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.

Application of Reaction-Bonded Silicon Carbide in Manufacturing of Spacecraft Combustion Chamber

Silicon carbide (SiC) ceramics is a good structural ceramics material, which have a lot of excellent properties such as superior high-temperature strength up to a temperature of 1 350 ℃, chemical stability, good resistance to thermal shock and high abrasion resistance. The silicon carbide ceramics material has so far been used widely for manufacturing various components such as heat exchangers, rolls, rockets combustion chamber. Sintering of ceramics structural parts have many technological method, the reaction-bonded is one of important sintering technology of ceramics structural parts. The preparation of reaction-bonded silicon carbide (RBSC) is based on a reaction sintering process, whereby a compacted body of α-SiC and carbon (graphite) powders is heated in contact with liquid silicon or gas silicon, which impregnates the body, converting the carbon (graphite) to β-SiC which bonds the original alpha grain. This process is characterized by low temperature and a short time sintering, and being appropriate to the preparation of large size and complex-shaped components, and so on. Besides, during compacting process of reaction sintering, it can maintain a stable dimension of ceramics parts. Therefore, the method of reaction-bonded silicon carbide ceramics has been identified as a technology suitable for producing complicated and highly exact dimensions’ ceramics parts. In this paper, the method of reaction-bonded silicon carbide was applied to the manufacturing of a complex-shaped spacecraft combustion chamber of SiC ceramics. SiC and carbon powder of 4～30 μm were chosen as the raw materials, green compacts containing appropriate wt.% carbon were formed using the mold press method, sintering was performed in a graphite electric furnace under an argon atmosphere. It was introduced in detail that the technological parameters and technological flow of reaction sintering silicon carbide ceramics. At the same time, physical and mechanical experiments such as bending strength, coefficient of thermal expansion, coefficient of thermal conductivity, gastight property, heat resisting property etc. have been carried out. The results demonstrated that spacecraft combustion chamber made from reaction sintering of silicon carbide ceramics is feasible and the results of experiment is satisfactory. The strength of high-temperature structural parts made by reaction sintered SiC varied with silicon content; Under the this article testing condition, the optimum silicon content is 10.5% for the part investigated. The method of reaction sintered SiC ceramics is suitable for manufacturing of complicated spacecraft parts with a working temperature of 1 500 ℃.

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.

Effect of SiO_2 on the Preparation and Properties of Pure Carbon Reaction Bonded Silicon Carbide Ceramics

Effect of SiO 2 content and sintering process on the composition and properties of Pure Carbon Reaction Bonded Silicon Carbide (PCRBSC) ceramics prepared with C-SiO 2 green body by infiltrating silicon was presented.The infiltrating mechanism of C-SiO 2 preform was also explored.The experimental results indicate that the shaping pressure increases with the addition of SiO 2 to the preform,and the pore size of the body turned finer and distributed in a narrower range,which is beneficial to decreasing the residual silicon content in the sintered materials and to avoiding shock off,thus increasing the conversion rate of SiC.SiO 2 was deoxidized by carbon at a high temperature and the gaseous SiO and CO produced are the main reason to the crack of the body at an elevated temperature.If the green body is deposited at 1800℃ in vacuum before infiltration crack will not be produced in the preform and fully dense RBSC can be obtained.The ultimate material has the following properties:a density of 3.05-3.12g/cm3,a strength of 580±32MPa and a hardness of (HRA)91-92.3.

Effects of substitution of group-V atoms for carbon or silicon atoms on optical properties of silicon carbide nanotubes

Silicon carbide nanotubes(SiCNTs) have broad application prospects in the field of micro-nanodevices due to their excellent physical properties. Based on first-principles, the difference between optical properties of SiCNTs where C atom or Si atom is replaced by group-V element is studied. The results show that the optical absorptions of SiCNTs doped by different elements are significantly different in the band of 600 nm–1500 nm. The differences in photoconductivity, caused by different doping elements, are reflected mainly in the band above 620 nm, the difference in dielectric function and refractive index of SiCNTs are reflected mainly in the band above 500 nm. Further analysis shows that SiCNTs doped with different elements change their band structures, resulting in the differences among their optical properties. The calculation of formation energy shows that SiCNTs are more stable when group-V element replaces Si atom, except N atom. These research results will be beneficial to the applications of SiC nanomaterials in optoelectronic devices and provide a theoretical basis for selecting the SiCNTs' dopants.

