Prediction on thermodynamic properties of ternary molten salts from wilson equation
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ee energy of formation of CaZrO3 obtained by Tanabe and Nagata suggests that this is indeed the case. However, the transport properties of Ca3P2O8 need to be established by independent measurements before cells based on the Ca4P2O9/Ca3P2O8 couple can be validated for the measurement of the activity of CaO over a wide range. As the activity of CaO is lowered further at the measuring electrode, Ca3P2O8 will in turn decompose to other phosphates (Ca2P2O7, Ca7P10O32, CaP2O6, Ca2P6O17, and CaP4O11).[15] The critical activity of CaO for the decomposition of Ca3P2O8 to Ca2P2O7 at 1700 K evaluated from the thermodynamic data[20] is 9.6 3 1025. Unfortunately, accurate values for the free energies of formation of the other phosphates are not available for estimating their thermodynamic stability ranges. Even when these calcium-deficient phosphates form, correct results for activity of CaO can be obtained if the phosphates are ionic conductors. As discussed by Jacob et al.,[21] thermodynamic stability and transport properties of phases in the microsystem near the electrolyte/electrode interface can impose constraints on the design of galvanic cells. The melting points of the phosphates also decrease with depletion of CaO. In summary, further research is needed on transport properties of other calcium phosphates, especially Ca3P2O8, in order to specify the range of activity of CaO that be measured using Ca4P2O9/Ca3P2O8 bielectrolyte combination. The behavior of the cell based on Ca4P2O7 is more complex than that outlined by Tanabe and Nagata.[1]
REFERENCES 1. J. Tanabe and K. Nagata: Metall. Mater. Trans. B, 1996, vol. 27B, pp. 658-62. 2. M. Allibert, C. Chatillon, and K.T. Jacob: J. Am. Ceram. Soc., 1981, vol. 64, pp. 307-14. METALLURGICAL AND MATERIALS TRANSACTIONS B
3. T. Mathews, J.P. Hajra, and K.T. Jacob: Chem. Mater., 1993, vol. 5, pp. 1669-75. 4. K.T. Jacob and Y. Waseda: Thermochim. Acta, 1994, vol. 239, pp. 233-41. 5. R.V. Kumar and D.A.R. Kay: Metall. Trans. B, 1985, vol. 16B, pp. 107-12. 6. G. Rog and A. Kozlowska-Rog: J. Chem. Thermodyn., 1996, vol. 28, pp. 357-62. 7. E.M. Levin, C.R. Robbins, and H.F. McMurdie: Phase Diagrams for Ceramists, American Ceramic Society, Columbus, OH, 1964, Figure 231. 8. J. Tanabe, K. Nagata, and K.S. Goto: Mater. Trans. JIM, 1990, vol. 31, pp. 69-74. 9. L.B. Pankratz: Thermodynamic Properties of Elements and Oxides, United States Bureau of Mines Bulletin 672, Government Printing Office, Washington DC, 1982, p. 487. 10. E.G. King and W.W. Weller: United States Bureau of Mines Rep. Invest. 5571, Government Printing Office, Washington, DC, 1960. 11. A.S. L’vova and N.N. Fedosev: Russ. J. Phys. Chem., 1964, vol. 38, pp. 28-35. 12. V.R. Korneev, V.B. Glushkova, and E.K. Keler: Izv. Akad. Nauk SSSR, Neorg. Mater., 1971, vol. 1, pp. 886-89. 13. E.T. Muromachi and A. Navrotsky: J. Solid State Chem., 1988, vol. 72, pp. 244-56. 14. R.R. Brown and K.O. Bennington: Thermochim. Acta, 1986, vol. 106, pp. 183-88. 15. E.M. Levin, C.R. Robbins, and H.F. McMurdie: Phase Diagrams for Ceramists: 1969 S
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