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Thermal-physical performances of novel pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) oxides
Journal Pre-proof Thermal-physical performances of novel pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) oxides Zhang Hongsong, Yang Shusen, Tong Yuping, Sang Weiwei, Zhao Yongtao, Yan Xianfeng, Tang An PII:
S0272-8842(20)30027-4
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.027
Reference:
CERI 23959
To appear in:
Ceramics International
Received Date: 17 October 2019 Revised Date:
13 December 2019
Accepted Date: 6 January 2020
Please cite this article as: Z. Hongsong, Y. Shusen, T. Yuping, S. Weiwei, Z. Yongtao, Y. Xianfeng, T. An, Thermal-physical performances of novel pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) oxides, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.027. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Thermal-Physical Performances of Novel Pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) Oxides Zhang Hongsong1, Yang Shusen2, Tong Yuping3*, Sang Weiwei1, Zhao Yongtao1, Yan Xianfeng4, Tang An1 (1. Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou 451191, PR China) (2. Department of Railway Safety and Security, Railway Police College, Zhengzhou 450002, China) (3. School of Materials Science and Engineering, North China University of Water Resources and Electric Powder, Zhengzhou 450011, China) (4. College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha 410014, PR China) Abstract:To identify candidates for thermal barrier coatings, two complicated oxides, Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5, were prepared via a high-temperature sintering method. The lattice-type and thermal physical properties of these two oxides were studied. The obtained products have a single pyrochlore-type lattice. Because of the high atomic weights, complex element compositions, and complicate lattice structures, the thermal conductivities of the obtained products are lower than that of the yttria-stabilized zirconia (YSZ). The thermal conductivity of Ca3Dy3Ce7Ta2O26.5 is lower than that of Ca3Nd3Ce7Ta2O26.5. The coefficients of thermal expansion for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides at 1473 K are approximately 11.92 × 10-6/K and 12.1 × 10-6/K, respectively. The thermal conductivities and expansion coefficients of these two oxides meet the demands for thermal barrier coatings. Keywords: High-temperature applications; Pyrochlore lattices; Coefficients of thermal expansion; Thermal conductivities
1. Introduction To increase the efficiency of gas turbine engines in aircraft operations and electricity generations, thermal
barrier coatings prepared by electron beam physical vapor deposition or air plasma spraying method have been widely adopted to protect the metallic parts in gas turbine [1-3]. The 8.wt% yttria-stabilized zirconia (8YSZ) ceramics are currently applied as thermal barrier coatings due to their excellent thermal-physical performances [4-7]. However, 8YSZ coatings cannot be applied above 1473 K owing to the connaturalphase and microstructure transformations, which can cause excessive increase in volume, reduction in thermal insulation ability, and formation of cracks [8-10]. To overcome the shortcomings of the zirconia-based coatings, a preferred strategy is to develop alternative materials possessing high coefficient of thermal expansion, low thermal conductivity, high melting point, and phase stability above 1473 K [11-13]. In recent years, some novel promising candidates for thermal barrier coatings have been reported in the open literatures. For example, Chen et al. reported that the forsterite-type Mg2SiO4 ceramics exhibit good lattice stability up to 1573 K, thermal insulating ability, and relatively high thermal expansion coefficient [14]. The minimum thermal conductivity for a TaZr2.75O8 ceramic is approximately 30% lower than that of a full dense YSZ ceramic, which also shows super-high micro-hardness and fracture toughness [15]. The thermal conductivity of the perovskite-type LaAlO3 ranges from 2.2 to 2.4 W/m K, and its coefficient of thermal expansion varies between 5.5 × 10-6/K and 6.5 × 10-6/K [16]. The Ba(Sr1/3Ta2/3)O3 oxide displays a single hexagonal perovskite lattice up to 1873 K, and an anisotropic growth of the grains above 1673 K is observed [17]. The elastic modulus for RE2SiO5 (RE represents a tombarthite element) oxides is proportional to the ionic size of the tombarthite cations, and these ceramics with a large rare-earth ionic radius exhibit excellent resistance to molten salt corrosion[18]. The GdPO4 exhibits ultralow thermal conductivity (0.98 W/m K at 1273 K), and its toughness is about 1.16 MPa m1/2 [19]. The materials reported above have complicated crystal lattices, high molecular masses, and distorted and disordered structures, which are consistent with those features of low thermal conductivity suggested by Clarke [20].
Recently, several novel rare-earth titanium oxides with complicated chemical formulas, such as Ca3Ln3Ti7Ta2O26.5 (Ln=Nd , Gd) [21] and Ca3RE3Ti7Ta2O26.5 (RE=La, Sm) [22], have been proposed as promising materials for thermal barrier coatings. It is advised by Clarke that addition of atoms with high molecular weights can reduce thermal conductivity of ceramics [20]. Due to the higher atomic mass of Ce4+ than that of Ti4+, it can be deduced that the Ca3Ln3Ce7Ta2O26.5 (Ln= rare earth elements) exhibit low thermal conductivities. Thus, Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5 were fabricated via a solid-state sintering technique, and the lattice structures, thermal conductivities, and thermal expansion behaviorswere evaluated.
2. Experimental Bulk Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides were fabricated by employing a high-temperature solid-state sintering method. The CaCO3, Nd2O3, Dy2O3, CeO2, and Ta2O5 oxides with a purity greater than 99.9% were selected as raw materials. Except CaCO3, each raw material wasaforehandcalcined at 1073 K for 2 h in order to eliminate the adsorptive humidity and CO2. Subsequently, the raw materials were dissolved separately in ethyl alcohol and mechanically ball-milled for 6 h. Then, the ball-milled mixtures were desiccated at 473 K for 24 h and dry-pressed at 260 MPa to produce green samples, which were further cold isostatically pressed at 200 MPa. Finally, the green samples were sintered at 1873 K for 10 h, and the sintering procedure was repeated three times with intervening re-grinding to ensure complete reaction. The identifications of the lattice structures of the sintered bulk sampleswere performed by the X-ray diffraction (XRD; D8 Advance, Bruker, Germany) and Raman spectroscopy (inVia, Renishaw, England). The densities (ρ) of densified ceramics were measured with the Archimedes drainagemethod. Due to the excellent computational accuracy, the Neumann-Kopp principle was adopted to calculate the specific heat capacities (Cp) for the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. The thermal diffusivities (λ) of obtained samples (Φ12.7×1mm)were measured using the laser flash method (LFA 1000, Linseis, Germany).
