Carbon Carbide Research Articles

Effects of temperature and duration on oxidation of ceramic composites with silicon carbide matrix and carbon nanoparticles

This paper was to investigate the oxidation behavior of three pressureless sintered ceramic composites of silicon carbide (SiC) matrix with various contents of carbon nanoparticles (Cp) (i.e., 0, 15 and 25wt%) at various temperatures (i.e., 600–1100°C) and durations (i.e., 2–12h). The results indicate that the oxidation of SiC/Cp ceramics in air is uniformly controlled by the oxidation of Cp at <700°C, and the strength of the samples reaches a minimum value at 700°C. At 700–900°C, the oxidation is controlled via the unequal diffusion of O2 through the micro-cracks. The oxidation of SiC/Cp is controlled by O2 diffusion through the structural defects of the matrix at 900–1100°C, and the strength of the composites increases at 1000°C and reaches a maximum value during the initial period of 2h due to the formation of SiO2 layer and the healing of the matrix defects.

Microwave-Assisted Synthesis of Mesoporous Tungsten Carbide/Carbon for Fuel Cell Applications

Different template routes were applied for the fabrication of two types of mesoporous tungsten carbide/carbon (WC/C) assisted with a microwave heating method. One was WC/C–F using triblock copolymer Pluronic F127 (PEO106PPO70PEO106) as soft template and the other was WC/C–S using mesoporous silica SBA-15 as hard template. In our study, sucrose was used as the carbon source for both tungsten carbide and carrier carbon. With loading small amount of Pt, two catalysts (Pt–WC/C–F and Pt–WC/C–S) exhibit better performance than the commercial Pt/C for methanol oxidation. Meanwhile the Pt–WC/C–F catalyst presents higher activity and stability than does the Pt–WC/C–S which is ascribed to the appropriate pore size adjusted by the different template routes.

Electrochemical impedance spectroscopy and indentation studies of pure and composite electroless Ni–P coatings

Electroless Ni–P coatings offer excellent corrosion and wear resistance and ability to withstand acidic and salt solutions. Medium and high phosphorus Ni–P coatings were produced using plating baths with 10g/L or 25g/L of sodium hypophosphite as reducing agent (RA) with composition of the resulting deposits to be 91.5 Ni: 8.5 P and 87.6 Ni: 12.4 P, respectively. From field-emission scanning electron microscope (FE-SEM) examination, the deposit morphology was found to change from nodular with surface porosity and cracks to dense, smooth upon increasing the RA content. Addition of nanostructures such as nanoparticles of alumina (Al2O3) or silicon carbide (SiC) or multi-walled carbon nanotubes (CNT) into Ni–P matrix, at low loading levels, was investigated for their effect on corrosion resistance and hardness of Ni–P–Al2O3, Ni–P–SiC and Ni–P–CNT composite coatings. Electrochemical impedance spectroscopy (EIS) studies in 4wt.% NaCl solution revealed 91.5 Ni: 8.5 P coating to offer much superior corrosion resistance than 87.6 Ni: 12.4 P coating even after immersion for 42days. Among all composite coatings, however, Ni–P–Al2O3 produced from 1.0g/L Al2O3 in plating solution exhibits higher impedance values at low and intermediate frequencies. Nyquist plots for different frequencies were analyzed for comparison between different composite coatings. Microhardness tests indicate higher hardness value of 8.46GPa for Ni–P–SiC coating as compared to 7.42GPa for pure Ni–P coating.

Mechanism of formation and electrochemical performance of carbide-derived carbons obtained from different carbides

Porous carbide-derived carbons (CDCs) are synthesized from different carbide (VC, TiC, NbC) as electrode materials for electrochemical capacitors. The process of carbide–carbon transformation is investigated by observations at different carbide/CDC interfaces. It is found that the restructuring process has much influence on formation of microstructure as well as the resultant electrochemical performance. The carbon structure in the produced CDC is well in accordance with that formed at the carbide/CDC interface, indicating that the microstructure in the produced CDC is decided by re-bonding of the residual carbon atoms. It is further found that the internal stress during carbide–carbon transformation has much influence on the CDC microstructure. In addition, the microstructure in CDCs is dependent on the volumetric concentration of carbon atoms in carbide precursor. Lower volumetric concentration of carbon atoms facilitates the formation of CDC with short and curved graphene structure, which owns easily accessible pores and large specific surface area, and thus high electrochemical performance for ultracapacitor. A novel strategy that controlling microstructure of CDC through controlling the volumetric concentration of carbon atoms in carbide precursor is presented. This strategy is very effective to form designed microstructure of CDC for electrochemical applications.

