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.     

The capillary and thermal performance of the porous silicon nitride ceramics with nearly spherical pore structure

The capillary and thermal performance of porous Si₃N₄ ceramics with nearly spherical pore structure has been investigated by altering the addition and diameter of pore-forming agent polymethyl methacrylate (PMMA) in this work. An exponential model is used to evaluate the liquid uptake capacity of porous Si₃N₄ ceramics. Porous Si₃N₄ ceramics fabricated by 5 μm PMMA with 40 wt.% addition possess the lowest capillary time constant and show the best capillary performance owing to the perfect balance between friction resistance and capillary force. The thermal conductivity of porous Si₃N₄ ceramics is significantly impacted by their porosity. Alexander model with an exponent of .96 is suitable for predicting the thermal conductivity of porous Si₃N₄ ceramics due to its R-squared up to .99. Moreover, with the addition and diameter of PMMA decrease, the flexural strength of porous Si₃N₄ ceramics increases. These results support the application of porous Si₃N₄ ceramics in the field of mass and heat transfer.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

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 Si3N4 ceramics prepared via partial nitridation and SHS

Porous Si₃N₄ ceramics were prepared via partial nitridation and self-propagating high temperature synthesis (SHS) process. Raw Si and additive Y₂O₃ were mixed and molded under 10 MPa into a compact, the compact was partial nitridation at 1300 °C to form a porous Si/Si₃N₄, and then it was buried in a Si/Si₃N₄ bed for SHS to obtain porous Si₃N₄ with rod-like β-Si₃N₄ morphology. The processing combined the advantages of the nitridation of Si and SHS with low cost, low shrinkage and time saving. Porous Si₃N₄ with a porosity of 47%, a strength of 143 MPa were obtained by this method.     

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.     

Effects of N2 pressure and Si particle size on mechanical properties of porous Si3N4 ceramics prepared via SHS

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N₂ pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N₂ influences not only the thermodynamics but also the kinetics of the SHS as initial reactant. Flexural strength ranged between 67 MPa and 134 MPa with increasing N₂ pressure. On the other hand, flexural strength ranged from 213 MPa to 102 MPa with different Si particle sizes. This plays an important role on the final diameter and length of β-Si₃N₄ grains and the formation mechanism of porous Si₃N₄ ceramics.     

Porous Si3N4 ceramics fabricated through a modified incomplete gelcasting and freeze-drying method

Porous Si₃N₄ ceramics were fabricated via incomplete gelcasting and freeze-drying method followed by pressureless sintering at 1690 ℃. The specific way used in this work was to slow down the gelling process of ceramic suspensions by stopping water evaporation. Since water was retained and further frozen before the suspensions were totally gelled in normal way, pore structure was modified. Compared to direct freeze-casting, the polymer framework generated by polymerization of organic components prevented ice crystals from growing to large sizes. Performance of porous ceramics can also be controlled by the gelling time. Further on, porous Si₃N₄ ceramics with controlled pore structures can be obtained without using other organic additives. The gelling agent used in this study is a copolymer of isobutylene and maleic anhydride (IBMA). The small addition amount was only 0.2 wt%, but could greatly change the microstructure as well as mechanical performance of porous 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.     

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 gas permeation performance of porous silicon nitride ceramics with unidirectionally aligned channels

Porous Si₃N₄ ceramics with unidirectionally aligned channels were prepared via freezing ceramics suspension with distinct solid contents under different freezing temperatures. The samples obtained using lower solid content in ceramic suspension at higher freezing temperature exhibit larger Darcian and non-Darcian constants due to their higher open porosity, larger pore size and lower tortuosity. Moreover the investigation on individual contributions of viscous energy losses and inertial energy losses on the total pressure drop during permeation process indicates that with decreasing the solid content or the freezing temperature the viscous energy losses increase but the inertial energy losses decrease for samples owing to the differences in their pore structures. It is worth mentioning that porous Si₃N₄ ceramics with unidirectionally aligned channels exhibit larger Darcian and non-Darcian constants than those with similar pore size distributions and open porosity owing to their lower tortuosity, thus rendering them appropriate for filters and membrane supports.     

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.     

