Papers by Author: Jiang Tao Li

Abstract: Both single-phase and multiphase bulk ceramics were prepared by high-gravity combustion synthesis via melt solidification instead of conventional powder sintering. The synthesis process included three steps, namely, combustion reaction, phase separation, and melt solidification. In these steps, phase separation played a key role in producing bulk ceramic materials with high purities and low porosities. It was demonstrated that, in a high-gravity field the phase separation was greatly accelerated, compared with the case under common gravitational condition. In comparison with powder sintering, high-gravity combustion synthesis requires no furnace and can reduce the processing time for the fabrication of bulk ceramic materials.

Abstract: In this paper, the combined artificial bone made from α- calcium sulphate hemihydrate（α-CSH） and β- tricalcium phosphate (β-TCP) was investigated and at the same time the study explored: the method of preparing the combined artificial bone. Scanning electron microscopy (SEM), Powder X-ray diffraction (XRD), Thermogravimetric (TG), Differential scanning calorimetry (DSC), and Mercury porosimeter were employed to characterize the samples. Results indicate that the combined artificial bone can be prepared by α-CSH and β-TCP using dehydrated alcohol as a solvent; β-TCP was made from cancellous bone in the femur of cattle under special conditions, which contained the spatial structure of normal cancellous bone; α-CSH was made from CSD under special temperature and pressure, which is more pure and regulation in crystal form.

Abstract: Large scale translucent yttrium-doped α-sialon (80 mm in diameter) with the composition of Y0.4Si9.6Al2.4O1.2N14.8 (m=1.2 and n=1.0) was prepared by two-steps hot pressing sintering (first heating to 1850 °C holding for 30 min, then continue heating to 1900 °C holding for 30 min) at 25 MPa in nitrogen, and the optical property was approached over the wavelength range 900-5000 nm. The results showed that dense sample was obtained and its assemblage only consisted of single α-sialon phase. The large scale sialon had uniform and higher infrared transmittance property. The maximum infrared transmittance for 1.0 mm thick specimen could reach 66% at 3200 nm. It is attributed to the dense, uniform, equaxed microstructure. In addition to high optical transmission property, the sample showed higher hardness of 20 GPa and better fracture toughness of 5 MPa•m1/2.

Abstract: Yttrium-doped α-sialon ceramics with different compositions were prepared by two-steps hot pressing (first heating to 1750 °C holding for 60 min, then continue heating to 1900 °C holding for 60 min) at 25 MPa in nitrogen, their design compositions were Y0.4Si9.8Al2.2O1.0N15 (Y1210, m=1.2 and n=1.0), Y0.4Si9.7Al2.3O1.1N14.9 (Y1211, m=1.2 and n=1.1) and Y0.4Si9.6Al2.4O1.2N14.8 (Y1212, m=1.2 and n=1.2) respectively, and the effects of n value on phase composition, microstructure and optical property were approached. It is found that only α-sialon is detected in Y1210, however, there exist β-sialon besides of major phase α-sialon in Y1211 and Y1212. The density of sintered sample decreases with the increase of n value, higher n value contributes to elongated grain growth, and both the size and content of elongated grain increase with the increase of n value. The samples all show light transmission in the near infrared and infrared region, and their maximal optical transmittances decrease with the increase of n value. All those indicate that lower n value contribute to higher density, single α-sialon phase, smaller size and lower content of elongated grain, which is better for the preparation of higher optical transmittance Y-α-sialon.

Abstract: Combustion synthesis (CS) of Si3N4 was accomplished by using as-milled Si/NH4Cl as reactants at low nitrogen pressure. The additive of NH4Cl decreased the combustion temperature and promoted the Si nitridation. Full nitridation of Si was achieved by burning Si in pressurized nitrogen with 10 ~ 25 wt. % NH4Cl as additives while no Si3N4 diluent added. The maximum combustion temperature (Tc), the combustion velocity (u) together with the α-Si3N4 content and mean particle size (d50) of the powder products were found to be great dependent on the NH4Cl content added in the reactants. Fine Si3N4 powder products with α-phase content up to 85 wt. % were obtained via steady combustion mode. A mathematical approach named combustion wave velocity methods for the analysis of temperature profiles in CS was proposed and the reaction kinetics was discussed. The apparent activation energy calculated according to the temperature profile analysis method is 29.7 kJ/mol, which agrees well with the corresponding low temperature nitriding combustion of Si.

Abstract: β-Si3N4 and β-SiAlON powders were prepared by combustion synthesis with SrCO3 and NH4F used as additives. The resultant β-Si3N4 and β-SiAlON powders consisted of elongated prismatic microcrystals. By adding SrCO3, the anisotropic growth of β-Si3N4 and β-SiAlON crystals is improved and their aspect ratios increase. The addition of NH4F enhanced nitridation reactions and reduced the residual Si in combustion products. It was proposed that the elongated prismatic β-Si3N4 and β-SiAlON crystals grew from liquid phase and the composition and property of this liquid was affected by the addition of SrCO3.

Abstract: Selective laser sintering (SLS) of Si3N4 and Al2O3 powders were investigated in this work. Copolymer was synthesized through monomer MMA and BMA. Infrared spectrum was employed to characterize the as-synthesized copolymer. Results show that the main compositions of the copolymer are PMMA and PBMA. The binders were prepared by mixing the copolymer and the inorganic NH4H2PO4 with certain ratio. Binders were used to coat the Si3N4 and Al2O3 powders by spray drying. The coated powders were sintered successfully by SLS to make cubic part and rotor part.

Abstract: Multi-step hot pressing sintering was adopted to prepare Y-SiAlON (Y0.4Si9.6Al2.4O1.2N14.8) at 1950°C for 60 min under 25 Mpa in nitrogen atmosphere, and the microstructure was studied. The phase assemblages, microstructures and densification of the sintered samples were discussed in detail. In order to contrast to multi-step sintering, one step sintering also was adopted. The results reveal that single-phase α-SiAlON with elongated grain pattern is formed in the multi-step sintering specimens, because of holding at low temperature in multi-step sintering is helpful to reduce the kinetics of crystal growth, the mount of elongated grain in multi-step sintering samples is more than that of in one step sintering ones.

Abstract: In order to investigate the effect of sintering temperature on the microstructure and densification process of Y-SiAlON ceramics, by hot pressing at 1750°, 1800°, 1850° and 1900°C for 1 h, respectively, Y-α-SiAlON (m=1.2, n=1.2 according to formula Mm/3Si12-(m+n)Alm+nOnN16-n) was fabricated from α-Si3N4, AlN, Al2O3 and Y2O3 starting powers. The results showed that α-SiAlON phase is formed in all sintered specimens, and during Y-SiAlON sintering, all the samples begin to quickly shrink at 1550°C, and the shrinkage is finished at 1750°C, which indicate that 1550 ~ 1750 °C is very important for the densification process of Y-α-SiAlON ceramics, and 1750 °C is necessary for the Y-SiAlON densification. Higher sintering temperature contributes to α-SiAlON grain development and the SiAlON ceramics densification.

Abstract: Yttrium aluminum garnet (YAG) nanopowders were synthesized by n-amyl co-precipitation method. Transparent YAG ceramics were achieved by sintering the green body at 1800°C and then 1600°C for ten hours with TEOS as sintering additive. The transmittance of the sample was 75% at visible wavelength of 800nm.