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Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system
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Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system Jaqueline Lopez-Martinez, Antonio Romero-Serrano ∗ , Aurelio Hernandez-Ramirez, Beatriz Zeifert, Carlos Gomez-Ya˜nez, Manuel Hallen-Lopez Metallurgy and Materials Department, ESIQIE-IPN, UPALM, Zacatenco, C.P. 07738, Mexico D.F., Mexico Received 28 August 2013; received in revised form 24 March 2014; accepted 10 April 2014
Abstract Liquidus line in the high-Bi2 O3 -containing region of the TiO2 –Bi2 O3 system was determined experimentally. The equilibrating and quenching technique with subsequent electron probe microanalyser (SEM-EDS) microanalysis were employed. Based on the data, liquidus line was constructed between 60 and 92 mol% Bi2 O3 . The current results showed a higher solubility of Bi2 O3 in the liquid phase in equilibrium with the Bi4 Ti3 O12 compound compared with the existing phase diagram. In addition, differential scanning calorimetry (DSC) was used to estimate the transformations covering the composition range from 60 to 95 mol% Bi2 O3 . Further, the phase diagram of the TiO2 –Bi2 O3 system was calculated using a quasichemical model for the liquid phase. The thermodynamic properties of the intermediate compounds were estimated from the data of TiO2 and Bi2 O3 pure solids. © 2014 Elsevier Ltd. All rights reserved. Keywords: TiO2 –Bi2 O3 diagram; Liquidus; Thermodynamic prediction
1. Introduction The new optical-fibre communication has been using singlecrystal Bi12 TiO20 due to its large electro-optic constant1 making it useful also for photorefractive applications. However, to establish a standard method for growing reproducibly Bi12 TiO20 single crystals it is required a reliable TiO2 –Bi2 O3 binary phase diagram. According to the TiO2 –Bi2 O3 binary system reported by Bruton,2 Bi12 TiO20 is a line compound melting incongruently at 1146 K, thereby decomposing into liquid and Bi4 Ti3 O12 and the eutectic composition between Bi12 TiO20 and Bi2 O3 melts at 1068 K. This work also reported the liquidus in the TiO2 –Bi2 O3 system between 78 and 98 mol% Bi2 O3 obtained by a thermobalance technique and by DTA. Levin and Roth3 suggested that the bismuth-rich compound Bi12 TiO20 melts congruently. Morrison4 concluded that the congruent melting is favoured but that the departure from
∗
Corresponding author. Tel.: +52 5511134983. E-mail address: [email protected] (A. Romero-Serrano).
congruent melting is small. According to the published phase diagram data by Speranskaya et al.,5 three incongruentlymelting compounds exist in the TiO2 –Bi2 O3 system: Bi4 Ti3 O12 (peritectic melting temperature at 1483 K), the bismuth-rich phase Bi8 TiO14 (peritectic melting temperature at about 1138 K) and the titanium-rich compound Bi2 Ti4 O11 (peritectic melting temperature at 1548 K). Miyazawa and Tabata6 studied the hypoperitectic region of the Bi12 TiO20 compound by lattice constant measurements of crystals grown from Bi2 O3 -rich solution and found the existence of a solid-solution region with the retrograde solidus line. These authors showed that Bi12 TiO20 is a non-stoichiometric compound from 1096 K to 1138 K with the widest solubility range at about 1125 K. Masuda et al.7 also reported the bismuth-rich phases Bi8 TiO14 and Bi12 TiO20 . In a previous article8 we reported the thermodynamic and experimental study on the TiO2 –Bi2 O3 system carried out using differential thermal analysis (DTA) technique covering the composition range from 65 to 90 mol% Bi2 O3 . The DTA results show that the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 is at approximately 90 mol% Bi2 O3 . The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed
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Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