Before measurement, the front and back surfaces of samples were coated with a graphite film, respectively. The thermal conductivities (k′) were acquired by multiplying the density, specific heat capacity, and thermal diffusivity according to Eq. (1), and the computed values were transformed into ones (k) of fully densified ceramics via Eq. (2) to remove the influence of porosity (φ). The rates and coefficients of thermal expansion for the bulk samples (4mm×4mm×14mm) were investigated using a high-temperature dilatometer (DIL 402, NETZSCH, Germany).
k' = λ ⋅Cp ⋅ ρ
(1)
k' 4 = 1− ϕ k 3
(2)
3. Results and discussion 3.1 Identification of crystal lattice The XRD patterns of Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides are illustrated in Fig. 1, along with the data of Sm2Zr2O7. Clearly, the XRD profiles of the proposed ceramics are consistent with that of Sm2Zr2O7, and the sharpness and excellent intensity of the peaks imply an excellent degree of crystallization. For Sm2Zr2O7, two weak peaks in the 2θ range of 34°–48° can be used to distinguish between the pyrochlore and fluorite lattice structures. For the proposed ceramics, one weak peak between 34° and 47° is observed, and another weak peak is found near 50.9°. These two weak peaks represent the superstructure for a pyrochlore lattice, the similar phenomena can be found for Ca3Ce3-xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Gd, Sm, or Y) [23] and Ca3R3Ti7Nb2O26.5 (R = Sm, Gd, or Y) oxides [24]. Therefore, it can be concluded that the obtained products in this study have a single pyrochlore-type structure. It is well known that both pyrochlore and fluorite-type oxides have the chemical formula A2B2O7 [25].In Ca3Ce3-xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Gd, Sm, or Y) [23] and Ca3R3Ti7Nb2O26.5 (R = Sm, Gd, or Y) oxides [24],it is inferred that the Ca2+ and rare-earth cations with high ionic radii are thought to occupy the A sites, and the Ti4+ and Nb5+ ions with low ionic radii may occupy the B sites. Simultaneously, some Ti4+ and Nb5+ cations
can also occupy the A sites owing to the greater totality of Nb5+ and Ti4+ cations compared to that of the Ca2+ and trivalent rare-earth cations [23, 24]. Similarly, it can be deduced that the Ca2+, Nd3+, and Dy3+ cations occupy the A sites, some of the Ce4+ and Ta5+ cations are located at the A sites and the rest cations occupy the B sites. Because the ionic radii of Ca2+, Nd3+, Dy3+, Ce4+, and Ta5+ with eight-coordination are 1.00, 1.109, 1.027, 0.972, and 0.64 Å, respectively [20, 21], the synthesized bulk ceramics cannot crystallize into a pyrochlore-type lattice, which is contrary to the XRD profiles. One explanation for this apparent contradiction is that the Ce4+ (0.972 Å) was reduced to Ce3+ (1.14 Å) during the sintering process, and a similar phenomenon is also found for the Ca3Ce3-xMxTi7Nb2O26.5 oxides [23]. As shown in Fig. 1, the locations of peaks for the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides shift to a low angle, which means that cations with a larger ionic radius than Sm3+ are introduced into the lattice of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. The ionic radius of Nd3+ (1.109 Å) is close to that of Sm3+ (1.079 Å), and ionic radii of Ca2+, Dy3+, Ce4+, and Ta5+ are lower than that of Sm3+. Therefore, the relatively larger cations in Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides can only be attributed to the presence of Ce3+, which remains the reduced state without being oxidized to Ce4+ by crystal-lattice stabilization [25]. Owing to the excellent sensitivity of Raman spectroscopy to pyrochlore and fluorite-type lattices [26], the Raman spectra for the proposed oxides are shown in Fig. 2, together with data of Ca3Nd3Ce7Ta2O26.5 and La2Ce2O7. Obviously, one wide and asymmetric Raman peak at approximately 460–470 cm-1 can be found for the obtained samples, which means that the obtained samples are CeO2-based solid solutions [26]. The main Raman peak is attributed to F2g mode, and the broad and asymmetric features of the main Raman peak are attributed to the influence of rare-earth doping on the F2g mode [27]. Four distinct Raman bands are observed in the patterns of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides, which are similar to those of pyrochlore-type Ca3Ln3Ti7Ta2O26.5 (Ln=Nd, Gd) [21]. The shoulder at near 600 cm-1 is ascribed to a vertical optic fashion because of slacking of the
symmetrical principles [28], the mode near ~280 cm-1 is related to the A-O interaction [29]. Therefore, the Raman spectra also confirm that the proposed ceramics have a pyrochlore-type lattice.
3.2Thermal conductivity As shown in Fig. 3, the specific heat capacities of the proposed ceramics exhibit a positive dependence on temperature. The values of Ca3Dy3Ce7Ta2O26.5 are higher than those of Ca3Nd3Ce7Ta2O26.5 due to the relatively greater specific heat capacity of Dy2O3 compared to Nd2O3. The relationship between thermal diffusivity and temperature is plotted in Fig. 4. Clearly, the dependence of thermal diffusivity on temperature indicates that a typical phonon heat-conduction is foundforCa3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. Over the tested temperature range, the thermal diffusivity of Ca3Nd3Ce7Ta2O26.5 decreases from 0.40 to 0.21 mm2/s, and the value for Ca3Nd3Ce7Ta2O26.5is within the range of 0.65-0.41 mm2/s. As seen in Fig. 5, the thermal conductivity decreases with increasing temperature, and the Ca3Dy3Ce7Ta2O26.5 has a lower thermal conductivity than Ca3Nd3Ce7Ta2O26.5. According to the phonon theory, the lattice heat conductivity is chiefly governed by the average free path of phonons, which is decreased by phonon scattering caused by different point defects, such as substitutingionsand oxygen vacancies [30, 31]. Considering the analytical results of the lattice structure, it can be concluded that the synthesized oxides are CeO2-related solid solutions, and the Ca2+, Nd3+, Dy3+, and Ta5+ are substituting ions. The only different atoms between Ca3Dy3Ce7Ta2O26.5 and Ca3Nd3Ce7Ta2O26.5 are Dy and Nd, and the atomic mass difference between Dy and Ce is higher than that between Nd and Ce. Therefore, the scattering in Ca3Dy3Ce7Ta2O26.5 is more severe than that in Ca3Nd3Ce7Ta2O26.5, which contributes to the lower thermal conductivity of Ca3Dy3Ce7Ta2O26.5compared to Ca3Nd3Ce7Ta2O26.5. Over the tested temperature range, the average thermal conductivities for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides are 1.09 and 1.73 W/m. K, respectively, which are less than that of La2Zr2O7 (2.9 W/ m.K) [32]. While
thermal conductivity of Ca3Nd3Ce7Ta2O26.5 is higher than that of Ca3Nd3Ti7Ta2O26.5 (1.47 W/m .K) [21], and their thermal conductivities are lower than that of YSZ [33]. Considering the phonon heat-conduction rule, the factors effecting the low heat conductivity for the proposed oxides are explained as follows [20, 31]: First, the atomic weights for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) are 2373.54 and 2318.64, respectively, which are greater than that of YSZ. Second, the elemental compositions of the proposed oxides are more complex than that of YSZ. Third, the complexity of the lattice structure is higher than that of YSZ. The heat conductivity for the obtained ceramics satisfies the basic demand for thermal barrier coatings.
