The 2015 MRS Fall Meeting was held in Boston, MA from November 29 – December 4. Recorded sessions include several award and keynote talks, three tutorial sessions and highlighted talks from the technical program.
HCP metals are widely used as structural materials in many industries, ranging from transport and energy to biomedical applications due to their low density, high specific strength. However, HCP metals show pronounced anisotropy in their mechanical properties and less number of slip systems compared to FCC and BCC metals. In the absence of sufficient number of slip systems in HCP metals, twining is found to be one of the important deformation modes during plastic deformation. At present, we do not have very clear knowledge of properties of twin boundaries in HCP metals at atomic length scale. Understanding atomic structure and chemistry of twin-associated boundaries is crucial to improve mechanical properties of HCP metals. In this work, using first-principles density function theory, we study twinning-associated boundaries (TBs), coherent twin boundaries (CTBs) and coherent basal-prismatic boundary (CBP) in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be), with a focus on structure and solute’s solubility at twin boundaries. We find that the formation of TBs is associated with creation of an excess volume. All the six metals show positive excess volume associated with and CTBs, but the excess volume associated with CTBs and CBP can be positive or negative depending on metal. To understand solubility at TBs, we calculated solubility of solute atoms in Mg, Ti, and Zr for solute positions in bulk, CTB and CBP boundaries and show that, in general, solute atoms have better solubility at CTB and CBP than in bulk. We also found solubility of solute atoms linearly changes with normal strain at CBP, increasing with the normal strain for solute atom with a greater metallic radius than the matrix, and decreasing with the normal strain for solute atom with a smaller metallic radius than the matrix. This suggests that the distribution of solute atoms in bulk, CTB, and CPB varies with stress state, and in turn affects mobility of CTB and CPB boundaries.Speaker(s):
In many fields of science and technology, the chemistry of materials at the atomic scale is essential to the large-scale behaviour. In extreme environments, such as the high temperatures and stresses of a modern aerospace engine or the heat and irradiation of the walls of a nuclear reactor, even small changes in local composition can result in embrittlement, corrosion and cracking. Similarly in semiconductors, even a few thousand atoms in the wrong place can result in failure of a transistor or similar device.
Atom probe tomography (APT) is becoming a vital tool in analysing the chemical behaviour of materials at this scale. Using an advanced ultra-high vacuum instrument, with specimens cooled to 20-80K, a small needle of the material is characterised atom by atom. An electric field is established between the specimen and the detector, just below the threshold required to remove an atom. A further pulse of either higher voltage or laser energy is applied to field evaporate individual atoms. The detector is position sensitive, and the time between the pulse and the detector event gives the time-of-flight and hence the mass/charge ratio of the detected ion.
Using this method, specimens can be reconstructed into a 3-dimensional, chemically-sensitive dataset of anywhere up to hundreds of millions of atoms. With an analytical sensitivity up to 1ppm and resolutions of 0.3nm achievable in all three dimensions, this allows the chemistry of the material to be mapped to incredibly high precision.
This webinar explains the fundamentals of the technique, and outlines a number of the applications of atom probe tomography being undertaken at the UK National APT facility at the University of Oxford. Some of the work includes characterisation of the tips of stress corrosion cracks in steels, clustering of minor elements in steels, studying the reaction of catalysts and other materials to corrosion and oxidation, as well as the distribution of dopants in GaN quantum wells. This webinar aims to demonstrate why APT is a powerful technique that is growing in influence and importance to many materials science applications.
