Thermoelectric Properties of CoSb₃ Based Skutterudites Filled by Group 13 Elements

Abstract

Thermoelectric (TE) generators can directly generate electrical power from waste heat, and thus could be an important part of the solution to future power supply and sustainable energy management. The main obstacle to the widespread use of TEs in diverse industries, e.g., for exhaust heat recovery in automobiles, is the low efficiency of materials in converting heat to electricity. The conversion efficiency of TE materials is quantified by the dimensionless figure of merit, ZT, and the way to enhance ZT is to decrease the lattice thermal conductivity (κ _lat) of the material, while maintaining a high electrical conductivity, i.e., to create a situation in which phonons are scattered but electrons are unaffected. Various concepts have been used in the search for this situation, e.g., the use of rattling of atoms weakly bonded in crystals and nanostructuring of materials. Here we report TE properties of skutterudites filled by group 13 elements, i.e., Ga, In, and Tl. Our group has examined the high-temperature TE properties of various skutterudites filed by group 13 elements, viz., Ga-filled CoSb₃, Tl-filled CoSb₃, and In/Tl double-filled CoSb₃. All systems exhibit relatively high TE figure of merit, especially, Tl_0.1In _x Co₄Sb₁₂ achieves a dramatic reduction of κ _lat, resulting in the ZT = 1.20 at 700 K—very high for a bulk material. We have demonstrated that the reduction of κ _lat in Tl_0.1In _x Co₄Sb₁₂ is due to the effective phonon scattering both by rattling of two atoms: Tl and In and by naturally formed nano-sized In₂O₃ particles (<50 nm). Since the combined approach of double filling and self-formed nanostructures could be applicable to various clathrate compounds, our results suggest a new strategy in the improvement of bulk TE materials.

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10.1 Introduction

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 [1–6]. The main obstacle to the widespread use of TEs in diverse industries, e.g., for exhaust heat recovery in automobiles, is the low efficiency of materials in converting heat to electricity. The efficiency of a material used in TE devices is determined by the dimensionless figure of merit, ZT = S ² Tρ ⁻¹ κ ⁻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 = κ _lat + κ _el, where κ _lat and κ _el are the lattice and electronic contributions, respectively). Since the S, ρ, and κ _el in bulk materials are interrelated, it is very difficult to control them independently. Therefore, the reduction of κ _lat is essential to enhancing ZT. The ZT value of the materials currently used in commercial cooling devices is still limited to about 1 or less over the entire operating temperature range, corresponding to a device efficiency 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, including filled skutterudites.

The name of skutterudite is derived from a naturally occurring mineral with CoAs₃ structure, which was firstly discovered in Skutterud (Norway). The general formula of skutterudite compounds is MX ₃, where M is one of the group 9 transition metals such as Co, Rh, or Ir and X is a pnictogen atom such as P, As, and Sb. These compounds are body-centered-cubic structure that contains 32 atoms in the unit cell with space group Im3. The most important point of the skutterudite structure is that there are two voids in the unit cell which can be filled by filler atoms with an ionic radius lower than the cage radius. This generates a so-called filled skutterudite with formula RM ₄ X ₁₂, where R is electropositive element like rare earth or alkaline earth. Figure 10.1 displays the filled skutterudite structure where the R, M, and X are shown in red, blue, and green spheres, respectively.

Fig. 10.1

Filled-skutterudite structure of RM ₄ X ₁₂. R, M, and X are shown in red, blue, and green spheres, respectively

Early investigations on unfilled skutterudites dates back to the mid-1950s. Among skutterudites, CoSb₃ has attracted the greatest interest in waste heat to electricity conversion applications due to its reasonable band gap of ~0.2 eV, high carrier mobility, and the fact that it is composed of inexpensive and environmentally benign constituent elements as compared to other skutterudites such as CoAs₃. However, the κ _lat of the pure binary CoSb₃ is too high, which leads to low ZT and thus poor conversion efficiency for TE applications.

In 1977 La-filled [7] and in 1991 Ba-filled [8] skutterudites were synthesized. However, it has not become popular. In 1994, Slack [9] proposed the concept of “phonon–glass electron–crystal” (PGEC) as one of the desirable features a material should possess to maximize the ZT, in which a material conducts heat like a glass but behaves like a good crystal for the electronic properties. The typical example of the PGEC concept is filled skutterudites, such as CeFe₄Sb₁₂ [10]. Filled skutterudites realize the PGEC concept through the following strategies: a semiconductor-like behavior may lead to large S and low ρ; in addition, the loosely bound filler atoms in the skutterudite cages act as rattlers and thereby reduce the κ _lat [11].

