不同铝源对SiC/堇青石复相多孔陶瓷的制备及性能影响

以碳化硅、氮化铝、层析氧化铝、氢氧化铝、氟化铝、滑石为主要原料,石墨为造孔剂通过原位反应烧结技术制备碳化硅/堇青石复相多孔陶瓷.研究了含铝化合物种类、烧结温度、石墨含量对SiC/堇青石复相多孔陶瓷相组成、微观结构、气孔率和抗折强度的影响,同时对S0组在1200℃烧结温度下制得的SiC/堇青石复合多孔陶瓷的孔径分布进行了测试分析.结果表明:以AlN为铝源在1200℃下烧结,石墨含量在15％时,堇青石结合SiC多孔陶瓷的抗弯强度和气孔率两项综合性能达到最优,气孔率为31.99％,相应的弯曲强度86.20 MPa.S0组的平均孔径大小在3.0191 μm.     

Fabrication and characterization of cordierite-bonded porous SiC ceramics

A reaction bonding technique was used for the preparation of cordierite-bonded porous SiC ceramics in air from α-SiC, α-Al₂O₃ and MgO, using graphite as the pore-forming agent. Graphite was burned out to produce pores and the surface of SiC was oxidized to SiO₂ at high temperature. With further increasing the temperature, SiO₂ reacted with α-Al₂O₃ and MgO to form cordierite. SiC particles were bonded by the cordierite and oxidation-derived SiO₂. The reaction bonding characteristics, phase composition, open porosity, pore size distribution and mechanical strength as well as microstructure of porous SiC ceramics were investigated. The pore size and porosity were strongly dependent, respectively, on graphite particle size and volume fraction. The porous SiC ceramics sintered at 1350 °C for 2 h exhibited excellent combination properties, the flexural strength of 26.0 MPa was achieved at an open porosity of 44.51%.     

Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration

Mullite bonded SiC ceramic membranes were synthesized by a facile solid-state reaction process, using SiC, solid waste fly ash as raw materials and MoO₃ as catalyst for growth of mullite at 1000 °C. The effect of MoO₃ catalyst on mullitization reaction and mullite morphology was investigated. Different pore formers were used to enhance the porosity and to observe its effects on the permeability parameters and filtration characteristics. At room temperature Darcian (k₁) and non-Darcian (k₂) in both water and air flow were measured and clean water flux was determined. The porous SiC ceramics with addition of 5 wt.% MoO₃ exhibited a flexural strength of 38.4 MPa at porosity 36.4 vol% and showed 92% oil removal efficiency from oily wastewater. This technique, combining low-cost materials and the co-sintering at low temperature, can serve as a cost-effective method for the production of high-performance porous SiC ceramic membrnaes for filtration application.     

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.     

多孔碳化硅陶瓷的原位氧化反应制备及其性能

以SiC为陶瓷骨料,Al2O3作为添加剂,通过原位氧化反应制备了Sic多孔陶瓷,并对其氧化反应特性及性能进行了研究.结果表明:在1 300～1 500℃,随烧结温度的升高,SiC的氧化程度增加,SiC多孔陶瓷的强度逐渐增加,但开口孔隙率有所降低.莫来石相在1 500℃开始生成·当烧结温度升高到1 550℃时,莫来石大量生成,得到了孔结构相互贯通且颈部发育良好的莫来石结合SiC多孔陶瓷;由于在SiC颗粒表面上覆盖了致密的莫来石层,SiC的氧化受到抑制,开口孔隙率因而升高,SiC多孔陶瓷的强度因莫来石的大量生成而增加.由平均粒径为5.0um的SiC,并添加20%(质量分数)Al2O3,经1 550℃烧结2h制备的SiC多孔陶瓷具有良好的性能,其抗弯强度为158.7MPa、开口孔隙率为27.7%.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

