Si3N4 Powders Research Articles

Dielectric constant and flexural strength of micro porous slip cast partially sintered silicon nitride

Slip casting of commercial Si3N4 powder dispersed in de-ionized water with Y2O3 and Al2O3 as sintering aids was carried out using optimum slurry conditions arrived through zeta potential and viscosity measurements. The cast specimens were sintered in graphite furnace in the temperature range of 1600 °C to 1800 °C under 0.15 MPa static N2 pressure. The sintered Si3N4 show density variation from 59% to 82% of theoretical value in the sintering temperature range studied. The 3-point flexural strength varied between 80 MPa and 320 MPa and dielectric constant 3.9 to 5.7 as a function of decreasing porosity from 39% to 17%. The samples exhibit uniform distribution of micro porosity in the range 0.5 μm–2.0 μm and grain size of 0.5 μm–3.0 μm. The microstructure changed from equiaxed and platelet shaped α-Si3N4 grains to needle shaped β- Si3N4 with increase in sintering temperature from 1600 °C to 1800 °C. The dielectric and flexural strength properties found promising for high temperature radome applications.

Liquid-Phase-Assisted Catalytic Nitridation of Silicon and In Situ Growth of α-Si3N4.

Si3N4 powders were synthesized with Fe, Co, or Ni as catalysts using Si powder at 1250 °C in a nitrogen atmosphere by liquid-phase-assisted catalytic nitridation synthesis (LPA–CNS). The catalytic effects of the metals on the nitridation of silicon powder were investigated by mixing the powder with 2 wt% by mass of Fe, Co, or Ni in a high-temperature liquid phase in flowing nitrogen. The α-Si3N4 micro-morphology could be effectively changed by adjusting the type of catalyst in the initial reaction mixtures. Fe, Co, and Ni promoted the formation of α-Si3N4 at 1250 °C and controlled the morphology of the α-Si3N4 particles. The hexagonal flakes of α-Si3N4 with a better defined morphology were obtained using Ni as the catalyst, compared to that obtained from the other two catalysts.

Synthesis, characterization and growth mechanism of SiC fibers

The smooth 2H–SiC fibers with hexagonal structure and a small amount of tower-like 3C–SiC fibers with face-centered cubic structure were synthesized with a simple catalyst-free method. Specifically, SiC fibers were obtained by thermal treatment of the Si3N4 powder in a graphite crucible at temperature of 1800 °C and pressure of 190 MPa in Ar atmosphere. The synthesized SiC fibers consist of SiC wires with several tens to hundreds of micrometers in length and mostly 200–800 nm in diameter and some SiC whiskers with shorter length of 3–15 μm. The smooth 2H–SiC fibers and rough 2H–SiC fibers with tower-like growth traces grown along [1010] direction, and the tower-like 3C–SiC fibers may grow with orientation attachment patterns along [111] direction. Finally, the vapor-solid growth mechanism and detailed growth process of SiC fibers were discussed.

A Novel Method of Si and Si3N4 Powder Resources Recycling: Cold Bonding Briquettes.

Silicon nitride (Si3N4) and silicon powder (Si) are two kinds of harmful solid waste in industrial production. As an environmental and low-consumption method, the cold-bonding technique is a novel method to utilize the problem of powder resource cycling. In this experiment, mechanical and high-temperature properties of Si and Si3N4 briquettes were studied after cold bonding. The results are as follows: (1) The compressive strength of the Si and Si3N4 briquettes increased with the improvement of molding pressure. With the same binder (1 wt.%) and water (10 wt.%) addition, the compressive strength of the Si3N4 briquette arrived at 12,023.53 N under 40 Mpa molding pressure, which is much higher than that of the Si briquette (942.40 N). The Si particles are uneven and irregular, which leads to an intense arch bridge effect in the Si briquette and the compressive strength decrease. Compared with Si powder, the particle size and shape of Si3N4 is small, uniform, and regular. The influence of the arch bridge effect is smaller than that in the Si briquette. (2) After being treated at 1473 K for 1 h, the compressive strength of the Si briquette increased to 5049.83 N, and the compressive strength of the Si3N4 briquette had a slight change. The surface of the briquettes was contacted with oxygen and reacted to form an outer shell which mainly contains SiO2 in the high-temperature treatment. FT-IR results have shown there were no extra impurities in cold-bonded briquettes when using the organic binder. (3) The microstructure of the cross section of the Si and Si3N4 briquettes after high-temperature treatment presented that oxygen entered the briquette through the pores and continued to react with the Si and Si3N4. The outer shell of the Si briquette grew and thickened continuously with the oxygen spreading in the Si briquette. However, because of the smaller particle size and regular shape, little oxygen diffused in the Si3N4 briquette. The outer shell of the Si3N4 briquette is fairly thin, so the compressive strength did not change too much.

