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Study on the thermal interaction mechanism between C4F7N-N2 and copper, aluminum
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Study on the thermal interaction mechanism between C4F7N-N2 and copper, aluminum Yi Lia, Xiaoxing Zhanga,b, , Qi Chena, Ji Zhanga, Dachang Chena, Zhaolun Cuia, Song Xiaoa, , Ju Tanga ⁎
a b
⁎
School of Electrical Engineering and Automation, Wuhan University, Wuhan 430072, China State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China
ARTICLE INFO
ABSTRACT
Keywords: C4F7N-N2 Copper Aluminum Interaction SF6 alternative gas
The thermal interaction mechanism between C4F7N (fluorinated nitrile)-N2 gas mixture using as substitute to SF6 with metal was investigated. We found that interaction between C4F7N-N2 and copper leads to surface corrosion or gas decomposition with interface temperature at 120℃-220℃. The oxidation of copper and accumulation of F, CFx and CN can be found and gaseous by-products such as C3F6, CF3H, C3F6O is produced. C4F7N-N2 has better compatibility with aluminum than that of copper due to the existence of alumina. The interaction between C4F7N and alumina surface has the lowest adsorption energy, charge transfer compared with that of copper and aluminum.
1. Introduction Sulphur hexafluoride (SF6) is suitable for gaseous insulation due to its high dielectric strength, long-term stability, non-toxicity and great arc extinguishing properties. It has been used as insulation and arcextinguishing medium for various gas insulated equipment (GIE) such as gas insulated switchgear (GIS), gas insulated cable (GIC), and gas insulated line (GIL) since the early 1960s [1]. Nevertheless, SF6 shows a high global warming potential (GWP) of 23,500 and an atmospheric lifetime around 3200 years according to the Intergovernmental Panel on Climate Change (IPCC) 5th report [2]. Actually, electrical industry is the largest consumer of SF6 and about 10,000 tons were implemented yearly in all kinds of GIE [3]. Several countries have applied SF6 tax to reduce its emission and minimize the environmental impact nowadays. For example, Slovenia imposed a value of 15€/kg of SF6 pre-charged and 300€/kg of SF6 used during maintenance. Spain also imposed duties on SF6 for 66€/kg in 2015, then 100€/kg for SF6 used during service [3]. Therefore, seeking for SF6-free solutions for GIE has become a hot topic over the past decades. More recently, a new compound, C4F7N (fluorinated nitrile), is developed specifically for a SF6-free solution to high-voltage (HV) applications. The GWP of C4F7N is only 2090 and its atmospheric lifetime is 35 years [4,5]. The dielectric withstand strength of pure C4F7N is roughly twice as high as that of SF6. Due to the high liquefaction temperature (-4.7℃), it is necessary to mix C4F7N with CO2, N2, O2 or
⁎
air to meet the working pressure and liquefaction temperature requirements of equipment. Researches on the dielectric properties [6,7], switching (interrupting) performance [8,9], decomposition characteristics [10–12] and toxicological tests [13,14] of C4F7N gas mixture have been done over the past three years, which confirm that C4F7N gas mixture is suitable for most common performance and ambient conditions. As to engineering application, material compatibility between gas insulating medium and materials used in the equipment should be considered. Different components of the gas mixture themselves need to be chemically stable and should not has negative influence on the materials used inside the equipment. In addition, there might be aging processes caused by the combination of the used materials and one or more components of the gas insulating medium. The interaction might lead to changed properties of the material itself, but can also influence the quality of the gas mixture. Therefore, it is necessary to evaluate the impact of all materials in contact with gas insulating medium before engineering application. For C4F7N gas mixture, F. Kessler et al. pointed out that more than one hundred different materials were used in latest GIL installations, which can be classified into five groups as follows: metals and alloys, insulators and thermoplasts, lubricants, elastomers, desiccants. They built high pressure lab-scale autoclaves made of stainless steel and heated them up to a certain temperature in the range of 75–225 ℃ to conduct accelerated aging tests for different fluoro-organic compounds
Corresponding authors at: School of Electrical Engineering, Wuhan University, BaYi Street No.299, Wuhan, Hubei Province, PR China. E-mail addresses: [email protected] (X. Zhang), [email protected] (S. Xiao).
https://doi.org/10.1016/j.corsci.2019.03.031 Received 7 December 2018; Received in revised form 13 March 2019; Accepted 13 March 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yi Li, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.03.031
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monitoring and control system. The experimental gas chamber (as shown in Fig. 2) is made of 304 L stainless steel with a volume of 2 L to simulate the high pressure environment in real equipment. The lab-scale autoclaves and sample holders used in literature [15] were also made of stainless steel, and it was pointed out that there exists no interaction between the stainless steel and C4F7N after heated up the autoclaves and sample holders to a certain temperature in the range of 75–225 °C for 4 weeks. Thus, the potential interaction between the steel case of the experimental gas chamber and the gas mixture can be excluded. The gas-solid interface interaction simulation system is mainly composed of heating element and metallic bushing. The K-type thermocouple is used as the heating element and the metallic bushing is in close contact with it to ensure the generated heat could evenly transmitted to the outer surface. The metallic bushing is made of pure copper or aluminum supplied by China Metallurgical Research Institute with the purity of 99.999% according to the experimental requirements, and its outer surface serves as the gas-solid reaction interface. Two metallic sheets (0.2mm*5mm*100 mm, made of pure copper or aluminum) bundled on the surface of metallic bushing could participate in gas-solid interface reactions, which are used for further structural characterization and analysis. The temperature monitoring and control system is mainly composed of the temperature sensor, the proportional-integral-differential (PID) controller, the switching power supply and the solid-state relay. The temperature sensor installed on the surface of metallic bushing is used to monitor its actual temperature. The PID controller could control the solid-state relay according to the signal of temperature sensor, and then realizes the on-off control of the switching power supply to ensure the surface temperature is maintained at a stable set value. The metallic bushing and the metallic sheets were cleaned to remove the oil stain on their surface before the test and then installed. After vacuuming and filling the gas chamber with high-purity N2 for three times, the C4F7N-N2 gas mixture is finally injected to carry out relevant experiments. Moreover, the concentration of C4F7N in the gas mixture is usually lower than 10% to meet the requirements of gas pressure in GIE and liquefaction temperature for engineering application [21]. Therefore, the 10%C4F7N-90%N2 gas mixture is selected in this paper. At the same time, considering the current-carrying metallic conductor in the equipment has a temperature rise effect of about 70℃ normally, the surface temperature in this paper is set to 120℃ 170℃ and 220℃. The experimental process for each group lasted for 40 h. The gas mixture after each test was collected and analyzed using the gas chromatography-mass spectrometry (GC–MS, Shimadzu QP2010 Ultra) with the column CP-Sil5CB (60m*8um*0.32 mm). The SCAN method is conducted to obtain relevant gas components data. The heating procedure for GC is listed as follows: keeping the temperature at 32℃ for 10 min, then heating up to 150℃ at the rate of 60℃/min and retaining for 2 min. The morphology of the metallic sheet was recorded by digital camera and optical microscope. The surface microstructure and element composition of the metallic sheet were also analyzed by field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS).
Fig. 1. Structure of heating reactor system.