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using the quasichemical model used for the liquid phase. The Gibbs free energy functions for the intermediate compounds were estimated from the properties of the TiO2 and Bi2 O3 pure solids. The present work deals firstly with the liquidus curve and the phase relations in the Bi2 O3 -rich region near the stoichiometric Bi12 TiO20 composition in the TiO2 –Bi2 O3 binary system, and secondly differential scanning calorimetry is used to estimate the temperature and limit composition of the peritectic decomposition of Bi12 TiO20 . A thermodynamic calculation of the binary system is also carried out in this work using the quasichemical model for the liquid phase; then, the previous model parameters are adjusted in this work to fit the liquidus obtained experimentally. 2. Experimental 2.1. Equilibrating and quenching The oxide samples were prepared from reagent-grade TiO2 and Bi2 O3 oxides (Sigma–Aldrich Chemical Co., purity 99.9%) and ground to 74 m > Φ > 45 m particle size. To estimate the liquidus curve we prepared 6 samples with 40 mol% TiO2 and 60% Bi2 O3 . The powders were mixed in a ball mill with ZrO2 balls and ethanol and then dried at 80 ◦ C. 20 g of each sample was mixed again in agate mortar and then placed in a platinum crucible and heated at 1173, 1223, 1273, 1323, 1373 and 1423 K inside an electric furnace. Each mixture was heated up to the given temperature and remained in the furnace during 8 h in order to reach the equilibrium; then, the samples were quenched into iced water. A molybdenum disilicide heater furnace was used to obtain the experimental temperature, and the temperature was regulated through an automatic controller within ±3 K. A calibrated Rtype thermocouple (Pt–Pt, 13%Rh) in connection with a precise potentiometer measured the temperature close to the platinum crucible. The microstructural analysis was carried out by mounting, polishing and depositing gold on the surface of the samples to get a good electric conductivity for the EDS analysis. This analysis was carried using scanning electron microscopy coupled with an energy-dispersive spectra analyzer, JEOL 6300 with an accelerating voltage of 10 kV. It must be stressed that the experiments at 1173 K were repeated at longer times of melt equilibration; the SEM-EDS results were similar to those obtained in the samples equilibrated during 8 h, then it was assumed that the equilibrium was reached at 1173 K and at higher temperatures after 8 h. 2.2. Phase transformations Four different compositions were prepared in the binary system TiO2 –Bi2 O3 : 65, 75, 85.71 and 95 mol% Bi2 O3 . 20 g of each oxide system were prepared as previously mentioned. Each mixture was heated up to 1473 K and remained in that temperature for 8 h, to ensure complete homogenization. The samples were left inside the furnace and the temperature was slowly decreased (2 K/min) up to room temperature.
Samples of each system were crushed into fine powders and characterized by X-ray diffraction (XRD Bruker D8 Focus) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS, FEI model Quanta 3D FEG). The calorimetry analysis was performed on a DSC TA Instruments SDT Q600. Alumina crucibles were used and measurements were performed under argon atmosphere. Samples weighting between 80 and 140 mg were investigated using the heating rate of 5 K/min. 3. Thermodynamic model for the TiO2 –Bi2 O3 liquid system For the liquid phase we used the modified quasichemical model proposed by Pelton and Blander.9 In this model the exchange of nearest-neighbour pairs in a binary A–B system is considered according to: (A − A) + (B − B) = 2(A − B)
(1)
The energy for this reaction is (ω − Tη). If (ω − Tη) is very negative, then (A–B) pairs are favoured. A “quasichemical” equilibrium constant can be written for reaction (1): KAB =
2 XAB ω − ηT = 4EXP − XAA + XBB RT
(2)
where Xij is the fraction of total nearest-neighbour pairs which are (i–j) pairs. When (ω − Tη) is very negative, the resultant expression for the Gibbs energy of the system goes through a sharp minimum at XA = XB = 0.5. If the minimum in the Gibbs energy is observed at a composition other than XA = XB = 0.5, “Equivalent Fractions”, YA and YB , can be used which are defined as: YA =
bA XA = 1 − YB bA XA + bB X B
(3)
where bA and bB are constants chosen so as that the Gibbs free energy minimum occurs at the experimental composition, and so that the configurational entropy is zero at this composition when (ω − Tη) = −∞. It was shown9 that the appropriate choice of constants are bBiO1.5 = 1.0331 and bTiO2 = 1.3774. In order to provide more flexibility in optimizing data, ω and η can be expanded as: ω = ω0 + ω1 YB + ω2 YB2 + ...
(4)
η = η0 + η1 YB + η2 YB2 + ...
(5)
The coefficients ωi and ηi are the parameters of the model which are obtained by optimization of the experimental data. The TiO2 –Bi2 O3 phase diagram was constructed using the FACTSage package, Thompson et al.10 using the quasichemical model for the liquid phase.
Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
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Table 1 Composition of the samples and the liquidus curve obtained by quench technique and EDS microanalysis. Sample no.
