V.N. Chernikov

Hydrogen adsorption on and solubility in graphites

The experimental data on sorption and solubility of hydrogen isotopes in graphite in a wide ranges of temperature and pressure are reviewed. The Langmuir type adsorption is proposed for the hydrogen -- graphites interaction with taking into account dangling sp{sup 2}{minus}bonds relaxation. Three kinds of traps are proposed: Carbon interstitial loops with the adsorption enthalpy of {minus}4.4 eV/H{sub 2} (Traps l); carbon network edge atoms with the adsorption enthalpy of {minus}2.3 eV/H{sub 2} (Traps 2): Basal planes adsorption sites with enthalpy of +2.43 eV/H{sub 2} (Traps 3). The sorption capacity of every kind of graphite could be described with its own unique set of traps. The number of potential sites for the ``true solubility`` (Traps 3) we assume as 1E+6 appm, or HC=l, but endothermic character of this solubility leads to negligible amount of inventory in comparison with Traps 1 and Traps 2. The irradiation with neutrons or carbon atoms increases the number of Traps 1 and Traps 2. At damage level of {approximately}1 dpa under room temperature irradiation the number of these traps was increased up to 1500 and 5000 appm respectively. Traps 1 and Traps 2 are stable under high temperature annealing.

Depth distribution of deuterium atoms and molecules in beryllium implanted with D ions

Abstract In-depth concentration profiles of deuterium atoms and molecules in beryllium implanted with 9 keV D ions to fluences, Φ, in the range from 6 × 10 19 to 9 × 10 22 D / m 2 at temperatures, T irr , of 300 and 700 K have been determined using SIMS and RGA (residual gas analysis) measurements in the course of surface sputtering. The microstructure of implanted specimens was studied by TEM. Implanted deuterium is retained in Be matrix in the form of both D atoms and D 2 molecules. The total amount of gas captured within the sub-surface layer of ∼ 700 nm in thickness as a result of implantation at 300 and 700 K reaches 4 × 10 21 and 1 × 10 21 D / m 2 , correspondingly. The ratio of deuterium quantities retained in the form of atoms and molecules, Q D : Q D 2 , varies from 1:3 for T irr = 300 K to 1:4 for T irr = 700 K . At T irr = 300 K the concentration of D 2 molecules at the depth of the ion mean range reaches its maximum of 4 × 10 27 molecules / m 3 at Φ ≈ 2 × 10 21 D / m 2 . The molecules are present in tiny bubbles which show a tendency toward interconnection at higher fluences. At T irr = 700 K , along with relatively small facetted bubbles (near the very surface), large oblate gas-filled cavities and channels forming extended labyrinths appear and they accumulate most of the injected gas. The maximum D 2 concentration in the latter case is of 1 × 10 27 molecules / m 3 . The high concentration of D atoms in the ion stopping zone after implantation at T irr = 300 and 700 K (about 2 × 10 27 and 1 × 10 27 atoms / m 3 , respectively) is attributed to deuterium (i) trapped in radiation vacancies, (ii) adsorbed on the walls of bubbles and channels and (iii) bonded to/by BeO formed on the surface and present in the form of metallurgical inclusions in the bulk.

Gas swelling and related phenomena in beryllium implanted with deuterium ions

Abstract An extensive TEM study of the microstructure of TIP-30 Be implanted with 3 and 10 keV D ions to fluences, Φ in the range from 3 × 1020 to 8 × 1021D/m2 at temperatures, Tirr = 300, 500 and 700 K has been carried out. Depth distributions of separate D atoms and D2 molecules have been investigated by means of SIMS and RGA methods, correspondingly. D ion irradiation, accompanied by blistering, gives rise to destructions dependent mainly on Tirr. Irradiation at 300 K leads to the formation of tiny D2 bubbles of 1 run in size (reminiscent of He bubbles in Be). At Tirr ≥ 500 K, along with small facetted bubbles, the development of larger oblate cavities occurs accumulating most of injected deuterium and providing for a much higher gas swelling compared to that at 300 K. D (He) ion implantation leads to the enhanced growth of microcrystalline layers of cph-BeO oxide with a microstructure differing from that on the electropolished Be surface. Based on the analysis of experimental data deuterium reemission, thermal desorption and trapping in defects are discussed.

