β-Si3N4 Seeds Research Articles

Improved mechanical properties of porous Si3N4 ceramics strengthened by β-Si3N4 seeds fabricated by vat photopolymerization

Porous Si3N4 ceramics are widely applied in aerospace and mechanical fields owing to their excellent properties. Furthermore, vat photopolymerization (VPP) technology can fabricate Si3N4 components with complicated structures and high precision, but its layer-by-layer printing method leads to poor mechanical properties of ceramics. In this study, porous Si3N4 ceramics with a porosity of 28.41 % strengthened by directional β-Si3N4 were fabricated by combining VPP technology and seeding method. Rheological behavior and curing properties of the slurry were explored, and the influence of β-Si3N4 content on the mechanical properties of printed Si3N4 ceramics was investigated systematically. With the increase of β-Si3N4 content, the orientation degree of β-Si3N4 grains increased gradually, while fracture toughness and flexural strength of the ceramics exhibited a trend of increased first and then decreased and Vickers hardness gradually decreased. As β-Si3N4 content increased to 5 wt%, the fracture toughness and flexural strength of porous Si3N4 ceramics were improved from 4.23 MPa m1/2 and 214.7 MPa–5.65 MPa m1/2 and 272.0 MPa, respectively. Therefore, this work indicates that vat photopolymerization combined with seeding method is a promising approach for the fabrication of porous Si3N4 ceramics with high performance and complex structures.

Role of β-Si3N4 seeds in microstructure development and properties of silicon nitride ceramics: a comprehensive review

Role of β-Si3N4 seeds in microstructure development and properties of silicon nitride ceramics: a comprehensive review

Lightweight and high-strength textured fibrous Si3N4 3D scaffold seeded with β-Si3N4 particles prepared via freeze casting

Highly porous Si3N4 ceramics with unidirectionally aligned pore channels are gaining significant attention across various fields due to their outstanding functional capabilities. However, achieving high strength in such unidirectional highly porous Si3N4 ceramics remains challenging. Herein, we design and fabricate a novel β-Si3N4 scaffold composed of directionally aligned lamellar walls with a textured microstructure by directionally freeze casting of α-Si3N4 suspensions with fine elongated β-Si3N4 seeds addition, followed by liquid phase sintering. During the sintering, the scaffold exhibited anisotropic shrinkage, and fibrous β-Si3N4 grains were synthesized through epitaxial growth on the seeds preferentially oriented or the nuclei originated from α-Si3N4 powders, resulting in the grains aligned parallel to lamellar walls and bridged the walls. Seed additions of 7 to 15 wt% were beneficial for the optimized distribution of the two types of β-Si3N4 grains, which contributed to the excellent resistance to bucking-induced fracture for the walls. Compared with other unidirectional porous Si3N4 prepared by freeze-casting in the literature, the Si3N4 scaffold exhibited outstanding compressive strength, ranging from 2.8 to 22.0 MPa, as the porosity decreased from 94.4 % to 88.0 % and the density increased from 175 to 365 mg/cm3. The lightweight and strong Si3N4 scaffolds are promising candidates for engineering applications in harsh environments.

Mechanical and thermal properties of Si3N4 ceramics prepared by gelcasting using high-solid-loading slurries

In this study, according to the Dinger model, aqueous Si3N4 slurries with a solid loading of 63 vol% and a viscosity lower than 1.0 Pa s were successfully prepared using two-grade Si3N4 powders with D50 = 7.2 μm and 9.4 μm as starting materials. These resulting slurries exhibited good rheological properties and suspension stability. Notably, the presence of β-Si3N4 seeds existed in the starting materials effectively reduced the nucleation barrier by transforming homogeneous nucleation into heterogeneous nucleation. Consequently, it facilitated the reconstructive phase transformation of α-Si3N4 to β-Si3N4, promoted the anisotropic growth of β-Si3N4 nucleation, and resulted in the final formation of the bimodal microstructure. This unique microstructure significantly enhanced the mechanical properties of Si3N4 ceramics, resulting in a high flexural strength of 1011.4 MPa and a fracture toughness of 14.8 MPa m1/2. These excellent properties were achieved through the combination of gelcasting and gas pressure sintering. Simultaneously, the Si3N4 ceramics exhibited a low thermal conductivity of 47.7 W m−1 K−1, mainly owing to the low density of 95.3 %, the presence of low-thermal-conductivity silicate-based noncrystalline substances, and the possible existence of lattice oxygen in Si3N4 crystals.

