Synthesis And Properties Of Azaboroles In Hypergolic Liquid Fuels

Azole-based compounds have been widely used as the parent structure in energetic materials due to their high densities and positive heats of formation(HOF).{B-H}-based compounds commonly own strong reducing ability showing the great application potential in bispropellant.To take advantage of the high energy capacity from azole ring and reducing ability from B-H bond,we have developed a series of novel hypergolic azaborole liquid propellant including azolylborate-based ionic liquids,azolylboronium-based ionic liquids(ILs)and azaborole complexes,which often exhibited high energy capacity,short ignition delay times,wide liquid range and remarkable low viscosity.Firstly,ten synthesized bishydrobis(tetrazol-1-yl)borate(BTB)based ionic liquids exhibited wide liquids range(T_m/T_goften<-70?,T_d>200?),show very high density(?1.20 g cm^-3)and HOF(?2.000 k J g^-1);With the white fuming nitric acid(WFNA)as an oxidant,they usually possess short ignition delay times(IDs<20 ms)and high density impulse(often?367 s g cm^-3).Secondly,eight synthesized cyanotetrazolylborohydride(CTB)anion-based ionic liquids exhibited wide liquids range(T_m/T_goften<-70?,T_d?200?),remarkable low viscosity(<20 m Pa·s)and show high density(often?1.10 g cm^-3)and HOF(up to 1.852 k J g^-1);With WFNA as an oxidant,they usually possess excellent ignition delay times(<5ms),moderate specific impulse(261-263 s)and good density impulse(354-366s·g·cm^-3).This work opens a new way to design new ultrafast-igniting hypergolic ionic liquids(HILs)with limited B-H bond and high energy capacity.Thirdly,six synthesized[imidazolyl-amine-BH₂]⁺-cation-based ionic liquids exhibited wide liquids range(T_m/T_goften<-78?,T_d>120?);With WFNA as an oxidant,they usually possess moderate specific impulse(263 s-269 s)and good density impulse(up to364 s·g·cm^-3).Among them,two BH₃CN^?-based ionic liquids had the shorter IDs(14 ms,15 ms,respectively)than them of DCA-based ionic liquids.Finally,six synthesized N-alkylimidazole-cyanoborane comlexes and two imidazolylidene cyanoborane complexes were prepared.Six synthesized N-alkylimidazole-cyanoborane adducts exhibited wide liquids range(T_m/T_goften<-60?,T_d>200?),and show high density(?1.00 g cm^-3)and HOF(up to 1.548 k J g^-1);With WFNA as an oxidant,they usually possess excellent ignition delay times(two of them<5ms),moderate specific impulse(261 s-264 s)and good density impulse(up to 358s·g·cm^-3).Compared with other ultrafast-igniting and hydrolytic-stable hypergolic ionic liquids,they exhibited higher energy capacity,and their synthetic method is more suitable for large-scale production(no toxic heavy metal);Compared the previous mostly effective ionic liquid fuel additive(triethylamine-borane,1-66),N-allylimidazole-cyanoborane complex(5-5)owned the comparative efficiency to decrease the IDs of fuels,but its mixture got higher energy capacity;Hydrophobic 3-propyl-1-methylimidazolylidene cyanoborane complexes(5-8)showed good liquids range(T_g=-22?),short ID time(13ms with WFNA);Compare with 1-66,5-8 possessed higher higher density,HOF,and density impulse;Compared with hypergolic fuel addictive such as NH₃BH₃,NH₂NH₂BH₃and H₃BNH₂NH₂BH₃,5-8 have higher effective.Overall,molecules bearing BH₂CN moiety have the potential to form hydrophobic,hydrolytic-stable hypergolic fuels,and also have advantage in density,HOF,IDs compared with BH₃-based borane complexes.