High resolution transmission electron microscopy of GeV heavy ion irradiated graphite

In this paper, we present some results relative to irradiations of natural and synthetic graphite with ∼1 GeV lead and uranium ions. High resolution transmission electron microscopy reveals no tracks in the bulk in either kinematic or diffraction contrast. Small surface patches are formed however random in distribution and proportional in number, but not equivalent to the total ion fluence. This suggests that some matter could be ejected from the entry and exit surfaces by electronic processes and are then redeposited on the surfaces.     

In situ characterization of micron-scale particles by nuclear microscopy

Nuclear microscopy is well suited for analyzing large particle populations as well as individual particles of interest in various types of collection samples. Unlike some commonly used in situ techniques, nuclear microscopy can quantitatively characterize particles embedded within the sample or particles that are too small to be reliably removed from their collection media. Additional advantages include its capability for the detection of minor and trace elements, its capability to simultaneously analyze multiple elements with little or no prior sample preparation, and its potential for automated analyses. Preliminary data are presented of particle size and composition measurements conducted on air filters. The potential of applying nuclear microscopy to the cataloging of micron-scale cometary remnants captured in low density aerogel collectors is also explored. Because of its importance to a variety of collection programs, an innovative high-throughput ion beam imaging and analysis system is being jointly developed by Sandia and Lawrence Livermore national laboratories for rapid quantitative particle characterization.     

Cross-section transmission electron microscopy of the ion implantation damage in annealed diamond

It has formerly been shown that low-damage levels, produced during the implantation doping of diamond as a semiconductor, anneal easily while high levels “graphitize” (above about 5.2 × 10¹⁵ ions/cm²). The difference in the defect types and their profiles, in the two cases, has never been directly observed. We have succeeded in using cross-section transmission electron microscopy to do so. The experiments were difficult because the specimens must be polished to ∼40 μm thickness, then implanted on edge and annealed, before final ion beam thinning to electron transparency. The low-damage micrographs reveal some deeply penetrating dislocations, whose existence had been predicted in earlier work.     

The influence of roughness on tribological properties of nuclear grade graphite

The influence of surface roughness on tribological properties of graphite IG-11 was investigated on a standard SRV tester. The experimental condition was selected as: 30 N normal load, room temperature and a 10 Hz frequency with different strokes. The experiments environments included helium and air. Five types of roughness were studied in the experiments. The experiments revealed that the surface roughness greatly affected the graphite friction behavior. When the friction surface was smooth, the friction coefficient was high because of intensive adhesion accompanied by many pits at the friction surface. When the friction surface was rough, the adhesion was very poor, but the wear was excessive and generated many graphite particles at the friction surface. These particles can separate the friction surfaces, which reduced the friction action between them. For very rough specimens, the friction coefficient decreased with sliding velocity at about 0.004 m/s and then increases gradually.     

Investigation of heavy ion tracks in polymers by transmission electron microscopy

Bulk samples and thin sections of polyethylene terephthalate (PET) and polyimide (PI) were irradiated with Se and Pb ions of 11.4 MeV per nucleon. The creation of latent tracks and structural changes were studied by means of transmission electron microscopy (TEM). In order to improve the radiation resistivity of the polymer under the electron beam and to enhance the contrast of the tracks, the samples were chemically stained with OsO₄ or RuO₄ before and/or after the ion irradiation. Depending on the preparation technique, the cross-sections of the tracks appear as circular objects of reduced or increased contrast. The diameter of the tracks depends on the ion species and on the sample preparation and varies between 9 and 17 nm.     