STUDY ON PREPARATION OF REACTION BURNING SILICON CARBIDE BY CARBON POWDER

The technology of preparing reaction burning silicon carbide (RBSC) by replacing SiC/C with entirely carbonaceous raw materials is investigated. Experimental results show the predominant factors of successfully preparing RBSC are as following:strictly controlling the porosity and pore diameter of biscuit, obtaining ideal carbon network permeating of Si and completely reaction between Si and beta-SiC.

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.

Thermal Conductivity of Carbon/Carbon Composites with the Fiber/Matrix Interface Modified by Silicon Carbide Nanofibers

Silicon carbide nanofibers grew on the surface of carbon fibers of a unidirectional carbon preform by CCVD and then chemical vapor infiltration was used to densify the preform to get the SiCNF-C/C composite. The effects of silicon carbide nanofibers on the microstructure of the pyrolytic carbon and the thermal conductivity of the SiCNF-C/C composite were investigated. Results show that silicon carbide nanofibers on the surface of carbon fibers induce the deposition of high texture pyrolytic carbon around them. The interface bonding between carbon fibers and pyrolytic carbon is well adjusted. So the efficiency of heat transfer in the interface of the composite is well enhanced. The thermal conductivity of the SiCNF-C/C composite is greater than that of the C/C composite, especially the thermal conductivity perpendicular to the fiber axis.

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.

Research on Silicon Carbide Dispersion-Reinforced Hypereutectic Aluminum-Silicon Electronic Packaging Materials

The objective of this study is to improve the mechanical properties and machining performance of high thermal conductivity and low expansion silicon carbide dispersion-strengthened hypereutectic aluminum-silicon electronic packaging materials to meet the needs of aviation,aerospace,and electronic packaging fields.We used the powder metallurgy method and high-temperature hot pressing technology to prepare SiC/Al-Si composite materials with different SiC contents(5vol%,10vol%,15vol%,and 20vol%).The results showed that as the SiC content increased,the tensile strength of the composite material first increased and then decreased.The tensile strength was the highest when the SiC content was 15%;the sintering temperature significantly affected the composite material’s structural density and mechanical properties.Findings indicated 700℃was the optimal sintering and the optimal SiC content of SiC/Al-Si composite materials was between 10%and 15%.Besides,the sintering temperature should be strictly controlled to improve the material’s structural density and mechanical properties.

Carbon nanotube@silicon carbide coaxial heterojunction nanotubes as metal-free photocatalysts for enhanced hydrogen evolution

Considerable research efforts have been devoted to developing novel photocatalysts with increased performances by hybridizing inorganic nanomaterials with carbon nanotubes.In this work,one-dimensional coaxial core-shell carbon nanotubes@SiC nanotubes were successfully synthesized via in situ growth of SiC coatings on carbon nanotubes by a vapor-solid reaction between silicon vapor and carbon nanotubes.High-resolution transmission electron microscope images show that SiC and carbon nanotubes link to form a robust heterojunction with intrinsic atomic contact,which results in efficient separation of the photogenerated electron-hole pairs on SiC and electron transfer from SiC to carbon nanotubes.Compared with those of similar materials such as pure SiC nanocrystals and SiC nanotubes,the metal-free carbon nanotubes@SiC exhibits an enhanced photocatalytic activity for hydrogen evolution,which is attributed to the enhanced light absorption and the efficient interfacial charge transfer/separation brought about by their one-dimensional coaxial nanoheterostructures.Moreover,the photocatalytic stability of the metal-free carbon nanotubes@SiC was tested for over 20 h without any obvious decay.

