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Novel nanostructure composite dielectric with high insulation performance: Silica-based nanometer-sized porous composite insulating paper reinforced by ceramic fibers
Scripta Materialia 181 (2020) 58–61
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Novel nanostructure composite dielectric with high insulation performance: Silica-based nanometer-sized porous composite insulating paper reinforced by ceramic fibers Wenxia Sima, Jiahui He, Potao Sun∗, Ming Yang, Ze Yin, Chuang Li State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400030, PR China
a r t i c l e
i n f o
Article history: Received 18 October 2019 Revised 30 December 2019 Accepted 6 February 2020
Keywords: Nanostructure Ceramic matrix composites (CMC) Dielectric Electrical properties
a b s t r a c t Nano-dielectrics, which were discovered more than 20 years ago, were once considered as promising insulators. Adding or hybridizing inorganic oxides into polymer matrices inevitably results in poor compatibility between multiphase materials and thus results in limited breakdown strength improvement (generally lower than 20%). Therefore, we bypassed the traditional nano-doping method and proposed the direct construction of matrix materials on nanostructures, that is, a ceramic fiber-porous silica composite insulating paper. The breakdown strength of prepared dielectric was remarkably improved by 120.5%. To the best of our knowledge, such a high improvement in electrical performance has not been reported in previous studies.
Insulating materials are widely used in various industries and fields, including electrical power, aerospace, rail transit, etc. [1]. In the past 25 years, scholars have tried to modify various insulating materials by nano-doping to improve the performance of the dielectric [2–7]. Unfortunately, most of the nano-doping insulating materials cannot be used in the power industry owing to the following constraints [8,9]: (1) the breakdown strength improvement by nano-doping is relatively limited (generally lower than 20%); (2) the long-term stability of the nano-modified insulating materials is poor due to the aggregation of nanoparticles [10]; and (3) a significant increase in dielectric loss due to nano-doping is unacceptable in industrial application. To address these problems, we proposed a novel method, that is, in situ hydrolyzation–condensation, to directly construct matrix materials into a nanostructure. Incompatibility between multiphase materials was avoided, and the function of nano-interface was preserved. Meanwhile, the intrinsic electrical properties of the dielectric were dramatically improved by constraining electron multiplication with the nanostructure. In this work, we focused on improving the electrical performance of a transformer insulating paper, which had been invented for more than 100 years. Ceramic fiber was used as a skeleton for enhancing the mechanical properties of the material. A porous silica medium (PSM) was constructed in the dielectric. The space barrier constructed by the PSM at nanoscale was used to sup∗
Corresponding author. E-mail address: [email protected] (P. Sun).
https://doi.org/10.1016/j.scriptamat.2020.02.016 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
press electron multiplication during the breakdown process. This method considerably improved the breakdown strength of the dielectric (2.2 times that of the dielectric without nanostructure and 62.3% higher than that of a traditional oil-paper insulation). Meanwhile the construction of nanostructure weakens the accumulation of space charges and accelerates the dissipation of space charges to some extent. The reagents for preparation were as follows: ethyl orthosilicate (TEOS), ethanol, trimethyl chlorosilane (TMCS), n-butanol, and nhexane (analytical reagents). HCL was 0.1 mol/L and NH4 OH was 1 mol/L. Deionized water was used in the study [11]. Ceramic fiber paper (CFP) was aluminum silicate ceramic fiber paper bought in Guangdong Xingwang Industrial Co. Ltd. Before the test, fiber paper was dipped into the ethyl alcohol, and dry at 100 °C for later use. CF-PSIP was prepared as follows: TEOS, ethanol, and deionized water were mixed together at room temperature. HCL was added to make the pH of the system acidic (pH=3.5). Silica sol was obtained several hours later. Ceramic fiber paper (CFP) was impregnated in silica sol and placed in a vacuum-drying box for 40 min. Then, NH4 OH was added for alkali catalysis. The container was stored at 80 °C for condensation polymerization. After the silica-fiber composite gels were formed, the composites gels were dipped into a TEOS/ethanol mixture with a volume ratio 6:4. The mixture was then aged for 2 days and soaked in ethanol for 1 h. After the composite gels were aged, they were dipped into n-butanol two times for 24 h each. A solution of n-butanol, n-hexane, and 3% TMCS was added, and the surface hydrophobic chemical modification of the gel was carried out for 24 h at 50 °C.
