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

doi:10.1016/j.jmst.2020.07.031

Abstract

Abstract:

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.

Key words: Rare earth oxides multi-doped hafnia, Fluorite, Pyrochlore, Sintering resistance, Thermo-physical properties

Chun Li, Chaolong Ren, Yue Ma, Jian He, Hongbo Guo. Effects of rare earth oxides on microstructures and thermo-physical properties of hafnia ceramics[J]. J. Mater. Sci. Technol., 2021, 72: 144-153.

Figures/Tables 17

Fig. 1. XRD patterns of (a) Gd-Y co-doped Hf0.76GdxY0.24-xO1.88, (b) Yb-Y co-doped Hf0.76YbxY0.24-xO1.88, (c) Gd-Yb-Y co-doped Hf0.76Gd0.5xYb0.5xY0.24-xO1.88 and (d) La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 ceramics.

Fig. 2. Raman spectra of La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 series.

Table 1 Phase compositions (vol.%) in the La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 series.

	Y24	La3Yb3Y18	La6Yb6Y12	La9Yb9Y6	La12Yb12
V_F	100	100	91.2	84.7	73.7
V_P	—	—	8.8	15.3	23.2
V_M	—	—	—	—	3.1

Fig. 3. SEM images of Gd-Y co-doped Hf0.76GdxY0.24-xO1.88 (left column), Yb-Y co-doped Hf0.76YbxY0.24-xO1.88 (middle column), and Gd-Yb-Y co-doped Hf0.76Gd0.5xYb0.5xY0.24-xO1.88 (right column) ceramics.

Fig. 4. SEM images of La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 ceramics.

Table 2 Chemical compositions of the A, B and C points in Fig. 4(e) by EDS (at.%).

Points	Hf	La	Yb	O
A	39.43	1.51	7.88	51.19
B	29.32	17.38	3.38	49.92
C	42.36	1.62	2.19	53.83

Fig. 5. Thermal conductivities of (a) Gd-Y co-doped Hf0.76GdxY0.24-xO1.88, (b) Yb-Y co-doped Hf0.76YbxY0.24-xO1.88, (c) Gd-Yb-Y co-doped Hf0.76Gd0.5xYb0.5xY0.24-xO1.88 and (d) La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 ceramics.

Fig. 6. Variation of linear thermal expansion of Hf0.76Ln0.06Y0.18O1.88 with temperature (Ln = Y, Gd, Yb, or Gd + Y).

Table 3 Effective ionic radius of the cations in the hafnia based solid solution [36].

Ions	Hf4+	Y³⁺	Gd³⁺	Yb³⁺	La³⁺
Radius (nm)	0.076	0.096	0.100	0.0925	0.110

Fig. 7. lattice parameters of (a) Gd-Y, Yb-Y and Gd-Yb-Y co-doped Hf0.76LnxY0.24-xO1.88 with single fluorite structure, Ln = Gd, Yb or Gd + Yb, and (b) La-Yb-Y co-doped Hf0.76LnxY0.24-xO1.88 with fluorite and pyrochlore structures, Ln = La + Yb.

Fig. 8. (a) Ion concentration in La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 ceramics and (b) element distributions in Hf0.76La0.09Yb0.09Y0.06O1.88, La9Yb9Y6.

Fig. 9. Grain size in the Gd-Y, Yb-Y and Gd-Yb-Y co-doped Hf0.76LnxY0.24-xO1.88 ceramics, Ln = Gd, Yb or Gd + Yb (a), and single Yb-Y co-doped Hf0.76YbxY0.24-xO1.88 (b).

Fig. 10. Thermal conductivities of Hf0.76Ln0.06Y0.18O1.88 ceramics, Ln = Gd, Yb or Gd + Yb.

Fig. 11. (a) Thermal radiation and (b) transmittance spectrum of La-Yb-Y co-doped Hf0.76La0.5xYb0.5xY0.24-xO1.88 ceramics with a thickness of ～135 μm.

Table 4 Cubic fitting coefficient A for the thermal radiation conductivity of the La-Yb-Y co-doped hafnia solid solution.

Parameters	Y24	La3Yb3Y18	La6Yb6Y12	La9Yb9Y6	La12Yb12
A (×10^-11 W m^-1 K^-4)	<1	8.15	61.10	36.87	25.63
R-square	—	0.900	0.998	0.969	0.977

Fig. 12. Linear thermal expansion coefficients of Hf0.76Ln0.06Y0.18O1.88, Ln = Y, Gd, Yb, or Gd + Y.

Table 5 Average linear expansion coefficients (TEC, 10-6 K-1) of Hf0.76Ln0.06Y0.18O1.88 between 200 and 1200 °C.

Y24	Gd6Y18	Yb6Y18	Gd3Yb3Y18
10.171	10.356	10.311	10.25

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