x (x = 0.23 to 0.29) solid electrolyte samples were prepared by direct synthesis method, and their impedance spectra were measured in air at different temperatures. Results show that the conductivity is on the order of 10 -5 to 10 -2 SAEcm -1 at 673 to 1023 K, and the activation energy ranges within 1.15 to 1.40 eV. The yttrium sensors were assembled with Y 0.75 Ca 0.25 F 2.75 solid electrolyte and used to determine the activity of yttrium dissolved in liquid Al-Y alloys at 1033 K, while the accuracy of the yttrium sensors was identified by simultaneously measuring the oxygen content with a counterpart oxygen sensor. The variations of measured EMF with yttrium concentration are well coupled to each other between the yttrium cell and the oxygen cell and comply with the deoxidation law of active metals. In liquid aluminum, the activity coefficients of solute yttrium in infinite dilution state and the standard free energy change of yttrium dissolved at 1033 K were assessed as follows:

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