Surface Of Graphite Substrate Research Articles

Oxidation protection performance of ZrB2-LaB6 modified SiC coating at 1700 ℃

In response to the problem of high-temperature oxidation and volatilization of SiC coatings on carbon materials at 1700 ℃, this paper proposes to prepare ZrB2-LaB6 modified SiC coatings on graphite substrate surface by spark plasma sintering. The oxidation protection performance of five different ZrB2-LaB6-SiC coatings with different ratios of ZrB2 and LaB6 was tested at 1700 ℃ isothermal environment and thermal shock environment. The results showed that the oxidation protection efficiency of the coated samples with LaB6 content of 0, 3, 5, 8 and 10 wt% after oxidation for 100 min at 1700 ℃ was 96.6%, 98.0%, 96.9%, 96.4% and 95.5%, respectively. The coating with 3 wt% LaB6 had the best oxidation protection effect with the coating still maintaining its integrity and high oxidation protection efficiency after 20 thermal shock cycles with an overall oxidation protection efficiency after thermal cycling of 97.7%.

The Microstructure, Hardness, Phase Transformation and Mechanical Properties of a NiTi Coating Applied to Graphite Substrate via a Plasma Spraying Process

In this study, Ni50Ti50 powder was coated on the surface of graphite substrate (C) via a plasma spraying process using a radio frequency inductively coupled plasma reactor. The coating was carried out using 12- and 9-kW power under Ar atmosphere. The cross-section of coating layers and the surface were examined with Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectrum (EDX), and X-ray Diffractometer (XRD) analyses and microhardness test. The thickness and quality of the coating increased with the input power. Many pores were detected in the cross-sectional surface areas. Higher input power caused a better coating layer of NiTi alloy. The hardness of the coating layer decreases with higher input power.

Preparation of Silicon Carbide Film by Composite Sintering of Silicon Nitride and Silicon Carbide

Graphite can be protected by coating it with a silicon carbide film. However, studies on the effects of different coating methods and process parameters on the thickness and bond strength of the films are not yet mature. In this paper, silicon nitride (Si3N4) and silicon carbide (SiC) are used as raw materials, and SiC films are successfully prepared on the surface of graphite substrate by composite sintering at high temperatures. The phase and microstructure of SiC films were characterized by X-ray diffractometer (XRD) and scanning electron microscope (SEM), respectively, and the effect and mechanism of sintering temperature on the formation of SiC films were investigated. The results show that Si3N4 and SiC decompose under high temperatures to generate silicon vapor and carbon-silicon gas molecules, which migrate to the graphite surface to react with C and recrystallize to form a SiC film. The main crystal phase of the SiC film at high temperature is 3C-SiC, the spherical SiC grains with smooth surfaces and small size gradually grow into regular hexagonal grains, and the SiC film is denser and thicker.

1,2,3-Triazole lamellar liquid crystal and its non-covalent palladium complex dimer: structure, mesomorphism and self-assembly properties

ABSTRACT 1,4-disubstituted 1H-1,2,3-triazoles L was successfully synthesised using a catalyst-free click reaction between an internal alkyne and an azide compound. Refluxing click product L and palladium chloride in a homogeneous mixed solvent led to the formation of the non-covalent palladium chloride complex PdCl2-L2 , which was characterised by X-ray crystallographic analysis of the template compound, indicating that the complex adopts predominant trans-trans isomeric orientations. The two compounds showed a typical fan-shaped texture of the lamellar mesophase. Because of weak Pd-N coordination bond connections, PdCl2-L2 is a monotropic liquid crystal with a narrower mesomorphic temperature range and relatively poorer thermal stability than ligand L. Furthermore, both L and the complex dimer can self-assemble into strip-shaped nanostructures on the oriented graphite substrate surface.

Formation of Ordered Silicon Structures on a Graphite Surface

The possibility of formation of ordered silicon structures, including silicene, on the graphite substrate surface has been studied. The effect of various conditions of depositing silicon atoms on a finite structure has also been studied. The data on the surface morphology has been obtained by atomic force microscopy, and the electronic structure has been studied by Auger electron spectroscopy.

