Mechanical and electrical evaluation and test results of composite insulating materials at low temperature
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https://www.eduzhai.net International Journal of Composite M aterials 2013, 3(6): 168-173 DOI: 10.5923/j.cmaterials.20130306.05 Mechanical, Electrical Evaluation and Test Results of Composite Insulation Materials at Cryogenic Temperature Rajiv Sharma1,*, V. L. Tanna1, S. Falnikar2, S. Pradhan1 1Institute for Plasma Research, Near Indira bridge, Bhat, Gandhinagar-382428, Gujarat, India 2Head, ISDE division, Electrical Research and Development Association, Vadodara-390010, Gujarat, India Abstract A steady state superconducting tokamak (SST-1) fusion machine comprises of superconducting magnets as toroidal field and poloidal field, currently in process for achieving fusion energy at Institute for Plasma Research, Ahmedabad. The composite materia ls as GFRP (Glass fiber re inforced plastic) of NEMA (National e lectrical manufacturer’s association) G-10 CR (Cryogenic radiat ion) cryogenic grade has been used for insulation in form o f boron free E-glass tapes for superconducting coil insulation, in sheet form to separate the conducting surfaces in heliu m and as well nitrogen systems and sub-systems and in tube form for co mponent fabrication as Electrical Insulation Breaks for superconducting magnet and current feeder system of SST-1 machine. To validate the insulation material selection, design consideration and quality assurance, control and acceptance aspects, we have carried out the mechanical and electrical evaluation of GFRP G-10 insulation material and cryogenic epoxy at 300 K and 77 K temperature as per the ASTM, IS and BS test standards at government test laboratory. The test results indicate that the mechanical p roperties as tensile strength, flexu ral, co mpressive, shear strength increases at cryogenic temperature. In this paper, the composites materials details, the different mechanical, electrical performance tests and scanning electron microscopic evaluation for material internal surface morphology, microstructure and its test results at room temperature and cryogenic temperature (77 K) will be presented. Keywords GFRP Co mposite, G-10 (CR) Insulation Material, Steady State Superconductor Tokamak (SST-1) 1. Introduction SST-1 machine co mp rises of sixteen TF (Toroidal field) and n ine PF (Po lo idal field ) superconduct ing co ils . To electrically isolate the hydraulics in terms of pancake of each coil, G-10 NEMA grade glas s fiber sheet form has been us ed for separation in between of each coil. Each of the hydraulics lines is elect rically iso lated fro m the h ead ers th rough specially designed cryo-compatible electrical b reaks shown in figure 1, is made of G-10 and stainless steel tubes, which are tested for 5 kV isolation and subjected to repeated cool down and warm up bet ween 4.2 K and 300 K. Figure 3 shows the G-10 grade sheet with stycast 2850GT epo xy over wrap on cantilever cold mass supports structure of all TF and PF co ils . Du ring the op erat ion th is co ld mass attains a cryogenic temperature of 4.2K in the hostile environ ment of high vacuum. Fo r current feeder system of superconducting magnet system, heliu m vapour electrical insulation breaks shown in figure 2 of G-10 material in tube form have been * Corresponding author: email@example.com (Rajiv Sharma) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved developed to electrically isolate fro m the magnet system and flanges of same materials has been used to isolate 10 kA current leads to heliu m cryogenic system. To support and thermal isolation G-10 spacer assembly in huge quantity has been used in between all panels of LN2 thermal shield of SST-1. 2. Description of Insulation Materials G-10 NEMA grade insulation material is used for electrica l and therma l insulation purpose in superconducting magnet based tokamak at required locations in different shapes as tube form, sheet form, and circular flange as per the needs for the system. G-10 CR (cryo-radiat ion) composite is the E-g lass fabric based on an alu mina-lime-borosilicate co mposition mat rix, it is a conventional, heat activated, amine catalyzed, b isphenol-A, solid type epoxy resin. Resin weight fraction is 32 to 36%. The selection criteria of G-10 CR grade[2, 4] insulation materials are its high mechanical strength, good electrical properties, low thermal and electrical conductivity and ease of processing. Laminated form, which is fabricated by hydraulic press method using optimized control of pressure and temperature cycles of curing, whereas tube form is International Journal of Composite M aterials 2013, 3(6): 168-173 169 