Structural,thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sintering

Highly dense AlN–SiC composites with various SiC additions (0–50?wt-%) were fabricated at 1800°C by plasma activated sintering. The effect of SiC addition on structural, thermal and dielectric properties as well as microwave absorbing performance of the composites was investigated. The thermal conductivity decreases with increasing SiC addition, from 68.7 W (m?K)^?1 for 0?wt-% SiC to 19.38?W (m?K)^?1 for 50?wt-% SiC. On the contrary, the permittivity and dielectric loss increase gradually, from 7.6–8.5 to 22–26.7 and from 0.02–0.1 to 0.2–0.53, respectively. AlN–SiC composite with better thermal and dielectric properties in 30?wt-% SiC, whose thermal conductivity and dielectric loss are found to be 24.88?W (m?K)^?1 and 0.15–0.74, respectively. Furthermore, the composite exhibits microwave absorbing performance with the minimum reflection loss (RL) of ?16.5 dB at 15.5 GHz and the frequency range of 2.6 GHz for RL below ?10 dB (90% absorption).     

Superhydrophobic and photothermal sponge for effective oil/water separation and solar-assisted recovery of viscous oil

With the occurrence of offshore oil spills, the recovery of viscous oil continues to be a significant issue. The multi-functional sponge with superhydrophobic, absorption of high viscosity oil is an effective approach to settle this problem. In this work, the melamine-formaldehyde (MF) sponge was the backbone and doped with polydopamine (PDA) to mimic mussels. Subsequently, candle soot nanoparticles (CS) and polydimethylsiloxane (PDMS) were applied to modify the MF sponge into a superhydrophobic sponge (P-CS-PDA@MF) (WCA = 153.56°). The P-CS-PDA@MF sponge showed good absorption properties for many oils and organic solvents (33.87–78.90 times of its own weight). In addition, the P-CS-PDA@MF sponge also showed outstanding chemical stability, low density (0.013 g/cm³), emulsion-breaking ability and flame retardancy. More interestingly, the P-CS-PDA@MF sponge showed superior photothermal conversion capability, the temperature of it increased to approximately 91°C within 100 s under simulated sunlight from room temperature (about 25°C), and the sorption capacity of viscous oils was significantly increased. Moreover, the P-CS-PDA@MF sponge exhibited a high efficiency (over 97%) for separating viscous oils/water mixtures with the aid of sunlight. This versatile and efficient multifunctional P-CS-PDA@MF sponge via a simple and low-cost way shows promising prospect in oily wastewater treatment and water ecosystem remediation.     

Ultralight,tunable monolithic SiC aerogel for electromagnetic absorption with broad absorption band

SiC aerogel is a novel candidate for electromagnetic absorption (EMA) materials in extreme environments because of its ceramic nature and aerogel morphology. In this study, graphene oxide (GO), which having has been generally serving served as a “brick” in the material community, was used as a building block to prepare SiC aerogel precursors and further develop a novel and designable route for preparing SiC aerogel for EMA. The experimental results suggested that the GO aerogel was well converted into a SiC aerogel with ultra-low density (<4 mg cm^?3). The synthesized SiC aerogel inherited the precursor well in structure while having a wide absorption band (approximately 5.5GHz) and a high absorption frequency-to-thickness correlation different from traditional SiC aerogels. The above idea for designing and assembling SiC aerogels will significantly broaden the practical applications of GO and SiC in EMA, supercapacitors and sensors.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

Wire electrical discharge machinable SiC with GNPs and GO as the electrically conducting filler

SiC based composites filled with graphene nano-platelets (GNPs) or graphene oxide (GO) prepared by rapid hot-pressing exhibit sufficient electrical conductivity for their machinability by wire electro-discharge machining (WEDM). Composites microstructure anisotropy caused by graphene alignment as a consequence of rapid hot pressing was confirmed by measuring of electrical conductivity and thermal diffusivity. Electrical conductivity increased significantly with increased weight fraction of graphene in both measured directions. Highest value of 2031 S/m was obtained for composites with 15 wt. % of GNPs in parallel direction and only 1246 S/m in perpendicular direction to aligned GNPs. Thermal diffusivity is 63.3 mm²/s in parallel and only 23.3 mm²/s in perpendicular direction. The increase of the electrical conductivity has resulted in successful WEDM. The MRR was almost doubled when the filler concentration increased from 5 wt. % GNPs/GO to 15 wt. % GNPs. At the same time, the surface roughness decreased.     

