Lockheed Martin Corporation Research Articles

Thermal emission cooling of aircraft elements. Review of modern studies

The purpose of this article is to review the current state of research in the field of thermal emission cooling and the feasibility study effect and to highlight their potential applications in astronautics and rocket science, especially for reusable spacecraft and high-speed aircraft. The article summarizes the results of the work of several research groups from several universities in Russia, the USA and China. It discusses theoretical models and experimental developments of thermal emission cooling technology, without going into detailed methodologies. The main research in this area is carried out by teams from the universities of Michigan, Colorado, Vermont and Texas in the USA with the active support of Lockheed Martin Corporation. Experimental studies of this technology are already being actively carried out at these universities. Moreover, the results already obtained correlate well with the results of theoretical study, from which we can conclude about the prospects of research. The effect of thermoelectronic emission makes it possible to reduce the temperature of the leading edges of the spacecraft, which is especially important for reusable spacecraft and unmanned missions, where maintaining the optimal temperature of components becomes a key factor in the service life and reliability of the system.

Model-Based System Engineering and Software System Safety Workshop

The G-48 System Safety Committee sponsored a Model-Based System Engineering (MBSE) and Software System Safety (SSS) workshop, hosted by A-P-T Research, Inc. (APT) in Huntsville, Alabama, on May 2-3, 2017. The idea of this workshop evolved at the 34th International System Safety Conference (ISSC) in Orlando, Florida, during presentations and a paper by Barry Hendrix, which noted that the MBSE needs to include more system safety and software system safety processes. An action recorded under urgent-need topics by International System Safety Society (ISSS) Fellow Dave West at the G-48 meeting in Orlando resulted in volunteers to host and coordinate the workshop.
 The MBSE SSS workshop consisted of a panel of seven subject matter experts. Approximately 40 attendees were present and more than 70 people viewed the workshop via a NASA live video streaming feed. The MBSE SSS panel consisted of Barry Hendrix, APT; Dr. Fayssal Safie, APT; Dr. Donna Havrisik, Government Agency System Engineering; Josh McNeil, AMRDEC Software Engineering Directorate (SED); David Arterburn, University of Alabama Huntsville; Joe Hale, NASA; and Paul Gill, NASA. Many attendees were from local Redstone Arsenal agencies, such as AMCOM, PEO Missiles & Space, and the Missile Defense Agency. Several contractors from companies within Cummings Research Park also attended. Special out-of-town guests included Peggy Rogers, U.S. Navy Software System Safety Technical Review Panel (SSTRP); Bob McAlister, U.S. Air Force; and Lynece Pfledderer, Lockheed Martin (LM), along with five other LM attendees from Texas, Florida and Connecticut.

(Invited) Novel Electronic Transport in Topological Semimetals

Since the discovery of graphene, Dirac systems have attracted considerable attention due to their extraordinary properties. Recently, Dirac semimetals (DSs)-a new quantum state of matter-have been proposed. DSs can be viewed as an analogue of graphene but with linear energy dispersion in all three momentum directions. Due to the distinct electronic properties, DSs have been a fertile playground in discovering novel phenomena. In addition, it is possible to realize topological superconductivity in DSs, which plays a key role in next generation quantum computation. Shortly after theoretical predictions, several materials have been experimentally confirmed to be DSs, such as Cd3As2 and ZrTe5. Here, we report magneto-transport studies on Cd3As2 and ZrTe5. Shubnikov–de Haas oscillations were observed in both materials. A non-trivial Berry’s phase of π was obtained indicating the existence of Dirac fermions. However, angular dependence measurement shows different results in these two materials. Dirac fermions in ZrTe5 show a two-dimensional nature while in Cd3As2 they behave like three-dimensional particles. In addition, we observed supercurrent in superconductor (Al)–DS (Cd3As2)–superconductor (Al) junctions. Realizing topological superconductivity is an essential step towards finding Majorana fermions which are believed to be building blocks for quantum computers. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nanoporous-Carbon As Host Material for High-Performance Li-Ion Energy Storage

