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 high fabrication and operating temperatures of these devices. In this work, we present a detailed ex situ and in situ study of 8 mol% yttria-stabilized zirconia (8YSZ) nanostructured dense thin films with a thickness of 100 nm. These films were synthesised through the sol–gel method and deposited by dip-coating on fused glass. The microstructural and crystalline evolution in the 300–800 °C range was studied by synchrotron Grazing Incidence X-ray Diffraction (GIXRD). Crystallisation of the 8YSZ films was observed to start at 343 °C with 4–5 nm crystallites consisting only of the cubic phase. With increasing temperature, this phase is maintained and the crystallite size reaches 38 nm at 800 °C. Additionally, the evolution of the lattice parameter was studied, which allowed us to determine the variation of the thermal expansion coefficient (TEC) of the films during both heating and cooling. The TEC as a function of 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 of the material under thermal cycling, relevant to the performance and stability of SOC devices. Coupled with the crystallographic characterisation, the electrical properties of the thin films were studied through conductivity measurements, obtaining conductivities about 1.5 to 5 times higher than the conductivity of 8YSZ bulk samples, with values of 0.06 S cm⁻¹ at 700 °C. Thus, the conjunction of a reduced electrolyte thickness with the enhanced conductivity of nanostructured 8YSZ makes these films attractive for intermediate-temperature SOC applications.