[en] In the framework of the collaboration with NET/ITER the 3D silicon doped CFC material NS31 with an optimized thermal conductivity was investigated. NS31 is a candidate for the plasma facing material in fusion devices. The sense of the dopant is to decrease the chemical erosion yield and the tritium retention of graphite. In order to get a view of the morphology scanning electron microscopy (SEM) was used. The distribution and thermal stability of the silicon were controlled using backscattering spectroscopy (BS). Information about the erosion were obtained out of weight loss and CD₄ production measurements regarding monoenergetic deuterium ions. The material consists of areas of pure graphite, of Si covered fasers, and of Si or SiC crystallites. The areas cover about 0.01-1 mm². Inside the BS information depth (∼20 μm) the Si distribution is homogeneous, but the Si concentration varies drastically with the lateral position of the analysing spot (∼1 mm²) around the average value of 8-10 at.%

[en] Bonding structure of carbon and metal as well as nanostructural changes of metal-doped amorphous carbon films (a-C:Me) were investigated depending on metal type (W, Ti, V, and Zr), concentration (<25 at.%) and annealing temperature (< 1300 K, except W: < 2800 K). Pure C films exhibit ∼ 2 nm distorted aromatic and graphene-like regions. Both increase in size with annealing. After deposition the metals have carbide-like bonding and are mainly distributed atomically disperse in an amorphous environment. Annealing leads to the formation of carbide crystallites (TiC, VC, ZrC, WC, W₂C, and WC_1-x) of several nanometers. The VC particles reach the largest size up to 1300 K. All metal dopings reduce the erosion rate against oxidation (expect V) and hydrogen impact.

[en] Erosion yields of Si, C and SiC due to D ion impact were determined between 20 and 300 eV up to 1100 K. A temperature dependence of the erosion yield has been observed for Si and C. The temperature of the maximum increases with ion energy from about 350 K for 20 eV to 570 K above 100 eV for Si, which is always about 250 K lower than for graphite. The erosion yields of Si are in general 10 times smaller than those of graphite. Chemical erosion species were observed mass-spectroscopically: silane was found for Si, but not for SiC, where only hydrocarbons were observed

[en] Total erosion yields of graphite and carbon materials under hydrogen and deuterium bombardment measured with the weight-loss method are presented for ion energies between 15 eV and 8 keV in the temperature range 300 to above 1000 K. The temperature of the maximum of the chemical erosion increases from below 600 to above 850 K with ion energies from 15 to 300 eV. Chemical erosion yields obtained by weight-loss measurements exceed yields measured mass-spectrometrically always by a factor of about two. Collector experiments show that a fraction of the eroded particle sticks to walls and, therefore, reduces the yield measured by mass spectrometry. A synergistic effect of neutrals in the ion beam on the chemical erosion yield can be excluded

[en] The composition of co-deposited hydrocarbon-silicon layers (a-C:Si:D) with varying Si concentrations and their removal by heating in air were investigated using MeV ion beam techniques. The amount of trapped D per re-deposited target atom depends weakly on the Si concentration. For pure C and Si, the D concentrations are about 0.45 and 0.5 D atoms per re-deposited target atom at room temperature, respectively. A maximum of about 0.7 D/(Si+C) was found at Si/C∼1. For increasing deposition temperature the D concentration does not decrease significantly until about 600 K. At about 1000 K the D concentration for pure C layers is still about 30% of the concentration at room temperature. The removal rates of D and C by heating in air increase strongly at temperatures around 550 K for a-C:D layers. With increasing Si content, these temperatures rise to above 650 K for layers with Si concentrations higher than 0.2 Si/(Si+C). The C removal rate is always lower than the D removal rate. Si is not removed by this method. For comparison, the composition of co-deposited stainless steel layers and Ti-C mixtures were investigated

[en] Doping of carbon generally results in a reduction of its chemical reactivity during hydrogen ion bombardment. Due to preferential sputtering of carbon, the dopants may be enriched at the surface, resulting in an additional reduction of the erosion yield. Dopants in the form of sub-μm precipitates with a very homogeneous distribution lead to in a more effective reduction of both chemical erosion processes, Y_therm and Y_surf. However, dopants may degrade the thermal conductivity of graphite, which has to be avoided. First results on the development of carbon materials doped with different carbides and with optimized microstructure and thermomechanical properties show that VC acts as an effective catalyst for graphitization causing an improvement of the thermal conductivity. It leads further to a reduction of both, the Y_therm and the Y_surf chemical erosion processes, which is partly attributed to surface enrichment but is also the result of a chemical influence