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
59
Fig. 1. The microstructure of the sample. (a) Mesoporous distribution of CF-PSIP, (b) SEM of CFP and (c) SEM of CF-PSIP.
After the surface modification, the composite gels were finally dried in a vacuum-drying box for 36 h, and CF-PSIP was obtained. We measured the pore size distribution of CF-PSIP (Fig 1a). The size of the mesopores mainly ranged from 10 nm to 22 nm, with an average of 14 nm. The result shows that the nanostructure was successfully constructed in the CFP. We used a needle tip to remove the surface material of the sample and sprayed the internal tear surface with gold. Then, we observed the internal morphology by SEM. Fig. 1b shows the CFP micromorphology with a large number of fibers. Numerous large cavities exist between the fibers, and their size ranged from microns to tens of microns. Fig. 1c shows the micromorphology of CF-PSIP. PSM coats most of the surface of the fiber and maintains a spongy porous structure with an aperture of nm. A nanostructure was successfully constructed in the CFP. In order to study the effects of the pore structure of the dielectric on its electrical properties, we adjusted its pore size distribution by changing the ratio of TEOS and H2 O. The mol ratio of TEOS and H2 O are 1:2, 1:3, 1:4, called as CF-PSIP (1#), (2#) and (3#), respectively. And the average pore size of CFP and the CF-PSIP samples are 8571, 1338, 14 and 127 nm, respectively which obtained by nitrogen adsorption and mercury porosimetry method. We determined the DC breakdown voltage of dry and oilimpregnated samples. We conducted a DC breakdown test in accordance with the IEC 60243–2 standard [12]. The test procedure and data processing method were based on our previous works [13].
The construction of nanostructure has a positive effect on the DC breakdown performance of the sample, as shown in Fig. 2. The breakdown voltages of the dry and oil-impregnated samples simultaneously improve. It can be seen that the insulation performance of samples is greatly improved whether it is impregnated with insulating oil or not. The breakdown strength of the dielectric increases with the decrease in the aperture. Meanwhile, the lifting range of oilimmersed samples is larger than that of dry samples. For sample CF-PSIP (2#), the breakdown strength increases the most whether it is impregnated with insulating oil or not. Notably, the breakdown strength of oil impregnated CF-PSIP (2#) is 2.2 times that before treatment. This finding indicates that the insulation performance of the composite dielectric can be remarkably improved by constructing the nanostructure and is far better than the performance of the currently known nano-doping technology. The performance of CF-PSIP (2#) is 62.3% higher than that of the traditional 0.46 mm oil-paper insulation (Taizhou Jinsong Paper Industrial Co. Ltd.). The electrical conductivity, thermal conductivity and mechanical properties of samples were measured, as shown in Table 1. To sum up, the porous structure in the composite dielectric will have a significant impact on the breakdown field strength, a certain impact on the thermal conductivity and resistivity, and a very limited impact on the mechanical properties. In the following text, we focus on the improving mechanisms of the breakdown characteristics of the dielectric. Generally, the
60
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
1.0
Breakdown Probability P
Breakdown Probability P
1.0
0.8 7.5
6.4
8.53
10.36
63.1%
0.6
0.4 CFP CF-PSIP(1#) CF-PSIP(2#) CF-PSIP(3#)
0.2
7
8
9
10
44.6
33.6
62.2
74.1
63.1%
0.6
0.4 CFP CF-PSIP(1#) CF-PSIP(2#) CF-PSIP(3#)
0.2
0.0
0.0 6
0.8
30
11
40
50
60
70
Electric Strength E (kV/mm)
Electric Strength E (kV/mm)
(a)
(b)
80
Cathode
4
Anode 10s 1min 10min 20min 30min
8
Charge density (C/m3)
8
Charge density (C/m3)
Cathode
Anode 10s 1min 10min 20min 30min
0
-4
4
0
-4
-8 -8
0
0
200
Position (um) (a)
200
Average bulk density of space charge (C/m3)
Fig. 2. Weibull distribution of the DC breakdown strength. (a) Weibull distribution of DC breakdown strength for the dry sample and (b) Weibull distribution of DC breakdown strength for oil-impregnated sample.
