Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped

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Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped with Metals for Emulation of Spent Fuels Weilin Jiang, Michele A. Conroy, Karen Kruska, Nicole R. Overman, Timothy C. Droubay, Jonathan Gigax, Lin Shao, and Ram Devanathan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06188 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped with Metals for Emulation of Spent Fuels Weilin Jiang,*,† Michele A. Conroy,† Karen Kruska,† Nicole R. Overman,† Timothy C. Droubay,† Jonathan Gigax,‡ Lin Shao,‡ and Ram Devanathan† †

Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States Texas A&M University, 3380 University Drive East, College Station, TX 77845, United States

‡

ABSTRACT: Epsilon-phase alloy precipitates have been observed with varied compositions and sizes in spent nuclear fuels, such as UO2. Presence of the inclusions, along with other oxide precipitates, gas bubbles and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel. To predict fuel performance, a fundamental study of the precipitation processes is needed. This study uses ceria (CeO2) as a surrogate for UO2. Polycrystalline CeO2 films doped with Mo, Ru, Rh, Pd and Re (surrogate for Tc) were grown at 823 K using pulsed laser deposition, irradiated at 673 K with He+ ions, and subsequently annealed at higher temperatures. A number of methods, including transmission electron microscopy and atom probe tomography, were applied to characterize the samples. The results indicate that there is a uniform distribution of the doped metals in the as-grown CeO2 film. Pd particles of ~3 nm in size appear near the dislocation edges after He+ ion irradiation to ~13 dpa. Thermal annealing at 1073 K in air leads to formation of precipitates of Mo and Pd near the grain boundaries. Further annealing at 1373 K produces 70 nm sized precipitates consisting of nano-grains at cavities. Keywords: Precipitation, epsilon nanoparticles, ion irradiation, emulation of spent fuels, ceria 1. INTRODUCTION Nuclear fuels, such as UO2, are designed to perform under harsh conditions in a nuclear reactor, including high dose and high temperature. During reactor operation, the composition of the fuel gradually changes as the chain fission reactions starts from fissile nuclides (235U). A typical burnup of 40 MWd/kg U results in the conversion of 4% of the uranium to approximately 3% fission products.1 The resulting fission fragments exhibit a bimodal distribution with many hundreds of fission products having half-lives of only days to weeks.2 More than 20 fission products can be detected in the fuel even at a moderate burnup.3 However, there are only a few dominating families of fission product phases, including oxide solid solutions, perovskite oxide known as grey phase, metallic Mo-Tc-Ru-Rh-Pd white phase with Mo and Ru being the main components, and fission gas bubbles.3 The metallic phases exist due to their high content of noble metals with low oxygen affinity. In addition to a small fraction of body-centered cubic (bcc) βMo(Tc, Ru) phase and face-centered cubic (fcc) α-Pd(Ru, Rh) phase, the metallic particles are found primarily in the hexagonal close-packed (hcp) ε-Ru(Mo, Tc, Rh, Pd) phase with a broad variation in component concentrations.4 The metallic ε-phase has a melting point between 2073 and 2273 K and possesses high resistance to corrosion. Its composition depends on fission yield, oxygen potential, temperature gradient and fuel burnup. Pores and grain boundaries (GBs) are preferred sinks for the metallic precipitates through the diffusion process. The diffusion is more significant at the higher temperature zone near the central part of the fuel pellet, leading to the formation of larger metallic precipitates. For normally operated light water reactor (LWR) fuels, the central temperature rarely exceeds 1473 K and the ε particles are typically less than 1 µm in diameter.5 The microstructure of the irradiated fuel is complex due to various factors, including presence of various fission product precipitates, β-decay induced changes in charge imbalance,6-8 increase in oxygen potential, temperature gradient, irradiation-enhanced GB migration, and lattice disorder produced by Page 1 of 15

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elastic collisions with high-energy neutrons and energetic daughter nuclide recoils in the damage cascade process. There is a long history of effort devoted to characterization of spent nuclear fuels. Based on the knowledge available in 1967, the chemical states of fission products were categorized into soluble fission products, metallic and non-metallic fission products.9 More extensive analyses of irradiated fuels later led to a specific classification of four groups:3 fission gas (Kr and Xe) and volatile isotopes (Br and I), metallic precipitates consisting of Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb and Te, oxides with Rb, Cs, Ba, Zr, Nb, Mo, and Te, and dissolved oxides containing Sr, Zr, Nb, and 8 rare earth elements. A detailed microscopy study10 revealed that the GBs of spent fuels were decorated with fine gas bubbles and larger ε particles of ~30 nm in diameter, while 5-10 nm diameter particle-bubble aggregates were uniformly distributed in the interior of the irradiated UO2 grains. The average composition of the ε particles was determined to be 40% Mo, 30% Ru, 10% Tc, 15% Pd, and 5% Rh with lattice parameters a=b=0.273 nm and c/a=1.61. Analyses with improved accuracy made it possible to determine the compositions and lattice parameters of submicron- and nanometer-sized ε particles extracted from a spent fuel.11 More recently, the nanostructures of ε particles from a dissolved high-burnup spent fuel were examined12 using an aberration-corrected scanning transmission electron microscope (STEM). The results suggest that individual 10-300 nm sized ε particles consist of 1-3 nm crystallites in an amorphous matrix, and the smaller particles were found to be more Ru-rich. In addition, using Re as a surrogate for Tc in spent fuel, successful synthesis of the ε phase alloys as a potential waste form to immobilize 99Tc for permanent disposal was also reported.13 99Tc is an isotope with a long half-life of 2.13×105 years and a high solubility under oxidizing conditions.14 A case study of samples from Oklo natural fission reactors suggested release of Mo and most of the Tc from the ε phase.15 Presence of the ε particles, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects can significantly degrade the physical properties of the fuel, including thermal conductivity,16 swelling, creep, and melting point.3 In order to predict fuel performance, it is important to investigate critical parameters, including dose and temperature, under which precipitation occurs in the UO2 matrix. Ion irradiation combined with thermal annealing can play a significant role in this effort. It allows for simulation of different doses at different stages of particle precipitation as well as different temperatures for different locations in the pellets. Irradiated structures may be simulated within hours to days as opposed to weeks to months or longer in a nuclear reactor to achieve a required dose. In addition, there is no radiological activation in the ion-irradiated material, allowing for immediate release of samples for characterization. The new results could improve our understanding of the precipitation process as well as provide experimentally determined parameters needed for validation of model calculations. To date, there have been few reports on modelling metallic ε phases.17 In this study, cubic phase CeO2 is selected as a surrogate for UO2. The two materials have the same cubic crystal structure (space group 225, Laue grope 11) with nearly identical lattice parameters (0.5411 and 0.5471 nm for CeO2 and UO2, respectively). They also have similar physical properties, such as melting temperature (2873 K for CeO2 and 3143 K for UO2) and thermal diffusivity.18 A similar precipitation process in CeO2 and UO2 is generally expected. In this study, formation of nanoparticles in doped CeO2 is reported as a function of ion irradiation and thermal annealing. 2. EXPERIMENTAL PROCEDURES The samples used in this study were prepared using pulsed laser deposition (PLD) in a custom designed off-axis system (PVD Products Inc., Wilmington, Maryland). The commercial target was a 2” diameter and 0.125” thick disk synthesized by Plasmaterials, Inc. (Livermore, California) with a customized composition of 95 wt.% CeO2, 2 wt.% Mo, 0.75 wt.% Pd, 0.25 wt.% Rh, 1.5 wt.% Ru and 0.5 wt.% Re. The metal dopants have the weight percentages that are typical in the five-metal ε-phase particles found in spent fuels.10 The overall purity of the CeO2 target material was 99.9% with impurities on the level of ppm or less except for some elements (e.g., Si) in the tens of ppm. The substrate was polycrystalline yttrium stabilized zirconia (YSZ) with a purity of 99% with Si as a major impurity, obtained from Page 2 of 15


