Advanced Thermoelectric Materials in Electrical and Electronic Applications

Transcription

1 Advanced Thermoelectric Materials in Electrical and Electronic Applications Pratibha Tiwari 1, a, Nishu Gupta 2, b and K.M.Gupta 3, c 1 Assistant Professor, Department of Electrical and Electronics Engineering, Sam Higginbottom Institute of Agriculture, Technology and Sciences (Deemed University), Allahabad, INDIA 2 Visiting Faculty, Department of Electronics and Communication Engineering, Motilal Nehru National Institute of Technology, Allahabad , INDIA 3 Professor, Department of Applied Mechanics, Motilal Nehru National Institute of Technology, Allahabad , INDIA a pratibha.shiats@gmail.com, b dce.nishu@gmail.com, c kmgupta@mnnit.ac.in Keywords: Thermoelectric materials, Thermoelectric generator, Silicon germanium quantum well, Micro-thermoelectric cooler, Peltier device, Bi 2 Te 3, Telluride, Microcooler, Thermoelectric energy harvesting. Abstract: Thermoelectric materials are a novel class of materials having unique characteristics. They are Seebeck and Peltier effect materials and are used as thermocouples, for thermoelectric cooling of microelectronic products, as thermoelectric converter for energy conservation etc. Due to their versatility of behaviour, they are now used as construction materials for microelectronic equipment, wireless sensors also. This paper aims at elaborating the development of such materials by compiling the recent and ongoing researches. In this regard, the research developments of some newer materials by other investigators have been presented here. Brief details of the development of thermoelectric generation, thermoelectric cooling, thermoelectric power generator for wearable systems, nano-thermocouple, thermoelectric Peltier microcoolers etc. are presented. In these elaborations, it is shown by the respective investigators that these TE materials can be effectively used in heavily doped semiconductor, thin films, quantum well etc. Introduction Thermoelectricity is of strong scientific and technological interest due to its application possibilities ranging from clean energy to photon sensing devices. Recent developments in theoretical studies on the thermoelectric effects as well as the newly discovered thermoelectric materials provide new opportunities for wide applications. One type of these materials is based on the strongly correlated electron system; typical examples are the transition metal oxides, which were not regarded as very promising for thermoelectric applications. The thermoelectric effect is based on the anisotropic Seebeck components in crystals. Upon radiation of heat and/or light on the film surface, a voltage is induced and hence, device which can detect the heat and/or light radiation can be made. In this paper some recent progress in this field and special emphasizes on the new applications has been discussed. Conventional thermocouples based on metal wires are cheap, reliable and widely used for measuring high temperatures. This is the case of furnaces, which are widely used in the microelectronic industry. A thermocouple is a simple electric circuit, formed by two dissimilar conductors joined at both ends (i.e. the junctions). Opening the circuit by cutting one of the wires enables the measurement of a voltage, which is proportional to the difference in temperature at

2 the two junctions (the Seebeck effect). Consequently, the thermocouple can be used to generate a voltage proportional to temperature difference without the need of any external electrical bias [1]. Thermoelectric Generation and Thermoelectric Cooling The simplest thermoelectric generator consists of a thermocouple, comprising a p-type and n- type thermo element connected electrically in series and thermally in parallel, Fig. 1a. Heat is pumped into one side of the couple and rejected from the opposite side. An electrical current is produced, proportional to the temperature gradient between the hot and cold junctions. If an electric current is applied to the thermo couple as shown, heat is pumped from the cold junction to the hot junction, Fig. 1b. The cold junction will rapidly drop below ambient temperature provided heat is removed from the hot side. The temperature gradient will vary according to the magnitude of current applied. Electrical power output. (a) (b) Fig.1. Illustration of (a) thermoelectric generation (Seebeck effect), and (b) Thermoelectric cooling (Peltier effect). Thermoelectric Module A typical thermoelectric module is shown in Fig. 2. The module consists of pairs of p-type and n-type semiconductor thermo elements forming the thermocouples, which are connected electrically in series and thermally in parallel. In cooling mode, an electrical current is supplied to the module. Heat is pumped from one side to the other (peltier effect), the result is that one side of the module becomes cold. In generating mode, a temperature gradient is maintained across the module. The heat flux passing through the module is converted in to electrical power (Seebeck effect). Fig. 2. A thermoelectric module

