(A) study on the microstructure tailoring of sintered silicon nitride ceramics

Abstract

Silicon nitride ($Si_3N_4$) ceramics has been explored extensively for structural applications. $Si_3N_4$ exhibit superior properties such as high strength, fracture toughness, good thermal shock, good oxidation resistance at both room and high temperature. Recently, its application range is expanding toward various applications, such as power electric module, microwave window application with its potential high thermal conductivity and low dielectric properties. However, those properties have not reached their full potential and it is also inhibited for realizing of a wide applications by a scatter of the property data

strength, fracture toughness and thermal conductivity plus insufficient reproducibility. Properties of $Si_3N_4$ ceramics are strongly influenced by various factors and one of the important factors governing the final properties of $Si_3N_4$ is the microstructure. $Si_3N_4$ ceramic has a relatively complex microstructure and it is difficult to tailor and many of the investigations had been focused on the analysis of the influence of additives on properties. The microstructural evolution in $Si_3N_4$ ceramics is important to optimize the properties for specific applications. Therefore, a tailoring the microstructure is necessary to fulfil the requirement for various applications. This thesis aims to understand and tailor the microstructure of sintered $Si_3N_4$ ceramics for various applications. Through the microstructure tailoring, the sintered $Si_3N_4$ ceramics could have various enhanced properties for designated application. The structure of present thesis is as follows: The literature review in Chapter 2 consists of four parts. Firstly, the characterization and applications of silicon nitride ceramics are described. The crystallization structure of $Si_3N_4$ ceramics and the preparation method of powder synthesis and applications are included. The second section describes the sintering method such as liquid phase sintering, hot-pressing, spark plasma sintering and gas pressure sintering. The properties of $Si_3N_4$ ceramics are then followed. Thermal conductivity and dielectric properties of $Si_3N_4$ ceramics and their measurement method are also described. Chapter 3 describes about the dense $Si_3N_4$ ceramics regarding to the thermal conductive properties. Dense $Si_3N_4$ ceramics are prepared with various sintering additives, sintering process and different raw powder. In addition, the factors influencing the thermal conductivity are investigated. In order to reduce the oxygen-related defect in sintered $Si_3N_4$ ceramics, non-oxide sintering additives are utilized. In addition, the secondary phase is less remained with the sintering additives, which have high vapor pressure at high temperature, and post heat treatment process. During the post heat treatment process, the remaining secondary phases from added sintering additives are evaporated and induce the improvement of thermal conductivity of $Si_3N_4$ ceramics. $Si_3N_4$ raw powder also can be purified by heat treatment process. The impurities presenting in the raw powder, such as oxygen and carbon can reduce after heat treated process and consequentially the thermal conductivity is improved. In Chapter 4, the porous $Si_3N_4$ ceramic with low dielectric properties is presented. Simply, the effect of sintering additives on the dielectric properties is examined and the $Si_3N_4$ ceramics formed composite materials with BN and $SiO_2$. The dielectric constant decreases with the amount of second phase containing the composite with $Si_3N_4$ ceramics or porosity increases. On the other hand, the methods of formation of composites or porosity have affected to the microstructural evolution and the related properties also have been influenced. Pore-forming agent can introduce the pore in green body before the grain growth or densification occurred and these pores come into the space for growing of elongated $\beta$-$Si_3N_4$ grains. Fibrous $\beta$-$Si_3N_4$ grains are grown and formed the crosslinked structure in the pore introduced by pore-forming agent and this microstructural evolution is helpful to improve the strength while the low dielectric or porosity are maintaining. In Chapter 5, tailoring of microstructure of the $Si_3N_4$ ceramics and the related properties are described. The effects of starting powder on the microstructural evolution are investigated. The amount and morphology of pre-existing the $\alpha$/$\beta$ phase and in raw powder have affected the growth mechanism and behavior resulted in various microstructure of $Si_3N_4$ ceramics. Ostwald ripening or abnormal grain growth, or normal grain growth are observed according to raw powders with different $\alpha$/$\beta$ ratio. The effects of sintering additives on the densification and phase transformation are investigated. Non-oxide and transient sintering additives induce the rapid transformation while minimum densification occurred, resulted in fabrication of porous $Si_3N_4$ ceramics. Furthermore, the microstructural evolution regards of the amount of phase transformation are investigated. The amount of beta nuclei capable to grow has influenced the microstructure of final material. Highly improved the properties in $Si_3N_4$ ceramics is able to achieve by tailoring the microstructure and could be very helpful in the various applications. In Chapter 6, analyses of the properties of individual single $Si_3N_4$ grains were introduced with respect to the thermal conductivity and mechanical strength. A membrane-type platform device was employed to measure the single rod of $Si_3N_4$ grains at the temperature range of 300 - 450 K. However, measured the thermal conductivity of single rod have very low value of thermal conductivity due to the large thermal contact resistance between the material and electrode of device. It is still challenging to measure the thermal conductivity of single grain. Fixed-ends beam bending tests were performed for analysis of the strength behavior in porous $Si_3N_4$ ceramics which was composed with cross-linked elongated grains. The fracture stress of single $Si_3N_4$ ligament showed about 5 to 10 times higher values compared to the bulk sample. Size-induced strengthening was occurred in single $Si_3N_4$ ligaments. Smaller sized ligament showed higher strength compared to the $Si_3N_4$ ligament with long length. According to the result of measured the single lattice property, it could give a guideline to tailor the microstructure for each proper purpose.