Graphite Compacts Research Articles

Development of Binderless Graphite Compact for Bipolar Plate Production

Bipolar plates account for almost 29% of the fuel cell stack costs and hence cost-effective manufacturing methods for making the bipolar plates are of paramount importance. Current manufacturing technologies for graphite bipolar plates require consolidation of graphite precursor materials using compression or injection molding followed by high temperature treatment for baking and graphitization. In contrast, in this investigation, we demonstrate direct consolidation of graphite powders without the use of any coal, petroleum or polymer binders resulting in significant reduction of time during compression molding as well as elimination of the need for additional heat treatment. Graphite particles were electrolessly coated with copper particles and subject to uniaxial compression at 100°C to produce a highly dense compact with relative density of ~ 98%, The process resulted in improving the flexural strength of the compact by almost ~50%. TEM investigation of the graphite particle interfaces revealed that copper undergoes creep deformation that aids in strengthening the compacts. The paper will describe the underlying mechanisms for the binderless production of graphite compacts which could be potentially useful for energy and cost-effective production of bipolar plates for fuel cells and flow batteries.

Reduced anisotropic properties of Cold Isostatic Pressed graphite matrix compacts

Matrix graphite (MG) compacts were produced by conventional uniaxial pressing (UP) and Cold Isostatic Pressing (CIP). An additional premolding procedure with UP was also carried out before CIP to manufacture MG compacts with a cylinderical shape which was not achievable using CIP alone. It was noted that the MG compacts made by CIP revealed more isotropic microstructural and thermal properties compared to the MG compacts made by UP. The MG compacts made by UP had graphite grains aligned in the transverse direction and severe anisotropic thermal expansion and conductivity according to direction of orientation (axial and transverse). On the other hand, the MG compacts made by CIP showed randomly aligned graphite grains, almost identical linear thermal expansion and shrinkage behaviors, and a reduced thermal conductivity difference between the axial and transverse directions compared to that of the UP MG compacts. The properties of the premolded CIP MG compacts also revealed considerable anisotropic properties. Finally, three-point bending experiments showed that the MG compacts made by CIP obtained 230% higher fracture toughness to the transverse direction compared to that of the UP MG compacts.

Diffusion of cesium and iodine in compressed IG-110 graphite compacts

Nuclear graphite grade IG-110 is currently used in the High Temperature Engineering Test Reactor (HTTR) in Japan for certain permanent and replaceable core components, and is a material of interest in general. Therefore, transport parameters for fission products in this material are needed. Measurement of diffusion through pressed compacts of IG-110 graphite is experimentally attractive because they are easy to prepare with homogeneous distributions of fission product surrogates. In this work, we measured diffusion coefficients for Cs and I in pressed compacts made from IG-110 powder in the 1079–1290 K temperature range, and compared them to those obtained in as-received IG-110.

Experimental measurement and numerical modeling of the effective thermal conductivity of TRISO fuel compacts

Accurate modeling capability of thermal conductivity of tristructural-isotropic (TRISO) fuel compacts is important to fuel performance modeling and safety of Generation IV reactors. To date, the effective thermal conductivity (ETC) of TRISO fuel compacts has not been measured directly. The composite fuel is a complicated structure comprised of layered particles in a graphite matrix. In this work, finite element modeling is used to validate an analytic ETC model for application to the composite fuel material for particle-volume fractions up to 40%. The effect of each individual layer of a TRISO particle is analyzed showing that the overall ETC of the compact is most sensitive to the outer layer constituent. In conjunction with the modeling results, the thermal conductivity of matrix–graphite compacts and the ETC of surrogate TRISO fuel compacts have been successfully measured using a previously developed measurement system. The ETC of the surrogate fuel compacts varies between 50 and 30Wm−1K−1 over a temperature range of 50–600°C. As a result of the numerical modeling and experimental measurements of the fuel compacts, a new model and approach for analyzing the effect of compact constituent materials on ETC is proposed that can estimate the fuel compact ETC with approximately 15–20% more accuracy than the old method. Using the ETC model with measured thermal conductivity of the graphite matrix-only material indicate that, in the composite form, the matrix material has a much greater thermal conductivity, which is attributed to the high anisotropy of graphite thermal conductivity. Therefore, simpler measurements of individual TRISO compact constituents combined with an analytic ETC model, will not provide accurate predictions of overall ETC of the compacts emphasizing the need for measurements of composite, surrogate compacts.

