A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 3 4 0 iTSSe TSS iTSSe TSS Fig. 3 — Bottom electrode (aluminum 6061-T6 substrate + pure aluminumbond coat) and diffusive medium (PEEK – alumina) of the MEET devices. Fig. 4 — Output voltage of the MEET cell when exposed to water vapor. Fig. 5 — Schematic of the absorption mechanisms in the PEEK – alumina film. electrodes, using commercially available CS apparatus and feedstock powders. Development work was performed to determine appropriate spray parameters to overcome the thermal mismatch issue between PEEK and aluminum and to reach the appropriate level of adhesion between the two materials as well as ensuring a pitting free and uniformPEEK layer, as shown in Fig. 3. As such, alumina particles were introduced in the feedstock powder. CS aluminum was then sprayed as the top electrode. Subsequently, the PEEK-based MEET device was tested and characterized, with a typical output voltage shown in Fig. 4. The rapid increase and decrease in voltage observed in the early stages is attributed to the fast absorption kinetics of voids. It results in a quick process with a sharp output voltage. But because the voids get filled with water molecules rapidly, this diffusion mechanism comes to a halt soon after its start, and the voltage drops in a short period of time. After the voids have been filled, the diffusion mechanism is slowed down as it is limited by the kinetics of diffusion in the polymer matrix. This diffusion process occurs through the amorphous regions of the PEEK and the free volume between them. Water molecules react with the polar ketone group of the PEEK molecule, resulting in the relaxation of the polymer chains and swelling. This secondary diffusion mechanism is believed to be non-Fickian. The two mechanisms are illustrated in Fig. 5. SUMMARY The performance of the MEET devices manufactured by cold spray show great promise in comparison to other designs. Despite showing lower power and current densities, the MEET device exhibits a significantly longer power generation cycle, which is an important attribute for potentially useful applications, combined with relatively low manufacturing cost that could be scaled up easily compared to chemical routes. While the suitability of PEEK as a MEET material was proven and the deposition of PEEK using cold gas dynamic spray (CGDS) was achieved, there remains additional work that needs to be undertaken to fully characterize the manufacturing of PEEK-based MEET devices via CGDS. In addition, their properties need to be determined and other materials need to be explored. ~iTSSe For more information: Bertrand Jodoin, FASM, professor, mechanical engineering, University of Ottawa, 770 King Edward Ave., Ottawa, Ontario K1N6N5, Canada, 613.562.5800 ext. 6280, bertrand.jodoin@uottawa.ca. FEATURE 8
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