Reaction-bonded robust SiC ceramic membranes sintered at low temperature with coal gangue

Herein, we report an in-situ reaction-bonded SiC membrane sintered at low temperature using a solid waste (i.e. coal gangue) as the sintering aid to form strong neck connections. The effects of sintering temperature and coal gangue proportion on their properties regarding pore size, open porosity, bending strength and pure water permeability were investigated. The single-channel tubular SiC membrane sintered at 1300 °C with a coal gangue proportion of 12 wt% was optimal, exhibiting an average pore size of 2.78 μm, a open porosity of 47.08%, a bending strength of 34.01 ± 1.3 MPa and a high water permeability of 83967 L m⁻² h⁻¹ bar⁻¹. The membrane could completely reject D₅₀ = 0.87 μm SiC solids and presented a steady-state water permeability of 458 L m⁻² h⁻¹·bar⁻¹. The SiC membrane could be regenerated through ultrasonication and its steady-state water permeability was almost unchanged for 3 cycles, proving its mechanical robustness. This work may appeal to the practical low-cost production of high-performance SiC membranes.     

Green synthesis of blue-green photoluminescent β-SiC nanowires with core-shell structure using coconut shell as carbon source

This work proposes a green method for synthesizing SiC nanowires (NWs) via the chemical vapor deposition (CVD) technique using coconut shell and silicon as raw materials. Using coconut shell as carbon source decreases the synthesis temperature of SiC. A large number of core-shell SiC NWs were obtained after firing at 1200 °C, a thin SiO₂ layer is distributed on the outer shell of SiC NWs. The synthesized SiC NWs grow along the [111] direction, up to dozens of micrometers in length and diameters of 10–75 nm. However, the chain-bead structure of SiC NWs is formed after firing at 1400 °C due to the SiO₂ bead embedded in SiC NWs. The synthesized core-shell SiC NWs fired at 1200 °C emit strong violet-blue light, which has good application prospects in optoelectronic devices.     

The preparation of mullite foamed ceramics reinforced by in-situ SiC whiskers and their reinforcement mechanism

A SiC whisker-bonded mullite foamed ceramic was prepared by using white clay, industrial alumina and silicon powder as raw materials without solid carbon sources. The XRD, SEM, EDS, and Factsage® software were used to investigate the effect of sintering temperature on the phase composition, microstructure, compressive strength, and Young's modulus of foamed ceramics. Additionally, the synthesis reaction of in-situ SiC whiskers and the effect of their formation on the properties of ceramics were studied. The results showed that the in-situ SiC whiskers with dendrite shapes were formed after firing above 1300 °C at the expense of Si/SiO vapors as well as CO vapor, though there were no solid carbon sources in raw materials, which provided a new idea for the synthesis of SiC whiskers. The formation of SiC whiskers was helpful for improving the compressive strength and Young's modulus of mullite foamed ceramics remarkably. Furthermore, the reinforcement mechanism has been investigated systematically.     

Porous ceramics derived from steel slag/coal gangue mixtures using cigarette butts as the pore-forming agent

Fabrication of ceramic materials with interconnected pores is necessary to improve thermal energy storage efficiency in high-temperature infiltration technology. In the present study, industrial wastes such as coal gangue, steel slag, etc., were selected as the raw materials to prepare ceramics with interconnected pores. By adopting 50% cigarette butts as the pore-forming agent, steel slag–coal gangue mixtures with a mass ratio of steel slag to coal gangue of 1:9 were sintered at 1100°C, and ceramics with interconnected elongated pores were prepared successfully. The highest apparent porosity and lowest volume density of the as-prepared ceramics were ca. 73% and .74 g/cm³, respectively. Further measurements of the thermophysical properties indicated that no obvious mass loss was observed in the temperature range from ambient temperature to 800°C. The maximum values of specific heat and thermal conductivity were 1.38 J/(g K) and 1.661 W/(m K), respectively, and meanwhile the minimum compressive strength could exceed 3.5 MPa. These research results implied that the as-prepared steel slag–coal gangue ceramics can provide long-term service and offer excellent thermal stability over a wide temperature range. Therefore, they should have potential applications in high-temperature infiltration technology.     

