Graduate Student Seminar Series - 2017

December 5, 2017

The Effect of Heat Treatment on Secondary Phase Formation in Dissimilar Metals Welds

Elizabeth Veillette, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

This study proves that a bead-on-plate dissimilar metal gas metal arc weld, with ~40% dilution can be made using a volumetric feed rate greater than the maximum rate predicted in literature studies. The base material used was 316L plate and the filler metals studied with their matching base material in parentheses were ER308L (304L), ER385 (904L), ERNiCrMo-3 (625), and ER2209 (2205). While the dilution is much greater than the 0% dilution predicted, the weld shows a lack of mixing in the weld pool, micro- and macrosegregation, and a lack of a heat affected zone. All of these properties can be attributed to the low heat input produced by the welding input parameters. The as-welded material was heat treated at 750°C and 1205°C for 1-hour, 6-hours, and 24-hours to understand the precipitation and dissolution of secondary phases with relation to the thermodynamic predictions. Ferrite numbers (FN) of the welds were measured and compared to the values predicted by the WRC-1992 diagram, revealing that the diagram over-predicted the FN for the 308L and 2209 welds. Electron-probe microanalysis (EPMA) was performed on the as-welded samples to study the micro- and macrosegregation between the dendrites and across the weld fusion zone. The major findings of this study are that (1) the WRC-1992 diagram inaccurately predicts the solidification method for rapidly cooled gas metal arc welds, (2) sensitization of the 308L welds causes a maximum hardness at 6-hours at 750°C, indicating carbide embrittlement, (3) sigma phase rapidly grows along the interdendritic regions of the 904L, 625, and 2209 welds increasing the hardness by embrittlement, and (4) the 1205°C heat treatment initially dissolves ferrite and stabilizes austenite; NiO forms after the 24-hour heat treatment in the 308L, 904L, and 625 welds, stabilizing ferrite in the Ni depleted weld metal after this exposure. 

Thermodynamics of DNA-Nanoparticle Bonding

Scott Peters, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Much work has been done to exploit the programmable nature of DNA duplexing to create self-assembling nanoparticle crystals. As the bonding of these structures is not determined by the base material of the nanoparticle, but by the DNA strands bound to its surface, the base nanoparticle may be selected of a wide variety of materials, from nobles metals, to semiconductors, to some plastic nanospheres. The ability to engineer crystal lattices of almost arbitrary materials holds much promise for the fields of photonics and plasmonics, among others. However, predicting exactly what lattice will be observed continues to be an issue for modelling these structures. We discuss the structure of these nanoparticles, and how an earlier model, Robert MacFarlane's Complementary-Contact model, uses the nanoparticle's structure to predict what crystal lattices are observed. We then discuss limitations of this model, and then expand upon it by discussing three sources of energy within the system: DNA duplex energy, elastic bending energy, and electrostatic potential energy. These terms are used to qualitatively explain certain phenomena of these DNA-nanoparticle systems, as well as to make new predictions about what crystal lattices will be observed under which parameters.

October 24, 2017

Crystallization of Ultra-Thin Li-V-O Cathode Films as a Function of Annealing Temperature and Ambient

Varun Sarbada, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Electrode microstructure plays a vital role in the electrochemical performance of Li ion batteries. Understanding the crystallization process during annealing of electrode thin films can provide potential new avenues to control of electrode microstructure in thin film battery systems. Thermal annealing experiments both in the vacuum of a transmission electron microscope (TEM) and in an Ar environment were performed to understand the crystallization of amorphous Li-V-O thin films (50-100 nm) sputter deposited from a LiV3O8 target. It is observed that the annealing atmosphere (vacuum vs Ar) has a profound effect on the Li-V-O phase crystallized.  Depending on annealing atmosphere and temperature, delithiated phases such as V2O5, V2O3 and VO2 were generated, but not the desired LiV3O8 stoichiometry. Approaches to contain the Li during annealing of the 50-100 nm films, using nickel diffusion barrier coatings and/or coating with LiPON were unsuccessful in maintaining the desired LiV3O8 stoichiometry. These results demonstrate that both lithium diffusion from the Li-V-O film into the substrate and lithium loss from the thin film surface can limit the ability to maintain the Li concentration required for forming desired/expected Li-V-O phases (in this case LiV3O8) suitable for battery applications. X-ray diffraction, nuclear reaction analysis (NRA), focused ion beam (FIB) and TEM analysis showed that thicker Li-V-O films (~1μm) did form the LiV3O8 phase within the interior of the film upon annealing in an Ar environment, but formed the Li-deficient Li0.3V2O5 phase at the film surface. In this work, combined nano- and micro-scale diffraction, spectroscopic and imaging analysis provide the necessary understanding regarding the Li concentration depth profile and phase distributions in such films, for future development of thin film battery systems.

