Synthesis of boron nitride nanotubes using triple DC thermal plasma reactor with hydrogen injection

Abstract

Highly crystalline boron nitride nanotubes (BNNTs) having multi-walls (≤5 walls) and a small diameter (~7 nm) were synthesized using a triple DC thermal plasma reactor with hydrogen injection. Triple torch configuration not only generates larger high temperature regions than single torch but also allow precursors to directly penetrate the core of the plasma flame, the hottest area in the reactor. A triple torch increases the productivity of BNNT synthesis by preventing the flow of precursors into the rim of plasma flame, a problem that occurs with single torch due to the high velocity and viscosity of the central flame. The role of hydrogen in the growth of BNNTs was analyzed by thermodynamic equilibrium reaction calculations and numerical analysis of thermal flow in the system. The reaction calculations revealed that hydrogen facilitates the formation of NH, NH2, and N2H2 molecules by inhibiting N from recombining into N2 in the high temperature region. Hydrogen influences the formation of scattered high temperature and enthalpy regions due to its thermal properties and expands the vortex region, leading to longer residence times for B-N-H intermediates in the reactive region. As a result, 12.6 g of BNNTs are produced with 21.4 kW of input power. A comparison with other methods shows that this yield is superior and provides an economic perspective for the proposed pilot synthesis of BNNTs.