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High voltage breakdown studies of sol–gel MgO–ZrO2 insulation coatings under various pressures at 298 K and 77 K
Physica C 422 (2005) 117–126 www.elsevier.com/locate/physc
High voltage breakdown studies of sol–gel MgO–ZrO2 insulation coatings under various pressures at 298 K and 77 K O. Cakiroglu *, L. Arda 1, Y.S. Hascicek National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA Received 21 February 2005; received in revised form 11 March 2005; accepted 11 March 2005 Available online 22 April 2005
Abstract High voltage breakdown (HVbd) tests were performed to investigate electrical properties of high temperature MgO– ZrO2 insulation coatings on long-length stainless steel (SS) tapes under various pressures at room temperature (298 K) and liquid nitrogen temperature (77 K) for applications of HTS/LTS coils and magnets. After solutions were prepared from Mg and Zr based precursors, solvent and chelating agent, the coating were fabricated on SS substrates using reelto-reel sol–gel technique. Coating thicknesses for 4, 8, and 9 dippings were about 7, 12, and 13 lm, respectively, and thickness of epoxy-impregnated samples (stycast 2850 FT/24 LV) were measured to be 32 lm. The pressure from 0 GPa to 0.54 GPa was applied on to test couples, the stycast thicknesses between the layers were varied 32–20 lm. Thickness of the coatings and epoxy-impregnated using stycast were determined by using environmental scanning electron microscopy (ESEM). The resistance, capacitance, and HVbd of the samples were measured by using standard machines HP 439 a high resistance meter, 161 analog digital capacitance meter and model 200–02R high voltage power supply, respectively. Electric strength and dielectric constant were calculated at 298 and 77 K under various pressures. The high HVbd, and electric strength values of the samples were 2.84 kV and 45.91 kV/mm, respectively. ESEM observation revealed that arcing spots in the insulation coatings become larger and deeper for higher HVbdÕs. 2005 Elsevier B.V. All rights reserved. PACS: 85.50.+k; 77.55.+f; 72.80.Sk Keywords: High voltage breakdown; Sol–gel; MgO–ZrO2
*
Corresponding author. Present address: Istanbul University, Faculty of Education, Bayezit 34470, Istanbul, Turkey. Tel.: +90 536 5720005; fax: +90 212 5130561. E-mail address: [email protected] (O. Cakiroglu). 1 Present address: Marmara University, Faculty of Science, Department of Physics, Goztepe Campus, 34722 Kadikoy, Istanbul, Turkey.
1. Introduction MgO–ZrO2 coatings have been insulated on SS-304 tapes using a reel-to-reel continues sol–gel technique. The technique was developed to
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provide turn-to-turn electrical insulation for high temperature superconductor (HTS) coil at the National High Magnetic Field Laboratory (NHMFL) [1] and has been used extensively [2–8]. It is vitally essential to use high temperature well-matched insulating materials for insulation in wind and react (W&R) technique for making coils and magnet from the HTS. The details of various insulators, which are used to build coils, can be seen in Ref. [9]. Of these insulation methods, reel-to-reel sol–gel technique is the most suited one, as reported elsewhere [1,2,4]. Also the sol–gel technique is very attractive low temperature processing technique for preparation of complex oxide composition with high homogeneity [9–11]. ZrO2 based coatings are suitable for high temperature insulation due to its chemical stability, high resistivity, and large relative dielectric constant. The large dielectric constant of ZrO2 provides outstanding insulation strength. Dielectric constant of crystal phase ZrO2 is 19, 20, 37, and 38 for orthorhombic, monoclinic, cubic, and tetragonal phases, respectively [12]. The cubic phase of ZrO2, above 2370 C until the melt forms at 2706 C [13], demonstrates the better insulation properties among different ZrO2 modification, in that the cubic phase of ZrO2 can be stabilized successfully by adding appropriate dopents of MgO, CaO, CeO2, Y2O3, and other rare earth oxides [10,14]. Both ZrO2 phase transformation and stabilization has been the subject of wide scientific investigation including electric–electronic structure, optical studies, etc. [13,15]. Sol–gel MgO– ZrO2 insulation coatings are most suited to HVbd tests for HTS since MgO content in ZrO2 seems to increase HVbd, and hence positively affect electrical properties of insulation coating. HVbd tests of electrical insulation materials, insulation system, and magnet wire coatings have been carried out at 298 K and 77 K by some researchers at several laboratories [9,16–18]. Previous researchers at NHMFL studied electrical properties of MgO–ZrO2 coating, on Ag and AgMg/Bi-2212 superconducting tapes [3,4,9,19], Cu–Nb3Sn wires [5], stainless steel and Ni wires [4,11] which were applied by a reel-to-reel continues sol–gel technique at ambient pressure at 298 K.
In this study, sol–gel MgO–ZrO2 insulation was coated on SS-304 tapes with dips of 4, 8, and 9 times. Three kinds of test samples were made of each of these tapes. Test couples were prepared with the tape with stycast (WS), without stycast (WOS), and with stycast prepared under pressure (WSUP). HVbd and electric strength of two types of these samples were recently published [20]. Pressure in the WSUP samples varied from 0 GPa to 0.54 GPa, and stycast was left to cure for 8 h. The electrical properties of the test couples, such as HVbd and dielectric constant were then investigated at 298 K and 77 K, and under various pressures. Surface morphology of arcing spots of the samples was examined using ESEM.
