X-ray diffractional, spectroscopic and thermo-physical properties analyses on Eu-doped lanthanum zirconate ceramic for thermal barrier coatings

Accepted Manuscript X-ray diffractional, spectroscopic and thermo-physical properties analyses on Eudoped lanthanum zirconate ceramic for thermal barrier coatings Renbo Zhu, Jianpeng Zou, Duopei Wang, Ke Zou, Di Gao, Jie Mao, Min Liu PII:

S0925-8388(18)30588-7

DOI:

10.1016/j.jallcom.2018.02.143

Reference:

JALCOM 45019

To appear in:

Journal of Alloys and Compounds

Received Date: 29 November 2017 Revised Date:

11 February 2018

Accepted Date: 12 February 2018

Please cite this article as: R. Zhu, J. Zou, D. Wang, K. Zou, D. Gao, J. Mao, M. Liu, X-ray diffractional, spectroscopic and thermo-physical properties analyses on Eu-doped lanthanum zirconate ceramic for thermal barrier coatings, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.02.143. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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X-ray diffractional, spectroscopic and thermo-physical properties analyses on Eu-doped lanthanum zirconate ceramic for thermal barrier coatings Renbo Zhu a, Jianpeng Zou a,*, Duopei Wang a, Ke Zou a, Di Gao a, Jie Mao b, Min Liu b a

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China National Engineering Laboratory for Modern Materials Surface Engineering Technology & the Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510650, China

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Abstract: To improve the protective temperature of thermal barrier coatings (TBCs), different Eu-doping ratios of La2Zr2O7 with pyrochlore structure, which are more stable in thermodynamics, were synthesized by coprecipitation-calcination method. Phase structure, grain growth kinetics, bond strength, thermo-physical properties of the doped La2Zr2O7 were investigated under different doping ratios. The results show that nano-sized

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doped La2Zr2O7 grains (<100 nm) grow rapidly under elevated synthesis temperature. Meanwhile the increasing Eu3+ ions lead to more imperfects, changes of bond strength, and slight disorder of the lattice. The coefficient of thermal expansion (CTE) of Eu-doped La2Zr2O7 is notably improved as Eu3+ ions increase. The thermal composition of (La0.6Eu0.4)2Zr2O7.

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conductivity decreases at initial period and increases afterwards, which reaches the lowest value for the

Keywords: Eu-doped lanthanum zirconate; thermal barrier coatings; phase structure; thermo-physical properties

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Introduction

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The operation efficiency and thrust-to-weight ratio of gas-turbine engines is directly related to the inlet gas temperature, which is much higher than the melting point of superalloy used in the turbines. Thermal barrier coatings (TBCs), composed of refractory oxide ceramics, are applied to the surface on hot parts of gas-turbine engines for thermal insulation and other protections of superalloy, thus enabling modern engines to operate at higher inlet gas temperature. Adding TBCs to turbine engines has the potential to increase operating temperature roughly as high as 150 °C, equivalent to the improvement of Ni-based superalloy and cooling technology in the last 20 years [1].The TBCs have emerged as one of the most critical materials for the next generation of gas turbine technology [2]. The conventional TBC is Y2O3-stabilized ZrO2 (YSZ), which is widely used for its low thermal conductivity, high melting point, and long service life [3-5], and its deposition technology is well established [6, 7]. But YSZ coatings could not be used at the temperature above 1200 °C for long-term application due to its relatively low sintering resistance and phase structure stability, which impose restrictions on the improvement of gas temperature and are less effective at higher temperature for the next generation of advanced engines [8, 9]. So it is urgently needed to develop new TBCs materials with better phase stability, greater sintering resistance and lower thermal conductivity than YSZ. Among the candidates of ceramic materials in recent studies, rare earth zirconates *

Corresponding author. E-mail address: [email protected] (J. Zou).

