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This study aims at comparing hot pressing (HP) and the electric field-assisted sintering technique (FAST) of 8 mol% yttriastabilized zirconia (8YSZ). Three different ionic conductive nano-and submicron powders were investigated, with a median particle size of 20-30, 51-65, and 150 nm, respectively. Processing parameters were kept identical for both sintering methods: sample geometry, heating schedule, applied pressure, and atmosphere. Investigations of densification and microstructural characterization reveal that under the same conditions FAST and HP sintered samples behave similarly. The analysis of the sintering curves shows that the densification mechanism is the same for both sintering methods (grain-boundary diffusion). An increase of the heating rate up to 150 K/min does not modify the densification mechanism. The sintering trajectory reveals that grain size depends only on density and not on the sintering method and grains have no preferential orientation. Secondary ion mass spectroscopy shows that both types of specimens are contaminated with carbon only superficially. Measurements of the electrical conductivity at high temperature show no significant difference between HP and FAST samples. II. Experimental Procedure(1) FAST and HP Experiments All experiments were made with HP (model HPW 150/200-2200-100-LA) and FAST (FCT HP D 25/1) equipments were D. Clarke-contributing editor This work was financially supported by the Deutsche Forschungsgemeinschaft (Emmy Noether Program GU993-1/1).

This study aims at comparing hot pressing (HP) and the electric field-assisted sintering (FAST, also known as spark plasma sintering, SPS) of zinc oxide. Two semiconductive nano-and submicrometer powders were investigated, with a particle size of 20-30 and 90-200 nm, respectively. Processing parameters were kept as identical as possible for both sintering methods: sample geometry, heating schedule (however affected by the temperature measurement method), applied pressure, and atmosphere. FAST and HP samples densify by grain-boundary diffusion. Zinc oxide is nevertheless very sensitive to transient overheating occurring in FAST with pyrometer temperature control, leading to different densification curves. The sintering trajectory reveals that grain size also depends on the temperature history. The electrical conductivity of ZnO dramatically increases at the sintering temperature, but not enough to affect significantly the densification behavior. Ex situ conductivity measurements do not reveal any difference between FAST and HP samples.

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Field-assisted sintering technique (FAST) and spark plasma sintering (SPS) apparatuses are based on the same technical principles, but differ in design. A current flows directly through the graphite pressing tool, which acts as heating element in both machines. The main differences are the pressing tool geometry and material, current pulsing and the position of the temperature measurements. Tests under identical conditions (same heating schedule, applied load, and pressing tool, as well as similar pulse patterns) were conducted on both types of set-ups. Three different oxide materials were tested: alumina, 8 mol% yttria-stabilized zirconia (8YSZ) and zinc oxide.