Search

Covalent organic frameworks (COFs) have potential applications in immobilized enzyme fields due to their unique chemical and mechanical properties. First, a novel COF material for immobilizing enzyme for preparing ginsenosides was obtained using 1,3,5-tricarbaldehyde and 3,3′-dihydroxybenzidine （BTDH）as organic monomers. Then the morphology, stability, pore size and the loading capacity of COF-BTDH were investigated using transmission electron microscopy, Fourier transform infrared (FT-IR), and loading capacity experiments. At last effect of reaction time on ginsenoside extraction efficiency and the maximum usage amount of material were explored. Results showed that pectinase was effectively immobilized on COF-BTDH. In addition the optimized reaction time of catalytic hydrolysis for extracting ginsenosides Rb1 and Rd was 2 h, and the mass ratio of material to enzyme was 31.25:1. Additionally, the immobilized enzyme remained more than 85% active according to the amount ratio of ginsenoside Rd after five consecutive cycles. [Display omitted] • A COF material with dihydroxybenzidine (BTDH) as monomer was used for immobilizing pectinase. • The mass ratio of material to enzyme was 31.25:1 for extracting ginsenoside Rb1 and Rd. • The immobilized pectinase remained more than 85% active after five consecutive cycles. [ABSTRACT FROM AUTHOR]

• Two CCAs demonstrated reduced void swelling compared to less complex materials. • Increasing temperature from 500 to 600 °C slightly reduced swelling levels in CCAs. • Swelling levels increased significantly with temperature in pure Ni and Fe 56 Ni 44. • Measured stacking-fault energy (SFE) increased with Mn-content between CCAs. • Increased swelling corresponded to lower faulted loop sink strength, higher SFE. This work attempts to link the microstructural evolution of single-phase compositionally complex alloy (CCA) compositions under dual-beam irradiation to their Mn-content via the stacking-fault energy (SFE) and vacancy migration energies. Two alloys, Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35 , along with less compositionally complex pure Ni and Fe 56 Ni 44 binary were irradiated at 500 and 600 °C under dual-beam 1 MeV Kr2+ and 16 keV He+ heavy-ions up to 7 displacements per atom (dpa) with a He/dpa ratio of 0.75 %/dpa using in situ transmission electron microscopy (TEM). Due to the bubble-stabilizing effect of implanted He, bubbles were observed in all irradiations, and populations of faulted interstitial loops were characterized in Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35. A reduction in swelling was observed in the two CCAs compared to pure Ni and Fe 56 Ni 44. Although swelling increased from 500 to 600 °C in Fe 56 Ni 44 , Cr 15 Fe 35 Mn 15 Ni 35 and Cr 18 Fe 27 Mn 27 Ni 28 both swelled slightly more at 500 °C. This was attributed to the difference in vacancy mobility, stronger pinning effect of vacancies on He, and the sink strength of faulted dislocation loops. Faulted interstitial loops nucleated with a higher number density and dislocation line density in Cr 15 Fe 35 Mn 15 Ni 35 at both temperatures, and at 600 °C in both materials. The differences in faulted loop population and temperature effect on swelling are correlated to the Mn-content and the measured SFE (20.2 ± 6.7 mJ/m2 for Cr 18 Fe 27 Mn 27 Ni 28 and 9.2 ± 3.4 mJ/m2 for Cr 15 Fe 35 Mn 15 Ni 35). [Display omitted] [ABSTRACT FROM AUTHOR]

