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Synthesis and thermophysical performances of complex Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides
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Synthesis and thermophysical performances of complex Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides Zhang Hongsonga,∗, Tong Yupingb, Yang Xianfengc, Sang Weiweia, Zhang Haominga, Zhao Yongtaoa a
Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou, 451191, China School of Materials Science and Engineering, North China University of Water Resources and Electric Powder, Zhengzhou, 450011, China c College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha, 410014, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal conductivity Thermal expansion coefficient Complex oxides Thermal barrier coatings
Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides were synthesized using a high-temperature solid-state fritting technique, and the thermophysical performances of these two oxides were investigated. Both Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 show a monophasic pyrochlore-type lattice. The thermal conductivity of Ca3Er3Ti7Ta2O26.5 is lower than that of Ca3Dy3Ti7Ta2O26.5. The oxides exhibit lower thermal conductivities than YSZ owing to their complex elemental compositions, large number of ions, and high oxygen vacancy concentrations. The thermal expansion coefficients of the obtained oxides are similar to that of YSZ.
1. Introduction In order to improve the overall efficiency of land-based gas turbines and aircraft engines and to realize their high-temperature applications, thermal barrier coatings (TBCs) are used to coat on their metallic parts. TBCs protect these metallic parts from the heat-flux generated at high temperatures [1,2]. TBCs are generally composed of a nickel-based superalloy-substrate, NiCrAlY bonding layer, and Y2O3 stabilized ZrO2 (YSZ) ceramic-layer [3–5]. YSZ cannot operate above 1473 K owing to its accelerated sintering and connatural phase-transformation between tetragonal zirconia and monoclinic zirconia at these temperatures [4–6]. In addition, YSZ coatings show poor corrosion resistance to molten salt contaminants generated from fuel impurities. This poor corrosion resistance causes premature spalling of YSZ coatings [5,6]. Hence, in order to overcome these limitations, various efforts have been made to develop alternative materials for TBCs [7,8]. A2B2O7 (B = Zr, Sn, Hf, Ti) oxides with the pyrochlore-type lattice are considered to be promising TBCs owing to their excellent thermophysical properties [8–17]. It has been reported that the thermal conductivities of La2Zr2O7, Nd2Zr2O7, and Sm2Zr2O7 are much lower than that of YSZ, and the thermal conductivities of these oxides can be improved by incorporating rare earth atoms [8–10]. First-principle computations have revealed that at 1273 K, pyrochlore-type Ln2Sn2O7 (Ln = Gd, Sm, Nd, La, Er and Yb) shows a thermal conductivity of 1.9–2.3 W/m. K [11,12]. (La1-xYbx)2Sn2O7 (x = 0, 0.3, and 0.7) oxides
∗
exhibit thermal conductivities and thermal expansion coefficients lower than those of 8YSZ. On the other hand, (La0.5Yb0,5)2Sn2O7 shows the lowest heat conductivity (0.85 W/m.K) and the highest expansion coefficient among the (La1-xYbx)2Sn2O7 oxides (13.5 × 10−6/K at 1223 K) [13]. La2Hf2O7 shows the lowest computed thermal conductivity among La2Zr2O7, La2Ti2O7, La2Sn2O7, and La2Ge2O7 [14]. The addition of Al2O3 can increase the thermal expansion coefficient and reduce the thermal conductivity of La2Hf2O7 [15]. The introduction of Ti4+ and Gd3+ can improve the heat radiation property of (Sm1xGdx)2(Hf1-xTix)2O7 oxides by improving their frequency and mode of lattice vibration and increasing the free carrier concentration [16]. The thermal conductivity of Y2Ti2O7 ranges from 2.25 to 2.6 W/m. K [17], while the coefficient of thermal expansion for Ln2Ti2O7 (Ln= Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Yb, and Lu) ranges from 10.1 × 10−6/K to 11.2 × 10−6/K [18]. Recently, various novel pyrochlore-type oxides have been developed as potential candidates for TBCs. For example, pyrochlore-type Y2GaSbO7 shows a heat conductivity of about 2 W/m.K at room temperature, and hence is considered as a promising candidate for TBCs [19]. At 1073 K, pyrochlore-type LaGdTaO7 and Gd3TaO7 show thermal conductivities of about 1.05 and 1.23 W (m.K)−1 [20], respectively. The substitution of Gd3+ with Al3+ causes the pyrochlore to weberite lattice-phase transformation [21]. The thermal conductivity of pyrochlore-type Ca3RE3Ti7Ta2O26.5 (RE = La, Sm, Nd, Gd) is lower than that of YSZ, while the thermal expansion coefficient is comparable to
Corresponding author. E-mail address: [email protected] (Z. Hongsong).
