Graduate Student Seminar Series - 2016

December 8, 2016

Development and Applications of Microstructured Solid-State Neutron Detectors

Adam Weltz, Department of Mechanical, Aeronautical, and Nuclear Engineering, Rensselaer Polytechnic Institute

Neutron detectors are instruments used for a wide-variety of purposes, including: nuclear reactor operation, homeland security efforts (including the identification of special nuclear material), and radiological protection and monitoring.  The industry standard for many applications of neutron detection are gas-filled proportional counters which utilize He-3 of BF3 as a fill gas.  These devices achieve excellent neutron detection properties, including: detection efficiency, gamma insensitivity, and surface area; however, there are certain drawbacks related to He-3 cost and BF3 toxicity.  Additionally, these devices are bulky, require large bias voltage for operation, and are poorly adaptable for portable applications.  Solid-state neutron detectors (SSNDs), on the other hand, are compact, require zero bias, and are suitable for portable applications.  The SSND devices developed at RPI have achieved an intrinsic thermal neutron efficiency approaching 30%, adequate gamma insensitivity, and scalability to large surface areas using a single amplification channel.  These microstructured solid-state neutron detectors have a heterogeneous honeycomb microstructure and have been configured in various systems in order to demonstrate the novel applications made available by the development of SSNDs.  Two application developed include a modular directional and spectral neutron detection system and a real-time personal neutron dosimeter have been developed and tested using the devices.

Image Driven Machine Learning Methods for Microstructure Recognition

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

Computer vision and machine learning methods were applied to the challenge of automatic microstructure recognition. Here, a case study on dendritic morphologies was performed. Two classification tasks were completed, and involved distinguishing between micrographs that depict dendritic morphologies from those that do not contain this particular microstructural feature (Task 1), and from those micrographs identified as depicting dendrites, different cross-sectional views (longitudinal or transverse) were identified (Task 2). Data sets were comprised of images taken over a range of magnifications, from materials with different compositions and varying orientations of microstructural features. Feature extraction and dimensionality reduction were performed prior to training machine learning algorithms to classify microstructural image data. Visual bag of words, texture and shape statistics, and pre-trained convolutional neural networks (deep learning algorithms) were used for feature extraction. Classification was then performed using support vector machine, voting, nearest neighbors, and random forest models. For each model, classification was completed using full (original size) and reduced feature vectors for each feature extraction method tested.

Performance comparisons were done to evaluate all possible combinations of feature extraction, selection, and classifiers for the task of micrograph classification. Results demonstrate that pre-trained neural networks represent microstructure image data well, and when used for feature extraction yield the highest classification accuracies for the majority of classifier and feature selection methods tested. Classification accuracies of 91.85 ± 4.25% and 95.74± 3.73% for Tasks 1 and 2 respectively, were achieved using pre-trained neural networks for feature extraction. Thus, deep learning algorithms can successfully be applied to the task of micrograph recognition. This work is a broad investigation of computer vision and machine learning methods that acts as a step towards applying these established methods to more sophisticated materials recognition or characterization tasks. The approach presented here could offer improvements over established stereological measurements by removing the requirement of expert knowledge (bias) for interpretation of image data prior to characterization.

October 20, 2016

ScN-based solid solution thin film for thermoelectric application

Sit Kerdsongpanya, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Clean and sustainable energy sources are an important concern in today’s society. Thermoelectric devices have the potential to contribute to solve this problem since they can directly convert heat into electricity or vice versa. The heat source can be solar, geothermal or a waste that comes from automobiles exhaust or industrial processes. However, the conversion efficiency of thermoelectric device is highly material-dependent, through a parameter called figure of merit (ZT = σS2/κ, where σ is the electrical conductivity, S is the Seebeck coefficient and κ is the total thermal conductivity). Recently, we have demonstrated ScN thin films as a promising thermoelectric material because of its high power factor (σS2) of 2.5×10-3 W/mK2 at 800 K. This result can be explained by nitrogen vacancies generating an asymmetric sharp feature in the ScN density of states which allows low electrical resistivity with retained relatively large Seebeck coefficient. However, ScN has still high thermal conductivity, thus its figure of merit of is low, about 0.2 at 800 K. In this circumstance, there are suggestions to reduce lattice thermal conductivity part through nanostructuring (alloying or nanoinclusion). Therefore, we have investigated the trends in mixing thermodynamics of ScN-based solid solutions in the cubic B1 structure by first-principle calculations. 13 different Sc1-xMxN (M = Y, La, Ti, Zr, Hf, V, Nb, Ta, Gd, Lu, Al, Ga, In) and three different ScN1-xAx (A = P, As, Sb) solid solutions are investigated and their trends for forming disordered or ordered solid solutions or to phase separate are revealed. Moreover, the experimental studies have been carried out in ScN-based solid solution thin films which were prepared by reactive magnetron sputtering. The results are used to discuss suitable candidate materials for different strategies to reduce the high thermal conductivity in ScN-based systems, a material having otherwise promising thermoelectric properties for medium and high temperature applications.

