Key Engineering Materials Vols. 336-338

Abstract: The amorphous nano-sized silicon nitride powders were sintered by liquid phase sintering method. Si3N4-Si2N2O composites were in-situ fabricated. The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% was obtained at a sintering temperature of 1650°C and reduced when the sintering temperature increased or decreased, which indicates that the reaction is reversible. The mass loss, relative density and average grain size increase with increasing of sintering temperature. The average grain size is less than 500nm when the sintering temperature is below 1700°C. During the sintering procedure, there is a complex crystallization and phase transition: amorphous Si3N4 → equiaxial α-Si3N4→ equiaxial β-Si3N4 → rod-like
Si2N2O → needle-like β-Si3N4. Small round-shaped β-Si3N4 particles are entrapped in the Si2N2O grains and a high density of staking faults are situated in the middle of Si2N2O grains at a sintering temperature of 1650°C.

Abstract: Densification behavior, microstructural developments and mechanical properties of 5 wt% SiC nano particulate toughened Al2O3 composites were prepared by using microwave sintering at different sintering temperatures. Densities of up to 97.5% of the theoretical value were achieved at temperature as low as 1380°C. Improvements in fracture toughness and bending strengths are of the order of 99% theoretical dense conventionally sintered composites. Surface heating was observed at the final stages of densification. No thermal runaway was observed in the nanocomposites due to presence of SiC spheroids.

Abstract: Porous TiB2 ceramics with a three-dimensional interconnected skeleton were fabricated by high temperature pressureless sintering from fine TiB2 powders. The microstructure of the porous TiB2 ceramic was characterized by the enhanced neck growth between the initially touching particles. This neck growth was ascribed to the selective heating of TiB2 particles with different dimension. The porous structure prepared by the high-temperature sintering exhibited higher bending strength and fracture toughness in the present experiment. The improved mechanical properties of the sintered composites were attributable to the enhanced neck growth by surface diffusion.

Abstract: An organic precursor was synthesized with C3N6H6 and H3BO3 as raw materials in aqueous solution. A novel amorphous BCN compound was obtained by thermolysis of the precursor at 1900°C in flowing N2 atmosphere. Single crystal X-ray diffraction analysis reveals that the precursor is a supramolecular compound with a formula C3N6H6(H3BO3)2. The pyrolysis product was characterized by XRD, XPS, FTIR, and SEM. XRD results reveal that the pyrolysis product has a turbostratic graphite structure. XPS analysis confirms the formation of nitrogen-rich BCN ternary compound with an approximate composition of B3CN3. Both the deconvoluted XPS spectra and FTIR spectrum indicate that B-N, B-C, and C-N chemical bonds have been established and there is no phase separation of graphite and h-BN occurred, which suggests an atomic-level hybrid of B, C, and N in the compound. SEM images show that the crystal shape of the B3CN3 compound is columnar, which is similar to the precursor.

Abstract: Si3N4-TiN-SiC multiphase ceramics was synthesized from TiSi2 and SiC powders by combustion synthesis. The effect of dilute content on conversion percentage was calculated in theory assuming that there is no penetration of nitrogen gas, and combustion synthesis was carried under different conditions. The results show that the increase of diluent content in certain extent is helpful for nitrification of TiSi2 and free silicon were detected in the 30wt% and 40wt% SiC-diluted samples with 45vol% porosity while no silicon were found in 50vol% porosity green parts. The increase in diluent content and porosity of the compacts are favorable for the development of rod-like β-Si3N4 and the long-diameter ratio of β-Si3N4 is increase as the increase of these two values.

Abstract: This paper presents the results of combustion synthesis of porous Si3N4 based ceramics from silicon powders, with fine α-Si3N4 and BN powders as diluents, under high nitrogen pressure. The effect of addition of BN on the morphology is investigated. The results revealed that the strength decreased and the porosity increased with the addition of BN, which is strongly depended on the final morphology of the products.

Abstract: Polycarbosilane (PCS) and SiC powders, in which PCS acted as binder, were adopted as starting materials to fabricate porous SiC ceramics. During pyrolysis, PCS experienced an organicinorganic transformation and bonded SiC powders at a low temperature of 1273K. The flexural strength of porous SiC ceramics increased with increasing PCS content and shaping pressure, while the porosity decreased with increasing the PCS content, shaping pressure and particle size of SiC powders. The fracture surface of porous SiC ceramics was observed.

Abstract: The kaolinite-polyacrylamide intercalation compound was prepared by the displacement reaction of the kaolinite-formamide intercalation precursor with acrylamide and the polymerization under 140°C for 15h with the catalysis of dibenzoyl peroxide. Sialon was synthesized from kaolinite-polyacrylamide intercalation compound by carbothermal reduction and nitridation (CRN) processing at 1400°C. The kaolinite-carbon mixture and its CRN product were also prepared for the comparison with the CRN result of intercalation compound. XRD, FT-IR, and TEM were used to characterize the phases, structure and morphology of products. In the CRN product from the intercalation compound, β′-sialon is the main phase. And the reduction and nitridation rate from the intercalation compound was greater than that from the mixture. There was higher sialon phase’s content and lower mullite phase’s content in the CRN product from the intercalation compound than that from the mixture. The CRN processing of kaolinite intercalation compound is a novel and effective method of sialon synthesis.

Abstract: Reticulated porous ceramics (RPCs) with uniform structure were fabricated by a two-step centrifuging process. The two-step centrifuging process includes two centrifuging steps. In the first stage, the polyurethane sponges are coated with thin slurry through high speed centrifuging process. In the second stage, the as-prepared dried green body was recoated with thixotropic slurry through centrifuging process with an appropriate centrifuging speed. According to the results, the RPCs with controlled structure can be fabricated by modulating the rotation speed in the second centrifuging process.