https://doi.org/10.1016/j.ceramint.2019.09.279 Received 14 September 2019; Received in revised form 25 September 2019; Accepted 28 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang Hongsong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.279
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that of YSZ [22,23]. However, the thermophysical performances of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 have not been investigated till date. In this study, Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides were synthesized using a high-temperature solid-state reaction method. The lattice-structures, thermal conductivities, and expansion behaviours of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 were investigated. 2. Experimental Bulk Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 were synthesized using a high-temperature solid-state sintering technique from highpurity oxides CaCO3, Dy2O3, Er2O3, TiO2, and Ta2O5. The desired amounts of these oxides were blended in an agate mortar. The grinding and blending processes were repeated thrice in order to obtain a homogeneous mixture. The blended powders were compressed into cylindrical samples, which were subsequently sintered at 1773 K for 10 h to obtain densified samples. The lattice-structures of the obtained bulk specimens were investigated using X-ray diffraction technique (XRD; Bruker D8Advance). Since the infrared (IR) active optic modes and vibrations of metal-oxygen bonds are closely related, the IR spectra for the densified samples were obtained using IR spectroscopy (Nicolet 6700, Thermo Fisher, USA). The Raman spectra of the synthesized oxides were obtained using a laser Raman spectrometer (inVia, Renishaw, England). Scanning electron microscopy (SEM, Quanta 250FEG, FEI, America) was used to examine the microstructures of bulk samples. The elemental compositions of the samples were analysed using energy dispersive spectroscopy (EDS, IE350). The densities (ρ) of the samples were measured using the Archimedes drainage method. The Neumann-Kopp principle was utilized to determine the specific heat capacities (Cp) of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 using the values for precursor oxides. The heat diffusivity (λ) of the samples from room temperature to 1473 K was gauged using a laser flash conductometer (LFA1000, Linseis, Germany), and the sample dimensions were about Ф12.7 × 1 mm. Prior to the measurements, two thin graphite films were coated on the front and back surfaces of the samples for laser heat absorption. Thus, the thermal conductivity (k’) of the samples was obtained by multiplying the density (ρ), heat diffusivity (λ), and specific heat capacity (Cp) (Eq. (1)). In order to avoid the effect of stomatal content (ϕ), the computed heat conductivity (k’) was converted into that of the completely dense samples (k) using Eq. (2). The thermal expansion behaviours of the samples were investigated using a high-temperature thermal expansion instrument (DIL 402, NETZSCH, Germany).
k ′ = λ⋅Cp⋅ρ
(1)
4 k′ =1− φ 3 k
(2)
Fig. 1. XRD patterns of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides.
