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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Impact of segregation energetics on oxygen conductivity at ionic grain boundaries

In pursuit of whether nanocrystallinity could lead to higher anion conductivity, research has revealed contradicting results exposing the limited understanding of point defect energetics at grain boundaries (GBs)/interfaces. Furthermore, by disentangling and addressing key GB energetics issues, i.e., segregation, migration and binding energies of oxygen vacancies in the presence and absence of dopants at the GBs, and the segregation energetics of dopants, we elucidate, using atomic simulations of doped ceria, that dopant segregation is the key factor leading to degradation of oxygen conductivity in nanocrystalline materials. Finally, a framework for designing enhanced conducting nanocrystalline materials is proposed where the focus of doping strategies shifts from bulk to segregation at GBs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Extraordinary role of Hg in enhancing the thermoelectric performance of p-type SnTe

We report several synergistic effects in Hg alloying of SnTe to enhance the power factor and overall figure of merit ZT. Hg alloying decreases the energy separation between the two valence bands, leading to pronounced band convergence that improves the Seebeck coefficient. Hg alloying of SnTe also significantly enlarges the band gap thereby effectively suppressing the bipolar diffusion. Collectively, this results in high ZT of similar to 1.35 at 910 K for 2% Bi-doped SnTe with 3% HgTe. The solubility limit of Hg in SnTe is less than 3 mol%, and above this level we observe HgTe precipitates in the SnTe matrix, typically trapped at grain boundary triple junctions. The strong point defect scattering of phonons caused by Hg alloying coupled with mesoscale scattering via grain boundaries contributes to a great reduction of lattice thermal conductivity. The multiple synergistic roles that Hg plays in regulating the electron and phonon transport in SnTe provide important new insights into continued optimization of SnTe-based and related materials.

Tan, Gangjian↗
DOE OSTI2014

SnTe–AgBiTe 2 as an efficient thermoelectric material with low thermal conductivity

SnTe is an intriguing alternative to its sister compound PbTe in thermoelectric energy conversion because of their electronic and structural similarity; however, it is challenging to optimize its thermoelectric performance to the level of PbTe because of the difficulties in decreasing its intrinsically large hole population and high thermal conductivity arising from the tin vacancies. We demonstrate in this work that by alloying some AgBiTe 2 in SnTe, thus forming AgSn x BiTe x +2 compositions the hole concentration can be duly decreased because of the high efficiency of Bi as an electron donor. The lattice thermal conductivity is also decreased due to the strong scattering of phonons (by point defect scattering as well as Ag-rich nanostructures) to achieve a value of ~0.7 W m –1 K –1 at ~750 K. As a result, a high thermoelectric figure ZT of merit of ~1.1 at 775 K is achieved by chemical composition optimization (×~15), making lead free SnTe–AgBiTe 2 a promising thermoelectric material.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Superior thermoelectric performance in PbTe-PbS pseudo-binary. Extremely low thermal conductivity and modulated carrier concentration

Lead chalcogenides have exhibited their irreplaceable role as thermoelectric materials at the medium temperature range, owing to highly degenerate electronic bands and intrinsically low thermal conductivities. PbTe-PbS pseudo-binary has been paid extensive attentions due to the even lower thermal conductivity which originates largely from the coexistence of both alloying and phase-separated precipitations. To investigate the competition between alloying and phase separation and its pronounced effect on the thermoelectric performance in PbTe-PbS, we systematically studied Spark Plasma Sintered (SPSed), 3 at% Na- doped (PbTe) 1-x (PbS)x samples with x=10%, 15%, 20%, 25%, 30% and 35% by means of transmission electron microscopy (TEM) observations and theoretical calculations. Corresponding to the lowest lattice thermal conductivity as a result of the balance between point defect- and precipitates- scattering, the highest figure of merit ZT~2.3 was obtained at 923 K when PbS phase fraction x is at 20%. The consistently lower lattice thermal conductivities in SPSed samples compared with corresponding ingots, resulting from the powdering and follow-up consolidation processes, also contribute to the observed superior ZT. Notably, the onset of carrier concentration modulation ~600 K due to excessive Na’s diffusion and re-dissolution leads to the observed saturations of electrical transport properties, which is believed equally crucial to the outstanding thermoelectric performance of SPSed PbTe-PbS samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synergistically optimized electrical and thermal transport properties of SnTe via alloying high-solubility MnTe

