Microwave heating characteristics of graphite based powder mixtures ☆ Sekaran Chandrasekaran, Tanmay Basak, Ramanathan Srinivasan ⁎ Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India abstract article info Available online 18 September 2013 Keywords: Microwave heating Graphite–silicon carbide mixture Graphite–magnetite mixture Microplasma The choice of material for a crucible is essential in hybrid microwave heating and graphite is one of the strong microwave absorbers which can sustain high temperatures. Experiments were carried out to investigate the mi- crowave heating behavior of graphite combined with other materials to enhance efficiency. Mixtures comprising of graphite and magnetite or graphite and silicon carbide were found to have better microwave heating proper- ties than pure components. The enhancement was likely due to microplasma generated by graphite in a micro- wave environment. Microwave heating of graphite–silicon carbide mixtures provided the best results in terms of high heating rates without arcing. The optimal combination of graphite–silicon carbide was identified and can be used as a starting material for making efficient crucibles and susceptors. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Microwave heating had profound applications in food, ceramic and chemical processes in the past few decades. Microwave heating is caused by the conversion of electromagnetic energy to thermal energy and the efficiency of the conversion depends upon the dielectric nature of materials. With high heating rates and reduced processing time, the application of microwave heating is investigated for processes such as drying [1], thawing [2], freeze drying [3,4], ceramic sintering [5], accel- erated curing of cement and rubber [6,7], microwave heating using sup- ports [8–10], waste water treatment [11] and preparation of activated carbon [12]. In many cases, the energy consumption in microwave heating is less than that of other heating processes and the processing time is shorter [13]. Microwave assisted processes are shown to have a high heating rate as well as a high reaction rate whereas it is not so for samples subjected to conventional processes [14]. Recently, there is an interest in characterizing the process of metals such as sintering [15], brazing [16], joining [17] and melting using mi- crowaves [18,19]. Conducting materials such as metals do not efficiently convert microwave energy to heat since they reflect microwaves [20]. There is a difference in the response of bulk metals and metal nanoparticles to microwaves. In microwave processing, skin depth is a measure of the depth of microwave penetration in which the incident radiation decreases to 36.8% of its value from the surface [21]. If the skin depth value of a material is less than its thickness, then the micro- wave heating occurs only on its surface. It is known that the skin depth value of bulk metals is in the order of microns [22]. However, the skin depth of the metals increases with a decrease in its particle size and hence metal powders in the range of sub-micron and nano-sizes interact well with microwaves [23]. Although bulk metals cannot be di- rectly heated by microwaves, it is possible to heat nano-sized metal powders. On the other hand, hybrid microwave heating can be used to heat bulk metals. Compared to conventional heating, the energy re- quirement of a metal melting process can be reduced by a factor of four by hybrid microwave heating [19]. In this process, microwave ab- sorbing materials such as SiC is used as cladding or susceptor, which supplies heat to the metal sample by means of conduction and radiation [19,24]. The use of microwave susceptors results in relatively lower heating efficiency. The efficiency can be improved if the crucible itself can be made of materials which convert microwaves to heat very well and thus the choice of material for a crucible is important for hybrid mi- crowave heating. Graphite is a strong microwave absorbing material which can sus- tain high temperatures and provide high heating rates. Carbon materials such as graphite are reported to generate microplasma, when they are heated in the presence of microwaves [25,26]. “During the process, some of the electrons might jump out of the material, due to increase in their kinetic energy. This results in ionizing the surrounding atmo- sphere which is visualized as sparks or electric arc formation. Locally, they are called as microplasma, since they exist for a very short duration (fraction of a second) and over tiny space region” [25,26]. Although, graphite or other carbon products were used to assist in microwave py- rolysis [27,28] and microwave carbothermal reactions [29–33], the microplasma may tend to generate arcs which can adversely affect equipment. By combining graphite with other materials, it may be pos- sible to enhance the heating efficiency while reducing the tendency to arc. Thus, the microwave heating characteristics of graphite combined with other materials require a detailed study. An optimal combination of graphite powder mixed with either microwave absorbers or non- microwave absorbers can be used as a starting material for making cru- cibles. With these crucibles, the energy efficiency of hybrid microwave heating could be enhanced. There are reports of adding a microwave International Communications in Heat and Mass Transfer 48 (2013) 22–27 ☆ Communicated by A.R. Balakrishnan and T. Basak. ⁎ Corresponding author. E-mail addresses: srinivar@iitm.ac.in, ramanathan.srinivar@gmail.com (R. Srinivasan). 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.09.008 Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt