Thermal radiation and cycling properties of (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings

Journal Pre-proof Thermal radiation and cycling properties of (Ca, Fe) or (Sr, Mn) co-doped La2 Ce2 O7 coatings Shujuan Dong, Fengning Zhang, Neng Li, Jinyan Zeng, Panpan Liang, Hao Zhang, Huiqi Liao, Jianing Jiang, Longhui Deng, Xueqiang Cao

PII:

S0955-2219(20)30011-X

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2020.01.012

Reference:

JECS 12989

To appear in:

Journal of the European Ceramic Society

Received Date:

26 May 2019

Revised Date:

28 December 2019

Accepted Date:

6 January 2020

Please cite this article as: Dong S, Zhang F, Li N, Zeng J, Liang P, Zhang H, Liao H, Jiang J, Deng L, Cao X, Thermal radiation and cycling properties of (Ca, Fe) or (Sr, Mn) co-doped La2 Ce2 O7 coatings, Journal of the European Ceramic Society (2020), doi: https://doi.org/10.1016/j.jeurceramsoc.2020.01.012

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.

Thermal radiation and cycling properties of (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings Shujuan Dong*, Fengning Zhang, Neng Li, Jinyan Zeng, Panpan Liang, Hao Zhang, Huiqi Liao, Jianing Jiang, Longhui Deng, Xueqiang Cao State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China *

First corresponding author. E-mail address: [email protected] (S. Dong) Tel.: +86 27 87651856. E-mail address: [email protected] (X. Cao).

ro of



-p

Abstract: La2Ce2O7 with low thermal conductivity as a potential candidate of thermal

re

barrier coatings (TBCs) was co-doped with (Ca, Fe) or (Sr, Mn) in order to further improve its thermal radiation at high temperatures. The microstructure, chemical

lP

composition, infrared emission properties (reflection and absorption properties) and thermal cycling lifetime of the coatings were respectively investigated. The results

na

revealed that La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ coatings had defected fluorite structure and their infrared emittances were much higher than that of the

ur

parent La2Ce2O7. The superior infrared emission could be ascribed to the

Jo

enhancement of the intrinsic absorption (electron transition absorption), free-carrier absorption and impurity absorption as well as lattice vibration absorption. However, the thermal cycling lifetime of La2Ce2O7 coatings presented a reduction after the (Ca, Fe) or (Sr, Mn) substitution, primarily due to the decrease in the fracture toughness

*

First corresponding author. E-mail address: [email protected] (S. Dong) Tel.: +86 27 87651856. E-mail address: [email protected] (X. Cao). 1



and the increase in the thermal conductivity. Keywords: La2Ce2O7 coatings; Thermal radiation; Infrared emittance; Co-doping; Thermal barrier coatings (TBCs) 1. Introduction Heat transfer by thermal radiation technology has been rapidly developed for the heat dissipation applications, in the field of supersonic weapons as well as for the energy

ro of

conservation in the field of civilian industries, etc. Especially, thermal radiation is the

only way to dissipate heat under space/vacuum conditions. According to Plank’s law,

90% of the electromagnetic waves emitted by the black body are concentrated in the

-p

infrared region, when its temperature reaches 1000oC. For example, missile warheads

re

coated with high infrared radiation materials could overcome the aerothermal problems. Therefore, thermal radiative coatings with high infrared emittance have

na

temperatures [1-7].

lP

attracted great attention as thermal protection systems (TPSs) used at high

Lanthanum cerium oxide (La2Ce2O7) [8-10], because of its low thermal conductivity

ur

and good thermal stability, etc., has been investigated as a candidate material of thermal barrier coatings (TBCs) to replace conventional Y2O3-stabilized ZrO2 (YSZ)

Jo

whose long-term reliable operation is limited at the temperatures higher than 1200oC due to the phase transformation. As reported in the literatures [10-12], La2Ce2O7 could be stable without phase transformation after long-term annealing at 1400oC and its single layer could similarly function thermal cycling performance to traditional YSZ. On this basis, (La1−xGdx)2Ce2O7 ceramics have been demonstrated to have 2

higher thermal expansion coefficients and lower thermal conductivities than 8YSZ, mainly being ascribed to the increase of the number of oxygen vacancies [13]. (La0.95Sr0.05)2Ce2O6.95

and

(La0.95Ca0.05)2Ce2O6.95

and

(La0.95Mg0.05)2Ce2O6.95

single-phase ceramics with fluorite structure have also been explored as a candidate material for TBCs [14, 15]. Besides these works about the improvement of the thermal

resistance

of

La2Ce2O7

by

substitution,

La2Ce2O7/8YSZ

and

ro of

La2(Zr0.7Ce0.3)2O7/La2Ce2O7 double-ceramic-layer TBCs prepared by atmospheric

plasma spraying (APS) [16-18] or electron beam-physical vapor deposition (EB-PVD) [19] have been found to have a much longer lifetime compared with the single layer

-p

La2Ce2O7 system due to the alleviation of the thermal expansion mismatch. Moreover,

re

segment-structured [20] and gradient-structured [21] La2Ce2O7/YSZ TBCs have been designed to improve the coating durability. These investigations have primarily

lP

focused on the thermal barrier/heat insulation properties of La2Ce2O7 coatings. Up to

na

now, reports on the infrared emitting properties of La2Ce2O7 coatings are very rare. In this work, La2Ce2O7 was chosen as the parent compound to be doped in order to

ur

develop high infrared radiation coatings. Their thermal radiation properties could be combined with the thermal barrier properties to finally improve the comprehensive

Jo

thermal protection performance. The heat energy not only can be emitted through thermal radiation but also be isolated by taking advantages of low thermal conductivity of the coatings. La2Ce2O7 (A2B2O7-type) ceramics were substituted by alkaline earth metal or transition metal atoms (Ca, Fe) or (Sr, Mn) at A and B sites. Their infrared emission properties (normal spectral emissivity, etc.) were investigated. 3

A series of plasma-sprayed coatings with the general formula of La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ were explored. In addition, their thermal cycling performance was evaluated and the failure mechanisms were analyzed in details.

