短链两亲分子活性剂制备氮化硅泡沫陶瓷

采用直接起泡法制备氮化硅泡沫陶瓷,研究了长链表面活性剂与短链两亲分子活性剂对泡沫稳定性的影响,分析了Si3N4泡沫陶瓷的微观结构。结果表明：与长链表面活性剂稳定的泡沫相比,短链两亲分子稳定的泡沫稳定性较好,泡体呈现球形或椭球形。通过控制发泡工艺制备出气孔尺寸分布均匀的开孔和闭孔两种不同孔结构的多孔氮化硅泡沫陶瓷,闭孔氮化硅泡沫陶瓷的气孔率和抗弯强度分别为40%和106MPa;开孔氮化硅泡沫陶瓷的气孔率和抗弯强度分别为80%和28MPa。     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent

In this paper, novel porous Si₃N₄ ceramics were prepared by aqueous gelcasting using Si₃N₄ poly-hollow microspheres as pore-forming agent. The effect of Si₃N₄ poly-hollow microsphere content on the phase composition, microstructure, shrinkage, porosity and mechanical properties of the prepared porous Si₃N₄ ceramics were investigated. It is found that there is only β-Si₃N₄ phase in all the prepared porous Si₃N₄ ceramics. Meanwhile, the SEM results show that the pores in the porous Si₃N₄ ceramics distribute uniformly, the added Si₃N₄ poly-hollow microspheres and the basal body contact closely. With the increase of Si₃N₄ poly-hollow microsphere content, the shrinkage of the porous Si₃N₄ ceramics decreases gradually, and the porosity of the porous Si₃N₄ ceramics decreases firstly and then increases. Furthermore, the flexural strength and fracture toughness of the porous Si₃N₄ ceramics decrease with the increase of the Si₃N₄ poly-hollow microsphere content.     

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.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

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.     

Microstructure and properties of Si3N4 foam ceramics modified by in-situ self-grown nanowires

In this study, Si₃N₄ nanowires were prepared by one-step method in Si₃N₄ foam ceramics. This was accomplished via simple and safe pre-embedded powder treatment combined with reaction sintering method. Nitriding temperature and time were determined to have influence on the production of nanowires. Under used sintering conditions with nitriding at 1400 °C for 4–6 h or at 1450 °C for 2–6 h, densely distributed nanowires were obtained in the sample, and the formed nanowires were either straight or bent. The straight nanowires were completely crystalline, while the bent nanowires had staggered crystalline-amorphous distribution at the bending position. Nanowires in the sample did not substantially affect open porosity and density of the sample. However, the analysis of typical samples with or without dense nanowires revealed that the existence of densely distributed nanowires reduced median pore diameter of windows on the pore walls from 60.7 μm to 21.4 μm and increased BET specific surface area of the sample by 46.7%. In addition, the presence of densely distributed nanowires in pores enhanced compressive strength of the sample by at least 10%.     

Bioactive silicon nitride by surface thermal treatment

Silicon nitride-based ceramics with SiO₂, CaO and Ca₃(PO₄)₂ as sintering additives, have been prepared in order to study the bioactivity. Dense ceramic bodies were oxidized by an oxy-acetylene flame at approx. 1475 °C for 60 s, in order to modify the surface in terms of bioactivity enhancement and the formation of optimal porosity for cell viability. During oxidation two concurrent processes occurred on the ceramic body surface: (i) formation of thin glassy layer with a composition close to that of grain boundary phase in ceramic body, and (ii) partial decomposition of silicon nitride matrix. The latter one resulted in the formation of gases (N₂ and SiO), which formed bubbles in the viscous surface glassy phase, resulting in porosity required for cell adhesion (small pores) and tissue ingrowth (large pores). The best bioactivity was obtained for oxy-acetylene flame treated Si₃N₄ ceramics with Ca₃(PO₄)₂ sintering additive.     

A novel method for preparing Si3N4 ceramics with unidirectional oriented pores from silicon aqueous slurries

We describe a low-cost and simple method to prepare porous silicon nitride (Si₃N₄) ceramics with unidirectional oriented pores. Using Si powders as raw material, porous ceramics were fabricated via solidification of the aqueous slurries followed by the nitridation and post-sintering. The formation mechanism is similar to the ordered porous metals that are fabricated under high temperature and high pressure. Differently, the green bodies with oriented pores were obtained at room temperature and atmospheric pressure. The nucleation and growth of the H₂ bubbles generated from hydrolyzed Si powders contributed to the highly oriented pores along the solidification direction of the aqueous slurries. Additionally, the pore microstructure was closely associated with the viscosity of the slurries, which can be well controlled by varying the concentration of organic additive and solid loading.     

