Zhengbo Yu

Laminated Si3N4/SiC Composites with Self-Sealed Structure

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.

Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure

One of the most creative achievements of researchers in the structural ceramics field is ceramic-matrix composite materials. Like so many materials science accomplishments, uni-directionally aligned continuous fibre reinforced ceramic composites [1–3] or laminated plate-form ceramic composites [4–12] actually partially imitate the structure of a natural material, in this case wood, either in one or two dimensions. The advantage of layered structures is that they perform the function of fibre-reinforced ceramic composites, but are much easier to fabricate. Plate-form laminated ceramic composites with a weak interface such as the SiC/graphite system, usually show a high work of fracture and fracture toughness as high as 14 MPam1/2 [4, 5]. However, the major problem associated with the plate-form laminate is that it possesses two delamination directions, which prevents the laminates from enjoying widespread applications as structural components. Therefore other structures are needed, either at the microor meso-/macro-level, to address this problem. From the meso-/macro-structure point of view, one possible solution to the intrinsic delamination problem in plate-form ceramic laminates is to once again look to a natural material and imitate the ring structure of wood in three-dimensions. This strategy reduces the potential delamination directions in a laminated ceramic material from two to zero when the structure of a laminate changes from plate form to a self-sealed ring, i.e., a highly anisotropic structure. The objective of the present work is to create a ceramic laminate by imitating the concentric cylinder tree trunk structure and to examine the delamination resistance and fracture behaviour of the structure. A simple shaping technique, a modified slip casting method, has been used to achieve a self-sealed ring structure for a variety of ceramic laminates. In our experiment, the silicon carbide/carbon system was chosen as an example to demonstrate this structure. This system has two characteristics: (1) the graphite not only happens to be a successful sintering aid for SiC but also provides a weak interface [4, 5] and (2) our previous research [4] showed that the carbon layers can be converted into porous silicon carbide layers. Hence a single-phase SiC ceramic with a better oxidation resistance can be obtained while at the same time retaining a suitably weak interface. SiC slurry was prepared by mixing 88 wt% of SiC, 8 wt% of Al2O3 and 4 wt% of Y2O3 with water with solid to liquid ratio of 30/70 by volume. The concentration of carbon in the water based graphite slurry was between 2.5 and 5% by volume. SiC/graphite laminates were slip-cast with alternate SiC and graphite layers in a plaster of Paris mould with a casting chamber 10 mm in diameter and 50 mm in length. All the laminates were fabricated in such a way that the outermost layer and core were SiC. The thickness of the SiC layer was varied from 50 to 600 μm and that of the graphite layer from 5–20 μm by adjusting the viscosity of the slurry and casting time. The number of SiC layers in the structure was varied from 20 to 25. After slip casting, the green bodies were slowly dried in air for 48 h and sintered at temperatures ranging from 1,800 to 1,850◦C for 1 h in an Ar atmosphere at 1 atm pressure. Bulk densities were measured by the Archimedes method. The theoretical density of the laminates was calculated on the basis of the rule of mixtures. Three-point bending tests on specimens with a span of 25.4 mm were conducted using an Instron machine (8502) at room temperature. The crosshead speed was 0.06 mm/min. Since ceramic rods with circular crosssections were used for the mechanical property tests, the work of fracture/failure work was used to characterize the fracture resistance of the silicon carbide/carbon laminates. The total work per unit volume during a bending test can be written as:

J-phase structures in the Y-Si-Al-O-N system

Preparative studies carried out on compositions lying between Y4Si2O7N2 (J-phase) and Y4Al2O9 (YAM) show similar variations in structure to those already reported for related compositions in the analogous Dy-Si-Al-O-N system. Careful studies of X-ray diffraction patterns confirm that whereas all compositions have a cuspidene-like structure, there is a marked change in the a and b unit cell dimensions for x values in excess of approximately 0.8 in the general formula Y4Si2-xAlxO7+xN2-x, with much smaller changes in the c unit cell dimension and in the β angle. The results are compared with similar data previously presented for J-phase structures in the DySi-Al-O-N system.

Effects of Sintering Temperature and Annealing in N2 Atmosphere on the Dielectric Properties of BaTio3 Based X7R Dielectric Materials

Abstract Two commercial BaTiO3 based powders were selected, characterized and used for the fabrication of X7R dielectric ceramics. The effects of forming and sintering on the dielectric properties have been studied and processing parameters optimized. The annealing treatment in N2 atmosphere has been used in order to modify the dielectric properties of BaTiO3. The related dielectric properties have been tested and microstructures of BaTiO3 ceramics examined. The results show that by combining the optimized processing parameters such as cold isostatic pressure 225 MPa, sintering temperature from 1300 to 1400 °C and annealing treatment at 850 °C, a marked increase in dielectric constant from approximately 3000 to 3600 has been achieved with the commercial BaTiO3 based powder. This was accomplished while keeping the X7R capacitor requirements on the dissipation factor (<2%) and temperature coefficient of capacitance (<±15%).

Development of BaTiO3 Based X7R Wafers for Single-Layer Capacitors

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.

Structure and properties of high dielectric constant CaCu/sub 3/Ti/sub 4/O/sub 12/ ceramics

The structure and dielectric properties of CaCu/sub 3/Ti/sub 4/O/sub 12/ ceramics have been investigated. The starting CaCu/sub 3/Ti/sub 4/O/sub 12/ powders were synthesized by a modified glycine nitrate process. The ceramics obtained exhibit a cubic perovskite structure. Dielectric constant up to 57,200 and dielectric loss of 4.4% at 1 kHz have been observed at room temperature. The dielectric properties have been investigated using an impedance spectroscopy technique. The effect of processing and microstructure on the dielectric properties has been discussed.