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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor InTaO4 and its Implications for Water Splitting Activity in the Wolframite Structure Type

The mechanism of Ni substitution into the oxide semiconductor InTaO{sub 4} has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron X-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In{sub 1-x}Ni{sub 2x/3}Ta{sub x/3})TaO{sub 4} with a solubility limit of x {approx} 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temperatures of 1150 C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In{sub 1-x}Ni{sub 2x/3}Ta{sub x/3})TaO{sub 4} compounds with different x values. Undoped InTaO{sub 4} is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap reductions (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In{sub 1-x}Ni{sub 2x/3}Ta{sub x/3})TaO{sub 4} compounds instead results from a weak {sup 3}A{sub 2g} {yields} {sup 3}T{sub 1g} internal d {yields} d transition not associated with the conduction or valence band that is common to oxide compounds with Ni{sup 2+} in an octahedral environment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

The relative variational model - A topological view of matter and its properties: UO{sub 2}{sup {+-}x} density

Formal definitions of convergence, connected-ness and continuity were established to characterize and describe the crystalline solid and its properties as a unified notion in the topological space. The crystalline solid is a previously empty space that has been filled with atoms and phonons, i.e., the crystal is built with packages of matter and energy in a regular and orderly repetitive pattern along three orthogonal dimensions of the space. The spatial occupation of the atom in the crystal structure is determined by its mean vibrational volume. Thus, the changes of volume and the changes of internal energy are intrinsically linked. In fact, physical and material properties are the interdependent and bijective quantifications associated with variations of the internal energy. These properties are modeled by means of an intrinsic and invariable form function: the Relative Variational Model. In this paper, the lattice parameter and specific mass of UO{sub 2}{sup {+-}x} are depicted in dependence on the variations of the stoichiometry in the band of -0.016 {<=} x and <0.25 and temperature from 0 K up to melting point. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2016

In-pile investigation results of re-sintering for uranium dioxide fuel with large grain size at temperature 650-700 C

When determining the radiation creep of nuclear fuel, apart from deformation caused by axial compressive stress, there is a shrinkage process induced by radiation re-sintering, which may contribute in total value of in-pile deformation. The aim of this work was to study re-sintering for large grain size fuel of uranium dioxide. WWER fuel pellets with grain size greater than 25 μm have been piled up in a special device and irradiated in the channel number 7 of the RBT-6 pool-type reactor. The height of the fuel column was continuously measured during the irradiation. The average temperature value in the fuel column center was 682 Celsius degrees and the total time of irradiation reached 1800 hours which corresponds to a specific burnup of 2.5 MWd/KgU, taking into account fission density and initial fuel stoichiometry. The results show that re-sintering throughout the height equals to 0.08% which correlates well with thermal re-sintering test and does not have an impact on the steady-state creep rate.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Modeling thermochemistry of fuel and coupling to fuel performance codes

A thermochemical library, Thermochimica, utilizes new algorithms and solvers designed for calculating equilibria of multicomponent and multiphase systems and has been coupled to the nuclear fuel performance code BISON. Thermochimica utilizes a generalized thermochemical database for urania-based fuels that has been under development for a number of years and continues to be expanded. One of the key motivations for use of thermodynamic models is that they can better represent the physics of oxygen migration. The coupling of the BISON code with Thermochimica and examples of its use in representing fuel phenomena are described. In the current implementation in BISON, fluxes are driven by the elemental concentration gradient. However, equilibrium is characterized by the absence of spatial variations of chemical potentials of the system species/components. The driving force to equilibrium (species flux) is proportional to the deviation from equilibrium, i.e., the gradient in chemical potential. It is therefore necessary to integrate chemical potential driven diffusion in the BISON representation of oxygen transport. The diffusion kernels based on thermodynamic models have been developed to simulate multicomponent transport in light water reactor (LWR) materials. The Thermochimica library was used to calculate various thermodynamics properties needed for transport calculations. The library also can calculate properties that are specific to various transport mechanisms for different fuel materials. For example, it can calculate defect site fractions among the sublattices in oxide fuels as a function of temperature, burnup, and stoichiometry. The transport and thermodynamic models are being integrated with other simulation efforts such as calculation of mobility functions that couple fluxes and chemical gradients. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Mineral Dissolution upon Alkaline Treatment and Variable Redox Conditions

