Damage evolution in Au-implanted Ho2Ti2O7 titanate pyrochlore

Damage evolution at room temperature in Ho₂Ti₂O₇ single crystals is studied under 1 MeV Au²⁺ ion irradiation by Rutherford backscattering spectroscopy along the 〈0 0 1〉 direction. For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) were used to analyze ion channeling spectra. A disorder accumulation model, with contributions from the amorphous fraction and the crystalline disorder, is fit to the Ho damage accumulation data. The damage evolution behavior indicates that the relative disorder on the Ho sublattice follows a nonlinear dependence on dose and that defect-stimulated amorphization is the primary amorphization mechanism. Similar irradiation behavior previously was observed in Sm₂Ti₂O₇. A slower damage accumulation rate for Ho₂Ti₂O₇, as compared with damage evolution in Sm₂Ti₂O₇, is mainly attributed to a lower effective cross section for defect-stimulated amorphization.     

Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation

Pyrochlore pellets with the Gd₂(Ti_2−xZr_x)O₇ stoichiometry (x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline-amorphous transition for Gd₂Ti₂O₇ and Gd₂TiZrO₇, (ii) a phase transition towards an anion-deficient fluorite structure (order-disorder transition) for Gd₂Zr₂O₇. Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13-14 keV/nm, below which phase transformations do not occur.     

Review of A2B2O7 pyrochlore response to irradiation and pressure

This article reviews recent research on swift heavy-ion irradiations and high-pressure studies on pyrochlores of the Gd₂Zr_2−xTi_xO₇ binary [1], [2], [3] and [4]. Applying three complementary analytical techniques (synchrotron X-ray diffraction, Raman spectroscopy and transmission electron microscopy) allowed for the investigation of the response of pyrochlore to irradiation and/or pressure. The chemical composition of pyrochlore has a strong effect on the character and energetics of the type of structural modifications that can be obtained under pressure or irradiation: For Ti-rich pyrochlores, the crystalline-to-amorphous transition is the dominant process. When Zr is substituted for Ti, an order-disorder transformation to the defect-fluorite structure becomes the increasingly dominant process. Except for Gd₂Zr₂O₇, single ion tracks in pyrochlore consist of an amorphous core, surrounded by a crystalline, but disordered, defect-fluorite shell. This shell is surrounded by a defect-rich pyrochlore region. In contrast to similar effects observed when pressure or irradiation are applied separately, the response of the pyrochlore structure is significantly different when it is exposed simultaneously to pressure and irradiation. The combination of relativistic heavy ions with high pressure results in the formation of a new metastable pyrochlore phase. TEM and quantum-mechanical calculations suggest that these novel structural modifications are caused by the formation of nanocrystals and the modified energetics of nanomaterials.     

Mechanical properties of fluorite-related oxides subjected to swift ion irradiation: Pyrochlore and zirconia

The modifications of the mechanical properties of related-fluorite oxides (cubic zirconia [c-ZrO₂] and pyrochlores [Gd₂(Ti_1−xZr_x)₂O₇ with x = 0.5 and x = 1]) induced by swift heavy ion irradiation are investigated. Polycrystalline pellets of both materials were irradiated at room temperature with 940 MeV Pb or 870 MeV Xe ions at the GANIL accelerator in Caen at fluences ranging from 2 × 10¹¹ to 10¹³ cm⁻². Residual macroscopic stresses induced by irradiation were determined using X-ray diffraction and the sin²ψ method. The microhardness and the fracture toughness of irradiated samples were studied by Vickers micro-indentation. Amorphization occurs in Gd₂TiZrO₇ and not in Gd₂Zr₂O₇ and c-ZrO₂. The mechanical behavior of materials is found to be closely related to the residual stresses induced in the surface layer by irradiation. Compressive stresses are generated in c-ZrO₂ and Gd₂TiZrO₇ (leading to an increase of fracture toughness), whereas tensile stresses (inducing a large decrease of fracture toughness) are observed in Gd₂Zr₂O₇ due to the lattice contraction related to a pyrochlore fluorite→transition.     