Effects of compaction pressure and graphite content on hydrogen storage properties of Mg(NH2)2–2LiH hydride

Preparation of hydride–graphite compacts serves as an effective method to improve the volumetric hydrogen storage density and the effective thermal conductivity for light complex hydrides. This paper presents the effects of compaction pressure and expanded natural graphite (ENG) content on the hydrogen storage properties of the Mg(NH2)2–2LiH–0.07KOH compacts. The results show that the hydrogen desorption kinetics of the 1st sorption cycle decreases with the increase of the compaction pressure. However, the compacts exhibit the similar hydrogen desorption kinetics and capacities from the 2nd sorption cycles on regardless of the compaction pressure. The ENG addition significantly enhances the desorption kinetics because of the improvement of the heat transfer performance of the hydride. Furthermore, the volumetric hydrogen storage density of the hydride reaches 47 g/L after the compaction at 365 MPa, but it reduces by increasing the ENG content.

Elastomeric behavior of exfoliated graphite, as shown by instrumented indentation testing

This paper reports for the first time the elastomeric behavior of a non-polymeric material, as observed in exfoliated graphite compacts (⩽12vol.% solid, preferably 4vol.% solid) and enabled by the high-amplitude reversible and easy sliding of the graphite layers within the cell wall of exfoliated graphite. The reversibility is probably due to the cellular structure of exfoliated graphite. The elastomeric character is independent of the maximum load and essentially unchanged upon reloading. The total and reversible engineering shear strains of the cell wall (∼60 graphite layers, ∼20nm thick) are up to 40 and 35 respectively during instrumented indentation (in the compaction direction) without fracture of the layers, compared to corresponding values of 12 and 8 for flexible graphite (38vol.% solid). The fraction of displacement that is irreversible is as low as 12%, compared to 29% for flexible graphite. The modulus is as low as 83kPa, compared to 790kPa for flexible graphite. The greater the degree of compaction, the lower are the shear strain and reversibility, and the higher are the modulus and the displacement load. The ease of interlayer sliding is inadequate in flexible graphite or highly-oriented pyrolytic graphite (prior work) for substantial elastomeric behavior.

Experimental Study on Shear Failure Behaviors and Wear Resistance of Copper-Electroplated Graphite Compacts

This work examines the processing and some mechanical properties of copper-electroplated graphite compacts. Shear and wear tests were conducted. Natural graphite (NG) flakes were compressed to make graphite compacts. No polymer binder was added in the compact. For reinforcing and preventing the NG particles from peeling off, a copper layer was electroplated on the compact. The density of green graphite compact approached a limit when the forming pressure was higher than 100 MPa. The results indicated that the shear strengths increase 150% and the wear resistance is improved more than ten times because of the copper layer. The induced damage mechanisms were examined, and the influence of the copper layer on the damage is discussed.

Electrical conductivities of carbon powder nanofillers and their latex-based polymer composites

The electrical conductivity of graphene, multi-wall carbon nanotubes, carbon black nanopowders and graphite powder is characterized using paper-like films and by means of powder compression. The large difference in surface area of these materials results in different packing density and number of contact spots, influencing the macroscopic conductivity of the compacts during powder compression. The results are compared with the percolation threshold and final conductivity of polypropylene (PP) composites, using latex technology for the incorporation of the carbon fillers in the polymer. Even though the PP composites produced in this work exhibit percolation thresholds as low as 0.3wt.%, the final conductivity for all the composites is below 1.5S/m. Reasons why the high value of ∼103S/m, which is obtained for graphene- and nanotube-based paper films or graphite compacts, is not reached for the composites are investigated.

