Subject: Can metals be heated with microwave energy?
What needs to be considered when metals are heated using microwave radiation for thermal processing?
If bulk metals are subjected to microwaves, a high degree of reflection is observed, accompanied by the induction of surface eddy currents and a high risk of arcing discharges. The penetration depth of microwaves on conductive metal surfaces is typically less than one micron (c.f. a typical penetration depth of 1–2 cm for organic matter). In addition to the well-documented safety issues, direct microwave heating of non-ferromagnetic bulk metals is very inefficient and best avoided (1).
Metal powders are a different matter. A significant amount of research has been undertaken into microwave heating of metal powders. Energy absorption is considerably higher than for bulk metals, due to a combination of high surface area, poor inter-particle conductivity and (for some metals) non-conductive “dielectric” surface layers on the particles (e.g. oxides, hydroxides, adsorbed water). Thus, microwave heating of metal powders is often possible without serious arcing (1–4).
Complex relationships between heating efficiency and particle size have been reported, and vary from metal to metal (related in part to the presence or absence of surface dielectric layers) (2, 3).
If the desired result of heating a metal powder is sintering to form a bulk metal (e.g. to produce metal objects of particular shapes), reflection increases and heating efficiency is lost as the process advances (the risk of arcing also increases). Indirect heating, using a microwave-adsorbing “susceptor” such as silicon carbide, is advisable in such applications to ensure efficient conversion of microwave energy and drive the process to completion.
For example, Ceralink have developed a microwave furnace embedded with silicon carbide blocks, for melting scrap aluminium to form fresh ingots (4). The process is reported as being faster and more energy efficient than conventional heating for this application.
Where metals have been dispersed upon a support, in many cases the microwave heating properties of the dry material are dominated by the support, although the metal may have a significant effect (particularly if the metal loading is high). For the majority of carbonaceous supports, such as graphite or polymers, strong coupling and efficient heating are likely.
For inorganic supports, microwave heating is material-specific and dictated by the equation below:
(T = temperature; t = time; V= sample volume; m = sample mass; Cp = heat capacity; E = electric field strength; ν = microwave frequency; ε’ and ε” are material-specific permittivity terms)
A small number of commercial organisations and research groups in academia are equipped to measure the microwave susceptibility of materials (in the UK, research groups at the Universities of Huddersfield and Nottingham are suitably equipped). It should be noted that susceptibility varies with temperature and temperature dependence is not yet readily predictable.
In principle, microwave radiation can be used in the preparation and activation of supported metal catalysts. A limited amount of research has been published in this area, the majority relating to systems in which the metal species are dispersed upon a carbon or polymeric support (5).