Preparation and properties of silicon nitride-phosphate composites for application in microwave furnace

Si₃N₄ ceramic is one of the most promising microwave metallurgy furnace materials because of the outstanding mechanical, relatively low dielectric properties and excellent thermal shock resistance. However, the difficult sintering of Si₃N₄ ceramics extremely restrict their large-scale application in the field of refractories for microwave metallurgy. In this work, silicon nitride-phosphate ceramics were fabricated by introducing aluminum phosphate or chromium phosphate aluminum into Si₃N₄ ceramics at 1500 °C. The effect of the amount of aluminum phosphate and chromium phosphate aluminum on sintering performance and dielectric properties was investigated. The results showed that the addition of aluminum phosphate or chromium phosphate aluminum could promote sintering, and the mechanical and dielectric properties of Si₃N₄ ceramics were efficiently improved. The Si₃N₄-aluminum phosphate composites exhibited better sintering performance (higher density and mechanical property) than that of Si₃N₄-chromium phosphate aluminum composites. Meanwhile, the dielectric constant and dielectric loss of Si₃N₄-chromium phosphate aluminum composites were better than Si₃N₄-chromium phosphate aluminum composites.     

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.     

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.     

Preparation of high-strength Si3N4 antenna window using selective laser sintering

In this study, a high-strength silicon nitride (Si₃N₄) antenna window was successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding before final sintering. The effects of CIP after debinding and sintering aids on the bulk density, total porosity, bending strength and microstructure of Si₃N₄ ceramics were examined. The results show that the bending strength of SLS Si₃N₄ ceramics can be greatly improved by adding sintering aids between Si₃N₄ granules and by CIP after debinding. Optimal performance of ceramics is obtained by CIP after debinding and the use of inter-granule sintering aids. The porosity, bulk density, and bending strength are 18.7%, 3.11 g/cm³, and 685 MPa, respectively. Eliminating the pores by the CIP after debinding and by inter-granule sintering aids promotes the growth of rod-like β-Si₃N₄, which lock with each other contribute to the strengthening of Si₃N₄ ceramics.     

The effect of BaTiO3 addition on the dielectric constant of Si3N4 ceramics

Si₃N₄ ceramics with different BaTiO₃ contents have been fabricated by pressureless sintering in a N₂ atmosphere at 1680°C for 2 h. Al₂O₃ and Nd₂O₃ were used as sintering additives to promote the densification of Si₃N₄ ceramics. The effect of BaTiO₃ addition on the densification, mechanical properties, phase compositions, microstructure, and dielectric properties of Si₃N₄ ceramics was investigated. The relative density and flexural strength of Si₃N₄ ceramics increased with the addition of BaTiO₃ up to 15 wt% and then decreased, while the dielectric constant increased continuously as the BaTiO₃ contents increased. The dielectric constant of Si₃N₄ ceramics can be tailored in the range from 8.42 to 12.96 by the addition of 5 wt%‐20 wt% BaTiO₃. Meanwhile, these Si₃N₄ ceramics all had flexural strength higher than 500 MPa.     

Dielectric and mechanical properties of porous Si3N4 ceramics prepared via low temperature sintering

Borophosphosilicate bonded porous silicon nitride (Si₃N₄) ceramics were fabricated in air using a conventional ceramic process. The porous Si₃N₄ ceramics sintered at 1000–1200 °C shows a relatively high flexural strength and good dielectric properties. The influence of the sintering temperature and contents of additives on the flexural strength and dielectric properties of porous Si₃N₄ ceramics were investigated. Porous Si₃N₄ ceramics with a porosity of 30–55%, flexural strength of 40–130 MPa, as well as low dielectric constant of 3.5–4.6 were obtained.     

Effect of magnesium titanate content on microstructures,mechanical performances and dielectric properties of Si3N4-based composite ceramics

Silicon nitride-based composite ceramics with different contents of magnesium titanate have been fabricated via gas pressure sintering method. The phase compositions, microstructure, mechanical performances and dielectric properties of the composite ceramics were investigated. The density of the Si₃N₄-based composite ceramics firstly increased with additive of magnesium titanate powder up to 5 wt% and then gently decreased, and the mechanical properties firstly increased and then declined. Besides, the dielectric constant and dielectric loss increased with the increase of magnesium titanate contents. For the Si₃N₄-based composite ceramics with 5 wt% magnesium titanate powders, the flexural strength, elastic modulus, dielectric constant and dielectric loss reached 451 MPa, 274 GPa, 7.65, 0.0056, respectively. These results suggested that the magnesium titanate was beneficial for the improvement of mechanical performances and dielectric constant of Si₃N₄-based composite ceramics.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

