Tailoring the microstructure of high porosity Si3N4 foams by direct foaming with mixed surfactants

The mixed surfactants were successfully applied to fabricate the highly porous Si₃N₄ ceramic foams by the direct foaming method. The oppositely charged surfactants mixed in slurries could combined into catanionic surfactant by the electrostatic attraction and facilitate the formation of ultra-stable foams. The microstructure of the Si₃N₄ ceramic foams, including pore structure, mean pore size, pore size distribution and porosity were tailored by varying the mixing ratio of surfactant, mixed surfactants concentration and solid content of the initial slurries. Si₃N₄ ceramic foams with porosity of 92%-97%, mean pore size of 140-240 µm and compressive strength of 0.85-5.38 MPa were obtained by adjusting mixed surfactants between 0.1 and 0.4 wt% and solid content between 22 and 30 vol%. The compressive strength of Si₃N₄ ceramic foams in current work was much higher than most reported results.     

New consolidation process inspired from making steamed bread to prepare Si3N4 foams by protein foaming method

A new consolidation process had been developed for preparing Si₃N₄ ceramic foams by using protein foaming method, which was inspired from the preparation of steamed bread. The main advantage of this consolidation process was no crack development during foamed slurry consolidation process. By using this new consolidation, Si₃N₄ ceramic foams with open porosities of 79.6–87.3% and compressive strength of 2.5–22 MPa were prepared. Protein addition and solid content on mechanical properties and pore structures of the as-prepared ceramic foams were investigated. Results indicated that the open porosity decreases with protein addition and solid content while compressive strength increased with solid content. With the increase of solid content, pores of the ceramic foams became regular in shape and uniform in size while both size and number of windows on the walls decreased.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

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.     

Ultralight Silicon Nitride Ceramic Foams from Foams Stabilized by Partially Hydrophobic Particles

Ultralight Si₃N₄ ceramic foams have been successfully prepared through particle‐stabilized foams method, which is based on the adsorption of in situ hydrophobized Si₃N₄ particles to the liquid/air interface of the foams. Here, we firstly used a long‐chain surfactant cetyltrimethylammonium chloride to render the Si₃N₄ particles partially hydrophobic. By tailoring the surfactant concentration and pH values of the suspensions, the wet foams were stabilized to avoid coarsening and coalescence. SEM results show that the Si₃N₄ ceramic foams possess single strut walls with elongated β‐Si₃N₄ grains interlocking with each other, and their pores are uniform with an average pore size of 95 μm. The obtained ceramic foams maintain compressive strength of 1.34 ± 0.13 MPa with porosity of 92.0%, when the suspension contains 3 mmol/L surfactant at the pH of 11.0.     

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.     

Effect of rotating speed during foaming procedure on the pore size distribution and property of silicon nitride foam prepared by using protein foaming method

Silicon nitride (Si₃N₄) foams were prepared by using protein foaming method with varying rotating speed during the foaming process. The pore sizes of these as-fabricated Si₃N₄ foams were measured by means of the Image Pro Plus software and the as-measured pore size date was analyzed statistically by using the SPSS Statistics software. It was indicated that the pore size data of the as-prepared Si₃N₄ foams abided by the logarithmic normal distribution. With the increase of rotating speed, the pore structure of Si₃N₄ foam became more uniform. This was because of the enhancing shear stress at higher rotating speed, which increased frequency of collision between bubbles in foamed slurry and hence improved the uniformity of bubble size distribution. The porosity, density and flexural strength of these as-prepared Si₃N₄ foams fluctuated in a small range, indicating that the rotating speed had limited influence on these properties.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

Improving cure performance of Si3N4 suspension with a high refractive index resin for stereolithography-based additive manufacturing

Silicon nitride (Si₃N₄) slurries with high solid loading, low viscosity and good stability is difficulty prepared, due to low intrinsic surface charge and a large refractive index (RI) difference between Si₃N₄ powder and resin. In this paper, the curing behavior of Si₃N₄ slurry with different functional group and RI of resin monomer were systematically researched, and then the kind and optimum content of dispersant were investigated. Subsequently, a high solid loading Si₃N₄ slurry (44 vol%) with good curing behavior, low viscosity and favorable stability was successfully prepared. Lastly, the dense Si₃N₄ ceramic parts were fabricated by the suitable Si₃N₄ slurry (44 vol%) via stereolithography. After debinding and sintering process, the relative density and flexural strength of Si₃N₄ ceramic were 98.28% and 800 ± 27.28 Mpa, respectively.     

In situ synthesis of Si2N2O/Si3N4 composite ceramics using polysilyloxycarbodiimide precursors

In situ synthesis of Si₂N₂O/Si₃N₄ composite ceramics was conducted via thermolysis of novel polysilyloxycarbodiimide ([SiOSi(NCN)₃]_n) precursors between 1000 and 1500 °C in nitrogen atmosphere. The relative structures of Si₂N₂O/Si₃N₄ composite ceramics were explained by the structural evolution observed by electron energy-loss spectroscopy but also by Fourier transform infrared and ²⁹Si-NMR spectrometry. An amorphous single-phase Si₂N₂O ceramic with porous structure with pore size of 10–20 μm in diameter was obtained via a pyrolyzed process at 1000 °C. After heat-treatment at 1400 °C, a composite ceramic was obtained composed of 53.2 wt.% Si₂N₂O and 46.8 wt.% Si₃N₄ phases. The amount of Si₂N₂O phase in the composite ceramic decreased further after heat-treatment at 1500 °C and a crystalline product containing 12.8 wt.% Si₂N₂O and 87.2 wt.% Si₃N₄ phases was obtained. In addition, it is interesting that residual carbon in the ceramic composite nearly disappeared and no SiC phase was observed in the final Si₂N₂O/Si₃N₄ composite.     

