  The Y1-xLixBa2Cu3O7-d polycrystalline bulk superconductors doped with Li
substituting at the Y site at different concentrations (x = 0, 0.01, 0.02, 0.1)
were prepared using the citrate-gel method to study the effects of doping on
the superconducting temperature and critical current density. The question was
whether Li addition characterized by a high Debye frequency would have any
positive effects on Tc. The optimum citrate-gel and heat treatment conditions
were identified as those yielding samples with a maximum grain size on the
order of 50 micro.m (up to the optimum Li-doping level, x=0.01). Li
substitution at the Y site was verified by structural, electrical, and magnetic
measurements of the produced samples, whereas X-ray diffraction (XRD) analysis
revealed the formation of a pure phase with no visible impurity phases.
Moreover, AC magnetic susceptibility measurements showed no increase in the
superconducting transition temperature Tc, consistent with the predicted
results obtained by machine learning method, although it was theoretically
expected to increase owing to the high Debye frequency of Li. This observation
is consistent with magnetic coupling models for pairing mechanism in cuprates.
Finally, because of the optimum conditions of the preparation procedure, nearly
identical values of the critical current density (Jc) were recorded for samples
with different Li doping levels (up to the optimum Li doping level). It was
found that improved compound preparation conditions would have a critical and
extensive effect on Jc enhancement, with nearly no Tc suppression.
