Thermoelectric materials: Fundamental, applications and challenges

Vietnam Journal of Science and Technology 56 (1A) (2018) 1-13 THERMOELECTRIC MATERIALS: FUNDAMENTAL, APPLICATIONS AND CHALLENGES Bui Duc Long 1, * , Duong Ngoc Binh 1 , Le Minh Hai 1 , Le Hong Thang 2 , Tran Duc Huy 3 1 Department of Non-ferrous Metals and Composites, School of Materials Science and Engineering, HUST, 1 Dai Co Viet, Ha Noi 2 Central Laboratory of Metals Technology, School of Materials Science and Engineering, HUST, 1 Dai Co Viet, Ha Noi 3 Department of Materials and Foundry Technology, School of Materials Science and Engineering, HUST, 1 Dai Co Viet, Ha Noi * Email: long.buiduc@hust.edu.vn Received: 15 August 2017; Accepted for publication: 21 February 2018 ABSTRACT Energy and the environment are popular themes in the 21 st century because both are closely interlinked. The current technologies are focusing on finding new, clean, safe and renewable energy sources for a better environment. Thermoelectric (TE) materials are able to generate electricity when applied a temperature different at a junction of two dissimilar materials. This is a promising technology to directly convert waste heat into electricity without any gas emission, thus providing one of the most clean and safe energy. However, the applications of TE devices are still limited due to its low energy conversion efficiency and high material cost. As a result, researches in TE materials are mainly focusing on the improving of efficiency and developing cheap materials. In this paper, the fundamental, challenges and applications of thermoelectric materials were reviewed. In addition, currently research in thermoelectric materials and improving their efficiency will also be reviewed. Keywords: thermoelectric materials, energy conversion, energy materials, and clean energy. 1. INTRODUCTION The world projected population and rapid economic growth lead to dramatically increase the demand for energy in the next decades [1]. According to the U.S Energy Information Administration [2], total world energy consumption will be double rate from 2012 to 2040. It was also estimated that by the end of this century the world energy consumption would be triple increase compare to that of 2012 [3]. Currently, the world energy resources are mostly from fossil fuel [2, 4]. However, the use of fossil fuel has led to serious environmental problems such as global warming, greenhouse gas emission, and climate change [5, 6]. Besides, the safety of nuclear energy is recently raising question and many European countries have shut down their nuclear plants [7]. Therefore, broad societal needs focused attention on the discovery of new, clean and renewable energy sources, and improving the existing energy efficiency [1, 5-10]. It is Bui Duc Long, Duong Ngoc Binh, Le Minh Hai, Le Hong Thang, Tran Duc Huy 2 reported that the world’s hunger for coal to abate starting around 2026 as governments work to reduce emissions in step with promises under the Paris Agreement on climate change. By 2040 more than 60 % of electricity will come from solar, wind and other renewable resources, less than 40 % of the electricity will produce from the fossil fuel [11]. In Vietnam, the government has adopted various policies and measures to provide a reliable and clean energy supply including provisions for the promotion of renewable and sustainable energies from different sources such as wind energy, biomass and biogas, as well as solar and hydropower [12]. Thermoelectric (TE) devices or thermoelectric generators (TEGs), which are based on TE materials, are solid-state devices with no moving parts; they are silent, reliable, scalable and no gas emission, making them ideal for small power generation, as shown in Figure 1. TEGs have greatly attracted attention from scientists, academia and industrialists as having the potential to make important contributions to improving energy efficiency and providing cleaner forms of energy without gas emissions [9, 12, 13]. TEGs can be used to recover or directly convert waste heat from home heating, automotive exhaust, and industrial processes into electric power [7, 12, 14]. For instance, by developing an automotive TEG that can be used to convert waste heat into useable electricity, the engine will burn less fuel to power the vehicle's electrical components and resulting in releasing fewer emissions. Currently, leading car manufacturers are working on the development of this possibility [15]. Figure 1. Conversion heat energy from automobiles, industrial processes, home heating, sun and other waste heat resources into electricity. 