While the scaling of conventional memories based on floating gate MOSFETs is getting increasingly difficult, novel type of non-volatile memories, such as resistive switching memories, have lately found increased attention by both industry and academia. Resistive switching memory (ReRAM) is being considered one of the prime candidates for next-generation non-volatile memory due to relatively high switching speed, superior scalability, low power consumption, good retention and simplicity of its structure which does not require the expensive real estate structure of the silicon substrate. Furthermore, integration of ReRAM directly into a CMOS low-k/Cu interconnect module would not only reduce latency in connectivity constrained devices, but also would reduce chip's footprint by stacking memory layers on top of the logic circuits. One good candidate is the well-behaved Cu/TaOx/Pt resistive switching device. However, since platinum (Pt) acting as the inert electrode is not an economic choice for industrial production, a Back End of Line (BEOL)-compatible replacement of Pt is highly desirable. A systematic investigation has been conducted and metals such as Ru, Rh and Ir are found to be the best potential candidates to supplant Pt. The device properties of Ru, Rh and Ir based resistive switching devices have been explored in this work. However, the challenges of implementing ReRAM cell into BEOL of CMOS encompass not only the choice of materials of a CBRAM cell proper, but also the way the cell is embedded within BEOL. In case of the inert electrode, the metal interfacing the solid electrolyte (e.g. TaOx) has to be supplanted by a glue layer, and heat transport layer, leading to an engineering task of a composite electrode beyond the requirements of low miscibility with, and low surface diffusivity of the inert electrode with respect of the active metal atoms released by the active electrode (here Cu). The metal of the active electrode (Cu, Ag, Ni) is required to allow for a copious redox reaction but simultaneously preventing reactions with the dielectric. Finally, for the solid electrolyte, a dielectric with a moderate level of defects is preferred which may be controlled, for example by the deposition processes modulating the stoichiometry of the material. This research study begins with exploration of several devices derived from the benchmark device Cu/TaOx/Pt and manufacturing those in Micron nanofabrication and characterization laboratory at Virginia Tech with the latter device used as a benchmark for performance assessment. Electric characterization of the manufactured Cu/TaOx/Ru devices has shown some notable differences between them due to the different formation, shape and rupture of the conductive filament. The inferior switching properties of the Ru device have been attributed to the substantially degraded inertness properties of the Ru electrode as a stopping barrier for Cu as compared to the Pt electrode. To study this degradation effect further, two nominally identical devices however differently embedded on the Si wafer have been fabricated. The electric behavior of the two devices are found to be markedly different and is attributed to the difference in high local temperatures in the device during the switching that cause species interlayer diffusion and trigger undesired chemical reactions. Thus, the embedment of the device has a foremost impact on the intrinsic device performance. To investigate the impact of inert electrode on the endurance of ReRAM memory cells, baseline device Cu/TaOx/Pt/Ti is compared with six devices manufactured with different inert electrode constructions: Pt/Cr, Rh/Cr, Rh/Ti, Rh/Al2O3, Ir/Ti, and Ir/Cr, while the Cu electrode and the TaOx dielectric are identical. Although the glue layers Ti, Cr or Al2O3 are not an inherent part of the device proper, they have a tangible impact on the device endurance as well. It is experimentally demonstrated that inert electrodes with high thermal conductivities have superior endurance properties over an electrode with low thermal conductivity and the heat conductivity of inert electrode has a substantial impact on ReRAM cell performance. Since reset operation is a thermally driven process, frequent switching of resistive memory cell leads to a local accumulation of Joules heat, especially when the switching rate is faster than the heat removal rate. This investigation of local heating effects led to the exploration of non-local heat transfer within a memory array. In a crossbar arranged ReRAM cell array, heat generated in one device spreads via common electrode metal lines to the neighboring cells causing their performance degradation constituting non-local heat transfer mechanism leading to performance deterioration of neighboring cells. In addition to the electrical characterization of devices affected by the remote heat transfer, novel cell array architectures have been proposed and investigated with the goal to significantly mitigate the cell-to-cell thermal crosstalk. One of the possible mitigation measures would be modified cell erasure algorithm. / Doctor of Philosophy / Emerging memory technologies are being intensively investigated for extending Moore's scaling law in the next decade. The resistive random-access memory (ReRAM) is one of the most propitious contenders to replace the current ubiquitous FLASH memory. ReRAM shows unique nanoionics based filamentary switching mechanism. Compared to the current nonvolatile memory based on floating gate MOSFET transistor, the advantages of ReRAM include superior scalability, low power consumption, high OFF-/ON-state resistance ratio, excellent endurance, and long retention of the logic bit states. Besides the nonvolatile memory applications, resistive