Nitinol-reinforced shape-memory polymers

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 33).Reinforced shape-memory polymers have been developed from an acrylate based thermoset shape-memory polymer and nitinol wires. A rectangular shape-memory polymer measuring approximately 1 by 2 by 0.1 inches has a ten fold increase in actuation force under three-point bending when reinforced with two 0.02 inch diameter nitinol wires. A constitutive model for shape-memory polymers and nitinol has been used to predict with good correlation the actuation-versus-time and displacement-versustime behavior of the reinforced shape-memory polymer composites. It is possible then, using finite-element modeling, to design and manufacture reinforced shape-memory polymers tailored for use as thermally-activated actuators of specific force.by Claudio V. Di Leo.S.B

A coupled theory for diffusion of hydrogen and large elastic-plastic deformations of metals

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 101-103).A thermodynamically-consistent coupled-theory which accounts for diffusion of hydrogen, trapping of hydrogen, diffusion of heat, and large elastic-plastic deformations of metals is developed. Our theoretical framework places the widely-used notion of an "equilibrium" between hydrogen residing in normal interstitial lattice sites and hydrogen trapped at microstructural defects, within a thermodynamically-consistent framework. The theory has been numerically implemented in a finite element program. Using the numerical capability we study two important problems. First, we show the importance of using a prescribed chemical potential boundary condition in modeling the boundary between a metal system and a hydrogen atmosphere at a given partial pressure and temperature; specifically, we perform simulations using this boundary condition and compare our simulations to those in the published literature. Secondly, the effects of hydrogen on the plastic deformation of metals is studied through simulations of plane-strain tensile deformation and three-point bending of U-Notched specimens. Our simulations on the effects of hydrogen on three-point bending of U-notched specimens are shown to be in good qualitative agreement with published experiments.by Claudio V. Di Leo.S.M

Diffusion–deformation theory for amorphous silicon anodes: The role of plastic deformation on electrochemical performance

Amorphous silicon (a-Si) is a promising material for anodes in Li-ion batteries due to its increased capacity relative to the current generation of graphite-based anode materials. However, the intercalation of lithium into a-Si induces very large elastic–plastic deformations, including volume changes of approximately 300%. We have formulated and numerically implemented a fully-coupled diffusion–deformation theory, which accounts for transient diffusion of lithium and accompanying large elastic–plastic deformations. The material parameters in the theory have been calibrated to experiments of galvanostatic cycling of a half-cell composed of an a-Si thin-film anode deposited on a quartz substrate, which have been reported in the literature. We show that our calibrated theory satisfactorily reproduces the mechanical response of such an anode — as measured by the changes in curvature of the substrate, as well as the electrochemical response — as measured by the voltage versus state-of-charge (SOC) response. We have applied our numerical simulation capability to model galvanostatic charging of hollow a-Si nanotubes whose exterior walls have been oxidized to prevent outward expansion; such anodes have been recently experimentally-realized in the literature. We show that the results from our numerical simulations are in good agreement with the experimentally-measured voltage versus SOC behavior at various charging rates (C-rates). Through our simulations, we have identified two major effects of plasticity on the electrochemical performance of a-Si anodes: • First, for a given voltage cut-off, plasticity enables lithiation of the anode to a higher SOC. This is because plastic flow reduces the stresses generated in the material, and thus reduces the potential required to lithiate the material. • Second, plastic deformation accounts for a significant percentage of the energy dissipated during the cycling of the anode at low C-rates. Hence, plasticity can have either (a) a beneficial effect, that is, a higher SOC for a given voltage cut-off; or (b) a detrimental effect, that is significant energy dissipation at low C-rates.National Science Foundation (U.S.) Division of Civil, Mechanical and Manufacturing Innovation (Award CMMI-1063626

A Finite Element Implementation of a Coupled Diffusion-Deformation Theory for Elastomeric Gels

The theory of Chester and Anand (2011) for fluid diffusion and large deformations of elastomeric gels is implemented as a user-defined element (UEL) subroutine in the commercial finite element software package ABAQUS. A specialized form of the constitutive equations and the governing partial differential equations of the theory are summarized, and the numerical implementation is described in detail. To demonstrate the robustness of the numerical implementation a few illustrative numerical simulation examples for axisymmetric, plane strain, and three-dimensional geometries are shown. For educational purposes, and also to facilitate the numerical implementation of other coupled multiphysics theories, the source code for the UEL is provided as an online supplement to this paper.National Science Foundation (U.S.) (NSF CMMI-1063626

Interplay of phase boundary anisotropy and electro-autocatalytic surface reactions on the lithium intercalation dynamics in LiX_XFePO4_4 platelet-like nanoparticles

