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.

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.

Application of silicon carbide temperature monitors in 49-2 swimming-pool test reactor

High purity SiC crystal was used as a passive monitor to measure neutron irradiation temperature in the 49-2 research reactor.The SiC monitors were irradiated with fast neutrons at elevated temperatures to 3.2×10^(20)n/cm^(2).The isochronal and isothermal annealing behaviors of the irradiated SiC were investigated by x-ray diffraction and four-point probe techniques.Invisible point defects and defect clusters are found to be the dominating defect types in the neutron-irradiated SiC.The amount of defect recovery in SiC reaches a maximum value after isothermal annealing for 30 min.Based on the annealing temperature dependences of both lattice swelling and material resistivity,the irradiation temperature of the SiC monitors is determined to be~410℃,which is much higher than the thermocouple temperature of 275℃ recorded during neutron irradiation.The possible reasons for the difference are carefully discussed.

Unveiling phonon frequency-dependent mechanism of heat transport across stacking fault in silicon carbide

Stacking faults(SFs)are often present in silicon carbide(SiC)and affect its thermal and heat-transport properties.However,it is unclear how SFs influence thermal transport.Using non-equilibrium molecular dynamics and lattice dynamics simulations,we studied phonon transport in SiC materials with an SF.Compared to perfect SiC materials,the SF can reduce thermal conductivity.This is caused by the additional interface thermal resistance(ITR)of SF,which is difficult to capture by the previous phenomenological models.By analyzing the spectral heat flux,we find that SF reduces the contribution of low-frequency(7.5 THz-12 THz)phonons to the heat flux,which can be attributed to SF reducing the phonon lifetime and group velocity,especially in the low-frequency range.The SF hinders phonon transport and results in an effective interface thermal resistance around the SF.Our results provide insight into the microscopic mechanism of the effect of defects on heat transport and have guiding significance for the regulation of the thermal conductivity of materials.

High-pressure and high-temperature sintering of pure cubic silicon carbide:A study on stress-strain and densification

Silicon carbide(SiC)is a high-performance structural ceramic material with excellent comprehensive properties,and is unmatched by metals and other structural materials.In this paper,raw SiC powder with an average grain size of 5μm was sintered by an isothermal-compression process at 5.0 GPa and 1500?C;the maximum hardness of the sintered samples was31.3 GPa.Subsequently,scanning electron microscopy was used to observe the microscopic morphology of the recovered SiC samples treated in a temperature and extended pressure range of 0-1500?C and 0-16.0 GPa,respectively.Defects and plastic deformation in the SiC grains were further analyzed by transmission electron microscopy.Further,high-pressure in situ synchrotron radiation x-ray diffraction was used to study the intergranular stress distribution and yield strength under non-hydrostatic compression.This study provides a new viewpoint for the sintering of pure phase micron-sized SiC particles.

Steam Oxidation Resistance Comparison of Several Silicon Carbide Refractories at Elevated Temperatures

The oxidation tests of different SiC refractories including Si3N4-SiC bricks,Si3N4-Si2N2O-SiC bricks from China(SNO-1),Si3N4-Si2N2O-SiC bricks from overseas(SNO-2),SiAlON-SiC bricks,oxides bonded SiC bricks,self-bonded SiC bricks as well as SiC castables were carried out in the steam atmosphere(1 000 ℃,32 kg·m-3·h-1)for different durations(100,200,300,400 and 500 h).The mass,the volume,the bulk density and the apparent porosity before and after the oxidation were tested.The XRD and SEM analyses were conducted.The results indicate that:(1)under the steam atmosphere condition(1 000 ℃,32 kg·m-3·h-1),as the oxidation time increases from 0 to 500 h,the volume and the mass of the silicon carbide refractories increase,while the bulk density decreases;in terms of the apparent porosity,oxides bonded SiC bricks and SiC castables present an increasing trend,Si3N4-SiC bricks,SiAlON-SiC bricks and self-bonded SiC bricks present an increasing trend first and then a decreasing trend,and Si3N4-Si2N2O-SiC bricks present a decreasing trend or a trend of decreasing first and then increasing;(2)as for Si3N4-Si2N2O-SiC bricks,SNO-1 and SNO-2 have basically the same chemical and phase composition,SNO-2 has the lower mass change rate than SNO-1 during oxidation from 200-500 h,which indicates that SNO-2 has the better steam oxidation resistance than SNO-1.