2. Experimental The MgO–ZrO2 coatings were applied on commercial SS tapes by using a reel-to-reel sol–gel dipping process with speed of 0.65 m/min. As shown in Table 1, two types of solution were prepared, dilute and normal. The normal coating solution consists of 20 mol% MgO, and 80 mol% ZrO2, which was prepared using zirconium tetrabutoxide, isopropanal, and acetone. Dilute solution is obtained by adding 50/50 isopropanol to normal solution. Zirconium tetrabutoxide and several Mg precursors were used as precursors, which was prepared as clear homogenous solution by stirring for 8 h. The detailed information on solution preparation and coating process can be found in Refs. [1,6,9, 19,21]. SS tapes with 5 mm width and 0.0254 mm thickness were cleaned with acetone. Then SS tape was coated by MgO–ZrO2 solution withdrawal rate of 0.65 m/min. The as-deposited films with dilute and normal solutions were dried at 550 C and 580 C, respectively. They were then exposed to heat-treating temperature in the range of 570– 580 C for the dilute and 610–630 C for the normal solutions, using an in-line three zone furnace. The coating process was repeated 4, 8, and 9 times. As seen in Table 1, the dip coating is composed of dipping, withdrawal, and heating. Coating thickness is an important factor for electrical properties,
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Table 1 The detail sol–gel MgO–ZrO2 insulation coating parameters Insulation
Number of Dip
Furnace temp. (C)
MgO–ZrO2
Total dip#:9 3 Dip dilute 4 Dip normal 2 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
Total dip#:8 3 Dip dilute 3 Dip normal 2 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
Total dip#:4 1 Dip dilute 2 Dip normal 1 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
MgO–ZrO2
MgO–ZrO2
which is regulated with dip numbers, viscosity of solution, wetting, and withdrawal rate. Test samples were cut 10 cm in length from insulated SS tapes and the insulation layers were removed from both ends. Three kinds of test couples produced for each 4, 8, and 9 dip samples. First type of couples were made without stycast; the second type was made with stycast that is in the ratio of 2850 FT of 4 units and of catalyst 24 LV of 3 units at room temperature curing for 8 h; and the third type was prepared with stycast under pressure ranging from 0 GPa to 0.54 GPa, and was left to cure for 8 h under the pressure. Samples with stycast were cured for all dips in the same way. The thickness of coating and stycast were determined by using ESEM, as seen in Fig. 1. The test couples were placed for testing the experimental set up as shown in Fig. 2. The resistance, HVbd and capacitance were measured by using HP 4339 A high resistance meter, model 200– 02R high voltage power supply and 161 analog digital capacitance meter, respectively. For each measurement, the process was repeated at least three times. The dielectric constant of samples was calculated by using Eq. (1) without taking into account of effects of strain. There should be a very small change in the capacitor gap due to cooling and applied pressure. er ¼
Cðd i þ d s Þ e0 A
ð1Þ
Withdrawal speed (m/min)
Thickness (lm)
0.65
13
0.65
12
0.65
7
Fig. 1. The insulation, stycast, and stainless steel of eight times dipped substrates. The scale bar is 50 lm.
Fig. 2. Experimental set up for HVbd tests.
where, er is dielectric constant of insulation coatings, e0 permittivity of free space (8.854 ·
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1012 F/m), A is coated area in the sandwich, di is twice the coating thickness, and ds is the stycast thickness. Insulation and stycast thickness were measured as shown in Fig. 2. HVbd were measured samples with and without epoxy at 298 K and 77 K. The width and depth of arcing spots were determined by ESEM and Tencor Alpha-step 200 profilemeter.
