Thermal Kinetics And Application In Solid Propellant Of Energetic Azolium Nitrates

As a novel kind of high energy nitrogen-rich compound, energetic azolium ion saltpossesses excellent physical and chemical properties, so it has a broad application prospectin the field of propellants and explosives, expecially in the field of solid propellant.Energetic azolium ion salt usually has very high possitive enthalpy of formation, because ithas a lot of N-N and C-N bonds besides stronger ring tension. Also the oxygen balance anddensity of energetic azolium ion salt are very high because of high nitrogen content and lowcontent of carbon and hydrogen. The main combustion gaseous product of energeticazolium ion salt (except azolium perchlorate) is N2, which does no harm to environment.Comparing with molecular compounds, energetic azolium ion salt has many advantages,such as high density, low vapor pressure, strong polarity, good solubility, excellentthermostability, adjustable property and so on, therefore, it attracts widespread attention bythe researchers both at home and abroad.For now, the research on energetic azolium ion salt is still based in the laboratory andfocused on such3aspects as molecule design, synthesis and structural characterization ofnew salt. However, there are no related reports about applied research targeting on energeticazolium ion salt, especially its application in solid propellant, at home and abroad.This paper took3kinds of energetic azolium nitrates as research subject including1-amino-1,2,3-triazolium nitrate (1-ATZN),4-amino-1,2,4-triazolium nitrate (4-ATZN) and5-amino-tetrazolium nitrate (5-ATZN). Thermal decomposition kinetics, thermaldecomposition mechanism and heat-resistance stability of energetic azolium nitrates werestudied by means of many kinds of thermal analysis techniques including TG, DSC,TG-DTG and TG-FTIR, which could provide theretical foundation for the application ofenergetic azolium nitrates. And on this basis, energetic azolium nitrates were applied intosolid propellant. Energy characteristic, combustion performance and catalytic combustionof azolium nitrate monopropellant and composite propellant were also researched.According to standard methods provided by GJB, mechanical sensitivity,hygroscopicity and compatibility of3kinds of energetic azolium nitrates were tested. Theresults show that impact sensitivity of energetic azolium nitrates is at about the same levelas that of RDX, but friction sensitivity is much lower than that of HMX and RDX.5-ATZNis basically non-hygroscopic and hygroscopicity of1-ATZN is about equal to that of AN. Hygroscopicity of4-ATZN, which is easily affected by ambient temperature, is evenstronger than that of ADN in a short time.3kinds of energetic azolium nitrates haveexcellent comptibility with common binders, oxidizers, high energy additives and estersplasticizers, but are incompatible with ethers plasticizers and isocyanate curing agents.The thermal decomposition kinetics of3kinds of energetic azolium nitrates wasinvestigated by means of TG-DTG and DSC at various heating rates (2,5,10,15and20℃min-1). And a systematic method processing thermal analysis data was given in order toobtain thermal decomposition kinetic triplet (apparent activation energy, pre-exponentialfactor and the most probable mechanism function). Even more important, the reasonabilitycondition for single heating rate method was proposed, which was also verified byiso-conversional method. The results show that the thermal decomposition process of1-ATZN consists of two decomposition stages. The most probable mechanism function forthe first stage can be described as f a231a ln1a12and reaction model israndom-into-nucleation and random-growth mechanism (n=3/2). But in the second stagethe vaules of apparent activation energy (E) change greatly with conversion (a), whichindicates that it is difficult to describe the second decomposition stage accurately only usingone mechanism function. So there is on the most probable mechanism function for thesecond stage.Combining thermal decomposition kinetic parameters of energetic azolium nitrate withother kinds of nitrogen-rich compounds, empirical formula of the kinetic compensationeffect for energetic nitrogen-rich compound was derived and its expression waslgA=0.1201E-2.8234. A new evaluated method of combining the evaluated way for thefundamental stability test with the evaluated way for the self-accelerating