Efficient fabrication of carbon/silicon carbide composite for electromagnetic interference shielding applications

Ceramic matrix composites are typically prepared by a costly, time-consuming process under severe conditions. Herein, a cost-effective C/SiC composite was fabricated from a silicon gel-derived source by Joule heating. The β-SiC phase was generated via carbothermal reduction, and the carbon fabric showed a well-developed graphitic structure, promoting its thermal and anti-oxidation stabilities. Owing to the excellent dielectric loss in carbon fabric, SiC and SiO₂ as well as the micropore structure of the ceramic matrix, the absolute electromagnetic interference shielding (EMI) effectiveness (SSE/t) reached 948.18 dB?cm²?g^-1 in the X-band, exhibiting an excellent EMI SE. After oxidation at 1000 °C for 10 h in the air, the SSE/t of the composite was only reduced to 846.02 dB?cm²?g^-1. The C/SiC composite promises the efficient fabrication of high-temperature resistant materials for electromagnetic shielding applications.     

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (C_f/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiC_f/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in C_f/C–SiC composites. Tensile strength and interlaminar shear strength of C_f/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiC_f/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of C_f/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in C_f/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared C_f/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.     

Synthesis of C/SiC core-shell fibers through siliconization of carbon fibers with SiO gas in semi-closed reactor

Novel C/SiC core-shell fibers have been synthesized through incomplete conversion of carbon fibers by their siliconization with SiO gas. The synthesis was performed in the laboratory-made semi-closed batch-type reactor at 1380 °C for 3 h using a 9:1 M ratio mixture of Si and SiO₂ powders as a solid source of SiO gas. The conversion rate of carbon into SiC was 34.0%. All synthesized fibers had a distinct C/SiC core-shell composite structure. The fiber product was of fairly good uniformity in respect of the shell thickness which varied approximately from 0.6 μm to 0.8 μm depending on the location of fibers inside the reactor. It was revealed that the formation of the shell was the result of inward growth of the SiC product layer. The effectiveness of the proposed semi-closed reactor for the synthesis of C/SiC core-shell fibers has been demonstrated.     

Phase, microstructure, and oxidation resistance of yttrium silicates coatings prepared by a hydrothermal electrophoretic deposition process for C/C composites

Oxidation-resistant yttrium silicates coatings for SiC precoated carbon/carbon composites were prepared by a novel hydrothermal electrophoretic deposition process. Sonochemical-synthesized yttrium silicates nanocrystallites, isopropanol, and iodine were respectively used as source materials, solvent, and charging agent during the deposition. Phase compositions, surface and cross-section microstructures of the as-prepared multilayer coatings were characterized by an X-ray diffractometer (XRD) and a scanning electron microscopy (SEM). The influence of deposition temperatures on the phase, microstructure, and oxidation resistance of the multilayer coated C/C composites was particularly investigated. Results show that the as-prepared outer coatings are composed of yttrium silicates crystallites with a main phase of Y₂Si₂O₇ and Y₂SiO₅. The thickness and density of the yttrium silicates coatings are improved with the increase of deposition temperature. Compared with SiC coating prepared by pack cementation, the multilayer coatings prepared by pack cementation with a later hydrothermal electrophoretic deposition process exhibit better antioxidation properties. The as-prepared multilayer coatings can effectively protect C/C composites from oxidation at 1773 K in air for 35 h with a weight loss of 0.32 × 10⁻³ g/cm².     

