Manufacture of Si3N4-SiCN composite bulks with hierarchical pore structure

Si₃N₄-SiCN ceramic foams with hierarchical pore architecture were formed by protein-based gelcasting and precursor infiltration and pyrolysis. The primary pore structure (>100 μm) was generated by protein gelation and precursor ceramization, while the secondary pore structure (10–50 μm) originated from the cell windows after pyrolysis. The network of Si₃N₄ nanowires and the voids among ceramic particles formed the tertiary pore structure (<2 μm). The obtained Si₃N₄-SiCN ceramics had a density of 0.45–0.66 g/cm³ and an open porosity of 72.7–82.8 vol.%. The porous bulks possessed a compressive strength of up to 16.9 ± 1.1 MPa (72.7 vol.% open porosity) at room temperature and 8.6 ± 0.2 MPa at 800 °C. A good gas permeability of the ceramics was indicated with a tested value of 3.27 cm³cm/(cm²·s·kPa). The excellent mechanical property, permeability together with the hierarchical pore structure enabled the Si₃N₄-SiCN composite bulks promising for industrial filtration applications.     

Microstructure and permeability of porous Si3N4 supports prepared via SHS

Porous Si₃N₄ ceramics with tailored pore structures were fabricated via self-propagating high temperature synthesis (SHS) using Polymethylmethacrylate (PMMA) as pore forming agent. The pore structures, mechanical properties and permeation performance of porous Si₃N₄ ceramics were investigated by altering the particle sizes and amount of PMMA. With the increasing content of PMMA, the flexural strength of samples decreased from 102.5 MPa to 9.4 MPa. The tortuosity which showed irregular variation affected gas permeability directly. The samples with 20 wt% content of PMMA exhibited the maximum Darcian and non-Darcian constants with the smallest tortuosity. Moreover, the comparison of permeability coefficients with other ceramics via different pore forming methods in literature was presented. The specimens exhibited great permeability due to the large pore sizes created by the elongated and coarsened β-Si₃N₄ grains during the SHS process, providing a low-cost and environmentally friendly method for preparing high permeability porous Si₃N₄ supports.     

Effects of solid loading and calcination temperature on microstructure and properties of porous Si3N4 ceramics by aqueous gelcasting using DMAA system

Herein, a low–toxic N, N–dimethylacrylamide (DMAA) system was used in preparation of porous Si₃N₄ ceramics by aqueous gelcasting, and variations in microstructure and properties with solid loading and calcination temperature were systematically investigated. In the considered solid loading range of 28–44 vol%, all the slurries exhibited superior rheological properties (≤145 mPa⋅s at 95.40 s⁻¹ for 44 vol% solid loading) perfectly suitable for casting. With increasing solid loading, a decreased bulk density (1.71–1.69 g/cm³), volume shrinkage (37.73–13.77%) and flexural strength (46.56–26.75 MPa) of green bodies were obtained, exhibiting better mechanical properties than those derived from the conventional acrylamide (AM) system. Regarding Si₃N₄ ceramics with various solid loadings, the increase in calcination temperature favored the phase transformation α→β–Si₃N₄ and β–Si₃N₄ growth, however, the increased solid loading exhibited an inhibiting effect on those since mass transport in gas phase was blocked due to the disruption of pore connectivity. The resulting microstructure changes imparted Si₃N₄ ceramics increasing flexural strength (110.36–367.88 MPa), fracture toughness (2.54–5.03 MPa⋅m^1/2), as well as decreasing porosity (54.21–41.05%) and pore size (0.38–0.33 μm). This work demonstrates the potential research value of DMAA system in preparing high–performance porous Si₃N₄ ceramics through gelcasting technique.     

Mechanical property and gas filtration behavior of porous reaction bonded Si3N4 with monodispersed PMMA as pore former

Porous Si₃N₄ ceramics were prepared by gel casting combined with a reaction bonding route using monodispersed PMMA as the pore former, and Isobam was used as a gel agent. With the PMMA addition varying from 0 to 20 wt.%, the bending strength was degraded from 94.0 to 39.1 MPa owing to the increased porosity and decreased bulk density. The β-Si₃N₄ prismatic grains and round pores introduced by the monodispersed PMMA micro balls would endow the samples with high strength, and the permeability of the resulting samples was increased obviously with the increase of PMMA addition. Flue gas filtration test exhibited that the filtration efficiency of the porous ceramics filter was not degraded with introducing of the PMMA pore former, even though the permeability was increased obviously. The block type of the sample with 20-wt.% PMMA additions during filtration was cake filtration, which indicates that the sample has the characteristic of being reusable after back-blowing in flue gas filtration applications. Porous Si₃N₄ with high strength and permeability fabricated via the reaction bonding route exhibits great potential for low-cost high-performance ceramics filters.     

