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Thermodynamic Optimization of the Na2O-B2O3 Pseudo-Binary System Chong Wang, Hao Yu, Huashan Liu, and Zhanpeng Jin

(Submitted 16 October 2000; in revised form 24 September 2002) The Na2O-B2O3 system is thermodynamically optimized by means of the CALPHAD method. A two-sublattice ionic solution model, (Na+1)P(O−2,BO3−3,B4O7−2,B3O4.5)Q, has been used to describe the liquid phase. All the solid phases were treated as stoichiometric compounds. A set of thermodynamic parameters, which can reproduce most experimental data of both phase diagram and thermodynamic properties, was obtained. Comparisons between the calculated results and experimental data are presented.

1. Introduction

2. Review of Experimental Data

Interest in borate systems originates from the discovery of functional crystals such as beta barium borate [BBO (␤BaB2O4)] and lithium triborate (LBO).[1] Precise phase diagrams are of vital importance to design a means for maintaining a stable growing interface during the growth process and avoiding the occurrence of different kinds of defects including impurities. However, the study of borate phase diagrams is a challenging problem due not only to experimental difficulties, but also to complex physical and chemical properties of borate systems. Both phase equilibrium data and thermochemical information involving these systems are limited and widely dispersed. Thermodynamic assessments of these systems are thus indispensable for a systematic analysis of borate phase diagrams. Fortunately, the CALPHAD approach offers a powerful method for collecting, analyzing, and assessing experimental information, and most importantly, for providing a reasonable means to predict undetermined or metastable phase relations.[2] A series of thermodynamic assessments of alkali and alkaline earth borate systems have been performed in our group, and a primary thermodynamic database has been successfully constructed.[3-6] Na2O is widely used in many novel multicomponent borate glasses, wherein any metastable immiscibility gap must be well defined to avoid phase separation when the borate melt is quenched from a high temperature. Also during BBO crystal growth, Na2O is often adopted as a “flux” to depress the viscosity of the melts and enlarge the primary crystal field of BBO. Systems involving Na2O are obviously important, with the Na2O-B2O3 pseudo-binary system being the most important. This article provides a set of self-consistent thermodynamic parameters for the Na2O-B2O3 binary system by thermodynamic modeling.

2.1 Phase Diagram Data

Chong Wang, Hao Yu, Huashan Liu, and Zhanpeng Jin, Department of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, P.R. China (current address of Hao Yu: Helsinki University of Technology, Laboratory of Electronics Production Technology, P.O. Box 3000, 02015 TKK, Finland). Contact e-mail: [email protected].

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In 1936, Morey and Merwin reported five compounds in the B2O3-rich part of the phase diagram of the Na2O-B2O3 system[7]: 2Na2O · B2O3, Na2O · B2O3, Na