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Preparation, thermophysical performances of Ca3Ln3Ti7Ta2O26.5 (Ln=Yb and Y) oxides for thermal barrier coating applications
Ceramics International xxx (xxxx) xxx–xxx
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Preparation, thermophysical performances of Ca3Ln3Ti7Ta2O26.5 (Ln=Yb and Y) oxides for thermal barrier coating applications Zhang Hongsonga,∗, Yang Shusenb, Tong Yupingc, Yang Xianfenga,d, Sang Weiweia, Zhao Yongtaoa a
Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou, 451191, China Rail Transit Security Department of Railway Police College, China c School of Materials Science and Engineering, North China University of Water Sources and Electric Powder, Zhengzhou, 450011, China d College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha, 410014, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare earth titanate Thermal-physical performances Pyrochlore complex oxides
In this manuscript, the Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides were prepared by employing a high-temperature solid-phase reaction method at 1773 K for 10 h. The phase compositions and thermal-physical performances were studied. The results show that the synthesized oxides are of a single pyrochlore-type lattice, the thermal conductivity of Ca3Y3Ti7Ta2O26.5 is larger than that of Ca3Yb3Ti7Ta2O26.5, and the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5. The thermal-physical properties match well with the requirements for thermal barrier coatings.
1. Introduction It is well known that thermal barrier coating is one of key technologies in advanced airplane engines. In particular, thermal barrier coatings can be utilized to reduce the surface temperature of key hotend components and lengthen the operating life under complex environments [1–3]. Presently, Y2O3-stabilized ZrO2 (YSZ) is still the most successful ceramic material for thermal barrier coatings [2–5]. However, YSZ coatings cannot be operated for any appreciable length of time above 1473 K due to its imminent phase transformation, relatively poor sintering resistance, and corrosion from molten silicate deposits [6–8]. These disadvantageous reasons can bring about a reduction of heat-insulation and formation of cracks, which quicken the peeling of thermal barrier coatings. Therefore, the search for novel candidate ceramics for thermal barrier coatings is of great importance. Over the last 17 years, the A2B2O7 (B]Zr, Ti, Sn, Hf) oxides with pyrochlore-type lattice have been suggested as prospective novel surface-layer materials [9–17]. For instance, the doped La2Zr2O7 and Sm2Zr2O7 exhibit much higher thermal insulation than YSZ, and the coefficients of thermal expansion are of the same order as YSZ [9–11]. After irradiation with fast heavy-ions for Y2Ti2O7 and Gd2Ti2O7, a pyrochlore-amorphous phase transformation can be experienced, and the thermal conductivity is decreased [12]. For Y2Ti2O7, the phase transformation from pyrochlore to disordered fluorite is caused by
∗
substitution of Hf4+ for Ti4+ [13]. The computed minimum thermal conductivity of La2Sn2O7 is approximately 1.24 W/m. K [14], and the addition of Yb2O3 in (La1-xYbx)2Sn2O7 oxides can improve the heatinsulation ability [15]. La2Hf2O7 has lower theoretical mechanical and thermal performances than those of other pyrochlores, and the thermal conductivity for RE2Hf2O7 (RE = Nd, Ce, Pr, La, Pm and Sm) varies from 1.38 W/m. K to 1.62 W/m. K [16], while the experimental thermal conductivity ranges between 4 and 7 W/m. K [17]. Recently, several novel oxides with the pyrochlore lattice were regarded as potential candidates for high-temperature applications of thermal barrier coatings [18–21]. For instance, the room-temperature thermal conductivity of Y2GaSbO7 is approximately 2 W/m. K [18]. The thermal insulation performances of Gd3TaO7 and LaGd2TaO7 are greater than that of YSZ, which simultaneously exhibit favourable lattice stability up to 1473 K [19]. The coefficients of the thermal expansion of Ca3RE3Ti7Ta2O26.5 (RE = La, Sm, Nd, Gd) oxides are in the same order with that of YSZ, while the heat-insulation abilities are much higher than that of YSZ [20,21]. However, investigation about the thermal-physical properties of Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) has not been reported in the open literatures. To evaluate the potentials of Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides for thermal barrier coatings, these two types of oxides were prepared, and the phase-structures, the thermal expansion properties and heat-insulation abilities were studied.