MoSe2/graphite composite with excellent hydrogen evolution reaction performance fabricated by rapid selenization method

MoSe2/graphite composite was fabricated by rapid selenization method for the hydrogen evolution reaction (HER) application. The microstructure, surface composition and HER performance of MoSe2/graphite composite were evaluated by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and linear sweep voltammetry. The results indicate that the internal and external surface of graphite substrate is covered with MoSe2 nanosheets. The size, number and crystallinity of MoSe2 nanosheets can be controlled by adjusting the selenization parameters to achieve better HER activity of the electrodes. MoSe2/graphite composite prepared at 500 °C for 15 min exhibits the best HER activity and great stability in acidic solution with an over potential of −125.8 mV vs reversible hydrogen electrode (RHE) at the cathodic current density of 10 mA cm−2. The Tafel slope of MoSe2/graphite electrodes is 52.1 mV dec−1, which shows the best performance among the reported literatures of MoSe2 materials.

Oxidation protective ZrB2-MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

In order to protect graphite materials from oxidation, a dense single layer ZrB2-MoSi2-SiC-Si coating was for the first time prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. The phase compositions and microstructure of the prepared coating were analyzed by XRD and SEM. The isothermal thermal cycle oxidation tests were conducted at 1600 °C in air. The results showed that the prepared coating exhibited excellent oxidation protection ability, after oxidation for 150 h, the weight loss was only 0.21%, which was considered to be the depletion of the outer coating and the appearance of defects in the coating. The excellent protection ability was attributed to the dense structure of the coating and the formation of thick oxide layer with immiscible ZrO2 on the outer surface of coating. Two layers could be observed in the coating after oxidation, namely, glassy oxide layer and ZrB2-MoSi2-SiC-Si layer. After thermal shock between 1200 °C and room temperature for 100 cycles, the weight gain was 0.11%. The good thermal shock resistance could be ascribed to the gradient thermal expansion coefficients of different components in the coating and the C/SiC interface layer between the substrate and coating.

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi2-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi2-SiC-Si coating with dense structure consisting of Si, MoSi2 and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600°C and TGA test from room temperature to 1500°C were used to evaluate the oxidation resistance ability of the MoSi2-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600°C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500°C due to the formed SiO2 layer on the surface of coating. After oxidation for 220h at 1600°C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO2 layer and MoSi2-SiC-Si layer.

Black rhenium coating prepared on graphite substrate by electrodeposition in NaCl-KCl-CsCl-K2ReCl6 molten salts

A compact and continuous rhenium coating with black appearance was obtained on the surface of graphite substrate in chloride molten salts with high efficiency and low cost. The rhenium coating prepared at a cathodic current density of 50mA/cm2 at 850°C has a preferential growth orientation of 〈110〉 and a high deposition rate of ~40μm/h. The rhenium coating is composed of a fine equiaxed-grain zone and a spired columnar-grain zone. The surface emittance is as high as 0.73, which is comparable with black rhenium coating reported by Ultramet. The microhardness of the coating as deposited is 350±16.4kg/mm2. The bond strength of rhenium/graphite interface tested at room temperature is ~10.2MPa and is greatly improved after heat treatment at 1200°C for 3h in vacuum.

Rectifying performance and negative differential behavior in graphite—chain—carbon nanotube junctions

In this paper, the (5, 5) capped carbon nanotubes (CNTs) in contact with different lengths of sp monoatomic chains grown on the surface of graphite substrate are fabricated and its electronic transport properties sandwiched between CNT and graphite electrodes are investigated. The first-principles calculations based on nonequilibrium Green's function in combination with density-functional theory show that their rectifying performance and negative differential resistance behavior are observed under very low biases and obviously are enhanced when the length increases. From our analysis, the charge transfer, transmission spectra, projected density of states and evolutions of molecular orbitals are responsible for these phenomena.

Adhesion, hardness and structure of thermal sprayed Al/SiC composite coat on graphite

Combustion powder spraying has been used in the coating of graphite by a layer of Al and SiC composite utilizing the Box–Wilson design that fixes the optimum spraying parameters of Al/SiC wt.% composition, substrate temperature and spraying distance. Graphite substrate surface was conditioned by subjecting to sand blast roughing and nitrogen flushed furnace treating. Under the spraying conditions considered, a single spraying pass attains a coating layer 65 ± 15 µm of average thickness. Two optimum spraying conditions that concern adhesion of coating to graphite and its hardness are obtained. Upon spraying for adhesion purpose, the coat/graphite adhesion strength is found to depend mainly on graphite surface conditioning, but for hardness purpose, it is found to depend mainly on SiC wt.% composition. Microstructure of transversal coating section indicates that adhesion efficiency is due to physical interlocking of molten sprayed materials on the roughened and treated graphite surface. Phases identified from fractured coatings by XRD analysis show individual phase of Al, SiC and graphite in addition to small amount of new extra phases that combine the three constituents such as Al 4C 3 and Al 4Si 2C 5. These phases play a role with the good physical interlocking in achieving adhesion strength that exceeds that of graphite.