Nanostructured metallic multilayers provide unique opportunity to investigate the influence of layer interfaces on mechanical properties of metallic nanocomposites. High strength is often achieved at small (several nm) individual layer thickness (h). Recently we discovered high-density stacking faults in FCC Co in highly (100) textured Cu/Co multilayers. In contrast in (111) textured Cu/Co nanolayers, Co remained its stable HCP structure at large h. The two Cu/Co systems have very different size dependent strengthening behavior. HCP Cu/Co has much greater peak strength than FCC Cu/Co. The large discrepancy in their strengthening mechanisms is discussed and compared to those of highly textured Cu/Ni multilayer systems. In another highly textured nanolayers system, Ag/Al, epitaxial interfaces were observed across various h (1-200 nm). High-density nanotwins and stacking faults appear in both Ag and Al layers, and stacking fault density in Al increases sharply with decreasing h. At smaller h, hardness of Ag/Al nanolayers increases monotonically and no softening was observed. These studies allow us to investigate the influence of layer interfaces, stacking faults and nanotwins on strengthening mechanisms of metallic nanolayers. This research is funded by DOE-OBES.Speaker(s):
Rapid increases in global energy use and growing environmental concerns have prompted the development of clean, sustainable, alternative energy technologies. Electrical energy storage (EES) is critical to efficiently utilize electricity produced from intermittent, renewable sources like solar and wind as well as to electrify the transportation sector. Rechargeable batteries are prime candidates for EES, but widespread adoption requires optimization of cost, cycle life, safety, energy density, power density, and environmental impact, all of which are directly linked to materials challenges. After providing a brief account of the current status of battery technologies, this presentation will focus on the development of new materials, cell chemistry, and cell configurations to overcome current problems. Specifically, the challenges and approaches of transitioning from the current insertion-compound electrodes in lithium-ion batteries to new conversion-reaction electrodes with multi-electron transfer per atom will be presented. The systems include safer antimony-based anodes, lithium-sulfur cells, and hybrid lithium-air cells with a solid electrolyte. Biography Arumugam Manthiram is currently the Cockrell Family Regents Chair in Engineering and Director of the Texas Materials Institute and the Materials Science and Engineering Graduate Program at the University of Texas at Austin (UT-Austin). He received his Ph.D. degree in chemistry from the Indian Institute of Technology at Madras in 1981. After working as a postdoctoral researcher at the University of Oxford and at UT-Austin, he became a faculty member in the Department of Mechanical Engineering at UT-Austin in 1991. Dr. Manthiram’s research is focused on clean energy technologies: rechargeable batteries, fuel cells, supercapacitors, and solar cells. He has authored 530 journal articles with 17,000 citations and an h-index of 68. He is the Regional (USA) Editor of Solid State Ionics. He is a Fellow of the American Ceramic Society, the Electrochemical Society, and the American Association for the Advancement of Science. He received the Battery Division Research Award from the Electrochemical Society in 2014.Speaker(s):
In the classical picture of nucleation, density fluctuations that are inherent at finite temperature form unstable clusters of the new phase through monomer-by-monomer addition. Clusters transition from unstable to stable if they exceed a critical size, beyond which the free energy cost of creating the new phase boundary is compensated by the drop in chemical potential. In recent years, hierarchical pathways involving assembly of species more complex than monomers have been proposed for numerous systems. Amongst these pathways, a “two-step nucleation” process was proposed whereby macromolecular crystals nucleate within monomer-rich, non-crystalline clusters. However, reports of such pathways are almost entirely based on computational models or interpretations of indirect observations. Moreover, little is known about two-step nucleation dynamics, and whether the monomers in the clusters are one and the same as those that comprise the crystal nucleus or are act instead to provide an environment for heterogeneous nucleation is uncertain, as is the extent to which two-step pathways are general features of either macromolecular or inorganic materials. To address these knowledge gaps, we have used in situ TEM and AFM to investigate nucleation in numerous systems. To examine nucleation pathways of macromolecules, we synthesized a biomimetic polymer sequence that forms 2D ordered structures and used in situ AFM to observe nucleation. Our results show that the nucleation occurs along a two-step pathway that begins with creation of disordered clusters containing ~ 10-20 molecules. These clusters transform directly into ordered nuclei that grow via molecule-by-molecule addition, with the kinetics of transformation strongly dependent on Ca concentration. However, when a small aggregation-promoting hydrophobic region is deleted, even though the same final structure is obtained, nucleation occurs in a single step and the kinetics are dramatically altered. To investigate nucleation of simple inorganic materials, we used in situ TEM to observe nucleation of CaCO3. Formation pathways are confirmed in most cases by collecting diffraction information of the observed phases. We find that amorphous calcium carbonate (ACC), as well as the three predominant crystalline phases: calcite, vaterite, and aragonite, can form directly even under conditions in which ACC readily forms. In addition to these direct formation pathways, we observe two-step nucleation of aragonite and vaterite from ACC. Here, ACC transforms directly to the crystalline phases through distinct nucleation events on or just beneath the surface followed by consumption of the parent ACC particle. The results demonstrate that two-step pathways are possible in both inorganic and macromolecular systems, but are not universal. They can be accompanied by direct nucleation pathways and, in the case of macromolecules, their existence can depend on the specific sequence of the molecule.Speaker(s):
