Graphite Matrix Research Articles

Current Status of Purification of Indonesian Natural Graphite as a Candidate for Nuclear Fuel Matrix by Hydrometallurgy and Pyrometallurgy Methods

Indonesia has natural graphite reserved in Sulawesi, Sumatera and Kalimantan islands. The highest graphite content was observed in Sanggau Region, West Kalimantan. Indonesian natural graphite has the potential to be used as fuel matrix in Pebble Bed Reactor (PBR) type - High Temperature Gas Cooled Reactor (HTGR). To increase self-reliance of graphite supply and decrease graphite import for fuel matrix purposes, it is necessary to master the purification technology of graphite. Graphite matrix in Pebble Bed Reactor (PBR) fuel has an important role. It is not only used as neutron moderator and structural material to protect nuclear fuel but also as heat transfer media. Therefore, the graphite matrix must meet the physical and chemical criteria specified for PBR fuel. This paper focuses on current status of purification of Indonesian natural graphite as a candidate for nuclear fuel matrix using hydrometallurgy and pyrometallurgy. Acid and acid mixtures such as HF, HNO3+H2SO4 and HF+HCl+H2SO4 were used for the hydromet-allurgy purification process, while Arc Plasma System were used for pyrometallurgy methods. Characterization for the purified graphite includes physical and chemical properties. The result showed that Indonesian natural graphite obtained from hydrometallurgy was classified into low nuclear grade graphite but it still could be used as nuclear fuel matrix PBR - HTGR. The graphitization degree needs to be increased and the impurity content needs to be decreased for the purified Indonesian graphite using hydrometallurgy method. The Indonesian natural graphite obtained from preliminary pyrometallurgy methods has 96.78 % graphite content according to SEM (surface) observation and the graphite content needs to be increased into nuclear grade graphite.

The Release of C-14 of HTR Graphite Spheres during High Temperature Oxidation

As the key components of the HTGR fuel, the intact TRISO-coated particles contain almost all fission products under the normal and accident conditions in the reactor. On the other hand, the high temperature oxidation of irradiated fuel pebbles is one of feasible processing methods for HTR spent fuel pebbles. The knowledge of the C-14 release from irradiated fuels and the migration behavior during high temperature oxidation is of great importance for the post-irradiation examinations (PIE) and long-term storage and reprocessing of HTR-PM fuel pebbles. In this work, the C-14 release behavior from one irradiated HTR graphite sphere, which was used as a substitute for a irradiated HTR fuel, was determined by out-of-pile examination. The irradiated HTR fuel was oxidized in the hot lab of INET, and the substantial migration of fission products from graphite matrix was adsorbed and systematically studied by γ-scanning. Specifically, the C-14 release during oxidation was analyzed and illustrated. This work will provide theory and experiment basement for spent fuel disposal of HTGR, as well as a new sample preparation method for C-14 measurement by liquid scintillation counter (LSC).

Comparison of oxidation resistance behavior between Y3+ and Zr4+ modified aluminum phosphate impregnated graphite

To provide an oxidation protection system for graphite at low and medium temperatures (<1000 °C), aluminum metaphosphate (Al(PO3)3) coating was prepared on graphite surface by solution impregnation technology. Y3+ and Zr4+ were added respectively to improve the oxidation resistance of the coating, and the optimum additive amount of them was explored. The results indicate that the addition of Y3+ and Zr4+ can further inhibit the volatilization of P2O5. Furthermore, the Y3+ can improve the wettability of the aluminum phosphate impregnation solution on graphite surface, and the oxidation resistance of the modified coating is better below 800 °C; the Zr4+ can reduce the coefficient of thermal expansion of the aluminum phosphate coating, which shows better oxidation resistance above 800 °C. The impregnation solution modified by Y3+ diffuses into the graphite, and the surface coating plays the role of antioxidants together with the graphite matrix. While the Zr4+ modified impregnation solution mainly forms a coating on the graphite surface, and the coating plays the major antioxidation effect.

Transient Finite Element Simulation of a Lithium-Ion Battery Pack Thermal Management System Based on Latent Heat System Materials

High-intensity use of lithium-ion battery packs can lead to overheating. When heat generated from battery use exceeds the rate at which its surroundings dissipate the heat, thermal runaway occurs. 12-cell and 84-cell assemblies of lithium-ion NCR18650B batteries exceeded the manufacturer maximum temperature after a discharge at constant power, 1P. To passively regulate the temperature of the battery packs, a phase-change material was used as a latent heat system, transferring heat from the cells to the material during operation. Utilizing ANSYS, finite element analysis simulations were performed to show the advantage of an expanded graphite matrix infiltrated with paraffin wax. Material properties of the mixture were studied with differential scanning calorimetry and an ideal mixture of 25% EG and 75% PureTemp 60 paraffin wax was found. The results verified that the mixture significantly lowered the temperature of the cells; providing a cost efficient, accessible solution to counter increased operating temperatures.

Consecutive Heat Treatment of Pre-lathed Pebble Fuel Elements for HTGR

In current manufacture process of pebble fuel elements for HTGR, the green pebbles were first carbonized, then lathed and purified at high temperature. The whole heat treatment process included two cycles of heating and cooling and corresponding two loading and unloading processes, which took approximately 90 hours in total. In order to reduce the time and energy consumed and improve the efficiency during the heat treatment process, a consecutive heat treatment process of pebble fuel elements for HTGR was established. In the newly established consecutive heat treatment process, the pebble fuel elements were carbonized and high temperature purified continuously in a specific furnace. The consecutive heat treatment process took less than 60 hours, which included only one cycle of heating and cooling and corresponding one loading and unloading process. Moreover, in order to recycle the matrix graphite powder after lathing, the green pebbles were machined to a certain size before suffering the heat treatment. However, in order to make the size of pebble fuel elements after the consecutive heat treatment meet the technical requirements, it is necessary to figure out their dimension changes during the heat treatment. As the pebbles were prepared by cold quasi-isostatic molding method, their dimension changes parallel and perpendicular to the molding direction were different. In order to better control the dimension changes of pebbles in the heat treatment process, the effects of physical properties such as apparent density, and particle size distribution of matrix graphite powder on the prepared pebbles were comparatively studied. Finally, the consecutive heat treatment of pre-lathed pebble fuel elements for HTGR was established. The comprehensive properties of pebbles prepared with the newly established consecutive heat treatment process satisfied all the technical requirements.

Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon

Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon

Dynamic Response of Graphitic Targets with Tantalum Cores Impacted by Pulsed 440-GeV Proton Beams

Various graphite targets with a tantalum core were exposed to 440 GeV pulsed proton beams at the HiRadMat facility at CERN. The dynamic response was investigated by monitoring the surface velocity of the samples by laser Doppler vibrometry. The study comprises different graphite grades, such as polycrystalline, expanded and carbon-fiber reinforced graphite, and low-density graphitic foams, all candidates for beam-intercepting devices in high-power accelerators. The purpose of the tantalum core is to concentrate the large energy deposition in this high-density material that withstands the localized beam-induced temperature spike. The generated pressure waves are estimated to result in stresses of several hundred MPa which subsequently couple with the surrounding graphite materials where they are damped. Spatial energy deposition profiles were obtained by the Monte Carlo code FLUKA and the dynamic response was modelled using the implicit code ANSYS. Using advanced post-processing techniques, such as fast Fourier transformation and continuous wavelet transformation, different pressure wave components are identified and their contribution to the overall dynamic response of a two-body target and their failure mode are discussed. We show that selected low-intensity beam impacts can be simulated using straight-forward transient coupled thermal/structural implicit simulations. Carbon-fiber reinforced graphites exhibit large (macroscopic) mechanical strength, while their low-strength graphite matrix is identified as a potential source of failure. The dynamic response of low-density graphitic foams is surprisingly favourable, indicating promising properties for the application as high-power beam dump material.

Oxidation behaviors of matrix-grade graphite during water vapor ingress accidents for high temperature gas-cooled reactors

Water ingress into cores of high temperature gas-cooled reactors (HTGRs) can cause serious safety problems in graphitic components, especially for matrix graphite, which is more vulnerable than nuclear graphite. In this study, oxidation behaviors of a matrix-grade graphite, ARB-B1, were investigated under simulated water ingress accidental conditions. The oxidation tests were conducted in water vapor/helium mixed atmospheres at elevated temperatures up to 1200 °C. Oxidation rates and activation energy were evaluated by mass loss measurements. The activation energy was 121.5 kJ/mol in the considered temperature range, indicating only one oxidation regime−in-pore diffusion assisted by cracks. The cracks originated from the fabrication process, acting as penetration paths for the oxidant, played an important role in the oxidation behaviors. Ungraphitized binder oxidation occurred at all temperatures, while apparent oxidation of filler particles started from 1100 °C. Moreover, oxidation effects on compressive properties and hardness of the matrix graphite were discussed in terms of oxidized microstructures. This first detailed study on matrix graphite oxidation in water vapor provides important kinetics knowledge and microstructural and mechanical data for accidental scenarios of HTGRs as well as for development of new matrix graphite materials.

Formation of nanocrystalline graphite in polymer-derived SiCN by polymer infiltration and pyrolysis at a low temperature

The microstructure of polymer-derived ceramics (PDCs) was closely related to processing. This study demonstrated that SiCN matrix prepared by polymer infiltration and pyrolysis (PIP) at 900 °C inside a Si3N4 whisker (Si3N4w) preform with submicro-sized pores differed from its powder-consolidated analogue in both the content and structure of free carbon. Chemical analysis showed that PIP process had a higher free carbon yield. Raman spectroscopy and transmission electron microscopy (TEM) observation discovered a higher graphitization degree of free carbon and the existence of nanocrystalline graphite in SiCN matrix. Dielectric properties of Si3N4w/SiCN composites were greatly enhanced when volume fraction of SiCN matrix reached 24.5% due to dielectric percolation caused by highly-lossy free carbon. Reconsolidation of hydrocarbon released during pyrolysis by gas-state carbonization in Si3N4 whisker preform was supposed to account for the high yield and graphitization degree of free carbon in PIP process.

Modeling Interface Debonding in Coated Fuel Particles with BISON

This work introduces a meshed debonding model intended for use with tristructural isotropic (TRISO) fuel particles. The ability to better examine the potential effects of interface debonding between the coating layers will enable a host of investigations of the stress state when complex loading and interactions between coated fuel particles are taking place. In this work, the debonding model is discussed in detail. Then, simulations are presented to show the capabilities and limitations of the model. First, a traditional high-temperature gas reactor TRISO particle is considered. Then, a TRISO particle in a graphitic matrix is analyzed, followed by a TRISO particle in a silicon carbide matrix.

Brønsted acid functionalized carbon catalyst for synthesis of biologically active coumarin-substituted bis(indolyl)methanes

A novel carbon based catalyst has been prepared for the synthesis of potential anticancer compounds with low toxicity by an ionic liquid assisted doping process. Sulphonic acid functionalized nitrogen sulphur co-doped graphite (SO3H-NSG) based catalyst was prepared by coating the carbon surface with the sulphonic acid bearing ionic liquid and annealing the resulting compound. X-ray photoelectron spectroscopy revealed the successful doping of nitrogen and sulphur atoms in the graphitic matrix and the functionalization of the surface with sulphonic acidic sites. The catalyst was used for the synthesis of biologically active coumarin-substituted bis(indolyl)methanes that were finally evaluated for their toxicity and anticancer properties. The cytotoxicity results unveiled that the iodo derivative of the coumarin-substituted bis(indolyl)methanes showed best anti-cancer activity among the four derivatives synthesized.

Recycling of photovoltaic silicon waste for high-performance porous silicon/silver/carbon/graphite anode.

The rapid development photovoltaic industry has generated a huge amount of waste ultra-fine silicon cutting powder. The management and value-added recovery of silicon cutting waste is highly important for both environmental remediation and economic efficiency. In this work, silicon waste was used as a cost-effective raw material for the preparation of silicon/graphite anode for lithium-ion batteries. First, porous Si embedded with Ag particles (pSi/Ag) was produced by silver-assisted chemical etching (Ag-ACE). Then, pSi/Ag was loaded on a micron-sized graphite matrix (pSi/Ag/G), and organic carbon (C) produced by the pyrolysis of polyvinylpyrrolidone (PVP) acted as a link to closely connect pSi/Ag and graphite to form the pSi/Ag/C/G composite. The incorporated Ag particles and the porous structure improve electron transfer and mitigate the volume expansion effect of silicon. The novel design and structure of the anode can maintain the integrity of the electrode during cycling, and thus strongly improve cycling stability. The prepared pSi/Ag/C/G composite exhibited a large initial discharge capacity of 2353 mAh/g at 0.5 A/g and good initial coulombic efficiency of 83%, delivering a high capacity of 972 mAh/g at 1 A/g after 200 cycles. This work confirmed the possibility of the preparation of lithium battery silicon-carbon anode from silicon waste and provides a promising new avenue for value-added utilization of silicon cutting waste materials.

Nano germanium incorporated thin graphite nanoplatelets: A novel germanium based lithium-ion battery anode with enhanced electrochemical performance

First-principles calculations forecast that germanium incorporated graphite has potential to be used as high performance negative electrode for lithium-ion batteries due to large mobility of electrons at the heterointerface. Inspired by this first-principles calculations, a novel functional structure of thin graphite nanoplatelets embedding a finely dispersed germanium component is fabricated through a scalable ball milling technique for the first time. Nitrogen adsorption-desorption isotherms and electron microscopy investigation demonstrate that the obtained electrode materials exhibit mesoporous features where germanium nanoparticles are implanted into the graphitic matrix of thin flakes. Such a novel structure provides several advantages including large surface area, high accessibility of lithium ions and shortened lithium ion diffusion distances, which enable fast ion and electron transfer at the interface of electrolyte and electrode. As results, the obtained electrode demonstrates promising electrochemical performance with a charge capacity of 822 mAh g−1 after 200 cycles at 0.1 C, high rate capacity of 325 mAh g−1 at 10 C and good cyclic stability with a capacity retention of 97%. This work delivers a robust electrode design strategy as well as the most practical route for the improvement of germanium-based anodes for application in lithium-ion batteries.

Structural and radiological characterization of irradiated RBMK-1500 reactor graphite

This study aims to characterize the irradiated RBMK-1500 nuclear graphite in terms of both structural and radiological properties. The experimental results of morphological and structural analysis of the irradiated graphite samples by using SEM, Raman spectroscopy as well as the theoretical evaluation of primary displacement damage are presented. Moreover, the experimental and theoretical evaluation of the neutron flux is provided and the presence of several γ emitters in the analyzed graphite samples is assessed. Furthermore, the improved version of rapid analysis method for 14C activity determination is applied and the experimentally obtained results are compared with calculated ones. Results indicate that structural changes are uniform enough in all the analyzed samples. However, the distribution of radionuclides is non-homogeneous in the irradiated RBMK-1500 reactor graphite matrix. The comprehensive understanding of both structural and radiological characteristics of nuclear graphite is very important when dealing with decision about irradiated graphite waste management strategy or treatment options prior to its final disposal.

Preliminary three-dimensional neutronic analysis of IFBA coated TRISO fuel particles in prismatic-core advanced high temperature reactor

In this study, preliminary neutronic calculations were performed in order to provide a basic design of a prismatic-core advanced high temperature reactor (PAHTR) that adapts a mixture of lithium fluoride and beryllium fluoride (Li2BeF4, known as FLiBe) as a coolant. This work investigates the effectiveness of utilizing an integral fuel burnable absorber (IFBA) as an additional coating layer for the outer surface of the fuel kernel of the tri-structural isotropic (TRISO) fuel particle. A parametric study was performed to attain a targeted cycle length of 60 months and a fuel burnup greater than 90 MWd/kgU for fuel unit cell, and 70 MWd/kgU and 45 MWd/kgU for 2D and 3D full reactor core, respectively, with a uranium enrichment of 19.75 w/o to support non-proliferation goals. This paper discusses the idea of applying homogeneous distribution of IFBA as an additional coating layer with different thicknesses through the reactor core and perform preliminary neutronics analysis on the reactivity control mechanism for both two- and three-dimensionals core. The Monte Carlo Serpent code has been utilized to estimate the core reactivity balance using the cycle length by reducing the burnup swing between the beginning of cycle (BOC) and end of cycle (EOC). This can be done by adopting the innovative idea of covering the outer surface of the fuel kernel with a thin coating layer of IFBA. This technique is utilized in the 3D evaluation of the PAHTR reactor. Additionally, the neutron spectrum, peaking factor (axial and radial), and multiplication factor for the 3D PAHTR reactor core at the BOC, middle of the cycle (MOC), and EOC were compared by taking into account the double heterogeneity nature of the TRISO fuel particles embedded in the graphite matrix.

Highly conductive graphite matrix/copper composites by a pressureless infiltration method

Graphite matrix/copper composites have found significant potential uses in numerous areas. Until now, a high ambient gas pressure has to be applied to overcome the non-wettability between the copper melt and graphite matrix, which has restricted the technological and economic efficiencies, as well as the composite performance. In this paper, we have proposed a pressureless infiltration strategy to realize the preparation of highly conductive graphite matrix/copper composites. A tungsten carbide network is first in situ constructed in the graphite matrix, transforming the capillary force from the resistance mode to the impetus mode. Results show that the contact angle of graphite/copper decreased from 138.5° to 23° due to the constructed tungsten carbide network. The composites exhibit good interfacial bonding, and the electrical conductivity reaches 15.1 × 105 S/m, far higher than the requirement of the industry standard.

