Solid-Phase Synthesis of Calcium Carbide in a Plasma Reactor

A laboratory-scale spout-fluid bed reactor with a dc plasma torch was used to study the solid-phase synthesis of calcium carbide. Calcium oxide powder with a mean particle size of 170 m was reacted with graphite powder (130 m). Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reaction took place in the vicinity of the plasma jet and that conversion to calcium carbide increased linearly with reaction time. The rate of conversion increased exponentially with plasma jet temperature, indicating that chemical reaction was the controlling mechanism. Microscopic analysis of the solid product showed that calcium carbide was formed around both reactants, and that the reaction followed a shrinking core model. Although melting and agglomeration of partially reacted particles occurred at high temperature, resulting in instability of the bed and impeding the reaction progress, high conversions are expected in a continuous process with optimized reactor design.     

Synthesis of Mixed-Phase TiO<Subscript>2</Subscript> Nanopowders Using Atmospheric Pressure Plasma Jet Driven by Dual-Frequency Power Sources

Mixed-phase TiO₂ nanopowders with different ratios of anatase and rutile have been successfully synthesized using atmospheric pressure plasma jet driven by dual-frequency power sources. The crystal structures of the TiO₂ nanopowders were characterized by X-ray diffraction, SAED, HRTEM, and Raman shift spectroscopy. These results indicated that samples possessed anatase and rutile structure, in addition, the crystallinity of the TiO₂ nanopowders increased and the chlorine contamination decreased with discharge RF power increasing. The photocatalytic activity of the TiO₂ nanopowders was evaluated by decomposition methylene blue solution. The TiO₂ nanopowders which were produced at the discharge RF power of 110 W had the highest photocatalytic activity. Optical emission spectroscopy (OES) was used to detect various excited species in the plasma jet. The results indicate that the various RF power significantly changes the intensities of emission lines (Ar, Ar⁺, Ti, Ti⁺, Ti²⁺, Ti³⁺ and O), which results in the TiO₂ nanopowders a mixture of anatase and rutile phases. The nonequilibrium chemical composition could be formed in one step without anneal. It may have potential applications for synthesizing nanosized particles of high crystallinity by reactive nonthermal plasma processing.     

Synthesis of titanium nitride and carbonitride nanopowders in confined-jet flow plasma reactor

The synthesis of titanium nitride and carbonitride nanopowders from titanium tetrachloride vapor in a stream of hydrogen–nitrogen plasma, generated by an arc torch, in confined-jet flow reactor has been experimentally studied. Single-phase nanopowders with a NaCl-type cubic crystal lattice as assemblies of preferably cube-shaped nanoparticles of a 20–150 nm size and aggregates based on them have been obtained in the experiments. By varying the synthesis parameters, it has been possible to prepare titanium nitride nanopowders with a specific surface area in the range of 11–39 m²/g containing 18.8–22.5 wt % nitrogen, which corresponds to the empirical formula TiN_0.79–TiN_0.99. The titanium carbonitride nanopowders had a specific surface area of 13–23 m²/g and carbon and nitrogen contents of 7.5–13.6 and 13.5–5.1 wt %, respectively.     

核壳结构碳化钨/碳化钨铁复合材料的制备与电催化活性

以铁黄为载体,偏钨酸铵为钨源,将直接包覆与原位还原碳化技术相结合制备了碳化钨/碳化钨铁复合材料.经X射线衍射(XRD)分析和透射电子显微镜(TEM)观察,复合材料的主要物相为碳化钨铁(Fe3W3C)、碳化钨(WC)和碳化二钨(W2C),且构成了以Fe3W3C为核、WC和W2C为壳的核壳结构.采用三电极体系循环伏安法测试了复合材料在酸性、中性和碱性体系中对甲醇的电催化氧化活性.结果表明,与颗粒状碳化钨和介孔空心球状碳化钨相比,复合材料的电催化活性有了明显的提高;进一步研究发现,复合材料的电催化活性不仅受到体系性质的影响,还与其物相组成和微结构相关.上述结果说明,通过控制复合材料的物相组成及微结构,以及反应体系的性质可实现对其电催化活性的调控;同时表明,核壳结构是提高碳化钨催化材料活性的有效途径之一.     