2. Experimental procedures 2.1 Sample preparation

ro of

The raw materials of lanthanum oxide, cerium oxide, calcium oxide, iron oxide, strontium carbonate and manganese oxide, with mass fraction greater than 99.99%,

were mixed with deionized water and zirconia balls in a nylon tank and ball milled for

-p

24 hours, according to the designed contents of different elements in the molecular

re

formula La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ (x=0.1, 0.2, 0.4) (Table 1). These mixtures were designed following the substitution principles of unequal

lP

valence or different ionic radii. The abbreviated LCCF1, LCCF2, LCCF4 and LCSM1,

na

LCSM2, LCSM4 were respectively used to mark La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ as x changed from 0.1 to 0.4 (the corresponding

ur

over-stoichiometry δ changes from -0.1 to -0.4). La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ powders were synthesized by solid-state reaction at 1500°C for

Jo

12 h and 1200°C for 12 h, respectively. Afterwards these powders were spray-dried to obtain the free-flowing powders to prepare coatings. The spray-dried powders were then sprayed onto AISI 316L stainless steel and GH4169 superalloy substrates using Multi-Coat plasma spraying unit with a F4-MB gun (Oerlikon Mecto, Wohlen, Switzerland) to deposit coatings. For improving the 4

bonding strength of the ceramic coatings on the substrates, bond coats using commercial gas-atomized NiCrAlY powders (Amdry 9624, Oerlikon Mecto) were prepared between the substrates and the ceramic coatings. All the substrates were sandblasted prior to spraying. The optimized plasma spraying parameters for depositing bond and ceramic coatings are as follows: arc current (585A), arc voltage (72 V), primary plasma gas (Ar, 35 NLPM), secondary plasma gas (H2, 12 NLPM),

ro of

powder carrier gas (Ar, 2.8 NLPM).

2.2 Characterization techniques

-p

The microstructural phases of the as-sprayed coatings were analyzed using X-ray

re

diffractometer (XRD, D8 Discover, Bruker) with Cu Kα radiation. The scan rate was 2°/min. The PDF2-2004 reference database was used to match/identify peaks by

lP

inputting a chemical filter. The cross-sectional microstructures of the coatings were

FEG-450).

na

observed using a field emission scanning electron microscopy (FE-SEM, QUANTA

ur

The UV-visible-NIR total reflectance and UV-visible absorption spectra of the as-sprayed coatings were measured using a Lambda 750S spectrophotometer. The

Jo

molecular spectra in MIR wavelength of the powders were analyzed by a Nicolet 6700/Nicolet 6700 Fourier transform infrared spectrometer. Since the Fourier transform used by the infrared spectrometer processed the signals of the lights, which avoided the error caused by the grating beam splitting, the repeatability of the absorbance was better. 5

The normal spectral emittance in the infrared wavelength range of 1-22 μm of the as-sprayed coatings was evaluated at different temperatures, based on the measurement principle of reflectance, using an IR-2 infrared radiation instrument equipped with several detectors. Heating of the samples is carried out in a tubular furnace which was used upto 600oC. The infrared emittance at the temperatures from 600 to 1000oC was extrapolated, based on the data trend below 600oC. The spectral

ro of

emittance was deduced by the reflectance measurements, called indirect method. According to the second Kirchhoff law:     1

(1)



the transmittance and  the reflectance.

-p

Where  denotes the absorbance,

re

The ceramics could be regarded as opaque materials as the thickness is higher than 2.5 μm, i.e., transition ratio is 0, the formula (1) could be expressed as follows:

lP

   1

(2)



= 

Thus, the infrared emittance



could be obtained:

=1- 

(4)

Jo



(3)

is the infrared emittance.

ur

Where

na

According to Kirchhoff’s law:

To further reveal the hetero-element doping effect on the thermal properties of La2Ce2O7, well resolved density functional theory (DFT) calculation was performed. The projected augmented wave (PAW) pseudopotentials implemented in the Vienna Ab-initio Simulation Package (VASP) code [22-24]. The Perdew-Burke-Ernzerhof 6

(PBE)

generalized

gradient

approximation

(GGA)

was

employed

for

exchange-correlation functions [25]. The cut-off energy for plane wave basis was set to 400 eV, the energy and force convergence criteria per atom of 10−4 eV and 0.02/Å, and the Monkhorst-Pack scheme kpoint mesh with 2×2×2 point for the optimization and density of states (DOS) calculations. In addition, thermal cycling/shock tests were performed for the coatings in an electric

ro of

furnace. The samples were isothermally held at 1050℃ for 34 min and subsequently quenched to room temperature within 2 min by compressed air. This procedure was

considered as one thermal cycle. The thermal shock lifetimes were defined as the

-p

cycling number when the spallation surface area of the coatings reached 20%.