Pr) shows similar ionic arrangement [27]. In Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5, Ca2+, Dy3+, and Er3+ ions occupy the A-sites, some of the Ti4+ and Ta5+ ions may also occupy the A-sites and the remaining ones occupy the B-sites. Because of the close relationship between the infrared-active optic mode and vibration of metal-oxygen bonds, Fourier transform infrared spectroscopy is considered as an efficient method to examine the lattice of complex metal oxides [28]. The infrared spectra of the Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides are shown in Fig. 2. The spectra show four distinct absorption bands between 200 and 1000 cm−1 corresponding to Dy3+ and Er3+ [25,26]. The bands near 700–730 cm−1 and 870–900 cm−1 can be attributed to the Ta-O vibration of TaO6 octahedron and Ti-O stretching vibration of TiO6 octahedron, respectively [25]. The first band at 400–440 cm−1 corresponds to the stretching vibration of the Ln-O bond, while the second band near 550–600 cm−1 corresponds to both Ln-O’ and Ca-O’ bonds [25]. These results are consistent with those reported previously [29,30]. These four absorption bands confirm that the formed oxides are pyrochlore-type compounds. This is consistent with the XRD results. The Raman spectra of the oxides are shown in Fig. 3. Pyrochloretype oxides show six Raman active bands, which are expressed as follows:
3. Results and discussion 3.1. Analysis of lattice-structure The XRD patterns of the samples and Sm2Zr2O7 are shown in Fig. 1. The bulk samples showed sharp and intense peaks. This indicates that these samples show high crystallization degrees. The XRD patterns of Ca3Dy3Ti7Ta2O26.5, Ca3Er3Ti7Ta2O26.5, and Sm2Zr2O7 are similar, indicating that the synthesized oxides show the pyrochlore-type crystallattice structure belonging to the A2B2O7 category [24]. In Ca3Ln3Ti7Nb2O26.5 (Ln=Y, Sm, Gd) oxides, Ca2+, Y3+, Sm3+, and Gd3+ are located at the A-sites owing to their relatively large ionic radii, and some Ti4+ and Nb5+ ions possibly inhabit the B-locations because of their large ionic radii [25]. However, some Ti4+ and Nb5+ ions may occupy the A positions because of their larger number than Ca2+ and rare-earth ions [25]. Pb3R3Ti7Nb2O26.5 (R= Dy, Gd, Nd, or
Fig. 2. Ft-IR spectra of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides. 2
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Fig. 3. Raman charts of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides.
Γ (Raman) = A1g + Eg + 4F2g
(3)
Normally, the A1g and Eg bands are observed at around 510 and 350 cm−1, and the other four modes are observed at ~750, ~585, ~400, and ~280 cm−1 [31,32]. The band at ~510 cm−1 in Fig. 3 corresponds to the bending vibration of the B-O bond, while the mode near 280 cm−1 corresponds to the A-O interaction and the broadening of this mode can be attributed to the increase in the concentration of oxygen vacancies [33]. The F2g mode near 400 cm−1 corresponding to 8b site oxygen overlaps with the Eg mode corresponding to the bending vibrations of B-O6 bonds [34]. This indicates that the synthesized Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides show single pyrochlore-type structure, which is consistent with the results of XRD and infrared charts. The calculated lattice parameters of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides from Fig. 1 are 10.2001 Å and 10.1474 Å, which are in the same order with those of pyrochlore-type Ln2Zr2O7 oxides [34].
Fig. 4. Typical micro-morphologies of (a) Ca3Dy3Ti7Ta2O26.5 and (b) Ca3Er3Ti7Ta2O26.5.
the oxides are comparable to that of YSZ (~9.0 × 10−6/K) [7] or La2Zr2O7 (~9.6 × 10−6/K) [10].
3.2. Micro-morphology and elemental composition
3.4. Thermal conductivity
Fig. 4 shows the surface micro-morphologies of the bulk Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides. The bulk samples show well-formed single-phase grains with a size of 3–8 μm. The oxides exhibit two crystalline forms, i.e. cuboid and round. These results are consistent with those reported for Pb3R3Ti7Nb2O26.5 (R=Dy, Gd, Nd, Pr, or Y) oxides [27]. The element compositions of the Ca3Er3Ti7Ta2O26.5 and Ca3Dy3Ti7Ta2O26.5 oxides are shown in Fig. 5. The elemental composition results of the oxides are consistent with their chemical formulae.