Direct laser sintering of reaction bonded silicon carbide with low residual silicon content

Additive manufacturing (AM) techniques are promising manufacturing methods for the production of complex parts in small series. In this work, laser sintering (LS) was used to fabricate reaction bonded silicon carbide (RBSC) parts. First, silicon carbide (SiC) and silicon (Si) powders were mixed in order to obtain a homogeneous powder. This powder mixture was subsequently laser sintered, where the Si melts and re-solidifies to bind the primary SiC particles. Afterwards, these SiSiC preforms were impregnated with a phenolic resin. This phenolic resin was pyrolysed yielding porous carbon, which was transformed into secondary reaction formed SiC when the preforms were infiltrated with molten silicon in the final step. This resulted in fully dense RBSC parts with up to 84?vol% SiC. The optimized SiSiC combined a Vickers hardness of 2045?HV, an electrical conductivity of 5.3?×?10³?S/m, a Young's modulus of 285?GPa and a 4-point bending strength of 162?MPa.     

Sintering and thermomechanical behavior of a Low Temperature Co-fired Ceramic

The thermoelastic, viscoplastic and sintering properties of a commercial Low Temperature Co-fired Ceramic (LTCC, ref. ESL 41111-G) were carefully characterized and correlated to its microstructural features. This LTCC was made of rhombohedral quartz and an amorphous phase composed of silicon, oxygen and some trace amounts of aluminum, carbon and potassium. The dependences of the elastic response vs. temperature and density were characterized by various techniques from nano- to macro-scales. A stable Young's modulus around 54.0?±?3.5?GPa was measured up to 500?°C before a sharp fall. It was associated with the exceeding of the glass transition temperature (around 576?±?4?°C). Based on constant heating rate and master sintering curve methodologies, it was shown that the apparent activation energy for free sintering was stable during the whole densification process, with an average value of approximately 291?±?11?kJ?mol^?1. The uniaxial viscosity ranged from 0.1 to 2.0?GPa?s for relative densities between 0.56 and 0.92. The activation energy identified for viscous flow was similar to the one for free sintering. Moreover, a low and stable value of viscous Poisson's ratio was determined, with an average value around 0.14?±?0.01 during the initial and intermediate stages of sintering.     

Microwave synthesis of worm-like SiC nanowires for thin electromagnetic wave absorbing materials

Thin thickness is always the pursuit of excellent electromagnetic wave absorbing materials. Herein, SiC nanowires with worm-like morphology were synthesized by microwave heating the mixture of expanded graphite and silica. The worm-like SiC nanowires exhibit an excellent microwave absorption ability at a thin thickness. With the filling ratio of SiC nanowires increases in the matrix, the dielectric loss and microwave absorbing ability are significantly enhanced; meanwhile the number of absorption peaks is gradually increased, and the absorption peaks also move toward a thinner thickness. When the nanowires filling ratio was 40?wt%, the minimum reflection loss reached down to ?35.2?dB and the effective absorption (RL?<??10?dB) bandwidth was 1.8?GHz?at a thickness of 1.3?mm. The possible growth mechanism of the worm-like SiC nanowires is that the intermediate reaction gas phases, SiO and CO, were confined in the relatively independent tiny pores of expanded graphite. This resulting in an excessive local gas phase pressure, which causes the nanowire growth direction changes randomly.     

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO₂). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30?×?3?mm². The pre-forming pressure was selected at 0?MPa, 1?MPa, 2?MPa, 3?MPa, 4?MPa and 5?MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4?MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.     

Grain-growth-induced high electrical conductivity in SiC–BN composites

SiC–BN composites were fabricated by conventional hot-pressing from β-SiC and h-BN nanopowders with 2?vol% yttria as a sintering additive. Electrical and thermal properties of the composites were investigated as a function of initial BN content. Owing to the nanosize of the starting powders, the grain-growth-assisted N-doping of the SiC lattice was significantly enhanced during liquid-phase sintering, yielding the highest-reported electrical conductivity of ~124 (Ω?cm)^?1 for a SiC–4-vol% BN composite. The typical values of electrical resistivity and thermal conductivity of the SiC–4-vol% BN composite at room temperature were 8.1?×?10^?3 Ω?cm and 92.4?W?m^?1 K^?1, respectively.     

