Experimental investigation of phase relations and thermodynamic properties in the system ZrO2–Eu2O3–Al2O3

Experimental investigation of phase relations and thermodynamic properties in the system ZrO2–Eu2O3–Al2O3

G Model ARTICLE IN PRESS JECS-10413; No. of Pages 14 Journal of the European Ceramic Society xxx (2015) xxx–xxx Contents lists available at www.sc...

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ARTICLE IN PRESS

JECS-10413; No. of Pages 14

Journal of the European Ceramic Society xxx (2015) xxx–xxx

Contents lists available at www.sciencedirect.com

Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Experimental investigation of phase relations and thermodynamic properties in the system ZrO2 –Eu2 O3 –Al2 O3 O. Fabrichnaya a,∗ , I. Saenko a,b , M.J. Kriegel a , J. Seidel c , T. Zienert a , G. Savinykh a , G. Schreiber a a

Institute of Materials Science, Technical University Bergakademie Freiberg, Gustav-Zeuner-Str. 5, D-09599 Freiberg, Germany A.A Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskiy Prospect 49, 119991 Moscow, Russia c Institute of Physical Chemistry, Technical University Bergakademie Freiberg, Leipziger-Str. 29, 09599 Freiberg, Germany b

a r t i c l e

i n f o

Article history: Received 9 October 2015 Received in revised form 7 December 2015 Accepted 12 December 2015 Available online xxx Keywords: Phase diagram Electron microscopy X-ray methods Thermal analysis Thermodynamic modeling

a b s t r a c t Phase relations in the Eu2 O3 –Al2 O3 and ZrO2 –Eu2 O3 –Al2 O3 systems were studied experimentally using X-ray diffraction (XRD), scanning electron microscopy combined with dispersive X-ray spectrometry (SEM/EDX) and differential thermal analysis (DTA). The stability ranges of Eu4 Al2 O9 phase were established as 1748–2095 K. Peritectic character of Eu4 Al2 O9 melting was confirmed. Temperatures and compositions of eutectic reactions were measured. Heat capacities of EuAlO3 and Eu4 Al2 O9 were measured by differential scanning calorimetry (DSC) in the range 298.15–1400 K. Thermodynamic description of Eu2 O3 –Al2 O3 system has been derived based on own results and data from literature. Isothermal sections of the ZrO2 –Eu2 O3 –Al2 O3 system at temperatures 1523–2073 K were constructed based on experimental study and thermodynamic calculations based on binary extrapolations. Temperatures and compositions of two eutectic reactions were measured. Based on obtained experimental results thermodynamic parameters in the ZrO2 –Eu2 O3 –Al2 O3 system have been optimized using CALPHAD approach. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The phase relations in the ZrO2 –Eu2 O3 –Al2 O3 system and thermodynamic properties of constituent phases present interest for several industrial applications. One of possible application of this system is thermal barrier coating. The co-doping of yttria stabilized ZrO2 with various rare earths is the way to produce coatings having lower thermal conductivity and therefore to increase efficiencies of gas turbines. The other possible application of the ZrO2 –Eu2 O3 –Al2 O3 and Eu2 O3 –Al2 O3 systems is the directionally solidified eutectics [1]. These in situ composites are of interest because of their excellent mechanical properties at elevated temperatures and homogeneous microstructure being stable and resistant to corrosion [2]. The Al2 O3 -based directionally solidified eutectics find applications as structural ceramics at high temperatures, as optical and electronic materials. The pyrochlore phase

∗ Corresponding author. Fax: +49 3731 393657. E-mail addresses: [email protected], [email protected] (O. Fabrichnaya).

Eu2 Zr2 O7 in the ZrO2 –Eu2 O3 system is interesting for a variety of applications such as thermal barrier coating [3], temperature sensors, host materials for fluorescence centers, nuclear materials, matrices for immobilization of actinides and fuel cell electrolyte materials [3–7]. The phase relations in the ZrO2 –Eu2 O3 system have been recently studied and thermodynamic description of the system was developed [8]. The thermodynamic description of ZrO2 –Al2 O3 system was derived by [9,10] based on phase equilibrium data. The found in literature information about phase relations in the Eu2 O3 –Al2 O3 system is contradictory [11,12]. Timofeeva et al. [11] identified phase EuAl11 O18 with ␤-alumina structure, while Mizuno et al. [12] found the Eu4 Al2 O9 phase with monoclinic structure. Melting temperature of perovskite EuAlO3 phase was substantially lower according to [11] compared to [12]. Two eutectic reactions were found in the work of Timofeeva et al. [11] as well as in Mizuno et al. [12]. However the reactions found in these works were different that probably caused substantial differences in measured temperatures. Thermodynamic description obtained by Wu and Pelton [13] was based mainly on data [12]. Wu and Pelton assumed that the Eu3 Al5 O12 phase with garnet structure was stable

http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.010 0955-2219/© 2015 Elsevier Ltd. All rights reserved.

