Table 1 - uploaded by Assel Aitkaliyeva
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Preparation of highly radioactive and irradiated nuclear fuels and materials for transmission electron microscopy (TEM) is accompanied with a set of unique challenges. The paper evaluates three specimen preparation techniques for preparation of irradiated materials and determines which technique yields to the most reliable characterization of radia...
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... ent contrast and lines going across micrographs depict thickness variations in the specimen. Table 1 summarizes PIPS-2 parameters, which were selected in preparation of this specimen. The specimen was initially thinned in FIB system at 30 keV to the thickness of 220 nm and then post-pro- cessed in PIPS-2 unit to the thickness of 50 nm. ...
Citations
... To track the precipitation sequence, one prepares samples which are heated to the temperature corresponding to the desired exothermic or endothermic peaks at the same rate, before quenching the sample. Subsequently, approximately 100 µm thin foils are prepared for TEM investigations at each temperature of interest [17,18]. This methodology is herein referred to as "ex situ TEM from bulk heating" analysis in this work, and necessitates a new sample for each temperature (DSC peak) of interest. ...
... The Ga ion beam could generate these defects and defect clusters during FIB sample preparation. [74][75][76] At 600 and 800 • C, these defects may have some influence; however, they are considerably more significant at 1000 • C. The dislocation loops grew much quicker at 1000 • C than at lower temperatures at the expense of the number density. The dislocation number density increased remarkably when the ion beam was first turned on, and then it decreased with continued exposure to the ion beam. ...
The early stage of microstructural evolution of ThO2, under krypton irradiation at 600, 800, and 1000°C, was investigated using in situ transmission electron microscopy (TEM). Dislocation loops grew faster, whereas their number density decreased with increasing irradiation temperature. Loop density was found to decrease with ion dose. Interstitial dislocation loops, including Frank loops with Burgers vector of a/3〈111〉 and perfect loops with Burgers vector of a/2〈110〉, were determined by traditional TEM and atomic resolution–scanning TEM techniques. Atomistic and mesoscale level modeling are performed to interpret experimental observations. The migration energy barriers of defects in ThO2 were calculated using density‐functional theory. The energetics of different dislocation loop types were studied using molecular dynamics simulations. Loop density and diameter were analyzed using a kinetic rate theory model that considers stoichiometric loop evolution. This analysis reveals that loop growth is governed by the mobility of cation interstitials, whereas loop nucleation is determined by the mobility of anion defects. Lastly, a rate theory model was used to extract the diffusion coefficients of thorium interstitials, oxygen interstitials, and vacancies.
... Therefore, as a post-FIB processing step, nanomilling can not only remove a surface amorphous layer and implanted Ga + layer effectively but avoids additional damage and contamination [136]. The final TEM specimen will be clean, thin (<100 nm), and flat [137,138]. Figure 14 shows a clear comparison between two TEM specimens with and without the nanomilling as post-FIB processing step. The specimen that went through 500 V nanomilling exhibited clear lattice fringes even at the edge of the sample, while the specimen that went through 5 kV FIB polishing showed obvious redeposition and damaged layers which impeded imaging and energy spectrum analysis [134]. ...
Interfaces such as grain boundaries (GBs) and heterointerfaces (HIs) are known to play a crucial role in structure-property relationships of polycrystalline materials. While several methods have been used to characterize such interfaces, advanced transmission electron microscopy (TEM) and scanning TEM (STEM) techniques have proven to be uniquely powerful tools, enabling quantification of atomic structure, electronic structure, chemistry, order/disorder, and point defect distributions below the atomic scale. This review focuses on recent progress in characterization of polycrystalline oxide interfaces using S/TEM techniques including imaging, analytical spectroscopies such as energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) and scanning diffraction methods such as precession electron nano diffraction (PEND) and 4D-STEM. First, a brief introduction to interfaces, GBs, HIs, and relevant techniques is given. Then, experimental studies which directly correlate GB/HI S/TEM characterization with measured properties of polycrystalline oxides are presented to both strengthen our understanding of these interfaces, and to demonstrate the instrumental capabilities available in the S/TEM. Finally, existing challenges and future development opportunities are discussed. In summary, this article is prepared as a guide for scientists and engineers interested in learning about, and/or using advanced S/TEM techniques to characterize interfaces in polycrystalline materials, particularly ceramic oxides.
... In the case when bulk information about precipitates forming well-dispersed in the microstructure is desired or large area of interest for good statistics is required, thin-foil specimen prepared by twin-jet electro-polishing method is the most efficient. The general procedures for preparing thin foils from bulk metal samples are [51,52] : (i) cutting the bulk material into approximately 0.3-0.5 mm thickness slices, grinding and polishing on different grade silicon carbide papers to approximately 50-200 lm thickness, while ensuring that the damaged layer can be fully removed in the subsequent electro-polishing; (ii) punching out 3 mm-diameter disks; (iii) finally twin-jet electro-polishing using appropriate conditions for the studied material, i.e., proper electrolyte, voltage and temperature in a dedicated electro-polishing equipment, for instance, a Struers Tenupol-5 or Fischione 110 polisher (the procedure is shown schematically in Figure 2a). In some cases, other techniques may be preferred for specimen preparation to study precipitation in bulk metallic materials. ...
