Study On Preparation, Combustion Property And Application Of Borides And Coated Boron

Boron is considered a primary candidate as an energetic additive in the formulations of solid fuel-rich propellants for volume-limited propulsion applications because of its potential for providing high volumetric and gravimetric heating values. However, the energy release during the combustion of boron-based fuel-rich propellant is significantly lowered due to its low combustion efficiency in application. Therefore, realizing high combustion efficiency of boron particles is the most important objective for the researchers in the field of combustion. Modification of boron particles is useful way to achieve this goal.The objective of this research is to improve the combustion efficiency of boron-based fuel-rich propellant using modified boron as energetic additive instead of boron. The preparation, characteristics of modified boron and its influences on the performance of boron-based fuel-rich propellant were systematically investigated. Borides and surface-coated boron were taken as the research objects in this dissertation. Borides included Mg B2, Mg B4 and organic boride. Surface coated boron included AP-coated boron, Li F-coated boron and Mg-coated(MB15) boron. Among the modified boron, magnesium borides were the most important one. The decomposition and oxidation characteristics of modified boron under slow heating rate were studied. The combustion behavior of modified boron in high temperature flame was systematically researched. The combustion mechanism of modified boron and combustion characteristics of its agglomerates were investigated. The energy property and primary combustion property of propellants containing modified boron were tested. The influences of modified boron on the performance of boron-based fuel-rich propellant were clarified.The synthesis condition of magnesium borides were obtained based on Mg-B phase equilibria diagram, thermodynamic analysis and experiments. Mg B2 and Mg B4 were produced by high temperature sintering under argon atmosphere. The purity of Mg B2 and Mg B4 were 93.6% and 93.4%, respectively. The density of Mg B2 and Mg B4 were 2.55 g·cm-3 and 2.47 g.cm-3, respectively, higher than that of boron(2.34g·cm-3).Results showed that the combustion heat of organic boride, 7.12%Li F-coated boron, Mg B2 and MB15 were higher than that of boron. And the combustion efficiencies of modified boron were all higher than that of boron. Especially, combustion efficiency of organic boron was the highest of all samples, which was 95%. The combustion efficiencies of Mg B2 and Mg B4 were 62% and 48%, respectively, which were higher than that of boron(38%).The first oxidation stage of boron occurred at 1053 K, and then its oxidation rate slowed. Under inert atmosphere, Mg B2 decomposed to Mg B4, Mg B7 and boron-rich phase in turn with elevated temperature, and released magnesium. Mg B4 decomposed to Mg B7 and boron-rich phase in turn. Namely, Mg B2 decomposed to Mg B4 firstly, and then experienced the same decomposition reaction. Both the primary decomposition and primary oxidation stage of magnesium boride occurred between 1200 K and 1665 K. At 1665 K, Mg B2 was fully oxidated, and oxidation percentage of Mg B4 was 86.8%. Compared with boron and surface coated boron, complete oxidation of magnesium boride could be achieved at lower temperature, indicating its superiority.The decomposition reaction of Organic boride was endothermic, but it could release large energy in oxidation reaction. Li F coating and Mg coating lowered the temperature of the first oxidation stage of boron.Emission spectra results showed that there were continuum emission signal around 546 nm during the combustion process of boron, indicating the existence of BO2. The combustion emission spectra of Mg B2 were considered as the combination of the combustion emission spectra of boron and magnesium. Magnesium boride decomposed in combustion, and released magnesium gas. The combustion of Mg B2 changed from magnesium-dominated combustion to boron-dominated combustion as time went on. However, the combustion emission spectra of organic boride, AP-coated boron, Li F-coated boron and Mg-coated boron were similar to that of boron. The combustion time of magnesium boride agglomerates was the lowest among the seven kinds of agglomerates at the same experimental condition. The combustion time of AP-coated boron agglomerates was shorter than that of boron agglomerates at low temperature. Organic boride could promote the combustion of boron at low temperature and low oxygen mole fraction. The promotion of other modified boron on boron combustion was ambiguous.The results showed that the combustion heat of propellants containing modified boron were all higher than that of propellant containing boron. Except for propellant containing AP-coated boron, the combustion efficiencies of propellants containing modified boron were higher than that of propellant containing boron. Especially, the combustion efficiency of propellants containing magnesium boride was the highest. The isotonic heat of explosion of propellants containing Mg-coated boron, Mg B2, Mg B4, and Li F-coated boron were higher than that of propellant containing boron. The gas evolution properties of propellant containing modified boron in primary combustion were better than that of propellant containing boron. Magnesium borides could elevate the energy release of boron-based fuel-rich propellant. Organic boride, Li F-coated boron and Mg-coated boron could promote the energy release in some extent. The burning rates of propellants containing magnesium borides were the highest among all the propellants at 0.5MPa, and the propellant containing Mg-coated boron was the lowest. The propellant containing boron agglomerates and organic boride burned with the lowest expulsion efficiency, whereas the other propellants burned with high expulsion efficiency(above 95%).The decomposition process of the propellants was consisted of the reaction between AP and GFP, the decomposition of AP, HTPB, and the melt of magnesium. The contribution of boron and modified boron to condensed reaction zone in propellants were neglectable. Compared with boron-based propellant, the comprehensive performance of propellants containing Mg B2 and Mg B4 were the best, propellants containing(boron/organic) agglomerates and Li F-coated boron took the second place,The burning rates of the propellants were elevated with decreasing decomposition temperature of AP and shortening combustion time of agglomerates. It was found that there was a linear relationship between the isotonic heat of explosion and the final flame temperature, the same to the weight loss in propellant decomposition and gas evolution property.