Porous Silicon Nitride Ceramics Research Articles

Investigations on porous silicon nitride ceramics prepared by the gel-casting method

Abstract Due to their special structure and many excellent properties, porous Si3N4 ceramics have a wide range of potential applications as a new structural and functional integrated material. In the present study, porous Si3N4 ceramics with excellent bending properties and satisfactory porosity were prepared by the gel-casting method. The variations in porosity, bending strength, fracture morphology and phase composition of the porous Si3N4 ceramics with different weight fractions of La2O3–MgO sintering additive were investigated. The results showed that the density of porous Si3N4 ceramics increased linearly with the increase of sintering additive content. When the content of sintering additive increased from 2.5 wt.% to 10 wt.%, the porosity of porous ceramics decreased from 30 % to 10 %. With the increase of sintering additive content, the bending strength of the samples first increased and then decreased. When the sintering additive content was 5.0 wt.%, the average bending strength of the samples reached a maximum of 240.88 MPa, and scanning electron microscopy and X-ray diffractometry results showed that β-Si3N4 particles with well-developed pores and a maximum aspect ratio of 5.8 were formed in the porous silicon nitride ceramics. This study provides a reference for the preparation of porous Si3N4 ceramics with good bending properties and porosity.

Progress in Preparation and Properties of Porous Silicon Nitride Ceramics

Porous silicon nitride ceramics is a promising functional ceramic material. In recent years, the research on the preparation of porous silicon nitride ceramics within different methods has been widely investigated. First, this work reviews the main synthesis methods of Si3N4 porous ceramics in detail, and compares the differences between strength and porosity caused by each method. The characteristics and advantages of different technologies under the current conditions were evaluated. Second, the dielectric properties, sound absorption properties and permeability properties of silicon nitride ceramics were compared and summarized based on the experimental results. Third, the applications fields of porous silicon nitride ceramics, such as smelting industry, catalyst carrier, sound absorption, wave-transparent, and biomedical fields were explored. Finally, the assessment of different silicon nitride ceramics preparation technologies was elaborated. This review gives an outlook on the porous silicon nitride ceramics, which shows great potential for further research in this field.

Effect of pH, milling time, and Isobam content on porous silicon nitride ceramics prepared by gel casting

As a non-toxic copolymer of isobutylene and maleic anhydride, Isobam was successfully used as a dispersant and a gelling agent for fabricating porous Si 3 N 4 ceramics by gel casting. The dispersity and rheological properties of the Si 3 N 4 slurry were influenced by the pH, milling time, and Isobam content, which varied from 0.1 wt.% to 0.6 wt.%, and these factors were investigated. The slurry with 40 vol.% solid content and milled for 4 h had a high zeta potential at pH 12 (adjusted by Tetramethyl ammonium hydroxide (TMAH)), which meant that the particles were well dispersed. The mechanisms of TMAH were electrostatic repulsion and steric hindrance. The viscosity of the slurry increased with increasing Isobam content. After pressureless sintering at 1700 °C for 2 h, a uniform unique interlocking microstructure of rod-like β-Si 3 N 4 grains was observed, which may improve the flexural strength of the ceramics by intergranular fracture and particle pullout of β-Si 3 N 4 grains. The density and porosity of the samples fluctuated negligibly with increasing Isobam content, and the Si 3 N 4 ceramic with 0.1 wt.% Isobam exhibited the highest bending strength of 251.6 MPa among all samples.

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

AbstractThe 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.

Progress of Porous Silicon Nitride Ceramics Prepared via Self-propagating High Temperature Synthesis

Progress of Porous Silicon Nitride Ceramics Prepared via Self-propagating High Temperature Synthesis

Microstructure and properties of porous Si3N4 ceramics by gelcasting-self-propagating high-temperature synthesis (SHS)

Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance, but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application. Herein, as a three-in-one solution for the above issues, for the first time we develop a novel approach that integrates the merits of gelcasting-SHS (self-propagating high-temperature synthesis) to prepare porous Si3N4 ceramics to simultaneously achieve high porosity, high strength, high toughness, and low thermal conductivity across a wide temperature range. By regulating the solid content, porous Si3N4 ceramics with homogeneous pore structure are obtained, where the pore size falls inbetween 1.61 and 4.41 µm, and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores. At the same time, porous Si3N4 ceramics with porosity of 67.83% to 78.03% are obtained, where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa·m1/2.

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.

Preparation, Properties and Growth Mechanism of Low-Cost Porous Si3N4 Ceramics with High Levels of β-Si3N4 Powders

Nowadays, porous Si3N4 ceramics are fabricated by high purity α-Si3N4 powders, resulting in a higher cost of production. To reduce cost and save energy, in this research, high levels of β-Si3N4 powders were effectively utilized to produce excellent and low-cost porous Si3N4 ceramics. The effects of high levels of β-Si3N4 powders on the microstructural evolution and properties were investigated in detail. The results suggested that, with the increase in β-Si3N4 powders from 0 wt.% to 80 wt.%, the aspect ratio of β-Si3N4 grains gradually decreased because of the anisotropic growth of grains significantly impinged by adjacent other β-Si3N4 grains. Additionally, the bending strength and dielectric constant of porous silicon nitride ceramics declined and their values were 685 MPa - 220 MPa, and 7.58–5.57, respectively, while the porosity increased from 5.9% to 28.5%. Similarly, the residual bending strength of Si3N4 ceramics degraded from 615 MPa to 172 MPa at 1000 °C for 20 h owing to the formation of SiO2 on the surface of silicon nitride ceramics after oxidizing.

Influence of Y2O3 particle size on the microstructure and corrosion behaviour of porous Si3N4 ceramics

In this study, porous silicon nitride ceramics were fabricated by pressureless with different particle size of Y2O3 using as sintering additive. The effects of particle size of Y2O3 particles on sintering behaviour, microstructure and corrosion behaviour of the β-Si3N4 ceramics were studied in detail. As the Y2O3 particles size decreasing from 5 μm to 0.5 μm, the linear shrinkage of porous β-Si3N4 ceramics increased from 14.3% to 18.3%, and the sintering porosity decreased from 25.3% to 17.6%. The smaller Y2O3 particles additions were consisted of finer β-Si3N4 grains and demonstrated more accordant microstructure due to the well-dispersed Y2O3. The finer microstructure resulted in the improvement of mechanical properties, the sintered sample with 0.5 μm Y2O3 addition shown flexural strength of 450 MPa. Significant weight loss was observed after corrosion in 1 M HCl solution at 70°C for different time, due to the assumed formation of the grain boundary phase added to the Y2O3.

Fabrication of porous filter material for the integrated gasification combined cycle

The porous ceramic filter material is the most effective filter materials in the integrated gasification combined cycle. The porous silicon nitride due to its higher mechanical strength, as well as good corrosion resistance, is considered as a promising material in the integrated gasification combined cycle. In this paper, porous silicon nitride ceramics with high porosity and uniform pore structure was prepared by carbothermal reduction reaction between low-cost diatomite and carbon black. The influence of the type of sintering additives and the size of diatomite particles was investigated. Microstructures and mechanical properties of porous silicon nitride ceramics were studied by XRD, SEM and three-point bending measurement. XRD analysis showed that β-Si3N4 phase, α-Si3N4 phase, Sialon phase Si5AlON7 and glass phase Y2Si3O3N4 were detected. SEM analysis showed that porous silicon nitride ceramics was composed of rod-like (β-Si3N4 grains and uniform pores.

Effective Thermal Conductivity for Low Density Silicon Nitride Porous Ceramics

Thermal conduction in porous materials generally includes four heat transfer forms, i.e. heat transfer via the gas phase, heat conduction along the solid matrix/skeleton, thermal radiation and gas-solid coupled heat transfer. The size effects on the gas and solid thermal conductivities are introduced to predict the effective thermal conductivity of the silicon nitride porous ceramic at room temperature, and the predictions from the random model present good agreements with the experimental data. The effect of the porosity on the effective thermal conductivity of the low density silicon nitride porous ceramic are studied as well, and the low density porous ceramics can achieve a low thermal conductivity in the range of 0.08-1.2 W/m K for a good performance of thermal insulation.

A facile strategy for fabricating self-sealing dense coating grown in-situ on porous silicon nitride ceramics

This present work explores initially the feasibility of producing in-situ surface oxidized coating on porous silicon nitride (Si3N4) ceramics. Theoretical prediction identifies the applied conditions of self-sealing strategy and oxidation time required to form dense coating. Experimentally, the porous Si3N4 ceramics with different pore structures were selected to fabricate in-situ oxidized coatings. The phase compositions, microstructures and mechanical properties of the porous Si3N4 ceramics were investigated before and after oxidation. The results show that flat and dense coatings are prevailed in all samples, which consist of amorphous SiO2 and its precipitates besides dominant Si3N4 phase. The strengthened substrate and strengthening effect of coating are the essential mechanisms associated with the improved mechanical properties. Self-sealing method seems to offer an inexpensive and efficient route to prepare coating on porous Si3N4 ceramics.

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

AbstractPorous 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.

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.

