Graphite Balls Research Articles

Simple, low-cost and high-quality fabrication of GNPs@Ti6Al4V powders for high-performance composites Inspired by pearl polishing

Uniformly dispersing graphene in matrices is challenging, which is the key to achieving high performance for metal matrix composites (MMCs) reinforced with graphene. Inspired by the process of polishing pearls, this work utilizes a vibratory polisher to mix high-purity graphite balls and Ti6Al4V powders for in-situ fabrication and homogeneous dispersion of graphene nanoplatelets (GNPs), by which the GNPs are exfoliated from the graphite balls via the friction force, and synchronously stick to the powders in one step. Whereby, the composite powders consisting of Ti6Al4V powders patched with GNPs were formed and then consolidated by spark plasma sintering. The 0.36 wt% GNPs reinforced Ti6Al4V matrix composite exhibits a tensile yield strength of 1010.8 MPa and an ultimate strength of 1075.3 MPa respectively, while possessing good ductility as high as 14.0 %. This work provides a simple and practical approach for the in-situ fabrication and uniform dispersion of GNPs in MMCs.

Spherical image acquisition and defect detection of nuclear fuel based on the line scan

Graphite balls are used as matrix materials of nuclear fuel. To ensure the safe use of nuclear energy, it is necessary to detect the surface defects of graphite balls to screen out the balls that do not meet safety requirements. However, due to the small surface defects and complex background of graphite balls, the detection accuracy based on traditional detection methods is difficult to meet engineering requirements. To solve the above problems, this paper designs an image acquisition device for graphite balls based on line scanning to collect spherical images and designs an improved algorithm based on the U2-Netp network to solve the problem of low accuracy of defect detection. Firstly, the algorithm replaces the up-sampling in the RSU module with cubic linear interpolation to enhance the smoothness of the image. Secondly, spatial channel attention and multi-feature fusion modules are built between the encoder and the decoder to improve the accuracy of the model to obtain interesting information. Finally, the prediction output of Sigmoid and Threshold fusion is designed to solve the problems caused by the gradient disappearance and the uneven polarity of positive and negative samples. Tested on the actual collected data set, the improved U2-Netp algorithm improved the average accuracy of detecting surface defects on graphite balls by 8.1% compared to the original U2-Netp algorithm, reaching 85.3%.

Microstructure evolution and tensile properties tailoring of graphene nanoplatelets/nickel composites fabricated by two-step 3D vibration milling

Uniformly dispersing graphene nanoplatelets (GNPs) with well-preserved structure into matrix is essential for pursuing high-performance nickel (Ni) based composites reinforced by GNPs. In this study, a two-step 3D vibration milling strategy was proposed to prepare GNPs/Ni composites with evenly-dispersed and slightly damaged GNPs. During step I milling, GNPs were exfoliated by Ni powders from graphite balls and initially distributed on powder surfaces synchronously. Followed by step II milling with zirconia balls, the uniform dispersion and thinner thickness of exfoliated GNPs were promoted, while the increased damage to GNPs was induced at higher vibration frequencies. The improved milling frequency boosted deformation of Ni powders and resulted in fine-grained matrix. The GNPs/Ni composites milled at 15 Hz in step II demonstrated a combination between homogeneously distributed GNPs with less defects and refined grains, thus actualizing the superior tensile strength (592 MPa) and ductility (30.6%). Therefore, it is feasible to tailor the microstructure and achieve exceptional mechanical properties for GNPs reinforced metal matrix composites by adjusting proper frequency in the two-step 3D vibration milling strategy.

