Phase relations in the ZrO2–La2O3–Y2O3–Al2O3 system: Experimental studies and phase modelling

Phase relations in the ZrO2–La2O3–Y2O3–Al2O3 system: Experimental studies and phase modelling

Available online at www.sciencedirect.com Journal of the European Ceramic Society 33 (2013) 37–49 Phase relations in the ZrO2–La2O3–Y2O3–Al2O3 syste...

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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 33 (2013) 37–49

Phase relations in the ZrO2–La2O3–Y2O3–Al2O3 system: Experimental studies and phase modelling O. Fabrichnaya ∗ , G. Savinykh, G. Schreiber Technical University of Freiberg, Institute of Materials Sciences, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany Received 22 June 2012; received in revised form 20 August 2012; accepted 21 August 2012 Available online 13 September 2012

Abstract Thermodynamic parameters of the La2 O3 –Y2 O3 and La2 O3 –Al2 O3 binary systems were re-assessed based on new experimental information. Phase relations in the La2 O3 –Y2 O3 –Al2 O3 system were investigated at temperatures 1523, 1673 and 1873 K by phase equilibration followed by phase identification by X-ray diffraction and microstructure investigation by scanning electron microscopy. Melting relations in the system were studied by differential thermal analysis. Temperature and liquid composition for two invariant reactions were determined. The obtained results were used to developed thermodynamic database for the La2 O3 –Y2 O3 –Al2 O3 system. Mutual solubility of La and Y in aluminates was taken into account. The obtained thermodynamic description was combined with already available databases of ternary systems and thermodynamic database of the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 was derived. © 2012 Elsevier Ltd. All rights reserved. Keywords: Phase diagrams; Electron microscopy; X-ray methods; Thermal analysis; Thermodynamic modelling

1. Introduction Addition of Rare Earths to yttria-stabilized zirconia (YSZ) decreases the thermal conductivity of the material and therefore investigation of these materials is important for development of advanced thermal barrier coatings (TBC). Two main groups of candidate materials for new TBC are reported; the first group is based on co-doping of YSZ with one or more Rare Earths (RE2 O3 ), and the second group includes the zirconates with pyrochlore structure.1,2 It was reported in several works3,4 that co-doping of YSZ with La2 O3 increased its resistance to sintering, reduced thermal conductivity and increased structural stability. The formation of a secondary phase, La2 Zr2 O7 , with pyrochlore structure results in significant suppression of densification. The La2 Zr2 O7 compound has been proposed recently as promising TBC material and its properties and stability in thermal cycling were investigated.5,6 However it was found that La2 Zr2 O7 reacts with Al2 O3 forming perovskite LaAlO3 , which has thermal expansion misfit with TBC material. Possible solution of this problem could be the



Corresponding author. Tel.: +49 3731 393156; fax: +49 3731 393657. E-mail address: [email protected] (O. Fabrichnaya).

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double-layer systems consisting of pyrochlore as top coat and YSZ, i.e. pyrochlore/YSZ and (1%La4%Y)SZ/YSZ, which revealed an excellent thermal cycling behaviour at temperatures up to 1450 ◦ C.7,8 Therefore investigations of chemical interactions in the ZrO2 –La2 O3 – Y2 O3 –Al2 O3 system are important for development of TBC materials. Another possible application of this database is solid oxide fuel cell (SOFC).9 The cubic ZrO2 stabilized by Y2 O3 is a fast oxide ionic conductor at high temperatures. Due to high stability in both reducing and oxidizing environments it is used as electrolyte material for solid oxide fuel cells and oxygen sensors.10 The doped LaFeO3 and LaAlO3 perovskites are alternative cathode materials for the possible use in SOFC.11 The phase relations on the ZrO2 –La2 O3 –Al2 O3 system are therefore important to understand interaction between solid electrolyte and cathode material. In this case, the formation of the La2 Zr2 O7 should be avoided because it significantly enhances the resistance of the cell. preliminary description of the The ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system was published by Fabrichnaya et al.12 using available thermodynamic descriptions of binary systems and two ternary systems ZrO2 –La2 O3 –Al2 O3 and ZrO2 –Y2 O3 –Al2 O3 . Phase equilibria in the ZrO2 –La2 O3 –Al2 O3 system were experimentally studied

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by Lakiza and Lopato.13 Additional experimental study and assessment of thermodynamic parameters of this system was performed in work of Fabrichnaya et al.14 This study allowed to resolve contradiction between experiments and calculations and to establish phase diagram of the ZrO2 –La2 O3 –Al2 O3 system. The thermodynamic description of the ZrO2 –Y2 O3 –Al2 O3 system consistent with experimental data of Lakiza and Lopato15 and own experimental results was developed by Lakiza et al.16 and re-optimized in work of Fabrichnaya et al.12 Phase diagram of the ZrO2 –La2 O3 –Y2 O3 and La2 O3 –Y2 O3 –Al2 O3 system were calculated based on binary extrapolations.12 Later phase equilibria in the ZrO2 –La2 O3 –Y2 O3 were experimentally studied in works of Fabrichnaya et al.17,18 and advanced thermodynamic modelling of pyrochlore and fluorite phase was performed to derive thermodynamic description compatible with experimental results.18 The phase relations in the La2 O3 –Y2 O3 –Al2 O3 system were not experimentally studied so far. Beside of the fact that this system is a part of the ZrO2 -based system important for thermal barrier coating application phase relations in the La2 O3 –Y2 O3 –Al2 O3 system present interest because these oxides are used as additives for liquid phase sintering of SiC and Si3 N4 ceramics.19–21 The aim of this work is experimental investigation of phase relations in the La2 O3 –Y2 O3 –Al2 O3 system and thermodynamic database development for this ternary system. The derived database will be combined with already available descriptions of ZrO2 –La2 O3 –Al2 O3 and ZrO2 –La2 O3 –Y2 O3 system into thermodynamic database of the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system after adjustments necessary due to taking into account new experimental data for binary systems. 2. Experimental 2.1. Sample preparation Samples were synthesized from precursor solution in a similar way to that described by Fabrichnaya et al.18 The starting chemical were zirconium acetate solution in acetic acid, La(NO3 )3 ·6H2 O (99.99%, Alfa Aesar), Y(NO3 )3 ·6H2 O (99.9%, Alfa Aesar) and Al(NO3 )3 ·6H2 O (99.9%, Alfa Aesar). In the first step, the Nd(NO3 )3 ·6H2 O, Y(NO3 )3 ·6H2 O and Al(NO3 )3 ·6H2 O were dissolved in distilled water. Concentration of the prepared solutions was determined by inductively coupled plasma (ICP) spectrometry. Initial solutions were mixed according to the selected ratios. The obtained precursor solution was dropped from the buret at a low speed (around 1 ml min−1 ) into a big beaker containing about 500 ml of deionized water with the pH value maintained above 9.0 by adding ammonium hydrate. The precipitation occurred during dropping and stirring. The obtained suspension was heated up and held at 333 K for 1–2 h. The precipitate was filtered and dried at 353 K. During pyrolysis at 1073 K proceeding for 3 h in air, hydroxides transform to oxides releasing water. Filtrates and samples dissolved in diluted solution of H2 SO4 were analysed by ICP, with an accuracy of ±2%.

