Preparation and thermal properties of Sm₂AlTaO₇

Abstract

In this work, the Sm₂AlTaO₇ was synthesized by solid reaction method, and its phase composition, microstructure, and thermophysical properties were investigated. XRD results show that pure Sm₂AlTaO₇ with single pyrochlore-type structure is prepared successfully. The thermal conductivity of Sm₂AlTaO₇ at 1,273 K is about 1.13 W m⁻¹ K⁻¹, which is much lower than that of YSZ. The low thermal conductivity can be attributed to the phonon scattering caused by substituting atoms. Its thermal expansion coefficient is lower than that of Sm₂Ce₂O₇, but close to that of YSZ. There is no phase transformation occurred between 293 and 1,673 K. The excellent thermophysical property means that the Sm₂AlTaO₇ has potential to be used as candidate ceramic material for thermal barrier coatings.

Keywords:

• Sm₂AlTaO₇
• thermal barrier coatings
• thermal properties
• phase stability

Public Interest Statement

In this paper, a novel pyrochlore-type Sm₂AlTaO₇ oxide which can be used for thermal barrier coatings was prepared by solid state reaction method. The synthesized Sm₂AlTaO₇ has low thermal conductivity, moderate thermal expansion coefficient, and excellent phase stability.

1. Introduction

The 7–8 wt. % yttria (7–8 YSZ) thermal barrier coatings (TBCs) by electron-beam physical vapor deposition (EB-PVD) or air plasma spraying (APS) have attracted extensive interest owing to their applications in metal components of advanced gas turbines (Hardwicke & Lau, 2013; Kumar & Balasubramanian, 2016). However, the YSZ coating has limited temperature capability due to accelerated sintering and phase transformation during long-term operation above 1,473°K (Angle, Steppan, & Thompson, 2014; Limarga, Shian, & Leckie, 2014). As a result, it is very urgent to identify novel ceramic materials with remarkably lower thermal conductivity, proper thermal expansion coefficient, and excellent phase stability between room temperature and operating temperature for TBCs applications (Ren, Wan, Zhao, Yang, & Pan, 2015; Wang & Wang, 2015; Zhang et al., 2015).

During the last decades, the Ln₂B₂O₇-type (Ln = rare earth elements, B = Zr, Ce, Sn) oxides have been proposed as potential TBC candidates due to their excellent thermophysical properties (Feng, Xiao, Zhou, & Pan, 2013; Fergus, 2014; Joulia, Vardelle, & Rossignol, 2013; Liu, Wang, Li, & Zhou, 2010; Wang, Guo, Zhang, Gong, & Guo, 2015; Wang & Wang, 2015; Zhang, Shi, Zhao, Li, & Li, 2015). For example, the thermal conductivities of element doped Ln₂Zr₂O₇ lie in the range of 1.4–1.65 W m⁻¹ K, and their thermal expansion coefficients are close to that of YSZ (Fergus, 2014; Joulia et al., 2013; Ren et al., 2015; Wang & Wang, 2015; Wang et al., 2015; Zhang et al., 2015). The thermal expansion coefficients of Ln₂Sn₂O₇ (Ln = La, Nd, Sm, Gd, Er and Yb) oxides are in the same order with that of YSZ, while their thermal conductivities are greater than that of YSZ (Feng et al., 2013). The thermophyscial properties of Ln₂Hf₂O₇ were only studied by first principles calculations, and experimental results are still rare (Liu et al., 2010). The Ln₂Ce₂O₇ oxides have low thermal conductivities, and their expansion coefficients are higher than that of corresponding Ln₂Zr₂O₇ oxides (Zhang, Lv, Li, Zhang, & Wang, 2012; Zhang et al., 2015). In light of the judgmental criterion proposed by Clarke, the low thermal conductivity of the Ln₂B₂O₇ oxides can be mainly ascribed to the complex crystal structure, relatively high atomic weight (Clarke, 2003).