T (K)
This work mol% Bi2 O3
Masuda et al.7 mol% Bi2 O3
Bruton2 mol% Bi2 O3
1 2 3 4 5 6
1173 1223 1273 1323 1373 1423
91.40 90.54 90.30 88.60 85.38 79.45
84.50 83.50 81.00 78.80 74.20 71.00
83.90 80.40 77.70 –
4. Results and discussion 4.1. Liquidus The experimental temperature and the analysed composition of the liquid phase for all samples are listed in Table 1. In all the samples two-phase equilibrium was found, supercooled liquid and a crystalline phase. Semiquantitative EDS analysis of these crystals was carried out and the Bi/Ti ratio was calculated of at least five measurements. The results show a value that corresponds to crystals of Bi4 Ti3 O12 . The composition of the supercooled liquid phase in Table 1 are the average values calculated from five different analysis points in the same sample and correspond to the liquidus curve of the TiO2 –Bi2 O3 phase diagram; the obtained standard deviation was about σ = 0.14. Low accelerating voltage (10 kV) together with an EDS pointanalysis was preferred from the area-analysis in order to ensure that the chemical analysis corresponded to the chosen phase. The experimental points are compared with the results from Masuda et al.7 and Bruton.2 The current results show higher solubility of Bi2 O3 in the liquid phase in equilibrium with the Bi4 Ti3 O12 compound than that previously reported. Fig. 1 presents two SEM micrographs of the two-phase region, liquid-Bi4 Ti3 O12 , equilibrated at 1273 K and rapidly cooled down (sample 3). The white matrix in the upper micrograph is supercooled liquid, whereas the plate-like morphology corresponds to the Bi4 Ti3 O12 compound with crystals of about 3–6 m in width and between 15 and 60 m in length. These micrographs also show the dendritic structure which is obtained when a solid phase freezes and grows rapidly into a supercooled liquid. 5. Phase transformation Fig. 2 shows the DSC patterns for the system from 60 to 95% Bi2 O3 . Samples with 60 and 70% Bi2 O3 showed three endothermic peaks. The first peak is about at 945 K which probably corresponds to the transition of the crystalline structure of Bi4 Ti3 O12 . Pookmanee et al.11 reported that the Bi4 Ti3 O12 compound presents a monoclinic structure at low temperatures, but above the Curie temperature, at about 948 K, the symmetry becomes tetragonal. The second is a peak at about 1134 K which can be ascribed to the peritectic reaction: Liquid + Bi4 Ti3 O12 = Bi12 TiO20
(6)
Fig. 1. SEM micrographs showing the coexistence of supercooled liquid and Bi4 Ti3 O12 of a sample with 60 mol% Bi2 O3 .
The third peak is at 1457 K and 1440 K for the samples with 60 and 70 mol% Bi2 O3 , respectively. These peaks correspond to the liquidus curve. The sample with 85.71% Bi2 O3 corresponds with the stoichiometric composition of the Bi12 TiO20 compound and the thermogram of Fig. 2c must present only the peak for the peritectic transformation, at about 1134 K. However, we obtained another small peak about at 940 K which corresponds to the transition of the crystalline structure of Bi4 Ti3 O12 . This confirms that there exits impurities of Bi4 Ti3 O12 in this sample even though it has a very high amount of the Bi12 TiO20 phase. Fig. 2d shows two thermal effects in the thermogram for the sample with 95% Bi2 O3 : a thermal arrest corresponds to the transformation of Bi2 O3 from monoclinic to cubic structure at 1002 K, the second one at about 1092 K corresponds to the eutectic transformation Liquid = Bi12 TiO20 + Bi2 O3
(7)
The DSC results for the sample with 95% Bi2 O3 help us to locate approximately the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 . The results of this sample show that the peritectic transformation takes place below 95% Bi2 O3 . We concluded from this result and the experimentally liquidus curve that the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 is at approximately 90 mol% Bi2 O3 .
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Fig. 2. DSC curves for TiO2 –Bi2 O3 mixtures with compositions from 60 to 95 mol% Bi2 O3 .
Levin and Roth,3 and Morrison4 reported that the peritectic line is from 40 mol % Bi2 O3 (the composition of Bi4 Ti3 O12 ) up to about 86% Bi2 O3 . Miyazawa and Tabata6 reported that the peritectic composition must lie close to 89.25 mol% Bi2 O3 . Fig. 3 shows a SEM micrograph of the sample with 60% Bi2 O3 . This figure shows plate-like morphology, which
according to the semiquantitative EDS analysis corresponds to crystals of Bi4 Ti3 O12 , which is in agreement with Jardiel et al.12 who reported that the bismuth titanate (Bi4 Ti3 O12 ) ceramics reflects the microstructure showing big platelets-like grains growing preferentially in a given plane. Fig. 4 shows the X-Ray diffraction pattern of sample with 85.71 mol% Bi2 O3 . This pattern shows that exits a high amount of the Bi12 TiO20 (JCPDS file 34-0097) but there is also small amount of the Bi4 Ti3 O12 compound (JCPDS file 12-0213),
Fig. 3. SEM micrograph of sample with 60 mol% Bi2 O3 . The platelet-like grains correspond to Bi4 Ti3 O12 .
Fig. 4. XRD pattern of TiO2 –Bi2 O3 system with 85.71 mol% Bi2 O3 and 14.79% TiO2 .