Corresponding author. E-mail address: [email protected] (Z. Hongsong).
https://doi.org/10.1016/j.ceramint.2019.11.136 Received 20 September 2019; Received in revised form 13 October 2019; Accepted 15 November 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.2019.11.136
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2. Experimental In order to prepare Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides, the high-purity oxides including CaCO3, Y2O3, Yb2O3, TiO2 and Ta2O5, were selected as raw chemicals. The selected raw chemicals were weighed and blended fully in an agate mortar. For obtaining a homogenous mixture, acetone was added, and the admixture was subsequently dried at 373 K. The final mixed powders were compressed into cylindrical samples of a thickness of 2 mm and 17 mm in diameter, and the quadrangular green samples were also compressed. The compressed green samples were sintered at 1773 K for 10 h to obtain the bulk samples. The X-ray diffraction technique (D8-Advance, Bruker, Germany) was employed to identify the phase lattice of the bulk samples. The infrared and Raman spectra were also gathered via an infrared spectroscopy (Nicolet 6700, Thermo Fisher, USA) and a laser Raman spectrometer (inVia, Renishaw, England), respectively. The micro-morphology of the bulk samples was observed using a scanning electron microscope (Quanta 250FEG, FEI, America), and the energy dispersive spectroscopy (EDS, IE350) was employed to analyse the element distribution. The actual density (ρ) was measured via the Archimedes drainage technology in distilled water. Based on the specific heat capacities (Cp) of the selected raw oxides, the corresponding specific heat values for bulk-specimens were predicted via the Neumann-Kopp rule [22]. Thermomechanical analysis (DIL 402, NETZSCH, Germany) was adopted to study the thermal expansion behaviour between room temperature and 1273 K. Every quadrangular sample was machined into the size of 4 mm × 4 mm × 14 mm to suit the holder, and the measuring process was performed under flowing argon gas. To inspect the heat-insulating ability, the cylindrical sample of appropriate dimension (Φ10 × 1 mm) was coated with a thin graphite film on the front and back surfaces, respectively. The measurement of thermal diffusivity (λ) from room temperature to 1473 K was carried out by using a laser-flash instrument (LFA1000, Linseis, Germany), and three individual tests were repeated at identical temperature points. Thus, the thermal conductivity (k’) can be derived from the multiplication of density (ρ), specific capacity (Cp) and thermal diffusivity (λ) according to Eq. (1). The obtained values of (k’) were further converted into those of fully dense samples (k) to remove the effect of porosity (ϕ) on the heat-insulation performance.
k ′ = λ⋅Cp⋅ρ
(1)
k′ 4 =1− ϕ k 3
(2)
Fig. 1. XRD patterns of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides.
that some Ti4+ and Nb5+ might occupy the positions of A sites due to the higher totality of Nb5+ and Ti4+ ions compared to Ca2+ and Ln3+ ions [23]. For Pb3R3Ti7Nb2O26.5 (R = Dy, Gd, Nd, Pr or Y) and Ca3Ce3xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Y, Sm or Gd), a similar reasoning can also be found [25,26]. Therefore, the Ca3+, Y3+ and Yb3+ ions may occupy the A sites in Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides and one part of Ta5+ and Ti4+ ions occupy the B sites, while the other part Ta5+ and Ti4+ ions maybe occupy the A sites. The lattice constants calculated from Fig. 1 for Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 are 10.163 Å and 10.121 Å, respectively, which are of the same order as the reported values of La2Zr2O7 [9] and Sm2Zr2O7 [10]. The Fourier infrared spectrum has been utilized to study the metaloxygen bonds in the pyrochlore-type lattice [27,28]. From Fig. 2, four typical absorption bands for Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 can be found between 200 cm−1 and 1000 cm−1, which exhibit slight differences in position and intensity due to the nature of Y3+ and Yb3+ [23]. The band at 700-730 cm−1 is due to the Ta–O vibration in a TaO6 octahedron, and the band near 870-900 cm−1 is assigned to the Ti–O stretching vibration in a TiO6 octahedron [23]. The first band at 400440 cm−1 is attributed to the stretching vibration of the Ln-O bond, and the second band near 550-600 cm−1 is related to both Ln-O’ and Ca–O’ bonds [23]. The current analytical results are similar to results reported earlier [28], and these four absorption bands also confirm that the
