Phase equilibria of the TiO2–Y2O3 system

Phase equilibria of the TiO2–Y2O3 system

CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 33 (2009) 624–627 Contents lists available at ScienceDirect CALPHAD: Computer Coupl...

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CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 33 (2009) 624–627

Contents lists available at ScienceDirect

CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry journal homepage: www.elsevier.com/locate/calphad

Phase equilibria of the TiO2 –Y2 O3 system Weiping Gong ∗ , Dajian Li, Zhongshen Chen, Feng Zheng, Yong Liu, Yong Du, Baiyun Huang State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, PR China

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Article history: Received 30 April 2009 Received in revised form 24 June 2009 Accepted 28 June 2009 Available online 15 July 2009 Keywords: TiO2 –Y2 O3 phase diagram Thermodynamic calculation X-ray diffraction

abstract A thermodynamic description for the TiO2 –Y2 O3 system is performed by considering reliable literature data and newly measured phase equilibria. Using X-ray diffraction, the phase composition range and the decomposition temperature of fluorite are detected. The present description of the TiO2 –Y2 O3 system yields a set of critical thermodynamic parameters as well as the complete TiO2 –Y2 O3 phase diagram. Comparison between the calculated and measured phase equilibria in the TiO2 –Y2 O3 system shows that the experimental information is satisfactorily accounted for and some new equilibria at higher temperature are predicted by the present calculation. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction The TiO2 –Y2 O3 system is important in many industrial applications. In the field of RAFs (reduced activation ferritic/martensitic steels), small Ti–Y–O complex oxide particles are reported to greatly increase the mechanical properties of 9Cr-ODS steel to high temperature [1–3]. Additionally, Ti2 Y2 O7 has recently attracted attention both in basic science and in engineering, since several rare earth pyrochlore A2 B2 O7 compounds are intrinsic ionic conductors to be considered as potential electrolytes in solid-oxide fuel cells [4] and as candidate materials for application in nuclear engineering [5,6]. Prior to real technological applications for proposed targets, basic knowledge on the thermodynamical and transport properties of TiO2 –Y2 O3 system is required for process development and optimization. However these data are scarce and not easily accessible in literature. The objective of this work is then to resolve the above-mentioned problems in the literature, by investigating the phase equilibria of TiO2 –Y2 O3 pseudo-binary system. All the work was done under 1 atm of air to take away the effect of various titanium suboxides. 2. Experimental data in the literature for the TiO2 –Y2 O3 system 2.1. Phase diagram experimental data The phase diagram data for the TiO2 –Y2 O3 system were very limited. The only experimental information was the phase relations in the temperature range between 1000 and 2100 K



Corresponding author. Tel.: +86 731 8877824. E-mail address: [email protected] (W. Gong).

0364-5916/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.calphad.2009.06.005

determined by Mizutai et al. [7]. The major experimental results of Mizutai et al. [7] can be summarized as follows: (1) Three ternary phases, TiY2 O5 , Ti2 Y2 O7 and fluorite-type solid solution exist in the TiO2 –Y2 O3 system. (2) TiY2 O5 undergoes an orthorhombic (α -TiY2 O5 ) to hexagonal structure (β -TiY2 O5 ) transition at 1603 K and β -TiY2 O5 is stable between 1603 and about 1793 K. (3) Ti2 Y2 O7 has pyrochlore-type structure. (4) The fluorite-type solid solution decomposes to Y2 O3 and β -TiY2 O5 by a eutectoid reaction at 1663 K. (5) Neither TiO2 nor Y2 O3 is soluble in TiY2 O5 or Ti2 Y2 O7 . (6) The eutectoid between Ti2 Y2 O7 and TiO2 occurs at 90 mol% TiO2 at about 1853 K. These results were confirmed by Yamaguchi et al. [8]. As a consequence, all of the experimental phase diagram data from Mizutai et al. [7] are utilized in this optimization. 2.2. Thermodynamic data There was considerable interest in pyrochlore phase, however few data can be found on the thermodynamic properties of Ti2 Y2 O7 . By using a high temperature differential calorimeter, Babu and Nagarajan [9] measured the enthalpy increment of rare earth pyrohafnates RE2 Hf2 O7 in the temperature range 980–1740 K. Several groups of investigators [10–13] have measured the data of the enthalpy of formation of rare earth zirconates pyrochlore. Based on the measured heat capacity values from 4 to 1000 K, Bolech et al. [14] derived the thermodynamic function for pyrochlore Zr2 La2 O7 and Zr2 Ce2 O7 . Very recently, Wang [15] evaluated the thermodynamic functions of a series of rare earth zirconate pyrochlore phases by using the literature experimental data. Ti2 Y2 O7 belongs to the family of ternary metallic oxides with ¯ the pyrochlore structure (space group Fm3m). All of the abovementioned thermodynamic information about the pyrochlore structures [9–14] are taken into account during the optimization procedure, but given a low weight for the assessment. There were no thermodynamic information about TiY2 O5 .