3.3 CFP CF-PSIP
3.0 2.7 2.4 2.1 1.8 10
60
600
1200
1800
t /s
Position (um) (b)
(c)
Fig. 3. Space charges accumulation characteristics for (a) CFP; (b) CF-PSIP; (c) Average bulk density of space charges during polarization. Table 1 The experiment results of the samples with different amounts of porous silica. Sample
Volume resistivity (/cm)
CFP CF-PSIP (1#) CF-PSIP (2#) CF-PSIP (3#)
4.3 5.1 5.5 4.9
× × × ×
1019 1019 1019 1019
Heat conductivity 25 °C (W/m.K)
Mechanical property (kN/m)
0.05905 0.05580 0.04841 0.05239
1.90 1.80 1.91 1.85
breakdown of dielectric is closely related to space charge behavior and the discharge process. Therefore, we analyzed the improving mechanism considering space charge behavior and discharge process respectively. Considering the space charges transport characteristic, the space charge characteristics of composite dielectric with and without nanostructure were obtained by the Pulsed Electro-Acoustic (PEA) method as shown in Fig. 3. Note that large amount of negative charges were detected in the CFP under the motivation of DC electric field. However, in the CF-PSIP some positive charges were found in the dielectric. The average bulk charge density of CF-PSIP is much lower than that of CFP. We infer from above results that the nanostructure constructed in the dielectric may change the electron injection barrier which contributes to inhibiting the injection of negative charges and hindering the accumulation of charges in the dielectric. This is also considered a possible reason for the improving of the breakdown voltage.
Considering the breakdown process, the above results show that the insulation performance of CF-PSIP is considerably improved whether it is impregnated with insulating oil or not. Given that the discharge of insulating oil is complicated and has not been fully revealed yet, we obtained a dry sample and used it to analyze the mechanism responsible for the improvement in CF-PSIP performance. The electric strength of the air or insulating oil in the dielectric is lower than that of a solid fiber. In general, the breakdown of a dielectric under strong electric field in a dry sample is initially caused by the discharge in air. Under the effects of external field strength, gas ionization produces electrons, which multiply and develop toward the anode. In the formation of electron avalanche, the development of electron avalanche is limited by the following three important parameters: the mean free path of gas electron, the molecular number of the gas, and the size of the head of the electron avalanche [14,15].
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
61
Fig. 4. Development of electron avalanches in air and nanopores.
Mean free path of gas electron Gas electrons constantly collide with one another as they move through gas, and λe determines whether the electron avalanche can start and continue to expand. The average free path of a gas electron can be expressed as
λe =
1 n×σ
(1)
where n is the volume density of gas molecules, σ is the collision cross-section, σ =π r2 , and r is the gas molecular ra˚ At normal temperature and atmospheric presdius, r = 3 A. sure, n = 2.68 × 1019 /cm3 . By substituting n and σ into Eq. (1), λe = 150 nm. The pore diameter of PSM ϕ n is less than the average free path of gas electron, that is, λe ≥ ϕ n = 14 nm. Therefore, the development of electron collision ionization in nanostructure is considerably reduced. Molecular number of gas
Declaration of Competing Interest
The number of gas molecules is one of the most important factors restricting the further development of electron avalanche. When the number of gas molecules is insufficient, the number of electrons caused by collision ionization becomes limited and thus the breakdown of the dielectric is restrained. The pore is assumed to be a regular cylinder, and the molecular number of the gas in the pore can be expressed as
Nn = V × n = π rφ 2 d × n
(2)
where V is pore volume. rϕ is a regular cylinder cross-section radius and d is pore height. Here, rϕ = ϕ n /2, d = 0.4 mm and n = 2.68 × 1019 /cm3 . By substituting rϕ , d, and n into Eq. (2), we obtain Nn = 1.65 × 106 . The number of molecules in the pore Nn is considerably smaller than that required in the avalanche breakdown according to the Seitz breakdown theory (Ns = 1017 ). Size of the head of the electron avalanche In the development of an electron avalanche, an electron avalanche advances along the direction of the electric field (Z) toward the anode. The electron cloud at the head of the electron avalanche is spherical, and r is the size of the head of electron avalanche which can be expressed as