Fig. 2. Structure of the experimental gas chamber.
which were mixed with N2 or CO2 as carrier gas. It was pointed out that for the gas mixture containing 10.1% C4F7N for −10 °C application, its lifetime in presence of metals has been estimated to 32 years [15]. Our team has explored the interaction mechanism between C4F7N and copper, aluminum, silver theoretically. It is found that the interaction between C4F7N gas and silver belongs to physisorption and the N atom in C4F7N shows strong interaction with copper and aluminum [16,17]. Generally, compartments and current-carrying metallic conductor of gas insulated switchgear are mainly produced of metal such as copper, aluminum [18]. In this paper, we tested the thermal stability of C4F7N-N2 gas mixture associated with copper, aluminum at different surface temperatures firstly. The impact of the interaction on the gas insulating mixture and metal materials were revealed in detail. Then the thermal interaction mechanism between C4F7N and copper, aluminum, alumina surface was revealed based on the density functional theory (DFT). The adsorption energy, charge transfer as well as the density of states (DOS) of the interaction system were obtained and compared. Relevant results provide important guidance for the manufacture of GIE using C4F7N mixture as the gas insulating medium and revealed the thermal stability of C4F7N-N2 mixture comprehensively. 2. Methods
2.2. Theoretical methods
2.1. Experimental methods
2.2.1. Computational methods In order to explore the interaction mechanism between C4F7N and copper, aluminum as well as alumina, we conducted density functional theory (DFT) calculations for C4F7N-metallic interface using Dmol3
The structure of the employed aging test device is given in Fig. 1. The platform is mainly composed of experimental gas chamber, gassolid interface interaction simulation system, and temperature
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Fig. 3. Relaxed geometric structure and possible adsorption sits of Cu (1 0 0), Al (1 0 0) andα-Al2O3(0 0 0 1) surface.
module in Material Studio. We used Perdew-Burke-Ernzerhof function (PBE) with generalized gradient approximation (GGA) to deal with the electron exchange and correlation [22]. The double numerical plus polarization (DNP) was chosen as the atomic orbital basis set and DFT semi-core pseudopotential (DSSP) method was also considered to conduct the relativistic effect correction [23]. In order to describe the van der Waals (VDW) interactions accurately, the Grimme’s scheme was employed [24]. The k-point sample of Monkhorst-Pack grid was set to 4 × 4×1 and the global orbital cut-off radius was implemented to 4.5 Å [25]. All the spin polarized calculations were performed with the tolerances of 1 × 10−5 Ha on energy, 2 × 10−3 Ha/Å on gradient, and 5 × 10−3 Å displacement on convergence, respectively. It should be noted that the numerical basis sets implemented in DMol3 code are more complete than the traditional Gaussian functions, thereby minimizing or even eliminating basis set superposition error (BSSE) [26,27]. The adsorption energy of the gas-metallic interaction system was defined as follows,
Ead = Emetal + EC4F7N-EC4F7N-metal Fig. 4. Relaxed geometric structure of C4F7N.
(1)
where Emetal and EC4F7N represents the energy of C4F7N molecule and
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2.2.2. Surface models The calculated lattice parameter of Cu, Al and α-Al2O3 bulk structure is 3.61 Å, 4.05 Å and 4.76 Å, respectively, which consists well with previous results [28–30]. Then the Cu (1 0 0) and Al (1 0 0) surface were built with a six layered 3 × 3×1 supercell separated by 20 Å vacuum slab. Three possible adsorption sites for Cu (1 0 0) and Al (1 0 0), listed as Top site, Bridge site and Hcp site (as shown in Fig. 3(a)), were considered. As for alumina, a 2 × 2 × 1 supercell of the α-Al2O3(0 0 0 1) surface separated by 20 Å vacuum slab was constructed. Fig. 3(c) demonstrates the five possible adsorption sites of the α-Al2O3(0 0 0 1) surface. Fig. 4 shows the optimized structure of C4F7N, which includes three characteristic group, listed as CN, F and CF3. In order to explore the interaction mechanism of C4F7N-metal surface comprehensively, we set nine initial adsorption structures for Cu (1 0 0) and Al (1 0 0) surface with the CN, CF3, F group located at the Top, Bridge and Hcp site of metal surface, respectively. Similarly, fifteen initial adsorption structures were conducted for α-Al2O3(0 0 0 1) surface. All the initial adsorption distance of C4F7N and the metal surface is set to 1.5 Å. Finally, geometric optimization calculations were performed and relevant electronic properties including the total charge transfer, partial densities of states (PDOS) of the system were obtained. 3. Results and discussion 3.1. Interaction between C4F7N-N2 and copper 3.1.1. Morphology characterization When heated metallic surface is exposed to C4F7N-N2 gas mixture environment, its surface color may be altered as a result of undergoing chemical reactions. Fig. 5 demonstrates the images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. It is apparent that the surface color of untreated copper is light orange with uniform color distribution. Optical microscope also shows clear veins on the copper sheet and no corrosive point exists. After contacted with C4F7N-N2 gas mixture for 40 h at 120℃, the surface color of the copper deepens from light orange to dark orange obviously, indicating that the metal surface has been corroded. Optical microscopy shows that the color distribution on the copper surface becomes nonuniform. Copper bushing at 170 ℃ exposed to C4F7N-N2 gas mixture lead to be further deepened surface color, and some area changes to purple even brown. Further magnification observation shows that its color becomes purple-red with uniformly distribution, indicating the corrosion is further deepened. As to the C4F7N-N2 gas mixture exposed to the copper bushing at 220℃, interaction between them results in dark purple or golden surface color change. Optical microscopy also shows that the original texture structure of the copper sheet has been destroyed with the golden corrosion layer evenly distributed. Overall, the optical characterization results show that there exists incompatibility between C4F7N-N2 gas mixture and heated copper. The increase of interface temperature leads to color modification from pale orange to deep orange, then purple (brown) and dark purple or golden due to the corrosion process. The morphology of the copper surface is also destroyed under high temperature condition. SEM detection can further reveal the influence of C4F7N-N2 gas mixture on the surface morphology of copper. According to the test results given in Fig. 6, we can find that the surface of untreated copper
Fig. 5. Photo images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
metal surface, respectively. EC4F7N-metal denotes the total energy of the gas-metallic system after interaction. The positive value of Ead indicates that the interaction process belongs to exothermic process. The negative value of Ead means that the interaction is an endothermic process. The Hirshfeld (HI) charge analysis was chosen to understand the electron transfer mechanism of the gas-metallic system. The total charge transfer is defined as follows,
Qt = QC4F7N
QC4F7N
metal
(2)
where QC4F7N and QC4F7N metal denotes the total charge of the gas molecule before and after interaction. The positive value of Qt indicates that C4F7N molecule gains electrons from the metal surface during the interaction process and the metal surface acts as the electron donor. While the negative value of Qt means C4F7N molecule loses electrons after interaction.
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Fig. 6. SEM images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures (Mag = 5KX).
(Cu2+) [22]. The Cu spectra of all samples treated with C4F7N-N2 gas mixture shows no obvious change. High-resolution spectra of O1s shows one peak located at 530.78 eV, which is attributed to the CuO component. The other peak located at 531.93 eV is assigned to the C = O component [23]. The fluorine component is not detected for the untreated copper. While the F1s peck located at 689.05 eV can be found for all the treated samples, indicating that the reaction brings fluorine to the copper surface. Moreover, the other peak located at 685.03 eV can be observed at 220℃, corresponding to the characteristic peak of CuF2 [24]. Thus, serious corrosion has happened at this temperature. The high resolution spectra of C1s shows one peak located at 284.84 eV belongs to the traditionally accepted position for aliphatic C. As to the sample treated at 120℃, the other peak centered around 288.48 eV is assigned to the COO component. The observation of the CN component (located at 285.94 eV) and C–F component indicates that the reaction between C4F7N-N2 gas mixture and copper at 170 ℃ and 220℃ generates CFx and CN particles on the metal surface. Above all, the reaction between C4F7N-N2 gas mixture and copper changes the element composition of metal surface to a certain extent. The oxidation of copper and accumulation of F, CFx and CN particles on the surface can be confirmed, indicating that the reaction between C4F7N-N2 gas mixture and copper will lead to metal corrosion, which consists well with the morphology analysis results.
is clear and no corrosion point exists. As for the copper heated to 120℃, its surface morphology does not change significantly with little corrosion point can be found. Thermal stress with heating mantle temperatures at 170℃ produces a small amount of granular corrosion spots on the copper surface after interaction. When the interface temperature reaches to 220℃, a large number of corrosive particles are formed and accumulated on the metal surface and the intrinsic veins of copper are obviously weakened, indicating that the thicker corrosion layer is formed, which is consistent well with the optical characterization results. According to the high-resolution image given in Fig. 6(d), we can find that the structure of corrosive particles shows irregularity and the accumulation effect exists among them.
