Preparation Of BN Micro/Nanomaterials From Ammonia Borane And Their Formation Mechanisms

As analogues of carbon micro/nanomaterials, BN micro/nanomaterials have low density, high-temperature stablity, oxidation resistance, good biocompatibility and a series of other advantages, which promises broad applications as UV laser devices, biosensors, composite materials, reinforcements and hydrogen storage materials. But compared with the carbon counterparts, the preparation of BN micro/nanomaterials are facing many difficulties such as demanding reaction conditions, low product yield, and poor purity. The main reason is that BN precursors which are suitable for the preparation of BN micro/nanomaterials is extremely limited, so finding the right BN precursor is the key to solving these problems. Ammonia borane (AB, BH₃NH₃) is a solid-state material and only contains B, N and H elements. Although it has been first synthesized in 1955, knowledge concerning the pyrolysis behavior of this substance and whether they can be used to prepare BN micro/nanomaterials are very limited. The thermal decomposition of AB, and preparation of BN micro/nanomaterials from AB have been systematically studied. It is proved that AB is an promising precursor for BN micro/nanomaterials. The main results obtained in this paper are summarized below.First-principles calculations show that B atoms and N atoms form covalent bonds with H atoms, while the B atoms and N atoms are connected by coordination bonds; The transfer of electrons from the N atom to B atom is confirmed, which led to the hydrogen bond interactions and dipole-dipole interactions among BH₃NH₃ units, The interaction energy was calculated to be 15.1kJ/mol, which is important for the structural stability of AB. TG-DSC-MS analysis show that prior to 1000°C, AB almost loss 50% of the initial mass, the lost part are converted to boranes, borazine, ammonia and other N and/or B containing small molecules, The remaining part was transformed to BN nanoplates, the fluorescent analysis showed that the emission bands of the BN nanoplates is in the 200-400nm region.Using AB as a precursor, BN nanotubes (BNNTs) have been successfully fabricated. The structure, magnetic property and optical properties of the BNNTs, and the effect of the catalyst, reaction temperature, pressure and other processing conditions on the growth of the BNNTs were investigated. The morphologies of the BNNTs can be divided into two kinds, one is the bamboo shaped, the other is cylindrical shaped. Studies on the growth process shows that iron, iron oxide, and ferrocene can be used as a catalyst for the fabrication of BNNTs under appropriate conditions. The catalytic effect of ferrocene is the best. Under appropriate conditions, BNNTs can be transformed into BN whiskers.According to thermodynamic theory and the VLS growth mechanism, the growth model for both BNNTs and whiskers is established. The morphology variations of the BNNTs along with the process conditions could be reasonably explained. Theoretical analysis shows that whether the catalyst particles are too small or too large is not conducive to the precipitation of BN layers. The magnetic properties of the BNNTs which encapsulate magnetic nanoparticles are investigated. It is shown that boron nitride nanotubes can effectively protect the metal nanoparticles in them. PL and CL spectra show that the BNNTs and BN whiskers are ultraviolet light emitting materials with excellent performance.Using AB and graphite paper, we prepared bowl-shaped and nest-shaped BN hollow microspheres. The structure, and the effects of the reaction temperature, pressure, atmosphere and other parameters on the growth of the BN microspheres are investigated. The growth mechanism of the BN microspheres was revealed. the average diameter of the BN hollow spheres is 3.4μm, and the thicknesses were about 200nm. Upon increasing the reaction temperature, the bowl-shaped BN hollow spheres gradually changes into nest-shaped BN microspheres. The effect of pressure on the growth of BN hollow microspheres was not significant. The bowl-shaped BN hollow microspheres show special resonance Raman spectroscopy. The CL emission bands of the BN hollow microspheres are in the region of 200-400nm, indicating that they are promising candidate for UV light-emitting devices.AB can be employed to form BN coatings via CVD method by which SiC/SiO₂/BN three-layered nanocables were prepared from SiC/SiO₂ two-layered nanocables. SiC/SiO₂/BN three-layered nanocables The structure, photoluminescence properties and the effects of vapour concentration on structure of SiC/SiO₂/BN nanocables are investigated. A growth model was proposed. The nanocables are about 100nm in diameter, the thicknesses of the SiO₂ and BN layers were 10nm and 5nm, respectively. The photoluminescence spectra of the original SiC/SiO₂ nanocables and that of the SiC/SiO₂/BN nanocables are basically the same, with only the 488.5nm emission peak blue shifting. Upon increase the concentration of reaction vapours, the original SiC/SiO₂ nanocables are transformed into nanotubes due to the etching of SiC and SiO₂ by hydrogen.