W. Gong et al. / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 33 (2009) 624–627

Fig. 1. X-ray diffraction patterns for TiO2 –Y2 O3 mixtures obtained by coprecipitation technique and heat treated at 1623 K for 10 h. The XRD traces are summarized in Table 1.

3. Present experimental investigation at 1623 and 1673 K for the TiO2 –Y2 O3 system 3.1. Experimental procedure Since there were only experimental data measured by Mizutani et al. [7] and the phase boundary for fluorite and pyrochlore was not very clear, critical experiments were performed in the present work to clarify the phase relationship in the composition of 30 and 70 mol% TiO2 . Another reason to conduct new experiments is that the experimental errors of the literature data required these data to be checked. The samples were prepared via reactive co-precipitation technique. Yttrium nitrate, Y(NO3 )3 ·6H2 O (99.99%) and TiCl4 (99.99%) were adopted as the starting materials and dissolved in distilled water. Magnetic stirring was employed during the dissolution. The two solutions were mixed in order to obtain required concentrations of cations while ammonium hydrate being added dropwise to maintain the pH value above 9.0 for precipitation. The gel-like precipitates were then filtered and washed with distilled water prior to drying at 393 K for 12 h and followed by heat-treatment at 873 K for 10 h to remove residual organics. Ten grams of the sample was grounded for 30 min in an agate mortar, and then pressed into small pellets. Finally, heat-treatment was carried out under air using an electric furnace, whose temperature was controlled for about 5 K by a conventional electronic controller. The samples were held at 1623 and 1673 K, respectively for long enough to establish equilibrium and then air quenched by rapid withdrawal from the furnace. The resultant phases were identified using an X-ray diffractometer. 3.2. Experimental results Figs. 1 and 2 show the phase relation of TiO2 –Y2 O3 system derived from the present XRD analysis. For clarity, the pyrochlore compound Ti2 Y2 O7 is marked as P, fluorite solid solution phase is marked as F, α -TiY2 O5 and β -TiY2 O5 marked as α and β respectively. The nominal compositions of samples together with the phase identification summaries are listed in Tables 1 and 2. As can be seen from both the Figures and Tables, the phase relations resulting from each set of data contradict each other. XRD observation indicates the existence of a phase field of Y2 O3 + α + β + P, in the composition range from 42.5 to 50 at.% TiO2 and

625

Fig. 2. X-ray diffraction patterns for TiO2 –Y2 O3 mixtures obtained by coprecipitation technique and heat treated at 1673 K for 10 h. The XRD traces was summarized in Table 2. Table 1 Summary of X-ray diffraction analysis of samples annealed at 1623 K. Number

Nominal composition (at.% TiO2 )

Identified phases

1 2 3 4 5 6 7 8

45.45 47.5 50 60 64.5 66.67 68.73 81.81

Y 2 O3 + α + β

α+β α+β α+β +P

P P P + TiO2 P + TiO2

1673 K. While according to the Gibbs rule, it is quite impossible to have such a four-phase range in a pseudo-binary system. Taking into account the experimental information of Mizutai et al. [7], we can believe that, for the sample in the composition range between 42.5 and 50 at.%. TiO2 , the structure transition between β and α and a eutectoid reaction, fluorite ←→Y2 O3 + β takes place during cooling. Furthermore, the temperature for the structure transition should be some lower than 1623 K and that for the eutectoid reaction should be lower than 1673 K but higher than 1623 K. XRD results indicate that the fluorite solid solution single-phase field exists no more than the composition range from 42.5 to 47.5 mol% TiO2 , which is in good agreement with the data 43.7 mol% TiO2 reported by Mizutai et al. [7]. 4. Thermodynamic modeling 4.1. CALPHAD approach In this section, the TiO2 –Y2 O3 system is assessed by incorporating the literature data and the present experimental data. There are six phases in the TiO2 –Y2 O3 system, they are liquid, TiO2 and Y2 O3 -based terminal solid solutions, TiY2 O5 , Ti2 Y2 O7 and fluorite solid solution ternary phases. In the following, the thermodynamic model and the analytical expressions for the Gibbs energy of every phase are briefly presented. The liquid phase is modelled with a two-sublattice ionic solution model, (Ti+4 , Y+3 )P :(O−2 )Q , where P and Q are the number of cation and anion sites, respectively. By applying the rules of mass and charge balance, P and Q are written as: P =