Microstructure and mechanical behaviour of TiCx-MLG-TiCx network reinforced Ti6Al4V fabricated with core-shell structure powders

In this work, the sandwiched TiCx-MLG-TiCx network reinforced Ti6Al4V composite (MLG@Ti6Al4V) was prepared with core-shell structure powders, in which Ti6Al4V core was anchored by multi-layer graphene (MLG) shell exfoliated from graphite balls via 3D vibration milling. After consolidation by spark plasma sintering (SPS), the robust interface is generated between MLG and Ti6Al4V matrix taking advantage of the in-situ synthesized TiCx (Ti8C5 + TiC), which favors an effective load transfer. The interlinked TiCx and MLG construct TiCx-MLG-TiCx network reinforcements that wrap around the matrix, endowing the composite with near equiaxed α phase and fine grains. The improved interface bonding, dislocation blocking and grain refinement provided by the TiCx-MLG-TiCx network result in an increased magnitude of 14.5% and 19.9% in the yield strength and ultimate strength, respectively, compared with monolithic Ti6Al4V. Also, the TiCx-MLG-TiCx network conduces to a well-maintained ductility of 10.7% for the composite associated with the deformation compatibility, the near equiaxed α phase as well as the fine grains.

Вплив нікелю і молібдену на структуру та хімічний склад поверхні руйнування чавуну з кулястим графітом

The results of the study of the influence of nickel and molybdenum on the structure and chemical composition of the fracture surface of cast iron with spherical graphite are presented in the paper. It is shown that the fracture of cast iron with spherical graphite occurs along the boundaries of the distribution of graphite balls with matrix, and the fracture of the matrix occurs both by the mechanism of intergranular and transgranular fractures. Molybdenum and nickel alloying changes the mechanism of transgranular fracture of the matrix from brittle for ordinary cast iron to viscous for molybdenum and nickel alloying. It is established that the fracture surfaces of cast iron, depending on the analysis places there are elements such as O, C, P, N, Cu, Ni, Si, Mg. Studies of the distribution of impurities in the near-surface layers of the destroyed samples have shown that the quantity of elements such as oxygen, phosphorus and nitrogen decreases with increasing distance from the fracture surface. The phosphorus quantity is reduced by 40 - 45% in the places of the cast iron matrix, where the graphite balls were located during the alloying of cast iron by molybdenum and nickel. Keywords: cast iron, nickel, molybdenum, alloying, Auger spectroscopy, chemical composition, structure, fracture, surface.

Uniformly dispersing GNPs for fabricating graphene-reinforced pure Ti matrix composites with enhanced strength and ductility

The uniform dispersion of graphene nanoplatelets (GNPs) in the titanium matrix is essential but challenging for preparing high-performance graphene-reinforced titanium matrix composites. This study conveniently dispersed GNPs uniformly in commercial pure titanium (CP Ti) powders by in-situ exfoliating GNPs from high-purity graphite balls via 3D vibration milling and fabricated graphene-reinforced titanium matrix composites (in-situ GNPs/Ti) with improved strength and ductility. Besides, the purchased GNPs were ex-situ introduced into CP Ti through 3D vibration milling as a comparison to investigate the effect of different carbon sources on the uniformity of GNPs distribution. Compared with CP Ti, the ultimate tensile strength, yield strength, elongation, and microhardness of the in-situ GNPs/Ti composite were improved by 42.8%, 20.2%, 12.6% and 9.6%, respectively. The improvements in mechanical properties were mainly attributed to the load transfer of the uniformly dispersed GNPs and the pinning effect of in-situ generated titanium carbide (TiC). Furthermore, it is benefited from considerable strengthening efficiency in this work, the high-purity graphite balls were verified to be a befitting carbon source for 3D vibration milling to introduce GNPs into composites. It could be generalized to fabricating other metal matrix composites reinforced with graphene.

Coaxial stacking disjoint model study of 10 MW high temperature gas-cooled reactor (HTR-10)

HTR-10 is quite different than conventional reactors due to its geometry, spherical fuel elements and random distribution of fuel elements and graphite balls. It is desired to built realistic and accurate reactor model. In this paper, the detailed mathematic geometrical relationships of HTR-10 are studied on HTR-10′s coaxial stacking disjoint model. The geometrical model of the full reactor is obtained by using lattice and universe facilities provided by SuperMC. Compared with available data, this model can achieve accurate results in terms of initial criticality of the HTR-10 reactor.

