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Keywords:

  • group 13 elements;
  • thermal conductivity;
  • thermoelectric;
  • telluride

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

Thermoelectric (TE) generators can directly generate electrical power from waste heat and are promising for use in power supplies and for realizing sustainable energy management. However, the low efficiencies of TE materials in converting heat to electricity is the main impediment to applying TE generators in many industries including exhaust heat recovery in automobiles. The efficiency of TE materials is quantified by a dimensionless figure of merit ZT. To enhance ZT, it is important to reduce the lattice thermal conductivity (κlat) of a material while maintaining a high electrical conductivity. Here, we review the TE properties of thallium-based compounds, mainly tellurides. Many thallium tellurides exhibit extremely low κlat below 0.5 W m−1 K−1, which is almost one third that of Bi2Te3 used in current TE devices. Of the thallium tellurides, Ag9TlTe5 has the highest ZT value of 1.2, which is higher than typical ZT values 0.8 of Bi2Te3; this is primarily due to the extremely low κlat of Ag9TlTe5. In addition, we briefly review the TE properties of tellurides of other group 13 elements that contain structural vacancies such as Ga2Te3. Tellurides exhibit various vacancy distributions and hence have interesting TE properties. Based on the results of the TE properties of these tellurides, we propose a strategy for improving TE materials.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

Thermoelectric (TE) materials can convert waste heat into electrical power, which is an effective way to reduce greenhouse gas emissions and contribute substantially to future power supply and sustainable energy management 1, 2. The main impediment to the widespread application of TE materials in diverse industries (e.g., exhaust heat recovery in automobiles) is the low efficiencies of TE materials in converting heat to electricity. The TE efficiency of a material used in TE devices is determined by the dimensionless figure of merit, ZT = S2−1κ−1, where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and κ is the total thermal conductivity (κ = κlat + κel, where κlat and κel are the lattice and electronic contributions, respectively). Since S, ρ, and κel are interrelated in bulk materials, it is very difficult to control them independently. It is thus essential to reduce κlat to enhance ZT.

The ZT values of materials currently used in commercial cooling devices are still limited to about 1 or less over the entire operating temperature range; this corresponds to device efficiencies of several percent. Recent improvements in TE materials have led to many advances and enhanced ZT values have been reported for several classes of bulk materials 3, including group 13 tellurides.

2 Low-thermal-conductivity thallium tellurides

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

2.1 A brief history of thermoelectric research on thallium tellurides

Since the 1960s, thallium tellurides have attracted interest as TE materials. They have recently been widely acknowledged as being a special group of compounds that exhibit extremely low thermal conductivities compared with those of the wide range of TE materials currently available. However, TE materials containing thallium are not limited to thallium tellurides. In recent years, extensive research has been conducted on skutterudite compounds filled with thallium 4, 5 and PbTe doped with small amounts of thallium 6, 7; and this research is steadily producing results. The following overview focuses on the TE properties of thallium tellurides.

The first paper on the TE properties of thallium tellurides is thought to have been published in 1965. It investigated the high-temperature TE properties of the liquid phase of binary compounds of thallium and tellurium 8. This study accurately analyzed liquid phases for various thallium telluride compositions and it reported relatively high ZT values (0.1–0.85). For some time after that study, thallium tellurides attracted little notice until Tedenac et al. published a series of reports on the crystal structure and physical properties of Cu–Tl–Te compounds 9 and Ag–Tl–Te compounds 10, 11 from the latter half of the 1970s and into the 1980s. However, the reported ZT values were not very high and perhaps due to the difficulty of handling thallium, thallium tellurides were not widely employed as TE materials.

Then in 1999, Sharp et al. 12 reported that Tl2SnTe5 exhibited an extremely low thermal conductivity and a high ZT (0.85 at 400 K), which was comparable to that of the commercial TE material Bi2Te3 near room temperature. In addition, Wolfing et al. 13 reported that Tl9BiTe6 exhibited an extremely low thermal conductivity and a high ZT (1.2 at 500 K) that were similar to those of Tl2SnTe5. These two findings represented a turning point. Subsequently, thallium tellurides started to attract a lot of attention as TE materials. Tedenac and co-workers 14 had reported the TE properties of Tl9BiTe6 as early as 1982.