Custom Small Batch Termination Strategies for Niche Market Capacitor Products

David Crist, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Work was carried out as a partnership between local electronics component manufacturer Custom Electronics Inc (CEI) and the RPI Center for Future Energy Systems (CFES) to bring a new Multi-Layer Ceramic Capacitor product manufacturing process online. Capacitor termination takes a focal role in reducing costly overhead for manufacturing as well as research and development of novel termination solutions. This collaborative work is reported and presented with context of the mature MLCC industrial manufacturing process and how this process is scaled down to fill the specialty market capitalized by CEI.

September 15, 2016

Crack Initiation in Metallic Glasses under Nanoindentation

Yongjian Yang, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Simulated nanoindentation tests on a model metallic glass reveal that the crack initiates inside a shear band via cavitation. The load-displacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation. The critical conditions for crack initiation were identified at both the macroscopic and microscopic levels. At the macroscopic level, the indenter geometry affects the overall critical load for crack initiation. Interestingly, the indentation volume at crack initiation appears to be a constant for different indenter geometries, based on which an analytical formula of the critical load as a function of the indenter geometry was derived. At the microscopic level, cavitation occurs once the normal stress perpendicular to the shear band exceeds a temperature-dependent critical cavitation stress. This critical cavitation stress was shown to reduce significantly upon shear deformation.

An Introduction to Polymer Nanocomposites

Marissa Giovino, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Polymer nanocomposites are a relatively new class of materials with applications in color converting LEDs and electric cable insulation.  These materials consist of a polymer matrix and a nanosized filler.  Here the filler is a spherical ceramic nanoparticle with d < 100 nm.  These materials have interesting phase separation behavior.  The interface is very important, the ceramic nanoparticle surface is hydrophilic, very polar like water, while the polymer matrix is hydrophobic, very nonpolar like oil.  Phase separation is favorable in this case.  It is possible to modify the surface of nanoparticles by attaching polymer to it.  In this case the filler acts hydrophobic, very nonpolar, so it is possible to prevent phase separation of filler and matrix.  Even in the case of nonpolar filler and nonpolar matrix there can still be phase separation.  Polymers are typically very large molecules this weakens the entropic driving force for mixing.  By considering carefully the polymer matrix parameters and the polymer grafted filler parameters it is possible to obtain well mixed systems.

May 26, 2016

Interference Lithography with Nanoscale Resolution for Scalable Fabrication of 3D Photonic Crystals

Harikrishnan Vijayamohanan, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Fabrication of three dimensional photonic crystals with bandgap in the visible spectrum has remained a coveted ambition for material scientists and physicists alike. The primary technological challenge lies in the ability to create precise variations in refractive index in three dimensions with nanoscale resolution, cheaply and rapidly. Current state of the art fabrication methods like electron beam lithography are serial in nature and are associated with a high setup cost, whereas the resolution of parallel techniques like optical interference lithography are limited by the diffraction limit. Here, taking inspiration from STED microscopy we propose using reversibly saturable photoreactions to direct-write 3D structures with subdiffraction resolution. Spirothiopyran, a photochromic molecule is best suited for this purpose, with photokinetic simulations of the writing scheme showing an exponential dependence of feature size on laser power. Intensity dependent control on writing was experimentally verified by in-situ NMR measurements. Such high resolution writing schemes can prove integral in reducing scattering losses and improving color mixing of contemporary LED platforms.