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Experimental Procedure

2.1

Preparation

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show promising thermo-physical properties, and La2Zr2O7 shows significant superiority over YSZ in terms of thermal conductivity, thermal and chemical stability, and sintering rate [10-12]. The general composition of rare earth zirconate is A2B2O7 (where A= rare earth element and B= Zr or Ce). It is established that A2Zr2O7 tends to be disordered fluorite structure if the cations radius ratio (A3+/Zr4+) is less than 1.46, which is less stable than pyrochlore structure in thermodynamics [13]. However, compared with YSZ, the coefficient of thermal expansion (CTE) of La2Zr2O7 is relatively low (9.1×10-6 K-1 at 1000 °C for La2Zr2O7, 10.1×10-6 K-1 at 1000 °C for YSZ) [14-16], which would lead to higher thermal stress than YSZ between the ceramic coating and its underlying alloy. To improve the CTE of La2Zr2O7, many attempts have been made to substitute La site and Zr site by other rare earth elements (e.g., (Sm0.5La0.5)2Zr2O7, (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7, (Y0.05La0.95)2(Zr0.7Ce0.3)2O7, La2(Zr1-x Bx)2O7 (B=Hf, Ce, 0
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Lanthanum nitrate, europium nitrate and zirconium oxychloride (Jining Zhongkai New Type Material Science Co., Ltd., purity≥99.99%) were dissolved into distilled water, respectively. The solutions were mixed in designed proportions and stirred for 30 min. Then, the homogenous solution was slowly added into ammonium hydroxide with pH=10 to obtain precipitates under stirring conditions, and the precipitates were washed with distilled water and ethanol until pH=7 was reached. The washed precipitates were dried at 100 °C for 12 h. After grinding and sieving, the obtained powders were calcined at 1300 °C for 2 h. The doped La2Zr2O7 bulks were produced by Spark Plasma Sintering (SPS) apparatus (FCT Group, Germany, D25/3) at 1450 °C, during which heating rate, sintering time and applied pressure were fixed to 50 °C/min, 5 min, 40 MPa, respectively. The obtained bulks were shaped by a low speed diamond blade, cleaned with ethanol, and dried for thermo-physical properties tests.

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2.2

Analyses and measurements

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Phase analyses of the synthesized powders calcined at 1300 °C for 2 h were carried out by using X-ray diffractometry (XRD, D/max 2550PC, Rigaku Ltd.) with Ni filtered Cu Kɑ radiation at a scan rate of 4 (°)/min. To figure out the variation of bond energy in the lattice, Fourier-transform infrared spectrum (FTIR, Nicolet 6700) was recorded in 4000-400 cm-1 at 2 cm-1 resolution. To further distinguish between disordered defect-fluorite and ordered pyrochlore structure, Raman spectroscopy (HORIBA Scientific, France, Labram Aramis, laser excitation source: λ=785 nm) was taken in 1000-100 cm-1. The linear CTE of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) bulks was determined by high-temperature dilatometry (PCY, Xiangyi) from 400 °C to 1200 °C at a heating rate of 5 °C/min in the air, the sample dimensions were approximately 20 mm×5 mm×5 mm. The thermal conductivity (λ) of the bulks, 10 mm in diameter and about 2 mm in thickness, is determined by Eq. (1), in which cp, ρ, and Dth represent heat capacity, density, and thermal diffusivity, respectively. The thermal diffusivity was measured in the range between 150 °C and 1000 °C by using laser-flash method (Model NETZSCH LFA 427, Germany) in argon atmosphere. The special heat capacity was measured by differential scanning calorimeter (Model NETZSCH DSC204, Germany) at a heating rate of 10 °C/min in the air between 150 °C and 1000 °C. The density was measured by Archimedes method.

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λ=cp ·Dth ·ρ

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Results and discussion

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3.1 XRD analysis and grain growth kinetics

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Fig. 1 shows the XRD patterns of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) powders calcinated at 1300 °C for 2h. It can be found that these samples have similar XRD pattern, indexed to the pure cubic phase of La2Zr2O7 with pyrochlore structure (JCPDS No. 73-0444, space group Fd-3m). From x=0 to x=0.5, no other distinct diffraction peaks are found from impurities. These facts indicate that Eu3+ ions are dissolved into La2Zr2O7 crystal lattice, and it forms substitutional solid solution. Meanwhile, the peaks shift a little to larger angle with increasing ratio of Eu3+ ions. It results from the different ion radius of La3+ (130 pm) and Eu3+ (118 pm), which would lead to the lattice contraction, with lattice parameter ranging from 10.80 Å (x=0) to 10.74 Å (x=0.5).

