Diffusion joining of Si3N4 ceramics by hot pressing under high nitrogen gas pressure

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Textured crystal growth of Si3N4 ceramics in high magnetic field

A systematic investigation has been made on the textured crystal growth of Si₃N₄ ceramics in high magnetic field with and without rotating of the gypsum mold during the slip casting. According to X-ray diffraction and microstructure analysis, a 10 T magnetic field is found to have a tendency to orient Si₃N₄ grains with their a- and b-axis parallel to the magnetic field. If the gypsum mold is rotated during the slip casting in the magnetic field, the crystal growth of Si₃N₄ grains has a tendency to align on a certain direction.     

Nanostructure defects in Si3N4

Microstructure defect studies of Si₃N₄ have contributed much to the development of Si₃N₄ ceramics material. In this paper we present some results of nanostructure defects by using high-resolution electron microscopy (HREM), showing new structural phenomena at the atomic level.     

Influence of powder characteristics on gas pressure sintering of Si3N4

The sintering behaviours of four kinds of Si₃N₄ powders were investigated by dilatometry in 10 atm N₂ at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si₃N₄ grains and reprecipitation as elongated -Si₃N₄ grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth.     

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 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.     

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.     

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.     

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.     

Iron impurities in Si3N4 processing

The atomic environment of iron impurities is investigated during the processing cycle of reaction-bonding silicon nitride (RBSN). Several analysis techniques are utilized, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR), to examine iron impurities in the starting silicon powder, in sintered silicon compacts, and in RBSN materials. Results indicate that iron impurities in as-received metallurgical grade silicon powder are incorporated in the silicon bulk as a highly distorted FeSi₂ compound. No surface iron or iron-based particulate is observed in the starting material. Upon sintering, the iron environment becomes an ordered FeSi₂ structure. In the RBNS material, the FeSi₂ structure is again distorted, as observed by both EXAFS and ESR.