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Den additiva tillverkningsindustrin associeras ofta med en hållbar hushållning av resurser. Trots detta har denna industriavfall med stor potential till vidare användning. Detta kandidatexamensarbete undersöker möjligheten att använda Polyamid 12 (PA12) avfall från selektiv lasersintring (SLS) i formen av filament till en annan additiv tillverkningsmetod, friformsframställning. Avfall från olika delar av SLS-processen försågs av life science företaget Cytiva. Olika blandningar av avfallen extruderades till filament. Innan extrudering undersöktes innehåll samt de termiska egenskaperna hos avfallet med hjälp av differentiell skanningskalometri (DSC), termogravimetrisk analys och fourier transform infraröd spektroskopi. Filamenten analyserades med dragprovning, DSC och svepelektronmikroskopi. Rent obearbetat PA12 pulver och ett kommersiellt PA12 filament användes som referenser vid jämförelse. Blandningarna innehållande avfall från SLS-printerns automatiska rengöringsprocess var kontaminerade med glaspartiklar och blev därför spröda och erhöll en skrovlig yta. Dessa filament gick inte att 3D-printa eftersom glaspartiklar ansamlades i munstycket av printern. Flera filament fick en ojäm och liten diameter och kunde därför inte användas i 3D-printern. Ojämn och liten diameter blev resultatet av bekymmer med sensorn som mäter diametern av filamenten. Dessa filament gjordes av granulerade utskrivna prototyper, pulver nära utskrivna delar och silat pulver långt ifrån de utskrivna delarna. Filament gjorda på blandningar innehållande avfall från dammsugaren som används för att rengöra SLS-printern kunde med framgång skrivas ut i 3D-printern. Dessa filament hade en jämn diameter och innehöll inga större kontamineringar. Det är därför genomförbart att tillverka filament av avfall från alla delar av SLS-processen även om inte alla filament kunde 3D-printas. / Additive manufacturing is often associated with sustainable use of resources. However, this industry still has material waste with great potential for further use. This bachelor thesis examines the opportunity of using Polyamide 12 (PA12) waste from Selective laser sintering (SLS) in the form of filaments for another additive manufacturing method, fused filament fabrication. Waste from different parts of the SLS process were provided from the life science company Cytiva. Several blends of the waste were made into filaments. Prior extrusion, the thermal properties and content of the waste were examined with differential scanning calorimetry (DSC), thermogravimetric analysis and fourier transform infrared spectroscopy. The filaments were analyzed by tensile testing, DSC and scanning electron microscopy. Pure virgin powder of PA12 and a commercial PA12 filament were used as a reference for comparison. The blends containing waste from the SLS printer’s automatic blasting was contaminated with glass beads which resulted in brittle filaments with a rough surface. These filaments were not possible to 3D print with due to accumulation of glass beads in the nozzle of the printer. Several filaments got an uneven and small diameter and could therefore not be 3D printed with. The small and uneven diameter was a result of issues with the sensor measuring the diameter. This includes filaments made of granulated printed prototypes, powder close to the printed parts and sieved powder further away from printed parts. The filaments made of blends including waste from the vacuum cleaner used to clean the SLS printer could successfully be used in printing. These filaments had an even diameter and did not contain any larger contaminations. It is therefore possible to make filaments from waste from all parts of the SLS process although not all filaments could be 3D printed.

Nonlinear dispersive partial differential equations occur in a variety of areas within mathematical physics and engineering. We study several classes of such equations, including scalar complex partial differential equations, vector partial differential equations, and finally non-local integro-differential equations. For physically interesting families of these equations, we demonstrate the existence (and, when possible, stability) of specific solutions which are relevant for applications. While multiple application areas are considered, the primary application that runs through the work would be the nonlinear dynamics of vortex filaments under a variety of physical models. For instance, we are able to determine the structure and time evolution of several physical solutions, including the planar, helical, self-similar and soliton vortex filament solutions in a quantum fluid. Properties of such solutions are determined analytically and numerically through a variety of approaches. Starting with complex scalar equations (often useful for studying two-dimensional motion), we progress through more complicated models involving vector partial differential equations and non-local equations (which permit motion in three dimensions). In many of the examples considered, the qualitative analytical results are used to verify behaviors previously observed only numerically or experimentally.

