Papers by Author: Ji Guang Li

Abstract: Ca₃Co₄O₉ powders were synthesized by a solid-state reaction method. Porous Ca₃Co₄O₉ ceramics with parallel sheet shaped pores were prepared by a template sacrifice method using epispastic polystyrend (EPS) hollow spheres as the templates. During compaction of the green body, the EPS hollow spheres change into EPS discs due to the pressing force. After sintering, the pores in the Ca₃Co₄O₉ ceramics are sheet shaped, well distributed and parallel to the pressing surface of compaction. The value of ZT merit of the porous Ca₃Co₄O₉ sample obtained with 10 wt% EPS spheres is 0.0489. It was found that the ZT merit value can be improved by changing the density of sample to achieve a high ratio of electrical conductivity to thermal conductivity.

Abstract: Magnesium has been recently recognized as a biodegradable metal for bone substitute applications. In the present work, a novel magnesium based scaffold with a specific two-layer structure was prepared for powder metallurgical process. The outer layer of the scaffold shows an interconnected porous structure, so that the fresh fluid can be easily sent into the material, allowing the ingrowth of new bone tissue. The inner compact structure reinforced by the salt particles can increase the strength of the material. Structural characterizations and mechanical tests of the materials demonstrate that the structural and mechanical properties of the magnesium-based scaffold with an appropriate salt content prepared by the current method are quite comparable to those of cancellous bone. Therefore, the magnesium-based scaffold with such a two-layer structure has the potential to serve as degradable implants for bone substitute applications.

Abstract: Fine yttrium stearate powder was produced at a relatively low temperature using yttrium nitrate hexahydrate, ammonia and stearic acid as the raw materials. Dispersed Y₂O₃ nanopowder was synthesized by calcining the yttrium stearate. The formation mechanism of the precursor and the Y₂O₃ nanopowder was studied by means of XRD, TG-DTA, FT-IR, BET, FE-SEM and HR-TEM. Pure and dispersed Y₂O₃ nanopowder with an average particle size of 30 nm was produced by calcining the precursor at 600 °C. The particle size increases to about 60 nm with the increase of the calcination temperature to 1000 °C. In the preparation of Y₂O₃ from yttrium stearate, no water medium is involved, thus capillarity force and bridging of adjacent particles by hydrogen bonds can be avoided, resulting in good dispersion of the particles. The dispersed Y₂O₃ nanopowder prepared in this work has potential application in phosphors and transparent ceramic materials.

Abstract: Basic carbonate monospheres of various lanthanide combinations are successfully synthesized by the urea-based homogeneous precipitation technique, which are then converted into well dispersed phosphor particles that emit diverse colours. Sequential precipitation is commonly observed for these mixed cation systems, calling for adequate annealing of the basic carbonate precursors to attain cation homogenization in the final oxide particles and thus better luminescence, through eliminating localized concentration quenching of luminescence. It is shown that, owing to their excellent dispersion and uniform size, the phosphor spheres are readily assembled into close-packed luminescent films, allowing their wide applications in white LEDs, plasma display panels (PDPs), and field emission displays (FEDs).

Abstract: In this work, hexagonal solid solutions of (Gd_0.95Eu_0.05)(OH)₃ with two distinctive morphologies of nanorods and nanotubes were successfully synthesized via hydrothermal treatment of mixed nitrate solutions in the presence of ammonium hydroxide. The hydroxide samples exhibited characteristic Eu³⁺ photoluminescence through the energy transfer from Gd³⁺ to Eu³⁺ and the self-excitation of Eu³⁺. The hydroxide precursors transformed into cubic (Gd_0.95Eu_0.05)₂O₃ at ~500 °C via an intermediate monoclinic (Gd_0.95Eu_0.05)OOH phase, and the Eu³⁺ coordination accordingly experienced symmetry changes from D_3h to C_2v, and then to C₂/S₆. The cubic (Gd_0.95Eu_0.05)₂O₃ well retained the original morphologies of their polycrystalline precursors and exhibited a single-crystalline character at 1000 °C. Greatly enhanced photoluminescence (~5000 times that of the hydroxides) was observed for the phase conversion.

Abstract: In this study, nano-hydroxyapatite (HA) powders were synthesized via a simple sol-gel method using Ca(NO3)2•4H2O and P2O5 as starting materials. Two different precursors, with and without citric acid (CA), were prepared. The transformation process of HA from precursors, purity and particle size of the obtained HA powders were evaluated. HA derived from the precursor with CA showed a different transformation process from that without CA. It was observed that the content of CaO as an unavoidable major impurity was reduced due to the addition of CA. In the calcined powders from the CA-free precursor, X-ray diffraction (XRD) patterns revealed an intense CaO peak. For the calcined powders from the CA-addition precursor, XRD analysis showed a very weak CaO peak. It was also found that the synthesized HA powders from precursor with CA were finer than those without CA. The mechanism of the influence of CA on the formation, purity and particle size distribution of HA powders was discussed.

Abstract: Hydroxyapatite (HA) powder was synthesized by a sol-gel method with Ca(OH)2 and H3PO4 as reactants. The HA granules were then coated with TiH2 powder using a mechanical mixing method. The HA-TiH2 material system produced HA-Ti composites after hot-pressing at 1050°C. The HA-Ti composites are mainly composed of HA and Ti, with small amounts of Ca2P2O7 and Ca3(PO4)2 phases. Fracture toughness and bending strength are 2.4 MPa·m1/2 and 54.3 MPa, respectively for the HA-20vol%Ti composite, higher than those of the pure HA ceramic. The improvement in properties is because of the unique 3D network structure of Ti, which is an ideal reinforcement structure for the weak and brittle HA. According to ISO/TR 7405-1984, hemolysis test was performed to evaluate the blood compatibility of the material. The results show that the hemolysis rate of the HA-20vol%Ti composite is 0.56%. Relative growth rates (RGR) of L-929 cells soaked after 6 days in the HA-20vol%Ti group, pure Ti group, black group and pure Pb group were 132%, 100%, 90% and 6% respectively, while the level of cytotoxicity was grade 0 in HA-Ti composite group. These results imply that the HA-20vol%Ti composite has good biocompatibility and bioactivity.