The thermophysical properties, including composition, thermodynamic properties, transport coefficients and net emission coefficients, of thermal plasmas formed from pure iso-C4 perfluoronitrile C4F7N and C4F7N–CO2 mixtures are calculated for temperatures from 300 to 30 000 K and pressures from 0.1 to 20 atm. These gases have received much attention as alternatives to SF6 for use in circuit breakers, due to the low global warming potential and good dielectric properties of C4F7N. Since the parameters of the large molecules formed in the dissociation of C4F7N are unavailable, the partition function and enthalpy of formation were calculated using computational chemistry methods. From the equilibrium composition calculations, it was found that when C4F7N is mixed with CO2, CO2 can capture C atoms from C4F7N, producing CO, since the system consisting of small molecules such as CF4 and CO has lower energy at room temperature. This is in agreement with previous experimental results, which show that CO dominates the decomposition products of C4F7N–CO2 mixtures; it could limit the repeated breaking performance of C4F7N. From the point of view of chemical stability, the mixing ratio of CO2 should therefore be chosen carefully. Through comparison with common arc quenching gases (including SF6, CF3I and C5F10O), it is found that for the temperature range for which electrical conductivity remains low, pure C4F7N has similar ρCp (product of mass density and specific heat) properties to SF6, and higher radiative emission coefficient, properties that are correlated with good arc extinguishing capability. For C4F7N–CO2 mixtures, the electrical conductivity is very close to that of SF6 while the ρCp peak at 7000 K caused by decomposition of CO implies inferior interruption capability to that of SF6. The calculated properties will be useful in arc simulations.

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C5F10O has recently been found to be a very promising alternative to SF6. This paper is devoted to the investigation of compositions, thermodynamic properties, and transport coefficients of high-temperature C5F10O mixed with CO2 and O2. Firstly, the partition functions and enthalpies of formation for a few molecules (CxFy and CxFyO) which are likely to exist in the mixtures, are calculated based on the G4(MP2) theory. The isomers of the above molecules are selected according to their Gibbs energy. The compositions of C5F10O-CO2-O2 mixtures are then determined using the minimization of the Gibbs free energy. Next, the thermodynamic properties (mass density, specific enthalpy, and specific heat) are derived from the previously calculated compositions. Lastly, the transport coefficients (electrical conductivity, viscosity, and thermal conductivity) are calculated based on Chapman-Enskog method. It is found that, as an arc quenching gas, C5F10O could not recombine into itself with the temperature decreasing down to room temperature after the arc extinction. Besides, the key species at room temperature are always CF4, CO2, and C4F6 if graphite is not considered. When taken into account, graphite will replace C4F6 as one of the dominate particles. The mixing of CO2 with C5F10O plasma significantly affects the thermodynamic properties (e.g. vanishing and/or shifting of the peaks in specific heat) and transport coefficients (e.g. reducing viscosity and changing the number of peaks in thermal conductivity), while the addition of O2 with C5F10O-CO2 mixtures has no remarkable influence on both thermodynamic and transport properties.

The influence of metal vapour on the arc behaviour during the arc-splitting process in the quenching chamber of a low-voltage circuit breaker is investigated numerically. A three-dimensional magnetohydrodynamic model of air arc plasma, taking into account the production of metal vapour from erosion of an iron splitter plate, is developed. An equation describing conservation of the iron vapour mass is added to the standard mass, momentum and energy conservation equations. The influence of the iron vapour on the thermodynamic and transport properties of the gas mixture is considered. The arc voltage, distributions of temperature, gas flow and mass fraction of iron vapour in the arc chamber are calculated. The formation of new arc roots on the splitter plate is examined. The simulation results indicate that this is strongly influenced by the presence of iron vapour from the splitter plate, due to the increased electrical conductivity in the arc root formation region. The consequences of this are dramatic. The presence of metal vapour causes the arc to attach first to the cathode side of the splitter plate, and electromagnetic forces then cause the arc on this side to move more rapidly than the arc on the anode side. The opposite occurs if metal vapour is neglected. High-speed photography of arc motion is used to confirm the arc motion predicted in the presence of metal vapour. Further, the calculated arc voltage taking into account metal vapour is lower than that calculated neglecting metal vapour, because of the increased electrical conductivity, and agrees much better with the measured voltage.

