Front. Energy 2011, 5(2): 125–136 DOI 10.1007/s11708-011-0150-1
FEATURE ARTICLE
Laifeng LI, Zhen CHEN, Min ZHOU, Rongjin HUANG
Developments in semiconductor thermoelectric materials
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2011
Abstract A surge in interest in developing alternative renewable energy technologies has been observed in recent years. In particular, thermoelectrics has drawn attention because thermoelectric effects enable direct conversion between thermal and electrical energy, and provide power generation and refrigeration alternatives. During the past decade, the performance of thermoelectric materials has been considerably improved; however, many challenges continue to exist. Developing thermoelectric materials with superior performance means tailoring interconnected thermoelectric physical parameters-electrical conductivities, Seebeck coefficients, and thermal conductivities for a crystalline system. The objectives of this paper are to introduce the recent developments in semiconductor thermoelectric materials, and briefly summarize the applications of such materials. Keywords thermoelectric materials, thermoelectric figure of merit, applications
1
Introduction
Providing a sustainable supply of energy is becoming a major societal problem as fossil fuel supplies decrease while demands increase. Hence, the past few years have witnessed a surge in interest in developing alternative renewable energy technologies. In particular, thermoelectric materials have drawn attention because thermoelectric effects enable direct conversion between thermal and electrical energy, and provide an alternative for power
Received January 19, 2011; accepted March 11, 2011
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Laifeng LI ( ), Zhen CHEN, Min ZHOU, Rongjin HUANG The Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China E-mail: lfl
[email protected] Zhen CHEN Graduate University of Chinese Academy of Sciences, Beijing 100049, China
generation and refrigeration. The efficiency of thermoelectric devices is strongly associated with the dimensionless figure of merit (ZT) of thermoelectric materials; the figure of merit is defined as ZT = (α2σ/κ)T, where α, σ, κ, and T are the Seebeck coefficient electrical conductivity, thermal conductivity, and temperature, respectively. To maximize the thermoelectric ZT value of a material, a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity are required. However, these transport characteristics depend on the interrelated physical properties of materials, such as carrier concentration, effective mass, electronic thermal conductivity, lattice thermal conductivity, Fermi level, etc. Consequently, a number of parameters require optimization to maximize the ZT value. High-performance thermoelectric materials have been pursued since Bi2Te3-based alloys were discovered in the 1960s. Until the end of the last century, moderate progress had been made in the development of thermoelectric materials. A ZT value benchmark of approximately 1 was achieved in the mid-1990s by two different research approaches: the exploration of new materials with complex crystalline structures, and the reduction of material dimensions. Figure 1 shows the highperformance bulk thermoelectric materials recently developed. The search for new materials, such as skutterudites [1,2] and clathrates [3], was mainly motivated by the suggestion of Slack [4]. In research on low-dimensional material systems, Dresselhaus et al. [5] suggested that the power factor (α2σ) can be enhanced using quantum confinement effects. Their pioneering work shed light on various low-dimensional systems, including superlattices, nanowires, and quantum dots. Venkatasubramanian et al. [6] reported Bi2Te3/Sb2Te3 superlattices with a high ZT value of up to 2.4. Subsequently, Harman et al. [7] reported PbTe/PbTeSe quantum dot superlattices with a ZT value greater than 3.0 at 600 K. Furthermore, nanostructures may improve thermoelectric efficiency through the introduction of some new scattering mechanisms. In recent years, considerably more work has been devoted to bulk samples that contain nanoscale constituents; these studies indicate that nanostructuring can effectively increase the ZT value [8–11].
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Fig. 1 Current state-of-the-art bulk thermoelectric materials [12]
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Semiconductor thermoelectric materials
2.1
Bi-Te alloys
In the 1950s, Bi2Te3 alloys were discovered to have a ZT value of approximately 1 near room temperature. To date, these alloys continue to dominate the field of thermoelectrics. As a class of materials with the best thermoelectric properties near room temperature, V-VI alloys, especially Bi2Te3 [13] and its alloys with Sb, Se, etc., have attracted interest for thermoelectric generation and refrigeration at both room temperature and lower temperatures. Bi2Te3 can be tailored as either n-type or p-type by varying the composition with slight deviations from its stoichiometric composition. Bi2 – xSbxTe3 – ySey alloys have been used for decades because their thermoelectric performance has reached optimal values through composition optimization [14–16], doping [17,18], and device design [19]. The Bi2Te3 single crystal has an anisotropy that is favorable to enhancements in thermoelectric properties. However, this compound has weak van der Waals bonding between the Te-Te layers. This bonding becomes a cleavage plane that leads to strength reduction in the single crystal. To overcome the disadvantages presented by single crystals, many synthetic methods were employed to obtain Bi2Te3-based alloy bulks with higher mechanical strength and good thermoelectric properties; these methods include the Bridgman-Stockbarger method [20], equal channel angular pressing [21], hot pressing [22], hot extrusion [23,24], shear extrusion [25], and physical vapor deposition [26]. Using a two-step spark plasma sintering technique, researchers prepared textured n-type Bi2Te3 with enhanced thermoelectric and mechanical properties; the authors indicated that controlling texture effectively improved the thermoelectric and
mechanical properties of Bi2Te3-based materials [27]. On the basis of the nanostructural concept, Poudel et al. [8] reported nanocomposite BiSbTe bulk with a peak ZT value of 1.4 at 373 K. These nanocrystalline bulk materials were created using hot pressing nanopowders that were ballmilled from crystalline ingots under inert conditions. Additionally, other methods were also used to form bulk nanostructured BiSbTe with a high ZT value [28,29]. 2.2
Pb-Te alloys
PbTe has been the premiere thermoelectric material for moderate-temperature (600–800 K) application as a thermoelectric generator. PbTe single crystals can be prepared by almost any standard method, including the Bridgeman, Czochralski, and vapor growth techniques, but preparing large single crystals with a high degree of perfection and homogeneous component distribution is difficult [4]. To overcome this drawback, the more general ternary systems were studied [30–32]. Instead of using pristine PbTe, Heremans et al. [33] used thallium-doped PbTe and succeeded in obtaining a higher ZT value and unreduced thermal conductivity by increasing the Seebeck coefficient. At a doping level of 2% thallium, the ZT value was increased to 1.5 at 773 K, and continued to increase with temperature. They reported that thallium-doped PbTe can lead to an enhanced density of states in the valence band, and therefore an improvement in the figure of merit. Enhanced densities of states may be obtained in other materials if suitable doping agents can be found. This observation provides a new direction in searching for new materials. Additionally, intensive research on doped PbTe samples has been underway. The thermoelectric properties of PbTeAg, GeTe-AgSbTe2 (TAGS), and PbTe-AgSbTe2 have also been studied [4,34]. As one of these remarkable materials,
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the Ag-Pb-Sb-Te system has been accorded increasing interest since Hsu et al. [35] reported their findings on this high-performance thermoelectric material system, discussed later in the paper. 2.3
Si-Ge alloys
Silicon-germanium (SixGe1 – x, where x indicates the mole fraction of silicon) alloy is a complete solid solution semiconductor with a cubic diamond-type structure. Such a structure is important for both electric and optoelectric applications at temperatures higher than 1000 K. Therefore, Si-Ge is most well-known as a material for thermoelectric power generators at elevated temperatures. Slack and Hussain suggested that a high-quality single crystal of SiGe alloy may be the most useful material for eliminating boundary scattering effects [36]. However, the purity and quality of Si-Ge bulk crystals are insufficiently high for actual applications. Many crystal growth techniques have been utilized in fabricating high-quality Si-Ge single crystals with uniform compositions; these approaches include the Bridgeman and Czochralski methods, as well as floating zone melting, liquid phase diffusion, and so on [37–39]. Both silicon and germanium have high lattice conductivities, and they can yield reasonably large values for the power factor because both elements have high carrier mobilities [40]. Therefore, reducing the lattice conductivity of Si-Ge alloys leads to a considerable increase in the ZT value. The possibility of reducing the thermal conductivity of thermoelectric semiconductor alloys by compacting fine-grained material was considered, and various estimates were made for thermal conductivity reduction with grain size [41]. Slack and Hussain conducted a complete review of the thermoelectric properties of n-type and p-type Si-Ge alloys to determine the maximum efficiency that can be achieved for a generator operating between 300 and 1300 K [36]. They demonstrated that if lattice thermal conductivity can be reduced to its minimum value without upsetting electrical properties, the efficiency of a generator of standard optimally doped and infinitely segmented material can be raised to a maximum level (from 12.1% to 23.3%). Efforts were also focused on enhancing carrier concentration levels, thereby increasing the power factor values of n-type materials [42,43]. With the help of nanotechnology, Si-Ge nanocomposite materials were prepared by various material synthesis processes and approaches, and the ZT values were enhanced [44–46]. The enhancement of the ZT value is due to a large reduction in thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures. This enhancement enables the fabrication of materials that are suitable for many applications such as solar, thermal, and waste heat conversion into electricity.
