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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

MnRhBi3: A Cleavable Antiferromagnetic Metal

This dataset contains DFT input and output files supporting the theoretical modeling in the associated publication (Chem. Mater. 2024, 36, 11306-11316). The calculations characterize MnRhBi3, an orthorhombic (Cmmm) van der Waals-layered intermetallic compound that cleaves easily between neighboring Bi layers. The dataset is organized into three calculation types: (i) Bulk: Structural relaxations of the periodic MnRhBi3 crystal in antiferromagnetic (AFM) and ferromagnetic (FM) configurations, using the vdW-DF-optB86b functional. These provide the equilibrium lattice constants, magnetic energy differences (AFM is 0.5 meV/f.u. lower than FM), and magnetic moments (4.4 µB/Mn, 0.17 µB/Rh, 0.18 µB/Bi) reported in Table 1 of the main text. (ii) Slab: Same magnetic configurations computed with an 18 Ang vacuum layer introduced between Bi layers, used to calculate the cleavage energy Ec = 0.56 J/m2 (AFM) and 0.57 J/m2 (FM), establishing MnRhBi3 as a van der Waals-layered material comparable to graphite, MoS2, and CrI3. (iii) ELF: Single-point calculation on the relaxed bulk AFM geometry with LELF=.TRUE., producing the ELFCAR file used to generate electron localization function isosurfaces and contour maps (Fig. 2, main text) showing Bi lone pairs directed into the van der Waals gaps. All folders contain CONTCAR, INCAR, KPOINTS, OUTCAR, and POSCAR. The ELF/ folder additionally contains ELFCAR. Calculations were performed using VASP 6.3.2 with PBE + vdW-DF-optB86b, PAW potentials, and an energy cutoff of 800 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Transient Studies of Nonequilibrium Transport in Two-Dimensional Semiconductors

The overarching objective of research under DOE support has been to investigate emergent carrier phenomena in different two-dimensional (2D) systems, under conditions where they are driven far from equilibrium. The description of nonequilibrium transport has long represented one of the most enduring challenges in condensed-matter physics, in large part due to the need to invoke the role of complex many-body phenomena (electron-phonon and electron-electron coupling) in this regime. In this project, we have explored aspects of this problem in a variety of different two-dimensional systems. These include the modulation-doped two-dimensional electrons gas, or 2DEG, in GaAs/AlGaAs heterostructures; the electron and hole systems in graphene and its related hetero-structures, and; the strongly-confined electron layers in transition-metal dichalcogenide (MoS 2 and WS 2 ) and trichalcogenide (TiS 3 ) materials. Significant understanding has been achieved in a number of different areas, as we elaborate upon in subsequent sections of this proposal: Nanoscale spectroscopy of defect-related barriers in MoS2 Negative differential conductance due to inter-valley scattering in WS 2 under high fields Incipient localization due to scattering from frozen-phonon disorder in quantum point contacts High-field transient transport in graphene/hexagonal BN heterostructures Universal voltage scaling of quantum corrections in graphene under nonequilibrium A defining feature of much of this research has been an approach to problem solving in which novel experimental techniques (such as transient electrical pulsing and scanning-probe microscopy) are combined with the development (in collaboration with colleagues from condensed-matter theory) of sophisticated models, a strategy that we propose to continue in our future research. In recent years, we have developed an increasing capacity for handling atomically-thin materials, and their van der Waals systems, a capacity that is reflected in the balance of topics listed above.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials (Final Report)