3. Results and discussion 3.1. Investigation into phase-composition Fig. 1 displays the collected XRD patterns for the synthesized bulk samples together with the data of pyrochlore-type Sm2Zr2O7. It can be observed that the XRD peaks exhibit strong and sharp intensities, which means that the obtained bulk samples have a high degree of crystallinity. The collected XRD patterns show an analogous mode with that of Sm2Zr2O7, which means that the obtained oxides are of the cubic pyrochlore lattice of space group Fd3m [10]. Simultaneously, the XRD patterns of the obtained oxides are similar to those of Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides [23,24]. For the Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides, Rao et al. indicated that Ca, Y, Sm and Gd may occupy the A positions, and the Nb and Ti probably occupy the B positions in order to maintain the ionic radius ratio of r(A3+)/r(B4+) at the A and B sites within the range of 1.46–1.80 [23,24], which is an important criterion of judgement for pyrochlore-type compounds (A2B2O7) [24]. In the lattice of Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides, it was also pointed out
Fig. 2. Ft-IR spectra of Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides. 2
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Ca3Yb3Ti7Ta2O26.5 is in the range 3–8 μm. From Fig. 4(b), the grains of Ca3Yb3Ti7Ta2O26.5 are shaped in two appearances, i.e., spheroid and cuboid type, and a similar phenomenon can also be found for Pb3R3Ti7Nb2O26.5 (R = Dy, Gd, Nd, Pr or Y) oxides [26]. The analytical results of the elemental composition for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 are presented in Fig. 5. Clearly, the element type and atomic ratio are in agreement with the respective chemical formulas for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5. 3.3. Thermal expansion performance From Fig. 6, the linear relation between temperature and thermal expansion rate for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides indicates an excellent lattice stability up to 1473 K. The favourable phase stability over the entire temperature range is helpful for improving the performances of thermal barrier coatings [2]. The coefficient of thermal expansion for the obtained bulk samples is plotted in Fig. 7. Clearly, the coefficient of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5 at identical temperature points above 473 K. It is well known that the coefficient of thermal expansion is related to the electronegativity difference between metal ions and O2− for oxides given as Eq. (4) [33]. From Eq. (4), a low value of electronegativity difference xA − xB can improve the thermal expansion coefficient. The electro-negativities for Yb and Y are 1.20 and 1.22, respectively, which can lead to a lower value of xA − xB for Ca3Yb3Ti7Ta2O26.5 compared to Ca3Y3Ti7Ta2O26.5. Therefore, the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5. The average values of thermal expansion coefficients of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 between room temperature and 1273 K are 9.98 × 10−6/K and 9.55 × 10−6/K, respectively, which are of the same order as YSZ [2] or La2Zr2O7 [10].
Fig. 3. Raman spectra of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides.
fabricated oxides are of the pyrochlore-type lattice. Generally, the Raman spectrum is of greater sensitivity to distinguish the crystal lattice than XRD pattern. Thus, the Raman spectra of Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides are presented in Fig. 3. Normally, there are six broad Raman modes, which are given as follows [29]:
Γ (Raman) = A1g + Eg + 4F2g
(3)
Among the six broad Raman-modes, five of them can be clearly found in the spectra. Normally, the A1g and Eg bands are around ~510 cm−1 and ~350 cm−1; the other four modes are at ~750 cm−1, ~585 cm−1, ~400 cm−1 and ~280 cm−1 [30,31]. In Fig. 3, the mode near 510 cm−1 can be assigned to the B–O bending vibration, the mode near ~280 cm−1 is related to the A-O interaction, and the broadening of this mode is attributed to the increased oxygen vacancy concentration [32]. Therefore, the synthesized Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides possess a pyrochlore-type structure, which is consistent with those of the XRD patterns and infrared spectra.
−(xA − x B)2 4
IA−B = 1 − e
(4)
3.4. Thermal conductivity According to Fig. 8, the predicted specific heat capacity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 exhibits positive dependence on temperature, and the values of Ca3Yb3Ti7Ta2O26.5 are lower than those of Ca3Y3Ti7Ta2O26.5 at identical temperature points. The experimental thermal diffusivity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 is displayed in Fig. 9. It can be observed that the thermal diffusivity for Ca3Y3Ti7Ta2O26.5 exhibits a negative dependence on temperature, while the thermal diffusivity of Ca3Yb3Ti7Ta2O26.5 is
3.2. Microstructure and elemental composition The surface micrographs of the sintered pellets of Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides are revealed in Fig. 4. The photoprint exhibits uniphase micro-morphology for the oxides, and the size of well-formed crystal particles for Ca3Y3Ti7Ta2O26.5 and
Fig. 4. Typical micro-morphologies of (a) Ca3Yb3Ti7Ta2O26.5 and (b) Ca3Y3Ti7Ta2O26.5. 3
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Fig. 5. Analytical result of elemental compositions (a) Ca3Y3Ti7Ta2O26.5 and (b) Ca3Yb3Ti7Ta2O26.5.
