SiC Specimens Research Articles

Effects of novel carbon sources additives on the solid-state sintering behavior and properties of SiC ceramics

To enhance the density of SiC and improve the purity of the SiC, carbon is frequently used as a sintering additive. SiC ceramics were prepared in this study by using various carbon sources (graphitic carbon microsphere, carbon black, and flake graphite) combined with B4C through spark plasma sintering (SPS) at temperatures ranging from 1800 °C to 1900 °C. Among the above carbon sources, the graphitic carbon microspheres were created by hydrothermal carbonization combined with catalytic graphitization. The influence of the carbon sources on the phase compositions, microstructure, physical properties, and oxidation resistance of the SiC specimens was investigated. The findings revealed that the SiC specimens prepared with graphitic carbon microsphere had a smaller crystallite size than those prepared with other kinds of carbon. The SiC specimens prepared with graphitic carbon microsphere displayed high relative density, hardness, fracture toughness, and remarkable oxidation resistance. This was attributed to the lower oxygen content, better dispersion, and higher chemical reactivity of graphitic carbon spheres. In terms of the oxidation resistance of SiC specimens, factors such as purity (residual carbon) and porosity were found to have a more significant impact than grain size. In general, the use of carbon sources with high reactivity and good dispersion is an effective method in preparing SiC ceramics.

Evaluation of the interaction between SiC, pre-oxidized FeCrAlMo with aluminized and pre-oxidized Fe-8Cr-2W in flowing PbLi

Dissimilar materials interactions are a major concern when liquid metals are used as a working fluid. To simulate a dual-coolant PbLi blanket, chemical vapor deposited (CVD) SiC, pre-oxidized FeCrAlMo (APMT) and pre-oxidized aluminized coating on Fe-8Cr-2 W (F82H) specimens were exposed to flowing commercial Pb-17at.%Li with a peak temperature of 650 °C in a pre-oxidized APMT thermal convection loop (TCL) for 1000 h. Similar to prior results in static PbLi, the coated F82H showed small mass changes and limited degradation of room-temperature tensile properties. After exposure, the α-Al2O3 on the surface coated F82H reacted with the PbLi to form LiAlO2. However, no significant Al coating loss or interdiffusion was observed. Mass transfer was observed between the APMT, TCL and the SiC specimens. Fe, Cr and Ni were dissolved into the molten flowing PbLi and transported through the TCL to react with the SiC specimens. Compared to a prior experiment with a majority of bare FeCrAl specimens that were exposed in a TCL with a peak temperature of 700 °C for 1000 h, the SiC reaction was reduced to only a few microns of carbides and silicides. Oxygen dissolved in the molten PbLi sustained the formation of an Al2O3 scale on the surface of the coating and induced the formation of a thin surface SiO2 layer on the surface of the SiC.

Additive manufacturing of high mechanical strength continuous Cf/SiC composites using a 3D extrusion technique and polycarbosilane‐coated carbon fibers

AbstractA novel additive manufacturing approach is herein reported for manufacturing high mechanical strength continuous carbon fiber‐reinforced silicon carbide (Cf/SiC) composite materials. Continuous carbon fibers were coated with polycarbosilane (PCS) using a colloidal evaporative deposition process and then coextruded with high solid content SiC ink. The zeta potential of the SiC ink was adjusted to optimize the printing ability of the suspension. During sintering, small SiC grains and whiskers were generated in the gaps in and around the PCS‐coated carbon fibers, which led to the improved flexural strength and density of the composites. Meanwhile, the PCS coating on the surface of the carbon fibers prevented the carbon fibers from reacting with SiO gas generated by reactions between the SiC matrix and SiO2 and sintering additives (Al2O3 and Y2O3), effectively preserving the structural integrity of the carbon fibers. Compared to the SiC specimens containing uncoated carbon fibers, the density of the specimens fabricated with coated carbon fibers was increased from 2.51 to 2.85 g/cm3, and the strength was increased from 190 to 232 MPa.