switching devices implement the function of a memristor which is the fourth basic electrical component and can be used for neuromorphic computing. A ReRAM device is in essence a metal-insulator-metal structure. One of the metal electrodes is called the active electrode and provides the building material for the filamentary connection between the electrodes. An important requirement of the second electrode, called the inert electrode, is to be immiscible with the metal atoms of the active electrode and to exhibit a minimum of susceptibility to structural changes and chemical reactions. This research presents a thorough investigation of the role and properties of the inert electrode and offers guideline for the optimal selection of the inert electrode in a commercially viable product. It has been found out that one important property of the inert electrode is its heat conductivity and also the way the inert electrode is embedded on a substrate. Consequently, the concept of the inert electrode has been replaced by the concept of engineered inert electrode module which evolved from a single metal layer to a multilayer stack displaying glue layers, high thermal conductivity layers dissipating the heat quickly, and diffusion stop layers eliminating unwanted chemical reactions. The investigation of the electro-thermal effects led to the discovery of the cell-to-cell thermal cross talk within the memory array which can seriously affect the performance of cells impacted by the remote heat transfer. When a memory cell is switched repeatedly a considerable amount of heat is dissipated in the cell and the heat may spread to neighboring cells that share the same metal lines. This heat transfer causes degradation of electrical performance of the neighboring cells. A method has been developed to characterize quantitatively how the electrical performance is affected by the thermal cross-talk impacting the electric performance of neighboring cells. Several novel mitigation strategies of new memory array architectures have been proposed and investigated.

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<p>In the pursuit to develop safe and reliable lithium-ion batteries, it is imperative to understand all the variabilities that revolve around electrodes. Current cutting-edge physics-based simulations employ an image-based technique. This technique uses images of electrodes to extract effective properties that are used in these physics-based simulations or employ the simulation on the structure itself. Though the electrode images have spatial variability, various particle morphology, and aberrations that need to be accounted for. This work seeks out to help quantify these variabilities and pinpoint uncertainties that arise in image-based simulations by using machine learning and other data analytic techniques. First, we looked at eighteen graphite electrodes with various particle morphologies to gain a better understanding on how heterogeneity and anisotropy interplay with each other. Moreover, we wanted to see if higher anisotropic particles led to greater heterogeneity, and a higher propensity for changes in effective properties. Multiple image-based algorithms were used to extract tortuosity, conductivity, and elucidate particle shape without the need for segmentation of individual particles. What was found is highly anisotropic particles induces greater heterogeneity in the electrode images, but also tightly packed isotropic particles can do the same. These results arise from porous pathways becoming bottlenecked, resulting in greater likelihood to change values with minimal changes in particle arrangement. Next, a model was deployed to see how these anisotropies and heterogeneities impact electrochemical performance. The thought of whether particle morphology and directional dependencies would have impact on plating energy and heat generation, leading to poor electrochemical performance. By using a pseudo-2D model, we elucidated that the larger the tortuosity the greater the propensity to plate and generate heat. Throughout these studies, it became clear that the segmentation of the greyscale images became the origin for subjectiveness to appear in these studies. We sought to quantify this through machine learning techniques, which employed a Bayesian convolutional neural network. By doing so we aimed to see if image quality impacts uncertainties in our effective properties, and whether we might be able to predict this from image characteristics. Being able to predict effective property uncertainty through image quality did not prove possible, but the ability to predict physics properties based on geometric was able to be done. With the largest uncertain particles occurring at the phase boundaries, morphologies that have a large specific surface area presented with the highest structural uncertainty. Lastly, we wanted to see the impact carbon binder domain morphology uncertainty impacts our effective properties. By using a set of sixteen NMC electrodes, which specify the carbon binder domain weight percentage, we can see how uncertainties in morphology, segmentation, spatial variability, and manufacturing variability impact effective properties. We expected there to be an interplay on which uncertainty impacts various effective properties, and if manufacturing variability plays a large role in determining this. By using surrogate models and statistical methods, we show that there is an eb and flow in uncertainties and effective properties are dependent on which uncertainty is being changed.</p>

