Insulation Strength and Decomposition Characteristics of a C 6 F 12 O and N 2 Gas Mixture

This paper explores the decomposition characteristics of a new type of environmentally friendly insulating gas C6F12O and N2 mixed gas under AC voltage. The breakdown behavior of 3% C6F12O and N2 mixed gas in quasi-uniform field was investigated through a breakdown experiment. The self-recovery of the mixed gas was analyzed by 100 breakdown experiments. The decomposition products of C6F12O and N2 under breakdown voltage were determined by gas chromatography–mass spectrometer (GC-MS). Finally, the decomposition process of the products was calculated by density functional theory, and the ionization energy, affinity, and molecular orbital gap of the decomposition products were also calculated. The properties of the decomposition products were analyzed from the aspects of insulation and environmental protection. The experimental results show that the 3% C6F12O and N2 mixed gas did not show a downward trend over 100 breakdown tests under a 0.10 MPa breakdown voltage. The decomposition products after breakdown were CF4, C2F6, C3F6, C3F8, C4F10, and C5F12. The ionization energies of several decomposition products are more than 10 eV. The Global Warming Potential (GWP) values of the main products are lower than SF6. C2F6, C3F8, and C4F10 have better insulation properties.


Introduction
Conventional gases (N 2 , CO 2 , and air) are relatively stable in terms of physical and chemical properties with a low preparation cost.They are widely used as insulating gases in medium-and low-pressure gas insulation equipment such as gas-insulated switchgear (c-GIS) [1].However, applying reasonable measures to increase the insulation performance of conventional gases, such as increasing the pressure of the gas chamber, increasing the size of the equipment, and using a combination of gas solids, is necessary given their low insulation properties.
Toshiba researchers studied the thermal breaking performance of CO 2 gas by measuring the characteristics of arc current and arc time constant, and they reported the prototype of CO 2 tank circuit breaker with 72 kV rating and breaking capacity of 31.5 kA.However, its volume and weight is approximately 1.5 to 2 times that of the SF 6 circuit breaker at a pressure of 0.7 MPa [2].Compressed air and N 2 have also been used for 24 kV, 72 kV, and other medium voltage switchgear insulations.To obtain the equivalent of SF 6 gas dielectric strength, the pressure of N 2 must be increased to 3-4 times of SF 6 given that the insulation strength of pure N 2 is low [3].Korea Hyundai Heavy Industries [4], Toshiba, and other researchers have used air-solid composite insulation technology to enhance the role of its insulation [5].The results show that the appropriate insulation materials on the high electric field electrode surface coating can effectively improve its electrical strength.Therefore, the current use of conventional gas in the switch equipment must optimize the structure of the device and increase the pressure and the size of the equipment to increase the production cost of the equipment.
The use of SF 6 gas mixtures are considered given that SF 6 gas in the dielectric strength relative to other insulating medium has evident advantages.The addition of SF 6 can enhance the insulation performance and maintain the pressure to keep the equipment structure compact [6].However, the "Kyoto Protocol," which was formulated in 1997, emphasized SF 6 as one of the six greenhouse gases with GWP of 23,900 times that of CO 2 ; moreover, the use of SF 6 should be gradually reduced and an environmentally friendly insulation gas to replace the SF 6 should be selected.Therefore, the recent research and exploration of a new type of environmentally friendly gas has been considered [7].SF 6 is mainly composed of strong negative gases and mixed gases (CF 3 I, c-C 4 F 8 , C 3 F 8 , and C 2 F 6 ) [8][9][10].Among them, the GWP values of c-C 4 F 8 , C 3 F 8 , and C 2 F 6 are less than that of SF 6 , but greater than that of CO 2 ; thus, the greenhouse effect cannot be ignored.CF 3 I has acceptable insulation performance and a GWP value of 0. However, the gas in the discharge has a solid iodine precipitation, affecting insulation performance.
C 6 F 12 O is non-combustible and non-toxic and currently limited for use in fire extinguishing agent [11], magnesium treatment, and two-phase immersion cooling system covered gas [12].C 6 F 12 O molecule is an ozone depletion potential (ODP) gas with a GWP value close to 1 and dielectric strength of approximately 1.7 times that of pure SF 6 gas [13].The structure of C 6 F 12 O is mainly the combination of the characteristic double and single bonds of ketone carbonyl and its prominent dielectric strength can be attributed to the strong electron absorption capacity of high fluorine content in molecular structure.The mixing ratio in the mixed gas is relatively low (less than 10%) because gas liquefaction temperature is extremely high (liquefaction temperature of 49 • C at 0.1 MPa).The discharge of insulating gas under the insulation defect leads to decomposition, and the insulating properties of the decomposition products are important indexes to evaluate insulating gas.The current decomposition of C 6 F 12 O is limited to the interaction of C 6 F 12 O with oxygen or air in the extinguishing state [14], and the decomposition properties of C 6 F 12 O gas under discharge have not been reported.Therefore, detecting the decomposition products of the insulating gas to ensure its safety, analyzing the physical and chemical properties, and insulating properties of the decomposition products, and evaluating its influence on the gas insulation performance and the environment are necessary.
The present study initially explores the C 6 F 12 O mixed N 2 decomposition characteristics under frequency AC voltage breakdown.Firstly, the decomposition products of C 6 F 12 O and N 2 mixed gas after 0.10 MPa were investigated.Secondly, the change in the surface elements of the electrode was detected using photoelectron spectroscopy.Thirdly, gas decomposition products of the mixed gas are detected by GC-MS.Fourth, binding test results were used to calculate the decomposition process of the products based on density functional theory (DFT), and the ionization energy, affinity, and molecular orbital gap of the decomposition products were calculated.Finally, the properties of the decomposition products and the influence on the insulation properties of the mixed gas were analyzed.

