In this study, the waste silicon powder generated in the production of solar-grade polysilicon scrap was used as the raw material, and silicon nitride (Si 3 N 4 ) was directly efficient prepared by the microwave heating nitridation. The temperature raising characteristics of silicon powder by microwave heating and the influencing factors of the nitridation reaction process were studied. The thermogravimetric analysis was performed, and the temperature raising dielectric properties of silicon powder were studied. The electromagnetic field and temperature distributions of the microwave heating-induced silicon powder nitridation process were simulated using COMSOL software. The nitridation reaction of silicon powder induced by microwave heating has better temperature raising characteristics: the average heating rate can reach 135 °C/min, and the reaction time is significantly shortened (only 10−20 min). Microwave heating decreases the nitridation reaction temperature by more than 100 °C and greatly shortens the reaction time. With the increase of nitrogen pressure and reaction time, the nitridation reaction is better. In addition, the conversion of the nitridation reaction is more than 97%, and the products are mainly β-Si 3 N 4 with the uniform and columnar morphology. Finally, it is proved that the efficient recovery and utilization of industrial waste silicon powder are realized, and there is lower energy consumption by microwave heating technology.

Using the same bend waveguide with two different E-field polarization directions, the breakdown phenomena in a curved waveguide with different power capacities were studied. For the case of lower power capacity, breakdown happens at the curved arm; with increasing incident power, the reflected peak amplitude increases, the breakdown delay time becomes shorter, and the reflected pulse gradually overlaps with the incident one, forming a slower falling edge at the end of incident pulse. For the case of higher power capacity, discharge may occur at the connected flanges without the choke shot. Using the four-frame intensified-charged-coupled device diagnosing the nanosecond light emission in each of the vertical and horizontal polarization cases, the intensities in both of the surface layer and the space beyond were found to first become bright during the microwave pulse, and then get dark after the pulse. The multiple bright spots on the wall illustrate the nonuniform plasma development and local high-density plasma at the microprotrusions with field enhancement, triggering the intense high-field emission or explosive emission.Index Terms-High-power microwave, vacuum breakdown, waveguide discharge.