re

Vickers microhardness was examined by Vickers indentation (VH1202, Wilson) with

shown below [26]: K IC  0 .1 6 H v 

a  (c / a )

lP

a 200 g load. The fracture toughness was calculated as well based on the equation

3 / 2

(5)

na

Where K IC denotes the fracture toughness (MPa  m1/2), Hv is the microhardness

ur

(GPa), a is the diagonal length (μm) and c is the crack length (μm). The dimensions of the free-standing coatings for linear thermal expansion coefficient

Jo

(TEC or CTE) and thermal diffusivity measurements were (23-25)×(5-8)×(1-1.2) mm3 and 10×10×1 mm3, respectively. The CTE was determined with a high temperature dilatometry (NETZSCH DIL 402C, Germany), operating in a temperature range from 200℃ to 1200℃. The heating rate was 5℃/min. The measurement of thermal diffusivity was carried out in a nitrogen atmosphere using the laser flash method 7

(NETZSCH LFA 457, Germany) from room temperature to 1000℃ at a temperature interval of 200℃. To enhance the absorption and emission of the laser beam, both the front and the back faces of the specimen were coated with a thin layer of graphite before thermal-diffusivity measurement.

3. Result and discussion

ro of

3.1 Coating microstructure and phase analysis Fig. 1 shows typical cross-sectional morphologies of plasma-sprayed (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings as well as the parent coating. All the coatings

-p

consisted of a NiCrAlY bond coat with a thickness of about 100 μm and a single or

double ceramic coats. For samples with single ceramic layer, the ceramic thickness is

re

250~300 μm and for double ceramic coatings the thickness of each ceramic layer is

lP

controlled to be 125~150 μm. All the coatings have a similar laminated structure with a small amount of tiny pores less than 3 μm and good bonding between the layers.

na

Fig. 2 shows the XRD spectra of the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings in comparison with the parent coating. It can be seen that the XRD patterns of all the

ur

coatings are very similar to that of standard CeO2 (JCPDS Card No.34-0394), except

Jo

that the peak position is slightly shifted toward the low diffraction angle. It is reasonable because La2Ce2O7 ceramic is a solid solution of CeO2 dissolved by La2O3. The diffraction peaks of the parent La2Ce2O7 coating are successfully matched to the fluorite-type phase (only one diffraction peak appears in the range of 2θ=30°~45° for the fluorite structure and two peaks should be appear in the same range for the pyrochlore structure. Pyrochlore structure compounds can be regarded as ordered 8

fluorite structure oxides with oxygen vacancies). As for (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings, no other phase can be distinguished from XRD patterns, indicating that (Ca, Fe) or (Sr, Mn) have inserted into the La2Ce2O7 lattices and formed the La2-xCaxCe2-xFexO7+δ or La2-xSrxCe2-xMnxO7+δ solid solution. They are identified as the disordered defect fluorite-type structure with the space group Fm 3 m. As the dopant contents of (Ca, Fe) or (Sr, Mn) increase, the coatings keep the defected

ro of

fluorite-type structure unchanged. The lattice parameters and the full width at half

maximum (FWHM) (Table 2) decrease with increasing the dopant amount into La2Ce2O7. The decrease in the lattice parameter indicates the lattice shrinkage after

-p

co-doping. It should be noted that additional weak peaks corresponding to La2O3

re

(JCPDS Card No.05-0602) could sometimes be detected at around 2θ of 28°, 30°, 40°, 45° and 53°, especially at the spectra of LC, LCCF2 as well as LCSM1-4 samples.

lP

The appearance of La2O3 peaks indicates the excess La2O3 in the coatings owing to the partial decomposition of the coating material and the more CeO2 evaporation

na

(higher vapor pressure of CeO2 ~2×10−2 atm at 2773 K than La2O3 ~8×10−5 atm at

ur

2773 K [11]) during the plasma spraying. Similar patterns could be found in the literatures [11, 27, 28]. Such preferential evaporation of CeO2 in spray process

Jo

deviated the coating composition from the stoichiometry of the original powders, resulting in the degraded performance of LC-based TBCs.

3.2 Infrared emittance Fig. 3 shows the infrared emittances of the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 9

coatings as well as the parent coating in the band of 1-22 μm at different temperatures. It could be found by comparison that the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings have significantly higher infrared emittance than the parent one. In addition, with the increase of doped (Ca, Fe) or (Sr, Mn) (ranged from 0.1 to 0.5), the infrared emittance increases. Especially, the infrared emittance of (Sr, Mn) co-doped La2Ce2O7 coatings is basically higher than 0.85.

ro of

With the increase of the temperature from room temperature to 1000oC, the infrared emittance increases for both kinds of the co-doped coatings. The maximum infrared

emittance appears at 1000oC. The (Ca, Fe) co-doped series has the highest infrared

-p

emittance of 0.94 (LCCF4) at 1000oC and the (Sr, Mn) co-doped series has the

re

highest value of 0.97 (LCSM4) at 1000oC, while the infrared emittance of the parent La2Ce2O7 coating maintains at about 0.76.

lP

Fig. 4 shows the reflection spectra of (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7

na

coatings compared with the parent one in the UV-Vis-NIR regions at room temperature. It is obvious that La2Ce2O7 coating has excellent reflection properties in

ur

the near-infrared region (750 to 2500 nm). Since La3+ and Ce4+ show d0 and f0 configurations in their stable oxidation states, there are no f-f or f-d transitions [29].

