Highly conductive low nickel content nano-composite dense cermets from nano-powders made via a continuous hydrothermal synthesis route

Homogenously doped and mixed yttria stabilized zirconia, YSZ (with 3 and 10 mol% Y2O3 known as 3YSZ and 10YSZ) and NiO/10YSZ co-precipitates (nominally corresponding to 7, 12, 24, 30, 35 and 45 vol.% Ni metal), were synthesized using a continuous hydrothermal flow synthesis (CHFS) system which uses a stream of superheated water at 450 °C and 24.1 MPa as a reaction medium to cause rapid precipitation of metal oxide nanoparticle co-precipitates from a mixed metal salt solution. All products were obtained directly from the outlet of the CHFS reactor as a slurry, which was then cleaned and freeze-dried prior to further processing. The highly crystalline nano-powdered products were characterized using a range of analytical methods, including: powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), Raman spectroscopy and BET surface area measurements. Spherical primary particles of 10YSZ and 3YSZ were observed under the TEM and found to be 5.0 ± 0.8 nm (range 3.2–6.3 nm) and 6.2 ± 1.4 nm (range 3.3–8.4 nm) in size, with measured BET surface areas of 160.6 and 241.7 m2 g−1, respectively. Sintering of the nano-powder co-precipitates was performed via spark plasma sintering (SPS) at 1100 °C for 1 min, leading to densities of ca. 98% for 10YSZ and ca. 96% for Ni/10YSZ cermets (all NiO was converted into Ni due to the reducing atmosphere of the SPS). The 24% Ni/10YSZ cermet was subjected to focused ion beam tomography, which allowed the 3D arrangement of the conducting Ni metallic network of the dense cermet to be elucidated, and showed that a complete 3D network of Ni existed throughout the dense cermet disk. Electrical conductivity tests showed that the samples exhibited higher than expected electrical conductivity (for such low metal content), e.g., the 24 vol.% Ni-containing sample achieved an electrical conductivity of ∼ 200 S cm−1 at the fuel cell operating temperature, which corresponds to an effective conductivity of ∼ 117 S cm−1 if a porosity of 30% were to be introduced.