Search NASA⌕ Search

SEARCH · Search NASA

Results for “Graphite”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Simulated laser-induced breakdown spectra of graphite and synthetic shergottite glass under Martian conditions

We report the results of a simulation of the laser-induced breakdown spectra of graphite and synthetic shergottite glass in an atmosphere similar to that of Mars using a 1-D, Lagrangian hydrodynamic model, a spherical geometry description of the laser ablation and plume expansion and a local thermodynamic equilibrium (LTE) approach for the emission spectra. We compare the LIBS spectra of two calibration targets on board of the Curiosity rover to the simulated ones calculated under nominally the same conditions as those encountered on Mars. The simulations provide an additional way, to laboratory based comparative studies, to better understand the effects of laser parameters and atmospheric pressures. We report on the effect of laser irradiance and ambient pressure on the electron and ion temperature, electron number density, and fluid velocity parameters characterizing the plasma. Lastly, we show the effects of laser irradiance and ambient pressure on the simulated emission spectra of graphite and shergottite and compare them to those acquired by ChemCam. The agreement between the two is good, particularly for the prominent emission lines.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Rapid Production of Graphite Without Contamination for Biomedical AMS

The application of AMS to the detection of 14 C makes possible a new class of sensitive experiments in molecular biology. Such experiments inherently produce large numbers of samples for the determination of biological variability in molecular interactions. The samples vary in 14 C concentration over many orders of magnitude. We added TiH 2 to aid the reduction of CO 2 by zinc in a sealed tube to reproducibly make graphite without sample cross-contamination. The CO 2 is transferred from a combustion tube to the reaction tube through a disposable plastic manifold. The sealed tubes are heated to a single-reaction temperature in a muffle furnace. The process is complete within 5 h. Bulk isotopic fractionation in the finished graphite is less than 0.5%.

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

Electrolyte volume effects on electrochemical performance and solid electrolyte interphase in Si-graphite/NMC lithium-ion pouch cells

This study aims to explore the correlations between electrolyte volume, electrochemical performance, and properties of the solid electrolyte interphase in pouch cells with Si-graphite composite anodes. The electrolyte is 1.2 M LiPF 6 in ethylene carbonate:ethylmethyl carbonate with 10 wt.% fluoroethylene carbonate. Single layer pouch cells (100 mAh) were constructed with 15 wt.% Si-graphite/LiNi 0.5 Mn 0.3 CO 0.2 O 2 electrodes. It is found that a minimum electrolyte volume factor of 3.1 times the total pore volume of cell components (cathode, anode, and separator) is needed for better cycling stability. Less electrolyte causes increases in ohmic and charge transfer resistances. Lithium dendrites are observed when the electrolyte volume factor is low. The resistances from the anodes become significant as the cells are discharged. As a result, solid electrolyte interphase thickness grows as the electrolyte volume factor increases and is non-uniform after cycling.

25 ENERGY STORAGE↗
DOE OSTI2018

Chemical Evolution in Silicon–Graphite Composite Anodes Investigated by Vibrational Spectroscopy

Silicon–graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. In conclusion, the spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings.

25 ENERGY STORAGE↗
DOE OSTI2018

Highly Graphitic Mesoporous Fe,N-Doped Carbon Materials for Oxygen Reduction Electrochemical Catalysts

The synthesis, characterization, and electrocatalytic properties of mesoporous carbon materials doped with nitrogen atoms and iron are reported and compared for the catalyzed reduction of oxygen gas at fuel cell cathodes. Mixtures of common and inexpensive organic precursors, melamine, and formaldehyde were pyrolyzed in the presence of transition-metal salts (e.g., nitrates) within a mesoporous silica template to yield mesoporous carbon materials with greater extents of graphitization than those of others prepared from small-molecule precursors. In particular, Fe,N-doped carbon materials possessed high surface areas (~800 m 2 /g) and high electrical conductivities (~19 S/cm), which make them attractive for electrocatalyst applications. The surface compositions of the mesoporous Fe,N-doped carbon materials were postsynthetically modified by acid washing and followed by high-temperature thermal treatments, which were shown by X-ray photoelectron spectroscopy to favor the formation of graphitic and pyridinic nitrogen moieties. Such surface-modified materials exhibited high electrocatalytic oxygen reduction activities under alkaline conditions, as established by their high onset and half-wave potentials (1.04 and 0.87 V, respectively vs reversible hydrogen electrode) and low Tafel slope (53 mV/decade). These values are superior to many similar transition-metal- and N-doped carbon materials and compare favorably with commercially available precious-metal catalysts, e.g., 20 wt % Pt supported on activated carbon. As a result, the analyses indicate that inexpensive mesoporous Fe,N-doped carbon materials are promising alternatives to precious metal-containing catalysts for electrochemical reduction of oxygen in polymer electrolyte fuel cells.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Sublimation Kinetics for Individual Graphite and Graphene Nanoparticles (NPs): NP-to-NP Variations and Evolving Structure-Kinetics and Structure-Emissivity Relationships