The filiform-like corrosion of Magnesium (Mg) alloys AZ31B and AM30 was investigated with electrochemical and microanalytical techniques. Potentiodynamic polarization testing and scanning vibrating electrode technique (SVET) measurements confirmed the “differential electrocatalytic” mechanism previously reported for filiform and filiform-like corrosion on pure Mg and AZ31B. Transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) revealed that the MgO corrosion filaments on both alloys were likely a product of the direct reaction of Mg and water (H2O), responsible for the rapid hydrogen (H2) evolution observed at the propagating corrosion fronts. TEM analysis also revealed through-thickness cracks and noble intermetallic particles within the corrosion filaments and noble metal enrichment at the corrosion filament/metal interfaces, which were proposed to play significant roles in the cathodic activation of the corrosion filaments. The higher susceptibility of the AZ31B alloy to cathodic activation versus AM30 suggested that Zinc (Zn) has a detrimental effect on the resistance of Magnesium-Aluminum-Zinc (Mg-Al-Zn) alloys to filiform and filiform-like corrosion. / Thesis / Master of Applied Science (MASc)

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24925号 / エネ博第467号 / 新制||エネ||87(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 佐川 尚, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Wire electrical discharge machining (WEDM) is a precision machining process that uses electrical discharges struck between an axially moving wire electrode and the workpiece to remove material through melting and vaporization. WEDM is replacing traditional processes like broaching for machining safety-critical components such as the turbine disk in the manufacture of fuel-efficient jet engines. The main issue preventing the more widespread use of WEDM is that due to WEDM being less productive than broaching, it currently requires 6 WEDM machine tools to replace a single broaching machine to maintain the same throughput. The main factor limiting WEDM productivity is wire breakage. To increase the Cutting Rate (CR) more power is required, and increasing power also increases the likelihood of breakage. The goal of this research is to determine whether wires thicker than the conventional 0.25 mm diameter will both optimize the cutting rate and minimize breakage. Thicker wires will allow for an increase in the duty factor, with a significantly decreased incidence of wire breakage. Given that an increased wire diameter also increases the kerf width, this research seeks to identify the optimal wire diameter that maximizes the linear cutting rate. This research concluded that using wire of optimal diameter in WEDM increases the CR by as much as 400%. / Thesis / Master of Science in Mechanical Engineering (MSME) / In order to secure jet engine blades onto the engine, complex features called Firtree Root Forms (FTRF) are used. These features need to be very precisely cut in order for the engine to work at peak efficiency. Currently, industry is using a manufacturing process called broaching to machine these FTRFs, however broaches wear out over time, which causes imprecise cuts. The solution to this is to use Wire Electrical Discharge Machining (WEDM). The problem with WEDM is that it takes on average 6 machines to replace a single broaching machine in terms of productivity. The objective of this project is to increase the cutting speed (and thus productivity) of WEDM, and one of the ways to do that is to increase the electrode wire diameter. This allows for more power to be used in the machining process without the risk of wire breakage, which is a major problem when cutting with WEDM. The research presented in this thesis successfully demonstrates that using thicker wires in WEDM can cut as much as 400% faster than the wires currently in common use in industry.

<p>  </p> <p>As our ability to make more sensitive measurements increases, we begin to reach for the ultimate measurement sensitivity: a single entity. Mass spectrometry and fluorescence-based methods exist for single entity studies, and through these the intriguing effects of confinement begin to be observed. These mass spectrometry and fluorescence-based techniques are however, often destructive, which precludes measurements over time. The advent of nanoelectrochemical methods, however, allows for the creation of tools that can make measurements inside of confined volumes—a droplet, a cell, <em>etc</em>.—over extended periods of time. While most nanoelectrochemical methods are based on amperometry or voltammetry, potentiometry allows for minimal perturbation of the system’s homeostasis by passing negligible current.</p> <p>To overcome many of the issues involved in these other methods, we design potentiometric tools that can be easily miniaturized to fit within single entities, particularly within single cells. These tools include enzymatic biosensors designed to be minimally influenced by the cell’s O2 or NAD+ levels, as well as a novel reference electrode that can be made to be very low leakage or completely leakless to avoid contamination of the cell by the reference electrode’s internal filling solution. Both of these tools are designed such that they are easily modified to suit a number of different applications, including for use inside non-aqueous solutions.</p>

