Abstract
Carbon Combustion Synthesis of Oxides (CCSO) is a promising method to produce submicron- and nano- sized complex oxides. The CCSO was successfully utilized for producing several complex oxides, a complete theoretical model including the sample porosity, flow parameters and reaction energetics is needed to predict the combustion parameters for CCSO. In this work, we studied the ignition temperature and combustion wave axial temperature distribution, activation energy, combustion heat and thermal losses for a typical CCSO synthesis for cylindrical samples of Ni-Zn ferrites with high (>85%) porosity. We developed a two level combustion model of chemically active nano-dispersed mixture, using the experimentally measured ignition temperature and combustion parameter values utilizing the slipjump method for high Knudsen numbers. The theoretical predictions of highly porous samples when the fl ow resistivity is small and the gas can easily fl ow through the cylindrical sample are in good agreement with the experimental data. The calculation of combustion characteristics for the lower porosity values demonstrated that the surface combustion was dominated due to high gas fl ow resistivity of the sample. Finger combustion features were observed at this combustion mode.
Highlights
Recent experimental studies of convective and diffusion transfer in carbon nanotubes [1,2,3] showed acceleration of transfer on more than 2 orders of magnitude compared to the estimations based on theory of continuum media [4,5,6,7]
In this work we provide the theoretical and experimental study of the synthesis of Ni-Zn ferrite nanoparticles using the Carbon Combustion Synthesis of Oxides (CCSO) method [12,13,14]
The combustion initiation temperature and the axial temperature distribution during the combustion, as well as the activation energy and reaction heat values were estimated experimentally. These values were used to develop a two level model of chemically active nano-dispersed mixture [15,16,17,18,19] which can be applied to the synthesis of nano-sized particles of Ni0.35Zn0.65Fe2O4 based on kinetics [14] and processes of mass and thermal transfer in the isolated pore, which is a procedure of averaging the micro-scaled mass and thermal flows in the calculations of characteristic mixture in macro scale
Summary
Recent experimental studies of convective and diffusion transfer in carbon nanotubes [1,2,3] showed acceleration of transfer on more than 2 orders of magnitude compared to the estimations based on theory of continuum media [4,5,6,7]. The combustion initiation temperature and the axial temperature distribution during the combustion, as well as the activation energy and reaction heat values were estimated experimentally These values were used to develop a two level model of chemically active nano-dispersed mixture [15,16,17,18,19] which can be applied to the synthesis of nano-sized particles of Ni0.35Zn0.65Fe2O4 based on kinetics [14] and processes of mass and thermal transfer in the isolated pore (meso-scale), which is a procedure of averaging the micro-scaled mass and thermal flows in the calculations of characteristic mixture in macro scale. In other cases, in parallel to the thermal outflow from the combustion zone, we observed an effect of intensified combustion process, which was due to the increased inflow of the oxidizer to the combustion zone This intensification of combustion results in increase of temperature, during which the sample is intensively heated through the extreme gas convection. An analogous effect of temperature increase takes place when the heating time of the sample and the oxygen supply rate is increased
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