Development and Application of Multi-phase Nitrides Bonded Silicon Carbide Lintel Blocks for Dry Quenching Furnaces

Multi-phase nitrides bonded silicon carbide lintel blocks were prepared using industrial SiC(SiC≥98 mass%,3-0.5,≤0.5 and≤0.044 mm),Si powder(Si≥98 mass%,≤0.044 mm),and SiO2 micropowder(SiO2≥96 mass%,d50=0.15 pm)as raw materials,and calcium lignosulfonate as the additive,batching,mixing,and molding on a vibration pressure molding machine,drying and then firing at 1420℃for 10 h in high-purity N2.The apparent porosity,the bulk density,the cold modulus of rupture,the hot modulus of rupture,and the linear expansion coefficient of the samples were tested.The phase composition and the microstructure of the samples at different nitriding depths(50,100,and 150 mm)were analyzed by XRD and SEM.The field application effects of the blocks were studied.The results show that:(1)the multi-phase nitrides bonded silicon carbide refractories can dynamically adjust their own phase composition and minimize structural and thermal stresses,improving the service life of key parts of dry quenching furnaces;(2)calcium lignosulfonate can improve the nitriding micro-environment of multi-phase nitrides bonded silicon carbide lintel blocks,successfully increasing the effective nitriding thickness of the blocks to 300 mm;(3)Sinosteel LI RR provides a unique concept in the design of materials and block types as well as the stable and scientific overall structure,promoting the industrialization process of dry quenching furnaces with long service life in China.

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.

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.

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.

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.

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.

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.

Neutronic analysis of silicon carbide cladding accident-tolerant fuel assemblies in pressurized water reactors

In resonance with the Fukushima Daiichi Nuclear Power Plant accident lesson, a novel fuel design to enhance safety regarding severe accident scenarios has become increasingly appreciated in the nuclear power industry. This research focuses on analysis of the neutronic properties of a silicon carbide(SiC) cladding fuel assembly, which provides a greater safety margin as a type of accident-tolerant fuel for pressurized water reactors. The general physical performance of SiC cladding is explored to ascertain its neutronic performance. The neutron spectrum, accumulation of ^(239)Pu, physical characteristics,temperature reactivity coefficient, and power distribution are analyzed. Furthermore, the influences of a burnable poison rod and enrichment are explored. SiC cladding assemblies show a softer neutron spectrum and flatter power distribution than conventional Zr alloy cladding fuel assemblies. Lower enrichment fuel is required when SiC cladding is adopted. However, the positive reactivity coefficient associated with the SiC material remains to be offset. The results reveal that SiC cladding assemblies show broad agreement with the neutronic performance of conventional Zr alloy cladding fuel. In the meantime, its unique physical characteristics can lead to improved safety and economy.

Sintering and microstructure of silicon carbide ceramic with Y_3Al_5O_(12) added by sol-gel method

Silicon carbide (SiC) ceramic with YAG (Y3Al5O12) additive added by sol-gel method was liquid-phase sintered at different sintering temperatures, and the sintering mechanism and microstructural characteristics of resulting silicon carbide ceramics were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and elemental distribution of surface (EDS). YAG (yttrium aluminum garnet) phase formed before the sintering and its uniform distribution in the SiC/YAG composite powder decreased the sintering temperature and improved the densification of SiC ceramic. The suitable sintering temperature was 1860 °C with the specimen sintered at this temperature having superior sintering and mechanical properties, smaller crystal size and fewer microstructure defects. Three characteristics of improved toughness of SiC ceramic with YAG added by sol-gel method were microstructural densification, main-crack deflection and crystal ‘bridging’.

On the Strength of Silicon Carbide Particulate Reinforced Aluminium Alloy Matrix Composites

In the present study, the modified continuum model, quench strengthening and dislocation pile-up model was respectively used to estimate the yield strength of SiCp/AI composites. The experimental results showed that the modified shear lag model or quench strengthening model would underestimate the yield strength of SiCp/AI composites. However, the modified Hall-Petch correlation on the basis of the dislocation pile-up model, expressed as σcy = 244 + 371λ-1/2, fitted very well with the experimental data, which indicated that the strength increase of SiCp/AI composites might be due to the direct blocking of dislocation motion by the particulate-matrix interface. Namely, the dislocation pile-up is the most possible strengthening mechanism for SiCp/AI composites.

Research on thermal shock resistance of mullite-bauxite-silicon carbide castable refractory

Effects of different silicon carbide contents on thermal shock resistance properties of mullite-bauxite castable refractory were analyzed respectively by taking the mullite,bauxite as main raw materials and taking calcium aluminate cement as a binding system.After 24 h curing in mould and another 24 h curing at 110℃ after demoulding,the samples were heated at 1000,1300 and 1500℃for 3 h in air,respectively.The bulk density,thermal expansion coefficient and thermal shock resistance were examined.The results showed that there was a great effect of content of SiC on bulk density,thermal expansion coefficient and thermal shock resistance of mullite-bauxite castable.Under the experimental conditions,the castable performed the best thermal shock resistance when w(SiC)=5%-10%.

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.

Diffusion Bonding of Silicon Carbide Particulate Reinforced 2024 Al Composites

A study has been made on diffusion bonding of SiCp/2024Ai composites by means of pure Al interlayer. In the condition of TB=843 K, PB=16 MPa, tB= 60 min, the diffusion bonded joint, with a shear strength of 235 MPa, was obtained when a 15 μm thick interlayer was used. The results of the shear testing and SEM indicate that fracture of the joint presented characteristics of ductile rupture.

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.