Microwave Absorption Characteristics of Conventionally Heated Nonstoichiometric Ferrous Oxide
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INTRODUCTION
IN recent years, interest has grown rapidly in extending the application of microwave energy to the processing of a wide variety of new and engineered materials, including ceramics, polymers, composites, and chemicals.[1–3] However, the equations for the evaluation of the microwave absorption are complicated, especially for magnetic materials, and only a few investigators have paid close attention to the effect of temperature on the dielectric properties, which is extremely important for high-temperature materials processing under microwave irradiation.[4,5] As an important transitional material and precursor for various magnetic ceramic/metal nanocomposites or nanocrystals with high values of coercivity for the application as recording media, ferrous oxide has attracted much attention from researchers.[6–8] However, the absorption behavior of ferrous oxide under microwave irradiation has never been investigated.[9–11] Furthermore, some research claims that there is phase transformation due to thermal decomposition of ferrous oxide during the heating.[12,13] This interesting phenomenon may highly affect the characterization of the dielectric properties. Also, it is known that the microwave heating efficiency ZHIWEI PENG, PhD Student, and JIANN-YANG HWANG, Professor, are with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Contact e-mail: [email protected] JOE MOURIS and RON HUTCHEON, Partners, are with Microwave Properties North (MPN), Deep River, ON K0J1P0, Canada. XIANG SUN, Postdoctoral Research Associate, is with Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180. Manuscript submitted October 13, 2010. Article published online March 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
of materials relies not only the dielectric properties but also on the magnetic properties. Although the ferrous oxide is known to be paramagnetic, and thus not a strong microwave absorber, the thermally decomposing heated sample may change substantially. Thus, a reasonable evaluation of microwave absorption capability of the ferrous oxide and its dissociation products should be based on the measurements of both permittivity and permeability. The aim of this work is to measure the dielectric and magnetic properties of the nonstoichiometric ferrous oxide up to 1373 K and 823 K (1100 C and 550 C), respectively, using the cavity perturbation technique and to evaluate the temperature dependence of the microwave absorption capability of the ferrous oxide with consideration of the phase transformation during the heating.
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EXPERIMENTAL
Nonstoichiometric ferrous oxide powder samples (99 pct purity; they were fabricated starting with a higher iron oxide, then reducing the oxide by introducing iron metal) were purchased from Sigma-Aldrich (Sigma-Aldrich Corp., St. Louis, MO). The particle sizes, microstructure, and distribution of the oxides were characterized by a Hitachi S-4700 field-emission–scanning electr
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