A single nanoparticle (NP) mass spectrometry method was used to measure sublimation rates as a function of nanoparticle temperature (T NP ) for sets of individual graphite and graphene NPs. Initially, the NP sublimation rates were ~400 times faster than that for bulk graphite, and there were large NP-to-NP variations. Over time, the rates slowed substantially, though remaining well above the bulk rate. The initial activation energies (E a values) were correspondingly low and doubled as a few monolayer’s worth of material were sublimed from the surfaces. The high initial rates and low E a values are attributed to large numbers of edge, defect, and other low coordination sites on the NP surfaces, and the changes are attributed to atomic-scale “smoothing” of the surface by preferential sublimation of the less stable sites. The emissivity of the NPs also changed after heating, most frequently increasing. Here, the emissivity and sublimation rates were anti-correlated, leading to the conclusion that high densities of low-coordination sites on the NP surfaces enhances sublimation but suppresses emissivity.

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

Photoemission studies of fluorine functionalized porous graphitic carbon

Porous graphitic carbon (PGC) has unique properties desirable for liquid chromatography applications when used as a stationary phase. The polar retention effect on graphite (PREG) allows efficient separation of polar and non-polar solutes. Perfluorinated hydrocarbons however lack polarizabilty and display strong lipo- and hydrophobicity, hence common lipophilic and hydrophilic analytes have low partition coefficiency in fluorinated stationary phases. Attractive interaction between fluorinated stationary phase and fluorinated analytes results in strong retention compared to non-fluorinated analytes. In order to change the selectivities of PGC, it is necessary to develop a bonded PGC stationary phase. In this study, we have synthesized perfluorinated, PGC using hepatadecafluoro-1-iodooctane, under different temperature conditions. Surface functionalization of the raw material was studied using photoelectron spectroscopy (PES). Results indicate the existence of fluorine containing functional groups, -CF, -CF{sub 2} along with an intercalated electron donor species. Multiple oxygen functional groups were also observed, likely due to the presence of oxygen in the starting material. These oxygen species may be responsible for significant modifications to planer and tetrahedral carbon ratios.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Production of negative ions on graphite surface in H{sub 2}/D{sub 2} plasmas: Experiments and srim calculations

In previous works, surface-produced negative-ion distribution-functions have been measured in H{sub 2} and D{sub 2} plasmas using graphite surfaces (highly oriented pyrolitic graphite). In the present paper, we use the srim software to interpret the measured negative-ion distribution-functions. For this purpose, the distribution-functions of backscattered and sputtered atoms arising due to the impact of hydrogen ions on a-CH and a-CD surfaces are calculated. The srim calculations confirm the experimental deduction that backscattering and sputtering are the mechanisms of the origin of the creation of negative ions at the surface. It is shown that the srim calculations compare well with the experiments regarding the maximum energy of the negative ions and reproduce the experimentally observed isotopic effect. A discrepancy between calculations and measurements is found concerning the yields for backscattering and sputtering. An explanation is proposed based on a study of the emitted-particle angular-distributions as calculated by srim.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Efficient graphite ring heater suitable for diamond-anvil cells to 1300 K

In order to generate homogeneous high temperatures at high pressures, a ring-shaped graphite heater has been developed to resistively heat diamond-anvil cell (DAC) samples up to 1300 K. By putting the heater in direct contact with the diamond anvils, this graphite heater design features the following advantages: (1) efficient heating: sample can be heated to 1300 K while the DAC body temperature remains less than 800 K, eliminating the requirement of a special alloy for the DAC; (2) compact design: the sample can be analyzed with in situ measurements, e.g., x-ray, optical, and electrical probes are possible. In particular, the side access of the heater allows for radial x-ray diffraction (XRD) measurements in addition to traditional axial XRD.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Near-field thermal radiation between hyperbolic metamaterials: Graphite and carbon nanotubes

The near-field radiative heat transfer for two hyperbolic metamaterials, namely, graphite and vertically aligned carbon nanotubes (CNTs), is investigated. Graphite is a naturally existing uniaxial medium, while CNT arrays can be modeled as an effective anisotropic medium. Different hyperbolic modes can be separately supported by these materials in certain infrared regions, resulting in a strong enhancement in near-field heat transfer. It is predicted that the heat flux between two CNT arrays can exceed that between SiC plates at any vacuum gap distance and is about 10 times higher with a 10 nm gap.

