Effects Of Graphite Content Research Articles

Effect of graphite content on microstructure and properties of Al2O3 ceramic and 304 stainless steel brazed joints

Al2O3 ceramic and 304 stainless steel were brazed by Ag-Cu-Ti filler. Graphite was added into the Ag-Cu-Ti filler to improve the properties of the joint. The effects of graphite content on the microstructure and properties of Al2O3/304 joint were investigated. The microstructure of the joint was Al2O3/Cu3Ti3O + TiCu/Ag(s,s) + Cu(s,s) + TiC + TiCu/Cu0.8Fe0.2Ti/Fe-Cr/304. The shear strength was 130.54 MPa with Ag-Cu-Ti. When 0.1 wt% graphite was added, strength of joint reached 203.49 MPa. The improvement of shear strength was attributed to the formation of TiC, which reduced the coefficient of thermal expansion (CTE) difference and refined the microstructure. When the graphite content exceeded 0.2 wt%, graphite would agglomerate in the joint, which deteriorated the mechanical properties of the joint.

Preparation and characterization of a heat storage material: Shape-stabilized KNO3 using a modified diatomite-based porous ceramic as the skeleton

The applications of salts-based phase change materials (PCMs) are greatly restricted by their corrosion to containers and low thermal conductivity. To address these issues, a modified diatomite-based porous ceramic (MDPC) was prepared to shape-stabilize KNO3 salt in this paper. Particularly, effects of graphite content on the pore structure of diatomite-based porous ceramic (DPC) and MDPC on the thermophysical properties of shape-stabilized KNO3 were investigated. The results showed that 30 wt% graphite increased apparent porosity of DPC up to 67.21% and enlarged average pore diameter from 0.71 to 3.58 μm, while weakened its compressive strength from 19.12 to 5.51 MPa. The MDPC effectively prevented leakage of molten KNO3 to avoid its corrosion problem, and possessed an excellent chemical compatibility with KNO3 salt. Additionally, the MDPC hardly changed phase transition temperature of shape-stabilized KNO3, while increased latent heat of composites from 51.70 to 60.21 J/g due to its higher apparent porosity. The MDPC was also shown to enhance thermal conductivity of shape-stabilized KNO3 from 1.16 to 1.52 W/(m·K) at 25 °C, which was attributed to that the MDPC with a larger pore size was more easily infiltrated by molten salt, contributing to a lower apparent porosity of composites. Compared with pure KNO3, thermal conductivity and thermal stability of shape-stabilized KNO3 were seen to be significantly improved by 238% and 60 °C, respectively. This work therefore solved two key challenges for the application of KNO3 salt, and proposed an effective way to improve thermophysical properties of such heat storage material.

Porous/dense bilayer BaZr0.8Y0.2O3-δ electrolyte matrix fabricated by tape casting combined with solid-state reactive sintering for protonic ceramic fuel cells

Perovskite-type yttrium-doped barium zirconate (BZY) has been considered as attractive electrolyte material for protonic ceramic fuel cells (PCFCs) due to its high bulk proton conductivity, excellent chemical stability and mechanical robustness. However, it requires very hash sintering conditions to obtain a dense ceramic, which will be unfavourable to the fabrication of anode-supported PCFCs. In this study, new cell structure based on porous/dense bilayer BaZr0.8Y0.2O3-δ (BZY) electrolyte matrix was designed for PCFCs, facilitating the densification of electrolyte thin film and the nanostructured anode preparation by impregnation. The matrix was fabricated by bilayer co-tape casting combined with solid-state reactive sintering from the starting powders of BaCO3, ZrO2 and Y2O3 as well as NiO as sintering aid and graphite as pore former. The effects of graphite content on the microstructure of porous electrolyte layer (PEL) and the fuel cell performance were investigated. When the graphite content was 40 g in 165 g starting powders, the obtained PEL had the highest porosity and suitable shrinkage rate. With 20 wt% impregnated NiO anode, the single cell based on porous/dense bilayer BZY electrolyte matrix with 40 g graphite in 165 g PEL starting powders achieved the best performance with a maximum power density of 184 mW cm−2 at 650 °C and operated with a stable output voltage of 0.72 V for 20 h at a constant current density of 100 mA cm−2 during H2/ambient air operation. The results indicate that the design and fabrication of porous/dense bilayer BZY electrolyte matrix is promising for the development of cost-effective PCFCs.