manufactured by ro lling CR prepreg around hot mandrels held between heated pressure rolls and filament winding method. The supplier has provided the raw material test report of glass fabric tensile strength, weave pattern, yarn count, basic weight and on epoxy, weight per epo xide, viscosity, and nonvolatile content listed in table 1. Helium vapour insulati on bre ak s Figure 2. Vapour insulation breaks in 10 kA current lead Superconducting coils Electrical insulation breaks Fi gure 1. Elect rical breaks in helium cryogenic transfer lines G-10 insulation mate rials Figure 3. G-10 sheet insulation between coils and over coils supports Glass fabric woven from continuous filament yarn En 10 204 E-glass (64-68% wt) Epoxy (32-36%) DGEBA (diglycidyl ether bisphenol-A) R-glass fiber thread form Epoxy DGEBA & KH560 Table 1. The tests parameters of composite materials consists of glass fiber and epoxy Weaving pattern laminated form P lain Thread count warp 17.3 l/cm weft 12.8 l/cm Yarn Weight g/m2 EC 9-68 tex EC 9-68 tex 203.3 Tensile st ren gth N/cm (minimum) warp 214 weft 140 Hardener Accelerator Diamino Disphenyl Sulphone Weaving pattern plain for tube form Hardener (GY051 & DDM, Liquid aromatic amine, Anhydride) Diamino diphenyl methane white coloured flakes Warp 18±1 threads/cm weft 14±1 threads/cm 240 3000-4000 MP a Nominal value thickness (mm) 0.185 mm 0.2±0.022 mm 3. Performance Test: Composite Material at 300 K/77 K Mechanical and electrical characterization of G-10 insulation material has been investigated considering the technical requirements of the components and systems of SST-1. The co mposite materials have been tested separately and along with assembled condition of the machine. The technical requirement and the type of tests performed at 300 K and 77 K of the materials is summarized in table 2 and table 3. Table 2. Technical Requirement of Insulation Material Electrical isolation Helium leak rate Outgassing rate Hydraulic pressure requirement Breakdown strength kV/mm ~2 kV (DC) < 10-8 mbar l/s after 5 thermal shock test at 77 K < 10-7 mbar l/s/cm2 4 bar, 4.5 K In vacuums: ~ 20-30 and in air 4-6 170 Rajiv Sharma et al.: M echanical, Electrical Evaluation and Test Results of Composite Insulation M aterials at Cryogenic Temperature Table 3. Tests Performed on Insulation Material Test s P erformed Raw material test of fabric and yarn of compo sit e Tensile strength Flexural strength Compressive strength Shear strength Impact strength Dielectric strength Water absorption Volume resistivity Surface resistivity Martens heat deflect ion Glass content Lap shear strength of stycast 2850 GT epoxy Glass transition temperature measurement Thermal cycle test at LN2 temperature Specification No En 10 204 AST M D 638-1991 AST M D 790-2003 AST M D 695-1991 AST M IS: 4248-1967 AST M D 638-1991 AST M D 149-2009 AST M D 570-1998 AST M D257-2007 AST M D 257-2007 DIN 53462-1987 BS: 2782-Pt10-1997 AST M D 5868-01 AST M E 1640 ANSI/EIA-364-32C-200 4.1.2. Co mpressive and Interlaminar Shear Strength Test For co mpressive strength, sample d imension in cube form of 20x20x10 mm was tested and failure was observed at break loads of average130 KN1 at 300 K and 136 KN1 at 77 K respectively. The Interlaminar shear strength was determined on samples of 100x19.5x3 mm d imension and failure was observed at break load of average 30 KN1 at 300 K and 33.2 KN1 at 77 K respectively. The apparent Interlaminar shear strength calcu lated fro m 3xPf / (4xb xh), where Pf is the applied load, b is the specimen width, and h is the specimen thickness. Both comp ression and shear tests have been performed in same 200 KN1 capacity universal testing machine as shown in figure 5. 1 Kilo-Newton 4. Test Methods For the mechanical and electrical test all the specimens have fabricated as per the ASTM test specification the samples were investigated at room temperature and to perform the test at 77 K sample was immersed in liquid nitrogen for 15 minutes and test immediately on removal fro m it. Temperature variation was not observed in samples during the test. 4.1. Mechanical Tests 4.1.1. Ultimate Tensile Test For measurement in length wise and cross wise direction, samples of dimension 250x25-10x3 mm were gripped in the 100 KN1 ‘Shimadzu’ make UTS mach ine and load was applied gradually and increases till the samples break. The break load and stroke were determined 12.60 KN1 and 5.098 mm / min respectively at 300 K. The specimen failure was observed at break load of 13.74 KN1 at 77 K. The test shown in figure 4. Figure 5. Compressive and interlaminar strength 4.1.3. Flexural Strength Flexu ral strength obtained in three point method were determined at 300 K and 77 K and calculated