J. Microelectron. Packag. Soc., 24(1), 9-16 (2017) https://doi.org/10.6117/kmeps.2017.24.1.009 Print ISSN 1226-9360 Online ISSN 2287-7525 9 Recent Advances in Thermoelectric Power Generation Technology Ashutosh Sharma1, Jun Hyeong Lee2, Kyung Heum Kim2 and Jae Pil Jung1,† 1Dept. of Materials Science and Engineering, University of Seoul, 163, Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea 2Duksan Himetal Co. Ltd., 66, Muryong 1-ro, Buk-gu, Ulsan 44252, Korea (Received February 1, 2017: Corrected February 20, 2017: Accepted March 8, 2017) Abstract: Thermoelectric power generation (TEG) technology with high figure of merit (ZT) has become the need of the modern world. TEG is a potent technology which can tackle most of the environmental issues such as global warming, change in climatic conditions over the globe, and for burning out of various resources of non-renewable energy like as petroleum deposits and gasolines. Although thermoelectric materials generally convert the heat energy from wastes to elec- tricity according to the theories Seebeck and Peltier effects yet they have not been fully exploited to realize their potential. Researchers are focusing mainly on how to improve the current ZT value from 1 to 2 or even 3 by various approaches. However, a higher ZT value is found to be difficult due to complex thermoelectric properties of materials. Hence, there is a need for developing materials with high figure of merit. Recently, various nanotechnological approaches have been incorporated to improve the thermoelectric properties of materials. In this review paper, the authors have performed a thorough literature survey of various kinds of TEG technology. Keywords: thermoelectric, figure of merit (ZT), power generation, Seebeck effect, nanotechnology, energy 1. Introduction The modern world is facing various energy crisis such as limitation of non-renewable resources, global warming, air and water pollution that are affecting the life of common people.1) According to one report, approximately seventy percent of energy into the planet earth is dissipated as heat due to dangerous gaseous emissions. A major fraction of the energy wastes comes from the fuel in production, trans- port and public sectors.2) The whole world is looking for the clean and sustainable sources of energy. The develop- ment in clean energy sources has various obstacles like abundance, cost, efficiency and climate. Though the well- known energy from the Sun is a better source clean energy yet it is constrained by the efficiency and availability of the sunlight. The conversion of waste heat using various energy converters and storing the energy for future use can be a possible solution to solve the above problem up to some extent. TEG technology can be considered as a powerful alternative to tackle the energy crisis all over the world.3) The TEGs are generally the power devices which are used to convert the heat from waste to the useful electricity. These TEGs use temperature gradient as input and provide the electrical energy as output based on the thermoelectric principles like Seebeck and Peltier effects.4) 2. Thermoelectric Science and Technology The thermoelectric science is based on the principle of energy conversion, from thermal energy to electricity or vice versa by thermoelectric materials based on various principles (Seebeck and Peltier effects) identified by Thomas Johann Seebeck in 1821 and Jean Charles Athanase Peltier in 1834.1-4) 2.1. Seebeck effect The Seebeck effect tells us that if a thermal gradient is applied to a thermoelectric material, a voltage can be gen- erated across the material. For example, if we connect two thermocouples or dissimilar metals A and B as shown in Fig. 1(a).5,6-9,10-14) If the two junctions, hot and cold junctions have different temperatures (TA>TB), then there will be a voltage gener- ated (V) across junctions due to charge difference in the material. The relationship between the voltage developed and the temperature difference across the hot and cold †Corresponding author E-mail: jpjung@uos.ac.kr © 2017, The Korean