2.2. Sample treatment and characterization The pyrolysed powder was pressed into cylindrical pellets and sintered in air at temperatures of 1523, 1673 and 1873 K in Pt-crucibles to obtain the equilibrium microstructure. The duration of heat treatments was 336 h at 1523 K, 192 h at 1673 K and 96 h at 1873 K. The samples were then analysed by XRD and SEM/EDX. The XRD patterns of powdered specimen were recorded using the Präzisionsmechanic diffractometer (CuK␣ radiation; Freiberg, Germany). Lattice parameters for phases, their volume fractions and grain size were calculated by Rietveld analysis using BGMN program22 and MAUD.23 The microstructures of sintered samples were examined by SEM (Leo1530 GEMINI) and energy dispersive X-ray spectroscopy (EDX Bruker AXS Mikroanalysis GmbH) was employed to obtain the compositions of the phases (±3 mol.% REO1.5 ) in the equilibrium state. For most of samples SEM images were made in back scattered electrons, while for Al2 O3 rich samples both secondary electron and back scattered images were considered to distinguish between Al2 O3 phase and pores. Two different devices of SETARAM for differential thermal analysis (DTA) were used: SETSYS EVOLUTION 1750 and 2400 (TG-DTA). Melting of sample materials were investigated using SETATAM instrument SETSYS EVOLUTION 2400 (TG-DTA) in W crucibles in He atmosphere at temperatures up to 2273 K. The heating rate in both instruments was of 20 K min−1 up to 1473 K and then of 10 K min−1 ; cooling rate was of 30 K min−1 also the same in both instruments. Temperature calibration of SETSYS EVOLUTION 1750 was made using melting points of Al, Ag, Au, Cu and Ni. Temperature calibration of SETSYS EVOLUTION 2400 was made using melting points of Al, Au and Al2 O3 and temperature of transformation of LaYO3 to monoclinic phase B.18 It was found that temperatures according literature data and measured value can be expressed as linear function Tcorr (K) = 3.676 + 0.9865Tmeas (K). This equation was used for temperature correction. 3. Thermodynamic modelling The thermodynamic description of the La2 O3 and Y2 O3 polymorphs and liquid were accepted from work of Zinkevich,24 data for ZrO2 phases are from work of Wang et al.25 and Al2 O3 are from work of Hallstedt.26 The binary descriptions of ZrO2 –Y2 O3 , ZrO2 –La2 O3 , Y2 O3 –Al2 O3 and ZrO2 –Al2 O3 are accepted from previous works.12,14 It should be mentioned that that the new experimental data for La2 Zr2 O7 enthalpy of formation of obtained by solution calorimetry and melting enthalpy obtained by high temperature DTA in work27 are in a good agreement with calculations based on thermodynamic description.14 This confirms reliability of thermodynamic description14 accepted in the present work. The La2 O3 –Y2 O3 system was assessed in work18 taking into account homogeneity range in perovskite phase LaYO3 . The latest description of this system was obtained in work28 based on new results for stoichiometric LaYO3 : solution calorimetry data for enthalpy of formation and heat capacity measurements by differential

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scanning calorimetry. In present work homogeneity range in the LaYO3 was introduced in description28 using the same model as in work.18 The thermodynamic description of the La2 O3 –Al2 O3 system was slightly modified in present work taking into account heat capacity data for LaAlO3 29,30 and its standard entropy29 which were not taken into account in work.14 It should be mentioned that e.m.f. data of Wang et al.31 for the Gibbs energy of formation of LaAlO3 are incompatible with calorimetric results29,32–34 and therefore were not taken into account in the present work. Two versions of thermodynamic database for the La2 O3 –Y2 O3 –Al2 O3 system were derived in the present work: the first one based on binary extrapolations and aluminate phases LaAlO3 (LaAP), Y3 Al5 O12 (YAG), YAlO3 (YAP) and Y4 Al2 O9 (YAM) considered as stoichiometric phases and the second one taking into account small solubility of Y in LaAP phase and small solubility of La in the YAG, YAP and YAM phase. The Gibbs energies of fictive end members LaAlO3 with orthorhombic structure, YAlO3 with rhombohedral structure, La3 Al5 O12 and La4 Al2 O9 were estimated, while the solid solutions were described by ideal model. The thermodynamic description of the ZrO2 –Y2 O3 –Al2 O3 was accepted from work of Fabrichnaya et al.12 Changes due to new description of La2 O3 –Al2 O3 system were introduced into description of the ZrO2 –La2 O3 –Al2 O3 system. Thermodynamic description of the ZrO2 –La2 O3 –Y2 O3 system was derived in work18 using advanced model of pyrochlore phase which takes into account the solubility of Y2 O3 . The pyrochlore phase was described by model with five sublattices. The formula for pyrochlore solid solution was presented as (La3+ ,Y3+ ,Zr4+ )2 (Zr4+ ,Y3+ ,La3+ )2 (O2− ,Va)6 (O2− )(Va,O2− ). Solubility of ZrO2 in monoclinic phase B was modelled by Fabrichnaya et al.18 The ternary parameter in fluorite phase as well as La,Y interaction parameter were introduced for better reproducing experimentally observed large extension of fluorite phase into ternary system.18 The database was slightly modified in present work due to changes in the La2 O3 –Y2 O3 system description. Additionally, the Gibbs energy of Zr2 Y2 O7 end-member was changed for consistency with pyrochlore descriptions in the ZrO2 –RE2 O3 –Y2 O3 systems (RE = Nd, Sm). Ternary interaction parameters of fluorite were also re-assessed for better fit to experimental data. Phase name, abbreviation and models used for thermodynamic description are presented in Table 1.