Except A₂B₂O₇-type oxides, pyrochlore-type oxides and their related compounds with general formula A₂³⁺B₁³⁺B₂⁵⁺O₇ are an important class of materials that possess excellent physicalchemical properties and technological applications (Hinastu & Doi, 2016; Leticia et al., 2012; Li, Chen, Zhang, & Li, 2009; Shlyakhtina et al., 2014; Teixeira, Otubo, Gouveia, & Alves, 2010). For example, Leticia et al. reported the synthesis of Sm₂FeTaO₇ by both solid state reaction and sol–gel routes, and found that Sm₂FeTaO₇ prepared by sol–gel was obtained with lower particle sizes than the solid state produced compound (Leticia et al., 2012). Xuan et al. has synthesized the La₂AlTaO₇ and La₃TaO₇ by solid reaction method using La₂O₃, Al₂O₃, and Ta₂O₅ as raw materials, and found that the photocatalytic activity of La₂AlTaO₇ is higher than that of La₃TaO₇ (Li et al., 2009). Powders and films of Bi₂LnNbO₇ (Ln = In and Al) with pyrochlore-type structures obtained by metal-organic decomposition process are uniform, with nanometric dimensions and high UV–vis transmittance (Teixeira et al., 2010). Hinastu et al. studied the magnetic susceptibility and specific heat of Pr₂YRuO₇ and La₂TbRuO₇ (Hinastu & Doi, 2016). Shlykhtina et al. found that fluorite-like Ho₂ScNbO₇ was obtained at 1,773 and 1,873 K, fluorite-like Ho₂LuNbO₇ is stable in the temperature range 1,473–1,873 K, while pyrochlore-type Sm₂ScTaO₇ exists in the range 1,473–1,673 K (Shlyakhtina et al., 2014). However, the thermophysical properties of the A₂³⁺B₁³⁺B₂⁵⁺O₇ oxides have not been reported in open literatures. In the current manuscript, the Sm₂AlTaO₇ was prepared by solid state reaction, its phase structure and thermophyscial properties were investigated.

2. Experimental

The Sm₂AlTaO₇ was synthesized by solid reaction method using Sm₂O₃ (≥99.9%), Al₂O₃ (≥99.9%) and Ta₂O₅ (≥99.9%) as the starting reactants. Firstly, these raw oxides were fired at 1173 K for 3 h before weighting the stoichiometric constituent of Sm₂AlTaO₇. Secondly, these oxides were mixed by ball milling for 6 h and dried at 293 K overnight, and the obtained mixing powders were pressed into disk pellets at 100 PMa pressure. Thirdly, the sintering process of Sm₂AlTaO₇ pellets was repeated three times at 1,673 K for 10 h. Finally, the pressed pellets were calcined and reacted at 1,873 K for 30 h to achieve corresponding bulk samples.

The phase structure of bulk samples was characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer). The actual bulk density (ρ) was measured by Archimedes drainage method, and the microstructure of the bulk samples was observed via scanning electron microscope (SEM, FEI Quanta-250). In the range of 293–1,273 K, the thermal diffusivity (λ) testing of Sm₂AlTaO₇ was carried out using a laser flash apparatus (Linseis LFA1000, Germany) in an argon atmosphere. The sample size for thermal diffusivity measurement is about Φ 12.7 × 2 mm, and the front and back surfaces of the sample were coated with a graphic thin layer in order to prevent the laser penetrating through the translucent sample at high temperatures. The specific heat capacity (C_p) of Sm₂AlTaO₇ was calculated by the Neumann–Kopp rule based on the heat capacities of the constituent oxides (Sm₂O₃, Al₂O₃, and Ta₂O₅) (Swalin, 1972). The thermal conductivity (k) of Sm₂AlTaO₇ was achieved in light of Equation (1), which was further revised into the actual value (k₀) using Equation (2) in order to remove the influence of porosity (φ). Its thermal expansion coefficient (TEC) between ambient and 1,273 K was determined with a high temperature dilatometer (Netzsch DIL402C/7, Germany). The phase stability was investigated using differential scanning calorimetry (DSC, Netzsch STA449-F3, Germany) from ambient temperature to 1,673 K under high pure N₂.(1) $$k=C_p·\mathit{\rho }·\mathit{\lambda }$$(1) (2) $$\frac{k}{k_0}=1-(\frac{4}{3})\mathit{\varphi }$$(2)

3. Results and discussion

3.1. XRD

Figure 1 lists the XRD patterns of Sm₂AlTaO₇ together with the data of Sm₂Zr₂O₇. Clearly, it can be observed that the diffraction peaks of Sm₂AlTaO₇ match well with that of pyrochlore-type Sm₂Zr₂O₇. Therefore, the Sm₂AlTaO₇ shows a typical pyrochlore-type structure, which is also characterized by the presence of typical superstructure peaks at 2θ values of about 29.697 (311), 31.33 (222), 40.091 (331), and 46.658 (511). It also can be seen from Figure 1 that no other peaks can be found in the product, which means that pure pyrochlore-type Sm₂AlTaO₇ is synthesized successfully.