Effect of the diluent on combustion synthesis of silicon nitride

AbstractSi3N4 powders were prepared by combustion synthesis with 1‐ and 3‐μm α‐Si3N4, β‐Si3N4 diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α‐Si3N4 and β‐Si3N4 as diluents are about 1600–1800°C. Moreover, the newly formed α‐Si3N4 contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α‐Si3N4 and β‐Si3N4 as diluent about 20–40 wt%. The strip‐like α‐Si3N4, rod‐like β‐Si3N4 grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α‐phase content of combustion synthesized Si3N4 is discussed, which provides key guidance for preparing Si3N4 powders with high α‐phase content.

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

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

A Novel Antipathogenic Agent for Nonwoven Fabric

Medical-grade masks and N95 respirators containing non-woven fibers are designed to prevent the spread of airborne diseases. While they effectively trap respiratory droplets and aerosols, they cannot lyse entrapped pathogens. Embedded antimicrobial agents such as silver, copper, zinc, iodine, peptides, quaternary ammonium salts, or nanoparticles have been used to overcome this limitation. However, their effectiveness remains debatable because these materials can be toxins, allergens, irritants, and environmental hazards. Recently, silicon nitride (Si3N4) was found to be a potent antipathogenic compound, and it may be an ideal agent for masks. In powder or solid form, it is highly effective in inactivating bacteria, fungi, and viruses while leaving mammalian tissue unaffected. The purpose of this study was to serially assess the antiviral efficacy of Si3N4 against SARS-CoV-2 using powders, solids, and embedded nonwoven fabrics. Si3N4 powders and solids were prepared using conventional ceramic processing. The “pad-dry-cure” method was used to embed Si3N4 particles into polypropylene fibers. Fabric testing was subsequently conducted using industrial standards—ISO 18184 for antiviral effectiveness, ASTM F2299 and EN 13274-7 for filtration efficiency, EN 14683 for differential pressure drop, and ISO 18562-2 for particle shedding. A modification of ISO 18562-3 was also employed to detect ammonia release from the fabric. Antiviral effectiveness for Si3N4 powders, solids, and embedded fabrics were 99.99% at ≤ 5 min, ~ 93% in 24 h, and 87% to 92% in 120 min, respectively. Results of the standard mask tests were generally within prescribed safety limits. Further process optimization may lead to commercial Si3N4-based masks that not only “catch” but also “kill” pathogenic microbes.

The influence of Ti: Si3N4 ratio on the microstructure evolution of Ti–Si3N4 reaction in Cu melts

In this work, through the study of the obtained Cu–Ti–Si–N intermediate products by controlling the reaction degree of Ti and Si3N4 powder in Cu melts at 1250∼1300 °C, the effects of different Ti: Si3N4 mass ratios on the microstructure evolution of Ti–Si3N4 reaction in Cu melts were verified. When the mass ratio of Ti: Si3N4 is higher, such as 3.09:1, TiN will cooperate with Ti5Si3 and depend on each other to nucleate and grow to form TiN/Ti5Si3 composites. The formed TiN are spherical and wetted by Ti5Si3 to uniformly disperse in Cu melts. As a result, the TiN–Ti5Si3 hybrid reinforced Cu matrix composites will be formed. However, when the mass ratio of Ti: Si3N4 is lower, such as 1.37:1, Ti and Si3N4 will firstly react to form TiN and Ti–Si liquid. The formed TiN are irregularly polygonal and connect with each other. The Ti–Si liquid will combine with Cu melts to form Cu–Ti–Si liquid and finally form δCu4Si, ηCu3Si and τ1-CuSiTi phases during the colling stage. In this case, TiN are difficult to be wetted, and the Ti–Si3N4 compact will keep its original shape and not spread in Cu melts.

Preparation and properties of Si3N4 ceramics via digital light processing using Si3N4 powder coated with Al2O3-Y2O3 sintering additives

It is difficult to form Si 3 N 4 ceramics by DLP technology owing to the high refractive index and high ultraviolet (UV) absorption of Si 3 N 4 powder. In this paper, the Si 3 N 4 ceramics were successfully fabricated by DLP technology using Si 3 N 4 powder coated with Al 2 O 3 -Y 2 O 3 sintering additives. The effects of the coating amount of sintering additives on the properties of Si 3 N 4 ceramic slurry, green bodies and parts after sintering were systematically studied. When Al(NO 3 ) 3 and Y(NO 3 ) 3 solutions were added at a content of 20 mL/100 g Si 3 N 4 , the slurry with lowest viscosity (0.042 Pa·s at a shear rate of 30 s −1 ) and a higher monolayer curing depth (47.9 ± 1.2 µm) was obtained, the relative density and flexural strength of the Si 3 N 4 ceramics reached maximum values at 85.4% ± 0.3% and 149.2 ± 1.9 MPa. The shrinkage of the Si 3 N 4 ceramics remained almost unchanged after coating Al 2 O 3 -Y 2 O 3 sintering additives. Finally, the Si 3 N 4 ceramics with excellent quality and clear outline were well prepared. This research has laid a certain experimental basis for DLP forming Si 3 N 4 and other non-oxide ceramics, which is of great researching significance.