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Table 1 Phases in the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system and their models. Phase

Model

Fluorite (F) Tetragonal (T) Monoclinic (M)

(Zr4+ ,La3+ ,Y3+ ,Al3+ )1 (O2− ,Va)2

A H X C B

(La3+ ,Y3+ ,Zr4+ )2 (Va,O2− )1 (O2− )3

Pyrochlore (Pyr)

(La3+ ,Y3+ ,Zr4+ )2 (Zr4+ ,Y3+ ,La3+ )2 (O2− ,Va)6 (O2− )1 (Va,O2− )1

LaAlO3 (LaAP) YAlO3 (YAP) LaYO3 (LaYP) Y4 Al2 O9 (YAM) Y3 Al5 O12 (YAG) ␦-Zr3 Y4 O12 (␦) Al2 O3 corundum LaAl11 O18 (␤)

(La3+ ,Y3+ )1 (Al3+ )1 (O2− )3 (Y3+ ,La3+ )1 (Al3+ )1 (O2− )3 (La3+ ,Y3+ )1 (Y3+ ,La3+ )1 (O2− )3 (Y3+ ,La3+ )4 (Al3+ )2 (O2− )9 (Y3+ ,La3+ )5 (Al3+ )3 (O2− )12 (Y3+ )4 (Zr4+ )3 (O2− )12 (Al3+ )2 (O2− )3 (La3+ )1 (Al3+ )11 (O2− )18

Liquid (L)

(La3+ ,Y3+ ,Zr4+ )P (O2− ,AlO1.5 )Q

taken into account in the same way as in work.18 Calculated phase diagram of the La2 O3 –Y2 O3 system is presented in Fig. 1 along with experimental phase diagram data.18,35–42 Calculated data for temperature and phase compositions of invariant reactions are presented in Table 2 along with available experimental data.36–38,43 The thermodynamic data for the La2 O3 –Al2 O3 system were re-assessed taking into account standard entropy data,29 heat capacity data29,30 and solution calorimetric data32–34 for LaAlO3 , as well as phase equilibrium data.44–46 Calculated phase diagram is presented in Fig. 2. Calculated data for invariant reactions are compared with available experimental information44–47 in Table 3. The binary descriptions of La2 O3 –Y2 O3 and La2 O3 –Al2 O3 systems were combined with the description of Y2 O3 –Al2 O3

4. Results and discussion 4.1. System La2 O3 –Y2 O3 –Al2 O3 4.1.1. Re-assessment of binary systems La2 O3 –Y2 O3 and La2 O3 –Al2 O3 Phase diagram of the La2 O3 –Y2 O3 system was calculated in work of Fabrichnaya et al.18 taking into account homogeneity range in LaYO3 (LaYP) phase. Recently thermodynamic parameters of the La2 O3 –Y2 O3 system were re-assessed based on new calorimetric determinations of enthalpy of formation and heat capacity of LaYO3 (LaYP).28 These data were accepted in this work and narrow homogeneity range in LaYP phase was

Fig. 1. Phase diagram of the La2 O3 –Y2 O3 system along with experimental data.18,35–42

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Table 2 Invariant equilibria in La2 O3 –Y2 O3 system. Reaction

T (K), x(Y2 O3 ) in phases Reference

L + H = X (per) L = X (congr) X = H (congr) B = H (congr) B = H + C (per) H = B + C (eut) H = B + A (eut) B = LaYP (cogr) B = C + LaYP (eut) B = A + LaYP

Calc., this work

35

36

43

38

2596 L:0.825 X:0.874 H:0.890 2489 0.438 2231 0.339 – 2083 C:0.777 B:0.660 H:0.620 – 2045 H:0.45 B:0.5 A:0.39 1863 0.499 1846 LaYP:0.505 B:0.591 C:0.793 985 B:0.150 A:0.002 LaYP:0.497

2583 L:0.83 X:0.85 H:0.87 2488 0.3 2218 0.35 – 2173 C:0.85 B:0.67 H:0.71 – 2003 H:0.45 B:0.5 A:0.4 1858 0.5 1843 LaYP:0.5 B:0.55 C:0.85

2643 L:0.85 X:0.90 H:0.94 2483 0.35 2213 0.4 – 2033 C:0.80 B:0.5 H:0.6 – 2028 – 1823 0.5 1773 LaYP:0.5 B:0.57 C:0.8

2623 L:0.82 X:0.85 H:0.93 2303 0.5 2253 0.37 2177 0.5 – 2133 C:0.85 B:0.6 H:0.7 2133 H:0.25 B:0.27 H:0.25 2177 0.5 1703 LaYP:0.5 B:0.6 C:0.85