Effect of holding time on SiC whiskers growth of SiCw/SiC composites based on SLS technology and their mechanical properties

The in-situ SiC whisker and SiC particle composites were prepared by selective laser sintering (SLS) technology, and the longitudinal and transverse growth rates of crystal nuclei at the liquid-solid interface were calculated and analyzed under the traditional vapor-liquid-solid mechanism. A mathematical model of holding time on the number of in-Situ SiC whisker growth was established, and the prediction rate was 95%. The mechanical properties of in-situ SiC whisker and pure SiC samples with similar volume densities were calculated. The results showed that: The longitudinal growth rate of crystal nuclei at the liquid-solid interface was higher than the transverse growth rate. After precursor infiltration pyrolysis (PIP) four-cycle treatment, the fracture toughness per unit volume density of B-1, B-2, and B-3 specimens increased by 198.66%, 225.00%, and 221.05%, respectively, compared with pure SiC specimens, indicating that this method has a vital role in increasing the toughness of SiC ceramics.

Dielectric properties and thermal conductivity of Si3N4–SiC composite ceramics

In this work, Si3N4–SiC composite ceramics were prepared by hot pressing sintering with Y2O3–MgO–Al2O3 as sintering additives in a flowing nitrogen atmosphere. The crystal structure and microstructure of Si3N4–SiC specimens with different SiC content were studied. The XRD results show that the peaks of β-Si3N4 and SiC were displayed and matched well on XRD patterns. Si3N4 particles were basically long rod-shaped. Besides, the effects of SiC content on thermal conductivity and dielectric properties in W band (75–110 GHz) of sintered Si3N4–SiC ceramics were researched. The dielectric constant first decreased then increased with the increase of SiC content, while the trend of dielectric loss (tgδ) was opposite. The dielectric constant of Si3N4–SiC composites for 35 wt% SiC reached a minimum range of 13–20 and tgδ values reached a maximum range of 0.72–0.96 in the frequency of 75–110 GHz. Meanwhile, the thermal conductivity of Si3N4–SiC composites for 35 wt% SiC was 63.008 W/(m K) at room temperature. The variation of thermal conductivity as test temperature was also studied. The thermal conductivity decreased with increasing test temperature. When the test temperature was 300 °C, the thermal conductivity of the Si3N4–SiC ceramics for 35 wt% SiC content was 36.022 W/(m K). The Si3N4–SiC composite ceramics were expected to be applied as microwave absorbing materials.

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al2O3 and Y2O3 powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al2O3–Y2O3 mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al2O3–Y2O3 mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.

Development of a silicon carbide ceramic based counter-flow heat exchanger by binder jetting and liquid silicon infiltration for concentrating solar power

A silicon carbide ceramic counter-flow heat exchanger with integrated headers was printed by binder jetting additive manufacturing process. Multiple phenolic binder infiltration cycles (3 or 5) followed by pyrolysis were conducted to increase the net carbon content of the printed SiC specimens. Subsequently, to attain full densification, silicon melt infiltration was used. The microstructure and mechanical properties were comprehensively characterized on the densified material. The chemical compositions and visual distribution of the various regions in the specimens were determined via scanning electron microscopy, while X-ray diffraction and synchrotron μ-computed tomography were used to provide a quantitative assessment of the volume fractions of the identified phase regions. Microhardness measurements showed dependence on the local microstructure. The fracture strength of the material was correlated with the specimen density and agreed with the reported values in the literature. High-temperature exposure at 750 °C for up to 200 h did not degrade the strength for the specimens with three phenolic-binder infiltrations; however, the strengths degraded for ones with five phenolic-binder infiltrations. The associated fracture toughnesses of the specimens were ∼3.4 MPam1/2 at room temperature and 750 °C, and the thermal conductivities varied from >150 W/mK at room temperature to ∼45 W/mK at 750 °C. Hence, this study validated the use of the binder-jetting printed SiC ceramic materials for high-temperature heat exchanges. Finally, we also present in this work the first successful fabrication of a binder-jetting printed one-piece dense SiC ceramic heat exchanger body with unblocked channels that can be used for the flow of heat transfer fluids.