r=
4Dd
1 / 2
vd
(3)
where D is the diffusion coefficient. vd is the electron drift velocity in air and is as the function of E/p. E is the electric field strength and p is the atmospheric pressure. When E/p ≥ 100 V/(cm•mmHg), vd can be expressed as
vd = 1.55 × 106 (E/p)0.62 cm/s
The following result can be obtained: r = 2.6 × 10−3 cm. However, the size of the nanopole ϕ n is 14 nm and is far less than r. Therefore, the nanostructure can limit the development of electron avalanche as shown in Fig. 4. The following conclusions were drawn: We bypassed the traditional nano-doping method and constructed a dielectric material with a nanostructure. For the oil-impregnated CF-PSIP (2#), the breakdown strength under DC increased by 120.5%. To the best of our knowledge, such a high improvement in electrical performance has not been reported in previous studies. We speculate that it is a promising way to improve the intrinsic electrical properties of the composite dielectric by directly constructing matrix materials into nanostructures. Meanwhile, the construction of nanostructure weakens the accumulation of space charges and accelerates the dissipation of space charges to some extent. It also has the prospect of promoting other insulating materials, such as epoxy resin, crosslinked polyethylene, and GIS gas insulation.
(4)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding This research was supported National Natural Science Foundation of China (51837002 and 51707023). References [1] E. Celik, H. Mutlu I, Y.S. Hascicek, Scr. Mater 47 (2002) 315–320. [2] N. Jovic´ , D. Dudic´ , A. Montone, M.V. Antisari, M. Mitric´ , V. Djokovic´ , Scri. Mater 58 (2008) 846–849. [3] Z. Wang, Y. Fu, W. Meng, C. Zhi, Nanoscale Res. Lett 9 (2014) 643. [4] J. Chen, P. Sun, W. Sima, Q. Shao, L. Ye, C. Li, Nanomater 9 (2019) 788. [5] P. Sun, W. Sima, J. Chen, D. Zhang, X. Jiang, Appl. Phys. Lett 112 (2018) 142902. [6] S. Rul, F. Lefèvre-Schlick, E. Capria, Ch. Laurent, A. Peigne, Acta Mater 52 (2004) 1061–1067. [7] T. Suetsuna, S. Suenaga, K. Harada, Scr. Mater 113 (2016) 89–92. [8] J.K. Nelson, L.S. Schadler, IEEE Trans. Dielectr. Electr. Insul 21 (2014) 411 -411. [9] A.R. Alian, S. El-Borgi, S.A. Meguid, Comput. Mater. Sci 117 (2016) 195–204. [10] M. Roy, J.K. Nelson, R.K. MacCrone, L.S. Schadler, J. Mater. Sci 42 (2007) 3789–3799. [11] Y. Lei, X. Chen, Z. Hu, H. Song, B. Cao, Scr. Mater 139 (2017) 5–8. [12] “Electric strength of insulating materials - Test methods - Part 2: Additional requirements for tests using direct voltage”, IEC standard, pp. 60243–2. [13] P. Sun, W. Sima, D. Zhang, Q. Chen, L. Ye, J. Chen, IEEE Trans. Dielectr. Electr. Insul 25 (2018) 1651–1659. [14] M. Sparks, D.L. Mills, R. Warren, T. Holstein, A.A. Maradudin, L.J. Sham, E. Loh Jr., D.F. King, Phys. Rev. B 24 (1981) 3519. [15] M.A. Harrison, R. Geballe, Phys. Rev 91 (1953) 1~7.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Novel nanostructure composite dielectric with high insulation performance: Silica-based nanometer-sized porous composite insulating paper reinforced by ceramic fibers Wenxia Sima, Jiahui He, Potao Sun∗, Ming Yang, Ze Yin, Chuang Li State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400030, PR China
a r t i c l e
i n f o
Article history: Received 18 October 2019 Revised 30 December 2019 Accepted 6 February 2020
Keywords: Nanostructure Ceramic matrix composites (CMC) Dielectric Electrical properties
a b s t r a c t Nano-dielectrics, which were discovered more than 20 years ago, were once considered as promising insulators. Adding or hybridizing inorganic oxides into polymer matrices inevitably results in poor compatibility between multiphase materials and thus results in limited breakdown strength improvement (generally lower than 20%). Therefore, we bypassed the traditional nano-doping method and proposed the direct construction of matrix materials on nanostructures, that is, a ceramic fiber-porous silica composite insulating paper. The breakdown strength of prepared dielectric was remarkably improved by 120.5%. To the best of our knowledge, such a high improvement in electrical performance has not been reported in previous studies.