constant for WSUP samples, as expected. The average values of these parameters are given in Table 2. It is clearly seen in Fig. 3 that values of surface resistivities of all the WOS samples at 298 K decrease sharply at about 0.08 GPa. Note that in Figs. 3 and 4 the surface resistivities of WS and WSUP test couples do not vary much by pressure at 298 K or 77 K. The surface resistivities of WOS samples are about 4 and 3 times higher than WS and WSUP samples for 4, 8, and 9 dips at 77 K. As seen in Table 2, the surface resistivities of WOS test couples are 15.54, 18.02, and 18.84 GOhms/cm2 for 4, 8, and 9 dips, respectively. Figs. 5 and 6 show the HVbd at 298 K and 77 K for 4, 8, and 9 dip samples for all of samples, respectively. HVbd values increased with increasing coating thickness, stycast thickness, and temperature. It is clear from Fig. 6 and Table 2 that the HVbd values of WOS samples for 4 dips are lower than 8 dips, which are lower than 9 dips samples, as expected. Also, HVbd values were increased about 3 times with epoxy-impregnation, when the samples were cooled from 298 K to 77 K. Epoxy-impregnation increased the HVbd values 2.3 times for 4 dips and 1.9 times for the other dips at 298 K. These increments were about 2.5 for all of samples at 77 K. The values of the HVbd of WSUP samples slightly decreased with increasing
3. Results and discussion As expected, coating thickness, crack in coatings, dopant materials, stycast thickness, pressure, and temperature influence electrical, mechanical properties as well as surface morphology and structure of insulations. HVbd test results, such as surface resistivity, HVbd, electric strength, and dielectric constant for each sample type at 298 K and 77 K, are graphically shown. When the mass of 3 kg of G-10 block, which was assumed zero pressure, was put on the samples with stycast, the thicknesses of stycast were about 32 lm. When pressure (0–0.54 GPa) was applied on the samples with stycast the thickness of stycast decreased according to pressure from 32 lm to 20 lm. The change in the thickness of epoxy-impregnation affected the HVbd, electrical strength, and dielectric
Table 2 Average test parameters of sol–gel MgO–ZrO2 insulations coatings on SS Room temperature (298 K)
Liquid nitrogen temperature (77 K)
Dip number
4
8
9
4
8
9
Without stycast (WOS) Surface res. (GOhms/cm2) HVbd (kV) Elec. strength (kV/mm) Dielectric constant
6.43 ! 0.056 0.21 14.93 8.50 ! 17.60
7.71 ! 0.024 0.37 15.35 10.67 ! 23.27
7.87 ! 0.032 0.39 15.36 11.25 ! 24.30
15.54 0.61 44.24 7.28 ! 14.56
18.02 1.15 47.82 9.61 ! 17.51
18.84 1.24 48.92 10.15 ! 18.00
With stycast (WS) Surface res. (GOhms/cm2) HVbd (kV) Elec. strength (kV/mm) Dielectric constant
3.41 0.51 12.06 7.15
4.68 0.71 12.63 8.68
5.32 0.78 13.39 9.17
3.79 1.83 43.35 6.65
5.93 2.67 47.57 7.95
6.61 2.84 48.69 8.80
5.17 0.70 13.26 9.13
3.53 1.63 42.13 6.30
5.75 2.33 47.33 7.57
6.27 2.56 48.43 8.52
With stycast under pressure (WSUP) P = 0–0.54 GPa Surface res. (GOhms/cm2) 3.28 4.50 HVbd (kV) 0.46 0.62 Elec. strength (kV/mm) 11.54 12.50 Dielectric constant 7.08 8.47
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2
Surface Resistivity (GOhms/cm )
10.00
1.00
T=298 K
4D. WOS
8D. WOS
9D. WOS
4D. WS
8D. WS
9D. WS
4D. WSUP
8D. WSUP
9D. WSUP
0.10
0.01 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 3. Resistivity vs pressure at 298 K for the test samples. Note that resistivity drops significantly from 0 GPa to 0.08 GPa at 298 K.
2
Surface Resistivity (GOhms/cm )
22.00
18.00
14.00 T=77 K 10.00
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
6.00
2.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 4. Resistivity vs pressure at 77 K for the test samples. Resistivities of WOS samples are higher than WS and WSUP samples.
pressure as stycast thickness of ribbons decreased from 32 lm to 20 lm. The high HVbd values were 0.78 and 2.84 kV for 9 dip WS samples at 298 K and 77 K, respectively. Here HVbd was about 0.39 V at 9 dip samples of WOS at 298 K, which is consistent with the earlier results [5,19].
Electric strength values of MgO–ZrO2 insulation coating on SS-304 tapes were given in Figs. 7 and 8. As seen in Fig. 7 and Table 2, electric strength values of ribbons increase depending on dip numbers in order for 4, 8, and 9 dips samples. These increments are about 3 and 3.7 times for
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0.90
T=298 K
4D, WOS
8D,WO S
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
HVbd (kV)
0.70
0.50
0.30
0.10 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 5. HVbd vs pressure at 298 K for all of test samples. HVbd increases with increasing dip numbers.
3.60 T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
8D, WS
HVbd (kV)
2.80
2.00
1.20
0.40 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 6. HVbd vs pressure with stycast at 77 K. HVbd decreases with decreasing stycast thickness for WSUP samples.
WOS, WS, and WSUP samples, respectively, when the samples were cooled from 298 K to 77 K. Electric strength values are decreased at a small ratio with epoxy-impregnation owing to the epoxy thickness. Electric strength should be constant according to equation E = V (Volt)/d (m). How-
ever, it slightly decreases with decreasing stycast thickness for WSUP samples. Table 2 and Fig. 7 indicate that electric strength is about 14.93, 15.35, and 15.36 kV/mm for 4, 8, and 9 dips for WOS samples at 298 K, which is in accordance with earlier researchers [5,19].
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Electric Strength (kV/mm)
16.00
14.00
12.00
T=298 K
10.00 0.00
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 7. Electric strength as a function of pressure at 298 K. Electric strength increases with small ratio increasing dip numbers but it decreases with stycast thickness.
51.00 T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
8D, WS
Electric Strength (kV/mm)
49.00
47.00
45.00
43.00
41.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 8. Variation of electric strength of samples as a function of pressure at 77 K. Electric strength increases with coating thicknesses.
