Thermodynamic and experimental study of the interaction of silicon and carbon monoxide: Synthesis of silicon carbide nanofibers

The reaction of crystalline silicon with carbon monoxide to produce silicon carbide was studied. Thermodynamic simulation of the equilibrium phase composition of the nSi-mCO system was carried out in the range 300–2000 K (27–1727°C). Conditions required for silicon carbide was carried out applying various experimental modes (n, m, and T) and possible pathways of the reactions were determined. Interaction between crystalline silicon and carbon monoxide formation in a temperature range of 1000–1450°C. The order of the reaction in CO was found to be close to unity. Silicon carbide nanofibers with thicknesses of from 5 to 100 nm were synthesized and characterized by powder X-ray diffraction, mass-spectral elemental analysis, and scanning electron microscopy. A possibility of synthesizing high-purity silicon carbide fibers were experimentally evaluated.     

Hollow Silicon Carbide Nanostructures

Hollow silicon carbide nanostructures with length up to 1 m have been produced for the first time along with threadlike structures. The preparation of SiC nanostructures from silicon and carbon at 1000-1100 °C was carried out without prior gasification of these elements. The growth of SiC nanostructures involves a step featuring atomization of silicon and carbon at such low temperatures. The growth of SiC nanotubes upon the reduction of carbon by SiO₂ in the initial period of the preparation proceeds with their predominant formation as bundles. Silicon carbide may correspond to the highly textured -modification.     

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。     

碳化硅纳米材料的溶剂热合成

本文综述了溶剂热法制备一系列碳化硅纳米材料的研究,包括一维纳米线、纳米带、纳米棒、二维纳米片及空心球等;同时,碳源过量时可形成碳包覆碳化硅的复合材料。使用废塑料作为碳源合成了碳化硅纳米材料,为废塑料的回收再利用提供了新途径。通过使用碘、硫等添加剂,有效降低了合成温度,显示出溶剂热技术在制备碳化硅方面的独特优势。     

碳化硅纳米材料的溶剂热合成

本文综述了溶剂热法制备一系列碳化硅纳米材料的研究,包括一维纳米线、纳米带、纳米棒、二维纳米片及空心球等;同时,碳源过量时可形成碳包覆碳化硅的复合材料。使用废塑料作为碳源合成了碳化硅纳米材料,为废塑料的回收再利用提供了新途径。通过使用碘、硫等添加剂,有效降低了合成温度,显示出溶剂热技术在制备碳化硅方面的独特优势。     

Effect of pH of aqueous ceramic suspensions on colloidal stability and precision of analytical measurements using slurry nebulization inductively coupled plasma atomic emission spectrometry

Colloidal stability of silicon nitride, silicon carbide and boron carbide aqueous slurries used for slurry nebulization inductively coupled plasma atomic emission spectrometry has been investigated in the pH range 2–10 by electrophoretic mobility and particle size measurements, together with sedimentation tests. The mean particle size of silicon nitride and silicon carbide suspensions change with increasing pH showing a maximum at the isoelectric points (pH 7.5 and 5.5 respectively). The particle size distribution of boron carbide slurries remains practically constant and the zeta potential of suspended boron carbide particles shows a small change in the pH range investigated. The silicon nitride and silicon carbide slurries have good stability at pH below 5 and above 8, respectively. Boron carbide slurries show excellent stability in the whole pH range investigated. The time demand for stabilization of the emission line intensities from the start of nebulization strongly depends on the colloidal stability of slurries. Consequently, it is advantageous to nebulize aqueous suspensions with a pH as far from the isoelectric point of the solid as possible and with the ionic strength of the dispersion medium as low as possible. The RSD values of the line intensity measurements determined after 3 min stabilization time decrease with increasing stability of the aqueous slurries.     

Effect of Nb,Ti, and W ion implantation on surface properties and cell compatibility of silicon carbide

Silicon carbide is considered as a bio-inert semiconductor material; consequently, it has been proposed for potential applications in human body implantation. In this study, we study the effect of implanting different metal ions on the surface properties of silicon carbide single crystal. The valence states of the elements and the surface roughness of implanted SiC were studied using X-ray photoelectron spectroscopy and atomic force microscope, respectively. Osteoblastic MG-63 cells were utilized to characterize the cytocompatibility of ion implanted SiC. The results show that after Nb ion implantation on the SiC surface, it mainly exists in the form of Nb–C bond, Nb–O bond, and a small amount of metallic niobium. The titanium implanted on SiC primarily forms Ti-C bond and Ti-O bond. The tungsten implanted on SiC mostly presents as metallic tungsten and W–O bond. The roughness of silicon carbide single crystal is improved by ion implantation of all three metal ions. Ion implantation of titanium and niobium can improve the cell compatibility and hydrophilicity of silicon carbide, whereas ion implantation of tungsten reduces the cell compatibility and hydrophilicity of silicon carbide.     