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5.1. Thermodynamic results The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed in a previous work8 using the quasichemical model for the liquid phase. In that work we reported the model parameters (ω and η) which best reproduces the experimental data including the liquidus curve published by Masuda et al.7 and Speranskaya et al.5 The Gibbs free energy functions for the intermediate compounds were estimated from the properties of the TiO2 and Bi2 O3 pure solids, according to the following expression: o o o gxTiO = xgTiO + ygBi + E + FT 2 ·yBi2 O3 2 2 O3
Fig. 5. SEM micrograph of sample with 85.71 mol% Bi2 O3 . The crystals correspond to Bi12 TiO20 .
which is in agreement with the DSC results of the sample with 85.71% Bi2 O3 . The XPowder software13 was used to estimate the amount of Bi4 Ti3 O12 in this sample. This software carries out linear and no-linear least square analysis (crystalline and amorphous) using the full pattern profile and experimental standard diffractograms. The X-ray pattern of Fig. 4 gives about 4.3% Bi4 Ti3 O12 and 95.7% Bi12 TiO20 . Fig. 5 shows a micrograph for the nominally composition of the Bi12 TiO20 compound. The EDS analysis of these crystals shows an average composition of 87.03 mol% Bi2 O3 which is very close to the stoichiometric composition (85.71 mol% Bi2 O3 ).
(8)
where x and y are 4 and 1 for Bi2 Ti4 O11 , 2 and 1 for Bi2 Ti2 O7 , 3 and 2 for Bi4 Ti3 O12 , 1 and 6 for Bi12 TiO20 . E and F are optimizable coefficients chosen to fit the experimental phase diagram. The thermodynamic parameters for the TiO2 and Bi2 O3 as well as the optimized properties of the intermediate compounds have been reported by Lopez et al.8 In the present work we recalculate the model parameters to take into account the compositions and temperatures of the liquidus curve obtained experimentally: 7 ω = −10800 − 12000YBiO1.5 J mol−1 4 η = −3.2 − 8.0YBiO1.5 J mol−1 K−1
(9) (10)
Fig. 6 shows a comparison between the calculated diagram with the experimental liquidus curve and DSC results obtained in this work and the results from Masuda et al.7 and Bruton.2 Predicted results from this work are in good agreement with those published previously,2,5,7 except in the liquidus near pure Bi2 O3 .
Fig. 6. Calculated (line) and experimental (symbols) TiO2 –Bi2 O3 phase diagram.
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6. Conclusions
References
The liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system were determined experimentally using the quench technique with subsequent EDS microanalysis. In comparison with previous works, the solubility of Bi2 O3 in the liquid phase in equilibrium with Bi4 Ti3 O12 compound is higher; the limit of the two-phase region (Bi4 Ti3 O12 + liquid) is closer to the Bi2 O3 side of the diagram. The DSC results showed that the liquidus was at 1447 K and 1427 K for the samples with 60 and 70 mol% Bi2 O3 , respectively. DSC was also used to estimate the transformation temperatures for the systems with 60–95%Bi2 O3 . The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed in the present work using the quasichemical model for the liquid phase. The model parameters (ω and η) were recalculated from a previous work8 to take into account the experimental liquidus curve obtained in this work.
1. Bian Z, Ren J, Zhu J, Wang S, Lu Y, Li H. Self-assembly of Bix Ti1−x O2 visible photocatalyst core–shell structure and enhanced activity. Appl Catal B: Environ 2009;89:577–82. 2. Bruton TM. Study of the liquidus in the system Bi2 O3 –TiO2 . J Solid State Chem 1974;9:173–5. 3. Levin EM, Roth RS. Polymorphism of bismuth sesquioxide. II. Effect of oxide additions on the polymorphism of Bi2 O3 . J Res Natl Bur Stand Sect A 1964;68:197–206. 4. Morrison AD. Somer properties of Bi12 TiO20 in the system Bi2 O3 –TiO2 . Ferroelectrics 1971;2:59–62. 5. Speranskaya EI, Rez S, Kozlova LV, Skorikov VM, Slavov VI. Bismuth oxide – titanium dioxide system. Izv Akad Nauk SSSR, Neorg Mater 1965;1:232–5. 6. Miyazawa S, Tabata T. Bi2 O3 –TiO2 binary phase diagram study for TSSG pulling of Bi12 TiO20 single crystals. J Cryst Growth 1998;191:512–6. 7. Masuda Y, Masumoto H, Baba A, Goto T, Hirai T. Crystal growth dielectric and polarization reversal properties of Bi4 Ti3 O12 single crystal. Jpn J Appl Phys Part 1 1992;31:3108–12. 8. López J, Romero-Serrano A, Hernandez-Ramirez A, Zeifert B, GomezYanez C, Martinez R. Thermal analysis and prediction of phase equilibria in the TiO2 –Bi2 O3 system. Thermochim Acta 2011;516:35–9. 9. Pelton AD, Blander M. Thermodynamic analysis of ordered liquid solutions by a modified quasichemical approach. Application to silicate slags. Metall Trans B 1986;17B:805–15. 10. Thompson WT, Bale CW, Pelton AD. Facility for the analysis of chemical thermodynamics (FACTSage). Montreal: Ecole Polytechnique; 2010. www.crct.polymtl.ca 11. Pookmanee P, Uriwilast P, Phanichpant S. Hydrothermal synthesis of the bismuth titanate powders. Ceram Int 2004;30:1913–5. 12. Jardiel T, Caballero AC, Fernández JF, Villegas M. Domain structure of Bi4 Ti3 O12 ceramics revealed by chemical etching. J Eur Ceram Soc 2006;26:2823–6. 13. XPowder, Qualitative, Quantitative and Microtextural Powder X-Ray Diffraction Analysis. www.xpowder.com
Acknowledgments The authors wish to thank the Institutions CONACyT, SNI, COFAA and IPN for their permanent assistance to the Process Metallurgy Group at ESIQIE-Metallurgy and Materials Department. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2014.04.023.
Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
ARTICLE IN PRESS Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx
Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system Jaqueline Lopez-Martinez, Antonio Romero-Serrano ∗ , Aurelio Hernandez-Ramirez, Beatriz Zeifert, Carlos Gomez-Ya˜nez, Manuel Hallen-Lopez Metallurgy and Materials Department, ESIQIE-IPN, UPALM, Zacatenco, C.P. 07738, Mexico D.F., Mexico Received 28 August 2013; received in revised form 24 March 2014; accepted 10 April 2014
Abstract Liquidus line in the high-Bi2 O3 -containing region of the TiO2 –Bi2 O3 system was determined experimentally. The equilibrating and quenching technique with subsequent electron probe microanalyser (SEM-EDS) microanalysis were employed. Based on the data, liquidus line was constructed between 60 and 92 mol% Bi2 O3 . The current results showed a higher solubility of Bi2 O3 in the liquid phase in equilibrium with the Bi4 Ti3 O12 compound compared with the existing phase diagram. In addition, differential scanning calorimetry (DSC) was used to estimate the transformations covering the composition range from 60 to 95 mol% Bi2 O3 . Further, the phase diagram of the TiO2 –Bi2 O3 system was calculated using a quasichemical model for the liquid phase. The thermodynamic properties of the intermediate compounds were estimated from the data of TiO2 and Bi2 O3 pure solids. © 2014 Elsevier Ltd. All rights reserved. Keywords: TiO2 –Bi2 O3 diagram; Liquidus; Thermodynamic prediction
1. Introduction The new optical-fibre communication has been using singlecrystal Bi12 TiO20 due to its large electro-optic constant1 making it useful also for photorefractive applications. However, to establish a standard method for growing reproducibly Bi12 TiO20 single crystals it is required a reliable TiO2 –Bi2 O3 binary phase diagram. According to the TiO2 –Bi2 O3 binary system reported by Bruton,2 Bi12 TiO20 is a line compound melting incongruently at 1146 K, thereby decomposing into liquid and Bi4 Ti3 O12 and the eutectic composition between Bi12 TiO20 and Bi2 O3 melts at 1068 K. This work also reported the liquidus in the TiO2 –Bi2 O3 system between 78 and 98 mol% Bi2 O3 obtained by a thermobalance technique and by DTA. Levin and Roth3 suggested that the bismuth-rich compound Bi12 TiO20 melts congruently. Morrison4 concluded that the congruent melting is favoured but that the departure from
∗
Corresponding author. Tel.: +52 5511134983. E-mail address: [email protected] (A. Romero-Serrano).
