Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.     

Studies on processing of layered oxide-bonded porous sic ceramic filter materials

Oxide-bonded porous SiC ceramic filter supports were prepared using SiC powder (d₅₀ = 212 µm), Al₂O₃, and clay as bond forming additives and graphite as pore former following reaction bonding of powder compacts at 1400°C in air. Reaction bonding characteristics, phase composition, porosity, pore size, mechanical strength, and microstructure of porous SiC ceramic supports were investigated. Mullite bond phase formation kinetics was studied following the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model using non-isothermal differential thermal analysis (DTA) data. Compared to porous SiC ceramic filter supports having no needle-like mullite bond phase, materials processed by the mullite bonding technique exhibited higher average strength (22.1%) and elastic modulus (5.4%) at a similar porosity level of ~38%, with upper and lower bounds of their strength, modulus, and porosity being 39.1 MPa, 40.2 GPa, and 36.3% and 34.2 MPa, 31.3 GPa, and 33.0%, respectively. Spray coating method was applied for preparation of oxidation-bonded SiC filtration layer having thickness of ~150 µm and pore size of ~5–20 µm over the porous SiC support compacts using aqueous slurry made of fine SiC powder (d₅₀ = 15 µm) followed by sintering. The layered ceramics thus prepared are potential materials for gas filter applications.     

高温烟气过滤用多孔陶瓷材料的研制

以氧化铝纤维和玻璃粉为主体材料,活性炭粉为造孔剂,通过半干压成型工艺制备了高温烟气过滤陶瓷.详细研究了玻璃粉含量、造孔剂含量以及烧成温度对材料过滤阻力、抗折强度、显气孔率等性能的影响,并对原料配方和烧成制度进行了优化,最优配方为:氧化铝70wt%、玻璃粉30wt%、外加造孔剂25wt%、羧甲基纤维素钠8wt%,最佳的烧成温度为1100 ℃,制得的高温烟气过滤陶瓷抗折强度8.9 MPa,过滤阻力95 Pa,显气孔率达59%.     

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.     

Synthesis and characterization of porous ceramics from spodumene tailings and waste glass wool

Glass wool waste remains a challenging waste fraction with relatively little utilization prospects. The present study investigated the development of porous ceramic materials from glass wool waste and spodumene tailings mainly made of quartz feldspar sand (QFS), with 0.05–0.5% silica carbide (SiC) as a pore-forming agent. The formulated compositions were sintered at 950 °C and analyzed in terms of mechanical properties, phase composition, and microstructure using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray micro-computed tomography. The results showed that a synergetic effect of glass wool and SiC started to be significant from 15 wt% glass wool and 0.05 wt% SiC, the strength reducing and the porosity increasing with the increase of SiC. The porous ceramics were largely amorphous, with compressive strength ranging from 5 to 30 MPa while the water absorption and apparent density ranged from 2 to 10% and 0.7–1.2 g/cm³, respectively. The total porosity varied between 20 and 75%, and the wall thickness between 62 and 68 μm; besides, most of the prepared materials floated in water. These results are of interest for the repurposing of glass wool waste in the development of non-flammable lightweight materials for potential filtering or high-rise building applications.     

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m³ m⁻² h⁻¹ bar⁻¹, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.     

Preparation and properties of anorthite-based ceramics by using metallurgical solid waste and fly ash

A large amount of metallurgical solid waste accumulation poses a serious threat to the environment. Study on synergistic reinforcement of synthetic process of metallurgical solid waste-based ceramics with fly ash is of great significance in reducing environmental pollution and resource utilization. A metallurgical solid waste-based ceramic used as building ceramic was developed with the erosion part of used MgO–C bricks and fly ash as main raw materials, and the amount of solid waste added to the prepared ceramics was at least 60 wt% and up to 90 wt%. The effects of fly ash content and sintering temperature on the crystalline phase transitions, morphologies, and the main physical and mechanical properties of ceramics were investigated by X-ray diffraction, scanning electron microscopy, and mechanical testing. The results show that the obtained ceramics presented maximum bending strength and minimum water absorption, 80.14 MPa and 5.04%, respectively, when the raw material proportions were the erosion part of used refractories accounted for 60 wt%, fly ash 20 wt%, pyrophyllite 10 wt%, and quartz sands 10 wt%, and the process parameters were the sintering temperature 1150°C, sintering time 120 min, and molding pressure 15 MPa.     