Jo

Although the absorption edges have no significant shift along the wavelength (Fig. 4), the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings present a remarkable reduction in the reflectance in comparison with the parent coating. Moreover, the near-infrared reflectance decreases with the increase of the doping amount and those of the (Ca, Fe) doped series are always higher than those of the (Sr, Mn) doped series at the same 10

dopant concentrations. By integrating the reflection spectra of the coatings in the near-infrared band, the average reflectance could be obtained and listed in Table 1. The corresponding absorbance could be calculated according to Eq. (4) and also listed in Table 1. The absorbance of the (Ca, Fe) doped La2Ce2O7 coating could reach up to 71.6%, improved from 14.1% of the pure La2Ce2O7 coating. Similarly, the absorbance of the (Sr, Mn) doped La2Ce2O7 coating could reach up to 87.9%. Therefore, (Ca, Fe)

ro of

or (Sr, Mn) co-doping La2Ce2O7 coatings achieve a transition from a high-infrared reflectance to a high-infrared absorbance in the near-infrared region.

energy level to a high energy level is [30]:

(6)

re

 E   E ro ta tio n   E e le c tro n ic   E v ib ra tio n

-p

It is well-known that the total energy required for molecule transition from a low

The rotational energy (  E ro ta tio n ) generally corresponds to the spectrum in the far

lP

infrared band (FIR: 25-1000 μm). It has been proved that the intrinsic absorption,

na

corresponding to the electronic transition from the valence band (VB) to the conduction band (CB) is the most important absorption for semiconductors. Therefore,

ur

the improvement in the infrared emittance of the doped ceramic coatings in the band of 1-22 μm could be explained from the combined aspects of electron transition and

Jo

lattice vibration.

The energy band gap (Eg) required for the electron transition from the valence band to the conduction band, can be evaluated by using Tauc’s equation [31]: ( h v )  k ( h v  E g ) 2

2

(7)

Where α is the absorbance obtained using Eq.(4), h is the Plank’s constant (4.136 11



10-15 eV.s),

v

is photon frequency, k is a material-dependent constant.

Their corresponding absorption spectra are platted in Fig. 5 converted from Fig. 4 on basis of Eq.(4). Combined with Eq. (7), the relationship plots of (α h v )2 verses could be sketched out. According to the plot of (α h v )2 verses

hv

hv

of the coatings for

a direct transition in Figs. 5b and 5d, the value of Eg could be extrapolated from the coordinate intercept when α h v =0. The detailed band gap values of the coatings are

ro of

shown in Table 2. It can be found that co-doping (Ca, Fe) or (Sr, Mn) in La2Ce2O7 results in a systematic decrease in band gap, which is consistent with the calculated G

results in Fig. 6. The diffuse reflectance spectra of La2Ce2O7 are mainly derived

-p

from the shifts of charge transfer transitions between O2p valence band and Ce4f

re

conduction band. The lowest band gap of (Ca, Fe) co-doped La2Ce2O7 coatings is estimated to be 2.61 eV and that of (Sr, Mn) co-doped series is 2.75 eV, while the

lP

parent coating has a value of 3.02 eV. The corresponding maximum excitation

na

wavelength (λ) has been estimated as 475 nm and 451 nm, compared with 411 nm of the parent coating, according to the equation as follows: (8)

ur

λ=hc/Eg

Where c is the light speed, h is the Plank’s constant (4.136



10-15 eV.s), Eg is the

Jo

energy band gap.

As a consequence, it could be deduced that the co-doping of (Ca, Fe) or (Sr, Mn) has a promoting tendency to extend the spectral absorption band of La2Ce2O7 from the UV-visible light region to the near-infrared region. The reduction of band gap for co-doped La2Ce2O7 coatings is caused by the 12

introduction of the impurity energy. By doping La2Ce2O7 with Fe3+ or Mn4+ ions for Ce4+, an additional 3d electronic energy level, as impurity energy level, has been introduced between the O2p and Ce4f orbitals (Fig. 6b), resulting in the widening of valence band. In addition, the valence change of metal ions, the replaced Ca2+ or Sr2+ for La3+ as well as Fe2,3+ or Mn2,3,4+ for Ce4+, leads to the formation of oxygen vacancies, promoting the increase in the free carrier concentration. The free-carrier

ro of

absorption is closely related to the temperature. Therefore, the enhanced thermal

emittances of co-doped La2Ce2O7 coatings in the NIR band are dominantly attributed to the intrinsic absorption and free-carrier absorption. As the temperature increases,

-p

the probability of electron transition increases and consequently the infrared emission

re

increases.

Fig. 7 shows the Fourier transform infrared spectra of (Ca, Fe) or (Sr, Mn) co-doped

lP

La2Ce2O7 ceramics compared with the parent compound in the MIR band (2.5-20

na

μm/4000-500 cm-1) at room temperature. The Fourier transform infrared spectra of all the coatings are similar in overall shape and have two strong infrared vibration

ur

absorption peaks at about 500 and 3500 cm-1. It is related with the lattice vibration of the polyhedron composed of La-O or Ce-O in La2Ce2O7 ceramics. According to

