Silicon Carbide Specimens Research Articles

Controllable reduction of absorbance and two-step reaction for 3D-printed SiC ceramics with micron-level periodic structure

Digital light processing (DLP) is increasingly widely used to manufacture high-precision complex structural ceramic components quickly and efficiently. However, due to the high absorbance of silicon carbide (SiC) micron fine powder and the high refractive index difference between the powder and the photosensitive resin, it is extremely challenging to DLP print SiC with high precision complex structure. Here, the surface of SiC powder was coated with SiO2 nano-layer of different thickness by controllable oxidation coating method, thus realizing the photocurable printing of SiC@SiO2 green bodies with structural precision up to 80 μm. Through the two-step reaction method of carbothermal reduction combined with gas-phase siliconizing (GSI), not only the SiO2 coated layer was effectively removed, but also the reaction-bonded silicon carbide (RB-SiC) ceramics with 80–300 μm periodic structure was successfully realized. Ultimately, the comprehensive performance of the RB-SiC samples is effectively improved by chemical vapor infiltration of silicon carbide (CVI-SiC). It is noteworthy that the maximum elastic modulus and hardness of the ultima SiC specimens (RB-SiC@CVI-SiC) are as high as 394.599 GPa and 50.241 GPa, respectively. Shockingly, the nanomechanical properties of postgenetic RB-SiC bound to initial α-SiC on the cross-section of ultima specimens were much higher than those of CVI-SiC on the surface. This study fills the gap that liquid-phase siliconizing (LSI) or pyrolysis process could not achieve high-performance SiC ceramics with micron-level periodic structure.

Investigation of the prolonged effect of electric stress into the cement mortar incorporating silicon carbide

This study examines the impact of electric stress on the electrical, thermal, and phase composition properties of cement mortar with silicon carbide (SiC) as a conductive filler. The results indicate that SiC specimens enhance the electrical conductivity of the cement mortar by 4.35% to over 50% compared to the control. However, as the curing progresses, the electrical resistance increases due to reduced ionic conduction and an increase in conduction through free electrons. SiC significantly raises the surface temperature and thermal conductivity of the cement mortar, with SiC specimens experiencing a temperature rise exceeding 30 °C within the first hour of electric stress, while the control remains below 30 °C. SiC also enhances thermal conductivity up to eight-fold, attributed to improved particle movement and electron transport. The crystalline structure remains largely unaffected by electric stress after 28 days. Thermogravimetric analysis (TGA) indicates that SiC hinders the hydration process, leading to a reduction in weight loss associated with the depletion of hydrated cement phases. SEM images demonstrate a denser matrix in SiC specimens, while CT images show increased pore volume and decreased sphericity with higher SiC content. Under electric stress, the mechanical performance of SiC specimens exhibits lower compressive strength compared to the normal state, with decreasing strength as the SiC replacement ratio exceeds 50%. This study provides insights into the effects of electric stress on SiC-incorporated cement mortar properties, highlighting its potential for diverse construction industry applications.

Irradiation temperature monitoring with SiC for RPV steel at low fluence

Accurate knowledge of the irradiation temperature is a key concern in irradiations of reactor pressure vessel (RPV) steel. We report results of passive temperature monitoring of RPV steel with SiC. Two un-instrumented capsules containing RPV steel blocks were irradiated in Belgian Reactor 2, at SCK CEN in Belgium. Because un-instrumented capsules were used, the irradiation conditions (gamma heating and irradiation temperature) were calculated. To have experimental verification of the irradiation conditions each capsule contained a passive silicon carbide (SiC) temperature monitor. After irradiation the resistivity and the radiation-induced swelling (lattice parameter) is measured using x-ray diffraction (XRD). These measurements were repeated after annealing treatments to determine the peak irradiation temperature. The best method to determine the peak irradiation temperature with the lowest uncertainty in this study was the resistivity technique. Although lattice parameter measurements could be improved by using material without preferential orientation. Tensile tests of the RPV blocks were compared to a large available database of RPV material with the proper sensitivity towards neutron fluence and irradiation temperature to find the irradiation temperature of the RPV steel. The data showed a discrepancy between the temperature obtained from SiC and the temperature obtained from tensile tests. The temperature differences were discussed and rationalized by finite element modelling with respect to uncertainties in the helium gap between the capsule and the RPV steel block. The aluminium holder accounted for the temperature difference and the use of a steel holder is recommended for future similar irradiations to minimize the temperature difference between SiC and tensile specimens . In-depth analysis showed that when the irradiation temperature dropped at the end of the irradiation for a considerable time, the SiC temperature monitor had a memory effect. In this case the post-irradiation resistivity analysis showed two peak irradiation temperatures with a region of constant resistivity between the two.

