# Compatibility of Environmentally Friendly Insulating Gases CF3I and c-C4F8 with Cu Contacts

Authors: Can Ding; Zhenjiang Gao; Xing Hu; Zhao Yuan
Journal: Advances in Condensed Matter Physics
 (2022)

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## Abstract

The gas-solid compatibility between environmentally friendly insulating gas and copper contacts is worth studying. In this paper, based on density functional theory, the adsorption calculation of CF3I, c-C4F8, five typical decomposition gases, and Cu (1 1 1) surface was carried out. The adsorption energies, transferred charges, charge densities, and densities of states were calculated for different adsorption configurations. Research indicates that there is no obvious charge transfer between the I atom and the Cu atom in the four adsorption sites of Cu (1 1 1) for the CF3I molecule. There is a charge transfer between the F atoms and the Cu top surface. The electrons lost by Cu are transferred to F atoms. In the configurations of different adsorption positions on CF3I and Cu (1 1 1) planes, the top and bridge adsorption energies are −0.835 eV and −0.993 eV, respectively, which are chemical adsorption. Therefore, CF3I is most likely to form adsorption at the top or bridge site of the Cu (1 1 1) surface. The adsorption energy of c-C4F8 gas on Cu (1 1 1) surface is similar to that of CF3I at fcc and hcp sites. The absolute values are all less than 0.8 eV, and the van der Waals force is the main force. The adsorption energies of C2F4 and C3F6 in the five decomposed gases are −1.315 eV and −1.204 eV, respectively. The charge transfer is −0.32 eV and −0.45 eV, respectively. Their values are larger than those of the other gases studied, which belong to chemical adsorption. The smaller values of the remaining three gases belong to physical adsorption. All molecular structures and Cu (1 1 1) planes were not significantly deformed. From a microscopic point of view, the gas can better exist on the copper surface.


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## Body

## 1. Introduction

SF6 is widely used in the vacuum interrupter of mechanical DC circuit breakers because of its excellent insulation and arc-extinguishing properties. However, its strong greenhouse effect has a negative impact on the atmosphere [1–3]. An environmentally friendly insulating gas to replace SF6 has been developed. It is of great significance for alleviating environmental pollution, slowing global warming and industrial applications [4]. In recent years, environmentally friendly insulating media, such as C5F10O, C4F7N, CF3I, and c-C4F8 produced by 3M Company in the United States attracted widespread attention. A large number of domestic and foreign scholars have carried out related research on these gases [4–9]. Zhang et al. [5] studied the decomposition mechanism of C3F7CN/CO2 mixed gas. It is concluded that the main decomposition products of mixed gas after multiple breakdowns are CF4, C2F6, and CF3CN. These product molecules have relatively good insulating ability. It ensures that the insulation performance of the C3F7CN/CO2 mixed gas is not damaged. Li et al. [6] conducted experimental and theoretical studies on the interaction of C6F12O with copper, silver, and aluminum. It was found that the interaction mechanism of C6F12O with copper and aluminum belonged to chemical adsorption. The interaction of C6F12O with silver belongs to physical adsorption. The compatibility of C6F12O with silver is greater than that of copper and aluminum. Some scholars also take CF3I, c-C4F8, and other gases as research objects. CF3I and c-C4F8 gases are used as insulating mediums in engineering equipment. For example, it will coexist with circuit breaker contacts for a long time in the form of a gas-solid interface. One of the most widely used and highest withstand voltage components in the arc-extinguishing chamber of a mechanical DC circuit breaker is the copper contact. It involves the compatibility between the substitute gas and the copper metal material. The reduction in the gas-insulating medium content results in a reduction in the overall dielectric strength of the device. It will lead to serious failures, such as insulation failure. The chemical adsorption and decomposition of gases can accelerate the corrosion of metal materials. It leads to the loss of equipment function, which, in turn, leads to equipment failure [10]. In addition, when overheating or discharge failure occurs in gas-insulated equipment, CF3I and c-C4F8 will decompose to generate CF4, C2F4, C2F6, C3F6, C3F8, and other gas products. Compatibility issues inevitably exist between these decomposition products and contact materials inside the equipment [11]. In existing gas-insulated equipment, insulating gases, such as CF3I and c-C4F8, may interact with metal contacts. It will directly affect the operation, maintenance, and safety assessment of environmentally friendly alternative gases, such as CF3I and c-C4F8, in insulating equipment in the future. It is also a basic issue that power operating companies and complete equipment manufacturers pay special attention to. At present, there are few related research reports on the above-mentioned problems. Therefore, a lot of experimental and theoretical research is needed. The compatibility of environment-friendly insulating gases CF3I and c-C4F8 with metal materials largely depends on the adsorption mechanism of the gas-insulating medium on the surface of the metal material and the effect of adsorption interface on the related chemical reactions occurring on the metal surface [12]. Therefore, the compatibility of CF3I and c-C4F8 with metallic materials should be evaluated before engineering application. The stability of CF3I and c-C4F8 and their interaction mechanism with the material surface were also analyzed. When gas mixtures such as CF3I and c-C4F8 are used as insulating media, we need to investigate whether gas affects the reliability of electrical equipment under long-term operating conditions [13].Therefore, this paper studies the compatibility of CF3I, c-C4F8, and their main breakdown products with the copper contacts of circuit breakers. Based on the density functional theory, the adsorption properties of CF3I, c-C4F8, and their decomposition products on the surface of Cu (1 1 1) were analyzed. Firstly, the adsorption models of CF3I, c-C4F8, and their main decomposition products on the surface of Cu (1 1 1) were constructed. Then, based on the density functional theory, the adsorption mechanism of CF3I, c-C4F8, and their main decomposition products on the surface of metal material copper was studied. The compatibility of CF3I, c-C4F8, and the decomposition products with copper was also analyzed. The compatibility study provides theoretical guidance for the future application of environmentally friendly insulating gas in the arc-extinguishing chamber of mechanical DC circuit breakers.