From Fig. 8, it can be observed that the computed specific heat capacity of the obtained products increases with an increase in temperature. Ca3Dy3Ti7Ta2O26.5 shows a higher specific heat capacity than Ca3Er3Ti7Ta2O26.5 because of the higher heat capacity of Dy2O3 than that of Er2O3. The thermal diffusivities of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 are shown in Fig. 9. The relationship between the thermal diffusivity and temperature reveals that the obtained oxides exhibit the phonon heat-conduction behaviour. Additionally, Ca3Er3Ti7Ta2O26.5 show lower thermal diffusivity than Ca3Dy3Ti7Ta2O26.5. The thermal conductivities of the fully dense pellets were calculated from the specific heat capacity, measured density and thermal diffusivity (Fig. 10). The dependence of thermal conductivity on temperature is similar to that shown in Fig. 9. Ca3Er3Ti7Ta2O26.5 display lower thermal conductivity than Ca3Dy3Ti7Ta2O26.5 at all temperatures. According to the theory of phonon heat-conduction, high atomic mass of a material can reduce its phonon average free path, which in turn reduces its lattice thermal conductivity [37]. The atomic weight of Er (167.2) is higher than that of Dy (162.5). As a result, Ca3Er3Ti7Ta2O26.5 exhibits lower thermal conductivity than Ca3Dy3Ti7Ta2O26.5. The mean values of the thermal conductivities of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 over the temperature range used in this study are 1.44 and 1.28 W/m. K, respectively. These values are much lower than that of YSZ (2.0 W/m. K) [7]. Various factors contribute to the excellent
3.3. Thermal expansion property Fig. 6 shows the thermal expansion ratio of the oxides as a function of temperature. Up to 1473 K, the thermal expansion ratio of the products presents linear dependence on temperature. This indicates that the products display excellent stability, which is beneficial for TBCs. It can be observed from Fig. 7 that the thermal expansion coefficient of the synthesized oxides increases gradually with an increase in temperature because of the increased mean space between their atoms at high temperatures. The oxides exhibit similar thermal expansion coefficients, and the average values of the thermal expansion coefficients for the Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides over the entire temperature range are 9.78 × 10−6/K and 9.68 × 10−6/K, respectively. This can be attributed to the similar ionic radii and electro-negativities of the oxides [35,36]. The thermal expansion coefficients of 3
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Fig. 5. Analytical results of element compositions (a) Ca3Er3Ti7Ta2O26.5 and (b) Ca3Dy3Ti7Ta2O26.5.
Fig. 6. Thermal expansion rates for Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides.
Fig. 8. Calculated specific heat capacities for Ca3Ln3Ti7Ta2O26.5 (Ln=Er and Dy) oxides.
Fig. 7. Thermal expansion coefficients for Ca3Ln3Ti7Ta2O26.5 (Ln=Er and Dy) oxides.
Fig. 9. Thermal diffusivities for Ca3Ln3Ti7Ta2O26.5 (Ln= Er and Dy) oxides.
4
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[3]
[4]
[5]
[6]
[7] [8] [9]
[10]
[11]
Fig. 10. Thermal conductivities for Ca3Ln3Ti7Ta2O26.5 (Ln= Er and Dy) oxides.
[12]
heat-insulation ability of the oxides prepared in this study [37,38]. First, the elemental composition of the oxides is more complex than that of YSZ. Second, the synthesized products consist of a large number of ions. Third, the oxygen vacancy concentrations of the obtained oxides are higher than that of YSZ, as evident from the broad and intense Raman peaks shown in Fig. 3.
[13] [14]
[15]
[16]
4. Conclusions
[17]
(1) Two kinds of complex oxides, Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5, were synthesized via a high- temperature solidstate sintering method. The obtained oxides show pyrochlore-type lattices and consist of cuboid and round grains. (2) Because of the similar ionic radii and electro-negativities of Dy3+ and Er3+ ions, the oxides produced in the work exhibit similar thermal expansion coefficients, which are comparable to that of YSZ. (3) The thermal conductivity of Ca3Er3Ti7Ta2O26.5 is lower than that of Ca3Dy3Ti7Ta2O26.5. These oxides show lower thermal conductivities than YSZ owing to their complex elemental compositions, large number of ions, and high oxygen vacancy concentrations. (4) The thermal conductivity and expansion coefficients of the oxides are suitable for TBC applications.
[20]
Declaration of competing interest
[24]
[18]
[19]
[21]
[22]
[23]
[25]
In this paper, the obtained Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal barrier coatings.
[26]
[27]
Acknowledgments [28]
The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), and the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001).
