Preface

The disproportionate use of fossil fuels has turned into a serious environmental issue. Con‐ sequently, global warming, greenhouse gas emissions, climate change, ozone layer deple‐ tion, and acid rain are frequently heard terms from the media. Also, fossil fuel reserves are limited, and strict environmental regulations are appearing. Thus, we are encountering one of the biggest challenges of the twenty-first century, satisfying the energy demand with re‐ spect to the environment. This fact brings an intense activity directed to obtain a rational use of traditional fuels, reduce the greenhouse gas emissions, and stimulate the research of alter‐ native energy sources.

Despite the complexity of the actual energy problem, as scientific, technological, economic, environmental, sociological, and political aspects are involved, new intensive activities fo‐ cused on two directives have been encouraged. The intelligent use of energy, boosting sav‐ ings, avoiding overspending, and developing more efficient equipment; and the development of renewable energy production, an intense activity which has increased a 220% of the global renewable power capacity in the last 10 years.

Thermoelectricity is one of the technologies, which contributes to the reduction in the im‐ pact of the use of fossil fuels, as it contributes to the better use of traditional fuels, improving the efficiency of the processes. Thermoelectricity is an emerging technology, due to its ca‐ pacity to convert heat into electricity or produce cooling or heating effects out of electricity without the necessity of refrigerants. The solid state of thermoelectric devices eliminates the moving parts, chemical reactions, and the presence of refrigerants. Hence, maintenance as well as the harmful emissions to the environment is cancelled. Longer lives are achieved due to the safe operation.

Thermoelectric generators harvest heat, especially waste heat, to produce electricity. Waste heat is defined as the by-product heat, which is produced by a process that is not used by but is emitted to the ambient. Today, the amount of waste heat produced is disproportion‐ ate; 40% of the primary energy used in the industrialized countries is emitted to the ambient as waste heat. Most of this heat corresponds to low-grade heat, complicating its reuse. How‐ ever, thermoelectricity is a very promising technology to recover this kind of heat. Likewise, thermoelectric coolers and heaters do not need refrigerants and therefore eliminate green‐ house gas emissions and hence do not contribute to the global warming. Accordingly, the electrical energy generation from waste heat and the cooling or heating without the necessi‐ ty of refrigerants contribute to a more sustainable world.

Unfortunately, the efficiency of thermoelectric generators, coolers, and heaters is still very low, and therefore, great efforts are being made to improve their efficiency. The study of novel thermoelectric materials, the development of computational models, the design of

proper assemblies, and the optimization of thermal designs, among others, are currently be‐ ing studied by the scientists. This book includes the previously mentioned aspects catego‐ rized into four sections:


I believe that the information contained in this book will help researchers and scientists to develop the definitive thermoelectric application applied to everyday life, boosting thermo‐ electricity to the visible plane.

I am very thankful to all the authors who have contributed to this book, and without whom it would not have been possible to gather all the wisdom. I hope that the coordinating ef‐ forts would serve to bring thermoelectricity into reality.

> **Dr. Patricia Aranguren** Public University of Navarre Pamplona, Spain

**Section 1**

**Advanced Thermoelectric Materials**

**Advanced Thermoelectric Materials**

proper assemblies, and the optimization of thermal designs, among others, are currently be‐ ing studied by the scientists. This book includes the previously mentioned aspects catego‐

• Thermoelectric Cooling: Principles, Effects, Optimization and Applications

I believe that the information contained in this book will help researchers and scientists to develop the definitive thermoelectric application applied to everyday life, boosting thermo‐

I am very thankful to all the authors who have contributed to this book, and without whom it would not have been possible to gather all the wisdom. I hope that the coordinating ef‐

> **Dr. Patricia Aranguren** Public University of Navarre

> > Pamplona, Spain

• Thermoelectric Generation: Automotive Waste Heat Recovery

• Building Up the Devices: Material Properties, Modeling, Geometry, and Assembly

rized into four sections:

X Preface

Optimization

electricity to the visible plane.

• Advanced Thermoelectric Materials

forts would serve to bring thermoelectricity into reality.

