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Investigation about thermal conductivities of La2Ce2O7 doped with calcium or magnesium for thermal barrier coatings
Journal of Alloys and Compounds 537 (2012) 141–146
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Investigation about thermal conductivities of La2Ce2O7 doped with calcium or magnesium for thermal barrier coatings Zhang Hong-song a,⇑, Wei Yuan b, Li Gang a, Chen Xiao-ge c, Wang Xin-Li a a
Department of Mechanical Engineering, Henan Institute of Engineering, Zhengzhou 450007, China Department of Materials and Chemistry, Henan Institute of Engineering, Zhengzhou 450007, China c Department of Construction Engineering, Henan Institute of Engineering, Zhengzhou 450007, China b
a r t i c l e
i n f o
Article history: Received 26 March 2012 Received in revised form 5 May 2012 Accepted 7 May 2012 Available online 22 May 2012 Keywords: Cerium oxides Thermal conductivity Phonon scattering Thermal barrier coatings
a b s t r a c t The La2Ce2O7 powders doped with Ca and Mg were synthesized by sol–gel method in this paper and their dense bulk samples were also prepared by pressure-less sintering at 1600 °C for 10 h. Their phase compositions, microstructures and thermal conductivities were investigated, respectively. XRD results reveal that single-phase (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 ceramics with fluorite structure are successfully synthesized. SEM and EDS results show that their microstructures are very dense and no other unreacted oxides or interphases exist in the interfaces between grains. Their thermal conductivities are lower than that of YSZ, which can be attributed to the phonon scattering caused by vacancies in their crystal lattices. The larger differences in atomic weight and ionic radius between Mg and La lead to the lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 than that of (La0.95Ca0.05)2Ce2O6.95. The synthesized rare earth cerium oxides have potentials to be used as novel candidate materials for thermal barrier coatings in the future. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction For decades, thermal barrier coatings (TBC) have been used in the hot sections of advanced gas turbine engines to increase operating temperatures, resulting in improved fuel efficiency and prolonged lifetime of nickel-based superalloy engine components. The typical TBCs generally consist of a multilayered coating with a metallic bond coating of Ni, Co and/or Fe alloyed with Al, Cr and Y, and a ceramic topcoat which is typically yttria-stabilzed zirconia (YSZ). The main roles of the metallic bond coating are to promote adhesion strength of the YSZ to the substrate and provide oxidation protection for the turbine components. The YSZ top-coat provides the thermal protection due to its low thermal conductivity [1]. However, a further improvement of the efficiency of gas turbines is intended by further improvement of the combustion and cooling technology in combination with higher turbine inlet temperature. This also implies that the standard material YSZ is approaching certain limitation due to its sintering and phase transformation at elevated temperatures [2–5]. In order to overcome disadvantages of YSZ and meet the ambitious design goal, the feasible and economic method is to search for new candidate ceramic materials with even lower thermal conductivity, higher operating temperature, better sintering resistance and phase stability at ⇑ Corresponding author. Tel.: +86 0371 62508765. E-mail address: [email protected] (Z. Hong-song).
higher temperature on the premise of the usage of advanced superalloys and cooling technique [6,7]. However, the selection of new ceramic materials for TBC is restricted by some basic requirements, such as high melting point, low thermal conductivity, high thermal expansion coefficient, no phase transformation between room temperature and operation temperature, excellent chemical stability, good adherence to the metallic substrate and low sintering rate of the porous microstructure [8–10]. Because of the rigorous requirements, ceramic materials that can be used as new candidates for TBC are very finite. No single ceramic material can satisfy all requirements up to now according to our knowledge. Among these requirements, low thermal conductivity has the first importance. Now, the reported candidate ceramic materials for TBCs can be classified three groups, the first is the co-doped of yttria-stabilized zirconia (YSZ) with one or more metal oxides, the second is the A2B2O7-type rare-earth zircontae ceramics and the third can be named other new-ceramic materials, such as rare earth doped CeO2 or HfO2–Ln2Ce2O7 [11–15] or Ln2O3–HfO2 [17,18], LnInFeZnO4 [19], BaLa2Ti3O10 [20] and LaTi2Al9O19 [21]. Thermophysical properties of YSZ can be improved by co-doping with one or more metal oxides. For example, plasma sprayed Y2O3 and La2O3 co-doped zirconia TBCs has more lower thermal conductivity as compared to that of un-doped TBCs [22]. However, there still exists phase transformation if they work for a long time at temperature above 1300– 1500 °C, which limits their applications for new thermal barrier
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coatings. Ln2Zr2O7 ceramics with pyrochlore or fluorite structure have low thermal conductivities, high melting points and better phase stability at high temperatures, which are advantageous to TBCs. However, their thermal expansion coefficients (CTEs) are relatively low resulting in high thermal stress in TBC applications, which is harmful for TBC’s performance [8,23–26]. In order to overcome the shortcoming in thermal expansion coefficient of this type ceramics, the multilayer thermal barrier coatings of rare earth are being prepared by plasma-spraying method or EB-PVD technology, and the double-ceramic-layer coating has become an important development direction of thermal barrier coatings [27,28]. In the third group, rare earth doped CeO2 oxides has attracted extensive attentions due to its excellent electrical, catalytic, mechanical properties, low thermal conductivity and high thermal expansion coefficients at high temperatures. Thermophysical properties of several interesting rare earth cerium oxides, such as La2Ce2O7 [11], Nd2Ce2O7 [12], Gd2Ce2O7 [13,14], Y2Ce2O7 [15] and Dy2Ce2O7 [16], have been investigated, and results indicated that these rare earth cerium oxides have potential to be used as new candidate materials for future TBCs. However, the thermal conductivities of doped Ln2Ce2O7 oxides with bivalent metal ions have not been investigated in detail up to now. In this manuscript, two kinds of rare earth cerium oxides, (La0.95Ca0.05)2 Ce2O6.95 and (La0.95 Mg0.05)2Ce2O6.95, were synthesized, their phase structures and thermal conductivities were examined. 