Growth and in-situ fiber formation of boron nitride nanotubes by laser ablation using ammonia borane

Abstract

Boron nitride nanotubes (BNNTs) are a type of one-dimensional nanomaterial, structurally identical with carbon nanotubes (CNTs) in which carbon atoms are replaced by boron and nitrogen atoms composed of hexagonal B-N structure. BNNTs have excellent intrinsic characteristics, such as mechanical properties, high thermal conductivity and oxidation resistance like CNTs. Since BNNTs consist of half boron, which has high cross section for neutron absorption, they can be utilized for numerous aerospace applications. It is hard to supply boron and nitrogen atoms by 1:1 continuously because nitrogen atoms prefer to be a gas molecule which has triple bond. For these reasons, the current methods to synthesis high-quality BNNTs require high energy such as laser ablation and thermal plasma. Ammonia borane is a compound with a structure in which 6 hydrogen atoms are bonded to a B-N bond. Since the binding energy of B-H and N-H bonds are weaker than that of B-N bonds, when energy is applied to ammonia borane, ammonia borane gradually releases hydrogen gas and becomes hexagonal boron nitride (h-BN). If ammonia borane is used as a molecular precursor, it is possible to independently supply B-N bonds one by one, just as carbon atoms are supplied one by one for CNTs synthesis. A CO2 laser having an energy of 1000 W was applied to ammonia borane. The pressure inside the reaction chamber was 2, 4, 8 and 12 bar of nitrogen and 2, 4, 8, and 12 bar of argon. The synthesized BNNTs rise up in the up-flow and are collected in the form of fibers. Regardless of the type of gas inside the reaction chamber, h-BN was produced at 2 and 4 bar, and BNNTs were produced at 8 and 12 bar, no difference in the amount. From the experimental results, it was found that in the laser ablation synthesis of BNNT using ammonia borane as a precursor, nitrogen gas does not participate in the reaction, which means the nitrogen source is ammonia borane itself.