We study the feasibility of nanoporous-carbon (NPC) as an electrically conductive host material for reversible Li-ion energy storage. The efficiency of energy storage devices based on intercalation of Li-ions highly depends on the interstitial spacing of the host material, which can significantly limit the diffusion of ions. NPC offers with controlled mass density and surface area considerable advantages over other carbon allotropes. For example, the internal structure of NPC, which self-assembles during growth and consists of nano-fragments of aligned graphene sheet stacks, exhibits as much as a 55% larger interplanar spacing compared to crystalline graphite. Hence, NPC porosity is well controlled via its mass density as a result of defined process parameters, allowing for the NPC host material to accommodate efficient diffusion, as recently demonstrated for the reversible intercalation of Mg-ions. To demonstrate viability as a host material for Li-ion intercalation, we grow NPC directly on 16 mm diameter stainless-steel disks for study as coin cell anodes. The NPC mass density is chosen to range from 0.8 – 1.7 g/cm3 for a series of coin cells which are electrochemically characterized. By varying the mass densities, a correlation between interplanar spacing and electrochemical response is established, revealing the most favorable NPC structure for rapid Li-ion intercalation. The correlating surface area of the most efficient NPC configurations is obtained through Brunauer–Emmett–Teller measurements as a function of nitrogen adsorption in the NPC framework. Moreover, the various NPC structures are investigated via transmission electron microscopy to verify successful diffusion of Li-ions into the NPC host material. NPC represents a new approach for an efficient host material in Li-ion intercalation based energy storage devices with potentially faster diffusion rates. Using NPC as a host material may also enable implementing other atomic species with larger theoretical Li-storage capacity than graphitic carbons, such as Si. Si suffers from drastic volumetric expansion during cycling which leads to fast degradation as a result of embrittlement of the Si bulk, thus limiting its employment as a battery anode. The high porosity of NPC, however, may allow embedding Si atoms or clusters into an NPC host material. We will report on the ability to embed Si into NPC without collapsing the low mass density structure. The decoration of NPC structures with Si offers obvious advantages to crystalline graphite and Si composite architectures for battery anodes, as both the large Li-storage capacity of Si and rapid ionic intercalation into NPC can be made use of in one combined architecture. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

(Invited) Ultra-Wide-Bandgap Aluminum Gallium Nitride Power Switching Devices

SiC- and GaN-based power semiconductor devices have in the past few years enabled great improvements in the efficiency and power density of switching power converters. A wide variety of SiC devices (e.g. MOSFETs, JFETs, BJTs, thyristors, and PiN/Schottky/JBS/MPS diodes) are now available from a number of manufacturers, and the same is true for GaN HEMTs. Thus, while vertical GaN devices are not yet mature, new research in the field is increasingly turning to the “ultra” wide-bandgap (UWBG) semiconductors, including diamond, gallium oxide, and aluminum gallium nitride (AlGaN), due to the expected scaling of the critical electric field as the bandgap to the 2.0-2.5 power. Notably, AlGaN is an alloy system, so that heterostructures are available, and it is also a polar material, which enables polarization doping. Both of these benefits significantly expand the range of device architectures that may be considered, compared to materials that do not have these properties; this is especially significant for UWBG materials, all of which have energetically deep impurity dopants that do not fully ionize at room temperature. This talk will report on both vertical PiN diodes and lateral HEMTs composed of Al-rich Al x Ga1-x N (x ≥ 0.7, EG ≥ 5.2 eV) designed as prototype power switching devices. The talk will begin with an overview of the properties of AlGaN that motivate its application to power switching, as well as an analysis of conduction and switching loss mechanisms as a function of AlGaN bandgap for various device types (PiN diodes, Schottky diodes, etc.) These results will be compared to similar metrics for SiC, GaN, and UWBG materials other than AlGaN. The talk will then present results on several PiN diode structures. All of the structures to be presented have thick (5-8 mm) Al0.7Ga0.3N drift regions doped in the 1-3×1016 cm-3 range, and were grown on thick (1.3 mm) sapphire substrates; due to the insulating nature of the substrate, the cathode contact is to a heavily-doped n-layer on the front side of the wafer in a so-called “quasi-vertical” configuration. The diodes differ in the design of the p-type anode. Homojunction diodes suffer from low free carrier density in the p-Al0.7Ga0.3N due to the low fraction of Mg than is ionized in material of such a wide bandgap (EA > 400 meV). Thus, two alternate approaches to achieve appreciable hole density were examined: heterojunction diodes in with the anode is composed of Al0.3Ga0.7N, in which reasonable Mg activation can be achieved; and diodes utilizing anodes in which the composition is graded from Al0.7Ga0.3N down to GaN. In order to avoid premature edge and/or surface breakdown, all diodes studied utilized a junction termination extension formed by nitrogen implantation into the p-region. The talk will conclude with a presentation of a second class of device, which is a power switching HEMT utilizing an Al0.85Ga0.15N/Al0.70Ga0.30N heterostructure. This heterostructure, which as a 25 nm thick barrier, is characterized by a pinch-off voltage of -4.5 V, a channel mobility of 250 cm2/Vs, and a channel sheet charge density of 6×1012 cm-2. The key challenge for Al-rich HEMTs has been the formation of Ohmic source and drain contacts; the current density in previous HEMTs reported by our group has been limited by the quasi-rectifying nature of these contacts. The HEMTs in the present work utilize planar source and drain contacts, as opposed to the etched-and-regrown contacts that we have reported previously; for those previous contacts, the regrown material was heavily-doped n-type GaN. The planar contacts in this study are composed of a Ti/Al/Ni/Au stack deposited directly on the Al0.85Ga0.15N barrier. While the current-voltage characteristics of these contacts are still quasi-rectifying, they have enabled a current density of 46 mA/mm, which is more than ten times greater than that achieved in our previous-generation AlGaN-channel HEMT, which had comparable channel sheet resistance. An effective breakdown field of ~170 V/um was achieved in a field-plated device with 4 um gate-to-drain spacing. This effective breakdown field exceeds that typically reported for GaN-channel HEMTs (~100 V/um), demonstrating the potential of AlGaN-channel HEMTs for high-performance power switching devices. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nanoelectrode Arrays for in-Situ Identification and Quantification of Halogen Ions