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Marketech International (Port Townsend, Washington). Utilization of the polycrystalline substrate was intended to grow polycrystalline CeO2 films that contains GBs similar to real fuel. The lattice mismatch between CeO2 and cubic ZrO2 is ~5% along their corresponding axial directions. Instead of commonly used O2 gas during the growth of oxides, Ar gas was used to filter out possible large clusters in the plume while preventing oxidation of the dopant metals. The YSZ substrate was first heated in 10-2 Torr oxygen environment in the deposition chamber to minimize any carbon or hydrocarbon on the surface (surface cleaning). The chamber was then evacuated to 10-8 Torr, and the Ar gas was flushed into the chamber to reach 10-2 Torr. Deposition was performed at the substrate temperature of 823 K with a pulsed KrF excimer laser (248 nm) operating at 5 Hz (Coherent COMPexPro 102). The laser beam was focused onto the target with an energy density of ~2.4 J cm-2. The typical growth rate was 0.2 nm/s. The doped CeO2 films on YSZ were irradiated at normal incidence with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K. The irradiation was performed over an area of 10 mm × 10 mm, covering the entire film surface. A uniform irradiation over the large area was achieved using a magnetic beam rastering system. SRIM (Stopping and Range of Ions in Matter) simulation19 was performed to obtain the depth profiles of the displaced atoms and implanted He in CeO2 with a bulk density ρ=7.215 g/cm3. The surface sputtering/monolayer collision step mode was chosen to eliminate artifacts into the sample damage by the energetic light ion in the near surface region. In the simulation, the threshold displacement energies of Ed(Ce) = 56 eV and Ed(O) = 27 eV were adopted.20 For 90 keV He+ ion implantation at normal incidence, the peak disordering rate is 0.32 atomic displacements per atom (dpa) per 1016 He+/cm2 at 324 nm and the He profile is peaked at 372 nm with a maximum of 0.57 at.% He per 1016 He+/cm2, as shown in Figure 1. The ion fluence of 4×1017 He+/cm2 applied in this study corresponds to a maximum of ~13 dpa and ~23 at.% He at their respective peak maxima. Furnace annealing was performed for the irradiated films at 1073 and 1373 K for 10 h each in air. The thermal treatments had a ramp-up time of 2 h and a ramp-down rate of 2 °C/min in the high temperature range above ~673 K and smaller rates at lower temperatures to allow sufficient time for the samples to cool down without abrupt quenching. After the combination of He irradiation and thermal annealing, six samples were available for characterization, including as-grown, as irradiated, annealed at 1073 and 1373 K with irradiation between depths of 200 and 400 nm, and annealed at 1073 and 1373 K without irradiation at depths larger than 500 nm. Additional unirradiated sample was also annealed at 1073 K for 10 h in air. The samples were characterized using a number of methods. Both symmetric x-ray diffraction (XRD) and grazing-angle incidence XRD (GIXRD) were performed using a Philips X’Pert multipurpose diffractometer (MPD) with a fixed Cu anode (λKα = 0.154 nm) operating at 45 kV and 40 mA. The data was analyzed using Jade software (Materials Data, Inc., Livermore, California) and x-ray powder diffraction database PDF 4+. Both scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were performed on a carbon-coated sample surface using a JEOL JSM-7600FESEM. The EBSD data was collected at a specimen tilting angle of 70° with electron energy of 20 keV. The scan was conducted over an area of 56.3 µm × 38.8 µm for the CeO2 film and 34.1 µm × 28.0 µm for the YSZ substrate, both with a 75 nm step size. Indexing was accomplished using structural parameters for cubic zirconia,21 monoclinic zirconia,22 and cubic CeO2.23 Time-of-flight secondary ion mass spectrometry (ToF-SIMS, IONTOF, GmbH, Germany) was employed to map the metal dopants in the sputtering target as well as to detect the doped elements in the CeO2 film with the sputter beams of 1.0 keV O2+ molecular ions over an area of 300 µm × 300 µm and of 20 keV Arn+ ions over 200 µm × 200 µm, respectively. The analytical beam was 25 keV Bi+ over 50 µm × 50 µm in both cases. Cross-sectional transmission electron microscopy (TEM) specimens were prepared using both a FEI Helios NanoLab dual-beam focused ion beam (FIB) integrated scanning electron microscope (SEM) and a FEI Quanta FIB/SEM. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was performed using a JEM-ARM 200CF aberration-corrected STEM microscope (JEOL, Peabody, Massachusetts). Equipped with a cold field emitter, the microscope is capable of sub-Å spatial resolution under HAADF-STEM imaging. The high-brightness source coupled with a ~0.3 eV energy resolution allows for energy dispersive spectroscopy (STEM-EDS) analyses, utilizing a Gatan Page 3 of 15



