Breaking the 3000 °C melting temperature barrier of oxide ceramics

Oxide scales grown on carbides or borides based on ultrahigh thermal protection materials during service play crucial roles in the safe operation of systems in extreme environments, where advancing technologies are pushing temperature limits beyond 3000 °C, exceeding the melting points of all known nonradioactive oxides. Although cationic solid solutions offer a pathway to modulate melting behavior, conventional phase diagrams show that most solid solutions exhibit lower melting points than their parent components do. The mechanisms underlying melting point elevation in oxides have remained unclear. Here, we demonstrate a cationic design strategy for ultrahigh melting point oxides based on simultaneous control of the valence electron concentration, cation size, orbital overlap, coordination number and crystallographic symmetry. Using this approach, we developed a Ta-doped HfO2 solid solution with a melting point of 3006 °C, the highest reported nonradioactive oxide, which represents an increase of nearly 200 °C over the parent oxide. This approach should be universally applicable to designing various ceramics with high or ultrahigh melting points.