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All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Effective properties of a fly ash geopolymer: Synergistic application of X-ray synchrotron tomography, nanoindentation, and homogenization models

Microstructural and micromechanical investigation of a fly ash-based geopolymer using: (i) synchrotron X-ray tomography (XRT) to determine the volume fraction and tortuosity of pores that are influential in fluid transport, (ii) mercury intrusion porosimetry (MIP) to capture the volume fraction of smaller pores, (iii) scanning electron microscopy (SEM) combined with multi-label thresholding to identify and characterize the solid phases in the microstructure, and (iv) nanoindentation to determine the component phase elastic properties using statistical deconvolution, is reported in this paper. The phase volume fractions and elastic properties are used in multi-step mean field homogenization (Mori–Tanaka and double inclusion) models to determine the homogenized macroscale elastic modulus of the composite. The homogenized elastic moduli are in good agreement with the flexural elastic modulus determined on macroscale paste beams. The combined use of microstructural and micromechanical characterization tools at multiple scales provides valuable information towards the material design of fly ash geopolymers.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution During Controlled Solidification of “Fe{sub 2}O{sub 3}”-CaO-SiO{sub 2} Liquids in Air

The principal chemical components in iron ore sintering are Fe{sub 2}O{sub 3}, CaO, and SiO{sub 2}. This sintering process consists of three key steps: heating, holding at peak temperature, and cooling. During the cooling stage, a liquid oxide solidifies to form the final sinter microstructures. To investigate the fundamental processes taking place during the cooling of sinters, a new experimental technique has been developed that allows the stages of solidification to be determined in isolation, rather than inferred from the final microstructures. Fe{sub 2}O{sub 3}-CaO-SiO{sub 2} oxide samples of a bulk composition having a CaO/SiO{sub 2} mass ratio of 3.46 and 73.2 wt pct Fe{sub 2}O{sub 3} were cooled in air from 1623 K (1350 °C) at 2 K/s, quenched at 5 K temperature intervals from 1533 K to 1453 K (1260 °C to 1180 °C), and analyzed using Electron Probe Micro X-Ray Analysis (EPMA). During cooling, four distinct stages were observed, consisting of the phase assemblages Liquid + Hematite (I), Liquid + Hematite + C{sub 2}S(II), Liquid + C{sub 2}S + CF{sub 2}(III), and C{sub 2}S + CF{sub 2} + CF (IV). This solidification sequence differs from that predicted under equilibrium and Scheil–Gulliver Cooling. Importantly, no Silico-Ferrite of Calcium (SFC) phase was observed to form on solidification of the liquid. Based on the microstructures formed and liquid compositions, measured by EPMA, it was demonstrated that kinetic factors play a major role in determining the phases and microstructures formed under the conditions investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Giant Electrocaloric Effect in Ferroelectrics with Tailored Polaw-Nanostructures