3.3Thermal expansion behavior From Fig. 6, the linear relationship between thermal expansion rate and temperature means that the fine lattice structure is stable over the entire temperature range, which helps to improve the working life of thermal barrier coatings. In addition, the thermal expansion rate for Ca3Dy3Ce7Ta2O26.5 is slightly higher than that of Ca3Nd3Ce7Ta2O26.5. Fig. 7 shows that the coefficient of thermal expansion increases with increasing temperature because of the elevated average distance between atoms at high temperatures. The coefficient of thermal expansion for Ca3Dy3Ce7Ta2O26.5 above 673 K is slightly greater than that of Ca3Nd3Ce7Ta2O26.5. Thermal expansion performance is inversely proportional to electrovalent bonds intensityexpressed in Eq. (3) [34]. It can be inferred from Eq. (3) that a small difference in electronegativity ( x A − x B ) between A and B sites can improve the (
I
= 1−e
)
(3)
coefficient of thermal expansion. For the proposed oxides, the electronegativities of Ca, Nd, Dy, Ce, Ta, and O are 1.0, 1.14, 1.22, 1.12, 1.50, and 3.44, respectively, and the value of x A − x B for Ca3Dy3Ce7Ta2O26.5 (73.66) is noticeably lower than that for Ca3Nd3Ce7Ta2O26.5 (73.9). Therefore, the coefficient of Ca3Dy3Ce7Ta2O26.5 is higher than that of Ca3Nd3Ce7Ta2O26.5. The thermal expansion coefficients of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd)
oxides at 1473 K are 12.1 × 10-6/K and 11.92× 10-6/K, respectively, which are each higher than that of YSZ [35, 36].
4. Conclusions In this work, two novel oxides, Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd), were prepared via high-temperature sintering method. These two oxides exhibit a single pyrochlore-type lattice. Because the atomic weight of Dy is greater than that of Nd, the thermal conductivity of Ca3Dy3Ce7Ta2O26.5 is lower than that of Ca3Nd3Ce7Ta2O26.5. Due to the relatively low electronegativity difference between A and B sites for Ca3Dy3Ce7Ta2O26.5, its coefficient of thermal expansion is higher than that of Ca3Nd3Ce7Ta2O26.5. The thermal expansion coefficients and conductivities of the proposed ceramics meet the requirements for thermal barrier coatings.
Acknowledgments The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001) and the National Nature Science Foundation of China (21805085).
References [1] W. S. Li, H. Y. Zhao, X. H. Zhong, L. Wang, S. Y. Tao, Air plasmas-sprayed yttria and yttria-stabilized zirconia thermal barrier coatings subjected to calcium-magnesium aluminum-silicate (CMAS), J. Thermal Spray Technol. 23 (2014) 975-983. [3] M. Yang, Y. P. Zhu, X. Y. Wang, Q. Wang, L. Ai, L. L. Zhao, Y. Chu, S. N. Guo, J. Hu, Q. Zhang, Preparation and thermophysical properties of Ti4+ doped zirconia matrix thermal barrier coatings, J Alloys Compds. 777 (2019) 646-654. [4] S.P. Donegan, A. D. Rollett, Simulation of residual stress and elastic energy density in thermal barrier coatings
using fastfourier transition, Acta Mater. 96 (2015) 212-228. [5] K. Bobzin, L. D. Zhao, M. Ote, T. F. Linke, Deposition and characterization of thermal barrier coatings of ZrO2-4 mol.%Y2O3-1mol.% Gd2O3-1 mol.% Yb2O3, Surf. Coat. Technol. 268 (2015) 205-208. [6] L. S. Su, W. X. Zhang, X. Chen T. J. Wang, Experimental investigation of the biaxial strength of thermal barrier coating system, Ceram. Int. 41 (2015) 8945-8955. [7] B. X. Wang, C. Y. Zhao, Modeling radiative properties of air plasma sprayed thermal barrier coatings in the dependent scattering regime, Int. J. Heat. Mass Transfer. 89 (2015) 920-928. [8] C. D. Girolamo, F. Marra, M. Schioppa, C.Blasi, G. Pulci, T. Valente, Evolution of microstructure and mechanical properties of lanthanum zirconate thermal barrier coatings at high temperature, Surf. Coat. Technol. 269 (2015) 298-302. [9] E. Bakan, R. Vaβen, Ceramic top coats of plasmas-sprayed thermal barrier coatings: materials, process, and properties, J. Thermal Spray Technol. 26 (2017)992-1010. [10] M. G. Gok, G. Goller, Microstructure evaluation of laser re-melted gadolinium zirconia thermal barrier coatings, Surf. Coat. Technol. 276 (2015) 202-209. [11] H R. Lu, A. C. Wang, C. G. Zhang,, S. Y. Tong, Thermo-physical properties of rare-earth hexaaluminates LnMgAl11O19 (Ln: La, Pr, Nd, Sm, EuandGd) magnetoplumbite for advanced thermal barrier coatings, J. Eur. Ceram. Soc. 32 (2015) 1297-1306. [12] X. Y. Guo, L. Zhe, Y. G. Jung, L. Li, J. Knapp, J. Zhang, Thermal properties, thermal shock, and thermal cycling behavior of lanthanum zirconiate-based thermal barrier coatings, Metall. Mater. Trans. E 7 (2016) 64-70. [13] X. Zhou, Z.H. Xu, X. Z. Fan, S. M. Zhao, X. Q. Cao, L. M. He, Y4Al2O9 ceramics as a novel thermal barrier coating material for high-temperature applications, Mater. Lett. 134 (2014) 146-148. [14] S. Chen, X. Zhou, W. J. Song, J. B. Sun, H. Zhang, J. N. Jiang, L. H. Deng, S. J. Dong, X. Q. Cao, Mg2SiO4