Microstructure and strengthening behavior in high content SiC nanowires reinforced SiC composites

In this study, the high-content SiC_nw reinforced SiC ceramic matrix composites (SiC_nw/SiC CMC) were successfully fabricated by hot pressing β-SiC and sintering additive (Al₂O₃-Y₂O₃) with boron nitride interphase modification SiC_nw. The effects of sintering additive content and mass fraction (5–25 wt%) of SiC_nw on the density, microstructure, and mechanical properties of the composites were investigated. The results showed that with the increase of sintering additives from 10 wt% to 12 wt%, the relative density of the SiC_nw/SiC CMC increased from 97.3% to 98.9%, attributed to the generated Y₃Al₅O₁₂ (YAG) liquid phase from the Al₂O₃-Y₂O₃ that promotes the rearrangement and migration of SiC grains. The comprehensive performance of the obtained composite with 15 wt% SiC_nw possessed the optimal flexural strength and fracture toughness of 524 ± 30.24 MPa and 12.39 ± 0.49 MPa·m^1/2, respectively. Besides, the fracture mode of the composites with 25 wt% SiC_nw content revealed a pseudo-plastic fracture behavior. It concludes that the 25 wt% SiC_nw/SiC CMC was toughened by the fiber pull-outs, debonding, bridging, and crack deflection that can consume plenty of fracture energy. The strategy of SiC nanowires worked as a main bearing phase for the fabrication of SiC/SiC CMC providing critical information for understanding the mechanical behavior of high toughness and high strength SiC nanoceramic matrix composites.     

ZCAS-assisting low-temperature hot-press sintering of SiC ceramic for immobilizing simulated radioactive graphite

The safe treatment and disposal of high-level radioactive graphite is an essential challenge in the governance of irradiated graphite. SiC has an insurmountable defect in densification sintering at low temperatures, although it is an ideal host material for immobilizing high-level radioactive graphite. To solve this issue, we employ ZCAS (short for ZnO–CaO–Al₂O₃–SiO₂ glass) as a sintering aid to prepare SiC-ZCAS composite ceramic with the relative density up to 98% by vacuum hot-press sintering. For investigating the optimum formula and technological conditions of (1-x) SiC-x ZCAS composite ceramic, the effects of synthesis and sintering process on preparation (1-x) SiC-x ZCAS (x = 25-40 wt%) composites were investigated in detail. The results show that the SiC-ZCAS composite powder with x = 25-40 wt% can be synthesized at 1350 °C for 2 h when the Si/C mole ratio is 1.05:1. The relative density, Vickers hardness, and thermal conductivity of SiC-ZCAS composites ceramic increase rapidly by increasing sintering temperature and pressure. However, these properties will display different effects from the increase in the content of ZCAS, such as continuous elevation of the relative density and dramatic depression of thermal conductivity. the Vickers hardness starts to decline when the content of ZCAS is 30 wt%. Considering the above mentioned, we can conclude that the 0.7SiC-0.3ZCAS composite ceramic has choiceness comprehensive properties when sintered at 1550 °C and 60 MPa for 1 h, with the relative density, Vickers hardness and thermal conductivity of 95.5%, 1084 (HV 10) and 7.104 W/m·k, respectively.     

传统陶瓷工艺制备玻璃陶瓷/SiC晶须复合材料

针对传统陶瓷工艺难以直接用于制备玻璃陶瓷/SiC晶须复合材料问题,本文以白云鄂博尾矿基透辉石系玻璃粉和商用SiC晶须为主要原料,在石墨粉包埋条件下,采用传统陶瓷工艺成功制备出透辉石玻璃陶瓷/SiC晶须复合材料.在此基础上,研究了填加0～40wt％SiC晶须对所制备复合样品显微结构及性能的影响.结果证明了所制备复合材料样...     

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.     

Characterization of flat ceramic membrane supports prepared with kaolin-phosphoric acid-starch

The purpose of this work is the development of microporous ceramic materials based on kaolin for a filtration process. Flat ceramic membrane supports were prepared from the mixtures of kaolin, phosphoric acid and starch. Porosity, permeability and mechanical properties of those supports were studied as functions of the amount of phosphoric acid, the sintering temperature and the compaction pressure. The rupture strength and the permeability of the ceramic membrane, increase with the content of phosphoric acid until 5 mass%. The porosity decreases with both the sintering temperature rise and the addition of phosphoric acid. The addition of 5 to 10 mass% of phosphoric acid and 10 mass% of starch to the kaolin supports sintered at 1100 °C for 2 h leads to a satisfied permeability and mechanical proprieties in the filtration application. The elaborated support was characterized using two analytical methods: DRX, ²⁷Al and ³¹P MAS-NMR. The obtained analytical data indicate the presence of an AlPO₄ high temperature phase at 1100 °C.     