Experiment
The experimental wiring diagram of the power frequency voltage experiment platform is shown in Figure 1.The sphere electrodes are placed in the insulating gas chamber, and the electrode material is copper with a diameter of 50 mm and an electrode spacing of 5 mm.T 1 : Induction voltage regulator with an input voltage of 380 V and an output voltage of 0-400 V; T 2 : Non-corona test transformer can provide the power frequency test voltage of 100 kV; R: Protection resistance of R = 10 kΩ, which can protect the transformer from being damaged in breakdown; C V : Capacitive divider, in which the test voltage with high amplitude can be converted into the lower value which can be measured directly by the voltmeter; V: Voltmeter measures the voltage through the capacitive divider.C6F12O gas is liquid at room temperature because its liquefaction temperature is 49 °C.The saturated vapor pressure of the gas satisfies the Antoine equation, and the saturated vapor pressure of C6F12O is 40 kPa at 25 °C [13].The Antoine equation is used to obtain the relation between saturated vapor pressure and the liquefaction temperature of C6F12O: lgp = −3719.7385/T+ 18.180 (1) where T is the liquefaction temperature of gas, and p is the saturated vapor pressure of C6F12O.
According to Dalton's law, ideally, the partial pressure of a gas in the gas mixture is equal to the pressure produced at the same temperature when it occupies the entire container alone, and the total pressure of gas mixture is equal to the sum of the gas partial pressure.
where PT is the total pressure of gas, and Pi is the partial pressure of each component.The two-component mixed gas is computed as follows: where k is the mixing ratio of C6F12O in the mixed gas.The liquefaction temperature of N2 is −196 °C [15].The liquefaction temperature of mixed gas only depends on C6F12O because the liquefaction temperature of C6F12O is 49 °C, which is much higher than that of N2.The minimum operating temperature is −15 °C in the cold condition based on the operating temperature of the medium-and low-voltage equipment of −5 °C to 40 °C, and the main unit of the medium voltage equipment such as GIS must be at −25 °C [16].To ensure that the mixture gas at less than −15 °C at 0.10 MPa is not liquefied, the mixing ratio of C6F12O should be less than 5%.We selected 3% C6F12O mixed N2 to carry out the breakdown experiment, and the liquefaction temperature of mixed gas is −26 °C.The breakdown voltage is measured under quasi uniform electric field at 0.10 MPa.The measured breakdown voltage is given as the root mean square (RMS) value.The experimental results show that the breakdown voltage of the 3% C6F12O mixed N2 is approximately 1.9 times that of pure N2 (the breakdown voltage of pure N2 is approximately 7.6 kV).Thus, a small amount of C6F12O greatly improves the breakdown performance of N2.The breakdown voltage has a certain discreteness given the 100 times breakdowns of 3% C6F12O/N2 gas mixture.However, with the increase in the number of discharge, the breakdown voltage does not decrease, as shown in Figure 2.  where T is the liquefaction temperature of gas, and p is the saturated vapor pressure of C 6 F 12 O.According to Dalton's law, ideally, the partial pressure of a gas in the gas mixture is equal to the pressure produced at the same temperature when it occupies the entire container alone, and the total pressure of gas mixture is equal to the sum of the gas partial pressure.
where P T is the total pressure of gas, and P i is the partial pressure of each component.The two-component mixed gas is computed as follows: where k is the mixing ratio of C 6 F 12 O in the mixed gas.The liquefaction temperature of N 2 is −196 • C [15].The liquefaction temperature of mixed gas only depends on C 6 F 12 O because the liquefaction temperature of C 6 F 12 O is 49 • C, which is much higher than that of N 2 .The minimum operating temperature is −15 • C in the cold condition based on the operating temperature of the medium-and low-voltage equipment of −5 • C to 40 • C, and the main unit of the medium voltage equipment such as GIS must be at −25 • C [16].
