Pyrolysis And Combustion Characteristics Of Aluminum Hydride Coated By Stearic Acid

Aluminum hydride(AlH3)is considered as the most promising high-energy additive in the field of high-energy propellants due to its high combustion heat value,strong reducibility,high hydrogen content,and the ability to significantly improve the specific impulse of propellants.However,its application in propellants is severely limited due to its relatively high decomposition and hydrogen release.Therefore,to further improve the application performance of AlH3 and provide data support for its safe application.This paper used stearic acid to coat and modify AlH3,and the pyrolysis and combustion characteristics of AlH3 and AlH3/C18H36O2 through experiments and ReaxFF molecular dynamics.The changes of molecular motion tracks,chemical bonds and products were analyzed,and the combustion and pyrolysis paths were summarized.The reaction mechanism not involved in the experiments were supplemented and explained from the micro level.At the same time,the experiments have verified the molecular dynamics calculation results to a certain extent,which is of great significance for the promotion and safe application of AlH3/C18H36O2.The main conclusions of this paper are as follows:(1)The thermal decomposition and combustion characteristics of AlH3 and AlH3/C18H36O2 dust were studied at macroscopic level through dust ignition,minimum ignition temperature and TG test.The results indicated that AlH3/C18H36O2 dust had higher initial decomposition temperature,decomposition peak temperature,decomposition completion temperature and activation energy than AlH3.Therefore,the thermal stability of AlH3 dust after coating was improved.Both kinds of dust combustion flame can be divided into flame growth,stable combustion,and flame decay stages,and both kinds of flame showed bright white flame.Nevertheless,the time for the flame propagation of AlH3/C18H36O2 dust to reach the maximum was longer than that of AlH3.At the same time,the combustion duration of AlH3/C18H36O2 dust was longer than that of AlH3 dust at the same concentration.The minimum ignition temperature of AlH3 was 335℃,while the minimum ignition temperature of AlH3/AlH3/C18H36O2 dust was 340℃.(2)The pyrolysis characteristics of AlH3,AlH3/C18H36O2,and AlH3@Al2O3/C18H36O2 were contrastively studied.The time that the three systems tending to equilibrium was AlH3<AlH3/C18H36O2<AlH3@Al2O3/C18H36O2.During the pyrolysis,H-H and C-Al bonds showed an increased trend,while H-Al and C-H bonds emerged a decreased trend.C-C bond showed a decreased trend at 2000 K,while at 2500-3500 K,it firstly reduced and then augmented.The O-Al bond firstly augmented and then reduced,while the H-O bond was reversed.The main product of AlH3 pyrolysis was H2,while AlH3/C18H36O2 and AlH3@Al2O3/C18H36O2 pyrolysis would also produce a large amount of H2O.The pyrolysis path of AlH3 was mainly the fracture of Al-H bonds and produced H,followed by the combination of ·H and ·H,and produced H2.The pyrolysis paths of AlH3/C18H36O2 and AlH3@Al2O3/C18H36O2 mainly included the pyrolysis of C18H36O2,the thermal interpretation of hydrogen by AlH3 and the formation of clusters dominated by C-Al bonds.(3)The combustion characteristics of AlH3,AlH3/C18H36O2 and AlH3@Al2O3/C18H36O2 were comparatively studied.The potential energy of the three systems increased sharply at first and then decreased gradually.During the combustion process,the H-Al,O-O and C-H bonds diminished,the H-H bonds firstly augmented and then reduced,while the C-Al,C-O and H-O bonds continuously increased.The C-C bonds descended at 2000 and 2500 K,while at 3000 and 3500 K,it firstly decreased and then increased.The main product of the three systems combustion was H2O followed by H2.The maximum molecular weight of the clusters produced was AlH3<AlH3/C18H36O2<AlH3@Al2O3/C18H36O2.The combustion paths of AlH3 included the fracture of H-Al bonds,the combination of ·H and H,the combination of H2 and O2,and the oxidization of Al.The combustion paths of AlH3/C18H36O2 also included the above steps.In addition,it also mainly included the the decomposition of C18H36O2 to produce carboncontaining small molecular fragments under the induction of O2 and the carbon-containing small molecular fragments combined with O-Al clusters,and then C displaced O of OAlclusters,finally formed clusters dominated by C-Al bonds.The difference of the combustion path between AlH3@Al2O3/C18H36O2 and AlH3/C18H36O2 was the fracture of O-Al bond in the oxide layer.