The Influence of Secondary Processing Conditions on the Mechanical Properties and Microstructure of a Particle Reinforced Aluminium Metal Matrix Composite

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350???????C, 450???????C and 550???????C, at a ratio of20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450???????C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550???????C exhibited a lower yield strength than material extruded at 350???????C and 450???????C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350???????C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This'microstructure was found to offer the optimum mechanical properties.Imperial Users onl

The Boston University Photonics Center annual report 2015-2016

This repository item contains an annual report that summarizes activities of the Boston University Photonics Center in the 2015-2016 academic year. The report provides quantitative and descriptive information regarding photonics programs in education, interdisciplinary research, business innovation, and technology development. The Boston University Photonics Center (BUPC) is an interdisciplinary hub for education, research, scholarship, innovation, and technology development associated with practical uses of light.This has been a good year for the Photonics Center. In the following pages, you will see that this year the Center’s faculty received prodigious honors and awards, generated more than 100 notable scholarly publications in the leading journals in our field, and attracted $18.9M in new research grants/contracts. Faculty and staff also expanded their efforts in education and training, and cooperated in supporting National Science Foundation sponsored Sites for Research Experiences for Undergraduates and for Research Experiences for Teachers. As a community, we emphasized the theme of “Frontiers in Plasmonics as Enabling Science in Photonics and Beyond” at our annual symposium, hosted by Bjoern Reinhard. We continued to support the National Photonics Initiative, and contributed as a cooperating site in the American Institute for Manufacturing Integrated Photonics (AIM Photonics) which began this year as a new photonics-themed node in the National Network of Manufacturing Institutes. Highlights of our research achievements for the year include an ambitious new DoD-sponsored grant for Development of Less Toxic Treatment Strategies for Metastatic and Drug Resistant Breast Cancer Using Noninvasive Optical Monitoring led by Professor Darren Roblyer, continued support of our NIH-sponsored, Center for Innovation in Point of Care Technologies for the Future of Cancer Care led by Professor Cathy Klapperich, and an exciting confluence of new grant awards in the area of Neurophotonics led by Professors Christopher Gabel, Timothy Gardner, Xue Han, Jerome Mertz, Siddharth Ramachandran, Jason Ritt, and John White. Neurophotonics is fast becoming a leading area of strength of the Photonics Center. The Industry/University Collaborative Research Center, which has become the centerpiece of our translational biophotonics program, continues to focus onadvancing the health care and medical device industries, and has entered its sixth year of operation with a strong record of achievement and with the support of an enthusiastic industrial membership base

Environmental-Induced Damage in Tin (Sn) and Aluminum (Al) Alloys

abstract: Sn and Al alloys are widely used in various industries. Environmental-induced damage resulting in whiskering in Sn and corrosion in Al account for numerous failures globally every year. Therefore, for designing materials that can better withstand these failures, a comprehensive study on the characterization of the damage is necessary. This research implements advanced characterization techniques to study the above-mentioned environmental-induced damage in Sn and Al alloys. Tin based films are known to be susceptible to whisker growth resulting in numerous failures. While the mechanisms and factors affecting whisker growth have been studied extensively, not much has been reported on the mechanical properties of tin whiskers themselves. This study focuses on the tensile behavior of tin whiskers. Tensile tests of whiskers were conducted in situ a dual beam focused ion beam (FIB) with a scanning electron microscope (SEM) using a micro electro-mechanical system (MEMS) tensile testing stage. The deformation mechanisms of whiskers were analyzed using transmission electron microscopy (TEM). Due to the heterogenous nature of the microstructure of Al 7075, it is susceptible to corrosion forming corrosion products and pits. These can be sites for cracks nucleation and propagation resulting in stress corrosion cracking (SCC). Therefore, complete understanding of the corrosion damaged region and its effect on the strength of the alloy is necessary. Several studies have been performed to visualize pits and understand their effect on the mechanical performance of Al alloys using two-dimensional (2D) approaches which are often inadequate. To get a thorough understanding of the pits, it is necessary for three-dimensional (3D) studies. In this study, Al 7075 alloys were corroded in 3.5 wt.% NaCl solution and X-ray tomography was used to obtain the 3D microstructure of pits enabling the quantification of their dimensions accurately. Furthermore, microstructure and mechanical property correlations helped in a better understanding of the effect of corrosion. Apart from the pits, a surface corrosion layer also forms on Al. A subsurface damage layer has also been identified that forms due to the aggressive nature of NaCl. Energy dispersive X-ray spectroscopy (EDX) and nanoindentation helped in identifying this region and understanding the variation in properties.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

