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Inertial sensing technologies, including accelerometers and gyroscopes, have been invaluable in numerous fields ranging from consumer electronics to healthcare and clinical practices. Inertial measurement units, specifically accelerometers, represent the most widely used microelectromechanical systems (MEMS) devices with excellent and reliable performance. Although MEMS-based accelerometers have many attractive attributes, such as their tiny footprint, high sensitivity, high reliability, and multiple functionalities, they are limited by their complex and expensive microfabrication processes and cumbersome, fragile structures that suffer from mechanical fatigue over time. Moreover, the rigid nature of beams and spring-like structures of conventional accelerometers limit their applications for wearable devices and soft-human machine interfaces where physical compliance that is compatible with human skin is a priority. In this dissertation, the development of novel practical resistive and capacitive-type inertial sensors using liquid metal as a functional proof mass material is presented. Utilizing the unique electromechanical properties of liquid metal, the novel inertial sensor design confines a graphene-coated liquid metal droplet inside tubular and 3D architectures, enabling motion sensing in single and multiple directions. Combining the graphene-coated liquid metal droplet with printed sensing elements offers a robust fatigue-free alternative material for rigid, proof mass-based accelerometers. Resistive and capacitive sensing mechanisms were both developed, characterized, and evaluated. Emerging rapid fabrication technologies such as direct laser writing and 3D printing were mainly adopted, offering a scalable fabrication strategy independent of advanced microfabrication facilities. The developed inertial sensor was integrated with a programmable system on a chip (PSoC) to function as a stand-alone system and demonstrate its application for real-time- monitoring of human health/ physical activity and for soft human-machine interfaces. The developed inertial sensor architecture and materials in this work offer a new paradigm for manufacturing these widely used sensors that have the potential to complement the performance of their silicon-based counterparts and extend their applications.

Despite the high demand for press-hardenable steel (PHS) with coatings that provide sacrificial cathodic protection, Zn-based coatings have experienced limited use due to the significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection when using conventional PHS materials and processing techniques. The present research addresses these challenges by reducing the conventional direct hot press forming (DHPF) temperature to between 600–700 °C, such that forming and quenching occurs well below the Zn(Fe)liq → Г-Fe3Zn10 peritectic temperature of 782 °C, thereby removing the conditions necessary for LME to occur while allowing for formation of the cathodically-protective Г-Fe3Zn10 phase. The objective of this work was to define a process window for two galvanized prototype PHS alloys with compositions of 0.20C-2.01Mn-0.26Si-0.005B and 0.19C-2.5Mn-0.26Si-0.005B (wt%) that would result in fully martensitic microstructures, tensile strengths (TS) ≥ 1500 MPa, and robust cathodic protection, defined as attaining ≥ 15 vol% Г-Fe3Zn10 in the coating microstructure, while avoiding LME. Accomplishing this task involved characterizing both grades as a function of austenization time, stamping temperature, and strain imposed by the forming process in order to define process windows that resulted in parts that met the aforementioned property requirements. It was found that the approach of increasing the Mn content relative to conventional PHS grades was successful in improving the hardenability sufficiently to enable the formation of fully martensitic microstructures despite the lower effective cooling rates associated with the reduced DPHF temperatures. Microstructural imaging and tensile testing demonstrated that, for both prototype PHS grades, a process window exists for the production of parts that satisfy the targets of the formation of fully martensitic microstructures and TS ≥ 1500 MPa while exhibiting uniform elongation of about 0.05 followed by significant post-uniform elongation. The effect of DHPF temperature and strain imposed by the forming process on mechanical properties was found to be negligible. Tensile tests and fractography revealed that reducing the DHPF temperature to between 600–700 °C was successful in preventing LME, thereby allowing samples to fracture in a ductile manner. Micro-cracking in the coating of the DHPF part was observed; however, these cracks were arrested at the coating-substrate interface. For all tested conditions, the coating met the target of ≥ 15 vol% Г-Fe3Zn10, implying that robust cathodic protection is expected. Based on the results of these experiments, it was concluded that DHPF process windows that meet all of the property targets include austenization times and DHPF temperatures of 120–180 s and 600–700 °C, respectively, for the 2Mn grade, and 60–180 s and 600–700 °C, respectively, for the 2.5Mn grade. / Thesis / Master of Science (MSc)

