  Rare earth barium copper oxide (REBCO) coated conductor has emerged as one of
the high Tc superconductors suitable for future ultrahigh field superconducting
magnet applications. In the design and fabrication of such ultrahigh field
REBCO magnets, it is essential to understand the behavior of REBCO coated
conductor. The effect of heating on the properties of commercial REBCO coated
conductors is very important for many practical reasons. Nevertheless, a
comprehensive study on this effect have not yet been presented in the published
literature. This work studies a commercial REBCO coated conductor heat-treated
at temperatures between 175 {\deg}C and 300 {\deg}C for various durations.
Critical current and lap joint resistivity were measured at 77 K and 4.2 K for
the heat-treated samples. We found that critical current degrades with heat
treatment time and temperature. This degradation can be described by a
one-dimensional oxygen out-diffusion model with a diffusion coefficient of D =
2.5 x 10-6 exp (-1.17 eV/kT) m2/s. The heat treatment also causes appreciable
increase in joint resistivity. Comprehensive structural and chemical analyses
were performed on Cu/Ag/RECBO interfaces by transmission electron microscopy
(TEM). Our electron energy loss spectroscopy (EELS) study provided direct
evidence of oxygen deficiency in the heat treated REBCO samples. In addition,
it is found that the oxygen diffused out of the REBCO layer forms mostly Cu2O
at both Ag/REBCO and Cu/Ag interfaces. Cu2O is also observed at grain
boundaries of the Ag layer. The oxygen out-diffusion model proposed in this
work is used to predict REBCO thermal degradation in several engineering
scenarios.
