Theoretical analysis of defect formation processes in silicon nitride

Processes of defect formation in Si₃N₄ by the Schottky, Frenkel, and anti-Frenkel mechanisms are analyzed theoretically within the method of quasichemical approximation. Equations describing the formation of defects in Si₃N₄ are derived, analyzed, and solved for different conditions of electroneutrality, and dependences of the number density of point defects in Si₃N₄ on the partial pressure of nitrogen under equilibrium conditions are determined. It is assumed that nitrogen vacancies are the predominant kind of point defect of nonstoichiometric origin.     

Fabrication and properties of Si3N4 bioscaffolds with orderly–interconnected big pore channels and well–distributed small pores

This paper focuses on investigating the technical potential for fabricating porous ceramic bioscaffolds for the repair of osseous defects from trauma or disease by inverse replication of three–dimensional (3–D) printed polymer template. Si₃N₄ ceramics with pore structure comprising orderly–interconnected big pore channels and well–distributed small pores are successfully fabricated by a technique combining 3–D printing, vacuum suction filtration and oxidation sintering. The Si₃N₄ ceramics fabricated from the Si₃N₄ powder with addition of 10?wt% talcum by sintering at 1250?°C for 2?h have little deformation, uniform microstructure, low linear shrinkage of 4.1%, high open porosity of 58.2%, relatively high compression strength of 6.4?MPa, orderly–interconnected big pore channels and well–distributed small pores, which are promising bioscaffold in the field of bone tissue engineering.     

Experimental and theoretical study on air reaction wetting and brazing of Si3N4 ceramic by Ag-CuO filler metal: Performance and interfacial behavior

Reactive air brazing of Si₃N₄ ceramic was successfully achieved by using Ag-CuO filler metal. The effects of CuO content on the wettability of Ag-CuO/Si₃N₄ system and the shear strength of Si₃N₄/Si₃N₄ joint were investigated, and meanwhile the interfacial behavior was analyzed and discussed. Moreover, the work of adhesion, interfacial energy, and electronic properties of Ag/Si₃N₄, Ag/CuO and Ag/SiO₂ interfaces were evaluated by first-principles calculations. The Ag-CuO/Si₃N₄ system transforms from no-wetting into wetting due to the oxidation of Si₃N₄ substrate and the formation of SiO₂ layer on the substrate surface. The maximum average joint shear strength of over 50 MPa is obtained as the CuO content is 8 at.%. Compared with the Ag(111)/Si₃N₄(0001) interface, the Ag(110)/CuO(001) and Ag(110)/SiO₂(001) interfaces show stronger interfacial bonding due to the formation of Ag-O ionic bond, indicating that the addition of CuO and the formation of SiO₂ contribute to the enhancement of interfacial bonding.     

Effect of single-walled carbon nanotubes on thermal and electrical properties of silicon nitride processed using spark plasma sintering

Si₃N₄ nanocomposites reinforced with 1-, 2-, and 6-vol% single-walled carbon nanotubes (SWNTs) were processed using spark plasma sintering (SPS) in order to control the thermal and electrical properties of the ceramic. Only 2-vol% SWNTs additions were used to decrease the room temperature thermal conductivity by 62% over the monolith and 6-vol% SWNTs was used to transform the insulating ceramic into a metallic electrical conductor (92 S m⁻¹). We found that densification of the nanocomposites was inhibited with increasing SWNT concentration however, the phase transformation from α- to β-Si₃N₄ was not. After SPS, we found evidence of SWNT survival in addition to sintering induced defects detected by monitoring SWNT peak intensity ratios using Raman spectroscopy. Our results show that SWNTs can be used to effectively increase electrical conductivity and lower thermal conductivity of Si₃N₄ due to electrical transport enhancement and thermal scattering of phonons by SWNTs using SPS.     

Investigation of Native Point Defects and Nonstoichiometry Mechanisms of Two Yttrium Silicates by First‐Principles Calculations

