Preparation And Performance Modulation Of PVDF-modified Micron Aluminum High-activity Composite Powders

With the rapid development of science and technology in modern society,higher and higher performance of energetic materials are required.The new generation of aerospace weapons demands energetic materials with higher reactivity,reaction speed,reaction heat,reaction rate.Micron aluminum（Al）powders with extensive sources,high active Al content and high energy density is widely added to propellants,explosives and pyrotechnics as a metal fuel to contribute energy to the system.However,micron Al powders also present significant drawbacks in the field of energetic materials.The large specific surface area and high activity make Al powders easy to react with water vapor and oxygen during production and storage,resulting in a lower active Al content.The surface aluminum oxide（Al₂O₃）does not participate in exothermic reactions between metals and metal oxides;instead,it reduces the energy density and combustion performance.Therefore,the presence of oxide shell easily reduces the reaction efficiency,contributing to a heat of reaction lower than the theoretical value.Polyvinylidene fluoride（PVDF）is a low-cost and common fluoropolymer with high mechanical strength,thermal stability and strong oxidation properties.In recent years,PVDF has been used as an oxidizer and modifier to improve the performance of Al powders.The presence of PVDF layer protects the Al powders from oxidation at room temperature,the fluorine-containing groups can react exothermically with the Al₂O₃shell at high temperature to form aluminum fluoride（Al F₃）.The fluorination reaction transforms the passivated shell into an energetic component.The fluorine-containing groups have been shown to excite a surface pre-ignition reaction（PIR）with the Al₂O₃shell,promoting the formation of low-boiling Al F₃species.the reactivity of the internal Al is increased during Al F₃sublimation（1277℃）and a vigorous oxidation reaction occurs,accompanied by a higher energy release rate,heat of reaction,and superior combustion performance.In this paper,PVDF,a fluoropolymer,was used as the coating material to prepare PVDF-coated aluminum composite powder by solvent-nonsolvent method on the surface of micron aluminum powder.N,N-dimethylformamide（DMF）was selected as the solvent of PVDF,and water and ethanol solution were used as the non-solvent of PVDF.The effect of process parameters on the coating effect was investigated.The process parameters included non-solvent,solvent,PVDF solution dropping method and washing method,and interface regulation.From the experimental results,it is concluded that PVDF coated aluminum composite powder with core-shell structure can be prepared when the volume ratio of water and ethanol is 1:1,the process of PVDF solution drip addition by peristaltic pump with fixed rate,filtration of composite powder samples by pumping,ultrasonic homogenizer and pre-treatment of aluminum powder by HF solution.Next,the PVDF-coated A composite powders with a core-shell structure were analyzed for structural characterization and thermal properties.The TG-DSC results indicated that the weight and energy release of the Al@PVDF powders prepared by pretreatment of Al powders by using ultrasonic homogenizer were the highest at1300℃（166%and 11765 J/g）.a-Al@PVDF powders prepared by pretreatment of Al powders with HF solution also showed a uniformly coated PVDF layer with a weight and energy release of 150.17%and 10962 J/g at 1300℃.The Al@PVDF powders and a-Al@PVDF powders have higher mass and heat release compared with the raw Al powders（140.06%,6560 J/g）,which sufficiently reveals that the addition of PVDF can significantly improve the reactivity and heat release of the Al.Among the powders,the oxidation reaction of a-Al@PVDF powders prepared after removing the original oxide layer on the surface of Al powders was more complete.Base on test results,the oxidation mechanisms of Al@PVDF powders,a-Al@PVDF powders and Al powders were proposed.Finally,by comparing the thermal performance of Al@PVDF powders with core-shell structure（166.10%,11976 J/g）and Al/PVDF powders with inhomogeneously coating（148.78%,9452 J/g）,the results demonstrated the superiority of core-shell structure.The effect of PVDF content（8%,10%,12%,14%,16%,18%,20%）on the microstructure,thermal performance and pressure output capability of Al@PVDF powders was further investigated to achieve the optimization of performance.From TG-DSC results,it was obvious that both weight and energy release at 1300℃had a clear trend with the content of PVDF.As the content of PVDF increased,the mass and heat release of the composite powder had a decreasing trend.When the PVDF content is 8%,the values of both reach the maximum,165.90%and 11765 J/g,respectively.From the constant volume combustion test,with the increase of PVDF content,the maximum pressure(P_max)increases and then decreases.When the PVDF content is 10%,P_maxreaches maximum.The combustion time(t_max)decreases and then increases,and t_maxreaches the minimum when the PVDF content is 18%.The highest pressurization rate of the composite powder（23.98 MPa/s）was obtained when the added PVDF content was 12%,and the lowest pressurization rate（11.53 MPa/s）was obtained when the PVDF content was 8%.