A Study on Ceramic Heater Devices Based on Si3N4 with a MoO3 Protective Layer

Currently, the need for high performance heaters for industrial application is increasing, and repeated wear, rapid temperature change, and uneven temperature can cause deteriorating quality and reduced productivity. The ceramic heater in this study consists of a substrate, an electrode layer, and a protective layer, and low-melting temperature glass frit was used for all layers to allow low-temperature processing. As a substrate for the ceramic heater, Si3N4, which is twice as strong as AlN and has excellent wear resistance, was used. As the electrode material, Ag-Pd, which provides good adhesion to the Si3N4 substrate and high electrical conductivity was used, and MoO3 powder was mixed 8 : 2 to make a paste. The MoO3 adjusts the resistance of the Ag-Pd electrode, thereby supplementing the heater performance. Si3N4 powder and glass frit were used for the protective film layer that protects the electrode, and MoO3 was added to unify the materials between the layers.

Surface modification of Si3N4 powder for hydrolysis control

The hydrolysis of Si3N4 powder was controlled by surface modification with Al(H2PO4)3 and Mg(H2PO4)2 with TEOS. The existence of Al(H2PO4)3 and Mg(H2PO4)2 catalyzed the hydrolysis of TEOS. From experimental results, the Si3N4 powder after surface modification exhibits a greatly-improved hydrolysis resistance than the initial Si3N4 powder in water at 60 and 120 °C. The surface modification had no clear influence on the dielectric loss of Si3N4 powders. By XPS analysis, the powders after surface modification showed the presence of P, Al, and Mg elements. It was proposed that the surface modification agent forms a strong bonding with the surface of Si3N4 particles.

Improving cure performance of Si3N4 suspension with a high refractive index resin for stereolithography-based additive manufacturing

Silicon nitride (Si3N4) slurries with high solid loading, low viscosity and good stability is difficulty prepared, due to low intrinsic surface charge and a large refractive index (RI) difference between Si3N4 powder and resin. In this paper, the curing behavior of Si3N4 slurry with different functional group and RI of resin monomer were systematically researched, and then the kind and optimum content of dispersant were investigated. Subsequently, a high solid loading Si3N4 slurry (44 vol%) with good curing behavior, low viscosity and favorable stability was successfully prepared. Lastly, the dense Si3N4 ceramic parts were fabricated by the suitable Si3N4 slurry (44 vol%) via stereolithography. After debinding and sintering process, the relative density and flexural strength of Si3N4 ceramic were 98.28% and 800 ± 27.28 Mpa, respectively.

Digital light processing 3D printing of surface-oxidized Si3N4 coated by silane coupling agent

ABSTRACT Due to high light absorption and high refractive index of silicon nitride (Si3N4) ceramic, it is difficult to prepare Si3N4 slurry with favorable curing ability, high solid loading and low viscosity at the same time, and thus the high-quality Si3N4 parts are hard to fabricate via Digital Light Processing (DLP). In this paper, the oxidation process was used to enhance the curing behavior of Si3N4 slurry, and then silane coupling agent (KH560) was used to improve the rheological properties of the oxidized Si3N4 slurry. The effect of Si3N4 slurry with oxidation and modification on its rheological behavior, light absorption, curing ability and stability have been systematically investigated. A Si3N4 slurry with abundant photocuring ability, high solid loading, low viscosity and reliable stability was fabricated by the oxidized (1 h) and KH560 (1 wt.%) modified Si3N4 powders, and the dense Si3N4 ceramic parts were produced by this slurry via DLP 3D printing. Subsequently, the influence of oxidation and modification on microstructure, mechanical properties and thermal conductivity have been investigated. Finally, the microstructure and performances of Si3N4 ceramic were compared between different forming processes (including: DLP and cool isostatic pressing).

Effect of nano- and micro-sized Si3N4 powder on phase formation, microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si6-zAlzOzN8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si3N4 − 4/3(AlN·Al2O3) in the phase diagram using (A) nano-sized amorphous Si3N4 and (B) micro-sized β-Si3N4 precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis. Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si3N4 precursor, the nano-sized amorphous Si3N4 precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv10) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase. In general, for samples synthesized from nano-sized amorphous Si3N4, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m1/2. For equivalent samples synthesized from micro-sized β-Si3N4, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.

