TOTAL HEMISPHERICAL EMISSIVITY OF PRE-OXIDIZED AND UN-OXIDIZED ZR-2.5Nb PRESSURE-TUBE MATERIALS AT 600 °C TO 1000 °C UNDER VACUUM

Abstract

The emissivity of pre-oxidized and un-oxidized pressure-tube specimens has been measured at high temperatures under vacuum. The emissivity values of un-oxidized tube specimens decreased only slightly from 0.34 at 600 °C to 0.30 at 800 °C and changed gradually to 0.25 at 1000 °C. In comparison, the emissivity of pre-oxidized pressure-tube specimens decreased drastically from 0.70 at 600 °C to 0.35 at 800 °C, and gradually decreased to 0.25 at 1000 °C. The oxide layer of the pre-oxidized tube specimens dissolved into the metal matrix when heated to 700 °C and higher. Using these results, 2 linear correlations were obtained for emissivity with the oxide thickness measured by scanning electron microscopy and secondary ion mass spectroscopy analysis.

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“…These emissivity values agree with [12] but are not agreement with generally accepted emissivity values of metals prior to oxidation. According to literature reports, emissivity of smooth, unoxidized Zr4 cladding should be closer to ~0.2-0.3 [13], [14]. However, oxides as thin as 2 µm can significantly change emissivity [15], and it's accepted that as zirconium based alloys oxidize, emissivity rises to ~0.9 [16].…”