Activation characteristics of metal-to-insulator transition behaviour in vanadium trioxide insulator for REBCO conductor under turn-to-turn transport current

Abstract Vanadium trioxide (V 2 O 3 ) is an ideal candidate for electrical insulation materials in rare-earth barium copper oxide (REBCO) magnet applications owing to its metal-to-insulator transition (MIT) behaviour that is facilitated by its adjustable resistance, which shifts from high (at temperatures below 77 K) to low (above the transition temperature of ∼160 K). Optimising the turn-to-turn contact resistance ( R ct ) in both normal and quenched states provides the benefits of both insulated and non-insulated (NI) winding methods. To implement V 2 O 3 as a turn-to-turn insulator in REBCO magnets, ensuring responsive MIT activation between the turns is crucial. This study experimentally investigated the electrical behaviour of V 2 O 3 insulation, which undergoes an electrical phase transition thermally triggered by transport current flow between the turns. Current–voltage ( I – V ) tests were performed on sandwiched REBCO tape samples with and without V 2 O 3 insulation to analyse the electrical and thermal reactions. The I – V tests included stepped and pulsed current ramping to examine the repetitive behaviours under varying conditions. Furthermore, we verified the effect of mixing metal powder with V 2 O 3 on the I – V characteristics. The results suggest that V 2 O 3 insulation produces nonlinear I – V characteristics due to its negative temperature dependence, while the NI samples display linear I – V properties. The MIT characteristics of V 2 O 3 insulation were reproducible across different current transport patterns and repeatable over thousands of cycles. Moreover, I – V measurements revealed that increasing the metal powder concentration reduced the R ct values in the V 2 O 3 -insulated layer. Thus, maintaining high R ct values (∼10 7 −∼10 4 μ Ω cm 2 ) in the normal operating range of below 77 K in REBCO magnets can effectively mitigate leakage current issues in the no-insulation winding technique. Conversely, transitioning to low R ct values (∼10 3 μ Ω cm 2 ) in the transient operating range above 160 K is expected to minimise stability degradation caused by quenching or extended thermal runaway in the insulation winding technique. These findings provide valuable insights in designing V 2 O 3 -insulated REBCO magnets for diverse applications, such as rotating machines, accelerator and superconducting magnetic energy storage.