Tribological performances of graphite–MoS2 coating at various high temperatures

The tribological performances of the graphite–MoS2 coating deposited on the disc specimen, whose material is GCr15 steel, are experimentally investigated under high temperatures. Effects of graphite content, applied load, rotational speed and test temperature on the coefficient of friction and wear scar width of the coating are evaluated. The graphite–MoS2 coating with 20 wt% graphite addition exhibits the relatively small coefficient of friction and wear scar width at 200 ℃. At or beyond 400 ℃, the coating's lubricating effect is severely deteriorated with the testing time. At 200 ℃, there exists a critical applied load of 10 N and a critical rotational speed of 1250 rpm beyond which the MoS2 coating with 20 wt% graphite content is worn through and brings out a large coefficient of friction. The test results indicate that adding 20 wt% to 33 wt% graphite to MoS2 coating leads to an obvious reduction of the coefficient of friction and wear scar width for the coating at 200 ℃.

Tribological performances of Ti(C,N)-based cermets with different graphite contents in dry sliding condition

Ti(C,N)-based cermets with different graphite (Gr) contents were prepared to solve the problem that the cermets self-mated couples suffer severe wear in dry sliding condition. The microstructure and mechanical properties of cermets with varying graphite contents were observed and evaluated, and the effects of graphite content on the tribological performances of the cermets self-mated rubbing pairs were investigated using a block-on-ring apparatus at room temperature. The results show that the distribution of graphite phase in cermets transforms from segregated floc-like to uniform granular with the increase of graphite content. The hardness and fracture toughness of the cermets self-lubricating materials rise firstly and then fall, whereas the transverse rupture strength decreases gradually with graphite adding. Compared with the cermet without graphite, the tribological properties of the cermets containing graphite are improved significantly. When the graphite addition reaches 0.8wt%, the cermet exhibits the optimal wear resistance, which is attributed to the formation of a stable lubricating film composed of graphite and metal oxides (TiO2, MoO3, NiO) on the tribo-surface. Additionally, results reveal that the wear mechanism of cermets in dry condition is a complicated regime containing abrasion, adhesion and oxidation. After adding graphite, the adhesive wear of cermets is inhibited effectively, however the abrasive wear will be aggravated as the graphite content increases continuously.

Piezoresistive Characterization of Polyethylene Terephthalate–Graphite Composite

Abstract This study investigated the mechanical properties as well as electrical resistivity and piezoresistivity of the polyethylene terephthalate (PET)–graphite composite. The effects of graphite content, loading type, and loading rate on the performance of PET–graphite composite were investigated. Test results showed that the compressive and flexural strengths of the composite decreased rapidly as the graphite content was beyond 12 %. The electrical resistivity of the composite decreased from 3.5 × 108 Ω · cm to 2.2 × 103 Ω · cm as the graphite contents increased from 8 % to 12 %. The graphite content for the conductive percolation threshold of the PET–graphite composite was determined to be 13 %. The PET–graphite composite was sensitive to low compressive stress. When the compressive stress reached 0.7 MPa, the fractional change in the resistivity could reach 10.5 %. The piezoresistivity of the PET–graphite composite was significantly influenced by the loading rates. The field test indicated that the composite was able to detect the traffic effectively.