3 Thermoelectric Power Generator for Integration in Wearable Microsystems [2] The functional integration of efficient solid-state TE devices and microelectronic circuits offers many benefits. One is the implementation of local cooling for thermal stabilization of an on-chip reference element or for the reducing leakage current in a critical component such as a photo detector. Another implementation is in TE power generation to enable operation of a lowpower circuit without external electric power source, such as a battery. The TE effect is very inefficient in most materials. The best performance is obtained in the presence of heavily doped semiconductors, such as the bismuth telluride or the silicon germanium. When using semiconductors, the most desirable situation is when the base materials are both n- and p-doped, since this allows the use of essentially the same material system for fabrication of the two TE legs between the junctions. Due to its compatibility with IC technology, polycrystalline SiGe alloys and polycrystalline Si are commonly used in thermopile applications. Their use in microcoolers has shown that their performance is very low when compared to that of tellurium compounds. Tellurium compounds (n-type bismuth telluride, Bi 2 Te 3 and p-type antimony telluride, Sb 2 Te 3 ) are well-established room temperature TE materials and are widely employed by the industry in conventional TE generators and coolers. The techniques explored for the deposition of Bi 2 Te 3 (1μm thick) thin-films are the thermal co-evaporation, the electrochemical deposition, the co-sputtering, the flash evaporation and the metal-organic chemical vapour deposition (MOCVD). Although all these approaches are in principle suitable, the co-evaporation technique is employed in this work to obtain the n-type Bi 2 Te 3 and p-type Sb 2 Te 3 thin-films. It allows to precisely control the stoichiometry of the deposited thin-film with the lowest costs. Nano-Thermocouple in Thermoelectric Energy Harvesting [3] The efficiency of thermoelectric material is often described by a dimensionless number called the figure-of-merit ZT = σs 2 T/k, where T is the absolute temperature, σ and k are the electrical and thermal conductivity, respectively, and S is the Seebeck coefficient. Material with high ZT is desired in thermoelectric generator design. In practice, however, it is difficult to increase ZT because increasing S often leads to simultaneous decreasing σ. In bulk thermoelectric materials, Bi 2 Te 3 alloys have the highest ZT about 1.0 at 300 K. ZT > 2 can be achieved by lowdimensional semiconductors such as quantum well and quantum wire. The breakthrough of ZT > 2.4 had mainly benefited from the reduced thermal conductivity. Even with these advances, applications of low-dimensional thermoelectric materials and their integration with state-of-the-art semiconductor process have not been explored as yet. For thermoelectric materials with feature size (also called the characteristic length) a < 100 nm, the quantum confinement effect and spatial confinement effect have been known to increase the figure-of-merit ZT. The former eliminates some states that the electrons can occupy, since they do not obey the boundary conditions of electronic wave function. The latter reduces the phonon relaxation rate and lowers the thermal transport properties of lowdimensional materials. In quantum wells, the electrons are confined to move in two-dimension so that the electron motion perpendicular to the potential barrier is quantized. This change of the energy band structure and electronic density-of-states (DOS) can increase the asymmetry between the hot/cold electron transport, obtain large transport energy, and increase the number of carriers in the materials, thereby achieving higher Seebeck coefficient.