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up to 2080 K to fit liquidus data [12]. This assumption is in contradiction with experimental work [14], where metastable character of Eu3 Al5 O12 phase synthesized at 1098–1108 K was proved by its complete decomposition and formation of EuAlO3 phase at 1573 K. The EuAlO3 enthalpy of formation at room temperature obtained by drop solution calorimetry [15] is the only available thermodynamic information in the Eu2 O3 –Al2 O3 system. There is no available experimental information for the ZrO2 –Eu2 O3 –Al2 O3 system so far. The liquidus surface of this system was predicted by [16] based on the comparison of liquidus surfaces of ZrO2 –Sm2 O3 –Al2 O3 , ZrO2 –Gd2 O3 –Al2 O3 and other similar systems. Therefore the aim of the present work is investigation of phase relations and thermodynamic properties in the quasi-binary system Eu2 O3 –Al2 O3 and quasi-ternary system ZrO2 –Eu2 O3 –Al2 O3 . 2. Experimental 2.1. Sample preparation Co-precipitation procedure was described in previous work of Fabrichnaya et al. [10]. The zirconium acetate solution in acetic acid, Zr(CH3 COO)4 (99.99%, Sigma–Aldrich), Eu(NO3 )3 ·6H2 O (99.99%, Alfa Aesar) and Al(NO3 )3 ·6H2 O (99.99%, Alfa Aesar) were used as the starting chemicals. In the first step, the Eu(NO3 )3 ·6H2 O and Al(NO3 )3 ·6H2 O were separately dissolved in distilled water and the initial zirconium acetate solution was diluted. The concentration of initial solutions was determined by Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) spectrometry, with an experimental accuracy of ±2 at.%. The solutions were mixed to get ∼3 g of oxide sample of desired composition. The obtained precursor solution was dropped from a buret at a low speed (around 1 ml min−1 ) into a big beaker containing about 500 ml of distilled water. The pH value was maintained above 9.0 by adding ammonia aqueous solution. 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 then dried at 353 K. To control completeness of co-precipitation the filtrates and sample after drying were analysed by ICP-OES spectrometry. Finally, the dried precipitate powder was annealed at 1073 K for 3 h in air. The powder after pyrolysis was pressed into cylindrical pellets and sintered in air atmosphere in Pt-crucibles using NABERTHERM furnace to reach the equilibrium state. The duration of heat treatments was selected depending on temperature and it was in the range of 36 h at 1973 K and 240 h at 1523 K. Synthesis of EuAlO3 and Eu4 Al2 O9 phases for heat capacity measurements was performed for 100 h at 1523 and 1873 K, respectively. Lowest eutectic reaction in the ZrO2 –Eu2 O3 –Al2 O3 system was additionally studied in the NABERTHERM furnace at air conditions by heat treatments at several temperatures starting from 1898 K with the step 25 K for 4 h each time to observe at which temperature sample was melted. Purity of the obtained samples was ensured by purity of initial reagents (99.99%) and additionally checked by ICP-OES and SEM/EDX. 2.2. Sample treatment and characterization The samples were analysed by X-ray diffraction (XRD), scanning electron microscopy combined with an energy dispersive X-ray spectrometry (SEM/EDX) and differential thermal analysis (DTA). The XRD measurements of powdered specimen were recorded using the URD63 diffractometer (Seifert, FPM, Freiberg, Germany). The goniometer working in the Bragg–Brentano geometry equipped with the graphite monochromator in the diffracted beam and the CuK␣ radiation ( = 1.5418 Å) was used for the

measurements. All measured diffraction patterns were refined using the Rietveld algorithm to obtain the volume fractions of present phases as well as lattice parameters. For the Rietveld refinement, the programs BGMN [17] and Maud [18] were used. The microstructures of sintered samples were examined by SEM (Leo1530, Carl Zeiss) equipped with EDX (Bruker AXS Mikroanalysis GmbH) to obtain the chemical compositions of sample. Ratio of metal elements were recalculated into the Al2 O3 , Eu2 O3 and ZrO2 content with an accuracy of ±4 mol.%. Most of DTA investigations were performed using SETARAM instrument SETSYS EVOLUTION 2400 (DTA-TG) in W crucibles in He atmosphere at temperatures up to 2373 K. The heating rate 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 . Temperature calibration of SETSYS EVOLUTION 2400 was made using melting points of Al, Al2 O3 and solid phase transformation in LaYO3 as discussed elsewhere [8]. Linear equation was obtained for temperature correction. DTA curves resented in this work are obtained after taking into account temperature correction. Temperature of transformation was accepted as on-set point, because calibrations were made using on-set points. However temperatures of first deviations from base-line were also indicated. Solid state transformations in the range between 298 and 1973 K were investigated in SETSYS EVOLUTION 1750 (TG-DTA) device using PtRh10% crucible and Ar (or He) atmosphere.

2.3. Calorimetric measurements The heat capacity of samples with EuAlO3 (EAP) and Eu4 Al2 O9 (EAM) nominal compositions were measured in Ar atmosphere in the temperature range from 573 to 1373 K by differential scanning calorimetry (DSC; NETZSCH Pegasus 404C, Pt/Rh crucible). The classical three-step method (continuous method) with a constant heating rate of 10 K/min was used to measure specific heat. The system was calibrated using a certified sapphire standard material. The mass and radius of sample pellet was kept the same as for standard material 84.1 mg and 5 mm. The measurements of two different samples were repeated three times with maximal uncertainty 2%. It should be mentioned that the CP measurements at temperature above 1200 K by described DSC equipment are becoming less reliable due to increase of heat radiation which decrease registered signal. The heat capacity measurements in the temperature range from 298.15 K to 353.15 K were carried out using two different instruments C80 and SENSYS DSC. The measurements in the C80 calorimeter (SETARAM, France; stainless steel cells, sample weight ∼2 g) were made in static air. The instrument software assisted CP by step method (steps of 2 K at the measuring temperature for sample, blank and reference material) was applied. The measurements in the Sensys DSC (SETARAM, France; alumina crucibles, sample weight ∼600 mg) were performed in pure Ar gas at a flow rate of 20 ml/min. The instrument software assisted CP by step method (steps of 10 K at the measuring temperature for sample, blank and reference material) was applied. Synthetic sapphire was also used as reference material in both calorimeters (data were taken from Ref. [19]). It should be mentioned that samples were investigated by DTA before heat capacity measurements to check for phase transformations. According to review of Vasylechko et al. [20] the orthorhombic perovskite EuAlO3 is stable in the range of temperatures investigated in DSC and undergo reversible phase transformations at ∼1600 K, i.e., at temperatures above measured range. The phase Eu4 Al2 O9 is metastable in the range of measurements, but heat capacity measurements are possible if this phase does not transform to stable assemblage during heat capacity measurements. Thus DTA confirmed possibility for CP measurements.

