Papers by Keyword: α -Al₂O₃

Abstract: Mullite- zirconia composites have better mechanical properties than monolithic mullite ceramics and can be produced by reaction sintering of ZrSiO4 and  -Al2O3. The samples were prepared from high-purity (99.9%) α-alumina and fine zircon (ZrO2>65 wt.%) powders using PVA as binder. The powder mixtures were compacted under 80 MPa as coin shaped samples by uniaxial dry pressing and then sintered in a multimode microwave field of 2.45 GHz. The microwave effect on ZrSiO4 dissociation and mullite formation was evaluated by comparing the microwave sintered samples with those sintered conventionally. The as-reacted compacts were characterized by X-ray diffraction and scanning electron microscopy (SEM). The effects of sintering parameters on mullitization and mullite grain growth were investigated.

Abstract: Alumina powder is used to improve the heat conduction and physical properties of heat radiation sheets, these properties can be improved by increasing the filling rate of alumina. Spherical alumina powder is being actively developed to improve the filling rate of alumina in sheets. In this study, a high-temperature flame was created by the flame-fusion method using an oxygen burner to produce spherical alumina. We investigated how the chemical composition and particle size of starting materials affect their fusion behavior under flaming conditions during spherical alumina production using the flame-fusion method. Boehmite and aluminum hydroxide, which accompany the endothermic reaction due to the dehydration of the starting materials, showed lower spheroidization ratios than sintered alumina. The spheroidization ratio improved as the particle size of the starting materials decreased.

Abstract: α-Al2O3 platelets were successfully synthesized in molten NaCl-KCl salt using the pre-calcined Al(OH)3 powders as starting materials, and the effect of milling medium on the powder characteristics was investigated, with the purpose of obtaining high performance sintered ceramics. When anhydrous alcohol was used as milling medium, the synthesized product at 900°C was porous α-Al2O3 with a small amount of amorphous Al2O3. While deionized water was used, single-phase and dense α-Al2O3 platelets were developed at 900°C. It was related with different solubility of NaCl-KCl salt in anhydrous alcohol and deionized water during milling. When the synthesized α-Al2O3 platelets were used as seeds in sintered ceramics, fracture toughness was developed.

Abstract: Nanocrystalline α-Al2O3 powders were synthesized by low temperature combustion technique using aluminum nitrate-urea reactive system. The relationship between the color of powders and the mass ratio of aluminum nitrate-to-urea was investigated. X-ray results showed presence of the well-crystallized α-Al2O3 after calcined at 1100oC for 2h with 2.5:1 mass ratio. The grain size of α-Al2O3 powders was calculated by Scherrer equation. TEM observation and Clouter N4 Plus measurement results showed that the particle size distributed from 20 nm to 70 nm.

Abstract: Slurry iron aluminide coatings are very resistant to steam oxidation at 600-650º C. These coatings can be used to protect new generation Ultra Super Critical (USC) steam power plant ferritic/martensitic steel components. The microstructure of the initially deposited coating changes as a function of time, mainly due to coating-substrate interdiffusion, going from mostly Fe2Al5 to FeAl, causing the precipitation of AlN in those substrates containing a minimum content of N and moreover, developing Kirkendall porosity at the coating-substrate interface. Steam oxidation at 650º C causes the formation of a protective thin layer of hexagonal χ-Al2O3 phase along with some α- and γ-Al2O3 after the first few hours of exposure. However, despite the relatively low temperature, and after several thousands hours the protective layer was mostly composed of α-Al2O3. A study of the evolution of the microstructure of slurry aluminide coatings deposited on P92 and exposed to steam at 650º C has been carried out by scanning and transmission electron microscopy and X ray diffraction.

Abstract: Dispersants are selected and optimized by determining the height of emulsion in the colorimetric cylinder after standing in order to ensure the homogeneity of the powder and the stability of electroless plating solution. Experimental results show that non-ionic surfactants such as PEG6000,PEG10000,OP-10 have stronger emulsifying ability to α-Al2O3 powder in ordinary condition and IW has more stability at 80~90°C. The optimum conditions are suggested as PEG6000 8g, IW 0.95g and ethanol 3mL per 50mL solution. Moreover, this composite electrolessplating solution is stable for seven days under 90°C. Therefore it has more dispersing ability. The SEM images show that the powder in the coating is homogeneous. The electroless plating coating consists of α-Al2O3 and amorphous Ni-P that will change into crystal Ni3P and Ni-base solid solution after heat treatment at 400°C for one hour, the α-Al2O3 powders are dispersed evenly in the nickel matrix.

Abstract: The sintering activation energy of high-purity alumina powders with different particle sizes was evaluated under non-isothermal condition. It was found that, during sintering, the activation energy for the lower temperature stage is higher than that for higher temperature stage. The value of the activation energies for the powder compact with larger particle size was higher than that for the powder compact with smaller particle size. If the selected temperature interval for calculation was narrow enough, the evaluated activation energy values varied with the increasing temperature continuously.

Abstract: High-purity nanoscale α-alumina powders with high specific surface area were prepared from boehmite powders synthesized by the method of aluminium isopropoxide hydrolysis. The physiochemical transformation of boehmite powders under heat-treatment was investigated. The results showed that the reaction progress of boehmite under heat-treatment is as follows: AlO(OH) → Al2O3 (amorphous) → γ-Al2O3 → δ-Al2O3 → α-Al2O3. The phase transformation from δ-Al2O3 to α-Al2O3 begins at 1050oC, and the α-Al2O3 grains growth accelerates as temperature increases. High-purity α-Al2O3 nanometer powders with higher specific surface area can be obtained after incineration at 1100oC for 4 h.

Abstract: AlN powders were synthesized by gas-reduction- nitridation of γ-Al2O3 powders using NH3 and C3H8 as reactant gases. AlN was identified from the products that synthesized at 1100-1400 oC for 120 min in the NH3-C3H8 gas flow, and it was confirmed that AlN can be easily fabricated by the gas-reduction-nitridation of γ-Al2O3. The products synthesized at 1100oC for 120min contained unreacted γ-Al2O3. By the 27A1 MAS NMR spectra, Al-N bonding in the product increased with an increase in the nitridation ratio of the tetrahedral AlO4 shoulder which decreased prior to that of the octahedral AlO6 shoulder. It seems that γ-Al2O3 was preferentially nitrided from AlO4 rather than AlO6. AlN nano particles were easily converted directly from γ -Al2O3 at a low temperature because the AlO4 within γ-Al2O3was preferentially nitrided.

Abstract: Unagglomerated α-Al2O3 powders of 100 to 200nm were synthesized by combustion spray pyrolysis with droplet filtration. The ignition conditions of the oxidizer and fuel were well elucidated by the calculation of partial equilibrium species and thermal analysis. A metal screen filter with 500 mesh was employed to dilute the number concentration of the sprayed droplets. The Reynold’s number of the aerosol fluid was kept at the value of 1,200 to keep a short residence time and a laminar flow.