Microelectronics and Packaging Society This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/ licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 특집: Recent Advances in Thermoelectric 10 Ashutosh Sharma, Jun Hyeong Lee, Kyung Heum Kim and Jae Pil Jung 마이크로전자 및 패키징학회지 제24권 제1호 (2017) junctions can be expressed as.5-7) ⇒ V = S(TA-TB) (1) ⇒ V = SΔT (2) Where S = Seebeck coefficient between the two metals, V = thermoelectric voltage and ΔT = TA-TB, temperature difference across the junction (Fig. 1(b)). The sign of See- beck coefficient depends upon the direction of flow of the charge careers from one junction to other. 2.2. Peltier effect The Peltier effect is just reverse of the Seebeck effect. For example, one junction is cooled and other one is heated after a potential difference is set up in a circuit of metals A and B as shown in figure 1. Suppose, in a circuit, two pieces of a metal plate are connected to a semiconducting wire joined by a battery. Then the junction gets heated where the current flows from metal to semiconductor, and cooling is noticed at the junction where the current flows from semiconductor to metal. The Peltier coefficient is expressed in terms of heating flow at one junction and a rate of cooling at the other junction. In other words, Peltier coefficient can be expressed as mathematically,5-7) ⇒ П = I/Q, (3) Where, Q = heat flow, I = electrical current generated. 3. Factors affecting Thermoelectricity 3.1. Seebeck coefficient The Seebeck coefficient can be defined as a property of a material caused by Seebeck effect is the amount of voltage generated between the two thermocouples per unit tem- perature difference.5-6) Seebeck coefficient depends on the electronic band structure of the materials and the density of states (DOS) in the range of Fermi level. Higher DOS and moderate carrier concentration will produce a larger See- beck coefficient. The type of the charge careers (holes or electrons) will determine the nature of the sign of Seebeck coefficient.7) 3.2. Electrical conductivity The electrical conductivity of a material is expressed in the form of equation: σ = neμ, (4) Where, σ is the electrical conductivity, n is the charge carrier concentration per unit volume, e is the electronic charge, and μ is the mobility of the charge careers as deter- mined from the Hall experiments. The charge careers and mobility are dependent on the temperature. The conductiv- ity is further affected by various types of impurities in the material which act as a barrier to flow of the electrons and scatter them inside the crystal lattice. Metals are good con- ductors of electricity (107 mho/m), semiconductors (10-6 to 104 mho/m) and insulators (10-10 and 10-20 mho/m) and the values may vary along with temperature and impurity amount present in that materials.5-8) 3.3. Thermal conductivity The ability of a material to conduct heat by lattice vibra- tions or phonons.7) Therefore, materials of high thermal conductivity are used for heat sink source whereas low thermal conducting materials are used for insulation. Ther- mal conductivity is also a function of temperature and can be expressed as: (5) Where Q is the amount of heat flow through the mate- Q K 1 dT dx ------ ⎝ ⎠ ⎛ ⎞ –= Fig. 1. (a) Schematic diagram of a basic thermocouple, (b) Seebeck effect, and (c) Peltier effects.5,7,9,14) Recent Advances in Thermoelectric Power Generation Technology 11 J. Microelectron. Packag. Soc. Vol. 24, No. 1 (2017) rial, is the thermal gradient across the material and Kl is the coefficient of thermal conductivity of the material.9) The negative sign indicates that heat flows from hot to cold region. The total thermal conductivity of a material is given by the KT = Kel + Kl (6) Where, Kel is the electronic contribution tyo thermal con- ductivity of the material and Kl is the due to the lattice vibrations.7-8) The Weidemann Franz law tells us that electronic ther- mal conductivity is related to the Lorentz number accord- ing to the relation: Kel = LσT, (7) Where σ is the electronic conductivity, T is the absolute temperature and L is the