The time-resolved electrical conductivity of a short-pulse generated plasma filament in air was studied. Close-coupled metal electrodes were used to discharge the stored energy of a high-voltage capacitor and the resulting microsecond-scale electrical discharge was measured using fast current sensors. Significant differences in the time dependence of the current were seen with the two electrode geometries used. Using sharp-tipped electrodes additional peaks in the time-resolved conductivity were seen, relative to the single peak seen with spherical electrodes. We attribute these additional features to secondary electron collisional ionization brought about by field enhancement at the tips. Additional discrepancies in the currents measured leaving the high-voltage electrode and that returning to ground were also observed. Implications for potential laser-induced discharge applications will be discussed.

Laser filamentation is a non-diffracting propagation regime consisting of an intense core that is surrounded by an energy reservoir. For laser ablation based spectroscopy techniques such as Laser Induced Breakdown Spectroscopy (LIBS), laser filamentation enables the remote delivery of high power density laser radiation at long distances. This work shows a quasiconstant filament-induced mass ablation along a 35 m propagation distance. The mass ablated is sufficient for the application of laser filamentation as a sampling tool for plasma based spectroscopy techniques. Within the scope of this study, single-shot ablation was compared with multi-shot ablation. The dependence of ablated mass on the number of pulses was observed to have a quasi-linear dependence on the number of pulses, advantageous for applications such as spectroscopy. Sample metrology showed that both physical and optical material properties have significant effects on the filament-induced ablation behavior. A relatively slow filament-induced plasma expansion was observed, as compared with a focused beam. This suggests that less energy was transferred to the plasma during filamentinduced ablation. The effects of the filament core and the energy reservoir on the filamentinduced ablation and plasma formation were investigated. Goniometric measurements of the filament-induced plasma, along with radiometric calculations, provided the number of emitted photons from a specific atomic transition and sample material. This work advances the understanding of the effects of single filaments on the ablation of solid materials and the understanding of filament-induced plasma dynamics. It has lays the foundation for further quantitative studies of multiple filamentation. The implications of this iv work extend beyond spectroscopy and include any application of filamentation that involves the interaction with a solid material

Emerging memory technologies are being intensively investigated for extending Moore\'s law in the next decade. The conductive bridge random access memory (CBRAM) is one of the most promising candidates. CBRAM shows unique nanoionics-based filamentary switching mechanism. Compared to flash memory, the advantages of CBRAM include excellent scalability, low power consumption, high OFF-/ON-state resistance ratio, good endurance, and long retention. Besides the nonvolatile memory applications, resistive switching devices implement the function of memristor which is the fourth basic electrical component. This research presents the characterization and modeling of Cu/TaOx/Pt resistive switching devices. Both Cu and oxygen vacancy nanofilaments can conduct current according to the polarity of bias voltage. The volatile resistive switching phenomenon has been observed on Cu/TaOx/delta-Cu/Pt devices and explained by a flux balancing model. The resistive devices are also connected in series and in anti-parallel manner. These circuit elements are tested for chaotic neural circuit. The quantum conduction has been observed in the I-V characteristics of devices, evidencing the metallic contact between the nanofilament and electrodes. The model of filament radial growth has been developed to explain the transient I-V relation and multilevel switching in the metallic contact regime. The electroforming/SET and RESET processes have been simulated according to the mechanism of conductive filament formation and rupture and validated by experimental results. The Joule and Thomson heating effects have also been investigated for the RESET processes. / Ph. D.

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.

Resistive random access memory (RRAM), based on a two-terminal resistive switching device with a switching element sandwiched between two electrodes, has been an attractive candidate to replace flash memory owing to its simple structure, excellent scaling potential, low power consumption, high switching speed, and good retention and endurance properties. However, due to the current limited understanding of the device mechanism, RRAMs research are still facing several issues and challenges including instability of operation parameters, the relatively high reset current, the limited retention and the unsatisfactory endurance. In this study, we investigated the switching mechanisms, conditions and applications of oxygen vacancy (Vo) filament formation in resistive memories. By studying the behavior of conductive Vo nanofilaments in several metal/oxide/metal resistive devices of various thicknesses of oxides, a resulting model supported by the data postulates that there are two distinct modes of creating oxygen vacancies: i) a conventional bulk mode creation, and ii) surface mode of creating oxygen vacancies at the active metal-dielectric interface. A further investigation of conduction mechanism for the Vo CF only based memories is conducted through insertion of a thin layer of titanium into a Pt/ Ta2O5/Pt structure to form a Pt/Ti/ Ta2O5/Pt device. A space charge limited (SCL) conduction model is used to explain the experimental data regarding SET process at low voltage ranges. The evidence for existence of composite copper/oxygen vacancy nanofilaments is presented. The innovative use of hybrid Vo/Cu nanofilament will potentially overcome high forming voltage and gas accumulation issues. A resistive floating electrode device (RFED) is designed to allow the generation of current/voltage pulses that can be controlled by three independent technology parameters. Our recent research has demonstrated that in a Cu/TaOx/Pt resistive device multiple Cu conductive nanofilaments can be formed and ruptured successively. Near the end of the study, quantized and partial quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10. / Ph. D.