Over 200.000 kg of uranium released to vadose zone from historic leaks and improper disposal of wastes from plutonium production. U highly mobile due to presence of carbonates and oxidizing conditions with partition coefficients k{sub d} of 0.11-8 mL/g at pH 8. Ammonia (NH{sub 3}) gas injection investigated as an in situ remediation technique that will not introduce additional liquid to the subsurface. Objectives: Understand the impact of remediation with base injection on geochemical process in Hanford site vadose zone. Understand effects of different base treatments on mineral dissolution. Results: - incongruent dissolution based on mineral stoichiometry: if ratio = 1 congruent dissolution occurs, if ratio <1.0 greater leaching of Si and Al and incongruent dissolution occurs. Conclusions: incongruent dissolution occurred for both alkaline treatments and redox conditions across all minerals. By 1440 hr. all normalised Al:Si ratios were <1 representing less [Al]{sub aq} and suggesting formation of secondary Al precipitates as a function of time. [Si]{sub aq} increased and [Al]{sub aq} decreased with increasing contact time with both alkaline treatments. NaOH dissolved slightly more than NH{sub 4}OH especially for [Al]{sub aq} in all investigated minerals. No clear trend for muscovite and montmorillonite for redox conditions but slightly greater Si dissolved under anaerobic conditions for illite. Ammonia gas injection increases the pH and dissolves aluminosilicate minerals in the vadose zone. U is likely to coprecipitate with secondary minerals forming at elevated pH and as the system reequilibrates at natural neutral pH. Further, U solids may be coated with low solubility, secondary precipitates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Oxidation of Glycolate in the Defense Waste Processing Facility (DWPF) Recycle Collection Tank - 20305

The Savannah River Site's Defense Waste Processing Facility (DWPF) operations are being upgraded with the introduction of the Nitric-Glycolic Flowsheet. Glycolic acid has been shown superior to formic acid as the reducing acid used during chemical processing. The new flowsheet improves or maintains necessary parameters such as 1) reduction of mercury, 2) adjustment of feed rheology and 3) adjustment of melter oxidation/reduction potential. Further, the potential for catalytic hydrogen generation in DWPF processing is virtually eliminated. DWPF process condensates are collected and returned to the SRS Concentration, Storage and Transfer Facilities (CSTF). The Recycle Collection Tank (RCT) collects off-gas condensate during chemical processing, vitrification, and other unit operations performed in DWPF and is the singular return vessel delivering recycle effluent back to CSTF. Each batch of recycle may contain a small amount of glycolate from chemical processing and melter off-gas condensates. To avoid potential flammability issues due to thermolysis of glycolate in the CSTF, chemical oxidation within the RCT has been investigated as an option for mitigating the transfer of glycolate. Sodium permanganate has been down-selected as the best option for oxidation of glycolate. Testing was performed using both 2-L and 22-L reactors (16,800:1 and 1,530:1 scale by volume) with non-radioactive waste simulants to approximate the expected RCT compositions. RCT simulants were evaluated at various process pH and temperature conditions. Also, RCT operations, namely the sequence of addition of corrosion inhibitors (NaOH and NaNO{sub 2}) versus a permanganate strike, were evaluated. Glycolate was introduced via a sludge simulant to mimic both expected entrainment and abnormal process foam-over conditions - the range being between 68 and 5100 mg/kg glycolate. Glycolate destruction was monitored by ion chromatography (IC). The corresponding manganese behavior was monitored in real-time using in situ ultraviolet-visible (UV-Vis) spectroscopy. RCT glycolate content can be reduced to below the IC detection limit within 90 minutes for all concentrations investigated. Ion Chromatography analysis revealed that under alkaline conditions, glycolate is primarily oxidized to oxalate with no significant formation of CO{sub 2} or carbonate, and nitrite is not oxidized to nitrate. Initially, complete oxidation of organics species and nitrite was assumed. Determination of the mechanistic chemical reaction has allowed the required amount of permanganate to be more accurately predicted and the total addition to be significantly reduced. UV-Vis measurements reveal that permanganate (Mn{sup 7+}) is reduced to manganate (Mn{sup 6+}) in the RCT. The oxidant stoichiometry is defined by using the initial permanganate to glycolate (P/G) molar ratio. At low initial glycolate concentration (68 and 140 mg/kg), the minimum required initial permanganate to glycolate (P/G) molar ratio was found to be 5-6. With high initial glycolate concentrations (5100 mg/kg) a lower (P/G) molar ratio of ∼2.5 was needed. The final portion of this effort supporting the nitric/glycolic flowsheet will be to test actual (fully radioactive) RCT samples as per the above simulant tests. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

