Graphite Volume Research Articles

Monitoring and analyses of substrate surface in first stages of graphene growth in plasma-enhanced chemical vapor deposition

In situ ellipsometry was carried out as well as ex situ measurements by scanning electron microscopy and Raman spectroscopy for the analyses of substrate surface in the first stage of graphene growth in plasma-enhanced chemical vapor deposition. Evolutions of the ellipsometric parameters Ψ and Δ were precisely measured during the growth of graphene with the sensitivity far less than 1 nm in film thickness. By the fitting of the experimentally obtained trajectory of ellipsometric parameters on the Ψ–Δ coordinate plane to that of the calculated ones, we confirmed that the graphite volume fraction decreased with growth after a dense graphite material initially formed. This suggests that carbon nanowalls grew on a thin graphitic layer.

Numerical and Experimental Investigation of Piezoresistance of Asphalt Concrete Containing Graphite

Asphalt concrete is an insulating material. Conductive materials are added to asphalt concrete in order that improving the conductivity. Conductive asphalt concrete (CAC) has become a promising method to snow melting and self-monitoring. In this study, the piezoresistance of CAC which can be improved the conductivity through graphite are analyzed. Based on the interparticle separation and bitumen film-thickness of graphite particle, a model has been developed to predict the piezoresistance under the applied stress. The influences of applied stress, graphite diameter, graphite volume fraction, compressive modulus on the piezoresistance are interpreted through laboratory experiments. Both the numerical and experimental results show that the theoretical data obtained from the model are found to agree with the experimental ones fairly well. In addition, it was found that all these parameters influence the piezoresistance by altering the change process of interparticle separation of graphite.

Experimental Study on PCDD/Fs Adsorption onto Nano-Graphite

ABSTRACTThe adsorption of the 17 toxic 2,3,7,8-substituted dioxin congeners onto graphite was investigated using a laboratory-scale fixed-bed adsorption system. First, the morphology and microstructure of graphite were characterized by Scanning Electron Microscopy and Brunauer-Emmer-Teller surface estimation. Removal efficiency of the 17 toxic PCDD/Fs varies from 89.31% to 99.96% and the amount adsorbed on graphite is a linear function of the inlet concentration of PCDD/Fs, as it was varied from 2.3–20.7 ng I-TEQ Nm–3. Operating over 3 hours, it is observed that the saturation time of fixed-bed adsorbers is less when the inlet concentration is higher. The removal efficiency of dioxin depends on vapour pressure and rises strongly with increasing chlorine substitution number. Removal efficiencies strongly correlate with vapour pressure, with R2 0.96 for PCDDs and 0.99 for PCDFs, respectively. The removal efficiency of dioxin decreases linearly as the temperature increased (R2 = 0.99). Possibly, π-π interactions between PCDD/Fs and graphite sheets lead to a high adsorption capacity of dioxin. The high mesopores volume and pore structure of graphite are critical factors when adsorbing dioxin.

IMPACT OF COOLING CHARACTER ON STRUCTURE AND CTE OF INVARALLOY WITH 0,6% C

The qualitative and quantitative analysis of distribution of structural components (-phase + graphite) in the carbon-containing invar alloy with 0,6% C was conducted. It is demonstrated that velocity of crystallization produces a strong influence on the dispersion of graphite and on the volume fraction of graphite components in the alloy, i.e. at faster cooling the graphite volume fraction and its dispersion are noticeably higher than at slower cooling. The linear thermal expansion coefficient of both slowly cooled and fast cooled alloys maintains its values constant at heating temperatures lower 200 oC. However, in the alloy received as a result of fast cooling this coefficient values are lower than at slow cooling which can be explained by a lower content of carbon in the -phase, being a carrier of invar properties, of the slowly cooled alloy.

Effects of boron addition and austempering time on microstructure, hardness and tensile properties of ductile irons

The present work analyzes the effect of boron addition to an Austempered Ductile Iron, in amounts from zero to 120ppm. It has been found that boron has a strong effect on the equivalent carbon content, resulting in an increase on the precipitated graphite volume and a decrease in the dissolved carbon content in the matrix. This in turn, increases the ferrite volume fraction in the as-cast conditions from 0.24 in the base alloy to 0.78 for the iron with 120ppm of boron. Furthermore, a decrease in the nodularity from 100% in the base alloy to 83% with 120ppm of boron has been observed. During austempering, the transformation to ausferrite was faster and lower volumes of martensite and unstable austenite were detected when boron increased; this promoted lower hardness values, 239HV for the base iron and 189HV for the 120ppm boron alloy. The increase in hardness and strength, typical for the start of bainite formation, were not observed in the boron added irons, but just in the base alloy. Under this basis, it is assumed that at least the addition of 60ppm of boron extended the optimal processing window. The higher values of strength and ductility were obtained for the alloy with 60ppm of boron; these results are discussed in terms of the graphitizing effect of boron in these irons and the reduced amount of carbon dissolved in austenite.