In Situ Transmission Electron Microscopy Investigation of All Solid-state Sodium Batteries

All solid-state batteries (ASSBs) utilizing metal anodes such as lithium and sodium hold great promise for achieving high energy and power density, surpassing the safety limitations associated with liquid-electrolyte counterparts. However, the development of commercially viable ASSBs operating at room temperature remains limited. This is primarily due to the sluggish kinetics and solid-solid interfacial issues that impede the performance of batteries. Among the various interfacial challenges, the growth of dendritic structures leading to cell failure is a persistent problem that cannot be mitigated solely by the initially anticipated high elastic modulus of solid electrolytes (SEs) for ASSBs. Despite significant progress in understanding the filamentary growth mechanism in lithium metal based ASSBs using inorganic SEs, the understanding of sodium ASSBs remains far from complete. To gain insights into the microstructural influences on sodium filament growth and Na+ ion transport, polycrystalline Na-β′′-alumina SE was employed as a model material due to its outstanding stability with Na metal. In this work, in situ biasing transmission electron microscopy (TEM) measurements were conducted to realize the cathodic sodium deposition at the interface between the Na-β′′-alumina and the electrode, as well as grain boundaries (GBs) within Na-β′′-alumina TEM lamellas. Based on orientation analysis and composition distribution, the layered crystal structure induces anisotropic Na+ ion transport under the electric field, significantly facilitating the blockade of Na+ ion transport at some GBs and consequently influencing the position of Na filament growth. Furthermore, the microstructural evolution of the Au interlayer, which is believed to protect against dendrite growth, was explored during the inhomogeneous sodium deposition using the same in situ biasing TEM setup. Notably, while Na-Au alloy particle forms by cathodic sodium deposition, Na-Au interdiffusion occurs at the interface, rather than solely sodium diffusion along the Au interlayer. Sodium diffusion along the Au interlayer leads to alloy formation, while the diffusion of Au towards the sodium deposition site may result in the redistribution of the Au interlayer. Additionally, the Au interlayer exhibits distinct behavior under different conditions, e.g. different bias voltages and layer morphology including the Au interlayer thickness and gap between Au particles. In addition to investigating interfacial issues in sodium metal based ASSBs, the study on the influence of scanning electron microscopy (SEM) imaging and focused ion beam (FIB) processing on the SEs was conducted to ensure the reliable preparation of TEM samples for in situ TEM measurements. The irradiation damage mechanism and the corresponding solution were understood during this investigation

Understanding the Mechanical Behaviors of Lithium-Based Battery Anodes─Silicon and Lithium Metal

abstract: This dissertation will investigate two of the most promising high-capacity anode materials for lithium-based batteries: silicon (Si) and metal lithium (Li). It will focus on studying the mechanical behaviors of the two materials during charge and discharge and understanding how these mechanical behaviors may affect their electrochemical performance. In the first part, amorphous Si anode will be studied. Despite many existing studies on silicon (Si) anodes for lithium ion batteries (LIBs), many essential questions still exist on compound formation, composition, and properties. Here it is shown that some previously accepted findings do not truthfully reflect the actual lithiation mechanisms in realistic battery configurations. Furthermore the correlation between structure and mechanical properties in these materials has not been properly established. Here, a rigorous and thorough study is performed to comprehensively understand the electrochemical reaction mechanisms of amorphous-Si (a-Si) in a realistic LIB configuration. In-depth microstructural characterization was performed and correlations were established between Li-Si composition, volumetric expansion, and modulus/hardness. It is found that the lithiation process of a-Si in a real battery setup is a single-phase reaction rather than the accepted two-phase reaction obtained from in-situ TEM experiments. The findings in this dissertation establish a reference to quantitatively explain many key metrics for lithiated a Si as anodes in real LIBs, and can be used to rationally design a-Si based high-performance LIBs guided by high-fidelity modeling and simulations. In the second part, Li metal anode will be investigated. Problems related to dendrite growth on lithium metal anodes such as capacity loss and short circuit present major barriers to the next-generation high-energy-density batteries. The development of successful mitigation strategies is impeded by the incomplete understanding of the Li dendrite growth mechanisms. Here the enabling role of plating residual stress in dendrite initiation through novel experiments of Li electrodeposition on soft substrates is confirmed, and the observations is explained with a stress-driven dendrite growth model. Dendrite growth is mitigated on such soft substrates through surface-wrinkling-induced stress relaxation in deposited Li film. It is demonstrated that this new dendrite mitigation mechanism can be utilized synergistically with other existing approaches in the form of three-dimensional (3D) soft scaffolds for Li plating, which achieves superior coulombic efficiency over conventional hard copper current collectors under large current density.Dissertation/ThesisDoctoral Dissertation Mechanical Engineering 201