Figure 1. XRD patter of Sm₂AlTaO₇ oxide.

3.2. Microstructure

Figure 2(a) presents the typical micro-morphology of Sm₂AlTaO₇. SEM observation shows that its average grain size is several micrometers. The microstructure is very dense, only a few apparent micropores are detected. The grain boundaries of the synthesized oxide are very clean, no other phases or un-reacted oxides exist in the interfaces. Its actual density is 5.28 g cm⁻³, and the relative density is about 94.5%, which illustrates that the sintered sample can be used for further examination of thermophysical properties. The element compositions analyzed by EDS are displayed in Figure 2(b) and Table 1. Obviously, the Sm₂AlTaO₇ is composed of Sm–Al–Ta–O, and the atomic ratio value of n_Sm: n_Al: n_Ta is almost 2:1:1, which is consistent with the theoretical composition of Sm₂AlTaO₇.

Figure 2. (a) Microstructure of as-prepared Sm₂AlTaO₇ oxide; (b) EDS pattern of Sm₂AlTaO₇.

Table 1. Element atomic ratio of Sm₂AlTaO₇

Element atomic	Sm	Al	Ta	O
Sm2AlTaO7	22.06	11.45	12.94	53.55

3.3. Thermophysical property

As can be noted from Figure 3, the DSC curve of Sm₂AlTaO₇ is smooth, and there are no exothermic or endothermic peaks above 773 K. This phenomenon indicates that Sm₂AlTaO₇ displays excellent phase stability in the range of 773–1,673 K. The small peak at about 500°C may be ascribed to the vibration of the testing instrument. Figure 4 shows the temperature dependence of the specific heat capacity of Sm₂AlTaO₇. Clearly, the specific heat capacity increases with the increasing temperature up to 1,473 K, which can be fitted by the following equations.(3) $$C_P({\text{Sm}}_2{\text{AlTaO}}_7)=0.41356+0.00008\times T-7076.27528/T^2$$(3)

Figure 3. DSC curve of Sm₂AlTaO₇ oxide.

Figure 4. Specific heat capacity of Sm₂AlTaO₇ oxide.

The temperature-dependence curves of thermal diffusivity and thermal conductivity of Sm₂AlTaO₇ oxide are displayed in Figure 5. Clearly, the thermal diffusivity manifests an inverse temperature-dependence, and the T⁻¹-dependence thermal diffusivity suggests an obvious phonon conduction behavior. With the increasing temperature, its thermal conductivity decreases from 1.95 to 1.13 W m⁻¹ K⁻¹ in the range of 473–1,273 K. Its thermal conductivity is much lower than that of YSZ (Zhang et al., 2009) and Sm₂Ce₂O₇ (Chen, Yang, Zhang, Li, & Li, 2014). According to the phonon thermal conductivity theory, the lattice thermal conductivity of the electric-insulation ceramics can be reduced by the phonon scattering resulted by the lattice point defect, such as substituting atoms, vacancies, and dislocations (Klemens, 2002; Wachsman, 2004). The Sm₂AlTaO₇ can be regarded as the substitutional solid solution, which are formed by the substitution of Ta⁵⁺ cation by trivalent Sm³⁺ and Al³⁺ ions when trivalent cations are doped into Ta₂O₅ (Khaw, Tan, Lee, & West, 2012; Tan, Khaw, Lee, Zainal, & Miles, 2010; Vanderah, Guzman, Nino, & Roth, 2008). The substitution of Ta⁵⁺ cations with Ln³⁺ cations (Ln = Sm and Al) is accompanied by the oxygen vacancies, to maintain the local electro-neutrality of the lattice. The defect chemistry due to co-doping can be expressed using the Kröger–Vink notation by the following equation, ${\text{Ln}}_{Ta}^{″}$ represents an Ln³⁺ cation that occupies a Ta⁵⁺ cation site (divalent negative charge), $V_o^{..}$ is a doubly charged oxygen vacancy, and $O_o^x$ is an O^2- anion on an oxygen site (neutral charge). Ln in Equation (4) can be either Sm and Al. Clearly, the concentration of oxygen vacancies in(4) $${\text{Ln}}_2{\text{O}}_3\stackrel{{\text{Ta}}_2{\text{O}}_5}{⟶}2{\text{Ln}}_{\text{Ta}}^{″}+2V_o^{..}+3{\text{O}}_o^x$$(4)

Figure 5. Thermal diffusivity and thermal conductivity of Sm₂AlTaO₇ oxide.