As shown in Figs. 9 and 10 dielectric constant values for WOS test couples increased with increasing number of dipping and pressure at 298 K and 77 K. But, these increments at 77 K are lower than those at 298 K as seen in Fig. 10. When the samples were cooled from 298 K to
77 K, dielectric constant decreased at smaller ratio. Dielectric constants of WOS samples vary between 8.50 and 24.30 depending on dip numbers and pressure at 298 K and 77 K, as seen in Table 2. These increments are about 7.08–8.80 for the WS and WSUP samples.
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T=298 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
Dielectric Constant
21.00
16.00
11.00
6.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 9. Variation of dielectric constant of insulation of samples as functions of number of dipping, temperature and pressure. Dielectric constant increases with increasing pressure for WOS samples.
20.00
18.00
T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
Dielectric Constant
16.00
14.00
12.00
10.00
8.00
6.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 10. Variation of dielectric constant of insulation as a function of number of dipping at 77 K and pressure. Dielectric constant is invariable with pressure owing to epoxy-impregnation.
Figs. 11 and 12 show ESEM micrographs of the arcing spot of the MgO–ZrO2 insulated samples, which were used in HVbd tests at 298 K and 77 K. The pictures of the WOS samples were taken
to examine the arcing spots for HVbd test. Note that, molten area of insulation and SS substrate are larger with larger number of dipping. Figs. 11 and 12 show that as dip number increases,
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Fig. 11. ESEM micrographs of the arcing spots of 4, 8, and 9 dipped substrates with MgO–ZrO2. The test was performed by using samples without epoxy at 298 K. The scale bar is 50 m on the micrographs.
Fig. 12. ESEM micrographs of arcing spots of 4, 8, and 9 dipped samples without epoxy at zero applied pressure and 77 K. The scale bar is 50 lm on the micrographs.
HVbd increases. Consequently arcing spots and molten area of the insulation become larger and deeper as expected.
4. Conclusion MgO–ZrO2 was coated on SS-304 tapes at 4, 8, and 9 dips by using reel-to-reel sol–gel technique. Test couples were formed as WS, WOS, and WSUP. Thickness of insulation and stycast was determined by ESEM, which were about 7, 12, and 13 lm for insulation for 4, 8, and 9 dips, and between 32–20 lm for stycast, respectively. HVbd tests were performed with these samples under various stresses at 298 K and 77 K. Surface
resistivity of WOS samples decreases with increasing pressure, being sharper from 0 GPa to 0.08 GPa at 298 K. However, they are constant at 77 K. When samples were cooled from 298 K to 77 K, HVbd values increased. HVbd slightly decreases with decreasing stycast thickness as seen for WSUP samples. The high values of HVbd are 0.39, 1.24 kV for WOS, and 0.78 kV, 2.84 kV for WS samples for 9 dips, for the former 298 K and the latter 77 K. Electric strength values increase with increasing number of dipping for all of samples. When the samples were cooled from 298 K to 77 K, the electric strength increased. Electric strength of WSUP test couples slightly decreases with decreasing stycast thickness between the layers. Dielectric constant depends on capacitance,
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temperature, pressure, coating, and stycast thickness. Dielectric constant values for WOS ribbons gradually increased with increasing numbers of dipping as well as increasing pressure at 298 K. But this trend is much less at 77 K. The dielectric constant values for WS samples are constant with increasing pressure, but decreases with increasing number of dipping. Dielectric constant of WS test couple values is found to be 7.15–9.17 and 6.65– 8.80 for WS samples at 298 K and 77 K, respectively. ESEM micrographs show that arcing spots, which seem to be molten SS tape of insulation, become larger and deeper with increasing dip numbers and HVbd. Acknowledgements This work is based upon research carried out at the National High Magnetic Field Laboratory (NHMFL), which is supported by the National Science Foundation, under Award No. DMR9527035.
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[3] E. Celik, I.H. Mutlu, E. Avci, Y.S. Hascicek, Adv. Cryogenic Eng. (Mater.) 46 (2000) 291. [4] E. Celik, I.H. Mutlu, Y.S. Hascicek, IEEE Trans. Appl. Superconduct. 10 (1) (2000) 1341. [5] E. Celik, I.H. Mutlu, H. Okuyucu, Y.S. Hascicek, IEEE Trans. Appl. Superconduct. 11 (2001) 2881. [6] E. Celik, E. Avci, Y.S. Hascicek, Physica C 340 (2000) 193. [7] E. Celik, I.H. Mutlu, Y. Akin, H. Okuyucu, Y.S. Hascicek, Adv. Cryogenic Eng. (Mater.) 48 (2002) 277. [8] E. Celik, Y. Akin, W. Sigmund, Y.S. Hascicek, IEEE Trans. Appl. Superconduct. 13 (2) (2003). [9] E. Celik, I.H. Mutlu, E. Avci, Y.S. Hascicek, Scripta Mater. 47 (2002) 315. [10] G. Emig, R. Wirth, R. Zimmermann-Chopin, J. Mater. Sci. 29 (1994) 4559. [11] E. Celik, I.H. Mutlu, Y.S. Hascicek, Physica C 370 (2002) 125. [12] X. Zhao, D. Vanderbilt, cod-mat/0403131 v1 3 March 2004. [13] R.H. French, S.J. Glass, F.S. Ohuchi, Y.-N. Xu, W.Y. Ching, Phys. Rev. B 49 (1994) 5133. [14] J.K. Dewhurst, J.E. Lowter, Phys. Rev. B 57 (1998) 741. [15] H.J. Jansen, J.A. Gardner, Physica B 150 (1988) 10. [16] E.W. Gray, T.A. Uhrig, IEEE Trans. Appl. Superconduct. 3 (1980) 442. [17] V. Selvamanickan, D.W. Hazelton, L. Motowidlo, F. Krahula, J. Hoehn, M.S. Walker, P. Halder, JOM (1998) 27. [18] S.M. Baek, J.M. Joung, J.H. Lee, S.H. Kim, IEEE Trans. Appl. Superconduct. 13 (2003) 2317. [19] I.H. Mutlu, E. Celik, Y.S. Hascicek, Physica C 370 (2002) 113. [20] O. Cakiroglu, L. Arda, Z. Aslanoglu, Y. Akin, O. Dur, A. Kaplan, Y.S. Hascicek, Adv. Cryogenic Eng. Mater.) 50 (2004) 184. [21] L. Yang, J. Cheng, J. Non-Cryst. Solids 112 (1989) 422.
