Papers by Author: Hua Min Kou

Abstract: Based on the combined toughening principle, pure Al2O3/AlB12/Al composite ceramic powders have been synthesized using a new laser-induction complex heating method. This method starts from Al and B2O3 powder mixtures, after which Al2O3/AlB12/AlN composite ceramics were fabricated by hot-press sintering at 1600°C for 2h under the protection of a N2 atmosphere. XRD and SEM techniques were used to characterize the phases and morphologies of the powders and the ceramics. The bending strength and the fracture toughness of the ceramics were measured by the three-point bending method and the indentation fracture method, respectively. The results show that the pure Al2O3/AlB12/Al composite ceramic powders can be successfully synthesized by this new laser-induction complex heating method because the adiabatic temperature of Al-B2O3 system is more than 1800K. Al2O3 and AlB12 phases were formed by the liquid-liquid reaction mechanism and the liquid-solid reaction mechanism, respectively. The bending strength and the fracture toughness of the Al2O3/AlB12/AlN composite ceramics were 551.44MPa and 6.04MPa.m1/2, respectively. These values are 57.55% and 51% greater than those of the pure Al2O3 ceramic (350MPa and 4MPa.m1/2) due to the reinforcing and toughening in-situ formation of small AlN particles.

Abstract: The main problems for multiphase materials are the even mixing and the bonding or reacting among two or more phases, as well as the states of their grain boundaries. The mixing of two phases is usually effective by using liquid medium. The technology of second phase coating for the original particles or so call nucleo-shell structure is a well approach for the even mixing. For ceramic matrix multiphase materials low sintering temperature is the effective way to avoid the occurring of reaction between different phases. The design of grain boundary and its stress is important for their bonding states. The thinking for above problems would be described briefly in this paper. The purpose of multiphase materials design is expected that every phase will be playing its role, combining together and then given out a comprehensive performances. So it is necessary to consider the following problems. 1. Even mixing Even mixing between each other material is important for multiphase materials. It’s the better when the processing is carrying on in the liquid state for the even mixing of two phase materials. Using physical or chemical method could be formed the fine solid particles with even dispersion of two or more phases materials. Usually it could be obtained nano-scale particles. The application of nano-technology is necessary. The nano-scale mixing of two phases materials would be greatly increased the contacting surface of each other particles without agglomeration. Of course, if one phase of material is fine solid particle which disperses in another phase of material solution, the even mixing of two phases is obtained similarly. Making a coating to other phase particle is a very good approach for the two phases even mixing. Coating process is attracting more and more attention in the preparation of composite materials mainly due to its predominance in the improvement of the uniformity for different phases [1]. The thickness of coating material determines the amount proportion of two phases which is able to be control. Two examples of so-called “nucleo-shell structure” will be introduced in the following parts. Cermet composites have been widely studied due to their potential for achieving higher toughness and reasonable strength compared with ceramic matrix. But except for numbered systems, such as Co/WC and Ni/TiN, few systems have reached the people’s expectation mainly for the poor wettability between metal and ceramic. Al/Al2O3 cermet is a low density and high strength material, and it has many potential and actual applications in military, industrial and consumer regions. To improve the dispersive uniformity of the two phases, coating aluminum with alumina may be a good candidate since such coatings not only stabilize aluminum dispersions but also make it possible to control inter-particle and particle-matrix interactions [2]. Figure1a-c shows the images

Abstract: In this article stable multi-walled carbon nanotubes (MWNTs) aqueous suspension with a 1.0 wt.% concentration was obtained with a very small quantity of dispersant. Precursor of ceramics were synthesized in the suspension and densely deposited on the surface of MWNTs successfully by a simple and effective in-situ precipitation method. The most important advantage for the in-situ composite method is to make MWNTs homogeneously distributed in the matrix. The fully dense compacts were obtained by spark-plasma-sintering (SPS) the in-situ precipitated composite powders at temperature 200 oC lower than that of composite powders made from the traditional mixing method. Furthermore, the microstructure and the mechanical property of the composites are much better than that of traditional method. The in-situ precipitation could be a promising method to fabricate CNTs composites of ceramics matrix especially those hard to sinter.

Abstract: Carbon nanotube-mullite (Al2O3/SiO2=3/2) composites have been prepared by hot-pressing the corresponding composite powders, in which the multi-walled carbon nanotubes(1~10 vol%) are homogeneously dispersed between the mullite grains. The microstructure of the specimens has been studied and discussed in relation to the properties of the matrix, the bending strength and the fracture toughness, the dielectric constant and the dissipative factor. Carbon nanotube-mullite composites are potential electromagnetic wave absorbers owing to the percolation of the carbon nanotubes.

Abstract: Coating process is attracting more and more attention in the preparation of composite materials mainly due to its predominance in the improvement of the uniformity for different phases. The formation of nanosize alumina shells on the aluminum core particles by a wet-chemical based route was investigated and the composite particles were characterized using TG/DSC, XRD, SEM, TEM, XPS. An enhancement of cermet properties can be expected by using these coated particles as initial powders.

Abstract: Al/Al2O3 cermet composites were fabricated by hot-pressing at the temperature range of 1350~1450oC from commercial γ - Al2O3 reinforced by Al/Al2O3 core-shell particles, which were synthesized via calcining the Al/AlOOH·nH2O core-shell particles prepared by wet-chemical based method. This newly designed aluminum coated with alumina composite structure can not only reduce or resist the oxidization of Al at elevated temperature but also make it possible to control inter-particle and particle-matrix interactions, and this predesigned structure is expected to improve the following sintering ability and therefore the properties of final products.