a proposed technology must, at minimum, meet the following requirements: 1) Sensors must not affect performance or structural integrity of component 2) Sensors must be show potential for large volume installation 3) Sensors must be capable of inspecting large areas of structures without mechanically moving the sensors 4) Sensors must be able to withstand the typical in-service monitoring environments 5) sensors must be compatible with wireless technology. This thesis evaluates to what degree guided wave technology satisfies the above requirements.
Advisor: V. K. Varadan
Committee: J. Kollakompil, O. Awadelkarim, J. Xu, J. Ruzyllo, J. Todd
A Four Element Phased Array Antenna System Monolithically Implemented on Silicon
Steadily increasing need for wideband wireless communication services have promoted the development of wireless communication systems with higher data rates and increased functionality. Phased array antennas are well suited to satisfy the growing demand with its ability to increase channel capacity and steer multiple beams. Of the various types of antennas, microstrip antennas would be a good common element in constructing the array antenna due to their low cost, low weight, conformability, and easy integration into arrays or use with microwave integrated circuits.
In this research work, a four element phased array antenna aimed for 15GHz has been monolithically implemented on silicon substrate using monolithic microwave integrated circuits (MMICs) technology. The array fabricated herein consists mainly of microstrip radiating patches and feed networks including coplanar waveguide (CPW)-to-Microstrip (MS) line transition, phase shifters, Wilkinson power dividers, and DC blocking filters for CPW and MS lines. Each component of the fabricated array antenna was carefully designed for operational efficiency, and validated using a custom simulation tool. All circuits were realized on a high resistivity silicon (HRS) substrate surface-stabilized by polysilicon. This configuration achieved a significant reduction in RF losses by immobilizing the surface charges populated in the interface of SiO2/Si. The monolithic integration of the array antenna into silicon not only makes the whole circuitry compact, but also reduces the cost utilizing mature CMOS technology.
A single microstrip patch showing a resonance frequency of 14.8GHz with a return loss (S11) of 21db is connected to the feed networks based on CPW lines through a CPW-to-MS transition. This transition, as well as DC blocking filters for both CPW and MS lines exhibited the possibility for wideband applications by showing wide 3dB bandwidths of 168%, 123%, and 130%, respectively. Two types of phase shifter designs were constructed to compare performance: a microelectromechanical system (MEMS) phase shifter, and a ferroelectric phase shifter. Despite a high operating voltage of up to 300V, the ferroelectric phase shifter utilizing permittivity tunability of (Ba,Sr)TiO3 (BST) films was adopted as the phase shifting device in the array due to its high phase shift capability, low leakage current level and notably high operational reliability. The four element phased array antenna completely integrated on silicon showed a total scan capability of 10o measured at its resonance frequency, 14.85GHz with a return loss of 32dB.
The phased array antenna presented herein will provide a basic view and understanding of the process of monolithic integration into silicon using MMIC technology. Improvements of antenna performance in terms of steering capability, side lobe level (SLL), half power beam width (HPBW) and bandwidth could be accomplished by further research on design modification as well as on process optimization for array antennas.
Advisor: O. O. Awadelkarim
Committee: S. Ashok, M. Horn, J. Ruzyllo, C. Wronski, J. Todd
Study of Thin Silicon Oxides and High-K Materials for Gate Dielectrics in Metal-Insulator-Si Structures
Complementary metal-oxide-Si (CMOS) technology has become the dominant very-large-scale-integrated (VLSI) circuit technology in the last few decades. The promise of such dominance is to speed up the CMOS device while in the same time increase the integration density to reduce the overall cost. This is made possible by continuous miniaturization of transistor's dimensions, e.g. shorten the channel length to reduce the carrier traveling time from drain to source, reduce the gate oxide thickness, minimize the transistor area, etc.
However, it appears that the fundamental limits of the materials constituting the building blocks of the planar CMOS process will be reached soon, and the physics behind device operation will begin to change to accommodate quantum physics effects. Present research focuses on two important questions. The first question is what is the impact of ultra-thin gate oxide (<5 nm) on device fabrication and operation? Or in another word, given the role of the oxide in device characteristics, what is the ultimate limit of oxide thickness reduction? The second question is, if such limit exists, do we have an alternative way to further scale down the CMOS device?