Structural Properties of Ditechnetium Heptoxide, Tc{sub 2}O{sub 7}

Group 7 elements form unique binary oxides with the stoichiometry M{sub 2}O{sub 7}. The heptoxides of manganese and technetium crystallize as molecular solids while rhenium forms an extended solid with mixed valances, 4 and 6. Unlike manganese and rhenium heptoxides which are used in the chemical industry technetium heptoxide has found very few applications due to its radioactive nature; it has been primarily use as a chemical precursor. While both of the molecular solids, Mn{sub 2}O{sub 7}{sup 3} and Tc{sub 2}O{sub 7}{sup 4} can be described by two corner-sharing MO{sub 4} tetrahedron. The most distinctive difference is that the manganese's compound has a metal-oxygen-metal bond angle of 120.7 deg. while the technetium species is 180 deg.. Additionally, Tc{sub 2}O{sub 7} has the largest molar volume of the heptoxides which suggest that it will be more susceptible to external stimuli of pressure and temperature than the others heptoxides. A pressure induced twisting has been predicted by Fang et al. Anomalous behavior is seen in the c-axis where below 2 GPa the Van der Waals interactions between the Tc{sub 2}O{sub 7} molecules is over come and compression is observed but at higher pressures the twisted distortion forces the lattice to expand. The terminology diagonal-double tetrahedrons (DDTs) is introduced to discuss the molecules of Tc{sub 2}O{sub 7}. The rigid nature of the DDTs has been attributed to the cause of this twisted distortion mechanism. Recently, the solid state and gas phase behavior of technetium heptoxide was revisited by Childs et al. in light of {sup 99}Tc being an important dose contributor in the Hanford waste and vitrification being proposed for immobilization of nuclear waste. Technetium heptoxide is one of the proposed volatilized species of technetium found in the vitrification process. In this study the crystal structure of Tc{sub 2}O{sub 7} was reported at 100 K and compared to the structure first determined by Krebs at room temperature. Inspection of the metal-oxygen distances revealed the 100 K structure had longer bond lengths for the bridging oxygen than at high temperature. It is suggested that transverse thermal vibrational modes can be used to explain this negative thermal expansion behavior. This describes the average bond length of a central atom (i.e. the bridging oxygen atom) being measured as shorter at higher temperatures due to the larger thermal motion. Diffraction methods that measure the long range average structure are susceptible to observing this shorter average length as opposed to the true bond length, atom position at oscillation node that a method like Pair-Distribution Functional PDF Analysis would yield. In the case of Tc{sub 2}O{sub 7}, the magnitude of the transverse thermal vibrational mode is at the limit of being detectable by SCXRD and likely not observable by PDF analysis. Here we report the preliminary results of our study on the nature of the thermal expansion behavior of Tc{sub 2}O{sub 7}. The transverse thermal vibration mode compression mechanism is considered and compared to the pressure induced twisted distortion mechanism predicted at high pressure. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2017

Precipitation of Epsilon Particles in Metal Doped Ceria Irradiated with Energetic Ions

Metallic particles precipitated from fission products in oxide fuel matrix (e.g., UO{sub 2}) have a great impact on the thermal and mechanical properties of the fuel. A burn-up of 40 MWd/kg U results in the conversion of 4% of the uranium to approximately 3% fission products and 1% transuranium elements. Epsilon phase is the major metallic phase of the precipitates with a hexagonal-closed packed (HCP) crystal structure. It can exist in a variety of metallic forms in UO{sub 2}, but typically, it consists of five metals: Mo, Pd, Rh, Ru and Tc. Epsilon particles can form over a wide range of stoichiometries in the five metal system. Simulated compositions with Re as a surrogate for Tc have been investigated as a waste form for {sup 99}Tc, an isotope with a long half-life of 2.13 10{sup 5} years and a high solubility under oxidizing conditions in addition to Mo in the corrosion process. There is a pressing need to understand the factors influencing the formation and growth of the metallic phase in fuel. Efforts are currently underway to perform careful neutron irradiation of nuclear fuel under controlled conditions in a test reactor, recover metallic phases that may be present, and characterize the resulting structure and chemistry. Due to the long duration, high cost and radiological concerns associated with neutron irradiation and sample characterizations, ion irradiation studies of a surrogate material with a full control of the processing parameters can emulate different stages of the particle precipitation, which can add a tremendous value to reactor-based studies. This presentation reports our recent ion irradiation results on the metallic particle precipitation under various irradiation and thermal annealing conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Correlation between Grain Size Distribution and Grain Boundary Character in Polycrystalline Uranium Dioxide