Fig. 6. Thermal expansion rates for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 8. Calculated specific heat capacities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 7. Coefficients of thermal expansion for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 9. Thermal diffusivities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
The thermal conductivity for the fully dense samples is exhibited in Fig. 10. Clearly, the thermal conductivity for Ca3Y3Ti7Ta2O26.5 is inversely dependent on temperature, while the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 presents a positive dependence on temperature due
independent of temperature. Over the temperature range, the thermal diffusivity for Ca3Yb3Ti7Ta2O26.5 is within 0.41–0.43 mm2/s, and the thermal diffusivity for Ca3Y3Ti7Ta2O26.5 ranges from 0.63 mm2/s to 0.38 mm2/s. 4
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barrier coatings. Acknowledgments The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), and Program for Science & Technology Innovation Talents in Universities of Henan Province (13HASTIT018), the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001). References [1] K.P. Jonnalagadda, R. Eriksson, X.H. Li, R.L. Peng, Thermal barrier coatings: life model development and validation, Surf. Coat. Technol. 362 (2019) 293–301. [2] J.G. Thakare, R.S. Mulik, M.M. Mahapatra, Effect of carbon nanotubes and aluminum oxide on the properties of a plasma sprayed thermal barrier coating, Ceram. Int. 44 (2018) 438–451. [3] V. Sankar, P.B. Ramkumar, D. Sebastian, D. Joseph, J. Jose, A. Kurian, Optimized thermal barrier coating for gas turbine blades, Mater. Today 11 (2019) 912–919. [4] H.Y. Zhang, Z.W. Liu, X.B. Yang, H.M. 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Fig. 10. Thermal conductivities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
to the immobile thermal diffusivity. Meanwhile, the thermal conductivity of Ca3Y3Ti7Ta2O26.5 is greater than that of Ca3Yb3Ti7Ta2O26.5 over the entire temperature range. In light of the phonon heat-conduction rule, a high atomic weight can contribute to a low average free path of phonons, which can reduce the lattice thermal conductivity [34]. The molecular masses of Yb and Y are 173.0 and 88.9, respectively, which leads to a higher atomic weight of Ca3Yb3Ti7Ta2O26.5 compared to Ca3Y3Ti7Ta2O26.5. Thus, the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 is lower than that of Ca3Y3Ti7Ta2O26.5. The mean values of thermal conductivity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 in the present temperature range are 1.44 W/m. K and 1.95 W/m. K, respectively, which are lower than that of YSZ. The low thermal conductivity of the obtained products can be attributed the following reasons [34,35]. Firstly, the total number of ions for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 is higher than that of YSZ. Second, the elemental composition is more complex than that of YSZ. Thirdly, the synthesized oxides have a higher content of oxygen vacancies compared to YSZ, which is confirmed by the broad Raman peaks. 4. Summaries (1) Two kinds of novel complex oxides, Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5, were synthesized through a high-temperature solid-phase reaction technique. The obtained oxides are of single pyrochlore-type structure, and the grains are comprised of cuboid and spheroid types. (2) Due to the lower electronegativity of Yb compared to Y, the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is greater than that for Ca3Y3Ti7Ta2O26.5. The thermal expansion coefficients for the prepared bulk pellets are of similar order to that of YSZ. (3) Because of the larger atomic weight of Yb compared to Y, the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 is lower than that of Ca3Y3Ti7Ta2O26.5. The complex elemental composition, high number of total ions and fraction of oxygen vacancies contribute to the lower thermal conductivity compared to YSZ. (4) The thermal-physical performances of the prepared products match well with the requirements for thermal barrier coatings. Declaration of competing interest In this paper, the obtained Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal 5
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Preparation, thermophysical performances of Ca3Ln3Ti7Ta2O26.5 (Ln=Yb and Y) oxides for thermal barrier coating applications Zhang Hongsonga,∗, Yang Shusenb, Tong Yupingc, Yang Xianfenga,d, Sang Weiweia, Zhao Yongtaoa a
Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou, 451191, China Rail Transit Security Department of Railway Police College, China c School of Materials Science and Engineering, North China University of Water Sources and Electric Powder, Zhengzhou, 450011, China d College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha, 410014, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare earth titanate Thermal-physical performances Pyrochlore complex oxides
In this manuscript, the Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides were prepared by employing a high-temperature solid-phase reaction method at 1773 K for 10 h. The phase compositions and thermal-physical performances were studied. The results show that the synthesized oxides are of a single pyrochlore-type lattice, the thermal conductivity of Ca3Y3Ti7Ta2O26.5 is larger than that of Ca3Yb3Ti7Ta2O26.5, and the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5. The thermal-physical properties match well with the requirements for thermal barrier coatings.