Trying to answer the first question, the first part of this thesis research is on the study of the various issues related to miniaturization and scaling of MOS structures. We focused on thin silicon dioxide that is currently in use as a gate dielectric in CMOS but is steadily reduced in thickness to meet CMOS standards. The severity of damage to the silicon dioxide induced by processing, especially plasma etching is one of the research areas addressed in this thesis. Effective ways of characterizing the thin oxide and, hence, evaluate MOSFETs and MOS capacitors performance are discussed. The gate leakage current is demonstrated to be the only transistor parameter which can detect plasma-processing induced changes in oxide charge and interface states. Subsequently, we subjected the oxide to Fowler-Nordheim and hot carrier stresses, thereby mimicking device damage by processing and/or device aging.
To further extend the device scaling approach, high-k dielectric materials are introduced in the basic CMOS structure to replace and/or augment the existing ones. In the second phase of this thesis research we examined a number of high-k candidates to replace the oxide as a gate dielectric. These high-k dielectrics are investigated in terms of their dielectric, interface, leakage and breakdown properties. The interfacial layer between the high-k dielectrics and the silicon substrate, either grown intentionally or unintentionally, are examined and their effects on the electrical properties of the gate dielectric stack are analyzed in details. Also, the impact of high-k dielectrics deposition on the underneath silicon substrate is investigated in the course of thesis studies. It is revealed, for the first time, that the accumulation capacitance of the MIS with the high-k dielectric is strongly dependent on the measurement temperature and frequency.
In conclusion, we have examined capacitors and transistors with various gate dielectrics of different dimensions and prepared using several growth techniques. The new challenge of characterizing process-induced-damage in ultra-thin-oxide has been addressed and discussed. The new characteristic of high-k dielectric gate-stack-system capacitance has been observed and analyzed.
Advisor: C. J. Lissenden
Committee: C. Bakis, F. Costanzo, P. Michaleris, A. Segall, J. Todd
Plastic Flow in a Discontinuously Reinforced Aluminum Composite Under Combined Loads
Discontinuously reinforced aluminum (DRA), particularly an aluminum alloy reinforced with ceramic particles such as silicon carbide or alumina, has become a viable structural material system for applications in the automotive, aerospace, aeronautics, and recreation marketplaces. DRA composites combine the properties of a ductile metal with those of brittle particles. Their tailorable mechanical properties such as low density, high specific modulus, high specific strength, high thermal conductivity, control of thermal expansion, and excellent wear resistance enable them to potentially meet the highest requirements in a variety of engineering designs. Since many structural components are subjected to complex loading conditions when in service, a better understanding of plastic behavior of DRA under combined loads is demanded. This thesis aims to experimentally describe plastic deformation and particle fracture in a DRA composite, and hence improve the material modeling of DRA, which in turn will help design DRA products more safely and efficiently.
The effect that various loads have on a particle reinforced aluminum composite (6092/SiC/17.5p-T6) is first determined. The damage evolution is described by means of the variations of Young's modulus, shear modulus and Poisson's ratio due to cyclic loading. Cyclic tensile straining to increasingly higher amplitudes indicates a modulus reduction of 16 prior to fracture, strongly suggesting accumulation of internal damage, but no change in the elastic Poisson's ratio is observed. In contract, cyclic compressive loading results in no observable change in modulus, but an increase of 12 in the elastic Poisson's ratio. Cyclic shear loading leads to a small shear modulus reduction of approximately 6.
Afterwards, initial and subsequent yield surfaces in the axial-shear stress plane are determined. Typically, a yield surface can be constructed by connecting a locus of yield points that are detected in the laboratory. In doing so, a more comprehensive method of yield surface determination is developed for DRA. Changes in the yield surface due to mechanical loading helps describe how the material hardens due to plastic deformation. It is observed that the subsequent yield surface for DRA translates as a rigid body in the stress plane, maintaining the size and shape of the initial yield surface. Therefore, hardening of DRA is believed to be primarily kinematic.
