Advisor: V. K. Varadan
Committee: B. Tittmann, J. Kollakompil, J. Todd
Design of Inter-Digital Transducers and MEMS based Lamb Wave Sensors for Health Monitoring of Structures
Most people in the world have at some point of time used aircrafts as a means of transportation. Almost everyone has a roof over their heads. Every one of these people uses these engineering marvels without wondering if they were actually safe. It has been shown time and again that all of these structures are prone to develop cracks and may even begin to rust or corrode. Engineers refer to the developing of such cracks and corrosion as failure.
In the recent past the much publicized commercial jet-liner, the Concorde had to be pulled out of service. The reason for this was attributed to the fact that the aircraft had begun to develop cracks at vital locations and thus safe flight for the passengers could not be guaranteed. After much deliberation it was decided that the aircrafts would have to be pulled out of service because of extreme costs of repairing the damage. Many experts felt that if information of the damage was available earlier, such a dramatic action could have been avoided.
In order to avoid such incidents and to offset the financial setbacks of such occurrences, it is essential to develop an early warning system. This early warning system can be placed at multiple locations on every aircraft. Such a system would enable technicians to get to the root of the problem and tackle it before it was too late. The early warning system would also negate the need to examine each and every aircraft for the same problem. The theory proposed here is to design and build an early warning system.
The early warning system works based on the concept of a certain wave called "Lamb Wave" and extremely small devices, called Micro Electro-Mechanical Systems (MEMS). A MEMS device is nothing but a device built on a scale comparable to the diameter of a human hair. It is called an electro-mechanical device because it uses both electrical and mechanical means to meet its end requirements.
A lamb wave is a kind of mechanical wave, which travels through the body. It is similar to the motion of a string fixed at one end while it is vibrated from the other end. The pattern of the motion of the string is similar to the pattern of the lamb waves. These waves travel very well in solids. When these waves come across a crack they are reflected. By using certain equipment and software it is possible to pick up these reflected waves. The received signals are then interpreted, based on the time taken for the wave to be picked up, to pinpoint the exact location of the crack.
The design of the MEMS device proposed is intended to build a device that has a width of roughly 4 strands of human hair placed alongside each other. These devices will be made predominantly of Silicon along with a few other materials like gold. silver etc. The structure of the device is like that of a bridge. When the bridge undergoes an oscillation at certain rate or frequency, it generates the Lamb Wave. The oscillation of the bridge is controlled by the voltage applied to the bridge, acting as a top electrode, and another bottom electrode. The electrodes are usually made of gold which is layered very finely onto the silicon base.
The device may also be embedded with a unique ID number. When we use a large number of such devices it becomes difficult to find out which device is detecting the cracks or deformities. With the help of this unique ID number it is possible to identify the device that is detecting deformities and thus pinpoint the location of the deformity. The technicians can then take remedial action at that very site rather than wasting time and money in scanning the whole structure.
The MEMS device mentioned above requires a source of power in order to oscillate and generate the lamb waves. In aircrafts it is not practical to tap the on board electric supply or to extend wires along the entire body of the aircraft. In order to generate power without using wires we propose to use radio waves to generate power using a component called a rectenna. This component is capable of picking up radio waves and converting them to voltages.
Advisor: V. K. Varadan
Committee: B. Tittmann, J. Abraham, J. Todd
Ambulatory Gait Parameter Measurement System
Walking disorders could be neurological or physiological. Some of these conditions can be corrected by designing appropriate shoes. Researchers study the walking pattern of people with no known disabilities and then they study a patient or a person with some ailments and see if there is any difference in his walking pattern. By observing people with normal walk they arrive at certain parameters. These parameters are compared with a person with some health problem. Any variation from the normal is an indication of an abnormal condition. Using these parameters one can classify patients into various categories depending on the disability. If a subject has some walking disability, based on these parameters one can determine his/her medical condition.
One of the parameters measured is the timing in walking, which is the time required to complete a stride. Other parameter measured is the pressure experienced under the sole. These can be measured by attaching sensors on the leg and under the sole. For this purpose, researchers need an elaborate equipment to attach sensors to the body and record' data of it. This involves cost and space in addition to discomfort to the person undergoing the test. The measurements are carried out in the lab and in a constrained environment. It is possible that the walking pattern of the subject might not be natural in this setting. Hence it is necessary to develop a system that can monitor walking pattern in any terrain and outside a lab.
We have developed a way to overcome these set backs. We have designed a measurement system which is truly mobile and the person under test is not constrained any longer. This system developed is as effective as the systems currently being used in the laboratories. The subject is free to carry on his regular work as usual and at the same time, all his movements are monitored. In our system, sensors are attached to the insole of the footwear which monitors the parameters involved in walking. These physical events during walking are translated to electrical signals by the sensor unit we have developed. These signals are then transmitted wirelessly to a laptop where it is stored. By observing and analyzing the data collected, the parameters mentioned earlier are calculated. These results are then validated. Our experiments were carried out with people with no disorders. The results obtained were compared with those obtained using the conventional technique of monitoring walking pattern and they agree closely with each other. This system is not only compact, light weight but also effective in monitoring walking pattern. We believe that our system will pave way for better treatment of people by collecting more accurate data. Also this research opens avenues to develop more compact systems so that they can be even more portable and convenient to the patients.