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Quantum 965 spectrometer with a proprietary JEOL Centurio Si-drift EDS detector that has 0.98 sr collection angle from a detection area of 100 mm2. Elemental mapping based on STEM-EDS in selected areas with automatic sample drift corrections were performed at 200 or 80 kV. Needle-shaped APT specimens were prepared using the same Helios and Quanta FIB/SEM microscopes. While the irradiated and 1373 K annealed tips were lifted perpendicular to the film surface, the APT specimens for the unirradiated and 1073 K annealed sample were sliced parallel to the film surface (Figure S1 in the supporting information). The latter was intended to minimize the probability of fracturing the tip at the film/substrate interface during APT data acquisition. Standard Si micro-post coupons from Cameca Instruments were used to hold FIB lift-outs. Two or three APT tips with a diameter of ~110 nm at the apex were prepared from each of the 1073 and 1373 K annealed samples. Once finished, the APT needles were quickly transferred into the ultrahigh vacuum chamber of a local electrode atom probe system (CAMECA LEAP 4000XHR) to avoid possible contamination. APT examinations were carried out at a base temperature of 44 K. The typical applied bias voltage ranged between 4.0 and 6.5 kV with a pulsed laser beam (λ=355 nm) at an energy of 200 pJ/pulse and a pulse repetition rate of 125 kHz. The overall detection efficiency of the detector was 36% and tips were run at a target detection rate of 0.2%, i.e., 2 detected evaporation events in 1000 pulses. Typical individual datasets consisted of 3 to 10 million detected ions. Three-dimensional (3D) data reconstruction and analysis of the precipitates were performed with Cameca’s Integrated Visualization and Analysis Software (IVAS). 3. RESULTS AND DISCUSSION 3.1. As-grown CeO2 film (doped). Figure 2 shows an SEM image in plan view of a YSZ substrate (left side) and an as-grown film (right side). In addition to some dust particles on the surface (artifacts), extensive surface pitting is observed on both the substrate and film surfaces with varied sizes mostly on the order of 1 µm with a few as large as 10 µm. The observed pitting is a result of the film growth that follows the surface contour of the sintered YSZ substrate. The sharp border, distinguished by the varied contrast between the substrate and film at the center of Figure 2 is a result of a shadow mask on the sample surface during film growth near the edge of the substrate. A typical GIXRD pattern at a fixed incident angle ω=2° for an as-grown film on YSZ substrate is shown in Figure 3. The grazing-angle geometry was chosen to maximize the diffraction intensity from the film. From Figure 3, cubic CeO2 with space group Fm-3m (225) is identified as a single phase (PDF#00034-0394(RDB), a=0.5411 nm, ρ=7.215 g/cm3) in the film. All major diffraction peaks from the cubic CeO2 are observed, suggesting that the film is polycrystalline in nature. In spite of the grazing-angle geometry, cubic ZrO2 (PDF#00-049-1642(RDB), a=0.5128 nm, ρ=6.069 g/cm3) and monoclinic ZrO2 (PDF#00-037-1484(RDB), a=0.5313 nm, b=0.5213 nm, c=0.5147 nm, ρ=5.817 g/cm3) from the YSZ substrate also appear, as indicated in Figure 3. Existence of the dual-phase nature of the substrate has been confirmed by symmetric x-ray diffraction (XRD) from a blank as-received YSZ substrate (Figure S2 in the supporting information). Other crystalline phases, including those potentially from metal dopants, are not visible from the diffraction pattern. Since CeO2 (111) peak is overlapped with monoclinic ZrO2 (1ത11) peak, the well-resolved CeO2 (220) peak at 2θ=47.78° is chosen to estimate the average crystallite size in the film assuming that the lattice strain effect is small. The value is determined to be 23.6 nm based on Scherrer formula.24 It should be noted that the average crystallite size should not be larger than the mean grain size (defined below) because the former is the average inter-planar distance between the adjacent planar defects that discontinue the lattice periodicity in a crystalline grain. A large average crystallite size indicates a low defect concentration in the individual grains. Figure 4 shows EBSD grain phase and orientation maps along with SEM images for a polished polycrystalline YSZ substrate and an as-grown CeO2 film on YSZ. The SEM images clearly exhibit cavities on the substrate and pits on the film in the chosen areas. While CeO2 has only a cubic phase (in blue), there are both cubic (in blue) and monoclinic (in yellow) phases of ZrO2 in the YSZ substrate, which is consistent with the GIXRD data in Figure 3. The overall grain size of the cubic phase appears significantly larger than that of the monoclinic phase. The grain orientations are shown with the coloring Page 4 of 15


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schemes indicated for the cubic ZrO2 and CeO2. The black regions in the orientation map of the YSZ substrate are the intentional exclusion of the indexed monoclinic phase to allow for easier comparison of the cubic phase orientation of the substrate and film. Black regions in the SEM images and EBSD maps are primarily indicative of voids and grain boundaries. The data reveals that the cubic phase grain orientation in the substrate is randomly distributed without texture, indicating that the sintered ZrO2 pellet is polycrystalline in nature, as expected. It is known that single crystal CeO2 films grow epitaxially on single crystal YSZ substrates.25,26 In this study, polycrystalline YSZ substrates were intentionally selected to promote growth of polycrystalline CeO2 films via grain-on-grain epitaxy. Figure 4 shows large grains and localized patches of fine grains of cubic CeO2 in the film, which were likely grown on cubic and monoclinic ZrO2, respectively. Large {001} oriented CeO2 grains are not observed in the film. This is in contrast to the randomly distributed grain orientation suggested by the YSZ substrate map. The result from Figure 4 suggests that the large CeO2 grains have somewhat preferred orientations between {101} and {111}. The resulting film microstructure might be associated with epitaxial growth rates that could be different along different orientations or variations in the surface free energy. The grain size in this study is defined as the diameter of a circle with the same grain area in the two-dimensional map, similar to our previous studies.27,28 The grain area in the map was determined using a number of criteria, including a critical misorientation of 7°, boundary completion to 2°, and class width of 0.25 µm. Small grains are bridged through connections as long as they are within 7°. The results for the grain size distributions are shown in Figure 5 as histograms binned at 0.25 µm, excluding those smaller than 0.75 µm to better evaluate the profile of cubic CeO2 grains grown on cubic ZrO2. In general, small grains have a dominant population in the CeO2 film. The grain size distribution in the film appears to be sharper than that of the cubic ZrO2 grains in the YSZ substrate. Based on Figure 5, the mean grain diameters are estimated to be 1.6 µm for the cubic CeO2 grains and 2.5 µm for the cubic ZrO2 grains. ToF-SIMS with an extremely high detection sensitivity was used to verify the presence of the metal dopants in the film. The mass spectrum from the ToF-SIMS measurement is shown in Figure 6, where the mass to charge state ratios for the doped isotopes Mo, Ru, Rh, Pd and Re are indicated. The SIMS intensity is an integrated count over the entire film thickness for improved statistics. In general, SIMS yield of a specific element or its cluster depends on its relative sensitivity factor (RSF), which varies in different materials.29 Although the absolute quantification of the dopants in this study requires reference samples that were not available, the data in Figure 6 confirms that all the doped elements are present in the film. Major Ce isotopes are observed in the mass spectra of Ce+, CeO+ and CeO2+ with strong SIMS intensities, as expected. High-resolution HAADF STEM was also performed to characterize the as-grown CeO2 film. Figure 7(a) shows a HAADF STEM micrograph of the doped CeO2 along the [110] zone axis, where dislocation lines are observed. STEM-EDS was used to map the metal dopants as well as the major elements of the matrix. Figure 7(b) shows the distributions of elements in the film. The doped Mo, Ru, Rh, Pd and Re maps do not provide any evidence for local enrichment of the doped metals, indicating that the dopants are uniformly distributed in the CeO2 matrix and precipitates are not formed, as listed in Table 1. This result is confirmed by tilting different zone axes on the ceria and no areas of higher concentration were observed. The uniform dopant distribution may be attributed to the rastered laser beam over a continuously rotating sputter target. Apparently, the metal dopants are not mobile at the film growth temperature (827 K). The five-metal doped CeO2 film is the starting material for irradiation and annealing to study dopant precipitation. The Ce 4d core level photoemission from the as-grown, as-irradiated and as-annealed ceria films was investigated using x-ray photoelectron spectroscopy (XPS). The XPS spectra (data not shown) contain overlapping Ce3+ and Ce4+ peaks in the binding energy range from 100 to 130 eV. The overall shape of the spectra is similar to the previously reported data.30 Except for a very little change in the oxidation states in the as-irradiated film, all other films are nearly fully oxidized ceria (cubic CeO2 phase). It should be noted that the EBSD map in Figure 4 shows only cubic phase of CeO2 (Ce4+) in the as-grown film without large grains identified as hexagonal phase of Ce2O3 (Ce3+). However, the possibilities of O