2. Experimental details In the current work, La2O3 (Rare-Chem Hi-Tech, Co. LTD, purity P 99.9%), MgO (analytical grade) and CaCO3 (analytical grade) in stoichiometric amount were first converted to its nitrate by dissolving in concentrated nitric acid. The contents of cerium in Ce(NO3)3.6H2O (analytical grade) was estimated by decomposition to constant weight as CeO2. Calculated amounts of Ce(NO3)3.6H2O was dissolved in distilled water and all solutions were mixed with constant stirring. Subsequently, the citric acid was added to this resultant solution such that the mole ratio of citric acid/cerium as the case is 2:1. The pH-value of the resultant solution was adjusted to 6–7 by adding ammonia hydroxide drop wise. The solution was then slowly evaporated on a water bath till a viscous liquid was obtained. At this stage, ethylene glycol was added such that the mole ratio of ethylene glycol to cerium was 1.8:1. The viscous liquid was heated at 130 °C using air oven till a porous solid mass was obtained. This porous solid mass thus obtained was grained in an agate mortar and activated at 800 °C for 2 h in a muffle oven. The La2Ce2O7 powder was also synthesized using the same method. Then the obtained powders were uniaxially cold pressed into pellets, and the pellets were placed on cerium tiles and sintered at 1600 °C for 10 h. The bulk ceramic pellets were subsequently cooled in air from 1600 °C in the end. The infrared spectra of the obtained powders were recorded on Nicolet 380 series FT-IR spectrometer using KBr pellets. Phase analysis of the synthesized powders and the corresponding bulk samples were determined by X-ray diffractometry (XRD, X’Pert PRD MPD Holand) with Ni filtered CuKa radiation (0.1542 nm) at the scanning rate of 4°/min. The morphology and composition of the synthesized powders and the corresponding bulk samples were analyzed using scanning electron microscope (SEM, HITACHI S-4800) and energy dispersive spectroscopy (EDS, EDAX GENESIS), respectively. The thermal diffusivities (k) of synthesized samples were measured using laserflash method (Model FlashLine™3000, Anter USA) in the range between 200 and 800 °C in an argon atmosphere. The sample dimension for thermal diffusivity measurement was about 12.7 mm in diameter and about 1 mm in thickness. Before thermal diffusivity measurement, both the front and back faces of the samples were coated with a thin layer of graphite. These coatings were done to prevent direct transmission of laser beam through the translucent specimens at high temperatures. The thermal diffusivity measurement of the specimens was carried out three times at 200, 400, 600 and 800 °C, respectively. The specific heat capacity (Cp) as a function of temperature was calculated from the heat capacity data of the constituent oxides of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95, in conjunction with the Neumann–Kopp rule [29]. The thermal conductivity (k) of the specimen was calculated by Eq. (1) with specific heat capacity (Cp), density (q) and thermal diffusivity (k):
k ¼ k q CP
3. Results and discussion 3.1. Characterization of powders Fig. 1 lists the XRD patterns of (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 powders. It can be observed that the XRD patterns of the synthesized powders are very similar to that of La2Ce2O7. Research results of C.X. Qiang indicate that the La2Ce2O7 ceramic has a fluorite crystal structure because of the absence of the additional two diffraction peaks in the 2h range from 30° to 45°in its XRD pattern compared to that of La2Zr2O7 [31]. Thus, it can be concluded from Fig. 1 that (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 powders synthesized in this paper still remain the fluorite crystalline-structure according to the research result of C.X. Qiang. The previous research about the A2B2O7 (B@Zr, Ti, Ce, Hf) oxides indicated that their crystal structures depends on (a) the relative ionic radius ratio of A and B cations and (b) sample processing conditions [32–34].The pyrochlore oxides were found to be stable when the radius ratio (rA/rB) of the cations lies in range 1.46–1.78 [35]. Mandal et al. have reported this radius ratio as 1.2–1.6 [32]. Oxides of general formula A2B2O7 crystallize in ordered pyrochlore (cubic, Fd3m) and defect fluorite (cubic, Fm3m) structures when the radius ratio (rA/rB) is in the upper and lower limits of the above range, respectively. In the present investigation, the radius ratios (rA/rB) of (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 are 1.177 and 1.193, respectively. Therefore, both these samples are expected to crystallize in the defect fluorite structures. It can also be noted form Fig. 1 that the diffraction peaks of the doped powders slightly shift to the higher angle side compared with La2Ce2O7. It is well known that the effective ionic radiuses of Mg2+ (0.89 Å) and Ca2+ (1.12 Å) are lower than that of La3+, (1.16 Å), which lead to increase of diffraction angle according to the Brug equation when Mg2+ and Ca2+ ions entered the crystal lattice of La2Ce2O7 and partially substituted sites of La3+ ions. FT-IR spectra of the synthesized powders are presented in Fig. 2 in order to further confirm their phase structures. There are three strong infrared absorptions at about 1360–1400 cm1, 1460– 1500 and 3400–3500 cm1 in their FT-IR spectrums. Besides these three absorption bands, another absorption band at 1000– 1100 cm1 can be clearly seen in FT-IR patterns of the doped La2Ce2O7 powders. The strong bands observed at 1460–1500 cm1 and 3400–3500 cm1 correspond to the water molecule absorbed in the samples, while the band at 1360–1400 cm1 could be due
ð1Þ
Because the sintered specimens were not full dense, the measured values of thermal conductivity were modified for the actual value k0 using Eq. (2), where / is the fractional porosity and the coefficient 4/3 is used to eliminate the effect of porosity on actual thermal conductivity [30]:
4 k=k0 ¼ 1 / 3
ð2Þ
Fig. 1. XRD patterns of the synthesized powders.