[29]
[30] [31]
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Synthesis and thermophysical performances of complex Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides Zhang Hongsonga,∗, Tong Yupingb, Yang Xianfengc, Sang Weiweia, Zhang Haominga, Zhao Yongtaoa a
Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou, 451191, China School of Materials Science and Engineering, North China University of Water Resources and Electric Powder, Zhengzhou, 450011, China c College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha, 410014, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal conductivity Thermal expansion coefficient Complex oxides Thermal barrier coatings
Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides were synthesized using a high-temperature solid-state fritting technique, and the thermophysical performances of these two oxides were investigated. Both Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 show a monophasic pyrochlore-type lattice. The thermal conductivity of Ca3Er3Ti7Ta2O26.5 is lower than that of Ca3Dy3Ti7Ta2O26.5. The oxides exhibit lower thermal conductivities than YSZ owing to their complex elemental compositions, large number of ions, and high oxygen vacancy concentrations. The thermal expansion coefficients of the obtained oxides are similar to that of YSZ.
1. Introduction In order to improve the overall efficiency of land-based gas turbines and aircraft engines and to realize their high-temperature applications, thermal barrier coatings (TBCs) are used to coat on their metallic parts. TBCs protect these metallic parts from the heat-flux generated at high temperatures [1,2]. TBCs are generally composed of a nickel-based superalloy-substrate, NiCrAlY bonding layer, and Y2O3 stabilized ZrO2 (YSZ) ceramic-layer [3–5]. YSZ cannot operate above 1473 K owing to its accelerated sintering and connatural phase-transformation between tetragonal zirconia and monoclinic zirconia at these temperatures [4–6]. In addition, YSZ coatings show poor corrosion resistance to molten salt contaminants generated from fuel impurities. This poor corrosion resistance causes premature spalling of YSZ coatings [5,6]. Hence, in order to overcome these limitations, various efforts have been made to develop alternative materials for TBCs [7,8]. A2B2O7 (B = Zr, Sn, Hf, Ti) oxides with the pyrochlore-type lattice are considered to be promising TBCs owing to their excellent thermophysical properties [8–17]. It has been reported that the thermal conductivities of La2Zr2O7, Nd2Zr2O7, and Sm2Zr2O7 are much lower than that of YSZ, and the thermal conductivities of these oxides can be improved by incorporating rare earth atoms [8–10]. First-principle computations have revealed that at 1273 K, pyrochlore-type Ln2Sn2O7 (Ln = Gd, Sm, Nd, La, Er and Yb) shows a thermal conductivity of 1.9–2.3 W/m. K [11,12]. (La1-xYbx)2Sn2O7 (x = 0, 0.3, and 0.7) oxides
∗
exhibit thermal conductivities and thermal expansion coefficients lower than those of 8YSZ. On the other hand, (La0.5Yb0,5)2Sn2O7 shows the lowest heat conductivity (0.85 W/m.K) and the highest expansion coefficient among the (La1-xYbx)2Sn2O7 oxides (13.5 × 10−6/K at 1223 K) [13]. La2Hf2O7 shows the lowest computed thermal conductivity among La2Zr2O7, La2Ti2O7, La2Sn2O7, and La2Ge2O7 [14]. The addition of Al2O3 can increase the thermal expansion coefficient and reduce the thermal conductivity of La2Hf2O7 [15]. The introduction of Ti4+ and Gd3+ can improve the heat radiation property of (Sm1xGdx)2(Hf1-xTix)2O7 oxides by improving their frequency and mode of lattice vibration and increasing the free carrier concentration [16]. The thermal conductivity of Y2Ti2O7 ranges from 2.25 to 2.6 W/m. K [17], while the coefficient of thermal expansion for Ln2Ti2O7 (Ln= Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Yb, and Lu) ranges from 10.1 × 10−6/K to 11.2 × 10−6/K [18]. Recently, various novel pyrochlore-type oxides have been developed as potential candidates for TBCs. For example, pyrochlore-type Y2GaSbO7 shows a heat conductivity of about 2 W/m.K at room temperature, and hence is considered as a promising candidate for TBCs [19]. At 1073 K, pyrochlore-type LaGdTaO7 and Gd3TaO7 show thermal conductivities of about 1.05 and 1.23 W (m.K)−1 [20], respectively. The substitution of Gd3+ with Al3+ causes the pyrochlore to weberite lattice-phase transformation [21]. The thermal conductivity of pyrochlore-type Ca3RE3Ti7Ta2O26.5 (RE = La, Sm, Nd, Gd) is lower than that of YSZ, while the thermal expansion coefficient is comparable to
Corresponding author. E-mail address: [email protected] (Z. Hongsong).