Lead chalcogenides are the most efficient thermoelectric materials. In comparison, SnTe, a lead-free analogue of PbTe, exhibits inferior thermoelectric performance due to low Seebeck coefficient and high thermal conductivity. In this report, we show that we can synergistically optimize the electrical and thermal transport properties of SnTe via alloying Mn. Further, we report that the introduction of Mn (0–50%) induces multiple effects on the band structure and microstructure of SnTe: for the former, it can tune the Fermi level and promote the convergence of the two valence bands, concurrently enhancing the Seebeck coefficient; for the latter, it can profoundly modify the microstructure into an all-scale hierarchical architecture (including nanoscale precipitates/MnTe laminates, stacking faults, layered structure, atomic-scale point defects, etc.) to scatter phonons with a broad range of mean free paths, strongly reducing the lattice thermal conductivity. Meanwhile, most significantly, the Mn alloying enlarges the energy gap of the conduction band (C band) and the light valence band (L band), thereby suppressing the bipolar thermal conductivity by increasing the band gap. The integration of these effects yields a high ZT of 1.3 at 900 K for 17% Mn alloyed SnTe.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electronegative Guests in CoSb 3

Introducing guests into a host framework to form a so called inclusion compound can be used to design materials with new and fascinating functionalities. The vast majority of inclusion compounds have electropositive guests with neutral or negatively charged frameworks. Here, we show a series of electronegative guest filled skutterudites with inverse polarity. The strong covalent guest-host interactions observed for the electronegative group VIA guests, i.e., S and Se, feature a unique localized cluster vibration which significantly influences the lattice dynamics, together with the point-defect scattering caused by element substitutions, resulting in very low lattice thermal conductivity values. The findings of electronegative guests provide a new perspective for guest-filling in skutterudites, and the covalent filler/lattice interactions lead to an unusual lattice dynamics phenomenon which can be used for designing high-efficiency thermoelectric materials and novel functional inclusion compounds with open structures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Thickness-dependence of block copolymer coarsening kinetics

In spite of active research, many fundamental aspects of block copolymer ordering remain unresolved. We studied the thickness-dependence of block copolymer grain coarsening kinetics, and find that thinner films order more rapidly than thicker films. Bilayer films, or monolayers with partial layers of islands, order more slowly than monolayers because of the greater amount of material that must rearrange in a coordinated fashion. Sub-monolayer films order much more rapidly than monolayers, exhibiting considerably smaller activation energies, as well as larger exponents for the time-growth power-law. Furthermore, by using molecular dynamics simulations, we directly study the motion of defects in these film regimes. Here, we attribute the enhanced grain growth in sub-monolayers to the film boundaries, where defects can be spontaneously eliminated. The boundaries thus act as efficient sinks for morphological defects, pointing towards methods for engineering rapid ordering of self-assembling thin films.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Large reduction of thermal conductivity leading to enhanced thermoelectric performance in p-type Mg 3 Bi 2 –YbMg 2 Bi 2 solid solutions

Intensifying the phonon scattering via point defect engineering has been demonstrated to be particularly effective in minimizing the lattice thermal conductivity for enhancing thermoelectric performance. In this work, significant phonon scattering has been realized by alloying YbMg 2 Bi 2 with Mg 3 Bi 2 . The substantial mass difference between the host atom Yb and the alloying atom Mg leads to an intense phonon scattering effect that significantly reduces the lattice thermal conductivity. The room-temperature lattice thermal conductivity decreases from ~2.7 W m -1 K -1 for YbMg 2 Bi 1.96 to ~0.8 W m -1 K -1 for Yb 0.7 Mg 0.3 Mg 2 Bi 1.96 , a reduction of ~70%. Benefiting from the greatly reduced thermal conductivity, the average ZT has been effectively improved from ~0.46 for YbMg 2 Bi 1.96 to ~0.61 for Yb 0.8 Mg 0.2 Mg 2 Bi 1.96 , an enhancement of ~33%. In addition, the predicted maximum heat-to-electricity conversion efficiency can be increased from ~7% for YbMg 2 Bi 1.96 to ~10% for Yb 0.8 Mg 0.2 Mg 2 Bi 1.96 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

