The effect of the 2D internal strain state on the critical current in GdBCO coated conductors

A reversible effect of strain on the critical current (I _c) has been reported for REBa ₂Cu ₃O ₇ (REBCO) coated conductors. In this study, the strain sensitivity of I _c was compared for GdBCO coated conductors with different crystal orientations. Extremely small strain sensitivity was confirmed from tensile and bending tests at 77K for a GdBCO film with the [110] direction parallel to the tape length (Gd-F), while a GdBCO film with the [100] or [010] direction parallel to the tape length (Gd-I) has much stronger strain dependence of I _c. To compare the strain sensitivity of I _c based on internal strain, the lattice strain was evaluated for a GdBCO film in a composite conductor by employing a diffraction technique using synchrotron radiation. The lattice strain was determined along both the a- and b-axes for the orthogonal domains that are generated by the twin structure of GdBCO film. A distinct difference in the 2D strain state that depended on the crystal orientation was revealed for the GdBCO coated conductors. In Gd-I, the lattice strains along the axial and lateral directions are tensile and compressive under tensile loading, respectively, that is, the 2D internal strain state is anisotropic. On the other hand, an almost isotropic 2D strain state was confirmed in Gd-F. In addition, the strain components along the crystal axes in Gd-F are much smaller than the axial strain components in Gd-I. This difference in the internal strain state is attributed to the difference in strain sensitivity between the GdBCO coated conductors with different crystal orientations. The contributions of the strain components along the a- and b-axes to I _c are discussed on the basis of the measured strain sensitivities and internal strains for two conductors.