We study the use of nanoelectrode arrays (NEAs), consisting of billions/cm2 of ~ 75 nm diameter electrodes, for the electrochemical analysis of ionic halogen species in water. Such NEA sensors were previously demonstrated to detect part-per-billion (ppb) levels of Pb-ions in aqueous solutions without supporting electrolytes. Briefly, the individual electrodes in the NEA can be tailored with sufficient separation coupled with pore-depth control so that each experiences non-overlapping hemispherical diffusion zones within which the ionic flux of an analyte is independent of time. This geometry is well-suited for trace-level detection, as fast voltage sweeps result in a large and steady-state current density. With optimized design, the additive response from the plethora of nanoelectrodes leads to an orders-of-magnitude analytical advantage for electrochemical detection compared to conventional (mm) sized disc electrodes using sweep or pulse voltammetric techniques. NEAs are fabricated by anodizing RF-sputter deposited Al films on a substrate-of-choice, resulting in billions/cm2 tailored with controllable pore diameters. Each nanopore is partially filled via electrochemical plating with an appropriate working electrode material, such as Au, Pt or Ni. A thin W film is beneath the nanopore template for both processing assistance and to provide a backside electrical contact in parallel to every nanoelectrode. In the case that the NEA is recessed within the porous alumina, a thin conducting film can be added to the topside to function as a counter/pseudo-reference electrode. In some situations, the counter electrode can also serve as an ion collector, preconcentrating ionic species near the pore openings prior to beginning an electrochemical measurement. Using such NEA sensors, we will demonstrate trace detection levels for both Cl and I ionic species in water, and compare the limits-of-detection with conventional electrodes. In addition, we explore the possibility of using NEA sensors to detect halogens in the gas phase. By manipulating the NEA structure, the monitoring capability may be extended to gas phase detection of Cl2 or I2. Such gas-phase detection is possible by utilizing the natural humidity in air condensing on the nanopore walls above the recessed working electrodes. Indirect sensing of such gases occurs after they dissolve in the pore water to form electrochemically detectable halogen species. Notably, the electrochemical measurement is achieved with simple, condensed humidity as the sole ionically conducting phase, without the need for a supporting electrolyte. This is because the high resistivity of water† is mitigated by the short ionic path length: the net solution resistance is small because of the sub-micron distance separating the NEA electrodes from the counter/pseudo-reference electrode sputtered upon the surface of the opposite (open) end of the pore. †The conductivity of humidity levels we consider in this study range from 10-6 S/m (humidity from DI water) to 10-3-10-4 S/m (conductivity range of natural humidity across the USA as monitored by the National Atmospheric Deposition Program’s National Trend Network). This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

(Invited) Metal Organic Frameworks for Thermoelectric Energy Conversion Applications

Motivated by low cost, low toxicity, mechanical flexibility, and conformability over complex shapes, organic semiconductors are attractive as thermoelectric (TE) materials to replace the costly, brittle, and non-eco-friendly inorganic TEs for near ambient temperature applications. Metal organic frameworks (MOFs) are extended, crystalline compounds consisting of metal ions interconnected by organic ligands forming a crystalline, nanoporous structure, share many of the attractive features of all-organic polymers, including solution processability and low thermal conductivity. A potential advantage of MOFs and Guest@MOF materials for TE applications is their synthetic and structural versatility, which allows both the electronic and geometric structure to be tuned through the choice of metal, ligand, and guest molecules. These unique features could solve the long-standing challenge of finding stable, high ZT n-type organic semiconductors, as well as promote high charge mobility as a result of the long-range crystalline order inherent in MOFs. In addition, the low atomic density and long, bridge-like bonding characteristic implies that MOFs should exhibit low thermal conductivity, which is an attractive feature for thermoelectric energy conversion. In my presentation I will review the recent advances in the synthesis of MOF and Guest@MOF TEs and discuss how the Seebeck coefficient, electrical conductivity and the thermal conductivity could be tuned to further optimize their TE performance. This work was supported by the Sandia Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Mitigation of Failure Propagation in Multi-Cell Lithium Ion Batteries

