Materials Design for Extreme Environment Applications Using Rare Earth Oxides and Silicates

The seventeen rare earths (Sc, Y, and the 15 lanthanides) provide an ideal materials design space for manipulation of phase, composition, and properties. The wide range of cation sizes results in oxygen coordination numbers varying from six to nine, a large number of polymorphs, (see the figures below) and many possible multicomponent rare earth compounds. This variety enables control of properties such as thermal expansion, thermal conductivity, and thermochemical stability in steam and molten siliceous deposits, all important for high temperature coating applications in extreme environments. In this presentation, control of properties through design of rare earth oxides, rare earth monosilicates, rare earth disilicates, and both rare earth apatites and calcium-stabilized rare earth apatite phases are presented with corresponding experimental validation of property control.