Boron carbide-copper infiltrated cermets

Porous boron carbide preforms, prepared with and without excess carbon, were infiltrated with a Cu-Si alloy. Contrary to unalloyed copper, the Cu-Si alloy wets and infiltrates the porous preforms. A thermodynamic analysis of the B-C-Cu-Si system indicated that a Si content of the alloy above 15 at% leads to the formation of SiC. At higher Si content, the composition of boron carbide in contact with the melt also changes towards higher boron content. Metallographic examination validated these conclusions. The SiC compound forms preferentially around the free carbon particles in preforms containing excess carbon, and also in the vicinity of carbide that did not contain any excess carbon. Eventually, SiC, a product of the reaction between the carbide and the melt, forms a continuous barrier that impedes completion of the reaction and accounts for the limited increase of hardness as a result of lengthy heat treatments.     

Structural investigations on pyrolysed polycarbosilanes

The combination of combustion analysis, IR-spectroscopy and Raman spectroscopy yields information about the chemical state of carbon in polycarbosilanes or silicon carbide. A stable polycarbosilane can be found where carbon is bonded in a network of silicon, carbon and hydrogen. In the temperature range between 700 and 800°C, the polycarbosilane is transformed into an amorphous silicon carbide with a small excess of carbon. During the crystallisation of the amorphous silicon carbide, which takes place at temperatures above 1100°C, glassy carbon is found by Raman-spectroscopy and combustion analysis. Finally after pyrolysis temperatures above 1500°C only silicon carbide exists; this may be caused by the reaction of free carbon with oxygen impurities in the samples.     

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.     

A DFT investigation of the interactions of Pd,Ag, Sn,and Cs with silicon carbide

With a view to understand the diffusion of radionuclides through the silicon carbide layers in tristructural isotropic coated fuel particles, density functional theory calculations are applied to assess the interaction of palladium, silver, tin, and caesium with silicon carbide. The silicon carbide molecule (Si₂C₂), crystalline cubic silicon carbide (β‐SiC), and the (120) ∑5 grain boundary of β‐SiC are investigated to elucidate the differences in the interactions of silicon carbide with these elements. The main stabilizing forces in the PdSi₂C₂ complex were found to be donor‐acceptor charge transfer (covalent) interactions, the Ag and Sn complexes involve significant contributions from both electrostatic and covalent interactions, while the Cs atom is bonded dominantly by electrostatic forces. For the unconstrained M? Si₂C₂ model, the following energetic ordering was obtained: Pd > Sn > Cs > Ag. The steric constraints in the bulk SiC and on the grain boundary change the order of binding energies to Pd > Ag > Sn > Cs in the interstitials and Pd > Sn > Ag > Cs in vacancies and at the grain boundary. By comparing the incorporation energies in the solid phases, it is possible to group these elements by similarities in the patterns of incorporation energies. Silver and palladium form a group with carbon, tin is grouped with silicon, and caesium is on its own. © 2014 European Commission. International Journal of Quantum Chemistry published by Wiley Periodicals, Inc.     

Synthesis and thermal studies of heterocyclic siloxanes containing boron

Eight- and six-membered cycloborasiloxanes containing two and one boron atoms respectively are strained ring compounds which undergo ring opening polymerization and ring–ring transformation reactions on thermolysis. Prolonged heating at 200°C results in volatilization of the cyclic boroxine (PhBO)₃, whereas rapid heating of either compound to 1500°C in an inert atmosphere does not result in loss of boron, but affords instead an amorphous residue containing silicon carbide, boron oxide and carbon. Upon further pyrolysis at 1700°C the final product consists of microcrystalline silicon carbide in which are embedded large crystals of boron carbide.     

Preparation, microstructure and mechanical properties of SiC-SiC and B4C-B4C laminates

Silicon carbide and boron carbide are high hardness materials with a low density but, like most ceramics, with a low toughness, that limits their use in various applications. One approach to reinforce ceramic materials consists in using crack deflection by weakening the interfaces in laminar materials. In our study, ceramic layers of different compositions were prepared by tape casting and stacked in predefined sequences. Different weak layers were tested: porous layers made with different pore forming agents for SiC; porous layers made with pore forming agent or by varying the quantity of sintering aid and weak interfaces made with graphite spray for B₄C. After debinding, SiC (Al₂O₃, Y₂O₃ additions) and B₄C (C addition) were pressureless sintered. For evaluation of the sintered parts, firstly the macrostructure and microstructure were characterized. Then, mechanical properties of multi-layered materials, obtained by stacking dense and porous layers that should contain enough porosity to initiate crack deflection, according to the models, were evaluated.     