Simultaneous improvement in porosity and strength of porous β-Si3N4 ceramics by formation of ultrafine fibrous grains

Si₃N₄ ceramic with ultrafine fibrous grains are expected to exhibit remarkable mechanical properties. In this work, highly porous Si₃N₄ ceramic monoliths composed of ultrafine fibrous grains were developed via a novel vapor-solid carbothermal reduction nitridation (V-S CRN) reaction between SiO vapor and green bodies comprised of carbon nanotubes (CNTs), α-Si₃N₄ diluents and Y₂O₃ in a N₂ atmosphere. The unique fibrous grains-interconnected structure was developed through in-situ formation of Si₃N₄ and following liquid phase sintering. The porous Si₃N₄ monoliths with porosity of 61–78% was developed by controlling the contents of α-Si₃N₄ diluents and densities of the CNT green bodies. With increasing of the α-Si₃N₄ contents, Si₃N₄ fibrous grains with an aspect ratio of approximate or higher than 20 could be achieved, and the grains were gradually refined. For the samples with 40 wt% α-Si₃N₄, the minimum mean grain diameter and pore size of 164 nm and 0.79 μm were achieved, respectively, and the resultant porous Si₃N₄ monolith exhibited a flexural strength of as high as 73–102 MPa with the porosity of 61–73%, which is much higher than that of the reported in literature. The improvement of mechanical strength could be attributed to the densely interconnected bird's nests structure formed by the ultrafine fibrous grains. The effects of the α-Si₃N₄ diluents on the resulting porous Si₃N₄ monolith via this method were analyzed.     

Photosensitive Si3N4 slurry with combined benefits of low viscosity and large cured depth for digital light processing 3D printing

Digital light processing 3D printing can be applied to fabricate complex silicon nitride (Si₃N₄) components. However, because of the surface hydroxyl groups and large refractive index, it is still a foremost challenge to realize a stable photosensitive Si₃N₄ slurry with combined benefits of low viscosity and large curing depth. In this study, we propose a new formulation strategy to prepare Si₃N₄ slurry. Starting from the optimization of monomer ratio, we have systematically optimized powder particle size, dispersant and photoinitiator on the rheological properties and curing properties of Si₃N₄ slurry. Specifically, we have fabricated a stable photosensitive Si₃N₄ slurry (48 vol%) with a viscosity of 2.09 Pa s (30 s^?1), a critical curing energy of 126.09 mJ/cm² and a maximum curing depth of 80 µm. Finally, based on this optimized slurry, we have successfully obtained complex Si₃N₄ green body with no defect, which demonstrates great potential to fabricate arbitrary complex ceramic components for various applications.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Fabrication,microstructural characterization and gas permeability behavior of porous silicon nitride ceramics with controllable pore structures

Porous Si₃N₄ ceramics with monomodal and bimodal pore structure were prepared by cold isostatic pressing and freeze-casting, respectively. Both the pore structure and permeability behavior of the porous Si₃N₄ ceramics were tailored by altering the pressure of cold isostatic pressing and the composition and content of solvent during freeze-casting. The specimens obtained by cold isostatic pressing exhibited smaller Darcian and non-Darcian permeability than those of freeze-casted samples due to their lower open porosity, smaller pore size and higher tortuosity. On the other hand, compared with the ice-templated specimens having the same solvent volume in the ceramic slurries as them during freeze-casting, the emulsion-ice templated samples showed smaller open porosity, macropore size and Dacian permeability, but the similar non-Darcian permeability because of their larger micropores and better pore interconnectivity.     

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.     

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.     

Mechanical and dielectric properties of Si3N4-SiO2 ceramics prepared by digital light processing based 3D printing and oxidation sintering

Si₃N₄-SiO₂ ceramics are considered as the preferred high-performance wave-transmitting material in the aerospace field. However, traditional fabrication methods for Si₃N₄-SiO₂ ceramics have the disadvantages of high cost and complicated fabrication process. In this paper, Si₃N₄-SiO₂ ceramics with excellent mechanical and dielectric properties were fabricated by digital light processing-based 3D printing combined with oxidation sintering. Firstly, the curing thickness and viscosity of slurries with different solid loadings for vat photopolymerization-based 3D printing were studied. Then, the effects of the sintering temperature on the linear shrinkage, phase composition, microstructure, flexural strength, and dielectric properties of Si₃N₄-SiO₂ ceramics, and the influences of solid loading on them were explored. The curing thickness and viscosity of the slurry with a solid loading of 55 vol% were 30 μm and ∼1.5 Pa‧s, respectively. The open porosity and the flexural strength of Si₃N₄-SiO₂ ceramic with a solid loading of 55 vol% were 4.3 ± 0.61% and 76 ± 5.6 MPa, respectively. In the electromagnetic wave band of 8–18 GHz, the dielectric constant of Si₃N₄-SiO₂ ceramics was within the range of less than 4, and the dielectric loss remained below 0.09. The method of digital light processing-based 3D printing combined with oxidation sintering can be further extended in the preparation of Si₃N₄-based structure-function integrated ceramics.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.