decompositiontemperature was proposed to evaluate the thermostability of energetic azolium nitrates,which overcomed shortcomings of original evaluated way for the self-acceleratingdecomposition temperature, such as long testing period, poor safety, high cost and so on.The results show that the heat-resistance stability of energetic azolium nitrates has closerelationship with the structural symmetry of nitrogen heterocycle. The better the structuralsymmetry of nitrogen heterocycle is, the better the heat-resistance stability of energeticazolium nitrates is. Amino substituent group shows contrary effect on the heat-resistancestability of1,2,4-triazolium nitrate and1,2,3-triazolium nitrate.The thermal decomposition mechanisms of3kinds of energetic azolium nitrates wereresearched by thermogravimetry-fourier transform-infrared spectroscopy (TG-FTIR). The results show that the main gaseous products from the thermal decomposition of1-ATZNand4-ATZN are the same (N2O, CO2and HCN), but their decomposition processes arequite different, which is caused by the positions of3nitrogen atoms on the nitrogenheterocycle. The tetrazole ring-opening reaction of5-ATZN has two differentdecomposition pathways. The gaseous products from two decomposition pathways are HN3and HCN, respectively; and the reaction producing HN3is the main decomposition pathway.Apparent activation energy for the decomposition reaction of4-ATZN was obtianed byKissinger method, which was applied to deal with IR data from TG-FTIR spectroscopy.This successful attempt could offer a new way for research on thermal decompositionkinetics by means of IR technique.The energy characteristic parameters of solid propellant containing energetic azoliumnitrates were calculated by using the least free energy method. The results show that thespecific impulses of1-ATEZN and5-ATZN monopropellants are2413.96Ns·kg-1and2371.38Ns·kg-1, respectively, which are both at about the same energy level as HMX andRDX monopropellants. The specific impulse of GAP composite propellant is characterizedby a parabola with substituting energetic azolium nitrates for ingredient AP, but the averagemolecular weight of geaeous products keeps becoming smaller. Therefore, it seems to behelpful to improve energy level of GAP propellant by replacing AP with energetic azoliumnitrates properly. Meanwhile, energetic azolium nitrates would also reduce thecharacteristic signal of GAP propellant as they don’t contain chlorine.The catalytic effect of many kinds of catalysts on the thermal decomposition ofenergetic azolium nitrates were investigated by means of TG. The resluts show that theselectivity on catalyst of energetic azolium nitrates is very high. Even the same catalystmay have quite different catalytic effect on the thermal decomposition of different azoliumnitrates. Copper acetate, nano-sized iron oxide, carbon black and lead salicylate can play apossitive role in the thermal decomposition of5-ATZN. And the catalysts that canaccelerate the thermal decomposition of4-ATZN include lead chromate, nano-sized ironoxide, bismuth oxide, ammonium dichromate, stannous sulfate and so on.The energetic azolium nitrate was applied into the solid propellant with a new cruingsystem consisting of alkynyl-terminated polyether and multi-azide curing agent, whichsolved the problem of poor compatibility between energetic azolium nitrates and traditionalisocyanate curing agents. A new kind of solid propellant containing5-ATZN was preparedby using the cacuum pouring process with propargyl-terminated PET4000(C≡PET4000) as binder, which was obtained by modifying terminal hydroxyl groups of ethyleneoxide-tetrahydrofuran copolyether with molecular weight of4000(PET4000). Thecombustion property of5-ATZN monopropellant and composite propellant was measuredby the CCD measurement system for the burning rate of solid propellant. The results showthat the burning rate of5-ATZN monopropellant is about1.94to2.67times that of CL-20monopropellant prepared in the same condition. The burning rate pressure exponent of5-ATZN monopropellant is about0.579, which is also lower than that of CL-20monopropellant. The addition of5-ATZN is helpful to increase the burning rate ofC≡PET4000propellant greatlly. In addition, the burning rate pressure exponent ofC≡PET4000propellant containing5-ATZN can also be reduced with the use of leadsalicylate catalyst, because lead salicylate could lower the decomposition temperature of5-ATZN.