X

Q =

X

yi (−γi ) = 2yO−2 = 2 yj γj = 4yTi+4 + 3yY+3

(1) (2)

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W. Gong et al. / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 33 (2009) 624–627

Table 2 Summary of X-ray diffraction analysis of samples annealed at 1673 K. Number

Nominal composition (at.% TiO2 )

Identified phases

1 2 3 4 5 6 7 8 9 10 11 12

30 40 42.5 45 47.5 50 52.5 60 63 66.67 70 81.81

+α+β +α+β +α+β +P +α+β +P +α+β +P +α+β +P α+β +P β +P Y2 O3 Y2 O3 Y2 O3 Y2 O3 Y2 O3 Y2 O3

P P P + TiO2 P + TiO2

Table 3 Summary of the thermodynamic parameters of the TiO2 –Y2 O3 system according to the present optimization. 0

GSTiO2 : −976 986.6+484.74037T −77.76175T ln T −67 156 800T −2 +1683 920T −1

0

GLTiO2 : 0 GSTiO2 + 61 022.4 − 28.2T

0

2 Gcubic Y2 O32 : −1976 462 + 731.6512T − 121.881T ln T − .00506T

+ 1090 000T −1 − 13 000 000T −2 : +25 100 − 9.654T + 0 Gcubic Y2 O32 0 liquid GY2 O32 : 107 700.3 − 40.509T + 0 Gcubic Y2 O32 0

Hexagonal

GY2 O32

Interaction parameters of the liquid phase: a0 = −206 435.27, a1 = 13 057.3 Gibbs energy of the compound Ti2 Y2 O7 (GSTi2 Y2 O7 ) S GCubic Y2 O3 + 2 ∗ GTiO2 − 65 569.9 − 37.19T

Temperature, K

Gibbs energy of the compound TiY2 O5 in high-temperature modification (GSTiY2 O5 ) S GCubic Y2 O3 + GTiO2 − 27 167.93 − 24.3T

2400

Gibbs energy of the compound TiY2 O5 in low-temperature modification GSTiY2 O5 − 4809 + 3T

2100

Gibbs energy of fluorite solid solution phase S 11GCubic Y2 O3 + 9GTiO2 − 365.6T

1800

In J/mol. Temperature (T ) in Kelvin. The Gibbs energies of Y2 O3 and TiO2 are the same as the new thermodynamic evaluation by Aldinger’s group [16,17].

1500 2400 1200 2100 Temperature, K

900 600 300

0 Y2O3

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Mole fraction TiO2

0.8

0.9

1.0 TiO2

1800 1500 1200

Fig. 3. The present calculated TiO2 –Y2 O3 phase diagram.

900

where yi and γi , yj and γj represent the site fraction and charges of anion i and cation j, respectively. The site fraction of O−2 (yO−2 ) is 1, those of the species of Ti+4 (yTi+4 ) and Y+3 (yY+3 ) are determined by the parameters and they are not necessarily 1. The Gibbs energy of the liquid is expressed by Eq. (3):

600

GLm = yTi+4 0 GLTi+4 :O−2 + yY+3 0 GLY+3 :O−2

where

GL +4 −2 Ti :O

and

0

GLY+3 :O−2 represent the Gibbs 0 L GTiO2 and Y2 O3 0 GLY2 O32 ,

energy of

Gm = yTi+4 yY+3 [(a0 + b0 T ) + (yTi+4 − yY+3 )(a1 + b1 T )

+ (yTi+4 − yY+3 )2 (a2 + b2 T ) + · · ·].