In this DOE Early Career project “Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials,” the PI’s group has combined ferroelectric field effect with nanoscale domain imaging and writing to design the electronic and optical properties of two-dimensional (2D) van der Waals materials, including graphene and transition metal dichalcogenides MoS2 and ReS2. The van der Waals materials have been prepared into field effect transistor (FET) devices with ferroelectric gates. Through domain patterning in a ferroelectric polymer PVDF-TrEF top-gate via conductive atomic force microscopy, the team has created programmable Schottky junctions in monolayer MoS 2 , where both barrier height and I-V rectifying polarity can be reconfigured. The transport anisotropy of monolayer to few-layer ReS 2 has been mapped out by defining the entire channel into an insulating state and creating nanoscale conducting paths along different directions through domain writing in the ferroelectric top-gate. The result shows that the conductivity along and perpendicular to the Re-chain can differ by >5.5x10 4 . Theoretical modeling points to the band origin of the transport anomaly and reveals the emergence of a flat band in few-layer ReS 2 . The interfacial epitaxial relation between ReS 2 and PVDF-TrFE further promotes the formation of close-packed, highly ordered PVDF-TrFE nanowires with width of 35 nm and 10 nm. Nonvolatile modulation of quantum Hall effect has been achieved in graphene FETs with a ferroelectric oxide Ba 0.4 Sr 0.6 TiO 3 back-gate. Scattering from the remote surface optical phonon in Ba 0.4 Sr 0.6 TiO 3 limits the room temperature mobility of graphene to be about 3x10 4 cm 2 /Vs. Steep-slope switching has been achieved in MoS 2 FETs back-gated by polycrystalline Pb(Zr,Ti)O 3 , which signals a static-state negative capacitance mode without involving an additional dielectric layer. Piezoresponse force microscopy studies show that the sub-threshold swing can be well correlated with the domain wall density in Pb(Zr,Ti)O 3 . The team also observes an unconventional filtering effect of the second harmonic generation response at the MoS 2 /Pb(Zr,Ti)O 3 heterointerface, which can be accounted for by the alignment between one of the polar axes of MoS 2 and the chiral dipole rotation at the surface of domain wall in Pb(Zr,Ti)O 3 . The research supported by this DOE grant has significantly advanced the fundamental understanding and functional design of ferroelectric/2D van der Waals heterostructures for their implementation towards energy applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anisotropic Thermal Conductivity of Inkjet-Printed 2D Crystal Films: Role of the Microstructure and Interfaces

Two-dimensional (2D) materials are uniquely suited for highly anisotropic thermal transport, which is important in thermoelectrics, thermal barrier coatings, and heat spreaders. Solution-processed 2D materials are attractive for simple, low-cost, and large-scale fabrication of devices on, virtually, any substrate. However, to date, there are only few reports with contrasting results on the thermal conductivity of graphene films, while thermal transport has been hardly measured for other types of solution-processed 2D material films. In this work, inkjet-printed graphene, h-BN and MoS2 films are demonstrated with thermal conductivities of ∼10 Wm−1K−1 and ∼0.3 Wm−1K−1 along and across the basal plane, respectively, giving rise to an anisotropy of ∼30, hardly dependent on the material type and annealing treatment. First-principles calculations indicate that portion of the phonon spectrum is cut-off by the quality of the thermal contact for transport along the plane, yet the ultra-low conductivity across the plane is associated with high-transmissivity interfaces. These findings can drive the design of highly anisotropic 2D material films for heat management applications.

2D materials↗
DOE OSTI2016

SYSTEMATIC UNCERTAINTIES IN THE SPECTROSCOPIC MEASUREMENTS OF NEUTRON STAR MASSES AND RADII FROM THERMONUCLEAR X-RAY BURSTS. III. ABSOLUTE FLUX CALIBRATION

Many techniques for measuring neutron star radii rely on absolute flux measurements in the X-rays. As a result, one of the fundamental uncertainties in these spectroscopic measurements arises from the absolute flux calibrations of the detectors being used. Using the stable X-ray burster, GS 1826–238, and its simultaneous observations by Chandra HETG/ACIS-S and RXTE /PCA as well as by XMM-Newton EPIC-pn and RXTE /PCA, we quantify the degree of uncertainty in the flux calibration by assessing the differences between the measured fluxes during bursts. We find that the RXTE /PCA and the Chandra gratings measurements agree with each other within their formal uncertainties, increasing our confidence in these flux measurements. In contrast, XMM-Newton EPIC-pn measures 14.0 ± 0.3% less flux than the RXTE /PCA. This is consistent with the previously reported discrepancy with the flux measurements of EPIC-pn, compared with EPIC MOS1, MOS2, and ACIS-S detectors. We also show that any intrinsic time-dependent systematic uncertainty that may exist in the calibration of the satellites has already been implicity taken into account in the neutron star radius measurements.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Superconductivity in the alkali metal intercalates of molybdenum disulphide

The complete series of alkali metals, lithium through cesium, have been intercalated into molybdenum disulphide, using both the liquid ammonia and vapor techniques. All the intercalates with the exception of lithium yielded full superconducting transitions with onset temperatures of 6 K for AxMoS2(Ax=K,Rb,Cs) and 4 K for BxMoS2(Bx=Li,Na). The superconducting transition for lithium was incomplete down to 1.5 K. Stoichiometries and unit cell parameters have been determined for the intercalation compounds. Both rhombohedral and hexagonal polymorphs of MoS2 have been intercalated and found to exhibit the same superconductivity behavior. The nature of the extraneous superconducting transition of some intercalated samples on exposure to air was elucidated.