MgO-pyrochlore composite as an inert matrix fuel: Neutronic and thermal characteristics

Inert matrix fuels are an important component of advanced nuclear fuel cycles, as they provide a means of utilizing plutonium and reducing the inventory of ‘minor’ actinides. We examine the neutronic and thermal characteristics of MgO-pyrochlore (A₂B₂O₇: La₂Zr₂O₇, Nd₂Zr₂O₇ and Y₂Sn₂O₇) composites as inert matrix fuels in boiling water reactors. By incorporating plutonium with resonance nuclides, such as Am, Np and Er, in the A-site of pyrochlore, the k_infvs. burn-up curves are shown to be similar to those of UO₂, although the Doppler coefficients are less negative than UO₂. The Pu depletion rates are 88-90% (²³⁹Pu) and 54-58% (total Pu) when the inert matrix fuels experience a burn-up equivalent of 45 GWd/tHM UO₂. Because of the high thermal conductivity of MgO, the center-line temperatures of the MgO-pyrochlore composites at 44.0 kW/m are lower than those of UO₂ pellets. After burn-up, the A-site cation composition is 15-35 at.% lower than that of the B-site cations in pyrochlore (e.g., A_1.84B_2.17O_7.00) due to the fission of Pu in the A-site and the presence of fission product elements in the A- and B-sites of the pyrochlore structure.     

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

Diffuse reflectance measurements were made over the wavenumber range of 4000-20,000 cm⁻¹ at room temperature on monoclinic and stabilised ZrO₂, together with Y₂Ti₂O₇ having the pyrochlore structure, all of which were doped with U and sintered in various atmospheres. X-ray photoelectron spectroscopy measurements were also carried out on selected samples. In monoclinic and stabilised zirconia, U exhibited valence states of +4 and/or +5, depending on the sintering atmosphere and the presence of appropriate charge compensators. Using both diffuse reflectance and X-ray photoelectron spectroscopy, U was also observed as mainly U⁴⁺ and/or U⁵⁺ in U-doped Y₂Ti₂O₇ sintered at 1400 °C in air or Ar, although a small amount of U⁶⁺ also appeared to be present in some U-doped Y₂Ti₂O₇ samples heated in air.     

Irradiation of synthetic garnet by heavy ions and α-decay of Cm

Garnet, A₃B₂X₃O₁₂, has a structure that can incorporate actinides. Hence, the susceptibility of the garnet structure to radiation damage has been investigated by comparing the results of self-radiation damage from α-decay of ²⁴⁴Cm and a 1 MeV Kr²⁺ ion irradiation. Gradual amorphization with increasing fluence was observed by X-ray diffraction analysis and in situ transmission electron microscopy. The critical dose, D_c, for an yttrium-aluminum garnet (Y₃Al₅O₁₂) doped with 3 wt.% ²⁴⁴Cm is calculated to be 0.4 displacements per atom (dpa). While the doses obtained by ion irradiation experiments of garnets with different compositions (Y_2.43Nd_0.57)(Al_4.43Si_0.44)O₁₂, (Ca_1.64Ce_0.41Nd_0.42La_0.18Pr_0.18Sm_0.14Gd_0.04)Zr_1.27Fe_3.71O₁₂, and (Ca_1.09Gd_1.23Ce_0.43)Sn_1.16Fe_3.84O₁₂, varied from 0.29 to 0.55 dpa at room temperature. The similarity in the amorphization dose at room temperature and critical temperature of the different garnet compositions suggest that the radiation response for the garnet structure is structurally constrained, rather than sensitive to composition, which is the case for the pyrochlore structure-type.     

In situ studies of ion irradiated inverse spinel compound magnesium stannate (Mg2SnO4)

Magnesium stannate spinel (Mg₂SnO₄) was synthesized through conventional solid state processing and then irradiated with 1.0 MeV Kr²⁺ ions at low temperatures 50 and 150 K. Structural evolutions during irradiation were monitored and recorded through bright field images and selected-area electron diffraction patterns using in situ transmission electron microscopy. The amorphization of Mg₂SnO₄ was achieved at an ion dose of 5 × 10¹⁹ Kr ions/m² at 50 K and 10²⁰ Kr ions/m² at 150 K, which is equivalent to an atomic displacement damage of 5.5 and 11.0 dpa, respectively. The spinel crystal structure was thermally recovered at room temperature from the amorphous phase caused by irradiation at 50 K. The calculated electronic and nuclear stopping powers suggest that the radiation damage caused by 1 MeV Kr²⁺ ions in Mg₂SnO₄ is mainly due to atomic displacement induced defect accumulation. The radiation tolerance of Mg₂SnO₄ was finally compared with normal spinel MgAl₂O₄.     