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to Ce–O vibration only [36]. The occurrence of the bands at 1000– 1100 cm1 in FT-IR pattern of doped powders can significantly be due to the doping of Mg2+ or Ca2+ ions into the crystal lattice of La2Ce2O7. Fig. 3 displays the micro-morphology of the doped powders synthesized by sol–gel method. From SEM observations. It can be noted that the size of the synthesized powders have a relatively uniform size and exhibit to a certain some agglomeration. 3.2. Characterization of the bulk ceramics In order to measure the thermal conductivities of doped ceramic materials, the synthesized powders were pressed into pellets and sintered at 1600 °C for 10 h. Fig. 4 presents the XRD patterns of the corresponding bulk ceramic samples of the doped powders. It can be clearly observed that the XRD patters of the bulk ceramic materials are very consistent to that of the synthesized powders, which means that there are no phase transformation occurred in the sintering process of powders at 1600 °C. Fig. 5 shows the typical microstructures of these synthesized ceramic bulk samples. These synthesized rare earth cerium samples have similar compact microstructures, the relative densities of the (La0.95Mg0.05)2 Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 bulk samples measured accurately by using Archimedes method is are 93.61% and 94.3%, respectively. The average grain size of these products is several micrometers, and their grain boundaries are very clean, no other interphases or unreacted oxides existed in the interfaces. The EDS patterns of the bulk samples were presented in Fig. 6. It can be noted that there are only La, Mg, Ca, Ce and O elements on the sample surface, the atomic ratios of (La + Mg):Ce:O and (La + Ca):Ce:O are (15.78 + 1.43 = 17.21):16.68:66.11 and (15.75 + 1.48 = 17.23):16.72:66.05(Table 1). The n(La+Mg):nCe or n(La+Ca):nCe is still 1:1, which is also close to the theoretical composition of the doped powders, and this analytical results imply that there is no element loss in the sintering process of powders at high temperature.
Fig. 3. Mico-morphology of the synthesized powders (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95Mg0.05)2Ce2O6.95.
3.3. Thermal conductivities The dependence of specific heat capacities of the synthesized ceramic materials calculated according to the the Neumann–Kopp rule was displayed in Fig. 7. It can be clearly seen form Fig. 7 that the specific heat capacities increase gradually with temperature increasing. The specific heat capacities of these samples can be fitted by the following equations, respectively:
Fig. 4. XRD patterns of sintered bulk samples.
C p ðLa1:9 Ca0:1 Ce2 O6:95 Þ ¼ 0:36275 þ 0:0009 T 55:91363 T 2
ð3Þ
C p ðLa1:9 Mg0:1 Ce2 O6:95 Þ ¼ 0:37791 þ 0:0009 T 55:37964 T 2
Fig. 2. FT-IR spectra of the synthesized powders.
ð4Þ
The variations in thermal diffusivities with of the temperature for bulk ceramic materials were plotted in Fig. 8. Their thermal diffusivities of the ceramic materials decrease slowly with
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temperature in the range between 200 and 800 °C, which shows typically phonon thermal conduction behavior, and this phenomena is very universal in the polycrystalline materials. In this investigation, their thermal diffusivities are located in the range of 0.42 to 0.76 mm2 s1 from 200 to 800 °C. Fig. 9 shows data for thermal conductivities versus temperature for (La0.95Mg0.05)2Ce2O6.95 and (La0.95 Ca0.05)2Ce2O6.95 ceramic materials. These data were corrected for porosity according to Eq. (2). Their thermal conductivities gradually decrease with the increase in temperature up to 800 °C. The thermal conductivities of (La0.95Mg0.05)2 Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 gradually decrease from 1.765 and 1.5 W/m K to 1.22 and 1.19 W/m K, respectively in the temperature range. These results are much lower than that of YSZ. However, the thermal conductivity of CeO2 is higher than YSZ at temperature below 1300 °C [12]. In order to explain the current result, the theory of phonon heat conduction has to be considered. In electrical insulate solids, the specific heat capacity, phonon velocity and density can be regarded as constants in the present temperature range. Thus, the thermal conductivity of electrical insulate solids is proportional to the mean free path of phonon. In real crystal structures, scattering of phonons occurs when they interact with lattice imperfections. The lattice imperfections include vacancies, dislocations, grain boundaries, substituting atoms with different atomic mass and other phonons. Ions and atoms of different ionic radius may also scatter phonons by locally distorting the bond length and thus, introducing elastic strain fields into the lattice. The effects caused by such imperfections can be quantified through their influence on the phonon mean free path (lp). This approach has been used by many workers, for which the phonon mean free path is defined by [37]:
Fig. 5. Mico-structure of the bulk samples (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95 Mg0.05)2Ce2O6.95.