Synergistic Reinforcement of Si3N4 Based Ceramics Fabricated via Multiphase Strengthening under Low Temperature and Short Holding Time.

Si3N4 ceramic as a tool material shows promising application prospects in high-speed machining fields; however, the required high mechanical properties and low-cost preparation of Si3N4 ceramic tool materials restrict its application. Herein, synergistic reinforced Si3N4 ceramic tool materials were fabricated by adding β-Si3N4 seeds, inexpensive Si3N4 whiskers and TiC particles into coarse commercial Si3N4 powder (D50 = 1.5 μm), then sintering by hot-pressing with low temperature and short holding time (1600 °C-30 min-40 MPa). The phase assemblage, microstructure evolution and toughening mechanisms were investigated. The results reveal that the sintered Si3N4 ceramics with synergistic reinforcement, compared to those with individual reinforcement, present an enhancement in relative density (from 94.92% to 97.15%), flexural strength (from 467.56 ± 36.48 to 809.10 ± 45.59 MPa), and fracture toughness (from 8.38 ± 0.19 to 10.67 ± 0.16 MPa·m1/2), as well as a fine Vickers hardness of 16.86 ± 0.19 GPa. Additionally, the various reinforcement modes of Si3N4 ceramics including intergranular fracture, crack deflection, crack bridging and whiskers extraction were observed in crack propagation, arising from the contributions of the added β-Si3N4 seeds, Si3N4 whiskers and TiC particles. This work is expected to serve as a reference for the production of ceramic cutting tools.

Preparation of graded silicon nitride ceramics with high mechanical performance using β-Si3N4 seeds

In this study, the graded silicon nitride ceramics were prepared by hot-pressing method combined with β-Si3N4 seeds. The composites with α-Si3N4 rich content were applied in the outer layers while the composites with elongated β-Si3N4 rich content were applied in the inner layers. Their mechanical properties and cutting performance of graded silicon nitride ceramics were evaluated in detail. The experimental results show that not only the level of hardness and cutting performance of silicon nitride ceramics can remain high, but also their flexural strength and fracture toughness can be significantly improved. Furthermore, the obtained graded silicon nitride ceramics were of a homogenous composition, and the interface between outer and inner layer was well bonded and free of defects such as cracks, pores, and agglomerates. These results indicate that this proposed method is a very promising and simple approach for the preparation of high mechanical performance of silicon nitride ceramics.

Effect of sintering additive composition on microstructure, thermal and mechanical property of multi-step spark plasma sintered silicon nitride

Dense silicon nitride composites were fabricated by multi-step spark plasma sintering (SPS). As the induced phase, 6 wt% Mo was added and related with the unadulterated β-Si3N4 seeds produced by the in-situ reaction during the sintering process. Si3N4 samples sintered with 7 mol% CeO2 and Yb2O3 by multi-step SPS exhibited a higher mechanical property, namely, a bending strength of 981 MPa and a fracture toughness of 8.85 MPa m1/2, while the thermal diffusivity was lower than that for Si3N4 samples sintered with 7 mol% CeO2 (27.77 mm2/s). The result was attributed primarily to the fully phase transformation and the abnormal growth of large β-Si3N4 grains, resulted from a longer time of solution-reprecipitation process during SPS.

Effects of β-Si3N4 Seeds on Microstructure and Performance of Si3N4 Ceramics in Semiconductor Package.

Among the various ceramic substrate materials, Si3N4 ceramics have demonstrated high thermal conductivity, good thermal shock resistance, and excellent corrosion resistance. As a result, they are well-suited for semiconductor substrates in high-power and harsh conditions encountered in automobiles, high-speed rail, aerospace, and wind power. In this work, Si3N4 ceramics with various ratios of α-Si3N4 and β-Si3N4 in raw powder form were prepared by spark plasma sintering (SPS) at 1650 °C for 30 min under 30 MPa. When the content of β-Si3N4 was lower than 20%, with the increase in β-Si3N4 content, the ceramic grain size changed gradually from 1.5 μm to 1 μm and finally resulted in 2 μm mixed grains. However, As the content of β-Si3N4 seed crystal increased from 20% to 50%, with the increase in β-Si3N4 content, the ceramic grain size changed gradually from 1 μm and 2 μm to 1.5 μm. Therefore, when the content of β-Si3N4 in the raw powder is 20%, the sintered ceramics exhibited a double-peak structure distribution and the best overall performance with a density of 97.5%, fracture toughness of 12.1 MPa·m1/2, and a Vickers hardness of 14.5 GPa. The results of this study are expected to provide a new way of studying the fracture toughness of silicon nitride ceramic substrates.