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vacancies in CeO2 and a small amount of ultrafine Ce2O3 grains (smaller than 75 nm) in the films cannot be ruled out. 3.2. General features. Low resolution cross-sectional SEM and HAADF STEM images are shown in Figure 8 for a general view of microstructural features before and after irradiation and annealing. From Figure 8, the film thickness is determined to be ~1.8 µm. The thickness is essentially unchanged following irradiation and thermal annealing. The as-grown sample in the doped CeO2 film (Figure 8(a)) at 823 K shows a uniform contrast with some microcracks and GBs. He+ ion irradiation at 673 K produces an observable damage band to the depth of ~310 nm, as indicated in Figure 8(b), which is consistent with the SRIM prediction shown in Figure 1. Some contrast features are observed in the irradiated region. At larger depths beyond 500 nm, the material appears to be unaffected, as expected. Thermal annealing at 1073 K for 10 h in air leads to development of extensive GB fracture and cracking in the film (Figure S3 in the supporting information) probably due to a large lattice mismatch (~5%) between CeO2 and YSZ or the difference in their thermal expansion coefficients. Formation of nanoscale features is also observed, as shown in the inset of Figure 8(c). Further annealing at 1373 K for 10 h in air promotes growth of the nano-features and produces dislocation lines and more cracks, as shown in Figure 8(d). In order to examine the nano-features, higher resolution microscopy combined with STEM-EDS mapping have been performed and the results are presented and discussed below. 3.3. As-irradiated CeO2 film (doped). The doped CeO2 film was irradiated to 4×1017 He+/cm2 at 673 K, corresponding to ~13 dpa at the depth of 324 nm. While an elevated temperature for irradiation is needed to reduce the residual defect concentration in CeO2, the irradiation temperature in this study is intentionally chosen to be lower than the film growth temperature (823 K) in order to minimize thermal effects during irradiation. A HAADF STEM micrograph around the damage peak region is shown in Figure 9(a), where a nanoparticle of ~3 nm in size near the edge of a linear dislocation (black line) is observed, as listed in Table 1. The dislocation line is likely produced during the film growth, but not necessarily as a result of He+ ion irradiation. However, lattice damage is produced in the CeO2 grains promoting polycrystallization in the depth region with highly disordered zones. Neither helium bubbles nor full amorphization of the crystal structure is evident. At depths greater than 500 nm where the material is not irradiated, precipitates are not observed. This is an expected result because the irradiation temperature (673 K) is significantly lower than the film growth temperature (823 K). Figure 9(b) shows concentration maps of Mo, Ru, Rh, Pd, Ce and Re from STEM-EDS in the same region as in Figure 9(a). The nanoparticle observed in Figure 9(a) is located in the area where only the Pd concentration is predominantly higher, as shown in Figure 9(b), indicating that the particle is enriched in Pd. It is also noticed that this same area is Ce deficient. The bright spot of the precipitate in the HAADF STEM image is a result of Z (atomic number) contrast. Based on the electron scattering yield31 I ∝ nZ1.7, where n is the atomic density, I(Pd)/I(CeO2)=1.6. Because of the small size of the Pd particles, it is challenging to determine its crystallographic structure by standard electron diffraction method. However, as bulk Pd has an fcc structure, it is possible that the nanoparticle has the same structure or its precursor structure. Absence of other metallic precipitates, including Mo and Ru that have higher doping concentrations in the CeO2 film, may suggest that Pd is the most mobile atom among the 5 metal dopants in CeO2. In addition, the location of the Pd particles near the edge of a linear dislocation may suggest a faster diffusion path through the defects and a preferred site for precipitation. Similarly, the rich GBs in the film are also expected to provide fast pathways for dopants to diffuse and precipitate, as observed in spent UO2 fuels.10 Precipitation of Pd should occur through irradiation-enhanced diffusion of the metallic fission product, involving ionization-enhanced diffusion, energy-release and recoil mechanisms,32 which are expected to play an important role in activating the diffusion process because thermal annealing by itself at the irradiation temperature (673 K) does not form Pd precipitates. Similar behavior was also observed from an in-situ study of loosely interconnected nanoclusters, where major nanostructural evolution was activated by ion irradiation.33 3.4. 1073 K annealed CeO2 film (doped). Post-irradiation thermal annealing was performed for the doped CeO2 films. This temperature is comparable to that at a location between the central core (~1573 Page 6 of 15


