Pt-Al₂O₃ Interfacial Bonding in Implantable Hermetic Feedthroughs: Morphology and Orientation

The present work examines the chemistry, microstructure, and crystallography of a Pt-Al₂O₃ joint used in implantable hermetic feedthrough designs in neural prostheses. Pt was joined to Al₂O₃ by passing Pt pins through green Al₂O₃ disks and then sintering in air. This created a Pt-Al₂O₃ joint that was prepared for the current investigation by gross sectioning and then polishing and sectioning into slices using focused ion beam milling. The slices were examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Two types of interfaces in the sintered material were identified: Vitreous-bonded Pt-glass-Al₂O₃ and direct-bonded Pt-Al₂O₃. In the case of the former, glass formation owing to the presence of glass-forming additives (to enhance densification and suppress grain growth) and consequent wetting of both the Pt and Al₂O₃ facilitated interfacial bonding. In the case of the latter, the interfacial planes were (002)_Pt // (0 2) [rhombohedral] or (002)_Pt // (0 2 2) [hexagonal]. The lattice mismatch was calculated to be 11% (based on the calculated d spacings) or 15% (based on the literature d spacings). Both of these suggest the establishment of semicoherent interfaces.