– – – – – – – 1778 0.5 –

Table 3 Invariant equilibria in La2 O3 –Al2 O3 system. Reaction

T (K), x(AlO2 O3 ) in liquid Reference

L = H + X (degen.) L = LAP (congr.) L = A + H (degen.) L + Al2 O3 = ␤ (per.) L = A + LaAP (eut.) L = LaAP + ␤ (eut)

Calc., this work

44

46

45

2373 0.1208 2368 0.5 2313 0.1489 2143 0.8638 2103 0.2525 2059 0.7794

– 2373 0.5 – 2198 0.8818 2148 0.2621 2103 0.7625

2373 0.12 2383 0.5 2303 0.15 2121 0.91 2113 0.275 2053 0.775

– –

system accepted from work12 and thus thermodynamic database for the La2 O3 –Y2 O3 –Al2 O3 system was derived based on binary extrapolations. The isothermal sections at 1523, 1673 and 1873 K as well as liquidus surface of phase diagram for the La2 O3 –Y2 O3 –Al2 O3 system were calculated and then used to select compositions for experimental investigations.

Fig. 2. Phase diagram of the La2 O3 –Al2 O3 system along with experimental data.44–47

2128 0.8503 – 2033 0.7719

4.1.2. Experimental study of ternary system La2 O3 –Y2 O3 –Al2 O3 The results of XRD investigations of selected compositions of the La2 O3 –Y2 O3 –Al2 O3 system after heat treatment at 1523, 1673 and 1873 K are presented in Table 4 and compared with calculated results. The experimental data are in a good agreement with calculations. The samples after heat treatments were also investigated by electron microscopy SEM/EDX and the obtained results are in agreement with XRD study. EDX determination of phase compositions indicated small mutual solubility of La and Y in LaAP, YAP, YAG and YAM phases. Practically no solubility of Y was found in ␤-LaAl11 O18 phase. It should be mentioned that accuracy of EDX determination of phase composition is not high enough for using these data for optimization of thermodynamic parameters. The sample #3 and #4 were investigated by DTA. Transformation of LaYP phase into B phase was observed in both samples at temperatures 1806 and 1832 K, respectively. The second transition due to partial transformation of B into C phase was observed in sample #3 at 1899 K. In heat treated sample #3 three-phase assemblage was already observed at 1873 K. The DTA heating curve and XRD for sample #3 heat treated at 1673 and 1873 K are presented in Fig. 3a–c. Melting of sample #9 was investigated by DTA in both devices SETSYS EVOLUTION 1750 and 2400 up to temperatures of 2003 and 2273 K, respectively. Temperature of

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Table 4 Phase assemblages in the La2 O3 –Y2 O3 –Al2 O3 system identified by XRD after heat treatments at 1523–1873 K and calculated using thermodynamic datasets. N

Composition, mole fraction

T (K)

XRD

Calculation dataset-0

Calculation dataset-1

La2 O3

Y2 O3

Al2 O3

1

0.7

0.1

0.2

1523 1673 1873

LaAP + A + B(tr.) LaAP + A LaAP + A

LaAP + A + B LaAP + A + B LaAP + A

LaAP + A + B LaAP + A + B LaAP + A

2

0.6

0.2

0.2

1523 1673 1873

LaAP + B + LaYP(tr.) LaAP + B LaAP + B

LaAP + B + LaYP LaAP + B + LaYP LaAP + B

LaAP + B + LaYP LaAP + B + LaYP LaAP + B

3

0.5

0.3

0.2

1523 1673 1873

LaAP + LaYP LaAP + LaYP LaAP + B + C(tr.)

LaAP + LaYP LaAP + LaYP LaAP + B

LaAP + LaYP LaAP + LaYP LaAP + B

4

0.4

0.4

0.2

1523 1673 1873

LaAP + LaYP + C LaAP + LaYP + C LaAP + B + C

LaAP + LaYP + C LaAP + LaYP + C LaAP + B + C

LaAP + LaYP + C LaAP + LaYP + C LaAP + B + C

5

0.2

0.6

0.2

1523 1673 1873

LaAP + C LaAP + C + YAM LaAP + C + YAM

LaAP + C + YAM LaAP + C + YAM LaAP + C + YAM

LaAP + C + YAM LaAP + C + YAM LaAP + C + YAM

6

0.1

0.7

0.2

1523 1673 1873

LaAP + C + YAM LaAP + C + YAM C + YAM

LaAP + C + YAM LaAP + C + YAM LaAP + C + YAM

LaAP + C + YAM LaAP + C + YAM LaAP + C + YAM

7

0.15

0.4

0.45

1523 1673 1873

LaAP + YAM + YAP LaAP + YAM + YAP LaAP + YAM + YAP

LaAP + YAM + YAP LaAP + YAM + YAP LaAP + YAM + YAP

LaAP + YAM + YAP LaAP + YAM + YAP LaAP + YAM + YAP

8

0.15

0.3

0.55

1523 1673 1873

LaAP + YAP + YAG LaAP + YAP + YAG LaAP + YAP + YAG

LaAP + YAP + YAG LaAP + YAP + YAG LaAP + YAP + YAG

LaAP + YAP + YAG LaAP + YAP + YAG LaAP + YAP + YAG

9

0.15

0.15

0.7

1523 1673 1873

LaAP + YAG + ␤ LaAP + YAG + ␤ LaAP + YAG + ␤

LaAP + YAG + ␤ LaAP + YAG + ␤ LaAP + YAG + ␤

LaAP + YAG + ␤ LaAP + YAG + ␤ LaAP + YAG + ␤

10

0.05

0.05

0.9

1523 1673 1873

YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤

YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤

YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤ YAG + Al2 O3 + ␤

eutectic reaction L = YAG + ␤ + LaAP equal to 1973 K was determined as mean value on heating and cooling obtained in SETSYS EVOLUTION 1750, because the measurement in this device is more precise than SETSYS EVOLUTION 2400. Melting temperature obtained on heating in SETSYS EVOLUTION 2400 was determined as 1978 K after correction that is in a good agreement with results obtained in SETSYS EVOLUTION 1750, while quite large overcooling effect was observed on cooling (on-set point 1930 K). The DTA curves on heating and cooling are presented in Fig. 4a and b. The SEM images after both DTA investigations are presented in Fig. 5a and b. SEM investigation did not indicate eutectic structure in sample #9 investigated by DTA up 2003 K, while the eutectic structure was observed in sample #9 after DTA investigation up to 2273 K. Different shape and grain orientation of ␤-phase was indicated in microstructure of sample heated up to 2003 K. This observation can be explained by the fact that sample was partially melted and some crystals of ␤-phase were crystallized from melt in the shape of needles while the others were growing during heat treatment. However EDX indicated approximately the same composition of different

