Enhanced thermal properties of epoxy composite containing cubic and hexagonal boron nitride fillers for superconducting magnet applications

Superconducting test coils impregnated with epoxy composites containing cubic boron nitride (cBN) particles, hexagonal boron nitride (hBN) particles, and a mixture of cBN/hBN particles were fabricated, and their thermal and electrical properties were investigated using cool-down, over-current, and repetitive-cooling tests. Micro-voids, which may act as major obstacles to the formation of thermally conductive passages, were observed in the epoxy composites containing the cBN or hBN particle fillers alone but were absent in that containing the cBN/hBN particle mixture. The coil impregnated with epoxy containing the cBN/hBN particle mixture also exhibited superior cooling performance and thermal/electrical stabilities, indicating that this composite effectively facilitated heat transfer between the coil and liquid nitrogen. Moreover, the addition of the cBN/hBN filler reduced the difference in thermal contraction between the superconducting tape and epoxy composite. Overall, the use of the epoxy composite containing the cBN/hBN filler shows potential for the development of highly stable superconducting coils with considerably enhanced thermal conductivity and low coefficients of thermal expansion.