Viscoelastic behavior of the cell wall of exfoliated graphite

The cell wall (∼60-graphite-layer thick, average) in the cellular structure of exfoliated graphite compacts is viscoelastic, due to the shear between graphite layers. The viscous character decreases with increasing solid content (volume fraction), due to the increasing difficulty of shear, which becomes limited at solid contents above 4vol.%. The elastic character is essentially independent of the solid content, weakening slightly with increasing solid content. The viscous character is stronger under in-plane flexure than compression in the compaction direction, due to the preferred orientation of the graphite layers. At the lowest solid content of 1.0 vol.%, the loss-tangent/solid-content is 35 and 25 under flexure and compression respectively, the storage-modulus/solid-content is 125MPa and 46kPa under flexure and compression respectively, and the loss-modulus/solid-content is 45MPa and 13kPa under flexure and compression respectively. The viscous character is strong under both flexure and compression, whereas the elastic character is much stronger under flexure than compression. The loss-tangent/solid-content decreases with increasing solid content, leveling off at 0.9 at 15vol.% solid. The loss-modulus/solid-content also decreases with increasing solid content. The highest values of the loss-tangent/solid-content, storage-modulus/solid-content and loss-modulus/solid-content are greater for exfoliated graphite compacts than rubber or carbon black compacts.

Production of nanoTiC–graphite composites using Ti-doped self-sintering carbon mesophase powder

This work reports the synthesis of nanoTiC–graphite composites using mesophase pitch containing titanium as TiC or TiO2 nanoparticles. NanoTiC–graphite composites have been prepared using Ti-doped self-sintering mesophase powders as starting materials without using any binders or a metal carbide-carbon mixing stage. The effect of manufacture variables on the graphite compacts properties was studied. Graphites were characterised using XRD and Raman spectroscopy, SEM and TEM, as well as by their mechanical, electrical and thermal properties. The presence of TiC promotes graphitisation producing materials with larger crystal sizes. The kind of titanium source and mesophase content of the starting pitch affects to the final properties. Mesophase pitch with higher amount of mesophase content produces graphites with higher degree of graphitisation. The incorporation of TiC nanoparticles to the graphites composites improved thermal conductivity more than four times, and mechanical properties are not significantly modified by the presence of TiC.

The use of carbon nanotubes to induce osteogenic differentiation of human adipose-derived MSCs in vitro and ectopic bone formation in vivo

Carbon nanotubes (CNTs), one of the most concerned nanomaterials, with unique electrical, mechanical and surface properties, have been shown suitable for biomedical application. In this study, we evaluated attachment, proliferation, osteogenic gene expression, ALP/DNA, protein/DNA and mineralization of human adipose-derived stem cells cultured in vitro on multi-walled carbon nanotubes (MWNTs) and graphite (GP) compacts with the same dimension. Moreover, we assessed the effect of these two kinds of compacts on ectopic bone formation in vivo. First of all, higher ability of the MWNTs compacts to adsorb proteins, comparing with the GP compacts, was shown. During the conventional culture, it was shown that MWNTs could induce the expression of ALP, cbfa1 and COLIA1 genes while GP could not. Furthermore, alkaline phosphatase (ALP)/DNA and protein/DNA of the cell on the MWNTs compacts, was significantly higher than those of the cells on the GP compacts. With the adsorption of the proteins in culture medium with 50% fetal bovine serum (FBS) in advance, the increments of the ALP/DNA and protein/DNA for the MWNTs compacts were found respectively significantly more than the increments of those for the GP compacts, suggesting that the larger amount of protein adsorbed on the MWNTs was crucial. More results showed that ALP/DNA and protein/DNA of the cells on the two kinds of compacts pre-soaked in culture medium having additional rhBMP-2 were both higher than those of the cells on the samples re-soaked in culture medium with 50% FBS, and that those values for the MWNTs compacts increased much more. Larger mineral content was found on the MWNTs compacts than on the GP compacts at day 7. In vivo experiment showed that the MWNTs could induce ectopic bone formation in the dorsal musculature of ddy mice while GP could not. The results indicated that MWNTs might stimulate inducible cells in soft tissues to form inductive bone by concentrating more proteins, including bone-inducing proteins.