Somoano, R. B.↗
NASA NTRS1973

High temperature solid lubricants: When and where to use them

The state of the art of solid lubrication for moderate to extremely high temperature lubrication (to 1600 F) is reviewed. Lubricating characteristics, stability in various environments, and relevant machine design considerations are discussed. Lubricating materials discussed include the layer lattice compounds: MoS2, WS2, graphite and graphite fluoride, the high temperature polyimide polymer, and calcium fluoride based coating and composites. The scope of the information includes results from wear testers, ball bearing, and journal bearings.

Sliney, H. E.↗
NASA NTRS1973

High temperature solid lubricants - When and where to use them.

This paper reviews the state of the art of solid lubrication for moderate to extremely high temperature lubrication (to 1600 F). Lubricating characteristics, stability in various environments, and relevant machine design considerations are discussed. Lubricating materials discussed include MoS2, WS2, graphite, graphite fluoride, the high temperature polymide polymer, and calcium fluoride based coatings and composites. The scope of the information includes results from wear testers, ball bearings, and journal bearings.

Sliney, H. E.↗
NASA NTRS1974

Effectiveness of solid lubricant coatings for friction in hard vacuum (10-9 tor)

A study was made of the efficiency of solid lubricating coatings, based on MoS2 with various binders, during friction and under highvacuum conditions. Mass spectrometry was used for an analysis of the composition of the gas evolved from the coatings in the friction process. It is shown that the vacuum level, loading, and sliding velocity influence coating effectiveness. In the friction process the solid lubricant coatings yield characteristic decay products associated with the chemical nature of the binders. The mechanism of coating breakdown during friction is associated with the binder breakdown mechanism.

Verkin, B. I.↗
NASA NTRS1976

An evaluation of dry film lubricants and substrate materials for use on SSME gimbal bearings

Failure of the spherical bearing shaft of the Space Shuttle Main Engine (SSME) gimbal bearing assembly was encountered during Design Verification Specification testing of the full scale engine. Investigation revealed that the failure was caused by a deficiency in the lubrication system. Based upon the materials and gimbal operating conditions, a lubricant of MoS2 and graphite with a ceramic binder was the best lubricant candidate for this particular application; however, the decision to implement the change was not made without verification testing. Scaled down simulation testing was performed. Four different substrate materials and eight different dry film lubricants were subjected to tests under simulated SSME environmental and stress load conditions. The test specimens were evaluated for friction and operating life. Each test specimen was subjected to cyclic operation under load until failure. The force required to move the bearing surfaces relative to each other was monitored throughout the test, thus providing analytical data for derivation of the coefficient of friction. Results indicate that the MoS2/graphite lubricant with ceramic binder proved to be superior from the standpoint of endurance and also from the standpoint of friction reducing capabilities when applied to the titanium substrate material used on SSME. Endurance of this lubricant was approximately 16 times that of the lubricant which was being used when the SSME gimbal failed.

Harp, J. A.↗
NASA NTRS1977

X-ray photoelectron spectroscopy study of radiofrequency sputtered chromium bromide, molybdenum disilicide, and molybdenum disulfide coatings and their friction properties

Radiofrequency sputtered coatings of CRB2, MOSI2, and MOS2 were examined by X-ray photoelectron spectroscopy. The effects of sputtering target history, deposition time, RF power level, and substrate bias on film composition were studied. Friction tests were run on RF sputtered surfaces of 440-C steel to correlate XPS data with lubricating properties. Significant deviations from stoichiometry and high oxide levels for all three compounds were related to target outgassing. The effect of biasing on these two factors depended on the compound. Improved stoichiometry correlated well with good friction and wear properties.

Wheeler, D. R.↗
NASA NTRS1977

Study of the lubricant properties of carbon nonfluoride: (CF subx) subn

Friction and wear tests were performed with rubbed carbon monofluorides derived from different types of carbon. Best results were given by fluorated artificial graphite and coke. The coefficient of friction is practically independent of the fluorine-to-carbon ratio x where 0 x 1, but wear decreases by a factor of 6 as x tends toward 1. For a range of variation of the severity factor (PV) expressed in Newtons/cm. sec (where P is the average pressure and V the velocity) contained between 1,000 and 200,000, the wear measured with carbon monofluoride is significantly lower than that obtained with MoS2. A transfer film or third body which protects the surfaces is formed with carbon monofluoride, and beyond 400 C, thick and fragile oxide coatings are also formed.

