Papers by Author: Chang An Wang

Abstract: LaMgAl₁₁O₁₉ was synthesized at 1550 °C using La₂O₃, MgO and Al₂O₃ as raw materials. The samples were characterization by XRD and SEM. The tablet shaped crystals free of impurity phase formed under this condition. The thermal diffusivities were measured by laser flash method and the determined intrinsic thermal conductivities decreased as temperature increases from 25 °C to 1000 °C. As comparison, intrinsic thermal conductivities of LaMgAl₁₁O₁₉ are lower than that of 7YSZ. The synthesized LaMgAl₁₁O₁₉ was heat treated at higher temperature from 1600 °C to 1700 °C and no change in the phase indicates that the LaMgAl₁₁O₁₉ phase is stable under 1700 °C, which is very important for thermal barrier coatings (TBCs) serving at elevated temperature.

Abstract: Porous Al₂O₃-SiO₂-ZrO₂ ceramic composite membranes were fabricated by the sol-gel method. The effects of constitution of the composite sol, dip-coating parameters and calcination parameters on the morphology, phase composition, pore size distribution and BET surface area of the membranes were investigated extensively. The thermogravimetric analysis (TGA)/differential scanning calorimeters (TG/DSC), scanning Electron Microscopy (SEM), XRD analysis and N₂ adsorption-desorption analysis was employed for the membranes characterization. TG/DSC curve shows that the suitable calcination temperature of Al₂O₃-SiO₂-ZrO₂ membranes is 1000°C. The SEM morphology displays a smooth and free-crack layer of Al₂O₃-SiO₂-ZrO₂ composite membranes on the surface of the support after calcination at 1000°C. Furthermore, the membranes were also successfully coated with a good adhesion to the support. The optimal composite membranes were obtained when the moral ratio of Al₂O₃-SiO₂-ZrO₂ was 10:2.3:1, the dip-coating parameters were four times dip-coating with every time of 60s respectively.

Abstract: A high molecular weight block copolymer dispersant with pigment affinic groups — DISPERBYK163 (BYK163) was chosen to disperse submicron-sized Si3N4 in tert-butyl alcohol (TBA) solvent. The stability of Si3N4 powders in TBA with dispersant BYK163 was studied by sedimentation. To achieve good casting behavior, rheological measurement was employed to find a reasonable dispersant content in the slurry. The adsorption isotherm of BYK163 on Si₃N₄ particles surface was obtained by thermogravimetric analysis (TGA). Based on the surface chemical properties of Si₃N₄ particles, the dispersing phenomena and mechanism of BYK163 in TBA were summarized. BYK163 could form a steric layer on the surface of Si₃N₄ particles, and effectively prevent particles agglomeration. Therefore, porous Si₃N₄ samples with porosity of 40% to 80% could be prepared from Si₃N₄ slurries with low and high solid loadings due to well dispersion with BYK163 addition.

Abstract: Porous mullite ceramics reinforced by mullite fiber have been prepared by vacuum filtering with MgO as sintering aid and starch as pore-forming agent. The influence of different sintering temperature and mullite fiber content on porosity and strength of porous mullite ceramics was investigated. The results showed that the compressive strength of the mullite ceramics increased with increasing sintering temperature. With the increase of mullite fiber content, the compressive strength of porous mullite ceramics appeared to increase at first and then decrease. And the peaking strength could reach 211MPa with 36.8% porosity when 7 vol% mullite fiber was added and sintering temperature of 1500 oC was adopted.

Abstract: Porous Si3N4/SiO2/BN composite ceramics with high strength and low dielectric constant were prepared by dry-pressing process and pressureless sintering at 1750°C for 1.5 h in flow nitrogen. The influences of BN content on microstructure, porosity, mechanical and dielectric properties of the porous Si3N4/SiO2/BN composite ceramics were discussed. The results showed that the porous Si3N4/SiO2/BN composite ceramics with porosity ranging from 29% to 48% were fabricated by adjusting the content of BN. The flexural strength of the porous Si3N4/SiO2/BN composite ceramics was 78215 MPa. The dielectric constant of the porous Si3N4/SiO2/BN composite ceramics was 3.9~5 at 1 MHz.