Experimental study on the dynamic mechanical behaviors of silicon carbide ceramic after thermal shock

Abstract As a potential substitute material for metal and concrete in producing nuclear waste storage canisters, silicon carbide (SiC) can be subjected to various mechanical and thermal influences during its lifetime. To investigate the reliability of SiC in situ, especially in unusual cases involving impact load and high temperature, dynamic mechanical tests are performed on heated SiC utilizing the Split Hopkinson Pressure Bar (SHPB) system. Before the mechanical tests, thermal shock (TS) treatments are applied on the SiC specimens before the mechanical tests, where the heated specimens are cooled in air and water to provide different cooling rates. The test results indicate no discernable variation of dynamic compressive strength after heating at 100 °C. Evident drop of strength value is observed at heating levels higher than 200 °C. It is also found that with approximately the same incident energy, the energy absorbed by the specimen decreases with ascending cooling rate. The scanning electron microscopy (SEM) technique is also utilized to provide explanations for the corresponding test results whereby the damage mechanisms of thermal shock on SiC are analyzed.

An investigation on structures and strains of gas-ion-implanted and post-implantation-annealed SiC

Silicon carbide (SiC) single crystals were implanted by 5.3-MeV Kr ion, 2.3-MeV Ne ion, and 100-keV He ion with the fluence of 2.0 × 1014, 3.75 × 1015, 3.0 × 1016 ions/cm2, respectively. After ion implantation, SiC specimens were annealed at different temperatures in a high vacuum. The HRXRD measurements were used to characterize the structure damage, strain generation and their recovery processes. HRXRD results reveal that the full width at half maximum (FWHM) of the main diffraction peak broadens and new diffraction peaks appear at lower angles besides the main diffraction peak due to lattice expansions and strain occurrences caused by the implantation. Meanwhile, with increasing annealing temperature, the lattice recovery and strain relaxation occur gradually and several stages of the strain release are presented. Moreover, based on the Arrhenius model, the thermal relaxation activated energy of strains at different temperature ranges was estimated. The mechanisms on strain releases were discussed in detail.

Corrosion Resistance of Ceramics Based on SiC under Hydrothermal Conditions

We study the possibility of improvement of the corrosion resistance of ceramics based on silicon carbide (SiC) under the conditions of high-temperature steam corresponding to the standard operating conditions of fuel shells in WWER-1000 (water-moderated water-cooled) power reactors. We formed and sintered SiC specimens with and without admixtures of Cr and Si by the method of high-speed hot pressing in a graphite mold in a vacuum. The corrosion tests were carried out at a temperature of 350°C under a pressure of 16.8 MPa for 1000 h in a water medium used as the heat carrier of the WWER-1000 reactor. The physicomechanical characteristics of SiC ceramics prior to and after the tests were analyzed by the methods of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Hardness was measured with the help of a PMT-3 microhardness tester. It is demonstrated that the procedure of alloying of silicon carbide with chromium guarantees the most pronounced elevation of its corrosion resistance without decreasing its microhardness and crack resistance.

Low‐temperature joining of SiC ceramics using NITE phase with Al2O3‐Ho2O3 additive

AbstractIn order to avoid the property degradation resulting from high‐temperature joining process, nano‐infiltrated transient eutectoid (NITE) phase with the Al2O3‐Ho2O3 as the joining adhesives was adopted to join silicon carbide (SiC) ceramics with the attempts to lower down the joining temperature. The liquid‐phase‐sintered silicon carbide (LPS‐SiC) specimens were joined at 1500‐1800°C by spark plasma sintering (SPS) under the pressure of 20 MPa. The results of the shear test and microstructure observation showed that the joining process could be finished at a relatively lower temperature (1700°C) compared to other NITE‐phase joining. In contrast to the shear strength of 186.4 MPa derived from the SiC substrate, the joint exhibited the shear strength of 157.8 MPa with the fully densified interlayer.