## 2. Simulation Calculation

### 2.1. Calculation Method

The paper is based on density functional theory. The calculation was performed using the Dmol3 module in the material calculation. In the process of calculating the adsorption, the generalized gradient approximation GGA-PBE algorithm and dual numerical basis set + polarization function (DNP) [14] are used. The Monkhorst-Pack method is selected for the Brillouin zone integral K point. Spin polarization is not considered. To obtain more accurate results, the Grimme method is introduced to describe weak interaction forces [15], such as van der Waals forces. The calculated lattice parameter of metallic Cu is a = 3.6417 Å, and the deviation from the experimental value [15, 16] a = 3.615 Å is about 0.2%. It shows that the calculation method is reliable. According to the relevant literature, the most stable densely packed surface of metal deposited in face-centered cubic is (1 1 1), which is the main exposed crystal surface [1]. Firstly, a surface model of Cu (1 1 1) (4 × 4) was constructed. The thickness of the vacuum layer is taken as 20 Å, and the number of layers of the model is tested. In this paper, the structural and energy parameters of the 3-layer Cu (1 1 1) surface are calculated. The three layers of Cu atoms are marked with different colors. During the calculation, the top two atomic layers were allowed to relax and the bottom Cu atoms were fixed [16]. When the number of layers on the Cu (1 1 1) surface is 3, the bond length and adsorption energy change with the increase of the number of layers are less than 0.025 Å and 0.005 eV, respectively. Therefore, it is more reasonable to select the model of the 3-layer Cu (1 1 1) surface. Secondly, the structure optimization analysis of CF3I, c-C4F8, and five typical decomposition gases were carried out, respectively. The optimized structure parameters have a small gap with the literature [17, 18], which ensures the correctness of the results. The paper also calculates the HOMO and LOMO orbitals of each gas. Identify where the gas is most likely to react with the copper surface. The initial structures of CF3I, c-C4F8, and five typical decomposition gases adsorbed on the surface of Cu (1 1 1) were constructed. Finally, the adsorption simulation calculation is carried out on the constructed model. The formulas for calculating the adsorption energy Ead and the transfer charge Qad are as follows:(1)Ead=Egas/Cu−Egas−ECu,Qad=Qgas/Cu−Qgas.In the formula,Ead and Qad are the adsorption energy and transfer charge generated during the adsorption process. Egas/Cu and Qgas/Cu are the energy and transfer charges generated by each gas after adsorption on the Cu (1 1 1) crystal plane. Egas, Qgas, and ECu are the energy of each gas, the charge of each gas, and the energy of the Cu (1 1 1) crystal plane configuration, respectively.


### 2.2. Computational Model

Based on relevant scholars’ research on environmentally friendly alternative gases, in this paper, the adsorption reactions of CF3I, c-C4F8, and decomposed gas molecules on the surface of Cu (1 1 1) were studied. It can be known from the related scholars [19–24] and the activity analysis of functional groups in Figure 1 in this paper. The C-I bond in CF3I has the highest activity. The electron activity around the C ring is the highest in c-C4F8. Therefore, the adsorption reactions of CF3I at four typical positions on the Cu (1 1 1) surface were further studied in this paper. As shown in Figures 2(a) and 2(b), there are four adsorption positions: top position, bridge position, face-centered cubic (fcc), and hexagonal close-packed (hcp). Figures 2(c) and 2(d) show the molecular models of CF3I and c-C4F8. The electronic properties of c-C4F8 and five decomposed gases when stably adsorbed on the Cu surface were also investigated. The adsorption model of CF3I, c-C4F8, and decomposed gas molecules on the Cu (1 1 1) surface is shown in Figure 3. It can be seen in Figure 3 that the C-I bonds of CF3I molecules in different adsorption configurations are all perpendicular to the Cu (1 1 1) crystal plane. There is a certain distance between the I atom and the Cu (1 1 1) crystal plane. It is ensured that the CF3I molecules do not contact with the Cu (1 1 1) crystal plane in the initial state. When I atoms and F atoms are adsorbed on different positions on the copper surface, respectively, the range of van der Waals forces is limited because of the different distances of the gas from the surface Cu atoms. Therefore, the forces generated between the initial atoms of several adsorption models are also different. Considering that the C-I functional group in the molecular structure of CF3I has strong reactivity, the initial structures of I atoms perpendicular to the copper surface with different adsorption sites were constructed. These structures are called CF3I-top, CF3I-bridge, CF3I-hcp (hexagonal-close-packed), and CF3I-fcc (face-centered-cubic). It is derived from the knowledge of solid-state physics and materials chemistry [25–29]. The initial adsorption site of c-C4F8 in the center of the copper atom can make the calculation converge faster. For the CF3I-top adsorption model, it can be concluded that the main interaction occurs between the C-I bond at the top position and the Cu atom. For the CF3I-bridge adsorption model, the copper atoms are subject to the interaction of two copper atoms on both sides of the bridge. A better adsorption site can be formed. For the CF3I-hcp and CF3I-fcc adsorption models, the three Cu atoms next to the three vacancies interact with the C-I bond. The CF3I and c-C4F8 are subject to overheating or discharge failures in the DC circuit breaker equipment, thereby decomposing to generate CF4, C2F4, C2F6, C3F6, C3F8, and other decomposition products. Therefore, this paper also lists the adsorption model of decomposed small molecular gas on the copper surface in detail. It was constructed on the stable adsorption site on the Cu (1 1 1) face. We investigated the possible reactions between the copper contacts and the insulating gas in more detail.Figure 1
HUMO and LOMO orbitals of each gas molecule studied. (a) CF3I-HOMO, (b) CF3I-LOMO, (c) c-C4F8-HOMO, (d) c-C4F8-LOMO, (e) CF4-HOMO, (f) CF4-LOMO, (g) C2F4-HOMO, (h) C2F4-LOMO, (i) C2F6-HOMO, (j) C2F6-LOMO, (k) C3F6-HOMO, (l) C3F6-LOMO, (m) C3F8-HOMO, and (n) C3F8-LOMO.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)Figure 2
Adsorption sites of Cu (1 1 1) surface and substitute gas molecular parameters. (a) Adsorption sites, (b) Cu (1 1 1) crystal face, (c) CF3I, and (d) c-C4F8.
(a)(b)(c)(d)Figure 3
Initial structure of gas on Cu (1 1 1). (a) CF3I-top, (b) CF3I-bridge, (c) CF3I-hcp, (d) CF3I-fcc, (e) c-C4F8, (f) CF4, (g) C2F4, (h) C2F6, (i) C3F6, and (j) C3F8.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


