A series of Ag-doped Tl2Ba2Ca2Cu3Oy superconducting oxides were prepared by a solid-state reaction and the influence of Ag2O content, with a varying molar ratio which ranged from 0 to 1.0, was also investigated. The normal-state resistance of the samples decreased with an increase in the content of Ag2O additive, while the critical superconducting temperatures of the Ag-doped samples increased once the Ag2O content was increased. The doping of the silver oxide was indicated from the results of the x-ray diffraction pattern to be favorable to the formation of the three CuO layers with a high-Tc phase Tl2Ba2Ca2Cu3Oy. The diamagnetic property of the samples was also confirmed by the magnetic measurements to become enhanced by doping with silver oxide. The platelike grains of Tl2 Ba2CaCu2Oy phase were observed from scanning electron micrographs to be primarily present in the undoped sample, while the amount of the needlelike Tl2Ba2Ca2Cu3Oy grain increased with an increase in the doping amount of Ag2O.
The effects of Ca, Cu, and Ca–Cu richness on the formation
behavior and physical characteristics of high-T
c Tl-based superconductors
were investigated systematically. In addition to sintering temperature and
time, intermediate phases which proved important to the formation and
properties of nearly single-phase, triple-CuO-layers Tl2Ba2Ca2Cu3O
y
superconductors and optimal processing conditions were also investigated.
Ca and Ca–Cu richness introduced positive effects on the properties of
nearly single-phase Tl2Ba2Ca2Cu3O
y
superconductors and helps in the
formation of the appropriate intermediate phases, whereas Cu richness
produced a negative effect. The higher sintering temperatures of the
specimens accounted for the tailing of electrical resistance before the critical
zero resistance temperature was approached, and for the instability of the
superconducting phase. Lower post-annealing temperatures of the bulk
samples slightly improved the superconducting properties. The resultant
bulk Tl-based superconductors can be used as a Tl vapor source for the
formation of superconducting thin films within a closed system.
Electron paramagnetic resonance (EPR) and luminescence
studies were carried out for the sol–gel-derived YAG:Cr powders
subjected to three kinds of heat treatment processes. The results
show that powders processed using the two-step heat treatment
process with the addition of 20 wt% LiCl and by direct heat treatment
at 1500°C exhibit good luminescence
properties. The emission spectrum for the powders obtained by the
two-step process shows the R-line at 688 nm, whereas the powders
directly heat treated at 1500°C
display an additional R-line of Al2O3:Cr at 694 nm. EPR
measurements show that the presence or absence of absorption peaks
depends on the Cr3+ environment in the host lattice resulting from the
different methods of heat treatment of the powder. The EPR spectra
of YAG:Cr3+ phosphors were interpreted to indicate both Cr3+ single
ion and Cr3+–Cr3+ pair (with spin three) transitions.
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