Study of Characteristcs and Mechanism of Pyrolysis and Combustion of Aluminum Based Composite Fuel
Author:Liu Xing Hua
Supervisor:li shu yuan
In this dissertation,characteristcs and mechanism of pyrolysis and combustion of aluminum-based composite fuel initialized by chlorine trifluoride oxide（ClF3O）were systematically investigated both theoretically and experimentally.The components of the composite fule were isopropyl nitrate（IPN）,hexogen（RDX）,ammonium perchlorate（AP）,n-decane and aluminum powder etc..Three types of reactions were found during thermal decomposition and combustion.First type is initial reaction of ClF3O with n-decane via electrophilic substitution to form fluorinated product along with large amount of released energy.The roaming of HF intermediate and the formation of carbocation were involved in this reaction.The second is thermal decomposition of composite fuel to form free radical intermediates and final pyrolysates.Final type is the combustion of composite fuel.Under the oxygen atmosphere,thermal decomposition and combustion were carried out to produce corresponding radicals and end products.The weak interactions among components of composite fuel were determined by reduced density gradient method（RDG）.AP was easy to form H-bond with other components,while RDX favored to form van der Waals interaction.The interactions among n-decane and others were negligible.Stable complexes could be formed among AP,RDX,IPN and n-decane.The active site of ClF3O was determined using dual descriptor method.The active sites of composite fuel were C-H bonds.The reaction pathways were calculated at B3WP91/6-31+G（d,p）level of theory by the density functional theory method（DFT）.The results showed that IPN,RDX and n-decane could react with ClF3O spontaneously by overcoming small energy barriers to generate HF,ClFO and fluorine-substituted products.ClFO continued to react with n-decane to form HF,HClO and decene.AP and aluminum were unreactive to ClF3O.The reactions among ClF3O and components of composite fuel were electrophilic substitution.The reactivity of n-decane>RDX>IPN was found based on the calculated results.N-decane was the most reactive component with averaged released heat of 421.1 kJ/mol,which would lead to thermal decomposition of n-decane.The products from reaction of ClF3O with n-decane were analyzed.The main gaseous products included H2、CO、C2H2、CH4、C3H6、C3H8,etc.,indicating the occurrence of thermal decomposition.Five types of main liquid products were found,including chain alkanes（10%）,cyclic alkanes（>30%）,alkenes（10%）,aromatics（>30%）and halohydrocarbons（<10%）,showing that besides thermal decomposition,fluorinated reactions took place as well.It was also suggested that thermal decomposition was a predominant reaction compared with fluorinated reaction.The elements of C and F were determined in solid products.C existed mainly in the form of（CH2）n,small amount of C in different C-F forms were also detected.F existed mainly in the form of C-F bond.The thermal decomposition mechanism of alkane was investigated using n-decane as a surrogate by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry（VUV-SI-TOFMS）.The n-decane pyrolysis began at 1040 K.The initial reaction started from the cleavage of C-C bond in the middle of n-decane to form the free radical of?C5H11.?C5H111 and its isomers from radical rearrangement were decomposed viaβ-C-C andβ-C-H fission to yield small alkenes and alkyl radicals.The thermal decomposition of alkene was studied using 1-hexene as a surrogate by combining the VUV-SI-TOFMS technique and the complete active space self-consistent field method（CASSCF）.The initial temperature of 1-hexene pyrolysis was lower than that of n-decane.The preliminary reaction occurred via C-C bond atβposition.The subsequent decompositions were similar with that of n-decane.Diradical retro-ene mechanism was important to the pyrolysis of 1-hexene.The emission spectra of n-decane combustion initialized by ClF3O were obtained by intensified CCD technique.The radicals of CH,C2 and OH were observed,illustrating that ClF3O served as an initiator for n-decane pyrolysis.No radicals related to ClF3O were found.The thermal decomposition behavior of composite fuel was investigated using thermogravimetric analyzer（TG-DTA）.N-decane and IPN vaporized below 200℃.RDX phase transformation and decomposition were carried out at the temperature of200300℃.The thermal decomposition of AP occurred at 300-580℃.The phase transformation and reaction with oxygen for aluminum took place above 580℃.The microstructure and surface element distribution of composite fuel,and the residues of thermal decomposition were studied by scanning electron microscope and energy disperse spectroscopy（SEM-EDS）.The composite fuel possessed flaky,blocky and spherical structures,and its surface was covered by hydrocarbons.The main elements on the surface of residue were Al,O and C.The surface of the residue was covered with Al2O3 film.From emission spectra analysis results,it was found that the typical radical intermediates（CH,C2 and OH）were observed from hydrocarbon combustion.New radicals were also found,including NO,CHO and AlO.NO and CHO were considered as the typical radicals from emission spectra during combustion of IPN and RDX,while AlO was the characteristic radical during aluminum combustion.