1. Introduction It is well known that thermal barrier coating is one of key technologies in advanced airplane engines. In particular, thermal barrier coatings can be utilized to reduce the surface temperature of key hotend components and lengthen the operating life under complex environments [1–3]. Presently, Y2O3-stabilized ZrO2 (YSZ) is still the most successful ceramic material for thermal barrier coatings [2–5]. However, YSZ coatings cannot be operated for any appreciable length of time above 1473 K due to its imminent phase transformation, relatively poor sintering resistance, and corrosion from molten silicate deposits [6–8]. These disadvantageous reasons can bring about a reduction of heat-insulation and formation of cracks, which quicken the peeling of thermal barrier coatings. Therefore, the search for novel candidate ceramics for thermal barrier coatings is of great importance. Over the last 17 years, the A2B2O7 (B]Zr, Ti, Sn, Hf) oxides with pyrochlore-type lattice have been suggested as prospective novel surface-layer materials [9–17]. For instance, the doped La2Zr2O7 and Sm2Zr2O7 exhibit much higher thermal insulation than YSZ, and the coefficients of thermal expansion are of the same order as YSZ [9–11]. After irradiation with fast heavy-ions for Y2Ti2O7 and Gd2Ti2O7, a pyrochlore-amorphous phase transformation can be experienced, and the thermal conductivity is decreased [12]. For Y2Ti2O7, the phase transformation from pyrochlore to disordered fluorite is caused by
∗
substitution of Hf4+ for Ti4+ [13]. The computed minimum thermal conductivity of La2Sn2O7 is approximately 1.24 W/m. K [14], and the addition of Yb2O3 in (La1-xYbx)2Sn2O7 oxides can improve the heatinsulation ability [15]. La2Hf2O7 has lower theoretical mechanical and thermal performances than those of other pyrochlores, and the thermal conductivity for RE2Hf2O7 (RE = Nd, Ce, Pr, La, Pm and Sm) varies from 1.38 W/m. K to 1.62 W/m. K [16], while the experimental thermal conductivity ranges between 4 and 7 W/m. K [17]. Recently, several novel oxides with the pyrochlore lattice were regarded as potential candidates for high-temperature applications of thermal barrier coatings [18–21]. For instance, the room-temperature thermal conductivity of Y2GaSbO7 is approximately 2 W/m. K [18]. The thermal insulation performances of Gd3TaO7 and LaGd2TaO7 are greater than that of YSZ, which simultaneously exhibit favourable lattice stability up to 1473 K [19]. The coefficients of the thermal expansion of Ca3RE3Ti7Ta2O26.5 (RE = La, Sm, Nd, Gd) oxides are in the same order with that of YSZ, while the heat-insulation abilities are much higher than that of YSZ [20,21]. However, investigation about the thermal-physical properties of Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) has not been reported in the open literatures. To evaluate the potentials of Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides for thermal barrier coatings, these two types of oxides were prepared, and the phase-structures, the thermal expansion properties and heat-insulation abilities were studied.
