Phase analysis, thermal and thermoelectric properties of nanocrystalline Na0.15Cu1.85S, Na0.17Cu1.80S, Na0.20Cu1.77S alloys

Abstract

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 Na 2 Cu 4 S 3 , the hexagonal phase of Cu 2 S and the cubic phase of Cu 9 S 5 (digenite). The predominant phase is Na 2 Cu 4 S 3 (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 Na 0.15 Cu 1.85 S 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.

Some Thermoelectric Phenomena in Copper Chalcogenides Replaced by Lithium and Sodium Alkaline Metals

“…Sodium doping in copper chalcogenides was studied in works [31,[36][37][38][39][40]53,138]. Copper chalcogenides with a high sodium content are not homogeneous and form a mixture of phases with different conducting and thermoelectric properties, but deserve attention as nanocomposite thermoelectric materials with low thermal conductivity.…”

“…In work [36], phase transitions and thermal effects of solid Na x Cu 2-x S samples were investigated by DSC in an argon atmosphere in the temperature range (30-430 • C). DSC was detected at 103 • C with an enthalpy area of 5.234 µW mg −1 .…”

“…In this respect, lithium-substituted copper sulfides and selenides look more suitable, since they can be single-phase even at room temperature [25]. Sodium-substituted sulfides, with a noticeable sodium content, are mixtures of various sulfide phases at room temperature [36][37][38][39][40], or individual chemical substances are formed with properties far from properties of initial thermoelectric material; for example, NaCu 4 S 4 [41] represents the behavior of an ideal metal. However, multiphase sodium-containing copper sulfides obtained in the nanocrystalline state turn out to be nanocomposites, which favorably affects their thermoelectric and thermal properties.…”

Thermal and electrical properties of nanocrystalline superionic NaxCu1.75S (x=0.1, 0.15, 0.2, 25) compounds

“…In works [13,14] were presented the results of studies of 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 compositions. The alloys were heterophasic and contained such copper sulfide phases as digenite Cu 9 S 5 , copper disulfide CuS 2 , covellite CuS, anilite Cu 7 S 4 in various combinations.…”