Ionic conductivity of YSZ/CZO multilayers with variable lattice mismatch

Abstract

Heterostructured multilayers have been controversially reported to alter the oxygen ion conductivity of solid electrolytes by inducing interfacial mechanical strain. Here, we fabricated thin film multilayers composed of 9 mol% Y₂O₃ doped ZrO₂ (YSZ) and Ce_1−xZr_xO₂ (CZO) to systematically quantify the effects of tensile strain on the oxygen-ion conduction behavior in YSZ. A significant advantage of using CZO is that its lattice parameter can be continuously varied by adjusting the Ce/Zr atomic ratio, simplifying the strain control over the neighboring YSZ layers. Three different sets of multilayers composed of YSZ with CeO₂, or with Ce_0.70Zr_0.30O₂ (CZO30), or with Ce_0.55Zr_0.45O₂ (CZO45) were prepared on Al₂O₃ substrates with interfacial lattice mismatch of +5.2%, +3.7%, and +2.9%, respectively. When decreasing the individual layer thicknesses from 35 nm to 5 nm, all of the multilayers exhibited little change of the conductivity, with values consistently near that of bulk YSZ. X-ray diffraction results indicate that the interfacial strains were largely relaxed. Suggestions that multilayers are unable to effect ionic conductivity changes must therefore consider the difficulties in obtaining lattice mismatch-based elastic strain, even at <3% mismatch.