Electrocaloric effect (ECE) is the temperature and/or entropy change in a dielectric material caused by an electric field induced polarization change. Although ECE has been studied since 1930s, the very small ECE observed in earlier studies in bulk materials before 2007 makes it not attractive for practical cooling applications. The objectives of this DOE program are to carry out a systematical scientific research on the entropy change and ECE in polar-dielectrics, especially ferroelectrics based on several fundamental hypotheses and to search for answers on a few scientific questions. Especially, this research program developed a series of polar-dielectric materials with controlled nano- and meso-structures and carried out studies on how these structures affect the polar-ordering, correlations, energy landscapes, and consequently the entropy states at different phases and ECE. The key hypotheses of the program include: (i) Whether a large ECE can be obtained near the ferroelectric-paraelectric (FE-PE) transition in properly designed ferroelectrics which possess large polarization P and large ß (the coefficient in the thermodynamic Landau theory where the Gibbs free energy G = G = G 0 + ½ a P 2 +1/4 b P 4 + 1/6 c P 6 – EP, and a = ß (T-T c ), where b,c,ß and Tc are constants)? (ii) What determines/determine ß? Whether a ferroelectric material with built-in disorders, which disrupt the polar-correlations and enabling a large number of local polar-states, such as a properly designed ferroelectric relaxor, can achieve a large ECE? (iii) How to design a ferroelectric material which has flat energy landscape so that the energy barriers for switching among different phases are vanishingly small? What are the necessary conditions to maximize the number of coexisting phases? (iv) How to design ferroelectric materials with a large tunable dielectric response? That is, at zero electric field, the material possesses very low polar-correlation and hence a very small dielectric constant, under the application of electric field, the material develops long range polar-correlation and hence a high dielectric response. Studying and developing these materials will deepen our understanding on the polarization responses in strongly coupled materials and the roles of molecular and nano, meso-, and micro-scale defects and structures on the polarization responses. On the application front, besides ECE, these dielectrics will also have great impact on micro-electronics and communications. (v) The multi-field effect, besides the electric, elastic and even magnetic effects, could be made use of to tune the energy landscape of polar-materials and hence enhance the ECE. Hence the question is what are the suitable material systems to develop and maximize the multi-field effects? (vi) Besides solid dielectric, liquid dielectrics with properly designed molecular structures and dipolar coupling can also exhibit a large ECE near the dipolar order-disorder transition. The study of order-disorder transition and their influence on entropy change and ECE will provide additional avenue to study dielectrics and understand relationship between the polar-ordering and dipolar entropy in dielectrics. (vii) Besides the regular ECE in which applying an electric field will induce dipolar ordering, there are dielectric material systems which can exhibit negative ECE in which the applied field will reduce the dipolar ordering and anomalous ECE in which applying an electric field pulse will generate cooling only. The question is how to control and balance the nano- and meso-scale polar coupling in dielectrics to achieve such effects? ECE in dielectrics provides an interesting and effective avenue to probe the polar-correlation in dielectrics. Thus the study of ECE in polar-dielectrics, besides the application values, will also deepen our understanding of strongly coupled materials systems, phase transitions, and materials systems with nano- and meso-scale disorders. Through the efforts of this DoE program, we have developed understandings for many questions and materials approaches for many hypotheses listed above. The major accomplishments include: (i) The first one to show that a giant ECE can be obtained in bulk materials of ferroelectric P(VDF-TrFE) copolymer, which has a large ß coefficient and high polarization, near FE-PE transition.[1,3,12] (ii) The first who developed the theoretical analyses on the upper bound of dipolar entropy change in polar-materials and the general approach to maximize the coexisting phases with vanishingly small switching fields among the coexisting phases[10,23] Experimental results confirm these theoretical predictions.[24] (iii) The first to show that the relaxor ferroelectrics, due to built-in defects structures at nano- and meso scale, exhibit a giant ECE over a broad temperature range.[1,3,7,14] (iv) The first to show that a large ECE can be obtained near order-disorder transition in dielectric fluids such as liquid crystals with large dielectric anisotropy. Also the study developed a general approach for developing dielectric fluids to achieve a large electric field induced entropy change.[26] (v) We are starting to explore the multi-field effect (multiferroic effect) in nanocomposites in which there exist large dielectric contrasts between the matrix and nanofilelrs and showed that a significantly enhanced ECE compared with polymer matrix.[36] (vi) By facially tuning the nano- and meso-scale dipolar coupling, we are the first to show that an anomalous ECE can be obtained in a relaxor/normal ferroelectric blend.[39] (vii) Introduced and demonstrated that the internal bias field approach can be effective in enhancing the EC response at low electric field. The result is significant since for practical applications, a low applied field is highly desired. (viii) A high sensitivity ECE characterization system has been developed. This program has made major contributions to the advancement of the EC materials and understandings of EC phenomena. To reflect the advancement in the EC materials development and scientific understandings on ECE through in this time period (from Sept. 1, 2007 to May 2015), this final report is written based on the reports complied each year through the program. Some early works on the ECE which were obtained using the indirect method are not included in this report.