聚碳硅烷粘结法低温制备碳化硅多孔陶瓷

分别以平均粒径为10μm和20μm的两种规格碳化硅(SiC)粉末为原料、聚碳硅烷(PCS)为粘结剂,通过包混、过筛、模压成型、1000℃热解等工序制备了SiC多孔陶瓷,研究了PCS含量对SiC多孔陶瓷微观形貌、线收缩率、孔隙率与抗弯强度的影响,并对两种规格粉末制备的SiC多孔陶瓷性能进行了对比。结果表明:随着PCS含量的增加,两种规格粉末制备的SiC多孔陶瓷微观形貌都逐渐变得致密,当PCS含量为13%时,两种规格粉末制备的多孔陶瓷都出现了微观裂纹。随着PCS含量的增加,两种规格粉末制备的SiC多孔陶瓷孔隙率都逐渐降低,线收缩率都逐渐增大,抗弯强度先增大后降低,在PCS含量为10%时,平均粒径为10μm与20μm的SiC粉末制备的多孔陶瓷抗弯强度取得最大值,分别为31.6MPa与29.0MPa。     

Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting

Porous mullite ceramics were prepared at 1300–1600°C for 2?h via a foam-gelcasting route using industrial-grade mullite powders as the main raw material, Isobam 104 as the dispersing and gelling agent, triethanolamine lauryl sulphate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilising agent. The effects of firing temperature on the sintering behaviour of green samples as well as microstructures and properties of final porous mullite products were investigated. With increasing the temperature from 1300 to 1600°C, linear shrinkage and bulk density values of fired samples increased, whereas their porosity decreased. Mechanical strength and thermal conductivity values of fired samples decreased with increasing their porosities. Even at a porosity level as high as 79.4%, compressive and flexural strengths of fired samples (with average pore size of 314?μm) remained as high as 9.0 and 3.7?MPa, respectively, and their thermal conductivity (at 200°C) remained as low as 0.21?W?(m^?1?K^?1).     

Preparation of porous SiC-Al2O3 ceramics via gelcasting utilising a shrinkable pore-forming agent and oxidised coarse-grained SiC

By utilising soaked millet as a shrinkable pore-forming agent, porous silicon carbide-alumina (SiC-Al₂O₃) ceramics were prepared via gelcasting. The fabrication of SiC-Al₂O₃ ceramics based on oxidised and unoxidised coarse-grained SiC was also studied. The water swelling, drying shrinkage, and low-temperature carbonisation of the millet were investigated. We found that the shrinkage of the soaked millet was greater than that of gel body during drying, which left large gaps that prevented shrinkage stresses from destroying the gel body. Low-temperature carbonisation of the millet should be performed slowly at 220–240?°C because its expansion rate increases to 45% at 250?°C, resulting in the cracking of samples. At a constant sintering temperature, the flexural strength of the SiC-Al₂O₃ ceramics prepared with SiC powders oxidised at 1000?°C was the highest, indicating that oxidised powders can successfully decrease the required sintering temperature and improve the flexural strength of composite ceramics. Based on our optimised process, porous SiC-Al₂O₃ ceramics were sintered at 1500?°C for 2?h. When their skeletons were fully developed, their pore sizes were in the range of 1.5–2?mm. Their porosity and flexural strength were 60.2–65.1% and 8.3–10.5?MPa, respectively.     

Preparation of Mullite Ceramics from Coprecipitated Aluminum Hydroxide and Kaolinite Using Hexamethylenediamine

Mullite ceramics have been prepared from an aqueous suspension of kaolinite (raw or ground) and aluminum hydroxide. The precursor was coprecipitated in the mixture using hexamethylenediamine (HMDA) or ammonium hydroxide, and a solution of aluminum chloride from acid dissolution of wastes of aluminum metal. The precursor and the resulting materials were characterized and studied by X-ray diffraction, thermal methods, and mechanical strength and porosity measurements. The feasibility of the proposed chemical processing route for mullite preparation was demonstrated, in particular using HMDA as a precipitating agent for aluminum hydroxide instead of ammonium hydroxide, which adversely affects the system reactivity. The use of HMDA, as compared with ammonium hydroxide, and ground kaolinite produces single-phase mullite and enhances the flexural strength (maximum of 49 MPa) of the resultant ceramic porous bodies (porosity ca. 52–45 vol%) fired at 1550–1600°C for 30 min.     

