The dissolution of potassium sodium niobate (KNN) in aqueous media towards sustainable electroceramics sintering

K0.5Na0.5NbO3 (KNN) is as a relevant lead-free piezoelectric material due to its piezoelectric performance and a high Curie temperature. Although high-sintering temperature is required for obtaining dense KNN ceramics, it favours alkalis volatilization with deleterious effects on the physical properties. Together with the need of more environmentally-friendly technologies for ceramic manufacturing, cold sintering (CSP), a low temperature sintering process, is a promising technique. CSP of incongruently dissolving materials (with different dissolution rates of constituents) is, however, a challenge. In this work, a systematic study of the interaction of KNN particles with water under different pH conditions and temperatures is used to elucidate the nature and mechanisms of KNN dissolution. The KNN dissolution follows the trend [K+] > [Na+] >> [Nb5+]. Differences are demonstrated between acidic, neutral, and alkaline conditions, with the latter exhibiting less incongruence. Acidic pH increases solubility of KNN and promotes the incongruence of dissolution. Oppositely, alkaline pH decreases solubility of KNN and prevents dissolution incongruence. The incongruence of KNN dissolution in aqueous media is quantified. These observations have implications for our understanding of powders dissolution and offer opportunities to control the stoichiometry of the powders, that might be the key for CSP of incongruently dissolving materials.