| Nano-sized alpha alumina power, which has a very high surface or interfacial area, exhibits dramatic properties, such as enhanced sinterability at low temperatures, improved UV scattering, very high hardness, smaller particle suspensions, superior optoelectronic properties, has become a kind of favorable material in applications such as high insulation material, high performance cutting tools, micro-electronics and fine porcelain.There are a number of techniques presently available for the synthesis of nano-sized gamma alumina power, while it is relatively difficult to make perfect nano-sized alpha alumina particle with fine size, narrow distribution and high purity. The main fact is because of that the transformation process of crystal phase from gamma to alpha must be performed under high temperature above 1200-Celsius degree, at which it is very easy for nano-sized particle to agglomerate and grow. Maintaining a narrow size distribution without obvious agglomeration in the power, which is important in many applications, is not possible with a majority of conventional techniques and has become a world aim that many scientists are going in for. In this paper, the particularity of nano-crystal growing was discussed firstly on the base of traditional crystal theory. Comparing with general directional and regular crystallization, during the nucleating of nano-alumina precursor, molecule or ion takes on out-of-order stacking structure because of the higher concreting rate; during the growing of crystal nucleus, out-of-order stacking makes most of crystal nucleus contact closely and stacking fault is formed as a result of Brown movement and chemistry potential difference among crystal interfaces, which makes atoms in crystal take on out-of-order or sub-boundary structure. This kind of special crystal growing model can also be proved by shell structure, out-of-order property of atom array and porous surface structure of nano-particles. Through analyzing existent agglomeration mechanisms, the main effect factors on agglomeration of nano-particles were discussed systematically and thoroughly. The results show that the interface energy difference is the main driving force and hydrogen bond, chemical bond and capillary action force can improve the grow of the particles. The common action of intrinsic factor and exterior factor results in agglomeration. What's more, contact among particles is the precondition of above various actions. Therefore, preventing direct contact among particles is the effective way to restrain agglomeration and grow of particles and the uniformity of microenvironment is also very important for attaining nano-sized power with narrow distribution.On base of above theory studies, the micro-reaction unit, including mainly template medium, solvent and ultrasonic, was built in order to attain uniform nano-sized powder through combining soft-chemistry and template-chemistry technique. The micro-reaction unit has the following double functions: the one is controlling the nucleation and grow of particles during preparing process of the precursor; the other is preventing agglomeration and abnormal grow of particles during the subsequent heat-treating process acting as a kind of obstruct. Furthermore, the action mechanism of micro-reaction unit was also studied, especially on the cavitations action of ultrasonic and obstruct action of template medium .the former can accelerate nucleating and control growing of nucleus, which assures the uniformity of grow and prevent the form of hard agglomerates during preparing process of the precursor; the later exerts entire obstruct action during the whole reaction process in the advantage of its special structure, which assures the uniformity of the last nano-sized powder. What's more, the surface characteristic of template medium also enhances the cavitations action of ultrasonic and the eliminable property of template medium also ensures the purity of final product. Thereafter, the nano-sized alpha alumina powder with controllable size, narrow distrib... |
