type-VIII Clathrate Ba8Ga16Sn30 Research Articles

Atomistic explanation of failure mechanisms of thermoelectric type-VIII clathrate Ba8Ga16Sn30

The type-VIII clathrate Ba8Ga16Sn30 (BGS) is a promising thermoelectric material with a cage-guest structure. The failure mechanisms behind the tetrahedrally bonded framework, which are vital for understanding the structure - mechanical property relationship, are still unrevealed. Here we conduct DFT calculations to investigate the intrinsic mechanical behavior of BGS. BGS has the ideal strength of 2.65 GPa under (100)/[010] shearing. The fast stretching and breakage of Sn-Sn bond give rise to the sudden failure of Ga-Sn bond, resulting in the collapse of void space and the structural damage. Under [100] tension, the structural failure is due to the breakage of Ga-Sn bonds that are nearly parallel to the tensile direction. Besides, Ga-Sn bonds are dominant for maintaining the local rigidity since they contribute the vast majority of structural resistance.

Thermoelectric Properties of p-Type Clathrate Ba8.0Ga15.9ZnySn30.1 Single Crystals with Various Carrier Concentrations

To match the excellent performance of n-type type-VIII clathrate Ba8Ga16Sn30 single crystals in thermoelectric (TE) generator design, synthesizing a large size p-type counterpart with a high TE figure of merit ZT continues to be a challenge for many years. In this work, we obtained 7-mm-diameter single crystals of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 (Zn-BGS) with various carrier concentrations by Ga-flux method. Though the actual substituted compositions of the single crystals Ba8Ga15.9ZnySn30.1 (y = 0–0.019) are much lower than the nominal ones, the Zn atoms work as a “spur” and effectively suppress the nucleation rate of BGS in the Ga-flux, which is vital to achieving a similar large size crystal with the n-type counterpart. The y = 0.006 and 0.007 crystals share nearly the same or a slightly lower value of resistivity with a pristine crystal in the whole temperature. However, the Hall coefficient measurement revealed that Zn substitution triggers a profound transition from a “low-carrier-concentration high-carrier-mobility” regime to a “high-carrier-concentration low-carrier-mobility” one in p-type BGS. As a consequence, both the Seebeck coefficient and thermal conductivity benefit from the relief of the negative impact of minority carrier by shifting the “trigger temperature” of intrinsic excitation to a higher temperature. Figure of merit ZTs of 0.97 and 1.07 are observed for x = 0.006, 0.007 at 500 K, which are more than 25% and 40% higher than the figure of merit for pristine BGS at the same temperature zone, respectively. The improvement demonstrates the potential of making TE modules from n- and p-type single crystal BGS for waste heat recovery at medium temperatures.

Thermoelectric and transport properties of sintered n-type K8Ba16Ga40Sn96 with type-II clathrate structure

This clathrate had a maximum dimensionless figure-of-merit, ZT, of 0.93 at 637 K, which was slightly higher than that of 0.83 for the sintered type-VIII clathrate Ba8Ga16Sn30. We investigated the high-temperature thermoelectric properties, transport properties, electronic structures, and thermal stabilities of the clathrates. The type-II clathrate was found to be superior to the type-VIII clathrate as a thermoelectric material; it had a high thermal stability and melting point, 859 K, high mobility, 141 cm2V−1s−1 at 300 K, because of its low inertial mass, and low high-temperature lattice thermal conductivity, approximately 4 mW cm−1K−1, resulting from a larger unit cell and weaker bipolar thermal conduction. We discuss these properties in terms of the electronic structure and the differences between the two types of clathrate.

Vertical Bridgman growth of thermoelectric clathrate Ba8Ga16Sn30 with a type-VIII structure

Type-VIII clathrate Ba8Ga16Sn30 (BGS) is a promising thermoelectric material with tunable carrier types. We have studied the Bridgman growth conditions by combining differential thermal analysis (DTA) and powder X-ray diffraction analysis. It is found that the raw elements Ba, Ga, and Sn react at 465°C to form the type-VIII BGS compound. The DTA indicates the presence of a liquidus line above the peritectic point of BGS at 495°C. By using the vertical Bridgman method, single crystals of n-type and p-type BGS have been grown from Sn-rich and Ga-rich initial compositions, respectively. The obtained crystals of n-type Ba8Ga16Sn30 and p-type Ba8Ga16Sn30−xGex (x≈0.7) are 10mm sized. On several parts cut from the grown crystals, the Seebeck coefficient and electrical resistivity have been measured up to 330°C. The values weakly change along the vertical direction of the crystals, keeping the thermoelectric power factors as high as 12 and 9μW/K2cm, respectively, for n- and p-type crystals.

