Anisotropy of functional properties of SiC composites with GNPs,GO and in-situ formed graphene

This paper reports on anisotropy of functional properties of different silicon carbide-graphene composites due to preferential orientation of graphene layers during sintering. Dense silicon carbide/graphene nanoplatelets (SiC/GNPs) and silicon carbide/graphene oxide (SiC/GO) composites were sintered in the presence of yttria (Y₂O₃) and alumina (Al₂O₃) sintering additives at 1800 °C in vacuum by the rapid hot pressing (RHP) technique. It is found that electrical conductivity of SiC/GNPs and SiC/GO composites increases significantly in the perpendicular direction to the RHP pressing axis, reached up to 1775 S/m in the case of SiC/GO (for 3.15 wt.% of rGO). Also, thermal diffusivity was found to increase slightly by the addition of GNPs in the SiC/GNPs composites in the perpendicular direction to the RHP pressing axis. But, in the parallel direction, the addition of GNPs showed a negative effect. The formation of graphene domains was observed in reference sample SiC-Y₂O₃-Al₂O₃ sintered by RHP, without any addition of graphene. Their presence was confirmed indirectly by increasing electrical conductivity about three orders of magnitude in comparison to the reference sample sintered by conventional hot press (HP). Raman, SEM and TEM analysis were used for direct evidence of presence of graphene domains in RHP reference sample.     

Tailoring the properties of spark plasma sintered SiAlON containing graphene nanoplatelets by using different exfoliation and size reduction techniques: Anisotropic electrical properties

Insulating SiAlON ceramics can be machined into complex shapes if the electrical conductivities can be increased with additives. Therefore, the microfluidization technique was used as an alternative to traditional sonication for exfoliation and homogenization of GNPs to investigate the effects of different exfoliation, size reduction and homogenization techniques on the microstructure, electrical conductivity and percolation threshold values of GNPs-SiAlON composites. 2.85, 5.70 and 11.40?vol. % sonicated and sonicated?+?microfluidized GNPs added into SiAlON matrix and were densified by using SPS technique. Due to their thinner and smaller platelet size, microfluidized GNPs dispersed more homogeneously compared to the sonicated GNPs in the SiAlON matrix. Electrical conductivities of the microfluidized GNPs-SiAlON composites were ～70–200% higher than sonicated GNPs-SiAlON in the both measured directions. Lower percolation thresholds were achieved when sonicated?+?microfluidized GNPs used in comparison to sonicated GNPs containing composites. Electrical conductivities in the in-plane direction were also higher than through-plane direction.     

Effects of porosity on electrical and thermal conductivities of porous SiC ceramics

The effects of porosity on the electrical and thermal conductivities of porous SiC ceramics, containing Y₂O₃–AlN additives, were investigated. The porosity of the porous SiC ceramic could be controlled in the range of 28–64 % by adjusting the sacrificial template (polymer microbead) content (0–30 wt%) and sintering temperature (1800–2000 °C). Both electrical and thermal conductivities of the porous SiC ceramics decreased, from 7.7 to 1.7 Ω⁻¹ cm⁻¹ and from 37.9 to 5.8 W/(m·K), respectively, with the increase in porosity from 30 to 63 %. The porous SiC ceramic with a coarser microstructure exhibited higher electrical and thermal conductivities than those of the ceramic with a finer microstructure at the equivalent porosity because of the smaller number of grain boundaries per unit volume. The decoupling of the electrical conductivity from the thermal conductivity was possible to some extent by adjusting the sintering temperature, i.e., microstructure, of the porous SiC ceramic.     

Solid state reactions between SiC and Ir

Reactivity between SiC and Ir as a function of SiC-crystallinity was investigated by diffusion bonding technique under a vacuum and over the temperature range of 1200–1450 °C. As reaction products, various Ir-silicides and free unreacted-C were detected. Reactivity is strongly affected by the temperature and SiC-crystallinity involving a series of interactions, from “no reaction” to “massive exothermic reactions”. In particular, interfacial phenomena are more pronounced by the presence of defects and grain boundaries.Solid state reactions result in formation of fine C-precipitates rearranged in a quasi-periodic microstructure. On the contrary, clustering of highly ordered C-precipitates (C-graphitized) occurs after “massive reactions” take place.A relationship between the degree of graphitization (from 1 to multi-layers of graphene), temperature and SiC crystallinity was found by Raman spectroscopy. 2D-layering phenomenon is enhanced in polycrystalline SiC at high temperature.     