36 MATERIALS SCIENCE↗
NASA NTRS2005

Monitoring the Intrinsic Absorption Complex Toward RXJ123.8+0115

The main focus of this study was changed after it became clear that the object to be observed by the Far Ultraviolet Spectroscopic Explorer (FUSE) could not be observed because of spacecraft attitude control system limitations. Therefore, we replaced the prime object for the program (RXJ1230.8+0115) with a secondary object (H1821+643) and began an analysis of a similar quasar spectrum for comparison with the extant RXJ1230.8+0115 spectrum obtained previously by FUSE. Using high-resolution spectra of the radio-quiet, X-ray bright quasar HE0226-4110 obtained with the Hubble Space Telescope (HST) and FUSE, we have examined the remarkable associated absorption line system. The redshift of the quasar is roughly 0.5 so the FUSE+HST spectra cover the rest-frame wavelength in the extreme ultraviolet range 610-1 160A. In this range, we detect transitions from a wide range of ionization species, including the H I Lyman series, Ne VIII, and, for the first time at this resolution, four adjacent stages of oxygen O III-VI. The high quality of these spectra allow us to disentangle the structure of the gas using the complex kinematics to guide the assessments of the multi-component and multi-phase nature of the system. The kinematics of the absorber indicate that all the H I, C 111, and 0 III reside in a single component, while higher ionization species largely arise in a separate, more kinematically complex structure. An analysis of a ground-based spectrum of the quasar covering the H-beta and [0 III] 5007 emission lines reveal excellent agreement in redshift between the 0 III absorption and emission. Thus, for the first time, we are able to use absorption line diagnostics to place interesting constraints on the location and structure of the narrow emission line region around a quasar. In addition, we detect a narrow absorption line that is only seen O VI and lies roughly 50 km/s redward of the narrow emission line component. Absorption from 0 V, and Ne VIII is also detected at this velocity, but as part of broader, more redshifted, component. The line width of this narrow component imply a maximum temperature that is too small to produce O VI through collisional process, suggesting instead that photoionization from the quasar is the dominant ionization mechanism. The photoionization constraints imply that O VII should be the dominant ionization stage in the component.

Sembach, Kenneth↗
DOE OSTI2022

Ultra-power-efficient heterogeneous III–V/Si MOSCAP (de-)interleavers for DWDM optical links

We discuss the design and demonstration of various III–V/Si asymmetric Mach–Zehnder interferometer (AMZI) and ring-assisted AMZI (de-)interleavers operating at O-band wavelengths with 65 GHz channel spacing. The wafer-bonded III–V/Si metal-oxide-semiconductor capacitor (MOSCAP) structure facilitates ultra-low-power phase tuning on a heterogeneous platform that allows for complete monolithic transceiver photonic integration. The second- and third-order MOSCAP AMZI (de-)interleavers exhibit cross-talk (XT) levels down to − 22 dB and − 32 dB with tuning powers of 83.0 nW and 53.0 nW, respectively. The one-, two-, and three-ring-assisted MOSCAP AMZI (de-)interleavers have XT levels down to − 27 dB, − 22 dB, and − 20 dB for tuning powers of 10.0 nW, 7220.0 nW, and 33.6 nW, respectively. The leakage current density is measured to be in the range of 1.6 – 27 μA / cm 2 . To the best of our knowledge, we have demonstrated for the first time, athermal III–V/Si MOSCAP (de-)interleavers with the lowest XT and reconfiguration power consumption on a silicon platform.