To ensure that the mixture gas at less than −15 • C at 0.10 MPa is not liquefied, the mixing ratio of C 6 F 12 O should be less than 5%.We selected 3% C 6 F 12 O mixed N 2 to carry out the breakdown experiment, and the liquefaction temperature of mixed gas is −26 • C. The breakdown voltage is measured under quasi uniform electric field at 0.10 MPa.The measured breakdown voltage is given as the root mean square (RMS) value.The experimental results show that the breakdown voltage of the 3% C 6 F 12 O mixed N 2 is approximately 1.9 times that of pure N 2 (the breakdown voltage of pure N 2 is approximately 7.6 kV).Thus, a small amount of C 6 F 12 O greatly improves the breakdown performance of N 2 .The breakdown voltage has a certain discreteness given the 100 times breakdowns of 3% C 6 F 12 O/N 2 gas mixture.However, with the increase in the number of discharge, the breakdown voltage does not decrease, as shown in At the end of the experiment, the surface elements of the sphere electrode are tested using Thermo Fisher's ESCALAB 250Xi photoelectron spectroscopy.The gas chromatographic mass spectrometer (GC-MS) equipped with a special capillary column (CP-Sil5CB) is used to separately and qualitatively analyze the decomposition of the characteristic components.A total of 99.999% of the high purity He is used as the GC-MS carrier.The working conditions are as follows: the inlet temperature is 220 °C, the injection volume is 1 mL, the split ratio is 10, the ion source temperature is 200 °C, and the chromatographic mass interface temperature is 220 °C.The mode of ionization is the electron impact ionization (EI).
The sphere electrode surface of the three kinds of elements (C, O, and F) before and after breakdown is scanned using photoelectron spectroscopy.The results are shown in Figure 3.The abscissa is the energy value of the binding energy, whereas the ordinate is the number of photoelectrons per second.After the breakdown, the distribution of the three kinds of elements on the surface of the electrode evidently changed.The C atom increases, and the increase in the F atom is not evident.Before the experiment, the percentage of C, O, and F atoms on the electrode surface is 70.12%, 28.64%, and 1.24%, respectively, and the percentage of atoms on the surface of the electrode after the breakdown is 78.09%, 20.33%, and 1.58%, respectively.The content of C atoms increased by nearly 8%; thus, a small amount of C atoms from the gas are deposited on the surface of the electrode.At the end of the experiment, the surface elements of the sphere electrode are tested using Thermo Fisher's ESCALAB 250Xi photoelectron spectroscopy.The gas chromatographic mass spectrometer (GC-MS) equipped with a special capillary column (CP-Sil5CB) is used to separately and qualitatively analyze the decomposition of the characteristic components.A total of 99.999% of the high purity He is used as the GC-MS carrier.The working conditions are as follows: the inlet temperature is 220 • C, the injection volume is 1 mL, the split ratio is 10, the ion source temperature is 200 • C, and the chromatographic mass interface temperature is 220 • C. The mode of ionization is the electron impact ionization (EI).
The sphere electrode surface of the three kinds of elements (C, O, and F) before and after breakdown is scanned using photoelectron spectroscopy.The results are shown in Figure 3.The abscissa is the energy value of the binding energy, whereas the ordinate is the number of photoelectrons per second.After the breakdown, the distribution of the three kinds of elements on the surface of the electrode evidently changed.The C atom increases, and the increase in the F atom is not evident.Before the experiment, the percentage of C, O, and F atoms on the electrode surface is 70.12%, 28.64%, and 1.24%, respectively, and the percentage of atoms on the surface of the electrode after the breakdown is 78.09%, 20.33%, and 1.58%, respectively.The content of C atoms increased by nearly 8%; thus, a