## 2.1. Calculation Method

The paper is based on density functional theory. The calculation was performed using the Dmol3 module in the material calculation. In the process of calculating the adsorption, the generalized gradient approximation GGA-PBE algorithm and dual numerical basis set + polarization function (DNP) [14] are used. The Monkhorst-Pack method is selected for the Brillouin zone integral K point. Spin polarization is not considered. To obtain more accurate results, the Grimme method is introduced to describe weak interaction forces [15], such as van der Waals forces. The calculated lattice parameter of metallic Cu is a = 3.6417 Å, and the deviation from the experimental value [15, 16] a = 3.615 Å is about 0.2%. It shows that the calculation method is reliable. According to the relevant literature, the most stable densely packed surface of metal deposited in face-centered cubic is (1 1 1), which is the main exposed crystal surface [1]. Firstly, a surface model of Cu (1 1 1) (4 × 4) was constructed. The thickness of the vacuum layer is taken as 20 Å, and the number of layers of the model is tested. In this paper, the structural and energy parameters of the 3-layer Cu (1 1 1) surface are calculated. The three layers of Cu atoms are marked with different colors. During the calculation, the top two atomic layers were allowed to relax and the bottom Cu atoms were fixed [16]. When the number of layers on the Cu (1 1 1) surface is 3, the bond length and adsorption energy change with the increase of the number of layers are less than 0.025 Å and 0.005 eV, respectively. Therefore, it is more reasonable to select the model of the 3-layer Cu (1 1 1) surface. Secondly, the structure optimization analysis of CF3I, c-C4F8, and five typical decomposition gases were carried out, respectively. The optimized structure parameters have a small gap with the literature [17, 18], which ensures the correctness of the results. The paper also calculates the HOMO and LOMO orbitals of each gas. Identify where the gas is most likely to react with the copper surface. The initial structures of CF3I, c-C4F8, and five typical decomposition gases adsorbed on the surface of Cu (1 1 1) were constructed. Finally, the adsorption simulation calculation is carried out on the constructed model. The formulas for calculating the adsorption energy Ead and the transfer charge Qad are as follows:(1)Ead=Egas/Cu−Egas−ECu,Qad=Qgas/Cu−Qgas.In the formula,Ead and Qad are the adsorption energy and transfer charge generated during the adsorption process. Egas/Cu and Qgas/Cu are the energy and transfer charges generated by each gas after adsorption on the Cu (1 1 1) crystal plane. Egas, Qgas, and ECu are the energy of each gas, the charge of each gas, and the energy of the Cu (1 1 1) crystal plane configuration, respectively.


## 2.2. Computational Model

Based on relevant scholars’ research on environmentally friendly alternative gases, in this paper, the adsorption reactions of CF3I, c-C4F8, and decomposed gas molecules on the surface of Cu (1 1 1) were studied. It can be known from the related scholars [19–24] and the activity analysis of functional groups in Figure 1 in this paper. The C-I bond in CF3I has the highest activity. The electron activity around the C ring is the highest in c-C4F8. Therefore, the adsorption reactions of CF3I at four typical positions on the Cu (1 1 1) surface were further studied in this paper. As shown in Figures 2(a) and 2(b), there are four adsorption positions: top position, bridge position, face-centered cubic (fcc), and hexagonal close-packed (hcp). Figures 2(c) and 2(d) show the molecular models of CF3I and c-C4F8. The electronic properties of c-C4F8 and five decomposed gases when stably adsorbed on the Cu surface were also investigated. The adsorption model of CF3I, c-C4F8, and decomposed gas molecules on the Cu (1 1 1) surface is shown in Figure 3. It can be seen in Figure 3 that the C-I bonds of CF3I molecules in different adsorption configurations are all perpendicular to the Cu (1 1 1) crystal plane. There is a certain distance between the I atom and the Cu (1 1 1) crystal plane. It is ensured that the CF3I molecules do not contact with the Cu (1 1 1) crystal plane in the initial state. When I atoms and F atoms are adsorbed on different positions on the copper surface, respectively, the range of van der Waals forces is limited because of the different distances of the gas from the surface Cu atoms. Therefore, the forces generated between the initial atoms of several adsorption models are also different. Considering that the C-I functional group in the molecular structure of CF3I has strong reactivity, the initial structures of I atoms perpendicular to the copper surface with different adsorption sites were constructed. These structures are called CF3I-top, CF3I-bridge, CF3I-hcp (hexagonal-close-packed), and CF3I-fcc (face-centered-cubic). It is derived from the knowledge of solid-state physics and materials chemistry [25–29]. The initial adsorption site of c-C4F8 in the center of the copper atom can make the calculation converge faster. For the CF3I-top adsorption model, it can be concluded that the main interaction occurs between the C-I bond at the top position and the Cu atom. For the CF3I-bridge adsorption model, the copper atoms are subject to the interaction of two copper atoms on both sides of the bridge. A better adsorption site can be formed. For the CF3I-hcp and CF3I-fcc adsorption models, the three Cu atoms next to the three vacancies interact with the C-I bond. The CF3I and c-C4F8 are subject to overheating or discharge failures in the DC circuit breaker equipment, thereby decomposing to generate CF4, C2F4, C2F6, C3F6, C3F8, and other decomposition products. Therefore, this paper also lists the adsorption model of decomposed small molecular gas on the copper surface in detail. It was constructed on the stable adsorption site on the Cu (1 1 1) face. We investigated the possible reactions between the copper contacts and the insulating gas in more detail.Figure 1
HUMO and LOMO orbitals of each gas molecule studied. (a) CF3I-HOMO, (b) CF3I-LOMO, (c) c-C4F8-HOMO, (d) c-C4F8-LOMO, (e) CF4-HOMO, (f) CF4-LOMO, (g) C2F4-HOMO, (h) C2F4-LOMO, (i) C2F6-HOMO, (j) C2F6-LOMO, (k) C3F6-HOMO, (l) C3F6-LOMO, (m) C3F8-HOMO, and (n) C3F8-LOMO.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)Figure 2
Adsorption sites of Cu (1 1 1) surface and substitute gas molecular parameters. (a) Adsorption sites, (b) Cu (1 1 1) crystal face, (c) CF3I, and (d) c-C4F8.
(a)(b)(c)(d)Figure 3
Initial structure of gas on Cu (1 1 1). (a) CF3I-top, (b) CF3I-bridge, (c) CF3I-hcp, (d) CF3I-fcc, (e) c-C4F8, (f) CF4, (g) C2F4, (h) C2F6, (i) C3F6, and (j) C3F8.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