Effect of rotating speed during foaming procedure on the pore size distribution and property of silicon nitride foam prepared by using protein foaming method

Silicon nitride (Si₃N₄) foams were prepared by using protein foaming method with varying rotating speed during the foaming process. The pore sizes of these as-fabricated Si₃N₄ foams were measured by means of the Image Pro Plus software and the as-measured pore size date was analyzed statistically by using the SPSS Statistics software. It was indicated that the pore size data of the as-prepared Si₃N₄ foams abided by the logarithmic normal distribution. With the increase of rotating speed, the pore structure of Si₃N₄ foam became more uniform. This was because of the enhancing shear stress at higher rotating speed, which increased frequency of collision between bubbles in foamed slurry and hence improved the uniformity of bubble size distribution. The porosity, density and flexural strength of these as-prepared Si₃N₄ foams fluctuated in a small range, indicating that the rotating speed had limited influence on these properties.     

Extrusion of highly porous silicon nitride ceramics with bimodal pore structure and improved gas permeability

Highly porous Si₃N₄ ceramics with bimodal pore structure were prepared by the extrusion processing with petroleum coke of 30 μm as pore‐maker. The microstructure, mechanical strength, and gas permeability were investigated. The microstructure with petroleum coke contained not only numerous fine pores by interlocking the high aspect ratio β‐Si₃N₄ grains, but also some large pores of 15‐25 μm left by the burnout of petroleum coke. The resultant samples obtained an improved gas permeability of 1.2 × 10^?12 m², which is approximately two times that of samples without petroleum coke addition. Furthermore, the mechanical strength is still superior even at a porosity of 67% in comparison with the other porous ceramics used in the current diesel particulate filter.     

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

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

Self-propagating high temperature synthesis (SHS) of porous Si3N4-based ceramics with considerable dimensions and study on mechanical properties and oxidation behavior

The aim of present work is to fabricate porous Si₃N₄ ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ diluent and Y₂O₃ as raw materials. The results indicate that Si₃N₄ diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si₃N₄ ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si₃N₄ ceramics possessed excellent flexural strength of 168 MPa～259 MPa, and high Weibull modulus of 11.0～17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si₃N₄-based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C～1300 °C, second phase-doped Si₃N₄ ceramics also presented higher residual strength than pure Si₃N₄ ceramics.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

Stress distribution around Fe5Si3 and its effect on interface status and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics with different amount of Fe₅Si₃ were prepared by adding FeSi₂. Residual thermal stress distribution and elastic energy around Fe₅Si₃ particles in various depths were calculated. The interface status between second phase particles and matrix was analyzed in terms of stress and energy. High tangential compressive stresses and low radial tensile stresses were generated along the surface of the ceramics. Elastic strain energy caused by unit interface was high around big particles in deep area of the ceramics. Microcracks are observed around the big Fe₅Si₃ particles. Furthermore, accord to our calculation, microcracks are easily generated around particles in superficial layer of matrix when second phase particles have lower thermal expansion coefficient than the matrix, while microcracks tend to be generated in deep layer of matrix preferentially when the thermal expansion coefficient of second phase particles is higher than matrix. Residual stresses and microcracks around Fe₅Si₃ particles greatly influenced mechanical properties. Fracture toughness of Si₃N₄ ceramics with similar Si₃N₄ particle size distribution increased with the amount of Fe₅Si₃, and fine Fe₅Si₃ particles could enhance the strength of Si₃N₄ ceramics. Si₃N₄ ceramics exceeding 1.2 GPa strength were prepared.     

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.     

Cellular silica-based ceramics prepared by direct foaming at high temperature

Cellular silica-based ceramics, including Si₃N₄/SiO₂ composite ceramics and monolithic silica ceramics, with dense shell and closed cells with dense and crack-free cell wall inside was prepared by the direct foaming of the green-compacts at 1310–1370 °C. The influences of the heat-treatment temperature on the relative density as well as the mechanism of the cell formation were investigated. The porosity of the obtained cellular silica and Si₃N₄/SiO₂ ceramics was within 60.0–84.0%, the cell size distribution was in the range of 10–120 μm, and the flexural strength was 9.7–16.3 MPa.     

Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

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.     

A Novel Si3N4–SiC Ceramic Used for Volumetric Receivers

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