Oxidation resistance of SiC/SiC composites containing SiBC matrix fabricated by liquid silicon infiltration

The mechanical behavior and oxidation resistance of SiC/SiC-SiBC composites were studied in this work. According to the debonding criterion of He and Hutchinson, the debonding could occur at the BN interphase, which insures that the fibers can well play the strengthening and toughening performance. The oxidation resistance of SiC/SiC-SiBC composites consisting of SiC fibers with thermal expansion coefficients (CTE) of 5.1 × 10^?6 K^?1 and 4.0 × 10^?6 K^?1 was compared. The composites consisting of SiC fibers with higher CTE show slight weight changes at 800, 1000, and 1200 °C, and the corresponding strength retention ratios are 109.6%, 103.2% and 102.9%, exhibiting excellent oxidation resistance. The CTE of composites consisting of SiC fibers with higher CTE matches well with the CTE of SiC coating, so rarely no cracks can be formed in the coating, which inhibits the inward diffusion of oxidizing medium and leads to high strength retention ratios after oxidation tests.     

Highly porous nano-SiC with very low thermal conductivity and excellent high temperature behavior

Highly porous nano-SiC is fabricated by partial sintering and decarburizing process using SiC nano-powders as starting materials and graphite flakes as pore forming agents. The prepared porous nano-SiC ceramics possess multiple pore structures, including well-distributed meso-pores in the skeleton and interconnected flakelike micro-pores. The samples prepared at 1800 °C have relatively low thermal conductivities of 5.61 ～ 0.25 W m^?1 K^?1 with porosities of 55.5–76.1%. While the samples sintered at 1500 °C with porosities between 54.0% to 76.3% show very low thermal conductivities of 0.74 ～ 0.14 W m^?1 K^?1, which is attributed to the integrated nano-scale phonon-scattering mechanisms and duplex pore structures. Porous nano-SiC ceramics also show good retention of elastic stiffness up to 1350 °C and low thermal conductivity at 1400 °C. Our results shed light on porous nano-SiC as a promising thermal insulator used in extreme thermal and chemical environments.     

A comparative study to evaluate the corrosion performance of Zr incorporated Cr3C2-(NiCr) coating at 900 °C

In this study zirconium incorporated Cr₃C₂-(NiCr) coating has been sprayed on three superalloys viz. Superni 718, Superni 600 and Superco 605 using D-gun technique. A comparative study has been carried out to check the cyclic oxidation in air and hot corrosion in simulated incinerator environment (40%Na₂SO₄-40%K₂SO₄-10%NaCl-10%KCl) for the coated specimens at 900 °C for 100 cycles. Oxidation kinetics has been established for all the specimens using weight change measurements. Corrosion products have been characterized using X-ray diffractometer (XRD) and scanning electron microscopy/energy-dispersive analysis (SEM/EDAX). Cr₃C₂-(NiCr) + 0.2%wtZr coating provides very good corrosion resistance in air oxidation for all the three coated superalloys. As all the three coated superalloys shows parabolic behaviour with parabolic rate constant as 0.07 × 10^?10 (g² cm^?4 s^?1) for Superni 718, 0.43 × 10^?10 (g² cm^?4 s^?1) for Superni 600 and 0.3 × 10^?10 (g² cm^?4 s^?1) for Superco 605 This coating is also effective in the molten salt environment but coating on Co-based superalloy Superco 605 did not perform satisfactorily. The parabolic rate constants for coated Superni 718 is 0.61 × 10^?10 (g² cm^?4 s^?1), for coated Superni 600 is 6.72 × 10^?10 (g² cm^?4 s^?1) and for coated Superco 605 is 17.5 × 10^?10 (g² cm^?4 s^?1).     

