Uranium (U) exhibits a high temperature body-centered cubic (bcc) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U-Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free bcc uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr interstitials, and Zr substitutional defects for the γ allotrope. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the (110) dumbbell, and the most probable defect location for dilute Zr in γ-U is the substitutional site. This is the first detailed study of self-defects in the bcc allotrope of U and also the first comprehensive study of dilute Zr defects in γ-U.

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Thin film Ho-YAG and EroyAG emitters with a platinum substrate exhibit high spectral emitlance in the emission band ( _.'_0.75,4115/2 "4113/2, for Er -YAG and E_. _ 0.65, 517 -518 for Ho-YAG) at 1500K. In addition, low out-of-band spectral emittance, ¢_. < 0.2, suggest these materials would be excellent candidates for high efficiency selective emitters in thermophotovoltaic (TPV) systems operating at moderate temperatures (1200-1500K).Spectral emittance measurements of the thin films were made (1.2<_.<3.0p.m) and compared to the theoretical emittances calculated using measured values of the spectral extinction coefficient.In this paper we present the results for a new class of rare earth ion selective emitters. These emitters are thin sections ( < I mm) of yttrium aluminum garnet (YAG) single crystal with a rare earth substitutional impurity. Selective emitters in the near IR are of special interest for thermophotovoltaic (TPV) energy conversion 1,2. The most promising solid selective emitters for use in a TPV system are rare earth oxides. Early spectral emittance work 3 on rare earth oxides showed strong emission bands in the infrared (.9 -3 microns). However, the emittance outside the emission band was also significant and the efficiency of these emitters was low. Recent improvements in efficiency 4,5 have been made with emitters fabricated from fine (5-10_m) rare earth oxide fibers similar to the Welsbach mantle used in gas lanterns. However, the rare earth garnet emitters are more rugged than the mantle type emitters.A thin film selective emitter 6 on a low emissivity substrate such as gold, platinum etc., is rugged and easily adapted to a wide variety of thermal sources. The garnet structure and its many subgroups have been successfully used as hosts for rare earth ions, introduced as substitutional impurities, in the development of solid state laser crystals 7. Doping, dependent on the particular ion and crystal structure, may be as high as 100 at. % (complete substitution of yttrium ion with the rare earth ion). These materials have high melting points, 1940 C for YAG (Yttrium Aluminum Gamet), and low emissivity in the near infrared making them excellent candidates for a thin film selective emitter.As previously stated, the spectral emittance of a rare earth emitter is characterized by one or more well defined emission bands. Outside the emission band the emittance(absorptance) is much lower. Therefore, it is expected that emission outside the band for a thin film selective emitter will be dominated by the emitter substrate. For an efficient emitter (power in the emission band/total emitted power) the substrate must have low emittance, _s" Within the emission band the spectral emittance is governed by the index of refraction, nf, and the spectral extinction coefficient, _. (sum of the absorption coefficient, aX, and scattering coefficient, c_X),the emitter temperature, T E, and thickness, d. In reference 6 the emitter emittance and efficiency are derived as a function of the optical depth, K_,= _d...