Development Of ReaxFF Reactive Force Field And Several Applications In Materials Science

ThisdissertationfocusesonthedevelopmentsandapplicationsofReaxFFreactiveforce feld method. ReaxFF force feld method is the new generation method of molec-ular mechanism, which could be used in the reactive systems. This work includes fveparts, as the corrections of long range interactions of ReaxFF reactive force feld, themechanisms of pyrolysis and combustion of fuel additives, the pyrolysis mechanismsof coal associated with the swelling process, the cracking mechanisms of ettringite, thestructure analysis of multi-metal oxides and the ammoxidation mechanism of propeneon its surface. ReaxFF force feld method is diferent with the quantum mechanismmethod and classical molecular simulation method, the reactions in condensed phaseor on the surface of catalysts at diferent temperatures and pressures can be predicted.The major conclusions obtained in this thesis are:1. Thelow-gradientmodelfordispersioncorrectionshasbeenproposedtoagainstthe underestimation of long range vdW interactions of ReaxFF. Graphite, polyethy-lene, carbon dioxide, and nitrogen solids are used to determine correction parametersto reproduce the low temperature density and heat of sublimation. ReaxFF-lg predictsequations of states consistent with experiment, making signifcant improvements of in-termolecular interactions compared with original ReaxFF. Moreover, ReaxFF-lg pre-dicts successfully the phase transition of RDX at high pressure. In addition, ReaxFF-lgleads to negligible efect on chemical reaction energies, indicating the extensibility ofReaxFF-lg method.2. The pyrolysis and combustion mechanisms of1,6-Dicyclopropane-2,4-hexynehave been investigated by the ReaxFF-RD method. Through the simulations of uni-molecular model, the initial states of pyrolysis and combustion were confrmed start- ing from the single molecule reactions. The complicated reaction mechanisms wereinvestigated from the multi-molecular models and the distributions of products wereunderstood from the view of molecular scale. The results point out the feasibility of in-vestigatingtheunknownreactionsincondensedphasethroughab-initiobasedReaxFF-RD method.3. Through the development of ReaxFF-MC-RD method, the unexplained de-tailed structures of M2phase of multi-metal oxides (MoVTeNbO) were studied. Themolecular structures in the bulk phase have been resolved frst time. In addition, theefects of Nb for the formation of M2phase have been described. The ammoxidationmechanisms of propene on the M2phase were resolved from the testing H on eachactive site of M2surface, and the activation model of propene has been proposed frommolecular scale.4. The ReaxFF force feld for concrete system has been developed based on thefrst principle calculations. The mechanical properties and the failure modes of de-formation have been investigated. Through the2pt analysis of ettringite at diferentwater content, the chemical potentials of water at ettringite have been obtained and theswelling mechanism have been carried out based on these chemical potentials. Thehydrogen bond network have been confrmed to be the driven force of swelling of et-tringite and the delayed ettringite formation cracking mechanism of concrete.5. ReaxFF has been extended to the coal system and the pyrolysis mechanismassociated with the swelling of Bituminous coal has been investigated by building updiferent density coal models. The swelling driven force has been point out to be theformation of H2, CH4and NH3in the initial stage. The free volume increase alongwith the swelling and it efects on the pyrolysis products that more unsaturated smallmolecular gases were produced such as C2H, C2H2, C3H3and CO, these new gasesaccelerate the swelling and pyrolysis of coal。6. Besides of the work on ReaxFF force feld, the desorptions of ammonia inzeolite were investigated based on the frst principle based classical force feld. Theinteraction between ammonia and zeolite were considered using the RI-MP2/TZVPPmethod, and the force feld was developed based on these calculations. Desorptionof ammonia were carried out by the grand-canonical monte carlo simulations and the network morphology of ammonia in the zeolite were confrmed in efecting on theacidity measurement via temperature-programmed desorption of ammonia in zeolite.