Based on experimental observations, a continuum plasticity model that represents a material with a tension-compression strength differential effect, pressure sensitivity and nonlinear kinematic hardening is proposed to predict plastic deformation. This type of material model is typically used in structural analysis of load bearing components. In order to implement the proposed model with the finite element method, a corresponding algorithm is developed as a user material subroutine of ABAQUS, a general-purpose finite element analysis tool. Model predictions are in good agreement with experimental results.
Advisor: S. J. Fonash
Committee: R. Messier, J. Xu, M. Horn, J. Ruzyllo, J. Todd
Nano-Structured PECVD Silicon Films and their Device Applications
As the foundation of the modern microelectronics, silicon is the most studied of all materials. Depending on the amount of the atomic order possessed, the silicon materials can be divided into three classifications:
The single crystal silicon is produced using Czochraski growth and float zone growth techniques. Most of the current secrete electronic devices and integrated circuits are built on crystal silicon wafers. Amorphous silicon and polycrystalline silicon are normally in the form of thin films deposited by physical vapor deposition (PVD) (including evaporation and sputtering) and chemical vapor deposition (CVD) (including various plasma enhanced CVDs and low pressure/air pressure CVD). For electronic device applications, these materials normally need to have as much structural order as possible. For example, in uses where polycrystalline silicon film is the active material of thin film transistors (TFT), the poly-silicon film needs to have large crystal fraction, large crystal size and few grain boundaries. Amorphous Si represents an extreme where order is not attained by hydro9gen is used to chemically tie-up all the defects arising from this total lack of order. It is really a Si-H alloy with a band gap different from that of single and poly-crystal silicon.
The micro/nano-structures formed by the crystalline and poly-crystalline Si materials have showed some unique properties and found numerous new applications in addition to the traditional ones. In 1990 Canham discovered that electrochemically prepared porous Si can emit visible light in room temperature. This stimulated intense research on this material for its potential in realization of Si phototonic devices. Later people found this material can also be widely used in vapor sensing, desorption-ionization mass spectrometry, bio-active material applications, and micro-machining. Under a narrow processing window, a LPCVD deposited poly-Si thin film can be permeable to HF-based etching solutions and it is being used as the sacrificial material in microfabrication. Sculptured Si thin films grown by physical vapor deposition with the glancing angle deposition technique have also been proposed to have optical, chemical, and biological applications.
In this thesis study, a type of Si thin film deposited by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition at low temperature is explored. These films show notable nanometer scale column/void morphology and have a deposition condition dependent large surface to volume ratio and widely spread void sizes. This study demonstrates demonstrate these films have applications in sensors and microfabrication.
First, the ECR-PECVD deposition conditions of the column-void nano-Si films are correlated with the resulting morphology. It is found that these ECRPECVD deposited Si films have a column/void structure consisting of three levels of nano-structure: a first level of tapered agglomerates (clusters) with diameters of 70-80nm in the studied thickness range and second structure level of columns, which make up the clusters, each with a constant diameter of 10-20nm depending on deposition conditions. The second level columns are built-up of the third level structure which is 10-20nm diameter elongated nano-crystalline Si blocks with various degree of crystal fraction depending on deposition conditions. The clusters are surrounded by interconnected nano-voids with radii up to 50nm. The films have large surface to volume ratios and the nano-crystal size, void size, void content, and specific surface area are tunable through adjusting deposition conditions. The void content and specific surface area are as high as 55% and 180 square meter per gram, respectively. Highest deposition pressure/lowest microwave power deposition conditions give largest void content and specific surface area. A growth model is also proposed based on the experimental evidence. This model is used to explain the formation of the tapering first level clusters. The depositions use low temperature processing and, therefore, a wide range of substrates can be used for different applications.