Investigating the Use of C/SiC Ceramic Composites in an Innovative Light-Water Reactor Core Grouping Catcher

China Nuclear Power Engineering, Northwestern Polytechnical University, and Beijing Institute of Technology have undertaken a joint research work with the goal of developing corium retention containers for use in an innovative light-water reactor core grouping catcher (CGC). In Serious Accidents (SAs), the corium will be retained in the C/SiC ceramic composite spherical shell container and thrown into the pool for passive cooling. In order to investigate the capability of the C/SiC ceramic composites to withstand the high temperature of 1800°C and the huge thermal shock at the initial stage of cooling, in this work, the high-temperature performance of these composites is studied. Furthermore, the compatibility of these composites with corium is investigated, and a retention container design prototype is proposed. The thermal conductivity, thermal expansion coefficient, high-temperature tensile strength, and thermal shock resistance of the candidate C/SiC specimens to be used in the manufacturing of the detention container are measured. The FactSage software package is used for the chemical compatibility study and product composition analysis of the composite SiC deposition layer and corium. The corrosion process of the SiC deposition layer of the composite material caused by corium is investigated through high-temperature corrosion tests and the electron microscopy analysis of the SiC specimen with the incorporation of CeO2. The results show that the C/SiC ceramic composite can meet the design criteria of the CGC corium retention container, and it is impossible to cause a carbon reaction disaster through the failure of the composite SiC deposition layer.

Influence of oxygen content on structure and properties of pressurelessly sintered aluminum-nitride- and zirconium-boride-doped silicon-carbide ceramics

SiC is a widely used material. Understanding how oxygen content affects the SiC structure and properties is crucial. In this paper, heat treatment was used to prepare SiC powder samples with different oxygen contents, which were doped with AlN and ZrB2 and were densified by pressureless sintering at 2050 °C. The effect of oxygen content on the sintered SiC structure was determined by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicated that the oxygen content influenced the SiC phase composition, grain boundaries, and densification. Additionally, the interaction between oxygen defects and AlN played an important role in sintering. The nanoindentation, alternating-current impedance, and thermal conductivity of the densified SiC specimens were also evaluated to elucidate the influence of the oxygen content on the densified-SiC functional properties. The results revealed that the oxygen content affected all the measured mechanical, electrical, and thermal properties. Furthermore, surface oxygen impurities suggested that oxygen content had similar critical effects on both the densified SiC structure and properties.

Electron tomography for sintered ceramic materials by a neural network algebraic reconstruction technique

The missing wedge effect in electron tomography introduces various types of artifacts in the tomograms and lowers the reconstruction resolution and quality. The artifacts produced in tomographic reconstruction of bulk materials can be very severe, particularly for sintered bulk ceramic materials in which there are often nano-pores or pore-like microstructure features. Here, we report a neural network algebraic reconstruction algorithm with no prior knowledge to perform electron tomography for a sintered SiC material with nano carbon zones. The results show that the proposed algorithm has a great suppressive effect on the missing wedge artifacts and a high tolerance for noise. The information in the missing wedge can be partly recovered by this technique. Thus, both the shape of the bulk SiC specimen and its irregular inner pore-like features are correctly retrieved in the obtained 3D image. Our study shows the effectiveness of the neural network algorithm for improving the reconstruction accuracy of electron tomography, in order to reveal sophisticated 3D microstructures generally existing in sintered ceramic materials.

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC f /SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated. Comparing the microstructures of SiC f /C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC f /SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC f /SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process. In addition, among the SiC f /SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC f /SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.

Effect of SHI irradiation and high temperature annealing on the microstructure of SiC implanted with Ag

Scanning electron microscopy (SEM), Raman spectroscopy and Rutherford backscattering spectrometry (RBS) were used to study the influence of swift heavy ion (SHI) irradiation and annealing on the microstructure of polycrystalline SiC implanted with silver (Ag). Polycrystalline SiC specimens were first implanted with 360 keV Ag + ions at room temperature (RT) to a fluence of 2 × 1016cm−2. Thereafter, some of the implanted samples were irradiated with Xe ions of 167 MeV (SHI) at room temperature to a fluence of 3.4 × 1014 cm−2 and 8.4 × 1014 cm−2. Both the as-implanted and implanted then irradiated samples were annealed in vacuum at temperatures ranging from 1100 to 1400 °C in steps of 100 °C for 5 h. Raman and SEM results showed that implantation of silver (Ag) resulted in complete amorphization of the near surface region of the SiC substrates. However, SHI irradiation of the as-implanted SiC resulted in partial recrystallization of the initially amorphized layer. The as-implanted samples exhibited more crystallinity after annealing at 1100 °C as compared to SHI irradiated samples annealed at same conditions. This poor recrystallization of the SHI irradiated SiC samples was due to the amount of impurities (i.e. concentration of Ag atoms) retained after annealing at 1100 °C. Raman and SEM results showed that annealing of the as-implanted samples at 1100 °C resulted in larger average crystal size compared to the SHI irradiated samples annealed in the same conditions. The intensity of the longitudinal optical (LO) phonon in Raman spectra increases with the increasing the average crystal sizes of SiC.