GaSb doping facilitates conduction band convergence and improves thermoelectric performance in n-type PbS

P-type lead chalcogenides have superior thermoelectric performance because they exhibit the energy convergence of several valence bands. However, despite the existence of two conduction bands, there has been no report about conduction band (CB) convergence for n-type counterparts because of the large energy difference between them. Therefore, new strategies are required to manipulate the CBs if enhancing the electrical transport performance of n-type lead chalcogenides is to be achieved. PbS is a highly attractive member of the lead chalcogenides because of its high earth-abundance and low cost. Here, we report that the introduction of GaSb can successfully dope the PbS matrix with Ga and Sb atoms occupying the Pb site in its rock salt structure. GaSb doping leads to conduction band convergence and enlarged effective density of state mass for n-type PbS. This effect results in superior power factor and decreased lattice thermal conductivity caused by the soft phonon modes and point defect scattering of phonons. Consequently, a record-high average power factor PF avg of ~20.4 μW cm –1 K –2 and figure of merit ZT avg of ~0.84 in the temperature range of 400 K to 923 K were obtained, higher than any n- and p-type PbS-based thermoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Effect of growth dynamics on the structural, photophysical and pseudocapacitance properties of famatinite copper antimony sulphide colloidal nanostructures (including nanosheets)

Facile phase selective synthesis of copper antimony sulphide (CAS) nanostructures is important because of their tunable photoconductive and electrochemical properties. In this study, off-stoichiometric famatinite phase CAS (fCAS) quasi-spherical and quasi-hexagonal colloidal nanostructures (including nanosheets) of sizes, 2.4–18.0 nm were grown under variable conditions of temperature (60–200 °C), time and oleylamine capping ligand concentration using copper(II) acetylacetonate and antimony(III) diethyldithiocarbamate precursors. Data from powder X-ray diffraction, Raman spectroscopy and high-resolution scanning/transmission electron microscopy confirm the tetragonal structure of the famatinite phase. X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy data suggest a correlation of particle size, morphology and composition of the off-stoichiometric fCAS nanostructures with growth temperature and time, and oleylamine concentration. The off-stoichiometric Cu 3-a Sb 1+b S 4±c (a, b, c – mole fractions) nanostructures being severely copper-deficient and antimony-rich, exhibit shallow-lying acceptor copper vacancy states, deep-lying donor states of antimony interstitials, sulphur vacancies and antimony-copper antisites and shallow-lying acceptor surface trapping states. Further, these electronic states are likely implicated in tunable UV-visible absorption and bandgaps between 2.3 and 2.8 eV, and broad visible-NIR photoluminescence with fast recombination of radiative lifetimes between 0.2 and 6.2 ns, confirmed from absorption, steady-state and time-resolved photoluminescence spectroscopies. Additionally, cyclic voltammetry and electrochemical impedance spectroscopy confirm that electrodes of the fCAS nanostructures display slightly variable pseudocapacitance of charge-storage primarily via possible sodium ion intercalation with a high specific capacitance of ~84 F g -1 obtained at a scan rate of 5 mV s -1 . Overall, these results show the influence of composition, in particular point defects, phase quality and morphology on the optical and pseudocapacitance properties of fCAS nanostructures, suitable as solar absorbers or electrodes for energy storage devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Epitaxially Grown Single-Crystalline SrTiO 3 Membranes Using a Solution-Processed, Amorphous SrCa 2 Al 2 O 6 Sacrificial Layer

