**2.1. Preparation by arc-melting**

Tin selenide alloys with various dopings were prepared in an Edmund Buhler mini-arc MAM-1 furnace (**Figure 1a**). Stoichiometric amounts of ground mixture of reacting elements were pelletized in a glove box. Pellets were molten under Ar atmosphere in a watercooled Cu crucible (**Figure 1b**), leading to ingots of intermetallic characteristics (**Figure 1c**), which were partially ground to powder for structural characterization. The Seebeck coefficient and electrical and thermal conductivities were measured in disk-shaped specimens obtained by pressing the arc-molten ingots under 200 MPa at ambient temperature in a cylindrical die.

#### **2.2. Structural characterization**

Laboratory x-ray diffraction (XRD) was used to characterize the as grown samples (in powder form), for phase identification and for assessment of the phase purity, using a Brucker D8 diffractometer (40 kV, 30 mA), controlled by a DIFFRACTPLUS software, in Bragg-Brentano reflection geometry with CuKα radiation (λ = 1.5418 Å), between 10 and 64° in 2θ°. The resulting diffractograms were evaluated by Rietveld method with the FULLPROF program [41]. A pseudo-Voigt profile function was used for the line shape of the diffraction peaks. The following parameters were refined: half-width, pseudo-Voigt, zero shift, background points, and scale factor. Crucially, unit-cell parameters and positional and overall thermal displacement parameters were also refined for XRD data. A preferred orientation correction was applied, considering platelets perpendicular to [100] direction for SnSe-related alloys.

**Figure 1.** (a) Images of the mini arc furnace and (b) its water-cooled copper crucible, for synthesis of SnSe alloys materials. Pellets of powders of starting material are molten and then quenched in this crucible. (c) Typical disks of as-grown and then pressed intermetallic alloys.

#### **2.3. Microstructural characterization**

electrical conductivity at high temperatures (a much more laborious task), where SnSe would perform as a thermoelectric. In particular, we looked for n-type alloys with negative Seebeck

Tin selenide alloys with various dopings were prepared in an Edmund Buhler mini-arc MAM-1 furnace (**Figure 1a**). Stoichiometric amounts of ground mixture of reacting elements were pelletized in a glove box. Pellets were molten under Ar atmosphere in a watercooled Cu crucible (**Figure 1b**), leading to ingots of intermetallic characteristics (**Figure 1c**), which were partially ground to powder for structural characterization. The Seebeck coefficient and electrical and thermal conductivities were measured in disk-shaped specimens obtained by pressing the arc-molten ingots under 200 MPa at ambient temperature

Laboratory x-ray diffraction (XRD) was used to characterize the as grown samples (in powder form), for phase identification and for assessment of the phase purity, using a Brucker D8 diffractometer (40 kV, 30 mA), controlled by a DIFFRACTPLUS software, in Bragg-Brentano reflection geometry with CuKα radiation (λ = 1.5418 Å), between 10 and 64° in 2θ°. The resulting diffractograms were evaluated by Rietveld method with the FULLPROF program [41]. A pseudo-Voigt profile function was used for the line shape of the diffraction peaks. The following parameters were refined: half-width, pseudo-Voigt, zero shift, background points, and scale factor. Crucially, unit-cell parameters and positional and overall thermal displacement parameters were also refined for XRD data. A preferred orientation correction was applied, considering platelets perpendicular to [100] direction for

**Figure 1.** (a) Images of the mini arc furnace and (b) its water-cooled copper crucible, for synthesis of SnSe alloys materials. Pellets of powders of starting material are molten and then quenched in this crucible. (c) Typical disks of as-grown and

coefficient. We also characterized the thermal conductivity of a few selected samples.

**2. Experimental section**

6 Bringing Thermoelectricity into Reality

**2.1. Preparation by arc-melting**

**2.2. Structural characterization**

in a cylindrical die.

SnSe-related alloys.

then pressed intermetallic alloys.

Surface texture of as-grown pellets was studied by scanning electron microscopy (SEM) in a table-top Hitachi TM-1000 microscope. This microscope, best used for middle and low resolutions with high acceleration voltage, is chiefly used to scan with low magnification to select interesting zones and to study obtain topographical information, with large depth of field, thanks to its sensitivity to surface characteristics.