Gas-induced swelling of beryllium implanted with deuterium ions

Abstract An extensive TEM study of the microstructure of Be TIP-30 irradiated with 3 and 10 keV D ions up to fluences, Φ, in the range from 3 × 10 20 to 8 × 10 21 D/m 2 at temperatures, T irr = 300, 500 and 700 K has been carried out. Depth distributions of deuterium in a form of separate D atoms and D 2 molecules have been investigated by means of SIMS (secondary ion mass spectrometry) and RGA (residual gas analysis) methods, correspondingly. D ion implantation is accompanied by blistering and gives rise to processes of gas-induced cavitation which are very sensitive to the irradiation temperature. At T irr = 300 K tiny gas bubbles (about 1 nm in size) pressurized with molecular deuterium are developed with parameters resembling those of helium bubbles in Be. Irradiation at T irr ≥ 500 K leads to the appearance of coarse deuterium-filled cavities which can form in sub-surface layers different kinds of oblate labyrinth structures. Questions of reemission, thermal desorption and trapping of deuterium in Be have been discussed.

Helium bubbles in nickel annealed at T>0.7Tm

Helium-induced cavities in fusion materials are considered to be detrimental. Valuable information on He bubbles and on the basic mechanisms underlying their evolution is obtained by post-implantation annealing, subsequent to He implantation at about 300 K. For Ni and Cu, it has been shown that up to annealing temperatures Ta≤0.7Tm (Tm is the melting point), highly overpressurized bubbles form in the volume and coarsen very slowly by migration and coalescence, whereas near vacancy sources the overpressure relaxes and the coarsening occurs rapidly by Ostwald ripening (OR) which leads to the appearance of small and large He cavities. Annealing of He loaded Ni at Ta from 0.72Tm to 0.92Tm leads to the formation of only one population of nearly equilibrium bubbles which is related to the recovery of the ability of dislocations to emit vacancies into their surroundings. The effective activation energy of the increase of the mean bubble radius was found to be 0.60±0.02 eV, which is lower than that for the OR at lower temperatures (1.1 eV). Analysis shows that the mechanism covering the coarsening of He bubbles at very high temperatures is still OR, but limited by the rate of ledge nucleation on the bubble walls.

Trapping of deuterium in boron and titanium modified graphites before and after carbon ion irradiation

Nuclear reaction analysis was used to investigate the retention of deuterium (D) at radiation damage in new graphites USB-15, PGI, RG-Ti-91, RG-Ti-91 with 0.5 wt%B made in Russia and in POCO AXF-5Q graphite. Samples were irradiated with C+ ions to produce damage up to doses of ~ 10 displacements per atom (dpa), and then soaked in deuterium gas at 1473 K. The concentration of D in the damaged region increased with damage and saturated for damage levels above about 1 dpa. At saturation the smallest D concentration was found in USB-15 (~ 60 appm), and the largest D concentration was found in RG-Ti-91 (~ 1500 appm). Based on microstructural investigations done by TEM, an attempt was made to relate differences in concentrations of retained D to variations in radiation-induced trap concentration and to kinetic limitations for D atom access to traps available. The effect of boron on D trapping in graphite was also examined.

Gas-induced swelling in helium-implanted copper during annealing

A detailed transmission electron microscopy (TEM) investigation of the gas porosity (GP) in pure copper homogeneously implanted with helium at 323 K to a concentration of cHe ⋍ 300 appm as a result of postimplantation annealing for up to 0.5 h at temperatures between 838 and 1193 K has been carried out. General features of the GP development in the bulk and near vacancy sources (primary and secondary GP, correspondingly) are similar to those for nickel reported previously in detail. First, sizes of small overpressurized (primary) gas bubbles formed in Cu at T > THeVD ⋍ 785 K (dissociation temperature of HeV complexes) were determined and He density therein estimated: ρHe ⋍ 105 nm−3. Large, near-equilibrium (secondary) bubbles arise not only near the outer surface and some grain boundaries but also locally in the vicinity of very small — less than 100 to 150 nm in size — inclusions. Under conditions of vacancy deficit they act as vacancy “point” sources in the course of their formation and/or due to phase transformation therein. By means of X-ray microanalysis, microdiffraction and CBED these inclusions were identified as copper sulphide precipitates. A supposition was made why in fcc metals (Ni, Cu) implanted with helium up to a certain concentration dislocations fail to supply vacancies into the surroundings up to temperatures of about 0.75Tm (Tm — the melting point).

