Sustainability of CoNi alloy in nanostructured environment through thermogravimetric and corrosion studies

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Title: Sustainability of CoNi alloy in nanostructured environment through thermogravimetric and corrosion studies
Author: Chen, Xu
Abstract: The interaction of atomized Co, Ni and CoNi with multi-walled carbon nanotubes has been studied by thermogravimetric analysis (TGA) and electrochemical polarization studies. The composites of the above materials have been prepared in different atomic ratios and investigated for examining the spin polarization. The composites have also been sintered at 500O to 1000 oC in argon atmosphere. The uncomposited metals showed thermal oxidations by TGA in air. The characteristic values for atomized Co, Ni and CoNi are 360oC, 500oC and 500oC respectively. The composited materials showed no thermal oxidations when there is sufficient amount of spin polarizing agent; the spin polarizing agent in this study is multi-walled carbon nanotubes. CoNi alloy alone interestingly showed only one thermal oxidation at 500oC. The characteristic oxidation of Co was not observed at 360oC suggesting that there is spin polarization occurring due to ferromagnetic metal Ni. These behavioral patterns have been modeled based on the predictions of the density functional calculations in the literature. The prediction is that when a carbon nanotube interacts with Co and Ni the electron distribution is modified in the following ways Co [Ar] 3d74s2 to Co [Ar] 3d94s0 Ni [Ar] 3d84s2 to Ni [Ar] 3d104s0 As a consequence of this spin polarization, the bonding property with oxygen is modified. This spin polarization prevents the thermal oxidation occurring. With an alloy there has been no density functional calculations, however, the experimental results shows that the thermal oxidation can be prevented by compositing it with carbon nanotubes. The absence of spin polarization with graphite composited Co demonstrates that a tubular structure is required for the spin polarization. In a series of measurements made with different quantities of multi-walled carbon nanotubes, it is concluded that one ferromagnetic atom requires five carbon atoms to be surrounded by it. The TGA experimental data reveals that unlike cobalt composites the nickel composites form at 800oC a gaseous compound that is tentatively ascribed to nickel tetracarbonyl. Although cobalt has similar atomic structure, its carbonyl formation does not occur as revealed by TGA. The electrochemical polarization measurements carried out demonstrates that the Tafel plots showed the corrosion current density for the composited CoNi alloy is two orders of magnitude smaller than CoNi alloy. This is in agreement with the open circuit potentials of the two systems.
Record URI: http://hdl.handle.net/1850/15316
Date: 2012-08-09

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