#### **2.4. Seebeck measurements in home-made apparatus**

Measurement of the Seebeck coefficient seems simple: create a temperature gradient and measure the induced voltage. Yet, at elevated temperatures, this poses a challenge, mainly due to large and hard to control temperature gradients.

In many of the instruments used for Seebeck measurement, the most important systematic errors originate in the great difficulty to detect temperatures at exactly the same spot where the voltage difference is observed. Furthermore, the strong chemical and metallurgic reactivity of typical thermoelectric materials at elevated temperatures limits the choice of materials for constructing the instrument. For example, Pt is typically used at high temperatures as an inert and useful material without second thought, but it is out of the question for many thermoelectric alloys of heavy p-block elements. Instead, niobium or tungsten is recommended [42].

**Figure 2.** Scheme of the Seebeck measurement apparatus: Two blocks of niobium (Nb) with inner cartridge heaters (with built-in thermocouples), and home-made thermocouples (in contact with the sample).

We try to minimize all the possible errors that could arise during the measurement; for that, we designed a slightly modified device recently published [42]. Using this apparatus, we can measure the Seebeck coefficient from room temperature up to around 950 K. **Figures 2** and **3** show the design scheme of the measurement device. This instrument is composed of two home-made thermocouples, pressed into contact with the sample with springs outside the furnace, two blocks of niobium (Nb), also pressed to the sample by springs, with inner cartridge heaters (with built-in thermocouples), and a tubular furnace with yet another thermocouple.

useful to gauge the behavior of the conductivity at higher temperatures. For more reliable absolute conductivity values, we estimated the geometric factor of this setup using a pellet of typical dimensions of our samples (10 mm diameter and 2 mm height) of a commercial Bi<sup>2</sup>

ingot manufactured by TECTEG MFR, by comparing to a conventional in-line 4-point mea-

We use a Keithley-2700 scanning multimeter to measure the Seebeck voltage, the thermocouples monitoring the sample temperature, and a Pt sensor used for a numerical "cold" junction compensation. Three independent West-P6100 PID controllers are used to measure and control the cartridge heaters and the furnace. They provide an analog DC programming output of 0–5 V amplified by DeltaES150 DC power supplies. The advantage of this somewhat costlier power control is that the Seebeck voltages and resistivity are less affected by the electronic noise from the heaters. Measurement errors associated with the Seebeck coefficients are close to ~5%, while resistivity error is strongly dependent on the magnitude of the resistance, varying from 20% for low resistances in the mΩ range to negligible errors for values around 1 Ω. Since typical thermoelectric pellets have resistances in the mΩ range, they are measured using a combination of Keithley Current Source 6221A and a KeithleyNanovoltmeter 2182A. Data acquisition and temperature control are handled by LabVIEW program, which communicates via GPIB with multimeter/nanovoltmeter and via RS485 serial protocol with PID controllers.

**Figure 4.** High temperature Seebeck measurement system: The electronics in the top shelf, the vacuum chamber (with

the sample inside) in the back, and the computer with the control-software.

bar cut from the same pellet.

surement of a 10 × 2 × 2 mm3

The whole setup is shown in **Figure 4**.

Te<sup>3</sup>

9

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

When high temperature measurements are planned, the system is placed in a vacuum chamber equipped with a turbo-pump, which can reach a vacuum of around 1 × 10−6 mbar. The baseplate is water-cooled in order to provide better control at high temperatures.

Home-made thermocouples are used to measure the temperature of the sample at either side and also the Seebeck voltage. Their election is guided by the need to use inert chemical materials at high temperature against typical compounds in many thermoelectric materials, such as Sn, Pb, Te, and Se. This is the reason we chose thermocouples made of a niobium and chromel combination, ensuring that the Nb wires are in contact with the sample [27, 33], which allows us to reach temperatures near up to 950 K without reactions between samples and thermocouples.