An In situ neutron diffraction study of phase formation during Ti–Si3N4 powder sintering

This paper characterises and evaluates Si 3 N 4 /Ti composites during thermal treatment using an in situ neutron diffraction technique. The composites were developed using a conventional press and sinter technique. Pure titanium (Ti) was chosen as the matrix, and different concentrations of Si 3 N 4 were used as the reinforcement. The effects of sintering temperature and the concentration of Si 3 N 4 in the Ti matrix were investigated with respect to phase constituents. The Si 3 N 4 mass fraction in the Ti matrix was found to be the key parameter for the reaction. Because of its instability in Ti at higher temperatures, in situ reactions between the reinforcing particles and matrix led to the formation of intermetallic compounds, such as Ti 5 Si 3 and possibly Ti 3 Si, in the composites containing higher weight fractions of Si 3 N 4 .

Si3N4/Graphene binary particles reinforced hybrid titanium composites and their characterization

Abstract This study reports on silicon nitride (Si3N4) and graphene nanoplatelets binary powder reinforced hybrid titanium composites obtained by a powder metallurgy method. Si3N4 powder was added at 3 wt.% and graphene nanoplatelets were added in various amounts (0.15, 0.30, 0.45, 0.60 wt.%) in the titanium matrix. Density, micro-Vickers hardness, compressive behavior, wear properties and microstructure of the hybrid composites were evaluated. Addition of different percentages of graphene nanoplatelets and 3 wt.% Si3N4 to the titanium matrix composites significantly enhanced mechanical properties. The highest hardness (634 HV) and compressive strength (1458 MPa) values were measured for 0.15 wt.% graphene nanoplatelets and 3 wt.% Si3N4 added titanium hybrid composite. The lowest mass loss and wear rate (Δm = 4 mg, W = 6.1×10–5 mm3 (N m)–1) values were measured for the same 0.15 wt.% graphene nanoplatelets and 3 wt.% Si3N4 added titanium hybrid composite compared with pure Ti.

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si3N4 powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si3N4 seeds content on the products were studied. It was found that high-purity α-Si3N4 (>99.0 wt%) was synthesized from C/SiO2 = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO2 and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si3N4 increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si3N4. Furthermore, higher nitriding temperature and Si3N4 seeds content both decreased the grain size and increased β-Si3N4 content. The forming mechanism of non-agglomerated and submicron-sized α-Si3N4 was clarified.

Influence of compaction pressure and Si3N4/ZrO2 reinforcement on the properties of aluminium hybrid composites

ABSTRACT In the present study, the aluminium hybrid composites (AHCs) are developed by the powder metallurgy method. Pure aluminium is reinforced with varying weight percentages of ceramic reinforcements (Si3N4 and ZrO2). Pure aluminium powder is mixed with 3 wt% Si3N4 and (0, 1.5, 3 and 4.5 wt%) ZrO2 powder in a centrifugal ball mill at a high speed of 600 rpm for 2.5 hours. The XRD results of the AHC powders shows that no interfacial reaction takes place during the ball milling process. The powder composites are then compacted at three different compaction pressure (340,420 and 500MPa) and sintered in a tube furnace at 500 °C for 30 minutes under an Argon environment. The SEM results of different composites show that the reinforcement and matrix particles are distributed uniformly with the accumulation of nano-size ZrO2 particles. The density and micro-hardness improve with high compaction pressure and weight percentages of the reinforcements but the porosity decreases. The maximum hardness value is found as 72 VHN for sample E (Al+3 wt% Si3N4 + 4.5 wt% ZrO2) at 500 MPa.

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 m1/2. 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.

Porosity control in silicon nitride‐based support materials toward enhanced gas permeability

AbstractPorous Si3N4 materials in tubular geometry are prepared by slip casting before partial sintering. A variety of material‐ and process‐specific variables and their respective effects on densification and resulting pore morphology are systematically evaluated, focusing on starting powder type, amount of sintering additives, and sintering parameters including temperature and time. An increased β‐Si3N4 content in the starting powder was found to promote the formation of a network of elongated grains exhibiting increased pore diameters, as opposed to a more finely featured pore network obtained from starting materials consisting of α‐Si3N4. Following an iterative evaluation of processing variables, materials exhibiting a characteristic diametral compression strength (C‐ring test) of 163 MPa and a Darcian permeability of 4.7 ⋅ 10−15 m2 at a total porosity of 41% were obtained, corresponding to an increase of over 40% in strength and of over 600% in permeability in comparison to materials obtained by α‐Si3N4 powders at comparable porosities. These results demonstrate that the composition of Si3N4 powders significantly affects the resulting pore structure, and by combining the respective selection of starting materials with finely tuned sintering parameters, materials with superior performance in terms of mechanical properties as well as permeability characteristics are accessible.