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3.1.2. XPS analysis The elemental composition of the samples surface XPS analysis were investigated by XPS. Fig. 7 demonstrates the high-resolution XPS spectrum of copper before and after exposed to C4F7N-N2 gas mixture at different temperatures. All the peaks were fitted based on XPS Peak software employing Gauss-Lorentz curves after subtraction of a Shirleytype background [19–21]. According to the high-resolution spectra of Cu2p given in Fig. 7(a), the main peaks are observed at 932.72 eV, 934.96 eV, 952.57 eV and 954.21 eV. The 932.72 eV and 952.57 eV components are assigned to the Cu2p3/2 and Cu2p1/2, respectively, while the other two components located at higher energy positions belong to the CuO2p3/2 and CuO2p1/
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Fig. 7. XPS spectrum of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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3.1.3. Gas composition analysis Fig. 8 gives the gas chromatogram of the C4F7N-N2 gas mixture after aging test for 40 h at different temperatures. It is apparent that the interaction between the C4F7N-N2 gas mixture and copper at 120 °C does not cause significant change to the gas mixture composition (only a very low content of C3F6 can be detected). With the increase of interface temperature, the C4F7N-N2 gas mixture produced CF3H, C3F6O and C3F6 after 40 h aging test. The generation of CF3H and C3F6O may related to trace water inside the metallic material. The peak area of C3F6, CF3H and C3F6O is significantly increased at 220℃, indicating that the content of the above decomposition by-products is greatly increased. Moreover, literature [5] tested the thermal stability of C4F7N gas mixture and pointed out that the decomposition of C4F7N gas mixture occurs at 650℃. While the decomposition of C4F7N-N2 gas mixture can be confirmed when the interface temperature reaches to 170℃ according to the above test results, indicating that the presence of copper could catalyze the decomposition of C4F7N. Therefore, the long-term interaction between C4F7N-N2 gas mixture and copper at high-temperature will cause the decomposition of gas mixture and produce by-products such as C3F6. The gas-solid interface reaction changes the composition of gas insulating medium, which is unfavorable for the operation safety and service life of the equipment to a certain extent.
Fig. 8. Gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h (Copper).
3.2. Interaction between C4F7N-N2 and aluminum 3.2.1. Morphology characterization Fig. 9 reveals the images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. The color and texture distribution of the untreated aluminum is uniform and no corrosion spot exists on the surface according to the optical microscopy. The color of aluminum surface did not change significantly, which still keeps bright after interacted with C4F7N-N2 gas mixture at 120℃, 170℃ and 220℃ for 40 h. Optical microscopy also shows that the texture of aluminum surface is clear and no corrosion layer can be observed. Fig. 10 demonstrates the SEM images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. It is apparent that the surface morphology of the aluminum before and after interaction does not change significantly, and no corrosion point is produced under various temperature conditions, which is consistent well with the optical characterization results. Therefore, the interaction between C4F7N-N2 gas mixture and the heated aluminum does not change the macroscopic and microscopic morphology of the metal surface. 3.2.2. XPS analysis Fig. 11 shows the XPS high-resolution spectrum of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. The Al2p spectrum of all the samples contains two components centered around 72.32 eV and 74.76 eV, which are assigned to Al and Al2O3. The primary peak of O1s located at 531.82 eV belongs to Al2O3. The other peak centered around 533.46 eV which originates from COO component can be observed. As to the high-resolution XPS spectra of F1s, the primary peak located at 685.78 eV existed for all the treated
Fig. 9. Photo images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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Fig. 10. SEM images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures (Mag = 5KX).
samples, indicating that the reaction process causes fluorine deposition on the aluminum surface. According to the high-resolution XPS spectra of C1s, the traditional position for aliphatic C and COO components is located at 284.86 eV and 288.92 eV, respectively. In addition, the observation of the CN (located at 286.42 eV) and CF3 (located at 293.49 eV) components indicates the reaction between C4F7N-N2 gas mixture and aluminum at 170℃ and 220 ℃ causes the decomposition of C4F7N. Overall, the surface analysis presented above confirms that the reaction between C4F7N-N2 gas mixture and aluminum introduces some species such as F, CN and CF3 on the aluminum surface, and relevant process is accelerated at 170℃ and 220 ℃.
existence of trace water, which may come from the inorganic hydrated oxide on the aluminum surface. 4. Discussion 4.1. Interfacial reaction mechanism According to the above test results, the compatibility of C4F7N-N2 gas mixture with copper and aluminum widely used in the equipment is different. Actually, the molecular structure of C4F7N is complex and may not as inert as SF6. When it is in contact with heated copper or aluminum, the reactive portion and the weak chemical bond in the molecular structure will chemically react with the metal surface. Generally, aluminum is more active than copper, which can be oxidized and form a dense protective film alumina (Al2O3) easily. In order to understand the interaction mechanism between metal surface and C4F7N, we carried out DFT calculations. Table 1 gives the adsorption energy and total charge transfer of C4F7N interacted with Cu(1 0 0) based on different initial structures. The interaction process is exothermic due to the positive value of
3.2.3. Gas composition analysis Fig. 12 gives gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h. It can be found the decomposition of C4F7N-N2 gas mixture does not occur when the interface temperature is lower than 170℃. While the characteristic peak of CF3H, C2F5H and C3F6 can be detected at 220℃, indicating that C4F7N-N2 gas mixture is partially decomposed. The generation of CF3H, C2F5H at 220℃ is related to the
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Fig. 11. XPS spectrum of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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changes to 1.935 Å and the Cu4 s, 3d orbitals mainly hybridize with the N2 s, 2p orbitals near −5 eV˜−7 eV, −9 eV according to the PDOS. The interaction between C4F7N and Cu (1 0 0) with CF3-Bridge initial structure has the adsorption energy of 0.844 eV with 0.516e electrons transfer. The interaction dramatically changed the molecule structure of C4F7N, resulting in one of the F atoms in CF3 group and C atom possibly form new bonds with Cu atoms. Orbital overlaps between Cu4 s, 3d and F2 s, 2p can be seen near −4eV˜−6 eV and hybridizations happens between C2 s, 2p orbitals and Cu4 s, 3d orbitals in the range of −8 eV˜-−10 eV. Overall, the interaction between C4F7N and Cu (1 0 0) belongs to chemical adsorption with new formed chemical bonds. The accumulation of F, CFx and CN particles on the surface can be explained according to the above calculation results. That is to say, C4F7N molecule could react with the copper surface, undergoing exothermic chemical adsorption process and causing the decomposition of C4F7N as well as element composition change of copper surface. It should be noted that in our previous study, the initial interaction distance between C4F7N molecule and copper surface was set to 3 Å (larger than 1.5 Å in this paper), and the obtained geometric optimization results show that the F and CF3 group in C4F7N has the tendency to keep away from the Cu atoms [12]. That is to say, only the gas molecules near the copper surface can undergo the chemical adsorption process. Table 2 gives the adsorption energy and total charge transfer of C4F7N interacted with Al(1 0 0). Similar to that of Cu (1 0 0), the interaction process between them belongs to exothermic process. The highest adsorption energy reaches to 3.473 eV (CF3-Bridge initial structure), which is higher than that of Cu (1 0 0), indicating that the interaction between C4F7N and Al (1 0 0) is stronger. According to the most energetically favorable structure given in Fig. 14(a), two of the F atoms in CF3 group bonds with Al atoms after interaction with the distance of 1.845 Å, 1.892 Å, respectively. The distance between C atom and Al (1 0 0) surface changed to 2.079 Å. There also exists 0.732e electrons transfer during the interaction process. Overlaps between Al3s, 3p and F2 s, 2p as well as C2 s, 2p orbitals in the range of −5 eV˜10 eV can been seen according to the PDOS. Thus, the interaction process falls into the chemical adsorption with new chemical bond formed between C, F and Al atoms. As to the C4F7N interacted with Al (1 0 0) with F-Hcp initial structure, the adsorption energy is 3.39 eV and 0.577e electrons transfers from the Al (1 0 0) to C4F7N molecule. The F atom dissociates from C4F7N and bonds with the Al (1 0 0) surface. The distance between N atom and Al (1 0 0) surface changed to 1.865 Å. According to the PDOS calculation results, the Al3s, 3p orbitals mainly overlaps with F2 s, 2p orbitals in the range of −7 eV˜−9 eV and hybridization between N atom and Al atoms can also be found, indicating that the interaction changes the orbitals structures of the gas and metal. The interaction between C4F7N and Al (1 0 0) surface with N-Bridge initial structure has the adsorption energy of 1.022 eV with 0.385e electron transfer. The distance between N atom and Al atoms becomes to 1.834 Å, 2.042 Å after interaction. Obvious orbital hybridizations could
Fig. 12. Gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h (aluminum).