[en] The composition of co-deposited carbon-silicon layers (a-C:Si:D) with varying Si concentrations and their removal by heating in air were investigated using MeV ion beam techniques. The ion-induced release of D due to the analysing beam (1.2 MeV ³He) was determined. The removal rates of D and C by heating in air increase strongly at temperatures around 550 K for a-C:D layers. With increasing Si content, these temperatures rise to above 650 K for layers with Si concentrations larger than 0.2 Si/(Si+C). The C removal rate is always lower than the D removal rate. Si is not removed by this method. The observed properties of the layers are compared with those of hard and soft a-C:D films

[en] Retention and enrichment of a model system for mixed layers, tungsten-containing carbon films (a-C:W), were investigated with respect to the interaction with D ions. a-C:W was exposed to a mass-separated, mono-energetic D beam (200 eV/D, 1.2 × 10¹⁵ D cm⁻² s⁻¹). The W concentration in the films (0–7.5 at.%), the specimen temperature during D beam exposure (300–1300 K) and the fluence (Φ) of incident D (10¹⁵–10²⁰ D cm⁻²) were varied. Analysis of retention and enrichment were performed by nuclear reaction analysis and Rutherford backscattering spectrometry, respectively. At 300 K and fluences up to 10¹⁹ D cm⁻², the increase of the D inventory with fluence in a-C:W cannot be distinguished from a-C and pyrolytic graphite, e.g., above ∼10¹⁷ D cm⁻² the D inventory increases with fluence according to Φ^x (x = 0.1). Above a fluence of 10¹⁹ D cm⁻², however, the D inventory depends strongly on the W concentration. At a fluence of 10²⁰ D cm⁻² the D inventory is increased to the 1.5-fold of the D inventory of pyrolytic graphite for 1% and 2.5% a-C:W and it is decreased to the half value of the D inventory of pyrolytic graphite for 7.5% a-C:W. At temperatures above 300 K, following trends are observed: With increasing temperature, the D inventory increases more strongly with fluence and D reaches depths far beyond the width of the ion range. However, the D inventory does not increase with fluence according to Φ^x, especially at fluences above 10¹⁹ D cm⁻²

[en] The erosion yield of SiC due to D bombardment has been determined by the weight-loss method as a function of temperature and energy in the range of 20-300 eV up to 1100 K. A temperature dependence exists clearly below 100 eV. For 20 eV D, the yield is 0.5% at 300 K and 1.5% in the maximum at 500 K. Contrary to Si erosion, no formation of silane was observed with mass spectrometry. Hydrocarbons, predominantly CD₄ molecules, are only found at 20 eV. From the erosion of a thin SiC layer investigated using MeV ion beam analysis, segregation and preferential erosion of carbon are ruled out, because the layer thickness decreases while the composition stays constant. As no volatile silane production is detected, it is assumed that sputtering of Si or silane precursors with low binding energy occurs at low ion energies. The mechanism of the chemical erosion of Si in presence of C and absence of C is different

[en] Highlights: • D retention for used W grades varies by more than one order of magnitude. • Hydrogen loading-induced damaging manifests in surface modifications and retention. • Effects of plasma flux on retention and surface modifications are discussed. • Higher fluxes produce more severe hydrogen loading-induced damaging. - Abstract: Five tungsten (W) grades were simultaneously exposed to deuterium (D) plasma with 10²⁰ D/(m² s) of 38 eV/D up to 10²⁶ D/m² at 500 K specimen temperature. The D inventories and their depth profiles within the topmost 12 μm were determined by nuclear reaction analysis (D(³He, p)α). Morphological modifications at and below the surface were analysed by confocal laser scanning microscopy and scanning electron microscopy assisted by focused ion beam cross-sectioning. The observed variation of the D inventory by more than one order of magnitude (0.5–15 × 10²⁰ D/m²) is attributed only to the different properties of each W grade. Spherical blisters and stepped flat-topped extrusions are observed depending on the W grade. These modifications are interpreted as an indication for hydrogen loading-induced damaging. The exposure conditions and W grades were chosen to allow a comparison between published data sets