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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 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

A novel approach to obtain highly intense self-activated photoluminescence emissions in hydroxyapatite nanoparticles

Defect-related photoluminescence (PL) in materials have attracted interest for applications including near ultraviolet (NUV) excitable light-emitting diodes and in biomedical field. In this paper, hydroxyapatite [Ca{sub 10}(PO{sub 4}){sub 6}(OH){sub 2}] nanorods with intense PL bands (bluish- and yellowish-white emissions) were obtained when excited under NUV radiation at room temperature. These nanoparticles were synthesized via chemical precipitation at 90 °C followed by distinct heat treatments temperatures (200–800 °C). Intense and broad emission profiles were achieved at 350 °C (380–750 nm) and 400 °C (380–800 nm). UV–Vis spectroscopy revealed band gap energies (5.58–5.78 eV) higher than the excitation energies (~3.54 and ~2.98 eV at 350 and 415 nm, respectively), confirming the contribution of defect energy levels within the forbidden zone for PL emissions. The structural features were characterized by X-ray diffraction, Rietveld refinement, thermogravimetric analysis, and Fourier transform infrared spectroscopy. By means of these techniques, the relation between structural order-disorder induced by defects, chemical reactions at both lattice and surface of the materials as well as the PL, without activator centers, was discussed in details. - Graphical abstract: The self-activated photoluminescence emissions of chemically precipitated hydroxyapatite nanorods were improved by different heat treatment temperatures. - Highlights: • HA nanorods were synthesized with improved self-activated PL at room temperature. • PL profile and intensity dependents on the temperature of posterior heat treatments. • Bluish- and yellowish-white emissions under NUV excitation (350 and 415 nm). • Broad and intense profiles achieved at 350 °C (380–750 nm) and 400 °C (380–800 nm). • PL from the e′–h{sup •} recombination between defect energy levels within the band gap.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay

Calcined clays can be potential supplementary cementitious materials if effects of heat-treatment on their structure and reactivity are understood. This work reports structural characterization of an interstratified illite/smectite clay, including a quartz impurity, upon heating using {sup 27}Al and {sup 29}Si MAS NMR spectroscopy and ICP-OES analysis. During dehydroxylation (600–900 °C) the Q{sup 3}-type SiO{sub 4} sites become disordered and octahedral AlO{sub 6} sites transform into tetrahedral sites, resulting in an amorphous material with substantial pozzolanic properties, as demonstrated by reactivity tests and hydration studies of a Portland cement–calcined clay blend. At higher temperatures (above 950 °C), inert Q{sup 4}-type phases crystallize which radically reduce the reactivity. At optimum calcination temperature (900 °C), the amorphous material contains highly dissolvable elemental species as seen from complementary ICP-OES analysis. The quartz impurity exhibits a unique variation in {sup 29}Si spin–lattice relaxation times upon heat-treatment which is ascribed to changes in the concentration of impurity ions in quartz.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Redox Behaviors in Li-Excess Cation-Disordered Mn-Nb-O-F Rocksalt Cathode

Lithium-excess cation-disordered rocksalt oxide (DRX) is a new choice of high energy Li-ion cathode materials. DRX cathode can deliver an exceptionally high capacity of >250 mAh/g, implying the participation of cationic transition metal and anionic oxygen redox in the electrochemistry. Understanding the explicit roles of both redox reactions in the newly developed cathode is of critical importance to further development of these materials. Herein, we synthesize a representative Mn-redox-based fluorinated DRX material, Li1.2Mn0.625Nb0.175O1.95F0.05 (LMNOF). Using a comprehensive suite of electroanalytical methods, we probe the cationic and anionic redox processes and their evolution upon cycling exclusively from the electrochemical point of view. This work sheds light on understanding of cationic and anionic redox and their roles in the capacity loss for high-capacity fluorinated DRX cathode, meanwhile, showcases an explicit approach to study the electrochemistry of novel electrode materials using such non-disruptive electroanalytical methods.

Li-ion battery, high-capacity cathode, Li-excess m↗
DOE OSTI2018

Ultrahigh Power Factor in Thermoelectric System Nb 0.95 M 0.05 FeSb (M = Hf, Zr, and Ti)

Abstract Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb 0.95 Hf 0.05 FeSb has not only ultrahigh PF over ≈100 µW cm −1 K −2 at room temperature but also the highest ZT in a material system Nb 0.95 M 0.05 FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb 0.95 Hf 0.05 FeSb distinguishes itself from other outstanding NbFeSb‐based materials in both the PF and ZT . Additionally, a large output power density of ≈21.6 W cm −2 is achieved based on a single‐leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2015

Transparency through Structural Disorder: A New Concept for Innovative Transparent Ceramics