... [81] In addition, similar as described for bulk thin foils earlier, a combination of FIB-based lift-out technique with post-processing in BIB has been also proposed to minimize or eliminate this damage and ensure high quality analysis results. [52,63,64,82] This combination is particularly useful since only the most advanced FIB systems are capable of reducing the ion energy below 2 keV using energetic Ga þ ions. An additional advantage of combining these two techniques is the reduction of specimen preparation time. ...
Precipitation hardening is one of the most important strengthening mechanisms in metallic materials, and thus, controlling precipitation is often critical in optimizing mechanical performance. Also other performance requirements such as functional and degradation properties are critically depending on precipitation. Control of precipitation in metallic materials is, thus, vital, and the approach presently in the limelight for this purpose is an integrated approach of theory, computations and experimental characterization. An empirical understanding is essential to build physical models upon and, furthermore, quantitative experimental data is needed to build databases and to calibrate the models. The most versatile tool for precipitation characterization is the transmission electron microscope (TEM). The TEM has sufficient resolving power to image even the finest precipitates, and with TEM-based microanalysis, overall quantitative data such as particle size distribution, volume fraction and number density of particles can be gathered. Moreover, details of precipitate structure, morphology and chemistry, can be revealed. TEM-based postmortem and in situ analysis of precipitation has made significant progress over the last decade, largely stimulated by the widespread application of aberration corrected microscopes and accompanying novel analytics. The purpose of this report is to review these recent developments in precipitation analysis methodology, including sample preparation. Application examples are provided for precipitation analysis in metals, and future prospects are discussed.
... With the increasing popularity of focused ion beam (FIB)based TEM sample preparation, new insight into fuel-cladding interaction has been gained. Readers should be aware of the FIB limitations and associated artifacts such as ion-beam induced defects, curtaining, and amorphization [37] and should therefore research methodology to minimize said artifacts. For example, some of the artifacts can be eliminated with cleaning steps at lower currents using FIB, using the precision ion polishing system (PIPS) upon completion of FIB milling, and utilizing flash-polishing techniques. ...
The microstructure of fuels evolves with irradiation and extensive microstructural characterization has been performed both on as-fabricated and irradiated U-Mo fuels for both dispersion and monolithic designs since initiation of the program to transition research reactors around the world from using high enriched uranium to low enriched uranium fuels. Developing mechanistic understanding of the U-Mo fuel microstructure and properties, which ultimately govern fuel performance during reactor operation, is an important part of the fuel qualification effort. With recent progress in implementation of modern characterization techniques in nuclear fuels research, a critical review of these efforts is warranted. In this review article, the authors provide a compilation and impartial assessment of the available experimental data involving microstructural evolution of U-Mo fuels, gaps in fundamental understanding of irradiation-induced phenomena, and outline path forward for fuels research. This article is intended as a resource for fuel designers and modelers that recaps recent research trends and a guide for prioritizing future experimental work on U-Mo fuels.
... Methods and precautions necessary to prepare lift-outs specimens from SNF using FIB-SEM have been described by Teague et al. 10 and Aitkaliyeva et al. 51 In this case, the SEM mounts containing the polished spent fuel fragments were mounted in specially designed metallic blocks that reduced the dose during handling. These blocks also contained insertion points for placing the lift-out row-holder. ...
We have made observations of noble metal phase fission-product agglomerates and gaseous xenon within the fuel-cladding interaction (FCI) zone of a high-burnup UO2 fuel. The FCI is the boundary between the UO2 pellet outer surface and the inner wall of the oxidized Zr-liner/cladding of the fuel rod. These fission-product agglomerates are well known to occur within the spent fuel matrix, and although radionuclides have been reported by others, we reveal aspects of their speciation and morphology. That they occur as discrete particles in the oxidized Zr liner, suggests the occurrence of hitherto unknown processes in the FCI zone during reactor operation, and this may have implications for the long-term storage and disposal of these types of materials. As expected, the particle agglomerates, which ranged in size from the nanometer scale to the micrometer scale, contained mainly Mo, Ru, Tc, Rh, and Pd; however, we also found significant quantities of Te associated with Pd. Indeed, we found nanometer scale separation of the distinct Pd/Te phase from the other fission products within the particles. Often associated with the particles was concentrations of uranium, sometimes appearing as a “cloud” with a tail emanating from the fuel into the oxidized cladding liner. Many of the noble metal phase particles appeared as fractured clusters separated by Xe-gas-filled voids. Possible mechanisms of formation or transport in the cladding liner are presented.
... Note that preferential milling of the material in the FIB, combined with topography of the specimen (such as porosity of the irradiated fuel) can lead to appearance of curtaining/ streaking artefacts. Readers are encouraged to read about various strategies for curtaining minimization in the following reference (Aitkaliyeva et al., 2015). It is thus recommended to minimize such sample preparation artefacts prior to segmentation of fission gas pores using FIB curtaining removal tool. ...