Table 1 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Cu(1 0 0). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Top CF3-Bridge CF3-Hcp F-Top F-Bridge F-Hcp N-Top N-Bridge N-Hcp
0.432 0.844 0.464 0.573 2.467 0.569 0.967 0.962 0.965
−0.004 0.516 −0.011 0.002 0.612 0.011 0.038 0.039 0.039
adsorption energy. The most energetically favorable interaction structures can be obtained by comparing the adsorption energy, and Fig. 13 shows their final adsorption configurations. The adsorption of C4F7N on Cu (1 0 0) with F-Bridge initial structure exhibits the largest adsorption energy (2.467 eV) and 0.612e electrons transfers from Cu (0 0 1) surface to C4F7N gas molecule. The F atom in C4F7N dissociates and adsorbs on Cu (1 0 0) surface after interaction. The possibly new formed chemical bonds between the F atom and two Cu atoms with the bond length of 1.988 Å and 1.971 Å can be found. In addition, the distance between N atom and Cu atom shortens to 2.082 Å and 1.951 Å after interaction. According to the PDOS of the interaction system given in Fig. 13(b), overlaps between the orbitals of F atom and Cu atoms could be found near -4eV˜6 eV, -1.5 eV, indicating that the interaction between them changed the molecular orbitals. The 4 s orbitals of Cu atom and 2p, 2 s orbitals of N atom also obviously overlaps at −3 eV and −8 eV˜−9 eV, which confirms the new formed chemical bond between N atom and Cu atoms. As to the adsorption of C4F7N on Cu (1 0 0) with N-Top initial structure, there exists 0.967 eV adsorption energy with 0.0385e electrons transfer. The distance between N atom and Cu (1 0 0)
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Fig. 13. The most energetically favorable structure and PDOS of C4F7N interacted with Cu (1 0 0).
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dissociation of C4F7N, generating new chemical bonds between F, C, N and Cu, Al atoms. The new formed chemical bonds between the gasmetallic systems could result in metal corrosion and decomposition of C4F7N gas mixture at macro level. While the existence of alumina, which has greater compatibility with C4F7N, could prevent the metal corrosion and gas decomposition to a certain extent.
Table 2 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Al (1 0 0). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Top CF3-Bridge CF3-Hcp F-Top F-Bridge F-Hcp N-Top N-Bridge N-Hcp
0.619 3.473 1.736 3.349 3.378 3.390 0.440 1.022 0.301
0.628 0.732 0.490 0.594 0.579 0.577 0.072 0.385 0.010
4.2. Application potential of C4F7N-N2 gas mixture According to the above results, C4F7N-N2 gas mixture is incompatible with copper under certain conditions, while it has great compatibility with aluminum at 120-170℃. When the surface temperature of copper is lower than 120℃, the interaction between C4F7NN2 gas mixture and copper does not cause the decomposition of gas mixture. As for engineering application, the conductor temperature is generally in the range of 100℃-115℃ considering the temperature rise effect of the current carrying conductor inside the equipment during normal operation. Moreover, when the current-carrying conductor is locally overheated due to several faults such as poor contact inside the device, C4F7N-N2 gas mixture could react with copper, causing decomposition of gas mixture and corrosion of copper, which will pose a threat to the operation safety and service life of the equipment. Therefore, it is necessary to strengthen the internal temperature monitoring for equipment with C4F7N-N2 gas mixture in engineering application to avoid overheating failure. At the same time, some anticorrosion treatment should also be applied on the metal surface during the manufacturing process.
be seen between Al3s, 3p and F2 s, 2p as well as C2 s, 2p orbitals around −11.5 eV, −10.5 eV, −9.5 eV, −6 eV˜−8 eV. Above all, interaction between C4F7N and Al (1 0 0) surface results in chemical adsorption process with new bonds formed as well as decomposition of C4F7N. Table 3 gives the adsorption properties of C4F7N interacted with Al2O3 (0 0 0 1). It can be found that the highest adsorption energy among all the initial configurations reaches to 1.078 eV, which is the smallest value compared with that of Cu (1 0 0) and Al (1 0 0). That is to say, alumina shows greater compatibility with C4F7N than that of copper and aluminum. In addition, the charge transfer feature of C4F7N interacted with Al2O3 (0 0 0 1) during the interaction is different from that of Cu (1 0 0) and Al (1 0 0). C4F7N molecule acts as the electrons donors and transfer electrons to Al2O3 (0 0 0 1) surface after interaction. As to the N-O(1)-Top initial structure, the distance between N atom in C4F7N molecule and Al atom in Al2O3 (0 0 0 1) surface changes to 2.031 Å after interaction and the structure of C4F7N molecule has no obvious change before and after interaction. According to the PDOS shown in Fig. 15(a), the hybridizations between Al3s, 3p orbitals are much weaker compared to that of Cu (1 0 0) and Al (1 0 0), indicating that the orbital interaction is weaker. It should be noted that the CF3-O (2)-Top, F-Al(1)-Top, N-Al(1)-Top, N-Al(2)-Top, N-O(2)-Top and N-O (3)-Top initial structure shows similar geometric configurations to that of N-O(1)-Top initial structure after interaction, so relevant adsorption properties are not analyzed. For the C4F7N interacted with Al2O3 (0 0 0 1) with F-O(3)-Top initial structure, the adsorption energy is 0.501 eV with 0.23e electron transfer. The interaction distance between F atom and Al atom in Al2O3 (0 0 0 1) surface is 2.161 Å and PDOS shows no obvious orbital hybridization between F atom and Al atom after interaction. Thus, the interaction process between them belongs to physical adsorption. Overall, the interaction between C4F7N and Al (1 0 0), Cu (1 0 0) surface is stronger than that of Al2O3 (0 0 0 1), which could result in the
5. Conclusion In this paper, the compatibility of C4F7N-N2 gas mixture with copper and aluminum widely used in the GIE was tested and evaluated. The thermal aging tests at different temperatures were conducted and the characteristics of gas mixture and metal surface were analyzed using GC–MS, XPS and SEM. The interaction mechanism between C4F7N and copper, aluminum, alumina was also analyzed based on DFT. Relevant conclusions are summarized as follows, (1) C4F7N-N2 gas mixture is incompatible with copper at 120-220℃. Interaction between gas mixture and copper could lead to surface corrosion and gas decomposition, resulting in color change, generation of corrosion point and accumulation of F, CFx and CN particles on the surface. Some gaseous by-products such as C3F6, CF3H and C3F6O is also produced. (2) C4F7N-N2 gas mixture has better compatibility with heated aluminum (at 120℃-220℃) than that of copper. The interaction between them has no significant effect on the morphology of metal surface, which is related to the existence of Al2O3 to a certain
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Fig. 14. The most energetically favorable structure of C4F7N interacted with Al (1 0 0).
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extent. While some gaseous by-products such as C3F6, CF3H and C2F5H are generated at 220℃. (3) Interaction between C4F7N and Cu (1 0 0) and Al (1 0 0) surface belongs to chemical adsorption, which could result in the dissociation of C4F7N and generate new chemical bonds between F, C, N and Cu, Al atoms. The compatibility between Al2O3 with C4F7N is greater than that of Cu (1 0 0) and Al (1 0 0). The structure of C4F7N molecule has no obvious change after interacted with Al2O3. (4) High temperature caused by temperature rise effect and overheating faults may lead to copper corrosion and C4F7N-N2 gas decomposition. Therefore, anti-corrosion treatment should be applied on the copper surface in the devices during manufacturing process.
Table 3 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Al2O3 (0 0 0 1). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Al(1)-Top CF3-Al(2)-Top CF3-O(1)-Top CF3-O(2)-Top CF3-O(3)-Top F-Al(1)-Top F-Al(2)-Top F-O(1)-Top F-O(2)-Top F-O(3)-Top N-Al(1)-Top N-Al(2)-Top N-O(1)-Top N-O(2)-Top N-O(3)-Top
0.198 0.412 0.392 1.050 0.413 1.041 0.440 0.484 0.287 0.501 0.957 1.074 1.078 1.039 0.953
−0.058 −0.225 −0.103 −0.344 −0.223 −0.318 −0.146 −0.234 −0.050 −0.230 −0.310 −0.344 −0.344 −0.325 −0.310
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Fig. 15. The most energetically favorable structure of C4F7N interacted with Al2O3 (0 0 0 1).