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(b)

(a)

(La0.5Eu0.5)2Zr2O7

(La0.5Eu0.5)2Zr2O7

(La0.6Eu0.4)2Zr2O7

(La0.6Eu0.4)2Zr2O7

(La0.7Eu0.3)2Zr2O7

(La0.8Eu0.2)2Zr2O7

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Intensity/(a.u.)

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(La0.8Eu0.2)2Zr2O7

(La0.9Eu0.1)2Zr2O7

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(222)

(440)

La2Zr2O7

(622) (444)

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(800) (622) (840)

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(400)

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(222)

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Fig. 1 (a) XRD patterns of Eu-doped La2Zr2O7 and (b) the enlarged view of the diffraction peak corresponding to the (222) plane.

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Fig. 2 shows XRD patterns of (La0.6Eu0.4)2Zr2O7 calcined at various temperatures for 2h. Four main diffraction peaks including (222), (400), (440), (622) have formed at 800 °C but show relatively less crystallinity. With calcination temperature increasing, the four main diffraction peaks of (La0.6Eu0.4)2Zr2O7 become sharper, indicating the grain growth and increased crystallinity. (222)

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(La0.6Eu0.4)2Zr2O7 1300°C (444) (840) (800) (622)

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Fig. 2 XRD patterns of (La0.6Eu0.4)2Zr2O7 powders at different sintering temperature.

The thermal conductivity of the material is closely related to the grain size, and it is thought that smaller grain size contributes to the less thermal conductivity [29]. Meanwhile, the narrowing of XRD peaks mainly originates from the increase of the

ACCEPTED MANUSCRIPT grain size, so the full width at half maximum (FWHM) could be used as a method to estimate the grain size at different calcination temperature. Take the half width of (222) lattice plane as an example, the grain sizes of (La0.6Eu0.4)2Zr2O7 under different synthesis temperature were estimated by Scherrer equation below: G=

kλ Bcosθ

(2)

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Where G is average grain size, k is Scherrer constant (usually k is equal to 0.89), λ is incidence ray wavelength (0.15418 nm), B is FWHM of diffractive peak, and θ is diffractive angle. The grain size result is shown in Fig. 3, which varies from 7.4 nm to 88.5 nm. The grain size of (La0.6Eu0.4)2Zr2O7 grows much faster above 1000 °C. The reason is probably that large-scale lattice disorders still exist when the calcination temperature of (La0.6Eu0.4)2Zr2O7 stays below 1000 °C. When calcination temperature reaches to 1000 °C, the grain size grows much faster since the lattice dislocations, lattice aberrances and lattice disorders decrease with the elevating temperature. Fig. 4 shows TEM image of (La0.6Eu0.4)2Zr2O7 powders calcined at 1300 °C, with the average grain size about 90 nm, which is in line with the Scherrer equation calculation. 100

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Fig. 3 Grain size of (La0.6Eu0.4)2Zr2O7 powders with the calcinating temperature.

Fig. 4 TEM image of (La0.6Eu0.4)2Zr2O7 powders calcined at 1300 °C.

ACCEPTED MANUSCRIPT From classical grain growth treatments expressed as Eq. (3) below:

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, the rate of grain growth can be

dG
t c = G
t dt

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Where G and t are grain size and growth time, c represents the constant, which is related to temperature as follows: A Q exp() T RT

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c=

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G
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Where Q is the activation energy for grain growth, R and T mean gas constant and synthesis temperature, A and B represent the constants. Thus, grain growth formula can be expressed as follows: (5)

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In this paper, G0 approximately equals to 0 and t equals to 2h. Then, the formula can be simplified as follows: lnG=B-

Q 2RT

(6)

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As is shown in Fig. 5, lnG has linear relationship with 1/T. The grain growth activation energy can be calculated by the formula, in which the slope of lnG~1/T line is –Q/R. The result shows grain growth activation energy of (La0.6Eu0.4)2Zr2O7 is 136 kJ/mol, which is much less than that of YSZ in other research reports [33, 34]. The more lattice defects in doped zirconate, the less grain growth activation energy of (La0.6Eu0.4)2Zr2O7. It means that particle diffusion is more active, which accelerates grain growth.

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ln(D,m)

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Fig. 5 The relationship of lnD versus 1/T for (La0.6Eu0.4)2Zr2O7.