as novel thermal barrier coating material for gas turbine applications, J. Eur. Ceram. Soc. 39 (2019) 2397-2408. [15] J. Y. Yuan, W. J. Song, H. Zhang, X. Zhou, S. J. Dong, J. N. Jiang, L. H. Deng, X. Q. Cao, TaZr2.75O8 ceramics as a potential thermal barrier coating material for high-temperature applications, Mater. Lett. 247 (2019) 82-85. [16] N.Vourads, E. Marathonit, P. K. Pandis, C. Argirusis, G. Sourkouni, C. Legros, S. Mirza, V. N. Stathopoulos, Evaluation of LaAlO3 as top coat material for thermal barrier coatings, Trans. Nonferrous Met. Soc. China. 28 (2018) 1582-1592. [17] Y.P. Ca, Q. S. Wang, X. J. Ning, Y.B. Liu, Evaluation of the phase stability, and mechanical and thermal properties of Ba(Sr1/3Ta2/3)O3 as a potential ceramic material for thermal barrier coatings, Ceram.Int. 45 (2019) 12989-12993. [18] Z. L. Tian, J. Zhang, T. Y. Zhang, X.M. Ren, W. P. Hu, L. Y. Zheng, J. Y. Wang, Towards thermal barrier coating application for rare earth silicates RE2SiO5 (RE=La, Nd, Sm, Eu, and Gd), J. Eur. Ceram. Soc. 39 (2019) 1463-1476. [19] L. Guo, Z. Yan, Z.H. Li, J. X. Yu, Q. Wang, M. Z. Li, F. X. Ye, GdPO4 as a novel candidate for thermal barrier coating applications at elevated temperatures, Surf. Coat. Technol. 349 (2018) 400-406. [20] D.R. Clarke, Materials selection guidelines for low thermal conductivity thermal barrier coatings, Surf. Coat. Technol. 163-164 (2003) 67-64. [21] H. M. Zhang, C. X. Lu, F. Wang, Y. D. Zhao, K. Lu, H. S. Zhang, Heat conductivity and expansion property of Ca3Ln3Ti7Ta2O26.5 (Ln=Nd , Gd) sosloids, Ceram. Int. 44 (2018) 16076-16078. [22] H. M. Zhang, S. S. Yang, C. X. Lu, X. G. Chen, B. Ren, A. Tang, K. Lu, Thermophysical properties of novel pyrochlore-type Ca3RE3Ti7Ta2O26.5 (RE=La, Sm) oxides, Ceram. Int. 44 (2018) 10994-10996. [23] P. P. Rao, S. J. Liji, K. Ravindran, P. Koshy, Ca3Ce3-xMxTi7Nb2O26.5 (M=Y, Sm or Gd; x=0, 1, or
2)-pyrochlore-type ceramic oxide semiconductors, Phy. B. 349 (2004) 115-118. [24] P. P. Rao, S. J. Liji, K. Ravindaran, P. Koshy, New pyrochlore-type oxides in Ca-R-Ti-Nb-O system (R=Y,Sm or Gd)-structure, F-IR spectra and dielectric properties, Mater. Lett. 58 (2004) 1924-1927. [25] M. Deepa, P.P. Rao, A.N. Radhakishnan, K. S. Sibi, P. Koshy, Pyrochlore type semiconducting ceramic oxides in Ca-Ce-Ti-M-O system (M=Nb or Ta)—structural, microstructure and electrical properties, Mater. Res. Bull. 44 (2009) 1481-1488. [26] M. A. Subramanian, G. Aravindan, G. V. S. Rao, Oxides pyrochlores-a review, Prog. Solid State Chem. 15 (1983) 55-143. [27] C. J. Wang, W. Z. Huang, Y. Wang, Y. L. Cheng, B. L. Zou, X. Z. Fan, J. L. Yang, X. Q. Cao, Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: a study of crystal growth and sintering behavior, Int. J. Refract. Met. Hard Mater.2012, 31:242-246. [28] Z. H. Xu, S. M. He, L. M. He, R. D. Mu, G. D. Huang, X. Q. Cao, Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic layer systems deposited by electron beam physical vapor deposition, J. Alloys Compds. 509 (2011) 4273-4283. [29] I. Kosacki, T. Suzuki, H. Anderson, P. Colomban, Raman scattering and lattice defects in nanocrystalline CeO2thin films, Solid State Ionics. 149 (2002) 99-105. [30] Z. D. D. Mitrovic, M. J. Scepanovic, M. U. G. Brojcin, Z. V. Popvic, S. B. Boskovic, B. M. Matovic, The sizeand strain effects on the Raman spectra of Ce1-xNdxO2-ynanopowders, Solid State Comm. 137(2006) 387-390. [31] S. Patil, S. Seal, Y. Guo, A. Schulte, J. Norwood. Role of trivalent La and Nd dopants in lattice distortion and oxygen vacancy generation in cerium oxide nanoparticles. App. Phys. Lett. 88(2006): 243110-243112. [32] H. S Zhang, K. Sun, Q. Xu, F. C. Wang, L. Liu, Thermal conductivity of (Sm1-xLax)2Zr2O7 (x=0, 0.25, 0,5, 0.75 and 1) oxides for advanced thermal barrier coatings, J. Rare Earth. 27 (2009) 222-226.
[33] P. G. Klemens, Theory of the Thermal Conductivity of Solids, Academic Press, London and New York, 1969, pp. 2-65. [34] X. F. Yang, Z. T. Ni, X. L. Yang, Q. L. He, H. H. Xie, X. W. Xu, Z. P. Xie, Preparation and properties of anti-static coating on the 3Y-TZP ceramics, Ceram. Int. 44(2018) 16459-16463. [35] J. X. Wang, L. P. Li, B.J. Campbell, L.Zhe, Y. Ji, Y. F. Yan,W. H. Su, Structure, thermal expansion and transport properties of BaCe1-xEuxO3-δoxides, Mater. Chem.Phy. 86(2004) 150-155. [36] X. F. Yang, X. L. Yang, W. Liu, J. H. Yang, X. W. Xu, Surface resistivity regulation of zirconia ceramics for anti-static purpose by novel solution infiltration method, Ceram. Int. 42(2016) 18503-18506.
Captions of Figures Fig.1 XRD patterns of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 2 Raman spectra of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 3 Calculation results of specific heat capacities for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 4 Relationship between thermal diffusivity and temperature for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 5 Final values of thermal conductivities of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 6 Thermal expansion rates of synthesized oxides Fig.7 Coefficients of thermal expansion for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd)
Declaration of Interest Statement In this paper, the obtained Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5 oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal barrier coatings
S0272-8842(20)30027-4
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.027
Reference:
CERI 23959
To appear in:
Ceramics International
Received Date: 17 October 2019 Revised Date:
13 December 2019
Accepted Date: 6 January 2020
Please cite this article as: Z. Hongsong, Y. Shusen, T. Yuping, S. Weiwei, Z. Yongtao, Y. Xianfeng, T. An, Thermal-physical performances of novel pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) oxides, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.027. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Thermal-Physical Performances of Novel Pyrochlore-type Ca3Ln3Ce7Ta2O26.5(Ln=Nd and Dy) Oxides Zhang Hongsong1, Yang Shusen2, Tong Yuping3*, Sang Weiwei1, Zhao Yongtao1, Yan Xianfeng4, Tang An1 (1. Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou 451191, PR China) (2. Department of Railway Safety and Security, Railway Police College, Zhengzhou 450002, China) (3. School of Materials Science and Engineering, North China University of Water Resources and Electric Powder, Zhengzhou 450011, China) (4. College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha 410014, PR China) Abstract:To identify candidates for thermal barrier coatings, two complicated oxides, Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5, were prepared via a high-temperature sintering method. The lattice-type and thermal physical properties of these two oxides were studied. The obtained products have a single pyrochlore-type lattice. Because of the high atomic weights, complex element compositions, and complicate lattice structures, the thermal conductivities of the obtained products are lower than that of the yttria-stabilized zirconia (YSZ). The thermal conductivity of Ca3Dy3Ce7Ta2O26.5 is lower than that of Ca3Nd3Ce7Ta2O26.5. The coefficients of thermal expansion for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides at 1473 K are approximately 11.92 × 10-6/K and 12.1 × 10-6/K, respectively. The thermal conductivities and expansion coefficients of these two oxides meet the demands for thermal barrier coatings. Keywords: High-temperature applications; Pyrochlore lattices; Coefficients of thermal expansion; Thermal conductivities
1. Introduction To increase the efficiency of gas turbine engines in aircraft operations and electricity generations, thermal
barrier coatings prepared by electron beam physical vapor deposition or air plasma spraying method have been widely adopted to protect the metallic parts in gas turbine [1-3]. The 8.wt% yttria-stabilized zirconia (8YSZ) ceramics are currently applied as thermal barrier coatings due to their excellent thermal-physical performances [4-7]. However, 8YSZ coatings cannot be applied above 1473 K owing to the connaturalphase and microstructure transformations, which can cause excessive increase in volume, reduction in thermal insulation ability, and formation of cracks [8-10]. To overcome the shortcomings of the zirconia-based coatings, a preferred strategy is to develop alternative materials possessing high coefficient of thermal expansion, low thermal conductivity, high melting point, and phase stability above 1473 K [11-13]. In recent years, some novel promising candidates for thermal barrier coatings have been reported in the open literatures. For example, Chen et al. reported that the forsterite-type Mg2SiO4 ceramics exhibit good lattice stability up to 1573 K, thermal insulating ability, and relatively high thermal expansion coefficient [14]. The minimum thermal conductivity for a TaZr2.75O8 ceramic is approximately 30% lower than that of a full dense YSZ ceramic, which also shows super-high micro-hardness and fracture toughness [15]. The thermal conductivity of the perovskite-type LaAlO3 ranges from 2.2 to 2.4 W/m K, and its coefficient of thermal expansion varies between 5.5 × 10-6/K and 6.5 × 10-6/K [16]. The Ba(Sr1/3Ta2/3)O3 oxide displays a single hexagonal perovskite lattice up to 1873 K, and an anisotropic growth of the grains above 1673 K is observed [17]. The elastic modulus for RE2SiO5 (RE represents a tombarthite element) oxides is proportional to the ionic size of the tombarthite cations, and these ceramics with a large rare-earth ionic radius exhibit excellent resistance to molten salt corrosion[18]. The GdPO4 exhibits ultralow thermal conductivity (0.98 W/m K at 1273 K), and its toughness is about 1.16 MPa m1/2 [19]. The materials reported above have complicated crystal lattices, high molecular masses, and distorted and disordered structures, which are consistent with those features of low thermal conductivity suggested by Clarke [20].
Recently, several novel rare-earth titanium oxides with complicated chemical formulas, such as Ca3Ln3Ti7Ta2O26.5 (Ln=Nd , Gd) [21] and Ca3RE3Ti7Ta2O26.5 (RE=La, Sm) [22], have been proposed as promising materials for thermal barrier coatings. It is advised by Clarke that addition of atoms with high molecular weights can reduce thermal conductivity of ceramics [20]. Due to the higher atomic mass of Ce4+ than that of Ti4+, it can be deduced that the Ca3Ln3Ce7Ta2O26.5 (Ln= rare earth elements) exhibit low thermal conductivities. Thus, Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5 were fabricated via a solid-state sintering technique, and the lattice structures, thermal conductivities, and thermal expansion behaviorswere evaluated.
2. Experimental Bulk Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides were fabricated by employing a high-temperature solid-state sintering method. The CaCO3, Nd2O3, Dy2O3, CeO2, and Ta2O5 oxides with a purity greater than 99.9% were selected as raw materials. Except CaCO3, each raw material wasaforehandcalcined at 1073 K for 2 h in order to eliminate the adsorptive humidity and CO2. Subsequently, the raw materials were dissolved separately in ethyl alcohol and mechanically ball-milled for 6 h. Then, the ball-milled mixtures were desiccated at 473 K for 24 h and dry-pressed at 260 MPa to produce green samples, which were further cold isostatically pressed at 200 MPa. Finally, the green samples were sintered at 1873 K for 10 h, and the sintering procedure was repeated three times with intervening re-grinding to ensure complete reaction. The identifications of the lattice structures of the sintered bulk sampleswere performed by the X-ray diffraction (XRD; D8 Advance, Bruker, Germany) and Raman spectroscopy (inVia, Renishaw, England). The densities (ρ) of densified ceramics were measured with the Archimedes drainagemethod. Due to the excellent computational accuracy, the Neumann-Kopp principle was adopted to calculate the specific heat capacities (Cp) for the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. The thermal diffusivities (λ) of obtained samples (Φ12.7×1mm)were measured using the laser flash method (LFA 1000, Linseis, Germany).
