*3.3.6. Sn0.99Au0.01Se*

Gold doping is a yet-unexplored alternative to silver doping. The transport results corresponding to SnSe doped with 1% Au are shown in **Figure 10**. The Seebeck coefficient increases

**Figure 10.** Sn0.99Au0.01Se: (top) Seebeck coefficient, (middle) electrical conductivity, (bottom) power factor, and (insert) inverse semilog plot of electrical conductivity indicating a gap.

up to a maximum at 500 K reaching 500 μV/K. The ambient electrical conductivity is noticeably lower than for Ag, yet it steadily increases with temperature reaching 40,000 S/m at 950 K, with a maximum power factor around 800 K above 1 mW/mK<sup>2</sup> , thus implying a maximum zT up to 1.5 (this surprising value requires more careful verification of the resistivity measurements). The exponential increase of sigma indicates a gap of Eg = 0.25 eV, similar to the one seen in the Y-doped sample in **Figure 8**. Such a small gap, together with the relatively constant Seebeck coefficient, indicates that it is not the charge concentration increase, but thermally activated intergrain hopping limits the electrical conductivity in arc-molten SnSe.

#### **3.4. Thermal conductivity**

around 100 S/m, but for 10% Ag (itself difficult to stabilize by other methods [21]), while the Seebeck coefficient halves, σ shoots up to above 3000 S/m. That the Ag indeed enters the structure at such high doping levels is shown up by the continuous decrease of the unit-cell volume in **Figure 6a**. A σ increase of several orders of magnitude with respect to the RT value was observed above 800 K. The Seebeck coefficient remained stable between 150 and 200 μV/K (within the error bars), indicating that the conductivity is limited by intergrain boundaries. This effect is seen also in the small gap of a gold-doped alloy (**Figure 10**), as well

Gold doping is a yet-unexplored alternative to silver doping. The transport results corresponding to SnSe doped with 1% Au are shown in **Figure 10**. The Seebeck coefficient increases

**Figure 10.** Sn0.99Au0.01Se: (top) Seebeck coefficient, (middle) electrical conductivity, (bottom) power factor, and (insert)

inverse semilog plot of electrical conductivity indicating a gap.

as in the Y-doped sample in **Figure 8c**.

*3.3.6. Sn0.99Au0.01Se*

16 Bringing Thermoelectricity into Reality

**Figure 11** illustrates the total thermal conductivity (κ) obtained by laser-flash diffusivity method for two selected thermoelectric compounds: Sn0.99Mn0.01Se and Sn0.95Ti0.05Se; in our experience, SnSe alloys prepared by arc-melting have similar thermal conductivities, and these values are representative for other alloys and different compositions.

Both SnSe alloys display remarkably low thermal conductivities at high-temperature region. At room temperature the thermal conductivities are 0.95 and 0.88 W/mK for the Mn and Ti alloys, respectively. These decrease further with increasing temperature, reaching 0.4 W/mK at 675 K. Both electron and phonon transport contribute to the total thermal conductivity, denoted as lattice thermal conductivity (*κlat*) due to phonon transport, and charge thermal conductivity (*κch*) due to thermal transport of charges (electrons and/or holes). With the use

**Figure 11.** Thermal conductivity of Sn0.99Mn0.01Se and Sn0.95Ti0.05Se obtained by laser flash diffusivity. The black-dashed line indicates the classical behavior, κ ~ T−1, due to the phonon-phonon interaction.

of the Wiedemann-Franz law [7], we can estimate charge contribution to the thermal conductivity as a function of temperature, *κch = L0 Tσ*, with *L0* the Lorentz number and σ the electrical conductivity. For the highly resistive SnSe and its alloys, total thermal conductivity is dominated by the lattice thermal conductivity by a factor of 10,000:1. The temperature dependence of the lattice thermal conductivity follows the T−1 trend (black-dashed line in **Figure 11**), showing that the phonon-phonon interaction dominates in this temperature range.

**Acknowledgements**

ect MAT2017–84496-R.

, Federico Serrano-Sánchez<sup>1</sup>

, Norbert M. Nemes3

\*Address all correspondence to: ja.alonso@icmm.csic.es

2 National School of Engineers, Sfax University, Tunisia

Developments. New York: Springer; 2001

Macro to Nano. Boca Ratonp: CRC press; 2006

1 Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Madrid, Spain

**Author details**

Javier Gainza<sup>1</sup>

Spain

Spain

**References**

0182

3942H

049718.ch3

Félix Carrascoso<sup>1</sup>

We thank the financial support of the Spanish Ministry of Science and Innovation to the proj-

, Oscar J. Dura4

3 Departamento de Física de Materiales, Universidad Complutense de Madrid, Madrid,

4 Departamento Física Aplicada and INEI, Universidad de Castilla La Mancha, Ciudad Real,

[1] Rowe DM. CRC Handbook of Thermoelectrics. CRD Press; 1995. DOI: 10.1201/9781420

[2] Goldsmid HJ. Introduction to thermoelectricity. SSMATERIALS, volume 121. Springer-

[3] Chen G, Dresselhaus MS, Dresselhaus G, et al. Recent developments in thermoelectric materials. International Materials Review. 2003;**48**:45-66. DOI: 10.1179/09506600322501

[4] Nolas GS, Sharp J, Goldsmid HJ. Thermoelectrics: Basic Principles and New Materials

[5] Rull-Bravo M, Moure A, Fernández JF, Martín-González M. Skutterudites as thermoelectric materials: Revisited. RSC Advances. 2015;**5**:41653-41667. DOI: 10.1039/C5RA0

[7] Zaitsev VK, Fedorov MI, Eremin IS, Gurieva EA, Rowe DW. Thermoelectrics Handbook:

Verlag Berlin Heidelberg; 2016. DOI: 10.1007/978-3-662-49256-7

[6] Ioffe AF. Physics of Semiconductors. New York: Academic Press; 1960

, Mouna Gharsallah1,2, Manuel Funes<sup>1</sup>

, José L. Martínez<sup>1</sup>

,

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442

and José A. Alonso<sup>1</sup>

\*

19

The extremely low thermal conductivities observed in all the materials of the different doped series reported here afford considerably high figures of merit, zT.
