Phase analysis, thermal and thermoelectric properties of nanocrystalline Na_0.15Cu_1.85S, alloys Na_0.17Cu_1.80S, Na_0.20Cu_1.77S

Synthesis, X-ray phase analysis, electron microscopy and investigations of the thermoelectric and thermal properties of nanocrystalline copper sulfide alloys contained sodium are presented. At room temperature, the alloys are a mixture of three phases of copper sulfide - the monoclinic phase of Na2Cu4S3 , the hexagonal phase of Cu2S and the cubic phase of Cu9S5 (digenite). The predominant phase is Na2Cu4S3 (with content from 57 to 85 volume %). The particle sizes in the compacted samples lie in the range from 20 to 400 nm. For all samples DSC studies revealed a first-order phase transition in the (370-380) K region with enthalpies from 5234 to 11720 J/kgK. The heat capacity varies within the range (0.15-0.48) J/(gK). The electrical conductivity, Seebeck coefficient and thermal conductivity were measured in the temperature range from 290 to 590 K. A very low thermal conductivity of the samples was observed in the interval of (0.1-0.6) Wm -1 K -1 . The Seebeck coefficient has a value higher than 0.2 mV/K for Na0.15Cu1.85S composition, but a low electrical conductivity about 10 S/cm limits the maximum dimensionless thermoelectric efficiency ZT of the material at 0.3 in the temperature range 290-590 K.