Microstructure and tribological properties of ZrO2(Y2O3)–Al2O3-graphite composite ceramic fabricated by milling with graphite balls

Abstract In this study, ZrO 2 (Y 2 O 3 )–Al 2 O 3 -graphite composite powder was prepared by a mechanical milling method and a direct powder doping milling method, and sintered in a microwave. Microstructure and tribological properties of the composites were investigated. The results showed that uniformly dispersed fine graphite flakes were obtained using the mechanical milling method. ZrC phase was generated in the composite ceramics. The tribological properties of ZrO 2 (Y 2 O 3 )–Al 2 O 3 - graphite composites prepared by the mechanical milling method were better than those of ZrO 2 (Y 2 O 3 )–Al 2 O 3 -graphite composites prepared by the direct powder doping milling method. The friction coefficient and specific wear rate of ZrO 2 (Y 2 O 3 )–Al 2 O 3 -graphite composites by the mechanical milling method were 0.147 and 6.2 × 10 −6 mm 3 /(N m), respectively. The wear mechanisms of ZrO 2 (Y 2 O 3 ) ceramic matrix composites were discussed.

Percolative Composites with Carbon Nanohorns: Low-Frequency and Ultra-High Frequency Response.

We systematically studied the electromagnetic properties of carbon nanohorns (CNHs) and polystyrene composites filled with CNHs in static regime, low frequency and microwave regions. CNHs were synthesized using the direct current arc-discharge method using solid graphite rods and graphite rods filled by melamine mixed with graphite powder. Transmission electron microscopy and thermo-gravimetric analysis showed that CNH agglomerates are the main product, while the addition of melamine promotes the formation of graphite balls. Graphitic contamination causes the internal leakage of inter-agglomerate capacity, lowering the permittivity and enhancing the conductivity of composites. The permittivity of CNH/polystyrene composites increases with the filler fraction, and near the dielectric threshold electromagnetic characteristics of the composites exhibit critical behaviour. Our results suggest that CNHs with relatively high values of permittivity and contact resistance could be used as high-k materials.

Criticality calculations of the HTR-10 pebble-bed reactor with SCALE6/CSAS6 and MCNP5

HTR-10 is a 10MWt prototype pebble-bed reactor (PBR) that presents a doubly heterogeneous geometry for neutronics calculations. An appropriate unit-cell treatment for the associated fuel elements is vital for creating problem-dependent multigroup cross sections. Considering four unit-cell options for resonance self-shielding correction in SCALE6, a series of HTR-10 core models were established using the CSAS6 sequence to systematically investigate how they affected the computational accuracy and efficiency of PBR criticality calculations. Three core configurations, which ranged from simplified infinite lattices to a detailed geometry, were examined. Based on the same ENDF/B-VII.0 cross-section library, multigroup results were evaluated by comparing with continuous-energy SCALE6/CSAS6 and MCNP5 calculations. The comparison indicated that the INFHOMMEDIUM results overestimated the effective multiplication factor (keff) by about 2800pcm, whereas the LATTICECELL and MULTIREGION treatments overestimated keff values with similar biases at approximately 470–680pcm. The DOUBLEHET results attained further improvement, reducing the keff overestimation to approximately 280pcm. The comparison yielded two unexpected problems from using SCALE6/CSAS6 in HTR-10 criticality calculations. In particular, the continuous-energy CSAS6 calculations in this study present a non-negligible discrepancy with MCNP5, potentially causing a keff value overestimate of approximately 680pcm. Notably, using a cell-weighted mixture instead of an explicit model of individual TRISO particles in the pebble fuel zone does not shorten the CSAS6 computation time in the high-fidelity HTR-10 model because of the presence of graphite balls and the complex modeling of peripheral reflectors.

Preparation of spherical fuel elements for HTR-PM in INET

The spherical fuel elements were successfully manufactured in the period of HTR-10. In order to satisfy the mass production of fuel elements for HTR-PM, several measures have been taken in modifying and optimizing the manufacture process of fuel elements. The newly developed overcoater system and its corresponding parameters exhibited good stability and high efficiency in the preparation of overcoated particles. The optimized carbonization process could reduce the carbonization time from more than 70h to 20h and improve the manufacturing efficiency. Properties of the manufactured spherical fuel elements and matrix graphite balls met the design specifications for HTR-PM. The mean free uranium fraction of 5 consecutive batches was 8.7×10−6. The optimized fuel elements manufacturing process could meet the requirements of design specifications of spherical fuel elements for HTR-PM.