1 1 1 1 1 ¼ þ þ þ lp li lvac lgb lstrain
Fig. 6. EDS spectrums of bulk samples (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95Mg0.05)2Ce2O6.95.
ð5Þ
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Table 1 Atomic percentages of bulk samples. Element mole ratio
La
Mg
Ca
Ce
O
(La0.95Ca0.05)2Ce2O6.95 (La0.95Mg0.05)2Ce2O6.95
15.75 15.78
0 1.43
1.48 0
16.72 16.68
66.05 66.11
Fig. 9. Temperature dependence of thermal conductivities of (La0.95Ca0.05)2Ce2O6.95, (La0.95Mg0.05)2Ce2O6.95 and La2Ce2O7.
Fig. 7. Varaition of specific heat capacities of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 with temperature.
of the lattice. Much more oxygen vacancies in La2Ce2O7 lattice attributed its lower thermal conductivity compared with that of YSZ [31]. More oxygen vacancies can be incorporated when the sites of two La3+ cations were substituted with two Ca2+ or Mg2+ cations, which can be expressed using kroger-Vink notation by the following equation:
2LaxLa þ 2MO ! La2 O3 þ 2M0La þ V00o
ð6Þ
where, represents an Mg2+ or Ca2+ cation that occupies a La3+ site V00o is a doubly charged (positive) oxygen vacancy, and Oxo is an O2 anion on an oxygen site (neutral charge). This equation means that the concentrations of oxygen vacancies in these two doped ceramic materials are greater than that of La2Ce2O7. Much more oxygen vacancies in the lattice of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 ceramic materials lead to their lower thermal conductivities than that of La2Ce2O7. On the other hand, the phonon mean free path is inverse to the square of the differences in atomic weigh and ionic radius between the solute (Mg or Ca) and host (La) cations [39,40]. Because the atomic weights of Mg and Ca are about 24 and 40, respectively, which are much lower than 138.9 – the atomic weight of host (La) cations. The effective phonon scattering by solute cations is significantly, which also attributes the lower thermal conductivities of the synthesized ceramic materials. In these two solute cations, the differences in atomic weight and ionic radius of Mg are much greater than that of Ca. Therefore, the thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 is lower than that of (La0.95Ca0.05)2Ce2O6.95, which can be seen from Fig. 9. It is well known that low thermal conductivity is one of the most critical requirements for TBC [39–41]. The lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 in the temperature range between ambient and 800 °C indicates that these ceramic materials can be explored as novel candidate ceramic materials for use in TBCs. M0La
Fig. 8. Thermal diffusivities of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 for different temperatures.
where li , lvac , lgb and lstrain are the contributions to the mean path due to interstitials, vacancies, grain boundaries and lattice strain, respectively. For a significant decrease in the high-temperature range, the average grain size has to be in the nanometer region [37,38], whereas the current specimens have grain size in the micrometer range. The decrease of the thermal conductivity due to the phonon scattering at grain boundary is not expected in the case of the ceramic materials investigated here. Also, radiation heat transfer can be neglected because the maximum temperature considered here is only 800 °C. Thus, the decrease in the phonon conductivity is assumed to result solely from phonon scattering by point defect. In Ln2Ce2O7 solid solutions, the substitution of two Ce4+ cations with two Ln3+ cations is accompanied by the incorporation of one oxygen vacancy, to maintain the local electro-neutrality
4. Conclusions (1) Pure (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 with fluorite structures were successfully synthesized by sol–gel method in this work. The dense bulk samples can be prepared by sintering at 1600 °C for 10 h, and the bulk samples have uniform comparatively grain size. Interfaces between grains are very clean, no other phases or unreacted oxides existed. No phase transformation and element loss occurred in the sintering process of powders at high temperature.
