  We have prepared Na-doped YBa2Cu3Oy (YBa1.9Na0.1Cu3Oy +40mol%Y211) (YBNCO)
and Na-free YBa2Cu3Oy (YBCO) samples by the Melt-Textured Growth (MTG) method
to study the effect of doped Na ion on flux pinning. The ac susceptibility
curves (acs) as well as the hysteresis loops were measured for the samples.
Then the effective pinning energy (U(T,Hdc,J)), irreversibility line (Hirr(T))
and critical current density (jc(Hdc)) were determined, where T, Hdc and J are
temperature, dc magnetic field and current density, respectively. We found
that, with Na doping, the Hirr(T) line shifted to lower temperature while the
Jc(Hdc) and U(T,Hdc,J) became smaller. It indicates that the Na ions play a
negative role in the flux pinning of YBCO. The appearance of the second peak in
the Jc(Hdc) curves and the enhancement of anisotropy in YBNCO further support
this finding.