Lorenz number. It can be con- cluded that Kel is a function of σ and T and therefore, form equation (4), the Kel also depends upon charge career den- sity (n) and their mobility (μ).10) The lattice contribution of thermal conductivity depends upon the phonon vibration characteristics. According to the kinetic theory of gases (8) Where, C is the heat capacity of the metal, v is the veloc- ity of phonon vibrations and l is the mean scattering free path.7-10) 3.4. Figure of merit The conversion efficiency of the TEG device is measured in terms of the figure of merit (ZT). From Carnot equation, the maximum efficiency of TEG is given by:11) (9) Where Thot and Tcold are the hot and cold junction tem- peratures, ∆T is given by Thot − Tcold, and Tmean is given by (Thot + Tcold)/2, respectively. The Carnot efficiency is given by ΔT/Thot multiplied by a reduction factor: Z = S2 ρ−1κ−1, (10) Where S, ρ, and κ are the Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. The dimensionless figure of merit (ZT) has been utilized since 1950 as a design parameter to achieve highly efficient materials. High ZT value (ZT ≈ 3-4) materials convert a greater amount of waste heat to power.12) In the past vari- ous researchers have been discovered that exhibit ZT > 1 and still there is a need for increasing this ZT value for realizing full potential of this technology. The conventional thermoelectric materials includes Pb-Te or Bi-Te based compounds which are expensive, and pose a threat to our environment due to their toxicity. Other class of thermoelec- tric materials include Mg-based materials which are cheaper but they have a poor figure of merit which makes them unsuitable for electricity conversion in various applications. Figure of merit (ZT) can be enhanced by controlling various factors like materials electronic structure, charge career con- centration and lattice vibrations characteristics.11-13) Now a days, nanotechnology have played a major role in enhancing the ZT value to a larger extent due to the novel properties of nanoparticles by control of crystallographic texture, modula- tion by doping, creating nanograins, dispersion of nanoparti- cles into the thermoelectric materials.14) 4. Thermoelectric Materials There are three categories of thermoelectric materials depending on the temperature range of applications. 1. Low temperature range (< 177oC): Bi based alloys 2. Moderate temperature range (177oC-577oC): Pb based alloys 3. High temperature range (>1027˚oC): Si-Ge based alloys etc. 4.1. Conversion Efficiency Commercial thermoelectric materials have ZT≈1. On laboratory scale, thermoelectric materials have ZT≈2 or approach to 3. Only the materials (ZT values above 2 or approach to 3) have capability to convert into some useful electrical energy from waste heat with sufficient conver- sion efficiency as shown in figure 2.15) 4.2. Material selection Semiconductor materials are suitable for thermoelectric generators because of their high charge carrier mobility, moderate charge carrier concentration. A high conductivity of the metals causes lower Seebeck coefficient and hence the generated power is low. An increase in electronic ther- mal conductivity of metals when temperature rises due to high electron carrier concentration which may cross a threshold of 1025/cm and may degrade the ZT value.6-7,11-15) dT dx ------ ⎝ ⎠ ⎛ ⎞ K 1 1 3 ---Cvl= n max TΔ T hot --------- 1 ZT mean + 1– 1 ZT m + T cold T hot ----------+ -----------------------------------------= 12 Ashutosh Sharma, Jun Hyeong Lee, Kyung Heum Kim and Jae Pil Jung 마이크로전자 및 패키징학회지 제24권 제1호 (2017) Various approaches have been tried to optimize the thermo- power, for example, by alloying, doping, nanostructured thermoelectric, nanocomposites approach, nano thin films, segmented thermoelectric etc. The fabrication route also affects the thermoelectric characteristics. Various routes like mechanical alloying (MA), melting and casting process, microwave fabrication routes, hot pressing and spark plasma sintering (SPS) tech- niques. 