Thermophysical property and electrical conductivity of titanium isopropoxide – polysiloxane derived ceramics

In this study, high temperature resistant Si-O-C-Ti has been successfully prepared based on the pyrolysis of polysiloxane (PSO) and titanium (IV) isopropoxide (TTIP) at 1200–1400 °C. PSO can homogeneously mix with TTIP to enhance its conversion to TiC. The carbothermal reactions between TiO₂ (product of thermal decomposition of TTIP) and carbon result in the formation of TiC. All the Si-O-C-Ti composites pyrolyzed at 1200–1300 °C are stable up to 1000 °C in an oxidizing air atmosphere. TiC leads to high electrical conductivity at elevated temperatures; the maximum conductivity is 1176.55 S/m at 950 °C, which is the first reported value of >1000 S/m conductivity for Si-O-C-Ti ceramics. However, too high a pyrolysis temperature, such as 1400 °C, can potentially ‘destabilize’ the Si-O-C-Ti system by consuming the free carbon and result in lower conductivities.     

Boron carbide/graphene platelet ceramics with improved fracture toughness and electrical conductivity

Boron carbide/graphene platelet (B₄C/GPLs) composites have been prepared with a different weight percent of GPLs as sintering additive and reinforcing phase, hot pressed at 2100 °C in argon. The influence of the GPLs addition on fracture toughness (K_IC) and electrical conductivity was investigated. Single Edge V-Notched Beam (SEVNB) method was used for fracture toughness measurements and the four-point Van der Pauw method for electrical conductivity measurements. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48 MPa.m^1/2 was achieved at 10 wt.% of GPLs addition, which was ∼50% higher than the K_IC value of the reference material. The electrical conductivity increased with GPLs addition and reached the maximum value at 8 wt.% of GPLs, 1.526 × 10³ S/m in the perpendicular and 8.72 × 10² S/m in the parallel direction to the hot press direction, respectively.     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Production of porous SiC by liquid phase sintering using graphite as sacrificial phase: Influence of SiO2 and graphite on the sintering mechanisms

Porous silicon carbide (SiC) is a promising ceramic for high-temperature applications due to its unique combination of properties. In the present work, a fabrication route for porous SiC is presented using graphite spherical powder as sacrificial phase to introduce porosity. By varying the initial amount of sacrificial phase, high-performance SiC materials with porosities in the range 30–50% were manufactured and characterized in terms of microstructure, density, thermal conductivity and flexural strength. The materials were fabricated by liquid phase sintering in presence of 2.5 wt.% Al₂O₃ and Y₂O₃ as sintering additives. The results indicate that the SiO₂ present in the starting SiC powders interacts with the sintering additives forming liquid phases that promote densification and weight loss. Besides, an Al-Si liquid phase is formed at higher sintering temperatures, whose contribution to densification is inhibited in presence of graphite due to the formation of Al-rich carbides.     

Crosslinking kinetics of polycarbosilane precursor in ozone atmosphere and the formation mechanism of continuous hollow SiC fiber

Crosslinking is favorable for increasing the ceramic yield of polycarbosilane precursor (PCS), so PCS fiber with crosslinked skin-uncrosslinked core structure may contribute to formation of hollow SiC fiber. In this study, diffusion-controlled ozone oxidation crosslinking method was adopted. EDS verified the oxygen element contributing to crosslinking mainly distributed in the outer layer of ozone-crosslinked PCS fiber (O ? PCS_f). After pyrolysis, SEM confirmed the formation of continuous hollow fiber and XRD indicated the presence of β-SiC structure. N₂ adsorption-desorption measurement demonstrated that a lot of micropores and mesopores, which were the main escape paths for degradation product of uncrosslinked core part of O ? PCS_f and contributed to the formation of continuous hollow SiC fiber, formed along fiber during pyrolysis and disappeared after the pyrolysis temperature was increased. In addition, by adjusting ozone oxidation time and applying vacuum pyrolysis method, the morphology of resulting hollow SiC fiber could be further modified.     