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3. Thermodynamic modeling Solid phases in the Eu2 O3 –Al2 O3 system practically do not have any homogeneity regions and therefore they are modeled as stoichiometric compounds. Thermodynamic parameters of EAP, EAM and liquid phases were re-assessed based on new experimental data obtained in the present work. The thermodynamic descriptions for the ZrO2 –Eu2 O3 (set 1) and ZrO2 –Al2 O3 systems were accepted from works [8] and [10], respectively. Solid phases stable in the ZrO2 –Eu2 O3 –Al2 O3 system which have homogeneity regions were described by the sublattice model in the form of compound energy formalism [21]. Liquid phase was described by partially ionic model [21]. Model of phases and abbreviations are given in Table 1. Thermodynamic parameters were assessed based on experimental data obtained in the present work for the Eu2 O3 –Al2 O3 and ZrO2 –Eu2 O3 –Al2 O3 systems. The assessment of thermodynamic parameters and phase diagram calculations were performed using Thermo-Calc program set [22]. 4. Results and discussions Nominal sample compositions, method of synthesis, results of ICP measurements of co-precipitated sample and results of XRD examination are presented in Table 2. Lattice parameters of EAP and EAM phases after heat treatments at 1523, 1673 and 1873 K respectively are presented in Table 3. 4.1. Heat capacity for perovskite EuAlO3 and monoclinic phase Eu4 Al2 O9 Sample EA2 heat treated at 1523 K and EA4 heat treated at 1873 K have been used for heat capacity measurements. It can be seen from Table 2, that practically single phase samples were obtained in these conditions. This was additionally confirmed by SEM/EDX investigation of these samples. The heat capacity measurements of the EuAlO3 (EAP) and Eu4 Al2 O9 (EAM) performed in the present work in the temperature ranges range of 293–353 K and 600-1400 K are compared with calculations based on Neumann–Kopp rule [23] in Fig. 1a and b. The calculations based on Neumann–Kopp rule show very good agreement with experimental results at room temperature for both EuAlO3 and Eu4 Al2 O9 . For the EuAlO3 the experimental results are slightly below calculations using Neumann–Kopp rule, while the difference increases with the temperature and reaches 4% at 1400 K. The calculation using the Neumann–Kopp rule are slightly higher than experimental results for the Eu4 Al2 O9 phase in the range from room temperature to 700 K while at higher temperature the calculations are slightly below than measurements with maximal deviations 1% at 1400 K. Therefore the differences are within uncertainty limits for both compounds. The experimental results of the temperature dependence of the heat capacity in the temperature interval from 293 to 1300 K are fitted to the Mayer–Kelly expression



Fig. 1. Heat capacities of (a) EuAlO3 (EAP) and (b) Eu4 Al2 O9 (EAM).



EuAlO3 : CP Jmol−1 K−1 = 127.23 + 0.0104T − 2, 670, 000/T 2





Eu4 Al2 O9 : CP Jmol−1 K−1 = 381.49 + 0.0548T − 7, 080, 000/T 2 where temperature T is in K.

Fig. 2. Heating and cooling DTA curves for sample EA1.

4.2. Phase relations in the Eu2 O3 –Al2 O3 system The information for invariant reactions in the Eu2 O3 –Al2 O3 system was obtained from DTA and SEM/EDX investigations. The experimental results obtained in the present study are compared with data from literature [11,12] in Table 4. The results of

thermodynamic calculations performed in the present work are also presented in Table 4. The DTA heating and cooling curves for sample EA1 are shown in Fig. 2. The heat effect is observed at 1997 K. Slight undercooling

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O. Fabrichnaya et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx Table 1 Phases, their names abbreviations, crystal structure and model used for thermodynamic modelling. Phase (abr.)

Model

Space group

Fluorite (F) Tetragonal (T) Monoclinic (M) Pyrochlore (Pyr) C–Eu2 O3 (C) B–Eu2 O3 (B) A–Eu2 O3 (A) H–Eu2 O3 (H) X–Eu2 O3 (X) Al2 O3 (Al2 O3 ) Monoclinic (EAM) Perovskite (EAP) Liquid (L)

(Zr4+ , Eu3+ , Al3+ )2 (O2− , Va)4 (Zr4+ , Eu3+ , Al3+ )2 (O2− , Va)4 (Zr4+ , Eu3+ , Al3+ )2 (O2− , Va)4 (Eu3+ ,Zr4+ )2 (Eu3+ ,Zr4+ )2 (O2− ,Va)6 (O2− )1 (O2− ,Va)1 (Eu3+ , Zr4+ )2 (O2– )3 (O2– , Va)1 (Eu3+ , Zr4+ )2 (O2– )3 (O2– , Va)1 (Eu3+ , Zr4+ )2 (O2– )3 (O2– , Va)1 (Eu3+ , Zr4+ )2 (O2– )3 (O2– , Va)1 (Eu3+ , Zr4+ )2 (O2– )3 (O2– , Va)1 (Al3+ )2 (O2– )3 (Al3+ )2 (Eu3+ )4 (O2– )9 (Al3+ )(Eu3+ )(O2– )3 (Zr4+ , Eu3+ )P (O2– , AlO3/2 )Q

Fm-3m P42/nmc P121/c1 Fd-3m Ia-3 C2/m P-3m1 P63 /mmc Im-3m R-3cH P121/c1 Pnma

Fig. 3. Microstructure of sample EA1 after melting in DTA. Dark phase is primary Al2 O3 and bright phase is EAP. The white points correspond to the tungsten.