Jo

Hooke’s law, the smaller the constituent atomic mass is, the greater the vibration frequency. The absorption peak near 500 cm-1 should correspond to the vibration of the Ce-O ligand and that near 3500 cm-1 corresponds to the La-O. In addition, some small vibrations also occur between 1000 and 2000 cm−1. After co-doping (Ca, Fe) or (Sr, Mn) for La and Ce atoms, the vibration oscillations increase near 500 and 3500 13

cm-1. Considering the radii of different ions La3+(0.116 nm), Ca2+(0.112 nm), Sr2+(0.126 nm) as well as Ce4+(0.097 nm), Fe3+(0.078 nm), Mn4+(0.053 nm) [32], the dissolution of dopant atoms into the La2Ce2O7 lattices would result in an asymmetry of lattice structure (Fig. 6a). Moreover, the distortion of La2Ce2O7 lattice increases with the increase of the doping level. The lattice distortion leading to polarization introduces new transition possibility of optical phonon by changing the vibration

ro of

periodicity/activity and improves the lattice absorption at long wavelengths [33]. The oscillations in the 2.5-20 μm regions are attributed to the lattice absorption of the defected fluorite-type structure. The higher infrared emittance of the coatings in the

-p

MIR band at the higher temperatures (Fig. 3) may be attributed to the stronger lattice

re

vibration.

Therefore, the infrared emittance enhancement of the co-doped La2Ce2O7 coatings is

lP

attributed to the intrinsic absorption, the impurity absorption, the free carrier

na

absorption as well as the lattice absorption.

ur

3.3 Thermal cycling performance

The thermal shock cycling lifetimes of all the coatings are displayed in Fig. 8. Under

Jo

the same tested conditions, the parent La2Ce2O7 coating achieves more than 10-fold longer thermal cycling lifetime than the (Ca, Fe) or (Sr, Mn) co-doped coatings. Even for double-layered coatings of co-doped ceramics/YSZ systems, their lifetimes are only one-sixth or one-eighth of that of the parent coating. Fig. 9 shows the cross-section microstructures of the (Ca, Fe) or (Sr, Mn) co-doped 14

La2Ce2O7 coatings as well as the parent coating after the thermal shock tests. Although the La2Ce2O7 coating still attaches to the bond coat, a large crack occurs near the interface between the ceramic layer and the bond coat and many continuous microcracks appear inside the ceramic layer. The (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings after thermal shock tests present similar failure structure with the parent coating, for which only a thin ceramic layer still attaches on the surface or

ro of

large horizontal cracks appear within the non-spallation area of the ceramic layer. The spallation basically parallels to the surface of the bottom ceramic layer. However, no

microcrack can be found at the interface between the ceramic layer and the bond coat

-p

for all the coatings. In addition, the thermally grown oxide (TGO) layer is not obvious

re

for all the coatings after thermal shock tests. X-ray diffraction patterns of all the coatings after thermal shock tests are also shown in Fig. 2. It is clearly seen that the

lP

non-spalled ceramic layer of the doped coatings after thermal shock has still a defect

na

fluorite structure, indicating no presence of phase transition during thermal shock tests. Here, it should be mentioned that the weak La2O3 peaks of all the coatings disappear

ur

after thermal cycling tests. Such disappearance could be attributed to the incorporation of La2O3 into LC/LCCF/LCSM solid solution at high temperatures,

Jo

which has also been discovered for the annealed La2Ce2O7 coatings and verified by the larger d-values [27]. Therefore, the failure of all the coatings is neither caused by the TGO generation from the bond coat nor by the phase transition. It is generally believed that this type of spallation of TBCs can be mainly resulted from the low fracture toughness of the top ceramic layer and/or the thermal expansion 15

mismatch between the ceramic top coat and the metallic bond coat/substrate [34-36]. The CTE evolution profiles versus the temperature of the co-doped La2Ce2O7 coatings are shown in Fig. 10 in comparison with that of the parent coating. It can be seen that the average CTEs of the (Ca, Fe) or (Sr, Mn) co-doped series are higher than that of the parent one, especially for LCCF4 and LCSM4 samples. It could thus be concluded that the main reason for the remarkable reduction of the thermal cycling lifetimes of

ro of

the co-doped coatings is not CTE mismatch between the ceramic layer and the bond coat/substrate. In fact, the dopants dissolved in the fluorite crystal lattice of the parent

La2Ce2O7 would contribute to the reduction in fracture toughness (Table 2), therefore

-p

playing a negative influence on the thermal shock resistance. Moreover, it has been

re

observed that the degradation process becomes accelerated, once the crack in the co-doped coatings occurs during the thermal shock tests concerned. This dominantly

lP

resulted from the rapid reduction or release of the thermal stress or the strain energy

na

within the ceramic top-coat after the coating cracking [34]. As illustrated in Fig. 11, the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings have relatively lower thermal

ur

insulation ability than the parent coating. Accordingly, the relatively low fracture toughness is one of the important attributions

Jo

to premature failure of all the coatings. The relatively low fracture toughness and the low thermal insulation could be the primary reasons responsible for the reduction of thermal cycling lifetime for the (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings, compared with the parent coating.

16

4. Conclusions (1) La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ ceramic coatings with defected fluorite structure (space group Fm 3 m) were prepared. (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings have significantly higher infrared emittance than the parent La2Ce2O7 coating. With the increase of the doped (Ca, Fe) or (Sr, Mn) amount as well as the temperature, the infrared emittance increases. The highest infrared emittance

ro of

was obtained to be 0.97 for LCSM4 at 1000oC.

(2) The enhanced thermal emittances of co-doped La2Ce2O7 coatings in the infrared band are dominantly attributed to the interaction of the intrinsic absorption (electron

-p

transition absorption), free-carrier absorption and impurity absorption as well as

re

lattice vibration absorption.

