Preparation of SiC nanowires-filled cellular SiCO ceramics from polymeric precursor

SiC nanowires-filled cellular SiCO ceramics were prepared using polyurethane sponge as a porous template infiltrated with silicone resin by pyrolysis at 1400 °C under Ar atmosphere. The pyrolysis temperature was an important parameter affecting the formation of SiC nanowires. The as-prepared sample obtained at 1000 °C was composed of SiCO glasses and turbostratic carbon. The SiCO ceramic was further converted into SiO₂ crystals and amorphous carbon by pyrolysis at 1200 °C. With the increasing pyrolysis temperature, SiC nanocrystals embedded in the non-crystalline SiCO matrix were observed. Furthermore, the SiC nanowires were formed in the pores of the SiCO ceramic. The diameters of the SiC nanowires are in the range 80–150 nm and the lengths are up to several tens of micrometers. The growth mechanism of the nanowires was supported by the vapor-solid mechanism.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Reactive infiltration processing (RIP) of ultra high temperature ceramics (UHTC) into porous C/C composite tubes

A novel reactive infiltration processing (RIP) technique was employed to infiltrate porous carbon fibre reinforced carbon (C/C) composite hollow tubes with ultra high temperature ceramic (UHTC) particles such as ZrB₂. The C/C composite tubes had initial porosity of ∼60% with a bimodal (10 μm and 100 μm) pore size distribution. A slurry with 40-50% ZrB₂ solid loading particles was used to infiltrate the C/C tubes. Our approach combines in situ ZrB₂ formation with coating of fine ZrB₂ particles on carbon fibre surfaces by a reactive processing method. A Zr and B containing diphasic gel was first prepared using inorganic-organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. Then commercially available ZrB₂ powder was added to this diphasic gel and milled for 6 h. The resultant hybrid slurry was vacuum infiltrated into the porous hollow C/C tubes. The infiltrated tubes were dried and fired for 3 h at 1400 °C in flowing Ar atmosphere to form and coat ZrB₂ on the carbon fibres in situ by carbothermal reaction. Microstructural observation of infiltrated porous C/C composites revealed carbon fibres coating with fine nanosized (∼100 nm) ZrB₂ particles infiltrated to a depth exceeding 2 mm. Ultra high temperature ablation testing for 60 s at 2190 °C suggested formation of ZrO₂ around the inner bore of the downstream surface.     

Preparation of a reticulated ceramic using vegetal sponge as templating

This paper describes the preparation of a reticulated ceramic that combines the morphology of vegetal sponge with ceramic properties, such as thermal stability, resistance to chemical attack, elevated porous degree and reticulation. In this method sponge samples are dipped into a colloidal suspension of 50% clay, 35% feldspar and 15% sand (w/w), followed by drying and heat treatment at 1175 °C for 120 min. Thermogravimetric analysis (TGA) of the vegetal sponge showed that the organic material is completely eliminated at temperatures around 515 °C. X-ray diffraction (XRD) analysis of the reticulated ceramic indicated the presence of mullite and cordierite. Scanning electron microscopy (SEM) of the reticulated ceramic showed the presence of two groups of porous ceramics, one in the range of 5–10 μm which was formed along the wall of the filaments, and another formed as a negative structure of the sponge filaments, measuring approximately 300 μm of diameter.     

Fabrication of porous SiC ceramics having pores shaped with Si grain templates

SiC porous ceramics were prepared by heating mixtures of Si powder and carbon black at 900 °C for 24 h in Na vapor. The grains of the Si powder were not only the source of Si for SiC but also served as templates for the pores in the SiC porous ceramics. Angular-shaped pores with sizes of 2-10, 10-150 and 50-150 μm were formed by angular Si grains with sizes of ≤10, ≤50 and ≤150 μm, respectively. The porosity of the SiC porous ceramics was around 55-59%. Spherical pores were also formed when spherical Si grains were used. A bending strength of 14 MPa was measured for the SiC porous ceramics prepared with the Si grains (≤50 μm).     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