4.3. Developments in thermoelectric materials Various types of thermoelectric materials have been developed so far and still there is a need to approach ideal thermoelectric material. Thermoelectric materials accord- ing to their composition and structure are classified as chalco- genides, clathrates, skutterudites, Half-Heusler (HH) alloys, silicides, oxides and Zintl phase materials etc.31-33) Some examples of thermoelectric materials are shown in Table 1. Most of these are based on Bi, Sb, and Te which are highly expensive, toxic and unstable at high temperature. Polymers have also been investigated as cheaper alterna- tive due to their light weight and flexibility and abundance. However, the ZT value is rather lower due to their low electrical conductivity and Seebeck coefficient and their use is limited. 5. Nanotechnological approach to Thermoelectric Materials With the emergence of nanotechnology which operates when the size of the material reaches at one or its dimen- sions < 100 nm. For example, the nanomaterials can be produced in the form of nanopowders, or nanotubes/ Fig. 2. Conversion efficiency of thermoelectric materials at cold junction temperature of 300 K.15) Fig. 3. Figure of merit of a number of thermoelectric materials.16-31) Recent Advances in Thermoelectric Power Generation Technology 13 J. Microelectron. Packag. Soc. Vol. 24, No. 1 (2017) nanowires, nano-rods and nanocoatings where the grain size falls in the nano-regime. The nanostructured thermo- electric materials have better figure of merit and conver- sion efficiencies as compared to the bulk counterparts.31-32) The reason why the ZT value is enhanced is due to the large decrement in the thermal conductivity as a conse- quence of the increased phonon scattering at the grain boundaries and higher power at higher temperatures.30-31) Various kinds of nanostructured materials have been developed using nanotechnology, such as, nanocomposites, ultrafine grained materials and super lattices. In case of nanocomposites, the selection of the reinforcement and the dispersion process must be optimized before actually used in practice. In nanocomposites, at least one phase should be in the nanorange. The incorporation of nanoparticles can solve the problem due to the formation of new inter- face in the matrix. However, the nanoparticles are very high surface active entities. Their selection as a reinforce- ment is critical for avoiding the segregation and hence a depression in thermopower.10,14,31) 6. Thin Film thermoelectric Technology Due to the miniaturization of the electronic circuits and systems, the amount of heat dissipation is large enough to cause thermal breakdown of the device and eventually fail- ure of entire device. Traditional or bulk thermoelectric devices have been used for years to control the temperature of electronics where the cooling of the entire device is an efficient method for thermal management leading to the oversizing of the thermoelectric device. The modern thin film thermoelectric technology targets the heat flux source to provide thermal control. Such types of thermoelectric device incorporate nanostructured p and n type materials in the form of a coating, typically 5 to 20 μm thick, over 200 μm for traditional devices. This creates a huge heat flux around 20 times as compared to traditional ones. Gary et al synthesized Bi2Te3 superlattice based thin thermoelectric device using metal organic chemical vapor deposition tech- nique and obtained cooling fluxes of 200 W/cm2 over com- mercial 100 W/cm233-35) However, there are various barriers in thin film technology which need to be sorted out for full potential of this technology. For example, electrical contact resistance between metal electrodes and the semi- conductor layers can be comparable to that of the thermo- electric element itself, thus increasing the overall electric resistance of the thermoelectric device and reducing the cooling flux.34) 7. Magnesium based Thermoelectric Magnesium based thermoelectric materials have attracted the researchers in the past to overcome the issue of cost, toxicity and light weight compared to the bulk tra- ditional