Abstract
The spontaneous ignition and combustion of small boron lumps were studied experimentally by using the stagnation region of an impinging oxygen stream. The critical ignition temperature was measured and the ignition was observed to occur in the gas phase, but the ignition process was rather different from those of metals with high vapor pressures at low temperatures. It was also found that, during the combustion, the burning lumps were melted and gas-phase reactions of BO and BO 2, not boron vapor, proceeded in the green-white radiating zone close to the boron surface. There existed a critical oxygen concentration below which self-sustained combustion with the gas-phase reactions could not occur. A combustion model for the B O 2 system has been developed, including the surface reactions producing BO gas, the gas-phase reactions producing BO 2 gas and gaseous B 2O 3, and the condensation of B 2O 3 gas. The ignition mechanism is discussed, and the role of surface reactions and condensation of B 2O 3 on the surface is clarified.
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