Insulating materials are widely used in various industries and fields, including electrical power, aerospace, rail transit, etc. [1]. In the past 25 years, scholars have tried to modify various insulating materials by nano-doping to improve the performance of the dielectric [2–7]. Unfortunately, most of the nano-doping insulating materials cannot be used in the power industry owing to the following constraints [8,9]: (1) the breakdown strength improvement by nano-doping is relatively limited (generally lower than 20%); (2) the long-term stability of the nano-modified insulating materials is poor due to the aggregation of nanoparticles [10]; and (3) a significant increase in dielectric loss due to nano-doping is unacceptable in industrial application. To address these problems, we proposed a novel method, that is, in situ hydrolyzation–condensation, to directly construct matrix materials into a nanostructure. Incompatibility between multiphase materials was avoided, and the function of nano-interface was preserved. Meanwhile, the intrinsic electrical properties of the dielectric were dramatically improved by constraining electron multiplication with the nanostructure. In this work, we focused on improving the electrical performance of a transformer insulating paper, which had been invented for more than 100 years. Ceramic fiber was used as a skeleton for enhancing the mechanical properties of the material. A porous silica medium (PSM) was constructed in the dielectric. The space barrier constructed by the PSM at nanoscale was used to sup∗
Corresponding author. E-mail address: [email protected] (P. Sun).
https://doi.org/10.1016/j.scriptamat.2020.02.016 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
press electron multiplication during the breakdown process. This method considerably improved the breakdown strength of the dielectric (2.2 times that of the dielectric without nanostructure and 62.3% higher than that of a traditional oil-paper insulation). Meanwhile the construction of nanostructure weakens the accumulation of space charges and accelerates the dissipation of space charges to some extent. The reagents for preparation were as follows: ethyl orthosilicate (TEOS), ethanol, trimethyl chlorosilane (TMCS), n-butanol, and nhexane (analytical reagents). HCL was 0.1 mol/L and NH4 OH was 1 mol/L. Deionized water was used in the study [11]. Ceramic fiber paper (CFP) was aluminum silicate ceramic fiber paper bought in Guangdong Xingwang Industrial Co. Ltd. Before the test, fiber paper was dipped into the ethyl alcohol, and dry at 100 °C for later use. CF-PSIP was prepared as follows: TEOS, ethanol, and deionized water were mixed together at room temperature. HCL was added to make the pH of the system acidic (pH=3.5). Silica sol was obtained several hours later. Ceramic fiber paper (CFP) was impregnated in silica sol and placed in a vacuum-drying box for 40 min. Then, NH4 OH was added for alkali catalysis. The container was stored at 80 °C for condensation polymerization. After the silica-fiber composite gels were formed, the composites gels were dipped into a TEOS/ethanol mixture with a volume ratio 6:4. The mixture was then aged for 2 days and soaked in ethanol for 1 h. After the composite gels were aged, they were dipped into n-butanol two times for 24 h each. A solution of n-butanol, n-hexane, and 3% TMCS was added, and the surface hydrophobic chemical modification of the gel was carried out for 24 h at 50 °C.