Cheung, Stanley (ORCID:0000000248860013)↗
DOE OSTI2016

The relation between [O III]/Hβ and specific star formation in galaxies at z ∼ 2

Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [O iii]/Hβ emission line ratios at z > 1, although the nature of this phenomenon continues to be debated. The [O iii]/Hβ line ratio is of interest because it is a main component of the standard diagnostic tools used to differentiate between active galactic nuclei and star-forming galaxies as well as the gas-phase metallicity indicators O {sub 23} and R {sub 23}. Here, we investigate the primary driver of increased [O iii]/Hβ ratios by median-stacking rest-frame optical spectra for a sample of star-forming galaxies in the 3D-HST survey in the redshift range z ∼ 1.4–2.2. Using N = 4220 star-forming galaxies, we stack the data in bins of mass and specific star formation rates (sSFRs), respectively. After accounting for stellar Balmer absorption, we measure [O iii]λ5007 Å/Hβ down to M ∼ 10{sup 9.2} M {sub ⊙} and sSFR ∼ 10{sup −9.6} yr{sup −1}, greater than an order of magnitude lower than previous work at similar redshifts. We find an offset of 0.59 ± 0.05 dex between the median ratios at z ∼ 2 and z ∼ 0 at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the z ∼ 2 stacks all lie within 1σ of the median SDSS ratios, with an average offset of only −0.06 ± 0.05. We find that the excitation properties of galaxies are tightly correlated with their sSFR at both z ∼ 2 and z ∼ 0, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Selective Epitaxial Growth of III–N Structures Using Ion-Beam Nanolithography

The selective epitaxial growth of GaN by metalorganic vapor-phase epitaxy combined with ion-beam etching is investigated. To this end, partially masked GaN epitaxial layers are fabricated by depositing a thin Si{sub 3}N{sub 4} layer onto the surface in a single technological process with the growth of GaN and the subsequent opening of windows of different shapes in this layer by an ion beam. Selective epitaxial growth regimes are studied. It is shown that, in a situation where the total area of the windows in the mask is small relative to the total area of the sample, the required epitaxy duration should be 5–10 s, which impairs the reproducibility of the parameters of the epitaxial process. It is also shown that the mechanism of the selective growth of submicrometer objects differs significantly from that for planar layers and selectively grown layers with dimensions of ~1 μm or greater. The effect of precursor (trimethylgallium and ammonia) fluxes on the character of selective epitaxy is examined. To investigate the possibilities of varying mask topology for fabricating model objects with regard to photonic crystals, the impact of the shape and orientation of the windows in the Si{sub 3}N{sub 4} mask on the character of selective epitaxy is studied.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Stimulated Emission in the 1.3–1.5 μm Spectral Range from AlGaInAs Quantum Wells in Hybrid Light-Emitting III–V Heterostructures on Silicon Substrates

Hybrid laser structures with AlGaInAs quantum wells are grown by metalorganic vapor phase epitaxy on Ge/Si(100) “virtual” substrates using GaAs and InP buffer layers. Stimulated emission is achieved under optical pumping of the prepared samples in the range of 1.3–1.5 μm at liquid-nitrogen temperature. The stimulated-emission threshold is 30–70 kW/cm{sup 2}.

36 MATERIALS SCIENCE↗
NASA NTRS2016

Evaluation of Mid-Size Male Hybrid III Models for use in Spaceflight Occupant Protection Analysis

In an effort to improve occupant safety during dynamic phases of spaceflight, the National Aeronautics and Space Administration (NASA) has worked to develop occupant protection standards for future crewed spacecraft. One key aspect of these standards is the identification of injury mechanisms through anthropometric test devices (ATDs). Within this analysis, both physical and computational ATD evaluations are required to reasonably encompass the vast range of loading conditions any spaceflight crew may encounter. In this study the accuracy of publically available mid-size male HIII ATD finite element (FE) models are evaluated within applicable loading conditions against extensive sled testing performed on their physical counterparts. Methods: A series of sled tests were performed at the Wright Patterson Air force Base (WPAFB) employing variations of magnitude, duration, and impact direction to encompass the dynamic loading range for expected spaceflight. FE simulations were developed to the specifications of the test setup and driven using measured acceleration profiles. Both fast and detailed FE models of the mid-size male HIII were ran to quantify differences in their accuracy and thus assess the applicability of each within this field. Results: Preliminary results identify the dependence of model accuracy on loading direction, magnitude, and rate. Additionally the accuracy of individual response metrics are shown to vary across each model within evaluated test conditions. Causes for model inaccuracy are identified based on the observed relationships. Discussion: Computational modeling provides an essential component to ATD injury metric evaluation used to ensure the safety of future spaceflight occupants. The assessment of current ATD models lays the groundwork for how these models can be used appropriately in the future. Identification of limitations and possible paths for improvement aid in the development of these effective analysis tools.

