Development of laser-based joining technology for the fabrication of ceramic thermoelectric modules
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Bing Feng Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Dresden 01277, Germany
Wolfgang Lippmann Institute of Power Engineering, TU Dresden, Dresden 01062, Germany
Hans-Peter Martin and Alexander Michaelis Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Dresden 01277, Germany
Antonio Hurtado Institute of Power Engineering, TU Dresden, Dresden 01062, Germany (Received 6 April 2014; accepted 1 August 2014)
The process of laser-induced brazing constitutes a potential option for connecting several ceramic components (n- and p-type ceramic bars and ceramic substrate) of a thermoelectric generator (TEG) unit. For the construction of the TEGs, TiOx and BxC were used as thermoelectric bars and AlN was used as substrate material. The required process time for joining is well below that of conventional furnace brazing processes and, furthermore, establishes the possibility of using a uniform filler system for all contacting points within the thermoelectric unit. In the work reported here, the application-specific optimization of the laser-joining process is presented as well as the adapted design of the thermoelectric modules. The properties of the produced bonding were characterized by using fatigue strength and microstructural investigations. Furthermore, the operational reliability of the modules was verified.
I. INTRODUCTION
Thermal energy can be converted into electrical current by means of the Seebeck effect. With suitable n- and p-doped materials and a temperature gradient, a voltage (i.e., an electric potential difference) can be created between the contacts on a thermoelectric device. For this, the n- and p-type thermoelectric materials must be thermally connected in parallel and electrically in series.1 Thermoelectric modules are used in a wide range of applications. One example is energy harvesting to supply ultralow-power devices.2 Recent studies have demonstrated the potential for the use of thermoelectric modules in processes in which useful electrical energy can be generated through recovered waste heat.3 Coupling of the thermoelectric materials with the heat generated by solar collectors is another application of thermoelectric modules.4 The thermoelectric materials used until now can be categorized according to their application temperatures or material groups. Group V elements (Bi, Sb) and their alloys are suitable for low temperatures, whereas bismuth tellurides (Bi2Te3) and mixed crystals with compositions a)
Address all correspondence to this author. e-mail: fl[email protected] DOI: 10.1557/jmr.2014.216 J. Mater. Res., Vol. 29, No. 16, Aug 28, 2014
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of Bi2Te3, Bi2Se3, and Sb2Te3 are the best choices at room temperature.2,5,6 These materials are already commercially available, but their application temperature is limited up to 250 °C. Lead tellurides as well as rare earth oxides and alloys of SiGe and FeSi2 are used at temperatures up to about 700 °C.7,8 Due to ecological concerns, the use of leadcontain
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