Oxidation behavior of IG and NBG nuclear graphites

This work studies the oxidation-induced characteristics of four nuclear graphites (NBG-17, NBG-25, IG-110, and IG-430). The oxidation characteristics of the nuclear graphites were measured at 600 °C. The surface properties of the oxidation graphites were characterized by means of scanning electron microscopy, X-ray photoelectron spectroscopy, and contact angle methods. The N₂/77 K adsorption isotherm characteristics, including the specific surface area and micropore volume, were investigated by means of BET and t-plot methods. The experimental results show an increase in the average pore size of graphites; they also show that oxidation produces the surface functional groups on the graphite surfaces. The surface area of each graphite behaves in a unique manner. For example the surface area of NBG-17 increases slightly whereas the surface area of IG-110 increases significantly. This result confirms that the original surface state of each graphite is unique.     

Deuterium retention in plasma spray tungsten coatings exposed to low-energy, high flux D plasma

Two types of porous plasma spray tungsten coatings deposited onto stainless steel and graphite substrates were exposed to low-energy (76 eV ), high-flux (10²² D/m² s) D plasma to ion fluences of (3-4) × 10²⁶ D/m² at various temperatures. Deuterium retention in the W coatings was examined by thermal desorption spectroscopy and the D(³He,p)⁴He nuclear reaction, allowing determination of the D concentration at depths up to 7 μm. The relatively high D concentration (above 0.1 at.%) at depths of several micrometers observed after D plasma exposure at 340-560 K can be related to accumulation of D₂ molecules in pores, while at temperatures above 600 K deuterium is accumulated mainly in the form of D atoms chemisorbed on the inner pore surfaces. At exposure temperatures above 500 K, the D retention in the plasma spray W coating on graphite substrate increases significantly due to trapping of diffusing D atoms at carbon dangling bonds located at the edge of a graphite crystallite.     

Erosion of Pyrolytic Graphite and Ti-doped Graphite due to High Flux Irradiation

The erosion of pyrolytic graphite and titanium doped graphite RG-Ti above 1,780K was investigated by 5keV Ar beam irradiation with the flux from 4x10¹⁹ to 1x10²¹ m^?2·s^?1. The total erosion yields were significantly reduced with the flux. This reduction would be attributed to the reduction of RES (radiation enhanced sublimation) yield, which was observed in the case of isotropic graphite with the flux dependence of RES yield of φ^?0.26 (φ: flux) obtained in our previous work. The yield of pyrolytic graphite was roughly 30% higher than that of isotropic graphite below the flux of 10²⁰ m^?2·s^?1 whereas each yield approached to very close value at the highest flux of 1x10²¹ m^?2·s^?1. This result indicated that the effect of graphite structure on the RES yield, which was apparent in the low flux region, would disappear in the high flux region probably due to the disordering of crystal structure.

In the case of irradiation to RG-Ti at 1,780K, the surface undulations evolved with a mean height of about 3μm at 1.2×10²⁰ m^?2·s^?1, while at higher flux of 8.0×10²⁰ m^?2·s^?1 they were unrecognizable. These phenomena can be explained by the reduction of RES of graphite parts excluding Tic grains.     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

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.     

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10^?2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH₄ during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.     

Deuterium trapping in deep traps of differently oriented pyrolytic graphite exposed to D2 gas at 1473 K

Due to their importance for tritium inventories in future DT fueled fusion machines, experimental data on H isotope diffusion, absorption and retention in deep traps (E_b ≅ 4.3 eV) of graphites exposed to hydrogen at elevated temperatures have been reviewed. Deuterium retention was studied in edge- and basal-oriented pyrolytic graphite (PG) and polycrystalline RG-Ti-91 damaged by irradiation with 200 keV carbon ions. Deuterium loading was done by soaking in D₂ gas at 1473 K, and the resulting D retention was measured by nuclear reaction analysis. The microstructure was studied by cross-sectional TEM, SEM and microprofilometry. The concentration of strong traps created by irradiation and estimated by the amount of accumulated deuterium was shown to saturate with the damage above ≈1 dpa at about 1000 appm. In non-damaged and damaged graphites deuterium diffuses via porous grain boundaries and along basal planes within crystallites, while its migration through the graphite lattice along the c direction was found to be negligible. Radiation modifications of PG retard deuterium diffusion and decrease the rate of its chemical erosion by a factor of five. The amount of deuterium accumulated in strong traps in graphites is mainly influenced by their macro- and microstructure, while the degree of graphitization seems to be less important. Derivations are made of the susceptibility of damaged graphites, in particular, CFCs to the retention of hydrogen isotopes in deep traps.     