Subsequent to the above-mentioned studies, we began researching the TE properties of thallium tellurides from 2001 by systematically investigating various TE properties including their thermal conductivity. Research with a particular emphasis on the Ag–Tl–Te series resulted in the discovery of a compound with the composition Ag9TlTe5 that exhibited a ZT of 1.2 at 700 K 15. Next, the TE properties of thallium tellurides that have been the subject of our research are introduced along with a discussion of the origin of the low thermal conductivity of this series of thallium tellurides.

2.2 Thermoelectric properties of thallium tellurides

Figure 1 shows the temperature dependences of the thermal conductivities of various thallium tellurides 16. It shows that many thallium tellurides exhibit an extremely low thermal conductivity of 0.5 W m−1 K−1 or less across a wide temperature range (room temperature to 700 K). The densities of the samples shown in the manuscript are high enough for thermoelectric characterization. For example, the densities of the polycrystalline samples of Ag9TlTe5 are 95% of the theoretical densities 15. Zn4Sb3 and Bi2Te3 also exhibit low thermal conductivities of about 1 W m−1 K−1, which indicates how extremely low the thermal conductivities of thallium tellurides are.

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Figure 1. (online color at: www.pss-a.com) Temperature dependences of the thermal conductivities (κ) of various thallium tellurides.

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Figure 2 shows the temperature dependences of ZT for various thallium tellurides 16. Note that here the reproducibility as determined from TE measurements using different contacts or with different slices from the same pellet is about 10% with the largest uncertainty. Considering the uncertainty in the measurements of electrical resistivity, Seebeck coefficient, and thermal diffusivity, the error bars are a maximum of around 20% for ZT. These compounds all exhibit relatively high ZT values, while Ag9TlTe5, Tl9BiTe6, Tl9SbTe6, Tl4SnTe3, and Tl4PbTe3 exhibit particularly high ZT. Of the latter group, ZT for Ag9TlTe5 in particular attains a maximum of 1.23 at 700 K. High-performance TE materials (i.e., bulk materials with ZT > 1) researched to date possess excellent electrical properties and relatively low thermal conductivities. They have power factors (i.e., numerator of Z: S2/ρ), which determine the electrical properties, of at least 1 × 10−3 W m−1 K−2 and thermal conductivities of at least 1 W m−1 K−1. In contrast, Ag9TlTe5 has a power factor of only 0.4 × 10−3 W m−1 K−2, but an extremely low thermal conductivity of 0.25 W m−1 K−1. It thus has a ZT of greater than 1.

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Figure 2. (online color at: www.pss-a.com) Temperature dependences of dimensionless figures of merit (ZT) of various thallium tellurides.

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2.3 Mechanism for extremely low thermal conductivity of thallium tellurides

The mechanism for the extremely low thermal conductivity of thallium tellurides was investigated by examining and classifying various low-thermal-conductivity compound groups (including those that are not TE materials) and compiling the results obtained.

We classified groups of compounds that exhibit extremely low thermal conductivities (i.e., with lattice thermal conductivities close to the minimum thermal conductivity), into the following three categories.

  • (1)
    Amorphous compound groups, typified by glasses or crystals that are completely disordered.
  • (2)
    Compound groups whose crystals have partially disordered regions.
  • (3)
    Compound groups that exhibit phonon scattering by nanostructuring.

The thermal conductivities of amorphous silica and CdGeAs2 whose crystals are completely disordered (i.e., category 1) have been reported to be close to the minimum thermal conductivity 17. Unfortunately, these compound groups are not currently attracting attention as TE materials, because their amorphous state reduces the electron mobility.

In contrast, numerous compound groups whose crystals have regions that are partially disordered (i.e., category 2) have been reported to be promising TE materials. For filled skutterudite compounds, inserting a third element into sublattice vacancies of the crystal structure greatly disrupts the crystal structure, greatly reducing the lattice thermal conductivity without reducing the electron mobility 18. For Zn4Sb3, zinc atoms that enter the lattice in a disordered fashion with a large displacement parameter effectively scatter phonons to realize a low thermal conductivity 19. These compounds represent typical examples of an ideal system for TE materials, namely phonon glass electron crystals. Next, although it is not widely recognized as a TE material, AgI exhibits extremely interesting thermal conductivity characteristics. The thermal conductivity of AgI drops suddenly at 420 K or above; it has been reported to be as low as 0.2 W m−1 K−1 20. Since AgI becomes a superionic conductor of Ag+ ions at or above 420 K, its low thermal conductivity is thought originate from strong scattering of phonons by Ag+ ions. In addition, for Cu2–xSe, the disorderly arrangement of aqueous Cu+ ions between the crystal lattice that consists of selenium atoms was found to afford an extremely low thermal conductivity and an associated high ZT value 21.

Typical examples of the realization of a low thermal conductivity through nanostructuring (i.e., category 3) include the Bi2Te3/Sb2Te3 superlattice structure 22 and nanostructured silicon 23, 24. Artificial creation of nanostructures in a bulk material (as opposed to thin films or nanowires) has led to reduced thermal conductivity and accompanying ZT enhancement 25.

How does the foregoing apply to thallium tellurides? Currently, the particular crystallographic features of thallium telluride together with the ancillary effects of (2) above are believed to be relevant. Hence, many thallium tellurides share two major characteristics required for low thermal conductivity: they contain the heavy element thallium and have an extremely complex crystal structure. Note that AgTlTe and TlInTe2 do not have extremely complex crystal structures, but do have low lattice thermal conductivity. Heavy atoms reduce thermal conductivity by reducing phonon energies and sound speeds. If the number of atoms per unit cell is N, then the ratio of acoustic modes (vibration modes that effectively transports heat) to the total number of modes will be 1/N. A large N thus means a low proportion of acoustic modes and thus the lattice thermal conductivity is expected to be low. While its crystal structure has not been completely determined, Ag9TlTe5 possesses a complex crystal structure with 120 atoms per unit cell. Due to its arrangement of heavy elements, it has an extremely low Debye temperature of 120 K. In addition to such crystallographic properties, it may be in a partially disordered state within the crystal, as described by the effect in (2). For example, shifts of monovalent ions such as Ag+ or Tl+ or the presence of atoms between the lattice are conceivable. The extremely low thermal conductivities of certain thallium tellurides may be due to one of these factors or a combination of them.

3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

As discussed above, thallium tellurides possess extremely low thermal conductivities and thus exhibit high ZT values. Since thallium is highly toxic, it is desirable to realize a similarly high ZT for compound groups that do not contain thallium. Although the solid state chemistry of monovalent thallium is more similar to that of alkali metals than that of other group 13 elements, here we show the recent results on the TE properties of some chalcogenides containing gallium and indium. In recent years, chalcogenides containing elements (specifically, gallium and indium) belonging to group 13 (i.e., the same group as thallium), particularly compounds with crystal structures originating from the diamond structure such as the zinc-blende structure or the chalcopyrite structure are attracting attention as TE materials. This is because these compounds have a particular crystal structure in which large numbers of vacancies exist in the crystal and these vacancies greatly affect the TE properties. Of the various effects that the vacancies have on TE properties, two are reviewed here: the effect of the vacancy distribution on the thermal conductivity and the effect of the amount of vacancies on the thermal conductivity and the electrical properties. Ref. 26 describes the toxicity of thallium.

3.1 Relationship between vacancy distribution and thermal conductivity of Ga2Se3

Ga2Se3 basically has the zinc-blende structure, even though its molecular formula indicates the ratio of gallium to selenium is 2:3. However, the ratio of lattice points that each element can occupy within the crystal is 1:1. Thus, one-third of the gallium sites are vacancies. Optimized heat treatment conditions cause the vacancy distribution to be dramatically altered resulting in a large drop in the thermal conductivity, as outlined below.

Figure 3 shows high-resolution TEM images and electron diffraction patterns of single grains in sintered pellets of Ga2Se3 annealed under different conditions 27. The diffraction pattern in Fig. 3a is consistent with the (001) reciprocal lattice plane of Ga2Se3 (space group: Cc (No. 9), a = 0.6608 nm, b = 1.16516 nm, c = 0.66491 nm, α = 90°, β = 108.84°, γ = 90°) in which vacancies induced by the valence mismatch between the cation and the anion are regularly arranged in the Ga sublattice. The high-resolution TEM image in Fig. 3a shows that no vacancy planes are formed, suggesting that the vacancies in the sample exist as point defects. On the other hand, in the high-resolution TEM image of the sample annealed at high temperature (Fig. 3b), there are two-dimensional (2D) vacancy planes in the (111) plane with random periodicity. In addition, a streak between the fundamental Bragg reflections due to the zinc-blende structure is observed in the electron diffraction pattern of the sample annealed at high temperature.

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Figure 3. High-resolution TEM images and (insets) electron diffraction patterns of single grains in sintered pellets of Ga2Se3 annealed under different conditions 27 (a) for sample annealed at a low temperature (point vacancies) (b) for sample annealed at a high temperature (in-plane vacancies with random periodicity).

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Figure 4 shows the temperature dependences of thermal conductivity (κ) of Ga2Se3 with two different vacancy distributions 27. Here, κ can be considered to correspond to κlat because Ga2Se3 has quite high ρ values. As shown in this figure, the sample with point vacancies clearly has a higher κ than the sample with in-plane vacancies. In addition, these samples clearly have different temperature dependences of κ: κ of the sample with point vacancies decreases with increasing temperature according to approximately T−1, whereas κ of the sample with in-plane vacancies exhibits a rather flat temperature dependence. The following conclusions are obtained from these experimental results: the presence of vacancies alone does not result in effective phonon scattering; rather, vacancies should form an in-plane defect structure to realize effective phonon scattering. Therefore, introducing structural vacancies with in-plane defect structures is a promising new method for reducing κ of TE materials and increasing ZT.

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Figure 4. Temperature dependences of thermal conductivities (κ) of Ga2Se3 with point and in-plane vacancies.

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3.2 TE properties of Cu–Ga–Te ternary compounds

The previous section introduced the relationship between the structural vacancy distribution and the thermal conductivity for Ga2Se3 in which Ga2Se3 with in-plane defects clearly exhibited a lower lattice thermal conductivity than Ga2Se3 with point defects. The amounts of vacancies and their distribution state are thought to greatly affect the TE properties of compounds, particularly the transport properties of heat and electricity. The relationships between the amount of vacancies, the lattice thermal conductivity, and the carrier mobility are outlined below for Cu–Ga–Te ternary compounds.

Several Cu–Ga–Te ternary compounds have a crystal structure derived from the diamond structure, such as the zinc-blende structure or the chalcopyrite structure. There are five typical compounds: CuGaTe2, Cu3Ga5Te9, Cu2Ga4Te7, CuGa3Te5, and CuGa5Te8. The four compounds besides CuGaTe2 characteristically contain many vacancies in their crystal structures. For example, Cu2Ga4Te7 has the following crystal structure. Assuming charges of +1 for Cu, +3 for Ga, and −2 for Te, the total cationic valence will be 2 × (+1) + 4 × (+3) = +14 and similarly the total anionic valence will be −7 × (2) = −14. Thus, the valences of the cations and the anions are balanced for Cu2Ga4Te7. Although the ratio of lattice points of cations to anions is 1:1 in the adopted zinc-blende structure, the molecular formula indicates seven anions (seven Te) and six cations (two Cu and four Ga). Thus, 1/7th of cation sites are necessarily vacancies. A similar examination of the remaining compounds reveals the following: 1/9th of cation sites are vacancies in Cu3Ga5Te9, 1/7th in Cu2Ga4Te7, 1/5th in CuGa3Te5, and 1/4th in CuGa5Te8 28–31. CuGaTe2 contains no vacancies. Vacancies in Cu2Ga4Te7 have been found to be regularly arranged as point defects 32. However, it has not been confirmed whether the vacancies have in-plane or point arrangements in other Cu–Ga–Te ternary compounds. More extensive studies are needed to determine this.

Figure 5 shows the thermal conductivity (κ) of polycrystalline Cu–Ga–Te ternary compounds as a function of temperature 33. Because the Cu–Ga–Te ternary compounds have low ρ values, κlat is dominant, accounting for over 97% of the measured κ. The κ of CuGaTe2 containing no vacancies decreases with increasing temperature. This implies that the typical lattice contribution is dominant in the κ of CuGaTe2. As shown in Fig. 5, κ clearly decreases with increasing vacancy density. In addition, the temperature dependence of κ becomes flat with increasing vacancy density. These results indicate that the vacancies in the crystal scatter phonons efficiently, significantly reducing the temperature independence of κ.

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Figure 5. (online color at: www.pss-a.com) Temperature dependence of the thermal conductivity (κ) of Cu–Ga–Te ternary compounds 33.

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Figure 6 shows the relationships between the vacancy density and the Hall mobility (µH), κ, and µH/κ for Cu–Ga–Te ternary compounds 33. It reveals that the presence of vacancies reduces both µH and κlat, indicating that vacancies scatter both carriers and phonons. However, µH clearly decreases faster than κlat and CuGaTe2 with no vacancies has the highest µH/κlat. These results mean that the presence of vacancies degrades the TE performance of Cu–Ga–Te ternary compounds. Thus, CuGaTe2 with no vacancies is expected to exhibit the highest ZT value in Cu–Ga–Te ternary compounds.

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Figure 6. Relationships between the vacancy density and the Hall mobility (µH), lattice thermal conductivity (κlat), and µH/κlat of Cu–Ga–Te ternary compounds. Plotted data were obtained near room temperature 33.

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Most recently, CuGaTe2 that has a chalcopyrite crystal structure was found to have a very high ZT value (1.4 at 950 K) at high temperatures 34. In addition, CuInTe2 35, 36 and AgGaTe2 37, which have the same crystal structure, were found to exhibit a similarly high ZT value. Further enhancement of the ZT value has been achieved by substituting a portion of the gallium sites in CuGaTe2 with indium 38. Figure 7 shows the temperature dependences of ZT for CuGaTe2 and its associated chalcopyrite structure compounds. It shows that all the compounds exhibit very high ZT at high temperatures. However, the mechanism that gives rise to these high ZT values is currently not completely clear. In a similar manner as AgI 20 and Cu2–xSe 21 introduced in Section 2.3, Cu+ and Ag+ ions may exhibit superionic conductivity at high temperatures, which reduces the lattice thermal conductivity, resulting in high ZT values. Evaluations of ionic conductivity and specific heat at high temperature range are thought to be effective to verify this possibility.

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Figure 7. (online color at: www.pss-a.com) Temperature dependence of the dimensionless figure of merit (ZT) of CuGaTe2, CuInTe2, AgGaTe2, and Cu(Ga,In)Te2 with chalcopyrite structure.

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4 Summary

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

Thallium tellurides possess extremely low thermal conductivities, making this compound group promising for high-performance TE materials. Silver–thallium–tellurium ternary compounds typified by Ag9TlTe5 exhibit extraordinarily high ZT values, but this compound group does not have superior electrical properties. In other words, much room for enhancement of their electrical properties remains. In practice, slightly altering the composition of Ag9TlTe5 causes phenomena that were confirmed to greatly alter the TE properties 39. Further enhancement to ZT is anticipated in future research.

In recent years, chalcogenides containing elements from group 13 (i.e., the same group as thallium) have attracted attention as TE materials, particularly compounds with crystal structures derived from the diamond structure, such as the zinc-blende structure and the chalcopyrite structure. Specifically, for Ga2Se3 that has the zinc-blende structure with structural vacancies, shifting the vacancy distribution (which is ordinarily a point state) to an in-plane state dramatically reduces the lattice thermal conductivity. For Cu–Ga–Te ternary compounds that have the same zinc-blende structure as Ga2Se3 or the chalcopyrite structure similar to the zinc-blende structure, the lattice thermal conductivity and mobility both decrease with increasing amount of vacancies, although the mobility is known to decrease more than lattice thermal conductivity. The presence of vacancies and the increase in their amounts for Cu–Ga–Te ternary compounds is thus detrimental to the enhancement of ZT. In other words, this means that CuGaTe2, which contains no vacancies, is the most promising high-performance TE material. In fact, very recently a series of compounds that include CuGaTe2, CuInTe2, and AgGaTe2 has been found to exhibit extremely high ZT values at high temperatures. These tellurides that contain group 13 elements and adopt the chalcopyrite structure are expected to attract increasing attention as novel high-performance TE materials in the future and to be the subject of much research aiming to determine the mechanism responsible for the high TE properties and to further enhance the performance.

Biographical Information

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Low-thermal-conductivity thallium tellurides
  5. 3 Chalcogenides (mainly tellurides) of group 13 elements with structural vacancies
  6. 4 Summary
  7. Biographical Information

Ken Kurosaki is an Associate Professor at the Graduate School of Engineering, Osaka University, Japan. He received his B.S. (1995), M.S. (1997), and Ph.D. (2003) in Nuclear Engineering from Osaka University. He worked as an Assistant Professor at Osaka University from 1998 to 2009. His current research focuses on nuclear fuels/materials and thermoelectric materials. He has over 200 publications.

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