Putnam, Jacob B.↗
DOE OSTI2016

Complexation Enhancement Drives Water-to-Oil Ion Transport: A Simulation Study

We address the structures and energetics of ion solvation in aqueous and organic solutions to understand liquid-liquid ion transport. Atomistic molecular dynamics (MD) simulations with polarizable force field are performed to study the coordination transformations driving lanthanide (Ln(III)) and nitrate ion transport between aqueous and an alkylamide-oil solution. An enhancement of the coordination behavior in the organic phase is achieved in contrast with the aqueous solution. In particular, the coordination number of Ce3+ increases from 8.9 in the aqueous to 9.9 in the organic solutions (from 8 in the aqueous to 8.8 in the organic systems for Yb3+). Moreover, the local coordination environ ment changes dramatically. Potential of mean force calculations show that the Ln(III)-ligand coordination interaction strengths follow the order of Ln(III-)nitrate> Ln(III)-water>Ln(III)-DMDBTDMA. They increase 2-fold in the lipophilic environment in comparison to the aqueous phase, and we attribute this to the shedding of the outer solvation shell. Our findings highlight the importance of outer sphere interactions on the competitive solvation energetics that cause ions to migrate between immiscible phases; an essential ingredient for advancing important applications such as rare earth metal separations. Some open questions in simulating the coordination behavior of heavy metals are also addressed.

Qiao, Baofu↗
DOE OSTI2025

In Situ MOVPE Smoothing of Acoustically Spalled GaAs for Substrate Reuse

High material costs, especially for substrates, have limited the widespread adoption of III-V photovoltaics. A potential to reduce this cost is to reuse the III-V substrate via acoustic spalling, however this technique can leave a rough surface, hindering subsequent device performance. This research investigates the potential of using metalorganic vapor-phase epitaxy growth as a buffer layer to smooth the surface of acoustically spalled germanium and gallium arsenide (GaAs) substrates for improved III-V photovoltaic cell yield and performance, while retaining the maximum number of reuses of a substrate. Three potential smoothing layers were explored: lightly doped C:GaAs, highly doped Se:GaInP, and lightly doped Se:GaInP. C:GaAs showed the most promise as a smoothing layer, while Se:GaInP tended to conform to the underlying morphology, potentially increasing roughness in some areas. Utilizing 5 ..mu..m of C:GaAs as a planarizing buffer increased the average efficiency (without an antireflection coating) from an as-spalled baseline from 2.1% to 4.9% and performing a 5-min 30 degrees C 8:1:1 H2SO4:H2O2:H2O etch prior to a 5 ..mu..m of C:GaAs as a planarizing buffer further increased efficiency to 11.1%.

14 SOLAR ENERGY↗
DOE OSTI2013

Crystal structure and phase transition mechanisms in CsFe{sub 2}F{sub 6}

For the first time, structural phase transitions induced by the temperature were found in A{sub x}M{sub x}{sup II}M{sub (1−x)}{sup III}F{sub 3} fluorides with the defect pyrochlore structure (Fd3{sup ¯}m, Z=8). The room temperature structure of CsFe{sub 2}F{sub 6} was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100 K and 700 K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I4{sub 1}/amd (Z=4)→Fd3{sup ¯}m (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500–560 K. The mechanism of structural transition has been mainly associated with the rotation of (MF{sub 6}) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations. - Graphical abstract: Mechanism of phase transition between the HT cubic form of CsFe{sub 2}F{sub 6} at 573 K (left) and the room temperature orthorhombic form at 298 K (right). The grey rectangles are clusters of five FeF{sub 6} octahedra. Highlights: ► Structural transition found for the first time in CsFe{sub 2}F{sub 6} with defect pyrochlore type. ► Fe{sup II} and Fe{sup III} atoms are ordered in room temperature Pnma form of CsFe{sub 2}F{sub 6}. ► Pnma(Z=4)→Imma(Z=4)→I4{sub 1}/amd(Z=4)→Fd-3m(Z=8) transition sequence is proposed. ► Structural transition due to rotation of MF{sub 6} groups+small displacements of Fe atoms. ► The low value of the entropy is in agreement with a displacive-type transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Tunable Bandgap GaInAsP Solar Cells With 18.7% Photoconversion Efficiency Synthesized by Low-Cost and High-Growth Rate Hydride Vapor Phase Epitaxy

