Carbon Reduction Kinetics of NiO by Microwave Heating of the Separated Electric and Magnetic Fields
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CROWAVE (MW) heating was discovered in 1946,[1] and it has been applied to various fields such as from a domestic cooking oven to immobilization of heavy metal containing soil.[2] Recently, MW processing has been taken into consideration as a special method for materials fabrication, for example, sintering,[3] synthesis of solid materials,[4] and some chemical reaction processes.[5] There are many studies on the chemical reaction kinetics by MW heating in the chemistry field such as organic/inorganic synthesis,[6,7] and some catalytic reactions.[8,9] On the other hand, some studies have been carried out for the high-temperature reduction reaction such as reduction of oxides used in chemical metallurgy.[10–12] These reduction reactions are not only the fundamental process of the industrial metal production but also applied for recycling, such as NiO reduction from battery[13] and from pickling sludge of stainless steel.[14] It is of great importance if the reaction kinetics is enhanced or the reaction temperature could be reduced, because an energy required for the reaction processes can be reduced. These are one of the motivations for this study on the high-temperature
reduction kinetics in various conditions of MW irradiation. Studies on the MW application in various fields were mostly performed so far, using a multimode applicator such as a domestic MW oven[15] or larger applicators[16] operated at higher power. Recently, on the other hand, separation of the E- and H-fields of MW has been attempted[17–19] using a single mode-wave guide. The purpose of these studies is to clarify the MW heating mechanisms of various materials using the different fields, such as electric conductor (metals), dielectric materials, or the magnetic materials. On the other hand, it is of interest to investigate the reduction kinetics dependence on the E- and H-fields, concerning the alteration of their reaction mechanisms. In this study, it is first intended to examine the parameter dependence of the kinetics such as the reaction temperature, the NiO/C molar ratios, and the grain size of NiO and graphite. They will be mostly examined in the E-field for the reasons mentioned later in the text. Then, the difference in the carbon reaction kinetics of NiO in the E- and Hfields will be examined, and the results will be discussed from various points of view.
II. NOBORU YOSHIKAWA, Associate Professor, KENICHI MASHIKO, Graduate Student, and SHOJI TANIGUCHI, Professor and Dean, are with the Graduate School of Environmental Studies, Tohoku University, Sendai, Japan 980-8579. Contact e-mail: yoshin@ material.tohoku.ac.jp ETSUKO ISHIZUKA, formerly Graduate Student, Graduate School of Environmental Studies, Tohoku University, is with the Division of Research and Development of Electronic Devices, Sharp Company Ltd., Nara, Japan. Manuscript submitted April 29, 2007. Article published online October 23, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
EXPERIMENTAL
A. Specimen The specimen powder particles having different sizes were prepared (NiO:500 lm, 7 lm,
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