The cold sintered samples were anal

The cold sintered samples were analyzed by X-ray diff raction (XRD) to verify the phase purity (Fig. 3(a)). Sharp diffraction peaks corresponding to BaTiO3 are detected in all cold sintered samples, yet with a small impurity peak of BaCO3, as magnified between 2θ angles of 22 and 26̊ in Fig. 3(a). It is reasonable to detect BaCO3, as the Ba(OH)2·8H2O flux can easily react with CO2 in the ambient environment, resulting in the formation of BaCO3. The steady decrease in intensity of the BaCO3 impurity diffraction peak is observed with increasing sintering temperatures. In the case of CSP at 150 °C, the intensity of the BaCO3 peak decreases with the sintering time until quasi-total disappearance after a 15 h dwell. Pressure solution, the main densification mechanism in CSP, is enabled by chemical reactions occurring at solid/liquid interfaces [13,14]. Considering the aforementioned mechanism and the presence of flux residues in final materials observed in previous studies [25], the presence of small amounts of residual Ba(OH)2·xH2O and its by-product BaCO3 (the limit of detection in XRD is approximately 5 % [31])should not be excluded. It is noted that no other phases such as Ba2TiO4, and Ba1.054Ti0.946O2.946, stable on the Ba-rich side of the BaO-TiO2 phase diagram [32], were detected in the Ba-rich environment used in this study. Based on the thermodynamic modeling of the Ti-Ba-H2O system [33], it is shown that the formation of BaTiO3 strongly depends on the concentrations of the dissolved solute ions, temperature, and pH. Importantly, as the pH becomes higher, the solute ions precipitate more easily at a lower temperature [34]. Therefore, mineralizers, including excess Ba(OH)2 to maintain a high pH, have been regarded as essential reagents for successful synthesis of BaTiO3 particles at low temperatures [35]. These previous findings are consistent with our XRD results, where the preferential formation of BaTiO3 is observed at low temperatures under an excess Ba-rich environment. Since a Ti source and its transport are also required for the densification of BaTiO3,soluble species such as [Ti(OH)x4–x] should be generated as part of the transient dissolution of the surface ions on the BaTiO3 particles under such a strong basic chemistry. Therefore, it implies that Ba(OH)2·xH2O is effective chemistry for both the dissolution and the precipitation mechanisms driving the sintering.