Effects of vacuum annealing on the particle size and phase composition of nanocrystalline tungsten carbide powders

The effects of the vacuum annealing temperature (400–1400°C) on the phase and chemical composition, particle size, and microstresses of the nanocrystalline powders of tungsten carbide WC with 20–60 nm particles were studied by X-ray diffraction and electron microscopy. Vacuum annealing of WC nano-powders at T _ann ≤ 1400°C was accompanied by decarbonization, resulting from the interaction of carbon with the oxygen impurity. Changes in the chemical composition of the nanocrystalline powder of tungsten carbide led to changes in its phase composition. The annealing was accompanied by growth of powder particles due to the aggregation of nanoparticles and by a decrease of microstresses.     

镍助剂对碳化钼催化剂的二苯并噻吩加氢脱硫性能的影响

 将MoO3和Ni-Mo混合氧化物在CH4/H2气氛中程序升温还原碳化制备了相应的碳化钼和碳化镍钼催化剂, X射线粉末衍射表征其物相分别为β-Mo2C和Ni-Mo2C. 考察了Ni助剂对碳化钼催化剂的制备及二苯并噻吩加氢脱硫反应性能的影响. 结果表明, Ni助剂的加入降低了碳化钼催化剂所需的还原碳化温度,提高了催化剂的比表面积,并对其二苯并噻吩加氢脱硫反应活性有明显的促进作用. Ni助剂添加量以Ni/Mo原子比为0.3为宜,此时Ni和Mo之间的催化协同效应达到最佳. 当反应压力为3.0 MPa, 反应温度为330 ℃, 空速8 h-1, H2/原料液体积比为500∶1时, 625 ℃还原碳化制备的碳化镍钼催化剂对0.6%二苯并噻吩/环己烷溶液的二苯并噻吩转化率达到96.25%, 较相应的碳化钼催化剂提高了1.57倍.     

Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction

Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β‐Mo₂C with an Fe₂N structure. Here, four phases of Mo‐C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine–metal oxide composite material including γ‐MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X‐ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ‐MoC. γ‐MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.     

碳化硅和碳化硼在电催化中的应用

燃料电池是具有广泛应用前景的新能源技术。碳载铂基催化剂（Pt/C）是最常用的燃料电池电极催化剂,不过Pt/C稳定性较差、且成本高昂,严重限制了燃料电池的规模化应用。共价型碳化物碳化硅和碳化硼,由于具有极强的共价键,其物化稳定性优异,成为制备高稳定性、低成本的燃料电池催化剂的重要基础材料。本文总结了相关研究成果,介绍了碳化硅和碳化硼的独特优势,讨论了相关研究的发展方向。     

碳化硅和碳化硼在电催化中的应用

燃料电池是具有广泛应用前景的新能源技术。碳载铂基催化剂(Pt/C)是最常用的燃料电池电极催化剂,不过Pt/C稳定性较差、且成本高昂,严重限制了燃料电池的规模化应用。共价型碳化物碳化硅和碳化硼,由于具有极强的共价键,其物化稳定性优异,成为制备高稳定性、低成本的燃料电池催化剂的重要基础材料。本文总结了相关研究成果,介绍了碳化硅和碳化硼的独特优势,讨论了相关研究的发展方向。     

Preparation of Boron Carbide from BF<Subscript>3</Subscript> and BCl<Subscript>3</Subscript> in Hydrogen Plasma of Arc RF Discharge

A comparative study was carried out of the process of plasma chemical deposition of boron carbide from hydrogen plasma containing the mixtures of BF₃ + CH₄ and BCl₃ + CH₄ sustained by RF arc (13.56 MHz) discharge. It was shown that in the case of synthesis of B₄C from a mixture of BF₃ + CH₄, carbon and complex coordination compound [X₃B]^?H⁺ (R₃B·FH) are formed as the by-products of condensed products. In the case of synthesis of B₄C from the BCl₃ + CH₄ mixture, the only condensed product is carbon. Mechanisms for the formation of boron carbide on the surface of heated electrodes are proposed. The main feature of these mechanisms is the preliminary deposition of a graphite layer from CH₄ and then the precipitation of boron with the participation of the radicals BF₂, BF and BCl. B₄C samples were obtained and the impurity composition, morphology and structure of bulk boron carbide samples obtained using both of its halides were studied. It was found that in both cases a carbon phase is present in boron carbide samples. The main impurities entering the B₄C, in the case of using a mixture of BF₃ + CH₄, is silicon, and in the case of a mixture of BCl₃ + CH₄, is tungsten.     

