A REVIEW ON THERMOELECTRIC MATERIALS PHENOMENA, TYPES AND APPLICATION

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1 A REVIEW ON THERMOELECTRIC MATERIALS PHENOMENA, TYPES AND APPLICATION PrathvirajUpadhyaya Mechanical Department, SMVITM,Bantakal Abstract -Thermoelectric materials have drawn vast attentions for centuries, as thermoelectric effects enable direct conversion between thermal and electrical energy.the thermoelectric effect refers to phenomena of Seebeckeffect (converting temperature to current).while all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. However, highefficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical energy. They can also be used in solid-state refrigeration devices. The conversion of waste heat into electrical energy may play an important role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels and reduce greenhouse gas emissions. A commonly used thermoelectric material in such applications is Bismuth telluride(bi2te3). In the present paper an overview of TE phenomena,several of currently used TE materials their properties and its applications are defined and discussed. A number of different systems of potential TE materials are currently under investigation by various research groups around the world such as quantum well TE materials, and these materials are reviewed in the article in this issue. At the end, a discussion of future possible strategies is proposed, aiming at further thermoelectric material performanceenhancements. Keywords- Thermoelectric materials, thermoelectric effect, Seebeck effect, Solid-state refrigeration, Alternative energy, bismuth telluride. I. INTODUCTION Current annual global energy consumption is 4.1 x 1020J (equivalent to 13 terawatts (TW)). By the end of the century, the projected population and economic growth will more than triple this global energy consumption rate [1].Statistical results show that more than 60% of energy is lost in vain worldwide, most in the form of waste heat [2]. High performance thermoelectric (TE) materials that can directly and reversibly convert heat to electrical energy have thus draw growing attentions of governments and research institutesdriven by the need for high temperature energy harvesting via the direct recovery of waste heat and its conversion into useful electrical energy and also for more efficient materials for electronic refrigeration and power generation [3]. Some of the research efforts focus on minimizing the lattice thermal conductivity, while other efforts focus on materials that exhibit large power factors [4]. Power-generation applications are currently being investigated by the automotive industry as a means to develop electrical power from waste engine heat from the radiator and exhaust systems for use in next-generation vehicles. Thermoelectric refrigeration is an environmentally green method of small scale, localized cooling in computers, infrared detectors, electronics, and optoelectronics as well as many other applications [4]. If significant economical cooling can be achieved, the resulting cold computing could produce speed gains of % in some computer processors based on CMOS technology. II. THERMOELECTRIC PHENOMENA The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa.a discussion of thermoelectric effects and devices should start with one of the most fundamental TE phenomena,the Seebeck effect [5,6].Seebeck effect mainly states that when All Rights Reserved 521

2 dissimilar materials are joined together and the junctions are held at different temperatures (T and T+T), a voltage difference (V) develops that is proportional to the temperature difference (T) and can be represented as, V=T (1) The ratio of the voltage developed to the temperature gradient (V/T) is related to an intrinsic property of the materials calledseebeckcoefficient, [7]. T MATERIAL A T+T MATERIAL B V Figure 1. Simplified diagram of the Seebeck effect. The potential of a material for TE applications is determined in large part by a measure of the material s figure of merit,zt. ZT= ( 2 T)/K (2) Conceptually, to obtain a high ZT, both Seebeck coefficient () and electrical conductivity () must be large, while thermal conductivity(k) must be minimized so that the temperature difference producing Seebeck coefficient () can be maintained [8,9]. The power factor 2 T) is typically optimized in narrow-gap semiconducting materials as a function of carrier concentration (typically=10 19 carriers/cm3), through doping, to give the largest ZT.High-mobility carriers are most desirable, in order to have the highest electrical conductivity for a given carrier concentration. The ZT for a single material is somewhat