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Nanophosphor [Y.sub.2]Si[O.sub.5]:Ce (n-YSO), [Lu.sub.2]Si[O.sub.5]:Ce (n-LSO), and [Gd.sub.2]Si[O.sub.5]:Ce (n-GSO) were prepared by solution-combustion synthesis yielding nanophosphor crystallite sizes between 20 nm-80 nm. Ce dopant concentrations were varied between 0.1%-10% for each the nanophosphors and concentration quenching curves were measured by radioluminescence (RL) and photoluminescence (PL). n-YSO exhibits concentration quenching at 1 at% and 4 at% under uv and x-ray excitation, respectively. Red shifted emission with a larger Stokes shift is observed for nanophosphors as compared to bulk crystals. The measured PL lifetime depended on the refractive index of the media, indicating that the PL originates from the surface. Measurements of the RL/PL intensity indicate that the light output of these materials is comparable to the bulk crystal. Index Terms--Concentration quenching, fluorescence lifetime, luminescence, nanophosphor, oxyorthosilicates.

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy ([F.sup.+] center). We have used electron paramagnetic resonance spectroscopy to identify this defect in [Ln.sub.2]Si[O.sub.5] : Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu- and Y-based nanophosphors form seven- and nine-oxygen coordinated structures ([P2.sub.1]/c) whereas the bulk phosphors form six. and seven-oxygen coordinated structures (C2/c). In each case the [F.sup.+] center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperfine interaction with g-values near the free electron value and hyperfine coupling ~0.4 mT. The [F.sup.+] center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior. Index Terms--Electron paramagnetic resonance, [F.sup.+] center, luminescence, oxyorthosilicates.

The coating of nuclear fuel kernels with pyrolytic carbon (PyC) is a well-understood practice dating back over half a century. In spite of decades of studies related to these coatings, no study has yet investigated the effect of the PyC deposition coating process on the kernels themselves. In this study, the composition and crystallographic phase of kernel materials were observed to change after exposure to the thermal and chemical environment of the PyC coating process. Specifically, the coating process increased the fraction of high carbon content phase within carbide microsphere kernels, with W2C containing microspheres driven toward WC, and UC containing microspheres driven toward UC2. Oxide microspheres consisted of a mixture of two crystalline phases. The monoclinic phase within yttria-stabilized zirconia microspheres was eliminated by the coating process resulting in a purely tetragonal phase. Hafnium oxide microspheres were more stable showing no detectable change in composition or crystal structure after coating. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) spectroscopy has been used to study energy transport properties of both bulk and nanophosphor (nominally 30 nm) oxyorthosilicate samples, and we were able to separate the effects of crystal disorder and relaxation lifetime broadening on the EPR linewidths. The low temperature linewidths (T<10 K) were inhomogeneously broadened and dominated by crystal disorder effects and the nanophosphors showed an order of magnitude increase in crystal disorder. Both bulk and nanophosphor samples showed significant lifetime broadening involving direct relaxation via phonons and the Orbach relaxation process. At low temperatures, the lifetimes of the bulk samples displayed the influence of the lattice-bath relaxation time as well as the spin-lattice relaxation time while the lifetimes of the nanophosphor samples were not influenced by the lattice-bath relaxation time. The results imply that reduced dimensionality in insulators does reduce the lattice-bath relaxation time, although the exact relationship cannot be confirmed by this study largely due to the nonideal nature of the materials studied. [ABSTRACT FROM AUTHOR]

We used combustion synthesis to produce phase pure GdOBr:Ce with varying Ce concentration. Several combinations of NH4Br and fuel concentration were tested before phase pure GdOBr:Ce was produced by using a 50% excess of NH4Br and a 50% lean concentration of fuel. In addition to the expected emissions from Ce and Gd, we also detected emissions from Eu3+ and Tb3+. Further research indicated that these extra rare earth emissions were due to impurities in the Gd2O3 starting material. While not the desired effect, we were able to monitor both Eu and Ce intensities as a function of Ce concentration in GdOBr. The Ce emission is seen to reach a maximum between 0.1 and 0.5 mol%. At higher Ce content, the Ce luminescence is quenched because of the concentration quenching effect and the higher concentration of O-H and N-H groups as revealed by IR spectra. Eu emission progressively decreases as Ce content increases from a combination of competition with Ce for charge capture in the RL process and the effect of OH and NH groups. As Ce concentration increases the RL intensity ratio of the bands of the Ce doublet changes even though the energy difference between the two bands (0.24 ± 0.1 eV) is compatible with the spin-orbit splitting of the ground level of Ce3+ (0.25 eV). Fitting results indicate that this relative change is not due to changes in self-absorption, and the effect may be due to changes in local symmetry of the Ce ion. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy (F+ center). We have used electron paramagnetic resonance spectroscopy to identify this defect in Ln2SiO5 : Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu- and Y-based nanophosphors form seven- and nine-oxygen coordinated structures (P21/c) whereas the bulk phosphors form six- and seven-oxygen coordinated structures (C2/c). In each case the F+ center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperfine interaction with g-values near the free electron value and hyperfine coupling ~0.4 mT. The F+ center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior. [ABSTRACT FROM AUTHOR]