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3 New high-performance thermoelectric systems Over the past years, traditional thermoelectric materials have been extensively studied and optimized for their applications in solid-state refrigeration or power generation. Recently, substantial efforts have been directed toward the investigation of new high-performance thermoelectric materials. Some of these include phonon glass electron crystal (PGEC) materials, nanostructured Ag-PbSb-Te (LAST) system materials, low-dimensional thermoelectric systems, and other thermoelectric materials. 3.1
PGEC materials
Slack proposed the PGEC idea as a means to achieve record low lattice thermal conductivities without diminishing electronic performance [4,12]. Two new classes of thermoelectric materials, namely, filled skutterudites and clathrates, are widely regarded as PGEC [1,47,48]. Binary skutterudites possess a CoAs3-type structure with the general chemical formula AB3, where A = late transition metal, B = P, As, or Sb. The unit cell of binary skutterudites contains square radicals of the pnicogen atoms, [As4]4–, which, located in the center of the smaller cube, is surrounded by eight trivalent transition metal Co3+ cations. The unit cell consists of eight smaller cubes, or octants described above, but two of them do not have the [As4]4– anion at the center. A typical coordination structure results in Co8[As4]6 = 2Co4[As4]3 composition and 32 atoms per cell, thereby enabling the maintenance of charge conservation [49]. A schematic showing the basic structure of the skutterudite unit is illustrated in Fig. 2. In the skutterudite family, CoSb3 has been accorded the most attention to date because of its higher weighted mobility (m*)3/2μ compared with other family members; m* and μ are the carrier effective mass and mobility, respectively. Two chemical approaches are used to enhance the thermoelectric performance of CoSb3-based alloys: filling the lattice cage of the skutterudite structure, which results in the significant depression of lattice thermal conductivity because of the strong “rattling” phonon scattering by filler atoms, and elemental substitution. For rare-earth (Ce or La) filling, Sales et al. [1] reported a class of thermoelectric materials [CeFe 4 – x Co x Sb 12 or LaFe4 – xCoxSb12 (0 < x < 4)] with a ZT value of approximately 1 at 800 K. Their calculations indicated that the optimized material should have a ZT value of 1.4. Through mechanical alloying and spark plasma sintering, the effect of Te substitutions in CoSb3 was confirmed by Liu et al. [50]. The CoSb2.85Te0.15 sample had the highest power factor and the lowest thermal conductivity, resulting in the highest thermoelectric ZT value (ZT = 0.93 at 547°C). On the basis of this work, Liu et al. [51] chose IVB-group elements (Si, Ge, Sn, Pb) as the charge compensating
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Fig. 2 Crystal structure of skutterudite CoAs3 (adapted from Ref. [49]): 32 atoms, with a cubic frame with 8 Co atoms and 24 As atoms distributed in the six square radicals and located in only six of the eight octants. The other two octants are void, which are often found by other elements to form a completely filled structure
elements for Te to increase its solubility in CoSb3 – xTex. They realized a high ZT value of 1.1 at 550°C in Sn/Te-codoped CoSb3. Using the melting-quench-annealing and spark plasma sintering methods, Bai et al. [52] recently synthesized the single-phase polycrystalline dual-elementfilled skutterudites BaxCeyCo4Sb12. Through the Ba-Ce cofilling, the thermoelectric ZT value was enhanced, increasing up to 1.26 at 850 K for Ba0.18Ce0.05Co4Sb12.02. Clathrate compounds are periodic solids in which tetrahedrally bonded atoms form a framework of cages that enclose metal atoms (guest atoms). In 1998, Nolas et al. [3] measured the transport properties of semiconducting Sr8Ga16Ge30 polycrystalline samples and observed temperature dependence typical of amorphous materials. These results identified clathrates as potential thermoelectric materials, and triggered extensive theoretical and experimental research efforts to synthesize several of these materials and further understand their peculiar transport properties. The current record for a clathrate was observed in Ba8Ga16Ge30 single crystal, with a ZT value reaching 1.35 at 900 K, further extrapolated to reach 1.63 at 1100 K; the single crystal was grown by the Czochralski method [48,53]. 3.2
Nanostructured Ag-Pb-Sb-Te (LAST) system materials
On the basis of the nanostructural concept, researchers heavily promoted the development of thermoelectric materials. Nanostructured materials have been proposed to enhance the ZT value on account of both power factor enhancements caused by increased density of states near
the Fermi energy level, and thermal conductivity reduction [54]. In addition, the nano/micro composite has now become an important research direction. In 2004, Hsu et al. [35] reported that the AgPbmSbTem + 2 (LAST) system bulk materials, fabricated by melting and the slow cooling process, exhibited a high ZT value of 2.1 at 800 K. As demonstrated in Fig. 3, the LAST compounds possess a typical NaCl structure, and can therefore be considered antimony and silver co-doped PbTe. They suggested this serial compound to be one of the most competitive thermoelectric materials. The microstructure showed the presence of Ag+-Sb3+–rich quantum “nanodots” inside the matrix (Fig. 4(a)). Later, Zhou et al. fabricated n-type nanostructured Ag0.8Pbm + xSbTem + 2 (m = 18, x = 4.5) system thermoelectric materials by combining mechanical alloying and spark plasma sintering methods followed by annealing for several days [55]. They reported that the thermoelectric performance of these materials were tremendously enhanced by an appropriate annealing treatment, and a high ZT value of 1.5 was obtained for the sample annealed for 30 days at 700 K. The microstructure also displayed nanoscopic inhomogeneities inside the grains, resulting in enhanced phonon scattering and the considerable reduction in thermal conductivity (Figs. 4(b) and 4(c)).
Fig. 3 Average Fm3m structure of the LAST system (adapted from Ref. [35])
Numerous studies on the LAST system were performed by the Kanatzidis group, particularly with respect to microstructure characterization [56] and the exploration of p-type LAST systems [57–59]. They found that a high ZT value of 1.45 at 630 K was achievable in p-type Ag(Pb1 – xSnx)mSbTe2 + m by partially substituting the lead in the LAST system with tin [57]. Subsequent research showed that simply replacing silver in the LAST system with sodium led to a higher ZT value of 1.7 at 650 K in
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Fig. 4 Typical microstructure images of LAST materials (a) TEM image of a AgPb18SbTe20 sample showing a nano-sized region [35]; (b) HRTEM images of unannealed Ag0.8Pb22.5SbTe20 sample; (c) the sample annealed for 30 days [55]
p-type Na0.95Pb20SbTe22 [58]. More recent potassiumbased analogs of p-type K1 – xPbmSbyTem + 2 materials were reported. A maximum ZT value of 1.6 at 750 K was achieved for a system with the composition K0.95Pb20Sb1.2Te22 [59]. Both the n- and p-type LAST materials can be obtained by adjusting the chemical composition, making the system particularly promising for power generation applications [60]. 3.3
Low-dimensional thermoelectric materials
Reducing dimensions offers a new possibility for individually tuning thermoelectric parameters. When system size decreases and approaches a scale comparable to the feature length of electron behavior (e.g., mean free path, wavelength) in any direction, the density of electronic states considerably increases because of quantum confinement [5], resulting in the enhancement of the Seebeck coefficient [61]. Meanwhile, thermal conductivity is also reduced because the surface strongly scatters the
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propagation of phonons, given that any dimension is lower than the average free path of phonons. Using PbTe/Pb1 – xEuxTe multiple-quantum-well structures grown by molecular beam epitaxy, Hicks et al. [62] experimentally studied the effect of quantum-well structures on the thermoelectric ZT value. The results were found consistent with theoretical predictions. Via different methods, nanostructured films, nanowires, nanotubes, and nanoplates were obtained to maximize ZT values [63–69]. Venkatasubramanian et al. [70] reported that thin-film thermoelectric materials demonstrated a significant enhancement in ZT value at 300 K through the control of phonon and electron transport in the superlattices (ZT = 2.4 in thin film p-type Bi2Te3/Sb2Te3 semiconductors; the previous record for the ZT value at room temperature was around 1). The ZT values for PbSe0.98Te0.02/PbTe quantum-dot superlattices (QDSLs) in the range 1.3 to 1.6 were reported at room temperature [71]. Temperaturedependent measurements of the ZT value for a PbSe0.98Te0.02/PbTe QDSL sample showed increases in the ZT value with rising T, reaching a ZT value of 3 at 550 K [72]. Most recently, Shi et al. [54] used the first-principles electronic structure calculation and Boltzmann transport equation to investigate the composition effects on the thermoelectric properties of silicon-germanium nanowires. They found that n-type SiGe nanowires are promising thermoelectric materials in achieving a ZT value higher than 3. These results indicate that nanoscaled thermoelectric materials may have properties superior to their bulk counterparts. 3.4 Other thermoelectric materials (oxides and half-Heusler compounds)
