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.


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;.

PUBLIC INTEREST STATEMENT
In this paper, a novel pyrochlore-type Sm 2 AlTaO 7 oxide which can be used for thermal barrier coatings was prepared by solid state reaction method. The synthesized Sm 2 AlTaO 7 has low thermal conductivity, moderate thermal expansion coefficient, and excellent phase stability.
2010; Wang, Guo, Zhang, Gong, & Guo, 2015;Zhang, Shi, Zhao, Li, & Li, 2015). For example, the thermal conductivities of element doped Ln 2 Zr 2 O 7 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;. The thermal expansion coefficients of Ln 2 Sn 2 O 7 (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 Ln 2 Hf 2 O 7 were only studied by first principles calculations, and experimental results are still rare (Liu et al., 2010). The Ln 2 Ce 2 O 7 oxides have low thermal conductivities, and their expansion coefficients are higher than that of corresponding Ln 2 Zr 2 O 7 oxides (Zhang, Lv, Li, Zhang, & Wang, 2012;. In light of the judgmental criterion proposed by Clarke, the low thermal conductivity of the Ln 2 B 2 O 7 oxides can be mainly ascribed to the complex crystal structure, relatively high atomic weight (Clarke, 2003).
Except A 2 B 2 O 7 -type oxides, pyrochlore-type oxides and their related compounds with general formula A 2 O 7 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 Sm 2 FeTaO 7 by both solid state reaction and sol-gel routes, and found that Sm 2 FeTaO 7 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 La 2 AlTaO 7 and La 3 TaO 7 by solid reaction method using La 2 O 3 , Al 2 O 3 , and Ta 2 O 5 as raw materials, and found that the photocatalytic activity of La 2 AlTaO 7 is higher than that of La 3 TaO 7 (Li et al., 2009). Powders and films of Bi 2 LnNbO 7 (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 Pr 2 YRuO 7 and La 2 TbRuO 7 (Hinastu & Doi, 2016). Shlykhtina et al. found that fluorite-like Ho 2 ScNbO 7 was obtained at 1,773 and 1,873 K, fluorite-like Ho 2 LuNbO 7 is stable in the temperature range 1,473-1,873 K, while pyrochloretype Sm 2 ScTaO 7 exists in the range 1,473-1,673 K (Shlyakhtina et al., 2014). However, the thermophysical properties of the A 2 3+ B 1 3+ B 2 5+ O 7 oxides have not been reported in open literatures. In the current manuscript, the Sm 2 AlTaO 7 was prepared by solid state reaction, its phase structure and thermophyscial properties were investigated.

Experimental
The Sm 2 AlTaO 7 was synthesized by solid reaction method using Sm 2 O 3 (≥99.9%), Al 2 O 3 (≥99.9%) and Ta 2 O 5 (≥99.9%) as the starting reactants. Firstly, these raw oxides were fired at 1173 K for 3 h before weighting the stoichiometric constituent of Sm 2 AlTaO 7 . 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 Sm 2 AlTaO 7 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 Sm 2 AlTaO 7 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 Sm 2 AlTaO 7 was calculated by the Neumann-Kopp rule based on the heat capacities of the constituent oxides (Sm 2 O 3 , Al 2 O 3 , and Ta 2 O 5 ) (Swalin, 1972). The thermal conductivity (k) of Sm 2 AlTaO 7 was achieved in light of Equation (1), which was further revised into the actual value (k 0 ) 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 N 2 . Figure 1 lists the XRD patterns of Sm 2 AlTaO 7 together with the data of Sm 2 Zr 2 O 7 . Clearly, it can be observed that the diffraction peaks of Sm 2 AlTaO 7 match well with that of pyrochlore-type Sm 2 Zr 2 O 7 . Therefore, the Sm 2 AlTaO 7 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 Sm 2 AlTaO 7 is synthesized successfully.

Microstructure
Figure 2(a) presents the typical micro-morphology of Sm 2 AlTaO 7 . 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 Sm 2 AlTaO 7 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 Sm 2 AlTaO 7 .

Thermophysical property
As can be noted from Figure 3, the DSC curve of Sm 2 AlTaO 7 is smooth, and there are no exothermic or endothermic peaks above 773 K. This phenomenon indicates that Sm 2 AlTaO 7 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 Sm 2 AlTaO 7 . Clearly, the specific heat capacity increases with the increasing temperature up to 1,473 K, which can be fitted by the following equations.
(1) The temperature-dependence curves of thermal diffusivity and thermal conductivity of Sm 2 AlTaO 7 oxide are displayed in Figure 5. Clearly, the thermal diffusivity manifests an inverse temperaturedependence, 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  and Sm 2 Ce 2 O 7 (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 Sm 2 AlTaO 7 can be regarded as the substitutional solid solution, which are formed by the substitution of Ta 5+ cation by trivalent Sm 3+ and Al 3+ ions when trivalent cations are doped into Ta 2 O 5 (Khaw, Tan, Lee, & West, 2012;Tan, Khaw, Lee, Zainal, & Miles, 2010;Vanderah, Guzman, Nino, & Roth, 2008). The substitution of Ta 5+ cations   with Ln 3+ 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   Sm 2 AlTaO 7 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): where a 3 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 Sm 2 Ce 2 O 7 (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): 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 Sm 2 AlTaO 7 is higher than that of Sm 2 Ce 2 O 7 , which contributes to the lower thermal expansion coefficient as compared with Sm 2 Ce 2 O 7 .

Conclusions
(1) The pure Sm 2 AlTaO 7 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 Sm 2 Ce 2 O 7 , but close to that of YSZ. The moderate TEC can be ascribed to the relative higher electronegativities of Al and Ta cations.