Preparation and properties of porous silicon carbide ceramics through coat-mix and composite additives process

The core-shell structure silicon-resin precursor powders were synthesized through coat-mix process and addition of Al2O3-SiO2-Y2O3 composite additives.A series of porous silicon carbide ceramics were produced after molding,carbonization and sintering.The phase,morphology,porosity,thermal conductivity,thermal expansion coefficient,and thermal shock resistance were analyzed.The results show that porous silicon carbide ceramics can be produced at low temperature.The grain size of porous silicon carbide ceramic is small,and the thermal conductivity is enhanced significantly.Composite additives also improve the thermal shock resistance of porous ceramics.The bending strength loss rate after 30 times of thermal shock test of the porous ceramics which were added Al2O3-SiO2-Y2O3 and sintered at 1 650 ℃ is only 6.5%.Moreover,the pore inside of the sample is smooth,and the pore size distribution is uniform.Composite additives make little effect on the thermal expansion coefficient of the porous silicon carbide ceramics.

Effect of High Temperature Annealing on Characteristics of 4H Silicon Carbide MESFET

For very high temperature annealing (1620℃) after ion implantation for 4H silicon carbide (4H SiC),the residual components of Al and O in the alundum furnace impact seriously on the surface of material,which yields the derivation of SiOC.This causes a significant degradation of the 4H SiC surface characteristics according to the results of surface composition analysis.As validity,Ni/SiC ohmic contact measurement illustrates a higher specific contact resistance than the normal value by a factor of 2～3.Consequently the MESFET fabricated with this kind of 4H SiC material results in a degraded I V output performance compared with that of normal 4H SiC MESFET.

Fabrication of n^+ Polysilicon Ohmic Contacts with a Heterojunction Structure to n-Type 4H-Silicon Carbide

Polysilicon ohmic contacts to n-type 4H-SiC have been fabricated. TLM （transfer length method） test patterns with polysilicon structure are formed on n-wells created by phosphorus ion （P^＋） implantation into a Si-faced p-type 4H-SiC epilayer. The polysilicon is deposited using low-pressure chemical vapor deposition （LPCVD） and doped by phosphorous ions implantation followed by diffusion to obtain a sheet resistance of 22Ω/□. The specific contact resistance pc of n^＋ polysilicon contact to n-type 4H-SiC as low as 3.82 × 10^-5Ω· cm^2 is achieved. The result for sheet resistance Rsh of the phosphorous ion implanted layers in SiC is about 4.9kΩ/□. The mechanisms for n^＋ polysilicon ohmic contact to n-type SiC are discussed.

Fabrication and Pressureless Sintering of Silicon Carbide Refractories

This work studies the fabrication and pressureless sintering of silicon carbide(SiC)refractories.SiC particles were adopted as aggregates,introducing different amounts(20%,30%,40%,50%,and 60%,by mass)of submicron SiC powder,adding resin as the binder and the carbon source,and B4C as the sintering aid.It is found that when the mass ratio of B4C to the submicron SiC powder is 3%,the optimal sintering can be obtained.With the increase of the submicron powder addition,the sintering linear shrinkage increases and the mechanical properties enhance.The optimal sintering temperature is 2050-2100℃.

High Strength Silicon Carbide Foams and Their Deformation Behavior

Silicon carbide （SIC） foams with a continuously connected open-cell structure were prepared and characterized for their mechanical performance. The apparent densities of SiC foams were controlled between about 0.4 and 2.3 g/cm^3, with corresponding compressive strengths ranging from about 23 to 60 MPa and flexural strengths from about 8 to 30 MPa. Compressive testing of the SiC foams yielded stress-strain curves with only one linear-elastic region, which is different from those reported on ceramic foams in literature. This can possibly be attributed to the existence of filaments with fine, dense and high strength microstructures. The SiC and the filaments respond homogeneously to applied loading.