Advisor: C. E. Bakis
Committee: H. Hofmann, G. Gray, A. Belegundu, J. Todd
Design of Rotating Machinery Using Anisotropic Elasticity Solutions and Simulated Annealing
The role of electric machines in energy conversion and storage is historically significant and a subject of current research. This document explores the theoretical design and optimization of such rotating machinery with an in-house computer analysis and an optimization algorithm called simulated annealing. Electric machines under consideration for use in quiet hydrogen-fueled aircraft of the future are optimized such that they are as powerful per unit mass and per unit volume as possible.
Electromechanical flywheels that are used to store energy more efficiently than chemical batteries but without the use of toxic materials are optimized such that they can store the most energy per unit mass. A flywheel was designed to handle a speed range of 0 - 298,000 rpm and a calculated maximum energy storage capability of approximately 9.9 W.hr. An electric machine designed to operate at the temperature of liquid hydrogen fuel is presented which exhibits a very high theoretical power to mass ratio of approximately 1600 kW/kg (over 1000 hp/lb). The simulated annealing algorithm written as part of the research proved to be robust despite the complexity of these design problems.
Advisor: J. L. Rose
Committee: C. Lissenden, M. Horn, J. Todd
Ultrasonic Guided Wave Defect Detection Feasibility in a Carbon/Epoxy Composite Plate
A broad definition of a composite material is one that is made of at least two components that can be distinguished from one another. Many common materials fall into the classification of composite materials such as wood, cement, or even bone. Other composite materials are designed and manufactured to behave in a specific manner so that they ideally suit a special application. These types of materials, called engineered composites, have two parts: a matrix material and reinforcing fibers. The fibers are long and thin, about the thickness of a strand of hair, and carry most of the load in the material, thereby giving the composite exceptional strength. The matrix acts like glue and holds the fibers in place while also adding flexibility to the composite. The resulting material is stronger than the individual components alone and is also extremely lightweight, making these materials ideal for applications where a high strength to weight ratio is desirable. A well-known example of an engineered composite material is fiberglass, which consists of glass fibers in an epoxy matrix and is used in many applications ranging from boat and automobile bodies to fishing rods.
Despite the favorable properties that composites exhibit, damage still occurs in these materials just as with any other. This is an especially serious concern since many of these composites are critical components of air and space craft. Therefore methods must be developed to inspect composite structures for damage quickly and effectively to ensure safety.
Inspecting composite materials is not an easy task however. For one, most damage occurs on the inside of composite structures, making visual inspection techniques impossible. In addition, traditional inspection techniques used on steel or aluminum structures are not applicable because of the complexity of the composite material. Therefore new ways must be developed to efficiently evaluate the condition of composites.
One such inspection method is being developed in the Ultrasonics R&D lab at Penn State University. The current research attempts to detect damage in composite plates using ultrasonic waves. This is the same type of ultrasound used to obtain in utero images of babies, but in this case, the sound travels in a composite plate rather than in the human body. The principle is the same however in both cases. Ultrasonic waves are excited at one location, they travel though a medium and then the resultant waves are received at another. The received waves contain information about the medium in which they traveled, and from this information, the state of damage in a plate can be determined or in the case of medical ultrasound, an image can be generated.
Ultrasonic waves are actually waves of vibrating particles within a structure. These waves are excited by a transducer that "hits" the structure with energy that causes particles to vibrate and they in turn cause particles next to them to vibrate. In this way, ultrasonic waves can travel through a material. The waves are received in the same way that they are excited, but in reverse. A receiving transducer "feels" the vibration of the propagating ultrasonic wave and translates that into an electrical signal that can be displayed on a computer screen.
When these waves are excited in a thin plate, they bounce back and forth inside the plate and superimpose on one another to form packets of energy known as guided waves, so named because they are guided by the boundaries of the plate. There are an infinite number of guided wave types, or modes, that can by excited in any structure and each one travels along the plate at a certain speed and has specific propagation characteristics. Using this uniqueness of guided wave modes, a material can be inspected with ultrasound by exciting a mode with known characteristics, allowing the wave to propagate along the material that is being inspected, receiving the resultant wave, and then analyzing that wave to determine if any of the mode's characteristics have changed due to interaction with damage in the plate. One characteristic being examined that may be sensitive to damage is the wave speed. Think of a guided wave traveling in a plate like a car on the highway. If there are no problems with the road (damage in the plate) the car can travel at its usual speed, but if there is say a pothole or roadblock, then it will take the car longer to get to its destination. What happens is that the damage slows the progress of the wave and therefore if the wave speed is slower than it should be, damage must be present. This is just one characteristic being studies and there are countless more. It is the challenge of the researcher to extract the hidden information from a received wave and interpret what that information means.