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(2) The lower thermal conductivities of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 can be attributed to the significantly higher concentration of oxygen vacancies, and the significantly larger atomic weight of the solute cations in these two ceramic materials. The larger differences in atomic weight and ionic radius attribute the lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 than that of (La0.95Ca0.05)2 Ce2O6.95. (3) (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 can be explored as novel candidate ceramic materials for use in thermal barrier coatings.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Investigation about thermal conductivities of La2Ce2O7 doped with calcium or magnesium for thermal barrier coatings Zhang Hong-song a,⇑, Wei Yuan b, Li Gang a, Chen Xiao-ge c, Wang Xin-Li a a
Department of Mechanical Engineering, Henan Institute of Engineering, Zhengzhou 450007, China Department of Materials and Chemistry, Henan Institute of Engineering, Zhengzhou 450007, China c Department of Construction Engineering, Henan Institute of Engineering, Zhengzhou 450007, China b
a r t i c l e
i n f o
Article history: Received 26 March 2012 Received in revised form 5 May 2012 Accepted 7 May 2012 Available online 22 May 2012 Keywords: Cerium oxides Thermal conductivity Phonon scattering Thermal barrier coatings
a b s t r a c t The La2Ce2O7 powders doped with Ca and Mg were synthesized by sol–gel method in this paper and their dense bulk samples were also prepared by pressure-less sintering at 1600 °C for 10 h. Their phase compositions, microstructures and thermal conductivities were investigated, respectively. XRD results reveal that single-phase (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 ceramics with fluorite structure are successfully synthesized. SEM and EDS results show that their microstructures are very dense and no other unreacted oxides or interphases exist in the interfaces between grains. Their thermal conductivities are lower than that of YSZ, which can be attributed to the phonon scattering caused by vacancies in their crystal lattices. The larger differences in atomic weight and ionic radius between Mg and La lead to the lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 than that of (La0.95Ca0.05)2Ce2O6.95. The synthesized rare earth cerium oxides have potentials to be used as novel candidate materials for thermal barrier coatings in the future. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction For decades, thermal barrier coatings (TBC) have been used in the hot sections of advanced gas turbine engines to increase operating temperatures, resulting in improved fuel efficiency and prolonged lifetime of nickel-based superalloy engine components. The typical TBCs generally consist of a multilayered coating with a metallic bond coating of Ni, Co and/or Fe alloyed with Al, Cr and Y, and a ceramic topcoat which is typically yttria-stabilzed zirconia (YSZ). The main roles of the metallic bond coating are to promote adhesion strength of the YSZ to the substrate and provide oxidation protection for the turbine components. The YSZ top-coat provides the thermal protection due to its low thermal conductivity [1]. However, a further improvement of the efficiency of gas turbines is intended by further improvement of the combustion and cooling technology in combination with higher turbine inlet temperature. This also implies that the standard material YSZ is approaching certain limitation due to its sintering and phase transformation at elevated temperatures [2–5]. In order to overcome disadvantages of YSZ and meet the ambitious design goal, the feasible and economic method is to search for new candidate ceramic materials with even lower thermal conductivity, higher operating temperature, better sintering resistance and phase stability at ⇑ Corresponding author. Tel.: +86 0371 62508765. E-mail address: [email protected] (Z. Hong-song).
higher temperature on the premise of the usage of advanced superalloys and cooling technique [6,7]. However, the selection of new ceramic materials for TBC is restricted by some basic requirements, such as high melting point, low thermal conductivity, high thermal expansion coefficient, no phase transformation between room temperature and operation temperature, excellent chemical stability, good adherence to the metallic substrate and low sintering rate of the porous microstructure [8–10]. Because of the rigorous requirements, ceramic materials that can be used as new candidates for TBC are very finite. No single ceramic material can satisfy all requirements up to now according to our knowledge. Among these requirements, low thermal conductivity has the first importance. Now, the reported candidate ceramic materials for TBCs can be classified three groups, the first is the co-doped of yttria-stabilized zirconia (YSZ) with one or more metal oxides, the second is the A2B2O7-type rare-earth zircontae ceramics and the third can be named other new-ceramic materials, such as rare earth doped CeO2 or HfO2–Ln2Ce2O7 [11–15] or Ln2O3–HfO2 [17,18], LnInFeZnO4 [19], BaLa2Ti3O10 [20] and LaTi2Al9O19 [21]. Thermophysical properties of YSZ can be improved by co-doping with one or more metal oxides. For example, plasma sprayed Y2O3 and La2O3 co-doped zirconia TBCs has more lower thermal conductivity as compared to that of un-doped TBCs [22]. However, there still exists phase transformation if they work for a long time at temperature above 1300– 1500 °C, which limits their applications for new thermal barrier
0925-8388/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.036
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coatings. Ln2Zr2O7 ceramics with pyrochlore or fluorite structure have low thermal conductivities, high melting points and better phase stability at high temperatures, which are advantageous to TBCs. However, their thermal expansion coefficients (CTEs) are relatively low resulting in high thermal stress in TBC applications, which is harmful for TBC’s performance [8,23–26]. In order to overcome the shortcoming in thermal expansion coefficient of this type ceramics, the multilayer thermal barrier coatings of rare earth are being prepared by plasma-spraying method or EB-PVD technology, and the double-ceramic-layer coating has become an important development direction of thermal barrier coatings [27,28]. In the third group, rare earth doped CeO2 oxides has attracted extensive attentions due to its excellent electrical, catalytic, mechanical properties, low thermal conductivity and high thermal expansion coefficients at high temperatures. Thermophysical properties of several interesting rare earth cerium oxides, such as La2Ce2O7 [11], Nd2Ce2O7 [12], Gd2Ce2O7 [13,14], Y2Ce2O7 [15] and Dy2Ce2O7 [16], have been investigated, and results indicated that these rare earth cerium oxides have potential to be used as new candidate materials for future TBCs. However, the thermal conductivities of doped Ln2Ce2O7 oxides with bivalent metal ions have not been investigated in detail up to now. In this manuscript, two kinds of rare earth cerium oxides, (La0.95Ca0.05)2 Ce2O6.95 and (La0.95 Mg0.05)2Ce2O6.95, were synthesized, their phase structures and thermal conductivities were examined. 