Fabrication of uranium oxycarbide kernels and compacts for HTR fuel

Fabrication of uranium oxycarbide kernels and compacts for HTR fuel

On the thermally conductive behaviour of compacted graphite nanosheets

Thermally conductive properties of compacts composed of graphite nanosheets (GNSs) are studied. The GNSs were made by the intercalation and exfoliation of natural graphite (NG) flakes. The GNSs were then compressed to make graphite compacts. Thermal diffusivities along the in-plane and transverse directions are measured using a laser flash method. The anisotropic ratio of the thermal conductivities ranges between 20 and 25. The properties of graphite compacts based on NG flakes are also studied. The results reveal that the GNS-based compact can be consolidated at a lower density, whereas the NG-based counterpart is denser and has a higher thermal conductivity. To protect the graphite compact, a method for coating a copper layer on the graphite compact was developed. The results also indicate the copper coating enhances heat transfer into the graphite compact.

Electrical conductivity and microstructure of sintered ferrous materials: iron–graphite compacts

The dissolution of graphite during sintering of PM steels prepared from iron and graphite powder mixes was studied using electrical conductivity measurement. The effect of graphite content and grade on the carbon transport rate and related mechanisms was investigated by sintering in the range 500 to 1200°C in different atmospheres, including hydrogen, nitrogen, and vacuum. The electrical conductivity measurement method was accompanied by quantitative metallography and microfractography techniques for determining the extent of carbon dissolution and its influence on the formation of sintered contacts. Weight loss measurement and the dilatometric method were also performed. The effect of manufacturing parameters on the mechanical properties of sintered parts was studied to find a correlation between mechanical behaviour, microstructure, and the conductivity/resistivity in order to develop a nondestructive testing method. The results show that changes in the material structure occurring during processing of sintered materials can be precisely detected via electrical conductivity measurement. The application of the method for iron–graphite powder compacts is shown.

Sorption and Recovery of A-grade Heavy Oil by Using Exfoliated Graphite Packed in Plastic Bag

For a practical application to recover heavy oil, exfoliated graphite packed into a plastic bag and compacted exfoliated graphite were prepared. Their sorption behavior was studied. Exfoliated graphite packed into a polypropylene non-woven bag with a lower bulk density sorbed 40g A-grade heavy oil per 1g exfoliated graphite. The sorption of heavy oil by the exfoliated graphite compacts showed a little lower sorption capacity than exfoliated graphite packed in the plastic bag. The use of exfoliated graphite packed into a plastic bag seemed to be effective in recovering heavy oil spilled on water.

Sintered copper–graphite powder compacts forindustrial applications

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.

Processes in PM steel compacts during the initial stages of sintering

Abstract In classical sintering procedures of ferrous precision part, important processes occur during the heating period and/or during presintering stages. Apart from the de-waxing process, which is not discussed here, changes in surface chemistry, formation of metallic bridges, and dissolution of alloy elements added as powders take place. In this work, desorption of gases, reduction of surface oxides, and neck formation have been studied as well as the dissolution of carbon and of metallic alloy elements. It showed that surface chemistry is changed within fairly narrow temperature intervals the position of which depends on the composition of the powder particles and in part on pretreatment. Dissolution of carbon in iron and steel matrices is a slower process than might be anticipated from the thermodynamic viewpoint, in plain Fe complete dissolution occurring only at T >900°C and the atmosphere also playing a major role. Formation of stable necks in steel–graphite compacts is retarded until dissolution of graphite has occurred to a significant extent. Finally, dissolution of metallic alloying elements, also if promoted by transient liquid phase, starts at still higher temperatures, in the case of Mo even T >1200°C being necessary, and full homogenization is complete only after extended isothermal sintering.