Lithium ion batteries are one of the main energy storage devices that have the potential to be fully integrated in electric grid applications and electric vehicles. Despite years of research in battery safety, there are still challenges associated with the complexity of system-level safety for large battery systems. There is a major gap of safety knowledge at the module or pack level on the effects on adjacent cells in a large-scale battery pack as one cell experiences failure. After the impacts and consequences are identified it is also important to determine mechanisms to mitigate the effects of a single cell failure to the entire pack. In this study, we analyze the failure behavior of small battery packs with 5 stacked pouch cells (LiCoO2 cathode, 3Ah) after thermal runaway is induced to a single cell with nail penetration as the failure initiation method. The first part of the study was to determine the SOC in which full propagation was identified. Different states of charge were tested, including 50%, 75%, 80% and 100% SOC. Complete propagation was observed for 80% and 100 % SOC as shown in Figure 1. The second part of the investigation focused on identifying passive thermal management approaches to mitigating failure propagation. Figure 2 presents the preliminary results using an aluminum heat sink, in which partial propagation was achieved. The results of this study could provide insightful information of the impacts of single battery failure in a pack and a base for future battery designs. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

(Invited) Atomic Scale Investigations of Extended Crystal Defects in Tetradymite-Structured Thermoelectric Materials

Dislocations and other extended defects, such as grain boundaries and stacking faults, can impart strong effects on the thermal and electronic transport properties of thermoelectric materials. Yet, our understanding of the structure of such defects, even for such widely used materials as bismuth telluride, is still quite limited. In this presentation, I will discuss our work investigating the structure of extended crystal defects in Bi2Te3 and related tetradymite-structured thermoelectric materials. As will be discussed, the anisotropic, layered crystal structure of Bi2Te3 has important implications for the resulting atomic-scale core structures of defects in this material. I will begin with a brief overview of the tetradymite system, placing the structure of Bi2Te3 in relationship to other layered chalcogenide materials and discussing the dependence of the layering sequence on the metal to chalcogen ratio. Next, I will discuss HAADF-STEM observations of the atomic structure of dislocations in Bi2Te3. I will focus on two very different types of dislocation: one with Burgers vector of type (1/3)<2 -1 -1 0>, which lies parallel with the basal plane, and the other with Burgers vector of type (1/3)<0 1 -1 -1>, which has a large dislocation component normal to the basal plane of c/3, or one full quintuple unit. The first type of dislocation possesses a stoichiometric core that is glissile on the basal plane. The second type possesses a much more complex dislocation core consisting of bismuth-rich faulted region consistent with Bi3Te4. I will also discuss our current understanding of grain boundaries in this class of materials. Arrays of discrete dislocations can accommodate small misorientations between adjacent crystallites of a few degrees. Examples will be shown for low angle boundaries composed of both (1/3)<2 -1 -1 0> and (1/3)<0 1 -1 -1> type dislocations. For large misorientations, however, simple dislocation descriptions break down and it is more physically meaningful to consider a given interface with reference to a low-energy, reference grain boundary structure. Perhaps the simplest such interface in Bi2Te3 is the (0001) basal twin. Our electron microscopic observations and density functional theory calculations have shown that the structure of the Bi2Te3 basal twin consists simply of a reversal of the basal plane stacking sequence and that this reversal occurs at the Te(1) type planes. As I will discuss it is also possible to go beyond the simple structure of an ideal (0001) twin by considering the introduction of disconnections (defects possessing both dislocation and step content) to this interface. Finally, I will discuss a more complex mode of twinning, on {1 0 -1 5} and {0 1 -1 10}, which has previously been reported in electroplated films of Bi2Te3. Our HAADF-STEM observations provide insight concerning the atomic-scale structure of such twinning, pointing to the formation of structural units consistent with local packets of rock-salt structured material. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

(Invited) Low-Resistance Ohmic Contacts to Al0.45Ga0.55n/ Al0.3Ga0.7n HEMTS

RF amplifiers and power electronics transistors in III-N semiconductors are increasingly important in emerging applications, as are photo-transistors for the purpose of UV light detection. Increasing the Al fraction in AlGaN-based HEMT devices enables transistors with higher breakdown voltages and a potentially favorable breakdown - specific on-resistance tradeoff. However, the advantages of higher aluminum content comes with a major challenge: Ohmic contacts with acceptable contact resistance become increasingly problematic with increasing Al content in the channel and barrier layer. We report sub 10-4 W-cm2 specific contact resistivity (rc) for a 30% channel, improving on the best prior result of approximately 2x10-4 W-cm2for AlGaN channels of at least 30% Al. We investigated five different Ohmic contact metallizations consisting of X/Al/Y/Au where X = Ti, Nb/Ti, Zr, or V, and Y = Ni, Mo, or V (Table 1) as function of alloying conditions. The MOCVD-grown structure (Figure 1) is on sapphire substrates and consists of a 1.6 µm AlN buffer layer, a 4.15 µm thick Al0.3Ga0.7N channel layer and a 50 nm thick Al0.45Ga0.55N barrier with Rsheet of 3500 Ω/□ and ns= 5x1012/cm2. Representative current-voltage plots measured on circular TLM structures are shown in Figure 2. Ohmic contacts with average specific contact resistance (pc) of 7x10-5 Ω-cm2 (3.3x10-5 Ω-cm2) extracted from device (TLM) data were achieved with a conventional Ti/Al/Ni/Au metal stack and a multi-temperature stepped-anneal process. Structural characterization was carried out on the Ti/Al/Ni/Au contacts, to compare Ohmic and Schottky conditions. TEM was used to characterize the interaction between the contact metals and the AlGaN, revealing an interface consisting primarily of Ti-Au with limited indication of spiking behavior. Using the optimized Ohmic contact process, HEMTs with 10 µm source-to-drain spacing and 2 µm gate lengths were fabricated; current densities of 78 mA/mm were achieved, and parasitic loading of RON was reduced. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

