A new concept of the chemical composition design of ultra-low conducting thermal barrier coatings

The article presents a new concept of A₂B₂O₇ pyrochlore materials design dedicated to thermal barrier coating based on the simultaneous use of several structural mechanisms responsible for phonon wave scattering. The basis for these analyses is the La₂O₃-Gd₂O₃-ZrO₂ ternary system, in which a smaller Gd³⁺ gadolinium atom was introduced into the La₂Zr₂O₇ compound in the substitution position A, with the simultaneous introduction of an excess B atom (Zr⁴⁺) and its partial localisation in position A. This resulted in the cumulative effect of the difference in mass and radius of rare earth elements cations, with the simultaneous formation of a non-stoichiometric type of lattice and the rattling effect. Initially, this effect was achieved using substitution atoms with a much smaller radius, such as Tm³⁺ or Yb³⁺, but without creating non-stoichiometric structures. Such systems were also characterised by a strong tendency to decompose into pyrochlore and fluorite phases. Using a new approach, a phase-stable material with ultra-low thermal conductivity was obtained, with a two-phase structure, which, however, was not formed as a result of the compound's decomposition but is the result of the presence of excess Zr⁴⁺ and a shift in the ternary system to the two-phase region. As a result, a material with extremely low thermal conductivity and a course independent of temperature increase was obtained, which suggests the maximum level of crystal lattice defect. In addition, thermal conductivity was effectively reduced by using relatively cheap rare earth oxides, reducing their amount compared to pyrochlore compounds based on lanthanum with ytterbium or thulium.