High voltage breakdown studies of sol-gel MgO-ZrO₂ insulation coatings under various pressures at 298 K and 77 K

High voltage breakdown (HV_bd) 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 reel-to-reel sol-gel technique. Coating thicknesses for 4, 8, and 9 dippings were about 7, 12, and 13 μm, respectively, and thickness of epoxy-impregnated samples (stycast 2850 FT/24 LV) were measured to be 32 μm. 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 μm. Thickness of the coatings and epoxy-impregnated using stycast were determined by using environmental scanning electron microscopy (ESEM). The resistance, capacitance, and HV_bd 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 HV_bd, 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 HV_bd's.