Microstructure and thermal conductivity of gas-pressure-sintered Si3N4 ceramic: the effects of Y2O3 additive content

Si₃N₄ ceramics were sintered at 1900 °C under a nitrogen pressure of 1 MPa using Y₂O₃-MgO additives. The effects of Y₂O₃ content (0.5-4 mol%) on microstructure and thermal conductivity were systematically investigated. The increasing Y₂O₃ content led to increases in amount and viscosity of liquid phase during sintering, which induced a “bimodal to normal” transition in distribution of grain size, decreased Si₃N₄/Si₃N₄ contiguity and enhanced devitrification degree of intergranular phase in sintered bulks. Moreover, the decreasing Y₂O₃ content was found to improve the elimination efficiency of SiO₂ impurity during sintering, resulting in lower lattice oxygen content in densified specimens. The microstructure had a strong effect on thermal conductivity. The samples sintered for 3 h gained an increase of thermal conductivity from 65 to 73 W·m^-1 K^-1 with increasing Y₂O₃ content, while the samples sintered for 12 h obtained a substantial increase of thermal conductivity from 87 to 132 W·m^-1 K^-1 with decreasing Y₂O₃ content.     

Synthesis of well‐dispersed β‐Si3N4 with equiaxed structures using carbothermal reduction–nitridation method

Well‐dispersed β‐Si₃N₄ powders with a novel equiaxed structure and eminent crystal integrity were prepared by carbothermal reduction–nitridation (CRN) strategy with the assistance of CaF₂ additive. The growth mechanism of Si₃N₄ particles in the CRN process was elucidated. It is proposed that the liquid phase formed by SiO₂ and CaF₂ additive is crucial to the formation of equiaxed β‐Si₃N₄, and with an appropriate content of CaF₂, Si₃N₄ powders with pure β phase, superior dispersity and crystal integrity can be obtained.     

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。     

Synthesis and growth mechanism of approximate spherical β‐Si3N4 particles via carbothermal reduction‐nitridation method

In this work, an efficient carbothermal reduction‐nitridation (CRN) strategy was rationally designed to directly synthesize β‐Si₃N₄ powders with eminent dispersity and granularity uniformity. With the aid of CaO additive, the obtained β‐Si₃N₄ particles were endowed with approximate spherical morphology and smooth surface. The size of β‐Si₃N₄ particles could be regulated in submicro and microscale by altering N₂ pressures. More significantly, the underlying growth mechanism of the β‐Si₃N₄ under elevated N₂ pressure was comprehensively analyzed and tentatively put forward. Benefiting from the remarkable merits, the as‐synthesized β‐Si₃N₄ powders showed great potential for alternative fillers in the application of high thermal conductivity plastic packages.     

Synthesis and growth mechanism of single crystal β‐Si3N4 particles with a quasi‐spherical morphology

Single‐crystal β‐Si₃N₄ particles with a quasi‐spherical morphology were synthesized via an efficient carbothermal reduction‐nitridation (CRN) strategy. The β‐Si₃N₄ particles synthesized under an N₂ pressure of 0.3 MPa, at 1450°C and with 10 mol% unique CaF₂ additives showed good dispersity and an average size of about 650 nm. X‐ray photoelectron spectroscopy analysis revealed that there was no SiC or Si–C–N compounds in the β‐Si₃N₄ products. Selected‐area electron‐diffraction pattern and high‐resolution image indicated single crystalline structure of the typical β‐Si₃N₄ particles without an obvious amorphous oxidation layer on the surface. The growth mechanism of the quasi‐spherical β‐Si₃N₄ particles was proposed based on the transmission electron microscopy and energy dispersive X‐ray spectroscopy characterization, which was helpful for controllable synthesis of β‐Si₃N₄ particles by CRN method. Owing to the quasi‐spherical morphology, good dispersity, high purity, and single‐crystal structure, the submicro‐sized β‐Si₃N₄ particles were promising fillers for preparing resin‐based composites with high thermal conductivity.     