Effect of BN content on microstructures,mechanical and dielectric properties of porous BN/Si3N4 composite ceramics prepared by gel casting

Porous BN/Si₃N₄ composite ceramics with different BN contents have been fabricated by gel casting. The rheological behaviors of the suspensions, microstructure, mechanical properties, dielectric properties and critical temperature difference of thermal shock (ΔT_C) of porous BN/Si₃N₄ composite ceramics with different BN contents were investigated. With BN contents increasing, the mechanical properties of the porous BN/Si₃N₄ composite ceramics were partially declined, but the dielectric properties and thermal shock resistances were enhanced at the same time. For the porous Si₃N₄ ceramic without BN addition, the porosity, flexural strength, dielectric constant and critical temperature difference were 48.1%, 128 MPa, 4.1 and 395 °C, while for the 10 vol% BN/Si₃N₄ porous composite ceramics, they were 49.4%, 106.6 MPa, 3.8, and 445 °C, respectively. The overall performance of the obtained porous BN/Si₃N₄ composite ceramics indicated that it could be one of the ideal candidates for high-temperature wave-transparent applications.     

Preparation and characteristics of highly porous BN-Si3N4 composite ceramics by combustion synthesis

Nitride based ceramics are considered as a kind of promising material for structural and functional integration due to their robust structure, extreme environmental resistance and electromagnetically transparency. It is still challenging to prepare nitride based ceramics with homogeneous and controllable microstructure because of their low self-diffusion coefficient and difficulty in sintering. Here, we developed a gelcasting-SHS process by combining gelcasting forming and self-propagating high temperature synthesis (SHS) for the preparation of porous BN-Si₃N₄ composite ceramics. First, carbon residue problem in the gelcasting -SHS process was studied. Based on the result, porous BN-Si₃N₄ composite ceramics with high porosity (69.42 ~ 86.48%), high strength (21.7 ~ 81.0 MPa) and low dielectric constants (1.42 ~ 2.87) were synthesized. In addition, the thermal shock resistance of porous BN-Si₃N₄ composite ceramics until 1000 ℃ was evaluated.     

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.     

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.     

Preparation,mechanical, dielectric and microwave absorption properties of hierarchical porous SiCnw-Si3N4 composite ceramics

Herein, hierarchical porous SiC_nw-Si₃N₄ composite ceramics with good electromagnetic absorption properties were prepared. A porous Si₃N₄ matrix with different pore structures was first prepared by gelcasting-pressureless sintering (G-PLS) and gelcasting combined with particle stabilized foam-pressureless sintering (G-PSF-PLS). SiC_nw was then introduced by catalytic chemical vapor deposition (CCVD). An increase in solid loading (25–40 vol%) decreased apparent porosity (47.7–41.3%) and improved flexural strength (142.19–240.36 MPa) and fracture toughness (2.25–3.68 MPa·m^1/2). The addition of foam stabilizer propyl gallate (PG, 0.5–1.0 wt%) significantly increased apparent porosity (73.2–86.4%) and realized large-sized spherical pores, reducing flexural strength (58.23–38.56 MPa) and fracture toughness (0.75–0.41 MPa·m^1/2). High apparent porosity and large-sized pores facilitated the introduction of SiC_nw. The 25 vol% sample exhibited a reflection loss of ? 14.67 dB with an effective absorption bandwidth of 3.47 GHz, suggesting a development potential in the electromagnetic wave absorption field.     

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.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

Tailored pore structures and mechanical properties of porous alumina ceramics prepared with corn cob pore-forming agent

In this work, the effects of porosity and different particle sizes of pore-forming agent on the mechanical properties of porous alumina ceramics have been reported. Different grades of porous alumina ceramics were developed using corn cob (CC) of different weight contents (5, 10, 15, and 20 wt%) and particle sizes (<63 µm, 63-125 µm and 125-250 µm) as the pore-forming agent. Experimental results showed that total porosity and pore cavity size of the porous alumina ceramics increased with rising addition of CC pore former. Total porosity increased with increasing particle size of CC with the Al₂O₃-^<63CC₅ sample exhibiting the lowest total porosity of 41.3 vol% while the highest total porosity of 68.1 vol% was exhibited by the Al₂O₃-^125-250CC₂₀. The particle size effect of CC on the mechanical properties revealed that diametral tensile strength and hardness of the porous alumina ceramics deteriorated with increasing particle size of CC pore former. The Al₂O₃-^<63CC₅ sample exhibited the highest diametral tensile strength and hardness of 25.1 MPa and 768.2 HV, respectively, while Al₂O₃-^125-250CC₂₀ exhibited the lowest values of 1.1 MPa and 35.9 HV. Overall, porous alumina ceramics with the smallest pore sizes under each particle size category exhibited superior mechanical properties in their respective categories.