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。     

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.     

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.     

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.     

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.     

添加剂和升温速度对Si3N4陶瓷显微结构的影响

研究了添加剂组合与升温速度对气压烧结Ｓｉ３Ｎ４显微结构的影响。第二添加剂使晶体粒径整体增大，长径比降低，同时加入Ｓｃ２Ｏ３和ＺｒＯ２，并慢速升温，可有效提高长径比，放慢升温速度利于形成双峰粒度分布。     

群青粒子的微胶囊化及其分形表征

研究了群青粒子微胶囊化及改性的方法，并用表面分形维数对粒子的表面形貌予以量化表征。结果表明：群青粒子的微胶囊化是改善群青不耐酸性的有效途径；群青粒子的Ｄ值在２－３这间，微胶囊化后的粒有面分维大于原样品粒子的表面分维，且微胶囊化粒子的Ｄ值越大，生胶囊化改性效果越明显。     

常压烧结Si3N4-MgO-Y2O3-CeO2陶瓷的研究

本研究了Si3N4-MgO—Y2O3-CeO2陶瓷的烧结过程和微观结构,常压烧结氮化硅陶瓷的致密化主要通过液相烧结实现。微观分析结果表明,氮化硅烧结体的显微结构为等轴状的α—Si3N4和长柱状的β—Si3N4相互交织,这种结构有利于提高烧结体的强度和韧性。     

Si3N4/Si3N4扩散连接的力学性能研究

采用热压法进行氮化硅陶瓷材料的扩散连接.结果表明:在1520℃,15MPa,60min条件下,氮化硅连接体的最高强度为448.6MPa,超过母材强度;平均连接强度为401.5MPa,为母材强度的96%.     

氮化硅针状晶体的制备

采用Ｓｉ粉直接氮化的方法制备氮化硅针状晶体,通过热力不计算,选择了晶体 温度和压力范围,探讨了温度、压力、添加剂等对针状晶体晶相、形貌、产率的影响,并在１８５０℃、ＣａＯ质量分数为１０％条件下,制备了长径比为４￣１０的β－Ｓｉ３Ｎ４针状晶体,产率为６８％,采用ＸＲＤ,ＴＥＭ、ＳＥＭ对得到的氮化产物及针状晶体进行了表征。     

多孔Si3N4-SiO2复相陶瓷及其性能

采用近净尺寸成型制备工艺-氧化烧结结合溶胶浸渍再烧结法,制备了多孔Si3N4-SiO2复相陶瓷.讨论了制各工艺对材料的成分、微结构和性能的影响规律.研究表明:随着硅溶胶浸渍量的增加,材料的抗弯强度、硬度、断裂韧性、密度和介电常数均增大.分别采用压痕法和单边切口梁法对材料的断裂韧性进行了测定和比较.结果表明:采用压痕法测定断裂韧性时,多孔Si3N4-SiO2复相陶瓷的增韧机理有裂纹偏转、裂纹分叉、裂纹桥接以及孔的钝化.采用单边切口梁法测定断裂韧性时,多孔Si3N4-SiO2复相陶瓷的增韧机理只有裂纹偏转.     

Electrical resistivity of Si3N4 ceramics with Yb2O3 additive

Si₃N₄ ceramics with excellent mechanical properties are used for heat dissipation substrates and so on. In order to improve their reliability and expand their application fields, it is desirable to understand and control the electrical properties of Si₃N₄ ceramics. In this study, the electrical resistivity of Si₃N₄ ceramics with Yb₂O₃ additive was investigated by applying various voltages at temperatures ranging from 25°C to 300°C. When Yb₂O₃ was added as a sintering aid to Si₃N₄ ceramics, a crystalline J-phase (Yb₄Si₂O₇N₂) was formed and their electrical resistivity was significantly lower than that of Y₂O₃ additive. The electrical resistivity of the Yb₂O₃-added ceramics decreased with an increase in temperature and applied voltage. Yb existed in multiple valence states, Yb²⁺ and Yb³⁺, in the Si₃N₄ ceramics and the decrease in the electrical resistivity can be attributed hopping conduction through the J-phase. The J-phase in the Si₃N₄ ceramics was observed to be continuous, and percolation analysis suggested that the J-phase formed an infinite cluster. Therefore, the decrease in the electrical resistivity of the Yb₂O₃-added Si₃N₄ ceramics was found mainly to result from the formation of an infinite cluster of J-phase, which exhibits hopping conduction.     

初始硅粉粒度对自蔓延高温合成氮化硅的影响

研究了平均粒度分别为２,７．８和１５．４μｍ的３种初始硅粉在氮气中的燃烧氮化规律。初始硅粉粒度越细,则在氮气中的燃烧温度高越高,燃烧滤蔓延速度越快,激活能也越低;较细的硅粉表面的硅蒸发通量大,ｐｓｉ高,易于形成延长方向与硅粉表面垂直的针状或柱状、纤维状晶体;而较粗的硅粉则易于形成氮化硅包覆层,且可以通过“包覆爆裂”机制继续进行二次氮化。较细的硅粉在氮气中的燃烧温度曲线上只出现一次燃烧峰,而较粗的硅     

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.     

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 and characterization of Si3N4 whisker-reinforced SiO2 ceramic for radome materials

Fused silica ceramic has become one of the most widely used radome materials in the world since the 1970s. But its poor mechanical properties restricted its application to some extent. To improve the mechanical properties of the fused silica ceramic and keep its characteristic for radome materials, silicon nitride (Si₃N₄) whisker-reinforced fused silica ceramics were prepared by a slip-casting method in the work. The influence of Si₃N₄ whisker contents on the properties of the slurry was studied, indicating that the preferable pH values of the slurry were 4–6 and whisker contents were 10 wt.%. The flexural strength of as-prepared Si₃N_4w/SiO₂ ceramic was about 74.35 MPa, exhibiting an increase of 7.75% over that of the pure silica sample. Its dielectric constant in the range from 8 to 12 GHz and tanδ under 10 GHz were, respectively, 3.37 and .0011. It is of great interest to find that Si₃N_4w/SiO₂ has excellent oxidization resistance and its mass maintains even at 1270°C.     

Fabrication of Si3N4 ceramics with high thermal conductivity and flexural strength via novel two-step gas-pressure sintering

Si₃N₄ ceramic substrates serving as heat dissipater and supporting component are required to have excellent thermal and mechanical properties. To prepare Si₃N₄ with desirable properties, a novel two-step gas-pressure sintering route including a pre-sintering step followed by a high-temperature sintering step was devised. The effects of pre-sintering temperature (1500 – 1600 °C) on the phase transformation, microstructure, thermal and mechanical properties of the samples were studied. The pre-sintering temperature played an important role in adjusting the Si₃N₄ particles’ rearrangement and α→β transformation rate. Furthermore, the densification process for the Si₃N₄ ceramics prepared via the two-step gas-pressure sintering was revealed. After sintered at 1525 °C for 3 h followed by a high-temperature sintering at 1850 °C for another 3 h, the prepared Si₃N₄ compact with a bimodal microstructure presented the highest thermal conductivity and flexural strength of 79.42 W·m^?1·K^?1 and 801 MPa, respectively, which holds great application prospects as ceramic substrates.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Si3N4／Ti界面反应机理及界面层的显微结构

研究了采用熔盐热析出反在无压烧结氮化硅陶瓷表面生成的钛金属膜与陶瓷基体的界面反应机理和反应层显微结构，研究了不同反应温度下的界面反应产物及其分布规律，并探讨了界面反应的机理 。