Effects of surfactant and particle size on the microstructure and strength of Si3N4 foams with high porosity

The highly porous Si₃N₄ ceramic foams were prepared by direct foaming with mixed surfactants. The surface tension and viscosity of slurries were tailored by different carbon chain length of surfactants and different mean Si₃N₄ particle size to achieve the pore size controlling. The nearly linear relationship between the pore size and the ratio of surface tension to viscosity was observed, which indicates that the pore size could be precisely tailored by the slurry properties. Si₃N₄ ceramic foams with porosity of nearly 94%, mean pore size of 110–230 µm, and compressive strength of 1.24–3.51 MPa were obtained.     

Highly porous silica foams prepared via direct foaming with mixed surfactants and their sound absorption characteristics

The highly porous silica ceramics were fabricated by direct foaming with mixed surfactants and the influence of silicon nitride addition and solid content on the microstructures and properties were investigated. The results showed that silicon nitride can impede the formation of cristobalite and facilitates the sintering of silica ceramics. When the addition of silicon nitride powders reached 15 wt%, the highest compressive strength of silica ceramic foams could be obtained. The porosity of silica ceramic foams was tailored in the range of 84.61%–91.35% by adjusting the solid content, and the compressive strength of the obtained ceramic foams ranged from 5.89 MPa to 0.94 MPa. Sound absorption characteristics of silica ceramics foams were investigated. With the porosity of ceramic foams increased from 84.61% to 91.35%, the sound absorption coefficients in the entire sound wave frequency were enhanced due to the reduction of flow resistances, besides, the sound absorption peak varied from 4200 Hz to 2300 Hz, and became more intense and sharper.     

Microstructure and permeability of porous zirconia ceramic foams prepared via direct foaming with mixed surfactants

The permeability performance of porous ceramics from particle-stabilized foams is closely associated with the connectivity between bubble-evolved pores. In order to regulate the connectivity of pore structure, the zirconia ceramic foams were fabricated by direct foaming with mixed surfactants of cetyltrimethyl ammonium bromide (CTAB) and sodium N-lauroyl sarcosinate (SLS). Different solid loadings and CTAB:SLS mixing ratios were used in this study. The results indicate that the pore structures of zirconia ceramic foams were interconnected by open windows on the cell walls, and the porosity and average size of cell and cell windows could be tailored by adjusting the solid loading and CTAB:SLS mixing ratio. The decrease in solid loading and CTAB:SLS mixing ratio caused larger porosity and size of cells and cell windows, and thus resulted in the obvious augment of Darcian (k₁) and non-Darcian (k₂) permeability constants. The ranges of k₁ and k₂ of the as-fabricated zirconia ceramic foams are 6.92 × 10^?13-4.05 × 10^?10 m² and 2.09 × 10^?5-3.19 × 10^?9 m respectively.     

Rapid fabrication of Si3N4 ceramics with gradient appearance and microstructure

Si₃N₄ ceramics with tailored gradient in color and microstructure were prepared by a rapid cost-effective one-step approach. The gradient microstructure was obtained by the manipulation of the dissolution-reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si₃N₄ ceramics, the β-phase content gradually changed from 84% to 11%. The Si₃N₄ ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m^1/2 and may be suitable for bio-implantation applications.     

高起泡性和稳泡性蛋白基发泡剂的研制

以碱性蛋白酶催化水解明胶制备了发泡剂母液,并与十二烷基苯磺酸钠和脂肪醇聚氧乙烯醚硫酸钠复配,制得多种新型蛋白基发泡剂。基于气泡指标对这些发泡剂进行初选,并利用初选后的发泡剂制备气泡混合轻质土试样,测定了试样的密度。结合气泡性能指标和试样密度确定的性能最优的发泡剂配方是自制发泡剂F-6（明胶10%、十二烷基苯磺酸钠11.4%、脂肪醇聚氧乙烯醚硫酸钠12.6%）和F-7（明胶10%、十二烷基苯磺酸钠7.8%、脂肪醇聚氧乙烯醚硫酸钠15.4%）。这两种发泡剂的1 h消泡占比分别为52.52%和53.18%,发泡倍数分别为82.61和81.66,综合性能优于一般市售发泡剂,且制出的试样密度较小,强度可达4MPa,完全满足工程要求。     

Architectural design and cryogenic synthesis of Si3N4@(TiN–Si3N4) for high conductivity

Si₃N₄@(TiN–Si₃N₄) composites with heteroshelled structure were designed for enhanced conductivity and successfully synthesized through the simultaneous reduction and in‐situ cocoating process in liquid ammonia at around ?40°C. The heteroshells were composed of nanosized TiN and Si₃N₄ particles, which were amorphous with the size ranging from 10 to 40 nm. Using spark plasma sintering, dense bulk composite with >98.1% relative density of theoretical value were obtained and their electrical conductivity were increased to an adequate value (6.62 × 10² S·cm^?1) for electrical discharge machining by compositing 15 vol% TiN to Si₃N₄, which is superior to the previous reports. The excellent electric performance could be attributed to the heteroshelled structure which guarantees the conductive network can be formed and kept with minimal TiN content. The nanosized Si₃N₄ powders in the shells reduce the content of conductive powders and limit the growth of TiN particles.     

