Microwave heating behavior of nanocrystalline Au thin films in single-mode cavity
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In this work, microwave heating curves and microstructural evolution of nanocrystalline Au thin films were studied so the heating behavior of metals could be better understood. Films with different thickness were irradiated by the H and E maximums of microwave in a single-mode cavity. These samples were more efficiently heated by H field than by E field. With the increase of film thickness, the attained highest temperature increased. This phenomenon was compared with the theory of the transmission line analysis. Peaks appearing at the initial stage of the heating curves were also compared with similar heating behaviors observed in metal powder compacts, and the possible mechanisms related to the heating behavior were explored.
I. INTRODUCTION
Microwave heating has become an important method for chemical synthesis and materials processing. Its volumetric, fast, and selective heating allow great savings in energy and time during the heating process. Furthermore, microwave can generate some non-thermal effects such as enhancing chemical reaction rate and lowering sintering temperature. Numerous reports show that microwave heating can modify the microstructure and improve properties of materials.1–4 Conventional belief has been that microwave heating mainly results from the polarization dissipation of dipoles, so the vast majority of studies in this field were focused on nonmetallic materials with relatively high dielectric loss.1–4 In 1999, Roy et al. reported that metal powder compacts had been sintered in a multimode microwave cavity,5 which was inspiring because it had been thought that the skin effect caused by microwave would prevent metals from being heated well. Subsequently, microwave heating of metals became a new attractive field in which some exciting results have been achieved.6–14 The heating behavior of metal powder compacts such as Fe, Co, Al, Cu, and Ni–Zr–Nb–Ti–Pt alloy confirmed that microwave sintering of metals is predominantly derived from the H field.10–14 The theoretical studies indicated that the heating mechanism is involved in skin depth and particle size.6,8 However, nowadays information about the fundamental interaction between microwave and metals is still not sufficient to thoroughly describe the detailed mechanism of microwave processing of metals. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0030
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 1, Jan 2009 Downloaded: 03 Apr 2015
Skin depth of metals at microwave frequency is in the micro scale. The grain size of metal powder used in the aforementioned reports was submicro or micro scale,10–14 which is near skin depth. For metal thin films having a thickness on a nano scale, the grain size is also nano scale, which is much smaller than skin depth. Due to this unique feature, the heating behavior of metal thin films may be different from their metal powder compacts. Unfortunately, to date, little research about this aspect has been reported. In this study, it was ex
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