Thermodynamic properties of compounds of Na/sub 2/O with the oxides of chromium, nickel, and iron

Abstract

Results of emf measurements on Na/sub 2/O solid electrolytes in binary compounds with Cr/sub 2/O/sub 3/, FeO, and NiO are presented along with thermodynamic properties of these compounds. It was found that reliable thermodynamic data for compounds of NaCrO/sub 2/, Na/sub 2/FeO/sub 2/, NaFeO/sub 2/, Na/sub 2/NiO/sub 2/, and NaNiO/sub 2/ at 500 to 800/sup 0/ can be obtained by using emf measurements with solid electrolyte cells. The pretreatment of heating the cells in a vacuum of 10/sup -2/ torr at 500/sup 0/C or above for about 12 hours causes the emf dependence on temperature to be very small. The measurements were carried out over the temperature range in which no phase transformations occurred, the ..delta..C/sub p/ for the compounds involved was reasonably considered as approximately zero. Linear emf-temperature plots were therefore expected for these cells and the equation of ..delta..G/sup 0//sub f/ was indeed valid for constant values of ..delta..H/sup 0//sub f/ and ..delta..S/sup 0//sub f/. The formation of compound NaCrO/sub 2/ is thermodynamically favorable in a sodium loop made of austenic stainless steels. The critical oxygen concentration for the formation of NaCrO/sub 2/ shows that it is stable in liquid sodium in temperature range from 400 to 1100/supmore » 0/C and Cr/sub 2/O/sub 3/ does not exist with the double oxide in liquid sodium. The existence temperature for (Na/sub 2/O)/sub 2/.FeO in equilibrium with oxygen saturated liquid sodium is 693/sup 0/K or above, for Na/sub 2/FeO/sub 2/ it is 1141/sup 0/K or above and for NaFeO/sub 2/ it is greater than or equal to 1173/sup 0/K. The double oxides of nickel with sodium oxide are much less stable than the iron double oxides and do therefore not exist in liquid sodium. The nickel in austenitic stainless steel shows the least attack by oxygen saturated liquid sodium.« less