grains of ␤-phase. The composition of eutectic was measured by EDX as 17 mol.%La2 O3 –13 mol.%Y2 O3 –70 mol.%Al2 O3 . Both temperature and composition of eutectic are in agreement with calculations (1969 K 14 mol.%La2 O3 –13 mol.%Y2 O3 –73 mol.%Al2 O3 ). Melting of sample #10 was studied by DTA up to 2273 K. The temperature of transition reaction L + Al2 O3 = ␤ + YAG was determined as 2026 K, on-set point on heating. The lowest heat effect at was due to eutectic crystallization at 1913 K. The temperature of eutectic reaction was lower than for sample #9. The DTA curves on heating and cooling are presented in Fig. 6. Microstructure of sample #10 after DTA is presented in Fig. 7. It can be seen from microstructure that primary phase was Al2 O3 , while secondary phase was ␤. Composition of liquid participating in transition reaction was determined as 10 mol.%La2 O3 –16 mol.%Y2 O3 –74 mol.%Al2 O3 . These results are in good agreement with calculations (2014 K, 7 mol.%La2 O3 –15 mol.%Y2 O3 –78 mol.%Al2 O3 ). The composition of eutectic reaction determined by SEM/EDX for sample #10 was 11 mol.%La2 O3 –19 mol.%Y2 O3 –70 mol.%Al2 O3 . The temperature and compositions for this eutectic were

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Fig. 4. DTA heating and cooling curves for sample #9 (La2 O3 –15 mol.%Y2 O3 –15 mol.%Al2 O3 ). (a) Heating up to 2003 K; (b) 2273 K.

of melt in the Y2 O3 –Al2 O3 and ZrO2 –Y2 O3 –Al2 O3 systems, respectively.

Fig. 3. DTA and XRD investigations of sample #3 (La2 O3 –30 mol.% Y2 O3 –20 mol.%Al2 O3 ). (a) DTA heating curve; (b) XRD after heat treatment at 1523 K; (c) XRD after heat treatment at 1873 K.

different from results obtained for sample #9. Probable explanation for this observation is that eutectic crystallizing in sample #10 was metastable L = LaAP + YAP + ␤. The temperature (1913 K) and composition of this eutectic (11 mol.%La2 O3 –19 mol.%Y2 O3 –70 mol.% Al2 O3 ) is in perfect agreement with calculation for metastable reaction (1914 K 10 mol.%La2 O3 –19 mol.%Y2 O3 –71 mol.%Al2 O3 ). Phase composition determined by EDX indicates LaAP and YAP phases differed by La/Y ratio. Formation of metastable eutectics was observed in work of Lakiza and Lopato.15 According to their observation the metastable eutectics L = YAP + Al2 O3 and L = YAP + Al2 O3 + F were easily formed by overheating

4.1.3. Thermodynamic modelling of the La2 O3 –Y2 O3 –Al2 O3 system Isothermal section of phase diagram at 1873 K and liquidus surface calculated based on binary extrapolations are presented in Fig. 8a and b along with experimental results obtained in the present work. The calculations appeared to be in a good agreement with experimental data. However, EDX measurements indicated solubility of Y in the LaAP (up to ∼15 mol.%) phase and small solubility of La in the YAG (up to ∼4 mol.%), YAP (up to ∼5 mol.%) and YAM (up to ∼4 mol.%) phases. Therefore provisional description was developed describing mutual solubility of La and Y in aluminates. The calculated isothermal sections at 1523, 1673 and 1873 K as well as liquidus surface are presented in Figs. 9a–c and 10. Temperatures and liquid compositions calculated using both descriptions are presented in Table 5 in comparison with experimental data obtained in the present work. Calculated data for invariant reactions using both models of aluminates did not differ very much and agree with experimental data from present study (Table 5). Therefore it was not necessary to introduce a ternary parameter into description of liquid phase. Temperatures of invariant reactions were slightly higher in case when mutual solid solubility of Y and La in aluminates was taken into account. Also temperature of L = YAM + C + LaAP reaction appeared to be higher than

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Table 5 Invariant equilibria in the La2 O3 –Y2 O3 –Al2 O3 system. Reaction

Type

L+H=X

pmin1

H=L+B+C

P1

L = LaAP + YAM

emax1

L = LaAP + YAP

emax2

L = LaAP + YAP + YAM

E1

L = LaAP + YAG

emax3

L = LaAP + YAG + YAP

E2

H+L=B+A

U1

L = LaAP + C

emax4

L + A = LaAP + B

L + Al2 O3 = ␤ + YAG

U2 U2 E3 E4 E4 E3 U3

L = LaAP + ␤ + YAG

E5

L = C + LaAP + B L + YAM = C + LaAP

Temperature (K)

2231 2231 2083 2083 2071 2096 2062 2082 2056 2078 2051 2067 2049 2066 2045 2045 2032 2037 2028 2030 2026 2029 2024 2035 2014 2016 2026 1969 1977 1973

Composition of liquid

Source

x(Al2 O3 )

x(La2 O3 )

x(Y2 O3 )