https://doi.org/10.1016/j.ceramint.2019.09.279 Received 14 September 2019; Received in revised form 25 September 2019; Accepted 28 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang Hongsong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.279
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that of YSZ [22,23]. However, the thermophysical performances of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 have not been investigated till date. In this study, Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides were synthesized using a high-temperature solid-state reaction method. The lattice-structures, thermal conductivities, and expansion behaviours of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 were investigated. 2. Experimental Bulk Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 were synthesized using a high-temperature solid-state sintering technique from highpurity oxides CaCO3, Dy2O3, Er2O3, TiO2, and Ta2O5. The desired amounts of these oxides were blended in an agate mortar. The grinding and blending processes were repeated thrice in order to obtain a homogeneous mixture. The blended powders were compressed into cylindrical samples, which were subsequently sintered at 1773 K for 10 h to obtain densified samples. The lattice-structures of the obtained bulk specimens were investigated using X-ray diffraction technique (XRD; Bruker D8Advance). Since the infrared (IR) active optic modes and vibrations of metal-oxygen bonds are closely related, the IR spectra for the densified samples were obtained using IR spectroscopy (Nicolet 6700, Thermo Fisher, USA). The Raman spectra of the synthesized oxides were obtained using a laser Raman spectrometer (inVia, Renishaw, England). Scanning electron microscopy (SEM, Quanta 250FEG, FEI, America) was used to examine the microstructures of bulk samples. The elemental compositions of the samples were analysed using energy dispersive spectroscopy (EDS, IE350). The densities (ρ) of the samples were measured using the Archimedes drainage method. The Neumann-Kopp principle was utilized to determine the specific heat capacities (Cp) of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 using the values for precursor oxides. The heat diffusivity (λ) of the samples from room temperature to 1473 K was gauged using a laser flash conductometer (LFA1000, Linseis, Germany), and the sample dimensions were about Ф12.7 × 1 mm. Prior to the measurements, two thin graphite films were coated on the front and back surfaces of the samples for laser heat absorption. Thus, the thermal conductivity (k’) of the samples was obtained by multiplying the density (ρ), heat diffusivity (λ), and specific heat capacity (Cp) (Eq. (1)). In order to avoid the effect of stomatal content (ϕ), the computed heat conductivity (k’) was converted into that of the completely dense samples (k) using Eq. (2). The thermal expansion behaviours of the samples were investigated using a high-temperature thermal expansion instrument (DIL 402, NETZSCH, Germany).
k ′ = λ⋅Cp⋅ρ
(1)
4 k′ =1− φ 3 k
(2)
Fig. 1. XRD patterns of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides.
