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Design, fabrication and experimental characterization of mixed thick-/thin film thermoelectric microgenerators based on constantan/silver

Small and cost-effective thermoelectric microgenerators, based on Seebeck effect in semiconductors or metals, usually convert waste thermal energy directly into useable electrical energy. They are potential energy sources for low-power autonomous microsystems. The most commonly investigated thermoelectric microgenerators are based on metals, semimetals and various semiconductors. Thick-film technology is cheaper than thin-film one but so far the largest Seebeck coefficient for screen-printed metallic films was equal to 24 μν/Κ (Ag/Ni system [1,2]). On the other hand hybrid (thick/thin) film microgenerators fabricated and investigated at the Wrocław University of Science and Technology had larger effective Seebeck coefficient but simultaneously even much larger internal resistance and this caused their lower output power compared to thick-film metallic microgenerators [3]. This paper describes design, manufacturing and characterization of newly developed mixed thick-/thin film thermoelectric microgenerators based on magnetron sputtered constantan (copper-nickel alloy) and screen-printed silver layers. Thermoelectric microgenerator consists on sixteen thermocouples made on 27.5×34.2×0.25 mm3 alumina substrate. One of thermocouples arms was made of magnetron sputtered constantan (Cu-Ni alloy), second was Ag-based screen-printed films. The length of every thermocouple arms was equal to 27 mm whereas their width −0.3 mm. The distance between arms was equal to 0.3 mm. In the first step a pattern mask with thermocouples was designed and fabricated. Then, constantan layer was magnetron sputtered on the whole substrate and photolithography process was used to prepare the first thermocouple arms. The second arms were screen-printed on the substrate using a low temperature silver paste (Heraeus C8829A or ElectroScience Laboratories ESL 599-E). To avoid oxidation of constantan they were fired in a belt furnace in nitrogen atmosphere at 550/450 °C peak firing temperature. Thermoelectric and electrical measurements were performed using the self-made measuring system [4]. Two pyrometers included into the system were used for temperature measurement of hot and cold junctions. The estimated Seebeck coefficient was from the range 35÷41 μV/Κ whereas the total internal resistances were between 250 and 3200 ohms, depending on magnetron sputtering time and kind of silver ink (the resistance of single thermocouple was between 15.5 and 200 ohms, respectively).