Although oxides are thought to be very poor thermoelectric materials because of their low electrical conductivity, Terasaki et al. [73] reported that NaxCoO2 compounds simultaneously exhibited high thermoelectric power and low resistivity (at 300 K; the in-plane thermoelectric power was approximately 100 μV/K, nearly 10 times larger than that of typical metals, and the in-plane resistivity was as low as 200 μΩ$cm). Since this discovery, the series of thermoelectric layered oxide NaxCoO2 has attracted much attention because of its remarkable thermoelectric power and high ZT value. Numerous theoretical and experimental studies [74,75] have focused on this thermoelectric system. Compared with traditional thermoelectric materials, oxide-based thermoelectric materials have many advantages. They are nontoxic and nonpolluting, can be simply prepared in air without vacuum protection, and can operate at high temperatures under an oxygen atmosphere for long periods. These merits make oxide thermoelectric materials potential candidates for high-temperature devices. In NaxCoO2 thermoelectric oxide, the sodium layers are
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sandwiched between CoO2 blocks and stacked along the c axis (Fig. 5). The CoO2 layers are responsible for electrical conductivity and the Seebeck coefficient, while the Na layers act as insulators and determine the thermal conductivity of the oxide [76]. Because NaxCoO2 is a transition metal oxide, both Na and Co sites can be substituted by a variety of elements, and many efforts were made to improve the thermoelectric properties of the oxide. For Ca-substituted compounds, Kawata et al. [77] showed enhanced thermoelectric properties. The effects of the partial substitution of other metals for Na on the thermoelectric properties of NaxCo2O4 were investigated by Nagira et al. [78]. They prepared polycrystalline samples of (Na1 – yMy)Co2O4 using a solid-state reaction method, where M is K, Sr, Y, Nd, Sm, or Yb. K and Rb substitutions in the Na layer of Na0.75CoO2 oxide were reported by Peleckis et al. [79]. Moreover, Park and Jang [80] attempted to partially substitute Ni for Co in NaCo2O4 to improve the thermoelectric properties of the system. They found that the partial substitution of Ni for Co in NaCo2O4 was highly effective in enhancing high-temperature thermoelectric properties. Other layered cobalt oxides were also fabricated by researchers. The ZT value in Ca3Co4O9 exceeds 0.87 at 973 K [81] because of low thermal conductivity, which is most likely associated with the misfit structure between the CoO2 layer and the Ca2CoO3 rock-salt layer [82]. Using a thermoforging process, Prevel et al. successfully textured dense Ca3Co4O9 thermoelectric oxides [83]. The contribution of the electronic structure to the large thermoelectric power in layered cobalt oxides (Na0.6CoO2, Bi2Sr2Co2O9, and Ca3Co4O9) was investigated by Takeuchi et al. [84,85]. In addition to the characteristics of thermoelectric properties, Na x CoO 2 became a focus of interest because of the occurrence of superconductivity with the
de-intercalation of Na and the inclusion of water in the parent structure; these processes resulted in a superconducting transition temperature of around 4.5 K for Na0.35CoO2∶1.3H2O [86]. Na content is important in determining not only thermoelectric properties, but also the occurrence of superconductivity. An increasing number of researchers have been focusing on the fascinating and yet puzzling ground states of oxide thermoelectric materials. Other environmentally benign compounds are the halfHeusler compounds, which have also attracted increasing interest [87–90]. Uher et al. [87] found that the isoelectronic alloying of Zr0.5Hf0.5NiSn produced a higher ZT value than did ZrNiSn or HfNiSn alone because of a reduction in thermal conductivity. Shen et al. [91] investigated the effect of partial substitution of nickel with palladium on the thermoelectric properties of ZrNiSnbased half-Heusler compounds. This substitution resulted in a significant, beneficial reduction in thermal conductivity, as well as a small decrease in the Seebeck coefficient, thereby leading to an improvement in the ZT value of 0.7 at about 800 K for a compound having the composition Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01. Later, Culp et al. [92] reported that through the optimization of Sb doping at the Sn site and substitutions at the M and Ni sites of MNiSn alloys, the temperature of the peak thermoelectric efficiency increased. The highest ZT values observed were 0.81 at 1025 K and 0.78 at 1070 K for the Hf0.75Zr0.25Ni0.9Pd0.1Sn0.975Sb0.025 sample. Considerable improvements in the ZT value through the use of nanostructures is anticipated because most of the current studies focus on half-Heusler compounds in isoelectronic alloying. Experimental investigations on these new compounds should be strongly encouraged [60].
4 Application prospects of thermoelectric devices
Fig. 5 Crystal structure of NaxCoO2 [86]
Thermoelectric devices based on the thermoelectric effect can be used in various applications, from power generation for deep space science exploration probes, waste heat recovery in automobiles, and energy intensive industrial processes to electronic cooling and heating [93]. A practical thermoelectric device consists of numerous elements that are used to increase operating voltage and spread heat flow. In addition, a typical thermoelectric element is composed of two ceramic substrates that serve as foundation and electrical insulation for p-type and ntype thermoelements. The thermoelements are electrically connected in series and thermally in parallel between the ceramics. The device can harvest a heat source and begins working once connected to an external resistive load. Conversely, heat can be pumped across a temperature differential if the device is connected to an electrical power source (Figs. 6 and 7).
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Fig. 6 Typical thermoelectric elements (a) When current runs across a thermoelectric junction, the material is cooled or heated through the Peltier effect, depending on the direction of the current flow; (b) when heat flows across the junction, electrical current is generated through the Seebeck effect
Fig. 7 Schematics of conventional arrangement for thermoelectric devices (a), (b) Structure of thermoelectric modules; (c) practical thermoelectric devices [13]
Thermoelectric devices offer substantial distinct advantages over other technologies. Some of the more significant features of thermoelectric devices are as follows [94]: they work with no moving parts; the volume of the thermoelectric module is small; the thermoelectric modules exhibit very high reliability because of their solid-state construction; thermoelectric devices contain no chlorofluorocarbons or other chemical materials that may require
periodic replenishment or may be harmful to the environment; the thermoelectric module can be used for both heating and cooling; precise temperature control within 0.1 K can be achieved using thermoelectric devices and appropriate support circuitry; and thermoelectric devices are scalable and not position dependent. Given these advantages, thermoelectric devices have been used in many areas, especially with advancements in the ZT value
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in the past years. The main applications include power generation and cooling (or heating). 4.1
Thermoelectric generators
Thermoelectric devices can be used to convert heat into electricity. A temperature gradient across a thermoelectric material drives the flow of carriers from the hot side to the cold part of the material. Thermoelectric power generation devices have been in production since bismuth telluride-based room-temperature materials were developed in the late 1950s. Presently, thermoelectric materials for power generation are based on PbTe at moderate temperature gradient applications. At higher temperature gradients, the development of silicon germanium and skutterudite high-temperature powergeneration materials directly lead to the production of heat engines for space equipment that can operate in the absence of sunlight [95]. Given the discovery of oxide thermoelectric materials, thermoelectric materials are expected to be used under even higher temperatures. With the development of nanotechnology, thermoelectric modules based on thin film technology have also been studied for power generation and refrigeration [96,97]. For the past several decades, thermoelectric generators have reliably provided power in remote terrestrial and extraterrestrial locations, mostly on deep space probes such as Voyager [95,98]. Currently, a huge window of opportunity exists for thermoelectrics for low-grade waste heat recovery, such as in vehicles where vehicular thermoelectric generators can be used to improve fuel economy and reduce green house gas emissions [99]. The temperature range at which the PbTe material exhibits high ZT values is particularly appealing for power generation from waste heat sources, such as the automobile exhaust. Combined with photovoltaics, thermoelectrics can be used as a solar thermoelectric generator [100], a potential explored by Amatya and Ram [101]. The potential effect of thermoelectric materials with a ZT value of 4 on solar thermal energy conversation technologies was also discussed by Xie and Gruen [102]. 4.2
Thermoelectric coolers (heaters)
The practical uses of thermoelectric coolers are wideranging. Thermoelectric cooling devices are commonly used for cooling electronic devices [103], which often have specified cooling requirements. They are also used in other niche applications that have modest cooling demands (such as camping and portable coolers) or cases in which the energy cost is not the main consideration (such as military and aerospace applications). In electronic cooling devices, thermoelectric coolers can be used to cool the heat-producing device and maintain its normal operations. The cooling function reduces the thermal noise of the electric components and the leakage current of the