High voltage breakdown studies of sol–gel MgO–ZrO2 insulation coatings under various pressures at 298 K and 77 K O. Cakiroglu *, L. Arda 1, Y.S. Hascicek National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA Received 21 February 2005; received in revised form 11 March 2005; accepted 11 March 2005 Available online 22 April 2005
Abstract High voltage breakdown (HVbd) tests were performed to investigate electrical properties of high temperature MgO– ZrO2 insulation coatings on long-length stainless steel (SS) tapes under various pressures at room temperature (298 K) and liquid nitrogen temperature (77 K) for applications of HTS/LTS coils and magnets. After solutions were prepared from Mg and Zr based precursors, solvent and chelating agent, the coating were fabricated on SS substrates using reelto-reel sol–gel technique. Coating thicknesses for 4, 8, and 9 dippings were about 7, 12, and 13 lm, respectively, and thickness of epoxy-impregnated samples (stycast 2850 FT/24 LV) were measured to be 32 lm. The pressure from 0 GPa to 0.54 GPa was applied on to test couples, the stycast thicknesses between the layers were varied 32–20 lm. Thickness of the coatings and epoxy-impregnated using stycast were determined by using environmental scanning electron microscopy (ESEM). The resistance, capacitance, and HVbd of the samples were measured by using standard machines HP 439 a high resistance meter, 161 analog digital capacitance meter and model 200–02R high voltage power supply, respectively. Electric strength and dielectric constant were calculated at 298 and 77 K under various pressures. The high HVbd, and electric strength values of the samples were 2.84 kV and 45.91 kV/mm, respectively. ESEM observation revealed that arcing spots in the insulation coatings become larger and deeper for higher HVbdÕs. 2005 Elsevier B.V. All rights reserved. PACS: 85.50.+k; 77.55.+f; 72.80.Sk Keywords: High voltage breakdown; Sol–gel; MgO–ZrO2
*
Corresponding author. Present address: Istanbul University, Faculty of Education, Bayezit 34470, Istanbul, Turkey. Tel.: +90 536 5720005; fax: +90 212 5130561. E-mail address: [email protected] (O. Cakiroglu). 1 Present address: Marmara University, Faculty of Science, Department of Physics, Goztepe Campus, 34722 Kadikoy, Istanbul, Turkey.
1. Introduction MgO–ZrO2 coatings have been insulated on SS-304 tapes using a reel-to-reel continues sol–gel technique. The technique was developed to
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.03.013
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provide turn-to-turn electrical insulation for high temperature superconductor (HTS) coil at the National High Magnetic Field Laboratory (NHMFL) [1] and has been used extensively [2–8]. It is vitally essential to use high temperature well-matched insulating materials for insulation in wind and react (W&R) technique for making coils and magnet from the HTS. The details of various insulators, which are used to build coils, can be seen in Ref. [9]. Of these insulation methods, reel-to-reel sol–gel technique is the most suited one, as reported elsewhere [1,2,4]. Also the sol–gel technique is very attractive low temperature processing technique for preparation of complex oxide composition with high homogeneity [9–11]. ZrO2 based coatings are suitable for high temperature insulation due to its chemical stability, high resistivity, and large relative dielectric constant. The large dielectric constant of ZrO2 provides outstanding insulation strength. Dielectric constant of crystal phase ZrO2 is 19, 20, 37, and 38 for orthorhombic, monoclinic, cubic, and tetragonal phases, respectively [12]. The cubic phase of ZrO2, above 2370 C until the melt forms at 2706 C [13], demonstrates the better insulation properties among different ZrO2 modification, in that the cubic phase of ZrO2 can be stabilized successfully by adding appropriate dopents of MgO, CaO, CeO2, Y2O3, and other rare earth oxides [10,14]. Both ZrO2 phase transformation and stabilization has been the subject of wide scientific investigation including electric–electronic structure, optical studies, etc. [13,15]. Sol–gel MgO– ZrO2 insulation coatings are most suited to HVbd tests for HTS since MgO content in ZrO2 seems to increase HVbd, and hence positively affect electrical properties of insulation coating. HVbd tests of electrical insulation materials, insulation system, and magnet wire coatings have been carried out at 298 K and 77 K by some researchers at several laboratories [9,16–18]. Previous researchers at NHMFL studied electrical properties of MgO–ZrO2 coating, on Ag and AgMg/Bi-2212 superconducting tapes [3,4,9,19], Cu–Nb3Sn wires [5], stainless steel and Ni wires [4,11] which were applied by a reel-to-reel continues sol–gel technique at ambient pressure at 298 K.
