SiC Samples Research Articles

Microstructural characterization of TiB2–SiC–BN ceramics prepared by hot pressing

This research intended to evaluate the discrete and cumulative impacts of hexagonal SiC and BN additives on the microstructural features and consolidation behavior of TiB2-based materials. All specimens were sintered at the same hot-pressing conditions at 2000 °C under 50 MPa for 120 min. Both discrete and synergetic influences of SiC and hBN ingredients on the sintering behavior of TiB2 were noticeable so that three near fully dense ceramics were secured. Due to a chemical interaction between SiC and TiO2 phases, in-situ SiO2 was generated during the hot pressing of the TiB2–SiC sample. Although no in-situ phase could be detected in the composite introduced by hBN additive, the X-ray diffraction pattern and electron microscopy images uncovered the in-situ generation of the TiC phase in the ternary and binary systems of TiB2–SiC-hBN and TiB2–SiC. While both TiB2–SiC-hBN and TiB2–SiC samples were mostly fractured intergranularly, a mixed transgranular-intergranular fracture mode was seen in the fracture surface of the TiB2-hBN ceramic. The finest microstructure was attained when both SiC and hBN additives were incorporated into the TiB2 matrix at the same time. Finally, the sample introduced by 25 vol% SiC reinforcement reached the most significant Vickers hardness (24.3 GPa), while the TiB2–SiC-hBN composite offered the highest fracture toughness (6.7 MPa m1/2).

Recycling of Al chips and Al chips composites using high-pressure torsion

Pure Al chip, Al chip-20% Al2O3, and Al chip-20% SiC samples were recycled using high-pressure torsion (HPT). Influence of the HPT processing on the feasibility of the consolidation, the microstructure evolution, and the hardness of pure Al chip, Al chip-20% Al2O3, and Al chip-20% SiC samples was investigated and compared with those of the as-received Al and HPTed Al solid samples. The HPT processing successfully produces approximately fully dense ultrafine-grained (UFG) microstructure Al and Al composite samples with relative densities ranged from 99.7%–98.3%. Moreover, HPTed recycled UFG Al chip, Al chip-20% Al2O3, and Al chip-20% SiC samples cost was lower by 11%–2678% than that of Al and Al composites samples produced using micro and nano-powders processed by HPT or powder metallurgy (PM). The HPT processing of the Al chip and Al chip composites samples effectively refine and fragmented the Al matrix and reinforcement particles, and so the hardness obviously increases.

Preparation of graphene on SiC by laser-accelerated pulsed ion beams* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11875077, 11975037, and 11921006) and the National Grand Instrument Project of China (Grant Nos. 2019YFF01014400 and 2019YFF01014404).

Laser-accelerated ion beams (LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here we explore an application of using laser-accelerated ion beams to prepare graphene. The pulsed LIBs produced a great instantaneous beam current and thermal effect on the SiC samples with a shooting frequency of 1 Hz. In the experiment, we controlled the deposition dose by adjusting the number of shootings and the irradiating current by adjusting the distance between the sample and the ion source. During annealing at 1100 °C, we found that the 190 shots ion beams allowed more carbon atoms to self-assemble into graphene than the 10 shots case. By comparing with the controlled experiment based on ion beams from a traditional ion accelerator, we found that the laser-accelerated ion beams could cause greater damage in a very short time. Significant thermal effect was induced when the irradiation distance was reduced to less than 1 cm, which could make partial SiC self-annealing to prepare graphene dots directly. The special effects of LIBs indicate their vital role to change the structure of the irradiation sample.

High accurate multi-angular polarized spectrum thermometry for smooth surface

A new multi-angular polarized spectrum thermometry based on the optimal inversion wavelength selection method was proposed to accurately measure the surface temperature of optically smooth opaque surface. The surface temperature was inverted from the multi-angular polarized spectral radiance of sample by the least square method. The optimal inversion wavelength was iteratively determined by using the relation of λTS=2410 μm·K. The optimal inversion model was proved to be inverting the refractive index and the extinction coefficient by the ratio model P/S firstly and then calculating the surface temperature by the non-ratio model P+S. A proof-of-principle experiment with a high-purity SiC sample was carried out to validate the reliability of the inversion method. The result shows that the relative deviation of temperature is less than 1% at the optimal wavelength for the calculation performed by the optimal inversion model.