**Chapter 1**

**Provisional chapter**

Se, by alloying with

**Nanostructured Thermoelectric Chalcogenides**

**Nanostructured Thermoelectric Chalcogenides**

DOI: 10.5772/intechopen.75442

Thermoelectric materials are outstanding to transform temperature differences directly and reversibly into electrical voltage. Exploiting waste heat recovery as a source of power generation could help towards energy sustainability. Recently, the SnSe semiconductor was identified, in single-crystal form, as a mid-temperature thermoelectric material with record high figure of merit, high power factor and surprisingly low thermal conductivity. We describe the preparation of polycrystals of alloys of SnSe obtained by arc-melting; a rapid synthesis that results in strongly nanostructured samples with low thermal conductivity, advantageous for thermoelectricity, approaching the amorphous limit,

3d and 4d transition metals such as M = Mn, Y, Ag, Mo, Cd or Au, provides for a means to optimize the power factor. M=Mo, Ag, with excellent values, are described in detail with characterization by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electronic and thermal transport measurements. Rietveld analysis of XRD data demonstrates near-perfect stoichiometries of the above-mentioned alloys. SEM analysis shows stacking of nanosized sheets, with large surfaces parallel to layered slabs. An apparatus was developed for the simultaneous measurement of the Seebeck coefficient

around 0.3–0.5 W/mK. An initial screening of novel samples Sn1−xM<sup>x</sup>

**Keywords:** thermoelectrics, nanostructuration, lattice thermal conductivity,

and electric conductivity at elevated temperatures.

thermopower, SnSe alloying

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Javier Gainza, Federico Serrano-Sánchez,

Javier Gainza, Federico Serrano-Sánchez,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Norbert M. Nemes, Oscar J. Dura, José L. Martínez and José A. Alonso

and José A. Alonso

**Abstract**

http://dx.doi.org/10.5772/intechopen.75442

Mouna Gharsallah, Manuel Funes, Félix Carrascoso,

Mouna Gharsallah, Manuel Funes, Félix Carrascoso, Norbert M. Nemes, Oscar J. Dura, José L. Martínez

#### **Chapter 1 Provisional chapter**

#### **Nanostructured Thermoelectric Chalcogenides Nanostructured Thermoelectric Chalcogenides**

DOI: 10.5772/intechopen.75442

Javier Gainza, Federico Serrano-Sánchez, Mouna Gharsallah, Manuel Funes, Félix Carrascoso, Norbert M. Nemes, Oscar J. Dura, José L. Martínez and José A. Alonso Javier Gainza, Federico Serrano-Sánchez, Mouna Gharsallah, Manuel Funes, Félix Carrascoso, Norbert M. Nemes, Oscar J. Dura, José L. Martínez and José A. Alonso

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75442

#### **Abstract**

Thermoelectric materials are outstanding to transform temperature differences directly and reversibly into electrical voltage. Exploiting waste heat recovery as a source of power generation could help towards energy sustainability. Recently, the SnSe semiconductor was identified, in single-crystal form, as a mid-temperature thermoelectric material with record high figure of merit, high power factor and surprisingly low thermal conductivity. We describe the preparation of polycrystals of alloys of SnSe obtained by arc-melting; a rapid synthesis that results in strongly nanostructured samples with low thermal conductivity, advantageous for thermoelectricity, approaching the amorphous limit, around 0.3–0.5 W/mK. An initial screening of novel samples Sn1−xM<sup>x</sup> Se, by alloying with 3d and 4d transition metals such as M = Mn, Y, Ag, Mo, Cd or Au, provides for a means to optimize the power factor. M=Mo, Ag, with excellent values, are described in detail with characterization by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electronic and thermal transport measurements. Rietveld analysis of XRD data demonstrates near-perfect stoichiometries of the above-mentioned alloys. SEM analysis shows stacking of nanosized sheets, with large surfaces parallel to layered slabs. An apparatus was developed for the simultaneous measurement of the Seebeck coefficient and electric conductivity at elevated temperatures.