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
59
Fig. 1. The microstructure of the sample. (a) Mesoporous distribution of CF-PSIP, (b) SEM of CFP and (c) SEM of CF-PSIP.
After the surface modification, the composite gels were finally dried in a vacuum-drying box for 36 h, and CF-PSIP was obtained. We measured the pore size distribution of CF-PSIP (Fig 1a). The size of the mesopores mainly ranged from 10 nm to 22 nm, with an average of 14 nm. The result shows that the nanostructure was successfully constructed in the CFP. We used a needle tip to remove the surface material of the sample and sprayed the internal tear surface with gold. Then, we observed the internal morphology by SEM. Fig. 1b shows the CFP micromorphology with a large number of fibers. Numerous large cavities exist between the fibers, and their size ranged from microns to tens of microns. Fig. 1c shows the micromorphology of CF-PSIP. PSM coats most of the surface of the fiber and maintains a spongy porous structure with an aperture of nm. A nanostructure was successfully constructed in the CFP. In order to study the effects of the pore structure of the dielectric on its electrical properties, we adjusted its pore size distribution by changing the ratio of TEOS and H2 O. The mol ratio of TEOS and H2 O are 1:2, 1:3, 1:4, called as CF-PSIP (1#), (2#) and (3#), respectively. And the average pore size of CFP and the CF-PSIP samples are 8571, 1338, 14 and 127 nm, respectively which obtained by nitrogen adsorption and mercury porosimetry method. We determined the DC breakdown voltage of dry and oilimpregnated samples. We conducted a DC breakdown test in accordance with the IEC 60243–2 standard [12]. The test procedure and data processing method were based on our previous works [13].
The construction of nanostructure has a positive effect on the DC breakdown performance of the sample, as shown in Fig. 2. The breakdown voltages of the dry and oil-impregnated samples simultaneously improve. It can be seen that the insulation performance of samples is greatly improved whether it is impregnated with insulating oil or not. The breakdown strength of the dielectric increases with the decrease in the aperture. Meanwhile, the lifting range of oilimmersed samples is larger than that of dry samples. For sample CF-PSIP (2#), the breakdown strength increases the most whether it is impregnated with insulating oil or not. Notably, the breakdown strength of oil impregnated CF-PSIP (2#) is 2.2 times that before treatment. This finding indicates that the insulation performance of the composite dielectric can be remarkably improved by constructing the nanostructure and is far better than the performance of the currently known nano-doping technology. The performance of CF-PSIP (2#) is 62.3% higher than that of the traditional 0.46 mm oil-paper insulation (Taizhou Jinsong Paper Industrial Co. Ltd.). The electrical conductivity, thermal conductivity and mechanical properties of samples were measured, as shown in Table 1. To sum up, the porous structure in the composite dielectric will have a significant impact on the breakdown field strength, a certain impact on the thermal conductivity and resistivity, and a very limited impact on the mechanical properties. In the following text, we focus on the improving mechanisms of the breakdown characteristics of the dielectric. Generally, the
60
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
1.0
Breakdown Probability P
Breakdown Probability P
1.0
0.8 7.5
6.4
8.53
10.36
63.1%
0.6
0.4 CFP CF-PSIP(1#) CF-PSIP(2#) CF-PSIP(3#)
0.2
7
8
9
10
44.6
33.6
62.2
74.1
63.1%
0.6
0.4 CFP CF-PSIP(1#) CF-PSIP(2#) CF-PSIP(3#)
0.2
0.0
0.0 6
0.8
30
11
40
50
60
70
Electric Strength E (kV/mm)
Electric Strength E (kV/mm)
(a)
(b)
80
Cathode
4
Anode 10s 1min 10min 20min 30min
8
Charge density (C/m3)
8
Charge density (C/m3)
Cathode
Anode 10s 1min 10min 20min 30min
0
-4
4
0
-4
-8 -8
0
0
200
Position (um) (a)
200
Average bulk density of space charge (C/m3)
Fig. 2. Weibull distribution of the DC breakdown strength. (a) Weibull distribution of DC breakdown strength for the dry sample and (b) Weibull distribution of DC breakdown strength for oil-impregnated sample.