High energy electron induced displacement damage in silicon

New measurements of displacement damage factors for electron-irradiated (4 to 53 MeV) bipolar silicon transistors have extended the correlation between nonionizing energy loss and damage factors reported previously another three orders of magnitude downward, to cover a total of six decades. To first order, the correlation remains linear for both n- and p-type silicon, but deviations are observed and explained in terms of differences in the fraction of initial vacancy interstitial pairs that recombines. These differences correlate linearly with the low-energy component of the PKA spectrum. Deep level transient spectroscopy measurements show oxygen- and dopant-related defect levels as well as divacancies. Defect concentrations scaled linearly with gain degradation, and no differences were seen between electron and proton plus neutron irradiated material. The results are consistent with a damage mechanism involving migration of vacancies to form well-separated stable defects     

The configuration of atomic defects from high resolutton transmission electron microscopy

With current high resolution 100 kV electron microscopes the arrangements of atoms in projections of the structure of thin crystals may be seen directly with a resolution of 3–4 Å. This resolution has been shown to be sufficient for the direct imaging of metal atom positions in both well-ordered regions and around defects in crystals of many oxides, sulfides and minerals. The possibility of extending the application of these methods to the study of the atomic arrangement in defects in metals depends on the attainment of resolutions of 1.5–2 Å. This may best be done by the use of high resolution high voltage microscopes which could have the added advantages of showing increased contrast for defects and allowing thicker specimens to be used for the same high resolution.     

The aging of zirconium tritides: A transmission electron microscopy study

Young Zr tritides were investigated for aging times up to about 6 months' using analytical transmission electron microscopy. No isotopic differences between hydrides and tritides were seen as far as precipitation morphology and structures are concerned. Also, no low-temperature phase transitions were observed in the tritides γ and δ. ³He generated by tritium decay was found to precipitate in very small bubbles (1–2 nm in diameter and densities of about 5×10²³ m⁻³) which were first clearly visible after approximately 24 days of aging. In addition to the ³He bubbles, interstitial loop damage was observed. Acoustic emission techniques applied to 5 and 6 months old Zr-T samples did not reveal any above background acoustic activity.     

High-resolution transmission electron microscopy and electron backscatter diffraction in nanoscaled ferritic and ferritic-martensitic oxide dispersion strengthened-steels

For specific blanket and divertor applications in future fusion power reactors a replacement of presently considered reduced activation ferritic-martensitic (RAFM) steels as a structural material by suitable oxide dispersion strengthened ferritic-martensitic steels would allow a substantial increase of the operating temperature from ∼823 to about 923 K. Due to this reason the RAFM-alloy ODS-Eurofer has already been developed and produced with industrial partners. In the He-cooled modular divertor concept, where temperatures above 923 K will arise, an ODS-steel with a purely ferritic matrix is advantageous, because of missing phase transitions. Due to this reason, a special ferritic ODS-steel is being manufactured as well. In this work the microstructures of these two ODS-alloy types, analysed mainly by high resolution TEM are compared, with respect to different manufacturing processes. In addition first results of high resolution EBSD scans together with determined orientation maps of the RAFM steel ODS-Eurofer will also be presented.     

Damage, crack growth and fracture characteristics of nuclear grade graphite using the Double Torsion technique

The crack initiation and propagation characteristics of two medium grained polygranular graphites, nuclear block graphite (NBG10) and Gilsocarbon (GCMB grade) graphite, have been studied using the Double Torsion (DT) technique. The DT technique allows stable crack propagation and easy crack tip observation of such brittle materials. The linear elastic fracture mechanics (LEFM) methodology of the DT technique was adapted for elastic-plastic fracture mechanics (EPFM) in conjunction with a methodology for directly calculating the J-integral from in-plane displacement fields (JMAN) to account for the non-linearity of graphite deformation. The full field surface displacement measurement techniques of electronic speckle pattern interferometry (ESPI) and digital image correlation (DIC) were used to observe and measure crack initiation and propagation.Significant micro-cracking in the fracture process zone (FPZ) was observed as well as crack bridging in the wake of the crack tip. The R-curve behaviour was measured to determine the critical J-integral for crack propagation in both materials. Micro-cracks tended to nucleate at pores, causing deflection of the crack path. Rising R-curve behaviour was observed, which is attributed to the formation of the FPZ, while crack bridging and distributed micro-cracks are responsible for the increase in fracture resistance. Each contributes around 50% of the irreversible energy dissipation in both graphites.     

