Microscopic mechanism of α-rhombic crystal boron nanocluster oxidation in oxygen

The oxidation of boron (B) powder plays an important role in its ignition and combustion. However, the microscopic mechanism of boron oxidation is controversy, so the detailed oxidation process of α-rhombic crystalline boron nanocluster in oxygen (T < 1900 K) was investigated by a molecular dynamics method based on the reaction force field (ReaxFF). The chemical reaction mechanism, oxidation products, and their properties were clarified. The microscopic dynamic process of boron oxidation was explored. The results show that the initial oxidation products of boron are BO2 and BO, and the final oxidation products are diverse complex boron-oxygen polymers (BmOn) (m ≠ 1) instead of B2O3. Boron and oxygen atoms can react with the oxide layer of boron nanocluster, and simultaneously diffuse in both directions. Oxygen molecules can react with the outer boron first and then gradually diffuse to the inner core, while they cannot react directly with the inner boron core through the oxide layer. Compared with the reaction of oxygen atoms with the oxide layer, the reaction of boron atoms with the oxide layer is easier. The α-rhombic crystalline boron cluster exhibits a strong inertness at low temperature, but the oxidation rate of boron surface accelerates as the temperature increases. During the oxidation process, electrons are transferred from boron atoms to oxygen atoms and polar covalent bonds are formed between boron and oxygen. When the temperature is close to the ignition temperature of the oxide layer, the oxidation products in the oxide layer start to evaporate and leave the surface of boron nanocluster. Different oxygen concentrations result in different starting times of oxidation reaction and different types of later oxidation products.