meaningless, since an array of TE couples is utilized in a device or module. The efficiency () of the TE couple is given by the power input to the load (W) over the net heat flow rate (QH), where QH is positive for heat flow from the source to the sink. W/ QH(3) The Peltier effect is the basis for many modern-day TE refrigeration devices, and the Seebeck effect is the basis for TE power-generation devices. The versatility of TE materials is illustrated in Figure 1, which shows a TE couple composed of an n-type (negative thermopower and electron carriers) and a p-type (positive thermopower and hole carriers) semiconductor material connected through metallic electrical contact pads Both refrigeration and power generation may be accomplished using the same module [4]. Thermoelectric energy conversion utilizes the Seebeck effect, where in a temperature gradient is imposed across the device, resulting in a voltage that can be used to drive a current through a load resistance or device. This is the direct conversion of heat into electricity. Figure 2.Thermoelectirc phenomena using a thermoelectric material

3 Conversely, the Peltier heat generated when an electric current is passed through a TE material provides a temperature gradient, with heat being absorbed on the cold side, transferred through the TE materials, and rejected at the sink, thus providing a refrigeration capability. III. THERMOELECTRIC MATERIALS PROPERTIES As we mentioned before, the figure of merit of a material is influenced by its electronic structure.the usefulness of a material in thermoelectric systems is determined by some of the factors like device efficiency, power factor, state density and conductivity. 1. Device efficiency The efficiency of a thermoelectric device for electricity generation is given by, defined as, = energy provided to load/energy absorbed at hot junction The ability of a given material to efficiently produce thermoelectric power is related to its dimensionless figure of merit given by: ZT= ( 2 T)/K(4) This depends on the Seebeck coefficient, thermal conductivity k, electrical conductivity,andtemperature T. In an actual thermoelectric device, two materials are usedsince thermoelectric devices are heat engines their efficiency is limited by the Carnot efficiency, hence the T H and T C terms in max. 2.Power factor In order to determine the usefulness of a material in a thermoelectric generator or a thermoelectric cooler the power factor is calculated by its Seebeck coefficient and its electrical conductivity under a given temperature difference: Power factor= 2 (5) Where is the Seebeckcoefficient and σ is the electrical conductivity. Materials with a high power factor are able to 'generate' more energy (move more heat or extract more energy from that temperature difference) in a space-constrained application, but are not necessarily more efficient in generating this energy. 3.State density: metals vs semiconductors The band structure of semiconductors offers better thermoelectric effects than the band structure of metals. The Fermi energy is below the conduction band causing the state density to be asymmetric around the Fermi energy. Therefore, the average electron energy of the conduction band is higher than the Fermi energy, making the system conducive for charge motion into a lower energy state. By contrast, the Fermi energy lies in the conduction band in metals. This makes the state density symmetric about the Fermi energy so that the average conduction electron energy is close to the Fermi energy, reducing the forces pushing for charge transport. Therefore, semiconductors are ideal thermoelectric materials [10]. 4.Conductivity According to the Wiedemann Franz law, the higher the electrical conductivity, the higher κ electron becomes [10].Thus in metals the ratio of thermal to electrical conductivity is about fixed, as the electron part dominates. In semiconductors, the phonon part is important and cannot be neglect. It reduces the efficiency. For good efficiency a low ratio of κ phonon / κ electron is desired.therefore, it is necessary tominimize κ phonon and keep the electrical conductivity high. Thus semiconductors should be highly All Rights Reserved 523

