2.04.4.2.5.1 Uranium monocarbide UC

Freshly cleaved UC is bright gray with a metallic shine. It promptly darkens in contact with oxygen, due to the formation of a thin oxide layer on the surface.

Bober eta/.166 studied the spectral reflectivity p of liquid UC, using a laser sphere reflectometer with a polarized laser beam at different angles and wave­lengths (458, 514, 647, and 752 nm). The refractive index n and absorption constant к were obtained. n resulted to be around 2 at 458 and 514 nm, and 2.5 at 647 nm, slightly decreasing with temperature in both cases, up to1.7 at 4100 K and 458 nm and 2.1 at 647 nm. The value of n at 752 nm is 1.7, independent of temperature. The same trend was observed for к, which takes the value 2.5 at 458 and 514 nm and 3.1 at 647 nm, decreasing with T whereas к = 2.5 at 752 nm is independent of T

The normal spectral emissivity e2 of UC has been investigated at 650 nm in polycrystalline sam­ples. Results are certainly affected by oxidation of the sample surface. The most complete trend is the one proposed by Bober et a/.,167,168 based on reflectivity measurements on 96.5% th. d. UC (with 0.36 wt% O and 0.02 wt% N) between 300 and 2780 K and in liquid UC between 2780 and 4200 K:

e650 = 0.566 — 2.7209 x 10-6T + 2.7697 x 10-9 T2

— 2.7102 x 10-12T3 + 2.8618 x 10-16T4 [41]

for 300 K < T< 2780 K, and

e650 = 0.45 2 — 4.3 2 47 x 10-6(T — 2780)

+ 3.1967 x 10-9(T — 2780)2

— 1.6784 x 10-12(T — 2780)3

— 4.6641 x 10-16(T — 2780)4 [42]

for 2780 K < T< 4200 K. An experimental error of ±5% should be taken into account, leading to uncertainty bands larger than the proposed emissiv — ity variation as a function of temperature. Even considering those uncertainty bands, a marked dis­continuity of about 0.1 in ei upon melting remains clear. Such a gap is probably dependent on the oxygen-impurity content.169 For practical purposes, it is reasonable to assume for solid UC e650 = 0.55 ± 0.02 and for liquid UC e650 = 0.45 ± 0.02. De Bruycker169 provided the following wavelength depen­dence of solid and liquid UC spectral emissivity for 488 nm < 1 < 900 nm:

Solid UC: £i = 0.75746 — 0.467911 + 0.184912 [43]

Liquid UC: sx = 0.79998 — 0.755451 + 0.3903612 [44]

De Coninck et a/.170 measured the total hemispheri­cal emissivity eT of nearly stoichiometric UC, obtaining eT = 0.45 between 1400 and 2100 K.

2.04.4.2.5.2 Uranium sesquicarbide U2C3

Freshly cleaved uranium sesquicarbide is bright gray with a metallic luster. U2C3 optical functions have never been studied so far.

2.04.4.2.5.3 Uranium dicarbide UC2

a-UC2-x is bright gray with a metallic shine and darkens upon oxidation. The normal spectral emis — sivity of a — and p-UC2-x at 650 and 2300nm was

studied by Grossman171 and by De Croninck eta/.152 The results of this latter work, in agreement with Grossman’s, are shown in Figure 19. The total hemi­spherical emissivity was estimated to be around 0.55 between 1300 and 2350 K. These results suggest that the emissivity of UC2_x does not detectably vary in the visible range upon the a! p transformation, whereas it varies very little in the infrared range.