A Novel Si3N4–SiC Ceramic Used for Volumetric Receivers

Si₃N₄–SiC composite ceramics used for volumetric receivers were fabricated by pressureless sintering of micrometer SiC, Si₃N₄, andalusite, and other minor additions powders. Mechanical, thermal expansion, thermal conductivity, and thermal shock resistance properties were tested at different sintering temperatures. The best sintering temperature of optimum formula A2 is 1360°C, and the bending strength reaches 79.60 Mpa. And moreover, its thermal expansion coefficient is 6.401 × 10^?6/°C, thermal conductivity is 7.83 W/(m K), and no crack occurs even subjected to 30 cycles thermal shock with a bending strength increase rate of 4.72%. X‐ray diffraction results show that the phase constituents of the sintered products mainly consist of SiC, Si₃N₄, mullite, and quartz. Microstructure that is most appropriate and exhibits maximal thermal shock resistance was detected using SEM. The porosity of Si₃N₄–SiC ceramic foam prepared from formula A2 is 95%, which provides a rapid and steady action for the receiver. The evaluation of the present foam shows that Si₃N₄–SiC ceramic composite is a good candidate for volumetric receivers.     

Preparation and Performance Study of Sialon‐Si3N4‐SiC Composite Ceramics for Concentrated Solar Power

For lowering the sintering temperature of silicon carbide ceramics used for solar thermal energy storage technology, O'‐Sialon and silicon nitride were employed as composite phases to construct Sialon‐Si₃N₄‐SiC composite ceramics. The composite ceramics were synthesized using SiC, Si₃N₄, quartz, and different alumina sources as starting materials with noncontact graphite‐buried sintering method. Influences of alumina sources on the physical properties and thermal shock resistance of the composites were studied. The results revealed that the employment of O'‐Sialon and silicon nitride could decrease the sintering temperature greatly to 1540°C. The optimum formula G2 prepared from mullite as alumina source achieved the best performances: 66.7 MPa of bending strength, 10.0 W/(m·K) of thermal conductivity. The composition parameter x = 0.4 of O'‐Sialon decreased to 0.04 after 30 cycles thermal shock, and the bending strength increased with a rate of 11.0% due to the increase of O'‐Sialon grain size, and the optimization of microstructure caused by the transformation of O'‐Sialon grains and densification within the samples. The good thermal shock resistance makes the composites suitable for the use as thermal storage materials of concentrated solar power generation.     

Prediction of bending strength of Si3N4 using machine learning

The bending strength of silicon nitride (Si₃N₄) plays a vital role in its application and is influenced by various process factors. Current experimental methods for investigating Si₃N₄ ceramics exhibiting low efficiency and high cost are incapable of systematically analysing the effect of process factors on the bending strength of Si₃N₄ ceramics and quantitatively predicting the optimum process parameters. In this study, machine learning (ML) approaches based on extreme gradient boosting (XGBoost) were applied to predict and analyse the bending strength of Si₃N₄ ceramics. Because the classification model of XGBoost is easily interpretable, the factors affecting the bending strength could be quantitatively evaluated. The current model can provide a suitable order of adding sintering additives to obtain excellent bending strength in Si₃N₄ ceramics. Although this study focuses on the bending strength of Si₃N₄ ceramics, the new approach reported herein is applicable for the in silico design and analysis of other ceramic materials.     

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.     

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.     

Effect of TiO2 addition on the microstructures,mechanical and dielectric properties of porous Si3N4-based ceramics

Porous Si₃N₄-based ceramics with different TiO₂ contents were prepared by gas pressure sintering method. The effects of TiO₂ addition ranging from 0 to 25?wt-% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. The addition of TiO₂ significantly promoted the density which increased from 1.64 to about 2.3?g?cm^?3. The mechanical properties of porous Si₃N₄-based ceramics with TiO₂ addition decreased first and then increased with the increase of TiO₂ content, and the flexural strength and elastic modulus are more than 167.4?MPa and 72.8?GPa, respectively, which were higher than that of the Si₃N₄ ceramic without TiO₂ addition. With the increase of TiO₂ content, both the dielectric constant and dielectric loss increased, and the dielectric constant enhanced obviously. These results suggested that the TiO₂ was beneficial for the improvement of mechanical properties and dielectric constant of porous Si₃N₄-based ceramics.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

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.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy

In this study, the preparation of Si₃N₄ ceramics with high mechanical reliability is investigated. The influences of several processing steps on the bending strength and the Weibull modulus are reported including: (i) coating of the Si₃N₄ powder with its sintering aids, (ii) oxidation of the coated powder, (iii) cold isostatic pressing, (iv) gelcasting of the green bodies and (v) gas pressure sintering. It was found that all the aforementioned steps contribute to improvements of strength and reliability of Si₃N₄ ceramics. Via an optimised processing strategy, Si₃N₄ ceramics with a bending strength and a Weibull modulus as high as 944.7±29.5 MPa and 33.9, respectively, could be prepared. Additionally, it was also found that surface modifications, i.e. coating and oxidation of Si₃N₄ powder, increased the rheological properties of the powder suspension in aqueous media, which is favourable for in situ colloidal forming such as gelcasting.     

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.