A study of the sintering of diatomaceous earth to produce porous ceramic monoliths with bimodal porosity and high strength

Diatomite powder, a naturally occurring porous raw material, was used to fabricate ceramic materials with bimodal porosity and high strength. The effect of the sintering temperature on the density and porosity of dry pressed diatomite green bodies was evaluated using mercury porosimetry and water immersion measurements. It was found that the intrinsic porosity of the diatomite particles with a pore size around 0.2 µm was lost at sintering temperatures above 1200 °C. Maintaining the sintering temperature at around 1000 °C resulted in highly porous materials that also displayed a high compressive strength. Microstructural studies by scanning electron microscopy and energy-dispersive X-ray analysis suggested that the pore collapse was facilitated by the presence of low melting impurities like Na₂O and K₂O.     

Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process

Sub-micrometer SiC particles were firstly added to the preceramic solution in the first infiltration step to enhance the mechanical properties of 2D C_f/SiC composites fabricated via polymer infiltration and pyrolysis (PIP) process. The effects of pyrolysis temperature and SiC-filler content on microstructures and properties of the composites were systematically studied. The results show that the failure stress and fracture toughness increased with the increase of pyrolysis temperature. SiC filler of sub-micron scale infiltrated into the composites increased the mechanical properties. As a result, for the finally fabricated composite infiltrated with a slurry containing 40 wt.% SiC filler, the failure stress was doubled compared to that without SiC filler addition, and the fracture toughness reached ≈10 MPa m^1/2.     

Fabrication of Ceramics with Complex Porous Structures by the Impregnate–Freeze-Casting Process

The ceramics with complex porous structures were fabricated by a combination of impregnating and freeze-casting process. The polyurethane sponge was impregnated in the mold with 20 vol% of aqueous alumina slurry, and then the bottom of the cast body was kept at a constant cooling rate of 6°C/min to induce unidirectional solidification. After drying and sintering of the green part, porous ceramic with obvious lamellar architectures was prepared. The lamellae thickness and interlayer distance were as large as ∼9 and ∼15 μm, respectively. The large pores, which resulted from the burn-up of sponge struts were homogeneously distributed in the sample. The use of the porous template introduced some local interruption of the lamellar structures. However, high compression strength for the porous ceramic can still be obtained.     

Fabrication of CaSiO3 bioceramics with open and unidirectional macro-channels using an ice/fiber-templated method

Porous CaSiO₃ bioceramics with open and unidirectional macro-channels of pore size more than 200 μm are of particular interest for biomedical applications. An ice/fiber-templated method was employed for the fabrication of CaSiO₃ bioceramics with interconnected lamellar pores and macro-channels of pore size more than 200 μm. The pores formed by ice crystals transformed from cellular to lamellar, while the pores formed by fibers were aligned macro-channels, which were also in alignment with the lamellar pores. Keeping the initial slurry concentration constant and increasing the packing density of fibers, the volume fraction of macro-channels and open porosity increased, and the compressive strength decreased. Maintaining the packing density of fibers and increasing the initial slurry concentration, the pore sizes of lamellar pores and open porosity decreased, and the compressive strength increased. The results indicated that it was possible to manufacture porous CaSiO₃ bioceramics with the macro-channels of 250–350 μm, lamellae spacing of 50–100 μm, open porosity of 71.12–83.94% and compressive strength of 0.87–3.59 MPa, indicating the suitability for tissue engineering.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

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%.     

Production of highly aligned porous alumina ceramics by extruding frozen alumina/camphene body

We herein propose a new technique for producing highly aligned porous ceramics by extruding a frozen ceramic/camphene body. To accomplish this, an alumina/camphene slurry with an initial alumina content of 10 vol% was first frozen unidirectionally in a 20 mm × 20 mm mold and extruded through a reduction die with a cross-section of 5 mm × 5 mm at room-temperature. This simple process enabled the formation of porous alumina ceramics with highly aligned pores as a replica of the camphene dendrites with a preferential orientation parallel to the extrusion direction. The sample showed much higher compressive strength of 280 ± 80 kPa with a porosity of 83 vol% when tested parallel to the direction of pore alignment. In addition, these materials could be used as a valuable framework for the production of ceramic/epoxy composites, particularly with a lamellar structure, which would result in a remarkable increase in mechanical properties.     