Silicon carbide nanomaterial as a coating for solid‐phase microextraction

Silicon carbide has excellent properties, such as corrosion resistance, high strength, oxidation resistance, high temperature, and so on. Based on these properties, silicon carbide was coated on stainless‐steel wire and used as a solid‐phase microextraction coating, and polycyclic aromatic hydrocarbons were employed as model analytes. Using gas chromatography, some important factors that affect the extraction efficiency were optimized one by one, and an analytical method was established. The analytical method showed wide linear ranges (0.1–30, 0.03–30, and 0.01–30 μg/L) with satisfactory correlation coefficients (0.9922–0.9966) and low detection limits (0.003–0.03 μg/L). To investigate the practical application of the method, rainwater and cigarette ash aqueous solution were collected as real samples for extraction and detection. The results indicate that silicon carbide has excellent application in the field of solid‐phase microextraction.     

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation

We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6 eV for (8,0) SiCNT and decreases the band gap to 1.47 eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.     

碳化硅上生长Cu/SSZ-13分子筛及其NH3-SCR催化性能

汽车尾气和柴油不完全燃烧所释放的NOx严重污染了大气环境.为了降低对大气的污染,可将其催化还原成氮气.氨气选择性催化还原(NH3-SCR)是使用较广泛的机动车高效脱硝技术.用于催化脱硝反应的催化剂有多种类型,分子筛具有特殊的孔道结构和骨架结构及高比表面积,因而已广泛用作脱硝反应催化剂.与传统三效催化剂相比,分子筛催化剂总体表现出更好的脱硝催化活性,但在高温下不稳定,容易失活,不耐热冲击,水热稳定性差.SiC具有耐酸碱、耐腐蚀、抗氧化、耐磨及良好的热稳定性和导电性,因此作为催化剂载体近年来引起广泛关注.但是其本身也存在许多缺点,如比表面积低、表面性质不活泼、不利于金属物种分散等.因此,本文通过原位水热法将SSZ-13生长在SiC表面,制备出新型催化复合材料SSZ-13@SiC.采用X射线衍射(XRD)、扫描电子显微镜(SEM)和N2吸附-脱附等手段研究了不同碱量和晶化时间对SSZ-13在SiC表面生长的影响,负载Cu后获得Cu/SSZ-13@SiC作为催化剂,研究了SiC对Cu/SSZ-13中高温下脱硝活性的影响规律.结果表明,碱含量会影响SSZ-13在SiC表面的结晶程度.当SiO2/NaOH ≥ 0.1时,SSZ-13有杂相出现,并且结晶度都不高; 当SiO2/NaOH < 0.1时,SiC表面会生长成纯相的SSZ-13晶粒且具有较高的结晶度.晶化时间也会影响SSZ-13在SiC表面的覆盖程度: 反应1 d时,SiC表面会生长零星的SSZ-13晶粒; 2 d时,SSZ-13达到全面覆盖; 3 d后,SSZ-13在SiC上的生长达到饱和,其比表面积达到最大值,约为201.3 m2/g.通过离子交换将不同含量Cu离子交换到分子筛表面,其中Cu(1.71)/SSZ-13@SiC样品具有最佳的脱硝活性,接近200 °C时,NO转化率就达到90%以上,到高温500 °C时,NO转化率仍能保持在70%以上.相比于未负载的Cu/SSZ-13,Cu/SSZ-13@SiC催化剂在NH3-SCR测试中具有更高的高温催化活性,同时催化活性窗口明显拓宽.上述结果说明SiC对Cu/SSZ-13的高温催化活性具有一定的提高和稳定作用.     

Formation of silicon carbide nanotubes and nanowires via reaction of silicon (from disproportionation of silicon monoxide) with carbon nanotubes

One-dimensional silicon-carbon nanotubes and nanowires of various shapes and structures were synthesized via the reaction of silicon (produced by disproportionation reaction of SiO) with multiwalled carbon nanotubes (as templates) at different temperatures. A new type of multiwalled silicon carbide nanotube (SiCNT), with 3.5-4.5 A interlayer spacings, was observed in addition to the previously known beta-SiC (cubic zinc blende structure) nanowires and the biaxial SiC-SiO(x) nanowires. The SiCNT was identified by high-resolution transmission microscopy (HRTEM), elemental mapping, and electron energy loss spectroscopy (EELS). The multiwalled SiCNT was found to transform to a beta-SiC crystalline structure by electron beam annealing under TEM.     