<p dir="ltr">Increasing power density and power consumption in electronic devices require heat dissipating components with high thermal conductivity to prevent overheating and improve performance and reliability. Polymers offer the advantages of low cost and weight over conventional metallic components, but their intrinsic thermal conductivity is low. Previous studies have shown that the thermal conductivity of polymers can be enhanced by aligning the polymer chains or by adding high thermal conductivity fillers to create percolation paths within the polymeric matrix. To further enhance the in-plane thermal conductivity, the conductive fillers can be aligned preferentially, but this leads to a lower increase in performance in the cross-plane direction. Yet, the cross-plane thermal conductivity plays a vital role in dissipating heat from active devices and transmitting it to the surrounding environment. Alternatively, when the fillers are aligned to enhance cross-plane thermal transport, the enhancement in the in-plane direction is limited. There is a need to develop polymer composites with an approximately isotropic increase in thermal performance compared to their neat counterparts.</p><p dir="ltr">To achieve this goal, in this study, I combine conductive fibers and fillers to enhance thermal conductivity of polymers without significantly inducing thermal anisotropy while preserving the mechanical performance of the matrix. I employ three approaches to enhance the thermal conductivity () of thermoset polymeric matrices. In the first approach, I fabricate thermally conductive polymer composites by creating an emulsion consisting of eutectic gallium indium alloy (EGaIn) liquid metal in the uncured polydimethylsiloxane (PDMS) matrix. In the second approach, I infiltrate mats formed from chopped fibers of Ultra High Molecular Weight Polyethylene (UHMWPE) with an uncured epoxy resin. Finally, the third approach combines the two previous methods by infiltrating the UHMWPE fiber mat with an emulsion of the liquid metal and uncured epoxy matrix.</p><p dir="ltr">To evaluate the thermal performance of the composites, I use infrared thermal microscopy with two different experimental setups, enabling independent measurement of in-plane and cross-plane thermal conductivity. The results demonstrate that incorporating thermally conductive fillers enhances the overall conductivity of the polymer composite. Moreover, I demonstrate that the network structure achieved by the fiber mat, in combination with the presence of liquid metal, promotes a more uniform increase in the thermal conductivity of the composite in all directions. Additionally, I assess the impact of filler incorporation and filler concentration on matrix performance through tension, indentation, and bending tests for mechanical characterization of my materials.</p><p dir="ltr">This work demonstrates the potential of strategic composite design to achieve polymeric materials with isotropically high thermal conductivity. These new materials offer a solution to the challenges posed by higher power density and consumption in electronics and providing improved heat dissipation capabilities for more reliable devices.</p>

X-ray phase-contrast imaging has seen rapid development in recent decades due to its superior performance in imaging low-absorption objects, compared to traditional attenuation x-ray imaging. Having higher demand on coherence, x-ray phase-contrast imaging is performed mostly at synchrotrons. With the development of different imaging techniques, and the development of laboratory sources and x-ray optics, x-ray phase-contrast imaging can now be implemented on laboratory systems, which is promising and practical for broader range of applications. The subject of this thesis is the implementation, development and comparison of different laboratory phase-contrast methods using a liquid-metal-jet source. The three x-ray phase-contrast imaging methods included in this thesis are the propagation-, grating-, and speckle-based techniques. The grating-based method has been implemented on a laboratory system with a liquid-metal-jet source, which yields several times higher brightness than a standard solid-anode microfocus source. This allows shorter exposure time or a higher signal-to-noise ratio. The performance of the grating-based method has been experimentally and numerically compared with the propagation-based method, and the dose required to observe an object as a function of the object’s diameter has been investigated with simulations. The result indicates a lower dose requirement for the propagation-based method in this system but a potential advantage for the grating-based method to detect relatively large samples using a monochromatic beam. The speckle-based method, both the speckle-tracking and speckle-scanning techniques, has been implemented on a