Experiments on single crystal Li$_X$FePO$_4$ (LFP) nanoparticles indicate rich nonequilibrium phase behavior, such as suppression of phase separation at high lithiation rates, striped patterns of coherent phase boundaries, nucleation by binarysolid surface wetting and intercalation waves. These observations have been successfully predicted (prior to the experiments) by 1D depth-averaged phase-field models, which neglect any subsurface phase separation. In this paper, using an electro-chemo-mechanical phase-field model, we investigate the coherent non-equilibrium subsurface phase morphologies that develop in the $ab$- plane of platelet-like single-crystal platelet-like Li$_X$FePO$_4$ nanoparticles. Finite element simulations are performed for 2D plane-stress conditions in the $ab$- plane, and validated by 3D simulations, showing similar results. We show that the anisotropy of the interfacial tension tensor, coupled with electroautocatalytic surface intercalation reactions, plays a crucial role in determining the subsurface phase morphology. With isotropic interfacial tension, subsurface phase separation is observed, independent of the reaction kinetics, but for strong anisotropy, phase separation is controlled by surface reactions, as assumed in 1D models. Moreover, the driven intercalation reaction suppresses phase separation during lithiation, while enhancing it during delithiation, by electro-autocatalysis, in quantitative agreement with {\it in operando} imaging experiments in single-crystalline nanoparticles, given measured reaction rate constants

In Situ Lithiation–Delithiation of Mechanically Robust Cu–Si Core–Shell Nanolattices in a Scanning Electron Microscope

Nanoarchitected Cu–Si core–shell lattices were fabricated via two-photon lithography and tested as mechanically robust Li-ion battery electrodes which accommodate ∼250% Si volume expansion during lithiation. The superior mechanical performance of the nanolattice electrodes is directly observed using an in situ scanning electron microscope, which allows volume expansion and morphological changes to be imaged at multiple length scales, from single lattice beam to the architecture level, during electrochemical testing. Finite element modeling of lithiation-induced volume expansion in a core–shell structure reveals that geometry and plasticity mechanisms play a critical role in preventing damage in the nanolattice electrodes. The two-photon lithography-based fabrication method combined with computational modeling and in situ characterization capabilities would potentially enable the rational design and fast discovery of mechanically robust and kinetically agile electrode materials that independently optimize geometry, feature size, porosity, surface area, and chemical composition, as well as other functional devices in which mechanical and transport phenomena are important

A new methodology for characterizing traction-separation relations for interfacial delamination of thermal barrier coatings

The ability to characterize the interfacial delamination properties of thermal barrier coatings (TBCs) is of great technological importance for lifetime assessment of such coatings under service conditions. The purpose of this paper is to report on our novel experimental-plus-simulation-based approach to determine the relevant material parameters appearing in a traction-separation-type law which should be useful for modeling delamination failures in TBCs. We combine load–displacement measurements obtained from (i) a standard tension experiment; (ii) a novel shear experiment; and (iii) a novel asymmetric four-point bending mixed-mode experiment, with simulations of these experiments using a representative traction-separation law in a finite-element program, to extract the requisite material parameters for this traction-separation model. The methodology is applied to determine the material parameters for a TBC system (consisting of an air-plasma-sprayed yttria-stabilized-zirconia top-coat and an MCrAlY bond-coat sprayed on a superalloy substrate) which has been isothermally exposed to air at 1100 °°C for 144 h prior to testing.National Science Foundation (U.S.) (NSF (CMMI Award No. 1063626))King Fahd University of Petroleum and Minerals (Project Number R9-CE-08)Center for Clean Water and Clean Energy at MIT and KFUPM (Project Number R9-CE-08

Electrochemically Reconfigurable Architected Materials

Architected materials can actively respond to external stimuli—such as mechanical forces, hydration and magnetic fields—by changing their geometries and thereby achieve novel functionalities. Such transformations are usually binary and volatile because they toggle between ‘on’ and ‘off’ states and require persistent external stimuli. Here we develop three-dimensional silicon-coated tetragonal microlattices that transform into sinusoidal patterns via cooperative beam buckling in response to an electrochemically driven silicon-lithium alloying reaction. In situ microscopy reveals a controllable, non-volatile and reversible structural transformation that forms multiple ordered buckling domains separated by distorted domain boundaries. We investigate the mechanical dynamics of individual buckling beams, cooperative coupling among neighbouring beams, and lithiation-rate-dependent distributions of domain sizes through chemo-mechanical modelling and statistical mechanics analysis. Our results highlight the critical role of defects and energy fluctuations in the dynamic response of architected materials. We further demonstrate that domain boundaries can be programmed to form particular patterns by pre-designing artificial defects, and that a variety of reconfigurational degrees of freedom can be achieved through micro-architecture design. This framework enables the design, fabrication, modelling, behaviour prediction and programming of electrochemically reconfigurable architected materials, and could open the way to beyond-intercalation battery electrodes, tunable phononic crystals and bio-implantable devices

Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis

BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London