It has been reported that a large variety of guest atoms can be filled, such as rare earth elements [12–16], alkaline earth elements [17–19], alkali metals [20, 21], and others [22–26]. The filling atoms are loosely bound to the other atoms in the intrinsic cages, leading to strong phonon scattering and significant reduction of the κ _lat [27, 28]. In addition to the single-filled system, a double-filling approach has recently been attracting increasing attention. Introducing two filler types from different chemical groups into the cages of CoSb₃ could introduce two distinctive filler vibrational frequencies for a broader range lattice phonon scattering, leading to a further reduction of κ _lat [29, 30]. As the results, the maximum ZT values were improved to 1.3–1.4 in double-filled skutterudites as shown in Fig. 10.2 [12, 14, 16–21, 30–34].

Fig. 10.2

Temperature dependence of dimensionless figure of merit, ZT of various filled-skutterudites reported so far; (a) single-filled system and (b) double-filled system

As described above, it has been widely reported that the voids in CoSb₃ can be fully or partially filled with a variety of different atoms. However, reports on the TE properties of skutterudites filled by group 13 elements, such as Ga, In, and Tl, have been limited. Here we review the TE properties of CoSb₃-based skutterudites filled by group 13 elements, which are mainly obtained in our group. In particular, the TE properties of the single-filled system: Tl _x Co₄Sb₁₂ and Ga _x Co₄Sb₁₂ and the double-filled system: Tl_0.1In _x Co₄Sb₁₂ will be discussed.

10.2 Thermoelectric Properties of Tl-filled skutterudite: Tl _x Co₄Sb₁₂ [26]

Among CoSb₃-based skutterudites filled by group 13 elements, the authors have focused on Tl-filled skutterudites: Tl _x Co₄Sb₁₂, because of the following reasons. First, the electronegativity of Tl is close to that of Sb, which suggests that it may have a small effect on the electrical transport properties of CoSb₃. Second, the radius of the void in a 12-coordinated site is close to the radius of Tl¹⁺. Finally, Tl is heavier than other elements that have been recognized as compatible R atoms in RCo₄Sb₁₂. However, there is only one report [22], of Tl-filled skutterudites in which the TE properties below room temperature have been systematically investigated. In addition, in [22], it has been reported that Tl-filled skutterudites readily formed and up to around 22 % of the voids in CoSb₃ could be filled with Tl. Therefore, the authors prepared the polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) and examined their high-temperature TE properties from room temperature to 750 K [26].

Figure 10.3 shows powder x-ray diffraction (XRD) patterns of the polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) prepared in our group, together with the peak positions calculated from the crystal structure of CoSb₃. Basically, all samples are identified as skutterudite compounds, although they contain negligible amounts of CoSb₂ as an impurity phase. The lattice parameters of Tl _x Co₄Sb₁₂ increase almost linearly with the increase in the Tl content up to x = 0.20 and the lattice parameter of Tl_0.25Co₄Sb₁₂ is almost identical with that of Tl_0.20Co₄Sb₁₂, as shown in Fig. 10.4. Therefore, it can be concluded that the Tl-filling limit in CoSb₃ is between 20 and 25 at.%, which is consistent with the literature data [22].

Fig. 10.3

Powder XRD patterns of polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25), together with the peak positions calculated from the crystal structure of CoSb₃

Fig. 10.4

Lattice parameters of polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25)

Figure 10.5 shows the temperature dependence of (a) the electrical resistivity, ρ and (b) Seebeck coefficient, S for polycrystalline samples of Tl _x Co₄Sb₁₂. The magnitude of ρ for CoSb₃ is approximately 11 × 10⁻⁵ Ω m at 330 K and decreases with increasing temperature. On the other hand, the ρ values of Tl _x Co₄Sb₁₂ are rather low (1–3 × 10⁻⁵ Ω m at 330 K) and indicate slight-positive temperature dependency. The magnitude of ρ for Tl _x Co₄Sb₁₂ decreases with increase in the Tl content x. S for all samples is negative, which indicates that a majority of charge carriers are electrons. The absolute S of CoSb₃ is 330 μV K⁻¹ at 330 K and remains constant up to around 450 K, then decreases gradually with increasing temperature. The absolute S of Tl _x Co₄Sb₁₂ decreases with increase in the Tl content x. The results for both ρ and S imply that Tl-filling leads to an increase of the electron carrier concentration. Room temperature values of Hall carrier concentration (n _H) and Hall mobility (μ _H) for Tl _x Co₄Sb₁₂ are summarized in Table 10.1 and plotted in Fig. 10.6, as a function of the Tl content x. n _H for Tl _x Co₄Sb₁₂ increases with increasing Tl content, viz. n _H for Tl_0.25Co₄Sb₁₂ (23.9 × 10⁻¹⁹ cm⁻³) is more than ten times that of CoSb₃ (1.56 × 10⁻¹⁹ cm⁻³). On the other hand, the Tl _x Co₄Sb₁₂ samples exhibits similar μ _H values at all Tl-filling contents. These results indicate that the Tl-filling increases the n _H, but has no significant influence on μ _H.