2. Experimental details In the current work, La2O3 (Rare-Chem Hi-Tech, Co. LTD, purity P 99.9%), MgO (analytical grade) and CaCO3 (analytical grade) in stoichiometric amount were first converted to its nitrate by dissolving in concentrated nitric acid. The contents of cerium in Ce(NO3)3.6H2O (analytical grade) was estimated by decomposition to constant weight as CeO2. Calculated amounts of Ce(NO3)3.6H2O was dissolved in distilled water and all solutions were mixed with constant stirring. Subsequently, the citric acid was added to this resultant solution such that the mole ratio of citric acid/cerium as the case is 2:1. The pH-value of the resultant solution was adjusted to 6–7 by adding ammonia hydroxide drop wise. The solution was then slowly evaporated on a water bath till a viscous liquid was obtained. At this stage, ethylene glycol was added such that the mole ratio of ethylene glycol to cerium was 1.8:1. The viscous liquid was heated at 130 °C using air oven till a porous solid mass was obtained. This porous solid mass thus obtained was grained in an agate mortar and activated at 800 °C for 2 h in a muffle oven. The La2Ce2O7 powder was also synthesized using the same method. Then the obtained powders were uniaxially cold pressed into pellets, and the pellets were placed on cerium tiles and sintered at 1600 °C for 10 h. The bulk ceramic pellets were subsequently cooled in air from 1600 °C in the end. The infrared spectra of the obtained powders were recorded on Nicolet 380 series FT-IR spectrometer using KBr pellets. Phase analysis of the synthesized powders and the corresponding bulk samples were determined by X-ray diffractometry (XRD, X’Pert PRD MPD Holand) with Ni filtered CuKa radiation (0.1542 nm) at the scanning rate of 4°/min. The morphology and composition of the synthesized powders and the corresponding bulk samples were analyzed using scanning electron microscope (SEM, HITACHI S-4800) and energy dispersive spectroscopy (EDS, EDAX GENESIS), respectively. The thermal diffusivities (k) of synthesized samples were measured using laserflash method (Model FlashLine™3000, Anter USA) in the range between 200 and 800 °C in an argon atmosphere. The sample dimension for thermal diffusivity measurement was about 12.7 mm in diameter and about 1 mm in thickness. Before thermal diffusivity measurement, both the front and back faces of the samples were coated with a thin layer of graphite. These coatings were done to prevent direct transmission of laser beam through the translucent specimens at high temperatures. The thermal diffusivity measurement of the specimens was carried out three times at 200, 400, 600 and 800 °C, respectively. The specific heat capacity (Cp) as a function of temperature was calculated from the heat capacity data of the constituent oxides of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95, in conjunction with the Neumann–Kopp rule [29]. The thermal conductivity (k) of the specimen was calculated by Eq. (1) with specific heat capacity (Cp), density (q) and thermal diffusivity (k):
k ¼ k q CP
3. Results and discussion 3.1. Characterization of powders Fig. 1 lists the XRD patterns of (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 powders. It can be observed that the XRD patterns of the synthesized powders are very similar to that of La2Ce2O7. Research results of C.X. Qiang indicate that the La2Ce2O7 ceramic has a fluorite crystal structure because of the absence of the additional two diffraction peaks in the 2h range from 30° to 45°in its XRD pattern compared to that of La2Zr2O7 [31]. Thus, it can be concluded from Fig. 1 that (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 powders synthesized in this paper still remain the fluorite crystalline-structure according to the research result of C.X. Qiang. The previous research about the A2B2O7 (B@Zr, Ti, Ce, Hf) oxides indicated that their crystal structures depends on (a) the relative ionic radius ratio of A and B cations and (b) sample processing conditions [32–34].The pyrochlore oxides were found to be stable when the radius ratio (rA/rB) of the cations lies in range 1.46–1.78 [35]. Mandal et al. have reported this radius ratio as 1.2–1.6 [32]. Oxides of general formula A2B2O7 crystallize in ordered pyrochlore (cubic, Fd3m) and defect fluorite (cubic, Fm3m) structures when the radius ratio (rA/rB) is in the upper and lower limits of the above range, respectively. In the present investigation, the radius ratios (rA/rB) of (La0.95Ca0.05)2 Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 are 1.177 and 1.193, respectively. Therefore, both these samples are expected to crystallize in the defect fluorite structures. It can also be noted form Fig. 1 that the diffraction peaks of the doped powders slightly shift to the higher angle side compared with La2Ce2O7. It is well known that the effective ionic radiuses of Mg2+ (0.89 Å) and Ca2+ (1.12 Å) are lower than that of La3+, (1.16 Å), which lead to increase of diffraction angle according to the Brug equation when Mg2+ and Ca2+ ions entered the crystal lattice of La2Ce2O7 and partially substituted sites of La3+ ions. FT-IR spectra of the synthesized powders are presented in Fig. 2 in order to further confirm their phase structures. There are three strong infrared absorptions at about 1360–1400 cm1, 1460– 1500 and 3400–3500 cm1 in their FT-IR spectrums. Besides these three absorption bands, another absorption band at 1000– 1100 cm1 can be clearly seen in FT-IR patterns of the doped La2Ce2O7 powders. The strong bands observed at 1460–1500 cm1 and 3400–3500 cm1 correspond to the water molecule absorbed in the samples, while the band at 1360–1400 cm1 could be due
ð1Þ
Because the sintered specimens were not full dense, the measured values of thermal conductivity were modified for the actual value k0 using Eq. (2), where / is the fractional porosity and the coefficient 4/3 is used to eliminate the effect of porosity on actual thermal conductivity [30]:
4 k=k0 ¼ 1 / 3
ð2Þ
Fig. 1. XRD patterns of the synthesized powders.