## 3. Calculation Results and Analysis

### 3.1. Activity Analysis of Gas Molecular Functional Groups

The activity of the functional groups of the gas molecules determines the possibility of the adsorption reaction. CF3I, c-C4F8, and the decomposed gas were analyzed by frontier molecular orbital theory. The movement of electrons outside the nucleus has no definite direction and trajectory. Hence, to describe the possibility of electrons appearing somewhere in space outside the nucleus, the concept of electron cloud has been proposed [30]. The frontier molecular orbital theory divides the electron cloud distributed around the molecule into molecular orbitals of different energy levels according to the size of the energy. The orbital with the highest energy is called the highest occupied orbital (HOMO). The orbital with the lowest energy is called the lowest unoccupied orbital (LUMO) [31, 32]. The distribution of these two orbitals determines the activity of the molecule. The frontier molecular orbitals of CF3I, c-C4F8, and decomposed gas molecules were calculated. The wave function distributions of its HOMO and LUMO orbitals are obtained as shown in Figure 1. In Figures 1(a) and 1(b), the electron clouds of HOMO and LUMO orbitals are mainly distributed near the C-I bond of the CF3I molecule. The simulation results show that the electrons around the C-I bond are most likely to appear. When the adsorption reaction occurs, it is easier to exchange electrons with the outside world to cause the adsorption reaction. For C-I bonds, the electron density of I atoms is significantly higher than that of C atoms. Therefore, the I atom is more suitable as the adsorption site of CF3I on the Cu (1 1 1) crystal plane during the simulated adsorption reaction. In Figures 1(c) and 1(d), the electron clouds of HOMO and LUMO orbitals are mainly distributed around the C ring of the c-C4F8 molecule. It means that the electrons around it are more active and more likely to transfer charges to the copper surface. Figures 1(e)∼1(n) show the electron cloud distributions of the HOMO and LUMO orbitals of the decomposed gas molecules. It can be concluded that the electron cloud of CF4 is distributed near the four F atoms, which is relatively uniform and stable in nature. The electron clouds of C2F4, C2F6, C3F6, and C3F8 gases are mainly distributed around the C-C bond. The binding to the F atom is stronger. The properties are also relatively stable, and it is not easy to transfer electrons with the outside world. These properties can only give a rough idea of the reaction possibilities of gases with solid surfaces. More accurate conclusions can be drawn from the following analysis.


### 3.2. Surface Adsorption Energy and Transfer Charge

The adsorption energies, transfer charges, and Cu-F bond lengths before and after adsorption are shown in Table1. It can be seen from the table that the adsorption energies are all negative values, indicating that the adsorption configuration system is thermodynamically stable. The adsorption process is an exothermic reaction [33–35]. The adsorption reaction must be accompanied by the transfer of charge. The adsorption energy and transfer charge of the CF3I bridge configuration are −0.993 eV and −1.12 e, respectively. Its absolute value is the largest among the four adsorption positions. It is because of the fact that the I atom shares the electron pair with the copper atom to form two new chemical bonds in the bridge configuration. Therefore, the adsorption energy and transfer charge of the bridge site are larger than those of the other configurations. The adsorption energies are the top position, the face-centered cubic position, and the hexagonal close-packed position, and their values are −0.835 eV, −0.746 eV, and −0.691 eV, respectively. For the adsorption reaction, the adsorption energy generated during the adsorption process is an important criterion to measure the strength of the adsorption [36, 37]. The property of adsorption when the absolute value of adsorption energy is greater than 0.8 eV is defined as chemisorption. At this point, electron-sharing pairs are formed to form bonds. If it is less than 0.8 eV, it is physical adsorption. Adsorption on solid surfaces relies on van der Waals forces. Therefore, it is concluded that the apical and bridge adsorptions are chemical adsorptions. Bridge adsorption is stronger than top adsorption. Face-centered cubic and cubic close-packed sites can form strong physical adsorption. It shows that CF3I is easier to adsorb on the top and bridge sites of the Cu (1 1 1) surface. The adsorption energy of c-C4F8 is -0.633 eV, which is physical adsorption. There is a van der Waals force between the copper surface and the gas, and the transfer charge is −0.16 e. For the five decomposed gas molecules, the adsorption energy and transfer charge of C2F4 are the largest, and their values are −1.315 eV and −0.32 e, respectively. From this, it can be inferred that there is an obvious charge transfer between the F atoms in the molecule and the Cu surface. The electrons of the copper atoms are transferred to the LUMO orbital of C2F4. A strong F–Cu chemical bond is formed on the surface. The regions of high electron density around the CF4 and C2F6 molecules are some distance away from the copper atoms on the surface. There is no apparent electron transfer in this space. The adsorption energies are −0.514 eV and −0.674 eV, respectively. The charge transfer is −0.27 e and −0.29 e, respectively. Therefore, it is determined that it is physical adsorption, and no chemical bond is formed with the Cu atoms on the surface. The distance between Cu and F before and after adsorption is also listed in Table 1. It can be seen that the distance between Cu and F increased from 2.13 Å to 2.96 Å after the adsorption of c-C4F8 molecules is the distance after the van der Waals force between Cu and F stabilizes. The gas molecule c-C4F8 has good compatibility with the copper surface. The distance after adsorption at the top and bridge sites decreases. The initial C-I bond is perpendicular to the Cu (1 1 1) surface to form an angle of about 50° with the surface. It is because the I atom does not substantially transfer electrons to the copper surface. The electrons of copper are reduced and transferred to the F atomic orbitals. A Cu-F bond is formed. Therefore, the distance between Cu and F decreases, and the position of CF3I shifts. The bridge site has stronger adsorption. Molecules have more distance to the surface after adsorption. The chemical bonds formed in the four positions are the strongest. Adsorption in the face-centered cubic and cubic close-packed sites did not rotate. However, the distance between F and Cu increases. Since the outermost electrons of F and Cu are denser, the electrons are repelled after optimization. The surrounding electronic force is relatively uniform, and it reaches a stable state without position rotation. The distance between F and Cu increased after the stable adsorption of the five decomposed gases. After the adsorption of CF4, the distance between F and Cu is the largest. The weak electrostatic force makes it difficult for CF4 to react with the surface. The adsorption properties of C2F6 and CF4 are similar. The distance between F and Cu is stable at around 3 Å. The adsorption distances of the remaining three decomposed gases are around 2.8 Å. Relatively close distances make it easier for electrons to transfer and exchange between F and Cu. It can be speculated that the electrons of the surface copper atoms are mainly transferred to the F atoms during the formation of chemical bonds. It occupies an atomic orbital that would otherwise be paired with the C atom’s electron.Table 1
Adsorption energy, charge transfer, and Cu-F bond length.
StructureEad (eV)Qt (e)Cu-Fbefore (Å)Cu-Fafter (Å)c-C4F8−0.633−0.162.132.96CF3I-top−0.835−1.044.853.31CF3I-bridge−0.993−1.124.763.11CF3I-fcc−0.746−0.264.715.25CF3I-hcp−0.691−0.314.765.12CF4−0.514−0.272.033.12C2F4−1.315−0.322.562.84C2F6−0.674−0.292.343.01C3F6−1.204−0.452.492.86C3F8−0.783−0.492.242.79Figure4 is a graph of the electron density and deformed charge density of the gas adsorbed Cu (1 1 1) surface. In the electron density graph, red represents high electron density, and blue represents low electron density. In the deformed charge density plot, red represents the gained electrons and blue represents the lost electrons. In Figure 4(a), when CF3I is adsorbed on the Cu (1 1 1) surface, it can be seen that the electron density around the I atom is the highest, however, almost no charge transfer occurs. Because of the electron overlap repulsion between Cu and F, the I atoms are kept away from the Cu atoms. The charge density around the F atoms increases. The charge of the first layer of Cu atoms adsorbed with the gas decreases. It shows that the electrons of the copper atoms are mainly transferred to the F atoms, forming a strong F-Cu bond. In Figure 4(b), the CF3I bridge site is adsorbed on the Cu (1 1 1) surface. Compared with the top-adsorbed Cu atoms, more charges are transferred to the F atoms, forming stronger F-Cu bonds. The electron density around the adatoms is high and close to each other, similar to face-centered cubic and hexagonal close-packed sites. In this paper, the face-centered cubic orientation is selected as the analysis object, as shown in Figure 4(c). The electron increase of the F atom originates from the Cu atom, which is similar to the apical and bridge positions. The increased distance between the I and Cu surfaces is because of the denser electron density around I and Cu. I and Cu do not bond and repel each other. F and Cu undergo charge transfer to form a bond to achieve a stable adsorption state. In Figure 4(d), when c-C4F8 is adsorbed on the copper surface, the electrons of F atoms and Cu atoms are dense. The Cu atom, which is close to the F atom, loses electrons. The F atom has less electron gain and weak interaction, which is consistent with the above analysis. In Figures 4(e)∼4(i), the electron density changes of five kinds of decomposition gases are adsorbed. The comparison shows that the charge transfer of C2F4 is the most obvious. The F atom gets the electron transferred by the Cu atom. The charge around the C-F bond is slightly reduced. Since F gets more electrons from Cu, the nuclear charge neutralization of C is required. Therefore, the charge is reduced. The stability from large to small is CF4, C2F6, C3F8, C3F6, and C2F4.Figure 4
Electron density and deformation charge density of the gas adsorption model. (a) CF3I-top electron density and deformation charge density, (b) CF3I-bridge electron density and deformation, (c) CF3I-fcc electron density and deformation charge density, (d) c-C4F8 electron density and deformation charge density, (e) CF4 electron density and deformation charge density, (f) C2F4 electron density and deformation charge density, (g) C2F6 electron density and deformation charge density, (h) C3F6 electron density and deformation charge density, and (i) C3F8 electron density and deformation charge density.
(a)(b)(c)(d)(e)(f)(g)(h)(i)