The role of chemical disorder and structural freedom in radiation-induced amorphization of silicon carbide deduced from electron spectroscopy and ab initio simulations

Chemical disorder has previously been proposed as an explanation for the anomalously facile amorphization of silicon carbide (SiC), on the basis of topological connectivity arguments alone. In this exploratory study, “amorphous” (formally, aperiodic) SiC structures produced in ab initio molecular dynamics simulations were assessed for their connectivity topology and used to compute synthetic electron energy-loss spectra (EELS) using the ab initio real-space multiple scattering code FEFF. The synthesized spectra were compared to experimental EELS spectra collected from an ion-amorphized SiC specimen. A threshold level of chemical disorder χ (expressed as the ratio of the number of carbon-carbon bonds to the number of carbon-silicon bonds) was found to be χ ≈ 0.38, above which structural relaxation resulted in formally aperiodic structures. Different disordering methodologies resulted in identifiably different aperiodic structures, as assessed by local-cluster analysis and confirmed by collecting near-edge electron energy-loss spectra (ELNES). Such structural differences are predicted to arise for SiC crystals amorphized by irradiations involving different damage mechanisms—and therefore differing disordering mechanisms—for example, when contrasting the respective amorphized products of ion irradiation, neutron irradiation, and high-energy electron irradiation. Evidence for sp2-hybridized carbon bonding is observed, both experimentally in the irradiated sample and in simulations, and related to connectivity topology-based models for the amorphization of silicon carbide. New information about the probable intermediate-range structures present in amorphized silicon carbide is deduced from enumeration of primitive rings and evolution of local cluster configurations during the ab initio-modelled amorphization sequences.

A theoretical and experimental investigation on ultrasonic assisted grinding from the single-grain aspect

Ultrasonic assisted grinding (UAG) has been demonstrated to be effective for processing hard and brittle materials. In some of the current UAG models, the high efficiency of UAG was attributed to the great maximum momentary force brought by ultrasonic vibration. However, the models were built based on multi-grains behavior which lacks of necessary support by single-grain models and experiments. In this paper, a model for ultrasonic assisted scratching (UAS) was proposed to investigate the cutting behavior of single-grains. The experiments with and without ultrasonic assistance were both conducted on a reaction-bonded silicon carbide specimen to verify the developed model. Results showed that even with the same maximum momentary force, the material removal rate increased greatly with ultrasonic assistance. The factor kL and kH did not remain constants as in the original indentation fracture mechanics models, and were highly related to the amplitude of the ultrasonic vibration.

Optimization of a novel process for preparation of silicon carbide foams

Silicon carbide (SiC) was mixed with alumina and kaolin to obtain porous alumino-silicate bonded SiC specimens. These mixtures contain 40 to 80 mass% SiC while the alumina contents of their bonding materials are 60–80 mass%.Porous SiC specimens were prepared from mixtures of silicon carbide, calcined alumina and Klabsha kaolin using the foaming method. The employed technique of foaming introduces pores into SiC specimens as a result of chemical reaction between aluminum powder (Al) and calcium carbide (CaC2) in presence of water which liberates acetylene and hydrogen gases. The reaction and process are assisted by adding sodium meta-silicate and a stabilizer. The shaped specimens were dried and then fired up to 1450 °C for 2hrs.The chemical composition and firing temperature of foamed specimens are responsible for all changes occurring in their physical properties, microstructure characteristics and mineral composition.The result indicated that CaC2 mass% and CaC2/Al mass ratio of 1 and 3.3 are considered to be more suitable amounts for using in this foaming method and firing temperature as low as 1350 °C is used to prepare more porous specimens. Also on increasing the water content from 25 to 30 mass%, specimens of higher open porosities and lower bulk densities are obtained. Under these conditions, porous SiC specimens having bulk density, open porosity and mean pore size of 0.88–1 g/cm3, 65–70% and less than 25 μm are obtained, respectively.

