In-situ reaction synthesis of porous Si2N2O-Si3N4 multiphase ceramics with low dielectric constant via silica poly-hollow microspheres

In the present work, a novel and facile process has been proposed to fabricate porous Si₂N₂O-Si₃N₄ multiphase ceramics with low dielectric constant (ε_r＜4.0). Since silica poly-hollow microspheres could serve as the source of SiO₂ and the pore-forming agent, they have been introduced into Si₃N₄ slurry through the gelcasting technique. This process is benefited from the liquid phase sintering reaction between SiO₂ and Si₃N₄ with the aid of sintering additives, leading to in-situ synthesis of Si₂N₂O phase and porous structure. The content of silica poly-hollow microspheres has great influence on the properties of the final products. It indicates that Si₂N₂O phase would become the major phase when the content of silica poly-hollow microspheres was above 25 wt%. Furthermore, the micromorphology results reveal that the content of pores with many smaller aggregate microspheres increases as microspheres amount rises. As a result, along with the addition of silica poly-hollow microspheres, the bulk density decreases to 1.32±0.01 g/cm³, and open porosity ranges from 28.4±0.4% to 52.0±0.5%. Porous Si₂N₂O-Si₃N₄ multiphase ceramics prepared with 25 wt% silica poly-hollow microspheres addition possess flexural strength of 42.3±3.8 MPa, low dielectric constant of 3.31 and loss tangent of 1.93×10⁻³. It turns out to be an effective method to fabricate porous Si₂N₂O-Si₃N₄ composites with excellent mechanical and dielectric properties, which could be applied to radome materials.     

Fabrication and characterization of Si3N4 whisker-reinforced SiO2 ceramic for radome materials

Fused silica ceramic has become one of the most widely used radome materials in the world since the 1970s. But its poor mechanical properties restricted its application to some extent. To improve the mechanical properties of the fused silica ceramic and keep its characteristic for radome materials, silicon nitride (Si₃N₄) whisker-reinforced fused silica ceramics were prepared by a slip-casting method in the work. The influence of Si₃N₄ whisker contents on the properties of the slurry was studied, indicating that the preferable pH values of the slurry were 4–6 and whisker contents were 10 wt.%. The flexural strength of as-prepared Si₃N_4w/SiO₂ ceramic was about 74.35 MPa, exhibiting an increase of 7.75% over that of the pure silica sample. Its dielectric constant in the range from 8 to 12 GHz and tanδ under 10 GHz were, respectively, 3.37 and .0011. It is of great interest to find that Si₃N_4w/SiO₂ has excellent oxidization resistance and its mass maintains even at 1270°C.     

3D printing Si3N4-bonded SiC refractories fabricated using colloidal films-containing slurries based on non-spherical SiC and Si powders

Stable slurries for Si₃N₄-bonded SiC refractories for direct ink writing (DIW) were successfully prepared from a mixture of non-spherical silicon carbide (SiC) and silicon (Si) powders with an average particle size of D₅₀ = 41.98 μm. The rheological properties and printability of slurries prepared using polyvinyl alcohol (PVA; 4–16 wt %) or hydroxypropyl methylcellulose (HPMC, 0.5–2 wt.%) were investigated with the effect of sintering temperature on the mechanical performance, phase, and microstructure of Si₃N₄-bonded SiC refractory products. The results indicated that slurries prepared with the HPMC solution showed better printability than those prepared with the PVA solution because colloidal films formed by HPMC in slurries play a role in encasing particles, preventing solid−liquid separation and contributing to plasticity and lubrication, which guarantees the smooth extrusion and homogeneity of slurries. The successful printing of SiC–Si slurries is not only related to proper viscosity, yield value, and shear thinning characteristics but it is also crucial for maintaining the homogeneity of slurries under extrusion pressure. Optimal SiC–Si slurries containing 52 vol % SiC–Si and 1.5 wt% HPMC exhibited proper viscosity, shear thinning, and homogeneity characteristics during printing. The obtained specimens achieved the best printing performance with height and section retention rates of 98.7% and 97.6%, respectively. When sintered at 1450 °C, Si₃N₄ fibres grow further and reach a diameter of 342.5 nm, the nitriding rate is 92.43%, the fibres tend to form a full network structure, and the mechanical properties of Si₃N₄-bonded SiC products are the best.     

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.     