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K) and the rim (573 K) of the LWR oxide fuel under normal reactor operating conditions. Figure 10 shows atom maps of major elements, dopants and impurities in a CeO2 film annealed at 1073 K for 10 h in air. There are two micro-fracture-like features in the maps. These are likely associated with voids where the evaporation rates at the sharp edges are very high and some amount of materials was lost without being registered by the detector. The data acquisition returned automatically to normal after those transient events. Enrichment of Mo and Pd in some overlapping regions near a GB appears to occur after the thermal treatment (listed in Table 1), indicating that both Mo and Pd are mobile in polycrystalline CeO2 at 1073 K. A similar Mo-Pd phase was observed in irradiated oxide fuel as one of the hightemperature phases, which are the hexagonally stabilized intermediate ε phases.4 The other dopants of Ru, Rh and Re do not show any visible aggregation in the analyzed volume. A total of 206 Ru atoms are identified in the mass spectrum and they are distributed rather uniformly without association with any precipitates, probably because Ru is not mobile at this annealing temperature, and neither are Rh and Re. Impurities of H, Si, Na and Ca are also observed in the sample, some of which, such as Si and Ca, could originate from the CeO2 target material. In fact, the data of the ToF-SIMS mass spectrum for the asgrown film shows a Si peak (Figure S4) with a comparable peak intensity of Mo (Figure 6). Atomic diffusion of Si, a major impurity element in the polycrystalline YSZ substrate, could also contribute during film growth at 823 K, similar to Ti diffusion from SrTiO3 substrate to CeO2 film observed during epitaxial growth34 In addition, quartz furnace annealing at 1073 K could introduce impurities from the residue of previous samples, especially for Na. Hydrogen could be diffused into the polycrystalline CeO2 film through GBs when the sample was exposed to air after thermal annealing. It appears that a high density of SiO2Hx nanoparticles are formed, which distribute rather randomly in the sample. Both Na and Ca aggregate along the GBs, where Pd and Mo are also enriched. The depleted areas shown in Figure 10 along the GB line are not fully understood at this time, but helium cavities or crystallographic orientation dependence of APT yields might contribute to the profile effects. Figure 11 (a) shows Pd atom map combined with the isoconcentration surfaces at 10 at.% Si and 7 at.% Mo. Two lines (lines 1 and 2) along the GBs and two particles (P1 and P2) near the GBs are selected for composition analysis. The one-dimensional profiles of Mo, Pd, O and Na concentrations across lines 1 and 2, as indicated by arrows in Figure 11 (a), are shown in Figures 11 (b) and (c). In both cases, Mo is enriched with a maximum concentration of ~1 at.% at the GBs. While the dimension of the Mo-enriched region is over 30 nm across line 1, it is only ~10 nm across line 2. Pd across line 1 shows a smaller concentration with FWHM of ~5 nm and a maximum of ~0.4 at.%, where oxygen is slightly depleted, probably associated with the noble metal state of Pd. The Pd peak is overlapped with a Na impurity peak. The ratio of Mo to Pd is roughly 10:1 in the flat region. Across line 2, Pd peak is slightly wider than Na peak and the ratio of Pd to Mo concentration decreases from ~3:1 at the GB to ~6:1 at 30 nm from the GB. Figures 11 (d) and (e) show proximity histograms of Mo and Pd in P1 and Si and H in P2 in addition to Ce and O at a bin size of 0.1 nm. Proximity histograms for particles provide a concentration profile across a particle-matrix interface to the center of the particle, which is representative of the particle composition35 based on an isoconcentration surface. The distance is relative to the surface defined by the isoconcentration value and the concentrations are calculated within a bin. For P1, Mo is enriched to ~15 at.% with negligible Pd concentration. Ce is depleted to 15 at.% at the location of P1, and the O concentration is higher than in CeO2, suggesting that the particle is likely to contain mixed phases of Mo and Ce oxides with a size of ~8 nm (listed in Table 1). It should be noted that outside the particle, the O concentration is measured to be 54 at.%, which is less than supposedly 67 at.% of the perfect stoichiometry in CeO2. There could be oxygen vacancies in the film that show a dark contrast due to charged oxygen vacancies (F-center). Selective loss of oxygen in their flight trajectories towards the detector could be another probable contributor, which has been observed for other oxide materials at a high laser power, such as 200 J/pulse used in this study. Figure 11 (e) suggests that P2 is mainly composed of Si, O and H with ~5 nm in size. Although the data suggest formation of Si-bearing precipitates, it is difficult to accurately analyze the local compositions for similar reasons noted above. This is especially the case for H, which could migrate during APT data acquisition.

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3.5. 1373 K annealed CeO2 film (doped). Further annealing at 1373 K for 10 h in air leads to significant growth of the solid-state precipitates that are connected by dislocation lines and microfractures, as shown in Figure 12. The annealing temperature is comparable to the highest temperature near the central core of the LWR oxide fuel pellet during normal reactor operation. Precipitation along the dislocation lines and micro-fractures is not a surprise because these regions provide a faster diffusion pathway for dopants and impurities and are the preferred sites for precipitation, as discussed above. It is expected that GBs in CeO2 should also provide similar diffusion pathways and precipitation sites. There are also precipitates that form in the interiors of grains, but their size is significantly smaller due to slower diffusion of dopants and impurities. The white contrast in the bright-field STEM images in Figure 12 turns to black in STEM-EDS maps in Figure 13 (a). The STEM-EDS maps in Figure 13 (b) suggest that those areas are deficient in elements (white coloring), which is likely helium cavities or voids formed during thermal annealing. The grey areas are identified as solid-state precipitates. A majority of the large solid-state precipitates are found at the cavities, where nucleation and growth is most energetically favorable. From Figure 13 (b), the particle of ~70 nm in size (listed in Table 1) in the lower portion of the image consists of all the five doped metals (Mo, Pd, Rh, Ru and Re) in a Ce depleted region. The particle also contains concentrations of other impurity elements, including Na, Al, Si, and Ca. These impurities could diffuse into the polycrystalline CeO2 through various routes, including thermal annealing in a quartz furnace, contributions from impure YSZ substrate and sputtering target, as discussed above. It is also noticed that although the location of the particles are Ce depleted, O concentrations are non-zero, suggesting existence of some oxides. Oxidation likely occurred during thermal annealing in open air for an extended period of time (10 h). Annealing in oxygen environment was needed to prevent oxygen loss from CeO2 that could transform to Ce2O3 upon vacuum annealing. The highest-magnification image in Figure 12 (d) shows a fine structure with granular features in the precipitated particle. It is possible that the particle consists of both small metallic and oxide grains of a few nanometers in size. This observation is similar to that from a recent high-resolution STEM study that shows large ε particles containing nanometer-sized crystallites in amorphous matrix.12 The nanoparticle formation occurred throughout the entire film thickness, including at the film/substrate interface, where smaller precipitates were observed (Figure S5 in the supporting information), probably because the diffusion influx of the dopants and impurities came from one side (film) only. These particles are also connected by dislocation lines, but most of them are not associated with cavities because of a lower concentration of helium in this depth region. From the observations above, it may be concluded that precipitation is a diffusion-driven process and is preferred at locations in a decreasing order of cavities (voids, pores and open surfaces), microfractures (dislocation lines, GBs and small cracks) and grain interiors during thermal annealing with a corresponding decreasing precipitate size. Figure 14 compares precipitates in the irradiated and unirradiated CeO2 after thermal annealing at 1373 K. Precipitates in the unirradiated region resemble those observed in Figures 12 and 13. Compared to the merely annealed region, there is a very little difference of the dopant precipitates in the irradiated and annealed region, as shown in Figure 14. Figure 15 shows an example of a higher-magnification STEM image and STEM-EDS maps of the precipitates in the irradiated region. Enrichment of metal dopants is observed in the precipitates, in addition to major impurities, Na and Si. The HAADF STEM image in the center of Figure 15 (a) shows a cavity with multiple precipitates. The dopant concentrations exhibit a ring shape with the white area being the cavity. The average precipitate size near the surface is a little smaller for the same reasons as near the interface where the diffusion influx from the dopants and impurities is lower. In spite of their different shapes and sizes, the precipitates in the irradiated and unirradiated regions show similar compositions and preferred locations. The similarity could be attributed to a significant defect recovery of the irradiation defects. In fact, the damage band completely disappeared after the thermal annealing (Figure 14). Additional APT for this sample was also performed. The APT mass spectrum (Figure S6 in the supporting information) has contributions only from CeO2 and silicon oxide impurity without detectable dopant concentrations. The apparent absence of the dopants in the sample may be attributed to statistical variations in sampling. The results suggest that the dopants are so