Enhanced mechanical strength of SiC reticulated porous ceramics via addition of in-situ chopped carbon fibers

Due to the poor strength of silicon carbide reticulated porous ceramics prepared by polymer sponge replica method, vacuum infiltration using a low viscosity infiltration slurry is performed to fill the flaws created during pyrolysis of the polymer sponge to improve the compressive strength. However, the flexural strength and thermal shock resistance of the porous silicon carbide ceramic are not significantly improved through this method. This paper investigates the improvement in thermal shock resistance and flexural strength of porous silicon carbide ceramics through the addition of different lengths and amounts of carbon fibers to the infiltration slurry. With the addition of 3 mm long carbon fibers at a concentration of 0.5 wt% to the infiltration slurry, the flexural strength of the porous silicon carbide ceramic was improved by 142%. Moreover, the thermal shock resistance of the SiC specimens was effectively enhanced through the addition of carbon fibers, as evidenced by a 74.4% increase in the residual strength ratio of the specimens. Due to the low thermal expansion coefficient of the carbon fibers, compressive residual stresses were generated in coating layers of SiC RPCs after sintering, which was inhibited the propagation of cracks in the coating layers. Thus, adding carbon fibers to the infiltration slurry can open more possibilities for application of SiC reticulated porous ceramics.

Compatibility of SiC with ODS FeCrAl in flowing Pb-Li at 600°–700 °C

To establish the maximum operating temperature for the dual coolant lead lithium (DCLL) blanket concept, thermal convection loop (TCL) experiments were conducted for 1000 h with a peak temperature of 700 °C to determine compatibility with commercial eutectic Pb∼17at.%Li. Two TCLs were fabricated from alloy APMT (FeCrAlMo) and the baseline TCL experiment contained only APMT specimens. The second TCL experiment exposed more fusion relevant CVD SiC and ODS Fe-10Cr-6Al specimens in the hot and cold legs. In the second experiment, the mass losses were much larger for the ODS FeCrAl specimens and most of the CVD SiC specimens experienced a mass gain as SiC reacted to form an Fe- and Cr-rich carbide reaction product. Dissolved Fe and Cr reacted with the SiC and removing Fe and Cr from the liquid appeared to increase ODS FeCrAl dissolution. Such a dissimilar material interaction appears consistent with thermodynamic calculations. Significant attack at the top of the hot leg in both experiments suggests that the Pb-Li compatibility of FeCrAl alloys is inadequate at 700 °C in a flowing environment and the maximum PbLi temperature is < 675 °C.

Steam oxidation of ytterbium disilicate environmental barrier coatings with and without a silicon bond coat

AbstractThe current generation of multilayer Si/Yb2Si2O7 environmental barrier coatings (EBCs) are temperature limited by the melting point of Si, 1414°C. To investigate higher temperature EBCs, the cyclic steam oxidation of EBCs comprised of a single layer of ytterbium disilicate (YbDS) was compared to multilayered Si/YbDS EBCs, both deposited on SiC substrates using atmospheric plasma spray. After 500 1‐h cycles at 1300°C in 90 vol%H2O‐10 vol%air with a gas velocity of 1.5 cm/s, both multilayer Si/YbDS and single layer YbDS grew thinner silica scales than bare SiC, with the single layer YbDS forming the thinnest scale. Both coatings remained fully adherent and showed no signs of delamination. Silica scales formed on the single layer coating were significantly more homogeneous and possessed a markedly lower degree of cracking compared to the multilayered EBC. The single layer EBC also was exposed at 1425°C in steam with a gas velocity of 14 cm/s in an alumina reaction tube. The EBC reduced specimen mass loss compared to bare SiC but formed an extensive 2nd phase aluminosilicate reaction product. A similar reaction product was observed to form on some regions of the bare SiC specimen and appeared to partially inhibit silica volatilization. The 1425°C steam exposures were repeated with a SiC reaction tube and no 2nd phase reaction product was observed to form on the single layer EBC or bare SiC.