Thermo-mechanical behavior of oxide nuclear fuels is extremely important to overall fuel element performance. Critical to the understanding of Pellet-Cladding Mechanical Interactions (PCMI) are elasticity, plasticity and creep, as this interaction with the cladding can lead to fuel rod fracture and in turn affect its thermal performance significantly. In addition, sub-grain scale mechanical behavior, e.g. elastic anisotropy, as well as dislocation-driven plasticity and creep, can also play a significant role on microstructure evolution of the fuel elements and their cladding, failure of which would severely reduce the useful service life of the elements and allow the release of fission products out into the working fluid of the reactor. Therefore, careful measurements of mechanical properties are key to validate robust fuel performance codes able to predict this PCMI behavior from inputs at the micro-scale, e.g., MARMOT, and to quantify its effects on other aspects of fuel behavior. In uranium dioxide (UO{sub 2}), the thermomechanical response at the meso-scale depends strongly on crystallography of individual grains, and available datasets are far from complete in terms of temperature, stress, and stoichiometry in parameter space. Therefore, details of the fuel microstructure, such as porosity distribution and location, as well as grain boundary (GB) character, distribution, and location are needed to create accurate multiscale models to predict the fuel's evolution in-pile, and in turn aid in the design and development of more efficient fuels that can last longer cycles.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Lipocalin Blc is a potential heme-binding protein

Lipocalins are a superfamily of functionally diverse proteins defined by a well-conserved tertiary structure despite variation in sequence. Lipocalins bind and transport small hydrophobic molecules in organisms of all kingdoms. However, there is still uncertainty regarding the function of some members of the family, including bacterial lipocalin Blc from Escherichia coli. Here, we present evidence that lipocalin Blc may be involved in heme binding, trans-periplasmic transport, or heme storage. This conclusion is supported by a cocrystal structure, mass-spectrometric data, absorption titration, and in silico analysis. Binding of heme is observed at low micromolar range with one-to-one ligand-to-protein stoichiometry. However, the absence of classical coordination to the iron atom leaves the possibility that the primary ligand of Blc is another tetrapyrrole.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Long-term Kinetics of Uranyl Desorption from Sediments Under Advective Conditions

Long-term (> 4 months) column experiments were performed to investigate the kinetics of uranyl (U(VI)) desorption in sediments collected from the Integrated Field Research Challenge (IFRC) site at the US Department of Energy (DOE) Hanford 300 Area. The experimental results were used to evaluate alternative multi-rate surface complexation reaction (SCR) approaches to describe the short- and long-term kinetics of U(VI) desorption under flow conditions. The SCR stoichiometry, equilibrium constants, and multi-rate parameters were independently characterized in batch and stirred flow-cell reactors. Multi-rate SCR models that were either additively constructed using the SCRs for individual size fractions (e.g., Shang et al., 2011), or composite in nature could effectively describe short-term U(VI) desorption under flow conditions. The long-term desorption results, however, revealed that using a labile U concentration measured by carbonate extraction under-estimated desorbable U(VI) and the long-term rate of U(VI) desorption. An alternative modeling approach using total U as the desorbable U(VI) concentration was proposed to overcome this difficulty. This study also found that the gravel size fraction (2-8 mm), which is typically treated as non-reactive in modeling U(VI) reactive transport because of low external surface area, can have an important effect on the U(VI) desorption in the sediment. This study demonstrates an approach to effectively extrapolate U(VI) desorption kinetics for field-scale application, and identifies important parameters and uncertainties affecting model predictions.