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mobile that precipitation of all the doped five metallic elements is nearly complete in preferred locations during annealing at 1373 K for 10 h. 3.6. Summary of results. Table 1 summarizes the results of the precipitates formed in doped CeO2 under various conditions applied in this study. The data appear to suggest that among the 5 metal dopants (Mo, Ru, Rh, Pd, and Re), Pd precipitation occurs first, followed by Mo and then the rest of the dopants. This statement might not be accurate for Rh and Re as their concentrations in CeO2 are relatively low in this study and their precipitation to small atomic clusters might not be clearly invisible. Statistical error due to limited sampling of the precipitates could also be a contributing factor. It is found that precipitates prefer to form in a decreasing order of cavities, GBs, dislocation lines and grain interiors probably due to the degree of diffusion rapidity and the magnitude of formation energy. It is possible that precipitates and helium cavities nucleate and grow together as a complex aggregate during thermal annealing. The size and composition of the precipitates are strongly dependent of the annealing temperature. Precipitates are not formed at 823 K, although ~3 nm sized Pd particles appear near the dislocation edges after He+ ion irradiation to ~13 dpa at 673 K. Both Pd and Mo are mobile at 1073 K in CeO2 and aggregate to ~8 nm size (Mo oxide in this study) likely at cavities along the GBs. At 1373 K, precipitates of ~70 nm in size containing all the five metal dopants are formed at cavities. The precipitate size also depends on the amount of dopant supplies by diffusion. The average particle size appears to be larger in the middle of the film than near the surface or CeO2/YSZ interface. Larger size is also found at cavities or along the dislocation lines than in the grain interiors. Irradiation defects play a less important role in hightemperature annealed samples due to efficient defect recovery in CeO2. As UO2 and CeO2 have the same crystal structure and similar diffusion properties, precipitation processes in the two materials are not expected to be much different. The CeO2 results from this study will help understand how epsilon particles and other metallic precipitates are formed in UO2 fuels under neutron irradiation. However, the required conditions, such as temperature and dose, for the precipitation to occur in UO2 fuels may vary. Uranium is heavier than cerium; there is a lower efficiency of energy transfer to U in the ballistic collision process. Since the threshold displacement energies for U in UO2 (~50 eV)36 and Ce in CeO2 (56 eV)9 are comparable, a lower disordering rate on the U sublattice is expected. In fact, recent simulations show that the disordering rate of the cation relative to oxygen sublattice is much smaller in UO2 than CeO2.37 Dose rate effects and the presence of other fission products in real fuel UO2 could also play important roles in the precipitation processes. In addition, the CeO2 film in this study has patches of fine grains that are not present in real UO2 fuels. The richer grain boundaries could provide faster pathways for diffusion and preferred sites for precipitation of the doped metal elements. The small volume of the thin CeO2 films (~1.8 µm thick) could limit nanoparticle growth to larger sizes that are observed in bulk spent UO2 fuels.

CONCLUSIONS Polycrystalline CeO2 films, used as a surrogate for UO2, were grown on polycrystalline YSZ substrates at 823 K. The films were doped uniformly with 2 wt.% Mo, 0.75 wt.% Pd, 0.25 wt.% Rh, 1.5 wt.% Ru and 0.5 wt.% Re (surrogate for Tc). They were irradiated with 90 keV He+ ions at 673 K and subsequently annealed in air at 1073 and 1173 K for 10 h each. Pd particles of ~3 nm in size are precipitated near the dislocation edges as a result of He+ ion irradiation to ~13 dpa. The precipitation is mainly driven by irradiation-enhanced diffusion processes. The absence of other precipitates suggests that Pd is the most mobile atom among the five dopants under the irradiation conditions. Thermal annealing at 1073 K for 10 h leads to the segregation of larger nanoparticles (~8 nm across) at GBs. GBs in CeO2 serve as both faster diffusion paths and effective sinks for Mo and Pd to precipitate. Impurity particles containing SiO2Hx are also formed during the thermal process. Further annealing at 1373 K produces particles composed of all the doped elements with sizes up to ~70 nm. The large particle consists of nanometer-sized grains. At the high temperature, which is comparable to that near the central core of the fuel pellet, irradiation-induced damage in CeO2 becomes invisible due to significant defect recovery. As a result, similar precipitation Page 9 of 15



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behavior is observed in the irradiated and unirradiated CeO2. This study suggests that precipitation of the doped metallic elements in CeO2 is a diffusion-driven process and is preferred at locations in a decreasing order of cavities (voids, pores and open surfaces), micro-fractures (dislocation lines, GBs and small cracks) and grain interiors during thermal annealing with a corresponding decreasing precipitate size. Similar precipitation processes are expected in UO2 fuels, but the required temperatures and doses may vary. Further studies of particle formation and growth are underway, including in-situ annealing TEM and in-situ ion irradiation TEM. ASSOCIATED CONTENT Supporting Information Figure S1. APT specimen for a doped CeO2 film Figure S2. XRD pattern for a polycrystalline YSZ substrate Figure S3. Plan view of SEM images for doped CeO2 films on YSZ Figure S4. ToF-SIMS mass spectrum for a doped CeO2 film Figure S5. Bright-field STEM micrographs showing nanoparticles formation Figure S6. APT mass spectrum of a doped CeO2 film AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel.: (509) 371-6491 ORCID Weilin Jiang: 0000-0001-8302-8313 Ram Devanathan: 0000-0001-8125-4237 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank Dr. Dan Schreiber for help on APT sample preparation and discussion on data analysis, and Dr. Zihua Zhu for ToF-SIMS measurements. This work was supported by the Nuclear Process Science Initiative (NPSI) under the Laboratory Directed Research and Development (LDRD) program at the Pacific Northwest National Laboratory, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. Some of the experiments were performed under a general user proposal of EMSL, a national scientific user facility sponsored by DOE Office of Biological and Environmental Research and located at PNNL. Ion irradiation was performed at Texas A&M University. REFERENCES (1) Bruno, J.; Ewing, R. C. Spent nuclear fuel. Elements 2006, 2, 343-349. (2) Burns, P. C.; Ewing, R. C.; Navrotsky, A. Nuclear fuel in a reactor accident. Science 2012, 335, 1184-1188. (3) Kleykamp, H. The chemical state of the fission products in oxide fuels. J. Nucl. Mater. 1985, 131, 221-246. (4) Kleykamp, H.; Paschoal, J. O.; Pejsa, R.; Thommler, F. Composition and structure of fission product precipitates in irradiated oxide fuels. J. Nucl. Mater. 1985, 130, 426-433. (5) Spahiu, K.; Evins, L. Z. Metal alloy particles in spent nuclear fuel. Swedish Nuclear Fuel and Waste Management Co. document number 1415408, version 1.0, 2013, pp 1-6; available at: Page 10 of 15