Play, D.↗
NASA NTRS1978

Some effects of composition on friction and wear of graphite-fiber-reinforced polyimide liners in plain spherical bearings

Oscillating, plain spherical bearings with graphite-fiber-reinforced polyimide (GFRPI) liners were tested for friction and wear from 25 to 315 C. A condensation polymer was compared with an addition polymer, and a high-modulus fiber was compared with a lower cost, low-modulus fiber. All polymer-fiber combinations gave friction coefficients from 0.05 to 0.18 and low wear. Adding CdO and CdI2 reduced the wear of degassed bearings in dry air. These additives were not needed when the bearing liners contained adsorbed moisture. Although, at 25 C, MoS2 reduced the friction and wear of the base composite at unit loads above 70,000,000 N/m squared (10,000 psi), it had no beneficial effect at lighter loads.

Sliney, H. E.↗
NASA NTRS1978

Application of ESCA to the determination of stoichiometry in sputtered coatings and interface regions

X-ray photoelectron spectroscopy was used to characterize radiofrequency sputter deposited films of several refractory compounds. Both the bulk film properties such as purity and stoichiometry and the character of the interfacial region between the film and substrate were examined. The materials were CrB2, MoS2, Mo2C, and Mo2B5 deposited on 440C steel. It was found that oxygen from the sputtering target was the primary impurity in all cases. Biasing improves the film purity. The effect of biasing on film stoichiometry is different for each compound. Comparison of the interfacial composition with friction data suggests that adhesion of these films is improved if a region of mixed film and iron oxides can be formed.

Wheeler, D. R.↗
NASA NTRS1978

A Comparison of the Lubricating Mechanisms of Graphite Fluoride and Molybdenum Disulfide Films

A microscopic study of 440C steel sliding surfaces lubricated by graphite fluoride or molybdenum disulfide solid lubricant rubbed films was conducted. The sliding surfaces, along with the friction, wear, and wear life were observed as a function of the number of sliding revolutions in three different atmospheres: moist air, dry air, or dry argon. In general, the lubricating mechanisms of the two solid lubricants were found to be relatively similar; that is, a dynamic, thin, layer-like film was formed between the two metallic surfaces. The mechanisms of failure were found to be somewhat different, however. Failure of MoS2 films was very dependent on atmospheric degradation, while that of graphite fluoride films was more dependent on flow of the lubricant film out of the contact zone.

Fusaro, R. L.↗
NASA NTRS1978

Composition of RF-sputtered refractory compounds determined by X-ray photoelectron spectroscopy

RF-sputtered coatings of CrB2, MoSi2, Mo2C, TiC, and MoS2 were examined by X-ray photoelectron spectroscopy (XPS). Data on stoichiometry, impurity content, and chemical bonding were obtained. The influences of sputtering target history, deposition time, RF power level, and substrate bias were studied. Significant deviations from stoichiometry and high oxide levels were related to target outgassing. The effect of substrate bias depended on the particular coating material studied.

Wheeler, D. R.↗
NASA NTRS1978

A comparison of the lubricating mechanisms of graphite fluoride and molybdenum disulfide films

A microscopic study of 440 C steel sliding surfaces lubricated by graphite fluoride or molybdenum disulfide solid lubricant rubbed films was conducted. The sliding surfaces, along with the friction, wear, and wear life were observed as a function of the number of sliding revolutions in three different atmospheres: moist air (10,000 ppm H2O), dry air (less than 20 ppm H2O), or dry argon (less than 20 ppm H2O). In general, the lubricating mechanisms of the two solid lubricants were found to be relatively similar; that is, a dynamic, thin, layer-like film (which sheared on relative motion) was formed between the two metallic surfaces. The mechanisms of failure were found to be somewhat different, however. Failure of MoS2 films was very dependent on atmospheric degradation, while that of graphite fluoride films was more dependent on flow of the lubricant film out of the contact zone

Fusaro, R. L.↗
NASA NTRS1978

Application of ESCA to the determination of stoichiometry in sputtered coatings and interface regions

X-ray Photoelectron Spectroscopy (XPS) was used to characterize radiofrequency sputter deposited films of several refractory compounds. Both the bulk film properties such as purity and stoichiometry and the character of the interfacial region between the film and substrate were examined. The materials were CrB2, MoS2, Mo2C, and Mo2B5 deposited on 440C steel. It was found that oxygen from the sputtering target was the primary impurity in all cases. Biasing improves the film purity. The effect of biasing on film stoichiometry is different for each compound. Comparison of the interfacial composition with friction data suggests that adhesion of these films is improved if a region of mixed film and iron oxides can be formed.

Wheeler, D. R.↗