Extreme Thermal Stability and Dissociation Mechanisms of Purified Boron Nitride Nanotubes: Implications for High-Temperature Nanocomposites

A fundamental understanding of the thermal behavior of reinforcement materials is crucial to fully exploit their properties in composites. Boron nitride nanotubes (BNNTs), structural analogues to carbon nanotubes, are a strong candidate for nanofillers in high-temperature composites due to their high thermal stability, oxidation resistance, excellent mechanical properties, and high thermal conductivity. In this paper, samples of high quality, high-purity BNNTs were tested to thermal failure in an inert atmosphere for the first time up to 2500 degrees C. A significant fraction of the BNNTs survived temperatures as high as 2200 degrees, and the BNNT samples were completely undamaged at temperatures as high as 1800 degrees C. Boron nitride (BN) nanopowders were tested identically to perform a comparative study, as hexagonal BN is commonly found in purified BNNT samples. Observed color darkening, significant weight loss, an increased boron atomic level, significant weight gain upon oxidation, the presence of boron oxide compounds in an oxidized sample, and the observed boron clusters at the nanoscale indicate dissociation of B-N bonds in the BNNT sample at 2200 degrees C. The stability of BNNT structures was observed up to 2000 degrees C, with local/partial wall dissociation or unzipping, and complete survivability of highly crystalline BNNTs is demonstrated up to 1800 degrees C. This paper presents the first-ever study on extreme temperature thermal stability of purified BNNTs in an inert atmosphere analogous to manufacturing processes for high-temperature nanocomposites.