congruent melting is small. According to the published phase diagram data by Speranskaya et al.,5 three incongruentlymelting compounds exist in the TiO2 –Bi2 O3 system: Bi4 Ti3 O12 (peritectic melting temperature at 1483 K), the bismuth-rich phase Bi8 TiO14 (peritectic melting temperature at about 1138 K) and the titanium-rich compound Bi2 Ti4 O11 (peritectic melting temperature at 1548 K). Miyazawa and Tabata6 studied the hypoperitectic region of the Bi12 TiO20 compound by lattice constant measurements of crystals grown from Bi2 O3 -rich solution and found the existence of a solid-solution region with the retrograde solidus line. These authors showed that Bi12 TiO20 is a non-stoichiometric compound from 1096 K to 1138 K with the widest solubility range at about 1125 K. Masuda et al.7 also reported the bismuth-rich phases Bi8 TiO14 and Bi12 TiO20 . In a previous article8 we reported the thermodynamic and experimental study on the TiO2 –Bi2 O3 system carried out using differential thermal analysis (DTA) technique covering the composition range from 65 to 90 mol% Bi2 O3 . The DTA results show that the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 is at approximately 90 mol% Bi2 O3 . The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed
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Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
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using the quasichemical model used for the liquid phase. The Gibbs free energy functions for the intermediate compounds were estimated from the properties of the TiO2 and Bi2 O3 pure solids. The present work deals firstly with the liquidus curve and the phase relations in the Bi2 O3 -rich region near the stoichiometric Bi12 TiO20 composition in the TiO2 –Bi2 O3 binary system, and secondly differential scanning calorimetry is used to estimate the temperature and limit composition of the peritectic decomposition of Bi12 TiO20 . A thermodynamic calculation of the binary system is also carried out in this work using the quasichemical model for the liquid phase; then, the previous model parameters are adjusted in this work to fit the liquidus obtained experimentally. 2. Experimental 2.1. Equilibrating and quenching The oxide samples were prepared from reagent-grade TiO2 and Bi2 O3 oxides (Sigma–Aldrich Chemical Co., purity 99.9%) and ground to 74 m > Φ > 45 m particle size. To estimate the liquidus curve we prepared 6 samples with 40 mol% TiO2 and 60% Bi2 O3 . The powders were mixed in a ball mill with ZrO2 balls and ethanol and then dried at 80 ◦ C. 20 g of each sample was mixed again in agate mortar and then placed in a platinum crucible and heated at 1173, 1223, 1273, 1323, 1373 and 1423 K inside an electric furnace. Each mixture was heated up to the given temperature and remained in the furnace during 8 h in order to reach the equilibrium; then, the samples were quenched into iced water. A molybdenum disilicide heater furnace was used to obtain the experimental temperature, and the temperature was regulated through an automatic controller within ±3 K. A calibrated Rtype thermocouple (Pt–Pt, 13%Rh) in connection with a precise potentiometer measured the temperature close to the platinum crucible. The microstructural analysis was carried out by mounting, polishing and depositing gold on the surface of the samples to get a good electric conductivity for the EDS analysis. This analysis was carried using scanning electron microscopy coupled with an energy-dispersive spectra analyzer, JEOL 6300 with an accelerating voltage of 10 kV. It must be stressed that the experiments at 1173 K were repeated at longer times of melt equilibration; the SEM-EDS results were similar to those obtained in the samples equilibrated during 8 h, then it was assumed that the equilibrium was reached at 1173 K and at higher temperatures after 8 h. 2.2. Phase transformations Four different compositions were prepared in the binary system TiO2 –Bi2 O3 : 65, 75, 85.71 and 95 mol% Bi2 O3 . 20 g of each oxide system were prepared as previously mentioned. Each mixture was heated up to 1473 K and remained in that temperature for 8 h, to ensure complete homogenization. The samples were left inside the furnace and the temperature was slowly decreased (2 K/min) up to room temperature.
Samples of each system were crushed into fine powders and characterized by X-ray diffraction (XRD Bruker D8 Focus) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS, FEI model Quanta 3D FEG). The calorimetry analysis was performed on a DSC TA Instruments SDT Q600. Alumina crucibles were used and measurements were performed under argon atmosphere. Samples weighting between 80 and 140 mg were investigated using the heating rate of 5 K/min. 3. Thermodynamic model for the TiO2 –Bi2 O3 liquid system For the liquid phase we used the modified quasichemical model proposed by Pelton and Blander.9 In this model the exchange of nearest-neighbour pairs in a binary A–B system is considered according to: (A − A) + (B − B) = 2(A − B)
(1)
The energy for this reaction is (ω − Tη). If (ω − Tη) is very negative, then (A–B) pairs are favoured. A “quasichemical” equilibrium constant can be written for reaction (1): KAB =
2 XAB ω − ηT = 4EXP − XAA + XBB RT
(2)
where Xij is the fraction of total nearest-neighbour pairs which are (i–j) pairs. When (ω − Tη) is very negative, the resultant expression for the Gibbs energy of the system goes through a sharp minimum at XA = XB = 0.5. If the minimum in the Gibbs energy is observed at a composition other than XA = XB = 0.5, “Equivalent Fractions”, YA and YB , can be used which are defined as: YA =
bA XA = 1 − YB bA XA + bB X B
(3)
where bA and bB are constants chosen so as that the Gibbs free energy minimum occurs at the experimental composition, and so that the configurational entropy is zero at this composition when (ω − Tη) = −∞. It was shown9 that the appropriate choice of constants are bBiO1.5 = 1.0331 and bTiO2 = 1.3774. In order to provide more flexibility in optimizing data, ω and η can be expanded as: ω = ω0 + ω1 YB + ω2 YB2 + ...
(4)
η = η0 + η1 YB + η2 YB2 + ...