Reuse of mineral wool waste and recycled glass in ceramic foams

Novel ceramic foams have been prepared by high temperature sintering of waste mineral wool and waste glass using SiC as a foaming agent. The aim of the research was to understand the effects of composition and sintering conditions on the properties and microstructure and produce commercially exploitable ceramic foams. Optimum ceramic foams were formed from 40 wt% mineral wool waste and 2 wt% SiC, sintered at 1170 °C using a heating rate of 20 °C/min with a 20 min hold at peak temperature. The ceramic foams produced had a bulk density of 0.71 g/cm³ and a uniform pore size distribution. The research shows that ceramic foams can be formed from waste mineral wool and these can be used for thermal insulation with associated economic and environmental benefits.     

Phase Evolution of SiO2–Al2O3–ZnO–CaO–ZrO2–TiO2‐Based Glass with Added Y‐PSZ Particles

Yttria partially stabilized zirconia Y‐PSZ/glass‐ceramic composites were prepared by reaction sintering using powder mixtures of a SiO₂–Al₂O₃–ZnO–CaO–ZrO₂–TiO₂‐based glass and yttria partially stabilized zirconia (Y‐PSZ). The glass crystallized during sintering at temperatures of 1173, 1273, and 1373 K to give a glass‐ceramic matrix for high‐temperature protecting coatings. With the increasing firing time, the added zirconia reacted with the base glass and a glass‐ceramic material with dispersed zircon particles was prepared in situ. Furthermore, the added zirconia changed the crystallization behavior of the base glass, affecting the shape, amount, and distribution of zircon in the microstructure. The bipyramid‐like zircon grains with imbedded residual zirconia particles turned out to have two growth mechanisms: the inward growth and the outward growth, and its rapid growth was mainly dominated by the later one. For comparison, the referenced glass‐ceramic was prepared by sintering using exclusive glass granules and its crystallization behavior at 1173–1373 K was examined as well. Scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) were used to characterize the crystallization behavior of the base glass and the phase evolution of the Y‐PSZ/glass‐ceramic composites.     

Oxidation inhibition behaviors of HfB2-MoSi2-SiC oxygen blocking coating prepared by spark plasma sintering

The HfB₂-MoSi₂-SiC oxygen blocking coatings were prepared by the spark plasma sintering (SPS) technique, whose oxidation inhibition ability was further strengthened by the pre-oxidation treatment. The effect of MoSi₂ content and pre-oxidation treatment process on the oxygen blocking ability of the HfB₂-MoSi₂-SiC coating at 1973 K were conducted. After SPS, for the HfB₂-MoSi₂-SiC coatings with 20 wt%, 40 wt%, and 60 wt% MoSi₂, the relative density of the coatings are 92.6%, 93.9%, and 85.6%, respectively. Owing to the enhanced compactness of the coatings, increasing MoSi₂ content can significantly improve the protection efficiency of the coatings during the activation oxidation stage. However, due to the increased formation of gaseous by-products during the inerting oxidation stage, excessive MoSi₂ weaken the oxidation inhibition ability of the coatings. The sufficient dispersion of Hf-oxides nanocrystals in the glass layer conduces to enhance the oxygen blocking ability of the glass layer, making the 40HfB₂-40MoSi₂-20SiC coating present the best oxidation protective ability. The pre-oxidation treatment at 1773 K conduces to form the steady glass layer with fewer defects at the cost of a lower oxidation consumption of the coating, which enhanced the protection efficiency of the coating from 96.9% to 99.8% and reduced the oxygen permeability from 0.13% to 0.028%.     

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.     