Irradiation induced microstructural changes in the NF709 austenitic stainless steel were investigated under 1 MeV Kr ion irradiations to a dose of 3 dpa at room temperature (RT), 300, 400, 500 and 600 °C, and a dose of 20 dpa at RT. The irradiation-induced defects and the stability of precipitates were characterized with transmission electron microscopy (TEM). Frank dislocation loops were observed in all the irradiated samples, and the loop sizes were much larger at 600 °C than those at lower temperatures. “Raft” defect structures, formed through self-alignment of small dislocation loops, were also observed in all irradiated samples. M 23 C 6 precipitates were amorphorized under irradiations at RT and 300 °C, but remained to be crystalline at 400 °C and above. MX precipitates were stable under irradiations at RT up to 20 dpa, and at temperatures below 600 °C to 3 dpa. At 600 °C, some MX precipitates were observed to dissolve during in-situ irradiation, suggesting possible precipitate instability at this irradiation temperature. [ABSTRACT FROM AUTHOR]

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500 °C, and after irradiation, two of them were annealed for 1 h at 600 °C and 700 °C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600 °C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700 °C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700 °C PIAed sample, but not in the 600 °C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700 °C than at 600 °C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations. [ABSTRACT FROM AUTHOR]

Fe and Fe-Cr (Cr = 10–16 at.%) specimens were neutron-irradiated at 300 °C to 0.01, 0.1 and 1 dpa. The TEM observations indicated that the Cr significantly reduced the mobility of dislocation loops and suppressed vacancy clustering, leading to distinct damage microstructures between Fe and Fe-Cr. Irradiation-induced dislocation loops in Fe were heterogeneously observed in the vicinity of grown-in dislocations, whereas the loop distribution observed in Fe-Cr is much more uniform. Voids were observed in the irradiated Fe samples, but not in irradiated Fe-Cr samples. Increasing Cr content in Fe-Cr results in a higher density, and a smaller size of irradiation-induced dislocation loops. Orowan mechanism was used to correlate the observed microstructure and hardening, which showed that the hardening in Fe-Cr can be attributed to the formation of dislocation loops and α′ precipitates. [ABSTRACT FROM AUTHOR]

Thermally-aged cast austenitic stainless steels (CASS) CF8 was irradiated with 1 MeV Kr ions at 300, 350 and 400 °C to 1.88 × 10 19 ions/m 2 (∼3 dpa) at the IVEM-Tandem Facility at the Argonne National Laboratory. Before irradiation, the distribution of G-phase precipitates in the ferrite showed spatial variations, and both their size and density were affected by the ferrite–austenite phase boundary and presence of M 23 C 6 carbides. Under 300 °C irradiation, in situ TEM observation showed G-phase precipitates were relatively unchanged in the vicinity of the phase boundary M 23 C 6 carbides, while the density of G-phase precipitates increased with increasing dose within the ferrite matrix. Coarsening of G-phase precipitates was observed in the vicinity of phase boundary M 23 C 6 carbides at 350 °C and 400 °C. [ABSTRACT FROM AUTHOR]

Abstract: A gradient structure was produced in a pure copper plate by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer was reduced to nanoscale and the grain size increased gradually along the depth of the treated sample. In situ transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observation was performed on the nanocrystalline copper after implantation of carbon. Carbon atoms first precipitated along the edges of the copper substrate or at the surface, then formed amorphous carbon layers. Subsequently, onion-like fullerenes were formed under electron-beam irradiation. The effects of ion implantation, electron beam irradiation, nanostructure of the substrate and interaction of C and Cu atoms on the formation of the onion-like fullerenes are discussed. [Copyright &y& Elsevier]

Abstract: In this paper, the microstructures of a thin-wall die-cast commercial magnesium (Mg) alloy, aged at 200°C for various times, were investigated using transmission electron microscopy. A martensite-like phase was observed. The crystal lattice parameters of this phase are a =0.30948nm, c =0.50498nm, and axial ratio c/a =1.6314. Orientation relationship of the martensite-like phase and the parent has been determined as , and or . The driving force of the possible martensitic transformation (MT) might came from the residual microstructure stresses left during die-cast process. [Copyright &y& Elsevier]