(5)
The coefficients ωi and ηi are the parameters of the model which are obtained by optimization of the experimental data. The TiO2 –Bi2 O3 phase diagram was constructed using the FACTSage package, Thompson et al.10 using the quasichemical model for the liquid phase.
Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
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Table 1 Composition of the samples and the liquidus curve obtained by quench technique and EDS microanalysis. Sample no.
T (K)
This work mol% Bi2 O3
Masuda et al.7 mol% Bi2 O3
Bruton2 mol% Bi2 O3
1 2 3 4 5 6
1173 1223 1273 1323 1373 1423
91.40 90.54 90.30 88.60 85.38 79.45
84.50 83.50 81.00 78.80 74.20 71.00
83.90 80.40 77.70 –
4. Results and discussion 4.1. Liquidus The experimental temperature and the analysed composition of the liquid phase for all samples are listed in Table 1. In all the samples two-phase equilibrium was found, supercooled liquid and a crystalline phase. Semiquantitative EDS analysis of these crystals was carried out and the Bi/Ti ratio was calculated of at least five measurements. The results show a value that corresponds to crystals of Bi4 Ti3 O12 . The composition of the supercooled liquid phase in Table 1 are the average values calculated from five different analysis points in the same sample and correspond to the liquidus curve of the TiO2 –Bi2 O3 phase diagram; the obtained standard deviation was about σ = 0.14. Low accelerating voltage (10 kV) together with an EDS pointanalysis was preferred from the area-analysis in order to ensure that the chemical analysis corresponded to the chosen phase. The experimental points are compared with the results from Masuda et al.7 and Bruton.2 The current results show higher solubility of Bi2 O3 in the liquid phase in equilibrium with the Bi4 Ti3 O12 compound than that previously reported. Fig. 1 presents two SEM micrographs of the two-phase region, liquid-Bi4 Ti3 O12 , equilibrated at 1273 K and rapidly cooled down (sample 3). The white matrix in the upper micrograph is supercooled liquid, whereas the plate-like morphology corresponds to the Bi4 Ti3 O12 compound with crystals of about 3–6 m in width and between 15 and 60 m in length. These micrographs also show the dendritic structure which is obtained when a solid phase freezes and grows rapidly into a supercooled liquid. 5. Phase transformation Fig. 2 shows the DSC patterns for the system from 60 to 95% Bi2 O3 . Samples with 60 and 70% Bi2 O3 showed three endothermic peaks. The first peak is about at 945 K which probably corresponds to the transition of the crystalline structure of Bi4 Ti3 O12 . Pookmanee et al.11 reported that the Bi4 Ti3 O12 compound presents a monoclinic structure at low temperatures, but above the Curie temperature, at about 948 K, the symmetry becomes tetragonal. The second is a peak at about 1134 K which can be ascribed to the peritectic reaction: Liquid + Bi4 Ti3 O12 = Bi12 TiO20
(6)
Fig. 1. SEM micrographs showing the coexistence of supercooled liquid and Bi4 Ti3 O12 of a sample with 60 mol% Bi2 O3 .
The third peak is at 1457 K and 1440 K for the samples with 60 and 70 mol% Bi2 O3 , respectively. These peaks correspond to the liquidus curve. The sample with 85.71% Bi2 O3 corresponds with the stoichiometric composition of the Bi12 TiO20 compound and the thermogram of Fig. 2c must present only the peak for the peritectic transformation, at about 1134 K. However, we obtained another small peak about at 940 K which corresponds to the transition of the crystalline structure of Bi4 Ti3 O12 . This confirms that there exits impurities of Bi4 Ti3 O12 in this sample even though it has a very high amount of the Bi12 TiO20 phase. Fig. 2d shows two thermal effects in the thermogram for the sample with 95% Bi2 O3 : a thermal arrest corresponds to the transformation of Bi2 O3 from monoclinic to cubic structure at 1002 K, the second one at about 1092 K corresponds to the eutectic transformation Liquid = Bi12 TiO20 + Bi2 O3
(7)
The DSC results for the sample with 95% Bi2 O3 help us to locate approximately the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 . The results of this sample show that the peritectic transformation takes place below 95% Bi2 O3 . We concluded from this result and the experimentally liquidus curve that the limit of the peritectic reaction between liquid and Bi4 Ti3 O12 is at approximately 90 mol% Bi2 O3 .
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Fig. 2. DSC curves for TiO2 –Bi2 O3 mixtures with compositions from 60 to 95 mol% Bi2 O3 .
