Thermal conductivities of certain pyrochlore oxides can be reduced by doping.
Doped pyrochlore oxides of a type described below are under consideration as alternative materials for high temperature thermal barrier coatings (TBCs). In comparison with partially yttria stabilized zirconia (YSZ), which is the state of the art TBC material now in commercial use, these doped pyrochlore oxides exhibit lower thermal conductivities, which could be exploited to obtain the following advantages:
- For a given difference in temperature between an outer coating surface and the coating/substrate interface, the coating could be thinner. Reductions in coating thicknesses could translate to reductions in weight of hot-section components of turbine engines (e.g., combustor liners, blades, and vanes) to which TBCs are typically applied.
- For a given coating thickness, the difference in temperature between the outer coating surface and the coating/substrate interface could be greater. For turbine engines, this could translate to higher operating temperatures, with consequent increases in efficiency and reductions in polluting emissions.
The materials that are sought as alternatives to YSZ are required to have and retain lower thermal conductivities and to be better able to withstand temperatures that degrade TBCs made of YSZ. The undoped versions of the doped pyrochlore oxides of the type now under consideration as alternatives to YSZ are of general composition Ma2Mb2O7, where Ma denotes a 3+ cation (for example, La to Lu) and Mb a 4+ cation (for example, Zr, Hf, Ti). Doping has been investigated as a means of reducing thermal conductivities even further below those of YSZ coatings. In the doping approach investigated thus far, another cation is substituted for part of Ma, yielding a general composition of Ma2–x MxMb2O7, where x lies between 0 and 0.5 and M denotes a rare earth or other suitable element.