Large Scale Preparation Of Boron Nitride Nanotubes

In this dissertation, the structures, syntheses, properties and applicationprospects of the boron nitride nanotubes （BNNTs） are briefly reviewed. Onthe basis of these previous investigations, we proposed new routes to largescale preparation of BNNTs. We prepared amorphous boron andsubmicro-polycrystalline CaB6powders as boron-containing raw materialsand various catalytic boron-containing porous precursors byself-propagation high-temperature synthesis （SHS） method. Consequently,large scale preparation of four kinds of BNNTs have been carried outsuccessfully via chemical vapor deposition synthesis （CVD） using thesecatalyst boron-containing porous precursors as starting materials. The majorresults are summarized as follows,1. Catalytic boron-containing porous precursors were prepared by SHSmethod using boron-containing materials, alkalin earth metals and transitionmetal compounds as the raw materials. For example，the molar ratio of theB-Mg-Fe porous precursors was1:（0.10¹.65）:（0.33¹.95）. The pore size ofthe porous precursors is1nm to5mm with pressive strengths of1.02².50MPa and volume densities ranging from1.25to5.50g/cm³. Anovel route so called reactive-diluted SHS method were used for producingamorphous boron nanopowders with particle size ranging from20to80nm.Submicro-polycrystalline calcium hexaboride （CaB6） powders were alsoprepared via SHS method, which could effectively prevent productagglomeration.2. Four kinds of boron nitride nanotubes, i.e. cylinder, bamboo-like,bubble-chain and wave-like, were synthesized at1050¹180℃for about6hby self-propagation high-temperature and chemical vapor depositionsynthesis （SHS-CVD） method using the catalytic boron-containing porousprecursors B₁₈Ca₂（MgO）₉, B₁₈Fe₄Ca₃O₆, B₁₈Fe₂Mg（MgO）₂₁and B₁₂Mg₂（MgO）₉, respectively. The yields of BNNTs were more than80wt%with purities of BNNTs estimated ranging80to95wt%.3. Base, tip and base-tip models were proposed to explain the growthmechanism of BNNTs. We explained the relationship between the abruptlychanging of growth rates and the features of the BNNTs by apressure-induced mutation mechanism. A smooth rate is for cylinderstructure, while a periodically, abruptly changing rate for three otherstructures. Furthermore, the mechanism by annealing porous precursors forlarge batch production of BNNTs was also discussed.4. Annealing setup for large batch production of BN nanotubes wasdesigned. In a single-tuble vertical annealing furnace, four kinds of productsup to one kilogram including cylinder, bamboo-like, bubble-chain andwave-like BNNTs were synthesized respectively using the same porousprecursor B₃₁Fe₁₇（MgO）₂7as starting materials by turning syntheticparameters at1000¹200℃for6¹2h with the flowing rate of the NH₃/N₂was0.5¹0L/min. The as-synthesized BNNTs displayed average diametersand mean lengths are about100nm and more than10μm for the cylinder,50nm and more than5μm for the bamboo-like,80nm and more than5μmfor the bubble-chain, and about150nm and5μm for the wave-like BNNTs.The average purity of four kinds of BNNTs was estimated above80wt%with maximum purities reaching up to85wt%. The as-synthesized BNNTshave been successful used in researches by worldwide groups such as in theItalian Institute of Technology. Pilot scale manufacture of boron nitridenanotubes （BNNTs） are expected coming into true in the near future.