Ablation properties of C/C–SiC composites tested on an arc heater

Carbon fiber-reinforced carbon and silicon carbide (C/C–SiC) composites were fabricated by a combination of chemical vapor infiltration and liquid silicon infiltration. Ablation properties of C/C–SiC composites and C/C composites with similar technique were tested on a high-pressure arc heater. The results show that ablation properties of C/C–SiC composites are more severe than those of C/C composites. Ablation of C/C–SiC composites includes oxidation, sublimation of SiC (Si), and mechanical denudation. Oxidation and sublimation of SiC (Si) lead to the enlarged ablation rates between carbon fibers and matrices, which finally cause serious ablation of C/C–SiC composites.     

Identification of selective oxidation of TiC/SiC composite with X-ray diffraction and Raman spectroscopy

Open cell 3D titanium carbide/silicon carbide (TiC/SiC) composite was oxidised to titanium oxide/silicon carbide (TiO₂/SiC) following different temperature profiles in a thermal gravimetric analysis (TGA) instrument in continuous air-flow and static air (oven) environments. The TiC oxidation to anatase, starting at temperatures over 450°C, was confirmed by Raman spectroscopy and X-Ray diffraction (XRD). By increasing the temperature, the mass fraction of anatase diminished, while the mass fraction of rutile increased. SiC oxidation started at 650°C when a mixture of TiO₂/SiO₂/SiC could be observed by Raman, XRD and HRTEM.     

Rheological and thermal behaviours of a hyperbranched polycarbosilane

A hyperbranched polymer, a precursor of silicon carbide (SiC), was successfully synthesized using a hydrosilylation reaction with Karstedt's catalyst. This reaction was optimized with the use of a rheometer coupled with an infrared spectrometer. The polymeric precursor was characterized using NMR and Fourier transform infrared spectroscopies, and dynamic rheology. The polymerization reaction was followed in situ by combined rheological and infrared measurements, indicating a gel‐like behaviour for alkene conversions higher than 0.55. Overall second‐order kinetics was determined for the hydrosilylation reaction. Pyrolysis at 1400 °C led to porous materials with β‐SiC and free carbon.     

The effect of the sintering atmosphere on the densification of B4C ceramics

Densification of boron carbide during sintering may be improved by a two-stage process, namely heating to 2000°C under vacuum and sintering at 2190°C under argon. This sintering regime allows achieving a relative density of the ceramic bodies fabricated from a fine powder higher than 95%. The nitrogen treatment of the boron carbide phase at 1900°C leads to the formation of the BN phase and precipitation of graphite. Vacuum treatment of these samples at 2000°C leads to decomposition of the boron nitride phase. The liberated free boron may again react with graphite to form in situ boron carbide particles. The experimental investigations of the sintering behavior of the boron carbide phase under various atmospheres supported the thermodynamic predictions regarding the phase transformation. No evidence, however, was found for enhanced sintering under a nitrogen atmosphere.     

碳化硼纳米线的制备和结构

以碳纳米管为模板，通过加热碳纳米管与硼粉的混合物，获得了笔直的硼碳纳米线.对纳米线的结构和成分进行研究，结果表明纳米线主要为B4C纳米线.在部分B4C纳米线的端部存在Ni颗粒，这些端部具有Ni颗粒的纳米线构成了纳米磁针.讨论了B4C纳米线的生长机制，B4C纳米线的生长主要为硼原子在碳纳米管中扩散并发生化学反应，使得碳纳米管晶格结构发生重组，形成B4C纳米线.反应后，硼原子部分取代了碳纳米管中碳原子，修补了碳纳米管中的晶格缺陷，获得了形态笔直的B4C纳米线.     

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.     

Surface morphology of Ge‐modified 3C‐SiC/Si films

The influence of Ge deposition prior to carbon interaction with 3° off‐axis Si(111) substrates on the structural and morphological properties of the formed silicon carbide (SiC) layer is studied. In situ reflection high‐energy electron diffraction (RHEED) and X‐ray diffraction (XRD) revealed the formation of the cubic silicon carbide (3C‐SiC) modification. In situ spectroscopic ellipsometry measurements revealed a decreasing 3C‐SiC thickness with increasing Ge predeposition. Atomic force microscopy (AFM) studies revealed that the surface overlayer morphology is mainly formed by periodic step arrangements whose relevant geometric parameters, i.e. lateral separation, height and terrace width, depend on the Ge content. Besides the changes of the step morphology, the surface roughness and the grain size and the strain of the formed 3C‐SiC decreases with increasing germanium precoverage. Copyright © 2008 John Wiley & Sons, Ltd.     

