Preparation and Analysis of Carbon Fiber-Silicon Carbide Thermally Conductive Asphalt Concrete

An experimental investigation into the thermal conductivity of CF-SiC two-phase composite asphalt concrete is presented.The main objective of this study was to verify the possibility of using SiC powder instead of mineral powder as the thermal conductive filler to prepare a new type of asphalt concrete and improve the efficiency of electrothermal snow and ice melting systems accordingly.The thermal conductivity of asphalt concrete prepared with different thermally conductive fillers was tested by a transient plane source method,and the related performances were measured.Then the temperature rise rate and surface temperature were studied through field heating tests.Finally,the actual ice melting efficiency of the thermally conductive asphalt concrete was evaluated using an effective electrothermal system.As shown by the experimental results,the composite made of SiC powder and carbon fiber has a high thermal conductivity.When SiC replaces mineral powder,the thermal conductivity of the asphalt mixture increases first and then decreases with the increase of carbon fiber content.In the present study,in particular,the thermal conductivity attained a peak when the carbon fiber content was 0.2%of the aggregate mass.

Influence of Carbon Content on Element Diffusion in Silicon Carbide-Based TRISO Composite Fuel

The coating layers of Tri-structural Isotropic Particles(TRISO)serve to protect the kernel and act as barriers to fission products.Sintering aids in the silicon carbide matrix variably react with TRISO coating layers,leading to the destruction of the coating layers.Investigating how carbon content affects element diffusion in silicon carbide-based TRISO composite fuel is of great significance for predicting reactor safety.In this study,silicon carbide-based TRISO composite fuels with different carbon contents were prepared by adding varying amounts of phenolic resin to the silicon carbide matrix.X-ray Diffraction(XRD)and Scanning Electron Microscopy(SEM)were employed to characterize the phase composition,morphology,and microstructure of the composite fuels.The elemental content in each coating layer of TRISO was quantified using Energy-Dispersive X-ray Spectroscopy(EDS).The results demonstrated that the addition of phenolic resin promoted the uniform distribution of sintering aids in the silicon carbide matrix.The atomic percentage(at.%)of aluminum(Al)in the pyrolytic carbon layer of the TRISO particles reached its lowest value of 0.55%when the phenolic resin addition was 1%.This is because the addition of phenolic resin caused the Al and silicon(Si)in the matrix to preferentially react with the carbon in the phenolic resin to form a metastable liquid phase,rather than preferentially consuming the pyrolytic carbon in the outer coating layer of the TRISO particles.The findings suggest that carbon addition through phenolic resin incorporation can effectively mitigate the deleterious reactions between the TRISO coating layers and sintering aids,thereby enhancing the durability and safety of silicon carbide-based TRISO composite fuels.

Evidence of the Role of Carbon Vacancies in Nickel-Based Ohmic Contacts to n-Type Silicon Carbide

N-wells are created by P＋ ion implantation into Si-faced p-type 4H-SiC epilayer. Ti and Ni are deposited in sequence on the surface of the active regions. Ni2Si is identified as the dominant phase by X-ray diffraction （XRD） analysis after metallization annealing. An amorphous C film at the Ni2 Si/SiC interface is confirmed by an X-ray energy-dispersive spectrometer （XEDS）. The Ni2Si and amorphous C film are etched away selectively,followed by deposition of new metal films without annealing. Measurement of the current-voltage characteristics shows that the contacts are still ohmic after the Ni2 Si and amorphous C film are replaced by new metal films. The sheet resistance Rsh of the implanted layers decreases from 975 to 438f2/D, because carbon vacancies （Vc） appeared during annealing,which act as donors for electrons in SiC.

Structural feature and electronic property of an (8, 0) carbon-silicon carbide nanotube heterojunction

A supercell of a nanotube heterojunction formed by an （8, 0） carbon nanotube （CNT） and an （8, 0） silicon carbide nanotube （SiCNT） is established, in which 96 C atoms and 32 Si atoms are included. The geometry optimization and the electronic property of the heterojunction are implemented through the first-principles calculation based on the density functional theory （DFT）. The results indicate that the structural rearrangement takes place mainly on the interface and the energy gap of the heterojunction is 0.31 eV, which is narrower than those of the isolated CNT and the isolated SiCNT. By using the average bond energy method, the valence band offset and the conduction band offset are obtained as 0.71 and -0.03 eV, respectively.