0.1408 0.1408 0.2095 0.2095 0.3997 0.3985 0.5008 0.50003 0.4599 0.4643 0.5844 0.5846 0.5617 0.5696 0.2212 0.2212 0.2631 0.2656 0.2308 0.2299 0.2416 0.2399 0.2908 0.2778 0.7816 0.7826 0.74 0.7334 0.7380 0.70

0.5786 0.5786. . . 0.4157 0.4157 0.1977 0.2005 0.1617 0.1561 0.1643 0.1581 0.1566 0.1501 0.1532 0.1469 0.4996 0.4996 0.3375 0.3199 0.5069 0.5062 0.4142 0.4145 0.2559 0.2808 0.0669 0.0676 0.10 0.1395 0.1380 0.17

0.2806 0.2806 0.3747 0.3747 0.4026 0.4010 0.3375 0.3436 0.3758 0.3776 0.2590 0.2636 0.2850 0.2835 0.2793 0.2793 0.3994 0.4145 0.2623 0.2639 0.3441 0.3457 0.4533 0.4414 0.1517 0.1498 0.16 0.1270 0.1240 0.13

Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Calc. 0 Calc. 1 Exp. Calc. 0 Calc. 1 Exp.

Calculations 0 were made base on binary extrapolations into a ternary system, calculations 1 took into account mutual solubility of La and Y in aluminates, experimental data are from this study.

temperature of L = A + B + LaAP reaction and therefore numbers were changed for these reactions in case of using more advanced models for aluminates (see Table 5). Thermodynamic parameters of the La2 O3 –Y2 O3 –Al2 O3 system derived in the present work are listed in Table 6.

4.2. System ZrO2 –La2 O3 –Al2 O3 New parameters of the La2 O3 –Al2 O3 system were introduced into the thermodynamic description of the ZrO2 –La2 O3 –Al2 O3 system. The isothermal sections were calculated at 1523 and 1923 K and compared with experimental data of Lakiza and Lopato13 and previous assessment of Fabrichnaya et al.14 No substantial changes occurred. Ternary parameter of liquid phase 0 L(La3+ ,Zr4+ :O2− ,AlO3/2 ) was reassessed to be 1.9 × 105 for better reproducing of experimental liquidus surface data.13,14 Calculated liquidus surface of the ZrO2 –La2 O3 –Al2 O3 system is presented in Fig. 11. As a result of changes in the binary system La2 O3 –Al2 O3 invariant reactions were changed in the La2 O3 -rich composition. Instead of reactions L = A + Pyr + LaAP (1) and L + H = A + Pyr (2) obtained in work14 reactions L + A = H + LaAP (3) and L = H + Pyr + LaAP (4) become stable according to calculations

made in present work. It should be mentioned that temperatures and compositions of reactions (1) and (2) according to work14 were very close to each other as well as temperatures and compositions of reactions (3) and (4). However it was not possible to change the topology of liquidus surface without changing of thermodynamic parameters of binary system. It should be noted that in situ XRD study would be necessary to establish experimentally which reactions (1) and (2) or (3) and (4) occur because H phase is not quenchable. Temperatures and liquid compositions of invariant reactions calculated in the present work are compared with experimental data of Lakiza and Lopato13 in Table 7.

4.3. System ZrO2 –La2 O3 –Y2 O3 The thermodynamic parameters of the ZrO2 –La2 O3 –Y2 O3 system were published in work of Fabrichnaya et al.18 based on own experimental data.17,18 New description of the La2 O3 –Y2 O3 system was introduced into description of ternary system. Additionally parameter GANCA1in pyrochlore phase and ternary mixing parameters in fluorite phase were modified in present work. The Gibbs energy difference between Zr2 Y2 O7 and Y2 Zr2 O7 designated as parameter GANCA1

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O. Fabrichnaya et al. / Journal of the European Ceramic Society 33 (2013) 37–49

Table 6 Thermodynamic parameters of the La2 O3 –Y2 O3 –Al2 O3 system at 298.15–6000 K. Phase

Model/parameter

YAG

(La3+ ,Y3+ )3 (Al3+ )5 (O2− )12 0 GYAG (La3+ :Al3+ :O2− ) = 1.5·GLA2O3A + 2.5·GCORUND − 70,000 0 GYAG (Y3+ :Al3+ :O2− ) = GYAG − 1684.33

YAP

(La3+ ,Y3+ )1 (Al3+ )1 (O2− )3 0 GYAP (La3+ :Al3+ :O2− ) = 0.5·GCORUND + 0.5·GLa2O3A − 20,000

0 GYAP (Y3+ :Al3+ :O2− ) = −24515.2904 − 6.858584·T + 0.5·GCORUND + 0.5·GY2O3C

(La3+ ,Y3+ )4 (Al3+ )2 (O2− )9 0 GYAM (La3+ :Al3+ :O2− ) = 2·GLA2O3A + GCORUND − 63,460 + 20·T

YAM

0 GYAM (Y3+ :Al3+ :O2− ) = GYAM − 9035.18728 + 2.18809722·T

LaAP

(La3+ ,Y3+ )1 (Al3+ )1 (O2− )3 0 GLaAP (La3+ :Al3+ :O2− ) = GLAALO3 0 GLaAP (Y3+ :Al3+ :O2− ) = 0.5·GCORUND + 0.5·GY2O3C − 2000

LaYP

(La3+ ,Y3+ )1 (Y3+ ,La3+ )1 (O2− )3 0 GLaYP (La3+ :La3+ :O2− ) = GLA2O3B + 30,211 0 GLaYP (Y3+ :La3+ :O2− ) = GLAYP

+ 40,000

0 GLaYP (La3+ :Y3+ :O2− ) = GLAYP 0 GLaYP (Y3+ :Y3+ :O2− ) = GY2O3C + 31,087 − 3.967·T 0 LLaYP (Y3+ :La3+ ,Y3+ :O2− ) = 6000 0 LLaYP (La3+ :La3+ ,Y3+ :O2− ) = 6000 0 LLaYP (La3+ ,Y3+ :Y3+ :O2− ) = 26,811 0 LLaYP (La3+ ,Y3+ :La3+ :O2− ) = 26,811