Water-soluble sacrificial layers based on epitaxially-grown, single crystalline (Ca, Sr, Ba) 3 Al 2 O 6 layer are widely used for creating free-standing perovskite oxide membranes. However, obtaining these sacrificial layers with intricate stoichiometry remains a challenge, especially for molecular beam epitaxy (MBE). In this study, we demonstrate the hybrid MBE growth of epitaxial, single crystalline SrTiO 3 films using a solution processed, amorphous SrCa 2 Al 2 O 6 sacrificial layer onto SrTiO 3 (001) substrates. Prior to the growth, the oxygen plasma exposure was used to first create the crystalline SrCa 2 Al 2 O 6 layer with well-defined surface crystallinity. Utilizing reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy, we observe an atomic layer-by-layer growth of epitaxial, single crystalline SrTiO 3 film on the SrCa 2 Al 2 O 6 layer with atomically smooth surfaces. The SrCa 2 Al 2 O6 layer was subsequently dissolved in de-ionized water to create free-standing SrTiO3 membranes that were transferred onto a metal-coated Si wafer. Membranes created with Sr-deficiency revealed ferroelectric-like behavior measured using piezo force microscopy whereas stoichiometric films remained paraelectric-like. Furthermore, these findings underscore the viability of using ex-situ deposited amorphous SrCa2Al2O6 for epitaxial, single crystalline growth, as well as the importance of point defects in determining the ferroic properties in membranes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Interface magnetism in vanadium-doped MoS 2 /graphene heterostructures

Magnetism in two-dimensional materials is of great importance in discovering new physical phenomena and developing new devices at the nanoscale. In this paper, first-principles simulations are used to calculate the electronic and magnetic properties of heterostructures composed of graphene and MoS 2 considering the influence of point defects and vanadium doping. It is found that the concentration of the dopants and the types of defects can result in induced magnetic moments leading to ferromagnetically polarized systems with sharp interfaces. This provides a framework for interpreting the experimental observations of enhanced ferromagnetism in both MoS 2 /graphene and V-doped MoS 2 /graphene heterostructures. The computed electronic and spin polarizations give a microscopic understanding of the origin of ferromagnetism in these systems and illustrate how doping and defect engineering can lead to targeted property tunability. Our work has demonstrated that through defects engineering, ferromagnetism can be achieved in V-doped MoS2/graphene heterostructures, providing a potential way to induce magnetization in other TMDC/graphene materials and opening new opportunities for their applications in nano-spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2026

First-principles thermodynamic assessment of Sr-containing secondary phase formation in strontium-substituted lanthanum manganites for solid oxide cell applications

Sr-secondary phase formation is a potentially significant degradation mode with direct impact upon solid-oxide cell (SOC) commercial viability. A first-principles based thermodynamic study was performed for La 1−x Sr x MnO 3±δ (LSM) perovskites to assess their stability against formation of different Sr-secondary phases, including SrO, SrCrO 4 , SrSO 4 , SrCO 3 , and Sr(OH) 2 , for SOC applications. The Sr-secondary phase formation reaction free energies were determined via a thermodynamic model by combining ab initio lattice dynamics calculations for the solid phases and ab initio thermodynamic data for the gas phases. The current approach expands the previously reported thermodynamic modeling studies by integrating first-principles based point defect equilibria into the thermodynamic analysis. The modeling results obtained using this new approach indicate an increased tendency to form the SrO oxide upon decreasing the oxygen partial pressure. Additionally, the enhancing factors to form the Sr-related secondary phase from the associated SrO activity in LSM are further quantified by considering the equilibrium of SrO reacting with the contaminant gas species as a function of temperature and pressure.

defect thermodynamics modeling↗
DOE OSTI2013

Ultrafast switching in wetting properties of TiO{sub 2}/YSZ/Si(001) epitaxial heterostructures induced by laser irradiation

We have demonstrated dark hydrophilicity of single crystalline rutile TiO{sub 2}(100) thin films, in which rapid switching from a hydrophobic to a hydrophilic surface was achieved using nanosecond excimer laser irradiation. The TiO{sub 2}/YSZ/Si(001) single crystalline heterostructures were grown by pulsed laser deposition and were subsequently irradiated by a single pulse of a KrF excimer laser at several energies. The wettability of water on the surfaces of the samples was evaluated. The samples were hydrophobic prior to laser annealing and turned hydrophilic after laser annealing. Superhydrophilic surfaces were obtained at higher laser energy densities (e.g., 0.32 J.cm{sup -2}). The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies on the surface, which are surmised to be responsible for the observed superhydrophilic behavior. According to the AFM images, surface smoothening was greater in films that were annealed at higher laser energy densities. The samples exhibited hydrophobic behavior after being placed in ambient atmosphere. The origin of laser induced wetting behavior was qualitatively understood to stem from an increase of point defects near the surface, which lowered the film/water interfacial energy. This type of rapid hydrophobic/hydrophilic switching may be used to facilitate fabrication of electronic and photonic devices with novel properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Identification of As-vacancy complexes in Zn-diffused GaAs

