Phase selectivity of microwave heating evidenced by Raman spectroscopy

Phase-selective temperature evolution of silicon-diamond powder mixtures under 2.45 GHz microwave irradiation is followed using a Raman microscope fitted onto a microwave cavity. This spectroscopic method is shown to enable the collection of thermal information for both the silicon and the diamond phase selectively. An initial difference up to 100 ° C is found between the thermal trajectories extracted from silicon and diamond. A thermal equilibration is observed, subsequently suggesting that heat is flowing from hot silicon towards the colder diamonds. The existence of microscopic thermal gradients reflects the specific interaction of each of the two components with the applied electromagnetic field, governed here by the free carrier concentration. In addition to phase-specific temperature measurements, monitoring of chemical changes during microwave heating is also illustrated for magnetite powder. Raman spectroscopy turns out to be a powerful contact-free, noninterfering tool to follow and analyze on-line microwave heating processing of heterogeneous solid mixtures.