Charge Carrier Dynamics in Van der Waals Two-dimensional Soft Perovskite (C4H9NH3)2PbI4

Zhizhong Chen, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

The thriving of lead halide based perovskites (e.g. CH3NH3PbI3, CsPbBr3) and two-dimensional (2-D) materials (e.g. graphene, MoS2) have exhibited remarkable electro-optical properties, as witnessed by their impressive performance as solar cells, LEDs, FETs, lasers and photodetectors. With the merit of both, 2-D lead halide perovskite A2PbX4 will intuitively introduce even more interesting physics and probably excellent optoelectronic performance. In today’s talk, I will introduce our recent studies in van der Waals 2-D soft perovskite (C4H9NH3)2PbI4. For the first time, the vapor-phase growth of this material was realized, minimizing the kinetic defects induced by conventional solution chemistry approaches. The interlayer van der Waals interaction assembles the whole structure as alternating organic/inorganic slabs (Ruddlesdon-Popper phase). This unique bottom-up electronic multiple quantum well not only gives rise to huge exciton binding energy (~300meV) and unique carrier dynamics, but also interesting phononic and structural properties. The weak van der Waals interaction enables a weak size-dependence of electronic band structure, in sharp contrast with 3-D perovskites. The weak interaction between substrate and crystals was, surprisingly, strong enough to align crystals epitaxially and even lock their structural phase transition.  In addition, we observed an unexpected decrease in electron-phonon interactions in thin (C4H9NH3)2PbI4, which was attributed to the unique non-polar interface structure compared to 3-D counterparts. These new properties of 2-D hybrid perovskite might provide new solutions to low-cost, self-assembled quantum well structures with reduced phonon scattering and size dependence. I’ll also present the on-going studies about exciton-polariton condensate in (C4H9NH3)2PbI4 optical microcavities. 

September 21, 2017

Deformation Mechanisms in Ultra-Thin Polymer Glasses

Konane Bay, Department of Polymer Science and Engineering, University of Massachusetts Amherst

The physical properties of polymer thin films change as the polymer chains become confined. Similar changes in mechanical properties have been observed, though these critical properties have only been explored a limited extent and with indirect methods. Here, we use a recently developed method to measure the complete uniaxial stress strain relationship of polymer thin films of polystyrene films (PS, Mw=130 kg/mol) as a function of thickness (20 nm - 300 nm). In this method, we hold a ‘dog-bone’ shaped film on water between a flexible cantilever and a movable rigid boundary, measuring force-displacement from the cantilever deflection. From our measurements, we find that the modulus decreases as the PS chains become confined. The PS thin films exhibit “ideal perfectly plastic” behavior due to crazing, which differs from the typical brittle response of bulk PS. The failure mechanism transitions from 3D crazes to 2D crazes to shear deformation zones with decreasing thickness. These results provide new fundamental insight into how polymer behavior is altered due to structural changes in the entangled polymer network and surface mobility changes upon confinement.

Vacancy Induced Mechanical Stability of Cubic Tungsten Nitride

Karthik Balasubramanian, Department of Mechanical, Nuclear, and Aerospace Engineering, Rensselaer Polytechnic Institute

First principle calculations are employed to determine the mechanical stability and the formation energies Ef of point defects in rock-salt phase group III B – VI B transition metal nitrides and carbonitrides. Cation and anion vacancies are energetically most preferred with the vacancy formation energy decreasing when moving to the right and attaining negative values for group VI B nitrides, indicating thermodynamic favorability despite experimental observations of cubic rock-salt type WN and MoN.  This decrease is accompanied by a decrease in the single crystal shear modulus, from c44 = 120 GPa for HfN to -86 GPa, for WN, indicating a transition to mechanical instability of rock-salt nitrides. We investigate this discrepancy by comparing the energetic and mechanical properties of WN in the experimentally reported rocksalt and theoretically predicted NbO phases. Charge distribution and electronic density of states analyses reveal that the mechanical instability of rocksalt WN is due an increased overlap of t2g orbitals upon the application of shear strain along [100], resulting in electron migration from the expanded [110] to the shortened [110] direction and a negative shear modulus. The mechanical transition from the unstable NaCl to the stable NbO phase is further explored using supercell calculations of the NaCl structure containing Cv = 0 to 25 % of both cation and anion vacancies. The structure is mechanically unstable for Cv <5 %. At this critical vacancy concentration, the isotropic elastic modulus is zero but increases steeply to 445 GPa for Cv = 10 % and to 561 GPa for the NbO structure with Cv = 25 % which is in good agreement with experimentally measured elastic moduli ranging from 110 – 360 GPa. These results show that the experimental reports of a cubic WN phase can be explained by the mechanical stabilization of the rocksalt phase by a minimum of 5% anion and cation vacancies.

May 16, 2017

Residual-layer-free transfer molding facilitated by 1-D discontinuous dewetting at the mesoscale

Michael Deagen, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Residual-layer-free transfer molding offers a low-cost approach to nanofabrication of isolated features, useful for applications in printed electronics and photonics. For a blade meniscus coating process, a regime for discontinuous dewetting is proposed by combining both macro- and micro-scale wetting considerations. Patterns comprised of parallel channels showed vastly different meniscus morphologies based on the azimuthal angle of the stamp, suggesting a mechanism for 1-D discontinuous dewetting that differs from the qualitative picture often presented in the literature. By tuning ink viscosity, stamp surface energy and blade velocity, the critical dewetting velocity of the meniscus was quantified. Residual-layer-free filling and transfer are demonstrated with high resolution (300 nm) across relatively large areas (>1 cm2).

Optimizing interaction potentials for Molecular dynamics

Siddharth Sundararaman, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Atomistic simulations like molecular dynamics (MD) can give great insight into structure-property relations in materials. The accuracy and reliability of these properties depend on the interaction potential used. A dearth of satisfactory interaction potentials for multi-component oxide glasses that can reasonably describe a variety of properties is a major stumbling block facing the glass community. The method of optimizing potentials will be discussed along with some of the major challenges faced in this process. In this work, a new optimization scheme was developed to parameterize effective pairwise potentials for molecular dynamic (MD) simulations of these glasses. Our approach was to fit to results from accurate first principles calculations and explicitly incorporate the radial distribution function (RDF) of the equilibrium liquid at multiple temperatures, the vibration density of states (VDOS) and the density of glass into the cost function of the fitting scheme. Improved potentials for silica glass from this optimization scheme will be compared to existing potentials in literature.

April 26, 2017

This special graduate student seminar is delivered by the recipients of the 2017 Norman S. Stoloff Research Excellence Award recipients at 11:00 am in LOW 3051, at the time and location of the weekly department seminar.

Understanding Sodium Borate Glasses And Melts From Their Elastic Response To Temperature

Siva Priya Jaccani, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

In-situ Brillouin light scattering (BLS) experiments were carried out to measure the high temperature elastic moduli of sodium borate glasses and melts from room temperature to temperatures beyond the glass transition temperature (Tg) over a large composition range. Besides composition, we also varied the cooling rate to study the effect of thermal history on room temperature properties and on the elastic response to high temperature. On heating air-cooled glasses of lower Na2O content, elastic moduli increase anomalously with increasing temperature just below their Tg, whereas this behavior is absent in corresponding annealed glasses. This anomalous increase of elastic moduli with temperature was not observed in glasses of higher Na2O content.  These differences were explained by different structural relaxation mechanisms in the glass transition range in sodium borate glasses of different compositions based on Raman spectroscopy.

Fatigue Fracture Toughening And Stress-Corrosion Mitigation At Metal-Ceramic Interfaces Using A Molecular Nanolayer

Matthew Kwan, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Tailoring the chemistry of heterointerfaces is crucial to control the fracture toughness of a variety of composite materials. This talk will demonstrate that introducing a molecular nanolayer at a metal-ceramic interface can lead to multifold toughening under both static and dynamic loads in chemically reactive environments. I will first describe the interplay between molecular layer-induced interfacial strength and plasticity as a function of metal film thickness, temperature and moisture, during stress-corrosion. While the molecular nanolayer is key to transferring the elastic energy to deform the metal layer, we find that plasticity can increase due to temperature-induced yield stress decrease despite interfacial weakening caused by water-induced siloxane bond scission. I will then present our recent results on loading-frequency-dependent multifold toughening of a layered polymer-metal-ceramic structure with a molecular nanolayer at the metal-ceramic interface. The fracture energy peaks in the ~75-300 Hz regime are underpinned by nanolayer-induced interface strengthening that facilitates plasticity in the distal polymer layer. The fatigue fracture energy peak height, frequency and width, are sensitive to the interfacial strength and the temperature relative to the polymer glass transition, and hence, are tunable by appropriate choices of nanomolecular layer and polymer. Our findings open up a completely new set of possibilities for tailoring mechanical properties of layered composites and soft-hard heterointerfaces through nanomolecular functionalization for diverse applications, such as, load-bearing structures, flexible electronics and biomedicine.

