Preparation and Thermal Properties of Rare Earth Tantalates (RETaO4) High-Entropy Ceramics

doi:10.15541/jim20200426

Abstract

Abstract:

Single-phase solid solution monoclinic structure high-entropy tantalates (Nd_1/6Sm_1/6Eu_1/6Gd_1/6Dy_1/6Ho_1/6) TaO₄(6RETaO₄), (Nd_1/5Sm_1/5Eu_1/5Gd_1/5Dy_1/5)TaO₄(5RETaO₄), (Nd_1/4Sm_1/4Eu_1/4Gd_1/4)TaO₄(4RETaO₄) were prepared by solid state method. STEM-EDS result shows the rare earth elements are uniformly distributed without segregation. The ferroelastic domain observed through SEM derived from the second ferroelastic phase transition. The thermal expansion experiment suggests the good thermal stability below 1200 ℃, where the thermal expansion coefficient of 6RETaO₄ reaches 9.25×10^-6K^-1 at 1200 ℃. Due to the increase of phonon scattering derived from high-entropy effect, RETaO₄ ceramics exhibit lower intrinsic thermal conductivity (2.98-1.23 W·m^-1·K^-1, 100-1000 ℃) and enhanced mechanical properties (6RETaO₄, (9.97±2.2) GPa), which indicates that it is a potential material for thermal barrier coatings.

Key words: thermal barrier coating, rare earth tantalate, high-entropy ceramics, thermal property

CLC Number:

TQ174

ZHU Jiatong, LOU Zhihao, ZHANG Ping, ZHAO Jia, MENG Xuanyu, XU Jie, GAO Feng. Preparation and Thermal Properties of Rare Earth Tantalates (RETaO₄) High-Entropy Ceramics[J]. Journal of Inorganic Materials, 2021, 36(4): 411-417.

Figures/Tables 12

Table 1 Theoretical density, experimental density, porosity, and average grain size of high-entropy tantalates

Sample	ρ_thr/(g·cm^-3)	ρ_exp/(g·cm^-3)	Porosity/%	d_grain/μm
4RETaO₄	8.593	8.441	1.77	2.96
5RETaO₄	8.735	8.597	1.58	6.63
6RETaO₄	8.983	8.758	2.50	3.29

Fig. 1 XRD patterns of high-entropy RETaO4 ceramics after sintering at 1500 ℃ for 10 h (a) 2θ=10°-65°; (b) The main peak patterns of (ˉ121) and (121)

Table 2 Detail structure information of high-entropy RETaO4 ceramics and the average radii of doped rare-earth ions

Sample	a/nm	b/nm	c/nm	β/(°)	$r_{RE^{3+}}$/nm
4RETaO₄	0.544	1.112	0.508	95.65	10.767
5RETaO₄	0.542	1.108	0.508	95.64	10.668
6RETaO₄	0.537	1.099	0.504	95.57	10.581

Fig. 2 Raman spectra of high-entropy RETaO4 ceramics

Fig. 3 STEM-EDS of high-entropy tantalates (a) 6RETaO4; (b) 5RETaO4; (c) 4RETaO4

Fig. 4 SAED patterns of (a1) 4RETaO4, (b1) 5RETaO4 and (c1) 6RETaO4; HRTEM images of (a2) 4RETaO4, (b2) 5RETaO4 and (c2) 6RETaO4

Fig. 5 Schematic diagram of 5RETaO4 crystal structure

Fig. 6 SEM images of high-entropy tantalates (a,d) 4RETaO4; (b,e) 5RETaO4; (c,f) 6RETaO4

Table 3 HV hardness of high-entropy tantalates

Sample	HV/GPa
4RETaO₄	(7.92±2.33)
5RETaO₄	(6.15±1.60)
6RETaO₄	(9.97±2.20)

Fig. 7 Thermal expansion of high-entropy RETaO4 (a) Thermal expansion rates; (b) Thermal expansion coefficients

Fig. 8 Thermal properties of high-entropy RETaO4 (a) Thermal diffusivity; (b) Constant pressure heat capacity; (c) Thermal conductivity; (d) Reciprocal thermal conductivity; (e) Intrinsic thermal diffusivity; (f) Intrinsic thermal conductivity

Table 4 Thermal conductivity of 6RETaO4, single rare earth tantalates, and YSZ (100-900 ℃)

Material	6RETaO₄	NdTaO₄	SmTaO₄	EuTaO₄	GdTaO₄	DyTaO₄	HoTaO₄	YSZ
Thermal conductivity/(W·m^-1·K^-1)	2.78-1.30	3.41-1.75	2.70-1.45	3.23-1.26	3.94-1.28	3.45-1.80	3.35-1.47	3.02-2.38

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Preparation and Thermal Properties of Rare Earth Tantalates (RETaO₄) High-Entropy Ceramics

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