| Hexagonal boron nitride (h-BN), with similar crystal structure of graphite, is a kind of white loose, soft and smooth feeling powder. H-BN owns many unique physical and chemical properties, such as low density, high temperature stability, high thermal conductivity, low dielectric constant and chemical inertness. These properties make h-BN an interesting material for many applications in hydrogen storage material, catalytic carrier, high-temperature lubricant and advanced ceramic composites, etc.Now, h-BN can be prepared by various methods, such as O’Connor traditional high-temperature synthesis, chemical vapor deposition (CVD), no pressure sintering or hot pressure sintering, sol-gol, self-propagating high-temperature synthesis (SHS), pyrolysis of precursor. In the present industry, the modified O’Connor method with borax (boric acid) and urea as starting materials was chosen to synthesize h-BN due to its availability. However, owing to high temperature synthesis, raw material being mixed unevenly and needing inert gases or ammonia for protection in the traditional synthesis processes, h-BN is manufactured with high cost, low purity and poor particle size uniformity. It is note worthy that pyrolysis of precursor is an interesting preparation approach to solid inorganic compounds, which exhibits high efficiency, low energy consumption, easy operation and mixing starting materials in the molecular level. Therefore, we developed a synthetic process of h-BN with low cost, simple operation and mild conditions by using precursor method in this paper.In this paper, a precursor was prepared by a wet chemical approach using melamine (C3N6H6) and boric acid (H3BO3) as starting materials, and then hexagonal boron nitride was synthesized by the high temperature treatment of the precursor in the air. To get an insight into the synthesis of boron nitride, the effects of melamine to boron acid molar ratio, solution concentration, synthesis temperature and time on h-BN synthesis were respectively investigated. The precursor and product h-BN were respectively characterized by FT-IR, chemical composition, XRD, SEM and particle size analyses. The results indicated that a better precursor was synthesized by using the molar ratio of1:2for the melamine and boric acid and the solution concentration of0.4mol/L for the reactants, and the morphology of precursor exhibits a rod-like crystals and its molecular formula is C3N6H6.2H3BO3. Then the stoving rod-like precursor was treated at950oC for6h to afford good h-BN crystals with a primary particle size of17μm and a purity of93.98%.On the other hand, h-BN was prepared again by choosing melamine and ammonium borate (NH4HB4O7.3H2O) as raw material according to the above mentioned route. The results indicated that the precursor prepared by using ammonium borate as starting materials was the same compound prepared by using boric acid, and a better precursor was synthesized by using the molar ratio of1:1for the melamine and ammonium borate and the solution concentration of0.5mol/L for the reactants. Then the stoving rod-like precursor was treated at950oC for4h to form good h-BN microcrystals with a purity of88.68%.To explore the reaction mechanism of transforming precursor into h-BN at high temperatures, the melamine and prepared precursor were characterized by IR and TG-DTG. The results show that the reaction contains two steps. First, the triazine rings and B-O bonds of precursor were broken and B, C, N started to happen mutual combination forming new intermediate including B, C, N elements. In the meantime, the precursor was combined with oxygen in the air or itself oxygen to release H2O, NH3, CO2and NO2. Finally, the new intermediate was broken under high temperatures and formed h-BN. The reaction process was that the complicated organic precursor transformed into simple inorganic products, and its possible reaction equation as follows:4C3N6H6.2H3BO3+21O2=4BN+8NH3+12CO2+12NO2+12H2O+2B2O3... |
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