Irradiation of low enriched uranium-molybdenum fuel results in the production and agglomeration of fission gas bubbles that can potentially lead to fuel failure. Manual point volume fraction counting in accordance with ASTME562 standard has been historically used to conduct pore size distribution analysis. While effective, the manual methodology is not efficient and therefore not feasible for the characterization of several fuel plates in a timely manner. In this contribution, ImageJ and MATLAB software were investigated as suitable alternatives to manual counting. Validation and verification were performed to show that the results are reproducible. Image analysis revealed insignificant variation of fission gas pore morphology with fission density. In addition, the results from two different sample preparation techniques – vibratory polishing and focused ion beam milling were compared. Sample preparation has more than 1% influence on the results of pore size distribution analysis. Comprehensive comparison identified vibratory polishing as the preferred method for conducting fission gas pore size distribution analysis.
... Studies have shown that as fission density increases in fuel, so does porosity. Specifically, at around 4.5 × 10 21 fissions/cm 3 , depending on the materials, the fuel starts to undergo a recrystallization process (Aitkaliyeva et al., 2015). Following that recrystallization, porosity grows rapidly, aided by increased grain boundary area providing more sites for fission gas pore nucleation. ...
... Following that recrystallization, porosity grows rapidly, aided by increased grain boundary area providing more sites for fission gas pore nucleation. The result is a microstructure referred to as High Burn-up Structure (HBS) characterized by relatively small grains and increased average pore size and pore density (Aitkaliyeva et al., 2015;Collette et al., 2016a). Therefore, the modeling of formation and growth of fission gas porosity, as well as the characterization of post irradiation porosity, has become a primary concern for the qualification of U-Mo research reactor fuel (Wachs, 2007). ...
... Research is being conducted to characterize U-Mo plate fuel porosity development and growth employing the generic metallic sample preparation methods along with FIB sectioning and surface preparation Aitkaliyeva et al., 2015;Casella et al., 2017). FIB sample preparation techniques possess several drawbacks. ...
Uranium-Molybdenum (U-Mo) low enriched uranium (LEU) fuels are a promising candidate for the replacement of high enriched uranium (HEU) fuels currently in use in a high power research and test reactors around the world. Contemporary U-Mo fuel sample preparation uses focused ion beam (FIB) methods for analysis of fission gas porosity. However, FIB possess several drawbacks, including reduced area of analysis, curtaining effects, and increased FIB operation time and cost. Vibratory polishing is a well understood method for preparing large sample surfaces with very high surface quality. In this research, fission gas porosity image analysis results are compared between samples prepared using vibratory polishing and FIB milling to assess the effectiveness of vibratory polishing for irradiated fuel sample preparation. Scanning electron microscopy (SEM) imaging was performed on sections of irradiated U-Mo fuel plates and the micrographs were analyzed using a fission gas pore identification and measurement script written in MatLab. Results showed that the vibratory polishing method is preferentially removing material around the edges of the pores, causing the pores to become larger and more rounded, leading to overestimation of the fission gas porosity size. Whereas, FIB preparation tends to underestimate due to poor micrograph quality and surface damage leading to inaccurate segmentations. Despite the aforementioned drawbacks, vibratory polishing remains a valid method for porosity analysis sample preparation, however, improvements should be made to reduce the preferential removal of material surrounding pores in order to minimize the error in the porosity measurements.
... While there is some FIB damage in some regions (Fig. 2c), there is almost no damage in another regions (Fig. 2d). Aitkaliyeva et al. [30] have shown that careful cleaning with 2 kV ions can reduce the damage, but minor FIB damage is inevitable. Among the investigated foils, even though most of the foils are clean, some regions were found to have FIB defects having the size less than~4 nm and the density less than 10 21 m À3 . ...
FeCrAl ferritic alloys are excellent cladding candidates for accident tolerant fuel systems due to their high resistance to oxidation as a result of formation of a protective Al2O3 scale at high temperatures in steam. In this study, we report the irradiation response of the 10Cr and 13Cr FeCrAl cladding tubes under Fe²⁺ ion irradiation up to ∼16 dpa at 300 °C. Dislocation loop size, density and characteristics were determined using both two-beam bright field transmission electron microscopy and on-zone scanning transmission electron microscopy techniques. 10Cr (C06M2) tube has a lower dislocation density, larger grain size and a slightly weaker texture compared to the 13Cr (C36M3) tube before irradiation. After irradiation to 0.7 dpa and 16 dpa, the fraction of <100> type sessile dislocations decreases with increasing Cr amount in the alloys. It has been found that there is neither void formation nor α′ precipitation as a result of ion irradiations in either alloy. Therefore, dislocation loops were determined to be the only irradiation induced defects contributing to the hardening. Nanoindentation testing before the irradiation revealed that the average nanohardness of the C36M3 tube is higher than that of the C06M2 tube. The average nanohardness of irradiated tube samples saturated at 1.6–2.0 GPa hardening for both tubes between ∼3.4 dpa and ∼16 dpa. The hardening calculated based on transmission electron microscopy was found to be consistent with nanohardness measurements.
