THERMOELECTRIC GENERATOR MODULES FOR SMART ENERGY-AUTONOMOUS MICRONODES
Relevant for Research Area
Prof. Dr. Peter Woias (Contact PI)
The global project goal of SEAM-TEG is the fabrication of an implantable and fully autono- mous energy source based on thermoelectric energy harvesting. The system can be implant- ed into the skull and makes use of the temperature gradient between brain surface (dura) and scalp. The concept theoretically provides a perpetual energy source, thereby greatly improving patient comfort and implant reliability. After estimating boundary conditions for implantation of micro thermoelectric generators (µTEGs) in the beginning of the project, simulations showed that a ΔT of only 2 K would be suitable. A measurement using the neuroprobe during stereotactic neurosurgery, was used to generate an approximate temperature depth profile of the human brain. Additionally, a datalogger with wireless readout and programming for long term temperature measurement of the desired temperature gradient in sheep was developed, built and tested. An implantation attempt unfortunately failed last minute due to electronic and communication issues.
For the fabrication of µTEGs a novel, highly flexible process was developed using thick laminate photoresist templates, electrochemical deposition and a mechanical pick and placer for structuring the thermoelectric elements. For this purpose, the existing deposition setup had to be completely redesigned and modified to be able to overcome the numerous challenges posed by Bi2Te3 and Sb2Te3 deposition. After the successful fabrication of a first µTEG, based on n-type Bi2Te3 and copper, the electro deposition of Sb2Te3 or Bi0,5Sb1,5Te3 as p-type material turned out to be more challenging – no reliable deposition was possible. Even though a thermal annealing process was developed to significantly increase the thermoelectric conversion efficiency of the electrodeposited materials, alternative materials like Te (and Cu) had to be investigated. This resulted in two options for a final µTEG, of which the Cu-based - thermally matched and ultra-low resistance - solution was selected for the final design. During the whole project, four different µTEG platform generations have been designed, fabricated and tested – with platform changes usually resulting in major challenges and necessary adjustments to the fabrication process itself. The final modular 4th gen. µTEG was able to supply 0.16 µW at 5 K ΔT with an internal resistance of only 0.39 Ω, which allows many of these µTEGs to be connected in series to increase the output voltage. A proposed new fabrication process based on powdered materials instead of electrodeposition could not be realized during the shortened final project period, but is under development in another project right now.
For the electronic energy management, a novel low-voltage step-up converter with high efficiency at low voltages, low start-up voltage and small size was developed. This system allows a reliable energy extraction from µTEGs under small temperature gradients in the range of a few Kelvin. Due to the shortened final project period the proposed demonstrator including the µTEG and step-up converter system integrated together with some measurement electronic into a hermetic, biocompatible housing had to be canceled.
 Rostek, R., Electrochemical Deposition as a Fabrication Method for Micro Thermoelectric Generators, Dissertation, Faculty of Engineering, University of Freiburg (2016), DOI: 10.6094/UNIFR/11127
 Roth, R., Rostek, R., Cobry, K., Kohler, C., Groh, M., Woias, P.: Design and Characterization of Micro Thermoelectric Cross-Plane Generators With Electroplated Bi2Te3, SbxTey and Reflow Soldering, J. Microelectromech. Syst. 23(4), 961–971 (2014).
 Schumacher, C., Reinsberg, K.G., Rostek, R., Akinsinde, L., Baessler, S., Zastrow, S., Rampelberg, G., Woias, P., Detavernier, C., Broekaert, José A. C., Bachmann, J., Nielsch, K., Optimizations of Pulsed Plated p and n-type Bi2Te3 -Based Ternary Compounds by Annealing in Different Ambient Atmospheres, Adv. Energy Mater. 3(1), 95–104 (2013).
 Rostek, R., Sklyarenko, V., Woias, P., Influence of Vapor Annealing on the Thermoelectric Properties of Electrodeposited Bi2Te3, J. Mater. Res. 26(15), 1785–1790 (2011).
 Pelz, U., Jaklin, J., Rostek, R., Thoma, F., Kröner, M., Woias, P., Fabrication Process for Micro Thermoelectric Generators (μTEGs), J. Electron. Mater. 45 (3), 1502–1507 (2016).
 Pelz, U., Jaklin, J., Rostek, R., Kröner, M., Woias, P., Novel Fabrication Process for Micro Thermoelectric Generators (μTEGs), J. Phys.: Conf. Ser. 660, 12084 (2015).
 Woias, P., Islam, M., Heller, S., Roth, R.: A low-voltage boost converter using a forward converter with integrated Meissner oscillator, J. Phys.: Conf. Ser. 476, 012081 (2013).
 Heller, S., Allinger, K., Pelz, U., Woias, P., Implantable Wireless Ultra-low Power Data Logger for Temperature Measurements in Animal Brains. In: MikroSystemTechnik 2017.
Congress : 23-25 October 2017, 410–413. VDE, Berlin, Germany (2018).
 Stegmeir, S., Selbstversorgte hocheffiziente Spannungswandler für sehr geringe Eingangsspannungen, Masterthesis (2017)