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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Dynamic Deformation in Nuclear Graphite and Underlying Mechanisms

Time-dependent deformation in nuclear graphite is influenced by the creation and migration of radiation-induced defects in the reactor environment. This study investigates the role of pre-existing defects such as point defect clusters and Mrozowski cracks in nuclear graphite IG-110. Separate specimens were irradiated with a 2.8 MeV Au 2+ beam with a fluence of 4.38 × 10 14 cm −2 and an 8 MeV C 2+ beam with a fluence of 1.24 × 10 16 cm −2 . Microscopic specimens were either mechanically loaded inside a transmission electron microscope (TEM) or subjected to ex situ indentation-based creep loading. In situ TEM tests showed significant plasticity in regions highly localized around the Mrozowski cracks, resembling slip or ripplocation bands. Slip bands were also seen near regions without pre-existing defects but at very high stresses. Ex situ self-ion irradiation embrittled the specimens and decreased the creep displacement and rate, while heavy ion irradiation resulted in the opposite behavior. We hypothesize that the large-sized gold ions (compared to the carbon atoms) induced interplanar swelling as well as cross-plane channels for increased defect mobility. These findings illustrate the role of pre-existing defects in the dynamic relaxation of stresses during irradiation and the need for more studies into the radiation environment’s impact on the mechanical response of nuclear graphite.

creep deformation↗
DOE OSTI2018

Highly Graphitized Carbon Coating on SiO with a π–π Stacking Precursor Polymer for High Performance Lithium-Ion Batteries

A highly graphitized carbon on a silicon monoxide (SiO) surface coating at low temperature, based on polymer precursor π–π stacking, was developed. A novel conductive and electrochemically stable carbon coating was rationally designed to modify the SiO anode materials by controlling the sintering of a conductive polymer, a pyrene-based homopolymer poly (1-pyrenemethyl methacrylate; PPy), which achieved high graphitization of the carbon layers at a low temperature and avoided silicon carbide formation and possible SiO material transformation. When evaluated as the anode of a lithium-ion battery (LIB), the carbon-coated SiO composite delivered a high discharge capacity of 2058.6 mAh/g at 0.05 C of the first formation cycle with an initial Coulombic efficiency (ICE) of 62.2%. After 50 cycles at 0.1 C, this electrode capacity was 1090.2 mAh/g (~82% capacity retention, relative to the capacity of the second cycle at 0.1 °C rate), and a specific capacity of 514.7 mAh/g was attained at 0.3 C after 500 cycles. Furthermore, the coin-type full cell composed of the carbon coated SiO composite anode and the Li[Ni0.5Co0.2Mn0.3O2] cathode attained excellent cycling performance. The results show the potential applications for using a π–π stacking polymer precursor to generate a highly graphitize coating for next-generation high-energy-density LIBs.

25 ENERGY STORAGE↗
NASA NTRS1961

Structural and High-Temperature Tensile Properties of Special Pitch-Coke Graphites

The room-temperature structural properties and the tensile properties up to 5000 F (275O C) were determined for ten grades of specially prepared petroleum-coke coal-tar-pitch graphites which were graphitized at 5430 F (3000 C). One impregnation with coal-tar pitch increased the bulk density from 1.41 to 1.57 g/cm3 and the maximum strength at 4500 F (2500 C) from 4000 to 5700 psi. None of the processing parameters studied had a marked effect on the closed porosity or the X-ray structure or the per cent graphitization. The coarse-particle filler resulted in the lowest coefficient of thermal expansion and the fine-particle filler in the highest coefficient. A marked improvement in uniformity of tensile strength was observed. A standard-deviation analysis gave a one-sigma value of approximately 150 psi for one of these special grades and values of 340-420 psi for three commercial grades.

GRAPHITE↗
NASA NTRS1995

Kinetics of the Formation of Intercalation Compounds in Crystalline Graphite

Crystalline graphite has a structure that can be best described as an ordered stack of flat aromatic layers. It is known to form intercalation compounds with bromine and nitric acid. Their formation was studied using thermal measurements and analytical techniques. Samples of graphite treated with either bromine or nitric acid were prepared by contacting these reagents with powdered graphite.

crystalline graphite intercalation compounds kinet↗
DOE OSTI2020

Graphite Baseline Status

Subtask Relevancy Inherent variation exists in nuclear-grade graphite due to processing techniques and raw material sources Comprehensive graphite properties database does not exist that fully establishes physical and mechanical property relationships Predictability of long-term behavior in a nuclear environment requires full analysis of variability Technical Approach, Accomplishments/Results Perform comprehensive mechanical and physical properties testing to populate a fully-validated database Grade selection is based upon candidate graphites from four international suppliers 15,000+ property values measured to date Physical properties testing that is based upon AGC geometries is a key component to establishing baseline/AGC data link

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Machining Graphite Specimens from Sub- Blocks for Baseline Characterization