Construction of a Pt/g-C3N4/SiC Heterostructure for Enhanced Methanol Photoelectrooxidation

The performance of a direct methanol fuel cell (DMFC) highly depends on the anode catalyst. However, it is a grand challenge to construct a potential electrocatalyst featuring high intrinsic activity, a large electrochemical surface area and outstanding photoelectrochemical function for the methanol oxidation reaction (MOR). Herein, g-C3N4 composited with nano-SiC (g-C3N4/SiC) was prepared by evenly mixing urea and SiC followed by annealing at 550°C for different times. X-ray diffraction, thermogravimetric analysis, x-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller surface area tests showed that g-C3N4 and SiC are bonded together by chemical bonds. Ultraviolet-visible (UV-Vis) spectroscopy characterization showed that the UV-Vis absorption edge of the g-C3N4/SiC sample is much higher than that of g-C3N4 or SiC. In addition, it was observed that an appropriate thickness of the g-C3N4 coating on the SiC surface would increase the photogenerated charge separation efficiency. Therefore, the obtained Pt/g-C3N4/SiC-90 exhibits a higher photoelectrocatalytic current density and long-term stability in comparison with Pt/SiC and Pt/g-C3N4 under the same conditions. Such a catalyst with high photochemical catalytic performance is thus promising for assembly of high-performance DMFCs.

Accuracy improvement for directional polarized spectral emissivity measurement in the wavelength range of 4–20 μm

A new directional polarized spectral emissivity measurement apparatus with the wavelength range of 4–20 μm was established to promote the development of thermal radiation transfer and polarization engineering. The measurement accuracy was improved by eliminating the background radiation with a high emissive thermostatic shutter and precisely determining the sample surface temperature. An accurate expression for directional polarized spectral emissivity was obtained taking into account the multiple reflection between the sample and the chamber. The linearity of spectral response was calibrated respectively in parallel and perpendicular polarized directions. The directional polarized spectral emissivity of a high-purity SiC sample was measured in a controlled environment to validate the capability of the apparatus. The excellent agreement between the measurement result and the literature data proved the reliability of the experiment apparatus and the measurement method. The measurement uncertainty was evaluated for each experiment, and the relative uncertainty was estimated to be less than 4.1%.

Hot-pressing and characterization of TiB2–SiC composites with different amounts of BN additive

This research aimed to study the influence of different amounts of hBN additive on the mechanical properties and microstructure of TiB2-15 vol% SiC samples. All ceramics, containing 0, 3.5, and 7 vol% hBN, were sintered at 2000 °C using a hot-pressing route and reached their near full densities. Thanks to two different chemical reactions among the SiC reinforcement and the TiB2 surface oxides (B2O3 and TiO2), the in-situ phases of SiO2 and TiC were generated over the sintering process. The intergranular mode was identified as the predominant fracture type in all three composite samples. The hBN additive could contribute to grain refining of composites so that the sample containing 7 vol% hBN reached the finest microstructure. Finally, the highest Vickers hardness of 25.4 HV0.5 kg and flexural strength of 776 MPa were attained for the TiB2–SiC and TiB2–SiC-7 vol% hBN samples, respectively.