Corresponding author. E-mail address: [email protected] (Z. Hongsong).
https://doi.org/10.1016/j.ceramint.2019.11.136 Received 20 September 2019; Received in revised form 13 October 2019; Accepted 15 November 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.2019.11.136
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2. Experimental In order to prepare Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides, the high-purity oxides including CaCO3, Y2O3, Yb2O3, TiO2 and Ta2O5, were selected as raw chemicals. The selected raw chemicals were weighed and blended fully in an agate mortar. For obtaining a homogenous mixture, acetone was added, and the admixture was subsequently dried at 373 K. The final mixed powders were compressed into cylindrical samples of a thickness of 2 mm and 17 mm in diameter, and the quadrangular green samples were also compressed. The compressed green samples were sintered at 1773 K for 10 h to obtain the bulk samples. The X-ray diffraction technique (D8-Advance, Bruker, Germany) was employed to identify the phase lattice of the bulk samples. The infrared and Raman spectra were also gathered via an infrared spectroscopy (Nicolet 6700, Thermo Fisher, USA) and a laser Raman spectrometer (inVia, Renishaw, England), respectively. The micro-morphology of the bulk samples was observed using a scanning electron microscope (Quanta 250FEG, FEI, America), and the energy dispersive spectroscopy (EDS, IE350) was employed to analyse the element distribution. The actual density (ρ) was measured via the Archimedes drainage technology in distilled water. Based on the specific heat capacities (Cp) of the selected raw oxides, the corresponding specific heat values for bulk-specimens were predicted via the Neumann-Kopp rule [22]. Thermomechanical analysis (DIL 402, NETZSCH, Germany) was adopted to study the thermal expansion behaviour between room temperature and 1273 K. Every quadrangular sample was machined into the size of 4 mm × 4 mm × 14 mm to suit the holder, and the measuring process was performed under flowing argon gas. To inspect the heat-insulating ability, the cylindrical sample of appropriate dimension (Φ10 × 1 mm) was coated with a thin graphite film on the front and back surfaces, respectively. The measurement of thermal diffusivity (λ) from room temperature to 1473 K was carried out by using a laser-flash instrument (LFA1000, Linseis, Germany), and three individual tests were repeated at identical temperature points. Thus, the thermal conductivity (k’) can be derived from the multiplication of density (ρ), specific capacity (Cp) and thermal diffusivity (λ) according to Eq. (1). The obtained values of (k’) were further converted into those of fully dense samples (k) to remove the effect of porosity (ϕ) on the heat-insulation performance.
k ′ = λ⋅Cp⋅ρ
(1)
k′ 4 =1− ϕ k 3
(2)
Fig. 1. XRD patterns of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides.
that some Ti4+ and Nb5+ might occupy the positions of A sites due to the higher totality of Nb5+ and Ti4+ ions compared to Ca2+ and Ln3+ ions [23]. For Pb3R3Ti7Nb2O26.5 (R = Dy, Gd, Nd, Pr or Y) and Ca3Ce3xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Y, Sm or Gd), a similar reasoning can also be found [25,26]. Therefore, the Ca3+, Y3+ and Yb3+ ions may occupy the A sites in Ca3Ln3Ti7Ta2O26.5 (Ln = Y and Yb) oxides and one part of Ta5+ and Ti4+ ions occupy the B sites, while the other part Ta5+ and Ti4+ ions maybe occupy the A sites. The lattice constants calculated from Fig. 1 for Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 are 10.163 Å and 10.121 Å, respectively, which are of the same order as the reported values of La2Zr2O7 [9] and Sm2Zr2O7 [10]. The Fourier infrared spectrum has been utilized to study the metaloxygen bonds in the pyrochlore-type lattice [27,28]. From Fig. 2, four typical absorption bands for Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 can be found between 200 cm−1 and 1000 cm−1, which exhibit slight differences in position and intensity due to the nature of Y3+ and Yb3+ [23]. The band at 700-730 cm−1 is due to the Ta–O vibration in a TaO6 octahedron, and the band near 870-900 cm−1 is assigned to the Ti–O stretching vibration in a TiO6 octahedron [23]. The first band at 400440 cm−1 is attributed to the stretching vibration of the Ln-O bond, and the second band near 550-600 cm−1 is related to both Ln-O’ and Ca–O’ bonds [23]. The current analytical results are similar to results reported earlier [28], and these four absorption bands also confirm that the
