Original ArticleA coupled phase diagram experimental study and thermodynamic optimization of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems, and prediction of the phase diagrams of the Li2O-CaO-Al2O3-SiO2 system
Introduction
The Li2O-CaO-Al2O3-SiO2 system forms a basic constituent of the glasses, ceramic refractories and steelmaking fluxes. The thermodynamic information of this system can be helpful for the design of novel thermal shock resistant ceramics and luminescent materials [[1], [2], [3]], solid electrolyte [4] and glass and ceramic applications such as prostheses materials [[5], [6], [7], [8], [9]]. CaO-Al2O3-based mold fluxes containing Li2O have been developed for the casting of high Al containing steels [[10], [11], [12], [13], [14]]. Despite the importance of the Li2O-CaO-Al2O3-SiO2 system, no comprehensive investigation of phase diagrams of the sub-ternary systems like Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 system and the entire quaternary system are available. In the Li2O-CaO-Al2O3 system, the phase stability along the LiAlO2-CaO isopleth by Semenov and Zabolotskii [15] is only phase diagram study available in literature. Any subsolidus phase equilibria for ternary system to confirm the existence of ternary phase were studied. For example, as a ternary NaCa4Al3O9 was reported in the Na2O-CaO-Al2O3 system [16,17], there is possibility that similar ternary compound can exist in the Li2O-CaO-Al2O3 system. In the Li2O-CaO-SiO2 system, the phase diagram information is available only in the SiO2-rich region. However, the phase diagram in the alkali-rich region, specifically near Li2CaSiO4, has not been explored.
To understand the complex phase relationships and chemical reactions of the Li2O-containing oxide systems, a comprehensive and self-consistent thermodynamic database for the Li2O-Na2O-K2O-CaO-MgO-Al2O3-SiO2 system is being developed. The database is constructed based on the thermodynamic optimization using the CALculation of PHAse Diagrams (CALPHAD) method. In the FactSage FTOxid database [18], the thermodynamic description of the Na2O-CaO-MgO-Al2O3-SiO2 system was available. This has expanded to Li2O and K2O containing multicomponent system. Several binary [[19], [20], [21]] and ternary systems [22,23] containing Li2O and K2O have been optimized by the present authors.
In the thermodynamic “optimization (modeling)” of a system, all available crystal structure data, thermodynamic properties and phase equilibrium data of the phases are critically and simultaneously evaluated to obtain a set of Gibbs energy expressions of all phases as functions of temperature and composition. From the optimized Gibbs energy equations, all the thermodynamic properties and phase diagrams can be back calculated. In this way, all the data are rendered self-consistent and coherent with each other. The resultant thermodynamic database containing optimized Gibbs energy parameters can be used to calculate complex chemical reactions and phase diagrams in the ternary or multicomponent systems for new materials design and process development.
In the present study, thermodynamic optimization of the Li2O-CaO-Al2O3-SiO2 system is performed. A schematic diagram of the system is presented in Fig. 1 along with the known stable compounds (filled black circles) and solid solutions (thick dashed lines) reported in the literature. The constituent binaries Li2O-SiO2 [24], Li2O-CaO [19], Li2O-Al2O3 [21], CaO-Al2O3 [25], CaO-SiO2 [25] and Al2O3-SiO2 [25] systems and ternaries Li2O-Al2O3-SiO2 [22] and CaO-Al2O3-SiO2 [25] systems were optimized previously. In the present study, two ternary systems Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 were optimized. The crystallographic information of ternary compounds available in literature is listed Table 1. All the mineral abbreviations used in the present study are based on Whitney and Evans [26]. The dotted lines in Fig. 1 indicate the isopleths in which phase equilibrium information are available in the literature for the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems. Due to the lack of phase diagram information, several key phase diagram experiments were also performed in the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems.
Section snippets
Experimental phase diagram study
The key experimental compositions are depicted by hatched circles in Fig. 1 along with their respective sample names. For the Li2O-CaO-Al2O3 system, differential scanning calorimetry (DSC) experiments were performed in sealed 90Pt-10Rh (90 wt. % Pt-10 wt.% Rh) crucibles. Based on the DSC experimental results, equilibration/quenching experiments (QM) were then conducted at 1643 and 1743 K (1370 and 1470 °C). Quenched samples were characterized using XRD, backscattered electron (BSE) image
Thermodynamic models
In the quaternary Li2O-CaO-Al2O3-SiO2, two ternary systems Li2O-CaO-SiO2 and Li2O-CaO-Al2O3 were optimized. The optimized model parameters obtained in the present study are presented in Table 4.
Critical evaluation/optimization of the experimental data
All the six binary phase diagrams of the Li2O-CaO-Al2O3-SiO2 system optimized previously are presented in Fig. 5. In the present study, the ternary Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems are optimized based on the present key phase diagram experiments and literature data.
Prediction of the phase diagrams of the quaternary Li2O-CaO-Al2O3-SiO2 system
Combining the current thermodynamic optimization results of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems with the previous optimizations of the Li2O-Al2O3-SiO2 and CaO-Al2O3-SiO2 systems, the quaternary phase relations of the Li2O-CaO-Al2O3-SiO2 can be calculated, considering no quaternary solid phases. In particular, the liquidus projections will be essential to understand the solidification of oxide melts and chemical corrosion of alumina and mullite based refractories. The effects of Li2O on
Conclusion
A critical evaluation and optimization of the available phase diagram data of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 system was conducted to obtain a set of self-consistent Gibbs energy functions of all phases in the systems. In order to assist the optimization, new phase diagram experiments were performed using DSC and equilibration/quenching experiments. The present experimental study confirmed the absence of ternary phase in the Li2O-CaO-Al2O3 system, and the stability range of the Li2CaSiO4
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial support from Tata Steel Europe, POSCO, RIST, Hyundai Steel, Nucor Steel, RioTinto Iron and Titanium, Nippon Steel and Sumitomo Metals Corp., JFE Steel, Voestalpine Stahl, RHI, Schott AG and the Natural Sciences and Engineering Research Council of Canada (grant number: CRD-469115-14) are gratefully acknowledged. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF- 2015R1A5A1037627). B. Konar would like to
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