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.     

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 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 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.     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.     

Enhanced bending strength and microwave dielectric properties of Li2MgTi3O8 ceramics by adding Si3N4 reinforcing phase

In the 5G era, the dielectric materials used in microwave electronic components must have not only have good microwave dielectric characteristics but also excellent structural characteristics. Li₂MgTi₃O₈ (LMT) ceramics have excellent microwave dielectric properties; however, their low bending strength limits their further applications in the 5G era. In this work, the dielectric properties and bending strength of LMT ceramics were optimized by the addition of Si₃N₄ reinforcing phase using a solid-phase method, and the effects of Si₃N₄ addition on the sintering properties, microscopic structure, crystalline phase, dielectric properties and bending strength of ceramics were investigated. The X-ray diffraction pattern indicates that all ceramics exhibit spinel structure. Combined with the phenomenon of grain reduction in the SEM graph, it indicates that the addition of Si₃N₄ can inhibit the grain growth and achieve the purpose of fine-grain strengthening. The dispersion enhancement of second phase particles is also one of the reasons for the increase of bending strength. LMT ceramics doped with 0.5 wt% Si₃N₄ exhibited the maximum bending strength after sintering at 1050 °C for 4 h, which was 76.97% higher than that of pure LMT ceramics. In addition, the ceramics exhibited outstanding dielectric properties: a dielectric constant of 23.20, quality factor of 49344 GHz, and temperature coefficient of ?5.90 ppm/°C. The high bending strength and good microwave dielectric properties indicate that Si₃N₄-added LMT ceramics can be effectively applied in the 5G era.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ 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.     

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.     

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.     

Effect of the molecular weight and concentration of PEG solution on microstructure and mechanical properties of Si3N4 ceramics fabricated by gel-casting

Gel-casting is a promising preparation technology of Si₃N₄ structural ceramics. The process involves drying of the “green” gel-cast parts before densification. And the drying of green gel-cast bodies is an important step in the gel-casting manufacturing process. In this work, the Si₃N₄ gel-cast green bodies were dried in polyethylene glycol (PEG) solution with the purpose of obtaining Si₃N₄ ceramics with good mechanical properties. The effect of the molecular weight and concentration of PEG solution on drying rate, microstructure and mechanical properties of Si₃N₄ ceramics was studied. The results indicated that with the increase of molecular weight of PEG, the drying rate increased obviously and the structure became more uniform and dense when the concentration of solution was 20?wt%. The Si₃N₄ ceramics after sintering have the excellent flexural strength (662.6?MPa) under PEG600 drying condition. Furthermore, the concentration of PEG600 solution had a positive effect on drying and sintering of the green body. Therefore, the bending strength reached 871.1?MPa under 65?wt% PEG 600 solution drying condition. Overall, the drying process (drying in 65?wt% PEG600 solution) promotes the efficiency and quality of drying of Si₃N₄ gel-cast green bodies, which is beneficial for the subsequent drying and sintering process.     

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.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Low-temperature densification by plasma activated sintering of Mg2Si-added Si3N4

In this study, highly dense Si₃N₄ ceramics with excellent mechanical properties were fabricated using Mg₂Si as a sintering additive by plasma-activated sintering at 1400–1500 °C. The effects of the sintering temperature and content of Mg₂Si on the densification, microstructures, and mechanical properties of the Si₃N₄ ceramics were investigated. The mechanism responsible for the effect of Mg₂Si in the promotion of the sinterability of Si₃N₄ is discussed. The results showed that the addition of Mg₂Si could effectively remove the oxide layers on the Si₃N₄ particles and form a liquid phase during the sintering, promoting the densification and phase transition of the Si₃N₄ ceramics. The Si₃N₄ ceramic sintered at 1450 °C with 6.0 wt% of Mg₂Si exhibited the maximum strength of 1050 MPa.     

Effects of Gd2O3 and MgSiN2 sintering additives on the thermal conductivity and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics were prepared by gas pressure sintering at 1900°C for 12 h under a nitrogen pressure of 1 MPa using Gd₂O₃ and MgSiN₂ as sintering additives. The effects of the Gd₂O₃/MgSiN₂ ratio on the densification, microstructure, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were systematically investigated. It was found that a low Gd₂O₃/MgSiN₂ ratio facilitated the thermal diffusivity of Si₃N₄ ceramics while a high Gd₂O₃/MgSiN₂ ratio benefited the densification and mechanical properties. When the Gd₂O₃/MgSiN₂ ratio was 1:1, Si₃N₄ ceramics obtained an obvious exaggerated bimodal microstructure and the optimal properties. The thermal conductivity, flexural strength, and fracture toughness were 124 W·m⁻¹·k⁻¹, 648 MPa, and 9.12 MPa·m^1/2, respectively. Comparing with the results in the literature, it was shown that Gd₂O₃-MgSiN₂ was an effective additives system for obtaining Si₃N₄ ceramics with high thermal conductivity and superior mechanical properties.     

Low‐Temperature Preparation of β‐Si3N4 Porous Ceramics with a Small Amount of Li2O–Y2O3

Porous β‐Si₃N₄ ceramics are sintered at 1600°C in N₂ and postheat treated at 1500°C under vacuum using Li₂O and Y₂O₃ as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li₂O. The addition of Y₂O₃ is advantageous for the formation of high aspect ratio β‐Si₃N₄ grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β‐Si₃N₄ porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li₂O and 0.33 wt% Y₂O₃, and it is suitable for high‐temperature applications.     

Crystal structure and dielectric properties of barium titanate–kaolinite composites

Barium Titanate–Kaolinite composites were prepared systematically by conventional solid-state method. The crystal structure and dielectric properties of samples were investigated by XRD and dielectric measurements, respectively. XRD results show that new phase BaAl₂Si₂O₈ was formed as kaolinite added into BaTiO₃. The 10 wt% kaolinite addition led to a considerable reduction in sintering temperature and a strong densification. The dielectric constant of BaTiO₃–Kaolinite composites tended to be stable with increasing of kaolinite content.     

Microstructure and Mechanical Properties of Lu2O3-Doped Porous Silicon Nitride Ceramics Using Phenolic Resin as Pore-Forming Agent

The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si₃N₄ ceramics with controlled microstructure. Lu₂O₃ and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu₂O₃-doped ceramics using 12–57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 μm. The porous ceramic infiltrated with CVI Si₃N₄ had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

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.