Thermodynamic properties of oxygen in yttrium-oxygen solid solutions
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I.
INTRODUCTION
IN recent years 99.9999 pct yttrium* has been produced *Excluding carbon and gaseous elements.
commercially for use in electronics.m The major impurity in this material is oxygen, present in the amount of 0.1 to 0.5 pct (5000 ppm by mass). Oxygen removal from yttrium metal is extremely difficult owing to a combination of two factors. Yttrium has a very strong affinity for oxygen (Figure lt2]) and dissolves oxygen in large concentrations (Figure 2[3.4]). As part of a larger study of electrochemical deoxidation of yttrium,IS1 a technique was developed for measuring extremely low oxygen potentials, circa 10-44 atm at 1273 K, which is far beyond the range of solid oxide electrolyte sensors. II.
PRINCIPLES OF MEASUREMENT OF ULTRALOW OXYGEN POTENTIALS
Measurement of ultralow oxygen potential in solid solutions of yttrium-oxygen (Y-O) was accomplished by an indirect technique involving equilibration of Y-O specimens with a reference material, namely, a solid solution of titanium and oxygen. Titanium-oxygen (Ti-O) solid solutions were chosen for the reason that the relationship between oxygen concentration ([O]) and oxygen potential (Po_,) is well established in this system and can be extrapolated to extremely low levels of oxygen potential in accordance with Henry's law. The principle of the technique is illustrated in Figure 3, which gives the relationship between Po~ and [O] in Ti-O and Y-O at constant temperature. The two lines were determined as follows. The coordinates of point A are P~ and
T.H. OKABE, formerly Postdoctoral Associate, Department of Materials Science and Engineering. Massachusetts Institute of Technology, is Research Associate, Institute of Advanced Materials Processing, Tohoku University, Tohoku 980, Japan. T.N. DEURA, Postdoctoral Candidate, and K. ONO, Professor, are with the Department of Energy Engineering and Science, Kyoto University, Kyoto 606-01, Japan. T. OISHI, Professor, is with Department of Materials Science and Engineering, Kansai University, Osaka 564, Japan. D.R. SADOWAY, Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307. Manuscript submitted September 19, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS B
[O]i. P~ is the value of Po: in equilibrium with pure Ca and CaO (5 • 10-42 atmat T = 1273 K) and is well known from having been measured in several investigations.t21 [OL is the concentration of oxygen in titanium (500 ppm*) at *All expressions of concentration in ppm are by mass.
this Po~, a value confirmed by multiple researchers.tS-gJ On the assumption that Ti-O solutions are Henrian up to [OL, the relationship between Po~ and [O] in Ti-O solutions is represented by a straight line connecting point A and the origin. It is possible to measure [O] in titanium metal down to concentrations as low as 10 ppm. At these values of [O] in titanium, the corresponding Po2 is unmeasurable by direct methods but nevertheless can be accurately determined from the Ti-O line in Figure 3. For
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