Light ion irradiation effects on stuffed Lu2(Ti2−xLux)O7−x/2 (x = 0, 0.4 and 0.67) structures

We have recently synthesized “stuffed” (i.e., excess Lu) Lu₂(Ti_2−xLu_x)O_7−x/2 (x = 0, 0.4 and 0.67) compounds using conventional ceramic processing. X-ray diffraction measurements indicate that stuffing more Lu³⁺ cations into the oxide structure leads eventually to an order-to-disorder (O-D) transition, from an ordered pyrochlore to a disordered fluorite crystal structure. At the maximum deviation in stoichiometry (x = 0.67), the Lu³⁺ and Ti⁴⁺ ions become completely randomized on the cation sublattices, and the oxygen “vacancies” are randomized on the anion sublattice. Samples were irradiated with 400 keV Ne²⁺ ions to fluences ranging from 1 × 10¹⁵ to 1 × 10¹⁶ ions/cm² at cryogenic temperatures (∼77 K). Ion irradiation effects in these samples were examined by using grazing incident X-ray diffraction. The results show that the ion irradiation tolerance increases with disordering extent in the non-stoichiometric Lu₂(Ti_2−xLu_x)O_7−x/2.     

Alpha-decay induced amorphization of Cm-doped Gd2TiZrO7

The sample of pyrochlore-based ceramic doped with a ²⁴⁴Cm isotope with a target composition Gd_1.935Cm_0.065 TiZrO₇ was prepared by cold pressing and sintering. The pyrochlore structure phase was predominant in the sample but minor perovskite and gadolinium zirconate (ideally Gd₂Zr₂O_7−x) were also present. The Ti/Zr pyrochlore phase was rendered amorphous at a dose of 4.6 × 10¹⁸ α-decays/g (0.60 dpa). Volume expansion of the pyrochlore lattice was found to be 2.7 vol.% at a dose of 3.85 × 10¹⁸ α-decays/g.     

Characterization of high energy ion implantation into Ti-6Al-4V

Ion implantation is a surface modification process that can improve the wear, fatigue, and corrosion resistance for several metals and alloys. Much of the research to date has focused on ion energies less than 1 MeV. With this in mind, Ti-6Al-4V was implanted with Al²⁺, Au³⁺, and N⁺ ions at energies of 1.5 and 5 MeV and various doses to determine the effects on strengthening of a high energy beam. A post heat treatment on the specimens implanted with Al²⁺ samples was conducted to precipitate Ti_xAl type intermetallics near the surface. Novel techniques, such as nanoindentation, are available now to determine structure-mechanical property relationships in near-surface regions of the implanted samples. Thus, nanoindentation was performed on pre-implanted, as-implanted, and post heat treated samples to detect differences in elastic modulus and hardness at the sub-micron scale. In addition, sliding wear tests were performed to qualitatively determine the changes in wear performance. The effect of this processing was significant for samples implanted with Al²⁺ ions at 1.5 MeV with a dose higher than 1 × 10¹⁶ ions/cm² where precipitation hardening likely occurs and with N⁺ ions.     

EXAFS study of the structural phase transition in the americium zirconate pyrochlore

The ²⁴¹Am₂Zr₂O₇ phase undergoes a structural transition from pyrochlore to defect fluorite driven by alpha self-irradiation. In an effort to better understand the underlying phenomena of this order-disorder transition, powder X-ray diffraction (PXRD) and X-ray absorption fine-structure (XAFS) spectroscopy experiments were conducted on two samples aged for 40 days (0.02 dpa) and 370 days (0.21 dpa), respectively. While the XAFS data support the phase transition observed by XRD, they reveal different local coordinations of americium and zirconium. The transition occurs through oxygen Frenkel and cation antisite formation. The XAFS, clearly showed that the ZrO polyhedron is stable against irradiation, probably a main factor explaining the excellent resistance to amorphization observed for americium zirconia defect fluorite structures.     