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3.2

Infrared and Raman spectra

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The infrared spectra of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) powders are illustrated in Fig. 6. It can be seen that doping has not caused visible damage to the pyrochlore structure. Three different regions can be distinguished obviously: O-H vibration around 3440 cm-1, nitrate vibration around 1631 cm-1, and Zr-O vibration around 510 cm-1 [35]. However, frequency (in cm-1) of the Zr-O vibration increases with the elevating Eu3+ ions ratio, indicating that Zr-O bond strength is enhanced with higher lattice contraction. The change in the Zr-O bond strength might attribute to the decrease of ions distance since Eu3+ ions with smaller radius would lead to lattice contraction and shorten the distance from Zr4+ ion to O2- ion.

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Fig. 6 The infrared spectra of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) powders.

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Raman spectroscopy has been widely utilized as a tool to distinguish disordered defect-fluorite structured materials from ordered pyrochlore structured materials [36]. There is only one Raman active mode T2g in the disordered fluorite structure of La2Zr2O7, while six peaks exist within the range of 200-1000 cm-1 in ordered pyrochlore structure of La2Zr2O7 [37, 38]. As is shown in Fig. 7, the Raman spectrum from La2Zr2O7 consists of four bonds at approximately 320 cm-1, 397 cm-1, 497 cm-1, 517 cm-1, corresponding to the O(1)-B-O(1), B-O(1), A-O(2), and A-O(1) [39]. The A1g bond (about 800 cm-1 estimated) and the Eg bond (about 600 cm-1 estimated) are not found on Raman spectrum, which may result from the strong coupling of octahedral in the Zr4O framework. Overall, peak broadening does not change much with the increasing ratio of Eu3+ ions, indicating that Eu3+ ions are distributed relatively uniformly as replacing La3+ ions. However, there is a little peak broadening when Eu3+ ions increase, which indicates a trend of disorder within the La2Zr2O7. Raman peaks of A-O(1), A-O(2) are distinct in La2Zr2O7, but only one bond exists after Eu-doping, which means that it is also caused by the disorder in the lattice[40]. Different-cations radius ratios {r (La3+)/r (Zr4+) =1.51, r (Eu3+)/r (Zr4+) =1.37} of the lattice structure tending to be disordered fluorite structure when the radius ratio is below 1.46 [13], could explain the trend of

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(La0.5Eu0.5)2Zr2O7

Intensity/(a.u.)

(La0.6Eu0.4)2Zr2O7 (La0.7Eu0.3)2Zr2O7

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Fig. 7 The Raman spectra of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) powders.

Thermo-physical properties

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CTE/(10 K )

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The CTE of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4) is presented in Fig. 8. It can be seen that the CTE of all samples increases with the increasing temperature. The CTE of solid sample is related to average atomic distance between particles in the lattice, and the atomic distance would increase when the lattice vibration is intensified at elevated temperature. The CTE of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4) are 8.358×10-6 K-1, 8.55×10-6 K-1, 8.654×10-6 K-1, 8.847×10-6 K-1, 9.225×10-6 K-1 at 1000 °C, respectively. It indicates that the CTE increases as Eu3+ ions increase, which proves that Eu3+ doping can improve the CTE of La2Zr2O7. It can be rationalized in terms of the change in A-O bond strength since lowering bond strength is a contributing factor in improving the CTE of the material. The bond strength of La-O is the largest (8.3 eV) among lanthanide series, while bond strength of Eu-O is the smallest (4.9 eV) among light rare earth elements [28]. It demonstrates that doping with Eu3+ ions could optimize the CTE of La2Zr2O7, which is beneficial to the reduction of residual stress, since the mismatch of thermal expansion at interface between top coat and underlying layer would be much relaxed with the improved CTE.

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8 .0 L a 2Z r2O 7 ( L a 0 .9 E u 0 .1 ) 2 Z r 2 O 7 ( L a 0 .8 E u 0 .2 ) 2 Z r 2 O 7

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( L a 0 .7 E u 0 .3 ) 2 Z r 2 O 7 ( L a 0 .6 E u 0 .4 ) 2 Z r 2 O 7

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Fig. 8 The CTE of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4) at various temperature.