Trivalent chromium based conversion coatings containing cobalt on the zinc plated steel

Im Zuge neuer EU-Direktiven wurde die Verwendung von Cr(VI)-Verbindungen stark reglementiert. In der Oberflächentechnik wurde daraufhin sechswertiges Chrom durch sicherere und gleichzeitig effektive Passivierungen auf Cr(III)-Basis ersetzt. Der Korrosionsschutz von Cr(VI)-Beschichtungen ohne Wärmebehandlung ist im Allgemeinen besser. In bisher durchgeführten Untersuchungen zeigte sich, dass durch Zusatz von Übergangsmetallionen der Korrosionsschutz der Cr(III)-Passivierungsschicht verbessert werden kann. In dieser Arbeit wird der Einfluss der Zusammensetzung von Cr(III)-Passivierung mit Kobaltanteil auf die Bildung und die Struktur von Konversionsschichten auf verzinkten Substraten untersucht. Auf den verzinkten Stahl wurden Modelllösungen mit zwei verschiedenen Komplexbildnern, nämlich Fluorid und Oxalat, mit und ohne Kobalt aufgetragen. Rasterelektronenmikroskopie (REM) und Rasterkraftmikroskopie zeigten Oberflächenmorphologien mit mikrostrukturellen Defekten. In Anwesenheit von Kobalt wurden die Schichten gleichmäßiger. Die elementare Zusammensetzung der Schichten wurde mit der Augerelektronenspektroskopie (AES) untersucht. Die Mengen an Cr und Co in den Beschichtungen wurden mit Hilfe der optischen Plasma-Emissionsspektroskopie (ICP-OES) bestimmt. Sowohl AES als auch ICP-OES zeigten Co-Gehalte in den Schichten. Mit Hilfe eines thermodynamischen Modells wurde die Konzentration von Cr(III)-, Zn(II)- und Co(II)-Spezies in der Behandlungslösung im pH-Bereich von 0,0 bis 14,0 und auch der minimale pH-Wert für die Abscheidung der Metallionen in der entsprechenden Lösung berechnet. Die Ergebnisse der Korrosionstests (Polarisationsmessung und elektrochemische Impedanzspektroskopie) legen nahe, dass die Bildung einer dichten Schicht für eine gute Korrosionsbeständigkeit entscheidend ist. Außerdem wurde der Bildungsmechanismus von Cr(VI) in den Schichten untersucht. Die Anwesenheit von Cr(VI) wurde mittels Spektrophotometrie nachgewiesen. Die Morphologie und Struktur der Filme wurden per REM beobachtet. Die Gesamtwassermenge in den Schichten wurde mittels Karl-Fischer-Titration gemessen. Es zeigte sich, dass die Morphologie des fluoridhaltigen Films mit einer hohen Dichte an Mikroporen die Wahrscheinlichkeit eines Wassereinschlusses erhöht. Dies führte zu einer Oxidation von Cr(III) zu Cr(VI) durch Sauerstoff in Gegenwart von Wasser bei erhöhten Temperaturen.Since hexavalent chromium has been recognized as toxic and carcinogenic, its usage has been restricted. Thereafter, Cr(VI) was substituted by a safer, yet effective, trivalent chromium-based treatment solution. The addition of transition metal ions into the Cr(III)-based treatment solution was proposed to improve the corrosion resistance of the produced passivation film. The present study intends to elucidate the effect of treatment solution composition on the formation and structure of Cr(III)-based conversion coatings containing cobalt. Model solutions with two different complexing agents, viz. fluoride and oxalate, with and without cobalt were applied to the zinc-plated steel. The scanning electron microscopy (SEM) and atomic force microscopy images revealed a morphology with microstructural defects that can be improved to a more uniform and adherent structure by adding cobalt to the passivating bath. The elemental composition of the layer was investigated by Auger electron spectroscopy (AES). Furthermore, the amounts of Cr and Co in the coatings were measured with the aid of inductively coupled plasma optical emission spectroscopy (ICP-OES). AES and ICP-OES both detected cobalt in the layers. Using a thermodynamic model, the concentration of Cr(III), Zn(II), and Co(II) species in the pH ranges of 0.0 to 14.0 and the minimum pH for the deposition of each metal ion species in the relevant treatment solution were calculated. The results of accelerated corrosion tests (polarization measurement and electrochemical impedance spectroscopy) suggested that the formation of a dense layer is crucial for good corrosion resistance of the coating. Furthermore, the formation mechanism of Cr(VI) in the layers formed in Cr(III)-based treatment solutions was also studied. The presence of Cr(VI) was detected by means of spectrophotometry. The morphology and structure of the films were observed with SEM. Besides, Karl Fischer titration was used to measure the total amount of water in the layers. It was shown that the morphology of the fluoride-containing film with a high density of micropores increased the probability of water entrapment. This resulted in the oxidation of Cr(III) to Cr(VI) by oxygen in the presence of water at elevated temperatures