Preparation of high open porosity ceramic foams via direct foaming molded and dried at room temperature

Herein an alternative approach was considered for addressing one difficulty of ceramic foams that the foam slurry with a high content of bubbles which were obtained via direct foaming, cannot maintain well for a long time at room temperature. It is fascinating that the foam slurry mentioned above could stably mold and dry at room temperature, based on an animal protein as foaming agent, kaolin, talc powder and alumina as raw materials, alpha-tricalcium phosphate prepared via co-precipitation as curing agent, and hydrophobic activated carbon powders as stabilizing agent. Effects of the calcination temperatures, the contents of alpha-tricalcium phosphate and activated carbon powder on microstructures, crystal phases, compressive strength and open porosities of ceramic foams were studied systematically. The results indicated that ceramic foams with a high open porosity and uniform pore distribution and sizes sought for application in catalysts supports, could be produced by adjusting these parameters.     

多孔网状Si3N4陶瓷增强体的制备

选用具有连通气孔的网状聚氨酯海绵作为骨架,利用有机泡沫浸渍的Si3N4浆料工艺,以Al粉、Al2O3和ZrO2作为Si3N4陶瓷材料的烧结助剂,采用2次挂料及2步烧结工艺制备了性能良好的多孔陶瓷骨架。在相同烧结工艺条件下,多孔陶瓷的抗压强度随着孔隙率的下降逐渐增加。当烧结温度为1400℃时,多孔陶瓷具有最佳的烧结效果,既能保持很高的通孔率,又保证了材料具有较高的抗压强度。     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.     

Fabrication of novel BPO4 ceramic foams using the combination of the direct foaming method and freeze-drying techniques

In this study, ammonium phosphate monobasic and boric acid were used as the primary starting materials to produce BPO₄ powder by solid-state reaction. Using BPO₄ powders as the main raw material, BPO₄ ceramic foams were prepared for the first time using the direct foaming method and freeze-drying techniques. The effects of the additive content and solid loading on the slurry's rheological behavior were investigated, and the microstructures and properties of the as-prepared BPO₄ ceramic foams were examined. The results reveal that the porosity of the BPO₄ ceramic foams synthesized at 1223 K ranged from 84.2% to 90.4%, the compressive strength ranged from .12 MPa to .72 MPa, and the thermal conductivity ranged from .32 W/(m·K) to .74 W/(m·K) (298 K). The findings of this study have great significance for the development of new thermal insulation ceramic materials.     

DC—PCVD法快速制备Si3N4薄膜

采用ＤＣ－ＰＣＶＤ方法，控制工艺参数，在ＧＣｒ１５钢试样上获得４０μｍ厚的，以Ｓｉ３Ｎ４为主要成分的非晶态绝缘薄膜，沉积速率约为３７Ａ／ｓ，讨论了沉积速率高的原因。     

W/Si3N4 composite ceramics reinforced with core-shell W-crystallized oxynitride phase

The introduction of a reinforced phase is an effective way to toughen silicon nitride ceramics. However, the residual stress caused by thermal expansion mismatch between the reinforced phase and Si₃N₄ ceramics may generate interfacial defects and deteriorate the strength of silicon nitride ceramics. Therefore, a novel strategy for optimizing interface structure to reinforceW/Si₃N₄ composite ceramics has been proposed. After introducing (Y₂O₃, Al₂O₃, W) particles in the W/Si₃N₄ ceramics, the melted liquid phase encompasses the W particles at high temperature and in situ-generates the core-shell structural W-crystallized oxynitride phase as the material cools. As a result, the interfacial defect was inhibited after the core-shell W-crystallized oxynitride phase was introduced. With the increase of W addition from 0 to 4 wt%, the flexural strength of the W/Si₃N₄ ceramics with introducing core-shell structural W-crystallized oxynitride phase increases from 985 to 1168 MPa, and the fracture toughness improves from 6.5 to 9.0 MPa m^1/2.     