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https://www.stralsakerhetsmyndigheten.se/Global/Slutf%C3%B6rvar/SKB_MIKE/SSM2011-2426155%201415408%20%20Metal%20alloy%20particles%20in%20spent%20nuclear%20fuel.pdf%20649993_1_1.pdf (accessed October 15, 2015). (6) Jiang, W.; Bowden, M. E.; Zhu, Z.; Jozwik, P.; Jagielski, J.; Stonert, A. Defects and minor phases in O+ and Zr+ ion co-implanted SrTiO3. Ind. Eng. Chem Res. 2011, 51, 621-628. (7) Jiang, W.; Van Ginhoven, R. M.; Kovarik, L.; Jaffe, J. E.; Arey, B. W. Superlattice structure and precipitates in O+ and Zr+ ion coimplanted SrTiO3: a model waste form for 90Sr. J. Phys. Chem. C 2012, 116, 16709-16715. (8) Jiang, W.; Kovarik, L.; Zhu, Z.; Varga, T.; Engelhard, M. H.; Bowden, M. E. Microstructure and Cs behavior of Ba-doped aluminosilicate pollucite irradiated with F+ ions. J. Phys. Chem. C 2014, 118, 18160-18169. (9) Anselin, F.; Bailey, W. E.: The role of fission products in the swelling of irradiated UO2 and (U, Pu)O2 fuels. Trans. Am. Nucl. Soc. 1967, 10, 103-104. (10) Thomas, L.E.; Einziger, R.E.; Woodley, R.E. Microstructural examination of oxidized spent PWR fuel by transmission electron microscopy. J. Nucl. Mater. 1989, 166, 243-251. (11) Cui, D.; Rondinella, V. V.; Fortner, J. A.; Kropf, A. J.; Eriksson, L.; Wronkiewicz, D. J.; Spahiu, K. Characterization of alloy particles extracted from spent nuclear fuel. J. Nucl. Mater. 2012, 420, 328-333. (12) Buck, E. C,; Mausolf, E. J.; McNamara, B. K.; Soderquist, C. Z.; Schwantes, J. M. Nanostructure of metallic particles in light water reactor used nuclear fuel. J. Nucl. Mater. 2015, 461, 236-243. (13) Crum, J. V.; Strachan, D.; Rohatgi, A.; Zumhoff, M. Epsilon metal waste form for immobilization of noble metals from used nuclear fuel. J. Nucl. Mater. 2013, 441, 103-112. (14) Wronkiewicz, D. J.; Watkins, C. S.; Baughman, A. C.; Miller, F. S.; Wolf, S. F. Corrosion testing of a simulated five-metal epsilon particle in spent nuclear fuel. Mat. Res. Soc. Symp. 2002, 713, 625-632. (15) Utsunomiya, S.; Ewing, R. C. The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO2 of the Oklo natural fission reactors. Radiochim. Acta 2006, 94, 749-753. (16) Yamanaka, S.; Kurosaki, K. Thermophysical properties of Mo-Ru-Rh-Pd alloys. J. Alloy Comp. 2003, 353, 269-273. (17) Middleburgh, S. C.; King, D. M.; Lumpkin, G. R. Atomic scale modelling of hexagonal structured metallic fission product alloys. R. Soc. Open Sci. 2015, 2, 140292. (18) Yun, D.; Ye, B.; Oaks, A. J.; Chen, W.; Kirk, M. A.; Rest. J.; Yacout, A. M.; Stubbins, J. F. Fission gas transport and its interactions with irradiation-induced defects in lanthanum doped ceria. Nucl. Instr. Meth. Phys. Res. B 2002, 272, 239-243. (19) Ziegler, J. F.; Biearsack, J. P.; Littmark, U. The stopping and range of ions in solids. Pergamon Press, New York, 1985; available at: http://www.SRIM.org/ (accessed October 1, 1987). (20) Guglielmetti, A.; Chartier, A.; van Brutzel, L.; Crocombette, J.-P.; Yasuda, K.; Meis, C.; Matsumura, S. Atomistic simulation of point defects behavior in ceria. Nucl. Instr. Meth. Phys. Res. B 2008, 266, 5120-5125. (21) Duwez, P.; Odell, F. Phase relationships in the system zirconia-ceria. J. Am. Ceram. Soc. 1950, 33, 274-283. (22) Hill, R. J.; Cranswick, L. M. D. International union of crystallography. Commission on powder diffraction. Rietveld refinement round robin. II. Analysis of monoclinic ZrO2. J. App. Cryst. 1994, 27, 802-844. (23) Ami, T.; Ishida, Y.; Nagasawa, N.; Machida, A.; Suzuki, M. Room-temperature epitaxial growth of CeO2 (001) thin films on Si (001) substrates by electron beam evaporation. Appl. Phys. Lett. 2001, 78, 1361-1363. (24) Cullity, B. D.; Stock, S. R. Elements of x-ray diffraction (third edition). Prentice Hall, Upper Saddle River, 2001, p 170.

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(25) Liang, S.; Chern, C. S.; Shi, Z. Q.; Lu, P.; Lu, Y.; Kear, B. H. Control of CeO2 growth by metalorganic chemical vapor deposition with a special source evaporator. J. Cryst. Growth 1995, 151, 359-364. (26) Kim, Y. J.; Thevuthasan, S.; Shutthanandan, V.; Perkins, C. L.; McCready, D. E.; Herman, G. S.; Gao, Y.; Tran, T. T.; Chambers, S. A.; Peden, C. H. F. Growth and structure of epitaxial Ce1-xZrxO2 thin films on yttria-stabilized zirconia (111). J. Elec. Spectr. Related Phenomena 2002, 126, 177-190. (27) Jiang, W.; Henager, C. H. Jr.; Varga, T.; Jung, H. J.; Overman, N. R.; Zhang, C.; Gou, J. Diffusion of Ag, Au and Cs implants in MAX phase Ti3SiC2. J. Nucl. Mater. 2015, 462, 310-320. (28) Jiang, W.; Zhang, J.; Edwards; D. J.; Overman; N. R.; Zhu, Z.; Price, L.; Gigax, J.; Castanon, E.; Shao, L.; Senor, D. J. Nanostructural evolution and behavior of H and Li in ion-implanted γ-LiAlO2. J. Nucl. Mater. 2017, 494, 411-421. (29) Wilson, R. G. SIMS quantification in Si, GaAs, and diamond – an update. Intern. J. Mass Spec. Ion Proc. 1995, 143, 43-49. (30) Mullins, D. R. The surface chemistry of cerium oxide. Surf. Sci. Rep. 2015, 70, 42–85. (31) Muller, A. D. Structure and bonding at the atomic scale by scanning transmission electron microscopy. Nat. Mater. 2009, 8, 263-270. (32) Bourgoin, J. C.; Corbett, J. W. Enhanced diffusion mechanisms. Rad. Eff. 1978, 6, 157-188. (33) Jiang, W.; Sundararajan, J. A.; Varga, T.; Bowden, M. E.; Qiang, Y.; McCloy, J. S.; Henager, C. H. Jr.; Montgomery, R. O. In situ study of nanostructural and electrical resistance of nanocluster films irradiated with ion beams. Adv. Funct. Mater. 2014, 24, 6210-6218. (34) Gao, Y.; Herman, G. S.; Thevuthasan, S.; Peden, C. H. F.; Chambers, S. A. Epitaxial growth and characterization of Ce1−xZrxO2 thin films. J. Vac. Sci. Technol. A. 1999, 17, 961-969. (35) Hellman, O. C.; Vandenbroucke, J. A.; Rüsing, J.; Isheim, D.; Seidman, D.N.; Analysis of threedimensional atom-probe data by the proximity histogram. Microsc. Microanal. 2000, 6, 437-444. (36) JNM 2005, 341, 25-30 (37) R. Devanathan, Molecular dynamics simulation of fission fragment damage in nuclear fuel and surrogate materials. MRS Adv. 2017, 2, 1225-1230.