(4)

The coefficients ai , bi (i = 0, 1, 2) are to be optimized. Since the mutual solubility between TiO2 and Y2 O3 is quite limited, the terminal solid solutions are treated as pure compounds. Since neither TiO2 nor Y2 O3 is soluble in TiY2 O5 or Ti2 Y2 O7 , these two compounds are treated as stoichiometric, and their Gibbs energy are expressed by the following expression: 0

GSTii Y2j O2i+3j = A + BT + i 0 GSTiO2 + j 0 GCubic Y2 O3

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Mole fraction TiO2

0.8

0.9

1.0 TiO2

(3)

) which are the pure liquid Ti2 O4 (2 ∗ ) ( taken from the new thermodynamic assessments by Aldinger’s group [16,17]. R is the gas constant. E Gm is the excess Gibbs energy of the liquid phase. The excess Gibbs energy is expressed by the Redlich–Kister polynomial: E

0 Y2O3

Fig. 4. Comparison of the calculated TiO2 –Y2 O3 phase diagram with the corresponding experimental data [7, this work].

+ 2RT (yTi+4 ln yTi+4 + yY+3 ln yY+3 ) + E Gm 0

300

(5)

the coefficients A and B are to be assessed from the experimental phase diagram data.

Based on the present experimental measurement by XRD, the fluorite solid solution single-phase field exists at the composition range 42.5 and 47.5 mol% TiO2 , this work treats it as a stoichiometric compound with the composition at 45 mol% TiO2 (9TiO2 :11Y2 O3 ) for simplicity, i.e. with the formula Ti9 Y22 O51 . Its Gibbs energy is expressed by the following expression: 0

0 S GSTi9 Y22 O55 = A + BT + 11 0 GCubic Y2 O3 + 9 GTiO2 .

(6)

The coefficients A and B are to be assessed in the optimization procedure. 5. Results and discussion The model parameters are evaluated using the computeroperated optimization program PARROT [18], which works by minimizing the square sum of the differences between measured and calculated values. The step-by-step optimization procedure is utilized in the present assessment. The experimental data selected from the literature as well as the present experimental results are employed in the optimization. The optimized thermodynamic parameters are listed in Table 3. These parameters together with the

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6. Conclusions The TiO2 –Y2 O3 phase equilibria at the temperature between 1623 and 1673 K are reinvestigated by XRD techniques. It is confirmed that fluorite decomposes to Y2 O3 and TiY2 O5 by a reaction below 1673 K. The TiO2 –Y2 O3 phase diagram is thermodynamically assessed by using both the literature and present new measured data. Thermodynamic calculation provides a complete TiO2 –Y2 O3 phase diagram for the first time and gives a good explanation for both the literature and present experimental results.

Cp (T), kJ/mol– cation. K

75

70

65

60

Acknowledgement 55

50 300

600

900 1200 Temperature, K

1500

1800

Fig. 5. Calculated heat capacity of Ti2 Y2 O7 (per mole of cations) compared with the values from literature measured data [9,14].

H (T) – H298, kJ/mol– cation

120

100

80

60

40

20

0 300

600

900 1200 Temperature, K

1500

1800

Fig. 6. Comparison of the present calculated H(T )–H(298) of Ti2 Y2 O7 with measured data [9–11,14].

Gibbs energies of TiO2 and Y2 O3 evaluated by Aldinger’s group [16, 17] allow the calculation of thermodynamic properties and the phase diagram of the TiO2 –Y2 O3 system. Fig. 3 shows the complete TiO2 –Y2 O3 phase diagram calculated using the present set of thermodynamic parameters. Comparison of the calculated TiO2 –Y2 O3 phase diagram with the corresponding experimental data is shown in Fig. 4. The fit to the experimental data is excellent. Fig. 5 shows the calculated heat capacities of Ti2 Y2 O7 compared with the literature values [9,14].The calculated heat capacities of Ti2 Y2 O7 are in good agreement with those of La2 Zr2 O7 measured by Bolech et al. [14] and those of Gd2 Hf2 O7 measured by Babu et al. [9], while the measured data of La2 Hf2 O7 [9] show a much higher increased rate at temperature higher than 500 K. In Fig. 6, the calculated enthalpy increment H(T)–H(298) of Ti2 Y2 O7 is compared with the literature experimental values of rare earth pyrochlore [9–11,14]. The agreement is quite good.