small amount of C atoms from the gas are deposited on the surface of the electrode.At the end of the experiment, the surface elements of the sphere electrode are tested using Thermo Fisher's ESCALAB 250Xi photoelectron spectroscopy.The gas chromatographic mass spectrometer (GC-MS) equipped with a special capillary column (CP-Sil5CB) is used to separately and qualitatively analyze the decomposition of the characteristic components.A total of 99.999% of the high purity He is used as the GC-MS carrier.The working conditions are as follows: the inlet temperature is 220 °C, the injection volume is 1 mL, the split ratio is 10, the ion source temperature is 200 °C, and the chromatographic mass interface temperature is 220 °C.The mode of ionization is the electron impact ionization (EI).
The sphere electrode surface of the three kinds of elements (C, O, and F) before and after breakdown is scanned using photoelectron spectroscopy.The results are shown in Figure 3.The abscissa is the energy value of the binding energy, whereas the ordinate is the number of photoelectrons per second.After the breakdown, the distribution of the three kinds of elements on the surface of the electrode evidently changed.The C atom increases, and the increase in the F atom is not evident.Before the experiment, the percentage of C, O, and F atoms on the electrode surface is 70.12%, 28.64%, and 1.24%, respectively, and the percentage of atoms on the surface of the electrode after the breakdown is 78.09%, 20.33%, and 1.58%, respectively.The content of C atoms increased by nearly 8%; thus, a small amount of C atoms from the gas are deposited on the surface of the electrode.The gas in the gas chamber is tested by GC-MS.The detected results of the decomposition products are shown in Figure 4.The purity of C 6 F 12 O gas is 99.5%.The two kinds of chemical impurities are C 2 HF 6 P and C 6 F 12 , which are detected using a GC-MS spectrum library.The results, before and after the breakdown, are shown in Figure 4.According to standard gas and the corresponding GC-MS spectra similarity detection report (see Figure 5), the main decomposition products of compounds can be produced by the fluorocarbon compounds such as CF 4 , C 2 F 6 , C 3 F 8 , C 3 F 6 , C 4 F 10 , and C 5 F 12 .The characteristic peaks of C 3 F 6 and C 4 F 10 overlap.The acquisition time of GC-MS is set to 4 min, and the retention time of N 2 is approximately 3 min; thus, N 2 does not form part of the results in Figure 4.The breakdown voltage of the C 6 F 12 O gas decomposition product is mainly fluorocarbon.Although the gas contains N 2 , compounds containing N atoms are undetected.The molecular structure of C 6 F 12 O indicates that the formation of the products is mainly due to the formation of free radicals by the breaking of C-C bonds, and the free radicals are bonded to form the corresponding compounds.
Energies 2017, 10, 1170 5 of 12 The gas in the gas chamber is tested by GC-MS.The detected results of the decomposition products are shown in Figure 4.The purity of C6F12O gas is 99.5%.The two kinds of chemical impurities are C2HF6P and C6F12, which are detected using a GC-MS spectrum library.The results, before and after the breakdown, are shown in Figure 4.According to standard gas and the corresponding GC-MS spectra similarity detection report (see Figure 5), the main decomposition products of compounds can be produced by the fluorocarbon compounds such as CF4, C2F6, C3F8, C3F6, C4F10, and C5F12.The characteristic peaks of C3F6 and C4F10 overlap.The acquisition time of GC-MS is set to 4 min, and the retention time of N2 is approximately 3 min; thus, N2 does not form part of the results in Figure 4.The breakdown voltage of the C6F12O gas decomposition product is mainly fluorocarbon.Although the gas contains N2, compounds containing N atoms are undetected.The molecular structure of C6F12O indicates that the formation of the products is mainly due to the formation of free radicals by the breaking of C-C bonds, and the free radicals are bonded to form the corresponding compounds.