Mechanical and dielectric properties of direct ink writing Si2N2O composites

Si₂N₂O composites were achieved via direct ink writing technique and pressureless sintering. The design and optimization of inks and the effect of mass ratio of Si₃N₄ and SiO₂ on the phase composition, microstructure, mechanical properties and dielectric properties of composites were systematically investigated. The inks exhibit superior stability and printability. Increasing SiO₂ content facilitated densification and enhancement of mechanical property. The generation of β-Si₃N₄ and Si₂N₂O and the nucleation of cristobalite were restrained by high content SiO₂. The ceramic composites with the flexural strengths from 74.1MPa to 104.9MPa and low dielectric constant (≤4.68) were fabricated. This strategy provides a systematic reference for synthesis of high-performance porous Si₂N₂O composites based on the DIW.     

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.     

Investigation on properties of Al2O3–SiO2 complex shaped refractory fabricated by layered extrusion forming

As one of the 3D printing methods, layered extrusion forming (LEF) has distinct advantages to form complex configuration ceramics directly. The feasibility of using LEF to make refractory products with complex shapes was explored by this work, using water-based Al₂O₃–SiO₂ ceramic slurry and specially equipped device. By measuring rheological parameters, the effects of binder addition, dispersant addition and volume proportion of the solid portion composed of α-Al₂O₃ ultrafine powder (92 wt%) and silica fume (8 wt%) on rheological behavior of the slurry were investigated. The green body specimens prepared by the LEF were fired at 1400°C–1600 °C for 3h. The influence of firing temperature on phase composition, microstructure, sintering degree and comprehensive properties of the specimens was investigated. At 2.5 wt% addition of aluminum dihydrogen phosphate as binder, 0.2 wt% addition of sodium hexametaphosphate as dispersant and with solid portion between 56 vol% and 58 vol%, required pseudoplastic behavior of the slurry can be achieved, suitable for the LEF. With the increase of heating temperature, mullitization by the reaction between the α-Al₂O₃ ultrafine powder and silica fume becomes stronger and sintering gets enhanced, leading to improved comprehensive properties of the specimens. Fired at 1600 °C, properties in terms of bulk density 3.03g/cm³, cold compressive strength 190.5 MPa and refractoriness under load 1598 °C are achieved. Crucible slag test shows a good resistance to the glass melt corrosion. Good feasibility of fabricating some complex shaped refractory products by LEF as a novel forming approach has been confirmed by the present work.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength,oxidation resistance,and dielectric properties

The high-temperature mechanical and dielectric properties of Si₂N₂O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si₂N₂O was prepared at 1700 °C by hot-pressing oxidized amorphous Si₃N₄ powder without sintering additives. A homogeneous network with short-range order and a SiN₃O structure was formed in the oxidized amorphous Si₃N₄ powder during the hot-pressing process. Si₂N₂O crystals preferentially nucleated at positions within the SiN₃O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si₂N₂O and some residual amorphous phases were obtained. The as-prepared Si₂N₂O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10⁻³ at 1000 °C.     

Rheology of organics-free aqueous ceramic suspensions for additive manufacturing of dense silicon nitride ceramics

A dense structure of silicon nitride ceramic was fabricated by direct ink writing using aqueous suspensions. The rheology of the suspensions was carefully tailored by the particle chemical state and the ion concentrations, without the use of any organics. The surface chemical states of the Si₃N₄ powder were modified by calcination at 600 °C at various times. The minimum absolute value of zeta potential, calculated by the DLVO theory, was 19 mV at which the interaction was attractive interaction. Suspensions with solid volume fraction of 25–40 vol% exhibited pseudoplastic behavior with yield stresses ≥55 Pa and equilibrium storage moduli ≥ 10⁴ Pa. These enabled suspensions to flow through the nozzle and retain the shape of the printed parts. The flexural strength of Si₃N₄ ceramics produced using a 40 vol% suspension was 348 MPa. This strategy provides a simple process for fabricating high-performance ceramics based on the DIW.     

Mechanical and dielectric properties of porous Si2N2O–Si3N4 in situ composites

Mechanical and dielectric properties of porous Si₂N₂O–Si₃N₄ in situ composites fabricated for use as radome by gel-casting process were investigated. The flexural strength of the Si₂N₂O–Si₃N₄ ceramics is 230.46 ± 13.24 MPa, the complex permittivity of the composites varies from 4.34 to 4.59 and the dissipation factor varies from 0.00053 to 0.00092 from room temperature to elevated temperature (1150 °C) at the X-band. In the porous regions, some Si₂N₂O fibers (50–100 nm in diameter) are observed which may improve the materials properties.     