April 14, 2017

Stimuli responsive hydrogels and microgels - Basic concepts and prospective applications

Apostolos Karanastasis, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Stimuli responsive materials constitute key elements in contemporary optical, sensing, actuating and drug delivery systems and their research and development has seen a prolific level of growth over the past decade. In this talk, we will focus on the subclass of synthetic polymeric thermoresponsive materials, with emphasis given on hydrogels and microgels based on poly(N-isopropylacrylamide) (PNiPAm). A brief insight on the physicochemical principles that govern the stimuli-responsive nature will be provided as well as synthetic strategies for the preparation of bulk and nanosized materials. Selected examples will highlight the versatility of these materials as platforms for the development of multiresponsive hybrid systems. Current advances and future perspectives related to our ability to explore the nanostructural details of soft materials using super resolution microscopy techniques will also be discussed.

The Fatal Flaw

Emily Aaldenberg, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

From the windows in our labs to the screens of the mobile devices in our pockets, we are surrounded by glass. The demands for “unbreakable” glass are high because the failure of glass products can be dangerous or expensive to replace. The reliability and strength of glass products is limited by surface, edge, and internal defects or flaws. Flaws in the glass lengthen over time when loaded under tension and eventually cause fracture and product failure. The strength and fatigue of glass will be reviewed along with methods to test the mechanical strength and crack growth behavior of glass. The dynamic fatigue, or loading rate-dependent strength, of unstrengthened glass will be compared to ion-exchanged strengthened glass. Strengthening of glass fibers by a water-assisted surface stress relaxation will be presented along with a demonstration of continuous processing of optical fibers by this method.

March 23, 2017

Towards Building Structure, Property, Processing Relationships for Polymer Nanocomposites

Aditya Prasad, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

The development of polymer nanocomposites continues to be Edisonian in nature and models characterizing the relationship between structure-processing-properties of these materials are not well developed. Road blocks to developing such models that describes these relationships are the ability to control the dispersion of nanofillers in the polymer matrix, effectively quantifying the interphase polymer surrounding the nanofiller and determining the properties of the interphase polymer, in comparison to the bulk polymer. By preparing nanodielectric systems with tailored dispersion states, quantifying the dispersion through TEM imaging and image analysis, and measuring the dielectric spectroscopy and breakdown, we have found quantitative relationships between these parameters that can aid in the design process. 

Design of Experiments (from α to z*)

Michael Deagen, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Experimentation lies at the heart of the scientific method and provides us with a window into the physical laws that govern the natural world. The objective of this talk is to provide an overview of the concepts, language, best practices, and resources available for the design and analysis of experiments. Whether you are trying to understand a system or design a new process, experimental design can improve the robustness of the results while minimizing resources such as time and money. Freely available systems (e.g. R) can be useful tools for planning experiments or analyzing large data sets. While the talk will gravitate toward research in materials science and engineering, the concepts presented are broadly applicable across disciplines and career types.

February 16, 2017

Deformation at Triple Junctions in Columnar Structure Nickel

Mingjie Li, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Triple junctions play important roles in heterogeneous deformation in polycrystalline materials. In order to minimize three-dimensional grain constraints, this work was conducted using a columnar structure nickel. The local strain distribution was quantified using digital image correlation methods through array of platinum nanodots. Distortions of the array used as markers were analyzed near the triple junctions after 10% tensile strain. Electron back scattered diffraction (EBSD), scanning electron microscopy (SEM) and atomic force microscope (AFM) allow to determine activated slip systems and out-of-plane rotational distortions. The strain distribution near triple junctions was attributed to slip transfer across adjoining grain boundaries. This work gives an insight on deformation processes near triple junctions without complexity in 3D grain interactions.

Electron Scattering at Rough Metal Surfaces: First-Principles Results

Tianji Zhou, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

With the continual downscaling of semiconductor devices, the size dependent resistivity becomes a bottleneck in interconnect performance. The resistivity of sub-100 nm Cu thin films increases dramatically above that of bulk material, due to additional scattering at surfaces and interfaces.  Surface scattering is traditionally described with the Fuchs-Sondheimer (FS) model which underestimates the resistivity for films with thickness d<10 nm, as it neglects quantum mechanical and explicit surface morphological effects. Density functional theory(DFT) and non-equilibrium Green’s function(NEGF) are applied to model electron transport in 1-2 nm thick Cu films with atomic roughness, phonons, and surface steps. Zero temperature calculations indicate a residual surface scattering resistivity proportional to 1/d, due to quantum confinement effect, while at finite temperature an approximately additive contribution from phonon scattering is suggested. A Landauer formalism is applied to relate additional surface roughness resistivity to measurable surface morphological parameters, RMS roughness and correlation length.

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