Oxygen ion irradiation effect on corrosion behavior of titanium in nitric acid medium

The corrosion assessment and surface layer properties after O⁵⁺ ion irradiation of commercially pure titanium (CP-Ti) has been studied in 11.5 N HNO₃. CP-Ti specimen was irradiated at different fluences of 1 × 10¹³, 1 × 10¹⁴ and 1 × 10¹⁵ ions/cm² below 313 K, using 116 MeV O⁵⁺ ions source. The corrosion resistance and surface layer were evaluated by using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and glancing-angle X-ray diffraction (GXRD) methods. The potentiodynamic anodic polarization results of CP-Ti revealed that increased in ion fluence (1 × 10¹³-1 × 10¹⁵ ions/cm²) resulted in increased passive current density due to higher anodic dissolution. SEM micrographs and GXRD analysis corroborated these results showing irradiation damage after corrosion test and modified oxide layer by O⁵⁺ ion irradiation was observed. The EIS studies revealed that the stability and passive film resistance varied depending on the fluence of ion irradiation. The GXRD patterns of O⁵⁺ ion irradiated CP-Ti revealed the oxides formed are mostly TiO₂, Ti₂O₃ and TiO. In this paper, the effects of O⁵⁺ ion irradiation on material integrity and corrosion behavior of CP-Ti in nitric acid are described.     

The effects of alpha-radiolysis on UO2 dissolution determined from batch experiments with Pu-doped UO2

The effects of alpha dose-rate on UO₂ dissolution were investigated by performing dissolution experiments with ²³⁸Pu-doped UO₂ materials containing nominal alpha-activity levels of ∼1-100 Ci/kg UO₂ (actual levels 0.4-80 Ci/kg UO₂), in 0.1 M NaClO₄ and in 0.1 M NaClO₄ + 0.1 M carbonate. Dissolution rates increased less than 10-fold for an almost 100-fold increase in doping level and fall within the range of predictions of the Mixed Potential Model (a detailed mechanistic model for used fuel dissolution). Dissolution rates were lower in carbonate-free solutions and enrichment of ²³⁸Pu on the UO₂ surface was suggested in carbonate solutions. Effective G values, defined as the ratio of the total amount of U dissolved divided by the maximum possible amount of U dissolved by radiolytically produced H₂O₂, increased with decreasing doping levels. This suggests that the dissolution reaction at high dose rates is limited by the reaction rate between UO₂ and H₂O₂, but becomes increasingly limited by the rate of production of H₂O₂ at lower dose rates.     

Ion beam induced modifications in electron beam evaporated aluminum oxide thin films

Al₂O₃ thin films find wide applications in optoelectronics, sensors, tribology etc. In the present work, Al₂O₃ films prepared by electron beam evaporation technique are irradiated with 100 MeV swift Si⁷⁺ ions for the fluence in the range 1 × 10¹² to 1 × 10¹³ ions cm⁻² and the structural properties are studied by glancing angle X-ray diffraction. It shows a single diffraction peak at 38.2° which indicates the γ-phase of Al₂O₃. Further, it is observed that as the fluence increases up to 1 × 10¹³ ions cm⁻² the diffraction peak intensity decreases indicating amorphization. Surface morphology studies by atomic force microscopy show mean surface roughness of 34.73 nm and it decreases with increase in ion fluence. A strong photoluminescence (PL) emission with peak at 442 nm along with shoulder at 420 nm is observed when the samples are excited with 326 nm light. The PL emission is found to increase with increase in ion fluence and the results are discussed in detail.     

Microstructural investigation of the stability under irradiation of oxide dispersion strengthened ferritic steels

Some fuel pin cladding made from a ferritic steel reinforced by titanium and yttrium oxides were irradiated in the French experimental reactor Phénix. Microstructural examination of this alloy indicates that oxides undergo dissolution under irradiation. This irradiation shows the influence of dose and, in a smaller part, of temperature. In order to better understand the mechanisms of dissolution, three ferritic steels reinforced by Y₂O₃ or MgO were irradiated with different charged particles. Inelastic interactions induced by 1 MeV He ion irradiation do not lead to any modification, neither in their chemical composition, nor in their spatial and size distribution. In contrast, isolated Frenkel pairs created by electron irradiation lead to significant oxide dissolution with a radius decrease proportional to the dose. Moreover, the comparison between irradiation with ions (displacements cascades) and electrons (Frenkel pairs only) shows the importance of free point defects in the dissolution phenomena.     

