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

Abstract

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.

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 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.…”

Super–protonic conductors for solid acid fuel cells (SAFCs): a review

“…In order to convert the chemical energy from the fuel into electrical energy, a fuel cell uses chemical processes to generate electricity [48][49][50][51]. In the fuel cell system, an electrolyte connects the positively charged cathode to a negatively charged anode [52][53][54][55]. Unlike batteries, the fuel cell does not store energy and does not need the power to charge .…”

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.…”