Advisor: J. L. Rose
Committee: A. Segall, T. Simpson, J. Todd
Ultrasonic Guided Wave Inspection of Fiber Reinforced Composite Materials
Fiber-reinforced composites are a special type of material consisting of fabric-like, woven sheets of glass or carbon fibers that are held together with advanced epoxies or plastics. Composite materials are being used increasingly in a variety of applications, because they are very strong and lightweight. They are commonly used to build airplane structures or for other aeronautical and aerospace applications, as well as in sporting goods and automobiles. Although these materials can be used to build structures stronger and lighter than with steel or aluminum, they can have disadvantages. They are very expensive, difficult to work with, and although generally very strong, they can also be quite fragile. Since composites are very stiff, they also tend to be brittle. While they could be used to construct an air tank that could easily withstand many hundreds of pounds per square inch of pressure, something as simple as dropping a hammer on such a tank could significantly damage it. Additionally, with repeated use, they can develop very small cracks which can eventually lead to significant weakening of the part. Worst of all, this damage is often very difficult or impossible to detect visually.
These disadvantages are sufficient to limit the use of composites to applications that have very critical strength and weight requirements. Consequently, it is very important to frequently inspect these parts to ensure that they have not been accidentally damaged. This requirement for inspection leads to very high maintenance costs, since not only is the inspection process time consuming and costly, but an airplane or helicopter must be out of use while inspections are being performed.
The goal of this work was to develop a system of sensors that could be left in place on a composite part while it is in use, allowing for inspection without taking the structure out of service. The particular part that these sensors were designed for was a driveshaft, although results have been generalized to simplify transferring this technology to other structures. The sensors that have been developed are based on the principle of ultrasonic guided waves, a technology similar to that used for medical imaging like sonograms of babies. Small devices called transducers are attached to the surface of the driveshaft. When electrical current is applied to the transducers, they produce a vibration in the driveshaft. This vibration then travels along the length of the shaft to another transducer, which will reverse the process, converting the vibration back into electrical current. Special computer software and hardware can then be used to look at the vibration picked up by the second transducer, and analyze it to evaluate the condition of the structure.
Since these vibrations travel along the entire length of the shaft, they can be used to detect damage that has occurred. If the vibration hits a damaged area as it moves between the transducers, then part of its energy will bounce off of the damage, and thus less will get through to the receiving transducer. Computer analysis of the vibration picked up by the receiving transducer can thus be used to detect if damage has occurred. Additionally, through advanced analysis of the vibration, it is possible to determine the severity and type of damage.
Presented in this thesis are the results of several studies using multiple kinds of transducers to detect different types of damage. Experimental setups, test methods, and data analysis are described. The ability to detect two important types of defects (cracking and impact type damage) is demonstrated. Additionally, a technique is presented that could potentially be used to measure the torque in the driveshaft using the same types of sensors.
Advisor: S. J. Fonash
Committee: S. Hayek, R. Messier, J. Todd
The In-Situ Fabrication Process Flow of a Multilayered Micro-Fluidic Network Separated by a Plasma Etched Membrane that is used to Isolate and Immobilize Beads and/or Biological Entities
This thesis describes a way to build a structure that can be used to capture single particles such as biological cells from a solution. The liquid containing the particles is run through small channels that are separated by thin membranes. The membranes have tiny holes that allow the fluid to travel from one channel to another. Particles that are too large to pass through the holes become trapped in the membrane clogging the pathway between the two conduits. The size and shape of the holes can be varied when the membrane is made, so a variety of particles can be captured. Flow through the channels is controlled by pressure and electricity, and the direction of the flow can also be controlled by these methods. Experiments were performed to check the control of directional flow and to test the capturing ability of the device.
Once the desired particles are held in place, additional tests can be performed. Pre-positioned electrical devices and sensors in the pore region can be activated to monitor the status of the captured entity. Also, chemicals can be transferred to the capture location and the by-products can be examined. As the captured entity absorbs the chemical, some byproducts will be produced. These byproducts can be examined downstream for chemical composition to see what the entity is making. The final chapter of this thesis will explore these ideas in more detail along with additional applications using this structure.
Advisor: J. L. Rose
Committee: B. Tittmann, R. Engel, J. Todd
Phased Array Focusing Using Torsional Guided Waves for Pipe Inspection
Waves are everywhere from the ocean to your radio. The waves you can hear are called sonic waves. These sonic waves travel through the air to our ears and reach wavelengths bigger than fifty feet or as smaller than one inch. The waves too small for us to hear is called ultrasonic waves. Just because we cannot hear them, does not exclude them from being useful to us.
Ultrasonic waves can travel fast and far in materials like steel and aluminum. Structures like airplanes, railroads, and pipelines are all made with these materials. As our nation grows these resources do too, yet the old structures remain in use as they decay from the effects of time and corrosion. It is too expensive and time consuming replace all these structures at once. Ultrasonic waves can inspect these structures; predict which parts need replacement, confirm that other parts can remain, and do it before accidents occur.
The thin walls of structures, like pipe, guide the wave as it travels for hundreds of feet. Ultrasonic guided waves have become a popular way to inspect long lengths of pipe by sending waves down the walls from a single position and listening for echoes that indicate corrosion or damage. Echoes occur from changes in the pipe wall, such as, rust or holes.
