A Computational Research On Atmospheric Degradation Mechanisms And Chemical Properties For Several Replacement Gases Of SF₆

The SF₆ gas has been widely used in high voltage power transmission and transformation system due to the good insulation strength and arc extinguishing performance.However,SF₆ gas has been identified as controlled greenhouse gases because of the serious greenhouse effect.Thus,it is urgent to develop an environmentally friendly gas for the power system application although it is extremely difficult.Based on rational molecular design principle,this thesis is to investigate the potential alternative gaseous molecules to characterize their atmospheric degradation,physical properties,and mechanisms for the discharge-induced chemical reactions by means of ab initio quantum chemistry calculations and molecular dynamics.The data in this work servers as a sound theoretical basis for the development of environmental friendly dielectrics.Two alternatives were studied systematically,including the perflorinated nitrogen-sulfur molecule,CF₃NSF₂ and CF3 I.CF₃NSF₂ is of special interest because its structure is representative of the uniquely bonded collections of atoms that heretofore had not been well established yet.Electronic structures of [?trifluoromethyl?imino]sulfur difluoride and degradation mechanisms by hydroxyl radical have been investigated using density functional theory.It was found that CF₃NSF₂ exists as two conformations connected by the internal rotation of CF3 around the central NS bond.The degradation of CF₃NSF₂ by OH can be accelerated considerably in the presence of a single water molecule,which acts as a bridge for the consecutive proton migration within the floppy cyclic geometries.The half-life of CF₃NSF₂ was estimated to be 2.5 year,and the final products are exclusively CF3 NH and SF2 O.Theoretical calculation supports that CF₃NSF₂ is an environment-friendly green gas.It is worthy of testing its dielectric properties to replace SF for practical use.Trifluoroiodomethane?CF₃I?has been used as an alternative insulation gas to SF₆ due to its low global warming potential and excellent dielectric strength.However,the by-products of CF3 I reactions with reactive radicals such as atomic oxygen,which are generated due to arc discharge of CO2 medium or O2/H2 O impurities,lead to its low current interruption capability.Therefore,the detailed mechanistic and kinetic information on the O+CF₃I reaction is highly desired for the rational design of the breakdown performance of CF3 I as an insulation medium.Both singlet and bifurcated triplet?3A' and 3A"?potential energy surfaces for the O+CF₃I reaction were explored systematically using density functional M06-2X method with small-core energy-consistent relativistic pseudopotentials.Single-point energies were calculated using the explicitly correlated CCSD?T?-F12 method.The theoretical rate constants are in good agreement with the available experimental data.The overall rate constants exhibit positive temperature dependence in the range 200³000 K and could be expressed and no pressure dependence of the rate constant has been found.How to optimize the mixture ratio of two type gas mixture is another important topic for SF₆ alternative gas research.In this thesis,it was found that the intermolecular interaction,namely,the electronic structure of the weakly-bound complexes and binding modes could be the reliable criterions for the rational design of the gas-mixture formulas.Preliminary theoretical investigations in this thesis demonstrate that the intermolecular interaction based rational molecular design for the gas mixtures should be feasible and effective.Moreover,a reasonable explanation on the possible synergic effect could be offered as well.Although the quantum chemistry methods are capable of calculating the electronic structures,reaction mechanisms,and discharge characteristics at the microscopic molecular or atomic level,it is hard to obtain the phase-dependent bulk properties such as density,boiling point,and energy transfer.Using the force-field based in situ molecular dynamics simulations,the dielectric mixtures of CF3 I and CF₃NSF₂ with CO2 and N2 bath gases,respectively,have been investigated as functions of mixing ratios,pressures,and temperatures.The research results can provides theoretical guidance and experimental reference for practice application with different condition requirement.