**Keywords:** thermoelectrics, nanostructuration, lattice thermal conductivity, thermopower, SnSe alloying

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

Thermoelectric materials hold a tantalizing promise of greater energy efficiency by providing a robust and clean option for waste heat recovery and conversion to useful electrical energy through the Seebeck effect [1–5]. However, existing materials have poor thermoelectric efficiency. This is related to the inherent difficulty to obtain, at the same time, a high electrical conductivity, a low thermal one, and a very high Seebeck voltage. Thermoelectric materials are characterized, for research purposes, by the thermoelectric figure of merit, *zT* <sup>=</sup> *<sup>σ</sup> <sup>S</sup>*<sup>2</sup> \_\_\_ <sup>κ</sup> *<sup>T</sup>*, combining into a dimensionless number the Seebeck coefficient, S (the larger, the better), the electrical conductivity, *σ* (the larger, the better, to minimize waste through Joule-heating), the thermal conductivity, κ (the smaller, the better, to minimize thermally shorting the temperature gradient giving the Seebeck voltage), and the absolute temperature, T. Unfortunately, the character of these properties pulls against the optimization of the figure of merit. While σ is directly proportional to the carrier concentration *n,* the Seebeck coefficient is inversely proportional to it (*n*−2/3), a common rule for doped semiconductors known since the 1950s as the Pisarenko relation, after Mr. N. L. Pisarenko [6]. Moreover, the Wiedemann-Franz law establishes a direct relationship between σ and the electronic contribution to the thermal conductivity (*κele*) [7, 8]. Usually, the thermoelectric properties are described as:

$$S = \frac{8\pi^2 k\_y^2 m^\* T}{3e^2 h^2} \left(\frac{\pi}{3n}\right)^{\frac{2}{3}}\tag{1}$$

$$
\sigma = \mu \epsilon n \tag{2}
$$

large as 2.6 was reported above 900 K in an overlooked semiconductor, SnSe [18]. This value was found in a single crystal of SnSe along one crystallographic direction, above a structural phase transition, just below the melting point. Later reports in single crystals found smaller values, and in polycrystalline samples only zT = 1.0 [19–21]. Nevertheless, SnSe is a very promising thermoelectric. As a semiconductor, SnSe was disregarded due to its complicated orthorhombic structure. It consists of puckered layers, quite analogous to black phosphorus, and it has a small bandgap of 0.61 eV, quite sought after among optoelectronic 2D materials [22]. One reason for the high zT of SnSe has to do with its low thermal conductivity. Nevertheless, a further reduction by nanostructuring is highly desirable. According to the Wiedemann-Franz law [7], the thermal conductivity of SnSe is overwhelmingly determined by the lattice thermal conductivity because of its inherently low electrical conductivity. Thus, doping can be used to increase σ through the charge density, and in turn zT. However, following the Pisarenko relation [23], the Seebeck coefficient will decrease with increasing charge concen-

controversy arose about the actual thermal conductivity of the material, with values reported anywhere between 0.2 and 1.0 W m−1 K−1 [18, 19, 21, 24–31]. Numerous different explanations have been proposed for such values, as surface oxidation, exact stoichiometry, porosity, morphology, and crystal defects of the samples. Thermoelectric devices made of this material have not been described yet despite the fact that SnSe presents one of the highest figures of merit, and only a few attempts proving the interface bonding properties for high-temperature

There have been several reports on doped SnSe, with silver, iodine, bismuth, or sodium [19– 21, 24, 34, 35]. These can be used to select the type of carriers, since a useful thermoelectric device must contain both p- and n-type thermo-elements. What is common in these studies is that the dopant concentration reaches no more than a few percent. Our group has reported on alloying SnSe with p-block elements, such as Ge, In, and Pb, using arc-melting [36–39]. The arc-melting technique has several advantages for SnSe, and some drawbacks. It is a very fast, one-step synthesis method that avoids the costly and time-consuming steps of spark plasma sintering (SPS), yet produces dense pellets [40]. Crucially, it yields highly nanostructured materials because of the rapid melting and quenching. It also allows for alloying at much higher concentrations. As a drawback, we find our polycrystalline samples to have notoriously low electrical conductivity at room temperature, and also much lower charge carrier density than would be expected from the huge amount of dopants. These are likely related to

Here we report on a general survey of alloying SnSe with *d*-block elements. As a quick screening tool, we employed the room temperature Seebeck coefficient and electrical conductivity. As a rule of thumb, we looked for indications of the Pisarenko relation at work: dopants that, at some concentration, can yield highly conducting samples, usually with almost zero Seebeck coefficient, whereas the pure or lightly doped SnSe is a bad conductor with large Seebeck effect. We characterized every composition by laboratory x-ray diffraction to check for phase purity and changes in the lattice constant, a good indication that some of the dopants indeed entered the crystal structure. For some samples, we also measured the Seebeck coefficient and

surface oxidation of the grain boundaries and to charge-trapping at defects.

