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 °C.
A significant fraction of the BNNTs survived temperatures as high
as 2200°, and the BNNT samples were completely undamaged at temperatures
as high as 1800 °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 °C. The
stability of BNNT structures was observed up to 2000 °C, with
local/partial wall dissociation or unzipping, and complete survivability
of highly crystalline BNNTs is demonstrated up to 1800 °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.