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Study on the thermal interaction mechanism between C4F7N-N2 and copper, aluminum Yi Lia, Xiaoxing Zhanga,b, , Qi Chena, Ji Zhanga, Dachang Chena, Zhaolun Cuia, Song Xiaoa, , Ju Tanga ⁎
a b
⁎
School of Electrical Engineering and Automation, Wuhan University, Wuhan 430072, China State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China
ARTICLE INFO
ABSTRACT
Keywords: C4F7N-N2 Copper Aluminum Interaction SF6 alternative gas
The thermal interaction mechanism between C4F7N (fluorinated nitrile)-N2 gas mixture using as substitute to SF6 with metal was investigated. We found that interaction between C4F7N-N2 and copper leads to surface corrosion or gas decomposition with interface temperature at 120℃-220℃. The oxidation of copper and accumulation of F, CFx and CN can be found and gaseous by-products such as C3F6, CF3H, C3F6O is produced. C4F7N-N2 has better compatibility with aluminum than that of copper due to the existence of alumina. The interaction between C4F7N and alumina surface has the lowest adsorption energy, charge transfer compared with that of copper and aluminum.
1. Introduction Sulphur hexafluoride (SF6) is suitable for gaseous insulation due to its high dielectric strength, long-term stability, non-toxicity and great arc extinguishing properties. It has been used as insulation and arcextinguishing medium for various gas insulated equipment (GIE) such as gas insulated switchgear (GIS), gas insulated cable (GIC), and gas insulated line (GIL) since the early 1960s [1]. Nevertheless, SF6 shows a high global warming potential (GWP) of 23,500 and an atmospheric lifetime around 3200 years according to the Intergovernmental Panel on Climate Change (IPCC) 5th report [2]. Actually, electrical industry is the largest consumer of SF6 and about 10,000 tons were implemented yearly in all kinds of GIE [3]. Several countries have applied SF6 tax to reduce its emission and minimize the environmental impact nowadays. For example, Slovenia imposed a value of 15€/kg of SF6 pre-charged and 300€/kg of SF6 used during maintenance. Spain also imposed duties on SF6 for 66€/kg in 2015, then 100€/kg for SF6 used during service [3]. Therefore, seeking for SF6-free solutions for GIE has become a hot topic over the past decades. More recently, a new compound, C4F7N (fluorinated nitrile), is developed specifically for a SF6-free solution to high-voltage (HV) applications. The GWP of C4F7N is only 2090 and its atmospheric lifetime is 35 years [4,5]. The dielectric withstand strength of pure C4F7N is roughly twice as high as that of SF6. Due to the high liquefaction temperature (-4.7℃), it is necessary to mix C4F7N with CO2, N2, O2 or
⁎
air to meet the working pressure and liquefaction temperature requirements of equipment. Researches on the dielectric properties [6,7], switching (interrupting) performance [8,9], decomposition characteristics [10–12] and toxicological tests [13,14] of C4F7N gas mixture have been done over the past three years, which confirm that C4F7N gas mixture is suitable for most common performance and ambient conditions. As to engineering application, material compatibility between gas insulating medium and materials used in the equipment should be considered. Different components of the gas mixture themselves need to be chemically stable and should not has negative influence on the materials used inside the equipment. In addition, there might be aging processes caused by the combination of the used materials and one or more components of the gas insulating medium. The interaction might lead to changed properties of the material itself, but can also influence the quality of the gas mixture. Therefore, it is necessary to evaluate the impact of all materials in contact with gas insulating medium before engineering application. For C4F7N gas mixture, F. Kessler et al. pointed out that more than one hundred different materials were used in latest GIL installations, which can be classified into five groups as follows: metals and alloys, insulators and thermoplasts, lubricants, elastomers, desiccants. They built high pressure lab-scale autoclaves made of stainless steel and heated them up to a certain temperature in the range of 75–225 ℃ to conduct accelerated aging tests for different fluoro-organic compounds
Corresponding authors at: School of Electrical Engineering, Wuhan University, BaYi Street No.299, Wuhan, Hubei Province, PR China. E-mail addresses: [email protected] (X. Zhang), [email protected] (S. Xiao).
https://doi.org/10.1016/j.corsci.2019.03.031 Received 7 December 2018; Received in revised form 13 March 2019; Accepted 13 March 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yi Li, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.03.031
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monitoring and control system. The experimental gas chamber (as shown in Fig. 2) is made of 304 L stainless steel with a volume of 2 L to simulate the high pressure environment in real equipment. The lab-scale autoclaves and sample holders used in literature [15] were also made of stainless steel, and it was pointed out that there exists no interaction between the stainless steel and C4F7N after heated up the autoclaves and sample holders to a certain temperature in the range of 75–225 °C for 4 weeks. Thus, the potential interaction between the steel case of the experimental gas chamber and the gas mixture can be excluded. The gas-solid interface interaction simulation system is mainly composed of heating element and metallic bushing. The K-type thermocouple is used as the heating element and the metallic bushing is in close contact with it to ensure the generated heat could evenly transmitted to the outer surface. The metallic bushing is made of pure copper or aluminum supplied by China Metallurgical Research Institute with the purity of 99.999% according to the experimental requirements, and its outer surface serves as the gas-solid reaction interface. Two metallic sheets (0.2mm*5mm*100 mm, made of pure copper or aluminum) bundled on the surface of metallic bushing could participate in gas-solid interface reactions, which are used for further structural characterization and analysis. The temperature monitoring and control system is mainly composed of the temperature sensor, the proportional-integral-differential (PID) controller, the switching power supply and the solid-state relay. The temperature sensor installed on the surface of metallic bushing is used to monitor its actual temperature. The PID controller could control the solid-state relay according to the signal of temperature sensor, and then realizes the on-off control of the switching power supply to ensure the surface temperature is maintained at a stable set value. The metallic bushing and the metallic sheets were cleaned to remove the oil stain on their surface before the test and then installed. After vacuuming and filling the gas chamber with high-purity N2 for three times, the C4F7N-N2 gas mixture is finally injected to carry out relevant experiments. Moreover, the concentration of C4F7N in the gas mixture is usually lower than 10% to meet the requirements of gas pressure in GIE and liquefaction temperature for engineering application [21]. Therefore, the 10%C4F7N-90%N2 gas mixture is selected in this paper. At the same time, considering the current-carrying metallic conductor in the equipment has a temperature rise effect of about 70℃ normally, the surface temperature in this paper is set to 120℃ 170℃ and 220℃. The experimental process for each group lasted for 40 h. The gas mixture after each test was collected and analyzed using the gas chromatography-mass spectrometry (GC–MS, Shimadzu QP2010 Ultra) with the column CP-Sil5CB (60m*8um*0.32 mm). The SCAN method is conducted to obtain relevant gas components data. The heating procedure for GC is listed as follows: keeping the temperature at 32℃ for 10 min, then heating up to 150℃ at the rate of 60℃/min and retaining for 2 min. The morphology of the metallic sheet was recorded by digital camera and optical microscope. The surface microstructure and element composition of the metallic sheet were also analyzed by field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS).
Fig. 1. Structure of heating reactor system.
Fig. 2. Structure of the experimental gas chamber.
which were mixed with N2 or CO2 as carrier gas. It was pointed out that for the gas mixture containing 10.1% C4F7N for −10 °C application, its lifetime in presence of metals has been estimated to 32 years [15]. Our team has explored the interaction mechanism between C4F7N and copper, aluminum, silver theoretically. It is found that the interaction between C4F7N gas and silver belongs to physisorption and the N atom in C4F7N shows strong interaction with copper and aluminum [16,17]. Generally, compartments and current-carrying metallic conductor of gas insulated switchgear are mainly produced of metal such as copper, aluminum [18]. In this paper, we tested the thermal stability of C4F7N-N2 gas mixture associated with copper, aluminum at different surface temperatures firstly. The impact of the interaction on the gas insulating mixture and metal materials were revealed in detail. Then the thermal interaction mechanism between C4F7N and copper, aluminum, alumina surface was revealed based on the density functional theory (DFT). The adsorption energy, charge transfer as well as the density of states (DOS) of the interaction system were obtained and compared. Relevant results provide important guidance for the manufacture of GIE using C4F7N mixture as the gas insulating medium and revealed the thermal stability of C4F7N-N2 mixture comprehensively. 2. Methods
2.2. Theoretical methods
2.1. Experimental methods
2.2.1. Computational methods In order to explore the interaction mechanism between C4F7N and copper, aluminum as well as alumina, we conducted density functional theory (DFT) calculations for C4F7N-metallic interface using Dmol3
The structure of the employed aging test device is given in Fig. 1. The platform is mainly composed of experimental gas chamber, gassolid interface interaction simulation system, and temperature
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Fig. 3. Relaxed geometric structure and possible adsorption sits of Cu (1 0 0), Al (1 0 0) andα-Al2O3(0 0 0 1) surface.
