Papers by Author: Zhengbo Yu

Abstract: Self-sealed laminar Si3N4/SiC composites, with different cross-section shapes and various thickness ratios of Si3N4 to SiC, have been fabricated. The laminates consist alternately of thicker Si3N4 layers ranging from 100 to 500µm and thinner SiC layers ranging from 6 to 15µm after sintering. Preliminary results indicate that SiC thin layer forms during sintering according to the reaction Si3N4 + 3C ® 3SiC + 2N2, which is confirmed by X-ray diffraction. An excellent physical and chemical compatibility between Si3N4 and SiC layers was observed. The self-sealed Si3N4/SiC composites not only demonstrate a superb resistance to delamination, usually associated with the plate-form ones, but also show a high damage-tolerance behavior. The laminated Si3N4/ SiC composite with a layer thickness ratio of Si3N4 to SiC of approximately 40 gives the highest value of work of fracture (WOF) of approximately 406 kJ/m3, whereas the highest toughness of 21 MPam1/2 was achieved at the layer thickness ratio of 50. The effects of the relative thickness of Si3N4 and SiC layers on the densification of the laminates are examined and fracture behavior and microstructure of the Si3N4/SiC laminates discussed.

Abstract: Research on the development and characterization of X7R BaTiO3 dielectric wafers for high frequency single-layer capacitors has been carried out. Commercial BaTiO3 powders were processed and optimized in dielectric constant (er), dissipation factor (DF) and temperature coefficient of capacitance (TCC). The results indicate a strong effect of sintering temperature on dielectric properties as well as on mechanical properties of the BaTiO3. It has been shown that the highest dielectric constant is achieved at high sintering temperatures (>1400°C) and lowest TCC at lower sintering temperatures (<1300°C). By optimizing fabrication process e.g. dry pressing, cold isostatic pressing, sintering, and machining such as grinding and lapping, BaTiO3 wafers of dielectric constant from 3400 to 3600, with a diameter approximately 50mm and thickness 150µm, have been manufactured successfully. The fabricated thin wafers exhibit the X7R capacitor characteristics of the dissipation factor (<3%) and temperature coefficient of capacitance (<±15%) in the temperature range of –55°C to 125°C.