In order to increase on-chip communication bandwidth, optical interconnects can potentially meet the strict requirements on low power consumption at high data rates. CMOS processing limits the choice of materials and processes. Therefore, the development of on-chip interconnect systems has focused on Si compatible materials with near IR light. More recently, wafer bonding and through-silicon vias (TSV) have been implemented using a Si interconnect platform. Here we propose an on-chip optical interconnect system, based on a III-Nitride or III-V photonic platform that is implemented on Si with CMOS electronics on the top surface and TSVs to connect to the underlying optical interconnect system. To reduce power consumption, we plan to use direct-modulated light-emitting diodes (LEDs) grown on a silicon substrate. Unlike laser based designs, incoherent LED-based links can only function with network-on-chip (NoC) architectures that can multiplex traffic flows atop 1-to-1 connections, i.e., where control and switching needs to be done with electrical routers. Optical routers based on resonance such as microrings are not applicable for filtering, modulating or switching. Wavelength-division multiplexing (WDM) cannot be used to enable 1-to-many or many-to-many connections. Therefore, the LED enabled interconnects will be used for point-to-point connections at low power consumption. We are evaluating two materials system, III-Nitride and III-V based light emitters and detectors. The advantage of using InGaN/GaN LEDs is that the epitaxy technology of III-Nitrides on (111) silicon is more advanced than III-V epitaxy on Si substrates. Applications of III-Nitrides in solid-state lighting have been widely used and commercialized. For III-Nitrides, the optical devices and the link will be fabricated on Si (111) substrates before any electrical components are fabricated. Similarly, the III-V devices will be based on a Ge-on-Si substrate. CMOS processing and components will be integrated on the processed optical link wafer via wafer bonding technology and back-of-end-line processing. We will present system simulation, evaluation, and preliminary results of InGaN multiple-quantum-well (MQW) LEDs and photodetectors. Initial results indicate low power-consumption and promising application of this technology as on-chip optical interconnects for many-core processors.Speaker(s):
A promising route to protein-mimetic materials capable of complex functions, such as molecular recognition and catalysis, is provided by sequence-defined peptoid polymers, structural relatives of polypeptides. Peptoids, which are relatively non-toxic and resistant to degradation, can fold into defined structures through a combination of sequence-dependent interactions. However, the range of possible structures accessible to peptoids and other biomimetics is unknown, and our ability to design hierarchical protein-like architectures from these polymer classes is limited. I will describe our use of molecular dynamics simulations, together with scattering and microscopy data, to determine the atomic-resolution structure of the recently-discovered peptoid nanosheet, an ordered supramolecular assembly that extends macroscopically in only two dimensions. Our simulations show that nanosheets are structurally and dynamically heterogeneous, can be formed only from peptoids of certain lengths, and are potentially water- and ion-porous. Moreover, their formation is enabled by peptoids’ adoption of a secondary structure not seen in the natural world. This structure, a zig-zag pattern that we call a Sigma-strand, results from the ability of adjacent backbone monomers to adopt opposed rotational states, thereby allowing the backbone to remain linear and untwisted. Such a binary rotational state motif is not seen in protein regular structures, which are built generally from one type of rotational state. Linear backbones tiled in a brick-like way form an extended two-dimensional nanostructure.Speaker(s):
Crossing the millennium, physicists, chemists and materials scientists have increasingly applied their skills to understanding biomaterials and living systems. This talk reviews some insights from our studies of soft matter under confinement, whether topological, steric, geometric or interfacial, following a similar path: from polymer physics and confinement-induced phase transitions to biolubrication and its relation to joint diseases. Topics will include:
- Reptation of topologically confined, entangled polymer chains
- Surface-attached polymers and steric stabilization
- Entropic friction-modification by polymer brushes
- Confinement-induced liquid-to-solid transitions, and the curious case of water
- Water dipoles: hydration lubrication, tissue engineering and osteoarthritis