3. Results and discussion 3.1. Investigation into phase-composition Fig. 1 displays the collected XRD patterns for the synthesized bulk samples together with the data of pyrochlore-type Sm2Zr2O7. It can be observed that the XRD peaks exhibit strong and sharp intensities, which means that the obtained bulk samples have a high degree of crystallinity. The collected XRD patterns show an analogous mode with that of Sm2Zr2O7, which means that the obtained oxides are of the cubic pyrochlore lattice of space group Fd3m [10]. Simultaneously, the XRD patterns of the obtained oxides are similar to those of Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides [23,24]. For the Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides, Rao et al. indicated that Ca, Y, Sm and Gd may occupy the A positions, and the Nb and Ti probably occupy the B positions in order to maintain the ionic radius ratio of r(A3+)/r(B4+) at the A and B sites within the range of 1.46–1.80 [23,24], which is an important criterion of judgement for pyrochlore-type compounds (A2B2O7) [24]. In the lattice of Ca3Ln3Ti7Nb2O26.5 (Ln = Y, Sm, Gd) oxides, it was also pointed out
Fig. 2. Ft-IR spectra of Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides. 2
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Ca3Yb3Ti7Ta2O26.5 is in the range 3–8 μm. From Fig. 4(b), the grains of Ca3Yb3Ti7Ta2O26.5 are shaped in two appearances, i.e., spheroid and cuboid type, and a similar phenomenon can also be found for Pb3R3Ti7Nb2O26.5 (R = Dy, Gd, Nd, Pr or Y) oxides [26]. The analytical results of the elemental composition for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 are presented in Fig. 5. Clearly, the element type and atomic ratio are in agreement with the respective chemical formulas for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5. 3.3. Thermal expansion performance From Fig. 6, the linear relation between temperature and thermal expansion rate for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides indicates an excellent lattice stability up to 1473 K. The favourable phase stability over the entire temperature range is helpful for improving the performances of thermal barrier coatings [2]. The coefficient of thermal expansion for the obtained bulk samples is plotted in Fig. 7. Clearly, the coefficient of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5 at identical temperature points above 473 K. It is well known that the coefficient of thermal expansion is related to the electronegativity difference between metal ions and O2− for oxides given as Eq. (4) [33]. From Eq. (4), a low value of electronegativity difference xA − xB can improve the thermal expansion coefficient. The electro-negativities for Yb and Y are 1.20 and 1.22, respectively, which can lead to a lower value of xA − xB for Ca3Yb3Ti7Ta2O26.5 compared to Ca3Y3Ti7Ta2O26.5. Therefore, the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is higher than that of Ca3Y3Ti7Ta2O26.5. The average values of thermal expansion coefficients of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 between room temperature and 1273 K are 9.98 × 10−6/K and 9.55 × 10−6/K, respectively, which are of the same order as YSZ [2] or La2Zr2O7 [10].
Fig. 3. Raman spectra of Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 oxides.
fabricated oxides are of the pyrochlore-type lattice. Generally, the Raman spectrum is of greater sensitivity to distinguish the crystal lattice than XRD pattern. Thus, the Raman spectra of Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides are presented in Fig. 3. Normally, there are six broad Raman modes, which are given as follows [29]:
Γ (Raman) = A1g + Eg + 4F2g
(3)
Among the six broad Raman-modes, five of them can be clearly found in the spectra. Normally, the A1g and Eg bands are around ~510 cm−1 and ~350 cm−1; the other four modes are at ~750 cm−1, ~585 cm−1, ~400 cm−1 and ~280 cm−1 [30,31]. In Fig. 3, the mode near 510 cm−1 can be assigned to the B–O bending vibration, the mode near ~280 cm−1 is related to the A-O interaction, and the broadening of this mode is attributed to the increased oxygen vacancy concentration [32]. Therefore, the synthesized Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides possess a pyrochlore-type structure, which is consistent with those of the XRD patterns and infrared spectra.
−(xA − x B)2 4
IA−B = 1 − e
(4)
3.4. Thermal conductivity According to Fig. 8, the predicted specific heat capacity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 exhibits positive dependence on temperature, and the values of Ca3Yb3Ti7Ta2O26.5 are lower than those of Ca3Y3Ti7Ta2O26.5 at identical temperature points. The experimental thermal diffusivity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 is displayed in Fig. 9. It can be observed that the thermal diffusivity for Ca3Y3Ti7Ta2O26.5 exhibits a negative dependence on temperature, while the thermal diffusivity of Ca3Yb3Ti7Ta2O26.5 is
3.2. Microstructure and elemental composition The surface micrographs of the sintered pellets of Ca3Y3Ti7Ta2O26.5 and Ca3Yb3Ti7Ta2O26.5 oxides are revealed in Fig. 4. The photoprint exhibits uniphase micro-morphology for the oxides, and the size of well-formed crystal particles for Ca3Y3Ti7Ta2O26.5 and
Fig. 4. Typical micro-morphologies of (a) Ca3Yb3Ti7Ta2O26.5 and (b) Ca3Y3Ti7Ta2O26.5. 3
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Fig. 5. Analytical result of elemental compositions (a) Ca3Y3Ti7Ta2O26.5 and (b) Ca3Yb3Ti7Ta2O26.5.