2. WORKING PRINCIPLES Thermoelectric phenomenon or Seebeck effect was discovered by Thomas Johann Seebeck in 1821. The fundamental of thermoelectric is very simple, when the temperature difference is applied on a TE junction of two dissimilar metals, a voltage difference (∆V) will be produced proportionally to the temperature difference (∆T) [5, 13,16], as shown in Figure 2a. The proportional constant related to the intrinsic property of the material is known as the Seebeck coefficient (denoted as S or α) [5, 13], where α = ∆V/∆T. (1) Thermoelectric materials: fundamental, applications and challenges 3 Solid-state TE devices based on Seebeck effect can be used to generate electric power. TE devices or TEGs are solid-state devices which is composed both n-, p-types materials. For practical TEGs, a connection of large numbers of junctions in series is needed to increase operating voltage, as shown in Figure 3. Figure 2. A schematic of thermoelectric effect (a) Seebeck effect, ∆T = Thot –Tcold, (b) Peltier effect. Likewise, a current flowing across a TE junction, either cooling or heating can occur at the junction (Figure 2b), which depends on the direction of the current [5, 13]. This phenomenon is named as Peltier effect, which was discovered by Jeans Charles Athanase Peltier in 1843. The Peltier effect has been widely used in the applications such as thermal couple, air conditioning, and refrigeration. Figure 3. TE module composing of both n, p-types materials. This figure is reproduced from [9] with copyright permission. The performance of TE material is evaluated by a dimensionless figure of merit (ZT), Bui Duc Long, Duong Ngoc Binh, Le Minh Hai, Le Hong Thang, Tran Duc Huy 4 defined as [1, 13]: ZT = (S 2σ/k)T (2) where S, σ, k and T are the Seebeck coefficient (μVK-1), electrical conductivity (Ω cm-1), thermal conductivity (Wm -1 K -1 ) and absolute temperature (K). ZT is a function of temperature (T) and depending on the intrinsic material properties (S, σ, k). For commercialization of thermoelectric power generation (TEG), high-ZT value (≥3) materials are needed [1]. The efficiency (η) of a TE device for power generation is given by [1, 13]: η = (3) where Th, Tc are the temperatures of hot and cold sides of the TE material, respectively. ZTM is average figure-of-merit over the whole working temperature range. For practical application, a high ZTM value is required to have high efficiency TE devices or generators. 3. APPLICATIONS OF THERMOELECTRIC MATERIALS 3.1. Thermoelectric power for space applications Thermoelectric power generations have shown their extraordinary reliability and long-life for deep space science and exploration missions [17,18]. Radioisotope thermoelectric generators (RTGs) were developed in the United State during the late 1950s [19]. The first RTGs were lunched into space in 1961 and continuously operated for more than 30 years using high- temperature heat sources (up to 1000 o C). The RTGs used by the U.S. space program were made from alloys of lead telluride, TAGS, or SiGe [18]. 3.2. Potential application in automobiles Figure 4. Typical energy path for vehicles with gasoline-fueled internal combustion engines. This figure is reprinted from [22] with copyright permission. Extremely large amounts of waste heat energy are generated from transportation vehicles, typically, heat produced by automotive engine [20]. Yu and Chau [21] reported that as for internal combustion engine, only 25 % of the energy generated by fuel combustion was used to run vehicle, whilst 70 % of the energy was lost as the waste heat, and 5 % of energy was dissipated as friction, as shown in Figure 4. By recovering parts of the waste heat to produce Thermoelectric materials: fundamental, applications and challenges 5 electricity, which leads to improve fuel efficiency, reduce the fuel consumption and greenhouse gas emissions [17]. Many car manufacturers, for example Toyota, Honda, Nissan, BMW, Ford and GM, are interested in developing automotive TEGs with the target of improving 5 % of consumption efficiency [22-25]. For instance, in 2008, a German automotive engineering company developed a prototype TEG device for the Volkswagen Golf with an output power of 600 W [23]. It was reported that General Motors achieved TEGs with output of 350 W and 600 W in a Chevrolet Suburban under city and highway driving conditions, respectively [24]. Recently, BMW developed high temperature automotive TEGs which produces 750 W [25]. 