Porous Ni/ZrO₂ Cermet from Highly Concentrated Composite Colloid

Abstract

Rheology and shaping of concentrated cermet suspensions consisting of nickel (Ni) and yttria‐stabilized zirconia (YSZ) nanoparticles in water have been examined over a broad range of volumetric solids concentration (ϕ = 0.1–0.4) and Ni fraction (fNi = 0.15–0.45). Preferential adsorption of pyrogallol‐poly(ethylene glycol) polymer (i.e., Gallol‐PEG) on surface of the Ni and YSZ particles imparts steric hindrance between the suspending particles so that fluidity can be obtained under shear stress. The cermet suspensions exhibit shear‐thinning flow behavior under steady‐shear measurement over shear rates of 100–103 s−1. Yield stress and yield strain of the suspensions appear to vary pronouncedly with ϕ and fNi under oscillatory shear over a shear‐strain range of 10−1–103%. With the Gallol‐PEG adsorption, an apparent viscosity less than 6 × 10−1 Pa.s at a shear rate of 102 s−1 has been obtained for the highly concentrated composite suspension with ϕ of 0.40 and fNi of 0.25. A high solids concentration effectively prohibits phase segregation during wet‐shaping processes. Uniform green compacts have been obtained from slip casting of the concentrated cermet mixture (ϕ = 0.30) without use of binder and are then fired at 1200°C under reducing atmosphere to form porous Ni/YSZ compacts. Relative sintered density increases from 65% to 75% of the theoretical value when fNi was increased from 0.15 to 0.45, due mainly to the lower sintering temperature required for the Ni phase.