Hydroflux-assisted densification: applying flux crystal growth techniques to cold sintering

Hydroflux-assisted densification (HAD) is introduced as a method for low-temperature ceramic densification. HAD expands upon the cold sintering process, using an inorganic “hydroflux” secondary mass transport phase as opposed to the aqueous acidic, basic, or salt solutions that have been reported previously. Hydrofluxes combine ionic salts with sparing quantities of water to depress their melting points into the cold sintering range. The substantial solubility of many oxides in these hydrofluxes makes them appealing transport phases for an expanded cold-sinterable materials spectrum. This paper focuses on a hydroflux transport phase containing a eutectic mixture of NaOH and KOH (called “NaK”) for which particular nuances and properties are discussed. We demonstrate HAD in the ZnO system, highlighting the importance and impact of processing variables such as pressure, transport phase quantity, and water content. Additionally, we show densification of the oxide binaries Bi2O3, WO3, CuO, and MnO, and the functional ternaries Bi2WO6 and KxNa1-xNbO3 in the 200–300 °C range. This entire set is challenging to cold sinter using aqueous transport phases.