effect is observed in cooling curve. The microstructure presented in Fig. 3 indicates that Al2 O3 is primary crystallisation phase and large areas of eutectic crystallisation (L = Al2 O3 + EAP). The measured composition of eutectic composition is 76.6 mol.% Al2 O3 . The obtained results are in a good agreement with data of Mizuno et al. [12] for this reaction. The DTA investigation of sample EA2 in SETSYS EVOLUTION 1750 indicates reversible transformation at 1601 K. The heating and cooling curves are shown in Fig. 4a. This effect can be related with phase transformation of orthorhombic EuAlO3 to rhombohedral modification. Temperature measured using DTA is in perfect agreement with data of Coutures and Coutures [24] who observed this effect at 1603 K using high-temperature XRD. The same transformation was reported at lower temperature 1420 K based on polarised Raman spectra [25]. The results of DTA study at high temperatures are shown in Fig. 4b. The melting effect was observed at 2182 K. This result is in better agreement with data of Timofeeva et al. [11] who determined melting temperature of EuAlO3 (EAP) as 2213 K than with data of Mizuno at al. [12] who determined melting at 2320 K. The microstructure investigation show mostly EAP formation with small crystals of EAM due to small deviation from nominal composition (see Fig. 5). The DTA results (heating and cooling curves) for EA3 and EA4 samples are presented in Fig. 6a and b. The observed temperatures of heat effects in these samples are 2132 and 2085 K, respectively. However if we assume that liquidus curve of Mizuno et al. [12] between EAP and EAM is correct, the EA3 sample should melts at lower temperature than EA4. In case of peritectic character of EAM melting the temperature of heat effect should be the same in both

Fig. 4. Heating and cooling DTA curves of sample EA2 (a) heated up to 1773 K (b) heated up to 2373 K.

samples. It can be seen that crystallisation of both samples occurred with large undercooling and occurred at 1991 and 1979 K. The SEM images of samples EA3 and EA4 are presented in Fig. 7a and b. The microstructure investigation of sample EA3 shows two different eutectics. One of them (white dots in gray matrix marked as a) is stable eutectic EAM + Eu2 O3 and the second one (white dots in black matrix marked as b) EAP + Eu2 O3 is metastable. The eutectic between EAM and EAP was not found. Compositions of stable eutectic was determined as 29 mol.% Al2 O3 and metastable eutectic as 31 mol.%Al2 O3 . The microstructure of sample EA4 indicated

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Table 2 Sample compositions and results its characterization by ICP and XRD. Mixture

Nominal composition (mol.%)

NN

ZrO2

Al2 O3

Eu2 O3

Composition by ICP

T (K)

XRD (vol.%)

EA-1

0

76

24

0

77.8

22.2

1523 1673 1873

52 Al2 O3 + 48 EAP 53 Al2 O3 + 47 EAP 59 Al2 O3 + 41 EAP

EA-2

0

50

50

0

48.5

51.5

1523 1673 1873

99 EAP + 1 B 99 EAP + 1 B 91 EAP + 9 EAM

EA-3

0

38

62

0

36.3

63.7

1523 1673 1873

68 EAP + 32 B 67 EAP + 32 B + 1 EAM 18 EAP + 82 EAM

EA-4

0

33.3

66.7

0

33.8

66.2

1523 1673 1723 1773 1873

58 EAP + 38 B + 4 EAM 53 EAP + 39 B + 8 EAM 51 EAP + 34 B + 15 EAM 3 EuAP + 2 B + 95 EAM 100 EAM

EA-5

0

23

77

0

22.6

77.4

1523 1673 1873

34 EAP + 64 B + 2 EAM 31 EAP + 65 B + 4 EAM 28 B + 72 EAM

#1

30

40

30

32.2

36.8

31

1523 1673 1873 1948 2033

16 Al2 O3 + 37 F + 47 EAP 20 Al2 O3 + 29 F + 51 EAP 33 Al2 O3 + 34 F + 33 EAP 23 Al2 O3 + 35 F + 42 EAP 25 Al2 O3 + 34 F + 41 EAP

#2

10

10

80

11.9

9.1

79

1523 1673 1873 1973

15 EAP + 70C + 15 B 15 EAP + 59C + 26 B 22 F + 44 B + 34 EAM 23 F + 41 B + 36 EAM

#3

30

5

65

30.9

4.1

65

1523 1673 1873 1973 2073

6 EAP + 46C + 48 F 8 EAP + 52C + 40 F 53 F + 28C + 19 EAM 57 F + 14C + 19 EAM + 10 B 32 F + 24C + 26 EAM + 18 B

#4

40

5

55

40.8

3.3

55.9

1523 1673 1873 1973 2073

4 EAP + 18C + 78 F 6 EAP + 17C + 77 F 14 EAM + 86 F 11 EAM + 69 F + 10C + 9 Pyr + 1 EAP 17 EAM + 42 F + 33C + 8 Pyr

#5

60

10

30

59.8

8.3

31.9

1523 1673 1873 1973

19 EAP + 64 F + 17 Pyr 22 EAP + 78 Pyr 22 EAP + 78 Pyr 21 EAP + 79 Pyr

#6

80

15

5

77.1

17.5

5.4

1523 1673 1873

48 F + 21 Al2 O3 + 31 M 51 F + 20 Al2 O3 + 29 M 67 F + 26 Al2 O3 + 7 M

#7

75

10

15

72.4

12.7

14.9

1523 1673 1873

90 F + 10 Al2 O3 85 F + 15 Al2 O3 86 F + 14 Al2 O3

#8

60

25

15

56.4

29

14.6

1523 1673 1873

76 F + 24 Al2 O3 4 EAP + 66 F + 30 Al2 O3 3 EAP + 68 F + 29 Al2 O3

#9

10

30

60

11.1

29.4

59.5

1523 1673 1873

53 EAP + 5 F + 38C + 4 EAM 51 EAP + 5 F + 38C + 6 EAM 13 EAP + 26 F + 61 EAM

#10

20

68

13

17.5

71.8

10.7

1898 1958 2023

71 Al2 O3 + 18 F + 11 EAP 71 Al2 O3 + 16 F + 13 EAP 72 Al2 O3 + 18 F + 10 EAP

that it crystallised mostly as metastable eutectic EAP + Eu2 O3 . The obtained results can be interpreted in a following way. Due to nonequilibrium character of solidification the composition of liquid was shifted in Eu2 O3 rich composition and solidification finished in L = EAM + Eu2 O3 eutectic. The metastable eutectic L = EAP + Eu2 O3 appears if EAM phase did not form due to kinetic reasons. Therefore