Owing to its excellent scaling potential, low power consumption, high switching speed, and good retention, and endurance properties, Resistive Random Access Memory (RRAM) is one of the prime candidates to supplant current Nonvolatile Memory (NVM) based on the floating gate (FG) MOSFET transistor, which is at the end of its scaling capability. The RRAM technology comprises two subcategories: 1) the resistive phase change memory (PCM), which has been very recently deployed commercially, and 2) the filamentary conductive bridge RAM (CBRAM) which holds the promise of even better scaling potential, less power consumption, and faster access times. This thesis focuses on several aspects of the CBRAM technology. CBRAM devices are based on nanoionics transport and chemo-physical reactions to create filamentary conductive paths across a dielectric sandwiched between two metal electrodes. These nano-size filaments can be formed and ruptured reliably and repeatedly by application of appropriate voltages. Although, there exists a large body of literature on this topic, many aspects of the CBRAM mechanisms and are still poorly understood. In the next paragraph, the aspects of CBRAM studied in this thesis are spelled out in more detail. CBRAM cell is not only an attractive candidate for a memory cell but is also a good implementation of a new circuit element, called memristor, as postulated by Leon Chua. Basically, a memristor, is a resistor with a memory. Such an element holds the promise to mimic neurological switching of neuron and synapses in human brain that are much more efficient than the Neuman computer architecture with its current CMOS logic technology. A memristive circuitry can possibly lead to much more powerful neural computers in the future. In the course of the research undertaken in this thesis, many memristive properties of the resistive cells have been found and used in models to describe the behavior of the resistive switching devices. The research performed in this study has also an immediate commercial application. Currently, the semiconductor industry is faced with so-called latency scaling dilemma. In the past, the bottleneck for the signal propagation was the time delay of the transistor. Today, the transistors became so fast that the bottleneck for the signal propagation is now the RC time delay of the interconnecting metal lines. Scaling drives both, resistance and parasitic capacitance of the metal lines to very high values. In this context, one observes that resistive switching memory does not require a Si substrate. It is therefore an excellent candidate for its implementation as an o n-chip memory above the logic circuits in the CMOS back-end, thus making the signal paths between logic and memory extremely short. In the framework of a Semiconductor Research Corporation (SRC) project with Intel Corporation, this thesis investigated the breakdown and resistive switching properties of currently deployed low k interlayer dielectrics to understand the mechanisms and potential of different material choices for a realization of an RRAM memory to be implemented in the back-end of a CMOS process flow. / Master of Science

Ceramic additive manufacturing is gaining popularity with methods like selective laser sintering (SLS), binder jetting, direct ink writing and stereolithography, despite their disadvantages. Laser sintering and binder jetting are too expensive, while direct ink writing lacks resolution and stereolithography lacks scalability. The project aims to combine one of the most versatile, affordable, and readily available 3D printing methods: fused filament fabrication (FFF) with polymer derived ceramics to produce cellular ceramics to overcome the disadvantages posed by the other methods. The process uses a two-step approach. The first step is to 3D print the part using a polymer FFF 3D printer with a thermoplastic polyurethane filament and the second step is to impregnate the part in a polysilazane preceramic polymer and then pyrolyze it in an inert environment up to 1200C. The resulting product is a high-resolution cellular ceramic of the composition SiOC(N). This type of cellular ceramic can find an application in several fields such as scaffolds for bone tissue regeneration, liquid metal filtering, chemical and gas filtering, catalytic converters and electric applications. The process can provide an affordable alternative to the products used in these fields currently.