Preparation, microstructure, and properties of GPS silicon nitride ceramics with β‐Si 3 N 4 seeds and nanophase additives

Si3N4 ceramics with high thermal conductivity and outstanding mechanical properties were prepared by adding β-Si3N4 seeds and nanophase α-Si3N4 powders as modifiers. The introduction of β-Si3N4 seeds enhanced the growth of β-Si3N4 grains. Owing to the interlocked structure induced by the β-Si3N4 grains, the fracture toughness of Si3N4 ceramics reached a high value of 7.6 MPa·m1/2; also, the large-sized grains increased the contact possibility of Si3N4 grains, improving the thermal conductivity of Si3N4 ceramics (64 W/(m·K)). Because of the introduction of nanophase α-Si3N4, the flexural strength, fracture toughness, and thermal conductivity of the Si3N4 ceramics increased to 754 MPa, 7.2 MPa·m1/2, and 54 W/(m·K), respectively. According to the analysis of the growth kinetics of Si3N4 grains, the rapid growth of Si3N4 grains was ascribed to the reduction in the activation energy resulting from the introduction of β-Si3N4 seeds and nanophase α-Si3N4.

Self-reinforced and high-thermal conductivity silicon nitride by tailoring α-β phase ratio with pressureless multi-step sintering

A self-reinforced Si3N4 ceramic with high thermal conductivity was prepared through multi-step pressureless sintering with Y2O3 and MgO additives. The steps consisted of phase control and densification stages to provide additional crystallization sites using a partial α-β phase transformation, which also established a bimodal microstructure. The optimal temperature for the intermediate stage that exhibited the maximum flexural strength was determined by comparing the phase content and linear shrinkage. Thus, 1400 °C, which resulted in 14.4 wt% β-Si3N4, was found to be optimal. After final sintering, a flexural strength of 932 MPa and a thermal conductivity of 74 W/mK were achieved, which are 11.5% and 8.0% higher, respectively, than those obtained using conventional pressureless sintering. The rate constant and activation energy of the pressureless sintering with the Y2O3–MgO additive system were also calculated to evaluate the potential of the α-β transformation to obtain the proper quantity of β-Si3N4 seeds.

β-Si3N4 Microcrystals Prepared by Carbothermal Reduction-Nitridation of Quartz

Single phase β-Si3N4 with microcrystals was synthesized via carbothermal reduction-nitridation (CRN) of quartz and carbon coke powder as starting materials. The effects of reaction parameters, i.e., heating temperature, holding time, C/SiO2 ratio, Fe2O3 additive and β-Si3N4 seeds on the phase transformation and morphology of products were investigated and discussed. Rather than receiving a mixture of both α- and β- phases of Si3N4 in the products, we synthesized powders of β-Si3N4 single polymorph in this work. The mechanism for the CRN synthesis of β-Si3N4 from quartz and the formation mechanism of Fe3Si droplets were discussed. We also firstly reported the formation of Fe3Si Archimedean solids from a CRN process where Fe2O3 was introduced as additive. Comparing to the gear-like short columnar morphology observed in samples without β-Si3N4 seeding, the addition of β-Si3N4 seeds led to an elongated morphology of final products and much finer widths. In addition, the β-Si3N4 microcrystals exhibited a violet‒blue spectral emission range, which could be highly valuable for their future potential optoelectronic applications.

Fabrication and wear behaviors of graded Si3N4 ceramics by the combination of two-step sintering and β-Si3N4 seeds

Graded Si3N4 ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si3N4 seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si3N4 phase were detected in the outer layers, and only β-Si3N4 phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (∼21.2 GPa) was much higher than that of inner layer (∼16.1 GPa), whereas fracture toughness of outer layer (∼3.5 MPa m1/2) was much lower than that of inner layer (∼5.9 MPa m1/2), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si3N4 ceramics exhibited superior wear resistance with low wear rate.