Preparation and properties of continuous Al-containing silicon carbide fibers

Continuous SiC(OAl) fibers, named KD-A fibers, were prepared by the melt-spinning of ceramic precursor polyaluminocarbosilane, air-curing, and pyrolizing at 1 300 ℃. These fibers contained small amount of aluminum and 7%- 9% oxygen. The KD-A fibers were converted into sintered SiC(Al) fibers, named KD-SA, by sintering at 1 800 ℃. The fibers were characterized by chemical analysis, tensile strength test, SEM and XRD. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.6 GPa, 210 GPa, 12 - 14 μm, respectively. The KD-A fibers have higher thermal stability, more excellent oxidation resistance than the Nicalon fibers. The properties of the KD-A fibers have reached the level of Hi-Nicalon fibers. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.1 GPa, 405 GPa, 10 - 12 μm, respectively. The KD-SA fibers with nearly stoichiometric component have stable performance at high temperature, and better creep resistance than the Tyranno SA fibers.

Electrochemical behavior of insulin on pretreated carbon black electrode enhanced with silicon carbide nanostructure

We previously reported the direct electrochemical detection of insulin at bare carbon electrodes. Here a novel modified acetylene carbon black paste electrode(SiC/CB-CPE), based on the outstanding characteristics of silicon carbide nanostructure,was developed for the electrooxidation of insulin in alkaline solution and it was characterized by cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS) in 5 mmol/L Fe(CN)63-/4- solution. It is found that silicon carbide nanostructure doped into the CB-CPE greatly facilitates the redox electrochemistry of Fe(CN)63-/4- probe and the electrochemical oxidation of insulin. The electrooxidation of insulin is a one-electron and one-proton reaction and an irreversible adsorption-controlled electrode process. The anodic oxidation current increases linearly with the concentration of insulin from 1×10-7mol/L to1.2×10-6mol/L in 0.1 mol/L Na2CO3-NaHCO3 buffer solution(pH 10.0) and the detection limit was 50 nmol/L. In addition, the SiC/CB-CPE shows good sensitivity, reproducibility, renewability and capacity of resisting disturbance.

Density functional theory calculation of the properties of carbon vacancy defects in silicon carbide

As a promisingmaterial for quantumtechnology,silicon carbide(SiC)has attracted great interest inmaterials science.Carbon vacancy is a dominant defect in 4H-SiC.Thus,understanding the properties of this defect is critical to its application,and the atomic and electronic structures of the defects needs to be identified.In this study,density functional theorywas used to characterize the carbon vacancy defects in hexagonal(h)and cubic(k)lattice sites.The zero-phonon line energies,hyperfine tensors,and formation energies of carbon vacancies with different charge states(2−,−,0,+and 2+)in different supercells(72,128,400 and 576 atoms)were calculated using standard Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof methods.Results show that the zero-phonon line energies of carbon vacancy defects are much lower than those of divacancy defects,indicating that the former is more likely to reach the excited state than the latter.The hyperfine tensors of VC+(h)and VC+(k)were calculated.Comparison of the calculated hyperfine tensor with the experimental results indicates the existence of carbon vacancies in SiC lattice.The calculation of formation energy shows that the most stable carbon vacancy defects in the material are VC 2+(k),VC+(k),VC(k),VC−(k)and VC 2−(k)as the electronic chemical potential increases.

Preparation and Characterization of Axial Gradient Silicon Carbide Fibers with Sinusoidal Electrical Resistivity

Two types of silicon carbide fibers with sinusoidal electrical resistivity were prepared by using different pyrolysis technology. The relationship between the microstructure and the electrical resistivity of these fibers was investigated and compared. The results indicated that carbon layer was the main conductive phase in the SiC fibers obtained by means of one step pyrolysis, whereas a free carbon phase governed the conductivity of the SiC fibers prepared through two step pyrolysis mode.

Influence of pyrolysis temperature on structure and dielectric properties of polycarbosilane derived silicon carbide ceramic

β-SiC ceramic powders were obtained by pyrolyzing polycarbosilane in vacuum at 800-1200 °C. The β-SiC ceramic powders were characterized by TGA/DSC, XRD and Raman spectroscopy. The dielectric properties of β-SiC ceramic powders were investigated by measuring their complex permittivity by rectangle wave guide method in the frequency range of 8.2-18 GHz. The results show that both real part ε′ and imaginary part ε″ of complex permittivity increase with increasing pyrolysis temperature. The mechanism was proposed that order carbon formed at high temperature resulted in electron relaxation polarization and conductance loss, which contributes to the increase in complex permittivity.

Low‐temperature synthesis of graphitic carbon‐coated silicon anode materials

We report the synthesis of a high‐performance graphitic carbon‐coated silicon(Si@GC)composite material for lithium‐ion batteries via a scalable production route.Porous Si is produced from the magnesiothermic reduction of commercial silica(SiO2)precursor followed by low‐temperature graphitic carbon coating using glucose as the precursor.The obtained Si@GC composite achieves an excellent reversible specific capacity of 1195 mAh g−1 and outstanding cycle stability.The thick Si@GC anode(3.4 mg cm^−2)in full cells with commercial lithium iron phosphate cathode delivers a remarkable performance of 800 mAh g^−1 specific capacity and 2.7 mAh cm^−2 areal capacity as well as 93.6%capacity retention after 200 cycles.