Role of homogeneous distribution of SiC reinforcement on the characteristics of stir casted Al–SiC composites

Stir casting process is very popular for the fabrication of particle reinforced metal matrix composites. However, achieving a homogenous distribution of the reinforcement particles is very challenging, which directly affect the final mechanical performances. In this research, a detailed investigation was conducted to obtain a superior synthesis of Al–SiC composite materials and to detect the effect of SiC particles distribution on the mechanical characteristic of the stir casted Al matrix composites using glycerol–water based model. The glycerol–water based model study displays the impacts of viscosity of Al melt (1.04–1.24 mPa.s), impeller position (10–50%), stirring speed (100–300 rpm) and blade angle (0–90°) on the vortex height, dispersion time, settling time and clustering of particles. The results of the experiments using glycerol–water based model, 45° blade angles, impeller position of 40% from the base, stirring speed of 250 rpm showed the best uniform distribution of reinforcement particles. Confirmation experiments were carried out by fabricating Al–SiC PRMMC based on three different viscosities (1.04, 1.13, and 1.24 mPa.s) of Al melt and considering other parameters as constant. Fabricated Al–SiC samples were analysed by Scanning Electron Microscope (SEM) and mechanical testing such as tensile, hardness and wear tests. Based on the confirmation results, the viscosity of 1.13 mPa.s for Al melt (at 750 °C) resulted in uniform distribution of SiC particles, which enhanced the tensile strength by 4%, wear resistance by 21%, and led to a uniform hardness value of 47 VHN throughout the composites.

Influence of SiC microstructure on its corrosion behavior in molten FLiNaK salt

The influence of the microstructure on the corrosion rate of three monolithic SiC samples in FLiNaK salt at 900 °C for 250 h was studied. The SiC samples, labeled as SiC-1, SiC-2, and SiC-3, had corrosion rates of 0.137, 0.020, and 0.043 mg/cm2h, respectively. Compared with grain size and the presence of special grain boundaries (i.e., Σ3), the content of high-angle grain boundaries (HAGBs) appeared to have the strongest influence on the corrosion rate of SiC in FLiNaK salt, since the corrosion rate increased six times as the concentration of high-angle grain boundaries increased from 19 to 32% for SiC-2 and SiC-1, respectively. These results stress the importance of controlling the content of HAGBs during the production process of SiC.

Characterization of reactive spark plasma sintered (Zr,Ti)B2–ZrC–SiC composites

This investigation aims to evaluate the influence of SiC introduction on the microstructure development, mechanical characteristics, and densification behavior of (Zr,Ti)B2–ZrC materials. Two specimens, with/without SiC addition, were manufactured from the starting materials of ZrB2 and TiC by reactive spark plasma sintering (SPS) at 1800 °C under 30 MPa for 5 min. Both samples reached relative density values higher than 99%; however, the SiC addition improved this value by ~0.5%, standing at 99.9%. The SiC incorporation into the (Zr,Ti)B2–ZrC system led to an ultrahigh temperature ceramic (UHTC) with considerable hardness and elastic modulus. The TiC additive was completely consumed over the sintering process, participating in the in-situ formation of TiB2 and ZrC as a result of a chemical reaction with the ZrB2 matrix. Thanks to the diffusion of Ti and Zr atoms into the ZrB2 and TiB2 phases, respectively, a solid-solution of (Zr,Ti)B2 formed with various compositions in different areas. Thanks to the synergistic effects of grain refinement and solid-solution formation, the (Zr,Ti)B2–ZrC–SiC sample reached excellent mechanical properties such as an elastic modulus of 495 GPa and a hardness of 31.2 GPa.

Retracted] A Comparative Study on Crack‐Healing Ability of Al2O3/SiC Structural Ceramic Composites Synthesized by Microwave Sintering and Conventional Electrical Sintering