Axisymmetric mode guided waves are the type of waves commonly used in current inspections. Axisymmetric means the ultrasonic guided wave has an equal distribution around the circumference of the pipe wall. Flexural modes have an unequal distribution of the wave around the pipe wall. Flexural modes have more complex wave motion pattern than the axisymmetric modes and are only beginning to be used in inspections. There are significant advantages to using flexural modes. They can be used to focus the guided wave a predetermined location in the pipe. This technique is called the phased array focusing technique.
The phased array focusing technique is being developed with the intent of inspecting hundreds of feet of pipe from a single array position. The single array position is beneficial if access to a pipe is limited, e.g. steam pipes onboard U.S. Naval ships. The pipes often have a protective coating which would ordinarily be removed and replaced for an inspection. From a single array position, ultrasonic guided waves propagate under the coating, down the length of the pipe and then return information about potential defects. Focusing the ultrasonic energy at a predetermined location along the length of the pipe enhances the sensitivity to defects by increasing the amount of ultrasonic energy at that location.
In this thesis, phased array focusing in the 200 to 800 kilohertz frequency range is implemented using flexural torsional mode ultrasonic guided waves in a 2 inch diameter steel pipe with wall thickness 0.125 inch. Sensors capable of emitting and detecting ultrasonic waves are used to excite guided waves in the pipe. An introduction to contemporary ultrasonic long range pipe inspection techniques and commercial players is given in Chapter 1. Background on ultrasonic waves with an emphasis on guided waves in pipe and the phased array focusing technique can be found in Chapter 2. A description of the inspection equipment used in this work is given in Chapter 3. Results of the phased array focusing studies can be found in Chapter 4. Some concluding remarks and future work considerations are presented in Chapter 5.
Advisor: D. Heaney
Committee: C. Binet, J. Meckley, A. Segall, J. Todd
The PVT Effect on the Final Sintered Dimensions of Metal Injection Molded Components
Components made from the powder metal injection molding (MIM) process typically exhibit 12% - 25% linear shrinkage during the processing and densification steps of the MIM process. This thesis investigates the effect of pressure on the final densified component. A relationship between pressure, volume, and temperature is presented that increases the accuracy of linear shrinkage predictions.
Advisor: V. K. Varadan
Committee: J. Abraham, M. Urquidi-Macdonald, J. Todd
Microstereolithography of UV-Curable Polymers with Chemically Bonded Carbon Nanotubes
Nanotechnology is a broad term which is used to categorize the techniques for the nanometer scale control of materials, the resulting nanometer scale structures, and the devices utilizing such structures. Typically, the nanometer scale control involved with these techniques allows for structures with dimensions of 1 to 100 nanometers. To put that into perspective, a 100 nanometer structure is 1000 times smaller than the diameter of a human hair. When compared to atoms, 1 nanometer is approximately the length of 10 hydrogen atoms lined up in a row. Making a 1 nanometer structure requires the precise arrangement of atoms.
The intense interest in nanotechnology is due to the novel properties which occur at this small scale. Numerous devices which exploit these properties have been proposed. Examples of potential devices include chemical sensors, biological sensors, drug delivery systems, composite materials, and advanced electronics. This has driven nanotechnology to become one of today's hottest areas of research.
Carbon nanotubes are one class of nanometer scale structures which receive significant research interest. A carbon nanotube typically measures 20 to 30 nanometers in diameter and can be thought of as a drinking straw which is made out of carbon atoms. Carbon nanotubes possess several novel properties which make them attractive candidates for a variety of potential applications. For example, the first consumer application of nanotubes will likely be Samsung's nanotube based televisions which exploit the field emission properties of nanotubes. Also, the high mechanical strength of carbon nanotubes makes them good candidates for composites. One possible composite application is to add carbon nanotubes to the plastics which are used as structural materials in micrometer scale devices. The overall dimensions of the structures in such a device are comparable to the width of a human hair.
In this research, carbon nanotubes were first modified by a chemical process known as functionalization. This was done so that the carbon nanotubes could form chemical bonds with the plastic. Following the chemical treatment, nanotubes were mixed into two liquid plastics. The resulting composites were then tested to determine if useful structures could be made.
The manufacturing process which was used to test the new nanotube composites is called Microstereolithography (MSL). For this process, a liquid plastic which hardens upon exposure to ultraviolet light is used as the building material. To start, a thin layer of the liquid plastic is applied to a solid base. A pattern can then be formed in this liquid layer by shining ultraviolet light onto some but not all of the liquid. The areas hardened by the exposure become part of the final structure. After a layer is completed, the process can be repeated by applying a new layer of liquid and then exposing it. Each new layer can have a unique pattern, and three dimensional structures can be built. In this research, several single layer structures have been successfully fabricated with the composite materials. This shows great promise for potential use in devices like phase shifters where only one structural layer is necessary. Unfortunately, multilayer structures have not yet been successful. The black color of these composites and poor control of layer thickness has lead to problems with interlayer bonding.