Transparent polycrystalline ceramics present signi fi cant eco- nomical and functional advantages over single crystal materials for optical, communication, and laser technologies. To date, transparency in these ceramics is ensured either by an optical isotropy (i.e., cubic symmetry) or a nanometric crystallite size, and the main challenge remains to eliminate porosity through complex high pressure - high temperature synthesis. Here we introduce a new concept to achieve ultimate transparency reaching the theoretical limit. We use a controlled degree of chemical disorder in the structure to obtain optical isotropy at the micrometer length scale. This approach can be applied in the case of anisotropic structures and micrometer scale crystal size ceramics. We thus report Sr 1+ x /2 Al 2+ x Si 2 - x O 8 (0 < x ≤ 0.4) readily scalable polycrystalline ceramics elaborated by full and congruent crystallization from glass. These materials reach 90% transmittance. This innovative method should drive the development of new highly transparent materials with technologically relevant applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local ordering in disordered Nd x Zr 1-x O 2-0.5x pyrochlore as observed using neutron total scattering

Pyrochlore complex oxides (A 2 B 2 O 7 ) are an important class of fluorite-derivative ceramics with exceptional chemical and structural versatility which make them ideal model systems for studying disordering mechanisms over a range of spatial scales. Here, neutron total scattering methods were used to analyze the structural behavior in the non-stoichiometric series Nd x Zr 1-x O 2-0.5x (0.5 ≤ x ≤ 0.23) as a function of Nd content, x. Characterization of the structure functions using Rietveld refinement and the pair distribution functions with small-box refinement reveal complex disordering pathways; the average, long-range phase changes over a very narrow compositional range from pyrochlore ($Fd\bar{3}m$) to defect fluorite ($Fm\bar{3}m$) through full randomization of the cation and anion sublattices at x ~ 0.31, while the local, short-range structure continuously adopts a weberite-type atomic arrangement (C222 1 ). Comparison to a previously studied Ho 2 Ti 2-x Zr x O 7 solid solution series reveals how changes in chemical composition and stoichiometry modify defect formation and determine how disordering progresses across different length scales in pyrochlore oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Lattice vacancy migration barriers in Fe-Ni alloys, and an indication as to why Ni atoms diffuse slowly: A first-principles study

Lattice vacancy migration barriers in ferromagnetic Fe 𝑥 ⁢Ni 1−𝑥 alloys (0.4 ≤ 𝑥 ≤ 0.6) are accurately quantified within the framework of ab initio electronic structure calculations using the nudged elastic band (NEB) method. Both the atomically disordered (A1) fcc phase, as well as the atomically ordered, tetragonal L⁢1 0 phase—which is under consideration as a material for a rare-earth-free gap magnet for advanced engineering applications—are investigated. Across an ensemble of NEB calculations performed on supercell configurations spanning a range of compositions and containing disordered, partially ordered, and fully ordered structures, we find that Ni-vacancy interchanges encounter significantly higher energetic barriers than do Fe-vacancy interchanges. We contend that this aspect is a key factor in determining the differences in mobility between Fe and Ni atoms in this ferromagnetic alloy. Moreover, we are able to interpret these findings in terms of the ferromagnetic alloy's underlying spin-polarized electronic structure. Specifically, we report a coupling between the size of local lattice distortions and the magnitude of the local electronic spin polarization around vacancies. This causes Fe atoms to relax into lattice vacancies, while Ni atoms remain rigidly fixed to their original lattice positions. These results give atomic-scale insight into the longstanding experimental observation that Ni exhibits remarkably slow atomic diffusion in Fe-Ni alloys.

density functional theory↗
DOE OSTI2022

Unifying the order and disorder dynamics in photoexcited VO 2

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory method, here we track the motion of individual vanadium (V) ions during PIPT in VO 2 and uncover that their coherent or disordered dynamics can be manipulated by tuning the laser fluence. We find that the photoexcited holes generate a force on each V–V dimer to drive their collective coherent motion, in competing with the thermal-induced vibrations. If the laser fluence is so weak that the photoexcited hole density is too low to drive the phase transition alone, the PIPT is a disordered process due to the interference of thermal phonons. Finally, we also reveal that the photoexcited holes populated by the V–V dimerized bonding states will become saturated if the laser fluence is too strong, limiting the timescale of photoinduced phase transition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Optical Signatures of Transiently Disordered Semiconductor Nanocrystals

The remarkable optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state-lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects can impact optical properties. The physical integrity of the NC is indeed questionable as recent transient X-ray diffraction studies have suggested that NCs reversibly lose crystalline order, or melt, under high fluence photoexcitation. Informed by such studies, here, we examine CdSe NCs under elevated fluences to determine the impact of disordering on optical properties. To this end, we implement intensity-dependent transient absorption using both one- and two-pump methods where the latter effectively subtracts out the NC optical signatures associated with lower fluence photoexcitation, especially band-edge features. At elevated fluences, we observe a long-lived induced absorption to the red of the bandgap across a wide range of sizes that follows power dependent trends and kinetics consistent with the prior transient X-ray measurements. NC photoluminescence studies provide further evidence that melting influences optical properties. These methods of characterizing bandgap narrowing caused by NC disordering could facilitate improved optical amplification and band-edge emission at high excitation density.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Visualizing Current Flow at the Mesoscale in Disordered Assemblies of Touching Semiconductor Nanocrystals