In this study, sol–gel MgO–ZrO2 insulation was coated on SS-304 tapes with dips of 4, 8, and 9 times. Three kinds of test samples were made of each of these tapes. Test couples were prepared with the tape with stycast (WS), without stycast (WOS), and with stycast prepared under pressure (WSUP). HVbd and electric strength of two types of these samples were recently published [20]. Pressure in the WSUP samples varied from 0 GPa to 0.54 GPa, and stycast was left to cure for 8 h. The electrical properties of the test couples, such as HVbd and dielectric constant were then investigated at 298 K and 77 K, and under various pressures. Surface morphology of arcing spots of the samples was examined using ESEM.
2. Experimental The MgO–ZrO2 coatings were applied on commercial SS tapes by using a reel-to-reel sol–gel dipping process with speed of 0.65 m/min. As shown in Table 1, two types of solution were prepared, dilute and normal. The normal coating solution consists of 20 mol% MgO, and 80 mol% ZrO2, which was prepared using zirconium tetrabutoxide, isopropanal, and acetone. Dilute solution is obtained by adding 50/50 isopropanol to normal solution. Zirconium tetrabutoxide and several Mg precursors were used as precursors, which was prepared as clear homogenous solution by stirring for 8 h. The detailed information on solution preparation and coating process can be found in Refs. [1,6,9, 19,21]. SS tapes with 5 mm width and 0.0254 mm thickness were cleaned with acetone. Then SS tape was coated by MgO–ZrO2 solution withdrawal rate of 0.65 m/min. The as-deposited films with dilute and normal solutions were dried at 550 C and 580 C, respectively. They were then exposed to heat-treating temperature in the range of 570– 580 C for the dilute and 610–630 C for the normal solutions, using an in-line three zone furnace. The coating process was repeated 4, 8, and 9 times. As seen in Table 1, the dip coating is composed of dipping, withdrawal, and heating. Coating thickness is an important factor for electrical properties,
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Table 1 The detail sol–gel MgO–ZrO2 insulation coating parameters Insulation
Number of Dip
Furnace temp. (C)
MgO–ZrO2
Total dip#:9 3 Dip dilute 4 Dip normal 2 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
Total dip#:8 3 Dip dilute 3 Dip normal 2 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
Total dip#:4 1 Dip dilute 2 Dip normal 1 Dip dilute
550, 570, 580 580, 610, 630 550, 570, 580
MgO–ZrO2
MgO–ZrO2
which is regulated with dip numbers, viscosity of solution, wetting, and withdrawal rate. Test samples were cut 10 cm in length from insulated SS tapes and the insulation layers were removed from both ends. Three kinds of test couples produced for each 4, 8, and 9 dip samples. First type of couples were made without stycast; the second type was made with stycast that is in the ratio of 2850 FT of 4 units and of catalyst 24 LV of 3 units at room temperature curing for 8 h; and the third type was prepared with stycast under pressure ranging from 0 GPa to 0.54 GPa, and was left to cure for 8 h under the pressure. Samples with stycast were cured for all dips in the same way. The thickness of coating and stycast were determined by using ESEM, as seen in Fig. 1. The test couples were placed for testing the experimental set up as shown in Fig. 2. The resistance, HVbd and capacitance were measured by using HP 4339 A high resistance meter, model 200– 02R high voltage power supply and 161 analog digital capacitance meter, respectively. For each measurement, the process was repeated at least three times. The dielectric constant of samples was calculated by using Eq. (1) without taking into account of effects of strain. There should be a very small change in the capacitor gap due to cooling and applied pressure. er ¼
Cðd i þ d s Þ e0 A
ð1Þ
Withdrawal speed (m/min)
Thickness (lm)
0.65
13
0.65
12
0.65
7
Fig. 1. The insulation, stycast, and stainless steel of eight times dipped substrates. The scale bar is 50 lm.