Crystal growth and thermoelectric properties of type-VIII clathrate Ba8Ga15.9Sn30.1−xGex with p-type charge carriers

Type-VIII clathrate Ba8Ga16Sn30 is a promising thermoelectric material in the 400–600 K range. We report on the effects of Ge substitution for Sn on both crystal growth and thermoelectric properties. In a series of samples prepared from the initial composition of Ba8 Ga40 Sn30−X GeX (0 ⩽ X ⩽ 6), the size of single crystals reaches the maximum of 8 mm at X = 0.5. The atomic composition of single crystals is found to be described as Ba8Ga15.9Sn30.1−xGex (0 ⩽ x ⩽ 4.73). Ge substitution results in the increase in the positive Seebeck coefficient and electrical resistivity by 1.3 and 2 times at the maximum, respectively, whereas the hole carrier density does not change substantially. Taken together with the thermal conductivity data, the maximum of the dimensionless figure of merit ZT reaches 0.87 at 540 K for x = 0.07.

Thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30 doped with Cu

The intermetallic clathrate Ba8Ga16Sn30 with type-VIII structure is one of the promising thermoelectric materials. We report on significant effects of Cu doping on both the crystal growth and thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30. By using the Ga flux and Sn flux, respectively, the carrier type could be tuned as p- and n-types. The p-type single crystals were grown to approximately 6mm in diameter, which is twice as large as that grown without Cu. The crystals were characterized by electron-probe microanalysis and powder X-ray diffraction and used for the measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity. The values of the dimensionless figure of merit ZT of the p- and n-type crystals reached 0.88 and 1.45 at around 500K, respectively, the latter of which is the highest among all substituted systems of type-VIII Ba8Ga16Sn30. This enhancement of ZT is discussed in relation with the charge balance and modification of the band structure caused by Cu doping.

Electronic Structures and Thermoelectric Properties of Sb-Doped Type-VIII Clathrate Ba<sub>8</sub>Ga<sub>16</sub>Sn<sub>30</sub>

The type-VIII clathrate Ba8Ga16Sn30 shows a high figure of merit (ZT) in the middle temperature range 470–670 K. ZT enhancement through Sb doping has recently been reported in p-type Ba8Ga16Sn30. In this study, calculations to determine the electronic structures and thermoelectric properties of Sb-doped Ba8Ga16Sn30 are performed using the WIEN2k code. The energy derivative of density of states [∂ρ(e)/∂ee=EF for valence band is lower in the Sb-doped Ba8Ga16Sn30 than in non-Sb-doped one, though the density of states [ρ(e)]e=EF of both system are almost the same as that for Ba8Ga16Sn30. Except for the signs, the calculated Seebeck coefficients for the p- and n-type Sb-doped and non-Sb-doped Ba8Ga16Sn30 are similar.

Thermoelectric performance of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 single crystals

We have grown single-crystalline samples of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 with n-type carriers by Sn-flux method. The actual compositions of the single crystals were found to be described as Ba8Ga15.8−2yZnySn30.2+y (y = 0 ∼ 0.54), where the charge balance is well maintained. As y goes from 0 to 0.42, the resistivity at 300 K decreases from 5.3 to 3.0 mΩ cm gradually, but the effective mass is essentially constant at 1.2 ∼ 1.5m0, indicating intact band structure near the conduction band minima upon Zn substitution for Ga. At elevated temperatures, the ambipolar effect on the thermal conductivity becomes less pronounced upon Zn doping, and the dimensionless figure of merit ZT for y = 0.07 and 0.42 remains at rather high values compared with the nondoped sample.

Enhancement of thermoelectric efficiency in type-VIII clathrate Ba8Ga16Sn30 by Al substitution for Ga

Single-crystalline samples of type-VIII clathrate Ba8Ga16−xAlxSn30 (0≤x≤12) were grown from Sn flux to characterize the structural and thermoelectric properties from 300 to 600 K. The lattice parameter increases by 0.5% as x is increased to 10.5 whose value is the solubility limit of Al. The Seebeck coefficients of all samples are largely negative and the absolute values increase to approximately 300 μV/K on heating to 600 K. This large thermopower coexists with the metallic behavior in the electrical resistivity. The values of resistivity for 1≤x≤6 at 300 K are in the range 3.3–3.8 mΩ cm which is 70% of that for x=0. As a result, the power factor for x=4 and 6 has a rather large maximum of 1.83×10−3 W/m K2 at 480 K. The thermal conductivity stays at a low level of 0.72 W/mK up to 480 K, and the sample with x=6 reaches a ZT value of 1.2 at 500 K.

Structural, transport, and thermal properties of the single-crystalline type-VIII clathrateBa8Ga16Sn30

We report the electrical resistivity, Hall coefficient, thermoelectric power, specific heat, and thermal conductivity on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R_H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 uV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm^(-3). Hall mobility u_H shows a maximum of 62 cm^2/Vs around 70 K. The analysis of temperature dependence of u_H suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.