Multi-product steady-state isotopic transient kinetic analysis of CO hydrogenation over supported molybdenum carbide

Isotopic transient analysis of steady state of CO hydrogenation catalyzed by alumina-supported Mo2C nanoclusters was performed at 573K, 1.2bar syngas with H2/CO=1. An isotope switch from 12CO to 13CO during the steady-state reaction produced a rapid transient associated with CO hydrogenation and a very slow transient associated with turnover of ineffective catalytic sites or exchange of carbidic carbon with reaction intermediates. Although the intrinsic turnover frequency for hydrocarbon formation was similar to that observed on late transition metals, the coverage of reaction intermediates on the Mo2C clusters was very low (≪1%), which accounts for the overall low activity of Mo2C. Promotion of Mo2C with Rb2CO3 lowered the coverage of intermediates even further, but increased the selectivity toward alcohols. Very strong interaction of the alcohols with the promoter suggests that Rb may form alkoxides at reaction conditions.

Influence of operating conditions in a continuously stirred tank reactor on the formation of carbon species on alumina supported cobalt Fischer–Tropsch catalysts

The paper focuses on the identification of carbon species using a wide range of ex situ characterization techniques (TPH/MS, TGA-DTA, XPS, TOF-SIMS) in spent 15wt% Co/Al2O3 supported catalysts exposed to different conditions of Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR) at 20bar of total pressure, different gas-space velocities and H2/CO ratios. Three types of carbonaceous species were uncovered in the used catalysts: (i) hydrcarbobns (probably wax), (ii) strongly adsorbed hydrocarbons and (iii) amorphous polymeric carbon. The amounts of deposed carbon species strongly depend on the operating conditions. In agreement with the observed slow catalyst deactivation, only very small amounts of strongly adsorbed hydrocarbons and polymeric carbon were observed when the catalyst was exposed to a H2/CO=2 syngas ratio and moderate carbon monoxide conversions. Lower gas space velocity and lower H2/CO ratio in syngas lead to larger amounts of deposed carbon species. Strongly adsorbed hydrocarbons and amorphous polymeric carbon seem to contribute to catalyst deactivation. Cobalt carbide and graphitic carbon were not detected in the used catalysts by ex situ methods.

Graphitic encapsulation of MgO and Fe3C nanoparticles in the reaction of iron pentacarbonyl with magnesium

Abstract A simple method to produce highly ordered carbon nanostructures by combustion synthesis is presented. Graphite-encapsulated magnesium oxide, iron carbide nanoparticles and carbon nanobelts were synthesized by the one-step reduction of iron pentacarbonyl with magnesium. High-resolution transmission electron microscopy analysis of the products revealed nanocrystalline MgO and Fe 3 C particles surrounded by a well-crystallized, tight graphite film. The possible formation mechanism is presented and discussed.

수소연료전지용 탄탈륨 탄화물에 대한 암모니아 분해반응

Tantalum carbide crystallites which is to be used for <TEX>$H_2$</TEX> fuel cell has been synthesized via a temperature-programmed reduction of <TEX>$Ta_2O_5$</TEX> with pure <TEX>$CH_4$</TEX>. The resultant Ta carbide crystallites prepared using two different heating rates and space velocity exhibit the different surface areas. The <TEX>$O_2$</TEX> uptake has a linear relation with surface area, corresponding to an oxygen capacity of <TEX>$1.36{\times}10^{13}\;O\;cm^{-2}$</TEX>. Tantalum carbide crystallites are very active for hydrogen production form ammonia decomposition reaction. Tantalum carbides are as much as two orders of magnitude more active than Pt/C catalyst (Engelhard). The highest activity has been observed at a ratio of <TEX>$C_1/Ta^{{\delta}+}=0.85$</TEX>, suggesting the presence of electron transfer between metals and carbon in metal carbides.

Microstructure, mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Abstract Boron carbide (B 4 C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m 2 g −1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m 1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W 2 B 5 platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B 4 C–W 2 B 5 composites could be potential candidate materials for structural applications.

Nanomechanical and boundary lubrication properties of titanium carbide and diamond-like carbon nanoperiod multilayer and nanocomposite films

To develop a new low friction and wear resistant coating, nanometer period TiC and DLC multilayer films composed of titanium carbide and hydrogen-free diamond-like carbon (DLC) layers were deposited by bias rf sputtering. Nanoperiod multilayer films were deposited by controlling the opposing time of the substrate to each of the titanium carbide and graphite targets. Below the 6-nm target layer period, nanocomposite films with nm-scale particles were deposited. Above the 12-nm target layer period, multilayer films were deposited. The nanoindentation hardness and nanowear resistance of the nanoperiod multilayer (TiC/DLC)n films change with the layer period. The 12-nm-target-period (TiC/DLC)n film shows higher hardness and nanowear resistance. The dependence of boundary lubrication properties on target period under poly (alpha-olefin) (PAO), PAO with glycerol monooleate (GMO) and water was investigated by an oscillating tribotest. The boundary lubrication properties of the (TiC/DLC)n films are significantly improved compared with those of the DLC monolayer film. Moreover, the hard and nanowear-resistant 12-nm-target-period multilayer film shows the lowest friction coefficient and a small damage depth under boundary lubrication using PAO, PAO with GMO and water. It is deduced that the 12-nm-target-period multilayer (TiC/DLC)n film has the shortest period and the largest number of interfaces that prevent damage elongation, and it is hard and shows superior nanowear resistance.