3.3. Recovery waste heat from industrial sectors A huge amount of waste heat energy is generated from power plants and industries processes. It is reported that manufacturing industries overall reject about 33 % of their energy as waste heat directly to the atmosphere or to thermal management systems. In the U.S. manufacturing sector alone, more than 3,000 TWh of waste heat energy is lost each year, an amount equivalent to more than 1.72 billion barrels of oil [17]. TEGs can be used to recover waste heat from many manufacturing processes such as biomass boiler, furnaces, cement kiln, metal casting and steel manufacturing [1, 9, 17, 26, 27]. Recently, Alphabet energy developed E1-C TEG which is the world largest TEG for exhaust heat recovery generator as large as a container [28]. 3.4. Stove-powered thermoelectric generator Figure 5: A schematic of stove-power thermoelectric generation. It is reported that more than three billion people, especially in some parts of developing countries or rural area, are dependent on solid fuels, particularly biomass fuels, for their daily cooking, heating, and even lighting [29]. In addition, electric supply in these areas is also unreliable, and in some cases power can be failed due to natural disasters such as earthquake, snowstorm, etc. Small power generators would be very useful to convert housing waste heat into electricity, which is called the stove-powered thermoelectric generator. These stove-TEGs consist of three parts: the stove system, the TEG system and the load system, as shown in Figure 5. When the biomass is burned and apart of waste heat can transfer to the TEGs and is converted into electricity. This electric power can be stored or power the fan, light, radio or charge a Bui Duc Long, Duong Ngoc Binh, Le Minh Hai, Le Hong Thang, Tran Duc Huy 6 mobile phone. 3.5. New applications of thermoelectric generator Harvesting solar energy based - TEG is another important way to capture low temperature heat. The amount of energy emitted from sun is gigantic, around 3 × 1024J/year, which are a few hundred-fold times greater than what we use at the present. It is estimated that with a conversion of 0.1 % of solar energy into useful energy with 10 % efficiency is more than enough to meet our current energy needs [30]. Thus it has been proposed the concept of hybrid solar power systems, which combines solid-state photovoltaic (PV) and TEGs [17]. This concept consists of concentrating and splitting the solar energy spectrum into a low wavelength portion directed at PV cells and a high wavelength portion directed at TE modules. Based on this idea, PV will convert the ultraviolet and visible light and removing the infrared portion of the spectrum to maximize their conversion efficiency by maintaining low operating temperatures [17]. Recently, researchers at Massachusetts Institute of Technology (MIT) and their collaborators have developed a high-performance and possibly less expensive way to convert solar heat into electricity using flat-panel solar-thermoelectric power combined with hot water systems [30]. 4. RECENT RESEARCH ON THERMOELECTRIC MATERIALS Each TE material works with high performance at a certain temperature range. Thus in this paper we review different TE materials based on different temperature ranges. As mentioned previously, the available waste heat sources can be came from electronics, sun, geothermal energy, the exhausted waste heat of transportation vehicles, industrial heat-generating processes etc. [1, 34]. The waste heat sources can be classified at different temperature ranges, i.e. low temperature (< 250 o C), middle temperature (250-650 o C), and high temperature (> 650 o C) [1, 34]. Low temperature TE materials Bi2Te3 is among the most outstanding TE materials for application at the vicinity of the room temperature [35,36]. Besides, Bi2Te3-based alloys also can be good candidates for power generation applications from room temperature to 230 o C [37]. For a long time, ZT value of Bi2Te3 is around 1.0 [38]. Xie et al. applied melt