熔盐热析出反应金属化Si3N4与Si3N4的连接

在采用熔盐热析出反应在Si₃N₄陶瓷表面沉积钛金属膜的基础上,对CuAg合金在金属化表面的润湿性进行了研究,结果表明,CuAg合金能对采用该方法金属化的Si₃N₄陶瓷实现良好润湿．在此基础上,成功实现了钛金属化Si₃N₄陶瓷与Si₃N₄陶瓷的连接并对连接工艺进行了系统研究．连接界面的TEM研究发现,界面上广泛存在Ti-Cu－Si-N相并对这种相对连接强度的影响进行了讨论．     

Thermal conductivity of unidirectionally oriented Si3N4w/Si3N4 composites

-silicon nitride whiskers were aligned unidirectionally in silicon nitride sintered with 2 wt% Al₂O₃ and 6 wt% Y₂O₃. It was be densified by the Gas Pressure Sintering (GPS) method. Thermal conductivity of the sintered body with different amount of - silicon nitride whiskers was measured by the direct contact method from 298 K to 373 K. This unidirectionally oriented -silicon nitride whiskers grew into the large elongated grains, and improved also the thermal conductivity. The amount of -silicon nitride whiskers changed the microstrcuture, which changed the thermal conductivity.     

ICVI法制备Si3N4P/ Si3N4 复合材料致密化行为数值模拟

根据Si₃N₄ 颗粒增强体的结构特点及等温化学气相法( ICVI) 的工艺特点, 对Si₃N₄ 颗粒增强Si₃N₄ 复合材料的致密化过程进行了数值模拟。用球形孔隙模型表征Si₃N₄ 颗粒增强体的结构特征, 用传质连续方程表征先驱体在预制体中的浓度分布。为了检验模型的准确性和适用性, 进行了相应的实验验证。模拟结果与实验结果具有相似的致密化规律, 预测的渗透时间和孔隙率与实验结果均十分接近, 表明本文中建立的数学模型可以较好地表征Si₃N₄P / Si₃N₄ 复合材料的ICVI 过程。     

Fracture toughness of Si3N4 and its Si3N4 whisker composite without sintering aids

Si₃N₄ without sintering aids is studied with special interest to the fracture behaviour and its relation to microstructure. Cracks propagated almost transgranularly and no rising R-curve behaviour was found, because crack-wake region gave no contribution on toughening due to very high grain-boundary bonding strength. Microstructure with highly elongated grains was obtained by addition of 20%Si₃N₄ whisker, but fracture toughness was found to be similar to that of the monolithic Si₃IM₄ with equiaxed grains. It is recognized that fracture toughness is not determined simply by apparent microstructural parameters such as mean aspect ratio of grains when grain-boundary bonding is sufficiently strong. Detailed examination of microfracture behaviour is, therefore, necessary for the analysis of toughening in this kind of composites.     

Fracture energy of Si3N4

The fracture energy of Si₃N₄ made by hot pressing, reaction sintering, and chemical vapour deposition (CVD) was studied. Extrapolation of fracture energies to zero additive or porosity levels, as well as analysis of CVD Si₃N₄ all indicate an intrinsic fracture energy of 20–30J m^–2. Higher fracture energies in dense bodies with increasing additive content, or in some more porous bodies (relative to expected porosity dependence) are associated with crack branching. In dense bodies such branching may arise due to micro-cracking from combined effects of crack tip stresses and mismatch stresses due to differences in properties, especially thermal expansion, between Si₃N₄ and the additive or its reaction products. In porous bodies such branching appears to be due to spatial distribution of pores.     

SiC-nanoparticle-reinforced Si3N4 matrix composites

Hot-pressed nanosized SiC-particle (SiC_p-reinforced Si₃N₄ composites have been studied with respect to their microstructures, room temperature mechanical properties and thermal shock resistance. The experimental results indicate that the flexural strength, fracture toughness and thermal shock resistance are all increased by the addition of 5 vol% SiC_p. Further additions of SiC_p, however, have a detrimental effect on these properties. These changes are closely related to the effects of SiC_p on the matrix grain morphology. The mechanisms of nucleation and growth of -Si₃N₄ grains in the presence of SiC nano-particles are discussed.     