### 3.3. Electronic Structure Analysis

The adsorption type of gas molecules on the copper surface was preliminarily determined by adsorption energy, charge transfer, and electron density. Next, the bond analysis of the above chemisorption system can be performed by DOS and PDOS plots. Figure5 shows the DOS and PDOS of CF3I, c-C4F8, and decomposed gas molecules adsorbed on the Cu (1 1 1) surface. It can be used to explore the electronic properties of the adsorption system and explain the interaction mechanism in detail.Figure 5
DOS and PDOS of the absorption structure of gas-Cu (1 1 1). (a) CF3I-top adsorption structure, (b) CF3I-bridge adsorption structure, (c) CF3I-fcc adsorption structure, (d) c-C4F8 adsorption structure, (e) CF4 adsorption structure, (f) C2F4 adsorption structure, (g) C2F6 adsorption structure, (h) C3F6 adsorption structure, and (i) C3F8 adsorption structure.
(a)(b)(c)(d)(e)(f)(g)(h)(i)Comparing Figures5(a)∼5(c), it can be seen that after CF3I is adsorbed on the surface of Cu (1 1 1), most of the electrons of the system are distributed in the valence band region to the left of the Fermi level. It is mainly composed of 4s, 3d, and 3p orbital electrons of the Cu atom, with the 3d orbital contributing the most electrons. The DOS crosses the Fermi level, indicating the metallic nature of this system. The peak of the total density of states of the system appears near the valence band −2 eV, which is mainly contributed by the 3d orbital electrons of Cu. The Cu-F bond formed by the top position adsorption shows sharp peaks at −12.8 eV and −10.3 eV far from the Fermi level, which is formed by the 2p and 2s orbital electron hybridization of the adsorbed F atom. The peaks formed in the −7∼−5 eV region are derived from F-2p, Cu-4s, and a small number of Cu-3s orbital electrons. The spike at the near-Fermi level −2 eV is contributed by the 3d and 3p orbital electrons of Cu. The small peak at the Fermi level of 0 eV is formed by the electronic hybridization of the 4s, 3d, and 3p of Cu atoms and the 2p orbital of a small amount of F. There is also a peak at 1.3 eV in the conduction band region, and the contribution is similar to that at 0 eV. Compared with the adsorption at the top site, peaks were formed at 0.11 eV and 0.9 eV when the bridge site adsorbed on the Cu surface. Electrons are more concentrated near the Fermi level. The peak at −2 eV in the valence band region becomes sharper. It is also being verified that the electron transfer between F and Cu is more. The adsorption capacity of the gas and the surface is stronger. The spike appears to shift to the left when adsorbed at the fcc site. Moving away from the Fermi level makes the Cu-F bond weaker. It can be seen in Figure 5(d) that the total density of the states of c-C4F8 and CF3I are similar when adsorbed on the copper surface at the fcc site. The valence band peak near the Fermi level drops slightly. The position of the spike at the conduction band is shifted to the left. The adsorption force of the gas on the Cu (1 1 1) surface is smaller than that in other positions. It can be seen from the DOS plot after gas adsorption. The gas adsorption has little effect on the density of states near the Fermi level. It mainly affects the deep energy level valence band. The 2p and 2s electron orbitals of F broaden the range of the valence band. Figures 5(e)∼5(i) are the densities of the states of the adsorbed electrons of the decomposed gas on the Cu (1 1 1) surface. The comparison shows that the density of the states changes slightly before and after adsorption. During the adsorption of C2F4 and C3F6, the DOS shifted to the Fermi level, and the delocalization decreased. Compared with the other three gases, the adsorption capacity is slightly stronger, however, it does not affect the original properties of Cu. It indicates that the adsorption of the two gases CF3I and c-C4F8 on the copper surface has little effect on it. It is proved from a microscopic point of view that it meets the requirements of alternative gas selection.When top-side adsorption occurs, the PDOS peak of the Cu-F bond forms peaks at 1.32 eV to the right of the Fermi level and −2 eV to the left of the Fermi level, with two peaks on both sides of the Fermi level. When the DOS between the two peaks is not zero, it is called a “pseudo gap.” The pseudo gap directly reflects the strength of the bond. The wider the pseudo gap, the stronger the bond between the two atoms. It can be seen that the pseudo gap under the Cu-F interaction during the top-side adsorption is 3.32 eV. Correspondingly, it can be concluded that the pseudo gap after the adsorption of CF3I-bridge, CF3I-fcc, c-C4F8, CF4, C2F4, C2F6, C3F6, and C3F8 are 3.35 eV, 3.29 eV, 3.28 eV, 3.26 eV, 3.38 eV, 3.28 eV, 3.36 eV, and 3.35 eV, respectively. Therefore, the Cu-F chemical bond formed during the adsorption of CF3I bridge site is the strongest. The bonding force between Cu and F is small when c-C4F8 is adsorbed. Among the five decomposed gases, C2F4 has the largest pseudo gap, and the formed bonding force is also stronger. The adsorption force of CF4 on the Cu (1 1 1) surface is the weakest, and the van der Waals force is dominant. It is consistent with previous findings through orbital and charge distribution studies.