Air-sintered silicon (Si)-bonded silicon carbide (SiC) hollow fiber membranes for oil/water separation

In this work we evaluated the effect of adding Si as sintering additive into SiC for producing air-sintered hollow fiber membranes. According to crystallographic analyses, SiC and Si were converted to SiO₂ after sintering at 1350 °C. The addition of 30 wt% of Si into SiC ceramic material promoted the binding of SiC particles and improved the membrane mechanical resistance to 42.25 ± 3.39 MPa after air sintering at 1350 °C. The produced asymmetric ceramic membrane presented a packed pore-network and micro-voids with pore sizes of 1.73 and 5.29 μm, respectively. The filtration of an oil/water emulsion enabled oil retention 98.75 ± 0.95 %. Cake formation was the main fouling occurrence and membrane regeneration with equivalent oil retention and similar steady sate flux was achieved after water cleaning under ultrasound irradiation. Thus, the use of Si as air-sintering aid was favorable for producing Si-bonded SiC hollow fiber membranes with suitable application for oil/water separation.     

Preparation of Granular Ceramic Filter and Prediction of Its Collection Efficiency

The silicon carbide (SiC) ceramic filter is the most favorable component to remove particulate matter from hot flue gas due to its high filtration efficiency and high thermal durability. The effect of SiC powder size on the physical properties and filtration performance to prepare high-performance granular ceramic filter media was investigated in this study. Disk-type filter media were prepared by mixing ceramic components followed by physical compression and sintering. The porosity and average pore diameter in the filter media increased with increasing powder size. However, the mechanical strength decreased with increasing powder size, while it increased with increasing physical compressive force. The filter performance factor, q_FM was introduced to evaluate the ceramic filter properties, and the SiC50 filter was the best of the ceramic filters prepared in this study. We also found that diffusion was a dominant collection mechanism for particles smaller than 0.7 μm, and direct interception and inertia were dominant collection mechanisms for particles larger than 0.7 μm in the SiC50 filter based on a single collector efficiency model. In addition, the predicted collection efficiencies showed reasonably good agreement with the experimental ones.

Copyright 2014 American Association for Aerosol Research     

Preparation,mechanical, and leaching properties of CaZrO3 ceramic cores

The CaZrO₃ ceramic core materials with excellent mechanical and chemical properties were successfully prepared using single-phase CaZrO₃ powders. Effects of particle size ratio and sintering temperature on the mechanical and chemical properties of CaZrO₃ ceramic core materials were researched. The chemical property was analyzed by leaching research of core materials in 10 wt% and 20 wt% HNO₃ solution at the boiling point. Results showed that the suitable particle size ratio was important for the preparation of CaZrO₃ ceramic core materials with excellent comprehensive properties. The addition of fine particles in ceramic core materials promoted the densification process owing to the framework formed by coarser particles and sintering neck formed by fine particles between coarse particles, which was beneficial for further improving their bending strength. When the content of particles with 200 mesh size was 80wt%, the highest bending strength was obtained, 54.38 ± 5.28 MPa. The porosity was 17.45% and the volume density was 3.86 g/cm³. The increasing sintering temperature increased the densification of CaZrO₃ ceramic core materials by offering the sintering driving force, further leading to the improvement of bending strength. When the temperature was 1650℃, at the 20% content of particles with 200 mesh size, the highest bending strength of CaZrO₃ cores reached 51.01 ± 5.18 MPa. Meanwhile, the porosity was 18.65% and the volume density was 3.83 g/cm³. Additionally, the CaZrO₃ samples could be effectively leached in 10 wt% HNO₃ solution. Therefore, CaZrO₃ materials with good mechanical and leaching properties were believed to be a suitable candidate for ceramic core materials in the investment casting of alloys with high melting point.     

Laser additive manufacturing and homogeneous densification of complicated shape SiC ceramic parts

To improve the density of SiC ceramic components with complicated shape built by laser sintering (LS), cold isostatic pressing (CIP) and reaction sintering (RS) were incorporated into the process. In the process of LS/CIP/RS, Phenol formaldehyde resin (PF)-SiC composite powder was prepared by mechanical mixing and cold coating methods, with an optimized content of PF at 18?wt%. For the purpose of obtaining improved density of the sintered body after final reaction sintering, carbon black was added into the initial mixed powder. The material preparation, LS forming and densification steps were optimized throughout the whole fabrication process. The final sintered SiC bodies with the bending strength of 292 ~ 348?MPa and the density of 2.94–2.98?g?cm^{? 3} were prepared using the PF coated SiC-C composite powder and the LS / CIP / RS process. The study further showed a positive and practical approach to fabricate SiC ceramic parts with complicated shape using additive manufacturing technology.