Investigation of kinetic recovery process in low dose neutron-irradiated nuclear graphite by thermal annealing

To investigate the kinetic recovery process of low dose neutron-irradiated graphite, nuclear-grade isotropic graphite IG-110U and ETP-10 were neutron irradiated using the JMTR up to 1.38 × 10²³ n/m² (E_n > 1 MeV) at ~473 K. In-situ measurement of macroscopic length was conducted during the isothermal and isochronal annealing process from room temperature up to 1673 K. From room temperature to 773 K for IG-110U, and to 1023 K for ETP-10, macroscopic lengths, lattice parameters, and unit cell volumes of both specimens recovered to their pre-irradiation values, and this recovery process subdivided into two stages. The activation energies of macroscopic volume recovery at 523–673 K and 673–773 K were determined to be ~0.22 eV and ~0.57 eV for IG-110U, respectively; ~0.13 eV and ~2.59 eV at 523–923 K and 923–1023 K for ETP-10, respectively. The migration of not only single interstitials but also interstitials dissociated from submicroscopic interstitial groups along basal planes followed by vacancy-interstitial recombination play a dominant role in the first stage. The second stage is suggested to proceed via the motion of carbon groups along basal planes for IG-110U, and migration of single interstitials along the c-axis for ETP-10. During 773 K or 1023 K up to 1673 K, macroscopic length continuously shrank with decreasing shrinking rate, even with a turnaround to swell at 1173 K for IG-110U.     

Precision dilatometer analysis of neutron-irradiated nuclear graphite recovery process up to 1673 K

Specimens of two kinds of isotropic nuclear graphite, IG-110U and ETP-10, were neutron-irradiated at fluence of 1.92 × 10²⁴ n/m² (E > 1.0 MeV) at 473 K. The recoveries of the macroscopic lengths of these specimens during isothermal and isochronal annealing at temperatures of up to 1673 K were investigated in a step-wise manner by using a precision dilatometer. The macroscopic lengths after isochronal annealing for 6 h at each temperature decreased gradually as the temperature was increased to 1673 K. The recovery trends of the c-axis and a-axis lattice parameters differed from one another, and from the macroscopic length recovery trends. For the IG-110U specimen, the activation energies (E_a) of macroscopic volume recovery corresponding to annealing at 523–773, 773–923, 923–1073, and 1073–1173 K were found to be 0.15, 0.34, 0.73, and 2.59 eV, respectively. For the ETP-10 specimen, the E_a corresponding to 523–923, 923–1223, and 1223–1373 K were determined to be 0.15, 0.46, and 2.19 eV, respectively. These results indicate that both graphite specimens underwent three or four stages of macroscopic length recovery between 523 K and the annealing temperatures at which their initial lengths were recovered. It is suggested that during the first stage recovery proceeded via the migration of single interstitials along the basal plane and the resulting V-I recombination. In the middle stages, recovery occurred due to the migration of interstitial groups such as C₂ along the basal plane, while in the last stage, it proceeded via through-layer migration of interstitials or migration of single vacancies.     

Statistical analysis of helium bubbles in transmission electron microscopy images based on machine learning method

Helium bubbles, which are typical radiation microstructures observed in metals or alloys, are usually investigated using transmission electron microscopy (TEM)....     

Characterization of phases in ‘crud’ from boiling-water reactors by transmission electron microscopy

This paper reports phases identified in samples of crud (activated corrosion products) from two commercial boiling-water reactors using transmission and analytical electron microscopy and selected-area electron diffraction. Franklinite (ZnFe₂O₄) was observed in both samples. Hematite (α-Fe₂O₃), crystalline silica (SiO₂), a fine-grained mixture of iron oxides probably including magnetite (Fe₃O₄), hematite (α-Fe₂O₃), and goethite (α-FeOOH), and an unidentified high-Ba, high-S phase were observed in one of the samples. Willemite (Zn₂SiO₄), amorphous silica, and an unidentified iron-chromium phase were observed in the other. Chloride-bearing phases were found in both samples, and are assumed to represent sample contaminants. Because of the small sample volumes and numbers of particles studied and the possibility of contamination, it is not clear whether the differences between the phases observed in the two crud samples represent actual differences in the assemblages formed in the reactors.     

