July 1, 2020.
Today, an automobile is a necessity for everyone. For a long or short journey, people need car regard to the safety, environment and most important comfort. Many vehicles are equipped with heating, ventilating and air conditioning system. Better performance than refrigerators as only a smaller temperature difference than refrigerators is required due to vapour compression coolers have a high coefficient of performance (COP) and high cooling capacities. However, they have a noisy operation and use refrigerants with high global warming potential (GWP) such as R134a. R513a is a lower GWP alternative of R134a . Absorption coolers have moderate values of COP with the advantage of recovering waste heat. Hence, the applications of thermoelectric (TE) vary from small refrigerators and electronics package cooling to avionic instrumentation illumination control and thermal imaging cameras. Lately, a dramatic increase in the applications of TE coolers in the industry has been observed. It includes water chillers, cold plates, portable insulin coolers, portable beverage containers, etc.  In 2010, Luo et al.  have reported a novel thermoelectric air-conditioner was developed for a track cab; it is compatible with free spaces because of its special advantages, such as reliability, eco-friendship and simplicity. The coefficient of performance showed between 0.4-0.8 at ambient temperature range of 30 to 46 oC. Also, Hyeung-Sik et al.  has been developed to control the temperature of the car-seat surface: the warm temperature in the summer and cold temperature in the winter with a thermoelectric device. The reference temperature inputs to the cool side of the thermoelectric device in the controller were set to 10 oC for the cooling temperature and 50 oC for the warm temperature. However, the development of thermoelectric technology has been fast progress on the applications for thermoelectric generator and thermoelectric cooler. Now, a thermoelectric device could be developed from bulk (Figure 1) to thin film (Figure 2). Based on the TE thin film, it has been offered to convert of low-grade waste heat energy into electrical power application such as; vehicle mobility and accessories wearable electronics, biometric sensors, and autonomous robots. Moreover, the flexible thin film thermoelectric is interested extensively for using in the microscale industry and wearable electronics.
Fig. 1 (a) Diagram shows component of bulk thermoelectric  and (b) photo of one of the thermoelectric cooling applications .
Fig. 2 (a) Schematics and (b) photo of the flexible thin film thermoelectric module .
The main aim of this research project is to develope the materials and thermoelectric cooling system for vehicle air conditioning application. First, the materials we focused on are Bi2Te3 and Sb2Te3 based on p-type and n-type semiconductor, respectively. Next, both materials have been developed on a flexible substrate by multi-layer sputtering to be fabricated thermoelectric thin-film module. Finally, the thermoelectric thin film module fabrication used mask design within two models for in-plane and cross-plane of thin film to compare the performance of the cooling. However, The performance of thermoelectric material has primarily been directed toward increasing the dimensionless figure of merit () as defined; , where is the Seebeck coefficient, is the electrical resistivity, and is the total thermal conductivity, and is the absolute temperature, which term is called thermoelectric power factor (PF). Base on n-type thermoelectric as developed, bismuth telluride (Bi2Te3) alloy is the most commonly used TE materials because of its high efficiency at room temperature. Moreover, this material is also easily deposited in thin films to make the module flexible . N-type Bi2Te3 is V–VI compound semiconductor. The band gap of n-type Bi2Te3 is 0.15 eV . It is noteworthy that Bi2Te3 has been reported to have remarkable anisotropy in its transport properties. The orientation of (00l) plane in Bi2Te3 film show higher PF value than ordinary Bi2Te3 film and bulk materials . The several techniques for the preparation of polycrystalline Bi2Te3 based thin films with c-axis grain orientation, such as molecular beam epitaxy (MBE), metal–organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). However, compared with MBE and PLD, magnetron sputtering is simpler and cheaper, and hence is applicable for large-scale industrial applications. In the sputtering process, however, tellurium (Te) exhibits a lower sputtering yield than Bi ; therefore it is difficult to obtain stoichiometric Bi2Te3 thin films. Precise control of the composition is therefore particularly important in achieving good thermoelectric properties of Bi2Te3 thin films . In this work, simultaneous stoichiometric composition and highly (00l) orientation of flexible Bi2Te3 thin films were deposited under the DC magnetron sputtering parameters. Stoichiometric Bi2Te3 and highly (00l) orientation structure was obtained by sputtering conditions, preheat temperature at 350 °C, and working pressure of 1.8 × 10−3 mbar (as shown in Figure 3) This designed structure of compact layered feature with stoichiometry provides the relatively high mobility. The maximum carrier mobility of 118 cm2/V was observed for highly (00l) film. The electrical conductivity of thin film has been greatly enhanced, to a maximum of about 14.90 × 103 S/cm at 50 °C. This value is higher than those of hot-pressed n-type Bi2Te3 bulk alloys. The maximum power factor of 12.5 × 10−3 W/m.K2 was obtained at 300 °C.
Fig. 3 Illustration of the growth mechanism of highly (00l)-oriented Bi2Te3 thin films, (a) too low working pressure and (b) proper working pressure. 
The result of p-type thermoelectric thin film based on antimony telluride (Sb2Te3), it has been developed by Ag adding (Ag-Sb2Te3; AST) thin film. This material is phase change materials. The AST thin films were successfully deposited on the polyimide substrate using the DC magnetron sputtering method. The as-deposited thin films showed amorphous phase and then became to mix crystalline of Ag2Te and Sb2Te3 phases after annealing. The film annealed at 350 oC showed good TE properties with the maximum Seebeck coefficient and power factor around 186 µV/K and 10 mW/m.K2, respectively. Moreover, its TE module could generate the maximum power approximately 0.90 nW (at ΔT = 20 oC) and generator the cooling with the temperature difference approximately 5 oC as shown in Figure 4.
Fig. 4 Thermoelectricity of AST. 
One of the challenging issues regarding the design and fabrication of thin film thermoelectric cooling concerns small temperature difference across the module. Surface micromachining technique will be used to solve the problems associated with small temperature difference (ΔT) across the thin films. One possible utilization of the developing thermoelectric cooler for cars air conditioning is illustrated in Figure 5.
Fig. 5 An idea presentation for a way to utilize the thin film thermoelectric cooler for car air conditioning.
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Prof. Dr. Santi Maensiri
School of Physics, Institute of Science, Suranaree University of Technology,
Nakhon Ratchasima - 30000, Thailand