April 27, 2016

This special graduate student seminar is delivered by the recipients of the 2016 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 the Charge Dynamics in Dielectric Polymers and their Nanocomposites

Yanhui Huang, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

The electrical conduction and the linking charge dynamics in dielectric polymers and their nanocomposites are of particular interest as it is closely associated with material failure under high electrical stress. Many interesting phenomena emerge under high field, including rapid charge injection, packet-like space charge transport and negative field dependent space charge mobility, which however, cannot be readily explained within the band transport models. Based on the investigation into a silicone rubber system, it is argued that charge transport proceeds in a form of thermally assisted tunneling among a distribution of localized electronic states. The transport was found to critically depend on the energetic and geometrical distribution of these states and can be modeled by a Monte Carlo method. To further investigate the effect of nanofillers, monodispersed 5 nm ZrO2 and TiO2 nanofillers are synthesized and specifically modified to achieve a homogenous distribution in the polymer. The fillers were found to act as deep states and their trapping effects on the electrical conduction and breakdown of the polymer are thoroughly examined and discussed.  

Elastic Properties, Deformation Mechanisms, and Cracking Behavior of Na2O-TiO2-SiO2 Glasses

Garth Scannell, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

The effects of composition on indentation deformation and cracking behavior of Na2O-TiO2-SiO2 glasses were studied in the light of structural considerations and parameters such as the atomic packing density (Cg) and the network energy, using a combination of elastic measurements and micro-hardness indentation experiments.  Fracture toughness was measured using the single-edge prenotched beam method and the permanent deformation volumes were probed using atomic force microscopy.  Na2O-TiO2-SiO2 glasses with titania contents of 4–10 mol% and sodium oxide contents of 10–25 mol% were prepared through a traditional melt-quench process. Indentation experiments were conducted using a Vickers indenter with loads ranging from 10 mN to 49 N. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. In this ternary system concerning a relatively large range of Poisson's ratio (ν), a minimum in critical crack initiation load was observed at a ν of 0.21–0.22.  This minimum coincided with a large increase in the densification volume.  A transition from predominately densification to predominately shear flow was observed at a ν of 0.235.  This study brings to light the unusual role of titanium in the glass network, which gives birth to peculiar trends in the structural and mechanical properties.

April 21, 2016

Cooling Rate and Compositional Dependence of High Temperature Elasticity in Sodium Borate Glasses and Melts

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

Since glass is a non-equilibrium material, its structure and properties depend on both composition and thermal history. Brillouin light scattering experiments were carried out in an optical furnace to investigate high temperature elasticity in air-cooled and annealed sodium borate glasses. On heating the air-cooled glasses, elastic moduli increase anomalously with increasing temperature below Tg, whereas this behavior is absent in annealed glasses. We suspect that portions of the air-cooled glass network did not saturate in superstructural units when the glass was cooled. And these units formed when heated, enhancing network connectivity in three dimensions and increasing the elastic moduli. The structural basis for observing unique trends in the compositional dependence of properties in the sodium borate system will also be discussed.

“More is different”—a Multi-scale Modeling Approach to Predict the Dielectric Properties of Nanocomposites

Yanhui Huang, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

The dielectric phenomena in polymer nanocomposites involve complex physical processes on vastly different scales—from atomic electron transfer (Å) to macroscopic electric field distribution (mm). Capturing and linking these physical processes occurring on the hierarchical structure of the material is essential to accurately predicting the macroscopic properties like electrical conductivity and breakdown strength. To tackle this problem, an integrated modeling approach is developed to bridge the scales with a combined strength from density functional theory, machine learning, Monte Carlo simulation and finite element analysis.  Efforts in modeling the charge transport, polarization, and dielectric breakdown will be specifically discussed, showcasing how this approach can be used to predict the composite macroscopic properties from the basic compositional and geometrical information of each phase.

March 24, 2016

A Graduate Forum

Discussing the opportunities for professional development within the department and the possibility of new student awards for presentation and research via the Graduate Student Seminar and the RPI AVS chapter.

Viscoelastic Damping in Crystalline Composites and Alloys

Raghavan Ranganathan, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Viscoelasticity, an ubiquitous material property, can be tuned to engineer a wide range of fascinating applications such as mechanical dampers, artificial tissues, functional foams and optoelectronics, among others. In this work, we show show that viscoelasticity of crystalline composites comprised of a stiff inclusion in a soft matrix can be tuned to to obtain exceptional damping properties. Our framework for studying this property involves oscillatory shear deformation of model structures using molecular dynamics simulations. The two crystals within the composite, namely a soft and a stiff phase, individually show highly elastic behavior and a very small loss modulus. On the other hand, a composite with the two phases is seen to exhibit damping that is about 20 times larger than theoretical bounds. We attribute this behavior to the fact that the shear strain is highly inhomogeneous and mostly concentrated in the soft phase. Interestingly, the shear frequency at which the damping is greatest is observed to scale with the microstructural length scale of the composite. Finally, a critical comparison of the damping property of these composites with those exhibited by ordered and disordered alloys and superlattice structures will be made.

