James A. Valdez

Enhanced radiation tolerance in nanocrystalline MgGa2O4

The authors demonstrate a substantial enhancement in radiation-induced amorphization resistance for single-phased nanocrystalline (NC) versus large-grained polycrystalline MgGa2O4. NC and large-grained MgGa2O4 were irradiated at ∼100K with 300keV Kr++ ions to fluences ranging between 5×1019 and 4×1020Kr∕m2. Large-grained MgGa2O4 samples began to amorphize by a fluence of 5×1019Kr∕m2, while NC MgGa2O4 remained crystalline with no evidence for structural changes (other than moderate grain growth in the lowermost implanted region), to a fluence of 4×1020Kr∕m2. To our knowledge, this is the first experimental study to reveal enhanced amorphization resistance in an irradiated, single-phase, NC material.

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21∕c structure, rather than the B2∕b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼25–100nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21∕c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.

Glass matrix/pyrochlore phase composites for nuclear wastes encapsulation

Novel composite materials have been developed as alternative forms to immobilise nuclear solid waste. These composites are made of a lead-containing glass matrix, into which particles of lanthanum zirconate pyrochlore are embedded in 10 and 30 vol% concentrations. The fabrication involves powder mixing, pressing and pressureless sintering. The processing conditions were investigated with the aim of achieving the highest possible density. The best composites obtained showed a good distribution of the lanthanum zirconate particles in the glass matrix, strong bonding of the particles to the matrix and relatively low porosity (<10%). The best sintering temperature was 600°C for the 10 vol% composite and 650°C for 30 vol%. Sintering was carried out for an hour and a heating rate of 10°C · min−1 was shown to be superior to a heating rate of 2°C · min−1. At the relatively low sintering temperatures used, the pyrochlore crystalline structure of lanthanum zirconate, relevant for containment of radioactive nuclei, was stable.

Ion-irradiation-induced phase transformation in rare earth sesquioxides (Dy2O3,Er2O3,Lu2O3)

Polycrystalline pellets of cubic C-type rare earth structure (Ia3¯) Dy2O3, Er2O3, and Lu2O3 were irradiated at cryogenic temperature (120K) with 300keV Kr++ ions to a maximum fluence of 1×1020Kr∕m2. Irradiated specimens were examined using grazing incidence x-ray diffraction and transmission electron microscopy. Ion irradiation leads to different radiation effects in these three materials. First, Dy2O3 begins to transform to a monoclinic B-type rare earth structure (C2∕m) at a peak dose of ∼5 displacements per atom (dpa), (corresponding to a fluence of 2×1019Kr∕m2). This transformation is nearly complete at a peak dose of 25 dpa (a fluence of 1×1020Kr∕m2). Er2O3 also transforms to the B-type structure, but the transformation starts at a higher irradiation dose of about 15–20 dpa [a fluence of about (6–8)×1019Kr∕m2]. Lu2O3 was found to maintain the C-type structure even at the highest irradiation dose of 25 dpa (a fluence of 1×1020Kr∕m2). No C-to-B transformation was observed in Lu2O3. The irradiation dose...

Mechanical properties of bone-shaped-short-fiber reinforced composites

Abstract Short-fiber composites usually have low strength and toughness relative to continuous fiber composites, an intrinsic problem caused by discontinuities at fiber ends and interfacial debonding. In this work a model polyethylene bone-shaped-short (BSS) fiber-reinforced polyester–matrix composite was fabricated to prove that fiber morphology, instead of interfacial strength, solves this problem. Experimental tensile and fracture toughness test results show that BSS fibers can bridge matrix cracks more effectively, and consume many times more energy when pulled out, than conventional straight short (CSS) fibers. This leads to both higher strength and fracture toughness for the BSS-fiber composites. A computational model was developed to simulate crack propagation in both BSS- and CSS-fiber composites, accounting for stress concentrations, interface debonding, and fiber pull-out. Model predictions were validated by experimental results and will be useful in optimizing BSS-fiber morphology and other material system parameters.

The strength and toughness of cement reinforced with bone-shaped steel wires

In this study, we have experimentally evaluated the effectiveness of bone-shaped short (BSS) steel wire reinforcement in improving the mechanical properties of cement. Results from four-point bending tests revealed that BSS-steel-wire-reinforced cement is substantially stronger, tougher, and more crack resistant than conventional straight short (CSS) steel-wire-reinforced cement and unreinforced cement. The BSS steel wires provided effective crack bridging by a combination of their ductility and the mechanical interlocking between their enlarged spherical ends and the cement matrix. In the BSS-wire-reinforced cement specimens, multiple cracks formed along the length of the specimen before final failure, whereas in the CSS-wire-reinforced cement and unreinforced cement specimens, a single central crack initiated and propagated across the specimen. Some BSS wires bridged the main crack and were eventually plastically deformed to failure. Secondary matrix cracks also radiated away from the main fracture surfaces, providing significant contributions to the toughness of the composites. In contrast, the bridging CSS wires were easily pulled out of the matrix without much deformation in the wire itself and surrounding cement. Therefore, they were not nearly as effective in bridging cracks and improving toughness as the BSS wires.

Radiation Tolerance of Nanocrystalline Ceramics: Insights from Yttria Stabilized Zirconia

Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr+, 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces.

Structure and optical properties of Lu2SiO5:Ce phosphor thin films

Luminescent, cerium doped Lu 2SiO 5 thin films with C2/c symmetry have been prepared by pulsed laser deposition (PLD) at temperatures much lower than the crystallization temperature (2150°C) of the corresponding bulk crystals. The PLD grown films show the typical luminescence resulting from the Ce 3+ 5d-4f transition. Maximum luminescence efficiency was observed for films prepared at an oxygen partial pressure of 200 mTorr at 600°C. These conditions reflect a balance between Ce 4+/Ce 3+ interconversion and the crystalline quality of the films. The results indicate that PLD offers a low temperature deposition technique for complex oxide phosphor materials.

Ion-beam-induced phase transformations in δ‐Sc4Zr3O12

Structural changes in ion-beam-irradiated rhombohedral Sc4Zr3O12 (δ‐Sc4Zr3O12) have been examined using transmission electron microscopy (TEM). Polycrystalline δ‐Sc4Zr3O12 samples were irradiated at cryogenic temperature with 300keV Kr2+ ions to a fluence of 3×1016 Kr∕cm2 (equivalent to a peak dose of ∼70 displacements per target atom). High-resolution TEM and nanobeam electron diffraction experiments revealed a phase transformation to another ordered crystalline phase in the near-surface region of the irradiated sample. We propose an atomistic model for this crystalline phase, based on the bixbyite structure, and discuss its formation process. The phase transformation (occurring during irradiation) that produces this bixbyite structure is unusual in the fact that a more highly ordered structure is the product of the transformation.

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. These results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives.