Adhesion properties of MgO–ZrO 2 insulation coatings for 5 Tesla HTS coils

Abstract

Self insulating substrate tapes (SIST) is the most promising insulation technique for high temperature MgO–ZrO 2 coatings on Ag and AgMg sheathed Bi 2Sr 2Ca 1Cu 2O x (Bi-2212) superconducting tapes and wires in applications of HTS/LTS coils and magnets. We have already reported successful results as to the synthesis, characterizations and applications of the insulation coatings using the SIST. In order to provide no electrical short circuit in J c measurements of HTS/LTS coils, the bonding of the coatings onto the substrate is a very important issue. In this present research, the adhesion properties of high temperature MgO–ZrO 2 coatings were scrutinized for different processing parameters. Lap joints were fabricated by laying fresh sol–gel coated silver tape samples over each other and then by heat-treating at temperature range of 500–800 °C for several times in air. These joint samples were pulled to failure by using a mini tensile tester. MgO–ZrO 2 was coated on Ag tapes by sol–gel process using Mg and Zr based precursors. The obtained results obviously pointed out that the best Mg precursor is Mg(C 5H 7O 2)·2H 2O to prepare solution and there is a strong relationship between film growth and adhesion properties. Also, MgO content in ZrO 2 increased its bonding strength. The optimum heat treatment conditions are 600 °C and 15 min for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO–ZrO 2 coating.