Advisor: J. L. Rose
Committee: R. German, I. Smid, J. Todd
Crack Detection in Unsintered Powder Metal Parts Using Ultrasonic Rayleigh Waves
Powder metal processing allows for the production of parts through consolidation of metal particles into high quality complex parts with high tolerances in an economical manner. Powder metal parts are made by mixing elemental or alloy powders and compacting the powder mixture in a die to produce a "green" part. The fragile green part must then be sintered, or heated, in a furnace with a controlled atmosphere to thermally bond the particles, giving the part its full strength. Traditional metal working machining processes remove material from raw stock creating "chips" as waste. Powder metal processing is a favorable technique for the production of large volumes of parts with complex geometries due the conservation of material and energy.
In order for particles to metallurgically bond together during sintering, the particles must be in contact with the neighboring particles around them. Surface breaking cracks formed in a part during compaction or handling before sintering can result in the final sintered part having flaws that affect the integrity of the part. The inspection of green parts for cracks has been restricted to visual inspection due to the fragile nature of green parts and the requirement that the green parts are not contaminated with a liquid or other substance that affects the sintering process. Visual inspection is a subjective technique that can vary between individuals completing the task.
A new nondestructive technique using ultrasonic Rayleigh surface waves has been developed for the detection of surface breaking cracks in green, or unsintered, parts. This technique fills a void for quality control midway through sintered part production. It allows for cost reduction by eliminating additional processing completed on flawed parts, as well as giving equipment operators immediate feedback on the effect of changes in pressing parameters such as fill level, punch settings, and pressure.
The surface wave mediator technique induces ultrasonic Rayleigh waves on a part using couplant free mediators and minimal pressure. Rayleigh waves travel along the surface of a part having the unusual behavior of energy decay with increased depth. Ultrasonic Rayleigh waves also have the ability to travel along curved surfaces.
A surface wave mediator probe is constructed by placing a piezoelectric transducer, a device that converts electrical voltage to mechanical motion and vice versa, on a Plexiglas wedge set at an angle in which Rayleigh surface waves are generated in a steel mediator. The generated surface wave travels along the mediator and is imposed onto a part through a small contact area at a knife-edge tip.
Measurements are made by a through transmission, otherwise known as pitch-catch, setup using two surface wave mediator probes. Ultrasonic energy traveling between probes is either reflected or refracted by a crack thus reducing the amount of energy that would be transmitted to the receiving probe if a crack were not present. Cracked parts will have a different ultrasonic signature, the received signal transmitted between two transducers, as compared to defect free parts due to variations in frequency, energy content, and wave velocity. Once the determining features of the ultrasonic signature are identified, these features are used to analyze received ultrasonic signals to determine whether an inspected part is cracked or defect free.
The technique allows for the local inspection over complex part geometries such as gear teeth and multi-level parts. Through the design of a fixture for proper probe alignment, the technique yields highly reproducible results capable of implementation onto an assembly line for rapid part inspection. This makes the surface wave mediator technique a viable quality control tool for the inspection of automotive, aerospace, and industrial parts.
Advisor: P. M. Lenahan
Committee: S. Ashok, J. Ruzyllo, J. Todd
Electron Spin Resonance Observation of trapping Centers in Atomic Layer Deposited Hafnium Oxide on Silicon
This thesis employs electron spin resonance spectroscopy and conventional electrical measurements to study the structure and chemistry of technologically important defect centers in materials being considered for near future integration in metal-oxide-semiconductor devices. Several ''dangling bond'' centers were observed. These centers undoubtedly play in important role determining the performance and reliability of devices based on the most promising high-k candidate: HfO2.
Advisor: V. K. Varadan
Committee: M. Urquidi-Macdonald, J. Kollakompil, J. Todd
Carbon-Nanotube Based Air-Cathodes for Alkaline Fuel Cells
In today's industrial world, power plays an important part in our lives. Many of the power sources in use today either have limited life cycle or are hazardous to the environment. An Eco-friendly solution to this problem lies in the use of Fuel cells for everyday applications. This technology is already in use which encompasses application areas from household, automobiles to the portable devices. The scientists around the world are spending millions in research and development to use the abundantly available hydrogen as a fuel to run various applications. The concept behind this technology involves combining of natural oxygen with hydrogen to produce the useful electricity with water as a byproduct.
This concept is investigated with an eye on miniaturization to cater to portable devices and also to improve the current output. The researchers at Center for the Engineering of Electronic and Acoustic Materials and Devices (CEEAMD) at Pennsylvania State University have developed a fuel cell, which is as thick as a finger. Among the many types of fuel cells which differ in the electrolyte and the temperature of operation, the alkaline fuel cells are the cheapest as they do not use expensive materials. The experiments at the laboratory used a reformed alkaline solution to supply the hydrogen which enables the transfer of ions and a modified cathode which helps tap oxygen from the air to complete the cycle for the power generation.
