Additives Enhancing The Ignition And Combustion Characteristics Of Micron Aluminum Powder

Aluminum(Al)has broad application prospects in solid propellants,underwater propulsion,and Mars exploration due to its advantages such as high energy density,high combustion temperature,and low cost.Micron aluminum(m-Al)powder has been widely studied due to its high content of active Al and easy storage.However,m-Al powder has some problems such as difficulty in ignition and low combustion efficiency in applications.In this paper,to solve these problems of difficult ignition and low combustion efficiency of aluminum,sodium fluoroaluminate(Na₃AlF₆)and potassium fluoride(KF)were selected as additives,and applied to micron-aluminum(m-Al)powders.We conducted a combustion experiment to systemtically investigate the effects of theses additives on the ignition and combustion characteristics of m-Al powder in carbon dioxide(CO₂),water vapor,the mixture gas of carbon dioxide with water vapor,including ignition delay time,combustion phenomenon,and combustion efficiency,etc.The high-speed photographic system was used to record the ignition and combustion process of various samples.At the same time,the components,morphology,and Al combustion efficiencies of these different combustion products were analyzed by an X-ray diffraction analysis(XRD),a scanning electron microscopy-energy dispersive spectrometry(SEM-EDS),and a chemical analysis method.Based on the experimental results,the effect mechanisms of additives on the combustion reaction of Al powder in different atmospheres were further revealed.The main conclusions in present work are as follows:(1)Quality could affect the ignition and combustion of 1-2?m Al(m-Al₁)powder in CO₂.The larger mass sample is easier to ignite.However,the increase in sample mass caused the sample thickness to increase,during the combustion process of m-Al₁ powders,the Al powder inside the sample could not fully contact with CO₂,which would cause the sample combustion efficiency to decrease.The sample with the lowest mass has the longest ignition delay time and the highest combustion efficiency.(2)Na₃AlF₆ could destroy the oxide film on Al surface at high temperature,thereby promoting the reaction between Al and CO₂,so the ignition delay time of m-Al₁ powder and 5?m Al(m-Al₅)powder in CO₂ atmosphere significantly decrease,and combustion efficiency increase.The m-Al₁powder added with 15wt%Na₃AlF₆ has the shortest ignition delay time and the highest combustion efficiency.However,the addition of Na₃AlF₆ affect the heat transfer of the sample,coupled with the reduction in the ignition temperature of the sample,the propagation speed of the combustion area after Al powder ignition decrease with the increase of Na₃AlF₆ content.(3)30?m Al(m-Al₃₀)powder successfully is ignited in CO₂ atmosphere with Na₃AlF₆ addition,but its ignition delay time is longer than m-Al₁ and m-Al₅ powder.The ignition delay time of m-Al₃₀powder added with 15wt%Na₃AlF₆ is the shortest and the combustion efficiency is the highest.The product surface of m-Al powder added with Na₃AlF₆ is covered with flocs or flakes.The product morphology is related to the particle size of aluminum powder and the content of Na₃AlF₆.(4)The ignition delay time of m-Al₅ powder in water vapor is decrease,and its combustion efficiency is increase after Na₃AlF₆ addition.The product morphology and product composition are similar to that of m-Al₅ powder combustion product in CO₂,so it is conclude that the reaction between Na₃AlF₆ and Al powder oxide film is independent of the atmosphere,but water vapor would affect the reaction of Na₃AlF₆ and the oxide film,resulting in unstable combustion of m-Al₅ powder.(5)Increasing the KF content is beneficial to reducing the ignition delay time of m-Al₅ and m-Al₃₀ powders in water vapor.With the increase of the KF content,the combustion efficiency of Al powder is increase.Al powder without KF cannot be ignited in water vapor.The higher the KF content,the shorter the ignition delay time and the higher the combustion efficiency.Flocculant and needle-like structures are observed in the combustion products of m-Al₅ powder,and shells form by cracked oxide films exist in the combustion products of m-Al₃₀ powder.(6)In CO₂ and water vapor mix gas,KF and Na₃AlF₆ promote the combustion of m-Al₅ and m-Al₃₀ powders.These additives reduce the ignition delay time of Al powder and improve its combustion efficiency,but the promotion effect is not as good as in a single gas environment.(7)In water vapor,CO₂/water vapor mix gas,and CO₂ atmospheres,the ignition delay time of m-Al₁ powder gradually increase,and the combustion efficiency gradually decrease.Compared with the other two gases,water vapor is more oxidizing,so Al is more likely to react with it,which has a shorter ignition delay time and higher combustion efficiency.(8)In CO₂ and water vapor,m-Al powder(m-Al₅ and m-Al₃₀ powder)show different oxidation behaviors.In CO₂,the degree of reaction between Al and CO₂ is low,and the oxide film could not rupture during the diffusion oxidation process of Al powder.In water vapor,the m-Al powder undergoe a crystalline transformation during the reaction with water vapor,and eventually cause the oxide film of the m-Al powder to crack.