The influence of matrix alloy on the microstructure and properties of (flake graphite + diamond)/Cu composites by hot pressing

Cu matrix composites reinforced with flake graphite and diamond particles were prepared by a powder hot pressing process. In this paper, we assess the influence of particle size and the powder composition of the raw material on the microstructure and thermal properties of (Gf + Diamond)/Cu composites. Our results show that Cu powders with small particle sizes that are close to the size of the diamond beneficially increase the performance of prepared composites because their improved flowability allows parallel arrangement of the flake graphite during pressing. The optimum Cu powder composition has an equal volume of the matrix alloy to the combined volume of diamond and flake graphite. Additionally, we compared the experimentally determined thermal conductivity with the theoretically calculated value.

Sliding wear behavior of austempered ductile iron microalloyed with boron

This work analyses the wear resistance under dry sliding conditions by the pin-on-ring method of an austempered ductile iron (ADI) micro-alloyed with different amounts of boron. The main observed effect of boron was to accelerate the transformation of austenite to ausferrite during the austempering heat treatment; this caused lower values of retained austenite in the irons with boron additions. Lower retained austenite amounts, in turn, caused higher wear rates since the amount of austenite able to transform to martensite by plastic deformation is lower. It was observed that wear loss is a function of the iron hardness. For short austempering times higher values of hardness were obtained which produced lower wear; and the main observed wear mechanism was oxidation. On the other hand, for longer austempering times hardness decreased but the ductility was improved (more ausferrite, less martensite). With the increase in load, a higher deformation under the worn surface was observed and wear was slightly increased. It was observed that boron additions increased the carbon diffusion and accelerated the ausferrite transformation promoting a more homogeneous microstructure. Due to this, a higher ductility but lower values of hardness were obtained, which changed the wear behavior. The higher hardness values of the base alloy are related to a low friction coefficient and lower wear. On the other hand, boron added irons were softer than the base alloy, but the higher volume of graphite acting as a lubricant and the finer matrix contributed to give wear resistance, giving a good balance of mechanical properties.

Mechanical and Wear Properties of High Carbon Grey Cast Iron for Automotive Brake Application

The mechanical and wear properties of three high carbon grades of cast iron alloys for brake application has been investigated in this work. The internal specification is to achieve the hardness of 190–260 BHN with 3.6–3.8 % carbon content and other alloying elements. The results show that the addition of Cu (0.5 %) and Cr (0.33 %) has only produced hardness of 170–180 BHN. The tensile strength and hardness decline by 15–20 % when graphite volume increases from 30 to 50 %. All the samples show both mass loss as well mass gains with uniform friction coefficient due to carbon layer deposit at the interface.

Preparation and Applications of the Expandable Graphite Catalyst with Hierarchically Porous Structure in the Aldol Condensation

The expandable graphite catalyst with hierarchically porous structure was prepared by chemical oxidation. The catalyst samples were characterized by X-ray diffraction, scanning electron microscope, N2 adsorption-desorption, X-ray photoelectron spectroscopy, UV spectroscopy. The results indicated that the expandable graphite is hierarchically porous structure catalyst. The concentrated sulphuric acid and the acetic acid had been inserted into the hierarchically pores. As the volume of the expandable graphite expanded, the amount of mixed acid in the hierarchically pores increased,as well as the acid catalytic centre increased. It can be concluded that the catalytic activity increase. The aldol condensation showed the expandable graphite with hierarchically porous structure has excellent catalytic activity and reusability. The reaction conditions were moderate and pollutions had not been found.

Modification of Expandable Graphite and Adsorption for Methyl Orange

Results of preparing expandable graphite through Microwave Expansion method with the material of expandable graphite made through the Chemical Oxidation process show that in the temperature range of 700°C~900°C, the volume of expandable graphite will bigger with the rise of temperature, reaching the maximum 203 ml/g at the temperature of 900°C. The expandable graphite of 900°C can be modified in 0.5M hydrochloric acid; at the temperature of 40°C and 6h later, decolourization ratio of methyl orange will reach 95%. If the graphite is modified in concentrated nitric acid with the temperature of 50°C for 6 hours, the decolourization rate will be 96%. In terms of cost, it is more economical to modify expandable graphite in hydrochloric acid.