module in Material Studio. We used Perdew-Burke-Ernzerhof function (PBE) with generalized gradient approximation (GGA) to deal with the electron exchange and correlation [22]. The double numerical plus polarization (DNP) was chosen as the atomic orbital basis set and DFT semi-core pseudopotential (DSSP) method was also considered to conduct the relativistic effect correction [23]. In order to describe the van der Waals (VDW) interactions accurately, the Grimme’s scheme was employed [24]. The k-point sample of Monkhorst-Pack grid was set to 4 × 4×1 and the global orbital cut-off radius was implemented to 4.5 Å [25]. All the spin polarized calculations were performed with the tolerances of 1 × 10−5 Ha on energy, 2 × 10−3 Ha/Å on gradient, and 5 × 10−3 Å displacement on convergence, respectively. It should be noted that the numerical basis sets implemented in DMol3 code are more complete than the traditional Gaussian functions, thereby minimizing or even eliminating basis set superposition error (BSSE) [26,27]. The adsorption energy of the gas-metallic interaction system was defined as follows,
Ead = Emetal + EC4F7N-EC4F7N-metal Fig. 4. Relaxed geometric structure of C4F7N.
(1)
where Emetal and EC4F7N represents the energy of C4F7N molecule and
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2.2.2. Surface models The calculated lattice parameter of Cu, Al and α-Al2O3 bulk structure is 3.61 Å, 4.05 Å and 4.76 Å, respectively, which consists well with previous results [28–30]. Then the Cu (1 0 0) and Al (1 0 0) surface were built with a six layered 3 × 3×1 supercell separated by 20 Å vacuum slab. Three possible adsorption sites for Cu (1 0 0) and Al (1 0 0), listed as Top site, Bridge site and Hcp site (as shown in Fig. 3(a)), were considered. As for alumina, a 2 × 2 × 1 supercell of the α-Al2O3(0 0 0 1) surface separated by 20 Å vacuum slab was constructed. Fig. 3(c) demonstrates the five possible adsorption sites of the α-Al2O3(0 0 0 1) surface. Fig. 4 shows the optimized structure of C4F7N, which includes three characteristic group, listed as CN, F and CF3. In order to explore the interaction mechanism of C4F7N-metal surface comprehensively, we set nine initial adsorption structures for Cu (1 0 0) and Al (1 0 0) surface with the CN, CF3, F group located at the Top, Bridge and Hcp site of metal surface, respectively. Similarly, fifteen initial adsorption structures were conducted for α-Al2O3(0 0 0 1) surface. All the initial adsorption distance of C4F7N and the metal surface is set to 1.5 Å. Finally, geometric optimization calculations were performed and relevant electronic properties including the total charge transfer, partial densities of states (PDOS) of the system were obtained. 3. Results and discussion 3.1. Interaction between C4F7N-N2 and copper 3.1.1. Morphology characterization When heated metallic surface is exposed to C4F7N-N2 gas mixture environment, its surface color may be altered as a result of undergoing chemical reactions. Fig. 5 demonstrates the images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. It is apparent that the surface color of untreated copper is light orange with uniform color distribution. Optical microscope also shows clear veins on the copper sheet and no corrosive point exists. After contacted with C4F7N-N2 gas mixture for 40 h at 120℃, the surface color of the copper deepens from light orange to dark orange obviously, indicating that the metal surface has been corroded. Optical microscopy shows that the color distribution on the copper surface becomes nonuniform. Copper bushing at 170 ℃ exposed to C4F7N-N2 gas mixture lead to be further deepened surface color, and some area changes to purple even brown. Further magnification observation shows that its color becomes purple-red with uniformly distribution, indicating the corrosion is further deepened. As to the C4F7N-N2 gas mixture exposed to the copper bushing at 220℃, interaction between them results in dark purple or golden surface color change. Optical microscopy also shows that the original texture structure of the copper sheet has been destroyed with the golden corrosion layer evenly distributed. Overall, the optical characterization results show that there exists incompatibility between C4F7N-N2 gas mixture and heated copper. The increase of interface temperature leads to color modification from pale orange to deep orange, then purple (brown) and dark purple or golden due to the corrosion process. The morphology of the copper surface is also destroyed under high temperature condition. SEM detection can further reveal the influence of C4F7N-N2 gas mixture on the surface morphology of copper. According to the test results given in Fig. 6, we can find that the surface of untreated copper
Fig. 5. Photo images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
metal surface, respectively. EC4F7N-metal denotes the total energy of the gas-metallic system after interaction. The positive value of Ead indicates that the interaction process belongs to exothermic process. The negative value of Ead means that the interaction is an endothermic process. The Hirshfeld (HI) charge analysis was chosen to understand the electron transfer mechanism of the gas-metallic system. The total charge transfer is defined as follows,
Qt = QC4F7N
QC4F7N
metal
(2)
where QC4F7N and QC4F7N metal denotes the total charge of the gas molecule before and after interaction. The positive value of Qt indicates that C4F7N molecule gains electrons from the metal surface during the interaction process and the metal surface acts as the electron donor. While the negative value of Qt means C4F7N molecule loses electrons after interaction.
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Fig. 6. SEM images of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures (Mag = 5KX).
(Cu2+) [22]. The Cu spectra of all samples treated with C4F7N-N2 gas mixture shows no obvious change. High-resolution spectra of O1s shows one peak located at 530.78 eV, which is attributed to the CuO component. The other peak located at 531.93 eV is assigned to the C = O component [23]. The fluorine component is not detected for the untreated copper. While the F1s peck located at 689.05 eV can be found for all the treated samples, indicating that the reaction brings fluorine to the copper surface. Moreover, the other peak located at 685.03 eV can be observed at 220℃, corresponding to the characteristic peak of CuF2 [24]. Thus, serious corrosion has happened at this temperature. The high resolution spectra of C1s shows one peak located at 284.84 eV belongs to the traditionally accepted position for aliphatic C. As to the sample treated at 120℃, the other peak centered around 288.48 eV is assigned to the COO component. The observation of the CN component (located at 285.94 eV) and C–F component indicates that the reaction between C4F7N-N2 gas mixture and copper at 170 ℃ and 220℃ generates CFx and CN particles on the metal surface. Above all, the reaction between C4F7N-N2 gas mixture and copper changes the element composition of metal surface to a certain extent. The oxidation of copper and accumulation of F, CFx and CN particles on the surface can be confirmed, indicating that the reaction between C4F7N-N2 gas mixture and copper will lead to metal corrosion, which consists well with the morphology analysis results.
is clear and no corrosion point exists. As for the copper heated to 120℃, its surface morphology does not change significantly with little corrosion point can be found. Thermal stress with heating mantle temperatures at 170℃ produces a small amount of granular corrosion spots on the copper surface after interaction. When the interface temperature reaches to 220℃, a large number of corrosive particles are formed and accumulated on the metal surface and the intrinsic veins of copper are obviously weakened, indicating that the thicker corrosion layer is formed, which is consistent well with the optical characterization results. According to the high-resolution image given in Fig. 6(d), we can find that the structure of corrosive particles shows irregularity and the accumulation effect exists among them.