Tribological characteristics of Al6061, boron, and graphite hybrid metal matrix composites

ABSTRACT Tribological properties of any substance give an overview of the life of the substance with respect to its application. Here an attempt is made to investigate the effect of parameters of wear test equipment considering the material type on the wear rate of different composition of hybrid MMC considered. Al6061 is considered as the base matrix and Boron 2 wt% and Graphite 2 wt% are considered as the reinforcement to develop the hybrid MMC with the traditional technique of stir casting. As the work deals with optimisation process the design of experiments is done through Taguchi technique considering L27 orthogonal array with the varying parameters (input) as material (combination of Al-B, Al-Gr and Al-B-Gr), load and speed, to analyse the effects on wear rate as the response. A wear test was performed using pin-on disk apparatus at room temperature for constant load of 20 N, 30 N, 40 N at a fixed sliding speed of 400,600,800 RPM and wear rate increased as the weight percentage of reinforcement increased. The major contribution along with the ranks of the parameters was found out using the Analysis of Variance, from which it is clear that, Boron is the significant factor that affects the hardness and wear loss of hybrid composites followed by Graphite. Confirmatory test was performed for the optimised parameters and the results were within the acceptable range when compared with the experimental results.

Ablation mechanism of AlSi/graphite dissipative heat protective composites under an oxy-acetylene torch

To improve the ablation resistance of graphite, AlSi alloy was used as a dissipative agent to fill the pores of graphite matrix by pressure infiltration method to prepare AlSi/graphite composites. The microstructure evolution and ablation behavior of the obtained AlSi/graphite composites were investigated under oxy-acetylene flame after ablating for varying lengths of time. Subsequently, the effect of heat dissipation through the AlSi alloy was quantitatively analyzed by data processing technique. Experimental results showed that the AlSi alloy reduced the surface temperature by about 300 °C in the first 20 s, and the linear ablation rate of the AlSi/graphite composite is found to be 119.3% lower than that of the graphite itself ablated for 10 s. The strengthening of ablation resistance is mainly achieved by heat dissipation and oxygen dissipation. However, it gradually decreases with continuous consumption of the AlSi alloy during ablation. Among these, the contribution of heat dissipation to the ablation resistance gradually increased from 10.1% at 10 s to 62.1% at 60 s.

Solid-state phase transformation of ductile cast iron during electroslag remelting cladding

ABSTRACT A bimetallic composite roll with the cladding layer of high-speed steel (HSS) and the roll core of ductile cast iron (DCI) was manufactured by the electroslag remelting cladding (ESRC) technology, and the changes in the graphite morphology, matrix structure, and mechanical properties of the DCI core before and after the ESRC process were compared. The DCI core was first heated to a very high temperature and then cooled down to room temperature during ESRC which led to a deteriorative graphite morphology with a low spheroidisation rate, a large size, and a nonuniform distribution and also caused a solid-state phase transformation of the matrix structure from lamellar pearlite + few carbides to bainite + large amounts of carbides. Based on the changes of graphite sphericity, matrix structure, carbides content, the tensile strength and impact toughness of DCI before and after the ESRC process changed from 458 MPa, 3.02 J/cm2 to 417 MPa, 5.74 J/cm2, respectively.

Infiltration behavior of copper melt into porous graphite and saturation improvement by WC particles doping

ABSTRACT In graphite/copper composites, the intrinsic non-wettability of graphite and copper makes it difficult for the copper melt to realize effective infiltration into the porous graphite matrix. The resulted void defects have reduced the final performance of the composites. It is of great significance to improve the saturation of copper melt in the graphite skeleton, which requires a well understanding of the infiltration behavior of copper melt. In this paper, a two-phase flow finite element model was developed, and verified by previous published experimental data. Effects of the external pressure on the infiltration behavior has been studied, and then an optimization method based on WC particle doping was proposed. Results indicated that effective transfer of displacement pressure at the liquid flow front can reduce the void defects of composite materials. WC doping has reduced the capillary resistance, and effectively improved the copper saturation in the infiltration process from 80.4% to 90.7%. This work provided a new method to reduce the void defects of composites, which may be helpful for the development of high performance copper/graphite composites.