The modified cathode uses carbon nanotubes that improve the current output. Carbon nanotubes are small cylinders of carbon that are stronger than steel. These tiny tubes harbor the best properties and come under the realm of nanotechnology. They can act as insulators or conductors and are the best material for today's electronic devices. If we placed thousands of these tubes alongside each other, they would measure up to half the thickness of a strand of human hair. These cylindrical structures can store oxygen or hydrogen molecules! The other remarkable property is that they possess more storage capacity and have larger surface areas for reactions. The fuel cell designed at CEEAMD encompassed all of these properties to provide efficient results. The battery composite laboratory has provided the testing facility to prove the carbon nanotubes worth. The team has tried to establish the efficient working of a fuel cell which is not too bulky. As we all know the size of the battery is the determining factor for its capacity. This however, is not the case with a fuel cell. The other features of alkaline fuel cell systems are its low-cost, use of non-expensive materials and its operation at room temperatures.
Scientists mimic nature's gamut of ideas to observe and leam from it. The team at CEEAMD has intertwined the nanotechnology and the fuel cell technology to obtain cleaner power at micro sizes. California and London already has buses powered by fuel cells but at the cost of a bulky system. Various companies have come up with this system to power laptops. The only drawback is the use of "pure" hydrogen and oxygen, but the day isn't too far when we would go to a gas station to refuel our automobiles with pure hydrogen to facilitate reduce the green house effects.
Advisor: P. M. Lenahan
Committee: S. Ashok, J. Redwing, J. Todd
Spin Dependent Recombination Observation of Deep Level Defects in 4H and 6H Silicon Carbide Mosfets
For several decades the microelectronics industry has revolved around the fabrication of silicon based integrated circuits. However, the fundamental material limitations of silicon prohibit its use in harsh environments; for example, the high temperature ambient of an aircraft engine causes the electrical properties of silicon to change drastically leading to integrated circuit failure. One solution to this problem is to fabricate integrated circuits on silicon carbide, which is a semiconductor material that can function properly in high temperature and high power density applications.
While theoretical calculations suggest that silicon carbide would be an excellent candidate for high temperature and high power application, experimental results show that there are still large numbers of defects in silicon carbide devices that limit the electrical performance. Details about the defects that plague silicon carbide devices are still largely unknown. The purpose of this study was to use a non-destructive evaluation technique called spin dependent recombination to determine the atomic structure and chemical composition of the electrically limiting defects in silicon carbide devices. Spin dependent recombination is a highly sensitive form of electron spin resonance that utilizes quantum mechanical properties of electrons and nuclei in a transistor.
The outcome of this study was successful in determining two of the major defects present in silicon carbide devices today. With the knowledge gained as a result of this study, future processing changes can be made to further develop silicon carbide technology. Advancement of silicon carbide technology will bring the availability of new high power, high operating temperature integrated circuits.
Advisor: F. Costanzo
Committee: G. Gray, P. Michaleris, J. Todd
A Space-Time Finite Element Method for Second Order Hyperbolic Problems
During the past half a century, finite element methods (FEM) have evolved into one of the most useful tools for engineers. However, the FEM was originally applied to problems without time dependence. As the mathematical and computational tools for finite elements improve, the variety of problems to which the FEM is applied has also expanded. In particular, partial differential equations (PDEs) describing evolutionary phenomena with finite speed of information propagation are notoriously difficult to solve numerically due to singularities in solutions.
This thesis presents a formulation and implementation of a FEM taylored to solve PDEs in which changes in a solid's microstructure occurs with a rate comparable to the speed of sound in the material. Instead of discretizing the spatial and temporal domains separately, both are discretized together such that the computational domain is a space-time grid. The advantage of using finite elements in space and time together is adaptivity. Using more elements where the solution changes faster allows one to get a more accurate solution in areas of discontinuities without sacrificing computational time.
Specifically, this thesis presents an implementation of a discontinuous Galerkin finite element formulation. Results computed on a uniform space-time grid are presented and compared with those of an adaptively refined grid. It is shown that by adaptively refining the space-time finite element mesh one can more accurately capture surfaces of discontinuity for second-order hyperbolic PDEs. It is also shown that by using an evolution law one can track a surface of discontinuity in real-time.
Advisor: J. L. Rose
Committee: N. Salamon, A. Segall, J. Todd
Implementing Guided Wave Analysis Through Design
Observing how a task is performed is one of the most powerful methods of learning. We do this on a daily basis. As a student we sit in class, and observe a professor. In order to become a better designer the same technique applies. One of the most powerful tools in design is observation. Unfortunately, when looking at a coffee maker, you can't fully realize why the designer made the handle on the carafe the way they did. This thesis is a collection of designs, in the area of guided wave ultrasonics. In guided wave ultrasonics a disturbance travels along a rod, pipe, or any waveguide to a defect whereby a reflection occurs coming back to our sensor indicating that a defect was present. The objective here is to provide the answer to why and how things were done, in hopes that this knowledge and experience may be of benefit to others designing similar types of systems, or totally different systems for that matter.