Fig. 6. Thermal expansion rates for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 8. Calculated specific heat capacities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 7. Coefficients of thermal expansion for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
Fig. 9. Thermal diffusivities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
The thermal conductivity for the fully dense samples is exhibited in Fig. 10. Clearly, the thermal conductivity for Ca3Y3Ti7Ta2O26.5 is inversely dependent on temperature, while the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 presents a positive dependence on temperature due
independent of temperature. Over the temperature range, the thermal diffusivity for Ca3Yb3Ti7Ta2O26.5 is within 0.41–0.43 mm2/s, and the thermal diffusivity for Ca3Y3Ti7Ta2O26.5 ranges from 0.63 mm2/s to 0.38 mm2/s. 4
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barrier coatings. Acknowledgments The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), and Program for Science & Technology Innovation Talents in Universities of Henan Province (13HASTIT018), the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001). References [1] K.P. Jonnalagadda, R. Eriksson, X.H. Li, R.L. Peng, Thermal barrier coatings: life model development and validation, Surf. Coat. Technol. 362 (2019) 293–301. [2] J.G. Thakare, R.S. Mulik, M.M. Mahapatra, Effect of carbon nanotubes and aluminum oxide on the properties of a plasma sprayed thermal barrier coating, Ceram. Int. 44 (2018) 438–451. [3] V. Sankar, P.B. Ramkumar, D. Sebastian, D. Joseph, J. Jose, A. Kurian, Optimized thermal barrier coating for gas turbine blades, Mater. Today 11 (2019) 912–919. [4] H.Y. Zhang, Z.W. Liu, X.B. Yang, H.M. 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Fig. 10. Thermal conductivities for Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides.
to the immobile thermal diffusivity. Meanwhile, the thermal conductivity of Ca3Y3Ti7Ta2O26.5 is greater than that of Ca3Yb3Ti7Ta2O26.5 over the entire temperature range. In light of the phonon heat-conduction rule, a high atomic weight can contribute to a low average free path of phonons, which can reduce the lattice thermal conductivity [34]. The molecular masses of Yb and Y are 173.0 and 88.9, respectively, which leads to a higher atomic weight of Ca3Yb3Ti7Ta2O26.5 compared to Ca3Y3Ti7Ta2O26.5. Thus, the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 is lower than that of Ca3Y3Ti7Ta2O26.5. The mean values of thermal conductivity for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 in the present temperature range are 1.44 W/m. K and 1.95 W/m. K, respectively, which are lower than that of YSZ. The low thermal conductivity of the obtained products can be attributed the following reasons [34,35]. Firstly, the total number of ions for Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5 is higher than that of YSZ. Second, the elemental composition is more complex than that of YSZ. Thirdly, the synthesized oxides have a higher content of oxygen vacancies compared to YSZ, which is confirmed by the broad Raman peaks. 4. Summaries (1) Two kinds of novel complex oxides, Ca3Yb3Ti7Ta2O26.5 and Ca3Y3Ti7Ta2O26.5, were synthesized through a high-temperature solid-phase reaction technique. The obtained oxides are of single pyrochlore-type structure, and the grains are comprised of cuboid and spheroid types. (2) Due to the lower electronegativity of Yb compared to Y, the coefficient of thermal expansion of Ca3Yb3Ti7Ta2O26.5 is greater than that for Ca3Y3Ti7Ta2O26.5. The thermal expansion coefficients for the prepared bulk pellets are of similar order to that of YSZ. (3) Because of the larger atomic weight of Yb compared to Y, the thermal conductivity of Ca3Yb3Ti7Ta2O26.5 is lower than that of Ca3Y3Ti7Ta2O26.5. The complex elemental composition, high number of total ions and fraction of oxygen vacancies contribute to the lower thermal conductivity compared to YSZ. (4) The thermal-physical performances of the prepared products match well with the requirements for thermal barrier coatings. Declaration of competing interest In this paper, the obtained Ca3Ln3Ti7Ta2O26.5 (Ln = Yb and Y) oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal 5
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