2
3.1.2. XPS analysis The elemental composition of the samples surface XPS analysis were investigated by XPS. Fig. 7 demonstrates the high-resolution XPS spectrum of copper before and after exposed to C4F7N-N2 gas mixture at different temperatures. All the peaks were fitted based on XPS Peak software employing Gauss-Lorentz curves after subtraction of a Shirleytype background [19–21]. According to the high-resolution spectra of Cu2p given in Fig. 7(a), the main peaks are observed at 932.72 eV, 934.96 eV, 952.57 eV and 954.21 eV. The 932.72 eV and 952.57 eV components are assigned to the Cu2p3/2 and Cu2p1/2, respectively, while the other two components located at higher energy positions belong to the CuO2p3/2 and CuO2p1/
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Fig. 7. XPS spectrum of copper surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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3.1.3. Gas composition analysis Fig. 8 gives the gas chromatogram of the C4F7N-N2 gas mixture after aging test for 40 h at different temperatures. It is apparent that the interaction between the C4F7N-N2 gas mixture and copper at 120 °C does not cause significant change to the gas mixture composition (only a very low content of C3F6 can be detected). With the increase of interface temperature, the C4F7N-N2 gas mixture produced CF3H, C3F6O and C3F6 after 40 h aging test. The generation of CF3H and C3F6O may related to trace water inside the metallic material. The peak area of C3F6, CF3H and C3F6O is significantly increased at 220℃, indicating that the content of the above decomposition by-products is greatly increased. Moreover, literature [5] tested the thermal stability of C4F7N gas mixture and pointed out that the decomposition of C4F7N gas mixture occurs at 650℃. While the decomposition of C4F7N-N2 gas mixture can be confirmed when the interface temperature reaches to 170℃ according to the above test results, indicating that the presence of copper could catalyze the decomposition of C4F7N. Therefore, the long-term interaction between C4F7N-N2 gas mixture and copper at high-temperature will cause the decomposition of gas mixture and produce by-products such as C3F6. The gas-solid interface reaction changes the composition of gas insulating medium, which is unfavorable for the operation safety and service life of the equipment to a certain extent.
Fig. 8. Gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h (Copper).
3.2. Interaction between C4F7N-N2 and aluminum 3.2.1. Morphology characterization Fig. 9 reveals the images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. The color and texture distribution of the untreated aluminum is uniform and no corrosion spot exists on the surface according to the optical microscopy. The color of aluminum surface did not change significantly, which still keeps bright after interacted with C4F7N-N2 gas mixture at 120℃, 170℃ and 220℃ for 40 h. Optical microscopy also shows that the texture of aluminum surface is clear and no corrosion layer can be observed. Fig. 10 demonstrates the SEM images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. It is apparent that the surface morphology of the aluminum before and after interaction does not change significantly, and no corrosion point is produced under various temperature conditions, which is consistent well with the optical characterization results. Therefore, the interaction between C4F7N-N2 gas mixture and the heated aluminum does not change the macroscopic and microscopic morphology of the metal surface. 3.2.2. XPS analysis Fig. 11 shows the XPS high-resolution spectrum of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures. The Al2p spectrum of all the samples contains two components centered around 72.32 eV and 74.76 eV, which are assigned to Al and Al2O3. The primary peak of O1s located at 531.82 eV belongs to Al2O3. The other peak centered around 533.46 eV which originates from COO component can be observed. As to the high-resolution XPS spectra of F1s, the primary peak located at 685.78 eV existed for all the treated
Fig. 9. Photo images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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Fig. 10. SEM images of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures (Mag = 5KX).
samples, indicating that the reaction process causes fluorine deposition on the aluminum surface. According to the high-resolution XPS spectra of C1s, the traditional position for aliphatic C and COO components is located at 284.86 eV and 288.92 eV, respectively. In addition, the observation of the CN (located at 286.42 eV) and CF3 (located at 293.49 eV) components indicates the reaction between C4F7N-N2 gas mixture and aluminum at 170℃ and 220 ℃ causes the decomposition of C4F7N. Overall, the surface analysis presented above confirms that the reaction between C4F7N-N2 gas mixture and aluminum introduces some species such as F, CN and CF3 on the aluminum surface, and relevant process is accelerated at 170℃ and 220 ℃.
existence of trace water, which may come from the inorganic hydrated oxide on the aluminum surface. 4. Discussion 4.1. Interfacial reaction mechanism According to the above test results, the compatibility of C4F7N-N2 gas mixture with copper and aluminum widely used in the equipment is different. Actually, the molecular structure of C4F7N is complex and may not as inert as SF6. When it is in contact with heated copper or aluminum, the reactive portion and the weak chemical bond in the molecular structure will chemically react with the metal surface. Generally, aluminum is more active than copper, which can be oxidized and form a dense protective film alumina (Al2O3) easily. In order to understand the interaction mechanism between metal surface and C4F7N, we carried out DFT calculations. Table 1 gives the adsorption energy and total charge transfer of C4F7N interacted with Cu(1 0 0) based on different initial structures. The interaction process is exothermic due to the positive value of
3.2.3. Gas composition analysis Fig. 12 gives gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h. It can be found the decomposition of C4F7N-N2 gas mixture does not occur when the interface temperature is lower than 170℃. While the characteristic peak of CF3H, C2F5H and C3F6 can be detected at 220℃, indicating that C4F7N-N2 gas mixture is partially decomposed. The generation of CF3H, C2F5H at 220℃ is related to the
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Fig. 11. XPS spectrum of aluminum surface before and after exposed to C4F7N-N2 gas mixture at different temperatures.
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changes to 1.935 Å and the Cu4 s, 3d orbitals mainly hybridize with the N2 s, 2p orbitals near −5 eV˜−7 eV, −9 eV according to the PDOS. The interaction between C4F7N and Cu (1 0 0) with CF3-Bridge initial structure has the adsorption energy of 0.844 eV with 0.516e electrons transfer. The interaction dramatically changed the molecule structure of C4F7N, resulting in one of the F atoms in CF3 group and C atom possibly form new bonds with Cu atoms. Orbital overlaps between Cu4 s, 3d and F2 s, 2p can be seen near −4eV˜−6 eV and hybridizations happens between C2 s, 2p orbitals and Cu4 s, 3d orbitals in the range of −8 eV˜-−10 eV. Overall, the interaction between C4F7N and Cu (1 0 0) belongs to chemical adsorption with new formed chemical bonds. The accumulation of F, CFx and CN particles on the surface can be explained according to the above calculation results. That is to say, C4F7N molecule could react with the copper surface, undergoing exothermic chemical adsorption process and causing the decomposition of C4F7N as well as element composition change of copper surface. It should be noted that in our previous study, the initial interaction distance between C4F7N molecule and copper surface was set to 3 Å (larger than 1.5 Å in this paper), and the obtained geometric optimization results show that the F and CF3 group in C4F7N has the tendency to keep away from the Cu atoms [12]. That is to say, only the gas molecules near the copper surface can undergo the chemical adsorption process. Table 2 gives the adsorption energy and total charge transfer of C4F7N interacted with Al(1 0 0). Similar to that of Cu (1 0 0), the interaction process between them belongs to exothermic process. The highest adsorption energy reaches to 3.473 eV (CF3-Bridge initial structure), which is higher than that of Cu (1 0 0), indicating that the interaction between C4F7N and Al (1 0 0) is stronger. According to the most energetically favorable structure given in Fig. 14(a), two of the F atoms in CF3 group bonds with Al atoms after interaction with the distance of 1.845 Å, 1.892 Å, respectively. The distance between C atom and Al (1 0 0) surface changed to 2.079 Å. There also exists 0.732e electrons transfer during the interaction process. Overlaps between Al3s, 3p and F2 s, 2p as well as C2 s, 2p orbitals in the range of −5 eV˜10 eV can been seen according to the PDOS. Thus, the interaction process falls into the chemical adsorption with new chemical bond formed between C, F and Al atoms. As to the C4F7N interacted with Al (1 0 0) with F-Hcp initial structure, the adsorption energy is 3.39 eV and 0.577e electrons transfers from the Al (1 0 0) to C4F7N molecule. The F atom dissociates from C4F7N and bonds with the Al (1 0 0) surface. The distance between N atom and Al (1 0 0) surface changed to 1.865 Å. According to the PDOS calculation results, the Al3s, 3p orbitals mainly overlaps with F2 s, 2p orbitals in the range of −7 eV˜−9 eV and hybridization between N atom and Al atoms can also be found, indicating that the interaction changes the orbitals structures of the gas and metal. The interaction between C4F7N and Al (1 0 0) surface with N-Bridge initial structure has the adsorption energy of 1.022 eV with 0.385e electron transfer. The distance between N atom and Al atoms becomes to 1.834 Å, 2.042 Å after interaction. Obvious orbital hybridizations could
Fig. 12. Gas chromatograph of the C4F7N-N2 gas mixture after aging test for 40 h (aluminum).
