Thermal Energy Transport in Oxide Nuclear Fuel.

To efficiently capture the energy of the nuclear bond, advanced nuclear reactor concepts seek solid fuels that must withstand unprecedented temperature and radiation extremes. In these advanced fuels, thermal energy transport under irradiation is directly related to reactor performance as well as reactor safety. The science of thermal transport in nuclear fuel is a grand challenge as a result of both computational and experimental complexities. Here we provide a comprehensive review of thermal transport research on two actinide oxides: one currently in use in commercial nuclear reactors, uranium dioxide (UO2), and one advanced fuel candidate material, thorium dioxide (ThO2). In both materials, heat is carried by lattice waves or phonons. Crystalline defects caused by fission events effectively scatter phonons and lead to a degradation in fuel performance over time. Bolstered by new computational and experimental tools, researchers are now developing the foundational work necessary to accurately model and ultimately control thermal transport in advanced nuclear fuels. We begin by reviewing research aimed at understanding thermal transport in perfect single crystals. The absence of defects enables studies that focus on the fundamental aspects of phonon transport. Next, we review research that targets defect generation and evolution. Here the focus is on ion irradiation studies used as surrogates for damage caused by fission products. We end this review with a discussion of modeling and experimental efforts directed at predicting and validating mesoscale thermal transport in the presence of irradiation defects. While efforts in these research areas have been robust, challenging work remains in developing holistic tools to capture and predict thermal energy transport across widely varying environmental conditions.

Broadband emission Gd3Sc2Al3O12:Ce3+ transparent ceramics with a high color rendering index for high-power white LEDs/LDs.

The discovery of single structure Ce3+ doped garnet transparent ceramics (TCs) with a broad full width at half maximum (FWHM) is essential to realize a high CRI for high-power white light emitting diodes (LEDs) and laser diodes (LDs). In this work, by utilizing the ion substitution engineering strategy, pure phase Gd3Sc2Al3O12:Ce3+ (GSAG:Ce) TC with a broad FWHM of 132.4 nm and a high CRI value of 80.7 was fabricated through the vacuum sintering technique for the first time. The optimized in-line transmittance of TCs was 58.4% @ 800 nm. Notably, the GSAG:Ce TCs exhibited a remarkable red shift from 546 nm to 582 nm, with a high internal quantum efficiency (IQE) of 46.91%. The degraded thermal stability in Ce:GSAG TCs was observed compared with that of Ce:YAG TC, owing to the narrowed band gap of GSAG. Additionally, remote excitation white LEDs/LDs were constructed by combining GSAG:Ce TCs with blue LED chips or laser sources. A tunable color hue from yellow to shinning white was achieved in white LEDs, whereas the acquired CRI and CCT of the white LDs were 69.5 and 7766 K, respectively. This work provides a new perspective to develop TCs with high CRI for their real applications in high-power white LEDs/LDs.

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3.

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in ß-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10-4 Ω.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy measurements confirm this finding and the interpretation of the experimental results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time, it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices.

Persistent photoconductivity in strontium titanate.

Persistent photoconductivity was observed in strontium titanate (SrTiO(3)) single crystals. When exposed to sub-bandgap light (2.9 eV or higher) at room temperature, the free-electron concentration increases by over 2 orders of magnitude. After the light is turned off, the enhanced conductivity persists for several days, with negligible decay. From positron lifetime measurements, the persistent photoconductivity is attributed to the excitation of an electron from a titanium vacancy defect into the conduction band, with a very low recapture rate.