Detection of graphite balls for the fuel handling system in HTGR using eddy current testing

The spherical graphite fuel elements (graphite balls) are transported into and out of the core in high-temperature gas-cooled pebble bed reactors (HTGR) by a fuel handling system (FHS) during plant operation. In order to control the reactor, it is necessary to have information on the location and movement of the graphite balls at various points of the FHS. In this paper, a new through-transmission eddy current testing probe has been put forward to detect the graphite balls. The probe's electromagnetic finite element model (FEM) has been established so as to study the factors affecting the performance of the probe. On this basis, electromagnetic detection parameters and probe designs have been optimised. According to the results of analysis, a bidirectional detector for the fuel ball handling system of HTGR has been developed. The experimental results confirm the validity of the FEM, and show that the detector works accurately and reliably, which meets the requirements of application in HTGR.

Efficient Separation of Giant Graphite Balls from As-Grown Single-Wall Carbon Nanohorns

We show in this report an easy and efficient method of purifying single-wall carbon nanohorns (SWNHs). The major impurities of SWNHs are micrometer-order graphite-based particles, named giant graphite (GG) balls, the content of which is within 5–15 wt %. GG balls cannot be removed by combustion because they are burned at higher temperature than SWNHs. However, we found that gravitational sedimentation is very effective for the removal of GG balls: As-grown SWNHs were dispersed in ethanol using a bath-type ultrasonicator, and then the dispersion was left to stand for 17 h. GG balls settled down, while SWNHs dispersed in the supernatant. The purity of the SWNHs in the supernatant was almost 100%, as estimated by thermogravimetric analysis. This separation was understandable because Brownian motion more predominantly affects the motion of SWNH aggregates with a smaller size and a lower density, while gravitational sedimentation predominantly affects the movement of GG balls with a higher density and larger size. Since gravitational sedimentation is simple and convenient, it can be easily applied to the large-scale purification of SWNHs.

Purification and separation of carbon nanocapsules as a magnetic carrier for drug delivery systems

Nanoparticles of iron carbides wrapped in multilayered graphitic sheets (carbon nanocapsules) were synthesized by electric plasma discharge in an ultrasonic cavitation field in liquid ethanol. The as-prepared material contained carbon nanocapsules but also included impurities such as amorphous carbon, graphite balls and flakes. These impurities must be removed to allow the development of applications for the carbon nanocapsules and, it is necessary to develop a suitable purification method to obtain samples of monodispersed carbon nanocapsules. Two purification methods were used. The first used a hydrogen peroxide solution and acid etching to remove the amorphous carbon and iron particles. These impurities were easily oxidized and dissolved in the solution and the graphite layers of the carbon nanocapsules were not damaged. The second used magnetic separation using permanent magnets to remove the graphite balls and flakes. Here, centrifugation was applied to separate the purified carbon nanocapsules around 100–200 nm in size.

Origin of giant graphite balls produced together with carbon nanohorns prepared by pulsed arc-discharge and a method for their removal

Origin of giant graphite balls produced together with carbon nanohorns prepared by pulsed arc-discharge and a method for their removal

Origin of giant graphite balls including in the carbon nanohorns prepared by pulsed arc-discharge and their removal method

Large sized spherical graphitic particles with the diameter as large as several micrometers, so called giant graphite balls (GGBs), are more or less included in the sample of carbon nanohorns (CNHs) which are prepared by the method of pulsed arc-discharge or laser ablation. For the use of CNHs in the application, it is needed to decrease such impurity from the sample. In order to find a way to decrease the GGBs content, we investigate the origin of GGBs and find a simple method for increasing the sample purity. The origin of GGBs is the exfoliated materials from the carbon rod, and such an exfoliation can be nearly avoided by the preheat treatment of the carbon rod up to 1000°C just in advance of the arc vaporization. According to this preheat, the quantity of GGBs in the sample can be reduced from 30 to 0.01 % (100 ppm) by weight. In addition, it was found that the simple two-staged purification which is combined with the centrifugation and filtration effectively works for removing such a small quantity of GGBs.

