Investigation on Yttrium Activity in Liquid Aluminum by Yttrium Sensor
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DUCTION
RARE earth elements have played an important role in improving physical and mechanical properties of metals and alloys, due to their effects on purification and grain refinement.[1–7] Among them, yttrium is of particular interest as an excellent purificant and additive for property modification,[8–10] although its unusual physicochemical behavior in liquid systems still remains to be explored. According to the principle of metallurgical physicochemistry, rare earth atoms may exist in soluble state in molten metals[11] and their activity in metallic solution can be determined by solid electrolyte EMF method.[12–15] So far, some lanthanum sensors have been developed to investigate the activity of lanthanum in carbon-saturated iron and liquid aluminum,[16–18] and for yttrium sensors they have been reported only as an ion selective fluorescence sensor to detect photoinduced electron transfer.[19] Over the last decades, CaF2 or YF3-doped CaF2 has been used in galvanic cells as solid electrolyte to study the thermodynamic properties of binary compounds and alloys. Sobolev[20] et al. reported the interaction of LnF3 with MF2 (M = Ca, Sr, Ba) for 34 binary systems of the LnF3-MF2 type in the concentration range of 60 to 100 mole pct LnF3. Pastor et al.[21] investigated the congruent melting compounds of CaF2-RYF3 (R = Gd, Tb, Dy, Ho, Y, Er). Reau et al.[22] measured the complex impedance of Ca1-xYxF2+x (0 £ x £ 0.38) solid solutions and showed that the ionic conductivity of Y. LI and Y.J. YANG, Associate Professors, and C.Z. WANG, Professor, are with School of Materials and Metallurgy, Northeastern University, Shenyang 110004, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted August 21, 2007. Article published online April 4, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
Ca0.75Y0.25F2.25 was about 10-5 to 10-2 SÆcm-1 in the temperature range of 250 C to 400 C. Sobolev et al.[23] investigated the influence of oxygen on YF3 and found that YF3-2xOx oxyfluorides were formed when heating in air. Takashima et al.[24] found that the electrical conductivity of rare earth oxyfluorides of the YF3Ln2O3 system was on the order of 10-5 to 10-2 SÆcm-1 at the temperature range of 200 C to 700 C and could also be used as solid electrolyte. Xiao et al.[25] measured the ionic conductivities of La1-xCaxF3-x (x = 0.01 to 0.09) solid electrolytes at 423 to 773 K, which ranged within the order of 10-5 to 10-2 SÆcm-1. To date, there still remains some difficulty in generating accurate thermodynamic data for molten metal systems dealing with very active elements, such as yttrium. Here, an attempt was made to develop a yttrium sensor with Y0.75Ca0.25F2.75 as solid electrolyte and pure yttrium as reference electrode, and its applicability was demonstrated by successful detection of the activity of yttrium in liquid aluminum.
II.
EXPERIMENTAL
A. Principle The electrochemical galvanic cell of yttrium sensor can be expressed as Mo j YðsÞ j Y0:75 Ca0:25 F2:75 j ½YAl j Mo
½1
The cell reaction and the elec
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