Research On Promotion Methods Of Boron Particles Ignition And Combustion

Boron is an attractive fuel for propellants and explosives because of its high energy density. Although boron is exceptionally effective, its potential as a fuel or a fuel additive is yet to be realized partly because boron combustion is difficult to complete. The external surface of most boron particles are coated with a layer of oxide film. Boron ignition is thus hindered by this protective oxide layer. On heating, the oxide shell on the boron particle melts before the solid core does, thus initiating a diffusion-controlled process through the molten shell. So it is important to study on ignition and combustion of boron and find ways for promoting ignition and combustion property of boron particles.Based on the results of the previous research, we used a variety of analytical and testing instruments to obtain physical and chemical characteristics of boron particles. A large number of experimental studies of boron ignition and combustion were done. The effect of purity of boron powders, crystal type, particle size, pressure, atmosphere, heating rate and other factors on the burning characteristics of boron particles were systematically investigated. Because of the shortcoming of recrystallization method, a new way named two-solvent method for coating boron particles was proposed. The optimal process parameters of two-solvent method were obtained through orthogonal experiment. The effect of many coating agents on the burning characteristics of boron were studied and a variety of additives that can significantly promote boron ignition and combustion were found. We also revealed their mechanism of action.The experimental results show that the purity of boron powder, the more low, the more easily the ignition and combustion. In the same purity, amorphous boron is easier to ignite and combust than crystal boron. In a certain size range, with the increase of particle size, the ignition temperature of boron powder increased gradually, but the amplitude was small. And with the increase of particle size, reaction efficiency of boron powder increased silghtly.The thermal analysis results show that, with the increase of heating rate, thermal gravimetric curve of boron powder has a lag effect. The ignition temperature increases and the maximum weight gain decreases. The thermal oxidation process of boron powder can be divided into two stages below 1000℃. The first stage has the optimal mechanism of G(a)=-ln(1-α), and the best function mechanism of the second stage is G(a)= (1-α)-2. Increasing the concentration of oxygen in the environment is conducive to boron ignition. With the increasing environmental pressure, the ignition temperature of boron powder slightly decreases, and the maximum rate of weight gain and the reaction efficiency decreases gradually.The PH-method was used to verify boron coating effect. The results show that two-solvent method is superior to recrystallization method for coating boron particles. The optimal value of ultrasonic mixing time is 10 min. Using ultrasonic mixing can improve coating effect effectively. The optimal value of evaporation temperature is 350℃. Increasing evaporation temperature is beneficial to improving coating effect, but the promotion effect decreases with increasing evaporation temperature. The optimal value of stirring speed is 200 r/min. LiF and TMP can reduce the ignition temperature and increase heat release of the boron samples. AP can shorten the ignition delay time and improve the combustion intensity.The effect of a variety of additives on the ignition and combustion of boron particles were studied. The combustion flame intensity of a sample increases with the increasing magnesium content of boron particless. An increase in magnesium content results in a decrease and a subsequent increase in boron ignition delay time. Among the seven kinds of metal oxide additives used in the experiment, Bi2O3 is the most active catalyst:it can reduce the ignition temperature by 15.2% and decrease the ignition delay time by 46.2%. Addition of oxalic acid into boron powders can significantly promote boron ignition and combustion. The ignition delay time of the resulting mixture is reduced by 42.4%, the combustion intensity is raised by 16.7%, and the combustion efficiency of boron is increased by 21.5 percentage points. The thermal reaction of boron can be significantly promoted with LiH, which can reduce the initial temperature of the first violent reaction stage by-140℃. The starting temperature of the post-reacion stage also decreases by～260℃. Besides, LiH can shorten the ignition delay time of boron by～34.1% and enhance the combustion intensity of boron by～117.6%.