. Furthermore, an intense

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442 5

tration. Thus, an optimum must be found of the power-factor, *σ S*<sup>2</sup>

modules have been reported [32, 33].

$$
\kappa\_{\rm tot} = \kappa\_{\rm lat} + \kappa\_{\rm de} = L\sigma T = L\mu \epsilon m \, T \tag{3}
$$

A zT above 4 would be desirable for economically viable thermoelectric modules for waste heat recovery, for example, in automotive applications. Yet, current state-of-the-art materials have zT limited to around 2, with commercial materials limited to 1. Furthermore, pand n-type thermoelectric materials with similar figure of merit, and electrical, thermal, and mechanical characteristics, in the same temperature range are necessary for their implementation. Nonetheless, much effort has gone into developing cost-effective devices for different applications [9–12]. For instance, as potential candidates for gas heat recovery, automobile exhaust thermoelectric generators (AETEG) were first studied in 1963, and they have been thoroughly investigated since then reaching 400 W and 5% efficiency in current Bi<sup>2</sup> Te<sup>3</sup> -based modules [12–14]. A recent report on AETEG for military SUV applications shows an output power up to 646 W and 1.03% efficiency, which meets the electrical requirements for automotive applications [13]. Another applicability of thermoelectric modules is found as Solar-Heat-Pipe-Thermoelectric-Generator hybrid systems for combined power generation and hot water production in the high-temperature range [11].

Thermoelectric materials are typically degenerately doped semiconductors, often of heavy p-block elements, such as SiGe, Bi<sup>2</sup> Te<sup>3</sup> , PbTe, and CoSb3 [15–17]. However, recently, a zT as large as 2.6 was reported above 900 K in an overlooked semiconductor, SnSe [18]. This value was found in a single crystal of SnSe along one crystallographic direction, above a structural phase transition, just below the melting point. Later reports in single crystals found smaller values, and in polycrystalline samples only zT = 1.0 [19–21]. Nevertheless, SnSe is a very promising thermoelectric. As a semiconductor, SnSe was disregarded due to its complicated orthorhombic structure. It consists of puckered layers, quite analogous to black phosphorus, and it has a small bandgap of 0.61 eV, quite sought after among optoelectronic 2D materials [22].

**1. Introduction**

4 Bringing Thermoelectricity into Reality

Thermoelectric materials hold a tantalizing promise of greater energy efficiency by providing a robust and clean option for waste heat recovery and conversion to useful electrical energy through the Seebeck effect [1–5]. However, existing materials have poor thermoelectric efficiency. This is related to the inherent difficulty to obtain, at the same time, a high electrical conductivity, a low thermal one, and a very high Seebeck voltage. Thermoelectric materials are characterized, for research purposes, by the thermoelectric figure of merit, *zT* <sup>=</sup> *<sup>σ</sup> <sup>S</sup>*<sup>2</sup> \_\_\_

combining into a dimensionless number the Seebeck coefficient, S (the larger, the better), the electrical conductivity, *σ* (the larger, the better, to minimize waste through Joule-heating), the thermal conductivity, κ (the smaller, the better, to minimize thermally shorting the temperature gradient giving the Seebeck voltage), and the absolute temperature, T. Unfortunately, the character of these properties pulls against the optimization of the figure of merit. While σ is directly proportional to the carrier concentration *n,* the Seebeck coefficient is inversely proportional to it (*n*−2/3), a common rule for doped semiconductors known since the 1950s as the Pisarenko relation, after Mr. N. L. Pisarenko [6]. Moreover, the Wiedemann-Franz law establishes a direct relationship between σ and the electronic contribution to the thermal con-

> <sup>2</sup> *<sup>m</sup>*<sup>∗</sup> *<sup>T</sup>* \_\_\_\_\_\_\_\_\_ 3*e h*<sup>2</sup> (

*σ* = *en* (2)

*κtot* = *κlatt* + *κele* = *LT* = *LenT* (3)