Improved oxidation resistance of expanded graphite through nano SiC coating

Expanded graphite with nano SiC and amorphous SiC_xO_y coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiC_xO_y coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiC_xO_y coating.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

TaB2–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites

To improve the oxidation resistance of the carbon/carbon (C/C) composites, a TaB₂–SiC–Si multiphase oxidation protective ceramic coating was prepared on the surface of SiC coated C/C composites by pack cementation. Results showed that the outer multiphase coating was mainly composed of TaB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The coating could protect C/C from oxidation for 300 h with only 0.26 × 10^?2 g²/cm² mass loss at 1773 K in air. The formed silicate glass layer containing SiO₂ and tantalum oxides can not only seal the defects in the coating, but also reduce oxygen diffusion rates, thus improving the oxidation resistance.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

Ablation behavior of C/C-ZrC and C/SiC-ZrC composites fabricated by a joint process of slurry impregnation and chemical vapor infiltration

C/C-ZrC and C/SiC-ZrC composites were fabricated by a joint process of slurry impregnation and chemical vapor infiltration, in which ZrC matrix was obtained by slurry impregnation process, while C or SiC matrix was introduced by chemical vapor infiltration process. The as fabricated C/C-ZrC and C/SiC-ZrC composites have densities of 1.67 g cm^?3 and 1.91 g cm^?3 respectively. Tensile strength is 89.4±8.4 MPa and 182.2±14.0 MPa respectively for the as prepared C/C-ZrC and C/SiC-ZrC. Ablation behavior of the C/C-ZrC and C/SiC-ZrC composites under air plasma was studied and compared in detail. Due to different oxidation resistance and heat transfer capacity of the matrix, these two ZrC based composites showed various ablation behavior. The linear erosion rate is 48 µm s^?1 and 39 µm s^?1 respectively for C/C-ZrC and C/SiC-ZrC composites.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

A mullite/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a mullite coating was prepared on SiC precoated C/C composites by a hydrothermal electrophoretic deposition process. The phase composition, microstructure and oxidation resistance of the prepared mullite/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free mullite coatings. The mullite/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 322 h with a weight loss rate of only 4.89 × 10^?4 g/cm² h. The failure of the multi-layer coatings is considered to be caused by the volatilization of silicate glass layer, the formation of microholes and microcracks on the coating surface and the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 111.11 kJ/mol.     

In situ nanostructured (TiCr)CN coating by reactive plasma spraying

In situ nanostructured (TiCr)CN composite coating was prepared by reactive plasma spraying Ti-Cr-graphite powder under air atmosphere. The phase composition, microstructure, mechanical properties and wear performance were investigated. The results show that the coating consists of a mixture of TiN, Ti(CN), (TiCr)N, Cr, Ti₃O, and amorphous graphite and CrN phases. The grain size is about 70 nm and the grains present equiaxed and columnar crystal morphologies. Moreover, 5 nm-sized nanocrystals are embedded in an amorphous phase. The (TiCr)CN composite coating possesses high hardness (1325 ± 120 HV) and toughness (4.35 ± 0.53 MPa m^1/2). The friction coefficient and wear rate of the coating are 0.46 and 3.01 ± 0.17 × 10^?6 mm³ N^?1 m^?1, respectively. The inclusion of metallic phase Cr could improve the toughness and wear resistance of the (TiCr)CN coating.     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

Elaboration of new tubular ceramic membrane from local Moroccan Perlite for microfiltration process. Application to treatment of industrial wastewaters

In this study, an original microfiltration tubular membrane (M1) made from local Moroccan Perlite was used to treat three wastewater types: effluents coming from beamhouse section of tannery (effluent A), textile effluent coming from jeans washing process (effluent B), and dicing wafer effluent generated by electronic industries (effluent C). The prepared membrane is composed of two layers of Perlite with two different granulometries: a macroporous support with a pore diameter centered near 6.6 μm and porosity of about 42%, and a microfiltration layer, performed by slip casting method, with a mean pore size of 0.27 μm. The water permeability determined of the membrane is 815 L/h m² bar. Tangential microfiltration using Perlite membrane proved to be effective in removing pollutants from the three effluents with almost the same efficiencies than that obtained with a commercial Alumina membrane (M2) with a pore diameter of 0.2 μm and a water permeability of 1022 L/h m² bar. Tangential microfiltration process operated at lower pressure (1 bar) was seen to remove turbidity from the three feeds completely. Perlite membrane allowed significant reduction of Chemical Oxygen Demand COD (50–54%) and Total Kjeldahl Nitrogen TKN (56%) of beamhouse effluent. It showed a significant decrease of COD (54–57%) and a complete discoloration of textile wastewater.     

Advanced visible-light photocatalytic property of biologically structured carbon/ceria hybrid multilayer membranes prepared by bamboo leaves

Biologically structured carbon/cerium dioxide materials are synthesized by biological templates. The microscopic morphology, structure and the effects of different oxidation temperatures on materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) ultraviolet-visible light spectrum (UV–Vis) and X-ray Photoelectron Spectroscopy (XPS). Moreover, by splitting water under visible light irradiation, the hydrogen production is measured to test the photocatalytic property of these materials. The results show that materials made with bamboo biological templates which are immersed in 0.1 mol L^?1 of cerium nitrate solution, then carbonizated in nitrogen (700 °C) and oxidized in air (500–600 °C), can obtain the biological structure of bamboo leaves. The product is in the composition of hybrid multilayer membrane, which one is carbon membrane form plant cell carbonation and another is ceria membrane by nanoparticle self assembly. The best oxidation temperature is 550 °C and the band gap of carbon/cerium dioxide materials synthesized at this optimum oxidation temperature could be reduced to 2.75 eV. After exposure to visible light for 6 h, the optimal hydrogen production is about 302 μmol g^?1, which is much higher than that of pure CeO₂.     

Carbon-coated Li2S cathode for improving the electrochemical properties of an all-solid-state lithium-sulfur battery using Li2S-P2S5 solid electrolyte

Li₂S is coated with carbon to improve the electrical conductivity of the composite cathode in all-solid-state lithium-sulfur batteries. Carbon is applied by thermal evaporation from a polyacrylonitrile (PAN) source at 600 °C for 5 h. It is shown that the carbon coating is impurity free, and the crystallinity of Li₂S is well maintained. The electronic conductivity of Li₂S is dramatically improved from 9.21 × 10^?9 S cm^?1 to 2.39 × 10^?2 S cm^?1 upon carbon coating. An all-solid-state battery prepared with the carbon-coated Li₂S shows a high initial capacity of 585 mAh g^?1 (g of Li₂S) that increases up to 730 mAh g^?1 (g of carbon-coated Li₂S) by the 10th cycle. This high capacity is stable throughout the 25 cycles tested, with an excellent coulombic efficiency of 99%. Carbon-coated Li₂S is advantageous for all-solid-state batteries due to the increased electrical conductivity, while allowing a reduction of the total carbon content present in the composite cathode.     

Effects of oxidation curing and sintering temperature on the microstructure formation and heat transfer performance of freestanding polymer-derived SiC films for high-power LEDs

Effects of oxidation cross-linking and sintering temperature on the microstructure evolution, thermal conductivity and electrical resistivity of continuous freestanding polymer-derived SiC films were investigated. The as-received films consisting of β-SiC nanocrystals embedded in amorphous SiO_xC_y and free carbon nanosheets were fabricated via melt spinning of polycarbosilane (PCS) precursors and cured for 3 h/10 h followed by pyrolysis from 900 °C to 1200 °C. Results reveal that nanoscale structure (β-SiC/SiO_xC_y/C_free) provides an ingenious strategy for constructing highly thermal conductive, highly insulating and highly flexible complexes. In particular, the 3 h-cured films sintered at 1200 °C with satisfying thermal conductivity (46.8 W m^?1 K^?1) and electrical resistivity (2.1 × 10⁸ Ω m) are suitable for the realization of high-performance substrates. A remarkable synergistic effect (lattice vibration of β-SiC nanocrystals and close-packed SiO_xC_y, free-electron heat conduction of β-SiC and free carbon, and supporting role of oxygen vacancy) contributing to thermal conductivity improvement is proposed based on the analysis of microstructure, intrinsic properties and simulations. Eventually, the SiC films without additional dielectric layers are directly silk-screen printed with high-temperature silver paste and used as heat dissipation substrates for high-power LED devices via chip-on-board (COB) package. The final devices can emit bright light with low-junction temperature (52.6 °C) and good flexibility owing to the mono-layer SiC substrate with low thermal resistance and desirable mechanical properties. This work offers an effective approach to design and fabricate flexible heat dissipation ceramic substrates for thermal management in advanced electronic packaging fields.     