Synthesis of Poly(silyne-co-hydridocarbyne) for Silicon Carbide Production

Pre-ceramic polymers have previously been shown to be polymeric precursors to silicon carbide, diamond and diamond-like carbon. Here, we report the synthesis of a pre-ceramic polymer, poly(silyne-co-hydridocarbyne), which was electrochemically synthesized from one monomer containing both silicon and carbon in its structure. The polymer is soluble in common solvents such as CHCl3, CH2Cl2 and THF. Since the polymer contains both silyne and carbyne on its backbone, it can be easily converted to silicon carbide upon heating under an ambient inert atmosphere, or to SiO2 under ambient air atmosphere. Poly(silyne-co-hydridocarbyne) was characterized with UV/Vis spectroscopy, FTIR, 1H-NMR, GPC and Raman spectroscopy. Conversion of the polymer to SiC ceramic was accomplished by heating at 1000 and 750°C under an argon atmosphere and characterized with optical microscopy, SEM, X-Ray and Raman spectroscopies.

Ammonia decomposition over titanium carbides

Ammonia decomposition over titanium carbides were investigated using eight different samples which have been synthesized by TPR (temperature-programmed reduction) method of titanium oxide (<TEX>$TiO_2$</TEX>) with pure <TEX>$CH_4$</TEX>. The resulting materials which were synthesized using wo different heating rates and space velocity exhibited the different surface areas. These results indicated that the structural properties of these materials have been related to heating rates and space velocity employed. The titanium carbides prepared in this study proved to be active for ammonia decomposition, and the activity changed with the particle size/surface area. These showed the relationship between ammonia decomposition activity and the different active species. Compared to molybdenum carbide, the titanium carbides were one order of magnitude less active, suggesting the correlation between the activity difference and the degree of electron transfer between metals and carbon in metal carbides.

Hybrid materials based on MF-4SK membranes modified with silicon carbide and carbon nanotubes

The effect of hydrophobic silicon carbide nanoparticles and carbon nanotubes on the mechanical and transport properties of MF-4SK perfluorinated cation-exchange membranes has been studied. It has been shown that the modification of the membranes with a small amount of silicon carbide and carbon nanotubes enhances both their mechanical strength and selectivity of transport processes. An increase in the dopant concentration leads to deterioration of these parameters and the destruction of the membranes. This change in the properties is attributed to the implantation of nanoparticles into the hydrophobic membrane matrix, which results in a decrease in its elasticity.

Shear-induced phase transition of nanocrystalline hexagonal boron nitride to wurtzitic structure at room temperature and lower pressure

Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure-temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h)BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under hydrostatic compression to 52.8 GPa, the same hBN sample did not transform to wBN but probably underwent a reversible transformation to a high-pressure disordered phase with closed-packed buckled layers. The current phase-transition pressure is the lowest among all reported direct-phase transitions from hBN to wBN at room temperature. Usually, large plastic straining leads to disordering and amorphization; here, in contrast, highly disordered hBN transformed to crystalline wBN. The mechanisms of strain-induced phase transformation and the reasons for such a low transformation pressure are discussed. Our results demonstrate a potential of low pressure-room temperature synthesis of superhard materials under plastic shear from disordered or amorphous precursors. They also open a pathway of phase transformation of nanocrystalline materials and materials with disordered and amorphous structures under extensive shear.

Amorphous silicon carbon films prepared by hybrid plasma enhanced chemical vapor/sputtering deposition system: Effects of r.f. power

Silicon carbon films were deposited using a hybrid radio frequency (r.f.) plasma enhanced chemical vapor deposition (PECVD)/sputtering deposition system at different r.f. powers. This deposition system combines the advantages of r.f. PECVD and sputtering techniques for the deposition of silicon carbon films with the added advantage of eliminating the use of highly toxic silane gas in the deposition process. Silicon (Si) atoms were sputtered from a pure amorphous silicon (a-Si) target by argon (Ar) ions and carbon (C) atoms were incorporated into the film from C based growth radicals generated through the discharge of methane (CH4) gas. The effects of r.f. powers of 60, 80, 100, 120 and 150W applied during the deposition process on the structural and optical properties of the films were investigated. Raman spectroscopic studies showed that the silicon carbon films contain amorphous silicon carbide (SiC) and amorphous carbon (a-C) phases. The r.f. power showed significant influence on the C incorporation in the film structure. The a-C phases became more ordered in films with high C incorporation in the film structure. These films also produced high photoluminescence emission intensity at around 600nm wavelength as a result of quantum confinement effects from the presence of sp2 C clusters embedded in the a-SiC and a-C phases in the films.