The funding for this project came from the Natural Science Foundation of China (Grant No. 50634060) and the Creative research group of National Natural Science Foundation of China (Grant No. 50721003). References [1] M.J. Alinger, G.R. Odette, D.T. Hoelzer, The development and stability of Y–Ti–O nanoclusters in mechanically alloyed Fe–Cr based ferritic alloys, J. Nucl. Mater. 329–333 (2004) 382–386. [2] S. Ohtsuka, S. Ukai, M. Fujiwara, T. Kaito, T. Narita, Improvement of 9CrODS martensitic steel properties by controlling excess oxygen and titanium contents, J. Nucl. Mater. 329–333 (2004) 372–376. [3] H. Sakasegawa, S. Ohtsuka, S. Ukai, H. Tanigawa, M. Fujiwara, H. Ogiwara, A. Kohyama, Microstructural evolution during creep of 9Cr-ODS steels, Fusion Eng. Des. 81 (2006) 1013–1018. [4] S.A. Kramer, H.L. Tuller, A novel titanate-based oxygen ion conductor: Gd2 Ti2 O7 , Solid State Ion. 82 (1995) 15–23. [5] S. Lutique, D. Staicu, R.J.M. Konings, V.V. Rondinella, J. Somers, T. Wiss, Zirconate pyrochlore as a transmutation target: Thermal behaviour and radiation resistance against fission fragment impact, J. Nucl. Mater. 319 (2003) 59–64. [6] W.J. Weber, R.C. Ewing, Plutonium immobilization and radiation effects, Science 5487 (2000) 2051–2052. [7] N. Mizutani, Yo Tajima, M. Kato, Phase relations in the system Y2 O3 –TiO2 , J. Am. Ceram. Soc. 59 (3–4) (1976) 168. [8] O. Yamahuchi, A. Narai, H. Kudara, K. Shimizu, Crystallization of fluorite-type solid solution from alkoxides in the system Y2 O3 –TiO2 , J. Am. Ceram. Soc. 69 (1986) C166–167. [9] R. Babu, K. Nagarajan, Calorimetric measurements on rare earth pyrohafnates RE2 Hf2 O7 , J. Alloys Compound. 265 (1998) 137–139. [10] D. Sedmidubsky, O. Benes, R.J.M. Konings, High temperature heat capacity of Nd2 Zr2 O7 and La2 Zr2 O7 pyrochlores, J. Chem. Thermodyn. 37 (2005) 1098–1103. [11] M. Bolech, E.H.P. Cordfunke, F.J.J.G. Janssen, A. Navrostsky, Standard enthalpy of formation of lanthanum zirconate, J. Am. Ceram. Soc. 78 (8) (1995) 2257–2258. [12] K.T. Jacob, N. Dasgupta, Y. Waseda, Composition-graded solid electrolyte for determination of the Gibbs energy of formation of lanthanum zirconate, J. Am. Ceram. Soc. 81 (7) (1998) 1926–1930. [13] G. Rog, A. Kozöowska-Rog, Determination of the standard molar Gibbs energy of formation of lanthanum zirconate by a galvanic cell involving lanthanum β -alumina electrolyte, J. Chem. Thermodyn. 34 (2002) 1311–1315. [14] M. Bolech, E.H.P. Cordfunke, A.C.G. Van Genderen, R.R. Van Der Lann, F.J.J.G. Janssen, J.C. Van Miltenburg, The heat capacity and derived thermodynamic function of La2 Zr2 O7 and Ce2 Zr2 O7 from 4 to 1000 K, J. Phys. Chem. Solids 58 (1997) 433–439. [15] C. Wang, Experimental and computational phase studies of the ZrO2 -based systems for thermal barrier coatings, Ph.D. Thesis, Max-Planck-Institut fur Metallforschung, Stuttgart, Germany. [16] M. Cancarevic, M. Zinkevich, F. Aldinger, Thermodynamic description of the Ti–O system using the associate model for the liquid phase, CALPHAD 31 (2007) 330–342. [17] D. Djurovic, M. Zinkevich, F. Aldinger, Thermodynamic modeling of the yttrium–oxygen system, CALPHAD 31 (2007) 560–566. [18] J.-O. Anderson, T. Helander, L. Hoglund, P. Shi, B. Sundman, Thermo-Calc & Dictra, computational tools for materials science, CALPHAD 26 (2002) 273–312.