T.Y. Hill, T.L. Reitz, H. Huang
Air Force Research Laboratory,
Keywords: solid oxide fuel cell, energy conversion and storage devices, kinematics, drop formation, solid-solvent colloid solution
Summary:Energy conversion and storage devices (ECSDs) are facing the challenge of manufacturing miniature dimensions and/or unconventional shapes. Furthermore, reducing the total device thickness can significantly reduce the internal resistance and potentially improve interfacial electrochemical kinetics leading to high performances, which has been demonstrated in ultrathin solid oxide fuel cells (SOFCs). The state-of-the-art micro-SOFCs are currently fabricated using thin film deposition and micro-electro-mechanical-systems (MEMs) techniques. The latter involves a series patterning and etching processes that are rather complex, time-consuming, and cost prohibitive. Drop-on-demand (DOD) inkjet printing is a promising alternative for fabricating micro ECSDs, for its low cost, noncontact fabrication, high throughput, reproducibility, and capability of producing micrometer features through precise deposition of droplets onto a designated site. The objective of this work was to elucidate the influence of jet kinematics, inkjet process parameters and droplet properties on final drop deposition quality and resolution. By controlling droplet/substrate interactions, and judicious placement of the jetted droplets, and ink formulation, we have obtained micrometer patterns that are dense, porous, or networked, which are desired for the different components in ECSDs. For this study, a dilute solid-solvent colloidal ink suspension composed of a common SOFC cathode material, e.g. La0.6Sr0.4Fe0.8Co0.2O3 (LSFC), was printed using multiple sequential inkjet passes. The influence of the droplet kinematic properties on final drop deposition quality was studied by adjusting temperature and velocity to obtain a range of Weber number and Z numbers. The Weber number was found to be a good indicator for stable jetting. Above the Weber threshold, printing defects from splashes and satellites resulted which were detrimental to the final printing quality of micro patterns. Other critical ink jet process parameters were identified which were essential for achieving multilayer, sequential deposition accuracy and resolution when printing micro dot (0-D) and micro line (1-D) structures. These included print height, interlayer delay time, substrate smoothness, platen temperature, and, in the case of micro lines, the spacing between sequentially jetted drops. Using optimal conditions, micro 0-D dots and 1-D lines with x/y dimensions < 100 µm and z axis dimensions < 1 µm with dense, open and networked microstructures were demonstrated. In addition, the ink formulation was found to fundamentally alter the microstructure of printed patterns. Ethyl cellulose, incorporated into the ink as a polymeric dispersant and binder, resulted in a very dense ring of LSFC forming at the periphery of features but a more open structure at the center. This unique feature may be advantageous to micro-SOFCs since the variation in porosity and density could be controlled to promote more favorable electrode/electrolyte microstructures. This research demonstrated that the inkjet process has the potential to engineer micro ceramic features with tunability of thickness and density to promote greater electrical and ionic conductivity as well as gas diffusion. The knowledge gained from this research may be useful to other micro all solid ECSD systems to decrease the barriers for low-cost and high-throughput manufacturing of power generation technologies.