Authors: Yao-Wen Yeh, Yevgeny Raitses, Bruce E. Koee, and Nan Yao

Abstract:  Boron nitride nanotubes (BNNTs)  were successfully synthesized by a dc arc discharge using a boron-rich anode as synthesis feedstock in a nitrogen gas environment at near atmospheric  pressure. The synthesis was achieved independent of the cathode material suggesting  that under such conditions the arc operates in so-called anodic mode with the anode material being consumed by evaporation  due to the arc heating. To sustain the arc current by thermionic electron emission, the cathode has to be at sufficiently high temperature, which for a typical arc current density of -100  Alcm2, is above the boron melting point (2350 K). With both electrodes made from the same boron rich alloy, we found that the arc operation unstable due to frequent sticking between two molten electrodes and formation of molten droplets. Stable and reliable arc operation and arc synthesis were achieved with the boron-rich anode and the cathode made from a refractory metal which has a melting temperature  above the melting point of boron. Ex-situ characterization of synthesized  BNNTs with electron microscopy and Raman spectroscopy revealed that independent of the cathode material, the tubes are primarily single and double walled. The results also show evidence of root-growth of BNNTs produced  in the arc discharge.

Submitted to: Scientific Reports
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