The blocks are built of niobium because i) of its chemical inertness to chalcogenides and pnictides and ii) this metal has a good thermal conductivity, which is important since the heaters are inside them. As an added advantage, the Nb blocks can act as current injection contacts in a 4-point measurement of sample resistance, with the Nb wires of the thermocouples acting as voltage leads, providing an estimate of the electrical conductivity of the material. Although this is not the best geometry to ensure reliable absolute values (the samples have large cross section and are short, just the opposite of what one would desire for 4-point resistivity), it is

**Figure 3.** View of the measurement setup, showing the upper and lower Nb pistons and the radiation furnace, which would be shifted upwards during the measurements.

useful to gauge the behavior of the conductivity at higher temperatures. For more reliable absolute conductivity values, we estimated the geometric factor of this setup using a pellet of typical dimensions of our samples (10 mm diameter and 2 mm height) of a commercial Bi<sup>2</sup> Te<sup>3</sup> ingot manufactured by TECTEG MFR, by comparing to a conventional in-line 4-point measurement of a 10 × 2 × 2 mm3 bar cut from the same pellet.

We try to minimize all the possible errors that could arise during the measurement; for that, we designed a slightly modified device recently published [42]. Using this apparatus, we can measure the Seebeck coefficient from room temperature up to around 950 K. **Figures 2** and **3** show the design scheme of the measurement device. This instrument is composed of two home-made thermocouples, pressed into contact with the sample with springs outside the furnace, two blocks of niobium (Nb), also pressed to the sample by springs, with inner cartridge heaters (with built-in thermocouples), and a tubular furnace with yet another thermocouple. When high temperature measurements are planned, the system is placed in a vacuum chamber equipped with a turbo-pump, which can reach a vacuum of around 1 × 10−6 mbar. The

Home-made thermocouples are used to measure the temperature of the sample at either side and also the Seebeck voltage. Their election is guided by the need to use inert chemical materials at high temperature against typical compounds in many thermoelectric materials, such as Sn, Pb, Te, and Se. This is the reason we chose thermocouples made of a niobium and chromel combination, ensuring that the Nb wires are in contact with the sample [27, 33], which allows us to reach temperatures near up to 950 K without reactions between samples and thermocouples. The blocks are built of niobium because i) of its chemical inertness to chalcogenides and pnictides and ii) this metal has a good thermal conductivity, which is important since the heaters are inside them. As an added advantage, the Nb blocks can act as current injection contacts in a 4-point measurement of sample resistance, with the Nb wires of the thermocouples acting as voltage leads, providing an estimate of the electrical conductivity of the material. Although this is not the best geometry to ensure reliable absolute values (the samples have large cross section and are short, just the opposite of what one would desire for 4-point resistivity), it is

**Figure 3.** View of the measurement setup, showing the upper and lower Nb pistons and the radiation furnace, which

would be shifted upwards during the measurements.

baseplate is water-cooled in order to provide better control at high temperatures.

8 Bringing Thermoelectricity into Reality

We use a Keithley-2700 scanning multimeter to measure the Seebeck voltage, the thermocouples monitoring the sample temperature, and a Pt sensor used for a numerical "cold" junction compensation. Three independent West-P6100 PID controllers are used to measure and control the cartridge heaters and the furnace. They provide an analog DC programming output of 0–5 V amplified by DeltaES150 DC power supplies. The advantage of this somewhat costlier power control is that the Seebeck voltages and resistivity are less affected by the electronic noise from the heaters. Measurement errors associated with the Seebeck coefficients are close to ~5%, while resistivity error is strongly dependent on the magnitude of the resistance, varying from 20% for low resistances in the mΩ range to negligible errors for values around 1 Ω.

Since typical thermoelectric pellets have resistances in the mΩ range, they are measured using a combination of Keithley Current Source 6221A and a KeithleyNanovoltmeter 2182A. Data acquisition and temperature control are handled by LabVIEW program, which communicates via GPIB with multimeter/nanovoltmeter and via RS485 serial protocol with PID controllers. The whole setup is shown in **Figure 4**.

**Figure 4.** High temperature Seebeck measurement system: The electronics in the top shelf, the vacuum chamber (with the sample inside) in the back, and the computer with the control-software.
