Combustion Mechanism Of New Promising Energetic Materials

In this Ph.D.dissertation, the decomposition characteristics and the combustion mechanisms between binders such as GAP, PET and energetic compositions were investigated and based on these studies, the combustion physical models were established. At the same time, the combustion mechanisms for high energetic composite propellants with additives were studied. In addition, theoretical study on the reaction of aluminum with water in the gas phase was performed.The findings obtained from the above researches are summarized as following: 1. Thermal gravimetric analysis (TG)-differential thermal analysis (DTA), and in-situ Fourier Transform Infra-Red spectrometer (FTIR) experiments were used to investigate the thermal decomposition mechanism of glycidyl azide polymer (GAP) crosslinked by using the curing agent isocyanate compound N-100 and the cured GAP with addition of different contents NG/BTTN plasticizer, which are of potential interest for the development of high performance energetic propellants. The results of TG-DTA show the onset of thermal decomposition temperature of GAP shifted to lower temperatures in the presence of NG/BTTN. The decomposition peak temperatures of cured GAP/NG/BTTN (1:1:1, 1:0.5:0.5, 1:0.25:0.25) decrease by approximately 20°C, 33°C and 39°C compared with cured GAP, respectively. This indicates that plasticizer NG/BTTN has a good acceleration effect on the decomposition of cured GAP, especially for low content of NG/BTTN. At the same time, the result of DTA shows that the decomposition heat of cured GAP/NG/BTTN is larger than that of cured GAP. FTIR results show NG/BTTN not only accelerates the decomposition of -N3 andcharacteristic urethane link but also accelerates the decomposition of C-O-C group.The studies of pyrolysis products and flash pyrolysis products of GAP and GAP/NG/TEGDN were performed through the double shot Py-GC/MS. The results show that there are 7 identified products for pyrolysis of GAP at 300°C, and mainly butylene glyco, the compounds of aromatic properties of ketone, ester and so on. The GAP/NG/BTTN pyrolysis produces 5 identified products at 300°C, and the structure of the main product is which was not reported in previous literatures. At500°C, identified products for the flash pyrolysis of GAP and GAP/NG/BTTN are 43 species and 34 species, respectively. In addition, the mechanism of GAP flash pyrolysis is discussed. The homolysis takes place at the initial moment of the whole pyrolysis process of GAP. The radicals produced during the homolysis of GAP continue to take place free radical reactions, intermolecular crosslinking reactions, addition reactions and H-migration reactions et al.The combustion mechanisms of GAP based propellants were investigated by Single short distance photograph, CCD and scanning electron microscopy. The results of burning rates show that the burning rates increase with the contents of nitrate ester NG/BTTN within GAP based propellants. The burning rates of GAP/AP based propellants are higher than that of GAP/HMX based propellants. The flame of GAP/HMX propellant exists dark zone, however, the flames don't found obvious dark zone in GAP/NG/BTTN/HMX propellants. The SEM results show the quenched surface have a lot of pores in GAP/HMX based propellants, but there are no pores on the quenched surface of GAP/AP based propellants. Especially, the thickness of conversion zone was estimated by the burning surface longitudinal section of propellant. The results found that thickness of conversion zones of propellants GAP/NG/BTTN/HMX are thicker than that of GAP/NG/BTTN/AP propellants According to the above experimental results, the control zones of combustion reaction are presumed in GAP/HMX based propellants and GAP/AP based propellants.2. Thermal gravimetric analysis (TG)-differential thermal analysis (DTA), and in-situ Fourier Transform Infra-Red spectrometer (FTIR) experiments were used to investigate the thermal decomposition characteristics of PET, PET+N-100 and cured PET/NG/TEGDN. The results indicate that the decomposition of PET takes place two steps. The first step is the depolymerization of PET with exothermal reaction and the second step is the decomposition of PET with endothermal reaction. The decomposition of PET+N-100 takes place three steps. The results of DTA show the endothermal peak of PET disappears with addition of N-100 within PET. This is due to the temperature of the depolymerization reaction of PET shifting to high temperature. In-situ FTIR results show nitrate esters NG/TEGDN make the temperature of completely decomposition of PET advance. This is consistent with the results of TG.The studies of pyrolysis products and flash pyrolysis products of PET and PET/NG/BTTN were performed through the double shot Py-GC/MS. The results show that identified pyrolysis products of PET are 5 species and mainly esters, phenol derivatives and dihydroxy alcohol at 300°C. The identified products of PET/NG/TEGDN pyrolysis are only 2 species. However, it is presumed that NG/TEGDN have effects on the pyrolysis mechanism of PET. At 500°C, identified products of PET and PET/NG/TEGDN are 33 species, respectively and the products are mainly butyl aldehyde, ether, ethanol, and ester. In addition, PET/NG/TEGDN also produce oxime. The mechanism of PET flash pyrolysis is discussed. PET involves the cleavage at either C-C or C-0 bonds, and then takes place free radical reaction, hydrogen transfer reaction and hydrogen abstraction reaction.The combustion mechanisms of PET/NG/TEGDN/HMX propellants and PET/NG/TEGDN/AP propellants were investigated by Single short distance photograph, CCD and scanning electron microscopy. The results indicate that the effect of contents of nitrate ester NG/TEGDN on the burning rate of PET/HMX is not obvious, but on the burning rate of PET/AP is very notable. The"linkage-mutualism"mechanism was proposed. The SEM results show the quenched surface have a lot of pores in PET/NG/TEGDN/HMX based propellants, but there are no pores on the quenched surface of PET/NG/TEGDN/AP based propellants. Especially, the thickness of conversion zones was estimated by the burning surface longitudinal section of propellants. The results found that thickness of conversion zones of PET/NG/TEGDN/HMX propellants are thicker than that of PET/NG/TEGDN/AP propellants. The species of oxidizer greatly affect the flame structure of PET/NG/TEGDN based propellants. The flame structures of PET/NG/TEGDN/HMX propellants and PET/NG/TEGDN/AP propellants are the same as the flame structures of HMX-CMDB propellants and AP-CMDB propellants, respectively.3. Based on the models of HMX-CMDB propellants and AP-CMDB propellants, the combustion physical models of high energetic propellants were established according to the combustion characteristics of high energetic propellants. This provided helps to the building the mathematical model of high energetic propellants.4. The effects of coated nano-sized Al powder and micron-sized Al powder in propellants on the burning rate and pressure exponent have been investigated. The results show that the burning rates of propellants increase as the n-Al content increases, but the burning rate pressure exponents tend to decrease. Compared with propellants containing micron-sized Al powder, the increments of burning rate of propellants containing n-Al powder reduce gradually with increase in the pressure because of the differences of the combustion characteristics and ignition performances of n-Al powder and g-Al powder. Single short distance photograph, scanning electron microscopy, X-ray fluorescence analysis were used to characterize the flame image, combustion phenomena, the quenched surface image and surface elements. A substantial difference in combustion characteristics of n-Al powder has been found in comparison with micron-sized Al powder. In addition, oxygen-bomb combustion heat, ignition temperature and recovery ratio of residues were measured.5. The mechanisms for AP of the main compositions of composite propellants with additives such as ammonium oxalate (AO), strontium carbonate (SC) and AO/SC are studied by in-situ FTIR and DSC. Analysises for in-situ FTIR show that AO makes disappearance temperatures of absorb peak for AP delay. SC reacts with HClO₄ from AP decomposition and produces more stabilized Sr(ClO₄)₂ in the condense phase. in-situ FTIR proves the producing of Sr(ClO₄)₂. Analysises for DSC show that the high decomposition exothermic peak temperature of AP with AO is increased, but there is no effect on the low decomposition exothermic peak temperature of AP. Both the low and the high decomposition exothermic peak temperatures of AP are increased by addition of SC. Although both of decomposition exothermic peak temperatures of AP are increased with addition of AO/SC, the experimental result shows that SC and AO didn't produce synergetic effects for the high temperature decomposition of AP at low pressure. Based on the above experimental results, the mechanisms of inhibiting the decomposition of AP for AO and SC are discussed.AP/HTPB based composite propellants with additives such as ammonium oxalate (AO), mixture of ammonium oxalate and strontium carbonate (SC) was investigated by burning rate and TG-DTG experiments. The results show that the burning rates of these propellants are decreased significantly. TG-DTG experiments indicate that decomposition temperatures of AP with these additives are increased. Furthermore, the activation energy of the decomposition reaction of AP is also increased in the presence of AO or AO/SC. These results show that AO or AO/SC restrains the decomposition of AP. The burning rates of these propellants are decreased. The burning rate temperature sensitivity of AP/HTPB based propellants is reduced significantly by the addition of AO or AO/SC. But the effect of AO is less than that of AO/SC. AO/SC has better effect to reduce temperature sensitivity and at the same time, to reduce pressure exponent. The reduced heat release at the burning surface of AP/HTPB/AO is responsible for the reduced temperature sensitivity. Synergetic action is probably produced between AO and SC within AP/HTPB based propellants in the pressure range tested. This synergetic effect causes the heat release to reduce and the burning surface temperature to increase. Moreover, it makes the net exothermal reaction of condensed phase become little dependent on T₀. Thus, the burning rate temperature sensitivity is reduced.6. Theoretical study on the reaction of aluminum with water in the gas phase was performed using the hybrid density functional B3LYP and QCISD(T) methods in connection with the 6-311+G(d, p) and 6-311++G(d, p) basis sets. The theoretical results show that there are three possible reaction paths that involve 4 isomers, 7 transition structures and 2 possible products for reaction of aluminum with water. Two most favorable reaction paths have been found, which involved in intermediates and products agree quite well with experimental results. At the same time, the enthalpy and Gibbs free energies changes of the reaction between Al and H₂O at 298K and 2000K are calculated, respectively. Some results are also in good agreement with the previous calculations or experimental results.