as following σf = 3 Pf. l/2b. h2, where Pf is the applied load, b is the specimen width, h is the specimen thic kness and l is the span. The specimen failed at average break loads of 1050 KN1 at 300 K and 1190 KN1 at 77 K temperature. The test set up shown in figure 6. Sample Figure 4. Ultimate tensile test Figure 6. Flexural strength test 4.1.4. Impact Strength Impact strength test was performed on 6012-impact strength tester (Izod and charpy) of 0-25 joule capacity as shown in figure 7. The specimens were failed at applied average load of 21 joule at 300 K and 14 joule at 77 K respectively, generally impact strength of G-10 material is ~ 4 J/cm notched at 2.54 cm. International Journal of Composite M aterials 2013, 3(6): 168-173 171 figure 9. Transformer Electrode S ample Figure 7. Impact strength test Figure 9. Dielectric strength test 4.2. Electrical Tests 4.3. Thermal and Other Tests 4.2.1 Vo lu me Resistivity and Surface Resistivity Test The test performed on 5M-8000 series SM E 8310 Hioko company machine according to ASTM D257 2007 on five specimens of 80 mm d iameters with 3 mm th ickness. A dc 500 volt for one-minute electrification t ime in air was applied and resistances were obtained 31.2 Tera Oh m at 300 K and 31.5 Tera oh m at 77 K temperature to find the volume res is tiv ity . Surface resistivity test have been also performed on same samples in the volu me resistivity measuring instruments. The resistances were obtained 39.8 Tera oh m at 300 K and 26.0 Tera oh m at 77 K temperature. The test shown in figure 8. 4.3.1. Thermal Cycle Test The composite insulation material is to work in operating temperature condition of subjected to repeated cool down to 4.5 K and warm upto 300 K temperature. Five thermal cycles of temperature 300 K– 77 K -300 K as per the European standard ANSI/EIA-364-32C-200 of the insulation sheet, components as electrical insulation breaks, liquid nitrogen panels G-10 spacer assembly, etc of SST-1 mach ine were tested. 4.3.2. Heliu m Leak at 300 K, 77 K and 4.5 K The tests were carried out to ensure the heliu m leak tightness of components at cryogenic operating temperature condition. The components that have used G-10 insulation material for fabrication as electrical insulation breaks for heliu m and nitrogen service, heliu m vapour insulation breaks for current feeder system, the heliu m leak t ightness were investigated at 300 K, 77 K and 4.5 K individually and with the assembly. Heliu m leak rate was measured better than 10-8 mbar l/s. The test set up has shown in figure 10. Figure 8. Volume and surface resistivity test 4.2.2. Dielectric Strength Dielectric strength was investigated on five samples of 200 mm sheet in square form of 3 mm thickness at room temperature using 100 kV transformer as shown in figure 3 (b). No leakage or puncture was detected in air mediu m. In oil mediu m, puncture was observed in the insulation materials specimen of vo ltage stress at average breakdown 24kV/ mm in all samp les at 300 K, whereas 23.6 kV/ mm observed at 77 K temperature. The test set up shown in Figure 10. t emp erat ure Helium leak test of insulation material at cryogenic 172 Rajiv Sharma et al.: M echanical, Electrical Evaluation and Test Results of Composite Insulation M aterials at Cryogenic Temperature 4.3.3. Martens Heat Deflection (MDT) Temperature Test MDT test has been performed as per DIN-53462-1987 specification specimens of d imensions 120x15 mm and 10 mm th ickness. The test specimen is loaded in three-point bending in the edgewise direction as shown in figure 11. The outer fiber stress used for testing is either 0.455 M Pa or 1.82 MPa, and the temperature is increased at 2°C/ min until the sample is deflected more than 6 mm. For the test FWM 63210 of temperature range 0-250℃. No deflection at 220℃ beyond the limit was reported in a ll specimens at 300 K and 77 K. constant, and the degradation of the stress vs. time is measured. The temperature-heating rate of 2ºC/ min. is used on sample. 4.3.6. Scanning Electron Microscope (SEM) Test For surface structure and internal microstructure morphology, the scanning electron microscope (SEM) investigations were done on model LEO-440i of 50 KX magnificat ion factor. Go ld coating of ~10 Armstrong layers were applied on samples to imp rove image quality. The photographs of different test samples are presented in figure no. 13. No damage was observed in the insulation samples. Sample Figure 11. MDT test 4.3.4. Glass Content % and Water Absorption Test To determine the glass content % in the co mposite, 15 samples of dimension 25x25x3 have been used. The specific weight measurement method have used in air and water med iu m with heating in furnace at 650°C