Are Cu₂Te‐Based Compounds Excellent Thermoelectric Materials?

Abstract

With the increasingly serious environmental pollution and intensified energy crisis, exploitation and utilization of new kinds of clean energy resources are imperative. Among them, thermoelectric (TE) conversion technology based on highperformance TE materials enables direct energy conversion between heat and electricity through the movement of internal phonons and charge carriers. [1][2][3][4] It has shown extensive and important prospects in power generation using industrial waste heat and electronic refrigeration. [5] The energy conversion efficiency of a TE material is mainly determined by its dimensionless figure of merit, defined as zT = σS 2 T/(κ L + κ e ), where σ, S, T, κ L , and κ e are the electrical conductivity, Seebeck coefficient, absolute temperature, lattice thermal conductivity, and electronic thermal conductivity, respectively. The general criteria for high zTs require high crystal symmetry for materials, many valleys (carrier pockets) near the Fermi level, heavy elements with small electronegativity differences between the constituent elements, or complex crystal structure, etc. [6][7][8][9][10] For the constituent elements in the same group such as S, Se, and Te, the heavy one (Te and Se) always has large atomic mass for low κ L and more covalent bonding character for large carrier mobility (µ H ) and thus outstanding electrical transports. [10] Therefore, the zTs are usually high in tellurides and selenides, but they are low in sulfides. This is the general phenomenon that has been observed in those well-known TE materials such as Bi 2 X 3 -, SnX-, and PbX-based compounds (X = S, Se, and Te). As shown in Figure 1, the zT values gradually improve as the anion element change from S to Se and then to Te. However, the case is different in Cu 2 X-based liquidlike TE materials that are among the hottest materials in recent TE study. They possess exceptionally low thermal conductivity and excellent zTs with the values of 1.7-1.9 for Cu 2 S, 1.5-2.3 for Cu 2 Se, and 0.4-1.1 for Cu 2 Te (see Figure 1). [48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] It is quite abnormal and interesting that the zT in Cu 2 Te is lower than those in Cu 2 S and Cu 2 Se. As we known, tellurium is less electronegative, thus the chemical bonds between Cu and Te should be less ionic as compared with those in Cu 2 S and Cu 2 Se, which is beneficial for large µ H and electrical transports. Besides, the κ L in Cu 2 Te is expected lower than or similar to those in Cu 2 S and Cu 2 Se because tellurium is much heavier than sulfur and Most of the state-of-the-art thermoelectric (TE) materials exhibit high crystal symmetry, multiple valleys near the Fermi level, heavy constituent elements with small electronegativity differences, or complex crystal structure. Typically, such general features have been well observed in those well-known TE materials such as Bi 2 X 3 -, SnX-, and PbX-based compounds (X = S, Se, and Te). The performance is usually high in the materials with heavy constituent elements such as Te and Se, bu...

β‐CsCu5Se3: A Promising Thermoelectric Material going beyond Photovoltaic Application

“…Such a phenomenon has also been observed in many classic thermoelectric materials, including PbX, Bi 2 X 3 , Cu 2 X, SnX (X = S, Se). [ 16,17 ] Although β ‐CsCu 5 Se 3 shows great performance in photovoltaic applications, [ 15,18 ] the research on its lattice thermal conductivity and thermoelectric properties has not been reported yet, which motivates us to carry out this study. On the basis of density functional theory (DFT) combined with semiclassical Boltzmann transport theory, we do find that β ‐CsCu 5 Se 3 possesses ultralow lattice thermal conductivities and a high ZT value, besides the reported photovoltaic features, exhibits high thermoelectric performance.…”

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

“…The highest ZT reported by Zhao et al [185] for Cu 2 Te 50% Ag 2 Te is~1.8 at 727 • C. Similarly, inclusion of nanosized Ag 2 Se into Cu 2 Se increased the ZT to~0.9 at 315 • C and to~1.85 at 527 • C, according to report of Ballikaya et al [176]. In the work of Mukherjee S. [183], the incorporation of nanostructured Ag 2 Te into bulk Cu 2 Te obtained by the melting route also improved the thermoelectric properties of Cu 2 Te significantly.…”

“…Binary copper and silver chalcogenides have excellent thermoelectric characteristics, which continue to be the subjects of intense research [2,4,82,83,109,139,[164][165][166][167]. Over the past 5 years, as examples, we can mention the works [12,14,17,31,39,99,106,129,168,169] on Cu 2-δ S, the works [15,16,165,167,[170][171][172][173][174][175][176][177][178] on Cu 2−δ Se, the works [110,[179][180][181][182][183][184][185] on Cu 2−δ Te, the works [186][187][188][189][190] on Ag 2+δ Se, the works [191][192]…”

A material catalogue with glass-like thermal conductivity mediated by crystallographic occupancy for thermoelectric application

“…Ag alloying on the Cu site was further used to optimize the electrical transport properties. It is reported that the cation vacancy formation energy can be tuned in (Cu, Ag)2Te, 73,74 in which Cu2Te possesses the lowest formation energy probably due to the small Cu atomic radius. Here a similar tendency is also observed that Ag alloying in Cu6Te3S leads to the significantly increased  (Figure 8a), in coincidence with Ag alloying in Cu2Te 73 .…”

“…As a result, Ag alloying in Cu6Te3S enables a significant improvement of ZT (Figure 8d), the maximum ZT about 0.7 at 600 K that is comparable with the well-optimized Cu2Te based materials at the same temperature range. 73 It is highly possible that other donor dopants, such as doping with divalent ions on the Cu site or doping with halogen ions on the Te/S site, can be also effective to improve the ZT of Cu6Te3S. Besides, since Cu6Te3S based materials show an extremely low κlat, increasing the power factor by modulation doping 75 and magnetic effects 76 may be also promising for the overall enhancement of thermoelectric performance.…”