Microstructure-property relationships in composites of 8YSZ ceramics and in situ graphitized nanocellulose

Ceramic matrix composites (CMC) of 8 mol.% yttria-stabilized zirconia (8YSZ) mixed with natural fiber nanocellulose (0.75, 1, 2 wt%) were prepared by spark plasma sintering (SPS). Nanocellulose markedly improved the densification of the 8YSZ ceramic matrix and induced significant grain size refinement. It was demonstrated that in situ graphitization of nanocellulose during the SPS processing resulted in 6 nm thin turbostratic graphite layers homogeneously covering the 8YSZ ceramic grains. The dielectric properties were analyzed by electrical impedance spectroscopy suggesting a low percolation threshold near or below ≈ 1.6 vol% graphite, above which mixed ionic-electronic conduction dominates. The CMCs are stable under reducing conditions (5%H2/Ar atmosphere) at least until 800 °C with a high conductivity of σdc = 0.17 S∙cm−1 even at 900 °C (8YSZ-2%CNF). These features make the 8YSZ-nanocellulose CMCs promising candidates for application in medium- to high-temperature electrochemical devices.