Abstract: Nano-hydroxyapatite with different morphology was synthesized by the co-precipitation method coupled with biomineralization using Ca(NO₃)₂•4H₂O and (NH₄)₂HPO₄ as reagents, adding chondroitin sulfate, agarose and aspartic acid as template. The structure and morphology of the prepared powders were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM).

Abstract: Porous SiC ceramics with high porosity and high strength were fabricated by gelcasting, with tert-butyl alcohol (TBA) as solvent, acrylamide (AM) as monomer, and in-situ reaction bonding with a-Al2O3 as sintering additive. SiC suspension with 10 vol% solid loading was successfully solidified by gel-casting to form high strength green body. The results showed that the compressive strength of the porous SiC ceramics increased with sintering temperature from 1300 to 1450°C, but porosity had little change, due to formation of more volume of cristobalite and mullite phases on the surface of SiC grains, accompanied by a large volume expansion effect. Very narrow single-peak distributions with about 2 mm median pore diameter could be found for the porous SiC ceramics. The porosity and compressive strength of the porous SiC ceramics sintered at 1450°C were 71.21 % and 12.14 MPa, respectively.

Abstract: ZrB2 powder has been prepared through carbothermal reduction boronization of zirconia/boron carbide/carbon mixtures heating assisted by microwave. The powder characteristics were investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen absorption (BET model) and scanning electron microscope (SEM). The experiments indicated that excessive B4C is necessary and the carbothermic reaction reacts severely at a higher temperature and complete at 1600oC. The crystallite size has ranged from 50-100 nm, according to the calculated surface area. Highest purity of ZrB2 powder, which was synthesized at 1600oC, is 99.67 wt%. The surface area of ZrB2 powder synthesis at 1600oC is 18.33 m2/g. Vibration of temperature should affect the purity of ZrB2, as the sub reaction acted.

Abstract: Porous silica ceramics with above 70% porosity were fabricated by gelcasting process using tert-butyl alcohol (TBA) as solvent. Two types of starting powders, α-silica and fused silica, were chosen to fabricate porous silica ceramics. According to XRD analysis, only single α-cristobalite phase can be identified in the sintered samples. SEM observation showed that highly porous skeleton formed by the bonding of molten ceramic particles in porous silica ceramics. Comparing to the samples using α-silica as starting materials, a compressive strength of 6.30MPa was achieved at a porosity of 70.7% when using fused silica as starting materials. A relatively high specific surface area higher than 10m2/g was obtained for both samples, which should be suitable for catalysis applications as catalyst supports.

Abstract: Al4SiC4 is probably used as non-oxide raw material for high-temperature ceramics. Al4SiC4 was synthesized by using starting materials of metal-carbon, metal-oxide- carbon or carbides. In this paper, Al4SiC4 has been synthesized by using oxides (Al2O3 and SiO2) and carbon as starting materials through carbothermal reduction process. The oxidation properties in air and O2 atmosphere by TG method and the stability of the synthesized Al4SiC4 heated in moisture were investigated. Pure Al4SiC4 phase was synthesized when heating the mixture powder of Al2O3, SiO2 and C (C : Al2O3 : SiO2 = 8 : 2 : 0.8, in molar ratio) at 1700°C for 8h in flowing Ar atmosphere. The synthesized Al4SiC4 powder consists of platelet shape grains with size of 68μm length, 35μm width and 1μm thickness. Al4SiC4 obviously oxidized from 800°C when heated in flowing air or O2 atmosphere. When keeping Al4SiC4 powder in moisture, it was not observed to be reacted with water by XRD and SEM analysis.