(La3+ )1 (Al3+ )11 (O2− )18 = 5.5GCORUND + 0.5La2O3A − 60,910 − 10.0·T

LaAl11 O18 (␤)

0 G␤

IONIC LIQ (L)

(La3+ ,Y3+ )P (O2− ,AlO1.5 )Q 0 GL (La3+ :O2− ) = GLA2O3L 0 GL (AlO ) = 0.5·GAl2O3L 1.5 0 GL (Y3+ :O2− ) = GY2O3C + 108,779 − 40.509·T 0 LL (La3+ ,Y3+ :O2− ) = 0 0 LL (La3+ :O2− ,AlO ) = −188,378 1.5 1 LL (Nd3+ :O2− ,AlO ) = −68,714 1.5 0 LL (Y3+ :O2− ,AlO ) = −124,455 + 12.23067·T 1.5 1 LL (Y3+ :O2− ,AlO ) = 266,564 − 151.915942·T 1.5

GLAALO3 = −1,845,069 + 723.2027·T − 118.76439·T·ln(T) − 0.006198·T2 + 1,380,620/T GYAG = −7,363,443 + 2714.0547·T − 438.9177·T·ln(T) − 0.034315335·T2 + 4,800,831.7/T GYAM = −5,686,303 + 2239.26502·T − 368.158373·T·ln(T) − 0.018363228·T2 + 4310328.16/T

Functions

Functions GCORUND, GAL2O3L, GHSEROO can be found in work,26 GLA2O3A, GLA2O3L, GLa2O3B, GY2O3C in work.24

Table 7 Temperature and liquid composition for invariant reactions in the ZrO2 –La2 O3 –Al2 O3 system. Reaction

e1 (max): L = LaAP + Pyr U1 : L + X = H + Pyr U2 : L + F = T + Pyr U3 : L + A = H + LaAP E1 : L = LaAP + H + Pyr U4 : L + Al2 O3 = ␤ + T U5 : L + Pyr = T + LaAP E2 : L = Pyr + T + LaAP E2 : L = T + ␤ + LaAP

Temperature, K

2227 2258 2181 2168 2223 2058 2055 2019 2003 2001 1988 1943 1938

Liquid composition

Source

x(ZrO2 )

x(La2 O3 )

x(Al2 O3 )

0.2782 0.32 0.2538 0.4853 0.52 0.1398 0.1534 0.2988 0.31 0.3441 0.38 0.2795 0.27

0.4308 0.42 0.6555 0.2112 0.30 0.6714 0.6625 0.1179 0.13 0.2082 0.36 0.1843 0.2

0.2910 0.26 0.0908 0.3035 0.18 0.1890 0.1841 0.5832 0.56 0.4476 0.26 0.5362 0.53

Calculated; Exp.13 Calculated Calculated; Exp.13 Calculated Calculated Calculated; Exp.13 Calculated; Exp.13 Calculated; Exp.13

O. Fabrichnaya et al. / Journal of the European Ceramic Society 33 (2013) 37–49

45

Table 8 Calculated invariant equilibria in the ZrO2 –La2 O3 –Y2 O3 system. Reaction

L+H=X L+H=X+C L+C=F+X L + F = X + Pyr

Type

Pmax1 U1 U2 U3

Temperature (K)

2595 2580 2528 2306

Composition of liquid x(ZrO2 )

x(La2 O3 )

x(Y2 O3 )

0.6414 0.0401 0.0778 0.3348

0.3403 0.1805 0.2542 0.6423

0.0183 0.7794 0.6680 0.0732

Fig. 7. SEM image after DTA investigation of sample #10.

Fig. 5. SEM image after DTA investigation of sample #9 heated up to temperatures. (a) 2003 K (white phase is LaAP, grey phase is YAG and different contrast from dark grey to black are ␤-phase due to different conditions of crystal growth and grain orientation); (b) 2273 K (white phase is LaAP, grey phase is YAG, dark grey is ␤-phase and black are pores).

Fig. 6. DTA heating and cooling curves for sample #10 (La2 O3 –5 mol.%Y2 O3 –5 mol.%Al2 O3 ).

in work18 was accepted equal to 80,000 J/mol to be consistent with the descriptions of other ZrO2 –RE2 O3 –Y2 O3 systems (RE = Sm,Nd).48,49 The ternary parameters L(Flu, La3+ ,Y3+ ,Zr4+ :O2− ) and L(Flu, La3+ ,Y3+ ,Zr4+ :Va) were accepted equal to −52,700 − 100·T J/mol for better reproducing of experimental data.18 The calculated isothermal sections at 1523 and 1873 K are presented in Fig. 12a and b along with experimental data.18 It should be mentioned that isothermal sections at 1523 and 1873 K were also constructed in the works of Andrievskaya et al.50–52 The substantial difference between results of Fabrichnaya et al.,17 at 1523 K and Andrievskaya and Red’ko50 was already discussed in details in work17 and could be due to different methods used for sample preparation. Difference between results of Andrievskaya et al.51 and our works17,18 became even more serious at temperature 1873 K. According to experimental data obtained17,18 the LaYP phase was not stabilized in ternary system. The fact that this phase was observed in some samples after heat treatment was due to the reverse transformation of high-temperature B phase into LaYP phase during cooling. The reverse transformation was confirmed in work17 by DTA investigation. Another example of substantial difference between experimental results17,18 and results of Andrievskaya et al.51 was stability of Pyr + C phase assemblage. Our experiments indicated wide extension of fluorite phase into the ternary system and due to this fact two phase field Pyr + C was not stable at 1873 K (the transformation of Pyr + C into fluorite phase was confirmed by DTA and XRD investigations in work17 ), while according to Andrievskaya et al.51 this phase assemblage was stable up to melting. Solidus