Processing of alumina‐coated glass‐bonded silicon carbide membranes for oily wastewater treatment

Crack‐free γ‐Al₂O₃‐coated glass‐bonded SiC membranes were successfully prepared using a simple heat‐treatment and dip‐coating process at a temperature as low as 850°C in air. The changes in the porosity, flexural strength, flux, and oil rejection rate of the membranes were investigated while changing the initial SiC particle size. Larger SiC particles led to bigger pores, resulting in higher flux in the oily water and a lower oil rejection rate. The SiC membranes with a support prepared from 10 μm SiC powder showed an exceptionally high oil rejection rate (99.9%) with a feed oil concentration of 600 mg/L at an applied pressure of 101 kPa. The typical porosity, flexural strength, steady state flux, and oil rejection rate of the alumina‐coated SiC membrane were ~45%, ~81 MPa, 1.78×10^?5 m³ m^?2s^?1, and 99.9%, respectively.     

Corrosion resistant ZrO2/SiC ultrafiltration membranes for wastewater treatment and operation in harsh environments

This work focused on the fabrication of a ZrO₂/SiC ultrafiltration membrane by dip coating a high porous SiC support with a ZrO₂ slurry prepared by ceramic processing. The membranes were sintered in different temperatures (1000−1300 °C). With the optimal temperature, it was obtained a mechanically strong, homogenous, and defect free separation layer with 45 μm of thickness and average pore size of 60 nm. A pure water permeability of 360 L.m⁻² h⁻¹ bar^-1 and high retentions of humic acid, indigo dye, and hemoglobin were observed. In a pilot test with an olive oil/water emulsion, 99.91 % of oil was removed without fouling. Long-term corrosion tests at basic and acid baths did not cause change in pore size and morphology. In conclusion, the ZrO₂/SiC membrane has potential to operate in harsh conditions (e.g. heavily contaminated industrial effluents or urban wastewaters) and when severe membrane cleaning and disinfection are required, such as food and pharmaceutical industries.     

Enhancing toughness and strength of SiC ceramics with reduced graphene oxide by HP sintering

In this paper, the silicon carbide-reduced graphene oxide (SiC/rGO) composites with different content of rGO are investigated. The hot pressing (HP) at 2100?°C for 60?min under a uniaxial pressure of 40?M?Pa resulted in a near fully-dense SiC/rGO composite. In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (fracture toughness, bending strength, and Vickers hardness) of SiC/rGO composites is discussed. The fracture toughness of SiC/rGO composites (7.9MPam^1/2) was strongly enhanced by incorporating rGO into the SiC matrix, which was 97% higher than the solid-state sintering SiC ceramics (SSiC) by HP. Meanwhile, the bending strength of the composites reached 625?M?Pa, which was 17.3% higher than the reference materials (SSiC). The microstructure of the composites revealed that SiC grains were isolated by rGO platelets, which lead to the toughening of the composite through rGO pull out/debonding and crack bridging mechanisms.     

Fabrication of high performance macroporous tubular silicon carbide gas filters by extrusion method

Hot gas filtration requires high performance tubular filters, but low permeability, low strength, and high sintering temperature of silicon carbide (SiC) filters limit their use. In this work, a high permeability tubular SiC support was fabricated with high strength at a sintering temperature of 1200?°C, when 100?µm SiC particles were used as aggregate, sodium dodecyl benzene sulfonate (SDBS) was used as sintering aid and organic additives were used as binders. Plasticity of the mixed particles was optimized by adjusting the ratios of methylcellulose, paraffin, and glycerol. The porosity, pore diameter, gas permeation coefficient, and bending strength of the SiC ceramic support reached 45.0%, 34.2?µm, 4.6?×?10^–12 m², and 22.8?MPa, respectively. Furthermore, compared to the cold isostatic pressure (CIP) technique, the extrusion method led to sharper peak of the pore diameter distribution, achieved higher bending strength, and had a more homogeneous microstructure.     

Structural design and energy absorption mechanism of laminated SiC/BN ceramics

The laminated silicon carbide/boron nitride (SiC/BN) ceramics with different structural designs were fabricated by pressureless sintering at 1900?°C for 1?h in argon flow. The alumina (Al₂O₃)-and yttrium(III) oxide (Y₂O₃)-doped SiC ceramic exhibited a significant intergranular fracture behavior, which could be attributed to the yttrium aluminum garnet (YAG) phase located at the grains boundaries. The bending strength and fracture toughness were used to characterize the crack propagation including the delamination cracking, crack kinking, and crack deflection. The energy absorption in the process of crack propagation was characterized by the work of fracture (WOF) and damping capacity. The mode of crack propagation changed with the change in the structure and variation of BN content in the BN layer. The delamination cracks occurred inside the BN layer or at the interface between SiC and BN layers. The sample with a gradient structure exhibited the combination of delamination cracks occurring at the interface and inside the BN layer, which showed the maximum WOF of 2.43?KJ?m^?2, bending strength of 300?MPa, and fracture toughness of 8.5?MPa?m^1/2. The damping capacity varied with the change of the structure and the amplitude. The sample with a gradient structure exhibited the damping capacity of 0.088 and the maximum loss modulus of 9.758?GPa.     