(Invited) The Non-Volatile Redox Transistor for Neuromorphic Computing

Inspired by the efficiency of the brain, CMOS-based neural architectures and memristors are being developed for pattern recognition and machine learning. However, the volatility, design complexity and high supply voltages for CMOS architectures, and the stochastic and energy-costly switching of memristors complicate the path to achieve the interconnectivity, information density, and energy efficiency of the brain using either approach. In my talk I will describe a non-volatile redox transistor (NVRT): a device with a resistance switching mechanism fundamentally different from existing memristors, based on the concept of reversible, electrochemical reduction/oxidation of a material to tune its electronic conductivity. The first type of NVRT that I will describe is based upon the intercalation of Li-ion dopants into a channel of Li1−xCoO2. This Li-ion synaptic transistor for analog computing (LISTA) switches at low voltage (mVs) and energy, displays hundreds of distinct, non-volatile conductance states within a 1V range, and achieves high classification accuracy when implemented in neural network simulations1. The second type of NVRT I will describe operates on a similar principle but is based on the polymer system PEDOT:PSS, and which we call the electrochemical neuromorphic organic device (ENODe)2. Plastic ENODes are fabricated on flexible substrates enabling the integration of neuromorphic functionality in stretchable electronic systems. Mechanical flexibility makes ENODes compatible with three-dimensional architectures, opening a path towards extreme interconnectivity comparable to the human brain. (1) Fuller, E. J.; El Gabaly, F.; Leonard, F.; Agarwal, S.; Plimpton, S. J.; Jacobs-Gedrim, R. B.; James, C. D.; Marinella, M. J.; Talin, A. A. Li-Ion Synaptic Transistor for Low Power Analog Computing. Advanced Materials 2017, 29, 1604310. (2) van de Burgt, Y.; Lubberman, E.; Fuller, E. J.; Keene, S. T.; Faria, G. C.; Agarwal, S.; Marinella, M. J.; Talin, A. A.; Salleo, A. A non-volatile organic electrochemical device as a low-voltage artificial synapse for neuromorphic computing. Nature Materials 2017, 16, 414. This work was supported by the Sandia Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

(Invited) Scalable Planar Fabrication Processes for Chalcogenide-Based Topological Insulators

The long spin diffusion lengths predicted for surface transport in topological insulators could lead to numerous applications. Many of the physics experiments used to show promising transport properties were conducted on exfoliated flakes from bulk material, thin films on substrates of limited dimensions, or bulk material, with limited yield. A planar thin film-based technology is needed to make topological insulator devices at a large scale, which is advantageous for applications and could also lead to new device designs. We address two problems related to fabricating chalcogenide-based topological insulator devices on large 3” wafers in the modern microfabrication facility at Sandia National Laboratories using polycrystalline and epitaxial Bi2Te3 films as a prototypical example. (1) Selective inhomogeneous doping was accomplished using ion implantation. Implantation damage and its subsequent mitigation through annealing is characterized. (2) Dielectric layers are often used to manipulate surface potential for elucidating topological surface state transport. We found that the quality of the gate dielectric can show significant degradation with different processing conditions. The leakage current is characterized as a function of temperature. Solutions to these problems are explored and discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

Atmospheric Corrosion and Stress Corrosion Cracking of Corrosion Resistant Alloys Used for Dry Storage of Spent Nuclear Fuel