## 3.1. Activity Analysis of Gas Molecular Functional Groups

The activity of the functional groups of the gas molecules determines the possibility of the adsorption reaction. CF3I, c-C4F8, and the decomposed gas were analyzed by frontier molecular orbital theory. The movement of electrons outside the nucleus has no definite direction and trajectory. Hence, to describe the possibility of electrons appearing somewhere in space outside the nucleus, the concept of electron cloud has been proposed [30]. The frontier molecular orbital theory divides the electron cloud distributed around the molecule into molecular orbitals of different energy levels according to the size of the energy. The orbital with the highest energy is called the highest occupied orbital (HOMO). The orbital with the lowest energy is called the lowest unoccupied orbital (LUMO) [31, 32]. The distribution of these two orbitals determines the activity of the molecule. The frontier molecular orbitals of CF3I, c-C4F8, and decomposed gas molecules were calculated. The wave function distributions of its HOMO and LUMO orbitals are obtained as shown in Figure 1. In Figures 1(a) and 1(b), the electron clouds of HOMO and LUMO orbitals are mainly distributed near the C-I bond of the CF3I molecule. The simulation results show that the electrons around the C-I bond are most likely to appear. When the adsorption reaction occurs, it is easier to exchange electrons with the outside world to cause the adsorption reaction. For C-I bonds, the electron density of I atoms is significantly higher than that of C atoms. Therefore, the I atom is more suitable as the adsorption site of CF3I on the Cu (1 1 1) crystal plane during the simulated adsorption reaction. In Figures 1(c) and 1(d), the electron clouds of HOMO and LUMO orbitals are mainly distributed around the C ring of the c-C4F8 molecule. It means that the electrons around it are more active and more likely to transfer charges to the copper surface. Figures 1(e)∼1(n) show the electron cloud distributions of the HOMO and LUMO orbitals of the decomposed gas molecules. It can be concluded that the electron cloud of CF4 is distributed near the four F atoms, which is relatively uniform and stable in nature. The electron clouds of C2F4, C2F6, C3F6, and C3F8 gases are mainly distributed around the C-C bond. The binding to the F atom is stronger. The properties are also relatively stable, and it is not easy to transfer electrons with the outside world. These properties can only give a rough idea of the reaction possibilities of gases with solid surfaces. More accurate conclusions can be drawn from the following analysis.