Physical and Chemical Properties of Matrix in Primitive Chondrites

The origin and formation of our Solar System is an open research question, which the scientific community is trying to address. In this work was specifically investigated the fine grained matrix which is a mixture of fine-grained materials composed largely of amorphous silicate and sub-micrometre crystals forsterite and enstatite. This is thought to be the remnants of the dust and gas of the protoplanetary disk that allows us to better understand chemical and physical properties of this precursor material. Four pristine primitive meteorites were selected: Acfer 094 (C2-ung.), ALHA77307 (CO3), MIL 07687 (C3-ung.) and QUE 99177 (CR2). The ability of a new generation of SEM-EDX detector was tested, in order to acquire high-resolution element maps of fine grained matrix. This allowed the calculation of abundances, and size distribution of discrete grains of different phases (silicate vs. opaque). Data acquired suggest that the four meteorites can be split into two groups, ALHA77307-MIL 07687 and QUE 99177 and Acfer 094, based on differences in relative abundances and sizes of discrete grains in their matrix. Micro X-ray diffraction was also used for mineralogical phase identification of the matrix constituents allowing the estimation of their modal mineralogy. MIL 0768 and Acfer 094 were also investigated using Scanning Transmission X-ray Microscopy (STXM) which revealed predominantly oxidising conditions; some reducing conditions are also displayed by some grains, reflecting the mixed redox conditions of the solar nebula. Measurements of O-isotopic composition of matrix regions were performed, and revealed similarities to values previously reported for IDPs (Starkey et al. 2013,2014 Nakashima et al., 2012, Aleon et al., 2009), rather than those of bulk meteorites (Clayton & Mayeda, 1999). I infer that the observed differences between these matrix components within the meteorite reflect the heterogeneity of the protoplanetary disk. Although these meteorites are pristine, parent body processes have also affected the small matrix grains