Fig. 2. Experimental set up for HVbd tests.
where, er is dielectric constant of insulation coatings, e0 permittivity of free space (8.854 ·
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1012 F/m), A is coated area in the sandwich, di is twice the coating thickness, and ds is the stycast thickness. Insulation and stycast thickness were measured as shown in Fig. 2. HVbd were measured samples with and without epoxy at 298 K and 77 K. The width and depth of arcing spots were determined by ESEM and Tencor Alpha-step 200 profilemeter.
constant for WSUP samples, as expected. The average values of these parameters are given in Table 2. It is clearly seen in Fig. 3 that values of surface resistivities of all the WOS samples at 298 K decrease sharply at about 0.08 GPa. Note that in Figs. 3 and 4 the surface resistivities of WS and WSUP test couples do not vary much by pressure at 298 K or 77 K. The surface resistivities of WOS samples are about 4 and 3 times higher than WS and WSUP samples for 4, 8, and 9 dips at 77 K. As seen in Table 2, the surface resistivities of WOS test couples are 15.54, 18.02, and 18.84 GOhms/cm2 for 4, 8, and 9 dips, respectively. Figs. 5 and 6 show the HVbd at 298 K and 77 K for 4, 8, and 9 dip samples for all of samples, respectively. HVbd values increased with increasing coating thickness, stycast thickness, and temperature. It is clear from Fig. 6 and Table 2 that the HVbd values of WOS samples for 4 dips are lower than 8 dips, which are lower than 9 dips samples, as expected. Also, HVbd values were increased about 3 times with epoxy-impregnation, when the samples were cooled from 298 K to 77 K. Epoxy-impregnation increased the HVbd values 2.3 times for 4 dips and 1.9 times for the other dips at 298 K. These increments were about 2.5 for all of samples at 77 K. The values of the HVbd of WSUP samples slightly decreased with increasing
3. Results and discussion As expected, coating thickness, crack in coatings, dopant materials, stycast thickness, pressure, and temperature influence electrical, mechanical properties as well as surface morphology and structure of insulations. HVbd test results, such as surface resistivity, HVbd, electric strength, and dielectric constant for each sample type at 298 K and 77 K, are graphically shown. When the mass of 3 kg of G-10 block, which was assumed zero pressure, was put on the samples with stycast, the thicknesses of stycast were about 32 lm. When pressure (0–0.54 GPa) was applied on the samples with stycast the thickness of stycast decreased according to pressure from 32 lm to 20 lm. The change in the thickness of epoxy-impregnation affected the HVbd, electrical strength, and dielectric
Table 2 Average test parameters of sol–gel MgO–ZrO2 insulations coatings on SS Room temperature (298 K)
Liquid nitrogen temperature (77 K)
Dip number
4
8
9
4
8
9
Without stycast (WOS) Surface res. (GOhms/cm2) HVbd (kV) Elec. strength (kV/mm) Dielectric constant
6.43 ! 0.056 0.21 14.93 8.50 ! 17.60
7.71 ! 0.024 0.37 15.35 10.67 ! 23.27
7.87 ! 0.032 0.39 15.36 11.25 ! 24.30
15.54 0.61 44.24 7.28 ! 14.56
18.02 1.15 47.82 9.61 ! 17.51
18.84 1.24 48.92 10.15 ! 18.00
With stycast (WS) Surface res. (GOhms/cm2) HVbd (kV) Elec. strength (kV/mm) Dielectric constant
3.41 0.51 12.06 7.15
4.68 0.71 12.63 8.68
5.32 0.78 13.39 9.17
3.79 1.83 43.35 6.65
5.93 2.67 47.57 7.95
6.61 2.84 48.69 8.80
5.17 0.70 13.26 9.13
3.53 1.63 42.13 6.30
5.75 2.33 47.33 7.57
6.27 2.56 48.43 8.52
With stycast under pressure (WSUP) P = 0–0.54 GPa Surface res. (GOhms/cm2) 3.28 4.50 HVbd (kV) 0.46 0.62 Elec. strength (kV/mm) 11.54 12.50 Dielectric constant 7.08 8.47
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2
Surface Resistivity (GOhms/cm )
10.00
1.00
T=298 K
4D. WOS
8D. WOS
9D. WOS
4D. WS
8D. WS
9D. WS
4D. WSUP
8D. WSUP
9D. WSUP
0.10
0.01 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 3. Resistivity vs pressure at 298 K for the test samples. Note that resistivity drops significantly from 0 GPa to 0.08 GPa at 298 K.
2
Surface Resistivity (GOhms/cm )
22.00
18.00
14.00 T=77 K 10.00
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
6.00
2.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 4. Resistivity vs pressure at 77 K for the test samples. Resistivities of WOS samples are higher than WS and WSUP samples.
pressure as stycast thickness of ribbons decreased from 32 lm to 20 lm. The high HVbd values were 0.78 and 2.84 kV for 9 dip WS samples at 298 K and 77 K, respectively. Here HVbd was about 0.39 V at 9 dip samples of WOS at 298 K, which is consistent with the earlier results [5,19].
Electric strength values of MgO–ZrO2 insulation coating on SS-304 tapes were given in Figs. 7 and 8. As seen in Fig. 7 and Table 2, electric strength values of ribbons increase depending on dip numbers in order for 4, 8, and 9 dips samples. These increments are about 3 and 3.7 times for
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0.90
T=298 K
4D, WOS
8D,WO S
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
HVbd (kV)
0.70
0.50
0.30
0.10 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 5. HVbd vs pressure at 298 K for all of test samples. HVbd increases with increasing dip numbers.
3.60 T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
8D, WS
HVbd (kV)
2.80
2.00
1.20
0.40 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 6. HVbd vs pressure with stycast at 77 K. HVbd decreases with decreasing stycast thickness for WSUP samples.