Levin and Roth,3 and Morrison4 reported that the peritectic line is from 40 mol % Bi2 O3 (the composition of Bi4 Ti3 O12 ) up to about 86% Bi2 O3 . Miyazawa and Tabata6 reported that the peritectic composition must lie close to 89.25 mol% Bi2 O3 . Fig. 3 shows a SEM micrograph of the sample with 60% Bi2 O3 . This figure shows plate-like morphology, which
according to the semiquantitative EDS analysis corresponds to crystals of Bi4 Ti3 O12 , which is in agreement with Jardiel et al.12 who reported that the bismuth titanate (Bi4 Ti3 O12 ) ceramics reflects the microstructure showing big platelets-like grains growing preferentially in a given plane. Fig. 4 shows the X-Ray diffraction pattern of sample with 85.71 mol% Bi2 O3 . This pattern shows that exits a high amount of the Bi12 TiO20 (JCPDS file 34-0097) but there is also small amount of the Bi4 Ti3 O12 compound (JCPDS file 12-0213),
Fig. 3. SEM micrograph of sample with 60 mol% Bi2 O3 . The platelet-like grains correspond to Bi4 Ti3 O12 .
Fig. 4. XRD pattern of TiO2 –Bi2 O3 system with 85.71 mol% Bi2 O3 and 14.79% TiO2 .
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5.1. Thermodynamic results The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed in a previous work8 using the quasichemical model for the liquid phase. In that work we reported the model parameters (ω and η) which best reproduces the experimental data including the liquidus curve published by Masuda et al.7 and Speranskaya et al.5 The Gibbs free energy functions for the intermediate compounds were estimated from the properties of the TiO2 and Bi2 O3 pure solids, according to the following expression: o o o gxTiO = xgTiO + ygBi + E + FT 2 ·yBi2 O3 2 2 O3
Fig. 5. SEM micrograph of sample with 85.71 mol% Bi2 O3 . The crystals correspond to Bi12 TiO20 .
which is in agreement with the DSC results of the sample with 85.71% Bi2 O3 . The XPowder software13 was used to estimate the amount of Bi4 Ti3 O12 in this sample. This software carries out linear and no-linear least square analysis (crystalline and amorphous) using the full pattern profile and experimental standard diffractograms. The X-ray pattern of Fig. 4 gives about 4.3% Bi4 Ti3 O12 and 95.7% Bi12 TiO20 . Fig. 5 shows a micrograph for the nominally composition of the Bi12 TiO20 compound. The EDS analysis of these crystals shows an average composition of 87.03 mol% Bi2 O3 which is very close to the stoichiometric composition (85.71 mol% Bi2 O3 ).
(8)
where x and y are 4 and 1 for Bi2 Ti4 O11 , 2 and 1 for Bi2 Ti2 O7 , 3 and 2 for Bi4 Ti3 O12 , 1 and 6 for Bi12 TiO20 . E and F are optimizable coefficients chosen to fit the experimental phase diagram. The thermodynamic parameters for the TiO2 and Bi2 O3 as well as the optimized properties of the intermediate compounds have been reported by Lopez et al.8 In the present work we recalculate the model parameters to take into account the compositions and temperatures of the liquidus curve obtained experimentally: 7 ω = −10800 − 12000YBiO1.5 J mol−1 4 η = −3.2 − 8.0YBiO1.5 J mol−1 K−1
(9) (10)
Fig. 6 shows a comparison between the calculated diagram with the experimental liquidus curve and DSC results obtained in this work and the results from Masuda et al.7 and Bruton.2 Predicted results from this work are in good agreement with those published previously,2,5,7 except in the liquidus near pure Bi2 O3 .
Fig. 6. Calculated (line) and experimental (symbols) TiO2 –Bi2 O3 phase diagram.
Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023
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6. Conclusions
References
The liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system were determined experimentally using the quench technique with subsequent EDS microanalysis. In comparison with previous works, the solubility of Bi2 O3 in the liquid phase in equilibrium with Bi4 Ti3 O12 compound is higher; the limit of the two-phase region (Bi4 Ti3 O12 + liquid) is closer to the Bi2 O3 side of the diagram. The DSC results showed that the liquidus was at 1447 K and 1427 K for the samples with 60 and 70 mol% Bi2 O3 , respectively. DSC was also used to estimate the transformation temperatures for the systems with 60–95%Bi2 O3 . The TiO2 –Bi2 O3 phase diagram was thermodynamically assessed in the present work using the quasichemical model for the liquid phase. The model parameters (ω and η) were recalculated from a previous work8 to take into account the experimental liquidus curve obtained in this work.
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Acknowledgments The authors wish to thank the Institutions CONACyT, SNI, COFAA and IPN for their permanent assistance to the Process Metallurgy Group at ESIQIE-Metallurgy and Materials Department. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2014.04.023.
Please cite this article in press as: Lopez-Martinez J, et al. Experimental determination of the liquidus line in the high Bi2 O3 region in the TiO2 –Bi2 O3 system. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.023