4 IV. THERMOELECTRIC MATERIAL TYPES Materials under consideration for thermoelectric device applications include: 1.Nanomaterial s and super lattices 1.1. Bismuthchalcogenides and their nanostructures Materials such as Bi2Te3and Bi2Se3 comprise some of the best performing room temperature thermoelectric with a temperature-independent figure-of-merit, ZT, between 0.8 and 1.0 [11]. Nanostructuring these materials to produce a layered super lattice structure of alternating Bi2Te3 and Sb2Te3 layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type) [12]. Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and therefore suitable for refrigeration applications around 300 K. The Czochralski method has been used to grow single crystalline bismuth telluride compounds. These compounds are usually obtained with directional solidification from melt or powder metallurgy processes. Materials produced with these methods have lower efficiency than single crystalline ones due to the random orientation of crystal grains, but their mechanical properties are superior and the sensitivity to structural defects and impurities is lower due to high optimal carrier concentration. 2.Inorganic clathrates Inorganic clathrates have the general formula A x B y C 46-y (type I) and A x B y C 136-y (type II), where B and C are group III and IV elements, respectively, which form the framework where guest A atoms (alkali or alkaline earth metal) are encapsulated in two different polyhedra facing each other. The differences between types I and II come from the number and size of voids present in their unit cells. Transport properties depend on the framework's properties, but tuning is possible by changing the guest atoms [12]. The most direct approach to synthesize and optimize the thermoelectric properties of semiconducting type I clathrates is substitutional doping, where some framework atoms are replaced with dopant atoms. In addition, powder metallurgical and crystal growth techniques have been used in clathrate synthesis. The structural and chemical properties of clathrates enable the optimization of their transport properties as a function of stoichiometry. The structure of type II materials allows a partial filling of the polyhedra, enabling better tuning of the electrical properties and therefore better control of the doping level. Partially filled variants can be synthesized as semiconducting or even insulating. 3.Magnesium group IV compounds Mg 2 B IV (B IV =Si, Ge, Sn) compounds and their solid solutions are good thermoelectric materials and their ZT values are comparable with those of established materials. Due to a lack of systematic studies about their thermoelectric properties, however, the suitability of these materials and in particular their quasi-ternary solutions, for thermoelectric energy conversion remain in question. The appropriate production methods are based on direct co-melting, but mechanical alloying has also been used. During synthesis, magnesium losses due to evaporation and segregation of components (especially for Mg 2 Sn) need to be taken into account. 4.Silicide Higher silicides display ZT levels with current materials. They are mechanically and chemically strong and therefore can often be used in harsh environments without protection. 5.Skutteruditethermoelectrics Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectrics [13]. These structures are of the form (Co,Ni,Fe)(P,Sb,As)3 and are cubic with space group Im3. Unfilled, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted in order to alter thermal conductivity by producing sources for All Rights Reserved 524

5 phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity [14].These materials could be potential in multistage thermoelectric devices. 6.Oxide thermoelectrics Their layered superlattice structure gives homologous oxide compounds potential in hightemperature thermoelectric devices.these materials exhibit low thermal conductivity perpendicular to the layers while maintaining electrical conductivity within the layers. ZT is relatively low(0.34 at 1000k) [15], but their enhanced thermal stability, as compared to conventional high- ZT bismuth compounds, makes them superior for use in high-temperature applications [16].In addition to their thermal stability, other advantages of oxides are their nontoxicity and high oxidation resistance. 7.Electrically conducting organic materials Some electrically conducting organic materials may have a higher figure of merit than existing inorganic materials. Seebeck coefficient can be even millivolts per Kelvin but electrical conductivity is usually low, resulting in small ZT values. Quasi-one-dimensional organic crystals are formed from linear chains or stacks of molecules that are packed into a 3D crystal. 8.Silicon-germanium Silicon-germanium alloys are currently the best thermoelectric materials around 1000 and are therefore used in some radioisotope thermoelectric generators (RTG) and some other high temperature applications, such as waste heat recovery. Usability of silicon-germanium alloys is limited by their price. 