based on temperatures determined using DTA and microstructure investigation it can be concluded that EAM phase melted by peritectic reaction. It should be also noted that cooling curve of EA3 indicated small effect at 2090 K due to primary phase crystallisation of EAP, while EA4 sample crystallized all as metastable eutectic.

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6 Table 3 Lattice parameters of EAP and EAM phases. Sample

EA-2

EA-4

Temperature (K) of heat treatment

Composition (mol.%) Eu2 O3

Lattice parameters (nm)

Nominal

EDX

1523

50

49.74

1873

50

49.74

1873

66.7

66.97

Fig. 5. Microstructure of sample (a) EA2 after melting in DTA. Gray phase is primary EAP and bright gray phase is EAM. The light points correspond to the tungsten.

The heating curve of EA4 investigated in SETSYS EVOLUTION 1750 device is presented in Fig. 8a. Two heat effects are observed: the first one at 1591 K due to polymorphic transformation in EuAlO3 observed also in sample EA2 and the second one at 1874 K. SEM/EDX study after DTA indicated almost complete

a = 0.5275 b = 0.5295 c = 0.7464 a = 0.5277 b = 0.5230 c = 0.7468 a = 0.7591 b = 1.0690 c = 1.1211 ˇ = 109.124

transformation into EAM phase. Small fractions of EAP and Eu2 O3 were also observed (see Fig. 8b). Hence it can be concluded that the formation of EAM phase was slightly uncompleted during DTA. It should be noted that XRD and SEM/EDX investigation of sample EA4 heat treated at 1873 K indicated complete transformation into EAM phase. Small fraction of Eu2 O3 was observed due to small deviation of sample composition from the nominal. The prolonged heat treatments of sample EA4 at 1723 and 1773 K followed by XRD investigation of sample indicated that transformation occurred between these two temperatures. The transformation temperature difference between results of DTA and phase equilibrium study can be explained by slow kinetics of solid phase transformation. It should be noted that due to slow kinetics solid phase transformations in DTA often occur with overheating effect [8]. Therefore the low temperature limit of EAM stability was accepted as 1748 K based on phase equilibration study. The heating and cooling curves for sample EA5 are shown in Fig. 9a. A large heat effect was observed at 2053 K during heating. Two effects were observed during cooling: a small effect of primary crystallization was observed at 2041 K and then large effect at 1960 K. The SEM image for this sample is shown in Fig. 9b. Microstructure investigation of sample EA5 shows both stable and metastable eutectics similar to sample EA3. Primary crystals of B-Eu2 O3 can be seen in microstructure as phase with white contrast.

Table 4 Invariant reactions in the system Eu2 O3 –Al2 O3 . Reaction

Type

T (K)

Composition Al2 O3 (at.%)

References

Liq + X = H Liq + H = A Liq + A = B

Peritectic Peritectic Peritectic

2543 2413 2323

0.059 0.129 0.179

This work, calc.

Liq = EAP

Congruent

2233 2195 2320 2213

0.500 0.500 0.500 0.500

This work, calc. This work, exp. [1977Miz] [1969Tim]

Liq + EAP = EAM

Peritectic –

2081 2094

0.333 0.333

This work, calc. This work, exp.

Liq = EAM

Congruent

2223

0.333

[1977Miz]

Liq = B + EAM

Eutectic

2072 2053 2133 1903

0.292 0.290 0.220 0.290

This work, calc. This work, exp. [1977Miz] [1969Tim]

Liq = Cor + EAP

Eutectic

2005 1998 1983 1923

0.746 0.766 0.750 0.710

This work, calc. This work, exp. [1977Miz] [1969Tim]

EAM = EAP + B

Eutectoid

1748

0.333

This work, calc.

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Fig. 6. Heating and cooling DTA curves for sample (a) EA3 and (b) EA4.

4.3. Assessment of thermodynamic parameters in the Eu2 O3 –Al2 O3 system The thermodynamic parameters of the Eu2 O3 –Al2 O3 system were assessed based on the phase equilibrium data and DTA obtained in the present work. Data from other works [11,12] were used in case of consistency with the results of present work. Experimental data for thermodynamic values (i.e., heat capacity of EAP and EAM phases from the present work and enthalpy of formation of EAP [15]) were used in the present assessment. The optimized parameters were standard entropies of EAP and EAM phases as well as mixing parameters of liquid phase. The thermodynamic description for the Eu2 O3 –Al2 O3 system is presented in Table 5. The calculated phase diagram for the system Eu2 O3 –Al2 O3 together with experimental data is shown in Fig. 10 together with experimental data. Comparison of calculated invariant reactions (temperature and liquid composition) with experimental data from the present work and literature is presented in Table 4. It can be concluded from this comparison that reasonable consistency of calculated and experimental data was achieved within uncertainty limits. 4.4. Phase relations in the ZrO2 –Eu2 O3 –Al2 O3 system Preliminary thermodynamic database was combined based on binary extrapolations into a ternary description. The thermodynamic databases for ZrO2 –Al2 O3 [10], ZrO2 –Eu2 O3 [8] and Eu2 O3 –Al2 O3 from the present work were combined into description of the ZrO2 –Eu2 O3 –Al2 O3 system without including ternary