## 3.2. Surface Adsorption Energy and Transfer Charge

The adsorption energies, transfer charges, and Cu-F bond lengths before and after adsorption are shown in Table1. It can be seen from the table that the adsorption energies are all negative values, indicating that the adsorption configuration system is thermodynamically stable. The adsorption process is an exothermic reaction [33–35]. The adsorption reaction must be accompanied by the transfer of charge. The adsorption energy and transfer charge of the CF3I bridge configuration are −0.993 eV and −1.12 e, respectively. Its absolute value is the largest among the four adsorption positions. It is because of the fact that the I atom shares the electron pair with the copper atom to form two new chemical bonds in the bridge configuration. Therefore, the adsorption energy and transfer charge of the bridge site are larger than those of the other configurations. The adsorption energies are the top position, the face-centered cubic position, and the hexagonal close-packed position, and their values are −0.835 eV, −0.746 eV, and −0.691 eV, respectively. For the adsorption reaction, the adsorption energy generated during the adsorption process is an important criterion to measure the strength of the adsorption [36, 37]. The property of adsorption when the absolute value of adsorption energy is greater than 0.8 eV is defined as chemisorption. At this point, electron-sharing pairs are formed to form bonds. If it is less than 0.8 eV, it is physical adsorption. Adsorption on solid surfaces relies on van der Waals forces. Therefore, it is concluded that the apical and bridge adsorptions are chemical adsorptions. Bridge adsorption is stronger than top adsorption. Face-centered cubic and cubic close-packed sites can form strong physical adsorption. It shows that CF3I is easier to adsorb on the top and bridge sites of the Cu (1 1 1) surface. The adsorption energy of c-C4F8 is -0.633 eV, which is physical adsorption. There is a van der Waals force between the copper surface and the gas, and the transfer charge is −0.16 e. For the five decomposed gas molecules, the adsorption energy and transfer charge of C2F4 are the largest, and their values are −1.315 eV and −0.32 e, respectively. From this, it can be inferred that there is an obvious charge transfer between the F atoms in the molecule and the Cu surface. The electrons of the copper atoms are transferred to the LUMO orbital of C2F4. A strong F–Cu chemical bond is formed on the surface. The regions of high electron density around the CF4 and C2F6 molecules are some distance away from the copper atoms on the surface. There is no apparent electron transfer in this space. The adsorption energies are −0.514 eV and −0.674 eV, respectively. The charge transfer is −0.27 e and −0.29 e, respectively. Therefore, it is determined that it is physical adsorption, and no chemical bond is formed with the Cu atoms on the surface. The distance between Cu and F before and after adsorption is also listed in Table 1. It can be seen that the distance between Cu and F increased from 2.13 Å to 2.96 Å after the adsorption of c-C4F8 molecules is the distance after the van der Waals force between Cu and F stabilizes. The gas molecule c-C4F8 has good compatibility with the copper surface. The distance after adsorption at the top and bridge sites decreases. The initial C-I bond is perpendicular to the Cu (1 1 1) surface to form an angle of about 50° with the surface. It is because the I atom does not substantially transfer electrons to the copper surface. The electrons of copper are reduced and transferred to the F atomic orbitals. A Cu-F bond is formed. Therefore, the distance between Cu and F decreases, and the position of CF3I shifts. The bridge site has stronger adsorption. Molecules have more distance to the surface after adsorption. The chemical bonds formed in the four positions are the strongest. Adsorption in the face-centered cubic and cubic close-packed sites did not rotate. However, the distance between F and Cu increases. Since the outermost electrons of F and Cu are denser, the electrons are repelled after optimization. The surrounding electronic force is relatively uniform, and it reaches a stable state without position rotation. The distance between F and Cu increased after the stable adsorption of the five decomposed gases. After the adsorption of CF4, the distance between F and Cu is the largest. The weak electrostatic force makes it difficult for CF4 to react with the surface. The adsorption properties of C2F6 and CF4 are similar. The distance between F and Cu is stable at around 3 Å. The adsorption distances of the remaining three decomposed gases are around 2.8 Å. Relatively close distances make it easier for electrons to transfer and exchange between F and Cu. It can be speculated that the electrons of the surface copper atoms are mainly transferred to the F atoms during the formation of chemical bonds. It occupies an atomic orbital that would otherwise be paired with the C atom’s electron.Table 1
Adsorption energy, charge transfer, and Cu-F bond length.
StructureEad (eV)Qt (e)Cu-Fbefore (Å)Cu-Fafter (Å)c-C4F8−0.633−0.162.132.96CF3I-top−0.835−1.044.853.31CF3I-bridge−0.993−1.124.763.11CF3I-fcc−0.746−0.264.715.25CF3I-hcp−0.691−0.314.765.12CF4−0.514−0.272.033.12C2F4−1.315−0.322.562.84C2F6−0.674−0.292.343.01C3F6−1.204−0.452.492.86C3F8−0.783−0.492.242.79Figure4 is a graph of the electron density and deformed charge density of the gas adsorbed Cu (1 1 1) surface. In the electron density graph, red represents high electron density, and blue represents low electron density. In the deformed charge density plot, red represents the gained electrons and blue represents the lost electrons. In Figure 4(a), when CF3I is adsorbed on the Cu (1 1 1) surface, it can be seen that the electron density around the I atom is the highest, however, almost no charge transfer occurs. Because of the electron overlap repulsion between Cu and F, the I atoms are kept away from the Cu atoms. The charge density around the F atoms increases. The charge of the first layer of Cu atoms adsorbed with the gas decreases. It shows that the electrons of the copper atoms are mainly transferred to the F atoms, forming a strong F-Cu bond. In Figure 4(b), the CF3I bridge site is adsorbed on the Cu (1 1 1) surface. Compared with the top-adsorbed Cu atoms, more charges are transferred to the F atoms, forming stronger F-Cu bonds. The electron density around the adatoms is high and close to each other, similar to face-centered cubic and hexagonal close-packed sites. In this paper, the face-centered cubic orientation is selected as the analysis object, as shown in Figure 4(c). The electron increase of the F atom originates from the Cu atom, which is similar to the apical and bridge positions. The increased distance between the I and Cu surfaces is because of the denser electron density around I and Cu. I and Cu do not bond and repel each other. F and Cu undergo charge transfer to form a bond to achieve a stable adsorption state. In Figure 4(d), when c-C4F8 is adsorbed on the copper surface, the electrons of F atoms and Cu atoms are dense. The Cu atom, which is close to the F atom, loses electrons. The F atom has less electron gain and weak interaction, which is consistent with the above analysis. In Figures 4(e)∼4(i), the electron density changes of five kinds of decomposition gases are adsorbed. The comparison shows that the charge transfer of C2F4 is the most obvious. The F atom gets the electron transferred by the Cu atom. The charge around the C-F bond is slightly reduced. Since F gets more electrons from Cu, the nuclear charge neutralization of C is required. Therefore, the charge is reduced. The stability from large to small is CF4, C2F6, C3F8, C3F6, and C2F4.Figure 4
Electron density and deformation charge density of the gas adsorption model. (a) CF3I-top electron density and deformation charge density, (b) CF3I-bridge electron density and deformation, (c) CF3I-fcc electron density and deformation charge density, (d) c-C4F8 electron density and deformation charge density, (e) CF4 electron density and deformation charge density, (f) C2F4 electron density and deformation charge density, (g) C2F6 electron density and deformation charge density, (h) C3F6 electron density and deformation charge density, and (i) C3F8 electron density and deformation charge density.
(a)(b)(c)(d)(e)(f)(g)(h)(i)