9.Sodium cobaltate Experiments on crystals of sodium cobaltate, using X-ray and neutron scattering experiments carried out at the European Synchrotron Radiation Facility (ESRF) and the Institute Laue-Langevin (ILL) in Grenoble were able to suppress thermal conductivity by a factor of six compared to vacancy-free sodium cobaltate. 10.Tin selenide In 2014 a research group discovered that tin selenide (SnSe) has a ZT of 2.6 along the b axis of the unit cell [17]. This is the highest value reported to date. This high ZT figure of merit has been attributed to an extremely low thermal conductivity found in the SnSe lattice. This SnSe material also exhibited a ZT of 2.3±0.3 along the c-axis and 0.8±0.2 along the a-axis. These excellent figures of merit were obtained working at elevated temperatures, specifically 923 K (650 C). As shown by the figures below, SnSe performances significantly improve at higher temperatures. V. QUANTUM WELL THERMOELECTRIC TECHNOLOGY New QW thermoelectric materials are being developed that are expected to yield conversion efficiencies several times that of present day bulk materials. For over 35 years, the ZT stayed close to the value of 1. However, breakthroughs have occurred in the Figure of Merit by using the recent QW alternatives to bulk material, with the QW material reaching the remarkable ZT value of 4.1 in performance tests. These new materials, called Quantum Wells, are composed of alternating layers of 10 nm thick Si and Si 0.8 Ge 0.2. They can be deposited by various deposition techniques and Hi-Z uses magnetron sputtering to obtain uniform layered structures. The costs of QW materials are lower than of the bulk materials, particularly if QW materials are sputtered on Kapton which is very inexpensive. Fabrication of QW films and modules is a highly automated process, while the fabrication of bulk TE modules is very labour intensive. While the fabrication of QW films and modules on a laboratory scale is slow and expensive, large-scale commercial sputtering machines have been available All Rights Reserved 525

6 some time and with their use the fabrication costs will be drastically reduced, yielding a projected cost of less than 1 $/W, which compares favourably with the cost of photovoltaic. The cost per watt for the bulk TE modules would be considerably higher even if the fabrication were outsourced to countries with very low labour costs. Quantum Well materials have the best measured power factor and combined with low thermal conductivity substrates should provide very high efficiency modules. QW TE materials have ZTs of > 5 which correspond to conversion efficiencies of > 20% at the same ΔT, and which allows for much wider commercial applications, particularly in the applications, such as waste-heat recovery from truck engines, refrigeration and air conditioning, where the SOTA bulk TE modules were shown to be technically feasible but economically unjustified due to low conversion efficiencies. With higher efficiency QW materials, these applications become economically attractive. A business case can even be made for the waste heat recovery of low-grade waste heat in industrial plants and profitably converting it to electricity with QW TE modules; this cannot be done with the SOTA bulk TE technology. VI. THERMOELECTRIC MATERIAL APPLICATION 1.Refrigeration Thermoelectric materials can be used as refrigerators, called "thermoelectric coolers", or "Peltier coolers" after the Peltier effect that controls their operation. As a refrigeration technology, Peltier cooling is far less common than vapour-compression refrigeration. 2.Power generation Thermoelectric efficiency depends on the figure of merit ZT. There is no theoretical upper limit to ZT, and as ZT approaches infinity, the thermoelectric efficiency approaches the Carnot limit. However, no known thermoelectrics have a ZT>3 [4]. 3. Automobiles Internal combustion engines capture 20 25% of the energy released during fuel combustion [18]. Increasing the conversion rate can increase mileage and provide more electricity for on-board controls and creature comforts (stability controls, telematics, navigation systems, electronic braking, etc.) [19].It may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation [18]. 