This study was conducted to assess and compare the crack‐healing ability of conventional electrical sintered and microwave sintered Al2O3/x wt. % SiC (x = 5, 10, 15, and 20) structural ceramic composites. The crack‐healing ability of both conventional electrical sintered and microwave sintered specimens was studied by introducing a crack of ∼100 µm length by Vickers’s indentation and conducting a heat treatment at 1200°C for dwell time of 1 h in air. The flexural or bending strength of sintered, cracked, and crack‐healed specimens was determined by three‐point bending test, and the phase variations by X‐ray diffraction and SEM micrographs before and after crack‐healing of both the sintering methods were studied and compared. The results show that almost all the specimens recovered their strength after crack‐healing, but the strength of microwave sintered Al2O3/SiC structural ceramic composites has been shown to be better than that of conventional electrical sintered Al2O3/SiC structural ceramic composites. The microwave sintered crack‐healed Al2O3/10 wt. % SiC specimen shows higher flexural strength of 794 MPa, which was 105% when compared with conventional electrical sintered Al2O3/10 wt. % SiC and crack‐healed Al2O3/10 wt. % SiC specimen. It was found by X‐ray diffractogram that before crack‐healing, all the conventional electrical sintered samples have SiO2 phase which reduce the crack‐healing ability and microwave sintered samples with 15 and 20 wt. % SiC show lesser SiO2 phase and 5 and 10 wt. % SiC samples have no SiO2 phase before crack‐healing. However, after crack‐healing treatment, all the samples have distinct SiO2 phase along with Al2O3 and SiC phases. Microwave sintered Al2O3/10 wt. % SiC specimen cracks were fully healed which was evident in SEM micrographs.

The role of Mg2Si additive in sintering silicon carbide

A novel Mg2Si sintering additive was introduced to improve the relative density of SiC ceramic. The SiC samples were hot press sintered at 0.5, 2 and 4 h. The microstructure and relative density were measured, the distribution, microstructure and orientation of Mg2Si were characterized, and the interface of SiC/Mg2Si was observed. Results show that relative density of the ceramics increased with the prolongation of sintering time, and densification fulfilled when sintered at 2 h, resulting in the high relative density of 96.86%. With the prolongation of sintering time, the pores become coarsening and the densification process was protracted. Some Mg2Si react with SiC matrix, and the Mg element distributes into the SiC matrix. The fraction of deformed grains and the geometrically necessary dislocations decreased initially then increased slightly with the prolongation of sintering time. The fraction of low angle boundaries is small in all the samples and the transition from low angle boundaries to high angle boundaries is not smooth. The Mg2Si grains locate on the grain boundaries of SiC with little misorientation, resulting in a perfect lattice coherence between SiC and Mg2Si.

Dynamic oxidation of SiC with arc-heated plasma wind tunnel and laser heating

For the investigation of thermal protection system (TPS) for re-entry vehicles, arc-heated plasma wind tunnel was developed to simulate re-entry conditions. The surface temperatures of the samples were independently controlled of the plasma flow by installing a laser heating system. By using this system, first attempt of the dynamic oxidation of SiC was conducted. SiC sample surface was heated up to 2200 K and was exposed to high-speed atomic oxygen (O) flow during oxidation for 100–600 s. From the obtained results, active/passive transition border of SiC was suggested: 1700–1800 K at 100–1000 Pa area of oxygen partial pressure. Obtained A/P transition border is considered close to the actual re-entry condition wherein the oxidation occurs in the condition of highly dissociated and slightly ionized oxygen gas flow simulating the re-entry conditions.

Understanding efficiency differences of betavoltaic batteries measured by electron gun mimicked source and radioactive β source

Electron guns have been widely used to mimic the radiation of radioactive β sources in many fields, including radiation effects and material modification, due to their unique advantages such as steerable irradiation area, adjustable electron energy, and flux. However, it is still unclear whether the results and conclusions drawn from the mimicking experiments can provide effective guidance and reference for real radiation. In this work, we systematically investigated the difference of electron guns and radioactive β sources on the efficiency measurement of betavoltaic batteries and gave a quantitative analysis on the causes. Geant 4 simulations of the energy deposition of electrons emitted from electron guns and 63Ni sources demonstrate that the energy dispersion and the incident direction distribution of electrons are the major causes of the difference of the energy deposition ratio and profile in energy conversion units. The single emission direction of electrons leads to an overestimation around 30% in the conversion efficiency measured using an electron gun. The device structure also contributes to the efficiency difference. The efficiency difference of three SiC samples with different structures measured under electron guns and 63Ni sources varies from 20.6% to 32.0% in three samples, which agrees very well with the simulation results. This work provides a valuable guidance to evaluate the results generated from electron-gun mimicked β sources. The physics discussed here would provide good references for other applications using mimicked β sources.

Enhancement of Ca3Co4O9+δ thermoelectric properties by dispersing SiC nanoparticles

Misfit-layered oxides Ca3Co4O9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) samples were synthesized using solid-state sintering method and the effects of SiC nanoparticles diapersion on the thermoelectric properties were investigated. Thermoelectric properties of Ca3Co4O9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) were investigated up to 923 K. Compared with pure sample, the electrical resistivity of SiC-added samples reduces, for Ca3Co4O9+δ + z wt% SiC (z = 0.00, 0.025, 0.05, 0.1, 0.2) ceramic samples with x ≤ 0.05, the electrical resistivity decreases with increasing SiC nanoparticles adding amounts. And, the electrical resistivity exists transition from semiconductor to metal conduction mechanism. While, the thermal conductivity also decreases due to the addition of SiC nanoparticles. As a result, the Ca3Co4O9+δ + 0.2 wt% SiC sample had the lowest thermal conductivity of 1.47 W/Km at 923 K, which was 18.4% lower than that of the Ca3Co4O9+δ sample. The ZT value of Ca3Co4O9+δ + 0.05 wt% SiC can reach 0.218, which is 40.9% higher than the pure Ca3Co4O9+δ sample.

Pool boiling critical heat flux studies of accident tolerant fuel cladding materials

Pool boiling experiments have been performed at atmospheric pressure on potential accident tolerant fuel (ATF) cladding materials (Zirlo®, Cr coated Zirlo®, FeCrAl coated Zirlo®, and monolithic SiC) to evaluate the Critical Heat Flux (CHF) points. Corrosion resistant coatings of Cr and FeCrAl alloy were deposited on flat Zirlo® samples using cold spray technology. The as-prepared samples after uniform surface polishing were subjected to autoclave tests at 360 °C water and 18.6 MPa for 360 h to simulate prototypic corrosion of the ATF cladding materials in LWR normal operation. Surface characteristics potentially influencing CHF such as surface morphology, roughness, static contact angle, and surface chemistry were characterized using a suite of characterization methods including scanning electron microscopy, 3D optical profilometry, optical contact angle measurements, and x-ray photoelectron spectroscopy (XPS). Thermo-physical properties of the samples such as density, thermal conductivity, and heat capacity were measured by differential scanning calorimetry and laser flash thermal diffusivity measurement. FeCrAl coatings showed CHF values comparable to bare Zirlo® samples, while slightly lower CHF values were observed for Cr coated samples and bulk SiC flats (produced by chemical vapor deposition, CVD). As-deposited Cr coatings showed 67% higher CHF than the as-polished Cr coatings due to its higher surface roughness level. The autoclave testing for the prepared samples generally increased the CHF except for the surface polished Cr coatings. Similar CHF values of bare Zirlo®, FeCrAl coating, and CVD SiC samples were observed after autoclave test but there were negligible change CHF of Cr coatings. XPS studies indicated that the formation or deposition of a few monolayers of hydrophobic carbonaceous species on surface can affect CHF values. The trends in CHF data are discussed in terms of evolution of the surface characteristics of the ATF materials.

Correlation between crystal warpage and swelling of 4H-SiC through implantation and annealing

Crystal warpage and swelling of implanted 4H-SiC with both un-implanted and implanted regions on the surface were investigated using a white-light interferometer. In the SiC samples with an implantation temperature of approximately 30 °C, the warpage was small and the step at the un-implanted/implanted interface for swelling was large. In contrast, the SiC samples with implantation temperatures of over 300 °C had a large warpage and a small interface step. In the x-ray topographs, no diffraction at the interface was observed in the SiC implanted at a low temperature; however, the SiC implanted at a high temperature exhibited diffraction at the interface. Diffraction at the interface is evidence of lattice bonding between the un-implanted and implanted regions. The interfacial lattice bonding can consistently explain the correlation between the crystal warpage and the swelling of the implanted SiC. After the annealing, crystal recovery developed and the warpage and swelling returned to their initial state in all samples.

