Thermal evolution of sol–gel-synthesized 8YSZ thin films: insights from coupled in situ synchrotron diffraction and electrical conductivity measurements

Abstract

The thin film approach for Solid Oxide Cell (SOC) electrolytes offers a pathway to reduce the highfabrication and operating temperatures of these devices. In this work, we present a detailed ex situ andin situ study of 8 mol% yttria-stabilized zirconia (8YSZ) nanostructured dense thin films with a thicknessof 100 nm. These films were synthesised through the sol–gel method and deposited by dip-coating onfused glass. The microstructural and crystalline evolution in the 300–800 °C range was studied bysynchrotron Grazing Incidence X-ray Diffraction (GIXRD). Crystallisation of the 8YSZ films was observedto start at 343 °C with 4–5 nm crystallites consisting only of the cubic phase. With increasingtemperature, this phase is maintained and the crystallite size reaches 38 nm at 800 °C. Additionally, theevolution of the lattice parameter was studied, which allowed us to determine the variation of thethermal expansion coefficient (TEC) of the films during both heating and cooling. The TEC as a functionof temperature has three linear regions during heating and two during cooling, with values between 9.6× 10−7 K−1 and 3.7 × 10−5 K−1. These findings provide valuable insights into the structural response ofthe material under thermal cycling, relevant to the performance and stability of SOC devices. Coupledwith the crystallographic characterisation, the electrical properties of the thin films were studied throughconductivity measurements, obtaining conductivities about 1.5 to 5 times higher than the conductivity of8YSZ bulk samples, with values of 0.06 S cm−1 at 700 °C. Thus, the conjunction of a reduced electrolytethickness with the enhanced conductivity of nanostructured 8YSZ makes these films attractive forintermediate-temperature SOC applications.