laboratory system for the first time, showing its adaptability to radiation of low temporal coherence. Tomography has been performed and shows the potential applications of this method on quantitative analysis on both absorption and phase information of materials. As a basis for further optimization and comparisons to other methods, the noise properties of the differential phase contrast of the speckle-based method have been studied and an analytical expression for the noise variance introduced, showing a similarity to the grating-based method. / Faskontrastavbildning med röntgenstrålning är en teknik som har utvecklats kraftigt de senaste årtiondena, eftersom den fungerar bättre än traditionella, absorptionsbaserade röntgenundersökningar för objekt med låg absorption. Den har dock höga krav på koherens, vilket gjort att den huvudsakligen används vid stora synkrotron-anläggningar. Tack vare utveckligen av nya avbildningstekniker, laboratoriekällor och röntgenoptik kan numera faskontrast användas även med laboratoriesystem, vilket är lovande då tekniken kan användas vid ett större antal olika tillämpningsområden Denna avhandling syftar till att tillämpa, utveckla och jämföra olika faskontrastmetoder i laboratoriemiljö, med en metallstråleröntgenkälla. De tre faskontrastmetoderna som behandlas i denna avhandling är propogation, gitter och speckelbaserad faskontrast. Den gitterbaserade metoden har implementerats i ett laboratoriesystem med en metallstrålekälla som ger flera gånger högre radians än en vanlig, fast mikrofokuskälla. Den högre radians en möjliggör kortare exponeringstider eller högre signal-brusförhållande. Den gitterbaserade tekniken har jämförts experimentellt och numeriskt med den propageringsbaserade metoden. Den strålningsdos som krävs för observera ett objekt, som funktion av dess diameter, har jämförts för de båda teknikerna, den här gången via simuleringar. Resultaten visar på en lägre strålningsdos för den propagationsbaserade tekniken i detta fall, men även att det finns en potentiell fördel för den gitterbaserade tekniken för något större objekt med monokromatisk röntgenstrålning. Speckelbaserade tekniker, nämare bestämt den som bygger på att spåra speckel och den som bygger på att scanna diffusorn, har för första gången implementerats i laboratoriemiljö. Därmed har visats att de fungerar även för strålning med låg tidskoherens. Tekniken har även använts för tomografi och visar möjliga tillämpningar inom kvantitativ analys av material. För att förenkla framtida optimeringar och jämförelser av tekniken med andra metoder, har brusegenskaperna för den speckelbaserade metoden studerats och visat sig likna den gitterbaserade metoden. / <p>QC 20160921</p>

"New, economical semi-solid metal (SSM) processes rely on forced convection during solidification to influence non-dendritic growth. The fundamental mechanisms that produce SSM microstructures in the presence of forced convection (due to fluid flow) are not fully understood. The objective of this work is to elucidate these mechanisms through the use of a new semi-solid slurry-making technique. Employing an apparatus developed at WPI, two alloy melts are mixed within a static reactor that induces convection and rapid cooling. Experiments carried out with this apparatus, named the “Continuous Rheoconversion Process” (CRP), result in globular semi-solid microstructures throughout a wide range of processing conditions. These conditions include the superheat in the melts before mixing, cooling rate of the slurry through the SSM range, and the presence or absence of inoculants in the melts. The results comprise repeatable sets of semi-solid microstructures having fine particle size and shape factors approaching unity. Even in the absence of melt inoculants, uniform distributions of α-Al particle sizes of about 60µm are attainable. Entrapped liquid is not present in the majority of the samples obtained with the CRP, and irregular particles that form in the process are of a limited distribution. Variation of slurry analysis methods indicates that these structures can be obtained consistently for both thixocasting and rheocasting applications. The design of the mixing reactor leads to turbulent fluid flow just as solidification commences. The results suggest that the following factors must be considered in identifying the mechanisms operating under the above conditions: copious nucleation of the primary phase; dispersion of nuclei throughout the bulk liquid; and inhibited remelting of nuclei due to temperature uniformity. In the CRP, these factors consistently lead to suppression of dendritic growth, significant grain refinement, and globular slurries. The exact fundamental mechanism leading to this effect is yet to be uncovered; however it is clear that temperature gradients ahead of the liquid are such that a cellular, non-dendritic morphology is the most stable growth form. Through further exploration of the process and identification of the operating mechanisms, future development of economical, continuous rheocasting methods will be facilitated."