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to Ce–O vibration only [36]. The occurrence of the bands at 1000– 1100 cm1 in FT-IR pattern of doped powders can significantly be due to the doping of Mg2+ or Ca2+ ions into the crystal lattice of La2Ce2O7. Fig. 3 displays the micro-morphology of the doped powders synthesized by sol–gel method. From SEM observations. It can be noted that the size of the synthesized powders have a relatively uniform size and exhibit to a certain some agglomeration. 3.2. Characterization of the bulk ceramics In order to measure the thermal conductivities of doped ceramic materials, the synthesized powders were pressed into pellets and sintered at 1600 °C for 10 h. Fig. 4 presents the XRD patterns of the corresponding bulk ceramic samples of the doped powders. It can be clearly observed that the XRD patters of the bulk ceramic materials are very consistent to that of the synthesized powders, which means that there are no phase transformation occurred in the sintering process of powders at 1600 °C. Fig. 5 shows the typical microstructures of these synthesized ceramic bulk samples. These synthesized rare earth cerium samples have similar compact microstructures, the relative densities of the (La0.95Mg0.05)2 Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 bulk samples measured accurately by using Archimedes method is are 93.61% and 94.3%, respectively. The average grain size of these products is several micrometers, and their grain boundaries are very clean, no other interphases or unreacted oxides existed in the interfaces. The EDS patterns of the bulk samples were presented in Fig. 6. It can be noted that there are only La, Mg, Ca, Ce and O elements on the sample surface, the atomic ratios of (La + Mg):Ce:O and (La + Ca):Ce:O are (15.78 + 1.43 = 17.21):16.68:66.11 and (15.75 + 1.48 = 17.23):16.72:66.05(Table 1). The n(La+Mg):nCe or n(La+Ca):nCe is still 1:1, which is also close to the theoretical composition of the doped powders, and this analytical results imply that there is no element loss in the sintering process of powders at high temperature.
Fig. 3. Mico-morphology of the synthesized powders (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95Mg0.05)2Ce2O6.95.
3.3. Thermal conductivities The dependence of specific heat capacities of the synthesized ceramic materials calculated according to the the Neumann–Kopp rule was displayed in Fig. 7. It can be clearly seen form Fig. 7 that the specific heat capacities increase gradually with temperature increasing. The specific heat capacities of these samples can be fitted by the following equations, respectively:
Fig. 4. XRD patterns of sintered bulk samples.
C p ðLa1:9 Ca0:1 Ce2 O6:95 Þ ¼ 0:36275 þ 0:0009 T 55:91363 T 2
ð3Þ
C p ðLa1:9 Mg0:1 Ce2 O6:95 Þ ¼ 0:37791 þ 0:0009 T 55:37964 T 2
Fig. 2. FT-IR spectra of the synthesized powders.
ð4Þ
The variations in thermal diffusivities with of the temperature for bulk ceramic materials were plotted in Fig. 8. Their thermal diffusivities of the ceramic materials decrease slowly with
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temperature in the range between 200 and 800 °C, which shows typically phonon thermal conduction behavior, and this phenomena is very universal in the polycrystalline materials. In this investigation, their thermal diffusivities are located in the range of 0.42 to 0.76 mm2 s1 from 200 to 800 °C. Fig. 9 shows data for thermal conductivities versus temperature for (La0.95Mg0.05)2Ce2O6.95 and (La0.95 Ca0.05)2Ce2O6.95 ceramic materials. These data were corrected for porosity according to Eq. (2). Their thermal conductivities gradually decrease with the increase in temperature up to 800 °C. The thermal conductivities of (La0.95Mg0.05)2 Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 gradually decrease from 1.765 and 1.5 W/m K to 1.22 and 1.19 W/m K, respectively in the temperature range. These results are much lower than that of YSZ. However, the thermal conductivity of CeO2 is higher than YSZ at temperature below 1300 °C [12]. In order to explain the current result, the theory of phonon heat conduction has to be considered. In electrical insulate solids, the specific heat capacity, phonon velocity and density can be regarded as constants in the present temperature range. Thus, the thermal conductivity of electrical insulate solids is proportional to the mean free path of phonon. In real crystal structures, scattering of phonons occurs when they interact with lattice imperfections. The lattice imperfections include vacancies, dislocations, grain boundaries, substituting atoms with different atomic mass and other phonons. Ions and atoms of different ionic radius may also scatter phonons by locally distorting the bond length and thus, introducing elastic strain fields into the lattice. The effects caused by such imperfections can be quantified through their influence on the phonon mean free path (lp). This approach has been used by many workers, for which the phonon mean free path is defined by [37]:
Fig. 5. Mico-structure of the bulk samples (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95 Mg0.05)2Ce2O6.95.
1 1 1 1 1 ¼ þ þ þ lp li lvac lgb lstrain
Fig. 6. EDS spectrums of bulk samples (a) (La0.95Ca0.05)2Ce2O6.95 (b) (La0.95Mg0.05)2Ce2O6.95.
ð5Þ
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Table 1 Atomic percentages of bulk samples. Element mole ratio
La
Mg
Ca
Ce
O
(La0.95Ca0.05)2Ce2O6.95 (La0.95Mg0.05)2Ce2O6.95
15.75 15.78
0 1.43
1.48 0
16.72 16.68
66.05 66.11
Fig. 9. Temperature dependence of thermal conductivities of (La0.95Ca0.05)2Ce2O6.95, (La0.95Mg0.05)2Ce2O6.95 and La2Ce2O7.