4 Low-Cost Micro-Thermoelectric Coolers for Microelectronic Products [4] The development of a high-efficiency microcooler for cooling localized hot-spots is very desirable to many microelectronic products such as power amplifiers, laser diode, and microprocessors. Although conventional cooling devices such as miniature heat pipes, microheat dissipation and traditional heat sinks and fans have performed a high cooling performance; their inconveniences in packaging and refilling of the circulating fluid, IC incompatible process and large physical size have kept them from being used in cooling localized tiny hot-spots applications. Conventional macro-thermoelectric coolers (macro-tecs) were also reported in wide range of applications such as microelectronics and optoelectronics cooling, thermal stabilization and refrigeration/cryosurgery instrument. Two types of miniaturized μ-tecs are reported here [4] for small area hot-spots cooling applications. The basic operation principle of TEC devices is the Peltier effect, which can be observed at the junction of two different metals or n- and p-type semiconductors under appropriate current drive. Thermoelectric material properties of the n-type- and p-type-doped polysilicon (1.5μm thick) and electroplated telluride TE thin films (8μm thick) have been measured. Figure 3 presents the temperature distribution of the bridge-type polysilicon μ-tec under 80 ma driving current. A near linear relationship (R 2 = 99.08%) between the cooling performance of the bridgetype polysilicon μ-tec and the driving current can be obtained. This research has demonstrated that the bridge-type polysilicon μ-tec has a better cooling performance than the column-type telluride μ-tecs. Fig 3. Temperature distribution of polysilicon under driving current. Thermoelectric Water-Cooling Device for Electronic Equipment [5] With the development of microprocessors, the heat dissipation problems become more and more serious; this raises challenges in electronic cooling. Recently, thermoelectric cooler (TEC) has been applied to electronic cooling with its advantages of sensitive temperature control, quietness, reliability, and small size. A conventional TEC consists of p-type and n-type bulk semiconductor thermoelements connected electrically in series and sandwiched between two ceramic substrates. Thermoelectric cooler is regarded as a potential solution for improving the thermal performances of cooling devices on the package. Air-cooling devices will no longer satisfy the high power requirements of next generation electronic components. With high thermal capacity, liquid cooling techniques had been developed to solve the high power dissipation problem. Figure 4 shows a schematic illustration of the thermoelectric water-cooling device. It consists of a TEC, a cold plate, a heat exchanger, a fan, a water pump, and a control valve. The TEC cold side contacts the heater to absorb the heat dissipation of heater (Q c ), and the TEC hot

5 side touches the cold plate to release the heat from the TEC hot side. The rejected heat (Q h ) from the hot side conducts to the cold plate cooled by water flow. Finally, the rejected heat is carried to the heat exchanger by water flow, and is then transferred to the environment by the air flow caused by the fan. Fig.4. Schematic illustration of the thermoelectric water-cooling device. Conclusion A temperature sensor results if one of the junctions is maintained at a well-known temperature. The thermocouple can also be used as an actuator. The advantages of TE energy conversion is that moving mechanical parts are avoided which enables high system reliability, quiet operation, and it is usually environmentally friendly. It has been found that the tellurium compounds Bi 2 Te 3 and Sb 2 Te 3 having ZT > 2 are well established TE materials for futuristic uses. References [1] K.M. Gupta, Electrical and Electronic Engineering Materials, 5th ed., Umesh Publications, New Delhi 2012, ISBN: [2] Joao Paulo Carmo, Luis Miguel Goncalves, Reinoud F. Wolffenbuttel, José Higino Correia A planar thermoelectric power generator for integration in wearable microsystems Sensors and Actuators A 161 (2010) [3] S.M. Yanga, M. Cong,T. Lee Application of quantum well-like thermocouple to thermoelectric energyharvester by BiCMOS process Sensors and Actuators A 166 (2011) [4] I-Yu Huang, Jr-Ching Lin, Kun-Dian She, Ming-Chan Li, Jiann-Heng Chen, Jin-Shun Kuo Development of low-cost micro-thermoelectric coolers utilizing MEMS technology Sensors and Actuators A 148 (2008) [5] Hsiang-Shen Huang, Ying-Che Weng, Yu-Wei Chang, Sih-li Chen, Ming-Tsun Ke Thermoelectric water cooling device applied to electronic equipment International communications in heat and mass Transfer 37 (2010)