As market-dominant Si solar cell technology approaches its practical efficiency limit of 27.1%, a key challenge for the photovoltaic industry is to search for a low-cost ~1.7 eV top cell that can enable cost-competitive tandems with solar-to-electricity conversion efficiency exceeding 30%. III-V semiconductors offer tunable bandgap and unparalleled efficiencies for tandem devices. However, their high manufacturing cost has been the biggest impediment for market entry. Hydride vapor phase epitaxy (HVPE) has recently reemerged as a promising low-cost alternative to incumbent metalorganic chemical vapor deposition (MOCVD) for III-V solar cells. Here, we show the first demonstration of ~1.7 eV GaInAsP solar cells that achieve nearly 19% photoconversion efficiency synthesized by HVPE. The unprecedented growth rate of ~42 um/h achieved via HVPE (~10x faster than conventional MOCVD) allowed the device layers to be deposited in less than seven minutes. Nearly abrupt and chemically distinct heterointerfaces were attained in spite of high-growth rates, which are known to exacerbate phase separation in GaInAsP alloys. Demonstration of ideality factor of n=1 at max power point highlight the advancements in growth optimization and device design. The results presented here highlight a promising path toward tunable bandgap, high-growth rate, low-cost, and high-efficiency GaInAsP devices for direct solar-to-electricity and solar-to-hydrogen conversion.

14 SOLAR ENERGY↗
DOE OSTI2014

Geochemical and Mineralogical Investigation of Uranium in Multi–element Contaminated, Organic–rich Subsurface Sediment

Alluvial sediments characterized by an abundance of refractory or lignitic organic carbon compounds and reduced Fe and S bearing mineral phases have been identified through drilling activities at the U.S. Department of Energy’s (DOE) Integrated Field Research Challenge (IFRC) site at Rifle, CO. Regions of the subsurface from which such sediments are derived are referred to as Naturally Reduced Zones (NRZ). We conducted a study with NRZ sediments with the objective to: i.) Characterize solid phase contamination of U and other co-contaminants; ii.) Document the occurrence of potential U host minerals; iii.) Determine U valence state and micron scale spatial association with co-contaminants. Macroscopic (wet chemical batch extractions and a column experiment), microscopic (SEM-EDS), and spectroscopic (Mössbauer, µ-XRF and XANES) techniques were employed. Results showed that sediments’ solid phase had significant concentrations of U, S, As, Zn, V, Cr, Cu and Se, and a remarkable assortment of potential U hosts (sorbents and/or electron donors), such as Fe oxides (hematite, magnetite, Al-substituted goethite), siderite, reduced Fe(II) bearing clays, sulfides of different types, Zn sulfide framboids and multi – element sulfides. Multi-contaminants, micron size (ca. 5 to 30 µm) areas of mainly U(IV) and some U(VI), and/or other electron scavengers or donors such as Se, As, Cr, and V were discovered in the sediments, suggesting complex micron-scale system responses to transient redox conditions, and different extent and rates of competing U redox reactions than those of single contaminant systems. Collectively, the results improve our understanding and ability to predict U and NRZ’s complex behavior and will delineate future research directions to further study both the natural attenuation and persistence of contaminant plumes and their contribution to groundwater contamination.

bioavailability↗
DOE OSTI2019

Cleanup and Conversion of Biomass Liquefaction Aqueous Phase to C3–C5 Olefins over ZnxZryOz Catalyst