additivity approach↗
DOE OSTI2018

High-Pressure Geophysical Properties of Fcc Phase FeH X

Face centered cubic (fcc) FeHX was synthesized at pressures of 18-68 GPa and temperatures exceeding 1,500 K. Thermally quenched samples were evaluated using synchrotron X-ray diffraction (XRD) and nuclear resonant inelastic X-ray scattering (NRIXS) to determine sample composition and sound velocities to 82 GPa. To aid in the interpretation of nonideal (X≠1) stoichiometries, two equations of state for fcc FeH X were developed, combining an empirical equation of state for iron with two distinct synthetic compression curves for interstitial hydrogen. Matching the density deficit of the Earth's core using these equations of state requires 0.8-1.1 wt % hydrogen at the core-mantle boundary and 0.2-0.3 wt % hydrogen at the interface of the inner and outer cores. Furthermore, a comparison of Preliminary Reference Earth Model (PREM) to a Birch's law extrapolation of our experimental results suggests that an iron alloy containing approximate to 0.8-1.3 wt % hydrogen could reproduce both the density and compressional velocity (V P ) of the Earth's outer core.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

A parsimonious modular approach to building a mechanistic belowground carbon and nitrogen model

Abstract Soil decomposition models range from simple empirical functions to those that represent physical, chemical, and biological processes. Here we develop a parsimonious, modular C and N cycle model, the Dual Arrhenius Michaelis‐Menten‐Microbial Carbon and Nitrogen Phyisology (DAMM‐MCNiP), that generates testable hypotheses regarding the effect of temperature, moisture, and substrate supply on C and N cycling. We compared this model to DAMM alone and an empirical model of heterotrophic respiration based on Harvard Forest data. We show that while different model structures explain similar amounts of variation in respiration, they differ in their ability to infer processes that affect C flux. We applied DAMM‐MCNiP to explain an observed seasonal hysteresis in the relationship between respiration and temperature and show using an exudation simulation that the strength of the priming effect depended on the stoichiometry of the inputs. Low C:N inputs stimulated priming of soil organic matter decomposition, but high C:N inputs were preferentially utilized by microbes as a C source with limited priming. The simplicity of DAMM‐MCNiP's simultaneous representations of temperature, moisture, substrate supply, enzyme activity, and microbial growth processes is unique among microbial physiology models and is sufficiently parsimonious that it could be incorporated into larger‐scale models of C and N cycling.

Abramoff, Rose Z.↗
DOE OSTI2020

Impact of Processing on Structural and Compositional Evolution in Mixed Metal Halide Perovskites during Film Formation

Understanding crystallization processes and their pathways in metal-halide perovskites is of crucial importance as this strongly affects the film microstructure, its stability, and device performance. While many approaches are developed to control perovskite formation, the mechanisms of film formation are still poorly known. Using time-resolved in situ grazing incidence wideangle X-ray scattering, the film formation of perovskites is investigated with average stoichiometry Cs 0.15 FA 0.85 PbI 3 , where FA is formamidinium, using the popular antisolvent dropping and gas jet treatments and this is contrasted with untreated films. i) The crystallization pathways during spin coating, ii) the subsequent postdeposition thermal annealing, and iii) crystallization during blade coating are studied. The findings reveal that the formation of a nonperovskite FAPbI 3 phase during spin coating is initially dominant regardless of the processing and that the processing treatment (e.g., antisolvent dropping, gas jet) has a significant impact on the as-cast film structure and affects the phase evolution during subsequent thermal treatment. It is shown that blade coating can be used to overcome the nonperovskite phase formation via solvothermal direct crystallization of perovskite phase. This work shows how real-time investigation of perovskite formation can help to establish processing–microstructure–functionality relationships.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tailoring the Redox Reactions for High-Capacity Cycling of Cation-Disordered Rocksalt Cathodes

Cation-disordered rocksalts (DRXs) have emerged as a new class of high-capacity Li-ion cathode materials. One unique advantage of the DRX chemistry is the structural flexibility that substantially lessens the elemental constraints in the crystal lattice, such as Li content, choice of transition metal redox center paired with appropriate d 0 metal, and incorporation of F anion, which allows optimization of the key redox reactions. Herein, a series of the DRX oxyfluorides based on the Mn redox have been designed and synthesized. By tailoring the stoichiometry of the DRX compositions, high-capacity cycling by promoting the cationic Mn 2+ /Mn 4+ redox reactions while suppressing those from anionic O is successfully demonstrated. A highly fluorinated DRX compound, Li 1.2 Mn 0.625 Nb 0.175 O 1.325 F 0.675 (M 0.625 F 0.675 ), delivers a capacity of ≈170 mAh g -1 at C/3 for 100 cycles. Furthermore, this work showcases the concept of balancing the cationic and anionic redox reactions in the DRX cathodes for improved electrochemical performance through the rational composition design.