Table 1 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Cu(1 0 0). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Top CF3-Bridge CF3-Hcp F-Top F-Bridge F-Hcp N-Top N-Bridge N-Hcp
0.432 0.844 0.464 0.573 2.467 0.569 0.967 0.962 0.965
−0.004 0.516 −0.011 0.002 0.612 0.011 0.038 0.039 0.039
adsorption energy. The most energetically favorable interaction structures can be obtained by comparing the adsorption energy, and Fig. 13 shows their final adsorption configurations. The adsorption of C4F7N on Cu (1 0 0) with F-Bridge initial structure exhibits the largest adsorption energy (2.467 eV) and 0.612e electrons transfers from Cu (0 0 1) surface to C4F7N gas molecule. The F atom in C4F7N dissociates and adsorbs on Cu (1 0 0) surface after interaction. The possibly new formed chemical bonds between the F atom and two Cu atoms with the bond length of 1.988 Å and 1.971 Å can be found. In addition, the distance between N atom and Cu atom shortens to 2.082 Å and 1.951 Å after interaction. According to the PDOS of the interaction system given in Fig. 13(b), overlaps between the orbitals of F atom and Cu atoms could be found near -4eV˜6 eV, -1.5 eV, indicating that the interaction between them changed the molecular orbitals. The 4 s orbitals of Cu atom and 2p, 2 s orbitals of N atom also obviously overlaps at −3 eV and −8 eV˜−9 eV, which confirms the new formed chemical bond between N atom and Cu atoms. As to the adsorption of C4F7N on Cu (1 0 0) with N-Top initial structure, there exists 0.967 eV adsorption energy with 0.0385e electrons transfer. The distance between N atom and Cu (1 0 0)
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Fig. 13. The most energetically favorable structure and PDOS of C4F7N interacted with Cu (1 0 0).
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dissociation of C4F7N, generating new chemical bonds between F, C, N and Cu, Al atoms. The new formed chemical bonds between the gasmetallic systems could result in metal corrosion and decomposition of C4F7N gas mixture at macro level. While the existence of alumina, which has greater compatibility with C4F7N, could prevent the metal corrosion and gas decomposition to a certain extent.
Table 2 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Al (1 0 0). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Top CF3-Bridge CF3-Hcp F-Top F-Bridge F-Hcp N-Top N-Bridge N-Hcp
0.619 3.473 1.736 3.349 3.378 3.390 0.440 1.022 0.301
0.628 0.732 0.490 0.594 0.579 0.577 0.072 0.385 0.010
4.2. Application potential of C4F7N-N2 gas mixture According to the above results, C4F7N-N2 gas mixture is incompatible with copper under certain conditions, while it has great compatibility with aluminum at 120-170℃. When the surface temperature of copper is lower than 120℃, the interaction between C4F7NN2 gas mixture and copper does not cause the decomposition of gas mixture. As for engineering application, the conductor temperature is generally in the range of 100℃-115℃ considering the temperature rise effect of the current carrying conductor inside the equipment during normal operation. Moreover, when the current-carrying conductor is locally overheated due to several faults such as poor contact inside the device, C4F7N-N2 gas mixture could react with copper, causing decomposition of gas mixture and corrosion of copper, which will pose a threat to the operation safety and service life of the equipment. Therefore, it is necessary to strengthen the internal temperature monitoring for equipment with C4F7N-N2 gas mixture in engineering application to avoid overheating failure. At the same time, some anticorrosion treatment should also be applied on the metal surface during the manufacturing process.
be seen between Al3s, 3p and F2 s, 2p as well as C2 s, 2p orbitals around −11.5 eV, −10.5 eV, −9.5 eV, −6 eV˜−8 eV. Above all, interaction between C4F7N and Al (1 0 0) surface results in chemical adsorption process with new bonds formed as well as decomposition of C4F7N. Table 3 gives the adsorption properties of C4F7N interacted with Al2O3 (0 0 0 1). It can be found that the highest adsorption energy among all the initial configurations reaches to 1.078 eV, which is the smallest value compared with that of Cu (1 0 0) and Al (1 0 0). That is to say, alumina shows greater compatibility with C4F7N than that of copper and aluminum. In addition, the charge transfer feature of C4F7N interacted with Al2O3 (0 0 0 1) during the interaction is different from that of Cu (1 0 0) and Al (1 0 0). C4F7N molecule acts as the electrons donors and transfer electrons to Al2O3 (0 0 0 1) surface after interaction. As to the N-O(1)-Top initial structure, the distance between N atom in C4F7N molecule and Al atom in Al2O3 (0 0 0 1) surface changes to 2.031 Å after interaction and the structure of C4F7N molecule has no obvious change before and after interaction. According to the PDOS shown in Fig. 15(a), the hybridizations between Al3s, 3p orbitals are much weaker compared to that of Cu (1 0 0) and Al (1 0 0), indicating that the orbital interaction is weaker. It should be noted that the CF3-O (2)-Top, F-Al(1)-Top, N-Al(1)-Top, N-Al(2)-Top, N-O(2)-Top and N-O (3)-Top initial structure shows similar geometric configurations to that of N-O(1)-Top initial structure after interaction, so relevant adsorption properties are not analyzed. For the C4F7N interacted with Al2O3 (0 0 0 1) with F-O(3)-Top initial structure, the adsorption energy is 0.501 eV with 0.23e electron transfer. The interaction distance between F atom and Al atom in Al2O3 (0 0 0 1) surface is 2.161 Å and PDOS shows no obvious orbital hybridization between F atom and Al atom after interaction. Thus, the interaction process between them belongs to physical adsorption. Overall, the interaction between C4F7N and Al (1 0 0), Cu (1 0 0) surface is stronger than that of Al2O3 (0 0 0 1), which could result in the
5. Conclusion In this paper, the compatibility of C4F7N-N2 gas mixture with copper and aluminum widely used in the GIE was tested and evaluated. The thermal aging tests at different temperatures were conducted and the characteristics of gas mixture and metal surface were analyzed using GC–MS, XPS and SEM. The interaction mechanism between C4F7N and copper, aluminum, alumina was also analyzed based on DFT. Relevant conclusions are summarized as follows, (1) C4F7N-N2 gas mixture is incompatible with copper at 120-220℃. Interaction between gas mixture and copper could lead to surface corrosion and gas decomposition, resulting in color change, generation of corrosion point and accumulation of F, CFx and CN particles on the surface. Some gaseous by-products such as C3F6, CF3H and C3F6O is also produced. (2) C4F7N-N2 gas mixture has better compatibility with heated aluminum (at 120℃-220℃) than that of copper. The interaction between them has no significant effect on the morphology of metal surface, which is related to the existence of Al2O3 to a certain
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Fig. 14. The most energetically favorable structure of C4F7N interacted with Al (1 0 0).
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extent. While some gaseous by-products such as C3F6, CF3H and C2F5H are generated at 220℃. (3) Interaction between C4F7N and Cu (1 0 0) and Al (1 0 0) surface belongs to chemical adsorption, which could result in the dissociation of C4F7N and generate new chemical bonds between F, C, N and Cu, Al atoms. The compatibility between Al2O3 with C4F7N is greater than that of Cu (1 0 0) and Al (1 0 0). The structure of C4F7N molecule has no obvious change after interacted with Al2O3. (4) High temperature caused by temperature rise effect and overheating faults may lead to copper corrosion and C4F7N-N2 gas decomposition. Therefore, anti-corrosion treatment should be applied on the copper surface in the devices during manufacturing process.
Table 3 Adsorption energy (Ead) and total charge transfer (Qt) of C4F7N interacted with Al2O3 (0 0 0 1). Adsorption configuration
Ead (eV)
Qt (e)
CF3-Al(1)-Top CF3-Al(2)-Top CF3-O(1)-Top CF3-O(2)-Top CF3-O(3)-Top F-Al(1)-Top F-Al(2)-Top F-O(1)-Top F-O(2)-Top F-O(3)-Top N-Al(1)-Top N-Al(2)-Top N-O(1)-Top N-O(2)-Top N-O(3)-Top
0.198 0.412 0.392 1.050 0.413 1.041 0.440 0.484 0.287 0.501 0.957 1.074 1.078 1.039 0.953
−0.058 −0.225 −0.103 −0.344 −0.223 −0.318 −0.146 −0.234 −0.050 −0.230 −0.310 −0.344 −0.344 −0.325 −0.310
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Fig. 15. The most energetically favorable structure of C4F7N interacted with Al2O3 (0 0 0 1).
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