Dissertation > Industrial Technology > General industrial technology > Materials science and engineering > Special structural materials

Structural Evolution and Reaction Behavior of Metal Powder during Ball Milling in Organic Environment

Author XiaoZuo
Tutor ZhaoZhongWei;XiaoSongWen
School Central South University
Course Non-ferrous metallurgy
Keywords Ball milling metal powder flaking chlorinated organic contaminates de-chlorination
CLC TB383.1
Type PhD thesis
Year 2008
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Ball milling is widely used in the preparation of powder materials and identified as the standard preparation process for metal flake powder. With the use extending to environmental protection, ball milling is identified as the most promising technology for the destruction and decontamination of POPs in 1990s. In our country, the technologies for metal flake powder preparation lag far behind that in Euramerican country, for examples, the preparation technologies for nickel, copper and zinc flake haven’t been industrialized. Meanwhile, no research is developed for destruction and decontamination of POPs. etc. chlorinated organic contaminates by ball milling.In order to address the issues mentioned above, in this paper, three aspects have been investigated: 1) Preparation process of metal nickel, copper and zinc flake by ball milling; 2) Microstructure evolution and deformation mode of metal powder during the flaking process; 3) Mechano-chemical de-chlorination process and behavior of chlorinated organic contaminates by ball milling zinc powder. The following summaries are obtained:(1) The effect of ball milling parameters on the flaking level of metal powder was investigated. It was found that medium environment affect the flaking level of the metal powder in great degree, and metal powder was flaked better in organic medium than in air. The most appropriate conditions to prepare nickel, copper and zinc flake powder were determined and metal flake powder with a high flaking level, good flaky microshape and metal luster were successfully prepared. The properties such as flaking level and microshape of zinc flake powder prepared almost reach the standard of best production in the world.(2) The microstructures of copper, nickel and zinc powder during flaking process were investigated by XRD, SEM, and TEM. It was found that the grain size decreased, while the microstrain, dislocation density, lattice parameter, and unit cell volume increased with increasing flaking time steppedly, and the microstrain, dislocation density possessed anisotropic behaviors. The microstrain and dislocation density of copper were relatively less than that of nickel. The microstructures of nickel and copper flake powder were characterized by the layer/rod-shaped grains, as well as the preferred orientation. The preferred orientation of nickel and copper flake was <200>, and the flaking level increased with the higher <200> preferred orientation.(3) On the aspect of microstructure, flaking process of copper powder was characterized as the equiaxed grains were elongated along the direction of the flake radius to layer-shaped and then the layer-shaped grains were broken into rod-shaped. In this process, a high density of dislocations were introduced into the grains with the random cycling loading and resultant strain, then dislocations aggregated and rearranged, which resulted grain refinement and preferred orientation. The grain refinement modes included: (1) In the main body of grains, dislocation cells/walls formed during straining, which subdivided initial grains on a smaller and smaller scale; (2) In local area of grains, the dislocation arrangements introduced at the instant of the impact could be maintained and forming nanograins due to high strain and strain rate in these areas.(4) The new technology for mechano-chemical de-chlorination of chlorinated organic contaminates by ball milling with zinc powder was developed in this paper for the first time. The optimum conditions were examined and determined, and the dechlorination rate of PVC, PVDC, HCB were almost reach to 100% on the optimum condition. Therefore, mechano-chemical de-chlorination would become a new process for de-halogenation of halogenated organic contaminates in an effective and safe way.(5) On the basis of characterizing and investigating the de-chlorination products by XRD, IR, Raman, GC/MS and NMR, the de-chlorination reaction paths were determined and verified. The mechanism of mechano-chemical dechlorination of chlorinated organic contaminates with zinc powder was accordance with the free radical theory. During the milling process, free radical formed in zinc powder under the effect of impact, which then attacked Cl atom in the chlorinated organic contaminates. Cl atom was split off from the chlorinated organic contaminates and reacted with zinc powder to form Zn(OH)Cl and Zn2OCl2. A great part of HCB were transformed into amorphous carbon, through the pathway of benzene ring polymerized in the direction of plane and space, besides a slight part was transformed into fatty acid in the way of oxidation and opening of benzene ring.(6) PVC/PVDC can be de-chlorinated following the paths of de-chlorination, crosslink, de-hydrochlorination and oxidation and transformed to diamond-like carbon, carbyne fragments and polyacelytene. Through the path of de-hydrochlorination, carbyne fragment formed. If the process can be performed in the more appropriate milling conditions such as low milling temperature, the reductive atmosphere, the pure carbye would be obtained, and the mechano-chemical de-chlorination process of PVC/PVDC would become a novel and effective approach for carbyne synthesizing,

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