Abstract: By means of surface mechanical attrition treatment (SMAT), a nanocrystalline (NC) surface layer about 40μm was synthesized on an AZ91D Mg alloy. The microstructural features of the surface layer produced by SMAT were systematically characterized by optical microscopy (OM) observations, X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations, and hardness measurement was also carried out in order to examine the hardness variation along the depth. Experimental results show that the microstructure is inhomogeneous along the depth. In the region from top surface to about 10μm in depth, the grain size increases from about 50nm to 100nm. In the adjacent region of about 10–40μm in depth, the grain size increases from about 100nm to 400nm. The grain refinement can be attributed to the activity of dislocation and occurrence of dynamic recrystallization. Moreover, emergence of stacking faults in the grain interior is a novel discovery. After the SMAT, the micro-hardness of the surface layer was enhanced significantly compared with that of the original sample. [Copyright &y& Elsevier]

To study the helium effects and their interactions with displacement damage in irradiated materials, an in situ dual-ion beam irradiation capability with transmission electron microscopy was recently established at the IVEM-Tandem Facility at Argonne National Laboratory. Pure nickel was irradiated in situ individually with 1 MeV Kr ions for displacement damage study, 16 keV He ions for helium implantation, and simultaneously with 1 MeV Kr ions and 16 keV He ions at the rate of 1 at.% He/dpa to study the interaction of helium and displacement damage at 500 °C. Helium bubbles were formed and segregated within dislocation loops during the He implantation experiment and no bubbles were observed outside the loops. The loop size increased with increasing helium concentration. In contrast, helium bubbles were formed uniformly in the matrix when nickel was irradiated simultaneously with 1 MeV Kr ions and 16 keV He ions. It is suggested that the uniformly-distributed defect clusters formed under 1 MeV Kr ion irradiation serve as trapping sites of helium atoms resulting in homogeneous nucleation of helium bubbles under the dual-ion beam irradiation in nickel. • In situ TEM study of interaction of helium implantation and displacement damage under dual-ion beams. • Heterogenous distribution of helium bubbles was observed inside dislocation loops under 16 keV helium ion implantation. • Distinctly different distributions of helium bubbles and dislocation loops were observed by TEM in pure nickel under in situ single and dual-beam irradiations. [ABSTRACT FROM AUTHOR]

The shape and size of grains significantly impact the properties of polycrystalline materials. In particular, high temperature and radiation exposure in nuclear reactors can lead to considerable grain growth of UO 2 , thereby substantially modifying the fuel performance. Transmission electron microscopy (TEM) has been used to characterize the grain morphology of polycrystalline materials, but the manual analysis of TEM images is a time-consuming and labor-intensive process, which cannot meet the increasing demand for high-throughput data analytics. This study presents an automated approach we developed for characterizing grain morphology recorded in bright field TEM images during ion irradiations performed in situ. Our approach combines a machine learning model for detecting grain boundaries and a computer vision algorithm named CHAC for selecting well-labeled grains for statistical analysis. Using TEM images acquired from in-situ ion irradiation experiments on nanocrystalline UO 2 , we demonstrate that this automated approach can achieve comparable results to human experts while significantly reducing the analysis time. Moreover, the machine learning model functions as a "few-shot" model, requiring only a modest number of training images to perform effectively on a specific task. Consequently, researchers in need can efficiently train their own models following the procedures described in this study to automate grain morphology analysis of their own TEM images. [ABSTRACT FROM AUTHOR]

The thermal conductivity of UO 2 fuel needs to be high enough to dissipate the heat generated from the fission reaction. Since grain size affects thermal conductivity and grain size can evolve with irradiation, it is critical to understand in-reactor UO 2 grain growth. Most studies of grain growth in UO 2 are based on thermally driven processes at elevated temperatures. However, studies have shown that grain growth can occur even at cryogenic temperatures by ballistic processes. Such irradiation-induced grain growth in UO 2 is yet to be studied. Advanced in-situ Kr ion irradiation and transmission electron microscopy were systematically performed on nanocrystalline UO 2 thin films at temperatures ranging from 50 K to 1073 K; grain growth was observed at all temperatures. A combination of manual and machine learning techniques was used to measure and plot grain size evolution against irradiation fluence at various irradiation temperatures. The machine learning method has significantly improved the analysis efficiency and reduced human labors. The grain diameter data were fitted using classical grain growth and thermal spike models to describe grain growth kinetics with and without irradiation effect. Grain growth during low temperature irradiation (≤ 475 K) can be well described by the thermal spike model. Above 475 K, there were additional thermally assisted processes that further accelerate the grain growth. At the highest irradiation temperature about 1075 K, both irradiation-induced dislocation loops and cavities/bubbles were observed to form in the UO 2. The effects of irradiation-induced defects on grain growth kinetics are discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In order to understand cavity and bubble formation and growth in oxide nuclear fuel materials, ion beam irradiation experiments were conducted with two common fission gas species: Kr and Xe. Ceria (CeO2) was selected as a surrogate material for uranium dioxide (UO2) due to its many similar properties to UO2. Ion beam energies were chosen such that both cavities and gas bubbles structures were induced by ion irradiations. The ion irradiation experiments were carried out at 600°C, at which temperature, cavity/gas bubble structures are believed to be immobile in this material. Lanthanum (La) was chosen as a dopant in CeO2 to investigate the effect of impurities. The presence of La in the CeO2 lattice also introduces a predictable initial concentration of oxygen vacancies, similar to the introduction of oxygen vacancies by the existence of Pu3+ in MOX fuel . The influence of two La concentrations, 5% and 25%, were examined. The study focused on the high dose regime where cavity/gas bubble structures were clearly identifiable with their sizes and number densities readily measurable. Cavity/gas bubble coarsening by coalescence was identified with TEM (Transmission Electron Microscopy) characterizations of as-irradiated La doped CeO2 specimens. The results revealed that lanthanum trapping has significant influence on the cavity/bubble growth in the material lattice by comparing the cavity/gas bubble size distributions between 5% La doped ceria and 25% La doped ceria. Lattice and kinetic Monte Carlo calculations described in a previous work have provided insights to the interpretations of the experimental results . Solid state Xe precipitates were observed in low energy Xe implantation in 5% La doped ceria to a very high fluence of 1×1017 ions/cm2 at 600°C. The solid state Xe precipitate structures are represented by faceted morphology. Very similar observations of solid state/near solid state Xe bubbles were made by Nogita et al. in the outer region of UO2 pellet irradiated to a pellet average burnup of 49GWd/t . [Copyright &y& Elsevier]

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr 18 Fe 27 Mn 27 Ni 28 , Cr 15 Fe 35 Mn 15 Ni 35 , and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of displacements per atom (dpa) using in-situ transmission electron microscopy. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in face-centered cubic (FCC) CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect production is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr 18 Fe 27 Mn 27 Ni 28 and 10 nm in Cr 15 Fe 35 Mn 15 Ni 35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Single crystal CeO2, as a surrogate material to UO2, was irradiated with 500keV xenon ions at 800°C while being observed using in situ transmission electron microscopy (TEM). Experimental results show the formation and growth of defect clusters including dislocation loops and cavities as a function of increasing atomic displacement dose. At high dose, the dislocation loop structure evolves into an extended dislocation line structure, which appears to remain stable to the high dose levels examined in this study. A high concentration of cavities was also present in the microstructure. Despite high atomic displacement doses, the specimen remained crystalline to a cumulated dose of 5×1015 ions/cm2, which is consistent with the known stability of the fluorite structure under high dose irradiation. Kinetic Monte Carlo calculations show that oxygen mobility is substantially higher in hypo-stoichiometric UO2/CeO2 than hyper-stoichiometric systems. This result is consistent with the ability of irradiation damage to recover even at intermediate irradiation temperatures. [ABSTRACT FROM AUTHOR]