Effect of graphite content on the wear behavior of Al/2SiC/Gr hybrid nano-composites respectively in the ambient environment and an acidic solution

Effects of graphite (Gr) content on the dry sliding wear, solid-particle erosion and corrosive wear of Al6061based hybrid nano-composites fabricated via powder metallurgy route containing 2vol% of SiC nano-particles and 0,2, 5 and 7vol%of micron-sized Gr particles were investigated. The hardness of these composites decreased almost linearly with their Gr content, accompanied with decreased erosion resistance at 90° particle impingement. Also the conductive Gr particles as the micro-cathodes enhanced galvanic effect and resulted in an increase in the corrosion rate and thereby stronger corrosive wear of nano-composites. The Al6061/2SiC/2Gr nano-composite exhibited the minimum wear rate and friction coefficient.

Effects of graphite content and compaction pressure on hydrogen desorption properties of Mg(NH2)2–2LiH based tank

Cylindrical lab-scale tank based on compacts of Mg(NH2)2–2LiH–0.07KOH mixed with expanded natural graphite (ENG) was designed. And the influences of ENG content and compaction pressure on the hydrogen desorption properties of the tank, including bed temperature, hydrogen flow rate and desorption capacity were investigated under the constant flow rate mode. As the ENG content increases, the heat transfer property of the hydride bed improves significantly, which results in the rise of the central temperature of the hydride bed and the increase of the hydrogen desorption time at the constant flow rate. As the compaction pressure increases, the hydrogen permeability of the hydride bed decreases obviously, but the entire desorption process changes a little. The results indicate that the heat transfer behavior of the hydride bed dominates the hydrogen desorption process of the tank.

Effect of Sliding Speed on Tribological Properties of Microwave Sintered Copper-Graphite Composites

Effects of graphite content, and sliding speed on the tribological characteristics of copper-graphite composites under dry sliding condition were evaluated using a pin-on-disc tribometer. The worn surfaces of the composites were analyzed through Scanning Electron Microscopy (SEM). The experimental results revealed the improvement in wear resistance with increasing graphite content. The friction coefficient is also gradually decreasing upto 25 vol% graphite. Sliding speed has an effect on copper (5-15 vol%) graphite composites where as sliding speed has no effect in copper-(20-30 vol%) graphite composites. This difference is attributed to availability of self-lubricating graphite layer at the contact zone.

Resistivity measurement of conductive asphalt concrete based on two-electrode method

The objective of this work is to develop a novel methodology for determining real resistivity of conductive asphalt concrete based on two-electrode method. Due to an influence of contact resistance, the measured resistivity is always not equal to the real resistivity. To determine the real resistivity, a linear relationship of the measured resistivity, contact resistance and the real resistivity was established. Then experiments for six specimens with varying graphite contents were designed and performed to validate the formulation. Results of experiments demonstrate that the slope of the line represents contact resistance, and the intercept indicates the real resistivity. The effects of graphite content on contact resistance and real resistivity are also revealed. Finally, results show that the influence of contact resistance on accuracy of resisitvity measurement becomes more serious if graphite content is beyond 3%. Hence, it is the time to choose this novel methodology to determine the real resistivity of asphalt concrete by taking account of contact resistance.

A study on the physicochemical properties of a graphite/polybenzoxazine composite for bipolar plate of polymer electrolyte membrane fuel cells

The effects of graphite content, compression molding conditions, and types of catalysts on the mechanical and electrical properties and corrosion resistance of a graphite composite based on benzoxazine resin for the bipolar plate of polymer electrolyte membrane fuel cells (PEMFC) are provided in this study. Four kinds of catalysts based on imidazole (Im) acting as catalysts are investigated in order to enhance the physicochemical properties of the graphite/polybenzoxazine composites. The characteristics of the graphite composites based on benzoxazine resin with 85 wt% graphite content prepared via compression molding satisfy the US DOE targets for the bipolar plate of a PEMFC. A graphite composite based on polybenzoxazine with an Im-based catalyst having a relatively long alkyl chain shows the best performance in terms of flexural strength and corrosion resistance without sacrificing electrical conductivity. This graphite/polybenzoxazine composite can be successfully molded as a bipolar plate with excellent physicochemical properties through a compression molding process. Open image in new window

Effect of wear conditions on tribological properties of electrolessly-deposited Ni-P-Gr-SiC hybrid composite coating