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Fig. 7. Microstructure of samples after melting in DTA (a) EA3 and (b) EA4. Light phase is B-Eu2 O3 , gray phase is EAM and dark phase is EAP. The stable eutectic between B-Eu2 O3 and EAM phases is marked as a. The metastable eutectic between B-Eu2 O3 and EAMis marked b.

mixing parameters. It should be noted that according to preliminary calculations liquid appeared as stable phase already at 1873 K. Based on liquidus surface data for the systems ZrO2 –Sm2 O3 –Al2 O3 [26] and ZrO2 –Gd2 O3 –Al2 O3 [27] stability of liquid at such low temperature does not look very realistic. Therefore ternary mixing parameter should be introduced into liquid description to make liquid less stable. Several compositions in the ZrO2 –Eu2 O3 –Al2 O3 system were selected for experimental investigations based on preliminary calculations. The prolonged heat treatments were performed at 1523, 1673 and 1873 K. The results of XRD analysis are presented in Table 2. The sample microstructures after heat treatments were investigated by SEM/EDX and phase compositions were determined. They were used for additional phase identification and compared with XRD results. The calculated isothermal sections at 1523, 1673 and 1873 K with suspended liquid phase are presented in Fig. 11a–c together with results of XRD and SEM/EDX investigations. The agreement with calculations is quite good except for the two things: in samples #7 and #8 two phases were found in equilibrium at 1523 K instead of three as expected from calculations and phase assemblage found at 1873 K in sample #3 is in contradiction with phase assemblage found in sample #2. According to ZrO2 –Eu2 O3 phase diagram B phase is stable at 1873 K and therefore in both samples #2 and 3 B + EAM + F assemblage should be stable. Even if stabilisation of C phase in ternary system is assumed stability of both phase assemblages F + EAM + B and F + EAM + C is not possible. To reach equilibrium samples #2 and 3 were heat treated at higher temperatures of

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Table 5 Thermodynamic parameters assessed in the present work. Phase/temperature range in K

Model/parameter (J/mol)

Ionic Liq 298–6000 298–6000 298–6000

(Eu3+ , Zr4+ )P (O2– , AlO3/2 )Q 0 L (IONIC LIQ, Eu3+ :O2– , AlO3/2 ) = −170,628 1 L (IONIC LIQ, Eu3+ :O2– , AlO3/2 ) = −99,015 0 L (IONIC LIQ, Eu3+ ,Zr4+ :O2– , AlO3/2 ) = 554,000 − 240 × T

Eu4 Al2 O9 EAM 298–6000 EuAlO3 EAP 298–6000

(Al3+ )2 (Eu3+ )4 (O2– )9 G (EAM, Al3+ :Eu3+ :O2– ) = −5,169,736.7 + 2250.0929 × T − 381.48591 × T × ln(T) – 0.027406924 × T2 + 3,540,981.4/T (Al3+ )(Eu3+ )(O2– )3 G (EAP, Al3+ :Eu3+ :O2– ) = −1,743,297.6 + 762.49097 × T − 127.22575 × T × ln(T) − 0.0051984965 × T2 + 1,336,251.9/T

Fig. 8. (a) Heating and cooling DTA curves for sample EA4 heated up to 2073 K, (b) Microstructure of this sample after DTA. Light phase is B-Eu2 O3 , gray phase is EAM, dark phase is EAP and black areas are pores.

1973 and 2073 K. The formation of B phase was confirmed. However five phases instead of three were identified in sample #3 after heat treatment at 2073 K. It can be concluded that fluorite phase partially decomposed to pyrochlore and C-phase on cooling. The SEM image of sample #3 after heat treatment at 2073 K is presented in Fig. 12. White phase is B-Eu2 O3 , dark gray EAM, gray phase in several places covered by white dots is fluorite with C-Eu2 O3 . Area of eutectoid decomposition of fluorite to pyrochlore and C-Eu2 O3 are indicated as a in the microstructure. The composition of eutectoid reaction 53.45%

Fig. 9. (a) Heating and cooling DTA curves for sample EA5, (b) Microstructure of this sample after melting in DTA. Light phase is B-Eu2 O3 and dark phase is EAP. Stable eutectic L = EAM + Eu2 O3 is marked as a and metastable eutectic L = EAP + Eu2 O3 is marked as b.

Eu2 O3 and 46.15% ZrO2 is in a perfect agreement with calculations for the ZrO2 –Eu2 O3 system [8]. The SEM image of samples #3 and #4 are similar. DTA heating and cooling curves for sample #1 are presented in Fig. 13a and microstructure formed during solidification of sample #1 is shown in Fig. 13b. The first heat effect on heating curve was observed at 1935 K and the second one at 2014 K. It can be seen from cooling curve that crystallisation of sample #1 was complex and consisted of several stages. This was confirmed by microstructure examination. It can be seen that EAP (light gray phase) crystallised first. The fluorite phase is observed as inclusions in EAP and can be distinguished as slightly different gray contrast. SEM confirm that this phase was fluorite containing 1.22 mol.% of Al2 O3 , 20.73 mol.%

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Fig. 12. Microstructure of sample #3 after heat treatment at 2073 K: a in the eutectoid decomposition of fluorite into pyrochlore and C-Eu2 O3 .

Fig. 10. Calculated phase diagram of the Eu2 O3 –Al2 O3 system along with experimental data of the present study and from Refs. [11,12].

Eu2 O3 and 78.05 mol.% ZrO2 . Corundum, Al2 O3 was also present as black phase. Unexpectedly metastable garnet phase Eu3 Al5 O12 was also found as dark gray contrast. Composition of eutectic was determined as 66.67 mol.% Al2 O3 , 19.82 mol.% ZrO2 and 13.51 mol.%

Eu2 O3 . Sample #1 was also melted in air. It was heat treated at several temperatures and then investigated by XRD and SEM/EDX. The temperature of melting was found in agreement with DTA data and composition of eutectic was also consistent with the results obtained after DTA. However XRD and SEM/EDX did not show formation of garnet phase Eu3 Al5 O12 . The SEM image for sample melted in air is shown in Fig. 13c.