Nuclear tracks in garnets studied by means of Rutherford backscattering and X-ray diffraction

(Y, La)₃(Fe, Ga)₅O₁₂ epitaxial garnet films on (111) Gd₃Ga₅O₁₂ substrates irradiated with ²³⁸U ions of 1.4 MeV/u specific energy in the dose range 10¹⁰ cm^?2 to 3 × 10¹¹ cm^?2 were measured by means of Rutherford backscattering and double-crystal X-ray diffraction before and after thermal annealing in oxygen. The nuclear track diameter of 10 nm confining a cylindrical volume of highly disordered material caused by each ion impact has been deduced from the comparison of the backscattering spectra of the irradiated and unirradiated film areas. The fraction of randomly backscattered ions due to the irradiation-induced damage as well as the lattice expansion perpendicular to the crystal surface caused by irradiation-induced lateral compressive stress are proportional to the ion dose. After thermal annealing the comparison of the almost identical backscattering yield of the irradiated areas and the unirradiated film regions demonstrates a nearly perfect recrystallization of the damaged track volumes.     

Gd2Zr2-xCexO7(0.0≤x≤2.0)的制备与表征

为获得钆锆烧绿石基固化体固化Pu后的物理性能、物相变化及微观形貌,本研究用Ce⁴⁺模拟Pu⁴⁺,以Gd₂O₃、ZrO₂和CeO₂粉体为原料,采用高温固相法,制备不同固溶度（0～100%,按摩尔计）的系列钆锆烧绿石固化体,并对密度、硬度、物相和微观形貌等进行表征。结果表明：实验所得系列固化体的密度和硬度均随着x的增大而增大,硬度H_V与x满足关系式H_V＝661.272 73+223.936 36x（R²＝0.946 38）。在x＝0.0时,所得固化体为单一的烧绿石结构;在x＝0.2时,固化体从烧绿石结构转变为萤石型结构;在0.2≤x≤2.0范围内,固化体均为单一萤石型结构。固化体微观形貌不规则,呈板块状。     

Swift heavy ion induced structural modifications in zircon and scheelite phases of ThGeO4

Structural modifications in the zircon and scheelite phases of ThGeO₄ induced by swift heavy ions (93 MeV Ni⁷⁺) at different fluences as well as pressure quenching effects are reported. X-ray diffraction and Raman measurements at room temperature on the irradiated zircon phase of ThGeO₄ indicate the occurrence of stresses that lead to a reduction of the cell volume up to 2% followed by its transformation to a mixture of nano-crystalline and amorphous scheelite phases. Irradiation of the zircon phase at liquid nitrogen temperature induces amorphization at a lower fluence (7.5 × 10¹⁶ ions/m²), as compared to that at room temperature (6 × 10¹⁷ ions/m²). Scheelite type ThGeO₄ irradiated at room temperature undergoes complete amorphization at a lower fluence of 7.5 × 10¹⁶ ions/m² without any volume reduction. The track radii deduced from X-ray diffraction measurements on room temperature irradiated zircon, scheelite and low temperature irradiated zircon phases of ThGeO₄ are, 3.9, 3.5 and 4.5 nm, respectively. X-ray structural investigations on the zircon phase of ThGeO₄ recovered after pressurization to about 3.5 and 9 GPa at ambient temperature show the coexistence of zircon and disordered scheelite phases with a larger fraction of scheelite phase occurring at 9 GPa. On the other hand, the scheelite phase quenched from 9 GPa shows crystalline scheelite phase pattern.     

Effect of Ag and Li ion irradiation on superconducting Tl2Ca2Ba2Cu3O10 single crystals

The effect of irradiation by 50 MeV Li³⁺ and 200 MeV Ag¹⁵⁺ ions on single crystals of Tl₂Ca₂Ba₂Cu₃O₁₀ (Tl2223) superconductor has been investigated at different fluences. Isothermal magnetization hysteresis loops have been recorded at different temperatures using a SQUID magnetometer and the effect of irradiation on the critical current density, irreversible field, second magnetization peak and pinning force has been studied. Irradiation by 200 MeV Ag¹⁵⁺ ions resulted in increased hysteresis and irreversibility field while no change in second magnetization peak position and critical temperature was observed. A broadening in the hysteresis loop before the second magnetization peak was also observed for the crystals irradiated by Li³⁺ ions. Annealing of irradiated crystals at 500 °C resulted in reduction of point defects created by Li³⁺ ions.