Analysis of the anisotropy, stoichiometry and polytypes in pyrolytic carbon and silicon carbide coatings

Silicon carbide (SiC) and pyrolytic carbon (PyC) coatings in tristructural isotropic (TRISO) coated fuel particles were characterised by a combination of 2-modulator generalised ellipsometry microscopy (2-MGEM), Raman spectroscopy and transmission electron microscopy. We compared the values of anisotropy obtained from 2-MGEM and Raman spectroscopy to investigate the effect of sampling area and microstructure. No linear correlation in anisotropy was found between these two techniques despite both sampling areas of 2–5μm. The largest deviations were observed for highly anisotropic samples with optical anisotropy factors (OPTAFs) above 1.1. For medium and low anisotropy samples (OPTAF<1.1) the relationship is close to linear. The limited use of only the average value of diattenuation does not give an accurate representation of the characteristics of the coatings as significant areas can exist with considerably higher diattenuations that could increase the probability of failure under neutron irradiation. We also observe a change in the diattenuation of SiC due to the presence of stacking faults as confirmed by Raman spectroscopy. Raman spectroscopy was also used to perform semiquantitative analysis of the Si and carbon excess in SiC in four TRISO particles.

Boron carbide as a target for the SPES project

Within the framework of the research on targets for the SPES project (Selective Production of Exotic Species), porous boron carbide (B4C) based materials were produced from the carbothermal reduction of boric acid and two different carbon sources, i.e. citric acid and phenolic resin. Samples composition and microstructural morphology were studied by means of X-ray diffraction spectrometry (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS). The amount of total porosity was obtained from the comparison between the theoretical density and the measured bulk density. To better characterize the material microstructure, nitrogen physisorption measurements were performed in order to obtain data about the type of generated porosity and the specific surface area of the samples. Analysis performed on the samples show that after the final thermal treatment they are composed of boron carbide and residual free carbon, whose quantity is related to the processes involved in the two synthesis. Remarkable differences in the overall weight loss have been noticed for the two different reactions, resulting in different densities and pore size distributions, but in both cases similar values of specific surface area (SSA) were obtained.

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65μm, 33.70μm and 63.35μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74kg/mm2 to 1674.90±100.00kg/mm2 and fracture toughness drops from 5.76±0.26MPam1/2 to 3.4±0.37MPam1/2. However, flexural strength decreases from 403.41±5.70MPa to 256.15±25.05MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.

Spark plasma sintering of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ceramic composite

In this paper spark plasma sintering (SPS) of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ultra-high temperature ceramic matrix composites is reported. Systematic investigations on the effect of reinforcement type (SiC and CNTs) and content (10–40vol.% SiC and 2–6vol.% CNTs) on densification behavior, microstructure development, and mechanical properties (microhardness, bi-axial flexural strength, and indentation fracture toughness) are presented. With the similar SPS processing parameters (1900°C, 70MPa pressure, and 15min soaking time), near-full densification (>99% relative density) was achieved with 10–40% SiC (in ZrB2–SiC) and 4–6% CNT (in ZrB2–CNT) reinforced composites. The SiC and CNT reinforcement further improved the indentation fracture toughness of the composites through a range of toughening mechanisms, including particle shearing, crack deflection at the particle-matrix interface, and grain pull-outs for ZrB2–SiC composites, and CNT pull-outs and crack deflection in ZrB2–CNT composites.

Carbon Diffusion in Lanthanum Erbium Carbide Stabilized in the Cubic Fluorite Structure

Lanthanum erbium carbide, La0.5Er0.5C2, a salt-like carbide with a cubic fluorite phase structure, has been produced from 13C, allowing carbon diffusion rate to be determined using 12C. Carbon in salt-like carbides exhibits significant ionicity, and a high carbon diffusion rate would enable a new class of high temperature fuel cells based on carbon-ion transport. The complete lack of carbon diffusion data for salt-like carbides is the motivation for this work. The carbon diffusion rate in La0.5Er0.5C2 has now been determined to be 2.0 ± 0.8 × 10−13 cm2/s at 850 °C, increasing to 1.8 ± 0.8 × 10−11 cm2/s at 1150 °C, with an activation energy of about 95 kJ/mole. These diffusion rates are too low for a carbon-ion fuel cell, but a number of other salt-like carbides exist. Be2C, in particular, is a salt-like carbide with an antifluorite structure, and should have higher carbon-ion diffusion than cubic La0.5Er0.5C2 due to the unoccupied octahedral sites in the antifluorite structure, but Be2C presents special difficulties due to the toxic nature of its hydrolysis products.