(3) (Ca, Fe) or (Sr, Mn) co-doped La2Ce2O7 coatings have a reduction in the thermal

lP

cycling lifetime and present similar failure structure with the parent coating. The

na

relatively low fracture toughness and the low thermal insulation could be the primary reasons responsible for the reduction of thermal cycling lifetime.

ur

The co-doped La2Ce2O7 coatings have potentials to be used as thermal radiative materials on the inner walls of civilian boilers and aero-engine tailpipes without the

Jo

need of thermal cycling lifetime.

17

Declaration of Interest Statement Thermal radiation/infrared emission of La2Ce2O7 coatings with low thermal conductivity could be improved by co-doping with (Ca, Fe) or (Sr, Mn). The superior infrared emission could be ascribed to the enhancement of the intrinsic absorption, free-carrier absorption and impurity absorption as well as lattice vibration absorption. Their thermal radiation properties could be combined with the thermal barrier properties to comprehensively improve the heat resistance.

ro of

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant

-p

numbers 51875424 and 51501137] and the Excellent Dissertation Cultivation Funds of Wuhan University of Technology [grant number 2018-YS-009] as well as the

lP

re

Fundamental Research Funds for the Central Universities [WUT: 2019Ⅲ033].

References

na

[1] J.W. Mao, S.Y. Ding, Y.J. Li, S.H. Li, F.S. Liu, X. Zeng, X.D. Cheng, Preparation and investigation of MoSi2/SiC coating with high infrared emissivity at high

ur

temperature, Surface & Coatings Technology 358 (2019) 873-878.

Jo

[2] J.Q. Chen, Y. Xiao, B. Huang, X.Q. Sun, Sustainable cool pigments based on iron and tungsten co-doped lanthanum cerium oxide with high NIR reflectance for energy saving, Dyes and Pigments 154 (2018) 1-7. [3] Y.J. Li, T.Y. Ouyang, X.H. Wang, S.H. Li, J.W. Mao, X.D. Cheng, Effect of boron on enhancing infrared emissivity of Ni-Cr system coating, Infrared Physics & Technology 89 (2018) 20-26. 18

[4] B. Huang, Y. Xiao, C. Huang, J.Q. Chen, X.Q. Sun, Environment-friendly pigments based on praseodymium and terbium doped La2Ce2O7 with high near-infrared reflectance Synthesis and characterization, Dyes and Pigments 147 (2017) 225-233. [5] S.Y. Ding, J.W. Mao, X. Zeng, X.D. Cheng, Enhanced infrared emission property of NiCr spinel coating doped with MnO2 and rare-earth oxides, Surface & Coatings

ro of

Technology 344 (2018) 418-422.

[6] J.P. Huang, C.L. Fan, G.P. Song, Y.B. Li, X.D. He, X.J. Zhang, Y. Sun, S.Y. Du,

Y.J. Zhao, Enhanced infrared emissivity of CeO2 coatings by La doping, Applied

-p

Surface Science 280 (2013) 605-609.

re

[7] J.Y. Zhang, X. Fan, L. Lu, X.M. Hu, G.Q. Li, Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application

lP

on energy-saving kettle, Applied Surface Science 344 (2015) 223-229.

na

[8] H.Y. Dong, D.X. Wang, Y.L. Pei, H.Y. Li, P. Li, W. Ma, Optimization and thermal cycling behavior of La2Ce2O7 thermal barrier coatings, Ceramics International 39

ur

(2013) 1863-1870.

[9] K. Liu, J.J. Tang, Y. Bai, Q.Z. Yang, Y. Wang, Y.X. Kang, L. Zhao, P. Zhang, Z.H.

Jo

Han, Particle in-flight behavior and its influence on the microstructure and mechanical property of plasmasprayed La2Ce2O7 thermal barrier coatings, Materials Science & Engineering A 625 (2015) 177-185. [10] L.H. Gao, H.B. Guo, S.K. Gong, H.B. Xu, Plasma-sprayed La2Ce2O7 thermal barrier coatings against calcium-magnesium-alumina-silicate penetration, Journal of 19

the European Ceramic Society 34 (2014) 2553-2561. [11] L.S. Wang, S.L. Zhang, T. Liu, C.J. Li, C.X. Li, G.J. Yang, Dominant effect of particle size on the CeO2 preferential evaporation during plasma spraying of La2Ce2O7, Journal of the European Ceramic Society 37 (2017) 1577-1585.

[12] X.Q. Cao, R. Vassen, W. Fischer, F. Tietz, W. Jungen, D. Stover, Lanthanum–

ro of

cerium oxide as a thermal barrier-coating material for high-temperature applications, Advanced Materials 17 (2003) 1438-1442.

[13] H.S. Zhang, X.G. Chen, G. Li, X.L. Wang, X.D. Dang, Influence of Gd2O3

-p

addition on thermophysical properties of La2Ce2O7 ceramics for thermal barrier

re

coatings, Journal of the European Ceramic Society 32 (2012) 3693-3700. [14] H.S. Zhang, J.G. Lv, G. Li, X.G. Chen, X.L. Wang, Preparation, characterization,

lP

and thermophysical properties of (La0.95Sr0.05)2Ce2O6.95 ceramic for thermal barrier

na

coatings, Int. J. Appl. Ceram. Technol. 11 (2014) 350-358. [15] H.S. Zhang, Y. Wei, G. Li, X.G Chen, X.L. Wang, Investigation about thermal

ur

conductivities of La2Ce2O7 doped with calcium or magnesium for thermal barrier coatings, Journal of Alloys and Compounds 537 (2012) 141-146.