Tape casting of polysiloxane-derived ceramic with controlled porosity and surface properties

Tape casting has been applied to produce porous hybrid and SiOC ceramic tapes using ceramic precursors and commercially available polysiloxanes as polymeric binders. SiC particles of two different mean sizes (4.5 or 6.5?μm) were used as inert fillers to prevent shrinkage and increase mechanical stability. Macroporosity was adjusted by varying the azodicarbonamide (ADA) content from 0 to 30?wt.%. Decomposition of the polysiloxanes at 600?°C resulted in the generation of micropores with high specific surface area (187–267 m²?g^?1) and a predominant hydrophobic behavior. At 1000?°C mainly meso/macroporosity were observed (SSA: 32–162 m²?g^?1) accompanied by increased hydrophilicity. The influence of ADA content, SiC size, and pyrolysis temperature on open porosity (2.5–37%), average pore size (<0.01–1.76?μm), surface characteristics, and flexural strength (10.5–121?MPa) were investigated. The porous tapes with different surface characteristics and controlled structure are highly promising for applications involving membrane processes, particularly microfiltration systems (0.1–10?μm).     

The preparation of SiCP/Ag–Cu–Ti hybrid tapes and application in the joining of silicon carbide ceramics

Abstract

A novel method of tape casting to fabricate ceramic-particulate-reinforced composite filler alloy tapes with low organics (no more than 6?wt.%) was developed, with which SiC_P/Ag–Cu–Ti hybrid tapes were successfully prepared and used in joining of sintered silicon carbide ceramics. The stress rheometer, scanning electron microscopy and energy dispersive spectrometer were used to characterize the rheological properties of slurry and microstructure of green tapes and joints. The slurry for tape casting consists of dispersant, binder, solvent, a mixture of ceramics particulates and metal powders, and no plasticizer was added. Castor oil phosphate was proven the suitable dispersant for the slurry, and the content of dispersant, binder, solid loading was optimized as 3?wt.%, 2.6?wt.% and 26?vol.%, respectively. The fabricated hybrid tapes possess good ductility and uniform thickness. The SiC particulates were homogeneously distributed in the metal powders matrix in both sides of the green tapes, and the distribution was retained in the SiC/SiC joints.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

Tribological behaviors of polyurethane composites containing self‐lubricating microcapsules and reinforced by short carbon fibers

Self‐lubricating microcapsules containing methyl silicone oil as core materials, were prepared with poly(melamine‐formaldehyde) as shell material by in situ polymerization method. Combining with synergistic effect of the short carbon fibers (SCFs) which were systematically treated by liquid‐phase oxidation and chemical grafting, they were simultaneously adopted as reinforcing additives to improve the tribological and mechanical properties of polyurethane materials. The tribological behaviors and mechanical properties of the polyurethane composites have been investigated by a block‐on‐ring wear tester and electronic universal testing machine, respectively. The results indicate that the friction coefficient and wear rate of polyurethane composites without SCFs significantly decreased with increased self‐lubricating microcapsule concentration from 2.5 to 10 wt % due to the release of methyl silicone oil; meanwhile, the polyurethane composites filled with 10 wt % microcapsule and 15 wt % SCFs not only exhibited the lowest friction and wear behaviors, but also improved mechanical strength and thermal stability of polyurethane composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45331.     

Fabrication and properties of SiC porous ceramics using a polyurethane preparation process

SiC porous ceramics can be prepared by introducing the polyurethane preparation method into the production process of ceramic biscuits, followed by sintering at 1300?°C for 2?h under N₂ flux after the cross-linking of polycarbosilane at 220?°C for 4?h in air. The microstructures, mechanical properties and infiltrations of the SiC porous ceramics are investigated in detail. The best dispersal effect comes from the SiC slurry with xylene as the solvent and a mixture of Silok®7096 (1?wt%) and Anjeka®6041 (4?wt%) as the dispersant. The compressive strength of SiC porous ceramics with high porosity (69.53%) reaches 16.9?MPa. The heat treatment can increase infiltration, the rate of which (4.296?×?10^?7 mm²) after the heat treatment at 750?°C in air is approximately two times faster than that before the heat treatment. The SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.