Molten-salt-assisted combustion synthesis of B4C powders: Synthesis mechanism and dielectric and electromagnetic wave absorbing properties

A novel electromagnetic wave (EMW) absorber was prepared by combustion synthesis. Boron carbide (B4C) powders with different grain sizes using a molten-salt-assisted combustion technique with B2O3, CB (carbon black), and Mg powders as starting materials, and NaCl as an additives. The effects of the NaCl content on the phase compositions and the microstructure of the products were characterized. A combustion front quenching method was used to elucidate the mechanism for the B4C powders synthesis. The dielectric, and EMW absorbing properties in the X-band were also investigated. The results showed that the addition of NaCl significantly reduced the grain size of B4C powders. Nanoscale B4C powders with cubic polyhedral structures were synthesized using 6 wt% NaCl (labeled as N-6). According to the quenching test results can be obtained that the first step in the combustion synthesis was melting B2O3 into a glassy substance. At the same time, Mg melted and formed a liquid pool into which the NaCl dissolved, followed reduction of the B2O3 to B. The formed B eventually reacted with CB to form B4C, and the B4C particles precipitated from the matrices. The N-6 sample exhibits optimal dielectric and EMW absorbing properties, because of a high specific surface area that enhances interfacial and space charge polarization.