Synthesis of nanofibrous carbon with herringbone structure on Ni-supported SiC particles using hot CVD apparatus

A nanofibrous carbon material having a herringbone structure was synthesized using Ni-supported silicon carbide (SiC) particles as the catalyst for the hot chemical vapor deposition (CVD) process using CH₄ as carbon source. The amount of the deposits during the CVD process strongly depended on the CVD treatment temperature. The enhancement of weight and the TG data indicated that the quantity synthesis of the deposits was achieved at 823 K. The specific surface area of the deposits was estimated at ca. 120 m² g^− 1. It was confirmed from the TEM images that the deposits synthesized in this study had a herringbone-like structure. From the data of Raman spectra and XRD patterns, the herringbone-like structure started to deposit after 30 minutes in the case of 823 K. The Ni-supported SiC can be used as the catalyst for the synthesis of nanofibrous carbon materials.     

Basic principle,progress, and prospects of coal gangue ceramic proppants

Ceramic proppants are preferred in hydraulic fracturing because of their high strength, good sphericity, and excellent acid corrosion resistance. Bauxite is the main raw material of ceramic proppants, but with the increasing depletion of bauxite resources, substitutes for bauxite need to be sought. Coal gangue is a solid waste that is rich in SiO₂ and Al₂O₃, making it a potential substitute for the preparation of ceramic proppants. The use of coal gangue to prepare ceramic proppants can reduce the cost of ceramic proppants and benefit environmental protection. Studies on using coal gangue to prepare ceramic proppants have been conducted and achieved progress, but further improvement can still be obtained. Here, the basic principle, progress, and prospects of coal gangue ceramic proppants are reviewed. First, coal gangue ceramic proppants are divided into two categories, namely, proppants prepared using coal gangue as an auxiliary material and proppants prepared using coal gangue as the main material. Second, the progress of research of coal gangue ceramic proppants is summarized from the perspective of additives, and the influence of different kinds of additives on the sintering temperature and performance of proppants is discussed. Lastly, the future prospects of coal gangue ceramic proppants are presented.     

Fabrication and properties of Cf/ZrC‐SiC‐based composites by an improved reactive melt infiltration

C_f/ZrC‐SiC composites with a density of 2.52 g/cm³ and a porosity of 1.68% were fabricated via reactive melt infiltration (RMI) of Si into nano‐porous C_f/ZrC‐C preforms. The nano‐porous C_f/ZrC‐C preforms were prepared through a colloid process, with a ZrC “protective coating” formed surrounding the carbon fibers. Consequently, highly dense C_f/ZrC‐SiC composites without evident fiber/interphase degradation were obtained. Moreover, abundant needle‐shaped ZrSi₂ grains were formed in the composites. Benefiting from this unique microstructure, flexural strength, and elastic modulus of the composites are as high as 380 MPa and 61 GPa, respectively, which are much higher than C_f/ZrC‐SiC composites prepared by conventional RMI.     

Mechanical properties of in situ synthesized mullite-based composite ceramics with three-dimensional network structure

Needle-like nanocrystalline mullite powders were prepared through the molten salt process at the temperature of 900°C using coal gangue as raw material. Then, mullite-based composite ceramics were prepared by a conventional solid-state reaction between in situ synthesized mullite and Al₂O₃ powders. Effects of Al₂O₃ content and sintering temperatures on phase compositions, microstructure, and mechanical properties of the mullite-based composite ceramics were also studied. The results show that mullite content productivity increase from 72% to 95%, as the sintering temperature increased from 1480°C to 1580°C, which led to the improvement in the bulk density and flexural strength of the samples. The three-dimensional interlocking structure for mullite-based composite ceramics was obtained by the in situ solid-state reaction process. The maximum bulk density, flexural strength, and fracture toughness for the sample with 15 wt% Al₂O₃ content are 2.48 g/cm³, 139.79 MPa, and 5.62 MPa··m^1/2, respectively, as it was sintered at the temperature of 1560°C for 3 h. The improved mechanical properties of mullite-based composite ceramics maybe ascribed to good densification and increased mullite phase content, as well as to the in situ three-dimensional network structure. Therefore, the results would provide new ideas for high-value utilization of coal gangue.     