Fig. 10.5

Temperature dependences of (a) the electrical resistivity, ρ and (b) the Seebeck coefficient, S of polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25)

Table 10.1 Lattice parameter a, density d, Seebeck coefficient S (300 K), electrical resistivity ρ (300 K), Hall carrier concentration n _H (300 K), Hall mobility μ _H (300 K), average sound velocity v _ave, Young’s modulus E, and Debye temperature θ _D for polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25)

Fig. 10.6

The Hall carrier concentration, n _H and Hall mobility, μ _H at 300 K of polycrystalline samples of Tl _x Co₄Sb₁₂, as a function of the Tl filling fraction, x

The thermal conductivity (κ) for Tl _x Co₄Sb₁₂ are significantly reduced by Tl-filling, as shown in Fig. 10.7a. Tl_0.25Co₄Sb₁₂ exhibits the lowest κ over the entire temperature range; at room temperature, κ for Tl_0.25Co₄Sb₁₂ is 3.4 W m⁻¹ K⁻¹, which is less than half that of CoSb₃. Figure 10.7b shows the temperature dependence of the lattice thermal conductivity (κ _lat) for Tl _x Co₄Sb₁₂. κ _lat was obtained by subtracting the electronic thermal conductivity (κ _el  = LσT, L = 2.45 × 10⁻⁸ W Ω K⁻²) from the total (measured) thermal conductivity κ. κ _lat for Tl _x Co₄Sb₁₂ significantly decreases with increase in the Tl content. However, as summarized in Table 10.1, the experimental results for average sound velocity (v _ave), Young’s modulus (E), and Debye temperature (θ _D) obtained from the sound velocity measurements are similar at all Tl-filling contents, despite κ _lat being significantly reduced by Tl-filling. These results imply that Tl in CoSb₃ has no direct effect on the strength of the interatomic bonding, but that Tl is weakly bonded with the other atoms, which results in a reduction of κ _lat by rattling within the cage.

Fig. 10.7

Temperature dependences of the thermal conductivity of polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25); (a) total thermal conductivity, κ and (b) lattice thermal conductivity, κ _lat

The low κ, compared with CoSb₃, results in large TE figure of merit (ZT) for Tl-filled CoSb₃, as shown in Fig. 10.8. In particular, Tl_0.25Co₄Sb₁₂ exhibits the best ZT values; the maximum value of 0.90 is obtained at 600 K.

Fig. 10.8

Temperature dependences of the dimensionless figure of merit, ZT of polycrystalline samples of Tl _x Co₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25)

10.3 Thermoelectric Properties of Ga-Filled Skutterudite: Ga _x Co₄Sb₁₂ [35]

There have been various reports on the TE properties of skutterudites filled by various elements, such as Ba-, K-, Na-, Ca-, Nd-, Sr-, Eu-, Yb-, In-, and Tl-filled skutterudites [12, 14, 16–22, 26, 36–38]. Although the TE properties of In-, and Tl-filled skutterudites have been investigated, the Ga-CoSb₃ system has been scarcely investigated. Recently, Xiong et al. [39] has reported the TE properties of the Yb_0.26Co₄Sb₁₂/yGaSb system, where only very small amount of Ga can fill into the crystal voids of CoSb₃. Additionally, very recently, Qiu has reported that Ga atoms occupy both the void and Sb sites in CoSb₃ and couple with each other and the ZT quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co₄Sb₁₂ [40]. Here, the authors show the TE properties of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) in the temperature range from room temperature to 750 K, which are obtained in the author’s group [35].

The powder XRD patterns of the polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 10.9. All the peaks in the XRD patterns are identified as the peaks derived from the skutterudite phase. However, in the XRD patterns of the samples of x = 0.05, 0.10, and 0.15, negligible peaks of CoSb₂ as the impurity phase can be observed. The lattice parameters calculated from the XRD patterns are plotted in Fig. 10.10, together with the data for Tl-filled CoSb₃ [26]. The lattice parameters of the Ga-CoSb₃ samples slightly increase with Ga-addition up to around x = 0.05, and after that keep a constant. In contrast, the lattice parameters of the Tl-CoSb₃ samples increase almost linearly with increasing Tl-addition up to x = 0.20 and after that keep a constant. These results imply that Tl can be filled up to around x = 0.20 in Tl _x Co₄Sb₁₂ but the maximum filling ratio of Ga in CoSb₃ is lower than x = 0.05 in Ga _x Co₄Sb₁₂. This result is well consistent with the other Ga-filled skutterudite reported by Xiong et al. [39], in which only very small amount of Ga can fill into the crystal voids of CoSb₃. In addition, in [40], it has been reported that the maximum filling fraction of Ga in the voids of CoSb₃ is around 8 %.