Synthesis of helical van der Waals crystals with tunable twist

Helicity, a geometric property rendering an object nonsuperimposable on its own mirror image is ubiquitous in Nature. Inorganic crystals can grow into helical form, which is of great interest from the perspective of fundamental material science as well as application. Various inorganic crystals can be grown into helically twisted form on different scales ranging from nanoscale to mesoscale and to macroscale. The natural growth of twisted macroscopic quartz crystals in the bulk form has been documented and studied for centuries. On the nanoscale, quantum dots (QD), nanowires and carbon nanotube into helical structures. Those helical crystals have intriguing optoelectronics properties including rotatory optical activity and circular dichroisms in both absorption and photoluminescence as well as unique stereoselectivity in chemical reactions. These properties render inorganic crystals good potential of applications in polarization optics, chiroptical sensing, enantioselective catalyst and biomedical imaging. The material science responsible for forming these twisted inorganic crystals, nevertheless, remains largely mysterious and elusive.In recent years, twisted van der Waals materials with rotational stacking of two-dimensional materials have attracted tremendous attention. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric, excitonic and spintronic behaviors. In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behavior and superconductivity. Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides, black phosphorus and germanium monoselenide. In contrast to electronic band structure of the twisted bilayer graphene, unique features such as one-dimensional flat electronic band may emerge in those twisted 2D materials with different structures and symmetries.The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. Twisted van der Waals materials are usually created using mechanical exfoliation and a transfer-stacking method, but limitations exist to extend this method to a variety of two-dimensional materials. In contrast, bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. This dissertation explores the bottom-up synthesis of twisted van der Waals materials. We demonstrate that the Eshelby twist associated with a screw dislocation (a chiral topological defect), can drive the formation of twisted van der Waals materials. The Eshelby twist is a continuous crystallographic twist generated by the torsional force of an axial screw dislocation in a one-dimensional structure. It has been shown to result in growth of helically twisted nanowires of various materials. This mechanism potentially provides a means to create twisted van der Waals (vdW) structures. Materials such as germanium sulfide (GeS) can grow into nanowires along the vdW stacking direction (the cross-plane direction), and introducing Eshelby twist into such nanowires naturally leads to twist between the successive layers. We synthesized twisted GeS crystals at both nanoscale and mesocscale. In the synthesis method, GeS nanowires with axial screw dislocations are first grown along the stacking direction, yielding vdW nanowires with Eshelby twist. These wires possess continuous twists in which the total twist rates are defined by the radii of the nanowires, consistent with Eshelby’s theory. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps in the twisting profile. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles. The twisting profiles of the structures are tunable. We show that the twisting profile can be tailored by controlling the radial size of the structure. The twisting morphology gradually transitions from initial continuous twisting to intermediate twisting (consisting of both continuous twisting between the twist boundaries and discrete twisting at the boundaries) and eventually to discrete twisting with increasing radial size. This allows us to control the twisting profile and angles at twist interfaces by controlling the radial growth of the structure. We also demonstrate that the twist rate and period can be tailored by tailoring the radii of the dislocated nanowires first grown in the VLS process. This is achieved by adding GeSe into the growth, which modulates the size of the droplets catalyzing the VLS process, therefore modulating the radii of the nanowires. The chemical modulation demonstrates good potential to tailor the twist rate and period of helical vdW crystals, enabling a new freedom to modulate optoelectronic properties and chiral light-matter interactions.Last, we explored the anisotropic propagation of surface phonon polaritons in GeS, enabled by the strong in-plane anisotropy of the crystal structure. We demonstrate that GeS thin films at thickness of tens of nanometers support strongly anisotropic surface phonon polaritons in the far-infrared range. The dispersion relations of these phonon polaritons can be tuned by varying the thickness of the GeS film

New Materials and Methods towards High-Energy Lithium Metal Batteries

The sluggish progress of battery technologies has drastically hindered the rapid development of electric vehicles and next-generation portable electronics. Improving the energy density requires breakthroughs in materials for both cathode and anode, and new characterization methods to accurately correlate the materials with their performances.For cathodes, lithium (Li) rich layered oxides exhibit high reversible specific capacities over 300 mAh g-1, attributing to the oxygen redox reaction. However, oxygen activity comes with instability in the form of oxygen loss, which is associated with irreversible voltage decay and capacity fading. Calculations suggest that incorporating 4d elements, such as Mo, enhances the structural stability by altering the local band structure and impeding oxygen vacancy formation. Driven by these findings, Mo is co-doped with Co into Li[Li0.2Ni0.2Mn0.6]O2, showing notably reduced voltage decay and capacity fading without sacrificing energy density and cycle life.The Li metal anode is critical to break the energy-density bottleneck of current Li-ion chemistry. Inactive Li formation is the immediate cause of capacity loss and catastrophic failure of Li metal batteries. However, its composition has not yet been quantitatively studied due to the lack of effective diagnosis tools that can accurately differentiate Li+ in solid electrolyte interphase (SEI) components and the electrically isolated unreacted metallic Li0, which together comprise the inactive Li. By establishing a new analytical method, Titration Gas Chromatography (TGC), we accurately quantify the contribution from unreacted metallic Li0 to the total amount of inactive Li. We identify the Li0, rather than the (electro)chemically formed Li+ in SEI, as the dominating cause for the inactive Li and capacity loss. Coupling the measurements of the unreacted metallic Li0 global content to the observations of its local micro- and nano-structure by cryogenic electron microscopies, we also reveal the formation mechanism of inactive Li in different types of electrolytes, and determine the true underlying cause of low CE in Li metal deposition and stripping. We ultimately propose strategies for highly efficient Li deposition and stripping to enable Li metal anode for next generation high-energy batteries