Low-temperature fabrication and characterization of porous SiC ceramics using silicone resin as binder

Commercially available silicone resin and silicon carbide (SiC) powders were adopted as the starting materials for the fabrication of porous SiC ceramics. During the heat treatment process, silicone resin experienced an organic–inorganic transformation and acted as the bonding material between SiC particles at a low temperature of 1000 °C. The mean particle size of starting SiC powders and silicone resin content can control the pore size, open porosity and fracture strength. The flexural strength of porous SiC ceramics increases with increasing silicone resin content and decreasing mean particle size of SiC powders. Larger pores can be obtained with coarser starting SiC powders and higher silicone resin content. The fracture surface of porous SiC ceramics was observed.     

Functionally graded porous ceramics with dense surface layer produced by freeze-casting

Graded and porous Al₂O₃ ceramics with dense surface layer were fabricated by camphene-based freeze-casting process. The Al₂O₃/camphene/dispersant slurries were prepared by ball-milling at 60 °C for 24 h, before pouring into silicone rubber die. The warm slurry was cast into a mold at 25 °C, where the top surface of the cast body was exposed to air to allow for the controlled evaporation of molten camphene, and the bottom was attached to a copper plate before being placed on the top of a metal plate immersed in a water bath which can be cooled by ice-water (°C) and liquid nitrogen (−196 °C). This processing method can produce a graded porosity and pore structure distribution. A typical four pore structure, that is surface dense layer, transition layer, aligned pore distribution region and inner porous region is formed between the top surface and bottom surface. This technique is considered potentially useful in fabricating novel porous ceramics with special structure.     

Rheology of aqueous mullite-starch suspensions

One of the forming methods developed for the manufacture of porous materials by direct consolidation, in which a ceramic suspension consolidates into non-porous molds (e.g. metal molds) by thermogelation of an organic agent, uses starch as both consolidator/binder of the ceramic suspension and pore former at high temperature. Changes in the rheological behavior of the aqueous suspensions are produced by starch gelatinization thermal process. This process as well as the presence of both the ceramic particles and added processing additives, influences the kinetics of green ceramic body formation and its microstructural features.In this work, the thermogelling behavior of mullite aqueous suspensions (40 vol.%; 0.45 wt.% of a polyacrylic polyelectrolyte as dispersant) containing 10 vol.% of different native starches (potato, cassava, and corn) was studied by dynamic rheology in order to determine the experimental conditions that must be used for forming mullite green bodies by thermal consolidation. Viscoelastic properties (G′ and G″) as a function of temperature (30-95 °C) and deformation (0.1-625.0% at 40 °C) were determined by temperature sweep tests and dynamic strain sweep tests, respectively. From these tests, and considering previous results of the rheological behavior of starch suspensions, we analyzed the influence of ceramic particles on the starch gelatinization process and the strength of the developed gels. On the other hand, shear flow properties of aqueous mullite-starch suspensions were also analyzed to obtain information on the rheological behavior of the suspensions at room temperature.     

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.     

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.     

聚氨酯海绵表面改性对Ti2AlN多孔陶瓷挂浆量的影响

利用有机泡沫浸渍法制备Ti2AlN多孔陶瓷,研究了聚氨酯海绵表面改性对陶瓷挂浆量的影响。通过对不同浓度的NaOH预处理后的海绵增重、坯体堵孔率分析表明,浓度为15%的NaOH对聚氨酯海绵的挂浆量最好;扫描电镜研究发现,聚氨酯海绵用15%NaOH溶液浸泡4h之后,再用羧甲基纤维素(CMC)预处理24h,海绵挂浆效果最佳。