Fig. 10.9

Powder XRD patterns of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30), together with the peak positions calculated from the crystal structure of CoSb₃

Fig. 10.10

Lattice parameters of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30), together with the data of polycrystalline samples of Tl _x Co₄Sb₁₂

In order to confirm the filling limit of Ga into CoSb_3, the field emission scanning electron microscopy (FE-SEM) and energy dispersive x-ray (EDX) analysis was performed on the surface of the hot-pressed samples. The XRD pattern, FE-SEM image, and EDX mapping images of Ga_0.30Co₄Sb₁₂ are shown in Fig. 10.11. As shown in Fig. 10.11a, no peaks corresponding to GaSb are observed in the XRD pattern. Since the main peak position of Ga metal is close to a peak of CoSb₃, it is unclear that whether Ga metal exists or not in the sample. On the other hand, as shown in Fig. 10.11b, the area that Ga concentrated in small pores is observed in the FE-SEM image and the EDX mapping images. Therefore, it can be concluded that most of Ga exists as metal state in the Ga_0.30Co₄Sb₁₂ sample. This result is not consistent with the other Ga-filled skutterudite reported by Xiong et al. [39], in which some evidence of impurity phase of GaSb but not metallic Ga has been shown. At this point, the reason of the differences between our data and the literature data has not been clearly understood. From the quantitative EDX analysis, the amount of Ga in the matrix phase is confirmed to be approximately 0.1 at.%, i.e., x = ~0.02 in Ga _x Co₄Sb₁₂. Therefore, it can be said that the maximum filling ratio of Ga into CoSb₃ is x = 0.02 in Ga _x Co₄Sb₁₂, and when exceeding the filling limit, Ga exists as metal state. This is clearly different from the cases of In- and Tl-filled CoSb₃, in which both In and Tl can be filled up to approximately 0.05 < x < 0.1 in In _x Co₄Sb₁₂ [41] and x = 0.2 in Tl _x Co₄Sb₁₂ [26], respectively. The Ga metal may exist as liquid phase during high-temperature TE properties measurements.

Fig. 10.11

(a) Powder XRD pattern of polycrystalline sample of Ga_0.30Co₄Sb₁₂, together with the peak positions of Ga and GaSb. (b) SEM and EDX mapping images of polycrystalline sample of Ga_0.30Co₄Sb₁₂

In order to verify the maximum filling limit of Ga and Tl into CoSb₃, we performed low-temperature heat capacity analyses on the Co₄Sb₁₂, Ga_0.2Co₄Sb₁₂, and Tl_0.2Co₄Sb₁₂ samples. At low temperature (below 7 K), standard plots of C _P /T versus T ² was linear for Co₄Sb₁₂ sample and yielded Debye temperature of 306 K. This result is well consistent with the literature value (307 K for single crystals of Co₄Sb₁₂ reported by Caillat et al. [42]). The contribution of the filling atoms to the heat capacity would emerge as a form of excess heat capacity (ΔC). In the present case, we considered the following two values of ΔC: ΔC ₁ = C of Ga_0.2Co₄Sb₁₂ − C of Co₄Sb₁₂, and ΔC ₂ = C of Tl_0.2Co₄Sb₁₂ − C of Co₄Sb₁₂. Figures 10.12 and 10.13 plot ΔC ₁ and ΔC ₂ as a function of temperature, respectively. ΔC is well approximated by an Einstein contribution:

Fig. 10.12

The difference in the heat capacities between Ga_0.2Co₄Sb₁₂ and Co₄Sb₁₂, together with a line fitted by using the Einstein model

Fig. 10.13

The difference in the heat capacities between Tl_0.2Co₄Sb₁₂ and Co₄Sb₁₂, together with a line fitted by using the Einstein model

$$ \Delta C={C}_{\mathrm{E}}(T)=3 Ry{\left(\frac{\theta_{\mathrm{E}}}{T}\right)}^2\frac{\exp \left({\theta}_{\mathrm{E}}/T\right)}{{\left(\exp \left({\theta}_{\mathrm{E}}/T\right)-1\right)}^2}, $$

(10.1)

where R is the gas constant, y is the content of the filling atom in the formula of Ga _y Co₄Sb₁₂ or Tl _y Co₄Sb₁₂, and θ _E is the Einstein temperature. Below 20 K, both ΔC ₁ and ΔC ₂ are well described by (10.1), in which we set two unknown parameters: y and θ _E. In the case of ΔC ₁, y and θ _E were found to be 0.02 and 35 K, respectively. On the other hand, in the case of ΔC ₂, y and θ _E were found to be 0.15 and 56 K, respectively. Thus, it can be said that Ga or Tl can be filled up to y = 0.02 in Ga _y Co₄Sb₁₂ and y = 0.15 in Tl _y Co₄Sb₁₂, respectively. Note that the obtained Einstein temperature of 56 K is close to the value of 52 K estimated from the atomic displacement parameters for Tl_0.22Co₄Sb₁₂ [22] indicating the substantial rattling of Tl in the skutterudite structure. Similarly, in the case of Ga-CoSb₃ system, the maximum filling limit was calculated to be y = 0.02 in Ga _y Co₄Sb₁₂, well consistent with the XRD and FE-SEM/EDX results as described above.

Temperature dependences of the electrical resistivity (ρ) and the Seebeck coefficient (S) of the polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 10.14a, b, respectively. The ρ of all the samples decrease with temperature, showing semiconductor behavior. By adding Ga to CoSb₃, the ρ values increase with increasing x up to x = 0.15, and then decrease. The S values are negative for all the samples, indicating that the majority of charge carriers are electrons. The absolute S of all the samples are almost constant up to around 500 K, and decrease rapidly with increasing temperature, which is due to the thermal excitation of the charge carriers. At around room temperature, all the samples show the similar S values. However, at high temperatures, the absolute S decrease with increasing x in Ga _x Co₄Sb₁₂ up to x = 0.15, and then increase. As summarized in Table 10.2, the n _H slightly increases with increasing the amount of Ga, while the μ _H decreases, which is due to the combination of the Ga-filling into the voids in the CoSb₃ crystal and precipitation of Ga metal. In contrast to the Tl-filled CoSb₃, the n _H of the Ga-added CoSb₃ is not sufficiently increased because only a few Ga is filled into CoSb₃. The n _H of Tl_0.25Co₄Sb₁₂ is 23.90 × 10¹⁹ cm⁻³ [26], which is almost ten times larger than that of Ga_0.25Co₄Sb₁₂ (3.42 × 10¹⁹ cm⁻³).

Fig. 10.14

Temperature dependences of (a) the electrical resistivity, ρ and (b) the Seebeck coefficient, S of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30)

Table 10.2 Lattice parameter a, sample bulk density d, Hall carrier concentration n _H (300 K), and Hall mobility μ _H (300 K) for polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30)

Temperature dependences of the κ and the κ _lat for polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 10.15a, b, respectively. The κ decreases with increasing temperature first and then increases from around 550 K. This temperature dependence can be attributed to a bipolar conduction in semiconductors. The κ _lat of Ga _x Co₄Sb₁₂ drastically decreases with Ga-addition. Ga_0.25Co₄Sb₁₂ exhibits the lowest κ _lat (3.52 W m⁻¹ K⁻¹ at 500 K). These results imply that not only the filled Ga but also the precipitated Ga lead to the effective reduction of κ _lat.

Fig. 10.15

Temperature dependences of the thermal conductivity of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30); (a) total thermal conductivity, κ and (b) lattice thermal conductivity, κ _lat

Temperature dependence of the ZT for polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) is shown in Fig. 10.16. Although Ga can be filled only up to x = ~0.02 in Ga _x Co₄Sb₁₂, significant reduction of κ _lat is achieved, leading to enhancement of ZT. In particular, Ga_0.25Co₄Sb₁₂ exhibits the maximum ZT value of 0.18 at around 500 K. This value is lower than those of In- and Tl-filled skutterudites [22, 26, 37, 38]. This low ZT of the Ga-CoSb₃ system is likely due to the unoptimized carrier concentration. In case of the Tl-CoSb₃ system, the Tl-filling into CoSb₃ increases carrier concentration [26]. However, in case of the Ga-CoSb₃ system, the carriers are not sufficiently doped because only a few Ga is filled into CoSb₃. Nonetheless, since Ga-adding to CoSb₃ is effective method for reduction of κ _lat.

Fig. 10.16

Temperature dependences of the dimensionless figure of merit, ZT of polycrystalline samples of Ga _x Co₄Sb₁₂ (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30)

10.4 Thermoelectric Properties of Tl- and In Double-Filled Skutterudite: Tl_0.1In _x Co₄Sb₁₂ [43]

Skutterudites filled by group 13 elements, of Ga-, In-, and Tl-filled systems, have attracted much attention recently as high-performance TE materials. Among the compounds in the Tl-filled system, our group has examined the high-temperature TE properties of Tl _y Co₄Sb₁₂ and obtained a maximum ZT of 0.90 at 600 K for Tl_0.25Co₄Sb₁₂ [26]. The maximum ZT values for In- and Tl-filled skutterudites have been obtained at the compositions of the maximum filling limit [26, 37]. Thus, the enhancement of the ZT of these filled skutterudites is restricted by the filling limit of atoms into the voids of the skutterudite structure. In the case of In-filled skutterudites, there are a few reports in which In naturally forms nano-sized InSb inclusions when it exceeds the filling limit, and these nano-inclusions appear to play an important role in significantly reducing κ _lat [34, 44]. However, there have been a number of reports where no such nano-inclusions were observed in the same In-filled system [38, 45]. Thus, it remains unclear whether any nanostructures form in In-filled skutterudites.

Here, we review the results on the dramatically reduced κ _lat, and thus enhanced ZT, for CoSb₃-based skutterudites by a combined approach of double filling of group 13 elements (Tl and In) and self-forming of nanostructures [43].

In the XRD patterns of polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.1, 0.2, and 0.3), all the peaks indicate a skutterudite structure with the space group Im3, and there are no notable peaks corresponding to impurities as shown in Fig. 10.17. The XRD patterns confirm that all samples have nearly the same lattice parameter values, as summarized in Table 10.3. According to Vegard’s rule, under the assumption that Tl is fully filled while In is partly filled, the lattice parameter a of Tl_0.1In _x Co₄Sb₁₂ can be written as:

Fig. 10.17

Powder XRD patterns of polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30), together with the peak positions calculated from the crystal structure of CoSb₃

Table 10.3 Lattice parameter a, sample bulk density d, Hall carrier concentration n _H (300 K), and Hall mobility μ _H (300 K) for polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30)

$$ a\kern0.5em \mathrm{of}\ {\mathrm{Tl}}_{0.1}{\mathrm{In}}_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}=a\kern0.5em \mathrm{of}\ {\mathrm{Co}}_4{\mathrm{Sb}}_{12}+\Delta {a}_1+\Delta {a}_2, $$

(10.2)

where a of Co₄Sb₁₂ is 0.9034 nm [26], Δa ₁ = a of Tl_0.1Co₄Sb₁₂ − a of Co₄Sb₁₂, and Δa ₂ = a of In _x Co₄Sb₁₂ − a of Co₄Sb₁₂. The values of Δa ₁ and Δa ₂ can be calculated from the changes in the lattice parameters of Tl- and In-filled CoSb₃ using the filling ratio obtained by the present authors’ group. By fitting the calculated lattice parameter to the experimental one, we obtained x = 0.09. This indicates that the maximum filling limits of Tl and In in Tl_0.1In _x Co₄Sb₁₂ are 0.1 and 0.09, respectively. Therefore, it is believed that a secondary phase composed mainly of In may exist in addition to the skutterudite phase after exceeding the maximum filling limit of In.

The FE-SEM and EDX mapping images of the hot-pressed Tl_0.1In_0.3Co₄Sb₁₂ sample are shown in Fig. 10.18. In these images, an In-rich region can be clearly observed mainly at the grain boundaries, implying that In added after exceeding the maximum filling limit precipitates as a secondary phase. Since the particle size of the secondary phase is too small, the peaks of this impurity phase cannot be observed in the XRD patterns.

Fig. 10.18

FE-SEM and EDX mapping images of the polycrystalline sample of Tl_0.1In_0.3Co₄Sb₁₂

In order to identify the chemical state of the In-based secondary phase, we performed transmission electron microscopy (TEM) and X-ray absorption fine structure (XAFS) analyses on the Tl_0.1In_0.3Co₄Sb₁₂ sample. TEM image taken from a single grain of a sintered pellet of Tl_0.1In_0.3Co₄Sb₁₂ is shown in Fig. 10.19a. Nanoparticles (<50 nm) can be clearly observed. They have mostly formed along the grain boundaries. It is thought that they are composed mainly of In present in excess of its filling limit. The nanoparticles were analyzed by electron diffraction techniques. The electron diffraction patterns obtained are shown in Fig. 10.19b, c. The incident beam was focused to a diameter of ~10 nm. The electron diffraction patterns are entirely consistent with In₂O₃ (space group: Ia-3) viewed along the [110] and [112] directions.

Fig. 10.19

(a) TEM image taken from a single grain of the sintered pellet of Tl_0.1In_0.3Co₄Sb₁₂; (b) and (c) electron diffraction patterns obtained with the electron beam aligned along the [110] and [112] directions, respectively; (d) comparison of the measured In L3-edge normalized spectra of a Tl_0.1In_0.3Co₄Sb₁₂ bulk sample with the reference In metal and In₂O₃ powder

To identify the chemical state of In within the sample, In L ₃-edge x-ray absorption near edge structure (XANES) measurements were performed on a Tl_0.1In_0.3Co₄Sb₁₂ bulk sample at the Synchrotron Light Research Institute, Thailand. Figure 10.19d compares the measured In L ₃-edge spectra of this Tl_0.1In_0.3Co₄Sb₁₂ bulk sample and the reference In metal and In₂O₃ powder. Clearly, all the features of the XANES In L ₃-edge spectrum of the Tl_0.1In_0.3Co₄Sb₁₂ bulk sample agree very well with those of the reference In₂O₃ powder. This indicates that the In added in excess of the filling limit within the sample is in the form of In₂O₃. Thus, the XANES measurements confirm that the Tl_0.1In_0.3Co₄Sb₁₂ sample contains In₂O₃.

Temperature dependences of the electrical resistivity (ρ) and the Seebeck coefficient (S) of the polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 10.20a, b, respectively. The ρ of all the samples decrease dramatically with In-addition. By adding Tl and In to CoSb₃, the ρ values of all the samples are below 1.25 × 10⁻⁵ Ω m at room temperature and increase with elevated temperature, typical of heavily doped semiconductors. The Tl_0.1In_0.3Co₄Sb₁₂ sample exhibits the lowest ρ value, 7.5 × 10⁻⁶ Ω m at room temperature. The S values are negative for all the samples, indicating that the majority of charge carriers are electrons. The absolute S of all the samples increases up to around 700 K, and decreases with increasing temperature. As summarized in Table 10.3, there is a large increase in n _H, while μ _H shows a minimal increase with increasing total filling fraction, which means that Tl/In addition leads to a net increase in conduction electrons but has no significant influence on the scattering of carriers. As shown in Fig. 10.21, all the samples show the constant and large values of the power factor (S ² ρ ⁻¹) in the whole temperature range investigated in the present study. The Tl_0.1In_0.3Co₄Sb₁₂ sample exhibits the highest power factor, around 5.0 mW m⁻¹ K⁻² in relatively wide temperature range from 600 to 750 K.

Fig. 10.20

Temperature dependences of (a) the electrical resistivity, ρ and (b) the Seebeck coefficient, S of polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30)

Fig. 10.21

Temperature dependences of the power factor, S ² ρ ⁻¹ of polycrystalline samples of Tl_0.1Tl _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30)

Temperature dependences of the κ and the κ _lat for polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 10.22a, b, respectively. The κ decreases with increasing temperature first and then increases from around 600 to 700 K. This temperature dependence can be attributed to a bipolar conduction in semiconductors. The κ _lat of Tl_0.1In _x Co₄Sb₁₂ drastically decreases with In-addition due to the phonon scattering by Tl- and In-rattling and nano-sized In₂O₃ precipitated at grain boundaries.

Fig. 10.22

Temperature dependences of the thermal conductivity of polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30); (a) total thermal conductivity, κ and (b) lattice thermal conductivity, κ _lat

The temperature dependences of the κ _lat of polycrystalline samples of CoSb₃, Tl_0.1Co₄Sb₁₂, Tl_0.1In_0.1Co₄Sb₁₂, and Tl_0.1In_0.3Co₄Sb₁₂ are shown in Fig. 10.23. A comparison between the κ _lat values of CoSb₃ and Tl_0.1Co₄Sb₁₂ reveals a significant reduction of κ _lat. This is due to the effective phonon scattering by the rattling of Tl. Moreover, filling with In in addition to Tl yields a further reduction of κ _lat in the Tl_0.1In_0.1Co₄Sb₁₂ sample, suggesting that the double-filling approach is effective for phonon scattering, and hence in reducing κ _lat. Furthermore, it can be confirmed that the κ _lat of the Tl_0.1In_0.3Co₄Sb₁₂ sample is much lower than those of the other samples. This implies that the rattling of Tl/In, as well as the In₂O₃ nanoparticles formed at the grain boundaries, scatter heat-carrying phonons, leading to the dramatic reduction of κ _lat. Owing to the very low κ _lat, the Tl_0.1In_0.3Co₄Sb₁₂ sample exhibits an excellent ZT; its maximum value is 1.20 at around 700 K, as shown in Fig. 10.24.