Pressure sintering of Si3N4

The sintering of Si₃N₄ with 5% MgO was investigated at 1450 to 1900 C under a pressure of nitrogen. A maximum density of 95% of the theoretical value was obtained, which is greater than that obtained by pressureless sintering. The sintering process was inferred to be liquid-phase sintering and divided into two processes; rearrangement and solution-precipitation. The contribution of rearrangement to densification was about 10% in the present system, and the rest, up to 17% was due to solution-precipitation. Application of the present method of sintering Si₃N₄ with a high strength grain-boundary phase at high temperature is surveyed.     

Pressureless sintering of Si3N4

An investigation of the pressureless sintering of Si₃N₄ powder with the addition of 5 wt % MgO revealed that shrinkage by a liquid phase mechanism and bulk decomposition are two countervailing processes. Within the temperature range studied, i.e. between 1500 and 1750° C, high densities can be achieved when sintering is performed either for long periods at low temperatures or short periods at higher temperatures. A model is presented showing that pore growth due to decomposition causes a decrease in the driving force for sintering and causes shrinkage to cease.     

Si3N4/FePd/Si3N4薄膜的晶体结构和磁性能研究

采用磁控溅射的方法制备了Si₃N₄/FePd/Si₃N₄三层膜, 研究了非磁性材料Si₃N₄作为插入层对磁记录FePd薄膜结构与磁性能的影响。结果表明, 热处理后Si₃N₄分布在FePd纳米颗粒之间, 抑制了FePd晶粒的生长, 与纯FePd薄膜相比, Si₃N₄/FePd/Si₃N₄薄膜的颗粒明显得到细化; 通过添加Si₃N₄层, FePd薄膜的晶体学参数c/a从0.960减小到0.946, 表明Si₃N₄可以有效促进FePd薄膜的有序化进程, 同时提升了矫顽力和剩磁比, 分别提高到249 kA/m、0.86; 随着600℃退火时间的进一步延长, 添加Si₃N₄的薄膜磁性没有迅速下降, 在较宽的热处理时间范围内磁性能保持在比较高的水平, 提高了抗热影响的能力。Si₃N₄作为插入层对FePd薄膜的磁性能具有较大的提升作用, 这对磁记录薄膜的发展具有重要意义。     

Si_3N_4陶瓷与Si_3N_4陶瓷及金属连接的研究进展

对Si3N4陶瓷与Si3N4陶瓷、Si3N4陶瓷与金属的连接工艺进展进行了系统的介绍 ,重点评述了Si3N4直接钎焊法、Si3N4间接钎焊法、Si3N4陶瓷玻璃焊法的研究进展 ,并提出了今后研究的重点     

Investigation of Si and Ge growth on Si3N4/Si

The growth of Si and Ge on silicon nitride surfaces has been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectrometer (AES). In the early stages, Si or Ge nanoclusters appeared irrespective of the different substrates. When annealed, the Si clusters were more stable against coalescence than those of Ge. As these clusters continued to grow, crystalline facets started to form. Both Si and Ge islands grew predominantly with (111)-oriented top facets on the crystalline Si₃N₄(0001)/Si(111). By contrast, they both grew in random orientation on the amorphous Si₃N₄ surface. Low-index facets such as (111) and (001) coexisted with high-index facets such as (113).     

Fractography of chemical vapour-deposited Si3 N4

The fractography of massive amorphous and crystalline chemical vapour-deposited silicon nitride (Pyrolytic-Si₃ N₄) prepared under various deposition conditions using SiCl₄, NH₃ and H₂ as the source gases has been carried out at room temperature in order to clarify the relation between fracture surfaces and structural features. For amorphous Py-Si₃ N₄, three types of fracture surfaces are observed; i.e. (a) a clean contour-like fracture surface, (b) a contour-like fracture surface including black spots and (c) a cone boundary fracture surface. The fracture mode of the crystalline Py-Si₃ N₄ depends greatly on the microstructure. The fracture of fine-grained and low-density Py-Si₃ N₄ occurs intergranularly, probably due to the presence of the undetectable amorphous Py-Si₃ N₄ between grain boundaries, while the coarse-grained and high-density Py-Si₃ N₄ with preferred orientations shows transgranular fracture. The fracture surfaces of massive Py-Si₃ N₄ are made in comparison with those of the varieties of Si₃ N₄ and SiC.