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 A/cm2, 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.
Highlights
Much research has been devoted to boron nitride nanotube (BNNT) synthesis as they possess attractive properties such as high Young’s modulus[1, 2], resistance to oxidation[3], and high absorption energy for hydrogen molecules[4, 5]
Arc synthesis of boron nitride nanotubes involves the ablation of the boron anode where the evaporated boron reacts with the background nitrogen
The grey fibrous products contained an abundant amount of BNNTs along with unreacted anode materials wrapped by layers of boron nitride
Summary
Much research has been devoted to boron nitride nanotube (BNNT) synthesis as they possess attractive properties such as high Young’s modulus[1, 2], resistance to oxidation[3], and high absorption energy for hydrogen molecules[4, 5]. Electrodes such as HfB218 and ZrB219 in nitrogen gas environments and boron nitride encapsulated composite electrode[21] in helium gas environments In these studies BNNTs were only found in the cathode deposits. The arc was operated with anode and cathode electrodes made from the same boron-rich alloy in a nitrogen gas filled chamber at 380 torr After operation of this arc, a grey web-like material consisting of BNNTs was found in the reactor chamber suggesting a volumetric synthesis of BNNTs. among the materials synthesis arcs reported to date, the arcs are mostly anodic and the synthesis feedstock comes from the anodes except the arc used to produce BNNTs. Presumably, the purpose of using the same boron-containing materials for both cathode and anode is to increase the likelihood of BNNT production since both electrodes receive a significant amount of heat from the arcs.
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