3.3 CFP CF-PSIP
3.0 2.7 2.4 2.1 1.8 10
60
600
1200
1800
t /s
Position (um) (b)
(c)
Fig. 3. Space charges accumulation characteristics for (a) CFP; (b) CF-PSIP; (c) Average bulk density of space charges during polarization. Table 1 The experiment results of the samples with different amounts of porous silica. Sample
Volume resistivity (/cm)
CFP CF-PSIP (1#) CF-PSIP (2#) CF-PSIP (3#)
4.3 5.1 5.5 4.9
× × × ×
1019 1019 1019 1019
Heat conductivity 25 °C (W/m.K)
Mechanical property (kN/m)
0.05905 0.05580 0.04841 0.05239
1.90 1.80 1.91 1.85
breakdown of dielectric is closely related to space charge behavior and the discharge process. Therefore, we analyzed the improving mechanism considering space charge behavior and discharge process respectively. Considering the space charges transport characteristic, the space charge characteristics of composite dielectric with and without nanostructure were obtained by the Pulsed Electro-Acoustic (PEA) method as shown in Fig. 3. Note that large amount of negative charges were detected in the CFP under the motivation of DC electric field. However, in the CF-PSIP some positive charges were found in the dielectric. The average bulk charge density of CF-PSIP is much lower than that of CFP. We infer from above results that the nanostructure constructed in the dielectric may change the electron injection barrier which contributes to inhibiting the injection of negative charges and hindering the accumulation of charges in the dielectric. This is also considered a possible reason for the improving of the breakdown voltage.
Considering the breakdown process, the above results show that the insulation performance of CF-PSIP is considerably improved whether it is impregnated with insulating oil or not. Given that the discharge of insulating oil is complicated and has not been fully revealed yet, we obtained a dry sample and used it to analyze the mechanism responsible for the improvement in CF-PSIP performance. The electric strength of the air or insulating oil in the dielectric is lower than that of a solid fiber. In general, the breakdown of a dielectric under strong electric field in a dry sample is initially caused by the discharge in air. Under the effects of external field strength, gas ionization produces electrons, which multiply and develop toward the anode. In the formation of electron avalanche, the development of electron avalanche is limited by the following three important parameters: the mean free path of gas electron, the molecular number of the gas, and the size of the head of the electron avalanche [14,15].
W. Sima, J. He and P. Sun et al. / Scripta Materialia 181 (2020) 58–61
61
Fig. 4. Development of electron avalanches in air and nanopores.
Mean free path of gas electron Gas electrons constantly collide with one another as they move through gas, and λe determines whether the electron avalanche can start and continue to expand. The average free path of a gas electron can be expressed as
λe =
1 n×σ
(1)
where n is the volume density of gas molecules, σ is the collision cross-section, σ =π r2 , and r is the gas molecular ra˚ At normal temperature and atmospheric presdius, r = 3 A. sure, n = 2.68 × 1019 /cm3 . By substituting n and σ into Eq. (1), λe = 150 nm. The pore diameter of PSM ϕ n is less than the average free path of gas electron, that is, λe ≥ ϕ n = 14 nm. Therefore, the development of electron collision ionization in nanostructure is considerably reduced. Molecular number of gas
Declaration of Competing Interest
The number of gas molecules is one of the most important factors restricting the further development of electron avalanche. When the number of gas molecules is insufficient, the number of electrons caused by collision ionization becomes limited and thus the breakdown of the dielectric is restrained. The pore is assumed to be a regular cylinder, and the molecular number of the gas in the pore can be expressed as