The Advanced Reactor Technologies (ART) Baseline Graphite Characterization program at the Idaho National Laboratory (INL) is performing a comprehensive evaluation of inter-billet variations in as-produced nuclear-grade graphite. To accomplish this, over 500 mechanical test specimens are machined from a single billet. A typical graphite billet is approximately 6 feet long by 2 feet in diameter or 2 ft square and weighs approximately 1500 pounds. Initially the full-size billets are pre-sectioned into sub-blocks that can be easily handled and shipped. In this SOW, 44 pre-labeled sub-blocks will be machined into mechanical test specimens by the supplier. These sub-blocks are supplied and shipped by the purchaser to the supplier.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Sectioning of NBG-17 Graphite Billet

The Advanced Reactor Technologies (ART) Baseline Graphite Characterization program at the Idaho National Laboratory (INL) is performing a comprehensive evaluation of inter-billet variations in as-produced nuclear-grade graphite. Before graphite test specimens are machined, the full-size billets must be pre-sectioned into sub-blocks that can be more easily handled and shipped. As manufactured, NBG-17 billets are rectangular in shape, nearly 6 feet in length, and may exceed 1,500 pounds in weight. Sectioning of these billets must be accomplished via a supplier that is capable of handling and cutting the billets into individual slabs and quartering these slabs into sub-blocks. Billets will be pre-labeled by the Purchaser prior to leaving controlled storage such that traceability of any individual sub-block is never lost during sectioning operations.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Graphite for Advanced Nuclear Reactors: Deployment Readiness Review

Historically, graphite has been used in numerous reactor technologies, including research/test reactors and, commercially, in advanced gas reactors. These non-metallic materials can play a key role as internal core structures, reflectors, or neutron moderators, making them important for deployment of certain advanced reactor technologies. This report explores the industry readiness for graphite material deployment, documenting Codes and Standards applicable to their design, qualification, and manufacturing. Discussions also examine the manufacturing processes, aging/degradation, inspection techniques, and disposal options. The report covers the current status, documents gaps, and proposes some conclusions about approaches to managing some of the current gaps.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Interesting Graphite Facts

Interesting facts about graphite. Graphite within HTRs and how long they will last, potential issues prediction, model building to predict expected behavior, and frequent and thorough inspection and monitoring strategy.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Graphitic‐Shell Encapsulation of Metal Electrocatalysts for Oxygen Evolution, Oxygen Reduction, and Hydrogen Evolution in Alkaline Solution

Abstract Developing highly efficient, cost effective, and environmentally friendly electrocatalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) is of interest for sustainable and clean energy technologies, including metal–air batteries and fuel cells. In this work, the screening of electrocatalytic activities of a series of single metallic iron, cobalt, and nickel nanoparticles and their binary and ternary alloys encapsulated in a graphitic carbon shell toward the OER, ORR, and HER in alkaline media is reported. Synthesis of these compounds proceeds by a two‐step sol–gel and carbothermal reduction procedure. Various ex situ characterizations show that with harsh electrochemical activation, the graphitic shell undergoes an electrochemical exfoliation. The modified electronic properties of the remaining graphene layers prevent their exfoliation, protect the bulk of the metallic cores, and participate in the electrocatalysis. The amount of near‐surface, higher‐oxidation‐state metals in the as‐prepared samples increases with electrochemical cycling, indicating that some metallic nanoparticles are not adequately encased within the graphite shell. Such surface oxide species provide secondary active sites for the electrocatalytic activities. The Ni–Fe binary system gives the most promising results for the OER, and the Co–Fe binary system shows the most promise for the ORR and HER.

Khani, Hadi↗
DOE OSTI2024

A Comprehensive Experimental Study on Microstructure‐Graded Graphite Anodes for Enhancing Fast‐Charging Capability of Lithium‐Ion Batteries

Lithium‐ion batteries with high gravimetric capacity density and improved cycle life performance under fast‐charging conditions are crucial for widespread electric vehicle (EV) adoption. This study investigates how designing graphite anode microstructure, specifically porosity, and particle‐size gradients, improves lithium‐ion (Li + ) transport during fast‐charging conditions. Three‐layered graphite anodes with varying porosity (24%, 36%, 46%) and particle size gradients (3, 5, 10 μm) were compared to a conventional single‐layered electrode in half‐cell configurations. At room temperature and high discharge rate (2C), both gradient structures showed significantly enhanced capacity retention (80% and 67% vs. 50%) compared to the conventional electrode, highlighting the effectiveness of microstructure engineering for fast charging. The study also investigated the temperature's impact on cycle life. After 200 cycles at 2°C and 45°C, all gradient structures demonstrated superior capacity retention (≈80%) compared to the conventional electrode (35%), suggesting the gradients mitigate degradation rate at high temperatures. Electrochemical impedance spectroscopy confirmed superior Li+ diffusion and lower resistivity in gradient electrodes. Simulations explored the influence of gradient profiles on reaction kinetics across the electrode thickness. Overall, this research demonstrates that the fast‐charging capability of graphite electrodes can be greatly enhanced by engineering the electrode microstructure, thereby making EV technology more accessible and appealing.

Ahmadi, Soma↗
DOE OSTI2022

An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (k f or k b ) as a function of applied potential (E-E 0 '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k 0 , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.