electronic devices, which can improve the efficiency of the electronic instruments. They can also be employed in microelectronics to stabilize the temperature of laser diodes, cool infrared detectors and charge-coupled devices, and reduce unwanted noise from integrated circuits [94,104,105]. In addition, the prospects for thermoelectric cooling of superconducting electronics have been reported because the superconducting transition temperature in some of the cuprate superconductors can reach levels higher than 130 K [106]. Rising interest has been devoted to thermoelectric heating, ventilation, and air conditioning for improved fuel economy, as well as reduced toxic gas and greenhouse gas emissions [4,107,108]. Available air conditioners and refrigerators have gradually become staples of daily life. The most widely used thermoelectric material for refrigeration in the temperature range of – 120 to 230°C is a pseudo-binary alloy, (Bi,Sb)2(Te,Se)3, commonly referred to as bismuth telluride. This alloy exhibits good performance around room temperature. Improving the performance of thermocouple materials remains a challenge, and overcoming this issue can lead to a breakthrough in terms of the efficiency of thermoelectric devices. As Peltier thermoelectric modules exceed a coefficient of performance of 2 at a competitive cost, thermoelectric household refrigerators will gain ever-increasing market share. Despite the numerous advantages of thermoelectric devices, their applications remain limited. One reason is that their efficiency has been excessively low for them to be economically competitive, a situation similarly encountered in the conversion of waste heat into useful electrical power. Thus, enhancing the performance of such devices as thermoelectric elements is necessary. The other reason is that the cost of traditional thermoelectric modules per watt of cooling, heat, or power generation has been excessively high to enable the replacement of existing technologies, with the exception of a few applications in which the beneficial characteristic of being solid state outweighs cost and performance limitations [13]. A lack of design knowledge and design tools is the third factor that constrains thermoelectric applications. Therefore, the focus of future research is to improve the thermoelectric performance of materials and optimize the design of thermoelectric devices.
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Conclusions
Thermoelectric materials, which can be used to convert heat into electricity or vice versa, can play an important role in providing a global sustainable energy solution and protecting the environment. A ZT value of 1 has been maintained for the past several decades, and the efficiency of energy conversion in thermoelectric systems is much lower than that in existing electric power generation systems. Along with the theory of nanostructures, a higher
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ZT value of 2 was reported for bulk thermoelectric materials, while a ZT value of 3 was obtained for lowdimensional thermoelectric materials. However, the thermoelectrics community is targeting a ZT value of 3 or higher to make these solid-state systems competitive. At present, the main problems that require resolution are overcoming low efficiency and achieving cost reduction per watt of power conversion. One of the pathways could be the improvement of the intrinsic efficiencies of currently used thermoelectric materials, or the search for new thermoelectric materials with good thermoelectric performance, as well as low cost. Another solution is to optimize the efficiencies of thermoelectric devices. As efficiencies are improved and costs are reduced, the range of thermoelectric applications is expected to expand in terms of both commercial and household applications.
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11. Acknowledgements This project was supported by the National Natural Science Foundation of China (Grant No. 50802101).
Prof. Laifeng LI, Director of the Center for Cryogenic Materials and Engineering, Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS) since 1998, holds a PhD from the Institute of Physics, CAS in 1996, and postdoctoral in the University of Michigan at Ann Arbor in 1998, and becomes a full professor in the Cryogenic Laboratory of CAS in the same year. He has authored/co-authored over 100 major research papers in the journals such as Adv. Mater., Appl. Phys. Lett., J. Appl. Phys., and Chem. of Materials in the areas of thermo electrical materials, negative expansion materials, insulation polymer, toughening ceramics and applied superconductivity, 2 books and obtained 12 patents. He is the Editor of the Chinese Journal of Materials Research and the Journal of Cryogenics.
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