Spent nuclear fuel (SNF) in the United States is stored onsite at the commercial reactors where it is generated. The dry storage systems consist of welded 304/316 stainless steel canisters enclosed in passively-ventilated concrete overpacks. The ambient cooling air comes from the surrounding environment and contains typical atmospheric aerosols, including chloride-rich salts (e.g. Cl-), which accumulate on the storage canister surface over time. When the canister surface cools sufficiently, surface salts can absorb water from the air and form corrosive surface electrolyte droplets. Over time, these droplets may interact with weld regions that have susceptible microstructures and high through-thickness tensile stresses to promote stress corrosion cracking (SCC). A study is underway to understand atmospheric corrosion mechanisms and morphology that leads to SCC initiation on metals currently used for SNF dry storage canisters and on other corrosion-resistant alloys (CRA) that could be used for future storage. Austenitic SS304H (both annealed and sensitized), PH13-8 Mo SS, and Ni-based Alloy 600 corrosion test coupons have been coated with finely disseminated synthetic sea salt deposits using an ink-jet printer, simulating deposition of naturally occurring sea-salt aerosols, and exposed to expected atmospheric conditions with the aim of studying the resulting corrosion morphology, surface electrolyte chemistry, and SCC crack initiation behavior. Corrosion morphology (pit size distribution, pit number density, and shape) will be characterized as a function of salt surface load density, relative humidity, temperature, and exposure time using optical profilometry, FIB-SEM, microXCT, and robomet (a robotic cross-sectional metallographic examination technique). The surface electrolyte and corrosion products will be characterized by SEM-EDS, TOF-SIMS, and microRaman spectroscopy. Identical salt loading and atmospheric conditions will be applied to dog-bone style tensile samples used for SCC studies where intermittent high R ripple-style fatigue loading will be applied to mark the progression of fracture after crack initiation has occurred. These marks on the fracture surface will be used to identify the critical crack initiating features. Ultimately, the results of this study will be used for modeling initiatives aimed at designing new CRA alloys for next generation SNF tanks and to aid in lifetime assessments for current SNF temporary storage canisters. This work is supported at The Ohio State University by the Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD), and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. The work conducted under WastePD is all stress corrosion cracking analysis and corrosion morphology investigations utilizing optical profilometry. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Surface Analysis of AlGaN Channel Hemts

High electron mobility transistors (HEMTs) with AlGaN channels are excellent candidates for high-power applications due to increased bandgap and electric field as compared to GaN channel HEMTs. An increase in critical electric field is achieved by increasing the Al mole fraction of both the barrier and channel layer of the HEMT to AlyGa1-yN/AlxGa1-xN, where y>x≥0.3. However, the effects of the fabrication process on the surface of high Al content AlGaN has yet to be studied. As AlyGa1-yN/AlxGa1-xN HEMTs gain interest for a variety of applications, unintended modifications to the surface of the AlGaN can have unintended consequences to device performance. X-ray photoelectron spectroscopy (XPS) was performed on Al0.45Ga0.55N/Al0.3Ga0.7N heterostructure. XPS indicated that native surface oxidation of Al0.45Ga0.55N occurs in as-grown material stored at ambient of approximately 8 atomic %. A standard low power microwave O2 ash is a standard process performed to remove organic contamination and descum after photolithography. Though an Al0.45Ga0.55N sample exposed to an O2 ash indicated a slight decrease in C, an increase in the surface oxidation from approximately 8 atomic % (as grown) to approximately 20 atomic % after O2 plasma was observed (Fig. 1). XPS spectra show that after exposure to an O2 ash, oxygen is binding to both Ga and Al. This has substantial implications for device performance and integration schemes for heterostructures with increasing Al content. As the presence of an oxide interlayer can have large implications to Schottky contacts, the effect of an HCl clean to remove the native oxide was investigated. Even after exposure to a 35% HCl dilute in H2O, the O atomic % decreased from 8% to only 6%. Presuming that lower oxygen surface content is desirable, the HCl clean offers a tangible benefit for the surface of Al0.45Ga0.55N/Al0.3Ga0.7N heterostructures. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

