Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

Abstract

With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics. Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C. Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering. By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N=3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering. The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process.

Unveiling the mechanisms of cold sintering of ZnO at 250 °C by varying applied stress and characterizing grain boundaries by Kelvin Probe Force Microscopy

“…First, solubility of ZnO is enhanced in acidic pH conditions, so solution-precipitation stage can start at lower temperatures. This is easily observed for the onset temperature for densification (Figure 2), as well as already observed in ZnO by CSP [20]. Furthermore, the incorporation of zinc acetate promotes larger dissolution of Zn 2+ and O 2-ions into the water boundary layer, as well as more Zn 2+ ions by the dissolution of the zinc acetate.…”

“…However, Funahashi et al [20] reported recently a much lower activated energy of grain growth -43 kJ/mol -of ZnO under CSP, indicating a genuine sintering process at these low temperatures.…”

“…Slight dissolution of ZnO in water is well known, and it is required to promote this process. The dissolution process can be enhanced by acidifying the water media, as it was carried out by Funahashi et al [20] to trigger CSP using acetic acid, or in this work by using zinc acetate. Enhancement of the dissolution process by the acetate can be easily observed in the strong reduction of the onset temperature for densification ( Figure 2).…”

“…The drastically reduced temperatures enable to use simple warm hand press equipment with a heater holder, which is for sure an advantage. CSP was also demonstrated for ZnO [20], where addition of acetic acid into the aqueous solution dramatically changed the final density and microstructure. In that work, ZnO with theoretical densities higher than 90% was obtained at < 300 °C, showing equivalent electrical conductivities as for ZnO sintered at higher temperatures by conventional sintering.…”

“…Due to the low temperature and the precipitation phenomena of an amorphous phase, CSP often requires a further annealing treatment at higher temperature (~ 700 -900 °C) to crystallize the ceramic compound [19]. Nevertheless, this further thermal treatment was not required for the ZnO [20]. In the present work, we show that water plays a key role, but since the low amount of water (only 1.7 wt.%) is not enough to promote solution-precipitation as main densification mechanism, other mechanisms need to be activated.…”