Physicochemical properties OF

inorganic powder/ molten carbonates coexisting systems II

 

*Shigehito Deki, Minoru Mizuhata,

Yasuyuki Harada, Hajime Watanabe,

and Akihiko Kajinami

 

Department of Chemical Science and Engineering, Faculty of Engineering,

Kobe University

 

1-1 Rokkodai-cho, Nada, Kobe, 657-8501 Japan

 

Molten Carbonates have been applied in various electrochemical applications such as MCFC, CO₂ sensors, etc.¹⁾  Generally, the carbonates are utilized with the porous solid materials in order to support and stabilize.  In such systems, chemical and physical properties of carbonates are influenced by the interfacial interaction with the solid phase.  In this study, the electrical conductivity and melting behavior of molten carbonate, (Li,K)₂CO₃ and (Li, Na)₂CO₃ coexisting with g-LiAlO₂ powder was studied. 

High-pulity g-LiAlO₂ powder (9.3, and 19.4 m²/g of the specific surface area) was used as the solid phase.  Guaranteed reagents of Li₂CO_3, (Li, K)₂CO_3, and (Li, Na)₂CO₃, annealed at 1273K for 48 hours under N₂ gas flow are used as the liquid phase.  The composition of prepared mixed carbonates are (Li_xK_1-x)CO₃; x=1.00, 0.62(eutectic), 0.50(compound), 0.30, and 0.00.  After mixing and molding of the powder and carbonate, the electrical conductivity was measured by the ac impedance method.  The activation energy of the electrical conductivity, DE_a, was calculated from the temperature dependence of the conductivity by the basis of the Arrhenius equation as shown in Fig.1.  The melting behavior of the carbonates was measured with DTA.  The molar enthalpy of fusion, DH_m, is calculated from the quasi-equilibrium DTA data.

Observed transition temperatures were lower than that of the melting point of Li₂CO₃, and it lowered as the liquid content decreased.  The electrical conductivity did not diminish even below the temperature of the melting point.  Since the conductivity was much larger than that of pure g-LiAlO₂ powder, it can be presumed that a part of the carbonate does not frozen and the conduction path is kept through the carbonate.  DE_a depends on the parameter; “the apparent average thickness of the liquid phase”, obtained by the calculation; [total volume of the liquid phase]/[total surface area of the solid phase].  The value of DE_a increases as the apparent average thickness decreases; below 40 nm of the thickness.  It is suggested that the electrical conduction of the carbonate was intensively influenced by the solid phase.  The decrease of the liquid phase and the increase of the specific surface area of the solid phase relatively strengthen the influence of the solid phase.  It is expected that the application of the molten salts as the ionic conductor by coexisting the porous solid materials as the quasi-, or semi-solid materials at the temperature range below the melting point.

References

 1)F. Salam et al., Electrochem. Solid State Lett., 2, 201(1999).