46

O. Fabrichnaya et al. / Journal of the European Ceramic Society 33 (2013) 37–49

Fig. 8. Results of calculations based on binary extrapolations into ternary La2 O3 –Y2 O3 –Al2 O3 system. (a) Isothermal section at 1873 K (numbers are sample numbers from Table 4); (b) liquidus surface.

and liquidus of the ZrO2 –La2 O3 –Y2 O3 system constructed in the works of Andrievskaya and Lopato53 and Andrievskaya,54 therefore cannot be used in the present work to determine mixing parameters of liquid phase because their results are in contradiction with our data for subsolidus phase relations. Calculated liquidus surface is presented in Fig. 12c. Calculated temperatures and compositions of invariant reactions are presented in Table 8. It should be mentioned that liquidus surface calculated in our work is substantially different from liquidus surfaces predicted by Andrievskaya and Lopato53 and constructed based on experimental data by Andrievskaya.54 5. Thermodynamic database for the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system and application for thermal barrier coatings The ternary description of the La2 O3 –Y2 O3 –Al2 O3 system was combined with the description of the ZrO2 –Y2 O3 –Al2 O3 system from12,16 and thermodynamic descriptions of

Fig. 9. Calculated of isothermal sections of phase diagram of the La2 O3 –Y2 O3 –Al2 O3 system accounting mutual solubility of La and Y in aluminates. (a) 1523 K; (b) 1673 K; (c) 1873 K.

ZrO2 –La2 O3 –Al2 O3 and ZrO2 –La2 O3 –Y2 O3 systems modified in present study. The obtained thermodynamic database of the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system can be applied for modelling of phase reactions at the TBC–TGO interface. Therefore it can be estimated at which concentration of La- or Y-stabilizers formation of the LaAP, YAG or YAP phases occurs resulting in incompatibility between thermal barrier coating and

O. Fabrichnaya et al. / Journal of the European Ceramic Society 33 (2013) 37–49

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Fig. 10. Calculated liquidus surface of the La2 O3 –Y2 O3 –Al2 O3 system accounting mutual solubility of La and Y in aluminates (white phase is LaAP, light grey phase is ␤ and different contrast from grey to black are Al2 O3 due to different grain orientation).

thermally grown oxide. This database allows one to calculate reactions between La2 Zr2 O7 and YSZ layer in case of using double layer coating.7,8 Using thermodynamic database for the ZrO2 –La2 O3 –Y2 O3 –Al2 O3 system it is possible to evaluate the thermal stability of the coating and its stability during thermal cycling. T0 -lines for diffusionless transformations (e.g., fluorite = tetragonal) can be calculated using derived thermodynamic data and thus it is possible to estimate how much stabilizer can be introduced into the tetragonal phase under quenching.

Fig. 12. Calculated phase diagram of the ZrO2 –La2 O3 –Y2 O3 system based on thermodynamic description up-dated in the present work. (a and b) Isothermal sections along with experimental data points17,18 at 1523 K (a); 1873 K (b); (c) liquidus surface.

Fig. 11. Calculated liquidus surface of the ZrO2 –La2 O3 –Al2 O3 system based on thermodynamic description up-dated in the present work.

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O. Fabrichnaya et al. / Journal of the European Ceramic Society 33 (2013) 37–49

6. Conclusions Up-dated thermodynamic descriptions of the La2 O3 –Al2 O3 and La2 O3 –Y2 O3 systems were combined with available data for the Y2 O3 –Al2 O3 system12 into a ternary description. The calculated phase diagrams at 1523, 1673 and 1873 K were used to select compositions for experimental study. Phase diagrams of the La2 O3 –Y2 O3 –Al2 O3 system were experimentally investigated for the first time. Melting relations in the system were investigated using DTA and electron microscopy of microstructures formed on cooling. Temperature and compositions of eutectic reaction L = LaAP + YAG + ␤ and transition type reaction L + Al2 O3 = YAG + ␤ were determined. Calculations based on binary extrapolations into a ternary system La2 O3 –Y2 O3 –Al2 O3 appeared to be in a good agreement with experimental data obtained. To model mutual solubility of La and Y in aluminates, which was observed experimentally, the Gibbs energies of La3 Al5 O12 , LaAlO3 orthorhombic, La4 Al2 O9 and YAlO3 rhombohedral end-members were assessed and solid solutions were assumed to be ideal. Calculations based on this database were also in a good agreement with experimental data obtained in the present work. The binary descriptions of the La2 O3 –Al2 O3 and La2 O3 –Y2 O3 systems were also substituted by the up-dates in the ZrO2 –La2 O3 –Al2 O3 and ZrO2 –La2 O3 –Y2 O3 system descriptions and phase diagrams were re-calculated. As a result of these changes liquidus surface of the ZrO2 –La2 O3 –Al2 O3 system in La2 O3 rich compositions was slightly modified. However since La2 O3 H phase is not quenchable and this issue is difficult to resolve experimentally. The parameter GANCA1 in pyrochlore and ternary mixing parameters of fluorite phase in the ZrO2 –La2 O3 –Y2 O3 system were adjusted for consistency with other similar descriptions48,49 and better fit to experimental data,18 respectively. Liquidus surface and parameters of invariant reaction were calculated. However experimental data on melting in the ZrO2 –La2 O3 –Y2 O3 system are not available so far. The updated thermodynamic descriptions of the ZrO2 –La2 O3 –Al2 O3 and ZrO2 –La2 O3 –Y2 O3 systems were combined with ZrO2 –Y2 O3 –Al2 O3 system12,16 and new description of the La2 O3 –Y2 O3 –Al2 O3 system was introduces into a four oxide thermodynamic database. Several key experiments need to be performed for checking the reliability of this database. The derived database can be applied for TBC applications and other applications like SOFC, SiC liquid phase sintering etc.

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