Athira Kattiparambil Sivaprasad

In order to optimize their system design and manufacturing processes, it is crucial to undertake a thorough electrical and thermal characterization of micro thermoelectric generators (µTEGs). To address this need, a highly advanced and fully integrated in-situ measurement system has been developed. The main objectives of this system are to (1) enable the measurement of ZT and thereby of all thermoelectric (TE) properties of thermolegs made from powder-based TE materials and (2) at the same time accurately measure the contact resistance between the TE material and the electrical contacts. The µTEG fabrication concept used in this study is based on copper-cladded printed circuit board (PCB) material as a substrate, using the Cu layers for easy contact formation. In a first step, an innovative measurement concept, based on a distinctive vertical rendition of the well-established transfer length method, has been realized, allowing for the in-situ measurement of contact resistance between the TE material and the copper conductors on the PCB substrate. This enables a comprehensive assessment of the impact exerted by the applied force and temperature during e.g. a hot-pressing step for compacting the powder-based thermolegs during the manufacturing process. In a second step, a comprehensive measurement platform, referred to as the ZT-Card, has been devised to facilitate the evaluation of all relevant TE material properties—Seebeck voltage, electrical conductivity and thermal conductivity (all measured in vertical cross-plane orientation)—inherent to a highly miniaturized thermoleg. Additionally, the ZT-Card also allows for the assessment of contact resistance between the copper contacts and the TE material. Successful testing of this measurement system inspires confidence in the capabilities of the platform and will aid in future µTEG development.

The development of fabrication procedures for micro-thermoelectric generators (µTEGs) based on low-cost fabrication technologies, appropriate for mass production is discussed and demonstrated in this study. Simulations were carried out, two manufacturing processes were tested, and device performance is compared to simulation results for vertical µTEGs. The substrate for this device is a printed circuit board, and the thermoelectric materials are self-developed Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type) pastes. A square µTEG (15 mm × 15 mm × 500 µm) with eight thermocouples (TCs) was fabricated. The characterization of the fabricated 8-TC-µTEG was carried out and a power output of 1.23 µW was obtained for the fabricated 8-TC-µTEG. The measurement results of this 8-TC-µTEG closely match simulation results as well. Moreover, a novel vertical electrical contact resistance measurement setup is designed and implemented into the final evaluation of µTEG production to have a more accurate assessment. The simulation study is also applied for the designed electrical contact resistance measurement setup. As a result, the contact resistivity of Bi₂Te₃/Cu was calculated as 5.65 × 10⁻⁴ Ωcm².