Texture,microstructures, and mechanical properties of AlN‐based ceramics with Si3N4–Y2O3 additives

Textured AlN‐based ceramics with improved mechanical properties were prepared by hot pressing using Si₃N₄ and Y₂O₃ as additives. The introduction of Si₃N₄–Y₂O₃ into AlN matrix led to the formation of secondary Y₃AlSi₂O₇N₂ and fiber‐like 2H^δ AlN‐polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN‐based ceramics by the replacement of Y₂O₃ by Si₃N₄–Y₂O₃ increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m^½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m^½ and 389.5±45.5 MPa, respectively.     

Novel polycarbosilazanes with Si?C and Si?N bonds in the main chain

Novel polycarbosilazanes possessing Si–CH₂ Si N linkage in their backbone are described. Their preparation involves thermal cross-linking of poly[(dimethylsilylene)-co-(1,3-dimethyl-1,3-disilazane)]s at 300 350 C. Thus, under controlled conditions, soluble and fusible preceramic polymrs were obtained. Upon pyrolysis, these materials gave good yields of silicon carbonitride-based ceramics containing low free carbon percentages and controlled nitrogen proportions.     

Sintered reaction-bonded silicon nitrides with high thermal conductivity: The effect of the starting Si powder and Si3N4 diluents

Silicon nitride ceramics with high thermal conductivity were fabricated by employing the reaction bonding method. It was revealed that the addition of Si₃N₄ diluents affected both the nitriding reaction and the post-sintering behavior by changing the size of the silicon particle during the milling process. The reduced size of silicon particle led to an increased degree of nitridation. Further, narrower pore channels in the nitrided bodies caused by the reduced size of silicon particle enhanced the final density, by promoting the easier elimination of finer pores during post-sintering. The positive effect of the finer silicon particle was confirmed by a back-up experiment employing a variety of silicon particle sizes, produced by milling the raw silicon powder for different milling times. Thermal conductivity was dominated by material density rather than variation of the microstructure or oxygen content in the current research.     

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.     

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。     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.     

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。     

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。     

Si3N4／环氧树脂导热复合材料制备和性能研究

采用硅烷偶联剂KH-550对氮化硅（β—Si3N4）进行表面处理,浇注制备氮化硅／环氧树脂（Si3N4／EP-828）复合材料,研究了Si3N4粒径、用量和表面改性对复合材料导热性能和力学性能的影响。结果表明,Si3N4／EP-828的导热性能随Si3N4用量的增加而提高,当Si3N4体积分数为30％时,热导率为0．83W／mK,为纯环氧树脂4倍多;力学性能则随Si3N4用量的增加先增大后降低。表面改性有助于进一步提高复合材料的导热性能和力学性能。初步分析表明,Si3N4／EP-828热导率与Si3N4形成的导热网链和Si-O-Si键导热骨架有关。     

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     

以柠檬酸铵为TiO2分散剂控制Si3N4-TiN复相陶瓷的显微结构

以Si3N4,AlN和TiO2为原料,Y2O3和Al2O3为烧结助剂,通过添加柠檬酸铵作为TiO2的分散剂,采用原位反应合成法制备TiN体积分数为5%的Si3N4-TiN复相陶瓷,在高温烧结过程中原料中的TiO2和AlN反应生成TiN.通过扫描电子显微镜观察了柠檬酸铵分散剂用量对Si3N4-TiN复相陶瓷显微结构的影响.结果表明:添加柠檬酸铵分散剂降低原料混合粉体中TiO2的团聚,获得组分均匀的Si3N4-TiO2-AlN复合粉体,从而提高Si3N4-TiN复相陶瓷中TiN相在Si3N4基体中的分散性,烧结后获得显微结构均匀的Si3N4-TiN复相陶瓷.当在体系中添加0.20g柠檬酸铵分散剂可以显著改善Si3N4-TiN复相陶瓷的显微结构,TiN晶粒被控制在0.2～0.3μm.     

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 。     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

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.