Papers by Author: Ji Qiang Gao

Abstract: Two domestic silicon carbide powders with different particle size distribution and one petroleum coke powder were blended in proportion and then dispersed in aqueous medium. Green bodies were solidified from these suspensions via conventional slip casting. The effects of pH, solid loading, and the amount of dispersant on the formability and packing ability were evaluated. The results showed that the pH within 9.5-10.5, solid loading of 42vol%, aging time more than 24h, and 0.3wt% of dispersant were optimal. Complicated green bodies with height of 300mm and thickness of 3.5mm were obtained. The corresponding suspension viscosity was 1200mPa∙s and the relative packing density was 64.8%. The density is 3.01g∙cm-3 and the flexural strength is 305±15 MPa after reaction sintering. These results may be attributed to accurately using of dispersant and in-depth study of processing parameter.

Abstract: A novel method for preparing porous silicon carbide ceramics with high porosity had been developed by recrystallization of green bodies composed of α-SiC, β-SiC, remnant silicon and incompletely-reacted carbon. Fine microstructure and uniform pore structure of the resultant porous silicon carbide ceramics was obtained. The green bodies of porous ceramics were prepared by the precursor powder which contained α-SiC, carbon black and silicon powder. The precursor powder was sintered at 1600°C under Vacuum circumstance to obtain the green bodies; the sintering process is same with the reaction sintering silicon carbide. Then the green bodies were sintered to 2300°C for half an hour to recrystallization. The incompletely-reacted carbon was fully reacted with silicon. And the remnant silicon was excluded during the recrystallization process to create porous structures. The influence of composition of the precursor powder and the fabrication process (the moulding pressure) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the transformation of β-SiC to α-SiC during the recrystallization process. The density and the porosity of this material was 1.027g/cm3 and 67% respectively.

Abstract: In this paper, porous Si3N4 ceramics were fabricated by carbothermal reduction between carbon black and diatomite. Diatomite is a siliceous, sedimentary rock consisting principally of the fossilized skeletal remains of diatom, a unicellular aquatic plant related to the algae. The main ingredient of diatomite is the amorphous active silicon dioxide. The influence of diatomite particle size on the microstructure of sintering bodies was analyzed. XRD analysis demonstrated the formation of Si3N4 except for minor of glass phase. SEM analysis showed that the resultant porous β-Si3N4 ceramics occupied fine microstructure and uniform pore structure.

Abstract: Dense multiwall carbon nanotubes (MWNTs) reinforced barium aluminosilicate (BAS)–silicon nitride (Si3N4) composites were fabricated by hot-pressing sintering. The effect of MWNTs on the microstructure, compositional investigations, as well as mechanical characterization of the composites was investigated. The results show that MWNTs were preserved in the composites after sintering and present good adherence to matrix grains. The incorporation of 3% 1-2μm MWNTs effectively improved the fracture toughness of the BAS/Si3N4 composites from 8.02 to 8.6 MPa•m1/2.

Abstract: The self-propagating combustion reaction 0.741Mg + 0.247Fe2O3 + 0.188Ni + 0.318Cr → 0.741MgO + Fe0.494Ni0.188Cr0.318 was applied to prepare a nano-MgO reinforced Fe-Cr-Ni composite, by reactive hot pressing (RHP) under a condition of 700°C/30MPa/2h. The densification was enabled by the low temperatures produced by the exothermic reaction. According to TG-DTA and X-ray diffractometry (XRD), the highly-exothermic thermite reaction began at about 600°C and the in-situ formation of composites comprised predominantly of (FCC) Cr0.19Fe0.7Ni0.11, (FCC) Fe-Cr, (BCC) MgO and a small quantity of (BCC) MgFe2O4. The Vickers hardness was 3.67GPa, the three-point bending strength was 112.5±10MPa, and the fracture toughness was 3.28 MPa•m1/2. The microstructure of the composite was observed via scanning electron microscopy. This indicated that the distributions of in-situ-formed (BCC) MgO phases (~800 nanometers) were homogeneous into in a matrix of a fine-grained metallic alloy phases that gather together to form agglomerates in the composite.

Abstract: SiC/h-BN nano-composites powder was synthesized using an in-situ reaction method and SiC/h-BN nano-composites were fabricated by hot-pressing. Damage resistance and R-curve behavior of machinable SiC/h-BN nano-composites was evaluated using the indentation- strength-in-bending technique and compared with that of SiC/h-BN micro-composites and monolithic SiC. The results showed that SiC/h-BN nano-composites have better damage resistance and R-curve behavior than SiC/h-BN micro-composites and monolithic SiC.

Abstract: Effects of nitrogen content on hot ductility of duplex stainless steels have been investigated. With the increase of nitrogen content in the duplex stainless steels, mechanical strength increased, while hot ductility and elongation decreased. With the same strain rates and deformation degree, the high nitrogen content led to the high optimum hot ductility temperature for the high nitrogen DSS alloy. These results indicated the importance of control over the shape and volume fraction of phases in duplex stainless steels to achieve the optimum hot ductility.

Abstract: The barium aluminum silicate-silicon nitride (BAS-Si3N4) matrix-ceramic composite was fabricated using pressureless sintering, at temperatures ranging from 1720°C, which is below the melting point of BAS, to 1850°C. The effect of processing conditions on sinterability, crystalline structure, microstructure and mechanical properties was evaluated. It was demonstrated the BAS glass-ceramic served as an effective liquid-phase-sintering aid, to attain high densities and completed the α-Si3N4–β-Si3N4 phase transformation, and remained as a structural matrix that was reinforced by the rod-like β-Si3N4 grains. Si3N4 grains nucleated and grow in random directions in an almost completely crystallized matrix of hexacelsian BAS. High flexural strength (665±40 MPa) and fracture toughness (7.74 MPa•m1/2) could be obtained from 30wt%BAS-70wt%Si3N4 samples that have been sintered at 1800°C for 120 min with a fine-grained microstructure.

Abstract: In this paper, cordierite-mullite multiphase ceramics material was prepared using cordierite powder, mullite particles, fused silica, magnesia and alumina as main starting material. Effects of addition of 2%~10% SiC on the thermal expansion, flexural strength and thermal shock resistance were studied, and the fracture surface morphology was observed with Scanning Electron Microscope (SEM). The results showed that the multiphase ceramics material’s thermal expansion coefficient and flexural strength had little change. The thermal shock resistance of cordierite-mullite multiphase ceramics material varied as increasing-decreasing with the increase of SiC content, when the content of SiC was as high as 4%, the highest conservation rate of the flexural strength after 1100°C~water(3 times) was 72.08%, and the thermal shock resistance of cordierite-mullite multiphase ceramics material was superior.

Abstract: In order to obtain dense-porous laminated structure in green bodies of SiC ceramics, rapid aqueous electrophoretic deposition (EPD) was introduced. The suspension for the electrophoretic deposition was prepared using silicon carbide, silicon and carbon powders as the starting materials. During the electrophoretic deposition process, the intending dense and porous layers were deposited alternately to form the green body. After drying, the green bodies were reaction-bonded at 1550°C in vacuum atmosphere. Pore fraction of the porous layers could be adjusted by changing process preferences of EPD and suspension composition. Pore size and size distribution could be controlled by using different sized starting powders. Using this process, no additional substance is necessary to generate the pores via burnt-off, and the dense/porous laminated structure can be obtained by one-step sintering process.