Since vapors can easily condense in a small void (pore) due to a phenomena called capillary condensation, a humidity sensor can be fabricated and used based on these nano-structured column/void Si films. The study in this thesis shows that this sensor has very high-sensitivity (6 orders of magnitude of conductivity change in the studied relative humidity range), high-speed (response time less than 1 second), and low hysteresis. Its behavior is reproducible. It can be miniaturized and integrated with signal processing circuits. Its sensitivity and speed of response are the highest that we have seen in the open literature. This humidity sensor is especially suitable for the application in respiratory monitoring, where sensor's high sensitivity and fast response are essential. In fact, this sensor has already been tested in the breath monitoring on a prematurely born infant (his body weight less than 1.5 kg) and very encouraging results have been obtained.
These nano-column/void films have interconnected void network and large surface area, so they are very easy to be attacked by chemical etchants and can be used as sacrificial materials. Using these films as sacrificial material, a novel technology for transferring high temperature fabricated high performance poly-Si thin film transistors onto flexible plastic substrates is further developed. This technology can avoid the temperature limitation of the plastics on the highest temperature that can be used in thin film transistor fabrication. High temperature poly-Si thin film transistor fabrication and transfer tests with this technology have been done for the first time. Transferred high performance poly-Si thin film transistors on plastic substrate are demonstrated. The effects of temperature, light illumination, and mechanical strain on the released thin film transistors are studied. One of the applications for this technology is in light-weighted, rugged, and rollable flexible display.
Advisor: J. L. Rose
Committee: B. Tittmann, C. Lissenden, Q. Zhang, E. Ventsel, J. Todd
Phased Array Focusing Wave Mechanics in Tubular Structures
A long-term goal of acoustics is ''to see what you hear''. Acoustical waves are mechanical waves that can be divided into different frequency bands. Below 20 kHz it is sonic, which can be heard by human beings. Above 20 kHz it is ultrasonic, which is not audible to human ears but can be heard by some animals, like bats or detected by acoustic sensors. Ultrasonic waves have been applied to many areas, like underwater sonar, medical diagnosis and therapy, structural health and condition monitoring, and material elastic property measurement.
Ultrasonic waves can propagate in pipes for long distances with little attenuation. They are also called ultrasonic guided waves since pipe walls function like a wave guide. This dissertation investigated guided waves in pipe by theory, numerical analyses and experiments.
According to the symmetry in a pipe circumference, all propagating guided waves can be divided into axisymmetric and non-axisymmetric modes. Axisymmetry means uniform distribution in the pipe circumference. Axisymmetric guided wave theory and applications have been studied for over fifty years. Compared to axisymmetric guided waves, non-axisymmetric modes are more complicated because of three-dimensional displacements, although there are many similarities between them. People tried to avoid non-axisymmetric modes because of the complexities. Until the late 1990's the non-axisymmetric guided wave study has started. There are many situations where non-axisymmetric guided waves play an important role. Limited pipe surface access, for example boiler tubes generally have access to only one side, non-axisymmetric guided waves will be generated. Curves in pipe like elbows also generate non-axisymmetric waves. They also come out when waves impinge on irregular defects. To exploit wave energy focusing, non-axisymmetric waves have to be fully understood. According to the dominant displacement direction non-axisymmetric guided wave are categorized as longitudinal (radial and axial plane) and torsional (circumferential direction) modes similar to the axisymmetric case.
Excitation and propagation characteristics of non-axisymmetric guided waves are modeled by theory and verified by experiments. With an array of sensors / actuators guided wave energy can be focused anywhere in a pipe. This greatly enhances the possibility of finding defects or anomalies with increased signal to noise ratio and power penetration ability. Based on a commercial pipeline inspection system focusing pipe inspections were carried out and successfully focused energy in pipes, which increased signal to noise ratio of reflections from defects. Numerical simulations by finite element methods are also carried out to simulate guided wave excitation, propagation and interaction with different defects in pipes. By sweeping the focusing point through a pipe, detailed defect distribution maps can be generated. Combined with sensor and wireless technologies, this research can be applied to online structural health monitoring and possible medical applications.
This page was last updated on January 18, 2006.