A zT above 4 would be desirable for economically viable thermoelectric modules for waste heat recovery, for example, in automotive applications. Yet, current state-of-the-art materials have zT limited to around 2, with commercial materials limited to 1. Furthermore, pand n-type thermoelectric materials with similar figure of merit, and electrical, thermal, and mechanical characteristics, in the same temperature range are necessary for their implementation. Nonetheless, much effort has gone into developing cost-effective devices for different applications [9–12]. For instance, as potential candidates for gas heat recovery, automobile exhaust thermoelectric generators (AETEG) were first studied in 1963, and they have been

modules [12–14]. A recent report on AETEG for military SUV applications shows an output power up to 646 W and 1.03% efficiency, which meets the electrical requirements for automotive applications [13]. Another applicability of thermoelectric modules is found as Solar-Heat-Pipe-Thermoelectric-Generator hybrid systems for combined power generation and hot

Thermoelectric materials are typically degenerately doped semiconductors, often of heavy

, PbTe, and CoSb3

thoroughly investigated since then reaching 400 W and 5% efficiency in current Bi<sup>2</sup>

Te<sup>3</sup>

\_\_\_*π* 3*n*) \_\_2 3

ductivity (*κele*) [7, 8]. Usually, the thermoelectric properties are described as:

*<sup>S</sup>* <sup>=</sup> <sup>8</sup> *<sup>π</sup>*<sup>2</sup> *kB*

water production in the high-temperature range [11].

p-block elements, such as SiGe, Bi<sup>2</sup>

<sup>κ</sup> *<sup>T</sup>*,

(1)

Te<sup>3</sup>

[15–17]. However, recently, a zT as


One reason for the high zT of SnSe has to do with its low thermal conductivity. Nevertheless, a further reduction by nanostructuring is highly desirable. According to the Wiedemann-Franz law [7], the thermal conductivity of SnSe is overwhelmingly determined by the lattice thermal conductivity because of its inherently low electrical conductivity. Thus, doping can be used to increase σ through the charge density, and in turn zT. However, following the Pisarenko relation [23], the Seebeck coefficient will decrease with increasing charge concentration. Thus, an optimum must be found of the power-factor, *σ S*<sup>2</sup> . Furthermore, an intense controversy arose about the actual thermal conductivity of the material, with values reported anywhere between 0.2 and 1.0 W m−1 K−1 [18, 19, 21, 24–31]. Numerous different explanations have been proposed for such values, as surface oxidation, exact stoichiometry, porosity, morphology, and crystal defects of the samples. Thermoelectric devices made of this material have not been described yet despite the fact that SnSe presents one of the highest figures of merit, and only a few attempts proving the interface bonding properties for high-temperature modules have been reported [32, 33].

There have been several reports on doped SnSe, with silver, iodine, bismuth, or sodium [19– 21, 24, 34, 35]. These can be used to select the type of carriers, since a useful thermoelectric device must contain both p- and n-type thermo-elements. What is common in these studies is that the dopant concentration reaches no more than a few percent. Our group has reported on alloying SnSe with p-block elements, such as Ge, In, and Pb, using arc-melting [36–39]. The arc-melting technique has several advantages for SnSe, and some drawbacks. It is a very fast, one-step synthesis method that avoids the costly and time-consuming steps of spark plasma sintering (SPS), yet produces dense pellets [40]. Crucially, it yields highly nanostructured materials because of the rapid melting and quenching. It also allows for alloying at much higher concentrations. As a drawback, we find our polycrystalline samples to have notoriously low electrical conductivity at room temperature, and also much lower charge carrier density than would be expected from the huge amount of dopants. These are likely related to surface oxidation of the grain boundaries and to charge-trapping at defects.

Here we report on a general survey of alloying SnSe with *d*-block elements. As a quick screening tool, we employed the room temperature Seebeck coefficient and electrical conductivity. As a rule of thumb, we looked for indications of the Pisarenko relation at work: dopants that, at some concentration, can yield highly conducting samples, usually with almost zero Seebeck coefficient, whereas the pure or lightly doped SnSe is a bad conductor with large Seebeck effect. We characterized every composition by laboratory x-ray diffraction to check for phase purity and changes in the lattice constant, a good indication that some of the dopants indeed entered the crystal structure. For some samples, we also measured the Seebeck coefficient and 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 coefficient. We also characterized the thermal conductivity of a few selected samples.

**2.3. Microstructural characterization**

thanks to its sensitivity to surface characteristics.

large and hard to control temperature gradients.

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

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,

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442 7

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

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).
