Combustion Characteristics Of Al-Mg Alloy Hydride And Its Application In Thermite

Aluminum powder has a high volumetric combustion enthalpy and energy density,thus can be widely used as a metal combustion agent for propeller.The combustion performance of aluminum powder can be improved by preparation of binary alloy.Hydrogen is a new green energy carrier.The introduce of hydrogen into Al-based alloy combustion agent via hydride can promote the combustion of oxidant and the release of heat,which is helpful to enhance the ignition and combustion performance.In this dissertation,hydrogen was introduced into Al₂Mg alloy,and the combustion performance and mechanism of Al₂Mg alloy hydride and its ball-milled products were studied by means of thermal analysis?TG-DSC?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and energy dispersive spectrum analysis?EDS?.Moreover,the Al₂Mg alloy hydride was applied to prepare the thermite by mixing with oxidant to improve the combustion performance.After hydrogenation of Al₂Mg alloy,the particles was refined and porous.In the first combustion stage of the Al₂Mg alloy hydride,there was a high-temperature dehydrogenation process,which generated fresh Mg phase and a large amount of crystal defects.The existence of Mg steam also promoted the decomposition of oxygen molecules.As a result,the ignition point was lowered as much as about 150°C in comparison with the Al₂Mg alloy?not hydrogenated?.Because the Al₂Mg alloy hydride had a smaller grain size and more crystal defect after the first-stage combustion,its every sub-step of combustion was easier to occur in the second combustion stage as compared to the Al₂Mg alloy.Moreover,due to the difference in the phase distribution and defect degree after the first-stage combustion,combustion and heat release of the alloy hydride was more concentrate and violent in the second combustion stage,and it was more difficult to generate AlN phase,which was beneficial to the practical application.When the Al₂Mg Alloy hydride was subjected to ball milling,the particle size became small and smooth and the degree of them was proportional to the milling time.The ignition points of 2 h,5 h and 10 h ball-milled products?denoted as Al₂Mg-H?bm2?,Al₂Mg-H?bm5?and Al₂Mg-H?bm10?,respectively?were all decreased about 50°C with respect to the sample without ball milling,and the Al₂Mg-H?bm5?sample had the best reaction activity.With the increase of ball milling time,the specific surface werea of particles and the crystal defects introduced were increased.The Al₂Mg-H?bm10?sample had the lowest onset and final oxidation temperature for the second-stage oxidation reaction.However the lower MgO coverage degree on the surface made it easier to be agglomerated,which was harmful to the practical application.The Al₂Mg alloy hydride was mixed with ultrafine KMnO₄.It was found that the Al₂Mg/KMnO₄ and Al₂Mg-H?bm10?/KMnO₄ thermites had a better combustion performance than that of the Al₂Mg-H/KMnO₄ and Al₂Mg-H?bm2?/KMnO₄ thermites from the point of view of combustion phenomenon.The combustion performance was Al₂Mg-H?bm10?/KMnO₄>Al₂Mg-H/KMnO₄>Al₂Mg/KMnO₄ from the reaction activity.And the combustion performance was Al₂Mg-H?bm10?/KMnO₄>Al₂Mg/KMnO₄>Al₂Mg-H/KMnO₄ from the degree of solid-solid reaction and heat release.The particle size and surface morphology had a great influence on the mixing uniformity of metal and oxidant as well as the contact area,thus having affects on the ignition point,reaction degree,heat release and reaction rate.Based on the above analysis,Al₂Mg-H?bm10?/KMnO₄ thermite exhibits the best comprehensive combustion performance.The combustion process of the hydrogen-containing thermite was different from that without hydrogen.Firstly,MgH₂ and Al reacted with K₂Mn₄O₈?decomposition products of KMnO₄?to generate Al?OH?₃ and MgO.Then,K₂Mn₄O₈,MgO and Al?OH?₃ reacted to form MgAl₂O₄ and K_xMnO₂.Finally,K_xMnO₂ and Al reacted to form Mn.