4. Power plants Cogeneration power plants use the heat produced during electricity generation for alternative purposes. VII. CONCLUSION AND FUTURE SCOPE OF DEVELOPMENT The thermoelectric materials figure of merit and also their operational temperature are the most important factors to choose a TE material for a particular application. Higher figure of merit means higher efficiency. The Bi2Te3 was selected for the coolant system TE generator designing because its maximum figure of merit is in the temperature of the coolant system (about 360 Kelvin). On the other hand, the Si0.7Ge0.3 was selected for the exhaust TE generator designing. Its maximum figure of merit is in the temperature of 1000 Kelvin which is the average temperature of exhaust gas. It was shown that with the designed TE generators, about 18 kw of waste heat can be converted to electricity power, in a HEV with a 98 hp ICE. It means that we can use a lower power ICE in the HEV which reduces the fuel consumption and the pollution produced by the vehicle. As a result, the method improves the HEVs performance and efficiency. New and more efficient thermoelectric materials that make use of nanotechnology have been developed. These new materials, called quantum wells, are composed of alternating layers of 10 nm thick silicon and SiGe films. The QW TE materials with ZTs greater than 3 lead to All Rights Reserved 526

7 efficiencies greater than 20 percent, which allows for much wider commercial applications, particularly in the applications such as the waste-heat recovery from truck engines, refrigeration, and air conditioning The thermoelectric conversion efficiency is low and mainly limited by the performance of thermoelectric materials. New concepts and technologies were applied recently to enhance ZT, but accompanied difficulties need to be solved. Nanostructures are an effective approach to reduce the lattice thermal conductivity but also cause a stronger charge carrier scattering. Thermoelectric materials with intrinsically low thermal conductivity deemed promising are facing the problem of poor electrical transport properties hence the thermal conductivity has to be reduced. Many researchers have studied the properties of different thermoelectric materials. There are different types of thermoelectric materials, their performance and applications vary with their properties. In the present paper, different types of thermoelectric materials, their properties, thermoelectric phenomena and the applications were discussed. Finally, from the study it was concluded that, these materials are best suited for power generation, refrigeration and automobiles. REFERENCES [1] Basic research needs for solar energy utilization, Report of the basic energy sciences workshop on solar energy utilization, April 18-21, DOE, USA. [2] Kanatzidis M G Nanostructured thermoelectrics: the new paradigm? [3] Xiao Zhang and Li-Dong Zhao Thermoelectric materials: Energy conversion between heat and electricity, School of Materials Science and Engineering, Beihang University, Beijing, 20 February [4] TMTrittand MA Subramanian.Thermoelectric materials, phenomena, and applications: bird s eye view, MRS bulletin, Cambridge Univ Press. [5].T.J. Seebeck, Abh. K. Akad. Wiss.(Berlin, 1823) p [6].A.F. Ioffe SemiconductorThermoelements and Thermoelectric Cooling, (Infosearch, London, 1957). [7] ].T.J. Seebeck, Abh. K. Akad. Wiss.(Berlin, 1823) p [8]He JQ, Kanatzidis MG, Dravid VP High performance bulk thermoelectrics via a panoscopic approach. Matter today [9]Zhao LD, Dravid VP, Kanatzidis MG. The panoscopic approach to high performance thermoelectrics. Energy Environ Sci [10] Timothy D. Sands Designing Nanocomposite Thermoelectric Materials [11] Duck Young Chung, HoganT, Schindler J, Iordarridis L, Brazis,P, Kannewurf. C.R, BaoxingChen,Uher C, Kanatzidis, M.G 16th International Conference on Thermoelectrics [12] Gatti, C, Bertini, L., Blake, N. P. and Iversen, B. B. "Guest Framework Interaction in Type I Inorganic Clathrates with Promising Thermoelectric Properties: On the Ionic versus Neutral Nature of the Alkaline-Earth Metal Guest A in A8Ga16Ge30 (A=Sr, Ba)" September [13] Caillat,T, Borshchevsky, A., and Fleurial, JP. Proceedings of 7th International Conference TEs, University of Texas, Arlington, [14].Nolas G. S, Slack G. A, MorelliDT,Tritt, T. M, Ehrlich, A. C. The effect of rare-earth filling on the lattice thermal conductivity of skutterudites journal of applied physics,1996. [15].Wunderlich, W.; Ohta, S.; Ohta, H.; Koumoto, K. Effective mass and thermoelectric properties of SrTiO3-based natural superlattices evaluated by ab-initio calculations [16].Senthilkumar, Meenakshisundaram; Vijayaraghavan, Rajagopalan "High-temperature resistivity and thermoelectric properties of coupled substituted Ca3Co2O6". Science and Technology of Advanced Materials,2009. [17].Zhang, H. and Talapin, D. V, Thermoelectric Tin Selenide: The Beauty of Simplicity. Angew. Chem. Int, [18] Yang, J, "ICT th International Conference on Thermoelectrics",2005. [19].Fairbanks J., Thermoelectric Developments for Vehicular Applications, U.S. Department of Energy: Energy Efficiency and Renewable Energy. August 24, All Rights Reserved 527