Phase composition and thermoelectric properties of the nanocomposite alloys NaxCu2−x−yS

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.