Efficient Band-to-Trap Tunneling Model Including Heterojunction Band Offset

It is well-known that band-to-trap tunneling processes in semiconductor devices cause a significant increase in terminal currents under high electric fields [1-3]. Standard band-to-trap tunneling models such as those by Schenk [1-2] and Hurkx [3] are widely incorporated in TCAD device simulation tools [4]. However, these models were developed for silicon devices, so they did not include the effect of band offsets in heterojunction devices, though they are routinely used in simulating the latter. As shown recently by Myers [5], the band offsets at the emitter-base (E-B) junction in heterojunction bipolar transistors (HBTs) can greatly enhance the band-to-trap tunneling near the E-B junction and consequently exacerbate the degradation of gain due to radiation damage. In this paper, we propose an improved band-to-trap tunneling model based on Schenk’s approach, which includes heterojunction band offsets and can be easily implemented into device simulators. Within Schenk’s approach, the band-to-trap tunneling is modeled [1] as the Shockley-Read-Hall recombination with the field-dependent carrier lifetime τ(T, F) given by Fig. 1(a), where T and F denote temperature and electric field respectively, Et is the trap binding energy measured from the band edge, and I(z) denotes the modified Bessel function. Characteristics of band profile, including the effect of electric field, affect the field enhancement factor g(T, F) through the 3D tunneling density of states (DOS) ρF(E). Schenk used the constant field DOS expression in his original model, which is a reasonable approximation for homojunction devices. However, detailed calculations in Ref. [5] show that the E-B junction band offsets in HBTs can significantly increase the DOS near the junction. Myers [5] computes the DOS by numerically solving the Schrodinger equation for the actual potential profile in a NPN HBT. Different from Ref. [5], we approximate the actual potential (e.g., black curve in Fig. 1b) by a linear potential with band offset (red curve in Fig. 1b), and develop an analytic DOS model that includes the band offset, for efficiency and easy implementation in device simulation. The new DOS model is obtained through the following steps: (i) separate the plane parallel to the heterojunction from the direction perpendicular to the junction and use the zero-field 2D DOS for the parallel plane; (ii) approximate the 1D potential along the perpendicular direction using a piece-wise linear profile with band offsets and analytically solve for the wave function in each region with open boundaries at the two outmost ends; (iii) normalize the 1D wave functions according to the Dirac Delta normalization condition [6]; (iv) compute the 3D tunneling DOS that goes into the Schenk model from the zero-field 2D DOS and the normalized 1D wave functions. Figure 1(c) shows a comparison of the 3D tunneling DOS versus carrier energies, computed by several methods, for the red potential in Fig. 1(b). The constant field DOS expression depends only on the field strength, not on the spatial location, as shown by the black solid curve. The color solid curves were obtained using the Fortran code provided by Myers [5] that numerically solves the Schrodinger equation and computes the DOS for the red potential. The color dashed curves were computed using the analytic DOS model proposed in this work. As expected, our analytic model produces the same DOS as those of the numerical method for the given potential. It is clearly seen that, when taking account of the heterojunction band offset, the DOS can be much higher than that of the constant field approximation. In particular, the closer the trap is located to the heterojunction, the higher the DOS is than the constant field value. For example, at 5 nm from the junction, the computed DOS using our model is more than ten orders of magnitude higher than that of the constant field expression for energies of 0.25 eV below the band edge. The substantially increased DOS near the heterojunction is expected to significantly enhance the band-to-trap tunneling in the vicinity of the junction. It turns out that the proposed DOS model can be generalized to arbitrary piece-wise linear 1D potentials. The band-to-trap tunneling model with the new DOS is being implemented in CHARON, an open-source multi-dimensional parallel TCAD code developed at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. Figure 1

Abstract 2859: Sensitive, specific detection of Her-2 positive tumors in mice using superparamagnetic relaxometry (SPMR)

Abstract Superparamagnetic Relaxometry (SPMR) is a non-invasive technique that utilizes superconducting quantum interference device (SQUID) detectors to localize and quantify the magnetization of superparamagnetic iron oxide (Fe3O4) nanoparticles (NPs) specifically bound to cancerous tumors. In an SPMR measurement, polyethylene glycol (PEG) coated NPs are functionalized with a tumor-targeting monoclonal antibody and injected intravenously. NPs that reach and bind to the target tissue are measured by the MRX™ instrument, while unbound nanoparticles, such as those freely circulating in the bloodstream, are not detected. Here, we demonstrate the use of SPMR for specific cancer detection using long-circulating anti-Her2 antibody conjugated PrecisionMRX® NPs in vitro and in vivo. The stability and biofunctionality of conjugated nanoparticles were measured by dynamic light scattering, gel electrophoresis, and ELISA. A cell competition assay was developed to measure specific binding of NPs to Her2 positive (BT474) and Her2 negative (MCF7) cells in vitro. Specificity was defined by the ability of the native antibody to competitively block the binding of the anti-Her2 conjugated NPs to the Her-2 antigen expressed on the cell surface. For in vivo studies, nude mice with xenograft BT474 tumors were intravenously injected with anti-Her2 NPs at a dose of 20 mg/kg of body mass, while control mice were injected with PEG only NPs. Mice were measured individually on the MRX™ instrument at successive time points over the course of 24 hours. At selected intervals during the 24-hour period, blood, tumor, and organs were harvested and analyzed for SPMR signals and anti-Her2 content. In vitro, the anti-Her2 NPs exhibited specific binding to BT474 cells, with little to no binding in MCF7 cells. In vivo, MRX measurements of mice injected with anti-Her2 NPs showed a measurable magnetic signal in the tumor that reached a near maximum approximately four hours after injection. Conversely, mice injected with unconjugated nanoparticles had no measurable tumor uptake. Finally, 24 hours post-injection, 4 – 8% of NPs and anti-Her2 were measurable in the blood, indicating long-term stability of the NP construct in circulation. Together, these results suggest targeted delivery of conjugated NPs to cancerous tissue in vivo and the utility of SPMR for the sensitive and specific detection of cancer in vivo. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Citation Format: Erika C. Vreeland, Kayla E. Minser, Caroline L. Weldon, Andrew Gomez, Todor Karaulanov, Helen J. Hathaway, William H. Anderson, Christopher Nettles, Dale L. Huber, Giulio Paciotti. Sensitive, specific detection of Her-2 positive tumors in mice using superparamagnetic relaxometry (SPMR) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2859. doi:10.1158/1538-7445.AM2017-2859

Re: Prevalence of Metabolic Syndrome in an Employed Population as Determined by Analysis of Three Data Sources.

