Nickel-coated Graphite Research Articles

Study on the structure and properties of conductive silicone rubber filled with nickel‐coated graphite

AbstractIn this study, the conductive silicone rubber composites filled with nickel‐coated graphite (NCG) have been prepared, and their morphology structure, electrical conductivity, electromagnetic interference shielding efficiency (EMI SE), and mechanical properties have been investigated with reference to the NCG filler loading. The mechanical strength of NCG particle was poor that it can be easily ground into smaller particle during the mixing process if the shear force during mixing is large enough. The electrical conductivity of the composites existed an obvious threshold value with the variation of the loading amount of the conductive filler. EMI SE of the composites increases with the decrease of the volume electrical resistivity. The Payne effect can be used to characterize the intensity of the three‐dimensional conductive network structure in silicone rubber matrix, and the difference of storage modulus in the low and high shear strain has good linear correlation with the electrical conductivity. So, the electrical conductivity and EMI SE can be estimated by means of the difference of storage modulus obtained from rubber process analysis test. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Preparation and characterization of nickel-coated graphite nanosheets

Nickel-coated graphite nanosheets have been fabricated by simple oxidation and reduction process. The resulting products have a homogeneous and consecutive distribution of nickel coated on the surface of graphite nanosheets. The addition of polyvinylpyrrolidone (PVP) can control the size of nickel particles. Scanning electron microscopy (SEM) images, XRD pattern of the composites all confirm the deposition of nickel. The magnetic property of the coated GNs was characterized by Magnetic Property Measurement System (MPMS).

Comparison on the Properties of Nickel-Coated Graphite (NCG) and Graphite Particles as Conductive Fillers in Polypropylene (PP) Composites

The present study was carried out to evaluate the performance of nickel-coated graphite (NCG) in comparison with graphite as conductive fillers in polypropylene (PP) matrix. Graphite exhibits smaller particle size and higher aspect ratio (length/thickness) than NCG particles. The results showed that the additions of graphite filler in PP exhibits higher tensile properties and electrical conductivity compared to NCG filled PP composites. The electrical results showed that the percolation threshold of graphite and NCG filled PP composites occurred in the range of 10 to 20 vol.% and 15 to 25 vol.%, respectively.

Comparison of solid oxide fuel cell anode coatings prepared from different feedstock powders by atmospheric plasma spray method

Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300 nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock.

Properties of Nickel Products for SOFC Applications

The first part of this paper presents a review of all nickel products that find applications in SOFCs. The work covers nickel oxides, nickel and nickel alloy foams, nickel powders, and nickel-coated graphite. The second part of the paper reports experimental work in two areas. The first one is an experimental study on the creep properties of nickel foam under compressive stress at high temperatures. This would provide insights for using nickel and nickel alloy foam as GDL or anode current collector in SOFC's. Another experimental work is concerning the preparation of SOFC anodes using Ni based powders including nickel oxides and nickel powders. Results are reported in this paper on volume and dimensional changes of an anode during burnout stage and sintering stage when various nickel powders and a nickel oxide are used.

Development of Porous Anode Layers for the Solid Oxide Fuel Cell by Plasma Spraying

This article focuses on the development of the anode layer for solid oxide fuel cells by plasma spraying. The composite (cermet) anode, developed by thermal spraying, consisted of nickel and yttria-stabilized zirconia (YSZ). The effect of different plasma-spraying technologies on the microstructure characteristics and the electrochemical behavior of the anode layer were investigated. Coatings were fabricated by spraying nickel-coated graphite or nickel oxide with YSZ using a Triplex II plasma torch under atmospheric conditions as well as a standard F4 torch under atmospheric or soft-vacuum conditions. The investigations were directed to have an open microporous structure, higher electrical conductivity, and catalytic activity of anode deposits. Porosity was investigated by measuring the gas permeability. Scanning electron microscopy and x-ray diffraction technologies were applied to examine the morphology, microstructure, and composition of the layers. Electrical conductivity measurements were carried out to determine the ohmic losses within the anode layer. The most promising layers were analyzed by measuring the electrochemical behavior to obtain information about catalytic activity and performance.