42 ENGINEERING↗
DOE OSTI2013

Three-dimensional diamond detectors: Charge collection efficiency of graphitic electrodes

Implementation of 3D-architectures in diamond detectors promises to achieve unreached performances in the radiation-harsh environment of future high-energy physics experiments. This work reports on the collection efficiency under β-irradiation of graphitic 3D-electrodes, created by laser pulses in the domains of nanoseconds (ns-made-sensors) and femtoseconds (fs-made-sensors). Full collection is achieved with the fs-made-sensors, while a loss of 25%–30% is found for the ns-made-sensors. The peculiar behaviour of ns-made sensors has been explained by the presence of a nano-structured sp{sup 3}-carbon layer around the graphitic electrodes, evidenced by micro-Raman imaging, by means of a numerical model of the charge transport near the electrodes.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Identifying the distinct phases of THz waves from K-valley electrons in graphite

The polarity change of THz electromagnetic waves radiated from single-crystalline graphite and polycrystalline graphite films has been studied to identify the main generation mechanism in conventional reflective THz time-domain spectroscopy scheme. The excitation wavelength variation around the K-valley produces no significant changes in THz field strength. We further found that THz waves become fully dispersed without polarity change in lateral detection geometry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Revisiting the domain model for lithium intercalated graphite

In this Letter, we study the stability of the domain model for lithium intercalated graphite in stages III and II by means of Density Functional Theory and Kinetic Lattice Monte Carlo simulations. We find that the domain model is either thermodynamically or kinetically stable when compared to the standard model in stages III and II. The existence of domains in the intercalation sequence is well supported by recent high resolution transmission electron microscope observations in lithiated graphite. Moreover, we predict that such domain staging sequences leads to a wide range of diffusivity as reported in experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Rectifying performance and negative differential behavior in graphite—chain—carbon nanotube junctions

In this paper, the (5, 5) capped carbon nanotubes (CNTs) in contact with different lengths of sp monoatomic chains grown on the surface of graphite substrate are fabricated and its electronic transport properties sandwiched between CNT and graphite electrodes are investigated. The first-principles calculations based on nonequilibrium Green's function in combination with density-functional theory show that their rectifying performance and negative differential resistance behavior are observed under very low biases and obviously are enhanced when the length increases. From our analysis, the charge transfer, transmission spectra, projected density of states and evolutions of molecular orbitals are responsible for these phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Multiscale modeling of polyisoprene on graphite

The local dynamics and the conformational properties of polyisoprene next to a smooth graphite surface constructed by graphene layers are studied by a multiscale methodology. First, fully atomistic molecular dynamics simulations of oligomers next to the surface are performed. Subsequently, Monte Carlo simulations of a systematically derived coarse-grained model generate numerous uncorrelated structures for polymer systems. A new reverse backmapping strategy is presented that reintroduces atomistic detail. Finally, multiple extensive fully atomistic simulations with large systems of long macromolecules are employed to examine local dynamics in proximity to graphite. Polyisoprene repeat units arrange close to a parallel configuration with chains exhibiting a distribution of contact lengths. Efficient Monte Carlo algorithms with the coarse-grain model are capable of sampling these distributions for any molecular weight in quantitative agreement with predictions from atomistic models. Furthermore, molecular dynamics simulations with well-equilibrated systems at all length-scales support an increased dynamic heterogeneity that is emerging from both intermolecular interactions with the flat surface and intramolecular cooperativity. This study provides a detailed comprehensive picture of polyisoprene on a flat surface and consists of an effort to characterize such systems in atomistic detail.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Quantum modelling of hydrogen chemisorption on graphene and graphite

The chemisorption of hydrogen on graphene or graphite is studied within a quantum formalism involving a subsystem coupled to a phonon bath. The subsystem includes the hydrogen atom approaching the surface perpendicularly right on top of a carbon atom which puckers out of the surface. The bath includes the acoustic and optical phonon modes vibrating perpendicularly to the surface. Couplings between subsystem and bath are obtained with a periodic density functional theory calculation. Trapping probabilities are obtained as a function of the hydrogen atom kinetic energy. These results are discussed in the light of the experimental hydrogenation studies performed on graphite by Zecho et al. [J. Chem. Phys. 117, 8486 (2002)] and on graphene by Haberer et al. [Adv. Mater. 23, 4497 (2011)].

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Electronic properties of graphite in tilted magnetic fields

The minimal nearest-neighbor tight-binding model with the Peierls substitution is employed to describe the electronic structure of Bernal-stacked graphite subject to tilted magnetic fields. We show that while the presence of the in-plane component of the magnetic field has a negligible effect on the Landau level structure at the K point of the graphite Brillouin zone, at the H point it leads to the experimentally observable splitting of Landau levels which grows approximately linearly with the in-plane field intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interaction of the high energy deuterons with the graphite target in the plasma focus devices based on Lee model

Numerical experiments are systematically carried out using the Lee model code extended to compute the ion beams on various plasma focus devices operated with Deuterium gas. The deuteron beam properties of the plasma focus are studied for low and high energy plasma focus device. The energy spectral distribution for deuteron ions ejected from the pinch plasma is calculated and the ion numbers with energy around 1 MeV is then determined. The deuteron–graphite target interaction is studied for different conditions. The yield of the reaction {sup 12}C(d,n){sup 13}N and the induced radioactivity for one and multi shots plasma focus devices in the graphite solid target is investigated. Our results present the optimized high energy repetitive plasma focus devices as an alternative to accelerators for the production of {sup 13}N short lived radioisotopes. However, technical challenges await solutions on two fronts: (a) operation of plasma focus machines at high rep rates for a sufficient period of time (b) design of durable targets that can take the thermal load.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