In-situ optical diagnostics of boron nitride nanotube synthesis for impurity control in thermal plasma systems

Boron nitride nanotubes (BNNTs) exhibit outstanding electrical insulation, thermal stability, and piezoelectric properties, making them promising materials for aerospace, defense, and electronic applications. However, BNNT synthesis remains challenging due to the inevitable formation of impurities during the process. In this study, we introduce a novel approach to monitor and control impurity formation during BNNT synthesis in a thermal plasma system via in-situ optical diagnostics. High-integrity BNNTs were synthesized using h-BN and ammonia borane precursors, achieving a maximum yield of 50.4 g/hr with minimized impurities at an ammonia borane feeding rate of 2.2 g/min. Specifically, in-situ optical diagnostics were utilized to distinguish between BNNT growth and impurity formation: spectral lines from excited radicals, such as BH and NH, reflect the generation of reactive species contributing to BNNT growth, whereas the continuous spectrum originates from blackbody radiation emitted by micron-sized impurity particles. Complementary X-ray diffraction (XRD) and thermogravimetric analysis (TGA) confirmed the correlation between the optical background intensity and impurity content. Consequently, in-situ optical diagnostics provide a direct means to evaluate impurity formation through blackbody radiation, offering a pathway toward effective impurity control in thermal plasma systems. • BNNTs and impurity can be monitored through in-situ optical diagnostics during thermal-plasma synthesis • Impurities produce continuous blackbody spectra, whereas radicals yield distinct spectral lines. • Ammonia borane precursor yields less impurities at high feeding rates compared to h-BN