Material removal and wear mechanism in abrasive polishing of SiO2/SiC using molecular dynamics

The effect of the sliding and rolling motions of the abrasive particle on the silicon carbide substrate covered by a thin film of silica atoms is investigated in this paper using molecular dynamics simulation. The influence of the sliding depth, sliding speed, rolling depth and rolling speed on the material removal and wear mechanism are surveyed. The results represent that increasing the sliding depth gives rise to higher forces, a higher temperature, a deeper groove, deeper subsurface damage (SSD) layer, and a higher amount of atoms erased. Improving the sliding velocity results in a higher fraction of atoms removed out of the pathway and a larger number of atoms erased. In the rolling mechanism, improving the depth and the rolling speed leads to the same effect as in the sliding mechanism but with a lower rate. Notably, the number of atoms erased in the rolling motion is greatly lower than the sliding one. In the ploughing regime, the silica film is effectively removed out of the SiC substrate, especially at a high sliding speed. The ploughing regime in the rolling motion requires a deeper depth to achieve, and the ploughing indications are weaker than the sliding motion. This report also considers both mechanical and chemical effects as some models with pure SiC workpiece or with different silica thickness are constructed and investigated. With the same removing conditions, the pure SiC sample suffers higher forces, a higher level of deformation and a larger number of atoms erased as compared to the sample covered by a silica thin film. The ploughing regime does not only require a sufficient penetration depth but also needs a suitable silica thickness to happen.

Role of graphene nano-platelets on thermal conductivity and microstructure of TiB2–SiC ceramics

The current research investigates the influence of adding 2 wt% graphene nano-platelets (GNP) on microstructure and thermal conductivity of the TiB2–25 vol% SiC composite. The disk-shape pellets were fabricated using spark plasma sintering method at a temperature of 1800 °C for 7 min under a pressure of 40 MPa. Measured relative densities were about 95.5% and 96% for TiB2–SiC and graphene containing TiB2–SiC ceramics, respectively. According to the microstructure analysis of the polished area and fractured surfaces, captured via scanning electron microscopy (SEM), the addition of graphene nano-platelets to the TiB2–25 vol% SiC resulted in finer, denser, and more uniform microstructure. The laser flash method was employed to measure the thermal diffusivity of the specimens followed by evaluating the thermal conductivity of the sintered specimens. Despite a remarkable thermal conductivity of graphene (about 5 × 103 W/m.K), the TiB2–SiC–GNP composite showed lower thermal conductivity than the graphene-free sample. Based on SEM images, the finer grains with increased boundaries are the reason for decreased thermal conductivity of the TiB2–SiC sample doped with graphene nano-platelets.

Mapping of large structural defects in SiC Schottky contacts using internal photoemission microscopy

4H–SiC and 6H–SiC epitaxial layers have been characterized by the scanning internal photoemission microscopy (SIPM) method based on use of Ni Schottky contacts. Geometrical patterns with widths of a few tens of microns were observed in the photoyield (Y) maps obtained for the 4H–SiC samples with a slightly lower Schottky barrier and a larger ideality factor than those in the current-voltage (I–V) characteristics; these patterns are caused by stacking faults located in the depletion region. SIPM also provided clear images of small particles adhered to the sample, which appeared as dark spots in the Y maps. These particles behaved as insulating materials and blocked the photocurrent flow; this behavior could not be detected from the conventional I–V characteristics. On the 6H–SiC samples, the violet SIPM measurements revealed surface pits, which originated from screw dislocations and were also shown as dark spots. We consider SIPM to be a powerful tool for investigation in inhomogeneities in crystal quality in conjunction with the electrical characteristics.