Next generation nuclear power plants (GEN-IV) will be capable of not only producing energy in a reliable, safe and sustainable way, but they will also be capable of reducing the amount of nuclear waste, which has been accumulated over the lifetime of current-generation nuclear power plants, through transmutation. Due to the use of new and different coolants, existing computational tools need to be tested, further developed and improved in order to thermal-hydraulically design these power plants.This work covers two different non-unity Prandtl number fluids which are considered as coolants in GEN-IV reactors, liquid lead/lead-bismuth-eutectic and supercritical water. The study investigates different turbulence modeling strategies, such as Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) modeling, and their applicability to these proposed coolants. It is shown that RANS turbulence models are partly capable of predicting wall heat transfer in annular flow configurations. However, improvements in these prediction should be possible through the use of advanced turbulence modeling strategies, such as the use of separate thermal turbulence models. A large blind benchmark study of heat transfer in supercritical water showed that the available turbulence modeling strategies are not capable of predicting deteriorated heat transfer in a 7-rod bundle at supercritical pressures. New models which take into account the strong buoyancy forces and the rapid change of the molecular Prandtl number near the wall occurring during the transition of the fluid through the pseudocritical point need to be developed. One of these strategies to take into account near-wall buoyancy forces is the use of advanced wall functions, which cannot only help in modeling these kind of flows, but also decrease computational time by 1 to 2 orders of magnitude. Different advanced wall function models were implemented in the open-source CFD toolbox OpenFOAM and their performance for different flows in sub- and supercritical conditions were evaluated. Based on those results, the wall function model UMIST-A by Gerasimov is recommended for further investigation and specific modeling tactics are proposed.Near-wall temperature and velocity behavior is important to and influenced by the wall itself. The thermal inertia of the wall influences the temperature in the fluid. However, a more important issue is how temperature fluctuations at the wall can induce thermal fatigue. With the help of LES thermal mixing in a simplified model of a control rod guide tube was investigated, including the temperature field inside the control rod and guide tube walls. The WALE sub-grid turbulence model made it possible to perform LES computations in this complex geometry, because it automatically adapts to near-wall behavior close to the wall, without the use of ad-hoc functions. The results for critical values, such as the amplitude and frequency of the temperature fluctuations at the wall, obtained from the LES computations are in good agreement with experimental results.The knowledge gained from the aforementioned investigations is used to optimize the flow path in a small, passively liquid-metal-cooled pool-type GEN IV reactor, which was designed for training and education purposes, with the help of 3D CFD. The computations were carried out on 1/4 of the full geometry, where the small-detail regions of the heat exchangers and the core were modeled using a porous media approach. It was shown that in order to achieve optimal cooling of the core without changing the global geometry a ratio of close to unity of the pressure drop over the core and the heat exchanger needs to be achieved. This is done by designing a bottom plate which channels enough flow through the core without choking the flow in the core. Improved cooling is also achieved by reducing heat losses from the hot leg through the flow shroud to the cold leg by applying thermal barrier coating similar to methods used in gas turbine design. / Nästa generations kärnkraftverk (GEN-IV) kan inte bara producera el på ett pålitligt, säkert och hållbart sätt, utan det kan också reducera mängden kärnavfall, som har producerats under tiden som man använt nuvarande generationen kärnkraftverk, genom