Research on graphite powders used for HTR-PM fuel elements

Different batches of natural graphite powders and electrographite powders were characterized by impurity, degree of graphitization, particle size distribution, specific surface area, and shape characteristics. The graphite balls consist of proper mix-ratio of natural graphite, electrographite and phenolic resin were manufactured and characterized by thermal conductivity, anisotropy of thermal expansion, crush strength, and drop strength. Results show that some types of graphite powders possess very high purity, degree of graphitization, and sound size distribution and apparent density, which can serve for matrix graphite of HTR-PM. The graphite balls manufactured with reasonable mix-ratio of graphite powders and process method show very good properties. It is indicated that the properties of graphite balls can meet the design criterion of HTR-PM. We can provide a powerful candidate material for the future manufacture of HTR-PM fuel elements.

Thermal–hydraulic feasibility analysis on uprating the HTR–PM

In order to meet energy demand in China, the high temperature gas-cooled reactor–pebble-bed module (HTR–PM) is being developed. It adopts a two-zone core, in which graphite balls are loaded in the central zone and the outer part is fuel ball zone, and couple with a steam cycle. Outer diameter of the reactor core is 4.0 m and height of the core is 9.43 m. The helium inlet and outlet temperature are 250 and 750 °C, respectively. The reactor thermal power is 380 MW. Preliminary studies show that the HTR–PM is feasible technologically and economically. In order to increase the reactor thermal power of the HTR–PM, some efforts have been made. These include increasing the height of reactor core, optimizing the thickness of fuel zone and better selection of the scheme of central graphite zone, etc. Basic design concepts and thermal–hydraulic parameters of the HTR–PM are given. Measures to increase the thermal power are introduced. Thermal–hydraulic analysis results are presented. The results show that, from the viewpoint of thermal–hydraulics, it is possible to increase the reactor power.

05/01337 The experimental investigation of the impact of a particle or a rigid body on a cylindrical shell

HTR-10 is a 10 MWt prototype pebble-bed reactor (PBR) that presents a doubly heterogeneous geometry for neutronics calculations. An appropriate unit-cell treatment for the associated fuel elements is vital for creating problem-dependent multigroup cross sections. Considering four unit-cell options for resonance self-shielding correction in SCALE6, a series of HTR-10 core models were established using the CSAS6 sequence to systematically investigate how they affected the computational accuracy and efficiency of PBR criticality calculations. Three core configurations, which ranged from simplified infinite lattices to a detailed geometry, were examined. Based on the same ENDF/B-VII.0 cross-section library, multigroup results were evaluated by comparing with continuous-energy SCALE6/CSAS6 and MCNP5 calculations. The comparison indicated that the INFHOMMEDIUM results overestimated the effective multiplication factor (keff) by about 2800 pcm, whereas the LATTICECELL and MULTIREGION treatments overestimated keff values with similar biases at approximately 470–680 pcm. The DOUBLEHET results attained further improvement, reducing the keff overestimation to approximately 280 pcm. The comparison yielded two unexpected problems from using SCALE6/CSAS6 in HTR-10 criticality calculations. In particular, the continuous-energy CSAS6 calculations in this study present a non-negligible discrepancy with MCNP5, potentially causing a keff value overestimate of approximately 680 pcm. Notably, using a cell-weighted mixture instead of an explicit model of individual TRISO particles in the pebble fuel zone does not shorten the CSAS6 computation time in the high-fidelity HTR-10 model because of the presence of graphite balls and the complex modeling of peripheral reflectors.

Nano-grained Net-Shaped Fabrication of Re Components by EB-PVD

Cost-effective net-shaped forming components have brought considerable interest into the Department of Defense, the National Aeronautics & Space Administration, and the Department of Energy. Electron beam–physical vapor deposition (EB-PVD) offers flexibility in forming net-shaped components with tailored microstructure and chemistry. High-purity rhenium components, including rhenium-coated graphite balls and rhenium plates and tubes, have been successfully manufactured by EB-PVD. EB-PVD rhenium components exhibited submicron and nano-size microstructure with high hardness and strength as compared with chemical vapor deposition (CVD). It is estimated that the cost of rhenium components manufactured by EB-PVD would be less than the current CVD and powder-high-temperature isostatic pressure technologies.