Fig. 10.23

Temperature dependences of the dimensionless figure of merit, ZT of polycrystalline samples of Tl_0.1In _x Co₄Sb₁₂ (x = 0.15, 0.20, 0.25, and 0.30)

Fig. 10.24

Temperature dependence of the lattice thermal conductivity, κ _lat of polycrystalline samples of CoSb₃, Tl_0.1Co₄Sb₁₂, Tl_0.1In_0.1Co₄Sb₁₂, and Tl_0.1In_0.3Co₄Sb₁₂. The numbers 1, 2, and 3 in the figure represent phonon scattering by Tl-rattling, Tl- and In-rattling, and Tl- and In-rattling plus In₂O₃ nanoparticles, respectively

We can conclude that the Tl/In-double-filled skutterudite exhibits the excellent TE properties. Tl and In are filled up to x = ~0.09 in Tl_0.1Tl _x Co₄Sb₁₂ when exceeding the filling limit, In existed as nano-sized In₂O₃ (<50 nm) at grain boundaries. By investigating the TE properties of Tl- and In-double-filled CoSb₃-based skutterudites, we demonstrate that the reduction of κ _lat is due to the effective phonon scattering induced both by the rattling of Tl and In and by the naturally formed In₂O₃ nanoparticles (<50 nm). This yields a dramatic enhancement of ZT to be 1.20 at 700 K. The combined approach of double filling and self-formed nanostructures might be applicable to various clathrate compounds. Thus, these results point to a new strategy in the improvement of bulk TE materials.

10.5 Summary

In this chapter, we reviewed the TE properties of CoSb₃-based skutterudites filled by group 13 elements, i.e., the systems of Tl _x Co₄Sb₁₂, Ga _x Co₄Sb₁₂, and Tl_0.1In _x Co₄Sb₁₂. The temperature dependence of ZT of the best materials in the individual system is summarized in Fig. 10.25.

Fig. 10.25

The dimensionless figure of merit, ZT of polycrystalline samples of Tl_0.25Co₄Sb₁₂, Ga_0.25Co₄Sb₁₂, and Tl_0.1In_0.3Co₄Sb₁₂

Tl can be filled in CoSb₃ up to around x = 0.2 in Tl _x Co₄Sb₁₂. Since the valence state of Tl in CoSb₃ can be considered to be Tl¹⁺, the Tl-filling yields net increase in electron carriers. Therefore, the carrier concentration of CoSb₃ can be controlled by Tl-filling. On the other hand, Tl filled in CoSb₃ can act as rattlers and scatter heat carrying phonons efficiently. Therefore, Tl-filled CoSb₃ exhibits low κ _lat with optimized carrier concentration, and hence high ZT value; the maximum ZT value is 0.90 at 600 K obtained for Tl_0.25Co₄Sb₁₂.

In contrast to Tl-filled CoSb₃, Ga can be filled in CoSb₃ up to only x = 0.02 in Ga _x Co₄Sb₁₂, and when exceeding the filling limit, Ga exists as metal states mainly at grain boundaries in the polycrystalline samples of Ga _x Co₄Sb₁₂. Due to this low filling limit of Ga in CoSb₃, carriers are not sufficiently doped by Ga-filling. Nonetheless, not only very small amount of filled Ga but also precipitated Ga metals may scatter heat carrying phonons, leading to the reduction of κ _lat. Mainly due to the reduced κ _lat, the ZT is slightly enhanced by Ga-adding to CoSb₃; the maximum ZT value is 0.18 at 500 K obtained for Ga_0.25Co₄Sb₁₂.

In case of the double-filling system, i.e., Tl_0.1In _x Co₄Sb₁₂, all Tl is filled but In can be filled up to around x = 0.09 in Tl_0.1In _x Co₄Sb₁₂. In added in excess of the filling limit within the samples is in the form of In₂O₃ which is naturally formed as nanoparticles (<50 nm) mainly at the grain boundaries of the polycrystalline samples. Effective phonon scattering occurred both by the rattling of Tl and In and by the naturally formed In₂O₃ nanoparticles yield significant reduction of κ _lat and thereby dramatic enhancement of ZT. Tl_0.1In_0.3Co₄Sb₁₂ exhibits the maximum ZT of 1.20 at around 700 K, which is very high for bulk materials.

We consider the following two studies are important in this research for future. The first one is further enhancement of ZT by nanostructuring. Recently, various nanocrystalline bulk materials have been prepared by ball-milling followed by hot-pressing or spark plasma sintering and the enhancement of ZT have been achieved [46, 47]. The method is considered to be applied to the CoSb₃-based skutterudites. The second one is to develop p-type CoSb₃-based skutterudites filled by group 13 elements. Although the CoSb₃-based skutterudites filled by group 13 elements reported here have high ZT values, all the samples are n-type, i.e., they indicate negative S values. However, for a powerful TE module, the similar performances in both n-type and p-type TE materials is required. Therefore, the p-type CoSb₃-based skutterudites filled by group 13 elements should be developed. Very recently, our group has demonstrated that Tl_xFe_yCo_1−y Sb₁₂ exhibit p-type characteristics and relatively high ZT values. The details of the results will be reported in near future.