First‐principles method is used to study the native point defects in Y₂SiO₅ and Y₂Si₂O₇ silicates. The calculated defect formulation energies show similar native point defect behaviors in Y₂SiO₅ and Y₂Si₂O₇: the oxygen Frenkel defect is predominant; and it is followed by the cation antisite and Schottky defects. The possible chemical potential range of each constituent is further considered in the calculation of defect formation energy. Oxygen interstitial (O_i) and oxygen vacancy (V_O) are the predominant native point defects under O‐rich and O‐poor condition, respectively. In addition, the mechanisms of accommodating composition deviations from stoichiometric Y₂SiO₅ and Y₂Si₂O₇ are investigated. For Y₂SiO₅, Y₂Si₂O₇ impurity may appear, together with the defects of Si_Y antisite, O_i interstitial, and/or V_Y vacancy when SiO₂ is excess; while Y_Si antisite appears together with Y_i interstitial and/or V_O vacancy in Y₂SiO₅ when Y₂O₃ is excess. For Y₂Si₂O₇, the main process is the formation of Si_Y antisite accompanied by O_i interstitial and/or V_Y vacancy when SiO₂ is excess; but Y₂SiO₅ impurity forms, together with Y_Si antisite, V_O vacancy, and/or Y_i interstitial in Y₂Si₂O₇ when Y₂O₃ is excess. We expect that the results are useful to control of processing conditions and further to optimization of performance of the two silicates.     

Two-step strategy for improving the tribological performance of Si3N4 ceramics: Controlled addition of SiC nanoparticles and graphene-based nanostructures

The tribological behaviour of silicon nitride (Si₃N₄) ceramics is investigated using a two-step strategy. A set of ceramic composites containing silicon carbide nanoparticles (SiC_n) is developed and, subsequently, graphene-based fillers are added to the Si₃N₄/SiC composite with the best tribological performance. The friction coefficient and the wear rate of Si₃N₄ are reduced up to 22 % and 40 %, respectively, when a 10 vol.% of SiC_n is incorporated into the ceramic matrix due to its improved mechanical response. Si₃N₄/SiC composites containing 11 vol.% of graphene nanoplatelets (GNPs) or reduced graphene oxide sheets (rGOs) are analysed under isooctane lubrication and dry testing. rGOs composite leads to an important decrease of the friction coefficient (50 %) under lubricated conditions, and an enhancement of the wear resistance (44 %) under dry sliding tests, as compared to the reference Si₃N₄/SiC. The best performance of rGOs composite is due to the nature of the lubricating tribofilm and its excellent toughness.     

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.     

Local Fracture Toughness of Si3N4 Ceramics Measured using Single‐Edge Notched Microcantilever Beam Specimens

Local fracture toughness gives us useful and important information to understand and improve mechanical properties of bulk ceramics. In this study, the local fracture toughness of silicon nitride (Si₃N₄) ceramics was directly measured using single‐edge notched microcantilever beam specimens prepared by the focused ion beam technique. The measured fracture toughness of grain boundary of the Si₃N₄ ceramics is higher than the fracture toughness of SiAlON glass, which exists in the grain boundaries of Si₃N₄ ceramics. It is also shown that the fracture toughness of grain boundary depends on the rare earth oxide added as a sintering aid, which is expected in terms of the difference in the grain‐boundary structure. The fracture toughness of a single β‐Si₃N₄ grains is higher than the grain‐boundary fracture toughness. It was also higher than the value estimated from ab initio calculations and surface energy, which means that any dissipative energy should be included in the fracture toughness of a grain in spite of the brittle fracture in Si₃N₄. The fracture toughness of polycrystals of Si₃N₄ ceramics measured using single‐edge notched microcantilever beam specimens is intermediate between those of grains and grain boundaries, and it agrees with the estimated initial value of the Rcurve, K_I0, in Si₃N₄ ceramics.     

Laser-induced metallization of porous Si3N4 ceramic and its brazing to TiAl alloy

The poor wettability of traditional brazing filler alloys on the surface of ceramics always lead to the formation of defects in the joints and weaken the bonding strength eventually, especially the porous ceramics. Metallization on ceramics is an effective way to improve the wettability. In this work, laser-induced cladding process was applied to metalize the surface of porous Si₃N₄ ceramic, and the traditional AgCu eutectic filler alloy can wet on the metalized surface completely. The metalized porous Si₃N₄ ceramic brazed to TiAl alloy successfully using AgCu filler alloy. The interfacial microstructure and mechanical property of the porous Si₃N₄/TiAl alloy brazed joint was significantly improved by the novel laser-induced metallization process.     

Accurate Exploration of the Intrinsic Lattice Thermal Conductivity of Si2N2O by Combined Theoretical and Experimental Investigations

Si₂N₂O is a promising ceramic with various structural and functional applications. Precisely exploring its thermal conductivity is crucially important to evaluate its thermal transport reliability as high‐temperature structural component and electronic device. In this paper, temperature‐dependent lattice thermal conductivity of Si₂N₂O is studied based on a method integrating density functional theory calculations and experimental measurements. The relationship between the complex crystal structure (or heterogeneous chemical bonding) and lattice thermal conductivity of Si₂N₂O is studied. We herein show that Si₂N₂O intrinsically has moderately high lattice thermal conductivity [30.9 W·(m·K)^?1 at 373 K], but extrinsic phonon scattering mechanisms, such as phonon scattering by point defects and grain boundaries etc., might significantly degrade the magnitude in experimental measurement [15.0 W·(m·K)^?1 at 373 K]. This work suggests the significance that understanding the intrinsic thermal conductivity, namely the upper limit value, is a precursor to deciphering the more complicated heat transport behavior of Si₂N₂O.     