Porous Silicon Carbide Ceramics Produced by a Carbon Foam Derived from Mixtures of Mesophase Pitch and Si Particles

Carbon foam templates were prepared from a mixture of mesophase pitch (MP) and Si particles, followed by foaming and carbonization. Subsequent molten Si infiltrated into the carbon foam at 1500°C for 4 h in an inert atmosphere resulted in the formation of porous SiC ceramics. Micrographs were investigated by a scanning electron microscope (SEM), and phase identification of porous SiC ceramics was performed by X-ray diffraction (XRD). The flexural strength and bulk density of porous SiC ceramics were also measured and calculated. The results revealed that the flexural strength of porous SiC ceramics increases with increasing Si content and decreasing porosity. The addition of Si in MP results in an increased densification of porous SiC struts. With 50 wt% Si, porous SiC ceramics with a high flexural strength of 23.9 MPa and a porosity of 55% were obtained.     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.     

Corrosion behaviors of porous reaction-bonded silicon carbide ceramics incorporated with CaO

Reaction-bonded SiC (RBSC) porous ceramics were fabricated at 1450?°C in air by incorporating CaO using ZrO₂ as sintering aids, activated carbon as pore-forming agent, and mullite fibers as reinforcing agent. The effects of CaO content on the properties of the porous RBSC ceramics were studied. Corrosion behaviors of the prepared RBSC porous ceramics in different environments were also investigated. The optimal open porosity, bending strength, average pore size and gas permeability of the ceramics with 0.5% CaO were 40%, 22.5?MPa, 42.9?µm, and 2100?m³/m² h?kPa, respectively. A well-developed neck reaction-bonded by calcium zirconium silicate (Ca₃ZrSi₂O₉) was identified. The porous RBSC ceramics exhibited excellent corrosion resistance in acid and basic solutions. The anti-oxidation temperature of the porous RBSC ceramics could reach 1200?°C in air. The RBSC ceramics maintained the bending strength of 17.5?MPa after 60 cold-hot cycles in air (0–800?°C). The porous RBSC ceramics also exhibited relatively good corrosion resistance in molten salts (NaCl, Na₂SO₄ and CaCl₂). Melten NaOH can aggravate the reaction by breaking the SiO₂ layers on the SiC surface. Overall, these findings offer significant insights into expanding the applications porous RBSC ceramics incorporated with CaO.     

Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding

An in situ reaction bonding technique was developed to fabricate mullite-bonded porous silicon carbide (SiC) ceramics in air from SiC and α-Al₂O₃, using graphite as the pore-former. Graphite is burned out to produce pores and the surface of SiC is oxidized to SiO₂ at high temperature. With further increasing the temperature, the amorphous SiO₂ converts into cristobalite and reacts with α-Al₂O₃ to form mullite (3Al₂O₃·2SiO₂). SiC particles are bonded by the mullite and oxidation-derived SiO₂ to obtain porous SiC ceramics. The reaction bonding behavior, open porosity, pore size distribution and mechanical strength of porous SiC ceramics were investigated as a function of the sintering temperature, forming pressure and graphite content. In addition, the phase composition and microstructure were also studied.     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.     

Porous SiC ceramics prepared via freeze-casting and solid state sintering

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000 °C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100 °C showed a good flexural strength of 11.25 MPa with an open porosity as high as 66.46%.     

Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications

Near-net-shape mullite ceramics with high porosity were prepared from ultra-low cost natural aluminosilicate mineral kaolin as raw material and polystyrene micro-sphere (PS) as pore-forming agent. Microstructure, flexural strength, thermal conductivity and dielectric properties of the ceramics were systematically researched. Results show that the porous mullite ceramics possess fibrous skeleton structure formed by a large quantity of interlocked mullite whiskers, which results in good mechanical properties and low-to-zero sintering shrinkage. Flexural strength of the porous mullite ceramics can be up to 41.01 ± 1.12 MPa, even if the porosity is as high as 62.44%. The dielectric constant and loss tangent of the porous mullite ceramics at room temperature are lower than 2.61 and 5.9 × 10⁻³, respectively. Besides, dielectric constant is very stable with the rising of temperature, and the dielectric loss can be consistently lower than 10⁻² when the temperature is not higher than 800 °C. In addition, thermal conductivity at room temperature is as low as 0.163 W/m/K when the porosity of mullite ceramics is 80.05%. The infiltration of SiO₂ aerogels (SiO₂ AGs) can further decrease the thermal conductivity to 0.075 W/m/K, while has just little effects on the dielectric properties. Excellent mechanical, thermal and dielectric properties show that the porous mullite ceramics have potential applications in radome fields. The porous mullite ceramics prepared from kaolin not only have low cost, but also can achieve near-net-shape.     

Porosity control of porous silicon carbide ceramics

Porous SiC ceramics were fabricated by the carbothermal reduction of polysiloxane-derived SiOC containing polymer microbeads followed by sintering. The effect of the SiC powder:polysiloxane-derived SiC (SiC:PDSiC) ratio on the porosity and flexural strength of the porous SiC ceramics were investigated. The porosity generally increased with decreasing SiC:PDSiC ratio when sintered at the same temperature. It was possible to control the porosity of porous SiC ceramics within a range of 32–64% by adjusting the sintering temperature and SiC:PDSiC ratio while keeping the sacrificial template content to 50 vol%.The flexural strengths generally decreased with increasing porosity at the same SiC:PDSiC ratio. However, a SiC:PDSiC ratio of 9:1 and a sintering temperature of 1750 °C resulted in excellent strength of 57 MPa at 50% porosity. Judicious selection of the sintering temperature and SiC:PDSiC ratio is an efficient way of controlling the porosity and strength of porous SiC ceramics.     

Effects of Preheat-Treated Aluminosilicate Addition on the Phase Development, Microstructure, and Mechanical Properties of Mullitized Porous OBSC Ceramics

Mullitized porous oxidation-bonded SiC (OBSC) ceramics were fabricated with and without preheat-treated aluminosilicate by an in situ reaction-bonding process. The effects of the preheat-treated aluminosilicate on the phase development, microstructure, and mechanical properties of porous SiC ceramics were investigated. With 5.0 wt% preheat-treated aluminosilicate addition, the process of mullitization was significantly promoted. Owing to the enhancement of neck growth by the preheat-treated aluminosilicate, a high flexural strength of 86.9 MPa was achieved at an open porosity of 35.3%, and the porous SiC ceramics maintained good thermal shock resistance. With 5.0 wt% preheat-treated aluminosilicate addition, the average pore size of porous SiC ceramics was enlarged from 2.8 to 4.0 μm. The effects of preheat-treated aluminosilicate content on the oxidation degree of SiC, open porosity, and flexural strength of porous SiC ceramics were also investigated.     

Properties of multiple oxide-bonded porous SiC ceramics prepared by an infiltration technique

Multiple oxide-bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC and alumina with cordierite sol followed by sintering at 1300-1400°C in air for 3 hours. The microstructures, phase components, mechanical properties, and air permeation behavior of the developed porous ceramics were examined and compared with materials obtained by the traditional powder processing route. The porosity, average pore diameter, and flexural strength of the ceramics varied from 33 to 37 vol%, ~12-14 μm and ~23-39.6 MPa, respectively, with variation in sintering temperature. The X-ray diffraction results reveal that both the amount of cordierite and mullite as the binder increased with increase in sintering temperature. In addition, it was found that the addition of alumina in powder form effectively enhanced the strength due to formation of mullite in the bond phase in contrast to the samples prepared without alumina additive. To determine the suitability of the material in particulate filtration application, particle collection efficiency of the filter material was evaluated theoretically using single collector efficiency model.