Focusing effect of electromagnetic fields and its influence on sintering during the microwave processing of metallic par
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ng Xua) and Xiaofang Hu CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China, Hefei 230026, Anhui, China
Yunbo Luan, Zhijun Han, and Zhiyong Wang College of Mechanics, Shanxi Key Lab of Material Strength & Structural Impact, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China (Received 25 July 2015; accepted 19 October 2015)
Microwave sintering is a novel and efficient technology for the rapid preparation of metallic materials. In this paper, an investigation has been performed on the distribution of microwave electromagnetic fields in a metallic particle system and its influence on sintering behavior. The results show that the microstructure of the “metallic-void” will induce a nonuniform distribution and focusing effect of electromagnetic fields during microwave processing, which may accelerate the sintering process. However, further study shows that the focusing effect will decline as the neck grows larger, and will also decline from outside to inside within the loosely packed powder system, which will result in the slowdown of the sintering rate. These results were supported by the synchrotron radiation computed tomography experimental observation of the microstructure evolution of metallic powders during an entire uninterrupted microwave sintering process.
I. INTRODUCTION
Over the last decades, microwave sintering has been under constant development for the rapid preparation of high-performance powder materials, such as ceramics and ceramic matrix composites.1–6 Recently, Roy and his co-author reported for the first time that pure metals can be sintered to full density by microwave processing.7 Subsequently, experiments on microwave sintering of various kinds of metals and alloys were carried out. It shows that the microwave heating not only enhances the densification rate of metallic materials, but also makes the products have better mechanical properties than that prepared by the conventional sintering.8–10 Thus far, much work has been carried out to study the mechanisms of microwave sintering to understand its superiority over conventional approaches. Many have attributed the advantages of microwave sintering to the effect induced by high-frequency alternating microwave fields, such as the enhancement of the diffusion coefficient,8,9 the reduction in activation energy10,11 and the ion conductance,12 nonconventional diffusion mechanism,13 microplasma discharges,14 etc. Moreover, during microwave
Contributing Editor: Sanjay Mathur a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.344 J. Mater. Res., Vol. 30, No. 23, Dec 14, 2015
processing, the microwave provides the direct energy source for sintering. So, the distribution of electromagnetic microwave in the particle system plays an important role for the sintering process. Some researchers investigated the mechanisms of microwave sintering from the propagation and distribution characteristics of electro–magnetic mi
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