Formation mechanism of porous reaction-bonded silicon nitride with interconnected pores in the presence of MgO

In porous reaction bonded silicon nitride, whiskers normally grow in globular clusters as the dominant morphology and deteriorate the pore interconnectivity. However, the ceramic microstructure was significantly transformed with the addition of MgO; specifically, the morphology was modified to a combination of matte and hexagonal grains. Microstructural observation along with thermodynamic studies suggest that MgO interfered with the presence and nitridation of SiO_(g). Consequently, rather than being involved in the whiskers’ formation, surface silica instead reacted with volatile MgO to form intermediate products. Through these reactions, whisker formation was blocked, and a porous interconnected structure formed which was confirmed by 3D tomography. After heat-treatment at 1700 °C, β-Si₃N₄ crystallized in a glassy matrix containing magnesium. Resulting samples had an open-pore structure with porosity of 74–84 vol. %, and density of 0.48-0.75 g.cm^?3. Combination of high porosity and pore size of <40 μm led to compressive strengths of 1.1–1.6 MPa.     

Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites

SiC-nanowire-reinforced SiC_f/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of ?47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.     

Influence of carbon nanotubes as secondary phase addition on the mechanical properties and amorphization of boron carbide

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B₄C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B₄C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B₄C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.     

Processing,characterization and properties of novel gradient Si3N4/SiC fibers derived from polycarbosilanes

In this work, we report a novel kind of Si₃N₄/SiC composite fibers, which exhibit a controlled gradient Si₃N₄(shell)/SiC(core) structure. These composite fibers are fabricated through a controlled nitridation and pyrolysis process on electron irradiation-cured polycarbosilane fibers. Structural and chemical analysis based on Elemental Analyzer, FT-IR, Raman spectroscopy, electron probe micro-analyzer, X-ray photoelectron spectroscopy, and X-ray diffraction confirm the gradient structure of obtained fibers, which consist a shell with high Si₃N₄ content and a SiC core. The as-fabricated fibers exhibit dense and smooth surfaces, and no microscopic holes or defects were observed. The effects of nitridation temperature on mechanical properties and electrical resistivity were also investigated. Combined with high mechanical properties and lightweight, the present gradient Si₃N₄/SiC fibers open a new strategy to fabricate multifunctional and electromagnetic wave absorbing materials.     

Enhanced electrical conductivity of alumina/nano-carbon ceramic composite via iodine impregnation of gel-casted alumina body and reductive sintering

In this work, we present the fabrication of alumina/nano-carbon ceramic composite displaying enhanced electrical conductivity through facile iodine impregnation of gel-casted alumina body. Following the previously developed novel fabrication process of alumina/nano-carbon composite having a highly uniform nano-carbon network and superior semi-conductive properties via reductive sintering of gel-casted alumina body under inert atmosphere, we extended the process through the facile impregnation of gel-casted body in iodine solution resulting in enhancement of the amount of nano-carbon, graphitization degree, electrical conductivity and carrier density. The effect of iodine concentration on the properties of alumina/nano-carbon as well as the mechanism for enhanced electrical performance are also systematically investigated.     

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

This work aims to explore zirconium as a possible dopant to promote thermoelectric performance in bulk ZnO-based materials, both within the single-doping concept and on simultaneous co-doping with aluminum. At 1100–1223 K mixed-doped samples demonstrated around ～2.3 times increase in ZT as compared to single-doped materials, reaching ～0.12. The simultaneous presence of aluminum and zirconium imposes a synergistic effect on electrical properties provided by their mutual effects on the solubility in ZnO crystal lattice, while also allowing a moderate decrease of the thermal conductivity due to phonon scattering effects. At 1173 K the power factor of mixed-doped Zn_0.994Al_0.003Zr_0.003O was 2.2–2.5 times higher than for single-doped materials. Stability tests of the prepared materials under prospective operation conditions indicated that the gradual increase in both resistivity and Seebeck coefficient in mixed-doped compositions with time may partially compensate each other to maintain a relatively high power factor.