The Study Of Synthesizing Boron Nitride (BN) Powders By Network Method

BN material has significant potential applications in the fields of hydrogen storage materials and catalyst carriers owing to its excellent mechanical properties, stability chemical and resistance to oxidation and high thermal conductivity, etc.But until now, BN material has not been widely used in these fields. The main reason for this is that there is no method of synthesizing BN with high productivity, purity and specific surface. At present, there are many prepared methods of BN such as chemical vapor deposition, arc discharge, ball-milling, laser ablation, carbon substitution reaction, pyrolysis of the precursor and so on. Among them, the arc discharge method and chemical vapor deposition are complicated to operate with high cost of production and low productivity. Carbon substitution reaction and pyrolysis method are often used to synthesize BN with high specific surface. However, the former is difficult to synthesize BN and its productivity is low, while the latter is difficult to synthesize BN powders with high purity because it's hard to remove the residual carbon after reaction. For these reasons, it is an important task to find a simple, economical and efficient method of synthesizing BN with high purity and specific surface.BN powders with high purity and specific surface were prepared where the precursor of BN was firstly synthesized by the polymer network method and with borax and urea used as the main raw materials. The formation mechanism of polymer network, the chemical composition and its mixture ratio of precursor, network of sol-gel were investigated. Moreover, the influences of kinds of carbon source on sol-gel network and the reaction temperature on specific surface, porosity and microstructure were discussed. The prepared process, reaction mechanism and microstructure of BN powders were also studied. The BN powders were characterized by XRD, SEM, EDS, TEM and adsorption-desorption of N2 at low temperature. The results obtained by the optimal prepared process were as follows.The dry gel of polymer network with the homogeneous distribution B and N elements was synthesized by the network method. And the optimal ratio of its ingredients was acrylicamide: N,N'methylenebisacrylamide:ammonium peroxydisulfate=140:4:25. The BN precursor with favorable dispersivity was firstly obtained by improving the dispersivity based on the network method. Then the BN powders with high special specific and excellent dispersivity were successfully prepared by adjusting the reaction time and reaction temperature. The optimal preparing process was that keeping 300℃and 675℃for 2h in the atmosphere of NH3, respectively. And BN powders were prepared after the products above were cleared and decarbonized. The average pore diameter of synthesized BN powder was about 5.16 nm, the specific surface was 288.57 m2/g. It was much higher than the existing reports. The network method is efficient, simple and easy to operate and the reaction temperature is low.