Tribological behavior of aluminum/SiC/nickel-coated graphite hybrid composites

Tribological behavior of Al/SiC/Ni-coated graphite (Gr–Ni) hybrid composites with various amounts of Gr–Ni addition synthesized by the semi-solid powder densification method was studied. The SiC and nickel-coated graphite (Gr–Ni) particles are distributed quite uniformly in an aluminum matrix. The graphite agglomerates much less in composites with coated Gr–Ni particles than in composites with non-coated Gr particles. Coefficient of thermal expansion decreases significantly with the amount of Gr–Ni addition. The fracture energy decreases monotonously as the amount of Gr–Ni addition increases. Wear debris become smaller, which causes smaller electric contact resistance, as the amount of Gr–Ni addition increases. Seizure occurs for monolithic aluminum alloy, but no seizure occurs for Al/SiC and Al/SiC/Gr–Ni composites. The friction coefficient and its fluctuations decrease as the percentage of Gr–Ni addition increases. Graphite released from the composites bonds to the wear surfaces of the composites and counterparts. Due to the “composite effects”, wear rates of both the composite and counterpart increase as the amount of Gr–Ni addition increases up to 5% Gr–Ni addition, then drop for 8% Gr–Ni addition. The wear rates of all the composites with Gr–Ni additions are higher than the wear rate of base Al/SiC material with no Gr–Ni addition. Therefore, the composites incorporating graphite do not work for wear reduction, as the debit in mechanical properties is larger than the very small benefit in friction reduction.

Backscatter measurements of thin nickel-coated graphite fibers

Backscatter cross sections were measured for thin nickel-coated graphite fibers as a function of length. A radiation wavelength of 0.86 centimeters (35 GHz) and fiber lengths varying across the primary resonance spectrum were used. Comparison with previously developed theory shows good agreement, and implications with respect to the effective magnetic permeability are discussed.

Preparation of composite powders by pressure reduction with hydrogen

Preparation of nickel-coated composite powders is presented by pressure reduction with hydrogen of aqueous Ni(II) from acetate feed solutions containing various base powders (graphite, tungsten carbide, and aluminium). Particularly, the effect of major process variables (pressure of hydrogen, temperature, concentration of Ni(II), concentration of acetates) on both the course of reduction and on the quality of resultant composite powders were investigated. Optimum conditions were established for preparation of nickel-coated graphite, tungsten carbide, and aluminium composite powders. Moreover, the effect of the presence of Bi(III), Sb(III), and As(V) in the feed Ni(II) acetate solutions on the course of pressure reduction and on the quality of nickel coating was examined and determined. The best quality nickel-coated composites were obtained from feed solutions containing Bi(III) at concentrations less than 1.0 g/l.

Machinability of Graphitic Metal Matrix Composites as a Function of Reinforcing Particles

Aluminum Metal Matrix Composites (MMCs) reinforced with ceramic particles have been developed for high wear resistance applications such as cylinder liners and brakes as a replacement for gray cast iron. Ceramic particles in an aluminum matrix improve its wear resistance property, but also cause high abrasive wear on cutting tools, which results in poor tool life and inconsistent part quality. A new family of MMCs (GrA-Ni®) consisting of an aluminum matrix reinforced with nickel-coated graphite particles and SiC or Al 2O 3 particles was recently developed. This paper presents the results of machining tests conducted to assess the machinability of the new graphitic ceramic reinforced MMCs. It was found that graphitic aluminum MMC reinforced with alumina is easier to machine than those reinforced with both SiC and graphite or SiC particles only. The incorporation of graphite into these composites and the variation of hard particle content improve their machinability.

Microstructure and volume resistivity of composites of isotactic polypropylene reinforced with electrically conductive fibers

AbstractThe effect of processing method on the microstructure and volume resistivity of polypropylene reinforced with nickel‐coated graphite (NCG) fibers or stainless steel (SS) fibers is presented. Samples were produced by compression molding, extrusion, and injection molding. The volume resistivity of the composites was measured in three perpendicular test directions to determine anisotropy. The stress and thermal fields experienced during processing determine the ultimate microstructure. The measured resistivity is dependent on this processing‐induced microstructure as reflected by the distribution of fiber orientation, length, and concentration. Composites in which the fiber orientation is anisotropic also exhibit anisotropic resistivity. Volume resistivity is lowest in the principal direction of fiber orientation. Samples with the greatest fiber length become conductive at the lowest fiber loadings. Resistivity decreases with an increase in fiber loading, but concentration gradients are often produced, especially in the injection molded samples. High fiber concentrations generally resulted in poor dispersion and wetting. The intertwining and bending of the SS fibers make processing difficult. A comparison between the two types of fibers reveals that, for the systems considered in this study, the SS fibers impart conductivity at lower loadings, but that the NCG fiber composites are ultimately more conductive.

