Separate Determination of Microamounts of Bulk and Surface Oxygen in Ultra-Pure Iron by a Surface-Area-Variation Method.

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A microamount of bulk oxygen in ultra-pure iron was determined separately from surface-contaminated oxygen by a surface-area-variation method. In this method, the total oxygen values (μg/g) for specimens with different specific surface areas were determined by means of a vacuum fusion analysis, and were plotted against the specific surface areas (cm2/g); the quantity of oxygen in the bulk (μg/g) was determined from the intercept of the regression line and the oxygen quantity per unit area of surface (μg/cm2) from the slope. A surface treatment method capable of controlling the depth of the surface oxygen layer at an atomic level was developed in order to prevent scattered bulk oxygen values caused by an irregular surface oxygen layer. Specimens were etched in a mixed solution of hydrogen peroxide, hydrofluoric acid and de-ionized water (volume ratio 17:1:2) under an argon atmosphere. A high sensitivity vacuum fusion gas analyzer with a Pirani gauge was used in this experiment. Bulk oxygen over 0.03μg/g in ultra-pure iron could be analyzed independently from the influence of surface oxygen. Bulk oxygen in an electrolytic pure iron sample was determined as 0.08±0.004μg/g with surface oxygen of 0.31±0.001μg/cm2.
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