*3.3.5. Sn0.9Ag0.1Se*

The power factor reaches a maximum of 0.5 mW/mK<sup>2</sup>

*3.3.2. Sn0.97Cd0.03Se*

indicating a gap.

14 Bringing Thermoelectricity into Reality

and have published before [36–39], the thermal conductivity for SnSe alloys produced by arc-melting is less than 0.5 W/mK, indicating a possible figure of merit zT > 0.8 for Sn0.9Y0.1Se.

**Figure 8.** (a) Seebeck coefficient, (c) electrical conductivity, (e) power factor of Sn0.9Y0.1Se, and (b) Seebeck coefficient, (d) electrical conductivity, (f) power factor of Sn0.97Cd0.03Se and insert in (d) inverse semilog-plot of electrical conductivity

As an alloy with contrasting behavior, we show the transport properties of Sn0.97Cd0.03Se (**Figure 8b**, **d**, **f**). The cadmium-doped SnSe is the system with the highest Seebeck coefficients in this study (**Table 1**). Conversely, the electrical conductivity is very poor at RT, but there is an extraordinary increase with temperature, as illustrated in **Figure 8** on the right (middle panel). Both the large Seebeck coefficient and the low electrical conductivity indicate very low charge concentration. The steady decrease of the Seebeck coefficient (highest at 500 K with 570 μV/K) accompanied by the rapid increase of the electrical conductivity is another manifestation of the Pisarenko relationship [6]. The exponential temperature dependence of σ indicates a gap of E<sup>g</sup> = 0.6 eV. This is very similar to the intrinsic gap of SnSe. We can conclude from this study that Cd is unlikely to be a useful dopant for SnSe

at around 800 K. As we will show below

The Ag-doped SnSe system is an example where the interplay of the Seebeck coefficient and electrical conductivity follows the Pisarenko relation, in **Table 1**. For low Ag-doping the Seebeck coefficient is around 200 μV/K at room temperature with an electrical conductivity

**Figure 9.** Seebeck coefficient of (a) Sn0.95Mo0.05Se, (b) Sn0.99Mn0.01Se, and (c) Sn0.9Ag0.1Se.

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

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

mum 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.

**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

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.

, thus implying a maxi-

17

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

950 K, with a maximum power factor around 800 K above 1 mW/mK<sup>2</sup>

these values are representative for other alloys and different compositions.

**3.4. Thermal conductivity**