The transport of electrons through assemblies of nanocrystals is important to performance in optoelectronic applications for these materials. Previous work has primarily focused on single nanocrystals or transitions between pairs of nanocrystals. There is a gap in knowledge of how large numbers of nanocrystals in an assembly behave collectively, and how this collective behavior manifests at the mesoscale. In this work, the variable range hopping (VRH) transport of electrons in disordered assemblies of touching, heavily doped ZnO nanocrystals was visualized at the mesoscale as a function of temperature both theoretically, using the model of Skinner, Chen and Shklovskii (SCS), and experimentally, with conductive atomic force microscopy on ultrathin films only a few particle layers thick. Agreement was obtained between the model and experiments, with a few notable exceptions. The SCS model predicts that a single network within the nanocrystal assembly, comprised of sites connected by small resistances, dominates conduction - namely the optimum band from variable range hopping theory. However, our experiments revealed that in addition to the optimum band, there are subnetworks that appear as additional peaks in the resistance histogram of conductive atomic force microscopy (CAFM) maps. Furthermore, the connections of these subnetworks to the optimum band change in time, such that some subnetworks become connected to the optimum band while others become disconnected and isolated from the optimum band; this observation appears to be an experimental manifestation of the ‘blinking’ phenomenon in our images of mesoscale transport.

Chen, Qinyi↗
DOE OSTI2019

Negative Electrodes for Lithium-Ion Batteries Obtained by Photoanodization of Solar-Grade Silicon

Negative electrodes for lithium-ion batteries prepared by electrochemical etching of single-crystalline silicon crystals demonstrate a high specific capacity per gram of the material and per unit of nominal anode area as well as a high stability during several hundreds and even thousands of cycles. However, industrial application of such structures is inexpedient in view of the high cost of the material and technology used. In this work we have investigated anodes based on disordered macropores in solar-grade n-Si obtained by photoanodization in a 4% HF solution in dimethyl formamide. The use of this organic electrolyte leads to the formation of layers with a porosity higher than in an aqueous electrolyte and ensures spontaneous separation of these layers from the substrate. The anodes have been prepared from macroporous membranes with a thickness of 48–86 μm and a porosity of 52–75%, and their electrochemical parameters have been studied. It is found that the geometrical parameters of the porous structure and experimental conditions determine the charge and discharge capacity and the cycle life of the anodes. In the regime of the charge capacity limited by 1000 mA h/g and charge/discharge rate C/5, the obtained anodes can stably operate during several hundreds of cycles, preserving a high (greater than 98%) Coulombic efficiency.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

HGMS: Glasses and Nanocomposites for Hydrogen Storage

The primary goal of this project is to fabricate and investigate different glass systems and glass-derived nanocrystalline composite materials. These glass-based, two-phased materials will contain nanocrystals that can attract hydrogen and be of potential interest as hydrogen storage media. The glass materials with intrinsic void spaces that are able to precipitate functional nanocrystals capable to attract hydrogen are of particular interest. Proposed previously, but never practically implemented, one of promising concepts for storing hydrogen are micro-containers built of glass and shaped into hollow microspheres. The project expanded this concept to the exploration of glass-derived nanocrystalline composites as potential hydrogen storage media. It is known that the most desirable materials for hydrogen storage do not interact chemically with hydrogen and possess a high surface area to host substantial amounts of hydrogen. Glasses are built of disordered networks with ample void spaces that make them permeable to hydrogen even at room temperature. Glass-derived nanocrystalline composites (two-phased materials), combination of glasses (networks with ample voids) and functional nanocrystals (capable to attract hydrogen), appear to be promising candidates for hydrogen storage media. Key advantages of glass materials include simplicity of preparation, flexibility of composition, chemical durability, non-toxicity and mechanical strength, as well as low production costs and environmental friendliness. This project encompasses a fundamental research into physics and chemistry of glasses and nanocrystalline composite materials, derived from glass. Studies are aimed to answer questions essential for considering glass-based materials and composites as potential hydrogen storage media. Of particular interest are two-phased materials that combine glasses with intrinsic voids spaces for physisorption of hydrogen and nanocrystals capable of chemisorption. This project does not directly address any hydrogen storage technical barriers or targets in terms of numbers. Specifically, hydrogen sorption and desorption tests or kinetics measurements were not part of the project scope. However, the insights gained from these studies could help to answer fundamental questions necessary for considering glass-based materials as hydrogen storage media and could be applied indirectly towards the DOE hydrogen storage technical targets such as system weight and volume, system cost and energy density. Such questions are: Can specific macro-crystals, proven to attract hydrogen when in a macroscopic form (bulk), be nucleated in glass matrices as nanocrystals to create two-phased materials? What are suitable compositions that enable to synthetize glass-based, two-phase materials with nanocrystals that can attract hydrogen via surface or bulk interactions? What are the limits of controlling the microstructure of these materials, especially limits for nanocrystals density and size? Finally, from a technological point of view, the fabrication of glass-derived nanocomposites that we explore is a very simple, fast and inexpensive process that does not require costly or specialized equipment which is an important factor for practical applications.