Protecting nuclear graphite from liquid fluoride salt and oxidation by SiC coating derived from polycarbosilane

Modification process has been conducted on commercial nuclear graphite IG-110 (Toyo Tanso Co., Ltd., Japan) by impregnation and pyrolysis of polycarbosilane (PCS) solution for getting the modified IG-110 (M-IG-110) coated by dense SiC coating for molten salt reactor. The microstructure and properties of graphite were systematically investigated and compared before and after the modification process. Results indicated that the M-IG-110 possessed of more excellent integrated properties including molten salt barrier property and oxidation resistance than bare IG-110 due to the filling effect of SiC particles in the pores of M-IG-110 and dense SiC coating adhering to the surface of M-IG-110. The fluoride salt infiltration amount of M-IG-110 under 5 atm was only 1.1 wt%, which was much less than 14.9 wt% for bare IG-110. The SiC coating derived from PCS exhibited remarkable compatibility with graphite substrate under high temperature and gave rise to excellent oxidation resistance of M-IG-110.     

Improved electrical and thermal conductivities of polysiloxane-derived silicon oxycarbide ceramics by barium addition

The electrical and thermal conductivities of bulk barium-added silicon oxycarbide (SiOC-Ba) ceramics are investigated. The SiOC-Ba ceramics exhibited improved electrical and thermal conductivities upon increasing the sintering temperature from 1450 °C to 1650 °C. Precipitation of graphitic carbon clusters observed by Raman spectroscopy and high-resolution transmission electron microscopy is attributed to the phase separation during the fabrication process. The increase in the electrical conductivity can be rationalized in terms of an increase in the density of the sp² CC bonds within the carbon clusters. The increase in the thermal conductivity is mainly attributed to the formation of interconnected graphitic clusters in the SiOC matrix and SiC embedded in the clusters. The electrical and thermal conductivities of the SiOC-Ba ceramics sintered at 1650 °C are 14.0 Ω^?1 cm^?1 and 5.6 W/m K, respectively, at room temperature. The electrical conductivity of SiOC-Ba sintered at 1550 °C is 5.3 Ω^?1 cm^?1 and 7.0 Ω^?1 cm^?1 at 2 and 300 K, respectively.     

Study on energy consumption of Al2O3 coating prepared by cathode plasma electrolytic deposition

Al₂O₃ coatings with large specific surface were prepared on cast nickel-based superalloy K418 by cathode plasma electrolytic deposition (CPED) in aqueous solutions at different concentrations. The significance of energy consumption and a simple calculation method during CPED were proposed, and the influence of electrolyte concentration on current density-voltage curve, energy consumption, and microstructure of coatings were studied. It was found that increasing the concentration of electrolyte can effectively reduce the current density at the initial stage while prolonging the deposition time and stepping up the energy consumption of whole coating preparation. The morphology observation results showed that the pore size of Al₂O₃ coatings enlarges with the increase of the concentration, and the optimum electrolyte concentration is 0.5–1 mol L^?1. Under this condition, the new method of oxidation pretreatment at 950 ℃ on samples for 30 min can efficiently decrease the current density during the early stage of preparation, which is beneficial to the deposition of complex-shaped samples with large size.     

Preparation and characterization of low-cost ceramic microfiltration membranes for the removal of chromate from aqueous solutions

Low-cost ceramic microfiltration membranes were prepared using clay of IIT Guwahati. Two membranes were prepared by paste casting followed by sintering at different temperatures, the first one from clay only (membrane A) and the second one from clay with small amounts of sodium carbonate, sodium metasilicate and boric acid (membrane B). Both the membranes were characterized by TGA, SEM, XRD, water permeability test and acid–base treatment. With the increase of sintering temperature, pore size as well as permeability and flexural strength were increasing while porosity and pore density were decreasing. The overall performance of membrane B was better than membrane A. The average pore size, porosity, pore density and flexural strength of membrane B sintered at 1000 °C were 4.58 μm, 0.42, 2.06 × 10¹⁰ m^− 2 and 11.55 MPa respectively. This membrane was used for the removal of chromate from aqueous solutions by micellar enhanced microfiltration (MEMF) using cetylpyridinium chloride (CPC). 100% rejection of chromate ions were obtained at a feed ratio (CPC/chromate) of 10. Based on raw material prices, the membrane cost was estimated to be $19/m². The prepared low-cost membrane showed good promise for the treatment of wastewater containing such heavy metals.