Jo

[16] W. Ma, H.Y. Dong, H.B. Guo, S.K. Gong, X.B. Zheng, Thermal cycling behavior of La2Ce2O7/8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying, Surface & Coatings Technology 204 (2010) 3366-3370. [17] K. Liu, Y. Bai, J.R. Li, J. Ma, J.G. Du, Y. Cao, X.K. Li, Structure-property 20

relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades, Ceramics International 44 (2018) 13662-13673. [18] P.G. Lashmi , P.V. Ananthapadmanabhan , Y. Chakravarthy , G. Unnikrishnan , N. Balaji, S.T. Arun, Hot corrosion studies on plasma sprayed bi-layered YSZ/La2Ce2O7 thermal barrier coating fabricated from synthesized powders, Journal of Alloys and Compounds 711 (2017) 355-364.

ro of

[19] W. Ma , S.G. Gong, H.F. Li, H.B. Xu, Novel thermal barrier coatings based on

La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition, Surface & Coatings Technology 202 (2008) 2704-2708.

-p

[20] H.B. Guo, Y. Wang, L. Wang, S.K. Gong, Thermo-Physical Properties and

re

Thermal Shock Resistance of Segmented La2Ce2O7/YSZ Thermal Barrier Coatings, Journal of Thermal Spray Technology Volume 18 (2009) 665-671.

lP

[21] Y. Bai , W. Fan, K. Liu , Y.X. Kang, Y. Gao, F. Ma, Gradient La2Ce2O7/YSZ

na

thermal barrier coatings tailored by synchronous dual powder feeding system, Materials Letters 219 (2018) 55-58.

ur

[22] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comp. Mater. Sci 6 (1996)

Jo

15-50.

[23] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169-11186. [24] G. Kresse, D. Joubert, From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Phys. Rev. B 59 (1999) 1758-1775. 21

[25] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett 77 (1996) 3865-3868. [26] A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc 59 (1976) 371-372. [27] H. Zhang, J.S. Wang, S.J. Dong, J.Y. Yuan, X. Zhou, S.W. Duo, S. Chen, P.J. Huo, J.N. Jiang, L.H. Deng, X.Q. Cao, Mechanical properties and thermal cycling

ro of

behavior of Ta2O5 doped La2Ce2O7 thermal barrier coatings prepared by atmospheric plasma Spraying, Journal of Alloys and Compounds 785 (2019) 1068-1076.

[28] H. Zhang, J.B. Sun, S.W. Duo, X. Zhou, J.Y. Yuan, S.J. Dong, X. Yang, J.Y.

-p

Zeng, J.N. Jiang, L.H. Deng, X.Q. Cao, Thermal and mechanical properties of Ta2O5

re

doped La2Ce2O7 thermal barrier coatings prepared by atmospheric plasma spraying, Journal of the European Ceramic Society 39 (2019) 2379-2388.

lP

[29] V.S. Vishnu, M.L. Reddy, Near-infrared reflecting inorganic pigments based on

na

molybdenum and praseodymium doped yttrium cerate: synthesis, characterization and optical properties, Sol Energy Mater. Sol Cells 95 (2011) 2685-2692.

ur

[30] H Z Liu, J H Ouyang, Z G Liu, et a1, Microstructure thermal shock resistance and thermal emissivity of plasma sprayed LaMAl11O19(M=Mg,Fe) Coating for

Jo

metallic thermal protection systems, Applied Suace Science 271 (2013) 52-59. [31] D.P. Singh, J. Singh, P.R. Mishra, R.S. Tiwari, O.N. Srivastava, Synthesis, characterization and application of semiconducting oxide (Cu2O and ZnO) nanostructures, Bull. Mater. Sci 31 (2008) 319-325. [32] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic 22

distances in halides and chalcogenides, Acta Crystallogr A32 (1976) 751-67. [33] S.M. Wang, Effects of Fe on crystallization and properties of a new high infrared radiance glass-Ceramics, Environmental Science & Technology 44 (2010) 4816-4820. [34] X.H. Zhong, H.Y. Zhao, X.M. Zhou, C.G. Liu, L. Wang, F. Shao, K. Yang, S.Y. Tao, C.X. Ding, Thermal shock behavior of toughened gadolinium zirconate/YSZ double-ceramic-layered thermal barrier coating, Journal of Alloys and Compounds

ro of

593 (2014) 50-55.

[35] Z.G. Liu, W.H. Zhang, J.H. Ouyang, Y. Zhou, Novel double-ceramic-layer

Ceramics International 40 (2014) 11277-11282.

-p

(La0.8Eu0.2)2Zr2O7YSZ thermal barrier coatings deposited by plasma spraying,

re

[36] J.H. Yu, H.Y. Zhao, S.Y. Tao, X.M. Zhou, C.X. Ding, Thermal conductivity of

ur

na

(2010) 799-804.

lP

plasma sprayed Sm2Zr2O7 coatings, Journal of the European Ceramic Society 30

Figure captions:

Jo

Fig. 1 SEM cross-sectional morphologies of as-sprayed coatings: (a) LC, (b) LCCF1, (c) LCCF2, (d) LCCF4, (e) LCSM1, (f) LCSM1/YSZ bilayer, (g) LCSM2, (h) LCSM4. Fig. 2 XRD patterns of the coatings: (a) as-sprayed (Ca, Fe) co-doped series and (b) as-sprayed (Sr, Mn) co-doped series as well as (c) (Ca, Fe) co-doped series and (d) (Sr, 23

Mn) co-doped series of La2Ce2O7 after thermal cycling. Fig. 3 Infrared emittances of as-sprayed coatings: (a) (Ca, Fe) co-doped series and (b) (Sr, Mn) co-doped series of La2Ce2O7. Fig. 4 UV-Vis-NIR reflection spectra of as-sprayed coatings: (a) (Ca, Fe) co-doped series and (b) (Sr, Mn) co-doped series of La2Ce2O7. Fig. 5 UV-Visible absorption spectra and plots of hv~(αhv)2 of as-sprayed coatings: (a,

ro of

a’) (Ca, Fe) co-doped series and (b, b’) (Sr, Mn) co-doped series of La2Ce2O7.