We have used positron annihilation spectroscopy to study the introduction of point defects in Zn-diffused semi-insulating GaAs. The diffusion was performed by annealing the samples for 2 h at 950 Degree-Sign C. The samples were etched in steps of 7 {mu}m. Both Doppler broadening using slow positron beam and lifetime spectroscopy studies were performed after each etching step. Both techniques showed the existence of vacancy-type defects in a layer of about 45 {mu}m. Secondary ion mass spectroscopy measurements illustrated the presence of Zn at high level in the sample almost up to the same depth. Vacancy-like defects as well as shallow positron traps were observed by lifetime measurements. We distinguish two kinds of defects: As vacancy belongs to defect complex, bound to most likely one Zn atom incorporated on Ga sublattice, and negative-ion-type positron traps. Zn acceptors explained the observation of shallow traps. The effect of Zn was evidenced by probing GaAs samples annealed under similar conditions but without Zn treatment. A defect-free bulk lifetime value is detected in this sample. Moreover, our positron annihilation spectroscopy measurements demonstrate that Zn diffusion in GaAs system is governed by kick-out mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Enhanced photocatalytic efficiency in zirconia buffered n-NiO/p-NiO single crystalline heterostructures by nanosecond laser treatment

We report the formation of NiO based single crystalline p-n junctions with enhanced photocatalytic activity induced by pulsed laser irradiation. The NiO epilayers were grown on Si(001) substrates buffered with cubic yttria-stabilized zirconia (c-YSZ) by using pulsed laser deposition. The NiO/c-YSZ/Si heterostructures were subsequently laser treated by 5 pulses of KrF excimer laser (pulse duration = 25 Multiplication-Sign 10{sup -9} s) at lower energies. Microstructural studies, conducted by X-ray diffraction ({theta}-2{theta} and {phi} techniques) and high resolution transmission electron microscope, showed a cube-on-cube epitaxial relationship at the c-YSZ/Si interface; the epitaxial relationship across the NiO/c-YSZ interface was established as NiO<111 > Double-Vertical-Line Double-Vertical-Line c-YSZ<001> and in-plane NiO<110> Double-Vertical-Line Double-Vertical-Line c-YSZ<100>. Electron microscopy studies showed that the interface between the laser annealed and the pristine region as well as the NiO/c-YSZ interface was atomically sharp and crystallographically continuous. The formation of point defects, namely oxygen vacancies and NiO, due to the coupling of the laser photons with the NiO epilayers was confirmed by XPS. The p-type electrical characteristics of the pristine NiO epilayers turned to an n-type behavior and the electrical conductivity was increased by one order of magnitude after laser treatment. Photocatalytic activity of the pristine (p-NiO/c-YSZ/Si) and the laser-annealed (n-NiO/p-NiO/c-YSZ/Si) heterostructures were assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined to be 0.0059 and 0.0092 min{sup -1} for the as-deposited and the laser-treated samples, respectively. The enhanced photocatalytic efficiency was attributed to the suppressed charge carrier recombination in the NiO based p-n junctions and higher electrical conductivity. Besides, the oxygen vacancies ease the adsorption of 4-chlorophenol, hydroxyl, and water molecules to the surface. Thus, n-NiO/p-NiO single crystalline catalysts can be introduced as a potent candidate to remediate the environmental pollution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Mechanical properties of irradiated single-layer graphene

The mechanical properties of irradiated single-layer graphene sheets are determined as a function of inserted vacancy concentration. We find that the vacancy-induced crystalline-to-amorphous transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets for inserted vacancy concentrations of 8%-12%. While point defects and larger voids appreciably degrade the strength of pristine graphene, we find that even heavily damaged samples ({approx}20% vacancies) exhibit tensile strengths of {approx}30 GPa, in significant excess of those typical of engineering materials. Our results suggest that defect engineering of graphene is feasible without incurring a complete loss of its desirable mechanical properties.

36 MATERIALS SCIENCE↗