Synergic Exfoliation of Graphene with Organic Molecules and Inorganic Ions for the Electrochemical Production of Flexible Electrodes.

A facile and efficient method based on electrochemistry for the production of graphene-based materials for electronics is demonstrated. Uncharged acetonitrile molecules are intercalated in graphite by electrochemical treatment, owing to the synergic action of perchlorate ions dissolved in acetonitrile. Then, acetonitrile molecules are decomposed with microwave irradiation, which causes gas production and rapid graphite exfoliation, with an increase in the graphite volume of up to 600 %. Upon further processing and purification, highly dispersible nanosheets are obtained that can be processed into thin layers by roll-to-roll transfer or into thicker electrodes with excellent capacitance stability upon extensive charging/discharging cycles. The good exfoliation yield (>50 % of monolayers), minimal oxidation damage and good electrochemical stability of the nanosheets obtained were confirmed by scanning force and electron microscopy, as well as Raman spectroscopy and galvanostatic analyses.

Optimization of initial redox potential in the preparation of expandable graphite by chemical oxidation

The initial redox potential is employed to control the quality of expandable graphite prepared from natural flake graphite by chemical oxidation using KMnO4 as oxidant and sulfuric acid as intercalation agent. The effects of the initial redox potential on the layer structure and the expansion volume of the expandable graphite are investigated by scanning electron microscopy and X-ray diffraction. Results show that there is an optimum initial redox potential range between 1 340 and 1 819 mV, at which the extent of graphite layer oxidation is moderate and the intercalation reaction is complete. Low-order graphite intercalation compounds are formed and a high expansion volume of the expandable graphite can be obtained in this potential range. The redox potential can be used as a quality control parameter for the industrial preparation of expandable graphite.

Effective properties of nodular cast-iron: A multi-scale computational approach

A numerical approach based on a computational constitutive multi-scale model is used in this work to predict the effective Young’s modulus and Poisson’s ratio of a pearlitic nodular cast iron. The Representative Volume Element (RVE) is defined based on a set of micrographs acquired from an optical device. For each micrograph, two RVE with different shape, rectangular and hexagonal, are defined. The volume fraction of graphite and metal matrix and the boundary of each object were identified on each RVE by using a procedure of image enhanced and segmentation. The set of RVE obtained was meshed with triangular finite elements. The numerical results obtained with rectangular and hexagonal RVE are compared with results obtained by means of an analytical expression. The influence of graphite fractions, aspect ratios and nodularity are investigated. The results show that graphite fraction has the largest influence on the effective modulus E. This feature is independent of the external shape of the RVE. Only one multi-scale model show a good agreement with the analytical expression in the effective modulus E. Also, the influence of the graphite volume fraction on the effective Poisson’s ratio is investigated by means of multi-scale simulation.

Graphite nodules and fatigue crack propagation micromechanisms in a ferritic ductile cast iron

ABSTRACTSince 1948, ductile cast irons (DCIs) are able to combine the toughness of steels with the good castability of grey irons. They are widely used in a number of applications, for example wheels, gears, and crankshafts in cars and trucks. Their fatigue crack propagation resistance depends on loading conditions, chemical composition, matrix microstructure and graphite elements morphology (graphite volume fraction, graphite elements nodularity, graphite elements distribution and dimensions).In this work, a ferritic DCI was investigated focusing fatigue crack propagation micromechanisms: step by step fatigue crack propagation tests were performed considering compact‐type specimens, and crack propagation micromechanisms were investigated by means of scanning electron microscope and digital microscope lateral surfaces observations, considering different loading conditions. Furthermore, overloads were also considered during fatigue crack propagation, investigating their influence on DCI fatigue crack propagation micromechanisms.

Preparation of the Sulfur-Free Expanded Graphite Using Induction Heating Method

The sulfur-free expansible graphite and expanded graphite were prepared by induction heating method at different temperature using natural flake graphite, nitric acid and phosphoric acid as the raw materials. Expansion effect and heating mechanism has been analyzed. The microstructure of expansible graphite and expanded graphite has been analyzed by means of a scanning electron microscope and an energy spectrum analyzer. The result shows that the expanded volume of the expansible graphite can reach 200mL/g. Expanded graphite is characterized by vermicular structure and more developed pore structure where the pore of internal level basically maintains the original layer appearance of natural flake graphite.