Nn = V × n = π rφ 2 d × n
(2)
where V is pore volume. rϕ is a regular cylinder cross-section radius and d is pore height. Here, rϕ = ϕ n /2, d = 0.4 mm and n = 2.68 × 1019 /cm3 . By substituting rϕ , d, and n into Eq. (2), we obtain Nn = 1.65 × 106 . The number of molecules in the pore Nn is considerably smaller than that required in the avalanche breakdown according to the Seitz breakdown theory (Ns = 1017 ). Size of the head of the electron avalanche In the development of an electron avalanche, an electron avalanche advances along the direction of the electric field (Z) toward the anode. The electron cloud at the head of the electron avalanche is spherical, and r is the size of the head of electron avalanche which can be expressed as
r=
4Dd
1 / 2
vd
(3)
where D is the diffusion coefficient. vd is the electron drift velocity in air and is as the function of E/p. E is the electric field strength and p is the atmospheric pressure. When E/p ≥ 100 V/(cm•mmHg), vd can be expressed as
vd = 1.55 × 106 (E/p)0.62 cm/s
The following result can be obtained: r = 2.6 × 10−3 cm. However, the size of the nanopole ϕ n is 14 nm and is far less than r. Therefore, the nanostructure can limit the development of electron avalanche as shown in Fig. 4. The following conclusions were drawn: We bypassed the traditional nano-doping method and constructed a dielectric material with a nanostructure. For the oil-impregnated CF-PSIP (2#), the breakdown strength under DC increased by 120.5%. To the best of our knowledge, such a high improvement in electrical performance has not been reported in previous studies. We speculate that it is a promising way to improve the intrinsic electrical properties of the composite dielectric by directly constructing matrix materials into nanostructures. Meanwhile, the construction of nanostructure weakens the accumulation of space charges and accelerates the dissipation of space charges to some extent. It also has the prospect of promoting other insulating materials, such as epoxy resin, crosslinked polyethylene, and GIS gas insulation.
(4)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding This research was supported National Natural Science Foundation of China (51837002 and 51707023). References [1] E. Celik, H. Mutlu I, Y.S. Hascicek, Scr. Mater 47 (2002) 315–320. [2] N. Jovic´ , D. Dudic´ , A. Montone, M.V. Antisari, M. Mitric´ , V. Djokovic´ , Scri. Mater 58 (2008) 846–849. [3] Z. Wang, Y. Fu, W. Meng, C. Zhi, Nanoscale Res. Lett 9 (2014) 643. [4] J. Chen, P. Sun, W. Sima, Q. Shao, L. Ye, C. Li, Nanomater 9 (2019) 788. [5] P. Sun, W. Sima, J. Chen, D. Zhang, X. Jiang, Appl. Phys. Lett 112 (2018) 142902. [6] S. Rul, F. Lefèvre-Schlick, E. Capria, Ch. Laurent, A. Peigne, Acta Mater 52 (2004) 1061–1067. [7] T. Suetsuna, S. Suenaga, K. Harada, Scr. Mater 113 (2016) 89–92. [8] J.K. Nelson, L.S. Schadler, IEEE Trans. Dielectr. Electr. Insul 21 (2014) 411 -411. [9] A.R. Alian, S. El-Borgi, S.A. Meguid, Comput. Mater. Sci 117 (2016) 195–204. [10] M. Roy, J.K. Nelson, R.K. MacCrone, L.S. Schadler, J. Mater. Sci 42 (2007) 3789–3799. [11] Y. Lei, X. Chen, Z. Hu, H. Song, B. Cao, Scr. Mater 139 (2017) 5–8. [12] “Electric strength of insulating materials - Test methods - Part 2: Additional requirements for tests using direct voltage”, IEC standard, pp. 60243–2. [13] P. Sun, W. Sima, D. Zhang, Q. Chen, L. Ye, J. Chen, IEEE Trans. Dielectr. Electr. Insul 25 (2018) 1651–1659. [14] M. Sparks, D.L. Mills, R. Warren, T. Holstein, A.A. Maradudin, L.J. Sham, E. Loh Jr., D.F. King, Phys. Rev. B 24 (1981) 3519. [15] M.A. Harrison, R. Geballe, Phys. Rev 91 (1953) 1~7.