Over the past several years as interest in cleaner energy sources has grown nuclear power has come to the forefront. However, as interest in nuclear power grows so does the concern over the amount of high-level radioactive waste produced. Currently, the most popular way to deal with spent nuclear fuel is interim storage until a viable treatment option becomes available. Simply waiting for spent fuel to become safe to handle will take thousands of years and is not a reasonable long-term solution. We will soon run out of space in our spent fuel pools and while more dry storage space can be found it is not an ideal solution. One answer to this problem is the reprocessing of spent nuclear fuel. This could be done with either the plutonium uranium reduction extraction (PUREX) method or the pyroprocessing method. Since PUREX does not have the same level of built-in proliferation resistance as pyroprocessing, pyroprocessing is starting to be seen as a good alternative method. Pyroprocessing would take the spent nuclear fuel from a light water reactor and make it into a metal-based fuel that could be used in certain advanced reactors. Molten salt reactors are of particular interest when it comes to reprocessing spent nuclear fuel because of their unique property of using a liquid fuel. Molten salt reactors and spent fuel reprocessors could be directly connected which would save both time and money as little storage and transportation would need to be considered. Regardless of how and where the used nuclear fuel is being recycled it is important to be able to keep track of the major actinides and fission products in the fuel as it moves through the process. Electrochemical concentration measurements are straightforward and well understood in static cases when there is only a single element to consider. When additional elements are added, or the system is flowing rather than static, things get slightly more complicated but are still decently well understood. However, in the case of spent fuel reprocessing the system is both be flowing and contains much more than a single element. This case is not well understood and is what this study attempts to understand. Two different rotating electrodes were designed to simulate flowing conditions in an electrochemical cell. The first was a tungsten rotating disk electrode (RDE) and the second was a graphite RDE. We were not able to fully insulate the tungsten RDE and were therefore unable to achieve reliable results. Because of this the tungsten design was put aside in favor of the graphite design, which did prove to be sufficiently insulated. The graphite RDE was tested in two different salt systems: LiCl-KCl-NiCl2-CrCl2 and LiCl-KCl-EuCl3-SmCl3. In the nickel-chromium system the graphite RDE produced the expected results. The calculated nickel concentration was found to be within 10% of the measured concentration. Calculations of the chromium concentration, however, were not possible due to the deposition of nickel on the graphite surface, which increased the surface area of the working electrode. When the graphite RDE was tested in the second system it was first tested in the ternary salt LiCl-KCl-EuCl3 and was able to produce decent results. The concentration of europium calculated from the scan was within 10% of the measured value. When the RDE was tested in the LiCl-KCl-EuCl3-SmCl3 salt the results did not come out as expected. Several rather noisy CV curves were obtained and no alterations to the cell seemed to affect them. At this point it was determined that the reason for the confused scans was a connection problem that could not be remedied within the time frame of this study. While this study does not accomplish the task it set out to do, it is a good step in the direction toward understanding flowing systems containing more than a single element of interest and has successfully designed a reliable graphite RDE. / Master of Science / As interest in nuclear power continues to grow, so does the concern over the amount of high-level nuclear waste produced. More nuclear power means more nuclear reactors and thus more spent nuclear fuel to be dealt with. Currently most used nuclear fuel ends up in interim storage facilities where it is meant to wait until it is safe to handle, which could take several thousand years, or until a reliable disposal method is determined. On this path the amount of spent fuel that requires storage will quickly overrun the amount of storage space safely available. One way to reduce the amount of nuclear waste is to reprocess it to be used as fuel for different types of reactors. The pyroprocessing method takes the spent nuclear fuel from a typical light water reactor and recycles it into fuel that can be used in certain types of advanced reactors, such as molten salt reactors (MSR) and sodium-cooled fast reactors (SFR). The reprocessing system works to separate the usable actinide elements, such as uranium and plutonium, from any fission products or other contaminants. During these processes it is important to be able to keep track of the concentrations of each of these different elements to ensure proper separation. This study examines the use of two rotating disk electrode (RDE) designs that are meant to simulate the flowing conditions found in many reprocessing systems. These RDEs were to be used to measure the concentrations of different elements in molten salt systems. The first design, a tungsten RDE, could not be properly insulated and thus was unable to produce reliable results when tested in the electrochemical cell. The second design was a graphite RDE. This design did prove to be properly insulated and was able to produce good results when tested in the cell. The graphite RDE was tested in both LiCl-KCl-NiCl2-CrCl2 and LiCl-KCl-EuCl3-SmCl3. In the first system the concentration of nickel was correctly calculated using the data collected with the graphite RDE, while the chromium concentration could not be due to the nickel deposition on the graphite. In the second system, good results were obtained before the SmCl3 was added to the salt. At this point a connection error became apparent and reliable results were no longer possible. Further study is needed to understand the LiCl-KCl-EuCl3-SmCl3 system using the graphite RDE.

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