WOS, WS, and WSUP samples, respectively, when the samples were cooled from 298 K to 77 K. Electric strength values are decreased at a small ratio with epoxy-impregnation owing to the epoxy thickness. Electric strength should be constant according to equation E = V (Volt)/d (m). How-
ever, it slightly decreases with decreasing stycast thickness for WSUP samples. Table 2 and Fig. 7 indicate that electric strength is about 14.93, 15.35, and 15.36 kV/mm for 4, 8, and 9 dips for WOS samples at 298 K, which is in accordance with earlier researchers [5,19].
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Electric Strength (kV/mm)
16.00
14.00
12.00
T=298 K
10.00 0.00
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 7. Electric strength as a function of pressure at 298 K. Electric strength increases with small ratio increasing dip numbers but it decreases with stycast thickness.
51.00 T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
8D, WS
Electric Strength (kV/mm)
49.00
47.00
45.00
43.00
41.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 8. Variation of electric strength of samples as a function of pressure at 77 K. Electric strength increases with coating thicknesses.
As shown in Figs. 9 and 10 dielectric constant values for WOS test couples increased with increasing number of dipping and pressure at 298 K and 77 K. But, these increments at 77 K are lower than those at 298 K as seen in Fig. 10. When the samples were cooled from 298 K to
77 K, dielectric constant decreased at smaller ratio. Dielectric constants of WOS samples vary between 8.50 and 24.30 depending on dip numbers and pressure at 298 K and 77 K, as seen in Table 2. These increments are about 7.08–8.80 for the WS and WSUP samples.
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T=298 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
Dielectric Constant
21.00
16.00
11.00
6.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 9. Variation of dielectric constant of insulation of samples as functions of number of dipping, temperature and pressure. Dielectric constant increases with increasing pressure for WOS samples.
20.00
18.00
T=77 K
4D, WOS
8D, WOS
9D, WOS
4D, WS
8D, WS
9D, WS
4D, WSUP
8D, WSUP
9D, WSUP
Dielectric Constant
16.00
14.00
12.00
10.00
8.00
6.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
P (GPa)
Fig. 10. Variation of dielectric constant of insulation as a function of number of dipping at 77 K and pressure. Dielectric constant is invariable with pressure owing to epoxy-impregnation.
Figs. 11 and 12 show ESEM micrographs of the arcing spot of the MgO–ZrO2 insulated samples, which were used in HVbd tests at 298 K and 77 K. The pictures of the WOS samples were taken
to examine the arcing spots for HVbd test. Note that, molten area of insulation and SS substrate are larger with larger number of dipping. Figs. 11 and 12 show that as dip number increases,
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Fig. 11. ESEM micrographs of the arcing spots of 4, 8, and 9 dipped substrates with MgO–ZrO2. The test was performed by using samples without epoxy at 298 K. The scale bar is 50 m on the micrographs.
Fig. 12. ESEM micrographs of arcing spots of 4, 8, and 9 dipped samples without epoxy at zero applied pressure and 77 K. The scale bar is 50 lm on the micrographs.
HVbd increases. Consequently arcing spots and molten area of the insulation become larger and deeper as expected.
4. Conclusion MgO–ZrO2 was coated on SS-304 tapes at 4, 8, and 9 dips by using reel-to-reel sol–gel technique. Test couples were formed as WS, WOS, and WSUP. Thickness of insulation and stycast was determined by ESEM, which were about 7, 12, and 13 lm for insulation for 4, 8, and 9 dips, and between 32–20 lm for stycast, respectively. HVbd tests were performed with these samples under various stresses at 298 K and 77 K. Surface
resistivity of WOS samples decreases with increasing pressure, being sharper from 0 GPa to 0.08 GPa at 298 K. However, they are constant at 77 K. When samples were cooled from 298 K to 77 K, HVbd values increased. HVbd slightly decreases with decreasing stycast thickness as seen for WSUP samples. The high values of HVbd are 0.39, 1.24 kV for WOS, and 0.78 kV, 2.84 kV for WS samples for 9 dips, for the former 298 K and the latter 77 K. Electric strength values increase with increasing number of dipping for all of samples. When the samples were cooled from 298 K to 77 K, the electric strength increased. Electric strength of WSUP test couples slightly decreases with decreasing stycast thickness between the layers. Dielectric constant depends on capacitance,
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temperature, pressure, coating, and stycast thickness. Dielectric constant values for WOS ribbons gradually increased with increasing numbers of dipping as well as increasing pressure at 298 K. But this trend is much less at 77 K. The dielectric constant values for WS samples are constant with increasing pressure, but decreases with increasing number of dipping. Dielectric constant of WS test couple values is found to be 7.15–9.17 and 6.65– 8.80 for WS samples at 298 K and 77 K, respectively. ESEM micrographs show that arcing spots, which seem to be molten SS tape of insulation, become larger and deeper with increasing dip numbers and HVbd. Acknowledgements This work is based upon research carried out at the National High Magnetic Field Laboratory (NHMFL), which is supported by the National Science Foundation, under Award No. DMR9527035.
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