Pr) shows similar ionic arrangement [27]. In Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5, Ca2+, Dy3+, and Er3+ ions occupy the A-sites, some of the Ti4+ and Ta5+ ions may also occupy the A-sites and the remaining ones occupy the B-sites. Because of the close relationship between the infrared-active optic mode and vibration of metal-oxygen bonds, Fourier transform infrared spectroscopy is considered as an efficient method to examine the lattice of complex metal oxides [28]. The infrared spectra of the Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides are shown in Fig. 2. The spectra show four distinct absorption bands between 200 and 1000 cm−1 corresponding to Dy3+ and Er3+ [25,26]. The bands near 700–730 cm−1 and 870–900 cm−1 can be attributed to the Ta-O vibration of TaO6 octahedron and Ti-O stretching vibration of TiO6 octahedron, respectively [25]. The first band at 400–440 cm−1 corresponds to the stretching vibration of the Ln-O bond, while the second band near 550–600 cm−1 corresponds to both Ln-O’ and Ca-O’ bonds [25]. These results are consistent with those reported previously [29,30]. These four absorption bands confirm that the formed oxides are pyrochlore-type compounds. This is consistent with the XRD results. The Raman spectra of the oxides are shown in Fig. 3. Pyrochloretype oxides show six Raman active bands, which are expressed as follows:
3. Results and discussion 3.1. Analysis of lattice-structure The XRD patterns of the samples and Sm2Zr2O7 are shown in Fig. 1. The bulk samples showed sharp and intense peaks. This indicates that these samples show high crystallization degrees. The XRD patterns of Ca3Dy3Ti7Ta2O26.5, Ca3Er3Ti7Ta2O26.5, and Sm2Zr2O7 are similar, indicating that the synthesized oxides show the pyrochlore-type crystallattice structure belonging to the A2B2O7 category [24]. In Ca3Ln3Ti7Nb2O26.5 (Ln=Y, Sm, Gd) oxides, Ca2+, Y3+, Sm3+, and Gd3+ are located at the A-sites owing to their relatively large ionic radii, and some Ti4+ and Nb5+ ions possibly inhabit the B-locations because of their large ionic radii [25]. However, some Ti4+ and Nb5+ ions may occupy the A positions because of their larger number than Ca2+ and rare-earth ions [25]. Pb3R3Ti7Nb2O26.5 (R= Dy, Gd, Nd, or
Fig. 2. Ft-IR spectra of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides. 2
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Fig. 3. Raman charts of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides.
Γ (Raman) = A1g + Eg + 4F2g
(3)
Normally, the A1g and Eg bands are observed at around 510 and 350 cm−1, and the other four modes are observed at ~750, ~585, ~400, and ~280 cm−1 [31,32]. The band at ~510 cm−1 in Fig. 3 corresponds to the bending vibration of the B-O bond, while the mode near 280 cm−1 corresponds to the A-O interaction and the broadening of this mode can be attributed to the increase in the concentration of oxygen vacancies [33]. The F2g mode near 400 cm−1 corresponding to 8b site oxygen overlaps with the Eg mode corresponding to the bending vibrations of B-O6 bonds [34]. This indicates that the synthesized Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides show single pyrochlore-type structure, which is consistent with the results of XRD and infrared charts. The calculated lattice parameters of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides from Fig. 1 are 10.2001 Å and 10.1474 Å, which are in the same order with those of pyrochlore-type Ln2Zr2O7 oxides [34].
Fig. 4. Typical micro-morphologies of (a) Ca3Dy3Ti7Ta2O26.5 and (b) Ca3Er3Ti7Ta2O26.5.
the oxides are comparable to that of YSZ (~9.0 × 10−6/K) [7] or La2Zr2O7 (~9.6 × 10−6/K) [10].
3.2. Micro-morphology and elemental composition
3.4. Thermal conductivity
Fig. 4 shows the surface micro-morphologies of the bulk Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides. The bulk samples show well-formed single-phase grains with a size of 3–8 μm. The oxides exhibit two crystalline forms, i.e. cuboid and round. These results are consistent with those reported for Pb3R3Ti7Nb2O26.5 (R=Dy, Gd, Nd, Pr, or Y) oxides [27]. The element compositions of the Ca3Er3Ti7Ta2O26.5 and Ca3Dy3Ti7Ta2O26.5 oxides are shown in Fig. 5. The elemental composition results of the oxides are consistent with their chemical formulae.
