Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity (k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m−1 K−1. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m−1 K−1, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m−1 K−1). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.