Before measurement, the front and back surfaces of samples were coated with a graphite film, respectively. The thermal conductivities (k′) were acquired by multiplying the density, specific heat capacity, and thermal diffusivity according to Eq. (1), and the computed values were transformed into ones (k) of fully densified ceramics via Eq. (2) to remove the influence of porosity (φ). The rates and coefficients of thermal expansion for the bulk samples (4mm×4mm×14mm) were investigated using a high-temperature dilatometer (DIL 402, NETZSCH, Germany).
k' = λ ⋅Cp ⋅ ρ
(1)
k' 4 = 1− ϕ k 3
(2)
3. Results and discussion 3.1 Identification of crystal lattice The XRD patterns of Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides are illustrated in Fig. 1, along with the data of Sm2Zr2O7. Clearly, the XRD profiles of the proposed ceramics are consistent with that of Sm2Zr2O7, and the sharpness and excellent intensity of the peaks imply an excellent degree of crystallization. For Sm2Zr2O7, two weak peaks in the 2θ range of 34°–48° can be used to distinguish between the pyrochlore and fluorite lattice structures. For the proposed ceramics, one weak peak between 34° and 47° is observed, and another weak peak is found near 50.9°. These two weak peaks represent the superstructure for a pyrochlore lattice, the similar phenomena can be found for Ca3Ce3-xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Gd, Sm, or Y) [23] and Ca3R3Ti7Nb2O26.5 (R = Sm, Gd, or Y) oxides [24]. Therefore, it can be concluded that the obtained products in this study have a single pyrochlore-type structure. It is well known that both pyrochlore and fluorite-type oxides have the chemical formula A2B2O7 [25].In Ca3Ce3-xMxTi7Nb2O26.5 (x = 0, 1, or 2; M = Gd, Sm, or Y) [23] and Ca3R3Ti7Nb2O26.5 (R = Sm, Gd, or Y) oxides [24],it is inferred that the Ca2+ and rare-earth cations with high ionic radii are thought to occupy the A sites, and the Ti4+ and Nb5+ ions with low ionic radii may occupy the B sites. Simultaneously, some Ti4+ and Nb5+ cations
can also occupy the A sites owing to the greater totality of Nb5+ and Ti4+ cations compared to that of the Ca2+ and trivalent rare-earth cations [23, 24]. Similarly, it can be deduced that the Ca2+, Nd3+, and Dy3+ cations occupy the A sites, some of the Ce4+ and Ta5+ cations are located at the A sites and the rest cations occupy the B sites. Because the ionic radii of Ca2+, Nd3+, Dy3+, Ce4+, and Ta5+ with eight-coordination are 1.00, 1.109, 1.027, 0.972, and 0.64 Å, respectively [20, 21], the synthesized bulk ceramics cannot crystallize into a pyrochlore-type lattice, which is contrary to the XRD profiles. One explanation for this apparent contradiction is that the Ce4+ (0.972 Å) was reduced to Ce3+ (1.14 Å) during the sintering process, and a similar phenomenon is also found for the Ca3Ce3-xMxTi7Nb2O26.5 oxides [23]. As shown in Fig. 1, the locations of peaks for the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides shift to a low angle, which means that cations with a larger ionic radius than Sm3+ are introduced into the lattice of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. The ionic radius of Nd3+ (1.109 Å) is close to that of Sm3+ (1.079 Å), and ionic radii of Ca2+, Dy3+, Ce4+, and Ta5+ are lower than that of Sm3+. Therefore, the relatively larger cations in Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides can only be attributed to the presence of Ce3+, which remains the reduced state without being oxidized to Ce4+ by crystal-lattice stabilization [25]. Owing to the excellent sensitivity of Raman spectroscopy to pyrochlore and fluorite-type lattices [26], the Raman spectra for the proposed oxides are shown in Fig. 2, together with data of Ca3Nd3Ce7Ta2O26.5 and La2Ce2O7. Obviously, one wide and asymmetric Raman peak at approximately 460–470 cm-1 can be found for the obtained samples, which means that the obtained samples are CeO2-based solid solutions [26]. The main Raman peak is attributed to F2g mode, and the broad and asymmetric features of the main Raman peak are attributed to the influence of rare-earth doping on the F2g mode [27]. Four distinct Raman bands are observed in the patterns of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides, which are similar to those of pyrochlore-type Ca3Ln3Ti7Ta2O26.5 (Ln=Nd, Gd) [21]. The shoulder at near 600 cm-1 is ascribed to a vertical optic fashion because of slacking of the
symmetrical principles [28], the mode near ~280 cm-1 is related to the A-O interaction [29]. Therefore, the Raman spectra also confirm that the proposed ceramics have a pyrochlore-type lattice.
3.2Thermal conductivity As shown in Fig. 3, the specific heat capacities of the proposed ceramics exhibit a positive dependence on temperature. The values of Ca3Dy3Ce7Ta2O26.5 are higher than those of Ca3Nd3Ce7Ta2O26.5 due to the relatively greater specific heat capacity of Dy2O3 compared to Nd2O3. The relationship between thermal diffusivity and temperature is plotted in Fig. 4. Clearly, the dependence of thermal diffusivity on temperature indicates that a typical phonon heat-conduction is foundforCa3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides. Over the tested temperature range, the thermal diffusivity of Ca3Nd3Ce7Ta2O26.5 decreases from 0.40 to 0.21 mm2/s, and the value for Ca3Nd3Ce7Ta2O26.5is within the range of 0.65-0.41 mm2/s. As seen in Fig. 5, the thermal conductivity decreases with increasing temperature, and the Ca3Dy3Ce7Ta2O26.5 has a lower thermal conductivity than Ca3Nd3Ce7Ta2O26.5. According to the phonon theory, the lattice heat conductivity is chiefly governed by the average free path of phonons, which is decreased by phonon scattering caused by different point defects, such as substitutingionsand oxygen vacancies [30, 31]. Considering the analytical results of the lattice structure, it can be concluded that the synthesized oxides are CeO2-related solid solutions, and the Ca2+, Nd3+, Dy3+, and Ta5+ are substituting ions. The only different atoms between Ca3Dy3Ce7Ta2O26.5 and Ca3Nd3Ce7Ta2O26.5 are Dy and Nd, and the atomic mass difference between Dy and Ce is higher than that between Nd and Ce. Therefore, the scattering in Ca3Dy3Ce7Ta2O26.5 is more severe than that in Ca3Nd3Ce7Ta2O26.5, which contributes to the lower thermal conductivity of Ca3Dy3Ce7Ta2O26.5compared to Ca3Nd3Ce7Ta2O26.5. Over the tested temperature range, the average thermal conductivities for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) oxides are 1.09 and 1.73 W/m. K, respectively, which are less than that of La2Zr2O7 (2.9 W/ m.K) [32]. While
thermal conductivity of Ca3Nd3Ce7Ta2O26.5 is higher than that of Ca3Nd3Ti7Ta2O26.5 (1.47 W/m .K) [21], and their thermal conductivities are lower than that of YSZ [33]. Considering the phonon heat-conduction rule, the factors effecting the low heat conductivity for the proposed oxides are explained as follows [20, 31]: First, the atomic weights for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) are 2373.54 and 2318.64, respectively, which are greater than that of YSZ. Second, the elemental compositions of the proposed oxides are more complex than that of YSZ. Third, the complexity of the lattice structure is higher than that of YSZ. The heat conductivity for the obtained ceramics satisfies the basic demand for thermal barrier coatings.