MeV He+ bombardment of silicon carbide and carbon

Samples of carbon, graphite, and silicon carbide have been bombarded with 0.6 and 3.5 MeV He⁺ to doses ranging from 0.03 C/cm² to 1.5 C/cm² (ion currents ranged from 10 μA/cm² to 100 μA/cm²). All samples experience surface uplifting. At 0.6 MeV, blisters form on near-theoretical density silicon carbide whereas on permeable graphite the surface appears to expand outward by separating along the existing pore and crack structure. These surface features are not observed for bombardment at 3.5 MeV. For larger doses, large effective sputtering coefficients for silicon carbide result from flaking off of successive layers. Smaller erosion rates are observed for a pyrolytic carbon with intermediate porosity, and no net erosion has been observed for the permeable graphite. Surface porosity is thought to play an important role in determining the effects of the ion bombardment.     

Erosion of carbon fiber composites under high-fluence heavy ion irradiation

The ion-induced erosion, determining by sputtering yield Y and surface evolution including structure and morphology changes of the modified surface layers, of two commercial carbon fiber composites (CFC) with different reinforcement - KUP-VM (1D) and Desna 4 (4D) have been studied under 30 keV Ar⁺ high fluence (φt ∼ 10¹⁸-10²⁰ ion/cm²) irradiation in the temperature range from room temperature to 400 °C. Ion-induced erosion results in the changes of carbon fiber structure which depend on temperature and ion fluence. Monitoring of ion-induced structural changes using the temperature dependence of ion-induced electron emission yield has shown that for Desna 4 and KUP-VM at dynamic annealing temperature Т_а ≈ 170 °С the transition takes place from disordering at T < T_a to recrystallization at T > T_a. The annealing temperature Т_а is close to the one for polycrystalline graphites. Microscopy analysis has shown that at temperatures Т < T_a the etching of the fibers results in a formation of trough-like longitudinal cavities and hillocks. Irradiation at temperatures T > T_a leads to a crimped structure with the ribs perpendicular to fiber axis. After further sputtering of the crimps the fiber morphology is transformed to an isotropic globular structure. As a result the sputtering yield decreases for Desna 4 more than twice. This value is almost equal to that for KUP-VM, Desna 4, polycrystalline graphites and glassy carbons at room temperature.     

Electrical properties of annealed, fully metamict REE2FeBe2Si2O10

The electrical properties of annealed, fully metamict gadolinite REEFe²⁺Be₂Si₂O₁₀ are studied as a function of annealing temperature. Changes due to annealing are also probed by ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction. The electrical conductivity measured at f = 100 Hz between 110 and 750 K varies markedly, ranging from 10⁻¹⁰ to 10⁻⁶ S m⁻¹ for untreated samples and 10⁻⁹ to 10⁻³ S m⁻¹ for sample annealed in argon at 1373 K. Average measured activation energies for electrical conduction are 0.47 and 0.63 eV for ranges of 400-450 K and 500-600 K, respectively. The dielectric permittivity shows strong dispersion effects above 235 K. After high temperature annealing, the electrical conductivity shows a marked dispersion below 604 K. The combination of polaron hopping and hydroxyl anion migration is proposed for the electrical conduction mechanism.     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

An intense channeling radiation source

A non-conventional X-ray source which is based on the production of electron channeling radiation in a diamond crystal has been installed at the radiation source ELBE. The brilliant electron beam with an average current of up to 200 μA allows to reach photon rates of quasi-monochromatic channeling radiation between 10¹⁰ s⁻¹ and 10¹¹ s⁻¹ per 10% bandwidth. The photon energy can be tuned by variation of the beam energy. On-line X-ray monitoring was realised at high beam currents using a Compton spectrometer. Monochromisation of channeling radiation and bremsstrahlung background reduction has been investigated applying X-ray diffraction at a highly ordered pyrolytic graphite crystal.     