Molecular dynamics simulation of pressure-driven water flow in silicon-carbide nanotubes

Many properties of silicon carbide (SiC) nanotubes, such as their high mechanical strength and resistance to corrosive environments, are superior to those of their carboneous counterparts, namely, carbon nanotubes (CNTs) and, therefore, SiC nanotubes can be a viable alternative to CNTs in a variety of applications. We employ molecular dynamics simulations to examine flow of water in SiC nanotubes and to study the differences and similarities with the same phenomenon in the CNTs. The simulations indicate that SiC nanotubes always provide larger flow enhancements than those reported for the CNTs. Moreover, a given flow enhancement in SiC nanotubes requires an applied pressure gradient that is at least an order of magnitude smaller than the corresponding value in a CNT of the same size.     

A computational study of oxygen-termination of a (6,0) boron nitride nanotube

Abstract  

We performed density functional theory (DFT) calculations to investigate the properties of electronic structures of representative armchair and zigzag silicon carbide nanotubes (SiCNTs). The model structures were optimized and the NMR parameters were calculated at the sites of silicon-29 and carbon-13 atoms in these structures. Our results indicated that different electronic environments could be detected by using the atoms of nanotubes in which the atoms of tips, especially for zigzag SiCNT, exhibit distinctive properties among other atoms.     

高频红外碳硫分析仪测定含碳化硅耐火材料中碳化硅含量

采用对试样进行灼烧预处理的方法,除去游离碳,用锡粒、纯铁、钨粒作为助熔剂,并以钢铁标准样品校正仪器,高频燃烧红外吸收法测定含碳化硅耐火材料中的碳化硅含量,测定结果的相对标准偏差为1.07%(n=6),该法与化学法的测定结果较接近,精密度和准确度均满足化学分析的要求。     

碳化硅纳米管的制备及表征

Silicon carbide nanotubes (SiCNTs) were prepared by chemical vapor deposition (CVD). Methyltrichlorosilane (MTS) was selected as the SiC gaseous source and, ferrocence and thiophene as the catalyst and the cocatalyst, respectively. The influences of pyrolysis temperature, the content of catalyst and the cocatalyst, and the mole ratio between H₂ and MTS, on the shape of the pyrolysis products were investigated, respectively. The products were characterized by SEM, EDX, XRD and HRTEM, respectively. Novel type of multi-walled SiCNTs, with 30～80 nm and 15～20 nm outer and inner diameters, respectively, were observed.     

Determination of the saturation vapor pressure of silicon by Knudsen cell mass spectrometry

Silicon evaporation has been investigated by high-temperature mass spectrometry, and the saturation vapor pressure of silicon over its melt has been determined over the temperature range from 1739 and 2326 K. The saturation vapor pressure data obtained via silicon evaporation from Knudsen cells made of molybdenum, tungsten, molybdenum disilicide-lined molybdenum, and graphite silicided by the gas-phase method are compared. Among these materials, silicided graphite is the most inert toward silicon vapor. The silicon partial pressures measured in the silicided graphite cell are close to the recommended values.     

硅钼蓝光度法测定碳化硅中二氧化硅的含量

采用氯化钠-盐酸-氢氟酸溶解碳化硅样品,加钼酸铵使硅酸离子形成硅钼杂多酸,然后用1-氨基-2-萘酚-4-磺酸还原剂将其还原成硅钼蓝,在700nm波长处测定其吸光度,从而测得碳化硅中的二氧化硅含量。考察了称样量、水浴温度、水浴加热时间、样品储存容器、显色反应时间等因素对测定结果的影响,优化了测试条件。二氧化硅的质量浓度在0．25-12．5mg／（100mL）的范围内与吸光度呈良好的线性关系,线性相关系数r=0．99986。加标回收率为99．2％～100．9％,测定结果的相对标准偏差为0．76％（n=6）。     

A first principles study of NO2 chemisorption on silicon carbide nanotubes

Using methods based on first principles, we find that an NO₂ molecules can be chemisorbed on silicon carbide nanotubes (SiCNTs) with an appreciable binding energy (∼−1.0 eV), and that this is not the case for either carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs). A detailed analysis of the energetics, geometry, and electronic structure of various isomers of the complexes was performed. The SiCNT–NO₂ complex can be metallic or nonmetallic depending on the type of adsorption site and the chirality of the tube. However, our analysis of the electronic structure predicts that a strong p-type effect of the adsorption turns semiconducting systems into metallic ones at room temperature, irrespective of the chirality of the tube.     

Hybrid nonotubes BN-SiC: Theoretical studies

Nanotubes (NT) representing a combination of BN and SiC units were calculated. The energy spectra, charge distribution, total energies, and geometry optimization were calculated in the framework of the electron density functional theory using the Gaussian-98 program package. It was shown for chair-shaped NT(4,4), which are the combination of BN-SiC-BN and SiC-BN-SiC rings, can exist as corrugated tubes retaining the strength and conduction characteristic of silicon carbide nanotubes.