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.

Syntaxy and defect distribution during the bulk growth of 4H-SiC Single crystal

In the present study, the inclusion of 6H foreign polytype was seen during the initial growth of 4H polytype silicon carbide single crystal by PVT technique. The foreign polytype inclusion was gradually diminished with growth time; the mechanism is described in the present paper. In order to study the defects, the SiC specimen was wet etched using KOH solution at higher temperature and it was found that the foreign polytype inclusions i.e. 6H polytype introduced high density of defects at the polytype boundary. The defect distribution in different regions viz. 6H polytype foreign inclusion, 4H polytype region and polytype boundary were found to be different. It is interesting to note that the threading screw dislocation was not seen in 6H-polytype inclusion region.

Mechanical Properties and Machinability of Aluminium and Aluminium-Silicon Carbide Composites Processed by Equal Channel Angular Pressing (ECAP)

ABSTRACT The present work was aimed to investigate the mechanical and machining characteristics of the Equal Channel Angular Pressed (ECAP) aluminium and Al-SiC composites fabricated by stir casting process. Commercial pure aluminium, Al-5%SiC and Al-10%SiC were considered for the analysis. ECAP processes were performed with all the three varieties of materials in BC route. The microstructure, mechanical properties namely hardness, ductility, tensile strength and machinability of all the three materials were analysed before and after single and two passes of ECAP process. The hardness of the ECAPed pure aluminium with two passes was 4% higher than that of casted Al-5%SiC specimen. This result suggests that the ECAP of pure aluminium with suitable number of passes is recommended to replace the Al-SiC composites. Vickers hardness and tensile strength of the ECAPed Al-10%SiC composites were increased by 18% and 20% respectively when compared with as cast condition. Al-10%SiC outperformed the pure aluminium and Al-5%SiC as cast and ECAPed conditions in terms of tensile strength. However, the machining performance of Al-10%SiC was not as good as pure aluminium and Al-5%SiC. Severe plastic deformed Al-5%SiC produced good surface finish and lesser tool wear compared with the other two specimens at all conditions.

Synthesis of SiC Nanowires via Controllable Anodic Etching Time

Porous SiC (PSC) was successfully synthesized via UV-assisted pulsed current anodic etching of hexagonal n-type silicon carbide (6H–SiC) substrate using different etching times. The micromorphological and structural characterizations of PSC were reported. Field-emission scanning electron microscopy (FESEM) results established that the etching time can be considered as a significant etching parameter that controls the micromorphology aspects of the porous SiC specimens. Furthermore, the adjustment of etching time can convert SiC pores into nanowire structures. Raman spectroscopy characterization was performed as well, where the shape of the Raman spectra was analyzed, in precise the transverse optical E1 (TO) and the longitudinal optical A1 (LO) peaks, which correlate powerfully with the porous SiC micromorphology. This simplistic synthesis may be considered as a potential and affordable technique to synthesize SiC nanowires for extensive applications.

The effect of rod orientation on the strength of highly porous filament printed 3D SiC ceramic architectures

The present work explores the strength enhancement via minor modifications of the pattern design in pure light SiC woodpile structures created by filament printing. In particular, the effect of the filament stacking angle on both the compression resistance and the elastic modulus of these structures are evaluated. Different patterns were designed while maintaining the bulk structure density, therefore the differences in the mechanical data (strength and elastic modulus) are not attributable to density variations. All of these materials were partially sintered at intermediate temperature for additional porosity enhancement. Moreover, SiC specimens made by full filament overlapping were produced to serve as reference massive material. Remarkably, the massive SiC printed material displays ordered spherical porosity and a closed-pore foam appearance, thus revealing a novel route for producing these type of porosity. Results evidence that the structure robustness can be tuned through slight design modification, which thus offers the possibility of further structure lightening without reducing the target strength.