Preparation And Combustion Performance Of Micro/nano-structured Aluminum-based High Energy Fuel

With the development of technology,the aerospace field has gone through a rapid devlopment.As the power sources of the aerospace field,the selection for solid rocket propellants is important,so the choice of propellants components has become the focus of research.Since the rocket propulsion requires high energy,metal fuels with high heat release have been widely investigated for the advantages of low oxygen consumption,high combustion heat,high density,abudance and low cost.At present,micron aluminum powders are most widely used in solid rocket propellants.However,micron aluminum powders possess thick oxide layers,which makes the combustion rate and combustion efficiency of aluminum powders low,and may even cause twophase flow loss,wake heat radiation and nozzle pollution.Therefore,it is necessary to improve the activity of aluminum powders.In addition,newly produced aluminum nanoparticles have extremely high activity,and spontaneously ignite as soon as they encounter air or water.Therefore,the surfaces of aluminum nanoparticles need to be stabilized for preserving the energy.Finally,in order to solve the problem that the nanoparticles are difficult to uniformly mix with other components,their particles size,morphology and hydrophobicity are also required to be improved.Firstly,the ball milling technique was employed for the study of nanosizing the micron aluminum powders.Through modifying the types and amounts of the ballmilling aid,the ball-milling time and the balls to materials ratio to determine the preparation process of highly active aluminum nanoparticles.The core-shell structure nanoparticles with spherical shape,high specific surface area and fast burning were also obtained through stabilization.The results show that the activities of products are directly affected by ball-milling time.When the ball-milling time is 14 h,the prepared aluminum nanoparticles are nearly spherical with particles size of approximately 30 nm and specific surface area of more than 30 m2/g.In addition,the aluminum nanoparticles ball milled for 14 h are much easier to burn in comparison with micron aluminum powders,which could be ignited at 12 V and the temperature can reach 1000 °C within 2 s.Nevertheless,the micron aluminum powders show no obvious ignition signs at 12 V,20 V and even 25 V.Additionly,A171 was also employed to stabilize the aluminum nanoparticles and the corresponding thermal stability and hydrophobicity of the stabilized products were greatly improved.To further improve the combustion properties of ball-milled aluminum nanoparticles,doping oxidative substances into aluminum nanoparticles was used to modify the properties.High activity Al/Fe₂O₃ nanothermites were fabricated by insitu ball-milling technique and conventional ultrasonic blending method,and the structure and properties of the two groups of products are compared.The results show that the in-situ ball-milled Al/Fe₂O₃ nanothermites are mixed evenly,nevertheless,the ultrasonic-blended products present poor uniformity and even exist selfagglomeration phenomenon.In addition,the contact angle of the nanothermites prepared and stabilized by in-situ ball-milling technique is 113.01°,which is significantly larger than that of the ultrasonic-blended products（96.54°）,indicating that the nanothermites prepared by in-situ ball-milling method is more conducive for preservation than that prepared by conventional ultrasonic blending method.The optimum doping amount of Fe₂O₃ of nanothermites prepared by in-situ ball-milling method is 17 wt% and the oxidation rate of in-situ ball-milled nanothermites is faster than that of ultrasonic-blended nanothermites.The initial combustion temperature of in-situ ball-milled nanothermites is much lower than that of ultrasonic-blended nanothermites and the heating voltage of in-situ ball-milled nanothermites is 12 V,which is also lower than that of conventional ultrasonic-blended nanothermites（15 V）.Additionly,the flame of in-situ ball-milled nanothermites is more stable and homogeneous than that of ultrasonic-blended nanothermites.On the basis of Fe₂O₃ addition in aluminum,the nanothermites doped with FeF₃ was also studied.Highly active Al/FeF₃ nanothermites were prepared by in-situ ballmilling technique and the properties of the products were fully characterized,and the preparation process was determined by changing the adding amounts of FeF₃.The results show that the products doped with 17 wt% FeF₃ is optimal.The products are spheroidal,with an average diameter of 23.2 nm and a specific surface area of 26.33 m2/g.In addition,compared with the undoped aluminum nanoparticles and Al/Fe₂O₃ nanothermites,the hydrophobicity of the Al/FeF₃ nanothermites change little,but the initial oxidation temperature is lower and the burning rate is faster.It shows that doping oxidized elements can improve the combustion performance of aluminum nanoparticles,and the stronger the oxidizability of doped elements,the more obvious the effect of improving the combustion performance of aluminum nanoparticles.The micro/nano-structured particles were studied using a self-made solvent evaporation technique.The optimal products are spherical waxberry-like with particles size of approximately 10 μm.The prepared products not only retain the excellent properties of corresponding nanothermites in terms of thermal stability and combustion,but also possess reduced hydrophobicity by 54.7%（from 115.12°to 52.20°）as compared to the corresponding nanothermites,which will greatly increase the fluidity and wettability of the particles,thereby improving the overall performance of the propellants component.