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.     

Preparation of Si3N4 ceramic based on digital light processing 3D printing and precursor infiltration and pyrolysis

The emergence of digital light processing (DLP) 3D printing technology creates favorable conditions for the preparation of complex structure silicon nitride (Si₃N₄) ceramics. However, the introduction of photosensitive resin also makes the Si₃N₄ ceramics prepared by 3D printing have low density and poor mechanical properties. In this study, high-density Si₃N₄ ceramics were prepared at low temperatures by combining DLP 3D printing with precursor infiltration and pyrolysis (PIP). The Si₃N₄ photocurable slurry with high solid content and high stability was prepared based on the optimal design of slurry components. Si₃N₄ green parts were successfully printed and formed by setting appropriate printing parameters. The debinding process of printed green parts was further studied, and the results showed that samples without defects and obvious deformation can be obtained by setting the heating rate at .1°C/min. The effect of the PIP cycle on the microstructure and mechanical properties of the Si₃N₄ ceramics was studied. The experimental results showed that the mass change and open porosity of the samples tended to be stable after eight PIP cycles, and the open porosity, density, and bending strength of the Si₃N₄ ceramics were 1.30% (reduced by 97%), 2.64 g/cm³ (increased by 43.5%), and 162.35 MPa.     

Cup-stacked carbon nanotubes hybridized Si3N4/Si3N4 composite ceramics for high-effciency microwave absorption with excellent thermal stability

Hybridization between carbon nanotubes (CNTs) and Si₃N₄ is a promising strategy for developing high-temperature microwave absorption (MA) materials for military application. Toward long-life services, it's important to achieve strong MA at a filler loading as low as possible on account of antioxidant protection against CNTs wastage. Herein, cup-stacked CNTs (CSCNTs) have been prepared in porous Si₃N₄ ceramics by chemical vapor deposition (CVD) and then CVD Si₃N₄ has been coated on them, forming CSCNT-Si₃N₄/Si₃N₄ composite ceramics. Results show that CSCNTs possess abundant exposed atomic edges on the outer surface and in the inner channel. Such unique defects not only benefit the impedance match but also cause considerable conductive loss, which helps CSCNT-Si₃N₄/Si₃N₄ with a filler content of only 0.79 wt% to achieve an effective absorption bandwidth (EAB) of 3.74 GHz in the X band at a thickness of 3.5 mm coupled with a minimum reflection loss of ?43.3 dB and an EAB covering the entire Ku band at a thickness of 2.25 mm. The ultralow filler loading generates a high efficiency of CVD Si₃N₄ in protecting CSCNTs against high-temperature oxidation, leading to a steady MA performance for CSCNT-Si₃N₄/Si₃N₄ during 23–1200 °C thermal shock tests in air.     

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.     

Processing,characterization and properties of novel gradient Si3N4/SiC fibers derived from polycarbosilanes

In this work, we report a novel kind of Si₃N₄/SiC composite fibers, which exhibit a controlled gradient Si₃N₄(shell)/SiC(core) structure. These composite fibers are fabricated through a controlled nitridation and pyrolysis process on electron irradiation-cured polycarbosilane fibers. Structural and chemical analysis based on Elemental Analyzer, FT-IR, Raman spectroscopy, electron probe micro-analyzer, X-ray photoelectron spectroscopy, and X-ray diffraction confirm the gradient structure of obtained fibers, which consist a shell with high Si₃N₄ content and a SiC core. The as-fabricated fibers exhibit dense and smooth surfaces, and no microscopic holes or defects were observed. The effects of nitridation temperature on mechanical properties and electrical resistivity were also investigated. Combined with high mechanical properties and lightweight, the present gradient Si₃N₄/SiC fibers open a new strategy to fabricate multifunctional and electromagnetic wave absorbing materials.     