Preparation,dielectric property and microwave absorption property of Cu doped SiC nanopowder by combustion synthesis

Abstract

Cu doped SiC nanopowders have been prepared via combustion synthesis, using silicon powder and carbon black as the raw materials, copper powder as the doping source and polytetrafluoroethylene as the chemical activator respectively. The microstructure of prepared nanopowders has been characterised by X-ray diffraction and scanning electronic microscope. The electric permittivities of prepared SiC nanopowders in the frequency range of 8·2–12·4 GHz have been determined. Results show that prepared β-SiC nanopowders have fine spherical particles and narrow particle size distribution, and a quantity of SiC whisker increases with increasing Cu doping content. The Cu₃Si impurity has been generated when Cu content is up to 10%. The β-SiC doped with 10% Cu has the highest real part ?′ and dielectric loss tanδ values. The 5% Cu doped SiC nanopowder with matching thickness of 2 or 2·5 mm exhibits the best microwave absorption properties in the frequency range of 8·2–12·4 GHz.     

污泥用于水泥混合材的探讨试验

以城市污泥作辅材,煤矸石等其它材料为主要原料,开发制备低碳节能水泥用混合材料。通过对城市污泥与其它原材料共同混合制备水泥用混合材料的性能试验,优化不同种类原材料配比并选择适合的配比试验分析其理化性能、火山灰性评定。结果表明,利用污泥替代部分石灰石、煤矸石,其中污泥占其质量比10%~15%,石灰石、煤矸石分别占物料总质量比20%-60%,制备的水泥混合材料具有人工火山灰质材料特性,可实现以废治废及废弃物有效利用。     

Recrystallization sintering and characterization of composite powders composed of two types of SiC with dissimilar particle sizes

Pure silicon carbide (SiC) ceramics were prepared through recrystallization sintering by using two types of SiC powder, with different particle sizes, as the raw materials. The effects of the fine powder content on the bulk density, porosity, flexural strength, and grain morphology were investigated. In the synthesis process, silicon nitride (Si₃N₄) was used as the sintering additive that decomposed and transformed into SiC to promote the growth of SiC grains. The added fine powder was exploited in the evaporation and condensation process and grain amalgamation caused by the movement of the grain boundaries. Thus, a dissimilar fine powder content modulated the microstructure and mechanical strength of the SiC ceramics. The results indicate that the bulk density and flexural strength increase to a maximum of 2.12 g/cm³, 44.2 MPa, respectively, when the fine powder content is 40 wt.%. Three kinds of grain morphologies, that is, uniform equiaxed grains, round, equiaxed grains, and hexagonal platelet grains, and the maximum average pore size (3.62 μm) are obtained when the fine powder content is between 0 wt.% and 60 wt.%. In addition, the main crystal phase 6H-SiC is partially converted to 4H-SiC when the fine powder content is up to 60 wt.%.     

Morphology and anti-ablation properties of PIP Cf/C–SiC composites with different CVI carbon content under 4.2 MW/m2 heat flux oxy-acetylene test

In this work, the needled carbon fiber preforms were used to make seven groups of carbon/carbon composite billets with different matrix carbon contents by controlling the processing time of chemical vapor infiltration (CVI). C_f/C–SiC composites were prepared by infiltration of SiC into these C/C composites billets using polycarbosilane (PCS) through precursor infiltration and pyrolysis (PIP). After oxy-acetylene torch testing (heat flux of 4.2 MW/m²) for 200s, 300s and 400s, respectively, it revealed that the anti-ablation properties of the C_f/C–SiC composite samples were enhanced by a higher content of SiC matrix. Additionally, specimens bearing longer duration tests showed a trend of lower average ablation rates. The lowest linear ablation rate is 0.008 mm/s and the mass ablation rate is 0.0019 g/s for those high SiC content samples tested for 400s. The SEM images of the tested samples showed the mechanism and the non-linear process of ablation resistance progression.     