There are countless techniques to aid in the design of products. Many of these are intuitive for some people, but they may not be for others. Even if they are intuitive the formalization of these techniques can help a designer realize solutions beyond what may have been developed without their aid. The design process is discussed from conception to the point of manufacturing. The following topics are covered; engineering process measures, concurrent engineering, engineering requirements documents, the theory of inventive problem solving, design for manufacturing, and techniques for moving beyond the concept of a single product. This work seeks to introduce some of these techniques, which are both traditional and newer methods. The techniques introduced have been employed by countless companies to increase productivity and enhance their product design process. The purpose here is to demonstrate how some of these techniques can be simply integrated into the thinking process, for even the simplest designs, and to provide an introduction to the wealth of available aids in the design process.
As this thesis is directly tied to guided wave ultrasonics, the necessary background information to understand some of the wave mechanics behind the presented designs is presented. This includes bulk wave and guided wave ultrasonics. In addition, wave generation and flaw detection techniques are covered. The examples in this thesis cover a wide range of guided wave applications; therefore, due to the extreme wealth of information required, only a brief introduction is provided, along with references that provide more in-depth information regarding the specific wave mechanics theory relative to the respective application. The understanding of these principles is imperative in designing ultrasonics equipment, as these theories drive the design and determine the degree of positional accuracy required for each application.
The first design example discusses the redesign and further development of a fixture for performing Rayleigh surface wave analysis on green compacts. The theory behind the technique is introduced. An existing design is evaluated, and then the Quality Function Development (QFD) approach is employed. This project demonstrates how the QFD method can be used to generate a quantitative requirements document and set of test parameters. Finally, test data and examples are provided to demonstrate the capabilities of the setup.
A redesigned fixture for the purpose of producing laser based computed tomography (CT) images of green powder metal compacts is presented. CT images are utilized to study density gradients and to perform defect detection in green compacts. Special emphasis is placed on how proper alignment can be used to eliminate image artifacts. Furthermore, the design provides a real world example, demonstrating how the theory of innovative thinking (TRIZ) concepts and traditional design techniques are employed to develop creative and effective solutions to design challenges in the area of Ultrasonic Nondestructive Evaluation (UNDE).
A redesign of a test setup for high frequency phased array testing of pipes is discussed. An existing setup is analyzed and a new system is presented. In this project a product family is created. A top-down design approach is employed with a beachhead platform strategy. A platform is developed and through horizontal and vertical leveraging of the platform, derivative products are developed. Other product family design techniques are also presented.
Lastly, outlined is the realization of a testing system for the inspection of vaulting poles. The design demonstrates the application of guided wave ultrasound principles including group velocity measurement and through transmission scanning. The final product design is a multifunctional instrument, which employs modularity.
These design examples demonstrate how knowledge of ultrasonics can be joined with traditional and newer design techniques to effectively define problems and provide unique solutions. There are countless techniques available to aid in design. The awareness of these tools can be an invaluable tool when searching for solutions to problems in any discipline.
Advisors: M. Urquidi-MacDonald, D. Macdonald
Committee: J. Xu, J. Todd
The Application of the PDM (Point Defect Model) to the Oxidation of Zircaloy Fuel Cladding in Water-Cooled Nuclear Reactors
With the development of higher burn-up fuels for nuclear power reactors, much greater demands are being placed on the performance of Zircaloy fuel sheaths. The principal threat to the integrity of the sheath is oxidation/corrosion and hydriding, leading to more-or-less uniform thinning and, in some instances, to localized corrosion in the form of nodular attack and/or hydriding. Failure leads to the release of fission products into the coolant, which in turn contributes to the man-REM costs of operating the system. Extensive fuel failures may require shutdown, which results in the unit being unavailable for normal operation. Thus, strong operational and economic reasons exist for enhancing fuel reliability. The principal goal of the proposed work is to develop sophisticated physico-electrochemical models for corrosion of Zircaloy fuel sheath that can be used by reactor operators to actively manage the accumulation of damage and hence to minimize the risk of fuel cladding failure in operating reactors.
Advisor: M. W. Horn
Committee: R. Messier, S. Tadigandapa, J. Todd
Integration of Palladium Chemresistors on a Temperature Controlled MEMS Platform
Hydrogen has varied applications and is available in large quantities. Life and economy of the future could depend on the efficient storage, distribution and use of this versatile fuel. A compact device to detect hydrogen over a large range of concentrations would be required for safe handling of this explosive gas.
Simple sensors constructed from a thin film of palladium have been used to sense hydrogen but have limitations at room temperature and atmospheric conditions owing to interference by other gases, humidity and phase transformation. Operation of the sensors at high temperatures is known to reduce these effects and also increase the overall sensitivity of all hydrogen sensors. Micro-electro-mechanical systems (MEMS) offer a convenient platform for the control of a sensor element using low power.
This work is devoted to the development of MEMS hydrogen sensors that combine temperature control and temperature detection on the same platform. A simple palladium resistor was fabricated, and its response to various concentrations of hydrogen was analyzed. The temperature of this resistor was controlled, using the underlying heater and thermopile, and subjected to identical tests. These results were analyzed to reveal an improvement in the pattern of response and an increase in the range of detectable concentrations at elevated temperatures. The palladium films were found to be stable at higher temperatures, and the MEMS platform was found to be amenable for future work with similar devices.