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

Electronic structure changes upon lithium intercalation into graphite – Insights from ex situ and operando x-ray Raman spectroscopy

This study probes the electrochemical intercalation mechanism of Li into graphitic carbon using x-ray Raman spectroscopy (XRS), an inelastic x-ray scattering technique. Operando and high resolution spectra of electrochemically lithiated composite graphitic electrodes at discrete states of Li uptake (stages III, II and I) show gradually changing spectral features with Li intercalation, the end state agreeing well with chemically fully lithiated LiC 6 highly oriented pyrolytic graphite (HOPG). The two most dominant changes in the XRS spectrum of the C-K edge are the reduced intensity of the $\pi$* peak and shift the onset of the σ* states to lower energies upon full lithiation. The excellent instrumental energy resolution uncovered novel spectral features and, thus, electronic changes with varying lithium content. Thus, the general spectral changes with progressing Li intercalation agree well with our accompanying DFT calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

In-situ study of the carbon gasification reaction of highly oriented pyrolytic graphite promoted by cobalt oxides and the novel nanostructures appeared after reaction

Cobalt interaction and its effects on carbon-based systems at the nanoscale have recently attracted much attention in different fields, such as catalysis of carbon nanotubes or graphene and graphite nano-patterning taking advantage of its ferromagnetic behavior. Experiments performed in our laboratories show how the re-oxidation process of two equivalent monolayers of CoO deposited on highly oriented pyrolytic graphite at 400 °C leads to the formation of nanochannels at lower temperature than using other methods. Here we present the in-situ characterization of the carbon gasification reaction that drives this process by means of near ambient pressure X-ray photoelectron spectroscopy performed at the ALBA synchrotron facility. The reason why this reaction takes place at such low temperature compared to other methods is due to the weakening of the carbon σ bonds by the initial CoO wetting layer formed at the early stages of growth on the graphite surface. Besides nanochannels, ex-situ atomic force microscopy measurements also show the appearance of two more kinds of nanostructures: nano-strips and nano-rings. The appearance of these nanostructures reveals the impressive modification of the surface after the re-oxidation process mediated by the cobalt oxide.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Vacancy-driven variations in the phonon density of states of fast neutron irradiated nuclear graphite

Research examining radiation damage and its effects in graphite began in the 1940’s during the development of moderated nuclear reactors. Interest in this topic is expanding because of emerging applications associated with fullerenes and carbon nanostructures as well as its long-standing use as a fission moderator. In this work, we report the measurements of the full phonon density of states of irradiated nuclear graphite using inelastic neutron scattering experiments at room temperature. The samples were previously exposed to different levels of neutron doses and irradiation temperatures. The phonon density of states of perfect and defected graphite supercells including different configurations of vacancies and interstitials were calculated using first-principles direct method. Despite high neutron irradiation doses, the scattering measurements show that the induced damage is localized, and the layered structure is preserved. In conclusion, a comparison of the measured phonon densities of states of irradiated samples with those calculated of defected supercells indicates that the main changes observed are mainly attributed to formation of vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2024

3D printed carbon fiber reinforced carbon as an energy efficient alternative to graphite for EFAS tooling

As Electric Field Assisted Sintering (EFAS) gains more industrial acceptance and use, it becomes more important to develop more efficient means to implement this technology. To this aim, 3D printed continuous carbon fiber reinforced carbon (CCC) was manufactured and fabricated into tooling for EFAS systems as an alternative to traditional graphite tooling. The impact of fiber orientation on the thermal and electrical properties of the CCC was characterized. Sample material was sintered in Tokai G535 graphite tooling, under common processing conditions and compared with CCC tooling. There was nearly 50 % energy savings compared to graphite while maintaining equivalent sample density and microstructure plus keeping ram temperatures 39 % cooler. This is due to spatial control of generated heat and thermal diffusivity within the molds, by means of fiber orientation anisotropy. Finite element modeling of the tooling design supported the experimental results as well as displays the effect of optimization of this 3D printed CCC material.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Benchmarking GEANT4 and PHITS for 14.8-MeV neutron transport in polyethylene and graphite materials

The feasibility of GEANT4 and PHITS with ENDF/B-VIII.0, JEFF-3.3 and JENDL-4.0 evaluated nuclear data libraries in 14.8-MeV neutron transport simulation are verified by comparing with the MCNP calculations and the experimental data. Here the measured data are from neutron integral experiments including graphite and polyethylene samples. For 6 cm-thick polyethylene and 20 cm-thick graphite samples, the results from GEANT4 and PHITS with three evaluated nuclear data libraries agree well with the MCNP calculations. For a 2 cm-thick graphite sample, the simulated results from GEANT4 and PHITS agree with the MCNP calculations, and they all underestimate the experiment data in below 3 MeV and overestimate them in the energy range of 5–7 MeV. The difference between the simulation results and the experimental data depends on the evaluated nuclear data libraries. In overall, GEANT4 and PHITS are performing reasonable jobs for 14.8-MeV neutron transport simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