Abstract:
Nanocrystalline alloys of the compositions Na 0.05 Cu 1.95 S, Na 0.075 Cu 1.925 S,
Na 0.10 Cu 1.90 S, Na 0.125 Cu 1.750 S, Na 0.15 Cu 1.85 S, Na 0.17 Cu 1.80 S, Na 0.20 Cu 1.77 S were synthesized
in a melt medium mixtures of hydroxides NaOH and KOH at a temperature of about 165 ◦ C. X-ray phase
analysis showed that the alloys are heterophasic and consist of phases of Cu 9 S 5 digenite, CuS 2 copper
disulfide, Covellite CuS, Cu 7 S 4 anilite in various combinations. The crystallite sizes range from 16 to
160 nm. The degree of crystallinity of the alloys slightly increases with an increase in the sodium content
from 68% in Na 0.05 Cu 1.95 S to 81% in Na 0.20 Cu 1.77 S. A quasi-one-dimensional Na 2 Cu 4 S 3 phase
was detected in the composition of the Na 0.20 Cu 1.77 S alloy. The measured values of the conductivity
of the alloys are two orders of magnitude lower than in isolated pure Cu 9 S 5 , CuS 2 , CuS, Cu 7 S 4 , of
which the alloys consist. An activation temperature dependence of the conductivity is observed in the
region from 300 K to 360 K with an activation energy of 0.08 - 0.15 eV. The reason for the low conductivity
of the alloys is assumed to be the presence of weakly conducting interfacial layers and sodium doping
of non-stoichiometric phases Cu 9 S 5 (Cu 1.8 S) and Cu 7 S 4 (Cu 1.75 S), leading to the compensation of
holes by electrons of impurity sodium atoms. The measured values of the coefficient of thermo-emf alloys at room temperature lie in the range from 0.032 to 0.147 mV/K. Due to the low thermal conductivity of the order of 0.2 W/mK, a rather high dimensionless thermoelectric figure of merit ZT ≈ 0.28 at 570 K was obtained for the composition Na 0.15 Cu 1.85 S.