Fig. 11. Calculated isothermal section of the ZrO2 –Eu2 O3 –Al2 O3 system at (a) 1523, (b) 1673 and (c) 1873 K.

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Fig. 13. (a) Heating and cooling DTA curves for sample #1, (b) Microstructure of sample #1 after melting in DTA, (c) Microstructure of sample #1 after melting in air, (d) Heating and cooling DTA curves for sample #10, (e) Microstructure of sample #10 after melting in DTA, (f) Microstructure of sample #10 after melting in air. a is ternary eutectic Al2 O3 + EAP + Fluorite, b—binary eutectic Fluorite + Al2 O3 .

For additional investigation of eutectic reaction L = EAP + F + Al2 O3 new sample #10 with the same composition as determined for eutectic in sample #1 was prepared. It was heat treated at different temperatures in air and investigated by DTA. The DTA heating and cooling curves for sample #10 heat treated in air at 1873 K are shown in Fig. 13d. Only one heat effect at slightly lower temperature (1933 K) compared to sample #1 was detected on heating. Practically no undercooling effect was observed for sample #10—temperature of crystallization was only 8 K below than melting temperature. The SEM/EDX investigation of sample #10 melted in DTA and in air showed that it was mostly

crystallised as eutectic containing primary grains of Al2 O3 . XRD and SEM/EDX indicated three phases fluorite, EuAlO3 and Al2 O3 . Microstructures of sample #10 after melting in DTA and in air are shown in Fig. 13e and f, respectively. The eutectic composition was slightly different than for sample #1:59.29 mol.% Al2 O3 , 20.98 mol.% ZrO2 and 19.73 mol.% Eu2 O3 and agreed well with [16]. The composition determined in sample #10 was accepted as composition of eutectic reaction L = EAP + F + Al2 O3 . The same eutectic reaction was found in sample #6 after melting in DTA. DTA heating and cooling curves as well as SEM image after melting are presented in Fig. 14a and b, respectively. Melting

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Fig. 14. (a) Heating and cooling DTA curves for sample #6, (b) Microstructure of sample #6 after melting in DTA.

temperature was determined as 2048 K. Crystallization occurred with large undercooling effect at 1892 K. The large gray grains with traces of decomposition are tetragonal zirconia, light gray phase is fluorite phase. Probably these large grains of T and F phases were not completely melted. Black phase is corundum. At the edges of fluorite phase and around fields of eutectic crystallization the fluorite phase is substantially enriched by Eu2 O3 (the white phase). Also small grains of metastable EAG phase were found. Therefore it can be assumed that sample melted according to transitional reaction L + T-ZrO2 = F + Al2 O3 , while the crystallization was finished in eutectic reaction L = F + EAP + Al2 O3 . DTA heating curve for sample #9 and corresponding SEM image after DTA are presented in Fig. 15a and b. According to XRD the sample #9 heat-treated at 1873 K was tree phase assemblage EAP + EAM + F (Table 2) and the eutectic microstructure containing the same phases was found in the sample after DTA investigation. Temperature of melting was determined as 2080 K and melting occurred by eutectic reaction L = EAM + EAP + F. According to SEM/EDX data the composition of this eutectic was 15.92 mol.% ZrO2 , 61.55 mol.% Eu2 O3 and 22.53 mol.% Al2 O3 . DTA heating and cooling curve for sample #3 is shown in Fig. 16a. The transformation at 2184 K on heating can be attributed to transitional reaction L + B = EAM + F. Cooling occur with undercooling effect. In the microstructure of sample #3 after melting presented in Fig. 16b the same eutectic is observed as in sample #9. There are also areas with the fluorite decomposition to pyrochlore and Eu2 O3 according to eutectoid reaction F = Pyr + Eu2 O3 . Separate grains of

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Fig. 15. (a) Heating and cooling DTA curves for sample #9, (b) Microstructure of sample #9 after melting in DTA.

Eu2 O3 and Eu2 Zr2 O7 pyrochlore were also observed as well as black grains of EAP. The other fine area of small gray sports in Eu2 O3 matrix is probably due to U reaction L + B=F + EAM presents crystallization of liquid interacting with B-Eu2 O3 . Microstructure investigations of sample #4 are presented in Fig. 16c and d. Large area of fluorite phase with gray contrast can be observed. There are also areas with the fluorite decomposition according to eutectoid reaction F = Pyr + Eu2 O3 . Separate grains of Eu2 O3 and Eu2 Zr2 O7 pyrochlore were also observed as well as slightly darker grains of EAM and black grains of EAP. The small area of the same eutectic as in samples #3 and 9 was also found. Joint crystallization areas of EAM and fluorite are also present in the microstructure. 4.5. Assessment of thermodynamic parameters in the ZrO2 –Eu2 O3 –Al2 O3 system. The isothermal sections of ZrO2 –Eu2 O3 –Al2 O3 phase diagram at temperatures 1523–1873 K indicated good agreement with experimental phase equilibria (Fig. 11a–c) and therefore ternary parameters were not introduced in solid solutions. Ternary mixing parameter L0 (Zr+4 , Eu+3 :O−2 ,AlO1.5 ) was introduced into liquid description to increase melting temperature of invariant reactions. Experimental data for two eutectic reactions E2 and E1 (temperature and composition) as well as temperature of transitional reaction were used for optimization. However it was found that increasing of mixing parameter resulting in reduce of

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Table 6 Invariant reactions in the ZrO2 –Eu2 O3 –Al2 O3 system. Reaction

Type

T (K)

