Effects Of Metal Hydrides On The Safety Of Conventional Explosive Compounds

The use of metal hydrides in energetic materials has been the subject of intense research.Hence,the work to find the safety of energetic materials with metal hydrides is also becoming important.In this thesis,two potential metal hydrides?MgH₂ and Mg?BH₄?₂?were chosen to be the additives of three common explosive compounds.trinitrotoluene?TNT?,cyclotrimethylenetrinitramine?RDX?and ammonium nitrate?AN?.The stabilities of these mixtures were studied theoretically and experimentally.In theory,a mathematical model method was used to calculate the theoretical explosion heats of above-motioned mixtures of metal hydrides and explosive compounds.The conclusion is that both MgH₂ and Mg?BH₄?₂ increase the explosion heat of TNT,RDX and AN.Among these mixtures,RDX/Mg?BH₄?₂ has the highest theoretical explosion heat which is 9409.1 kJ·kg-1 when the mass fraction of Mg?BH₄?₂ is 26.7%.This result meets people's expectation.However,the higher explosion heat means the higher potential hazard.Moreover,conventional RDX/Al explosive still has the higher explosion heat than RDX/MgH₂ or RDX/Mg?BH₄?₂ with the same calculation.Except the explosion heat estimation,density functional theory?DFT?of quantum chemistry was used to simulate the absorption and decomposition processes of TNT,RDX,and AN on MgH₂?110?surface in order to give micro-explanations for the mechanisms of interaction between explosive molecules and the MgH₂ surface.Simulation results show that TNT molecule is not decomposed on the MgH₂?110?surface.RDX molecule is decomposed to an O atom and AN molecule is decomposed to NH3,NO,OH and O free radicals.During the absorption processes,the MgH₂?110?surface gives away a large number of electrons to the explosive molecules.The structure of MgH₂?110?surface changes significantly with a great number of free H atoms generated especially in the systems with RDX and AN.Therefore,it is predicted that the three kinds of explosive molecules could induce the releasing of hydrogen and against the storage of MgH₂.Experimentally,the mechanical sensitivities of metal hydrides and explosives mixtures were studied.The results show that MgH₂ has obvious effects on the mechanical sensitivities of TNT,RDX and AN.Mixture of pure RDX and MgH₂ is extremely dangerous.Even TNT and AN,which have low mechanic sensitivities themselves,after the MgH₂ is added in,both TNT and AN have higher impact and fraction sensitivities.Thus,these mixture explosives with MgH₂ cannot far meet the safety requirement.Mixture RDX/Mg?BH₄?₂ still has really high mechanical sensitivities.However,the Mg?BH₄?₂ has less impact than MgH₂ on the mechanical sensitivities of TNT and AN.The free hydrogen exists in MgH₂ lattice structure is thought to be the major cause.With the kinetic energy converting to het energy,free hydrogen creates more small bubbles and more hot spots.Differential Scanning Calorimetry?DSC?,Accelerating Rate Calorimeter?ARC?and Vacuum Stability Test?VST?are thermal analyses with different sample sizes and testing principles which were used in this thesis in order to study the mixtures of metal hydrides and explosives.The following conclusions were obtained:both of the two metal hydrides decrease the decomposition rate of TNT due to the inhibition of free H atoms to-CH3 oxidation.However,base on the heat releases,TD24,E and so on,the dangerous order is:TNT/MgH₂>TNT/Mg?BH₄?₂>TNT.The addition of MgH₂ has a trend to accelerate the second decomposition stage of RDX whereas Mg?BH₄?₂ does not have this effect.The additions of both MgH₂ and Mg?BH₄?₂ make the decomposition of RDX easier and MgH₂ has more influence than Mg?BH₄?₂.However,once the reaction runs away,mixture with Mg?BH₄?₂ may cause a more severe hazard.The mixture of MgH₂ and AN has one more exothermic peak than pure AN.A solid phase reaction is thought to be happened which means the mixture is extremely dangerous.The TD24 of AN/Mg?BH₄?₂ is 700? higher than AN and the reaction heat is lower,too.Base on that,Mg?BH₄?₂ has no negative effect on the thermal safety of AN.