Silicon nitridation mechanism in reaction‐bonded Si3N4–SiC and Si3N4‐bonded ferrosilicon nitride

Reaction‐bonded Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe₃Si–Si₃N₄ particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N₂ at 1723K. There is columnar β‐Si₃N₄ in both Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride. However, fibrous α‐Si₃N₄ is only observed in Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride contains much more Si₂N₂O than Si₃N₄–SiC. By analyzing the oxidation thermodynamics of Si and Si₃N₄, it is known that in the process of producing Si₃N₄–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si₃N₄ is generated with SiO and N₂. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si₃N₄‐bonded ferrosilicon nitride, Si₃N₄ is easier to be oxidized than Si and Si₂N₂O is generated on the surface of Si₃N₄ hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe₃Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors β‐Si₃N₄ generation from Si direct nitridation and fibrous α‐Si₃N₄ transformation, which used to exist in ferrosilicon nitride raw materials.     

Molecular dynamics simulation of color centers in silicon carbide by helium and dual ion implantation and subsequent annealing

Atomic and close-to-atomic scale fabrication with high yield for the color centers in silicon carbide is critical in developing its applications. Combined with Wigner-Seitz method and identify diamond structure method to consider the structure around the silicon vacancy (V_Si), it is found that He ion implantation is more likely to fabricate a small number of silicon vacancies with complete structure but locating deep, while Si ion implantation is more likely to introduce more silicon vacancies with incomplete structure but a large number and closer to the near surface. Therefore, a method of dual ion implantation is proposed in this paper. By adjusting the ratio of He to Si ion concentration for dual ion implantation, the fabrication yield of color centers with depth below 5 nm can be increased compared with that of He implantation after high temperature annealing. Molecular dynamics (MD) simulation is employed to discover the underlying mechanism of V_Si color center and damage evolution by helium ion and dual ion implantation into four-hexagonal silicon carbide (4H–SiC) with subsequent annealing. Density-functional theory (DFT) calculation proves that magnetic-spin polarization enhances the stability of carbon anti-vacancy pair (C_SiV_C) defect, which indicates C_SiV_C defects are more stable than V_Si defects. The evolution of the V_Si color centers of different defect models are also calculated at various temperatures by MD, and the dynamic process of V_Si defects to C_SiV_C defects is demonstrated. It is revealed that the decrease of the V_Si color center at high temperature annealing is partly due to the transformation of some silicon vacancies to C_SiV_C defects.     

Improved oxidation resistance properties of Si3N4/O′–SiAlON composite ceramics by a repeated sintering method

Si₃N₄/O′–SiAlON composite ceramics with superior oxidation resistance properties were fabricated by a repeated sintering method. The effects of sintering time on the phase evolution, microstructure, and oxidation resistance properties of the Si₃N₄/O′–SiAlON composite ceramics were investigated. The results indicated that the content of the O′–SiAlON phase and the densification of Si₃N₄/O′–SiAlON composite ceramics increased after two-time sintering. Furthermore, the thickness of the oxide layer of the Si₃N₄/O′–SiAlON composite ceramics after oxidation at 1100–1500°C for 30 h was not significant. Compared to the oxidation weight gain after the one-time sintering process, the oxidation weight gain of Si₃N₄/O′–SiAlON composite ceramics was 0.432 mg/cm² after two-time sintering when oxidized at 1500 C for 30 h, which was reduced by 43.3%. The mechanism of the improved oxidation resistance properties was ascribed to the formation of more O′–SiAlON and the enhancement of the densification.     

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si₃N₄) fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si₃N_4f/BN/Si₃N₄ composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m^1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m^1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si₃N_4f/BN/Si₃N₄ composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.     

Electromagnetic wave absorption properties of hot-pressed Si3N4-SiC ceramic nanocomposites

High-density Si₃N₄-SiC ceramic nanocomposites have exceptional mechanical properties, but little is known about their electromagnetic wave absorption (EMA) capabilities. In this paper, the effects of sintering temperature and starting material compositions on the dielectric and EMA properties of hot-pressed Si₃N₄-SiC ceramic nanocomposites were investigated. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites increase with increasing sintering temperature or SiC content, particularly at the sintering temperature of 1850°C and SiC content of 50 wt.%. This is primarily due to the improvement of interfacial and defect polarizations, which is caused by the doping of nitrogen into the SiC nanocrystals during the solution-precipitation process. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites show decreasing trends as sintering aid content increases. Si₃N₄-SiC ceramic composites have both good EMA and mechanical properties when they are sintered at 1850°C with 30 wt.% SiC and 5–8 wt.% sintering aids. The minimum reflection loss and maximum flexural strength reach -58 dB and 586 MPa, respectively. Materials with multilayered structural designs have both strong and broad EMA properties.