The friction and wear properties of the electrolessly-deposited Ni-P-Gr-SiC composites were investigated. The effects of graphite content, load and rotation speed on the friction coefficient and wear resistance of the composite coatings were mainly investigated. The worn surface and cross section of the coatings were characterized by scanning electron microscopy and energy-dispersive X-ray analysis. The results show that the composite coatings reveal good antifriction and wear resistance due to the synergic effect of graphite and SiC particles. The formation of graphite-rich mechanically mixed layer (GRMML) on the surface of Ni-P-Gr-SiC coating contributes to the good tribological behavior of the wear counterparts and SiC particles play a load bearing role in protecting GRMML from shearing easily.

Synthesis and characterization of BaFe12O19/Fe3O4 and BaFe12O19/Fe/Fe3O4 magnetic nano-composites

In this research mixtures of barium ferrite and graphite with different molar ratios were mixed and milled in a planetary ball mill to produce a BaFe12O19/Fe3O4 nano-composite. Some of the milled samples were then heat treated at 750 and 850°C under vacuum to synthesize BaFe12O19/Fe/Fe3O4 composite. The effects of graphite content in the mixtures (C:O molar ratio of 0.9, 1 and 1.1), milling time and heat treatment on the characteristics of powder mixtures were investigated by XRD, VSM and TEM. XRD results showed that barium ferrite was partially reduced to Fe3O4 during milling. It was also found that increasing the C:O molar ratio from 0.9 to 1.1 accelerated the reduction process. The results showed that the magnetic properties of the milled and heat treated samples were altered by changing the C:O molar ratio. The saturation magnetization of BaFe12O19/Fe3O4 nano-composite formed in the 40h milled sample with a C:O molar ratio of 0.9 was 27.86emu/g. This value increased to156.40emu/g after heat treatment at 850°C. Increasing the C:O molar ratio from 0.9 to 1.1 increased the saturation magnetization to 169.19emu/g in the heat treated sample. The VSM results also showed that the coercivity of milled and heat treated samples is influenced by C:O molar ratio. The lowest coercivity value of 24.36Oe was obtained in the sample milled for 40h and then heat treated at 850°C. This result is correlated well with the XRD results and confirmed mainly soft magnetic nature of the produced nano-composite.

Effects of graphite content on the microstructure and wear properties of an AISI 8620 steel surface modified by tungsten inert gas (TIG)

A surface modification process was used to modify the surface properties of a steel material by using the TIG method on AISI 8620 steel with a 135-A constant energy input and the addition of 0.5 g of constant SiC particle and variable C content. SEM and optical microscopy were used to characterize the microstructure of the materials tested. To evaluate the mechanical properties, the microhardness distribution was determined from the surface to the central region. Wear tests were performed under dry friction conditions with WC balls at a 5-N normal load in a ball-on-disk geometry. The results of this study indicated that the surface properties of the material could be modified with the TIG method and by using SiC/C particles and that the newly formed structure could improve the wear characteristics and the hardness of the surface.

Effects of graphite content on microstructure and tribological properties of graphite/TiC/Ni-base alloy composite coatings

In order to reduce the friction coefficients and further improve the anti-wear properties of Ni-base alloy coatings reinforced by TiC particles, graphite/TiC/Ni-base alloy (GTN) coatings were prepared on the surface of 45 carbon steel. The effects of graphite content on the microstructure and tribological properties of the GTN coatings were investigated. The results show that the addition of graphite to the GTN coatings may greatly reduce the friction coefficients and improve their wear resistance. The 6.56GTN and 12.71GTN coatings exhibit excellent integrated properties of anti-friction and wear resistance under low and high loads, respectively. Under a low load, the wear mechanisms of the GTN coatings are mainly multi-plastic deformation with slight abrasive wear and gradually change into mixture of multi-plastic deformation, delamination and micro-cutting wear with the increase of graphite fraction. As the load increases, the main wear mechanisms gradually change from micro-cracks, micro-cutting and adhesive wear to micro-cutting and micro-fracture with the increase of graphite fraction.