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Table 1. Precipitates in Five-Metal Doped Ceria through Ion Irradiation and Air Annealing sample

temperature (K)

fluence (He+/cm2)

peak dose (dpa)

duration (h)

precipitate

as-grown

823

0

0

4

none

as-irradiated

673

4×1017

~13

5

unirradiated/annealed

1073

0

0

10

unirradiated/annealed

1373

0

0

10

irradiated/annealed

1373

4×1017

~13

10

Pd; ~3 nm Mo, Pd and impurities; up to 8 nm Mo, Ru, Rh, Pd, Re and impurities; up to 70 nm Mo, Ru, Rh, Pd, Re and impurities; up to 70 nm

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Figure Captions: Figure 1. SRIM simulation of the He concentration and atomic displacement profiles in CeO2 implanted with 90 keV He+ ions at normal incidence. Figure 2. SEM image of a doped CeO2 film grown on a polycrystalline YSZ substrate across a shadow marked area. Figure 3. GIXRD pattern for a doped CeO2 film grown on a polycrystalline YSZ substrate. Figure 4. SEM images, EBSD phase maps and grain orientation maps for a polycrystalline YSZ substrate and a doped CeO2 film on YSZ. Figure 5. Histograms for the grain size distributions of cubic CeO2 in the film and cubic ZrO2 in the substrate from EBSD maps (Figure 4). Figure 6. ToF-SIMS mass spectrum for a doped CeO2 film grown on a polycrystalline YSZ substrate. Figure 7. (a) HAADF STEM micrograph of an as-grown CeO2 film (doped) and (b) STEM-EDS elemental maps of the film. Figure 8. Cross-sectional view of (a) SEM image of as-grown film (doped) at 823 K and low-resolution bright-field STEM images of (b) the film irradiated with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K, and the irradiated film annealed at (c) 1073 K and (d) 1373 K for 10 h each in air. Figure 9. (a) HAADF STEM micrograph of a doped CeO2 film irradiated with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K and (b) STEM-EDS maps of elements in the film. Figure 10. Atom maps of doped elements (Mo, Ru, Rh, Pd and Re) and impurities (H, Na, Si and Ca) in a CeO2 film annealed at 1073 K for 10 h in air. Figure 11. (a) Atom map of Pd and isoconcentration surfaces of Si at 10% and Mo at 7%, (b) and (c) concentration profiles of Mo, Pd, O and Na across grain boundaries, and (d) and (e) proximity histograms of various elements in selected particles in a doped CeO2 film annealed at 1073 K for 10 h in air. Figure 12. Bright-field STEM micrographs of a doped CeO2 film irradiated with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K and annealed at 1373 K for 10 h in air at (a) low, (b) medium, (c) high, and (d) the highest magnifications. Figure 13. (a) HAADF STEM micrograph of a doped CeO2 film annealed at 1373K for 10 h in air, and (b) STEM EDS elemental maps of the film. Figure 14. (a) HAADF STEM micrograph of a doped CeO2 film irradiated with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K and annealed at 1373K for 10 h in air, (b) a higher magnification of (a), and (c) STEM EDS elemental maps of the film. Figure 15. (a) HAADF STEM micrograph of a doped CeO2 film irradiated with 90 keV He+ ions to a fluence of 4×1017 He+/cm2 at 673 K and annealed at 1373K for 10 h in air, and (b) STEM EDS elemental maps of the film.

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TOC Graphic

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0.6

0.6 0.5 0.4 0.3

SRIM13 SIMULATION Ed(Ce) = 56 eV Ed(O) = 27 eV

0.5

Atomic Displacement (dpa)/(1016 He/cm2)

He Distribution (at.% He)/(1016 He/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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(372, 0.57)

0.4

CeO2 90 keV He+ Normal Incidence

0.3 (324, 0.32)

0.2

0.2 0.1

0.1 0.0 0

100

200

300

400

Depth (nm)


500

0.0 600

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


YSZ Substrate

Doped CeO2 Film

100 µm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Intensity (counts) 25000

20000

15000

10000

5000

0 20 30

Two Theta (deg.) 40

50


60 70

CeO2(331) CeO2(420)

CeO2(400)

CeO2(311) CeO2(222) Cubic ZrO2(311)

CeO2(220) Cubic ZrO2(220)

Cubic ZrO2(111) Monoclinic ZrO2 (111) Cubic ZrO2(200) Monoclinic ZrO2 (200) Monoclinic ZrO2 (112)

Monoclinic ZrO2 (110) CeO2(111), Monoclinic ZrO2 (111) The Journal of Physical Chemistry Page 18 of 30

80

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Normalized Relative Frequency


30

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Mean Grain Diameter (exluding grains smaller than 0.75 µm) Cubic CeO2: 1.6 µm Cubic ZrO2: 2.5 µm Binned at 0.25 µm

20

10

0 0

1

2

3

4

5

6

Grain Diameter (µm) ACS Paragon Plus Environment

7

8

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1000000

SIMS Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


100000

Mo,Re,Ru,R and Pd doped CeO2

10000 1000 100

CeO+ Ce+ CeO2+

Ru+ Mo+ Pd+ Rh+

Re+

10 1 85 90 95 100 105

140150 160 170180 190

Mass-to-Charge State Ratio ACS Paragon Plus Environment


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(a)

O

(b)

Pd

Page 22 of 30

Mo

Ru

Ce

Re


Rh

Page 23 of 30

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(a)

(b) C protection layer

Irradiated Region

CeO2

As-grown

YSZ

1 µm

As-irradiated

1 µm

(d)

(c)

1 µm

1 µm

Irrad. & 1073 K Ann.

Irrad. & 1373 K Ann.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(a)

O

Pd

(b)

Page 24 of 30

Mo

Ru

Ce

Re


Rh

Page 25 of 30

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Page 26 of 30

Page 27 of 30

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 28 of 30

Mo

Ru

Pd

Ce

Re

Na

Al

Si

O

(b)

Rh

(a)


Ca

Page 29 of 30

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(a)

(c)

Irradiated

(b)

O

Mo

Ru

Pd

Ce

Re


Rh


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 30 of 30

Mo

Ru

Pd

Ce

Re

Na

Al

Si

O

(b)

Rh

(a)


Ca

Nanoparticle Precipitation in Irradiated and Annealed Ceria Doped

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