Papers by Author: Juan Wu

Abstract: Mullite- corundum insulation material with nano crystals in matrix was fabricated by gel-casting process. The results showed that the volatilization temperature of organisms is 285°C- 600°C, there is no glass phase appeared in specimens; The mullitization begin at 1000°C, along with the raise of temperature, the quantity of mullite phase increases, especially during 1200°C- 1400°C; when the temperature is up to 1550°C, mullite becomes the main crystal phase and there are also some corundum crystals in the specimens; The specimen matrix sintered at 1550°C is consisted of nano crystals 80nm in length and 10-20nm in diameter.

Abstract: Using Al, MgO and Al₂O₃ as raw materials, Al₂O₃-MgAlON composite was synthesized in N₂ atmosphere by hot-press sintering at 900°C. Cracks prepared on the sample surface were healed at 1200°C×6hrs. Phase composition and element chemical state of sample were studied before and after healing. The results showed that: after healing process Al and MgO phase disappeare, and AlN and MgAlON phase appear, all kinds of elements in Al₂O₃-MgAlON material change their chemical state after the healing treatment, the changed electronic binding energy is Al 0.05ev, Mg 0.08ev, O 2.58ev or N 1.02ev respectively, and the chemical composition and crystal structure of MgAlON phase also change, all these change are helpful for crack healing process of Al₂O₃-MgAlON material.

Abstract: Cordierite material was prepared by talc, bauxite and kaolin, and the coefficient of thermal expansion (CTE) of cordierite crystals could be determined by high temperature X-ray diffractmeter: the volume and average liner CTE is 2.31×10^-5°C-1 and 0.84×10^-5°C-1 (200°C -800°C) respectively; With the temperature increasing, the CTE in a or b axis direction increases, and in c axis direction decreases rapidly below 400°C then slowly, cordierite crystal is of obvious thermal expansion anisotropy. The orientation of crystal in sample is random, expansion and shrinkage partly counteract, so that the CTE of cordierite materials is low.

Abstract: Cordierite is an excellent material with good thermal shock resistance and used at high temperature for its low thermal expansion coefficient. Cordierite ceramics were prepared by using talc, alumina and kaolin clay as starting materials. The thermal expansion coefficient, phase composition and microstructure were studied and the results showed that: in order to get samples with low thermal expansion coefficient, the optimum chemical composition was a little rich in Al2O3 compared with the theoretical composition, the optimum sintering temperature was 1350°C, and adding 10% starch as pore-forming agent could effectively decreased the thermal expansion coefficient of the samples even to 0.8×10-6/°C. The samples contained majority of cordierite phase, with trace mullite and glass, the acicular cordierite crystals in samples developed very well and there were 10% starch powder used as pore-forming agent in formula. All these were the reasons to decrease the thermal expansion coefficient of cordierite material.

Abstract: After preparing samples (3mm long×4mm wide×36mm high) of Al2O3-MgAlON composites and sintered at 1500°C for 2 h in N2 atmosphere, samples’ cracks were carved by a Vickers hardometer’s pressing head on the center of the sample surface (4 mm×36 mm). Subsequently, the cracks were healed at 1000°C-1550°C for 6 h respectively. Effects of healing temperature on sample’s strength, crack healing dynamics and its molecular dynamics simulation were investigated. The results suggested that: the optimum range of cracks healing temperature was 1300°C-1550°C, and the healing process accelerated at 1300°C, meanwhile, the strength of samples increased significantly. Cracks completely healing finished at 1550°C. The dynamics equation of crack healing was lnν = -Q/kT+lnC. Through characterizing the crack healing rate with the recovering rate of sample’s strength, the diffusion activation energy Q = 4.264 × 10-30 J•K-1 and diffusion constant C=7.359 were claimed. The result of the molecular dynamics simulation suggested that cracks healing process was caused by diffusion could be divided into five stages: passivation of crack tips, formation of salient island, crack shrinkage, generation of secondary crack, and complete healing.

Abstract: Mullite- corundum ceramic was fabricated using bauxite and silica fume as raw materials by gel-casting process. The fluidity of the slurry, as well as strength, apparent porosity, pore size distribution, phase composition and microstructure of sintered specimen were studied. The results showed that: the slurry had low viscosity, good fluidity for casting if pH=9, dispersant 0.33wt% and solid content 60vol%; the strength of material came up to 165MPa even if the apparent porosity was about 30%; the samples were characterized by mullite 70%, corundum 22%, rutile 3%, glass phase only 5%, the size of majority grains was about 2μm, and most crystals with high melting point bonded directly; the average aperture was 264nm, the pore size distribution were divided into 2~30nm, 45~60nm and 1500~4000nm three regions. The distribution probability of nano- aperture was the largest, the volume of micron -aperture was the maximum, but all pores were less than 4μm in diameter, which suggested that the materials had micro porous characteristics. The microcrystalline, crystals combined directly and micro porous structure leaded to high strength of the refractory materials together.