OBJECTIVE The aim of this study was to compare estimates of the prevalence and incidence of metabolic syndrome (MetS) using various data sources. METHODS We integrated health risk assessment (HRA), claims, and biometric screening data from Lockheed Martin Corporation. We measured the extent to which MetS risk factors measured using HRA and medical claims correlated with biometric screening data. RESULTS Using biometric data, 24.9% of employees were identified as having MetS. Prevalence estimates were much lower using HRA data (6.8%) and claims (3.7%). Between 2012 and 2014, 10.4% of the sample newly acquired MetS. The number of MetS risk factors per employee was predictive of diabetes, heart disease, health care costs, and utilization. CONCLUSION MetS is prevalent and associated with progression to disease. It is more easily tracked with biometric screening data than with HRA or claims data. Employers should consider efforts to manage and prevent this condition in their workforce.

Addressing Lithium-Ion Battery Reliability: A Survey of Electrochemical Behavior

Over the last three decades, lithium-ion batteries (LIBs) have emerged as premier electrochemical energy storage devices. Their widespread application includes cell phones, laptop computers, hybrid electric vehicles, etc. Today, this technology is applied to grid-level energy storage, supporting renewable energy integration into existing power grid infrastructure. To support increasing demand for long-term, large-scale LIB systems; a deeper understanding of commercial LIB safety and reliability is required. Despite thousands of published articles detailing development of LIB components, comparative studies of electrochemical performance as a function of application environments and conditions are scarce. Specifically, parameters such as: cycle stability, capacity, rate capability, cost, and safety implications must be defined as a function of reasonable operating temperatures. Herein, a survey of the electrochemical performance of several commercially available lithium-ion 18650 cells is reported. These studies describe the electrochemical response to temperatures and discharge currents within manufacturer specifications. Electrochemical impedance spectroscopy was used in conjunction with charge/discharge testing to help identify the mode of capacity fade under specific conditions. Several common phenomena occur within these systems, which are reviewed and discussed with a focus on their advantages and disadvantages. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND NO. SAND2016-12467 A

Modeling Thermal Runaway in Li-Ion Packs As a Function of Scale and Heat Source

As deployment of large-scale Li-Ion battery modules is contemplated, there is a need to understand the propensity for thermal runaway in individual cells and the large-scale thermal failure at the module level. Sources of thermal energy that can lead to runaway include short circuits (internal or external), exothermic processes from overcharge of imbalanced cells, the external environment, and other factors. With battery modules consisting of hundreds or even thousands of cells, it will be necessary to design tolerance to local heat release, regardless of the source. This work presents a chemistry-independent framework for analyzing and modeling thermal runaway that will be demonstrated by applying it to multiple lithium-ion cell chemistries to also demonstrate the sensitivity to the cell chemistry. The combined effects of module scale and the thermal environment surrounding the battery are considered in detail. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND NO. 2016-12691 C

Study of Polysulfide Speciation in Lithium Sulfur Batteries Using in Situ Confocal Raman Microscopy

Lithium sulfur (Li-S) batteries are a promising option for high energy density applications, such as electric vehicles and grid storage. In addition to a high theoretical specific energy, the Li-S system also boasts a lower cost and the use of relatively nontoxic, abundant materials compared to current lithium ion batteries. However, soluble, mobile polysulfide intermediates in the electrolyte lead to a polysulfide shuttling mechanism resulting in damage to the anode, loss of active material, and diminished cyclability. A better understanding of the polysulfide speciation as a function of depth of discharge is warranted to identify means to mitigate the polysulfide shuttle. In response, polysulfide speciation is examined, under relevant conditions, in an operational coin cell, at varying states of charge and position, using in situ confocal Raman microscopy. Specifically, a spectroscopic 2032 coin cell was designed for use in in situ experiments while maintaining a representative environment for discharge. The coin cell consisted of a carbon-sulfur cathode, lithium metal anode, and 1:1 1,2-dimethoxyethane to 1,3-dioxolane with 1M LiTFSI and 0.25M LiNO3electrolyte. To aid in comparison, polysulfide standards were synthesized and investigated using electrospray ionization-mass spectroscopy (ESI-MS). Raman and ESI-MS were utilized to map polysulfide speciation and relative abundances along the discharge curve. The electrolyte within the cell was probed as a function of position within the cell to determine the spatial distribution of differing polysulfide species. Challenges associated with performing Raman spectroscopy within this environment, to differentiate the varying polysulfide species in solution, were also investigated. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1