Auger Recombination and LED Droop

Mark Durniak, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Gallium-Indium Nitride (GaInN) based LEDs have long suffered from a decrease in performance at high current densities, known as droop.  For many years the origin of this droop was a mystery. While many theories existed, there were two that seemed post plausible: carrier overflow and Auger recombination.  However, none had any direct proof until recently when two landmark experiments were published proving not only the existence of Auger recombination in LEDs, but also its contribution to droop.  Auger recombination is a non-radiative process in ­which energy from an electron-hole recombination is given to a third carrier who is then ejected to a higher energy level.  We will explore why Auger recombination occurs in GaInN LEDs as well as go over the experiments that proved its existence.  Lastly, new approaches to combat Auger recombination will be discussed.

February 18, 2016

Band Gap Engineering of a Soft Inorganic Compound PbI2 by Incommensurate Van der Waals Epitaxy

Yiping Wang, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Van der Waals epitaxial growth had been thought to have trivial contribution on inducing substantial epitaxial strain in thin films due to its weak nature of Van der Waals interfacial energy. Due to this, electrical and optical structure engineering via Van der Waals epitaxial strain has been rarely studied. However, by appropriate film-substrate selection, we show that significant band structure engineering could be achieved in a soft thin film material PbI2 via Van der Waals epitaxy. The thickness dependent photoluminescence of single crystal PbI2 flakes was studied and attributed to the substrate-film coupling effect via incommensurate Van der Waals epitaxy. It is proposed that the Van der Waals strain is resulted from the soft nature of PbI2 and large Van der Waals interaction due to the involvement of heavy elements. Such strain plays vital roles in modifying the band gap of PbI2. The deformation potential theory is used to quantitatively unveil the correlation between thickness, strain and band gap change. Our hypothesis is confirmed by the subsequent mechanical bending test and Raman characterization.

Vacuum Technology for Airheads: An introduction on why it’s good to suck

Thomas Cardinal, Department of Mathematical Sciences, Rensselaer Polytechnic Institute

Vacuum-based characterization and processing techniques permeate the physical sciences and allow for an exceptional degree of control and precision in a wide variety of industrial applications.  It is therefore important to understand the motivation for having vacuum and the physical components which create vacuum.  This introductory presentation will highlight the desire to have vacuum based on the mean free path of gas particles and briefly comment on vacuum components such as pumps, valves, and gauges.  Finally, basic care and precautions related to vacuum systems will be covered to assist researchers in properly using and caring for instruments and tools which require high and ultrahigh vacuum.

January 21, 2016

Optimizing Molecular Dynamics Potentials for Multi-component Glasses

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

A major stumbling block in modelling multi-component oxide glasses is the lack of satisfactory potentials that can describe the wide range of amazing properties of these glasses. The main thrust of this work was to develop a new optimization scheme for parameterizing potentials for molecular dynamics (MD) simulations of multi-component glasses. The major motivation was to improve predictions of static properties like elastic moduli and other dynamic properties that MD simulations have generally not been able to estimate correctly while still using a simple functional form for computational efficiency. Our approach was to fit the potentials to results from accurate first principles calculations to predict both the static and dynamic properties correctly, by explicitly incorporating the radial distribution function (RDF) and the vibrational density of states (VDOS) into the cost function of the fitting scheme. We will first show the ability of this simple potential form to predict the properties for silica glass and then binary alkali silicate glasses, and then extend to multi-component oxide glasses. These newly developed potentials will be used to study mechanical properties of multi-component oxide glasses, especially the elastic moduli and their response to external stimuli, such as high temperatures, high pressures and high strains.

Dynamics of Discrete Solitary Waves and the Peierls-Nabarro Barrier

Michael Jenkinson, Department of Mathematical Sciences, Rensselaer Polytechnic Institute

We study the dynamics and stability of localized pulses propagating along a discrete lattice described by the discrete nonlinear Schroedinger equation (DNLS). We find that traveling pulses radiate energy away to infinity until becoming pinned at a lattice site as a stable standing wave; this phenomenon is known as the Peierls-Nabarro (PN) barrier, the energy required for a localized state to travel by one lattice site, and play a significant role in DNLS dynamics. Applications of DNLS range from optical waveguide arrays, tight binding limits in Bose-Einstein condensates, and the dynamics of crystals and biological molecules (phonons and intrinsic localized modes).