thermoelectric materials (Bi2Te3, PbTe and CoSb3). Mg is the light metal having density (1.73 g/cm3) versus most popular aluminum (2.70 g/cm3) and iron (7.86 g/ cm3). Mg based alloys are high strength and low weight, moderate melting points and their intermetallic compounds or alloys form Zintl phase with a large electronegativity gap.36) Zintl phases are the potential candidates for obtain- ing high ZT value in thermoelectric materials, e.g. (Eu1/ 2Yb1/2)1-xCaxMg2Bi2. Mg2X (X=Si, Ge, Sn, Si-Sn, Si-Ge, etc) compounds have shown higher figure of merit as com- pared to the Si-Ge and β-FeSi2 [37-41]. It is also notewor- thy point that band gap range of Mg based thermoelectric materials are close band gaps of semiconductor materials as shown in Table 2. However, the synthesis of these Mg2(Si, Sn) single phase is difficult due to the high vapor pressure and chemical Table 1. Figures of merit of developed bulk thermoelectric materials15,33) Materials Examples ZT at Temperature Metal Oxides Bi doped Ca3Co4O9 >1 at 727 oC Chalcogenides Ti9BiTe6 1.25 at 226 oC Clathrates Ba8Ga16Ge30 1.35 at 627 oC Half Heuslers Hf0.75Zr0.25NiSn0.975Sb0.025 0.81 at 752 oC Skutterudites Ba0.30Ni0.05Co3.95Sb12 1.25 at 627 oC Polymers Polyacetylene Polyprrole Polyaniline PEDOT 0.047-0.38 at 27oC 0.002 at 27oC 0.051 at 27oC 0.25 at 27oC Table 2. Band gaps for different Mg based thermoelectric materials.7,9) Material Band Gap (eV) Mg2Si 0.78 Mg2Ge 0.69 Mg2Sn 0.36 Mg2Si0.6Sn0.7 0.51 Mg2Si1-xGex 0.05-0.23 SiGe 0.7 PbTe 0.3 Bi2Te3 0.1 Ge 0.67 Si 1.14 14 Ashutosh Sharma, Jun Hyeong Lee, Kyung Heum Kim and Jae Pil Jung 마이크로전자 및 패키징학회지 제24권 제1호 (2017) activity of Mg as well as large gap in melting points of the constituents.42) The stability and performance of Mg2(Si,Ge) is better than Mg2(Si, Sn) but is highly expensive due to cost of Ge. Application of nanotechnology has been also found to increase the figure of merit of theses alloys via nanopar- ticles inclusion in the matrix. 8. Conclusion Thermoelectric materials are the backbone of energy sec- tor to avoid the energy crisis in the coming years worldwide. Materials with high figure of merit categorized as metals, polymers, chalcogenides, clathrates, skutterudites, half-Heu- sler alloys, and silicides. The task to develop the Mg based thermoelectric is challenging in view of the purity, melting point, light weight, cost, abundance, ecofreindly nature etc. Polymers have also gained importance in thermoelectric as a portable, light weight and flexible options and further inves- tigations are needed to bring them into market. Nanostruc- tured materials and thin films are also attractive ways to enhance the figure of merit of these materials. 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Chen, “Power Fac- tor Enhancement by Modulation Doping in Bulk Nanocom- posites”, Nano letters, 11, 2225 (2011). 42. G. B. Granger, C. Navone, J. Leforestier, M. Boidot, K. Romanjek, J. Carrete and J. Simon, “Microstructure Investi- gations and Thermoelectrical Properties of an N Type Mag- nesium–Silicon–Tin Alloy Sintered from a Gas-Phase Atomized Powder”, Acta Materialia, 96, 437 (2015). 16 Ashutosh Sharma, Jun Hyeong Lee, Kyung Heum Kim and Jae Pil Jung 마이크로전자 및 패키징학회지 제24권 제1호 (2017) • Ashutosh Sharma • Department of Materials Science and Engineering University of Seoul, Seoul- 02504, Korea • Research Interests: Pulse Electroplating, Lead Free Soldering, Brazing, Metal Matrix Nanocomposites • Email: stannum.ashu@gmail.com • Jun-Hyeong Lee • Duksan Himetal Co. Ltd., 66, Muryong 1-ro, Buk-gu, Ulsan, Korea • Research Interests: Electroplating, Thermoelectric module • Email: leewnsgud@naver.com • Kyung-Heum Kim • Duksan Himetal Co. Ltd., 66, Muryong 1- ro, Buk-gu, Ulsan, Korea • Research Interests: Soldering, Thermoelectric module • Email: khkim@oneduksan.com • Jae Pil Jung • Department of Materials Science and Engineering University of Seoul, Seoul- 02504, Korea • Research Interests: Microjoining, Electroplating, Brazing Fillers, Solder-Joint Reliability, Metal Matrix Nanocomposites, Lead Free Soldering • Email: jpjung@uos.ac.kr