Sm₂AlTaO₇ is significantly greater than that of 8YSZ. Therefore, the synthesized ceramics has lower thermal conductivity as compared to 8YSZ due to the scattering of the phonons by the oxygen vacancies.

According to the phonon thermal conduction theory, the thermal conductivity is proportional to the phonon mean free path, and the influence of substituting atoms on phonon free path (l_p) can be expressed by Equation (1) (Chen et al., 2014):(5) $$\frac{1}{l_p}=\frac{a^3}{4\mathit{\pi }{v}^4}{\mathit{\omega }}^{4}c{(\frac{\mathrm{\Delta }M}{M})}^2$$(5)

where, a³ is the volume per atom, v the transverse wave speed, ω the phonon frequency, c the concentration per atom, M the average mass of the host atom, ΔM the differences of masses between the substituted and substituting atoms, respectively. The atomic weights of samarium, aluminum, and tantalum are 150.9, 26.9, and 180.9, which means the atomic mass difference between A site and B site is very great. The effective phonon scattering by atomic mass difference contributes the low thermal conductivity. In addition, the low thermal conductivity can also be attributed to its high level of local disorder in crystal structure and bonding character, which can increase a harmonic interaction among phonons effectively (Klemens, 2002).

Its thermal expansion coefficient is presented in Figure 6. Its TEC increases smoothly with the temperature reaching 1,473 K. Its TEC at 1,200°C is about 10.1 × 10⁻⁶/K, which is obviously lower than that of Sm₂Ce₂O₇ (11.09 × 10⁻⁶/K) (Joulia et al., 2013), but close to that of YSZ (10.6 × 10⁻⁶/K). According to the thermal expansion theory, a low strength of ionic bond often leads to a great ionic distance, that is, a high TEC can be obtained. The strength of ionic bond is given as Equation (2) (Wachsman, 2004):(2) $$I_{A-B}=1-e^{-\frac{{(x_A-x_B)}^2}{4}}$$(2)

Figure 6. Thermal expansion coefficient of Sm₂AlTaO₇ ceramic.

Where I_A-B is the intensity of ionic bond between ions at A site and B site, x_A and x_B are the average electro-negativity of ions at A site and B site. Therefore, a low electronegativity difference between cations at site A and B contributes to the TEC reduction. Electronegativities of Sm, Al, Ta, and Ce are 1.17, 1.61, 1.5, and 1.12. Thus, the electronegativity difference between cations at A site for Sm₂AlTaO₇ is higher than that of Sm₂Ce₂O₇, which contributes to the lower thermal expansion coefficient as compared with Sm₂Ce₂O₇.

4. Conclusions

(1)	The pure Sm2AlTaO7 with single pyrochlore structure was prepared, this oxide is thermally stable in the temperature range of 293–1,673 K.
(2)	Its thermal conductivity is 1.13 W m−1 K−1 at 1,273 K, which is much lower than that of YSZ. The lower thermal conductivity can be attributed to the phonon scattering resulted by substituted atoms and local lattice distortion.
(3)	Its average TEC is moderate, which is lower than that of Sm2Ce2O7, but close to that of YSZ. The moderate TEC can be ascribed to the relative higher electronegativities of Al and Ta cations.
(4)	The synthesized Sm2AlTaO7 has a promising future for TBCs.

Funding

The authors would like to thank the financial support from the Projects National Nature Foundation of China [grant number U1304512]; Program for science &Technology Innovation Talents in Universities of Henan Province [grant number 13HASTIT018]; the Scientific and Technological Projects of Henan Province [grant number 132102210142] and the Postdoctoral Research Sponsorship in Henan Province [grant number 2014069].

Additional information

Notes on contributors

Yu Haipeng

Dr Zhang Hongsong, professor is in department of mechanical engineering at Henan Engineering Institute in China. He received his PhD degree in Materials Science at School of Materials Science and Engineering, Beijing Institute of Technology (2007). His current research interest focuses on advanced ceramic materials for thermal barrier coatings, novel ceramic photo-catalysts. The research reported in this paper relates to understanding of a novel ceramic candidate for thermal barrier coatings.