Advisor: J. L. Rose
Committee: B. Tittmann, I. Smid, J. Todd
Ultrasonic Sensor Systems for In-Situ Monitoring of Crystal Growth
Crystals of several hundred different materials are now commercially available [1]. Common processes for growing these crystals include: growth from melt, hydrothermal growth, flux growth, growth by physical vapour transport, epitaxial growth, and electrocrystallisation [2]. Currently, many research efforts are focusing on improving process monitoring of crystals while they are being produced. Existing in-situ monitoring methods provide no information on crystal interface shape. The ability to accurately monitor the growth of these crystals, in real time, will improve the quality and efficiency at which they are produced [2].
Ultrasonic sensor systems were developed for in-situ monitoring of crystals grown from the melt and by physical vapor transport. For the case of crystals grown from the melt, a dual-rod pulse-echo system was developed, lab tested, and successfully demonstrated to be capable of detecting the position of the solid/liquid interface during CaF2 crystal growth. A dual-rod pulse-echo technique has also been developed and bench tested to detect the solidifying interface for crystals grown using physical vapor transport. The ultrasonic sensor systems utilize changes in signal time of flight measurements to detect the growing crystal interface. Advantages of the systems are that they are relatively low cost, easy to install on most standard furnaces, and can provide excellent accuracy. For both systems, multiple sensors could provide interface contour information and will be the focus of future work.
Advisor: R. M. German
Committee: R. Engel, A. Segall, J. Todd
Pressing to Full Density-Fundamental Limitations and Capabilities of High Density Powder Metallurgy
Powder metallurgy (P/M) is a net shaping process to manufacture components from powders. The P/M process consists of three different stages. Firstly, the powders are mixed with lubricants or binders and shaped in a die by using pressure, termed compaction. Secondly, the lubricant or the binder is removed from the shaped component by using a thermal process, termed delubrication. Lastly, the sample is heated in a controlled atmosphere at high temperature to bond the particles, termed sintering. When properly performed, the properties of the final P/M components are similar to cast or wrought materials of similar composition.
The effect of density on the properties of the final P/M product is tremendous. A key difference between wrought and P/M components is the presence of porosity, which significantly affects the structural performance. High performance comes with full density. Achieving full density iron based P/M materials in a single compaction operation has long been a topic of study. Zero porosity (or full density) can be achieved by variation of the compaction and sintering conditions. Practically, there are three processing options for full densification in P/M components. The first route is pressure-based densification, performed at low speeds and at high pressures (quasistatic compaction) or at high speeds, using shock waves (dynamic compaction). The second route is simultaneous pressure and temperature based densification, such as via hot isostatic pressing. Low sintering temperatures and high pressures are typical for full density. Third route is sintering-based densification, which relies on small particles and high sintering temperatures.
Each of the methods stated above has advantages and disadvantages. Aim of pressure based densification is to obtain high density green compact. Producing components using a pressure-based technique has main advantage in that a high compacted density reduces the density gradients while improving the final properties.
The investigation focuses on obtaining full density through the compaction route. To obtain a full density sintered component, the powder is compacted to high green density. There are fundamental limitations and capabilities to both low and high speed methods to full density. The research studies the effects of pre-processing factors, which are the powder characteristics such as particle size, particle shape, packing density, and processing factors such as compaction pressure, compaction velocity, compaction energy, and lubrication on both quasistatic and dynamic compaction, to generate data useful for the design of high density systems. Quasistatic compaction was performed using a hydraulic press with a high pressure tool and die set. Dynamic compaction was performed using a gas gun system. Supplement dynamic compaction experiments were performed on a commercial press. Sound velocity measurements using ultrasonics were performed to confirm that the dynamic experiments reach shock wave velocities. Compaction energy, and not compaction velocity, was the density controlling processing factor. Density was dependent on particle shape, but not on particle size and packing density (apparent density or tap density).
Advisor: C. E. Bakis
Committee: C. Lissenden, A. Segall, J. Todd
Radial Tensile Test Method for Thick Composite Rings
Considerable effort is underway to find an alternative source of power to replace conventional chemical batteries. The main concern in using conventional batteries is that they produce hazardous waste on disposal, they have short lives at high temperatures, and they do not deliver or store power very well at low temperatures. Therefore, in an attempt to create a better energy storage medium, flywheels are under investigation. A flywheel energy storage device consists of a rotating wheel attached to a motor/generator. The motor/generator is used to store and extract energy from the flywheel. Energy extracted from the flywheel can be used to power various electrical appliances during power outages, for example. However, it needs to be determined how fast the flywheel can be rotated before it fails. The failure of the flywheel depends on the strength of the material used for its manufacture. In this investigation, a very simple test method is developed to determine the strength of the flywheel made of a fiber reinforced composite material. Composite materials are desirable due to their light weight and high strength. High strength is required to rotate flywheels at high speeds and, thereby, store more energy per unit weight. Experiments were conducted on thick flywheels to determine the strength of the material and understand the behavior of flywheels under various loading conditions. The simplicity of the test method, in conjunction with early promising results, suggests that the test method is suitable for widespread use and possible standardization that will ultimately facilitate the commercial viability of flywheel energy storage.
This page was last updated on January 18, 2006.