Bulg. J. Phys. vol.34 no.s1 (2007), pp. 030-045

Ferromagnetic Nanomaterials Obtained by Thermal Decomposition of Ferrocene in a Closed Chamber

N. Koprinarov1, M. Konstantinova1, T. Ruskov2, I. Spirov2, M. Marinov3, Ts. Tsacheva3
1SENES, Bulgarian Academy of Sciences, Tsarigradsko Chaussee 72, BG-1784, Sofia, Bulgaria
2Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Tsarigradsko Chaussee 72, BG-1784, Sofia, Bulgaria
3Laboratory of Electron Microscopy and Microanalysis, Institute of Physical Chemistry, Bulgarian Academy of Sciences, bl. 11 Acad. G. Bontchev St., Sof
Abstract. Ferromagnetic micro and nano particles that are chemically resistant have been obtained by thermal decomposition of ferrocenes in a tightly closed chamber at high pressures. Our investigation focused on the influence of the decomposition temperature, the work atmosphere (including organic-solvent vapor, water vapor, the presence or lack of reactive gases), the temperature-change rate, the process duration, and the influence of additional thermal treatments of the end materials. According to the conditions, Fe3C, Fe3O4, and pure α -Fe particles have been created. Their composition and structure have been studied by Mössbauer spectroscopy, TEM, SEM, EPXMA and EDX.
In a tightly closed chamber, all components obtained during the decomposition process remain there. This difference to the widely-used CVD method is very important. It inhibits the decomposition process and the growth of ordered structures, preventing the end materials to be separated from each other. During the process, the iron is liberated from the ferrocene molecule. Experiments have shown that it is highly chemically active to carbon and oxygen. For example, creation of carbide occurs in conditions that are not allowed according to the iron-carbon phase diagram valid for bulk iron. Water vapor has shown abnormal behavior as well.
Parallel to the reaction of iron with oxygen or carbon (according to the work atmosphere), the rest of the carbon and hydrogen atoms form a hydrocarbon substance, which covers the iron end product. This hydrocarbon cover is thermally and chemically resistant. Therefore, it can be used as a protection. During the syntheses, it also holds the particles together, building hollow agglomerations of different shapes (spherical, cylindrical, etc.). These are large structures with thin walls. In some experiments, even ideal hydrocarbon spheres without any iron products have been observed. When the chamber is kept at the ferrocene decomposition temperature, the particles with the highest load of α-Fe iron and the smallest sizes have been obtained. Increasing the temperature, especially in the presence of an organic solvent vapor, stimulates the iron to form carbide. The presence of water vapor or oxygen leads to iron oxides. Keeping high temperatures for longer periods of time increases the possibility of iron products to crystallize in highly symmetrical structures. The most common shape observed is cubical with cones growing at the sides.
The ferromagnetic particles promise to be very useful due to their very small sizes and chemical resistance. They can be used for transport or extraction of substances from liquids, for local heating of living organisms, for changing of the liquid viscosity by magnetic fields, etc.

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