spinning method combined with subsequent spark plasma sintering to fabricate nanostructured p-type (Bi,Sb)2Te3 that exhibits a maximum ZT of 1.5 at about 390 K [39]. However, Te is considered a toxic and expensive element. Thus, researchers are searching for other non-toxic TE such as binary (Bi2S3, CdS, TiS2, Ag2S, Mg3Sb2 etc.) [16, 40]. Recently, H. Zhao et al. reported the discovery of high performance room temperature TE material based on MgAgSb. As reported, the ZT values of this TE material are close to 1.0 at room temperature and reached a maximum of 1.4 at 475 K [41]. Middle temperature TE materials Cu - S based minerals appeared as potential candidates for TE materials due to their advantages: earth abundant, low cost, and less toxic constituent elements, and high TE performances at 400 o C. This trend in the research of TE materials was triggered by the pioneering works on kesterite Cu2ZnSnS4, digenite Cu1.8S, and chalcopyrite CuFeS2. These Thermoelectric materials: fundamental, applications and challenges 7 works were followed by several reports on tetrahedrites Cu12-xTrxSb4S13 (Tr: 3d transition metals and Zn) [42]. On other hand, lead telluride, clathrates, silicides, skutterudites have good performance at temperature range of ~500 - 600 °C [1, 34, 43, 44, 45]. Skutterudites systems have attracted great attention from TE community due to its high Seebeck coefficient, excellent electrical transport properties and special lattice structure. Skutterudites have the cubic structure, which has eight formula units per cubic cell. The two that are empty are called voids. Although it is still controversial hypothesis, it has been believed that thermal conductivity of skutterudites was reduced by filling impurity atoms in the voids. The “filler” atoms acted as the center scattering phonons, consequently thermal conductivity will be decrease while electrical conductivity can be increased due to the reduction of the band gap of the compound [1]. Recently, a remarkable ZT value of 1.7 was achieved for n-type filled skutterudites [46]. The following silicides have favorable properties as thermoelectric: CrSi2, MnSi1.75, β-FeSi2, Ru2Si3, ReSi1.75, and Mg2X (X = Si, Ge, and Sn) [42, 43]. These TE material have energy gaps and melting temperatures suitable for middle temperature range which is 300 - 600 °C. Silicides have generated interest due to their low cost, abundant and non-toxic materials [32, 43]. Among all semiconductor silicides, Mg2IV - based TE materials and higher manganese silicides (HMS) are the most popular choices due to their promising n-type and p-type TE performance, respectively [39, 47]. Currently, the highest ZT values obtained with the n-type and p-type Mg2IV-based TE materials ~ 800 K are about 1.5 and 0.7, respectively [44, 45]. High temperature TE materials Half-Heusler materials, denoted as XYZ, are semiconductors which consisting of a covalent and an ionic part. The X and Y atoms have a distinct cationic character, whereas Z can be seen as the anionic counterpart [48, 49]. Half-Heuslers have 18 valence electrons, they consist of a late transition metal, an early transition metal or a rare earth element, and a main group element [50, 51]. Half-Heusler compounds that can be used to directly convert the waste heat to clean electric energy at relatively high temperatures around 700 o C [51]. There are two types of half-Heusler which are n- (MNiSn where M = Ti, Zr, Hf, NbCoSn) [50, 51] and p-types (MCoSb, where M is Ti, Zr, or Hf) [52, 53]. Many research groups focused on the n-type half- Heusler MNiSn (M = Ti,Zr,Hf) system, since MNiSn based alloys due to their high power factor [50, 51, 54, 55]. Si-Ge system is well known for high temperature (up to 1000 o C) thermoelectric power generation, which is used for space mission [17]. Si-Ge system has high Seebeck coefficient but also has large thermal conductivity. Thus many research works have been focused on the reduction of lattice thermal conductivity [56-62]. Oxide semiconductors, which are thermally and chemically stable in air at high temperature, which are promising the candidates for high temperature TE applications. However, their ZT value has remained low, around 0.1–0.4 for more than 20 years. The poor performance in oxides is due to their low electrical conductivity and high thermal conductivity [63]. In Japan, the rapid increase in performance