Porous Si3N4 ceramics prepared by aqueous gelcasting using low–toxicity DMAA system: Regulatable microstructure and properties by monomer content

In this study, the low–toxicity monomer N, N–dimethylacrylamide (DMAA), serving as both gelling agent and pore–forming agent, was adopted to fabricate porous Si₃N₄ ceramics with a regulatable microstructure and property by aqueous gelcasting. Results indicate that monomer content played an important role in regulating and optimizing the properties of sintered bodies. With increasing monomer content (5.94–30.69?wt%), both slurry viscosity (maximum 0.14?Pa?s at 95.40 s^?1) and green body strength (11.35–49.23?MPa) exhibited monotonic increasing trends, demonstrating superior mechanical properties to those obtained using the neurovirulent acrylamide (AM) gelling system. The increased monomer content not only improved porosity, but also promoted α→β–Si₃N₄ transformation as well as β–Si₃N₄ grain growth through enhancing the connectivity of interlocking pores and accelerating the vapor phase transport during liquid–phase sintering. These variations in phase composition and microstructure derived from the varied monomer content further resulted in monotonic changes in porosity (40.32–51.50%), mean pore size (0.27–0.38?μm), flexural strength (202.77–132.15?MPa), fracture toughness (2.93–2.32?MPa?m^1/2), dielectric constant (3.48–2.78) and loss (3.52–3.09?×?10^?3) at 10?GHz for sintered bodies, displaying an excellent comprehensive properties. This study suggests a promising prospect for DMAA in preparation of high–performance porous Si₃N₄ ceramics by aqueous gelcasting.     

Pore structure control of Si3N4 ceramics based on particle-stabilized foams

Porous Si₃N₄ ceramics with open, closed pores and nest-like structures were prepared by direct foaming method, and the stability of bubbles and the microstructures of sintered Si₃N₄ foam ceramics were investigated. The bubbles produced by short-chain amphiphiles have higher stability as compared with that produced by long-chain surfactants. Si₃N₄ ceramic foams using short-chain amphiphiles are particle-stabilized one, porous Si₃N₄ ceramics with open and closed pores can be easily prepared with this method, and the nest-like microstructure in Si₃N₄ foam ceramics is achieved at high gas-pressure sintering conditions. The decrease of flexural strength due to the increase of porosity can be weakened by decreasing pore size.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Microstructure and Mechanical Properties of Lu2O3-Doped Porous Silicon Nitride Ceramics Using Phenolic Resin as Pore-Forming Agent

The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si₃N₄ ceramics with controlled microstructure. Lu₂O₃ and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu₂O₃-doped ceramics using 12–57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 μm. The porous ceramic infiltrated with CVI Si₃N₄ had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness.     

Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent

In this paper, novel porous Si₃N₄ ceramics were prepared by aqueous gelcasting using Si₃N₄ poly-hollow microspheres as pore-forming agent. The effect of Si₃N₄ poly-hollow microsphere content on the phase composition, microstructure, shrinkage, porosity and mechanical properties of the prepared porous Si₃N₄ ceramics were investigated. It is found that there is only β-Si₃N₄ phase in all the prepared porous Si₃N₄ ceramics. Meanwhile, the SEM results show that the pores in the porous Si₃N₄ ceramics distribute uniformly, the added Si₃N₄ poly-hollow microspheres and the basal body contact closely. With the increase of Si₃N₄ poly-hollow microsphere content, the shrinkage of the porous Si₃N₄ ceramics decreases gradually, and the porosity of the porous Si₃N₄ ceramics decreases firstly and then increases. Furthermore, the flexural strength and fracture toughness of the porous Si₃N₄ ceramics decrease with the increase of the Si₃N₄ poly-hollow microsphere content.     

Microstructure–permeability relation of porous β-Si3N4 ceramics

Porous β-Si₃N₄ ceramics with two distinct structures were produced by using two different Si₃N₄ sources to investigate the relationship between microstructure and permeability. Results showed that regardless of pore amount, size of pore channels, shape-distribution of β-Si₃N₄ grains are more effective on permeability of porous Si₃N₄ ceramics. Higher permeability and lower contribution of inertial forces was obtained by microstructure consists of coarse and equiaxed grains even at lower porosity amount. Calculated Forchheimer number (F_o) and measured the local breadth of a pore also supported the effect of microstructure on permeability.     

Preparation of AlN micro-honeycombs with high permeability via freeze-casting

Highly permeable AlN micro-honeycomb (AlN-H) ceramics with unidirectional macropore channels and porous struts were fabricated via tertiary butyl alcohol (TBA)-based freeze-casting. The effect of the AlN solid loading of the freezing slurries on the microstructure, open porosity, N₂ gas permeability, and compressive strength of the as-prepared AlN-Hs were systematically investigated. The results showed that the honeycomb structure and open porosity of the AlN-Hs can be adjusted by altering solid loading. With the increase in solid loading from 10 vol% to 30 vol%, the approximate pore channel size of the AlN-Hs decreased from 50.1 μm to 15.6 μm, strut thickness increased from 8.2 μm to 16.6 μm approximately, and the corresponding open porosity decreased from 87.6–56.6%. The as-fabricated AlN-H with an open porosity of 64.9% possessed high N₂ gas permeability and essential compressive strength and can be used as a catalyst support or filter in industries.     