3.3Thermal expansion behavior From Fig. 6, the linear relationship between thermal expansion rate and temperature means that the fine lattice structure is stable over the entire temperature range, which helps to improve the working life of thermal barrier coatings. In addition, the thermal expansion rate for Ca3Dy3Ce7Ta2O26.5 is slightly higher than that of Ca3Nd3Ce7Ta2O26.5. Fig. 7 shows that the coefficient of thermal expansion increases with increasing temperature because of the elevated average distance between atoms at high temperatures. The coefficient of thermal expansion for Ca3Dy3Ce7Ta2O26.5 above 673 K is slightly greater than that of Ca3Nd3Ce7Ta2O26.5. Thermal expansion performance is inversely proportional to electrovalent bonds intensityexpressed in Eq. (3) [34]. It can be inferred from Eq. (3) that a small difference in electronegativity ( x A − x B ) between A and B sites can improve the (
I
= 1−e
)
(3)
coefficient of thermal expansion. For the proposed oxides, the electronegativities of Ca, Nd, Dy, Ce, Ta, and O are 1.0, 1.14, 1.22, 1.12, 1.50, and 3.44, respectively, and the value of x A − x B for Ca3Dy3Ce7Ta2O26.5 (73.66) is noticeably lower than that for Ca3Nd3Ce7Ta2O26.5 (73.9). Therefore, the coefficient of Ca3Dy3Ce7Ta2O26.5 is higher than that of Ca3Nd3Ce7Ta2O26.5. The thermal expansion coefficients of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd)
oxides at 1473 K are 12.1 × 10-6/K and 11.92× 10-6/K, respectively, which are each higher than that of YSZ [35, 36].
4. Conclusions In this work, two novel oxides, Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd), were prepared via high-temperature sintering method. These two oxides exhibit a single pyrochlore-type lattice. Because the atomic weight of Dy is greater than that of Nd, the thermal conductivity of Ca3Dy3Ce7Ta2O26.5 is lower than that of Ca3Nd3Ce7Ta2O26.5. Due to the relatively low electronegativity difference between A and B sites for Ca3Dy3Ce7Ta2O26.5, its coefficient of thermal expansion is higher than that of Ca3Nd3Ce7Ta2O26.5. The thermal expansion coefficients and conductivities of the proposed ceramics meet the requirements for thermal barrier coatings.
Acknowledgments The authors are grateful for financial support from the Henan Province University Scientific and Technological Innovation Team (18IRTSHN005), the Key Project of Science and Technology Research at the Henan Province Department of Education (19A480001) and the National Nature Science Foundation of China (21805085).
References [1] W. S. Li, H. Y. Zhao, X. H. Zhong, L. Wang, S. Y. Tao, Air plasmas-sprayed yttria and yttria-stabilized zirconia thermal barrier coatings subjected to calcium-magnesium aluminum-silicate (CMAS), J. Thermal Spray Technol. 23 (2014) 975-983. [3] M. Yang, Y. P. Zhu, X. Y. Wang, Q. Wang, L. Ai, L. L. Zhao, Y. Chu, S. N. Guo, J. Hu, Q. Zhang, Preparation and thermophysical properties of Ti4+ doped zirconia matrix thermal barrier coatings, J Alloys Compds. 777 (2019) 646-654. [4] S.P. Donegan, A. D. Rollett, Simulation of residual stress and elastic energy density in thermal barrier coatings
using fastfourier transition, Acta Mater. 96 (2015) 212-228. [5] K. Bobzin, L. D. Zhao, M. Ote, T. F. Linke, Deposition and characterization of thermal barrier coatings of ZrO2-4 mol.%Y2O3-1mol.% Gd2O3-1 mol.% Yb2O3, Surf. Coat. Technol. 268 (2015) 205-208. [6] L. S. Su, W. X. Zhang, X. Chen T. J. Wang, Experimental investigation of the biaxial strength of thermal barrier coating system, Ceram. Int. 41 (2015) 8945-8955. [7] B. X. Wang, C. Y. Zhao, Modeling radiative properties of air plasma sprayed thermal barrier coatings in the dependent scattering regime, Int. J. Heat. Mass Transfer. 89 (2015) 920-928. [8] C. D. Girolamo, F. Marra, M. Schioppa, C.Blasi, G. Pulci, T. Valente, Evolution of microstructure and mechanical properties of lanthanum zirconate thermal barrier coatings at high temperature, Surf. Coat. Technol. 269 (2015) 298-302. [9] E. Bakan, R. Vaβen, Ceramic top coats of plasmas-sprayed thermal barrier coatings: materials, process, and properties, J. Thermal Spray Technol. 26 (2017)992-1010. [10] M. G. Gok, G. Goller, Microstructure evaluation of laser re-melted gadolinium zirconia thermal barrier coatings, Surf. Coat. Technol. 276 (2015) 202-209. [11] H R. Lu, A. C. Wang, C. G. Zhang,, S. Y. Tong, Thermo-physical properties of rare-earth hexaaluminates LnMgAl11O19 (Ln: La, Pr, Nd, Sm, EuandGd) magnetoplumbite for advanced thermal barrier coatings, J. Eur. Ceram. Soc. 32 (2015) 1297-1306. [12] X. Y. Guo, L. Zhe, Y. G. Jung, L. Li, J. Knapp, J. Zhang, Thermal properties, thermal shock, and thermal cycling behavior of lanthanum zirconiate-based thermal barrier coatings, Metall. Mater. Trans. E 7 (2016) 64-70. [13] X. Zhou, Z.H. Xu, X. Z. Fan, S. M. Zhao, X. Q. Cao, L. M. He, Y4Al2O9 ceramics as a novel thermal barrier coating material for high-temperature applications, Mater. Lett. 134 (2014) 146-148. [14] S. Chen, X. Zhou, W. J. Song, J. B. Sun, H. Zhang, J. N. Jiang, L. H. Deng, S. J. Dong, X. Q. Cao, Mg2SiO4
