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A high-yield production of high-quality boron-nitride nanotubes (BNNTs) wasreported recently in several publications. A boron-rich material is evaporatedby a laser or plasma in a nitrogen-rich atmosphere to supply precursor gaseousspecies for nucleation and growth of BNNTs. Either hydrogen was added orpressure was increased in the system to achieve high yield and high purity ofthe synthesized nanotubes. According to the widely-accepted "root grow"mechanism, upon the gas cooling, boron droplets form first, then they adsorbnitrogen from surrounding gas species, and BNNTs grow on their surfaces.However, what are these precursor species that provide nitrogen for the growthis still an open question. To answer this question, we performed thermodynamiccalculations of B-N mixture composition considering broad set of gas species.In enhancement of previous studies, the condensation of boron is now taken intoaccount and is shown to have drastic effect on the gas chemical composition.B2N molecules were identified to be a major source of nitrogen for growth ofBNNTs. Presence of B2N molecules in a B-N gas mixture was verified by ourspectroscopic measurements during a laser ablation of boron-rich targets innitrogen. It was shown that the increase of pressure has a quantitative effecton the mixture composition yielding increase of the precursor density. Thehydrogen addition might open an additional channel of nitrogen supply tosupport growth of BNNTs. Nitrogen atoms react with abundant H2 molecules toform NH2 and then NH3 precursor species, instead of just recombining back toinert N2 molecules, as in the no-hydrogen case. In addition, thermodynamics wasapplied in conjunction with agglomeration theory to predict the size of borondroplets upon growth of BNNTs. Analytical relations for identification ofcrucial species densities were derived.

Determining Gas Composition for Growth of BNNTs Using Thermodynamic Approach