From Fig. 8, it can be observed that the computed specific heat capacity of the obtained products increases with an increase in temperature. Ca3Dy3Ti7Ta2O26.5 shows a higher specific heat capacity than Ca3Er3Ti7Ta2O26.5 because of the higher heat capacity of Dy2O3 than that of Er2O3. The thermal diffusivities of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 are shown in Fig. 9. The relationship between the thermal diffusivity and temperature reveals that the obtained oxides exhibit the phonon heat-conduction behaviour. Additionally, Ca3Er3Ti7Ta2O26.5 show lower thermal diffusivity than Ca3Dy3Ti7Ta2O26.5. The thermal conductivities of the fully dense pellets were calculated from the specific heat capacity, measured density and thermal diffusivity (Fig. 10). The dependence of thermal conductivity on temperature is similar to that shown in Fig. 9. Ca3Er3Ti7Ta2O26.5 display lower thermal conductivity than Ca3Dy3Ti7Ta2O26.5 at all temperatures. According to the theory of phonon heat-conduction, high atomic mass of a material can reduce its phonon average free path, which in turn reduces its lattice thermal conductivity [37]. The atomic weight of Er (167.2) is higher than that of Dy (162.5). As a result, Ca3Er3Ti7Ta2O26.5 exhibits lower thermal conductivity than Ca3Dy3Ti7Ta2O26.5. The mean values of the thermal conductivities of Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 over the temperature range used in this study are 1.44 and 1.28 W/m. K, respectively. These values are much lower than that of YSZ (2.0 W/m. K) [7]. Various factors contribute to the excellent
3.3. Thermal expansion property Fig. 6 shows the thermal expansion ratio of the oxides as a function of temperature. Up to 1473 K, the thermal expansion ratio of the products presents linear dependence on temperature. This indicates that the products display excellent stability, which is beneficial for TBCs. It can be observed from Fig. 7 that the thermal expansion coefficient of the synthesized oxides increases gradually with an increase in temperature because of the increased mean space between their atoms at high temperatures. The oxides exhibit similar thermal expansion coefficients, and the average values of the thermal expansion coefficients for the Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5 oxides over the entire temperature range are 9.78 × 10−6/K and 9.68 × 10−6/K, respectively. This can be attributed to the similar ionic radii and electro-negativities of the oxides [35,36]. The thermal expansion coefficients of 3
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Fig. 5. Analytical results of element compositions (a) Ca3Er3Ti7Ta2O26.5 and (b) Ca3Dy3Ti7Ta2O26.5.
Fig. 6. Thermal expansion rates for Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides.
Fig. 8. Calculated specific heat capacities for Ca3Ln3Ti7Ta2O26.5 (Ln=Er and Dy) oxides.
Fig. 7. Thermal expansion coefficients for Ca3Ln3Ti7Ta2O26.5 (Ln=Er and Dy) oxides.
Fig. 9. Thermal diffusivities for Ca3Ln3Ti7Ta2O26.5 (Ln= Er and Dy) oxides.
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[3]
[4]
[5]
[6]
[7] [8] [9]
[10]
[11]
Fig. 10. Thermal conductivities for Ca3Ln3Ti7Ta2O26.5 (Ln= Er and Dy) oxides.
[12]
heat-insulation ability of the oxides prepared in this study [37,38]. First, the elemental composition of the oxides is more complex than that of YSZ. Second, the synthesized products consist of a large number of ions. Third, the oxygen vacancy concentrations of the obtained oxides are higher than that of YSZ, as evident from the broad and intense Raman peaks shown in Fig. 3.
[13] [14]
[15]
[16]
4. Conclusions
[17]
(1) Two kinds of complex oxides, Ca3Dy3Ti7Ta2O26.5 and Ca3Er3Ti7Ta2O26.5, were synthesized via a high- temperature solidstate sintering method. The obtained oxides show pyrochlore-type lattices and consist of cuboid and round grains. (2) Because of the similar ionic radii and electro-negativities of Dy3+ and Er3+ ions, the oxides produced in the work exhibit similar thermal expansion coefficients, which are comparable to that of YSZ. (3) The thermal conductivity of Ca3Er3Ti7Ta2O26.5 is lower than that of Ca3Dy3Ti7Ta2O26.5. These oxides show lower thermal conductivities than YSZ owing to their complex elemental compositions, large number of ions, and high oxygen vacancy concentrations. (4) The thermal conductivity and expansion coefficients of the oxides are suitable for TBC applications.
[20]
Declaration of competing interest
[24]
[18]
[19]
[21]
[22]
[23]
[25]
In this paper, the obtained Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal barrier coatings.
[26]
[27]
Acknowledgments [28]
The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), and the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001).
[29]
[30] [31]
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