A New Route to High Yield Mesostructure Boron Carbide Powder from SiC/Si3N4 Whiskers

Boric acid/Mg (magnesiothermic or metal sintering aid)/C (activated carbon)/N₂ or Ar (atmosphere)/additives (mesoporous SiO₂ or mesoporous SiC or SiC/Si₃N₄ whiskers) systems were used in the one‐step synthesis of mesostructured B₄C (221.04 m²/g). In this study, a mixture of the active precursors was allowed to react via a self‐sustaining reaction (high‐energy ball milling process). Also, the properties of the samples prepared using powdered activated carbon (PAC) and SiC/Si₃N₄ whiskers (concentration in the range 5–10 wt%) as sources of carbon were investigated. X‐ray diffraction results proved the presence of crystalline boron carbide in the peak positions of B₄C (B₁₂C₃). The advantage of the present route for yielding mesostructured B₄C powder seems to be limited by the growth of carbide crystals. This restriction is believed to be imposed by a lack of whisker additives around the pores where B₄C crystals grow. The results also show that the best mesoporous additive for the synthesis of nanoscale boron carbide is mesoporous SiC. The effect of the concentration of CO (reduction of α‐Fe₂O₃ to Fe by CO) on the B₄C synthesis suggests that, in addition to the concentration of CO, the pressure of the N₂ atmosphere is an important factor in the synthesis of mesostructured B₄C.     

TiC nanocrystal formation from carburization of laser-grown Ti/O/C nanopowders for nanostructured ceramics

Refractory carbide ceramics (TiC and ZrC) raise interest as promising materials for high-temperature applications such as structural materials for the future generation of nuclear reactors. In this context, nanostructured ceramics are expected to exhibit improved thermomechanical properties as well as better behavior under irradiation when compared to conventional materials. It is therefore necessary to synthesize carbide nanocrystals of such materials to elaborate the ceramics. We report here the formation study of TiC nanocrystals through the direct carburization of Ti/O/C nanopowders grown by laser pyrolysis. A spray of titanium tetraisopropoxide was laser pyrolyzed with ethylene as the sensitizer, leading to Ti/O/C nanopowders with various C contents controlled by the synthesis conditions. Annealing treatments performed on these nanopowders under an inert atmosphere without any C addition enabled the formation of TiC grains through the carburization of the oxide phase by free C incorporated during the synthesis. The powders were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The final TiC grain size was about 80 nm, and the grains were monocrystalline. The influence of the free C content on the grain growth during the annealing step, together with its effects on the densification of the ceramics after sintering by high-pressure flash sintering, was examined. A 93% densification was finally achieved.     

Surface Modification of Poly-ε-Caprolactone with an Atmospheric Pressure Plasma Jet

In this work, poly-ε-caprolactone samples are modified by an atmospheric pressure plasma jet in pure argon and argon/water vapour mixtures. In a first part of the paper, the chemical species present in the plasma jet are identified by optical emission spectroscopy and it was found that plasmas generated in argon/0.05 % water vapour mixtures show the highest emission intensity of OH (A–X) at 308 nm. In a subsequent section, plasma jet surface treatments in argon and argon/water vapour mixtures have been investigated using contact angle measurements and X-ray photoelectron spectroscopy. The polymer samples modified with the plasma jet show a significant decrease in water contact angle due to the incorporation of oxygen-containing groups, such as C–O, C=O and O–C=O. The most efficient oxygen inclusion was however found when 0.05 % of water vapour is added to the argon feeding gas, which correlates with the highest intensity of OH (X) radicals. By optimizing the OH (X) radical yield in the plasma jet, the highest polymer modification efficiency can thus be obtained.     

Mechanochemical activation of aluminum: 5. Formation of aluminum carbide upon heating of activated mixtures

The mechanical activation of thermal synthesis of aluminum carbide Al₄C₃ in Al-15 wt % C and Al-30 wt % C mixtures is studied with differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. It is found that the mechanical treatment of powders results in an essential reduction in the temperature of carbide synthesis. A correlation between the temperature of the onset of synthesis and size L of the coherent scattering region of aluminum is established. When the doses of absorbed mechanical energy exceed 15–20 kJ/g and, as a result, the L value decreases to 20 nm, the synthesis proceeds by a solid-phase mechanism at a temperature significantly lower than the melting point of aluminum and the synthesis temperature reduces by 800°C. The particle size of the formed aluminum carbide and unreacted aluminum after heating to 900°C is 20–40 nm. At doses D = 50–80 kJ/g, the heat of the formation of carbide from activated samples is about two times lower compared to the standard value. The possible sources of this discrepancy are discussed.     