Optical characteristics of recrystallized tungsten mirrors exposed to low-energy, high flux D plasmas

The surface topography and optical properties of recrystallized tungsten exposed to a low-energy (38 eV/D), high flux (10²² D/m² s) deuterium plasma with an ion fluence of 10²⁶ D/m² at various temperatures was investigated. It was found that the surface morphology weakly depends on the exposure temperature in the range 320-695 K with the exception of the narrow temperature region around 535 K, where large changes to all optical characteristics occurs. After plasma exposure at this temperature, the surface topography of the W sample is characterized by active blistering as has already been indicated in previous publications. The reflectance found in direct measurements at normal incidence drops in the wavelength interval 220-650 nm, whereas the estimations of reflectance using the ellipsometry data demonstrate some increase.     

Deuterium retention in tungsten exposed to low-energy, high-flux clean and carbon-seeded deuterium plasmas

Depth profiles of deuterium trapped in tungsten exposed to a low-energy (≈200 eV/D) and high deuterium ion flux (about 1 × 10²¹ D/m² s) in clean (We use the term ‘clean’ in quotation marks having in mind the impossibility to obtain absolutely clean plasma. In our case the conception ‘clean’ D plasma means the plasma without intentionally introduced carbon impurities.) and carbon-seeded D plasmas at an ion fluence of about 2 × 10²⁴ D/m² and various temperatures have been measured up to a depth of 7 μm using the D(³He, p)⁴He nuclear reaction at a ³He energy varied from 0.69 to 4.0 MeV. The deuterium retention in single-crystalline and polycrystalline W increases with the exposure temperature, reaching its maximum value at about 500 K (for ‘clean’ plasma) or about 600 K (for carbon-seeded plasma), and then decreases as the temperature grows further. It is assumed that tungsten carbide formed on the W surface under exposure to the carbon-seeded D plasmas serves as a barrier layer for diffusion and prevents the outward transport of deuterium, thus increasing the D retention in the bulk of tungsten.     

First-principles study on dissolution and diffusion properties of hydrogen in molybdenum

Employing a first-principles method, we have investigated dissolution and diffusion properties of hydrogen (H) in molybdenum (Mo), one of potential candidates for plasma facing materials in a nuclear fusion Tokamak. We show that single H atom is energetically favorable sitting at the tetrahedral interstitial site (TIS) instead of octahedral interstitial site and diagonal interstitial site. This can be confirmed by the electron localization function result. Bader charge analysis suggests that the bonding between H and surrounding Mo is mainly ionic mixed with slight covalent component. Double H atoms tend to be paired up at the two neighboring TIS’s along the 〈1 1 0〉 direction with the distance of ∼0.221 nm and the binding energy of 0.03 eV. This suggests a weak attractive interaction between H atoms, with the implication that self-trapping of H and thus formation of the H₂ molecules are quite difficult in an intrinsic Mo environment. We demonstrate that the diffusion barrier of H that jumps between the TIS’s is 0.16 eV, and the dissolved concentration of H in the intrinsic Mo is 2.6 × 10⁻⁸ at a typical temperature of 600 K. The diffusion coefficients of H, D, and T are different due to the different masses, which are calculated to be 1.3 × 10⁻⁷ m²/s, 9.2 × 10⁻⁸ m²/s, and 7.5 × 10⁻⁸ m²/s at 600 K.     

Behavior of carbon readsorbed on tungsten during low energy Ar ion irradiation at elevated temperatures

A study of the behavior of carbon sputtered and readsorbed after scattering collisions with particles of surrounding gas on the tungsten surface affected by Ar ion irradiation with the flux equal to 2 × 10¹⁶ cm⁻² s⁻¹ extracted from plasma under 300 V negative bias voltage in the temperature range 370-870 K was performed. The dependence of the W sample weight change on the working gas pressure in the range 0.1-10 Pa was registered and the information was deduced about prevailing sputtering-redeposition processes. The depth profiles of carbon at the tungsten surface were measured. We found that carbon distribution profiles in tungsten depend on the C redeposition rate for fixed ion irradiation parameters. Three regimes have been distinguished: (i) at working gas pressure equal to 5 Pa and more, the C redeposition rate prevails the sample surface erosion rate and the W surface is covered by continuous amorphous carbon film (the C film growth regime), (ii) at working gas pressure equal to about 1 Pa, the C redepostion rate is approximately equal to the erosion rate and the W surface is partially covered by redeposited carbon, and (iii) at working gas pressure less than 0.2 Pa, the erosion rate prevails the C redeposition rate (the W surface erosion regime). In the regime of balanced redeposition and erosion deep C penetration depth into nanocrystalline W was registered.It is suggested that under simultaneous C adsorption and ion irradiation at elevated temperature C adatoms are driven from the W surface into grain boundaries and into the bulk by the difference in chemical potentials between the activated W surface and grain boundaries. As the W surface is covered by amorphous C film, the grain boundaries are blocked and the efficiency of carbon transport decreases.     