A modified SHS method for Si2N2O elaboration

Silicon oxynitride (Si₂N₂O) has been prepared from a compacted mixture of silicon and silica under high nitrogen gas using the high-temperature synthesis (SHS) process. The aim of this work was to optimise this reaction by using a preliminary treatment of the raw materials before the combustion. The SHS samples were characterised by using X-ray diffraction and SEM analyses. Rietveld refinement was performed on recorded diffractograms and a very high formation rate for Si₂N₂O (more than 97 wt%) at low nitrogen pressure was revealed. By studying the influence of the compacting pressure, the nitrogen pressure and the reactant mixture, a mechanism of Si₂N₂O formation has been suggested.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.     

ZrSi2–MgO as novel additives for high thermal conductivity of β-Si3N4 ceramics

A novel ZrSi₂–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi₂ in the range of 0-7 mol%, the effect of ZrSi₂ addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi₂–MgO were effective for the densification of Si₃N₄ ceramics. XRD observations demonstrated that ZrSi₂ reacted with native silica on the Si₃N₄ surface to generate ZrO₂ and β-Si₃N₄ grains. TEM and in situ dilatometry confirmed that the as formed ZrO₂ collaborated with MgO and Si₃N₄ to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si₃N₄. Abnormal grain growth was promoted by in situ generated β-Si₃N₄ grains. Consequently, compared to ZrO₂-doped materials, the addition of ZrSi₂ led to enlarged grains, extremely thin grain boundary film and high contiguity of Si₃N₄–Si₃N₄ grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)⁻¹ when ZrO₂ was substituted by ZrSi₂.     

Microstructure,high-temperature mechanical and dielectric properties of novel Si3N4 f/SiNO wave-transparent composites

Continuous Si₃N₄ fiber reinforced SiNO matrix composites (Si₃N_4 f/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si₃N_4 f/SiNO composites were investigated in detail. The as-fabricated Si₃N_4 f/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si₃N_4 f/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 ℃ for the surface reactions between the SiNO matrix and Si₃N₄ fibers. After heat-treated at 1550 ℃, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si₃N₄ fibers at this temperature. Si₃N_4 f/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7–18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10^?3. The dielectric constants and dielectric loss of Si₃N_4 f/SiNO composites are relatively stable from RT to 800 ℃. The as-fabricated Si₃N_4 f/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.     

Robust super-hydrophobic inorganic nanoparticles modified layered structure Si2N2O membrane for membrane distillation

Robust super-hydrophobic ceramic membranes consisting of layered structure Si₂N₂O grains and organosilane-derived inorganic nanoparticles were successfully fabricated and employed for membrane distillation. First, phase inversion and sintering method were used to prepare porous Si₂N₂O membranes. The slurry composition and sintering temperature were optimized to obtain a pure phase Si₂N₂O membrane with high bending strength, tailored average pore size, and high permeability. Then, the Si₂N₂O membranes were modified with organosilane-derived inorganic nanoparticles through ammonolysis and pyrolysis reactions. Due to the micro and nano-hierarchical rough structures and the presence of -Si-CH₃ groups, the membranes showed super-hydrophobicity with a water contact angle of 152 ± 1°. Finally, the membranes were applied to desalinate seawater by sweeping gas membrane distillation. A stable water flux of 76 ± 0.9 L/(m² day) with a salt rejection of > 99% was recorded during 30 h distillation test at 75 °C, demonstrating the stability and durability of the membranes.     

Effect of SiO2 content on the microstructure,mechanical and dielectric properties of Si3N4 ceramics

This study investigated the effect of SiO₂ content in the Y₂O₃–Al₂O₃ additive system on the microstructure, mechanical and dielectric properties of silicon nitride (Si₃N₄) ceramics. The total sintering additive content was fixed at 8 wt% and the amount of SiO₂ was varied from 0 to 7 wt%. The crystalline phases of the samples were determined by X-ray diffraction analysis. Complete α-to-β transformation of the Si₃N₄ occurred during sintering of all of the samples, which indicated that the phase transformation was unaffected by the SiO₂ content. However, the microstructures showed that the aspect ratio of the β-Si₃N₄ grains decreased and the residual porosity increased with increasing SiO₂ content. Additionally, the flexural strength and the dielectric constant decreased with increasing SiO₂ content because of the residual porosity and the formation of the Si₂N₂O phase via a reaction of SiO₂ with Si₃N₄.