Analysis of Lateral Orientation of Single-Walled Carbon Nanotube on Graphite

We investigate the stable lateral orientation of the single-walled carbon nanotube (SW-CNT) physically adsorbed onto the graphite substrate surface using molecular mechanics simulation. The system of the (3,3) armchair-type SW-CNT comprised of 198 carbon atoms with a length of 40.3 A interacting with the rigid graphene sheet is considered. Effect of the initial lateral orientation on the final lateral orientation is discussed. The stability of the initial and the final stable orientations can be explained by analyzing the interaction energy between the SW-CNT and the substrate, as a function of the rotational angle θ and the center position rc=(xc,yc) of the SW-CNT, within the lateral (0001) plane of the graphite substrate. Molecular mechanics simulations for the perfect substrate surface under the condition T=0 give the final stable minima near the initial states, instead of the atomic-scale locking around the global minima. [DOI: 10.1380/ejssnt.2009.48]

Theoretical Simulation of Atomic-Scale Peeling of Single-Walled Carbon Nanotube from Graphite Surface

Molecular mechanics simulation of atomic-scale peeling of carbon nanotube (CNT) from the graphite substrate surface is performed. We have first obtained the theoretical 'peeling force curve' of the CNT, where the CNT physically adsorbed on the graphite substrate surface is gradually retracted or peeled. In the simulation the single-walled carbon nanotube (SW-CNT) of the (3, 3) armchair type with a length of 40.3 A comprised of 198 carbon atoms is used. It is clarified that the peeling force curve shows a characteristic behavior mainly dominated by the van der Waals interaction acting between the CNT and the substrate surface. The typical change of the CNT shape during the peeling process, shows a transition from the 'line contact' to the 'point contact', which reflects the covalent bonding interaction. The peeling force curve gives us information of an elementary process of peeling of the CNT. [DOI: 10.1380/ejssnt.2006.133]

Effect of the tip structure on atomic-force microscopy.

Theoretical simulation of atomic-force microscope (AFM) images is performed with a model of a single-atom tip or two kinds of multiple-atom diamond tips scanned on a graphite substrate surface. Several fundamental effects on AFM images are systematically investigated. First, it is made clear how the AFM images and the force distributions change as the load varies. Then, for the multiple-atom tips, the effects of the tip orientation and the tip apex structure, including the bond length, are examined. The characteristics of the AFM images, such as their detailed microscopic pattern, the symmetry, and the corrugation amplitude, depend strongly on these effects. In the cluster models, the interatomic potential within the tip and the surface is assumed to be harmonic, and that between the tip and the surface is taken as being of the Lennard-Jones type. It is clearly shown that AFM images reflect not merely the geometrical structure of the surface, but also various microscopic properties of the tip and the surface.

Theoretical Simulation of Atomic Force Microscope Based on Cluster Models

The theoretical simulation of atomic force microscope (AFM) images is performed using a model of a multiple-atom diamond tip scanned on a graphite substrate surface. It is clarified how the AFM images and the force distributions change as the load varies. The effect of the tip apex structure is examined by tilting the tip and by using two different tips. In the cluster models, the interatomic potential within the tip and the surface is assumed to be harmonic, and that between the tip and the surface is taken as Lennard-Jones type. It is clearly shown that AFM images are influenced not only by the geometrical structure of the surface, but also by microscopic elastic properties of the tip and the surface as well as the tip structure.

気相反応による炭化ジルコニウム析出

For the purpose of determining the optimum temperature and relating factors for vapor deposition of zirconium carbide, the deposition rates were measured with a thermo-balance.Deposition was observed at emperatures above 770°C and the (100) planes were developed in parallel with the surface of graphite substrate. The both planes of (110) and (100) were found relatively distinct at above 840 and 1200°C, respectively. The deposition took place within 0.4 sec. after the two reacting gas streams were mixed together. The deposition rates were proportional to the linear velocity of hydrogen and the concentrations of zirconium tetrachloride and hydrocarbons. The increase of the rates with temperature became slower at higher temperatures owing to the pyrolysis of hydrocarbons.Higher rates were given by paraffins and naphthenes. However, the highest rates were found in the range of their concentrations of 0.3-0.7vol.% (indicated as reduced value to propane); and the rates were rather decreased by carbon deposition beyond that range.Benzene was out of the above regularity, owing to its large anti-pyrolytic property. Its growth rate was 0.3-0.8μ/min. under the optimum condition at 1200-1300°C.