Fabrication of solid-phase-sintered Si C-based composites with short carbon fibers

Solid-phase-sintered Si C-based composites with short carbon fibers（Csf/SSi C） in concentrations ranging from 0 to 10wt% were prepared by pressureless sintering at 2100°C. The phase composition, microstructure, density, and flexural strength of the composites with different Csf contents were investigated. SEM micrographs showed that the Csf distributed in the SSi C matrix homogeneously with some gaps at the fiber/matrix interfaces. The densities of the composites decreased with increasing Csf content. However, the bending strength first increased and then decreased with increasing Csf content, reaching a maximum value of 390 MPa at a Csf content of 5wt%, which was 60 MPa higher than that of SSi C because of the pull-out strengthening mechanism. Notably, Csf was graphitized and damaged during the sintering process because of the high temperature and reaction with boron derived from the sintering additive B4C; this graphitization degraded the fiber strengthening effect.

Prefabrication of SiC whiskers through induction of carbon fiber

A new way to prepare SiC whiskers through the induction of carbon fiber was suggested. With the processing steps of opening furnace firstly and then shutting it, rice hall, as the only raw material, was carbonified to get excess quantity of Si in rice hull. After a certain catalyzer was added, SiC whiskers were prepared by means of the induction of carbon fiber. The component and morphology of the whisker were analyzed by XRD, SEM and TEM. The results reveal that the diameters of the whiskers range in 0.5-2μm and their lengths in 100-500μm. The whiskers are straightβ-SiC crystals with smooth surface. The whisker is homogeneous and its productivity is 100%. Two kinds of formation mechanisms, both VLS mechanism and vapor formation mechanism, are involved during the growth of the whiskers. But the vapor formation mechanism relatively plays a key role.

Effect of composition and structure on specific resistivity of SiC fibers

Four kinds of SiC fibers with different specific resistivities were prepared by the pyrolysis of cured polycarbosilane fiber. The results show that SiC fibers with different specific resistivities can be obtained by changing the curing and pyrolysis conditions. And the free carbon content and the ability to crystallize no longer affect the specific resistivities notably with the time when the fiber is covered with an excess carbon layer, and the fiber has a low specific resistivity. The excess carbon layer in the circular outer part is originated from the re-pyrolysis and deposition of hydrocarbon volatiles. The removal of the carbon by oxidative treatment may affect the surface property and also promote the magnitude of specific resistivity. The influence of the surface property on the specific resistivity can be considerable and should not be neglected.

Mesoporous silicon carbide nanofibers with in situ embedded carbon for co-catalyst free photocatalytic hydrogen production

Silicon carbide （SIC） has been considered a promising metal-free photocatalyst due to its unique photoelectrical properties and thermal/chemical stability. However, its performance suffers from the fast recombination of charge carriers. Herein, we report mesoporous SiC nanofibers with in situ embedded graphitic carbon （SiC NFs-Cx） synthesized via a one-step carbothermal reduction between electrospun carbon nanofibers and Si powders. In the absence of a noble metal co-catalyst, the hydrogen evolution efficiency of SiC NFs-Cx is significantly improved under both simulated solar light （180.2 μmol.g-1.h-1） and visible light irradiation （31.0 ~amol-g-l-h-~） in high-pH solution. The efficient simultaneous separation of charge carriers plays a critical role in the high photocatalytic activity. The embedded carbon can swiftly transfer the photogenerated electrons and improve light absorption, whereas the additional hydroxyl anions （OH-） in high- pH solution can accelerate the trapping of holes. Our results demonstrate that the production of SiC NFs-Cx, which contains exclusively earth-abundant elements, scaled up, and is environmentally friendly, has great potential for practical applications. This work may provide a new pathway for designing stable, low- cost, high efficiency, and co-catalyst-free photocatalysts.

Effect of Carbon Source on Crystal Morphology of SiC in-situ Formed in AI_2O_3-Si-C Matrix

Three kinds of Al2O3- Si- C matrix specimens were prepared using tabular corundum powder and Si powder as starting materials,ultrafine flake graphite,nano carbon black,and carbon nanotubes as carbon sources,respectively,to research the effect of micro or nano carbon materials on structure and morphology of formed Si C crystals. The specimens were fired at 1 000,1 200 and 1 400℃ for 3 h in carbon-embedded condition,respectively.The phase composition was studied by XRD and the crystal morphology of Si C was investigated by FESEM. The results show that：（ 1） the amount of Si C increases with the firing temperature rising;（ 2） the in-situ reaction mechanism and the formed Si C crystal morphology vary with carbon source： carbon nanotubes are generally converted into Si C whiskers by carbon nanotubes-confined reaction; Si and nano carbon black react to nucleate quickly,and the nucleated Si C crystals grow evenly in all directions forming Si C particles; Si C whiskers are produced from edge to internal of ultrafine flake graphite.