of oxide materials has made more than 10 times in the ZT value within the last two decades due to the change in the guiding principles of TE materials research [64]. Some strategies of lowering thermal conductivity of oxides materials are nanostructures, enhancing the phonon scattering by introducing ‘‘impurity’’ atoms which serve as scattering centers [63, 64]. Recently several oxide-based TE materials have been developed such as Ca3Co4O9 [65], CaMnO3 [66], SrTiO3 [67], BiCuSeO [68], ZnO [69], In2O3 [70] and NiO [71] due to their structural and chemical stabilities, oxidation resistance, easy processing, and low cost. Up-to-date, maximum ZT values of Bui Duc Long, Duong Ngoc Binh, Le Minh Hai, Le Hong Thang, Tran Duc Huy 8 0.5 - 0.7 have been achieved for oxide TE materials [72]. 5. IMPROVING EFFICIENCY APPROACHES The efficiency of TE device is mainly determined by the value ZT. Other factors also affect the efficiency of TE device such as resistances of the intermediate surface between TE material and substrate, heat exchanger etc. This review was focus on the challenges and improving approaches ZT of materials, which is led to improving the TE device performance. From Eq. (2), having a high-ZT value, TE material needs to have large Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, a material has large Seebeck coefficient, which has low carrier concentration (see Eq. (4)), will have low electrical conductivity (see Eq. (5)) [12]. (4) where κB is the Boltzmann constant, e is the charge carrier, h is Planck’s constant, m* is the effective mass of the charge carrier, and n is the carrier concentration. The relationship between electrical conductivity and carrier concentration can be defined as: (5) where µ is the carrier mobility. A material has high electrical conductivity, will also have high thermal conductivity (see Eq. (5) and (7)). Therefore, it is really a challenge to improve ZT or the efficiency of the TE material. Thermal conductivity of materials comes from two sources: (1) electrons and holes transporting heat (ke) and (2) phonons travelling through the lattice (kl) [12]. K = ke + kl (6) According to the Wiedemann-Franz law, the relationship between electrical conductivity and electric thermal conductivity is defined as: = (7) Despite of difficulty, recently TE technology has made a big improvement via nanostructuring approach, including nanocomposites [1, 7, 12, 36, 39, 63]. Here are the briefly review of some main improving ZT approaches. The traditional approach is finding new TE materials with high ZT values. In this method, the concept of “phonon-glass-electron-crystal” (PGEC) with the ideal that TE material should have low thermal conductivity like in glass, high Seebeck coefficient like in semiconductors, and high electrical conductivity like in crystal material such as metals. The PGEC concept can be found in the materials with complex symmetry and crystal structure such as skutterudites, zintl phases and clatharates [12, 13]. The second approach is to enhance the powder factor (S 2σ) (see Eq.2) by doping to optimize the carrier concentration in the range of 10 19 - 10 21 carriers/cm 3 [12, 73, 74]. The third approach is to reduce the thermal conductivity (k) (see Eq. 2) via nanostructuring. In fact, bot improvement in the power factor (S 2σ) and reduction in lattice thermal conductivity are possible in nanostructures. Particularly, theories and experiments indicated that a larger reduction in thermal conductivity can be achieved in nanometer-sized low-dimensional structures as well as bulk nanograined materials, arising from similar boundary and interface phonon-scattering mechanisms [75]. Thermoelectric materials: fundamental, applications and challenges 9 6. CONCLUSION Thermoelectric is a promising technology to convert waste heat into electricity. Thermoelectric generators, which are based on thermoelectric materials, are able to apply in many different areas such as automobile, industrial processes, space, home heating, sun energy etc. Although thermoelectric technology has made a big improvement in recent years, the application of the technology is still limited by the low energy conversion efficiency of the materials. There are a few enhanced efficiency approaches such as finding new materials with high ZT value, doping, and nanostructures. 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