SiCp/ Al复合材料与化学镀镍层结合机理研究

根据结构化学理论，用SEM, EDAX和XPS等测试手段研究了镀层和碳化硅颗粒增强铝基复合材料(SiC_p/ Al)的表面、断面形貌及界面的结合状态，分析了镀层和基体之间的结合机理。结果表明，Ni镀层与复合材料界面有良好的结合，在复合材料表面的SiC-Al界面，初期沉积物Ni按Al的晶格外延生长出现微晶层，然后吸附原子扩散迁移、碰撞结合并与界面上的SiC晶格匹配生长，在镍层中诱发了拉伸应力。镍晶格和基体粒子之间产生了键合作用，形成的键显示出共价键和离子键的混合性质。     

Microstructure and tribological properties of 3D needle-punched C/C–SiC brake composites

Carbon fibre reinforced carbon and silicon carbide dual matrix composites (C/C–SiC) show excellent tribological properties and are promising candidates for advanced friction materials. A pressure infiltration/carbonization combined with liquid silicon infiltration was developed for fabricating C/C–SiC composites. The carbon fabric preform was fabricated with the three-dimensional needling method. In the pressure infiltration process, the carbon fibre reinforced plastic was prepared by infiltration of the fabric preform with the furan resin. Then the carbon fibre reinforced plastic was carbonized which was pyrolysed to form a porous carbon/carbon composites. Finally, the porous carbon/carbon was infiltrated with molten silicon to obtain C/C–SiC composites. The composites exhibit excellent friction behavior, including a good stability of brake, and the average dynamic μ is 0.38 and static μ is 0.50, in combination with the linear wear rate of about 5.6 μm cycle^?1. Moreover, the friction surface was covered with friction film which is about 10 μm in thickness. These results show that the C/C–SiC brake composites are promising candidates for advanced brake and clutch systems.     

Evaporation of carbon atoms from the open surface of silicon carbide and through graphene cells: Semiempirical quantum-chemical modeling

The evaporation of silicon atoms during the epitaxial growth of graphene on the singular carbon and silicon faces of silicon carbide SiC was modeled by the semiempirical AM1 and PM3 methods. The analysis was performed for evaporation of atoms both from the open surface of SiC and through the surface of the formed graphene monolayers. The total activation barrier of the evaporation of the silicon atoms, their passage from the graphene cell, and further evaporation from graphene was shown to be lower than the barrier to evaporation of the silicon atom on a free surface of SiC. Passage through graphene is thus not the limiting stage of the process, but contributes significantly to the effective evaporation time.     

Imaging and diffusion structural diagnostics of silicon carbide-based composites and fibers

The aim of this study is to characterize the microstructure and high temperature induced structural changes within fiber reinforced silicon carbide (SiC_f/SiC) composites by means of non-destructive techniques. In order to understand their properties, the characterization of the microstructure of SiC_f/SiC composites is the crucial issue. Porosity within composites is unavoidable with currently available manufacturing processes, and reduces significantly the life time and performance of the composites under harsh environments. Moreover, the internal pores, created in the manufacturing process cause the degradation most of the outstanding properties such as thermal conductivity, mechanical properties at high temperature, and radiation stability. Cold neutron tomography and diffusion structural diagnostic techniques were applied in the investigation of the microstructure of SiC_f/SiC composites to gain complementary information. One of the main obstacles to using these composites in fusion technology and other applications are a change of the porous structure and a swelling at high temperatures and in a severe radiation environment. Cold neutron tomography enables visualization of the microstructure of the composite and consequently the pore distributions within the SiC_f/SiC composite were observed with a suitable resolution. The diffusion structural diagnostic technique was used to characterize the thermal behavior of SiC_f/SiC composites on heating up to 1300 °C.     

Gravimetric determination of free carbon and silicon carbide in silica fume

Silica fume is formed as a by-product in the manufacture of silicon from quartzite. This paper describes an analytical method for the determination of free carbon and silicon carbide in silica fume. The silicon carbide was determined after removal of free carbon, amorphous silica, crystalline silica, graphite and silicon from the fume. The free carbon content was found to vary from 2 to 8% while the silicon carbide content ranged from 1 to 5%. X-ray diffraction, thermal analysis, scanning electron microscopy and Fourier Transform infrared spectroscopy were used to validate the steps used in the analytical procedure. The purpose of determining the free carbon and silicon carbide content of the fume is to help understand the efficiency of the reduction process and mechanism of the reaction.     

Template‐Synthesized Porous Silicon Carbide as an Effective Host for Zeolite Catalysts

A facile method has been developed for the fabrication of porous silicon carbide (SiC) by means of sintering a mixture of SiC powder and carbon pellets at a relatively lower temperature, that is, 1450 °C, in air. The pore density and the total pore volume of the resulting porous SiC could be tuned by changing the initial SiC/C weight ratio. The structure evolution and the associated property changes during the preparation were examined through X‐ray diffraction, scanning electron microscopy, thermogravimetric analysis, ²⁹Si magic‐angle spinning (MAS) NMR spectroscopy, and mercury‐intrusion porosimetry analyses. Silica and SiO_xC_y ceramics formed in situ during the calcination process acted as binders of the porous SiC grains. The porous SiC can be used as a host for the growth of ZSM‐5 zeolite crystals to form the ZSM‐5/porous‐SiC composite material. After loading another catalytic active component of molybdenum, a novel catalytic material, Mo‐ZSM‐5/porous‐SiC, was obtained, which exhibited improved catalytic activity in the methane dehydroaromatization reaction.     

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.