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.

Novel primary amine diazeniumdiolates-Chemical and biological characterization.

Hit, Lead & Candidate Discovery Diazeniumdiolates, also known as NONOates, are extensively used in biochemical, physiological, and pharmacological studies due to their ability to release nitric oxide (NO. ) and/or their congeneric nitroxyl (HNO). The purpose of this work was to synthesize a series of primary amine-based diazeniumdiolates as HNO/NO donors and to determine their efficacy as anticancer and antifungal agents in vivo. The seven compounds (3a-3g) were successfully synthesized and characterized, one of which had been previously reported in the literature (3g). Two compounds showed anti-proliferative effects against ovarian (ES2 and SKOV3) and AML monocyte-derived cancer cells (THP-1) when tested with standard MTT assays. Compounds 3a and 3g demonstrated reduced ovarian cancer cell proliferation when treated at doses from 0.033 to 1.0 mg/mL at the 24 hr time point. These compounds also exhibited moderate and selective antifungal activity against Fusarium oxysporum f.sp. lycopersici, one cause of opportunistic infections of immunocompromised patients, inhibiting the growth of the fungi at LD50 at 10 mg/mL. A third compound (3e) did not exhibit similar activities, possibly due to the alkyl chain. Our results suggest that the primary amine diazeniumdiolates may offer a versatile platform for the development of HNO/NO donors for biomedical applications.

A Rutile Chevron Modulation in Delafossite-Like Ga3-xIn3TixO9+x/2.

The structure solution of the modulated, delafossite-related, orthorhombic Ga3-xIn3TixO9+x/2 for x = 1.5 is reported here in conjunction with a model describing the modulation as a function of x for the entire system. Previously reported structures in the related A3-xIn3TixO9+x/2 (A = Al, Cr, or Fe) systems use X-ray diffraction to determine that the anion lattice is the source of modulation. Neutron diffraction, with its enhanced sensitivity to light atoms, offers a route to solving the modulation and is used here, in combination with precession electron diffraction tomography (PEDT), to solve the structure of Ga1.5In3Ti1.5O9.75. We construct a model that describes the anion modulation through the formation of rutile chevrons as a function of x. This model accommodates the orthorhombic phase (1.5 ≤ x ≤ 2.1) in the Ga3-xIn3TixO9+x/2 system, which transitions to a biphasic mixture (2.2 ≤ x ≤ 2.3) with a monoclinic, delafossite-related phase (2.4 ≤ x ≤ 2.5). The optical band gaps of this system are determined, and are stable at ∼3.4 eV before a ∼0.4 eV decrease between x = 1.9 and 2.0. After this decrease, stability resumes at ∼3.0 eV. Resistance to oxidation and reduction is also presented.

Chemical Unit Cosubstitution and Tuning of Photoluminescence in the Ca2(Al(1-x)Mg(x))(Al(1-x)Si(1+x))O7:Eu(2+) Phosphor.

The union of structural and spectroscopic modeling can accelerate the discovery and improvement of phosphor materials if guided by an appropriate principle. Herein, we describe the concept of "chemical unit cosubstitution" as one such potential design scheme. We corroborate this strategy experimentally and computationally by applying it to the Ca2(Al(1-x)Mg(x))(Al(1-x)Si(1+x))O7:Eu(2+) solid solution phosphor. The cosubstitution is shown to be restricted to tetrahedral sites, which enables the tuning of luminescent properties. The emission peaks shift from 513 to 538 nm with a decreasing Stokes shift, which has been simulated by a crystal-field model. The correlation between the 5d crystal-field splitting of Eu(2+) ions and the local geometry structure of the substituted sites is also revealed. Moreover, an energy decrease of the electron-phonon coupling effect is explained on the basis of the configurational coordinate model.

Selective Crystal Growth and Structural, Optical, and Electronic Studies of Mn3Ta2O8.

Mn3Ta2O8, a stable targeted material with an unusual and complex cation topology in the complicated Mn-Ta-O phase space, has been grown as a ≈3-cm-long single crystal via the optical floating-zone technique. Single-crystal absorbance studies determine the band gap as 1.89 eV, which agrees with the value obtained from density functional theory electronic-band-structure calculations. The valence band consists of the hybridized Mn d-O p states, whereas the bottom of the conduction band is formed by the Ta d states. Furthermore, out of the three crystallographically distinct Mn atoms that are four-, seven-, or eight-coordinate, only the former two contribute their states near the top of the valence band and hence govern the electronic transitions across the band gap.

Tris(3,5-di-tert-butylcatecholato)molybdenum(VI): Lewis acidity and nonclassical oxygen atom transfer reactions.

In the solid state, tris(3,5-di-tert-butylcatecholato)molybdenum(VI) forms a dimer with seven-coordinate molybdenum and bridging catecholates. NMR spectroscopy indicates that the dimeric structure is retained in solution. The molybdenum center has a high affinity for Lewis bases such as pyridine or pyridine-N-oxide, forming seven-coordinate monomers with a capped octahedral geometry, as illustrated by the solid-state structure of (3,5-(t)Bu2Cat)3Mo(py). Structural data indicate that the complexes are best considered as Mo(VI) with substantial π donation from the nonbridging catecholates to molybdenum. Both the dimeric and the monomeric tris(catecholates) react rapidly with water to form free catechol and oxomolybdenum bis(catecholate) complexes. Monooxomolybdenum complexes are also obtained, more slowly, on reaction with dioxygen, with organic products consisting mostly of 3,5-di-tert-butyl-1,2-benzoquinone with minor amounts of the extradiol oxidation product 4,6-di-tert-butyl-1-oxacyclohepta-4,6-diene-2,3-dione. The pyridine-N-oxide complex reacts on heating (with excess pyO) to form initially (3,5-(t)Bu2Cat)2MoO(Opy) and ultimately MoO3(Opy), with quinone and free pyridine as the only organic products. The decay of (3,5-(t)Bu2Cat)3Mo(Opy) shows an accelerated, autocatalytic profile because the oxidation of its product, (3,5-(t)Bu2Cat)2MoO(Opy), produces an oxo-rich, catecholate-poor intermediate which rapidly conproportionates with (3,5-(t)Bu2Cat)3Mo(Opy), providing an additional pathway for its conversion to the mono-oxo product. The tris(catecholate) fragment Mo(3,5-(t)Bu2Cat)3 deoxygenates Opy in this nonclassical oxygen atom transfer reaction slightly less rapidly than does its oxidized product, MoO(3,5-(t)Bu2Cat)2.

Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate).

Mechanistic studies indicate that the oxomolybdenum(vi) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine-N-oxides in a reaction where the oxygen is delivered to molybdenum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2-benzoquinone.