Decomposition Process Calculation
The experimental detection of the C6F12O reaction with N2 shows that the main products are CF4, C2F6, C3F8, C3F6, C4F10, and C5F12, and these products also have certain insulation performance.The decomposition process is calculated using density functional theory (DFT), and the insulating property of several kinds of decomposition products is analyzed by calculating the ionization energy of the molecules, affinity energies, and molecular orbital gaps.The calculation is carried out by Dmol3 in the material studio (MS) computing software.The calculation method of the generalized gradient approximation (GGA) is selected and is based on the optimization of the local density approximation (LDA) functional; thus, GGA has higher accuracy than LDA.The GGA method and the Perdew, Burke, and Enzerhof (PBE) function are used to deal with the electron exchange interaction [17].The function fitting d and p orbital polarization are used in the atomic orbital calculations through the p polarization dual numerical basis set (Double Numerical Set Plus Polarization Functions, DNP).The energy convergence precision, atomic migration, and energy gradient are set to 1.0 × 10 −5 , 0.005, and 0.002 Ha, respectively, and the convergence precision of charge density is 1.0 × 10 −6 Ha [18].The Direct Inversion of Iterative Subspace (DIIS), which is used to improve the convergence rate of self-consistent field charge density, can be used to improve efficiency.

Decomposition Process Calculation
The experimental detection of the C 6 F 12 O reaction with N 2 shows that the main products are CF 4 , C 2 F 6 , C 3 F 8 , C 3 F 6 , C 4 F 10 , and C 5 F 12 , and these products also have certain insulation performance.The decomposition process is calculated using density functional theory (DFT), and the insulating property of several kinds of decomposition products is analyzed by calculating the ionization energy of the molecules, affinity energies, and molecular orbital gaps.The calculation is carried out by Dmol3 in the material studio (MS) computing software.The calculation method of the generalized gradient approximation (GGA) is selected and is based on the optimization of the local density approximation (LDA) functional; thus, GGA has higher accuracy than LDA.The GGA method and the Perdew, Burke, and Enzerhof (PBE) function are used to deal with the electron exchange interaction [17].The function fitting d and p orbital polarization are used in the atomic orbital calculations through the p polarization dual numerical basis set (Double Numerical Set Plus Polarization Functions, DNP).The energy convergence precision, atomic migration, and energy gradient are set to 1.0 × 10 −5 , 0.005, and 0.002 Ha, respectively, and the convergence precision of charge density is 1.0 × 10 −6 Ha [18].The Direct Inversion of Iterative Subspace (DIIS), which is used to improve the convergence rate of self-consistent field charge density, can be used to improve efficiency.

Formation of Decomposition Products
First, the molecular structure of C 6 F 12 O and several decomposition products are optimized to obtain the lowest stable state of molecular energy.To conveniently express the bond lengths and angles of each molecule, the structure of each molecule and the labeling of C atoms are presented in Figure 5.The optimized bond lengths and angles are shown in Table 1.The results of the references are listed in the table.By contrast, the results of C 3 F 8 , C 3 F 6 , C 2 F 6 , and CF 4 are compared with the references, and the validity and accuracy of the model are verified.

Formation of Decomposition Products
First, the molecular structure of C6F12O and several decomposition products are optimized to obtain the lowest stable state of molecular energy.To conveniently express the bond lengths and angles of each molecule, the structure of each molecule and the labeling of C atoms are presented in Figure 5.The optimized bond lengths and angles are shown in Table 1.The results of the references are listed in the table.By contrast, the results of C3F8, C3F6, C2F6, and CF4 are compared with the references, and the validity and accuracy of the model are verified.The structure of the C 6 F O is shown in Figure 6, and ( 1)-( 4) indicates the position of bond breaking of C-C bond in the molecule.
Energies 2017, 10, 1170 The structure of the C6F O is shown in Figure 6, and ( 1)-( 4) indicates the position of bond breaking of C-C bond in the molecule.Calculated by DFT, the four broken bond process and energy changes are as follows: (1 By calculating the energy of four C-C bonds, the absorbed energy in the four processes is found to be similar, that is, when the external energy is sufficiently large, the probability of the existence of several free radicals is similar.The free radicals, such as CF3•, C2F5• and C3F7•, can produce F atoms by bond breaking reaction and form relatively stable fluorocarbon compounds, such as CF4, C2F6, C3F8, C3F6, C4F10, and C5F12, between the radical combination process, as shown in Figure 7. CF3• can be derived from the C1-C2 breaking and the C4-C5 or the C4-C6 breaking (shown in Figure 5).Calculated by DFT, the four broken bond process and energy changes are as follows: By calculating the energy of four C-C bonds, the absorbed energy in the four processes is found to be similar, that is, when the external energy is sufficiently large, the probability of the existence of several free radicals is similar.The free radicals, such as CF 3 •, C 2 F 5 • and C 3 F 7 •, can produce F atoms by bond breaking reaction and form relatively stable fluorocarbon compounds, such as CF 4 , C 2 F 6 , C 3 F 8 , C 3 F 6 , C 4 F 10 , and C 5 F 12 , between the radical combination process, as shown in Figure 7. CF 3 • can be derived from the C 1 -C 2 breaking and the C 4 -C 5 or the C 4 -C 6 breaking (shown in Figure 5).CF3 can continue to form CF2 by breaking C-F, and the calculation in this paper is consistent with the literature [19] (the result in the literature is 84.92 kcal/mol).CF2 can continue to eliminate F and eventually form a C atom; thus, a significant increase in C elements on the surface of the electrode is observed.The process must absorb the energy of 234.768 kcal/mol.
The numbers ①-⑩ in Figure 7 indicate the reactions and the related reactions is shown in Table 2. C2F6 can be formed in two means, and the bond of the two free radicals, CF3 or C2F5 and F, can also be formed.The formation of C2F6 has two paths, as shown in the reaction ③ and ⑤.The formation of C3F8 also has two paths, as shown in the reaction ⑦ and ⑧ in Figure 7.The formation of C3F6 requires C3F7• to eliminate F, and requires the C-C single bond to form the C=C double bond.Table 2 presents the main forming process of fluorocarbon, and the barrier energy changes, and the generation process.The energy calculation includes the zero-point vibrational energy (ZPVE) correction value and enthalpy correction.Table 2 shows the C-C and C-F bond breaking of C6F12O to form the free radicals CF3, C2F5, and C3F7, and the F atoms, whereas the free combination reaction to form stable fluorocarbon.In addition to the production process of C3F6, the other decomposition products generated include an exothermic reaction.The change in energy is the difference between the energy of the reactants.The  The numbers 1 -10 in Figure 7 indicate the reactions and the related reactions is shown in Table 2. C 2 F 6 can be formed in two means, and the bond of the two free radicals, CF 3 or C 2 F 5 and F, can also be formed.The formation of C 2 F 6 has two paths, as shown in the reaction 3 and 5 .The formation of C 3 F 8 also has two paths, as shown in the reaction 7 and 8 in Figure 7.The formation of C 3 F 6 requires C 3 F 7 • to eliminate F, and requires the C-C single bond to form the C=C double bond.Table 2 presents the main forming process of fluorocarbon, and the barrier energy changes, and the generation process.The energy calculation includes the zero-point vibrational energy (ZPVE) correction value and enthalpy correction.Table 2 shows the C-C and C-F bond breaking of C 6 F 12 O to form the free radicals CF 3 , C 2 F 5 , and C 3 F 7 , and the F atoms, whereas the free combination reaction to form stable fluorocarbon.In addition to the production process of C 3 F 6 , the other decomposition products generated include an exothermic reaction.The change in energy is the difference between the energy of the reactants.The calculation is carried out in 0 K; thus, the temperature correction is introduced at a normal temperature (298 K).The Energies 2017, 10, 1170 9 of 11 formation process of C 3 F 6 not only includes the breaking of chemical bonds, but also the formation of chemical bonds.With the transition state search (TS search) activation energy calculation of the process, the energy of 61.135 kcal/mol must be absorbed to generate C 3 F 6 , and the reaction requires endothermic 54.296 kcal/mol to break the C-F bonds in C 3 F 7 .The formation processes of CF 4 , C 2 F 6 , C 3 F 8 , C 4 F 10 , and C 5 F 12 include simple bonding processes, compared with C 3 F 6 .

Basic Properties of C 6 F 12 O and Decomposition Products
To analyze the basic properties of C 6 F 12 O and the decomposition products of the molecule, DFT is used to calculate the adiabatic ionization energy of molecules E ion , the adiabatic affinity energy E aff , and the electron orbit distribution.The ionization and affinity energies are calculated by the following: where X is a molecule, and X-is a negative ion.X+ is a positive ion.E ion represents the energy required for the loss of electrons and the ability of the gas molecule to bind electrons.E aff is the energy released by the molecules adsorbed on the molecule and the ability of the molecule to adsorb electrons.The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are generally referred to as frontier orbitals.The most active electrons in the electron chemical reaction in the frontline are the core of the chemical reaction.The molecular orbital energy gap E O is used to describe the molecular activity.
where E HOMO is the energy of the highest occupied molecular orbital, and E LUMO is the energy of the lowest unoccupied molecular orbital.E O indicates that the electrons in the molecule must be involved in the chemical reaction, which is required to overcome the orbital energy, and in the ability to participate in the chemical reaction.More stable molecules can bind electrons to relatively low levels of energy, and electrons are not easily ionized away from the orbital.Several decomposition products are calculated using this method, and the comparative results are shown in Table 3.The vertical affinity energy of SF 6 is 0.438 eV, which is consistent with the measured value (0.32 ± 0.15 eV) in the literature [24].Table 3 shows that the ionization energy of the formation is relatively high; thus, the gas molecules have a strong ability to bind and to not easily lose electrons.The process of adsorption electrons of CF 4 , C 2 F 6 , C 3 F 8 , C 4 F 10 , and C 5 F 12 need absorb energy, whereas the adsorption electrons of C 3 F 6 need release energy, and these energy values are relatively small.From the view of a molecular orbital gap, the difference between various molecules is large.The comprehensive evaluation reveals that the decomposition products after the breakdown of C 6 F 12 O with larger ionization energy have a certain degree of electronic adsorption capacity.Table 3 shows that the ionization energy of the products, which is more than 10 eV, is large.The insulation strength of CF 4 gas is approximately 39% of SF 6 , and the insulation performance of C 2 F 6 is approximately 80% of SF 6 .The insulation performance of SF 6 is close to C 3 F 8 , whereas C 4 F 10 has insulation properties that are even better than SF 6 .The liquefaction temperature of the decomposition products is lower than that of C 6 F 12 O, and the concentration of the product is much lower than that of C 6 F 12 O.The main products are Perfluorinated compounds (PFCs), which impact the environment [29].However, the GWP of the product is less than that of SF 6 .Thus, the product does not damage the environment.

C 6 F
12 O gas is liquid at room temperature because its liquefaction temperature is 49 • C. The saturated vapor pressure of the gas satisfies the Antoine equation, and the saturated vapor pressure of C 6 F 12 O is 40 kPa at 25 • C [13].The Antoine equation is used to obtain the relation between saturated vapor pressure and the liquefaction temperature of C 6 F 12 O: lgp = −3719.7385/T+ 18.180 (1)

Figure 3 .
Figure 3. Detection results of the surface elements of the sphere electrode.

Figure 2 .
Figure 2. Breakdown voltage of the C 6 F 12 O/N 2 mixed gas.

Figure 3 .
Figure 3. Detection results of the surface elements of the sphere electrode.Figure 3. Detection results of the surface elements of the sphere electrode.

Figure 3 .
Figure 3. Detection results of the surface elements of the sphere electrode.Figure 3. Detection results of the surface elements of the sphere electrode.

Figure 4 .
Figure 4. Test results of C 6 F 12 O/N 2 mixed gas decomposition components.

Figure 6 .
Figure 6.Schematic of the C 6 F 12 O bond breaking.

Figure 7 .
Figure 7. Schematic of the decomposition process.

Figure 7 .
Figure 7. Schematic of the decomposition process.

CF 3
can continue to form CF 2 by breaking C-F, and the calculation in this paper is consistent with the literature [19] (the result in the literature is 84.92 kcal/mol).CF 2 can continue to eliminate F and eventually form a C atom; thus, a significant increase in C elements on the surface of the electrode is observed.The process must absorb the energy of 234.768 kcal/mol.

Table 1 .
Bond lengths and angles.

Table 1 .
Bond lengths and angles.

Table 2 .
Energy and barrier of decomposition product.

Table 2 .
Energy and barrier of decomposition product.

Table 3 .
Parameter comparison of C 6 F 12 O decomposition products.