Reaction mechanism and luminescence properties of orange-red SrAlSi4N7:Eu2+ phosphor for white-LEDs

The well-crystalline SrAlSi4N7:Eu2+ phosphor could be synthesized by the solid-state reaction method using different types of Si3N4 raw material. The corresponding reaction mechanism was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and fluorescence spectra. It was found that the structural difference between α-Si3N4 and β-Si3N4, and the rearrangement of Si3N4 microstructures at high temperature had an important influence on the formation of SrAlSi4N7 phase. Results showed that the α-β transformation of Si3N4 was included in the synthesis reaction of SrAlSi4N7. So the reaction mechanism of SrAlSi4N7:Eu2+ phosphor should be: α-Si3N4→β-Si3N4; β-Si3N4+AlN+Sr2N→SrAlSi4N7. The β-Si3N4 seeds addition were actually helpful to the grain growth of α-β transformation of Si3N4 and then increased the synthesis reaction rate of SrAlSi4N7. Pure SrAlSi4N7:Eu2+ phosphor with higher luminous efficacy and better crystal morphology was finally obtained, indicating that the as-synthesized phosphor was an attractive candidate material as an orange-red component for warm white light LEDs.

Simple fabrication of highly dispersive β-Si3N4 seeds with moderate aspect ratio

A powder mixture was prepared using α-phase-rich silicon nitride (Si3N4) as the major raw material, with 0.5wt% Yb2O3 as the additive. These powders were fired at 1650–1800°C for 0.5–2h at a nitrogen pressure of 0.1MPa. The effects of firing temperature and holding time on the crystalline phase and microstructure evolution of β-Si3N4 seeds were investigated in detail. The particle size distribution of the β-Si3N4 seeds before and after acid treatment was also evaluated using laser diffraction/scattering. The results show that, even with a low mass fraction of sintering additive, rod-like β-Si3N4 seeds with an average aspect ratio of ~4 could be successfully obtained after firing at 1750°C for 2h. Furthermore, the use of a dilute HF soak for a short period, the resultant powder showed a single peak frequency distribution with a narrow particle size range (D50≈2.5µm). This suggests that the residual crystalline/glassy phase contained in the as-prepared powder is readily attacked by acid rinse, resulting in the subsequent easy separation of the β-Si3N4 seed grains. The present method is therefore a simple and promising procedure for the preparation of highly dispersive rod-like β-Si3N4 seeds for use in the mechanical properties improvement of Si3N4 ceramics.

Fabrication of c-axis oriented Si3N4 ceramics using multilayered-graphene-coated β-Si3N4 seeds and their orientation in an innovative low magnetic field

c-axis oriented Si3N4 ceramics were fabricated by applying a low static magnetic field. Conventionally, c-axis oriented Si3N4 ceramics are fabricated using a rotating magnetic field of over 10T considering the anisotropy and the magnitude of diamagnetic susceptibility between c- and a-axes. In this work, multilayered-graphene was used for orienting rod-like β-Si3N4 particles as graphene exhibits a high diamagnetic anisotropy. The multilayered-graphene was coated with their orientation on the surface of rod-like β-Si3N4 particles by using a mechanical treatment process. As a result, the c-axis of multilayered-graphene-coated β-Si3N4 particles were oriented parallel direction to the applied magnetic field even at 0.4T. In addition, c-axis oriented Si3N4 ceramics were fabricated successfully by adding a small quantity of multilayered-graphene-coated β-Si3N4 particles as seeds. The thermal conductivity of the c-axis oriented Si3N4 ceramics prepared with a magnetic field of 0.4T exhibited anisotropy with a higher value achieved in the c-axis oriented direction.

High-toughness Lu2O3-doped Si3N4 ceramics by seeding

Previous reports showed that Lu2O3-doped Si3N4 ceramics had excellent high-temperature strength, but relatively coarse microstructure and low toughness. This work studied the effects of two kinds of β-Si3N4 seeds with similar small diameter and different aspect ratio on Lu2O3-doped Si3N4 ceramics. Results showed that the addition of seed with small diameter and low aspect ratio produced more bimodal and finer self-reinforced microstructure with the elongated grains of small diameter, and thus led to more significant improvement in fracture toughness, compared to the ceramics without seed or with seed of similar diameter and high aspect ratio.