Preparation and mechanical properties of Si3N4 nanocomposites reinforced by Si3N4@rGO particles

A new type of reduced graphene oxide-encapsulated silicon nitride (Si₃N₄@rGO) particle was synthesized via an electrostatic interaction between amino-functionalized Si₃N₄ particles and graphene oxide (GO). Subsequently, the Si₃N₄@rGO particles were incorporated into a Si₃N₄ matrix as a reinforcing phase to prepare nanocomposites, and their influence on the microstructure and mechanical properties of the Si₃N₄ ceramics was investigated in detail. The microstructure analysis showed that the rGO sheets were uniformly distributed throughout the matrix and firmly bonded to the Si₃N₄ grains to form a three-dimensional carbon network structure. This unique structure effectively increased the contact area and load transfer efficiency between the rGO sheets and the matrix, which in turn had a significant impact on the mechanical properties of the nanocomposites. The results showed that the nanocomposites with 2.25 wt.% rGO sheets exhibited mechanical properties that were superior to monolithic Si₃N₄; the flexural strength increased by 83.5% and reached a maximum value of 1116.4 MPa, and the fracture toughness increased by 67.7% to 10.35 MPa·m^1/2.     

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.     

Synthesis of Si3N4 whiskers by rapid nitridation of silicon droplets

Belt-like β-Si₃N whiskers were successfully synthesized by nitriding of liquid silicon without catalysts at 1500°C by using micron-sized silicon powders within 10 minutes. Silicon droplets formed by the melting of silicon particles greatly facilitates the diffusion of nitrogen. Several whiskers cling together to form a whisker-cluster. The whisker-clustermorphology results from nitriding of separate silicon droplets. The growth of the belt-like β-Si₃N₄ whisker was controlled by vapor-liquid-solid mechanism. The synthesis of silicon nitride whiskers can be effectively improved by nitriding liquid phase silicon.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

Effect of Nano-Al2O3 on Mechanical Properties and Microstructure of Si3N4 Ceramics

Si3N4ceramics were prepared by pressureless sintering at 1 650 ℃ in nitrogen atmosphere using Si3N4powder as main starting material and adding nanoAl2O3powder( 3%,6%,9%,12%,and 15% in mass,the same hereinafter). The bending strength and fracture toughness( KIC) of the specimens were detected.The microstructure and phase compositions of the specimens were analyzed. The results show that Si3N4ceramics can be prepared by pressureless sintering when adding9%- 12% nano-Al2O3as active reactant,which dissolves in Si3N4,in-situ forming non-oxide SiAlON. The obtained Si3N4ceramics have the maximum bending strength of 710. 86 MPa and KICof 8. 61 MPa·m1 /2.The excellent properties come from many interwoven structures distributed uniformly in the ceramics matrix,which is composed of big and firm plate-like β-Si3N4,hexagonal SiAlON and sheet Si2N2O.     

三维网络结构多孔氮化硅陶瓷增强体的制备

以Si3N4和Si粉为主要原料,Al2O3、Y2O3等为助剂,制备Si3N4料浆,用有机前驱体浸渍和二次烧成工艺来制备具有网络结构的多孔氮化硅陶瓷增强体.结果表明:二次烧成能显著提高材料性能,烧成温度在1600～1700℃为宜.用XRD、SEM、XEDS等对二次烧成材料的显微结构和晶相进行分析,研究二次烧成制度改善材料性能的原因,以利于更好的优化工艺.     

One-pot foam-gelcasting/nitridation synthesis of high porosity nano-whiskers based 3D Si3N4 porous ceramics

Nano-whiskers based 3D Si₃N₄ porous ceramics (3D-NWSNPC) with high-porosity (about 91–93 %), low density (0.17–0.25 g/cm³), low thermal conductivity, and a certain degree of recoverability under cyclic compressive loading and reasonably strengths were prepared at 1423–1523 K via a one-pot foam-gelcasting/nitridation route using inexpensive commercial Si powders as starting materials and hexadecyl trimethyl ammonium bromide as foaming agent. After nitridation at 1523 K, the sample with an original solid loading of 12.5 wt% exhibited the highest compressive strength of 1.9 MPa, even though its density was lowered to 0.25 g/cm³. The sample nitrided at 1473 K had a relative density of 7.3 %. Its compressive and specific strength were respectively 1.1 MPa and 5.5 MPa·cm³ g^?1, and its thermal conductivity was as low as 0.074 W/(m K) (measured at 323 K). These outstanding properties would make the as-prepared 3D-NWSNPC a promising candidate for applications in catalysis, filtration, thermal insulation and many other important areas.