Artificial dielectrics of conductive fibers in polymers: Effects of viscosity and matrix composition on permittivity and loss

We report the microwave dielectric properties of composites made of nickel-coated graphite fibers in two different types of silicone polymers. The influence of matrix viscosity on the fiber alignment, fiber filamentation in the composites, and how those affect the permittivity and loss tangent of the composite are discussed. The dependence of the permittivity and loss of the composites on the permittivity and loss of the matrix will also be compared.

The permittivity at X-band frequencies of nickel-coated graphite fibers in an epoxy matrix

In this study, we have investigated the microwave dielectric behavior of a composite formed by embedding nickel-coated graphite fibers in an epoxy matrix. Permittivities of composites in the X-band frequency range as a function of fiber concentration, fiber length, and the degree of fiber aggregation were studied. Fiber aggregation was reduced significantly by the addition of silica particles to the composite mixture before epoxy curing. Predictions from the mean field theory fit the experimental data well at dilute fiber concentrations.

Effects of particle shape and size distributions on the electrical and magnetic properties of nickel/polyethylene composites

AbstractElectrical and magnetic properties of composite materials prepared by incorporating various nickel‐based fillers of different shapes into polyethylene were investigated. The fillers used were nickel powders, nickel filamentary powders, nickel flakes, and nickel‐coated graphite fibers. The particle‐size distributions of the fillers were determined both before and after the processing of the composite samples. A wide range of filler volume fractions were used. In some cases, the volume fraction approached the maximum packing fraction of the solid phase to significantly exceed the percolation threshold. The composite samples were characterized in terms of their volume resistivity, dielectric constant, and magnetic permeability values. Filler particles of asymmetric shapes were very effective in terms of altering the electrical properties of the composite samples. At the highest loading levels of the nickel fillers, the volume resistivity values of the composites decreased by more than 17 orders of magnitude. At such high filler concentrations, the dielectric constant values of the composite samples increased considerably, to values that were greater than 1000. The permeability values of the samples increased linearly with the volume fraction of the nickel filler and were insensitive to the shape of the fillers. The highest relative permeability value measured was 5.8 for composites with 67% by volume of nickel powder. © 1993 John Wiley & Sons, Inc.

Volume Resistivity and Mechanical Properties of Electrically Conductive Long Fiber Composites

Long, electrically conductive nickel-coated graphite fibers were employed to produce thermoplastic composites by compounding and processing with polypropylene resin. The fibers were blended into the resin in a Brabender mixer and the resulting composite was compression molded into square plaques. The fiber loading was varied from 2 to 20 weight percent. The effects of fiber loading on electrical and impact properties were studied and related to the composite microstructure. A device was constructed to measure the volume resistivity of the samples in two planes, longitudinal and latitudinal. A large initial decrease in resistivity occurs as fibers are added to the insulating matrix. Beyond some critical loading, the resistivity reaches a minimum and begins to level off. The resistivity is identical in the two perpendicular planes. Examination of the microstructure reveals that the fiber orientation is fairly random in the composite, which results in a uniform resistivity in the two planes. Fiber attrition is more significant at low loadings where the mixing shear is greater. Impact tests show that the addition of the fibers results in an increase in sample stiffness, thereby reducing the force and energy needed to break the samples.

Dielectric Permittivities at X-Band Frequencies of Conductive Fibers Aligned in Polymer Matrices

AbstractComposites have been formed of Cycom, nickel-coated graphite fibers, aligned at concentrations of up to 6 weight percent in polymer matrices. Dielectric constants up to approximately 70 have been measured for these samples at x band frequencies. The results have been compared with a single non-interacting particle theory.

Friction and wear properties of aluminum-particulate graphite composites

Aluminum alloy matrix composites containing 3, 6 and 9 wt.% particulate graphite were produced and their friction, wear, and tensile properties were determined. The composite fabrication process consisted of adding nickel-coated graphite particles into a partially solidified and vigorously agitated Al-7%Si-1%Mg alloy, followed by remelting and casting into ingots. The coefficients of friction and wear of the as-cast and liquid-forged composites were determined by a Falex lubricant test machine. It was found that the friction and wear properties of both as-cast and liquid-forged alloys considerably improved with addition of graphite. While the tensile strength and ductility of composites containing 3% graphite were comparable with those of the matrix, increasing graphite content reduced these properties.

Production of cast aluminium-graphite particle composites using a pellet method

Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm−2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm−2 and the optimum size and percentage of aluminium powder were 400 to 1000μm and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.