ACCEPTED MANUSCRIPT It can be seen from Fig. 9 that thermal conductivity decreases with the increase the temperature, which can be explained based on the shaking of phonon. The thermal conductivity of phonon can be expressed by Eq. (6) below [41]: 1 λ = C ⋅ν⋅l 3

(7)

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Where Cv is the specific heat capacity and remains as a constant when temperature is higher than Debye temperature,ν is the average speed of phonon and can be also regarded as a constant when the effect of modulus of elasticity (E) and density toν can be ignored. ̅ Thus, thermal conductivity is mainly decided by the mean free path of phonon (l). With the increase of temperature, the shaking energy of phonon, the frequency, and the impact probability increase and the mean free path of phonon decreases, thus resulting in the decrease of the thermal conductivity [42]. The thermal conductivity of Eu-doped zirconate is below 1.0 Wm-1K-1, which is better than that of YSZ lying in the range of 1.2-1.8 Wm-1K-1. With the increase of Eu3+ ions ratio, thermal conductivity decreases at initial period and increases afterwards, reaches the lowest value for the composition of (La0.6Eu0.4)2Zr2O7. Compared with the YSZ, doping with Eu3+ ions brings about more lattice defects, including distortion of the lattice and oxygen vacancies which maintain electro-neutrality within the lattice. These defects would increase the scattering of phonons, shorten the mean free path of phonon and lower the thermal conductivity. The defects of lattice increase at initial period and decrease afterwards with the increasing temperature, which lead to the variation of mean free path of phonon and thermal conductivity mentioned above. 0.80

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Fig. 9 The thermal conductivity of (La1-xEux)2Zr2O7 (x=0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0) at various temperature.

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Conclusions 1. The La2Zr2O7 powders doped with Eu3+ ions were synthesized by the

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copreciptitation-calcination method. The Eu-doped La2Zr2O7 has a pyrochlore structure and exhibits lattice contraction. 2. The crystallization and grain size of the doped La2Zr2O7 increase with the elevated calcination temperature. The crystallization of (La0.6Eu0.4)2Zr2O7 is optimal with calcination temperature at 1300 °C for 2 h and the grain growth activation energy of (La0.6Eu0.4) 2Zr2O7 is 136 kJ/mol. 3. The CTE of Eu-doped La2Zr2O7 is improved as Eu3+ ions increase, which shows Eu3+ ions are beneficial to improvement of the CTE of zirconate and may reduce thermal stress between the ceramic coating and underlying alloy. With the increase of Eu3+ ions ratio, the thermal conductivity decreases at initial period and increases afterwards, reaches the lowest value for the composition of (La0.6Eu0.4)2Zr2O7.

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Acknowledgements

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We would like to acknowledge the financial support from the National Key Research and Development Program of China (Grant No. 2017YFB0306100).

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ACCEPTED MANUSCRIPT [40] M. Pokhrel, M. Alcoutlabi, Y. Mao, Optical and X-ray induced luminescence from Eu3+, doped La2Zr2O7, nanoparticles, Journal of Alloys and Compounds. 693 (2017) 719-729. [41] R. Mévrel, J.C. Laizet, A. Azzopardi, B. Leclercq, M. Poulain, O. Lavigne, D. Demange, Thermal diffusivity and conductivity of Zr1-xYxO2-x/2 (x=0, 0.084 and 0.179) single crystals, Journal of the European Ceramic Society. 24 (2004) 3081-3089. [42] H.M. Zhou, D.Q. Yi, Effect of rare earth doping on thermo-physical properties of lanthanum zirconate

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ceramic for thermal barrier coatings, Journal of Rare Earths. 26 (2008) 770-774.

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Highlights • The influence of doping with Eu3+ ions on the phase structure is investigated. • Growth kinetics of Eu-doped La2Zr2O7 is investigated. • Doping with Eu3+ ions is effective in improving the coefficient of expansion. • Thermal conductivity reaches the lowest value for the composition of (La0.6Eu0.4)2 Zr2O7.

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La2Zr2O7 shows significant superiority over YSZ in terms of thermal conductivity, thermal and chemical stability, and sintering rate. However, its thermo-physical properties still need to be optimized. In this paper, phase structure, grain growth kinetics, bond strength, thermo-physical properties of the were investigated under different doping ratios. Eu-doped La2Zr2O7 nanoparticles with ordered pyrochlore structure were synthesized, which is good for the application of thermal barrier coatings. The coefficient of thermal expansion is improved by doping with Eu3+ ions, and thermal conductivity reaches the lowest value for (La0.6Eu0.4)2Zr2O7.