Effects of rare earth oxides on microstructures and thermo-physical properties of hafnia ceramics

Rare earth oxides doped hafnia ceramics, with a formula of Hf_0.76Ln_xY_0.24-xO_1.88 (Ln = Gd, Yb, Gd + Yb or La + Yb), were prepared by solid state sintering at 1500 °C. The effects of the rare earth oxides on the microstructures, sintering resistance, and thermo-physical properties of the doped hafnia ceramics were investigated. Results show that the Gd-Y, Yb-Y or Gd-Yb-Y co-doped hafnia ceramics remain the same defect fluorite (F) structure, while the La-Yb-Y co-doped hafnia revealing coexistence of pyrochlore (P) and fluorite structures. Yb-Y co-doped samples exhibited much better sintering resistance compared with Gd-Y and Gd-Yb-Y co-doped samples. The coexistence of P and F phases is beneficial to improved sintering capability. The thermal conductivities of the Gd-Y, Yb-Y and Gd-Yb-Y doped samples are relatively lower (1.4-1.7 W m⁻¹ K⁻¹ at 1200 °C), but for the La-Yb-Y co-doped samples, the thermal conductivity increases dramatically with temperature due to increased thermal radiation at high-temperature. The average thermal expansion coefficients (TECs) of the Gd-Y, Yb-Y and Gd-Yb-Y co-doped samples are as high as ∼10.3 × 10⁻⁶ K⁻¹ in temperature range between 200−1200 °C.