## 3.3. Electronic Structure Analysis

The adsorption type of gas molecules on the copper surface was preliminarily determined by adsorption energy, charge transfer, and electron density. Next, the bond analysis of the above chemisorption system can be performed by DOS and PDOS plots. Figure5 shows the DOS and PDOS of CF3I, c-C4F8, and decomposed gas molecules adsorbed on the Cu (1 1 1) surface. It can be used to explore the electronic properties of the adsorption system and explain the interaction mechanism in detail.Figure 5
DOS and PDOS of the absorption structure of gas-Cu (1 1 1). (a) CF3I-top adsorption structure, (b) CF3I-bridge adsorption structure, (c) CF3I-fcc adsorption structure, (d) c-C4F8 adsorption structure, (e) CF4 adsorption structure, (f) C2F4 adsorption structure, (g) C2F6 adsorption structure, (h) C3F6 adsorption structure, and (i) C3F8 adsorption structure.
(a)(b)(c)(d)(e)(f)(g)(h)(i)Comparing Figures5(a)∼5(c), it can be seen that after CF3I is adsorbed on the surface of Cu (1 1 1), most of the electrons of the system are distributed in the valence band region to the left of the Fermi level. It is mainly composed of 4s, 3d, and 3p orbital electrons of the Cu atom, with the 3d orbital contributing the most electrons. The DOS crosses the Fermi level, indicating the metallic nature of this system. The peak of the total density of states of the system appears near the valence band −2 eV, which is mainly contributed by the 3d orbital electrons of Cu. The Cu-F bond formed by the top position adsorption shows sharp peaks at −12.8 eV and −10.3 eV far from the Fermi level, which is formed by the 2p and 2s orbital electron hybridization of the adsorbed F atom. The peaks formed in the −7∼−5 eV region are derived from F-2p, Cu-4s, and a small number of Cu-3s orbital electrons. The spike at the near-Fermi level −2 eV is contributed by the 3d and 3p orbital electrons of Cu. The small peak at the Fermi level of 0 eV is formed by the electronic hybridization of the 4s, 3d, and 3p of Cu atoms and the 2p orbital of a small amount of F. There is also a peak at 1.3 eV in the conduction band region, and the contribution is similar to that at 0 eV. Compared with the adsorption at the top site, peaks were formed at 0.11 eV and 0.9 eV when the bridge site adsorbed on the Cu surface. Electrons are more concentrated near the Fermi level. The peak at −2 eV in the valence band region becomes sharper. It is also being verified that the electron transfer between F and Cu is more. The adsorption capacity of the gas and the surface is stronger. The spike appears to shift to the left when adsorbed at the fcc site. Moving away from the Fermi level makes the Cu-F bond weaker. It can be seen in Figure 5(d) that the total density of the states of c-C4F8 and CF3I are similar when adsorbed on the copper surface at the fcc site. The valence band peak near the Fermi level drops slightly. The position of the spike at the conduction band is shifted to the left. The adsorption force of the gas on the Cu (1 1 1) surface is smaller than that in other positions. It can be seen from the DOS plot after gas adsorption. The gas adsorption has little effect on the density of states near the Fermi level. It mainly affects the deep energy level valence band. The 2p and 2s electron orbitals of F broaden the range of the valence band. Figures 5(e)∼5(i) are the densities of the states of the adsorbed electrons of the decomposed gas on the Cu (1 1 1) surface. The comparison shows that the density of the states changes slightly before and after adsorption. During the adsorption of C2F4 and C3F6, the DOS shifted to the Fermi level, and the delocalization decreased. Compared with the other three gases, the adsorption capacity is slightly stronger, however, it does not affect the original properties of Cu. It indicates that the adsorption of the two gases CF3I and c-C4F8 on the copper surface has little effect on it. It is proved from a microscopic point of view that it meets the requirements of alternative gas selection.When top-side adsorption occurs, the PDOS peak of the Cu-F bond forms peaks at 1.32 eV to the right of the Fermi level and −2 eV to the left of the Fermi level, with two peaks on both sides of the Fermi level. When the DOS between the two peaks is not zero, it is called a “pseudo gap.” The pseudo gap directly reflects the strength of the bond. The wider the pseudo gap, the stronger the bond between the two atoms. It can be seen that the pseudo gap under the Cu-F interaction during the top-side adsorption is 3.32 eV. Correspondingly, it can be concluded that the pseudo gap after the adsorption of CF3I-bridge, CF3I-fcc, c-C4F8, CF4, C2F4, C2F6, C3F6, and C3F8 are 3.35 eV, 3.29 eV, 3.28 eV, 3.26 eV, 3.38 eV, 3.28 eV, 3.36 eV, and 3.35 eV, respectively. Therefore, the Cu-F chemical bond formed during the adsorption of CF3I bridge site is the strongest. The bonding force between Cu and F is small when c-C4F8 is adsorbed. Among the five decomposed gases, C2F4 has the largest pseudo gap, and the formed bonding force is also stronger. The adsorption force of CF4 on the Cu (1 1 1) surface is the weakest, and the van der Waals force is dominant. It is consistent with previous findings through orbital and charge distribution studies.


## 4. Conclusion

In this paper, based on density functional theory, the relationship between CF3I, c-C4F8, and decomposed gas molecules and the gas-solid compatibility of Cu (1 1 1) crystal planes is studied. The main conclusions are as follows:(1)
There is no obvious charge transfer between the I atom and the Cu atom in the four adsorption sites of Cu (1 1 1) in the CF3I molecule. There is a charge transfer between the F atoms and the Cu top surface. The electrons lost by Cu are transferred to F atoms. CF3I is most likely to form adsorption at the top or bridge site of the Cu (1 1 1) surface. The adsorption energy of c-C4F8 gas on Cu (1 1 1) surface is less than 0.8 eV, and the van der Waals force is the main force. The adsorption energy and charge transfer of C2F4 and C3F6 in the five decomposed gases are larger, which belong to chemical adsorption. The rest of the gases have less force and belong to physical adsorption.(2)
The Cu-F chemical bond has the strongest effect on the adsorption at the top and bridge positions. The bonding force of fcc site, hcp site, and c-C4F8 is smaller when Cu (1 1 1) surface is adsorbed. Among the five decomposed gases, C2F4 and C3F6 have stronger adsorption force to the surface. The remaining gas Cu-F bonds are weaker.In conclusion, by calculating the adsorption characteristics of CF3I, c-C4F8, and decomposed gas on the Cu (1 1 1) crystal plane, it is concluded that CF3I and c-C4F8 are well-compatible on the Cu (1 1 1) crystal plane. The five possible decomposition gases listed are also compatible with the Cu (1 1 1) plane. It provides an idea for the application research of environmental protection gas in circuit breakers.



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