Atom probe tomography characterization of radiation-sensitive KS-01 weld

The microstructure of a radiation-sensitive KS-01 test weld has been characterized by atom probe tomography. The levels of copper, manganese, nickel and chromium in this weld were amongst the highest of all the steels used in Western reactor pressure vessels. After neutron irradiation to a fluence of 0.8 × 10²³ n m⁻² (E>1 MeV) at a temperature of 288 °C, this weld exhibited a large Charpy T_41J shift of 169 K, a large shift of the fracture toughness transition temperature of 160 K, a decrease in upper shelf energy from 118 to ∼78 J, and an increase in the yield strength from 600 to 826 MPa. However, the mechanical properties data conformed to the master curve. Atom probe tomography revealed a high number density (∼3 × 10²⁴ m⁻³) of Cu-, Mn-, Ni-, Si- and P-enriched precipitates and a lower number density (∼1  × 10²³ m⁻³) of P clusters.     

X-ray photoelectron and Raman spectroscopic studies of MeV proton irradiated graphite

Poly-crystalline graphite samples were irradiated using 2.25 MeV H⁺ ions with a fluence of 2 × 10¹⁷ ions/cm². Magnetic ordering in highly oriented pyrolytic graphite samples have been reported earlier under the similar irradiation conditions [Esquinazi et al., Phys. Rev. Lett. 91 (2003) 227201]. In that study, the authors attribute the observed irradiation induced magnetic ordering to the formation of a mixed sp²-sp³ hybridized carbon atoms. In the present study, we report the X-ray photoelectron and Raman spectroscopic studies on pristine and irradiated samples. Irradiated samples are found to show an increased number of sp³ hybridized carbon atoms. However, the Raman spectrum, specially the second order data, do indicate that the nature of the graphene lattice structure has been preserved in the irradiated samples. The mechanisms for the irradiation induced enhancement in sp³ hybridization are discussed.     

Thermal and structural properties of low-fluence irradiated graphite

The release of Wigner energy from graphite irradiated by fast neutrons at a TRIGA Mark II research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry / synchrotron powder X-ray diffraction between 25 and 725 °C at a heating rate of 10 °C min⁻¹. The graphite, having been subject to a fast-neutron fluence from 5.67 × 10²⁰ to 1.13 × 10²² n m⁻² at a fast-neutron flux (E > 0.1 MeV) of 7.88 × 10¹⁶ n m⁻² s⁻¹ and at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 1.2 to 21.8 J g⁻¹ and a Wigner energy accumulation rate of 1.9 × 10⁻²¹ J g⁻¹ n⁻¹ m². The differential-scanning-calorimeter curves exhibit, in addition to the well known peak at ∼200 °C, a pronounced fine structure consisting of additional peaks at ∼150, ∼230, and ∼280 °C. These peaks correspond to activation energies of 1.31, 1.47, 1.57, and 1.72 eV, respectively. Crystal structure of the samples is intact. The dependence of the c lattice parameter on temperature between 25 and 725 °C as determined by Rietveld refinement leads to the expected microscopic thermal expansion coefficient along the c axis of ∼26 × 10⁻⁶ °C⁻¹. At 200 °C, coinciding with the maximum in the differential-scanning-calorimeter curves, no measurable changes in the rate of thermal expansion have been detected - unlike its decrease previously seen in more highly irradiated graphite.     

A molecular dynamics study of diamond exposed to tritium bombardment for fusion applications

Diamond, with its low atomic number and high thermal conductivity, is being assessed as a possible plasma facing material within a fusion reactor. Molecular dynamics simulations using the AIREBO potential were performed simulating the exposure of diamond to a plasma in conditions similar to those of the divertor region of a tokamak. Diamond surfaces at temperatures of 300 and 600 K were bombarded with 15 eV tritium at a high flux (10²⁹ ions m⁻² s⁻¹). A layer-by-layer etching process was observed which, with the lack of any tritium diffusion though the remaining diamond structure, was responsible for limiting damage, and thus tritium retention, to the top 4-5 diamond layers. Analysis of this damaged region also showed a large amount of residual structure suggesting that bombardment below the physical sputtering threshold (∼30 eV) may not lead to amorphisation of the surface.