08 HYDROGEN↗
DOE OSTI2015

Disorder-induced structural transitions in topological insulating Ge-Sb-Te compounds

The mechanism for the fast switching between amorphous, metastable, and crystalline structures in chalcogenide phase-change materials has been a long-standing puzzle. Based on first-principles calculations, we study the atomic and electronic properties of metastable Ge{sub 2}Sb{sub 2}Te{sub 5} and investigate the atomic disorder to understand the transition between crystalline hexagonal and cubic structures. In addition, we study the topological insulating property embedded in these compounds and its evolution upon structural changes and atomic disorder. We also discuss the role of the surface-like states arising from the topological insulating property in the metal-insulator transition observed in the hexagonal structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Neural network-based order parameter for phase transitions and its applications in high-entropy alloys

Phase transition is one of the most important phenomena in nature and plays a central role in materials design. All phase transitions are characterized by suitable order parameters, including the order–disorder phase transition. However, finding a representative order parameter for complex systems is non-trivial, such as for high-entropy alloys. Given the strength of dimensionality reduction of a variational autoencoder (VAE), we introduce a VAE-based order parameter. Here, we propose that the Manhattan distance in the VAE latent space can serve as a generic order parameter for order–disorder phase transitions. The physical properties of our order parameter are quantitatively interpreted and demonstrated by multiple refractory high-entropy alloys. Using this order parameter, a generally applicable alloy design concept is proposed by mimicking the natural mixing process of elements. Our physically interpretable VAE-based order parameter provides a computational technique for understanding chemical ordering in alloys, which can facilitate the development of rational alloy design strategies.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Local Chemical Strain in PtFe Alloy Nanoparticles

Precise deciphering of the atomic-insight chemical strain and distribution in nanosolids, especially in nanoalloys with disordered structure, is significant to correlate the structure and chemical properties. In this report, we found an abnormally enhanced negative thermal expansion (NTE) along the c axis in a 3.5 nm PtFe magnetic L1 0 alloy, which was usually weakened by the reduced contribution of invar behavior in a nanosized magnetic material. 57 Fe Mössbauer spectra and extended X-ray absorption fine structure studies indicated the local disordered state of chemical coordination of the L1 0 phase. Pair distribution function and reverse Monte Carlo simulation revealed a surface A1-phase coating and atomic-level stretching chemical strain on the heteroatomic pair along the c axis. Local tensile chemical strain and increasing magnetic moments of Fe atoms on the surface of the L1 0 core made a dominant contribution to the enhanced NTE behavior. Furthermore, the present work brings an atomic reveal for the local chemical strain in bimetallic nanomaterials and provides an innovative perspective to tailoring the functional properties according to the local chemical environment.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Crystallization induced ordering of hard magnetic L1{sub 0} phase in melt-spun FeNi-based ribbons

The microstructure of newly developed hard magnetic Fe{sub 42}Ni{sub 41.3}Si{sub x}B{sub 12-x}P{sub 4}Cu{sub 0.7} (x = 2 to 8 at%) nanocrystalline alloy ribbons has been studied by transmission electron microscopy (TEM) and electron diffraction. A high-density polycrystalline grains, ∼30 nm in size, were formed in a ribbon after annealing at 673 K for 288 hours. Elemental mapping of the annealed specimen revealed the coexistence of three regions, Fe-rich, Ni-rich, and nearly equiatomic Fe-Ni, with areal fractions of 37%, 40%, and 23 %, respectively. The equiatomic L1{sub 0}-type ordered phase of FeNi was detected in between the Fe and Ni-rich phases. The presence of superlattice reflections in nanobeam electron diffraction patterns confirmed the formation of the hard magnetic L1{sub 0} phase beyond any doubt. The L1{sub 0} phase of FeNi was detected in alloys annealed in the temperature range of 673 to 813 K. The present results suggest that the order-disorder transition temperature of L1{sub 0} FeNi is higher than the previously reported value (593 K). The high diffusion rates of the constituent elements induced by the crystallization of an amorphous phase at relatively low temperature (∼673 K) are responsible for the development of atomic ordering in FeNi.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