Surface morphologies and microstructures of (Tl, Pb)(Sr, Ba)-1223 single crystals were
investigated by using a scanning electronic microscope (SEM) and an atomic
force microscope (AFM). The SEM images showed that a Tl-1223 single crystal
grown by self-flux obeys a layer-by-layer growth mechanism. Periodic modulation
structures 200 nm in width and about 0.7–0.8 nm in height were observed using
an AFM. Heat treatments in different atmospheres of argon and oxygen at
500 °C
were carried out with different treatment times.
Tc
and Jc
have been improved through optimized heat treatment for 3 h in argon
gas. Crystals were also heat treated in PbO. Results showed that the
magnetic properties were significantly improved, with large increases in
Tc
and Jc, indicating that the ratio of Tl to Pb in the crystal is another important factor in addition
to the oxygen content in affecting the superconducting properties. The modulation structure is
probably responsible for the strong flux pinning in (Tl, Pb)(Sr, Ba)-1223 crystals and the large
Jc.
Lead chalcogenide
quaternary systems have been shown to provide
high thermoelectric (TE) efficiency superior to those of binary and
ternary lead chalcogenides, arising from both altered electronic band
structures and a reduction in lattice thermal conductivity. Here,
we have synthesized single-phase samples of the quaternary compound
(PbTe)
0.55
(PbS)
0.1
(PbSe)
0.35
doped
with Na and characterized their TE properties. We show that the dopant
solubility is limited to 1 at. %. A very low lattice thermal conductivity
of ∼0.6 W m
–1
K
–1
at 850
K is achieved at all dopant concentrations because of phonon scattering
from point defects associated with solute atoms with high contrast
atomic mass. As a result, a high TE figure of merit of approximately
1.5 is achieved at 823 K in heavily doped samples. Moreover, the figure
of merit is greater than 1 over a wide temperature range above 675
K.
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