Preparation of rod-like β-Si3N4 seeds with tailored morphology

Microstructures of β-Si3N4 seeds prepared by the heat treatment of α-Si3N4 powders were tailored by controlling the amount of Y2O3 additives and the N2 pressure. Based on the previous studies on rare earth (RE) distribution in Si3N4 ceramics doped with RE2O3, the Y2O3 existed in the two forms: (1) Y-containing phase such as Y2Si3O3N4, and (2) the adsorption of Si3N4 grain surfaces. The Y-containing phase resulted in the agglomeration of the seeds, and the Y adsorption promoted the growth anisotropy. Both the increased Y2O3 content and the decreased N2 pressure led to the increased Y adsorption and the higher aspect ratio of β-Si3N4 seeds. By the combination of low N2 pressure and low Y2O3 content, the β-Si3N4 seeds with moderate and uniform aspect ratio (~4.23), and few agglomerations were obtained.

The influence of α- and β-Si3N4 seeds on the properties of reaction bonded silicon nitride foams produced via wet processing

This work presents the influence of α- and β-Si3N4 seeds on the properties of reaction bonded silicon nitride (RBSN) foams fabricated via wet processing of Si–PMMA–Si3N4 suspensions. Characterization of the seeded foams was performed in terms of porosity, density, and mechanical strength. Phase and microstructural analysis were also used to investigate the influence of α- and β-seeding on the α/β phase ratio and microstructure of the RBSN foams.It was observed that both α- and β-seeds can improve the foam strength by 30% and 85%, respectively, due to more uniform nitridation. A foam strength of 2.23MPa was obtained for RBSN foams seeded with 5wt% α-Si3N4 with a foam porosity of 72vol%. Higher contents of α-seeds create a loose structure of the Si3N4 seed particles within the nitrided matrix resulting in a considerable decrease in foam strength. The addition of 10wt% of β-Si3N4 resulted in the maximum strength of 2.31MPa at a porosity level of 72vol%. The addition of β-seeds also resulted in an increase in the vapor phase reactions during nitridation leading to a significant increase in the β-whisker content of the microstructure. It was observed that even 30wt% of β-seed addition increases the strength of the RBSN foams compared to the unseeded foams.

A strategy for fabricating textured silicon nitride with enhanced thermal conductivity

Abstract C-axis textured Si3N4 with a high thermal conductivity of 176 W m−1 K−1 along the grain alignment direction was fabricated by slip casting raw α-Si3N4 powder seeded with near-equiaxed β-Si3N4 particles and Y2O3–MgSiN2 as sintering additives in a rotating strong magnetic field of 12 T, followed by gas pressure sintering at 1900 °C for 12 h at a nitrogen pressure of 1 MPa. The green material reached a relative density of 57%, with slip casting and the sintered material exhibited a relative density of 99% and a Lotgering orientation factor of 0.98. The morphology of the β-Si3N4 seeds had little effect on the texture development and thermal anisotropy of textured Si3N4. The technique developed provides highly conductive Si3N4 with conductivity to the thickness direction, which is a major advantage in practical use. The technique is also simple, inexpensive and effective for producing textured Si3N4 with high thermal conductivity of over 170 W m−1 K−1.

Effects of whisker-like β-Si3N4 seeds on phase transformation and mechanical properties of α/β Si3N4 composites using MgSiN2 as additives

α/β Si3N4 composites with β-Si3N4 content ranging from 26% to 100% were hot-pressed with or without β-Si3N4 seeds, using MgSiN2 as additives, and their mechanical properties were investigated. When the α-Si3N4 content was over 58%, the microhardness of α/β Si3N4 composites was in the range of 23–24GPa, and then the indentation hardness decreases with decreasing the content of α-Si3N4, whether with β-Si3N4 seeds or not. The toughness increased with increasing elongated β-Si3N4 grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0MPam1/2 with 1wt% β-seeds. In comparison with α/β Si3N4 composite with a similar phase composition, the fracture strength was improved by adding β-Si3N4 seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si3N4 composite with 5wt% β-seeds showed a high strength of 1253MPa, a high hardness of 20.9GPa and a toughness of 6.9MPam1/2.