Effects of Graphite Content on Performance of Powder Injection Molding Copper-Based Antifriction Materials

Copper-based antifriction materials were prepared by powder injection molding. The wear rate of the antifriction material and the coupled parts, and the friction coefficient were studied under the friction condition by changing the content of graphite from 2.5 % to 25%. The results show that the wear rate and the friction coefficient decrease with the increase of the graphite content when the content of graphite is less than 10%. When the content of graphite is 10%~15%, the friction coefficient can be kept stably at low levels(0.25), thereby better density and hardness can be obtained by reducing the consumption of antifriction material and coupled parts, which improves the antifriction effect and protects the coupled parts at the same time. When the content of the graphite is larger than 15%, the wear rate of the antifriction material goes up and the friction coefficient is increased.

FABRICATION AND CHARACTERIZATION OF ALUMINUM- GRAPHITE COMPOSITES

ABSTRACT– In the present study, a proposed technique called “ex-situ and in-situ powder metallurgy” has been developed to produce aluminum–graphite (Al/Gr) composites. In order to avoid any interfacial reactions between the graphite and the aluminum, an isostatic pressing of material powders followed by hot extrusion techniques was used. Five weight percentages of graphite flakes were mixed with Al powder using a mechanical mixing stirrer. The thermal characteristics and microstructures of the composite were investigated. The effects of graphite content and SiC formation on structure characteristics and properties of composites were investigated. The results showed that some graphite flakes have reacted with silicon present in commercial aluminum powder and formed silicon carbide (SiC) at temperatures above 252 ◦ C. SiC has great effect on composite characteristics. The results also showed refinement and uniform distribution of graphite and SiC particles within the aluminum matrix. Keywords:

Tribological performance of graphite containing tin lead bronze–steel bimetal under reciprocal sliding test

As a solid self-lubricating material to serve under heavy load and low velocity, graphite containing tin lead bronze–steel bimetal composites were prepared using the powder metallurgy (P/M) technique. Effects of graphite content on tribological performance under reciprocal sliding were studied using the UMT-2MT tribo-meter. The optimal performance of average friction coefficient, maximum friction coefficient, friction coefficient amplitude and wear resistance can be achieved at the graphite content of ∼3 wt%. Appropriate graphite content and hardness are the two most crucial factors to achieve a good quality lubricating film on the worn surface and hence the desired solid lubrication performance.

Preparation and electrochemical properties of Co3O4/graphite composites as anodes of lithium ion batteries

Co3O4/graphite composites were synthesized by precipitation of cobalt oxalate on the surface of graphite and pyrolysis of the precipitate, and the effects of graphite content and calcination temperature on the electrochemical properties of the composites were investigated. The samples were characterized by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffractometry (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and charge/discharge measurements. With increasing the graphite content, the reversible capacity of the Co3O4/graphite composites decreases, while cycling stability improves dramatically, and the addition of graphite obviously decreases the average potential of lithium intercalation/deintercalation. The reversible capacity of the composites with 50% graphite rises from 583 to 725 mA·h/g as the calcination temperature increases from 300 to 500 °C, and the Co3O4/graphite composites synthesized at 400 °C show the best cycling stability without capacity loss in the initial 20 cycles. The CV profile of the composite presents two couples of redox peaks, corresponding to the lithium intercalaction/deintercalation for graphite and Co3O4, respectively. EIS studies indicate that the electrochemical impedance decreases with increasing the graphite content.

Mechanosynthesis of molybdenum carbides by ball milling at room temperature

β-Mo2C and high temperature phase η-Mo3C2 were mechanosynthesised by ball milling mixtures (MoO3+2Al+xC) with different molar graphite contents (x=0.5, 1.0, 1.25, 1.5, 1.75, 2.0) at room temperature. The experimental results and thermodynamic calculations reveal that the mechanosynthesis reactions were mechanically induced self-propagating reaction (MSR). The effects of graphite content on the phase constitution of the resultants were discussed. The structure and thermal stability of the molybdenum carbides were also studied.