Microstructure of RGT graphite before and after helium implantation

Abstract An investigation of the microstructure of RG-Ti-91 (RGT) graphite containing 7.5 wt% Ti, has been carried out, both before and after homogeneous implantation with He at 320 K up to about 0.08 at% causing a displacement damage of 0.1 dpa. Using TEM it can be shown that this material is textured and a relation has been found between its anisotropy and texture. The latter appears during synthesis when hot pressing of the furnace charge. At the same time grains of Ti powder transform into separated colonies of tiny TiC inclusions which provide for good strength and thermal conductivity across the laminated matrix structure. Helium implantation into specimens (0.6 mm in thickness) to a depth of 100 μm results in a partial loss of crystallinity in this layer, an increase of the c parameter by about 5%, swelling in excess of 10% and a remarkable strengthening of the matrix. High energy charged particle irradiation proved to be an adequate technique for the simulation of radiation effects due to fast neutrons. Inhomogeneous (in depth) swelling of anisotropic RGT results in internal stresses which may lead to crack formation and finally to rupture.

Deuterium trapping in graphites irradiated with C+ ions at 350 and 673 K

By means of NRA method the retention of D atoms was investigated in USB, RGT, PGI and POCO graphites after irradiation at 350 and 673 K with 200 keV C + ions to different doses up to about 10 dpa followed by deuterium loading from D 2 gas at 1473 K under pressure of 0.66 Pa for 1 h. The aim was to understand the nature of deep traps for D atoms with E D b ≃ 4.5 eV, differences in deuterium accumulation in different graphites and the role of the irradiation temperature. After irradiation at 673 K the concentration, C D , of D atoms trapped in the ion stopping range in all the graphites except for USB is 1.5-2.0 times as low as in those after irradiation at 350 K and amounts to 600-800 appm. USB demonstrates a minimum gas retention (≃ 100 appm), but it is more than that after implantation at 350 K. It was shown that deep traps are available in trace amounts in all graphites in their original state (about 10 appm) and can be created in the course of plastic deformation and/or fracture (up to about 100 appm). Deep traps are considered to be dangling bonds of C atoms along peripheral edges of interstitial clusters formed mainly due to radiation damage of graphites. Differences in the dose dependences of D retention, C D (Kt), for different graphites irradiated at 350 and 673 K were discussed in frames of available knowledge on the damage buildup and relevant effects which greatly depend on the irradiation temperature

Helium-induced swelling in tungsten during postimplantation annealing

Abstract A detailed transmission electron microscopy investigation of the gas porosity has been carried out in two types of tungsten specimens: 1. (1) hot-rolled W, homogeneously implanted with 600 appm He at 325 K 2. (2) W single crystals, implanted with up to 2 × 1021 He+ ions/m2 of 40 keV energy at 325 K, followed by annealing at temperatures between 1273 and 2373 K for 1 h (in case (2) after W vapor deposition onto implanted surface). In both types of specimens monomodal distributions of near-equilibrium bubbles, mostly attached to dislocations have been found for T ≥ 1523 K. This is in contrast to the bubble development in typical fcc metals (Ni, Cu) where dislocations are apparently unable to provide sufficient vacancies for the relaxation of the overpressure in the bubbles which leads to spatially separated bubble populations, i.e. large bubbles close to surfaces and some grain boundaries and small bubbles in the bulk. An evaluation of the temperature dependence of the mean bubble radii in W suggests migration and coalescence controlled by surface diffusion as the main coarsening mechanism during annealing.