for around 2 to 3 hours. For water absorption test, the samples of dimension 40x40x3 mm were put in water for 24 hours at room temperature to detect the water absorption in the specimens. The tests were also carried out on liquid nit rogen thermal cycled samples. The glass content % was obtained 71.0 % at 300 K and 69.7% at 77 K. Fi gure 13. SEM photographs of insulat ion material 5. Test Results and Discussion 4.3.5. Glass Transition Temperature (Tg) Sample Figure 14. Mechanical properties test results at 300 K and 77 K t emp erat ure DMA analyzer Figure 12. Tg test These tests were carried on co mposite specimen of dimension 57.59 x 14.29 x 3.19 mm thickness in dynamic mechanical analyzer Q800 DMA as shown in figure 12. A stress is applied on sample clamped at three bending point. To find Tg, a deformation is applied to the sample and held The mechanical and electrical test results are summarized in table 4 and table 5 and data shown in figure 14. The mechanical and electrical characterization has been performed of the one manufacturer of G-10 insulation materials at 300 K and 77 K. Specimens have been made fro m the h igh-pressure laminates. The data are the average reading of four or five specimens, wh ich were used for test as per the ASTM specification. The standard deviations among individual tests were about 4% of the average value. Since the composite materials show anisotropic behavior, the fiber International Journal of Composite M aterials 2013, 3(6): 168-173 173 yarn direction and counting ratio in warp and weft is the important consideration that changes the mechanical properties. The mechanical tests data presented are of warp orientation and warp/weft thread ratio of 0.75. At 77 K temperature tests data indicates that the mechanical properties increase to 15% high in tensile strength, 10% high in compressive strength, and 17% high in shear strength, whereas the impact strength reduced to ~10% at 77 K temperature. No significant changes observed in electrical properties of G-10 CR materials at 77 K. Table 4. Mechanical Characterization of G-10 CR Grade Insulation Mat erial Temp K 300 77 Tensile st ren gth MP a 397 457 Compres sive st ren gth MP a 306 336 Int erlaminar shear st ren gth MP a 135 158 Flexural st ren gth MP a 534 590 Impact st ren gth kJ/m2 227 200 Table 5. Electrical characterization of G-10 CR grade insulation material Temp K 300 77 Volume resistivity Ohm-cm 1.8 x 1015 1.8 x 1015 Surface resistivity Ohms 7.0 x 1014 4.4 x 1014 Dielectric St ren gth kV/mm 23.9 23.6 Thickness mm 3 3 6. Conclusions Electrical insulation play significant role in the large-scale superconducting magnets based fusion devices. Heliu m leak tightness, thermal cycling, electrical stress on the materials are the crit ical parameters which have to be satisfied under service operating conditions. As this machine operates under 4.5 K environ ment, all these components are housed in 77 K and vacuum space. Hence, repair of these components is too difficult task. Therefore, ease of installation and reliab ility of these components are crit ical and essential to have s mooth and reliable operation of any large-scale fusion machine. The tests result shows that the material has better mechanical strength and electrical insulation properties at room temperature as well as cryogenic temperature. This insulation material meets the necessary technical requirements at operating conditions of systems of SST-1. Deuteriu m-Tritiu m (D-T) based Tokamak will have an additional requirements of radioactive resistant insulation mater ia ls . ACKNOWLEDGEMENTS The authors are gratefully thank to S. Falnikar, Anil Patel of ISDE laboratory of Electrical Research and Develop ment Association, Baroda for their invaluable support for making arrangement and facilities development for the mechanical and electrica l characterization of insulation materia l at 77 K temperature. I also wish to thank to Dr. Raole, Naren Chauhan of FCIPT, SEM materials test laboratory, Gandhinagar for conducting the SEM test. REFERENCES  M . B. Kasen, G.R. M acDonald, D.H. Beekman, Jr., and R.E. Schramm “M echanical, Electrical, and Thermal Characterization of Glass-Xloth/Epoxy laminates” National Bureau of Standards, Boulder, Colorado.  J.R. Benzinger “M anufacturing Capabilities of CR-Grade Laminates” Spaulding Fibre Compnay, Tonawanda, New York.  Zhixiong Wu, Hao Zhang, Huihui Yang, Xinxin Chu, Yuntao Song, Weiyue Wu, Huajun Liu, Laifeng Li “ Properties of radiation stable, low viscosity impregnating resin for cryogenic insulation system” Cryogenics 51 (2011) 229-233.  R. P. Reed, “M aterial at Low Temperature” Composites, 415-441.  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