High temperature stability and photocatalytic activity of nanocrystalline anatase powders with Zr and Si co-dopants

TiO₂ nanopowders doped by Si and Zr were prepared by sol–gel method. The effects of Si and Zr doping on the structural, optical, and photo-catalytic properties of titania nanopowders have been studied by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and UV–Vis absorption spectroscopy. XRD results suggest that adding impurities has a significant effect on anatase phase stability, crystallinity, and particle size of TiO₂. Titania rutile phase formation in ternary system (Ti–Si–Zr) was inhibited by Zr⁴⁺ and Si⁴⁺ co-doped TiO₂ in high temperatures (500–900 °C) and 36 mol% anatase composition is retained even after calcination at 1,000 °C. The photocatalyst activity was evaluated by photocatalytic degradation kinetics of aqueous methylen orange under visible radiation. The results show that the photocatalytic activity of the 20 %Si and 15 %Zr co-doped TiO₂ nanopowders have a larger degradation efficiency than pure TiO₂ under visible light.     

Methane Activation by Diatomic Molybdenum Carbide Cations

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts.     

Engineering Non‐sintered,Metal‐Terminated Tungsten Carbide Nanoparticles for Catalysis

Transition‐metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide‐scale use as catalysts. A method is presented for the production of metal‐terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon‐supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo_xW_1?xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100‐fold higher than commercial WC and within an order of magnitude of platinum‐based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.     

Engineering Non‐sintered,Metal‐Terminated Tungsten Carbide Nanoparticles for Catalysis

Transition‐metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide‐scale use as catalysts. A method is presented for the production of metal‐terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon‐supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo_xW_1−xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100‐fold higher than commercial WC and within an order of magnitude of platinum‐based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.     

The structural, morphological, and surface properties of tungsten-doped TiO2 nanopowders and their contribution to the photocatalytic activity

Tungsten-doped TiO₂ nanopowders (W-TiO₂) were prepared by chemical vapor synthesis and the effects of a post-heat treatment on their physical, surface, and photocatalytic properties were investigated. The W-TiO₂ nanopowders containing about 1.0 mol % of tungsten were obtained and annealed from 400 to 700 °C. The as-synthesized and annealed W-TiO₂ nanopowders were carefully examined for their crystalline and opto-electronic structure and morphology by means of X-ray diffraction, UV–Vis spectroscopy, and transmission electron microscopy. In addition, the surface condition was investigated by X-ray photoelectron spectroscopy. The photocatalytic activities were studied by the oxidative degradation of 2-propanol under UV light irradiation. We found that the photocatalytic activity of W-TiO₂ varied significantly with the temperature of the heat treatment, exceeding the performance of P25 after annealing at 600 °C. Interestingly, the chemical composition of titanium and tungsten of W-TiO₂ played a crucial role to its photocatalytic activity as the mixed valence states, Ti ⁿ⁺ (n = 4, 3, 2, 0) and W ⁿ⁺ (n = 6, 5, 4), were found in accordance with altering the annealing temperature.     

Arc-Enhanced Plasma Machining Technology for High Efficiency Machining of Silicon Carbide

Atmosphere plasma etching methods have been demonstrated efficient in the etching of fused silica or ULE. However, because of the high chemical stability of silicon carbide (SiC), the conventional plasma etching methods seem incapable of obtaining a high material removal rate (MRR). We have found that MRR will be significantly improved while the electric spark appears between the plasma and the SiC surface. As a result, a new plasma source is designed to generate stable arc at the surface. Due to the generation of arc, the MRR of 0.35 mm³/min is obtained, about 10 times as high as the conventional method. In this paper, the removal characteristics and the thermal effect of this method are presented. MRR and the surface temperature are investigated in dependence on plasma parameters: RF power, travel speed of plasma source, SF₆ gas flow and O₂ gas flow. Due to the negligible thermal effect, the surface figuring can be achieved using the conventional dwell time method. The shape error of a flat SiC surface is corrected, verifying the figuring capability and the effectiveness of this method.