Effects of Impregnating and Heat Treatment on the Oxidation and Thermal Shock Properties of Pressureless Sintered SiC/Graphite Composites

SiC/0~20% graphite (volume fraction) composites were successfully fabricated by pressureless sintering at 1700 °C for 2 h in nitrogen atmosphere. The impregnating and heat treatment process for the samples with silica sol/phenolic impregnant was carried out at 1450 °C for 2 h in nitrogen atmosphere. The Vickers hardness, oxidation resistance and thermal shock property of the composites before and after treatment were tested, and the microstructure and phase composition were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results shown that the Vickers hardness of SiC/20% graphite was increased from 3.35 GPa to 4.90 GPa by heat treatment, and the oxidation resistance and thermal shock property of the heat treated samples were also improved significantly. The SEM and XRD results revealed that the formation of new SiC particles by the reaction of silica sol and phenolic was the main reason for the mechanical properties improvements.

Numerical analysis of a medium scale latent energy storage unit for district heating systems

The present paper describes the application of computational fluid-dynamics (CFD) to the design and characterization of a medium scale energy storage unit for district heating systems. The shell-and-tube LHTES unit contains a technical grade paraffin (RT100) as phase change material (PCM) and uses water as heat transfer fluid (HTF). The system has been designed to transfer heat from the district to the building heating networks. After an initial description of the LHTES unit and a wide literature overview on the subject, the paper discusses the need for thermal enhancement to improve the thermal conductivity of the PCM. A solution based on a paraffin-graphite composite with a 15% graphite volume fraction has been found to be well performing in this particular application. Several operating scenarios characterized by heat requests ranging between 130 kW and 400 kW have been explored and the main outputs presented as function of Re and St numbers. The timewise variations of other significant quantities such as liquid fraction, sensible and latent energy content, HFT outlet temperature and heat fluxes have been also presented and discussed. A final discussion on the possible system configurations shows that in comparison to traditional water storage systems for district heating, LHTES systems provide, depending on the chose alternative, higher energy storage densities.

Influence of Load and Sliding Speed on Friction and Interface Temperature of Hypereutectic Alloyed Gray Cast Iron

The influence of incremental sliding speeds and normal loads on friction, wear and interface temperature have been investigated on a series of hypereutectic alloyed gray cast irons. Mass loss and friction coefficient values show inverse trend with sliding speeds. The interfacial temperature increases with incremental speeds and loads while the friction coefficient decrease with the same. The alloys with higher volume of graphite show slightly lower interfacial temperature due to higher thermal transport capacity. The conjoint interaction of asperities and debris possibly predominates at lower speeds and higher mass loss predominates at higher sliding speeds.

Dielectric analysis of high‐density polyethylene‐graphite composites for capacitor and EMI shielding application

AbstractIn this article, we present and analyze the dielectric properties of high‐density polyethylene (HDPE)‐graphite(Gr) composites in low and radio frequency ranges for capacitor and electromagnetic interference (EMI) shielding application, respectively. The HDPE‐Gr composites were prepared by mixing and the hot compression mold technique. The dielectric constant and AC conductivity of HDPE‐Gr composites according to graphite volume fraction follow the power law model of percolation theory. The percolation threshold of the composites was estimated to be 0.039 (∼ 7 wt % of Gr). The HDPE‐Gr composites exhibited high dielectric constants, high dissipation factors, and high AC conductivities at a certain addition of graphite in low frequency and radio frequency regions. The AC conductivity of the composites with the higher content of graphite depicted frequency independency in low and radio frequency region. The EMI shielding properties of the HDPE‐Gr composites was evaluated in the radio frequency region and it was found that the maximum EMI shielding of the composites was obtained. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of expanded graphite (EG) dispersion on the mechanical and tribological properties of nitrile rubber/EG composites

In this work, in order to prepare expanded graphite (EG) and acrylonitrile-butadiene rubber (NBR) composites, two preparation methods were adopted, namely directly mechanical blending (micro-composites) and latex compounding technique (nanocomposites), and their resulting mechanical and tribological properties were compared. Meanwhile, the effects of the graphite dispersion state, volume fraction, load and sliding velocity on the friction and wear properties were also explored. As expected, nanocomposite showed better tensile properties, hardness, dynamic storage modulus and lower hysteresis loss peak. The coefficient of friction (COF) for both composites decreased as the load and sliding velocity increased, while the effect of the graphite volume fraction was subtle. The wear rate of nanocomposites decreased as the load and sliding velocity increased. As the load increased, the wear rate of micro-composites increased, but with increasing sliding velocity, the wear rate first increased, reached a maximum, and then decreased. The wear mechanism could be described as graphite sheets de-bonded from matrix, lubricant film formation and breaking, matrix wear, and lubricant film re-formation in cycles. Compared with micro dispersion, the nano-sized dispersion of graphite was beneficial to the formation of a continuous, uniform, and stable lubricant film under high sliding velocity and heavy load, contributing to reduce the COF and improve the wear resistance.