In-situ synthesis of ultra-fine ZrB2–ZrC–SiC nanopowders by sol-gel method

ZrB₂–ZrC–SiC nanopowders with uniform phase distribution were prepared from cost-effective ZrOCl₂·8H₂O by a simple sol-gel method. The synthesis route, ceramization mechanism and morphology evolution of the nanopowders were investigated. ZrB₂–ZrC–SiC ceramic precursor can be successfully obtained through hydrolysis and condensation reactions between the raw materials. Pyrolysis of the precursor was completed at 650 °C, and it produced ZrO₂, SiO₂, B₂O₃ and amorphous carbon with a yield of 39% at 1300 °C. By heat-treated at 1500 °C for 2 h, highly crystallized ZrB₂–ZrC–SiC ceramics with narrow size distribution were obtained. With the holding time of 2 h, both the crystal size and the particle size can be refined. Further prolonging the holding time can lead to serious particles coarsening. Studies on the microstructure evolution of the generated carbon during the ceramic conversion demonstrates the negative effect of the ceramic formation on the structure order improvement of the carbon, due to the large amount of defects generated in it by the boro/carbothermal reduction reactions.     

低变质烟煤与SiO2合成SiC研究

以3种低变质烟煤与SiO2为原料实现了SiC的合成。实验发现弱粘煤的合成效果最好。研究了煤质、煤的高温质量损失率和烟煤高温焦炭的微孔隙大小及微碳粉化对合成的影响，指出弱粘煤的高固定碳含量、低挥发份、低的高温质量损失率、高孔隙性的高温下易微碳粉化是其合成SiC产率高的主要原因。     

Direct ink writing of chopped carbon fibers reinforced polymer-derived SiC composites with low shrinkage and high strength

The chopped carbon fiber reinforced SiC (C_f/SiC) composite has been regarded as one of the excellent high-temperature structural materials for applications in aerospace and military fields. This paper presented a novel printing strategy using direct ink writing (DIW) of chopped fibers reinforced polymer-derived ceramics (PDCs) with polymer infiltration and pyrolysis (PIP) process for the fabrication of C_f/SiC composites with high strength and low shrinkage. Five types of PDCs printing inks with different C_f contents were prepared, their rheological properties and alignment of carbon fiber in the printing filament were studied. The 3D scaffold structures and bending test samples of C_f/SiC composites were fabricated with different C_f contents. The results found that the C_f/SiC composite with 30 wt% C_f content has high bending strength (～ 7.09 MPa) and negligible linear shrinkage (～ 0.48%). After the PIP process, the defects on the C_f/SiC composite structures were sufficiently filled, and the bending strength of C_f/SiC composite can reach up to about 100 MPa, which was about 30 times greater than that of the pure SiC matrix without C_f. This work demonstrated that the printed C_f/SiC composites by using this method is beneficial to the development of the precision and complex high-temperature structural members.     

Fabrication and properties of SiCnw-reinforced SiC composites by reactive melt infiltration with BN and PyC interface

To tailor the fiber–matrix interface of SiC nanowires-reinforced SiC (SiC_nw/SiC) ceramic matrix composites (CMCs) for improved mechanical properties, SiC nanowires were coated with BN and pyrolytic carbon (PyC) compound coatings prepared by the dip-coating process in boric acid and urea solution and the pyrolysis of phenolic resin. SiC_nw/SiC CMC with PyC/BN interfaces were fabricated by reactive melt infiltration (RMI) at 1680°C for 1 h. The influences of phenolic resin content on the microstructure and mechanical properties of the CMC were investigated. The results showed that the flexural strength and fracture toughness reach the maximum values of 294 MPa and 4.74 MPa m^1/2 as the phenolic resin content was 16 and 12 wt%, respectively. The displacement–load curve of the sample exhibited a gradient drop with increasing phenolic resin content up to 12 wt%. The results demonstrated that the PyC/BN compound coatings could play the role of protecting the SiC_nw from degradation as well as improving the more moderate interfacial bonding strengths during the RMI.