Fig. 6 (a) A schematic representation of the local structures of La2Ce2O7 and its

hetero-element doping structure; (b) and (c) are the total density of states and zoom-in

-p

of La2Ce2O7 and its hetero-element doping structure.

Mn) co-doped series of La2Ce2O7.

re

Fig. 7 IR spectra of as-synthesized powders: (a) (Ca, Fe) co-doped series and (b) (Sr,

na

of the parent coating.

lP

Fig. 8 Thermal shock lifetimes of the co-doped La2Ce2O7 coatings compared with that

Fig. 9 SEM cross-sectional morphologies of the coatings after thermal cycling tests: (a)

ur

LC, (b) LCCF1, (c) LCCF2, (d) LCCF4, (e) LCSM1, (f) LCSM1/YSZ bilayer, (g) LCSM2, (h) LCSM4.

Jo

Fig. 10 Thermal expansion coefficient curves of as-sprayed coatings: (a) (Ca, Fe) co-doped series and (b) (Sr, Mn) co-doped series of La2Ce2O7. Fig. 11 Thermal conductivities of as-sprayed coatings: (a) (Ca, Fe) co-doped series and (b) (Sr, Mn) co-doped series of La2Ce2O7.

24

(a)

(b)

Resin LC

LCCF1

NiCrAlY Substrate

NiCrAlY

LCCF2

LCCF4

ro of

Resin

(d)

(c)

NiCrAlY NiCrAlY

(f)

re

(e)

-p

Substrate

LCSM1

na

NiCrAlY

lP

LCSM1

NiCrAlY

(h)

ur

(g)

YSZ

LCSM2

Jo

LCSM4

NiCrAlY Substrate

NiCrAlY

Fig. 1

25

(b)

(a)

(d)

re

-p

ro of

(c)

Jo

ur

na

lP

Fig. 2

26

(b)

ro of

-p

re

lP

na

ur

Jo

(a)

Fig. 3

27

(b)

ro of

-p

re

lP

na

ur

Jo (a)

Fig. 4

28

(a’)

(a)

ro of

(b’)

Jo

ur

na

lP

re

Fig. 5

-p

(b)

29

30

ro of

-p

re

lP

na

ur

Jo Fig. 6

ro of

-p

re

lP

na

ur

Jo (a)

(b)

Fig. 7

31

32

ro of

-p

re

lP

na

ur

Jo Fig. 8

(a)

(b)

Spallation interface

LC Microcracks Horizontal cracks

LCCF1 NiCrAlY

NiCrAlY

Substrate

(c)

(d)

LCCF4 LCCF2

NiCrAlY

NiCrAlY

LCSM1

(f)

lP

YSZ

NiCrAlY

na

Substrate

Horizontal cracks

Substrate

LCSM4

Jo

LCSM2

NiCrAlY

(h)

ur

(g)

Spallation interface

re

Spallation interface Microcracks

-p

Substrate

Substrate

(e)

ro of

Spallation interface

Spallation interface

NiCrAlY

NiCrAlY

Substrate

Substrate

Fig. 9

33

Horizontal cracks

ro of

(b)

-p

re

lP

na

ur

Jo (a)

Fig. 10

34

(b)

ro of

-p

re

lP

na

ur

Jo (a)

Fig. 11

35

Table 1 Composition design of La2-xCaxCe2-xFexO7+δ and La2-xSrxCe2-xMnxO7+δ

x

δ

LC LCCF1 LCCF2 LCCF4 LCSM1 LCSM2 LCSM4

0 0.1 0.2 0.4 0.1 0.2 0.4

0 -0.1 -0.2 -0.4 -0.1 -0.2 -0.4

-p

Coating sample

ro of

coatings.

Table 2 Structural and optical characteristics of La2-xCaxCe2-xFexO7+δ and

LCCF2

10.00

LCCF4

9.98

LCSM1

10.02

Reflectan ce % 85.9

Absorban ce % 14.1

Band gap Eg (eV) 3.02

Excited wavelentgh (nm) 411

0.525

42.3

57.7

2.73

455

0.578

36.8

63.2

2.67

465

0.582

28.4

71.6

2.61

475

0.464

21.3

78.7

2.89

429

LCSM2

10.00

0.472

18.7

81.3

2.81

442

LCSM4

9.99

0.473

12.1

87.9

2.75

451

lP

10.02

0.23 6 0.17 1 0.20 6 0.19 4 0.26 0 0.25 5 0.17 0

Fracture toughness (MPa  m1/2) 0.832

na

LCCF1

FW HW

ur

LC

Lattice parameter (Ǻ) 10.04

Jo

Coating sample

re

La2-xSrxCe2-xMnxO7+δ coatings.

36