Surface morphology changes of silicon carbide by helium plasma irradiation

Silicon carbide (SiC) and its composites are candidate materials for the blanket components and for the first wall in a fusion reactor. If the SiC is used without any armor materials for the first wall, it is exposed by helium (He) plasma as well as hydrogen plasma. Characteristic surface morphology changes are reported for various materials by He plasma exposure. Thus, we exposed SiC specimens to He or simultaneous deuterium (D) and He (D + He) plasma by various conditions and then observed surface morphology changes by SEM. As a result, needle-like structures and whiskers-like structures at the tip were formed in He plasma and D + He irradiation, while only needle-like structures were formed in D plasma. Therefore, it indicated that the effects of He were attributed to form whiskers-like structures. Although the structures are different among He plasma, simultaneous D + He plasma and D plasma irradiations, sputtering is considered to be a dominant process for the formation of the structure formation. However, the effects of He atoms in the structure could also be attributed to form whiskers-like structures.

In-situ testing of surface evolution of SiC during thermal ablation: Mechanisms of formation, flowing and growth of liquid silica beads

In this work we use real-time and in-situ observation technique at high temperature to capture the surface evolution of SiC (silicon carbide) specimen subjected to oxyacetylene torch flame. The obtained real-time images reveal clearly the nucleation, flowing and growth of the liquid silica beads on the surface of the specimen during thermal ablation process. A detailed model is developed to qualitatively analyze and interpret the mechanisms of the nucleation, flowing and the growth of the liquid silica beads. The result is expected to provide a better understanding of the ablation mechanism of SiC materials and serve as a preparatory work for the design and modification of the surface properties and topography of the materials to improve the thermal ablation resistance.

Dynamic Impact Induced Residential Stress and Deformation in Silicon Carbide Ceramics

Silicon carbide ceramics with different grain sizes were tested at high strain rate using a small-scale gas gun with sharpened tungsten carbide projectiles impacting at a sub-ballistic velocity of 100 m/s. The resistance of ceramics to fracture was recorded by visual examination of the cracking on the impacted surface and the damage in the sub-surface region. The crack pattern and impact crater region size observed in silicon carbide specimens are generally due to their mechanical properties. Silicon carbide samples had good capacity to defeat the projectiles but the low toughness led to early failure. Raman SiC mapping was used to examine the residual stress and plastic deformation induced in each material. The silicon carbide samples showed different dynamic behaviours due to the extent of stress relaxation.

Concentration of point defects in 4H-SiC characterized by a magnetic measurement

A magnetic method is presented to characterize the concentration of point defects in silicon carbide. In this method, the concentration of common charged point defects, which is related to the density of paramagnetic centers, is determined by fitting the paramagnetic component of the specimen to the Brillouin function. Several parameters in the Brillouin function can be measured such as: the g-factor can be obtained from electron spin resonance spectroscopy, and the magnetic moment of paramagnetic centers can be obtained from positron lifetime spectroscopy combined with a first-principles calculation. To evaluate the characterization method, silicon carbide specimens with different concentrations of point defects are prepared with aluminum ion implantation. The fitting results of the densities of paramagnetic centers for the implanted doses of 1 × 1014 cm−2, 1 × 1015 cm−2 and 1 × 1016 cm−2 are 6.52 × 1014/g, 1.14 × 1015/g and 9.45 × 1014/g, respectively. The same trends are also observed for the S-parameters in the Doppler broadening spectra. It is shown that this method is an accurate and convenient way to obtain the concentration of point defects in 4H-SiC.

FLiNaK compatibility studies with Inconel 600 and silicon carbide

A small liquid fluoride salt test apparatus has been constructed and testing has been conducted to examine the compatibility of silicon carbide (SiC), Inconel 600 and a spiral wound gasket material in FLiNaK, the ternary eutectic alkaline metal fluoride salt mixture. These tests were conducted to evaluate materials and sealing systems that could be used in fluoride salt systems. Three months of testing at 700°C was conducted to assure that these materials and seals would be acceptable when operating under prototypic operating conditions. The SiC specimens showed little or no change over the test period, while the spiral wound gasket material did not show any degradation except that salt might have been seeping into the outermost spirals of the gasket. The Inconel 600 specimens showed regions of voiding which penetrated the specimen surface to about 250μm in depth. Analysis indicated that the salt had leached chrome from the Inconel surface, as was expected for this material.