Effect of Carbon Containing Materials on Pure Carbon Reaction-bonded SiC

Petroleum coke, graphite, gas carbon and lower sulfur carbon black were used to prepare reaction-bonded silicon carbide. The influences of different carbon containing materials on properties of carbonaceous precursors, sintering process, and microstructure of the prepared SiC were researched. The results show that ： （ 1 ） With the density of carbon containing materials increasing, the porosity of carbonaceous precursors decreases and the infiltrating process of liquid silicon is more difficult. （2） The reaction between carbon containing materials and liquid silicon, the volume effect is more obvious with the density of carbon containing materials increasing. （3） As the carbon containing materials density decreasing, residual carbon in reaction bonded SiC also decreases.

Effects of B_4 C and Si Antioxidant on Oxidation Resistance of Low-carbon MgO-C Brick

Low-carbon MgO - C specimens with dimension of Ф6 mm × 36 mm were prepared using fused magnesia, flake graphite and carbon black as main starting materials, thermoplastic phenolic resin as binder, hexamethyl ene tetramine as curing agent, Si powder （ w （Si） 〉 80%, particle size 〈0. 074 mm） and B℃ （ d90 =36. 5 μm ） as antioxidant, pressing under 200 MPa and drying at 200 ℃ for 24 h. The oxidation resistance tests were conducted at 600 ℃ , 1 000 ℃ and 1 400 ℃ for 2 h, respectively. Effects of B4C and Si antioxidant on oxidation resistance of low-carbon MgO - C brick were studied by comparing the areas of the decarburized layers. The results shaw that： （1） When Si powder addition is 3%, at 600 ℃ and 1 000 ℃, the specimens with 0. 5% B4C perform good oxidation resistance; at 1 400 ℃, the oxidation resistance of the specimens is improved with the addition of B4C increasing, and that of the specimen with 0. 7% B4 C is the best. （2） When B4C addition is 0. 5%, at 600 ℃ and 1 000 ℃ ,the oxidation resistances of the specimens with 3% and 5% Si powder are similar, which are better than that of the specimen with 1% Si powder; and at 1 400 ℃ , the oxidation resistance of the specimens is improved obviously with the addition of Si powder increasing, and that of the specimen with 5% Si powder is the best. （3） Bused on the results, it is believed that the low-carbon MgO - C brick with 0. 5% B4C and 3% Si powder performs the best oxidation resistance.

Investigation towards scalable processing of silicon/graphite nanocomposite anodes with good cycle stability and specific capacity

Silicon/graphite(Si/Gr)nanocomposites with controlled void spaces and encapsulated by a carbon shell(Si/Gr@void@C)are synthesized by utilizing high-energy ball milling to reduce micron-sized particles to nanoscale,followed by carbonization of polydopamine(PODA)to form a carbon shell,and finally partial etching of the nanostructured Si core by NaOH solution at elevated temperatures.In particular,the effects of ball milling time and NaOH etching temperature on the electrochemical properties of Si/Gr@void@C are investigated.Increasing the ball milling time results in the improved specific capacity of Si-based anodes.Carbon coating further enhances the specific capacity and capacity retention over charge/discharge cycles.The best cycle stability is achieved after partial etching of the Si core inside Si/Gr@void@C particles at either 70 or 80C,leading to little or no capacity decay over 130 cycles.However,it is found that both carbon coating and NaOH etching processes cause some surface oxidation of the nanostructured Si particles derived from high-energy ball milling.The surface oxidation of the nanostructured Si results in decreases in specific capacity and should be minimized in future studies.The mechanistic understanding developed in this study paves the way to further improve the electrochemical performance of Si/Gr@void@C nanocomposites in future.

Influence of Silicon Oxynitride on Slag Resistance of Al_2O_3-Si C-C Refractory

Silicon oxynitride was added in shaped Al_2O_3-SiC-C refractory material to improve the slag resistance in this paper.Optimum adding quantity of silicon oxynitride powder was also studied. The results show that the slag resistance of Al_2O_3-SiC-C shaped refractory is improved when 2% or 3% Si_2N_2O is added. A reasonable amount of Si_2N_2O added into Al_2O_3-Si C-C shaped refractory can produce silicon oxide into the slag, which can improve the viscosity of slag and prevent the slag erosion and penetration.