Composition of the liquid phase, in at.% Al2 O3

Eu2 O3

Reference ZrO2

Liq + X = H + F Liq + H = A + F Liq + A = B + F Liq + B = EAM + F Liq = EAP + EAM + F

Transitional U1 Transitional U2 Transitional U3 Transitional U4 Eutectic E1

2353 2282 2190 2020 2016 2080

4.54 10.01 16.26 25.81 27.08 22.53

72.49 71.16 68.36 63.83 61.54 61.55

22.96 18.83 15.38 10.35 11.39 15.92

Calc. this work Calc. this work Calc. this work Calc. this work Calc. this work Exp. This work

Liq + Pyr = EAP + F Pyr + Liq = EAP + F Liq = Cor + EAP + F

Transitional U6 Transitional U7 Eutectic E2

2063 2066 1890 1933

36.76 34.80 60.41 59.29

41.95 44.57 23.75 19.73

21.30 20.64 15.83 20.98

Calc. this work Calc. this work Calc. this work Exp. This work

Liq + TZrO2 = Cor + F

Transitional U5

1991 2048

17.45

19.40 –

Calc. this work Exp. This work

Liq = Pyr + F Liq = EAP + Pyr

Eutectic e1 Eutectic e2

2089 2063

43.75 42.07

21.91 21.27

Calc. this work Calc. this work

63.15 –

– 34.34 36.66

Fig. 16. (a) Heating and cooling DTA curves for sample #3, (b) Microstructure of sample #3 after melting in DTA. (c) and (d) Microstructure sample #4 after melting in DTA. a is the eutectoid reaction F = Pyr + C-Eu2 O3 ; b is the eutectic reaction Liq = EAP + EAM + F; c is the transition reaction L + B = EAM + F; d is EAM + F.

stability of liquid phase also results in miscibility gap formation in Al2 O3 rich compositions. This does not correspond to experimental observations in the ZrO2 –Eu2 O3 –Al2 O3 system and other similar systems. Therefore optimization was stopped at maximal mixing parameter which does not lead to miscibility gap formation. The calculated liquidus and solidus surfaces of ZrO2 –Eu2 O3 –Al2 O3

system are presented in Fig. 17a and b. The calculated temperatures and liquid composition are compared with experimental results in Table 6. As mentioned above temperature of invariant reactions are systematically below (∼40 K) than measured ones. Calculated compositions for E1 and E2 agree quite well with experimental data.

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system was established. Additionally heat capacities of EAP and EAM phases were measured in the range 298.15–1400 K. Thermodynamic parameters in the Eu2 O3 –Al2 O3 system were derived by CALPHAD methods. The phase relations in the ZrO2 –Eu2 O3 –Al2 O3 system have been investigated for the first time. The phase identification in samples after heat treatment at 1523, 1673 and 1873 K are generally in a good agreement with thermodynamic calculations based on binary extrapolations into ternary system. Based on melting investigation using DTA followed by SEM/EDX investigation temperature and composition of two eutectic reactions were established. The temperature of eutectic reaction L = EAP + Al2 O3 + F was determined at 1933 K and liquid composition was 59.29Al2 O3 –20.98ZrO2 –19.73Eu2 O3 (mol.%). Temperature and liquid composition in eutectic reactions L = EAP + EAM + F were determined as 2080 K and 61.55Eu2 O3 –22.53Al2 O3 –15.92ZrO2 (mol.%). The obtained experimental results were used to optimize thermodynamic parameters in the system ZrO2 –Eu2 O3 –Al2 O3 . The calculations with this dataset reproduce experimental results within uncertainty. The derived thermodynamic database can be used to calculate different phase diagrams which were not studied in the present work (e.g., vertical sections) and used for planning of new experimental studies. It should be noted that additional thermodynamic information e.g., melting enthalpy of EAP, enthalpy of formation of EAM, activity data for liquid phase would be useful to improve the thermodynamic description of Eu2 O3 –Al2 O3 system. Activity measurements for liquid phase in ternary system would be also important for better modeling of the system. Acknowledgments This work was performed in the frame of diploma thesis of I. Saenko and financially supported by TU Bergakademie Freiberg. Authors are thankful to Prof. Leineweber for discussions and to Mrs. Bleiber for technical support in SEM/EDX investigations.

Fig. 17. Calculated (a) liquidus and (b) solidus surface of the ZrO2 –Eu2 O3 –Al2 O3 system optimized based on experimental data of the present study.

5. Conclusions The phase relations in the Eu2 O3 –Al2 O3 system were investigated in the temperature range between 1523 and 1873 K using XRD and SEM/EDX. Formation of EAM phase from EAP and Eu2 O3 was observed during heating in DTA and by heat treatments. Low temperature stability limit of EAM was established as 1748 K. The DTA investigation showed the upper stability limit of EAM as 2095 K. SEM/EDX investigation of sample with EAM composition indicated peritectic character of its melting. Two eutectic reactions and were determined: L = Al2 O3 + EAP at 1997 and 76.6 mol.%Al2 O3 and L = EAM + Eu2 O3 at 2055 K and 29 mol.% Al2 O3 . Congruent melting of EAP was determined at 2182 K. Comparison with data of Mizuno et al. [12] confirms the results for eutectic reaction L = EAP + Al2 O3 . The melting temperature of EAP determined in the present work is consistent with data of Timofeeva et al. [11]. The phase with ␤-Al2 O3 structure found by [11] is not confirmed. Stability limits of EAM phase and character of its melting determined in the present work are in contrast with data of Mizuno et al. [12]. The temperature of eutectic reaction L = EAM + Eu2 O3 was determined at lower temperature than by Mizuno et al. [12]. Eutectic composition measured in the present work contained more Al2 O3 compared to Mizuno et al. [12]. Based on experimental studies performed in the present work phase diagram of the Eu2 O3 –Al2 O3

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