Effect of silicon carbide dispersion on the microwave absorbing properties of silicon carbide-epoxy composites in 2–40 GHz

In this study, silicon carbide powders were manufactured successfully by the method of preheating combustion synthesis in nitrogen atmosphere where it was introduced into an epoxy resin to produce a microwave absorber. The structure of the silicon carbide was characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Composite based on the various loadings of silicon carbide and epoxy resin specimens were prepared and the reflection losses of these composite samples were studied using the free space method. Based on the microwave measurements, microwave absorber specimens of silicon carbide with thermal plastic resin at frequencies between 2 and 18 and 18–40 GHz could be obtained from a matching thickness of 2.0 mm by controlling the content of silicon carbide.

Control of Electrical Resistivity in Silicon Carbide Ceramics Sintered with Aluminum Nitride and Yttria

The electrical properties of β‐SiC ceramics were found to be adjustable through appropriate AlN–Y2O3 codoping. Polycrystalline β‐SiC specimens were obtained by hot pressing silicon carbide (SiC) powder mixtures containing AlN and Y2O3 as sintering additives in a nitrogen atmosphere. The electrical resistivity of the SiC specimens, which exhibited n‐type character, increased with AlN doping and decreased with Y2O3 doping. The increase in resistivity is attributed to Al‐derived acceptors trapping carriers excited from the N‐derived donors. The results suggest that the electrical resistivity of the β‐SiC ceramics may be varied in the 104–10−3 Ω·cm range by manipulating the compensation of the two impurity states. The photoluminescence (PL) spectrum of the specimens was found to evolve with the addition of dopants. The presence of N‐donor and Al‐acceptor states within the band gap of 3C–SiC could be identified by analyzing the PL data.

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO2 scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO2 surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO2 scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO2 layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO2 layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition kl governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.

Ballistic performance of liquid-phase sintered silicon carbide

Abstract The ballistic performances of liquid-phase sintered silicon carbide (SiC) specimens prepared by pressureless sintering were evaluated. Solid-state sintering additives were also used in preparation of additional SiC specimens for comparison purposes. SiC specimens prepared by adding solid-state sintering additives were also prepared for the purpose of comparison. With the addition of 10–15 wt% of Al 2 O 3 and Y 2 O 3 , the strength and toughness of the liquid-phase sintered SiC (LSC) were higher than those of solid-state sintered SiC (SSC). Nevertheless, the hardness of LSC specimen tended to be lower. However, LSC specimens sintered at 1875 °C can pass the NIJ Level IV test. The microstructure observation provides direct evidence of the importance of hardness and toughness regarding ballistic performance. A performance index, the hardness/density ratio, is proposed to rank the ballistic performance on ceramics used for personnel armor applications.

Mechanical Properties and Defect Evolution of Kr-Implanted 6H-SiC

Specimens of silicon carbide (6H-SiC) were irradiated with 5 MeV Kr ions (84Kr19+) for three fluences of 5×1013, 2 ×1014 and 1× 1015 ions/cm2, and subsequently annealed at room temperature, 500°C, 700°C and 1000°C, respectively. The strain of the specimens was investigated with high resolution XRD and different defect evolution processes are revealed. An interpretation of the defect evolution and migration is given to explain the strain variation. The mechanical properties of the specimens were studied by using a nano-indentation technique in continuous stiffness measurement (GSM) mode with a diamond Berkovich indenter. For specimens irradiated with fluences of 5×1013 or 2×1014 ions/cm2, hardness values exceed that of un-implanted SiC. However, hardness sharply degrades for specimens irradiated with the highest fluence of 1×1015 ions/cm2. The specimens with fluences of 5×1013 and 2×1014 ions/cm2 and subsequently annealed at 700°C and 500°C, respectively, show the maximum hardness value.