Fabrication and properties of Si3N4 bioscaffolds with orderly–interconnected big pore channels and well–distributed small pores

This paper focuses on investigating the technical potential for fabricating porous ceramic bioscaffolds for the repair of osseous defects from trauma or disease by inverse replication of three–dimensional (3–D) printed polymer template. Si₃N₄ ceramics with pore structure comprising orderly–interconnected big pore channels and well–distributed small pores are successfully fabricated by a technique combining 3–D printing, vacuum suction filtration and oxidation sintering. The Si₃N₄ ceramics fabricated from the Si₃N₄ powder with addition of 10?wt% talcum by sintering at 1250?°C for 2?h have little deformation, uniform microstructure, low linear shrinkage of 4.1%, high open porosity of 58.2%, relatively high compression strength of 6.4?MPa, orderly–interconnected big pore channels and well–distributed small pores, which are promising bioscaffold in the field of bone tissue engineering.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

A non-sintering fabrication method for porous Si3N4 ceramics via sol hydrothermal process

A non-sintering fabrication method for porous Si₃N₄ ceramics with high porosity and high mechanical strength was proposed. Strength of the porous ceramics can be obtained by silica sol mass transfer process in hydrothermal conditions rather than a traditionally controlled high temperature sintering process. Under hydrothermal circumstances, silica sol is continuously transferred to the necks of Si₄N₃ powder compact, depositing there and thus consolidating the ceramic skeleton. The key of the method to obtain homogeneous microstructure and mechanical strength is how to keep the silica sol from gelatin during hydrothermal procedure. The stabilization of silica sol and its affecting factors were studied. The results indicated that ultrasonic treatment makes alkali-catalyzed silica sol remain stable even in 200?℃ hydrothermal condition, which insures consecutive silica transportation. The effect of hydrothermal time on open porosity/mechanical strength of the porous Si₄N₃ ceramics were also thoroughly investigated. The porous Si₄N₃ ceramics with open porosity above 42% and flexural strength of 45?MPa were obtained without any high temperature sintering process. This method can be widely employed for the preparation of other porous ceramics as well.     

Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

Preparation and characterization of lotus ceramics with different pore sizes and their implication for the generation of microbubbles for CO2 sequestration applications

Four kinds of porous mullite ceramics, named lotus ceramics because of the similarity of their microstructure with lotus roots, were prepared by an extrusion method using rayon fibers of four different diameters (8.1, 9.6, 16.8 and 37.6 μm) as the pore formers. The physicochemical properties of these samples were characterized to test their applicability for the generation of microbubbles. The lotus ceramic samples contained pores of 9.4, 10, 15.6 and 30 μm size and porosities of 45–48%. SEM micrographs confirmed that the cylindrical pores were oriented unidirectionally along the extrusion direction and the degree of alignment was greater with larger fiber diameter. The permeability for gaseous CO₂ increased with increasing pore size from 3×10^?13 to 8×10^?13 m². The four lotus ceramic samples, a commercial air stone (72 μm) and two simple tubes (1000 and 3500 μm) were used to generate microbubbles in water under ambient conditions from a gas mixture of CO₂ and air. It was found that the bubble size could be decreased with bubblers of smaller pore size. In the bubble size measurements for pure CO₂ and air, the air bubbles were larger than the CO₂ bubbles due to partial dissolution of CO₂ into the water during bubbling. In order to generate smaller size bubbles using porous ceramic bubblers, the liquid must penetrate through the pores of the lotus ceramics before the gas is introduced into the system.     

Permeability behavior of silicon carbide-based membrane and performance study for oily wastewater treatment

Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high-processing costs. In this study, mullite-bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7-6.5 µm, 38-50 vol. %, and 28-38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k₁) and non-Darcian (k₂) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%-93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.     

Self-propagating high temperature synthesis (SHS) of porous Si3N4-based ceramics with considerable dimensions and study on mechanical properties and oxidation behavior

The aim of present work is to fabricate porous Si₃N₄ ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ diluent and Y₂O₃ as raw materials. The results indicate that Si₃N₄ diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si₃N₄ ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si₃N₄ ceramics possessed excellent flexural strength of 168 MPa～259 MPa, and high Weibull modulus of 11.0～17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si₃N₄-based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C～1300 °C, second phase-doped Si₃N₄ ceramics also presented higher residual strength than pure Si₃N₄ ceramics.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.