att transmutera avfallen. Framtidens kärnkraftverk använder andra kylmedel än nuvarande kraftverk som t.ex. flytande bly, gas eller superkritiskt vatten. Det betyder att många beräkningsverktyg måste testas, utvecklas och förbättras så att man kan genomföra termohydrauliska designberäkningar. Den här avhandlingen omfattar två olika kylmedel, flytande bly och superkritiskt vatten, som har ett Prandtl-tal som skiljer sig från 1 och kommer att användas i GEN-IV reaktorer. Studien undersöker olika strategier för att modellera turbulens som Large Eddy Simulation (LES) och Reynolds-Averaged Navier-Stokes (RANS) och hur man kan använda dessa strategierna i beräkningar av strömning och värmetransfer i den nya kylvätskan. Undersökningen visar att RANS turbulensmodeller delvis kan förutsäga värmeöverföringen vid en vägg i en ringformad strömningsgeometri. Förbättringar av förutsägelsen ska vara möjlig genom användning av avancerade strategier för turbulensmodellering, t.ex. termiska turbulensmodeller. En stor prestandajämförelse för värmeöverföring i superkritiskt vatten visade att ingen av nuvarande strategier för turbulensmodellering kan förutsäga försämrad värmeöverföring i en 7-stavknippet under superkritiskt tryck. Nya modeller, som omfattar de starka flytkrafterna och den snabba förändringen av den molekulära Prandtl-tal vid väggen som uppstår när vätskan går genom pseudokritiska punkten, måste utvecklas. Avancerade väggfunktioner är en av strategierna som kan ta hänsyn till dessa fenomen. Väggfunktioner kan inte bara hjälpa till att modellera de typer av flöden som behövs utan kan också hjälpa till att sänka beräkningstiden med en eller två tiopotenser. Olika avancerade väggfunktioner i open-source beräkningsverktyget OpenFOAM implementerades och deras prestation i sub- och superkritiska vattenflödar värderades. Baserat på detta rekommenderas Gerasimovs modell för ytterligare utredning. Dessutom läggs olika strategier fram för att utöka modellens validitet till flöde med superkritiskt vatten i sammanband med försämrad och förbättrad värmeöverföring. Kunskap om beteendet av temperatur och hastighet i väggens närhet är viktigt för väggens integritet, detta då väggen även påverkar beteendet. Väggens termiska tröghet påverkar flödets temperatur och hastighet. Dock är ett ännu viktigare problem, som kan uppträda, är att temperaturfluktuationer kan framkalla termisk utmattning i en vägg. Med användning av LES utreds termisk blandning av varmt och kallt vatten i en simplifierad modell av ett styrstavsledrör, inklusive temperaturfältet i styrstaven och ledrörsväggen. Användningen av WALE LES-turbulensmodellen gör det möjligt att utföra beräkningar i den komplexa geometrin, detta eftersom modellen anpassar sig automatiskt till fenomenen nära väggen utan användning av ad-hoc funktioner. LES resultaten för alla värden som är viktiga för att bestämma utmattningsbeteende, som amplitud och frekvens av temperaturfluktuationer i väggens närhet och i väggen själv, är i god överensstämmelse med resultaten från experiment från KTH i samma geometri.Kunskapen som vunnits genom ovannämnda utredningar användes för att optimera den termohydrauliska designen av en liten, pool-typ GEN-IV reaktor som är passivt kyld med flytande bly. Reaktorn är designad som en utbildnings- och träningsreaktor och optimeringen genomfördes med hjälp av 3D CFD. Beräkningarna genomfördes på en fjärdedel av reaktorns hela geometrin. Regioner med små detaljer, som de åtta värmeväxlarna och reaktorns kärna, modellerades genom porösa material. Det visar sig att för att ha en optimal kylning av kärnan, utan att förändra reaktorns globala geometri, måste förhållandet mellan tryckförlust i reaktorkärnan och värmeväxlarna vara nära 1. Detta uppnås genom att designa plattan vid ingången till kärnan så att tillräckligt med bly flödar genom kärnan utan att kväva flödet i denna. Ytterligare en förbättring i reaktorkylningen uppnås genom att reducera värmeförlusten genom väggen som skiljer varm och kall vätska. Detta görs med en strategi som förekommer i gasturbinteknologin, genom att man lägger till ett tunt skikt av termiskt isolerande material på väggen, som reducerar värmeöverföring med ungefär 50%. / <p>QC 20151123</p> / THEMFA / GENIUS / THINS

