Thermomechanical properties and performance of microfabricated solid oxide fuel cell ([mu]SOFC) structures

Abstract

The mechanical properties of a ceramic electrolyte, sputtered yttria-stabilized zirconia (YSZ), in thin film (<1Clm) form were studied in order to design and fabricate thermomechanically stable microfabricated SOFCs (SOFCs) at high operation temperature. YSZ films of 70-600nm thickness were deposited at either room temperature or high temperature (500/600°C) on substrates of either silicon, or silicon nitride. The residual film stresses varied from -700 to -100MPa as-deposited, and exhibited tensile hysteresis reaching stresses of -300 to +400MPa with post-deposition annealing to 500°C. Mechanisms controlling the residual stress trends include tensile stress evolution with grain growth and compressive stresses due to "atomic peening". Young's modulus was obtained by center deflection measurement of square membranes for films in mild compression, and from bulge tests for films in tension. The modulus (24-105GPa) was found to be highly dependent on deposition conditions, and was less than half the bulk value (200GPa). Meanwhile, CTEs ( 10.5 x 10-6/oC) extracted from wafer curvature measurement during thermal cycling were independent of deposition conditions. Based on these properties, maximum in-plane stresses in the films were assessed with nonlinear plate theory, and used for uSOFC design.

(cont.) Tri-layer (Pt-YSZ/YSZ/Pt-YSZ) membranes designed in this way for operation in the post-buckling regime were fabricated with sidelengths up to 200/pm and with total thickness of 450nm. These large-area membranes buckled, but were structurally viable during repeated thermocycles to 6250C. These devices functioned and produced power of -O0.1mW/cm2 at 500°C, less than estimated (0.25W/cm2) due to lack/leakage of gases and other test set-up issues. This work experimentally verified the post-buckling design regime for functional electrolyte-supported pSOFCs. Future work includes refinement of thermomechanical property characterization, optimal design of other jSOFC systems, and controlled testing of SOFCs.