Preparation And Properties Of Aluminum Powder/Fluoride

Nano aluminum powder is a commonly used metal combustion agent in solid propellants,which can significantly improve the specific impulse.However,the number of atoms on the surface of nano aluminum powder is large,and the inert component on the surface increases with the decrease of particle size,with the inert component accounting for nearly 40%.It directly affects the energy release of nano aluminum powder,and the surface of nano aluminum powder is extremely easy to agglomerate.These all lead to incomplete combustion of nano aluminum powder.The addition of fluoropolymers can improve the properties of nano aluminum powder.Fluoropolymer can react with aluminum powder to form Al F₃,when the temperature reaches 1277℃,it can sublimate.The system is pressurized internally,and particles are broken,which can reduce aluminum powder agglomeration.Fluoropolymer coated nano aluminum powder can improve the safety of propellant preparation and use.This topic uses tetrafluoroethylene,hexafluoropropylene,and vinylidene fluoride copolymer（THV）to modify nano aluminum powder.THV/nano aluminum powder composite particles were prepared by solvent evaporation method.In order to compare the effect of fluoropolymers on the addition of nano aluminum powder,nitrocellulose/nano aluminum powder and F₂₆₀₄/nano aluminum powder composite particles were prepared by solvent evaporation method.The reaction mechanism of fluoropolymer/nano aluminum powder composite particles was discussed.The specific content is as follows:Firstly,the coating characteristics of THV,nitrocellulose,and F₂₆₀₄coating materials were studied using molecular dynamics methods.Amorphous cell modules for three types of coatings were established using the Material Studio software Amorphous cell module.The binding energy of alumina and these three amorphous crystal cells was calculated using the Forcite module.The final calculation shows that the binding energy of THV/nm Al composite particles is 118.62 kcal/mol,that of nitrocellulose/aluminum powder composite particles is27.82 kcal/mol,and that of F₂₆₀₄/aluminum powder composite particles is 70.45 kcal/mol.Molecular dynamics simulation results showed that the three amorphous crystal cells all adsorbed on alumina,with THV/nm Al composite particles having the highest adsorption energy.Secondly,THV/nm Al,nitrocotton/nm Al,and F₂₆₀₄/nm Al composite particles were prepared by solvent evaporation method to compare the effect of fluoropolymers on improving nano aluminum powder.The functional groups and components of nano aluminum powder before and after preparation were analyzed by Fourier transform infrared spectroscopy（FTIR）and X-ray diffraction（XRD）.Transmission electron microscopy（TEM）and scanning electron microscopy（SEM）were used to observe the surface morphology of nano aluminum powder.The results showed that the three kinds of coating materials were successfully coated on the surface of nano aluminum powder,and a coating layer was formed on the surface of aluminum powder.After coating,nano aluminum powder also had good dispersion.XRD and FTIR results showed that new functional groups and characteristic peaks appeared on the surface of THV/nm A composite particles.New functional group characteristic peaks appeared on the FTIR spectra of nitrocellulose and F₂₆₀₄.The two substances exist on the surface of nano aluminum powder in an amorphous form.Finally,the thermal decomposition properties of three composite particles were tested using synchronous thermal analysis（DSC-TG）,and the combustion heat data of the composite particles were measured using an oxygen bomb calorimeter.The reaction mechanism of aluminum powder fluoropolymer was discussed based on the results of DSC-TG.The results showed that the combustion heat of THV/nm Al composite particles was24.612 k J/g,the combustion heat of nitrocellulose/nm Al composite particles was 23.579 k J/g,and the combustion heat of F₂₆₀₄/nm Al composite particles was 22.125 k J/g when the coating ratio of the three coatings was 10%.20%THV coating ratio has the maximum combustion heat of 25.557 k J/g.Among the three coating materials,THV has the greatest effect on improving the combustion heat of nano aluminum powder.After coating,the combustion heat value of composite particles shows a trend of first increasing and then decreasing.When the coating ratio is 20%,the composite particles have the highest combustion heat value.The DSC-TG results show that pure nano aluminum powder will melt and absorb heat near 660℃,and the addition of fluoropolymer will advance the reaction of nano aluminum powder composite particles to about 550℃.Aluminum oxide can undergo a pre ignition reaction（PIR）with fluoropolymer,and aluminum oxide can no longer act as an inert medium and react with fluoropolymer to generate HF.The fluoropolymer also decomposes itself to generate intermediate products and HF,which will promote the reaction of aluminum powder to generate AlF₃.