X-ray diffraction and mechanical property studies of TiCx synthesized by shock-assisted reaction sintering

Abstract Elemental titanium and carbon (graphite) powder mixtures (∼65% dense) were shock-compressed to ∼88% density, using a single-tube cylindrical implosion system. X-ray diffraction line broadening analysis (based on the Williamson–Hall method) revealed microstrain in Ti particles in shock-densified Ti+C compacts as being of the order of ∼3×10 −3 , which corresponds to a dislocation density of the order of 10 11 cm −2 . The shocked Ti+0.95 graphite compacts were subsequently machined into triangular sections and reaction-sintered in an induction-heated hot press, under different reaction conditions, and X-ray diffraction (XRD) analysis was used to determine the lattice constant of reaction-synthesized TiC x compound, yielding values in the range of 4.279 to 4.309 A and suggesting formation of a non-stoichiometric TiC 0.35–0.58 compound. The reaction-sintered TiC x compound had a microhardness as high as 1900 KHN, approaching that of commercial fully-dense TiC ceramics (2000 KHN).

Structure and Properties of Boronated Graphite

Boronated natural graphite compacts were prepared at a temperature of 1750°C under pressures between 0.1 and 0.5 GPa with a nominal B4C content of 2 wt.% and at 2200°C under a pressure of 19.8 MPa with nominal B4C contents of 1, 2.5 and 50 wt.%. Their interlayer spacing d002 and lattice constant a0 were determined. The measurements of the electrical resistivity, magnetoresistance and Hall coefficient for the compacts were carried out mainly at liquid nitrogen temperature. With the increased substitution of boron atoms, the interlayer spacing d002 decreases from the values of the reference natural graphite compacts, showing a trend of the lattice constant a0 to increase slightly. At liquid nitrogen temperature, all of the boronated compacts show the positive Hall coefficient decreasing with the increase in substituted and ionized boron atoms. Below liquid nitrogen temperature, the boronated compacts except the slightly boronated ones exhibit the negative magnetoresistance and almost temperature independent resistivity.

Effect of Graphite with and Without Copper Coating on Consolidation Behaviour and Sintering of Copper–Graphite Composite

Copper-graphite composites were made by a powder metallurgical method. Two starting graphite powders, either with or without copper coating, were each mixed individually with copper metal powder in a conical flask using a mechanical stirrer at 3600 rev min−1 for a period of 6 min. The method of copper coating graphite particles is described. The copper-graphite powder mixture was of nominal composition 8 wt-% graphite. Small cylindrical specimens of 8mm diameter and 12mm length were obtained by die compaction using a unidirectional hydraulic press at ambient temperature over a pressure range of 400-1200 MPa. For comparison, parallel specimens made of pure copper powders were consolidated and pressed under identical conditions. In this investigation, an attempt has been made to study the particle properties and behaviour which influence the consolidation of mixes of copper and graphite (coated or uncoated) powders. The green density values of compacts at various pressures are reported. It was found that the Konopicky-Shapiro equation can be applied to the compaction of copper-graphite powder mixes. When sintering at 1000°C, it was found that copper coated graphite compacts were fully sintered in a shorter time than uncoated compacts. The sintered specimens were mirror polished using a standard technique before being examined metallographically. Copper coated graphite composites possessed high relative density and low electrical resistivity very close to the corresponding values of pure copper, however uncoated graphite composites showed lower relative density and higher electrical resistivity values. Compression measurements showed that the yield and ultimate strengths of coated graphite composites were much higher than those of uncoated specimens. PM/0745