25 ENERGY STORAGE↗
DOE OSTI2021

Exsolution Synthesis of Nanocomposite Perovskites with Tunable Electrical and Magnetic Properties

Nanostructured functional oxides play an important role in enabling clean energy technologies and novel memory and processor devices. Using thin-film La 0.6 Sr 0.4 FeO 3 (LSF) as a model system, the novel utility of exsolution in fabricating self-assembled metal oxide nanocomposites with tunable functionalities is shown. Exsolution triggers the formation of metallic iron (Fe 0 ) nanoparticles, Ruddlesden–Popper domains, and nm-scale percolated Fe-deficient channels in LSF. Combining multimodal characterization with numerical modeling, the chemical, magnetic, and electrical properties of the exsolution-synthesized nanocomposite at different stages of Fe 0 exsolution as well as during redox cycling are assessed. After exsolution, the electronic conductivity of the nanocomposite LSF increased by more than two orders of magnitude. Based on numerical analysis representing all the constituents, it is expected that this increase in conductivity originates mainly from the Fe-deficient percolating channels formed during exsolution. Moreover, the exsolved nanocomposite is redox-active even at moderate temperatures. Such redox capabilities can enable dynamic control of the nanocomposite functionality by tailoring the oxygen non-stoichiometry. This concept is demonstrated with a continuous modulation of magnetization between 0 and 110 emu cm -3 . These findings point out that exsolution may serve as a platform for scalable fabrication of complex metal oxide nanocomposites for electrochemical and electronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Surface-Driven Evolution of the Anomalous Hall Effect in Magnetic Topological Insulator MnBi 2 Te 4 Thin Films

We report understanding the effects of the interfacial modification to the functional properties of magnetic topological insulator thin films is crucial for developing novel technological applications from spintronics to quantum computing. Here, a large electronic and magnetic response is reported to be induced in the intrinsic magnetic topological insulator MnBi 2 Te 4 by controlling the propagation of surface oxidation. It is shown that the formation of the surface oxide layer is confined to the top 1–2 unit cells but drives large changes in the overall magnetic response. Specifically, a dramatic reversal of the sign of the anomalous Hall effect is observed to be driven by finite thickness magnetism, which indicates that the film splits into distinct magnetic layers each with a unique electronic signature. These data reveal a delicate dependence of the overall magnetic and electronic response of MnBi 2 Te 4 on the stoichiometry of the top layers. This study suggests that perturbations resulting from surface oxidation may play a non-trivial role in the stabilization of the quantum anomalous Hall effect in this system and that understanding targeted modifications to the surface may open new routes for engineering novel topological and magnetic responses in this fascinating material.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Iodine Electrochemistry Dictates Voltage‐Induced Halide Segregation Thresholds in Mixed‐Halide Perovskite Devices

Abstract Owing to straightforward stoichiometry–bandgap tunability, mixed‐halide perovskites are ideal for many optoelectronic devices. However, unwanted halide segregation under operational conditions, including light illumination and voltage bias, restricts practical use. Additionally, the origin of voltage‐induced halide segregation is still unclear. Herein, a systematic voltage threshold study in mixed bromide/iodide perovskite devices is performed and leads to observation of three distinct voltage thresholds corresponding to the doping of the hole transport material (0.7 ± 0.1 V), halide segregation (0.95 ± 0.05 V), and degradation (1.15 ± 0.05 V) for an optically stable mixed‐halide perovskite composition with a low bromide content (10%). These empirical threshold voltages are minimally affected by composition until very Br‐rich compositions, which reveals the dominant role of iodide/triiodide/iodine electrochemistry in voltage‐induced Br/I phase separation and transport layer doping reactions in halide perovskite devices.

36 MATERIALS SCIENCE↗