The viability of using a ZnxZryOz mixed oxide catalyst for the direct production of C4 olefins from the aqueous phase derived from three different bio-oils was explored. The aqueous phases derived from (i) hydrothermal liquefaction of corn stover, (ii) fluidized bed fast pyrolysis of horse litter, and (iii) screw pyrolysis of wood pellets were evaluated as feedstocks. While exact compositions vary, the primary constituents for each feedstock are acetic acid and propionic acid. Continuous processing, based on liquid–liquid extraction, for the cleanup of the inorganic contaminants contained in the aqueous phase was also demonstrated. Complete conversion of the carboxylic acids was achieved over ZnxZryOz catalyst for all the feedstocks investigated. The main reaction products from each of the feedstocks include isobutene (>30% selectivity) and CO2 (>23% selectivity). Activity loss from coking was also observed, thereby rendering deactivation of the ZnxZryOz catalyst, however, complete recovery of catalyst activity was observed following regeneration. Finally, the presence of H2 in the feed was found to facilitate hydrogenation of intermediate acetone, thereby increasing propene production and, consequently, decreasing isobutene production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Stable Light-Emitting Diodes Using Phase-Pure Ruddlesden-Popper Layered Perovskites

State-of-the-art light-emitting diodes (LEDs) are made from high-purity alloys of III–V semiconductors, but high fabrication cost has limited their widespread use for large area solid-state lighting. Here in this paper, efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure 2D Ruddlesden–Popper (RP) halide perovskites with a formula (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n-1 Pb n I 3n+1 are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr -1 cm -2 at 744 nm with an ultralow turn-on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Finally, phase-pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm -2 in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Distribution and speciation of rare earth elements in coal combustion by-products via synchrotron microscopy and spectroscopy

Rare earth element (REE) binding environments and associated mineral phases in coal and coal combustion byproducts (CCBs) are not well understood. Advanced characterization of REEs' binding environments before, during, and after production can be vital in establishing an economically viable recovery method for REEs from these materials. Here, this study characterized different REE distributions, cerium (Ce) oxidation states, and Ce binding environments in 11 CCBs collected from various locales via X-ray fluorescence (XRF) elemental mapping and X-ray absorption near edge structure (XANES) at Stanford Synchrotron Radiation Lightsource (SSRL). Bulk Ce LIII edge XANES identified Ce(III) (>85%) as the predominant Ce oxidation state in all studied samples. Ce speciation and XRF elemental map analyses further reveal the REEs in studied samples can be (1) dispersed throughout the aluminosilicate glass phase, (2) as micro-particles in large glass grain, or (3) as independent trace phases in studied samples. Both fly ash and bottom ash contain mainly Ce(III) micro-hotspots (<40 μm), co-localization with a range of elements, including, but not limited to: P, S, Ca, Ti, and Mn. At microscale, Ce can change from Ce(III) to mixture of Ce(III) and Ce(IV) and to 100% Ce(IV) oxide, revealing potential REE-phase decomposition and oxidation in coal and coal combustion process. Our study also observed intraparticle and interparticle distribution difference between light REEs and heavy REEs. Finally, the synchrotron-based characterization confirmed the presence of these trace, previously only theorized, phases during the combustion process and ultimately may inform the simplification of separation and extraction processes for REE from different CCBs.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Kinetic-controlled transformations of group-III arsenide nanocubes

Tracking the structural evolution of colloidal nanocrystals (NCs) facilitates the mechanistic studies of their materials chemistry. NC engineering via phase transformation reveals the chemical and physical determinants that drive lattice-scale dynamic processes such as cation exchange. Here, in this study, we employed NCs to demonstrate the cation exchange process from Cu 3 As to InAs and GaAs within nanocubes. The symmetry conversion in unit cells from cubic Cu 3 As to hexagonal InAs and GaAs can be described using a schematic cellular automaton model, which suggests a simplified cube-to-sphere transition. The strong covalent characteristics of III–V materials highlight the kinetic control that navigates the tailorable transformation through either an isotropic trajectory, leading to hollow structures, or a topotaxial trajectory, with abundant stacking faults. The reconstruction of complex covalent bonds is envisioned as the foundation for the synthesis of NCs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