Dual-purpose alumina ALD coatings for enhanced oxidation resistance and thermal conductivity of boron nitride nanotube fabrics

Boron nitride remains a critical material for myriad applications requiring high-temperature stability and thermal management. This work explores the utilization of Al2O3 atomic layer deposition (ALD) on boron nitride nanotube (BNNT) fabrics to push the stability limit of boron nitride above 900 °C. Long precursor exposures during 50 cycle ALD runs were used to completely infiltrate high-aspect-ratio BNNT fabrics, resulting in uniform, conformal coatings on the interior surfaces of the fabrics. Brunauer–Emmett–Teller method surface area measurements found that the BNNT fiber mat surface area was reduced from 434 ± 3 to 117 ± 1 m2 g−1 after ALD and scanning electron microscopy confirmed that the ALD coating was uniform throughout the thickness of the fabric. We report the first thermal conductivity measurements for Al2O3-coated BNNT fabrics where thermal conductivity increased by approximately 15% in the in-plane direction and 192% in the through-plane direction after ALD compared to uncoated BNNT fabrics. Fabric exposures in pure oxygen at 1000 °C demonstrated that coated BNNT fabrics had superior oxidation resistance over uncoated fabrics. Interestingly, the presence of alumina led to the formation of Al4B2O9 instead of liquid B2O3, resulting in the shape retention of the coated BNNT fabrics after the 2 h oxygen exposure. The work supports ALD as a promising method for improving the high-temperature performance of BNNT fabrics or BNNT-based composites.