Fig. 7. Varaition of specific heat capacities of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 with temperature.
of the lattice. Much more oxygen vacancies in La2Ce2O7 lattice attributed its lower thermal conductivity compared with that of YSZ [31]. More oxygen vacancies can be incorporated when the sites of two La3+ cations were substituted with two Ca2+ or Mg2+ cations, which can be expressed using kroger-Vink notation by the following equation:
2LaxLa þ 2MO ! La2 O3 þ 2M0La þ V00o
ð6Þ
where, represents an Mg2+ or Ca2+ cation that occupies a La3+ site V00o is a doubly charged (positive) oxygen vacancy, and Oxo is an O2 anion on an oxygen site (neutral charge). This equation means that the concentrations of oxygen vacancies in these two doped ceramic materials are greater than that of La2Ce2O7. Much more oxygen vacancies in the lattice of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 ceramic materials lead to their lower thermal conductivities than that of La2Ce2O7. On the other hand, the phonon mean free path is inverse to the square of the differences in atomic weigh and ionic radius between the solute (Mg or Ca) and host (La) cations [39,40]. Because the atomic weights of Mg and Ca are about 24 and 40, respectively, which are much lower than 138.9 – the atomic weight of host (La) cations. The effective phonon scattering by solute cations is significantly, which also attributes the lower thermal conductivities of the synthesized ceramic materials. In these two solute cations, the differences in atomic weight and ionic radius of Mg are much greater than that of Ca. Therefore, the thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 is lower than that of (La0.95Ca0.05)2Ce2O6.95, which can be seen from Fig. 9. It is well known that low thermal conductivity is one of the most critical requirements for TBC [39–41]. The lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 in the temperature range between ambient and 800 °C indicates that these ceramic materials can be explored as novel candidate ceramic materials for use in TBCs. M0La
Fig. 8. Thermal diffusivities of (La0.95Ca0.05)2Ce2O6.95 and (La0.95Mg0.05)2Ce2O6.95 for different temperatures.
where li , lvac , lgb and lstrain are the contributions to the mean path due to interstitials, vacancies, grain boundaries and lattice strain, respectively. For a significant decrease in the high-temperature range, the average grain size has to be in the nanometer region [37,38], whereas the current specimens have grain size in the micrometer range. The decrease of the thermal conductivity due to the phonon scattering at grain boundary is not expected in the case of the ceramic materials investigated here. Also, radiation heat transfer can be neglected because the maximum temperature considered here is only 800 °C. Thus, the decrease in the phonon conductivity is assumed to result solely from phonon scattering by point defect. In Ln2Ce2O7 solid solutions, the substitution of two Ce4+ cations with two Ln3+ cations is accompanied by the incorporation of one oxygen vacancy, to maintain the local electro-neutrality
4. Conclusions (1) Pure (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 with fluorite structures were successfully synthesized by sol–gel method in this work. The dense bulk samples can be prepared by sintering at 1600 °C for 10 h, and the bulk samples have uniform comparatively grain size. Interfaces between grains are very clean, no other phases or unreacted oxides existed. No phase transformation and element loss occurred in the sintering process of powders at high temperature.
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(2) The lower thermal conductivities of (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 can be attributed to the significantly higher concentration of oxygen vacancies, and the significantly larger atomic weight of the solute cations in these two ceramic materials. The larger differences in atomic weight and ionic radius attribute the lower thermal conductivity of (La0.95Mg0.05)2Ce2O6.95 than that of (La0.95Ca0.05)2 Ce2O6.95. (3) (La0.95Mg0.05)2Ce2O6.95 and (La0.95Ca0.05)2Ce2O6.95 can be explored as novel candidate ceramic materials for use in thermal barrier coatings.
Acknowledgments Great thanks to the financial support of Nature Science Research Project of Education Office of Henan Province in China (2010A430001) and Doctor Research Fund in Henan Institute of Engineering (D2007012). References [1] C.M. Weyant, J. Almer, K.T. Faber, Acta Mater. 58 (2010) 943. [2] R. Vaben, M.O. Jarligo, T. Steinke, D.E. Mack, D. Stover, Surf. Coat. Technol. 205 (2010) 938. [3] Z.H. Xu, S.M. He, L.M. He, R.D. Mu, G.H. Huang, X.Q. Cao, J. Alloys. Compd. 509 (2011) 4273. [4] X.Y. Xie, H.B. Guo, S.K. Gong, H.B. Xu, J. Eur. Ceram. Soc. 31 (2011) 1677. [5] M.H. Habibi, L. Wang, S.M. Guo, J. Eur. Ceram. Soc. 32 (2012) 1635. [6] O.I. Malyi, P. Wu, V.V. Kulish, K. Bai, Z. Chen, Solid State Ionics 212 (2012) 117. [7] X.L. Chen, L.J. Gu, B.L. Zou, Y. Wang, X.Q. Cao, Surf. Coat. Technol. 206 (2012) 2265. [8] Z.H. Song, X. Qiang, W.F. Chi, L. Ling, W. Yuan, C.X. Ge, J. Alloys Compd. 475 (2009) 624. [9] Z.H. Song, S. Kun, X. Qiang, W.F. Chi, L. Ling, J. Rare Earths 4 (2009) 222. [10] X.Q. Cao, R. Vassen, D. Stover, J. Eur. Ceram. Soc. 24 (2004) 1. [11] K.H. Kwak, B.C. Shim, S.M. Lee, Y.S. Oh, H.T. Kim, B.K. Jang, S. Kim, Mater. Lett. 65 (2011) 2937. [12] H. Dai, X.H. Zhong, J.Y. Li, J. Meng, X.Q. Cao, Surf. Coat. Technol. 201 (2006) 2527.
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