as novel thermal barrier coating material for gas turbine applications, J. Eur. Ceram. Soc. 39 (2019) 2397-2408. [15] J. Y. Yuan, W. J. Song, H. Zhang, X. Zhou, S. J. Dong, J. N. Jiang, L. H. Deng, X. Q. Cao, TaZr2.75O8 ceramics as a potential thermal barrier coating material for high-temperature applications, Mater. Lett. 247 (2019) 82-85. [16] N.Vourads, E. Marathonit, P. K. Pandis, C. Argirusis, G. Sourkouni, C. Legros, S. Mirza, V. N. Stathopoulos, Evaluation of LaAlO3 as top coat material for thermal barrier coatings, Trans. Nonferrous Met. Soc. China. 28 (2018) 1582-1592. [17] Y.P. Ca, Q. S. Wang, X. J. Ning, Y.B. Liu, Evaluation of the phase stability, and mechanical and thermal properties of Ba(Sr1/3Ta2/3)O3 as a potential ceramic material for thermal barrier coatings, Ceram.Int. 45 (2019) 12989-12993. [18] Z. L. Tian, J. Zhang, T. Y. Zhang, X.M. Ren, W. P. Hu, L. Y. Zheng, J. Y. Wang, Towards thermal barrier coating application for rare earth silicates RE2SiO5 (RE=La, Nd, Sm, Eu, and Gd), J. Eur. Ceram. Soc. 39 (2019) 1463-1476. [19] L. Guo, Z. Yan, Z.H. Li, J. X. Yu, Q. Wang, M. Z. Li, F. X. Ye, GdPO4 as a novel candidate for thermal barrier coating applications at elevated temperatures, Surf. Coat. Technol. 349 (2018) 400-406. [20] D.R. Clarke, Materials selection guidelines for low thermal conductivity thermal barrier coatings, Surf. Coat. Technol. 163-164 (2003) 67-64. [21] H. M. Zhang, C. X. Lu, F. Wang, Y. D. Zhao, K. Lu, H. S. Zhang, Heat conductivity and expansion property of Ca3Ln3Ti7Ta2O26.5 (Ln=Nd , Gd) sosloids, Ceram. Int. 44 (2018) 16076-16078. [22] H. M. Zhang, S. S. Yang, C. X. Lu, X. G. Chen, B. Ren, A. Tang, K. Lu, Thermophysical properties of novel pyrochlore-type Ca3RE3Ti7Ta2O26.5 (RE=La, Sm) oxides, Ceram. Int. 44 (2018) 10994-10996. [23] P. P. Rao, S. J. Liji, K. Ravindran, P. Koshy, Ca3Ce3-xMxTi7Nb2O26.5 (M=Y, Sm or Gd; x=0, 1, or
2)-pyrochlore-type ceramic oxide semiconductors, Phy. B. 349 (2004) 115-118. [24] P. P. Rao, S. J. Liji, K. Ravindaran, P. Koshy, New pyrochlore-type oxides in Ca-R-Ti-Nb-O system (R=Y,Sm or Gd)-structure, F-IR spectra and dielectric properties, Mater. Lett. 58 (2004) 1924-1927. [25] M. Deepa, P.P. Rao, A.N. Radhakishnan, K. S. Sibi, P. Koshy, Pyrochlore type semiconducting ceramic oxides in Ca-Ce-Ti-M-O system (M=Nb or Ta)—structural, microstructure and electrical properties, Mater. Res. Bull. 44 (2009) 1481-1488. [26] M. A. Subramanian, G. Aravindan, G. V. S. Rao, Oxides pyrochlores-a review, Prog. Solid State Chem. 15 (1983) 55-143. [27] C. J. Wang, W. Z. Huang, Y. Wang, Y. L. Cheng, B. L. Zou, X. Z. Fan, J. L. Yang, X. Q. Cao, Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: a study of crystal growth and sintering behavior, Int. J. Refract. Met. Hard Mater.2012, 31:242-246. [28] Z. H. Xu, S. M. He, L. M. He, R. D. Mu, G. D. Huang, X. Q. Cao, Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic layer systems deposited by electron beam physical vapor deposition, J. Alloys Compds. 509 (2011) 4273-4283. [29] I. Kosacki, T. Suzuki, H. Anderson, P. Colomban, Raman scattering and lattice defects in nanocrystalline CeO2thin films, Solid State Ionics. 149 (2002) 99-105. [30] Z. D. D. Mitrovic, M. J. Scepanovic, M. U. G. Brojcin, Z. V. Popvic, S. B. Boskovic, B. M. Matovic, The sizeand strain effects on the Raman spectra of Ce1-xNdxO2-ynanopowders, Solid State Comm. 137(2006) 387-390. [31] S. Patil, S. Seal, Y. Guo, A. Schulte, J. Norwood. Role of trivalent La and Nd dopants in lattice distortion and oxygen vacancy generation in cerium oxide nanoparticles. App. Phys. Lett. 88(2006): 243110-243112. [32] H. S Zhang, K. Sun, Q. Xu, F. C. Wang, L. Liu, Thermal conductivity of (Sm1-xLax)2Zr2O7 (x=0, 0.25, 0,5, 0.75 and 1) oxides for advanced thermal barrier coatings, J. Rare Earth. 27 (2009) 222-226.
[33] P. G. Klemens, Theory of the Thermal Conductivity of Solids, Academic Press, London and New York, 1969, pp. 2-65. [34] X. F. Yang, Z. T. Ni, X. L. Yang, Q. L. He, H. H. Xie, X. W. Xu, Z. P. Xie, Preparation and properties of anti-static coating on the 3Y-TZP ceramics, Ceram. Int. 44(2018) 16459-16463. [35] J. X. Wang, L. P. Li, B.J. Campbell, L.Zhe, Y. Ji, Y. F. Yan,W. H. Su, Structure, thermal expansion and transport properties of BaCe1-xEuxO3-δoxides, Mater. Chem.Phy. 86(2004) 150-155. [36] X. F. Yang, X. L. Yang, W. Liu, J. H. Yang, X. W. Xu, Surface resistivity regulation of zirconia ceramics for anti-static purpose by novel solution infiltration method, Ceram. Int. 42(2016) 18503-18506.
Captions of Figures Fig.1 XRD patterns of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 2 Raman spectra of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 3 Calculation results of specific heat capacities for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 4 Relationship between thermal diffusivity and temperature for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 5 Final values of thermal conductivities of the Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd) Fig. 6 Thermal expansion rates of synthesized oxides Fig.7 Coefficients of thermal expansion for Ca3Ln3Ce7Ta2O26.5 (Ln= Dy and Nd)
Declaration of Interest Statement In this paper, the obtained Ca3Nd3Ce7Ta2O26.5 and Ca3Dy3Ce7Ta2O26.5 oxides are of single pyrochlore-type lattice and excellent thermophysical properties, which have potential to be explored for thermal barrier coatings