Synthesis of micro- and nanostructured materials via oscillating reactions initiated by high-power microwave pulses

Herein we describe the results of our study of plasma chemical process initiated by high-power microwave pulses generated by 75/0.8 gyrotron (frequency 75 GHz, pulse duration up to 12 ms, power up to 550 kW) in mixtures of metal (Mg, Ti, Mo) and dielectric (Al2O3, AlN, B) powders. The process leads to formation of microand nanostructured materials and the yield is strongly affected by hydrogen-containing additives (C2B10H12, H3BO3, NaBH4) in the mixture. It was found that the process occurs in three main stages. The first initiation stage is very short (just slightly exceeds the pulse duration) and includes the absorption of microwave irradiation and the discharge proceeded as coulombic explosion. The main stage is much longer and includes decomposition of starting materials and reactions between all the components. All the tested additives prolong the main stage, in some cases by an order of magnitude or even more, and turn it into self-sustaining oscillating mode. The last one is realized most likely due to autothermic effect. As a result, the yield increases sufficiently. Finally, the decay stage is relatively short stage (up to 200 ms) following the main stage and includes irreversible cooling down of the reaction zone and deposition of the particles. The decay stage is not affected by additives. Owing to use the additives we were able to decrease the energy consumption for preparation of the materials below 1 kJ/g, which is 2-3 times lower compared to our previous results. The prepared materials were characterized by SEM, EDS and X-ray diffraction analysis and found to be mainly oxides, nitrides and borides.