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Due to the high global warming potential (GWP) and increasing environmental concerns, efforts on searching the alternative gases to SF6, which is predominantly used as insulating and interrupting medium in high-voltage equipment, have become a hot topic in recent decades. Overcoming the drawbacks of the existing candidate gases, C5- perfluorinated ketone (C5 PFK) was reported as a promising gas with remarkable insulation capacity and the low GWP of approximately 1. Experimental measurements of the dielectric strength of this novel gas and its mixtures have been carried out, but the chemical decomposition pathways and products of C5 PFK during breakdown are still unknown, which are the essential factors in evaluating the electric strength of this gas in high-voltage equipment. Therefore, this paper is devoted to exploring all the possible decomposition pathways and species of C5 PFK by density functional theory (DFT). The structural optimizations, vibrational frequency calculations and energy calculations of the species involved in a considered pathway were carried out with DFT-(U)B3LYP/6-311G(d,p) method. Detailed potential energy surface was then investigated thoroughly by the same method. Lastly, six decomposition pathways of C5 PFK decomposition involving fission reactions and the reactions with a transition states were obtained. Important intermediate products were also determined. Among all the pathways studied, the favorable decomposition reactions of C5 PFK were found, involving C-C bond ruptures producing Ia and Ib in pathway I, followed by subsequent C-C bond ruptures and internal F atom transfers in the decomposition of Ia and Ib presented in pathways II + III and IV + V, respectively. Possible routes were pointed out in pathway III and lead to the decomposition of IIa, which is the main intermediate product found in pathway II of Ia decomposition. We also investigated the decomposition of Ib, which can undergo unimolecular reactions to give the formation of IV a, IV b and products of CF3 + CF-CF3 in pathway IV. Although IV a is dominant to a lesser extent due to its relative high energy barrier, its complicated decomposition pathway V was also studied and CF3, C = CF2 as well as C-CF3 species were found as the ultimate products. To complete the decomposition of C5 PFK, pathway V I of Ic decomposition was fully explored and the final products were obtained. Therefore, the integrate decomposition scheme of C5 PFK was proposed, which contains six pathways and forty-eight species (including all the reactants, products and transition states). This work is hopeful to lay a theoretical basis for the insulating properties of C5 PFK.

This paper focuses on the numerical research of the influence of electrode erosion on the arc behavior during opening process of low-voltage circuit breakers. The mathematical model of three-dimensional air arc plasma considering electrode erosion is built based on magnetic hydrodynamics. The mass fraction equation of copper vapor is introduced to the model on the basis of traditional mass, momentum, and energy balance equations. The influence of copper vapor on the thermodynamic and transport properties of the gas mixture is considered in this paper. The distributions of temperature field, gas flow field, and mass fraction of copper vapor in the arc chamber are simulated. The arc root displacements and arc voltage, which takes account of the influence of electrode erosion, are calculated. The simulation results indicate that the immobility time of both moving contact and stationary contact is much longer considering electrodes erosion. The calculated voltage of the arc column during arc motion considering erosion is smaller because of the change in the electrical conductivity of air-copper vapor mixtures. Except for the numerical investigation on the arc motion considering electrode erosion, the experiment work is also carried out to support the simulation work.

The calculated values of thermodynamic and transport properties of mixtures of carbon and argon, and carbon and helium, at high temperatures are presented in this paper. The thermodynamic properties are determined by the method of Gibbs free energy minimization, using standard thermodynamic tables. The transport properties including electron diffusion coefficients, viscosity, thermal conductivity and electrical conductivity are evaluated using the Chapman–Enskog method expanded up to the third-order approximation (second order for viscosity). Collision integrals are obtained using the most accurate cross-section data that could be located. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30 000 K for different pressures between 0.1 and 10 atm. The results are compared with those of previously published studies. Good agreement is found for pure argon and helium. Larger discrepancies occur for carbon and mixtures of carbon and argon, and carbon and helium; these are explained in terms of the different values of the collision integrals that were used. The results presented here are expected to be more accurate because of the improved collision integrals employed.