Preparation and properties of porous silicon nitride ceramics with polymethyl methacrylate as pore-forming agent

Porous Si3N4 ceramics were prepared by adding polymethyl methacrylate (PMMA) as pore-forming agent, combined with cold isostatic pressing and pressureless sintering at nitrogen atmosphere. The composition of phases, microstructure, density and porosity, flexural strength, dielectric properties of samples with different content of PMMA were studied in this paper. With an increase of PMMA content, the density and flexural strength of samples were decreased, while the porosity and dielectric properties were improved. Two types of pores in the sample were found in the experiment, the micropores formed by β-Si3N4 bridging, while the macropores formed by the pore-forming agent, and the macropores were the main factor for reducing the flexural strength and improving the dielectric properties by increasing porosity. When the PMMA content was 15 wt%, the comprehensive performance of sample was optimal, the porosity of 33.49%, the flexural strength of 103.62 MPa, the dielectric constant of 5.82 and dielectric loss tangent of 6.0 × 10−3.

Preparation of porous silicon nitride ceramics by freeze drying

Silicon nitride ceramics have excellent mechanical properties, good thermal and chemical stability, low dielectric constant and dielectric loss. It is an ideal choice for high-speed aircraft electromagnetic wave transmission materials. In this paper, the preparation methods of porous silicon nitride materials with controllable dielectric constant and pore structure were systematically studied. Porous silicon nitride materials with controllable pore structure and directional growth were prepared by freeze-drying process and adjusting the solid content of slurry.

Preparation of porous silicon nitride ceramics by microwave sintering and its performance evaluation

In this paper, the preparation methods of porous silicon nitride materials with controllable dielectric constant and pore structure were systematically studied. By using microwave sintering technology, porous silicon nitride materials with high closed pore ratio were prepared by adjusting the content of sintering additives and sintering process parameters, and controlling the grain boundary phase and pore structure and size. The effects of sintering conditions on the total porosity, closed porosity, pore structure and dielectric properties of materials were systematically studied.

Pore structure, porosity and compressive strength of highly porous reaction-bonded silicon nitride ceramics with various grain morphologies

Complex characteristics of the pores and properties of porous reaction-bonded Si3N4 have been investigated and correlated with the microstructure of Si3N4 grains. Porous ceramics with porosities of ≤ 75 vol% and α-Si3N4 matte grains (α/β phase ratio of 1.5) or α-Si3N4 whiskers (α/β phase ratio of 0.36) were prepared by in situ nitridation of silicon powder. To obtain various microstructures by α → β-phase transformation and grain morphology modification, samples were heat-treated at 1700 °C while embedded in a Si3N4 powder bed containing MgO. By the growth of α-matte or β-Si3N4 grains on the pore walls, highly interconnected structures with spherical cavities and unimodal pore size distributions resulted with d50 ≈ 8.8 µm and ≈ 6.5 µm, respectively. In contrast, α-whiskers grew inside the pore cavities; thus, complex and irregular inter-particle pores appeared which generated an extra peak near d50 ≈ 1 µm forming a bimodal pore size distribution. Compared to the α-matte grains, α-whiskers densified upon heat treatment and produced a large drop in porosity, which resulted in a structure with less interconnectivity. As a consequence of growth of fine β-rods, pore walls became relatively smooth and whisker free; thus, inter-cluster channels were modified to spherical cavities with d50 ≈ 3.7 µm. Samples exhibiting networked whiskers and fine pores or low porosity demonstrated higher compressive strength than the interconnected structures with spherical cavities.

Dielectric Properties of Porous Reaction-Bonded Silicon Nitride with Different Additives

The influence of additives on the dielectric properties of porous silicon nitride ceramics with 0.6-mm apertures and 56% porosity prepared by reaction sintering has been studied. The results show that high porosity and large apertures in porous silicon nitride ceramics lower the dielectric constant and dielectric loss compared with dense silicon nitride ceramic. α-Si3N4 additive is beneficial to decrease the dielectric constant and dielectric loss. Addition of Y2O3 and La2O3 powders may result in formation of Y-Si-O-N and La-Si-O-N phases, and enhance the volume fraction of β-Si3N4 phase. The transition of Y3+ and La3+ weakly associated ions and vacancies in the Y-Si-O-N and La-Si-O-N systems leads to higher dielectric constant and dielectric loss.

Gas permeation performance of porous silicon nitride ceramics with controllable pore structures

Porous Si3N4 ceramics with controllable pore structures were prepared by polymethyl methacrylate (PMMA) as the pore agent combined with cold isostatic pressing (CIP) method. The pore structures of specimens were tailored via the changes in the content and particle size of PMMA. The higher the addition and particle size of PMMA, the larger the open porosity and pore size respectively. The increased Darcian and non-Darcian permeability coefficients are obtained as the open porosity and pore size of pore structures increase. The ratio of Darcian and non-Darcian permeability coefficient representing the individual contribution of viscous and inertial resistance during permeation process, that is strongly dependent on the pore size and open porosity, but there is distinct relationship for small and larger pores. Moreover, the comparison of the theoretical permeability coefficients with the experimental permeability constants of porous materials is presented in this work.