Behavior of graphite in nuclear reactor stacks

The results of a study of radiation effects in graphite and the influence of those effects on the design of graphite stacks for nuclear reactors are discussed in this article. Several of the manifestations of these effects may lead to serious complications in reactor performance. Measures used to avert such complications are considered. A unified approach to the physical nature of the radiation effects in graphite is suggested for a broad range of elevated temperatures. The problem of preventing oxidation of graphite is approached in the light of the high temperatures prevailing in the graphite stacks of reactors of recent design.     

In situ studies of the decomposition of ammonium uranyl carbonate in an electron microscope

An electron microscope has been used to study the reduction of ammonium uranyl carbonate (AUC) to UO₂. The reactions could be followed both in the diffraction and the lattice image mode. The first intermediate obtained was amorphous UO₃. Depending on the experimental conditions, either crystalline UO₂ was formed directly from this phase, or several other intermediates were indicated.Properties like crystallinity and crystallite size of final UO₂ were found to be a function of experimental conditions. Comparisons with properties of UO₂ from the industrial process have been made.     

In situ luminescence qualification of radiation damage in aluminas: F-aggregation and Al colloids

Recent work for in situ sequential measurement of ion beam induced luminescence and surface electrical conductivity has identified a correlation between surface electrical degradation and the luminescence for aluminas and sapphire during 45 keV He ion bombardment. Detailed measurements for the initial stages of degradation where rapid changes in the luminescence emission bands occur, have now identified processes related to oxygen vacancy (F centre) aggregation and aluminium colloid production as precursors to measurable surface electrical degradation in the irradiated region. This understanding enhances the possibility of using ion beam induced luminescence as a potential monitoring tool for material evolution and insulator surface degradation during irradiation, not only in ITER and future fusion devices, but also in present experimental reactor materials test programmes.     

Transport of caesium through a nuclear graphite. part I. Microgravimetric studies and electron probe microanalysis

Adsorption of caesium (≈40 mg/g maximum) on a gilsonite graphite at 473 K and subsequent desorption in the range 973–1373 K have been followed in a vacuum microbalance and caesium concentration profiles in the graphites have been measured by electron probe microanalysis at stages in the adsorption-desorption programme. An analysis is developed which enables apparent diffusion coefficients, D, to be determined from desorption rates. Electron probe microanalysis provides evidence for two-phase or multi-phase diffusion of caesium in the graphites during adsorption and desorption and there is evidence that diffusion of caesium is sensitive to the thermal history of graphite. The variation of all reported values of D (m²/s) with temperature is log $\text{D = ?6.34 ?}\text{5450}\text{T}$. The limitations of the method of analysis due to nonuniform caesium concentrations, finite surface exit resistance and multi-phase diffusion are also considered.     

Investigation of the nature of stage-IE interstitials in fcc metals by means of transmission electron microscopy

This paper reports on the use of transmission electron microscopy (TEM) for discriminating between the crowdion version of the two-interstitial model and other models of radiation damage. The method is based on the specific prediction of the crowdion model that, as a function of the irradiation temperature, vacancy clusters in heavy-ion-bombarded metals should show an increase in size within an extremely narrow temperature interval around the temperature at which the metastable crowdions undergo thermal conversion into the stable stage-III interstitial configuration. This was found in Cu⁺-bombarded Ni, Cu, and Ag, whereas the cluster sizes in Au are independent of the irradiation temperature. These results confirm the crowdion model for Ni, Cu, and Ag as well as the long-standing prediction that crowdions in Au are thermally converted below the temperature at which they would become mobile.