Preparation Of Fluoropolymer Coated Nano-Aluminum Powders And Its Application In Solid Propellants

So far aluminum has received widespread attention as a metal component in solid rocket fuel.However,during the combustion process of the propellant,aluminum and its oxides easily aggregate to form larger aggregates.The aluminum particles burn slowly after leaving the combustion surface,causing incomplete combustion of the aluminum particles,and the specific impulse of the propellant decreases by 2%to 10%.The introduction of nano-aluminum powder into the composite propellant system can reduce the loss of two-phase flow and improve the combustion efficiency of the propellant;the gasification of Al F₃ generated by the reaction of fluoropolymer decomposition products with aluminum at high temperatures can reduce the residence time of aluminum powder on the combustion surface Formation of a condensed phase product with a smaller particle size;in addition,fluoropolymer-coated nano-aluminum powder can improve the safety of propellant preparation and use.To this end,two nano-aluminum/fluoropolymer composite particles were prepared by in-situ solution polymerization and solvent volatilization.The surface morphology,dispersion,and stability of composite particles with different ratios were studied.The energy release characteristics were systematically studied,the aluminum-fluorine reaction mechanism was discussed,and the combustion properties and particle size distribution of the combustion products were analyzed.The details are as follows:First,two kinds of nano-aluminum/fluoropolymer composite particles were prepared by in-situ solution polymerization method and solvent evaporation method.X-ray powder diffraction(XRD)and Fourier infrared spectroscopy(FTIR)were used to analyze the composition and functional groups of the nano-aluminum powders before and after preparation.Scanning electron microscope-energy dispersive X-ray spectrometer(SEM-EDS)and transmission electron microscope(TEM)were used to study the surface morphology and dispersion of the composite particles.At the same time,the stability of the composite particles was characterized.The results show that the surface of the aluminum/fluorinated acrylate(Al/PFDMA)composite particles is continuously and uniformly covered with a polymer shell layer of 4 to 5nm,and the proportion of the F element in the surface elements reaches 10.4wt%.The particles are bonded to each other through the fluoropolymer to form micron-scale clusters.The aluminum/polyvinylidene fluoride(Al/PVDF)composite particles are also protected by a4 to 5nm PVDF layer.The F element accounts for 13.8wt%of the surface atoms.Thanks to the vacuum freeze-drying technology,the particles maintain good dispersibility.In addition,the two coating layers can significantly improve the stability of nano-aluminum in neutral,strong alkaline or strong acidic environments.The fluoropolymer shell can also greatly improve the hydrophobicity of nano-aluminum,and no change was found under the conditions of storage at25?for 5 days and storage at 50?for 1 hour.Furthermore,the thermal decomposition performance of composite particles was tested using simultaneous thermal analysis(TG/DSC),the burning rate of the samples was also characterized,and the products were characterized by XRD,and the aluminum-fluorine reaction mechanism in the open system was discussed.The results show that the maximum heat of reaction of the two composite particles of Al/PVDF and Al/PFDMA are 11908J/g and8203J/g,respectively,which are 68%and 15wt%higher than the aluminum nanomaterial.Except that the raw material nano-aluminum powder and the sample with 10%by weight of fluoropolymer cannot be ignited,the other samples can sustain self-sustaining combustion.Within a certain range,with the increase of the fluoropolymer content,the burning rate of the composite particles of Al/PVDF and Al/PFDMA gradually increased,and the maximum burning rates reached 86.2 mg/s and 26.6 mg/s,respectively.Finally,composite particles and other propellant components were made into pellets,and the effect of nano-aluminum/fluoropolymer composites on the detonation and burning rate of composite propellants and the effect on the particle size of combustion products were studied.The results show that the composite propellant samples containing Al/PFDMA and Al/PVDF have 97.7%and 73.7%higher combustion rates and 6123k J/kg and 6108k J/kg,respectively,compared to conventional micron aluminum composite propellants.Higher heating value levels.In addition,the combustion product agglomeration of both propellants was significantly reduced,and the particle size of the propellant combustion products containing Al/PVDF composite particles was even smaller than the initial nano-aluminum particle size.