Made available in DSpace on 2014-10-09T12:26:10Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:22Z (GMT). No. of bitstreams: 1 00951.pdf: 4937591 bytes, checksum: 160731d29ec9edf1fc78d0034f24638b (MD5) / Thesis (Doctorate) / IPEN/T / Massachusetts Institute of Technology - Cambridge, Mass - MIT

Nous avons étudié la sensibilité du T91 à la fragilisation par le sodium liquide. Une procédure expérimentale a été mise en place afin de procéder à des essais mécaniques en sodium sous atmosphère inerte. Grâce à l’introduction d’une étape préliminaire d’exposition au sodium, la mouillabilité du T91 par le sodium liquide et la structure de l’interface sodium/acier ont pu être étudiés en fonction des différents paramètres d’exposition. Une réduction significative des propriétés mécaniques est observée quand le mouillage de l’acier par le sodium est bon. L’utilisation de différentes teneurs en oxygène et en hydrogène suggère que l’oxygène joue un rôle primordial dans l’amélioration de la mouillabilité du T91. La sensibilité du phénomène de fragilisation à la vitesse de sollicitation et à la température a été caractérisée. A partir de ces résultats, l’existence d’une transition fragile/ductile, fonction de ces deux paramètres a été mis en évidence. L’étude de cette transition suggère qu’une étape de diffusion du sodium dans les joints de grains du T91 est limitante pour la fragilisation. Des analyses en microscopie électronique en transmission et par cartographies d’orientation de fissures arrêtées ont également permis de constater que le mode de rupture est interlatte ou intergranulaire, fournissant ainsi une image cohérente du processus de fragilisation. La même méthodologie a été appliquée à l’acier non allié XC10. Les résultats montrent un comportement en tous points similaire à celui de l’acier T91 et suggèrent donc un mécanisme commun pour les aciers cubiques centrés. De plus, ils confirment que la transition fragile ductile observée semble la conséquence d’une vitesse de propagation de fissure fragile relativement limitée. Cette propagation est activée thermiquement avec une énergie d’activation d’environ 50 kJ/mol. Enfin, il a été montré que l’acier austénitique 304L est également sensible à la fragilisation par le sodium liquide. Certaines surfaces de rupture témoignent clairement d’une rupture intergranulaire, cependant des interrogations persistent à propos du chemin de fissuration. / We studied the sensitivity of T91 steel to embrittlement by liquid sodium. An experimental procedure was set up to proceed to mechanical testing in sodium under an inert atmosphere. The introduction of a liquid sodium pre-exposure step prior to mechanical testing enabled the study of both the wettability of T91 by sodium and the structure of the sodium steel/interface as a function of the exposure parameters. The mechanical properties of T91 steel are significantly reduced in liquid sodium provided the wetting conditions are good. The use of varying oxygen and hydrogen concentrations suggests that oxygen plays a major role in enhancing the wettability of T91. The sensitivity of the embrittlement to strain rate and temperature was characterized. These results showed the existence of a ductile to brittle transition depending on both parameters. Its characterization suggests that a diffusion step is the limiting rate phenomenon of this embrittlement case. TEM and EBSD analysis of arrested cracks enabled us to establish that the fracture mode is interlath or intergranular. This characteristic is coherent with the crack path commonly reported in liquid metal embrittlement. A similar procedure was applied to the unalloyed XC10 steel. The results show a behavior which is similar to the one of T91 steel and suggest a common mechanism for liquid sodium embrittlement of body centered cubic steels. Moreover, they confirm that the ductile to brittle transition seems associated with a limited crack propagation rate. The propagation is thermally activated with activation energy of about 50 kJ/mol. Finally, it was shown that 304L austenitic steel is sensitive to liquid sodium embrittlement as well. Some fracture surfaces testify of an intergranular fracture mode, but some questions still remain about the crack path.