Activation and transmutation of tungsten boride shields in a spherical tokamak

The FISPACT-II code is used to compute the levels of activation and transmutation of tungsten borides for shielding the central High Temperature Superconductor (HTS) core of a spherical tokamak fusion power plant during operations at 200 MW fusion power for 30 years and after shutting down for 10 years. The materials considered were W2B, WB, W2B5 and WB4 along with a sintered borocarbide B0.329C0.074Cr0.024Fe0.274W0.299, monolithic W and WC. Calculations were made within shields composed of each material, for five reactor major radii from 1400 to 2200 mm, and for six 10B isotope concentrations and at five positions across the shield. The isotopic production and decay in each shield is detailed. The activation of boride materials is lower than for either W or WC and is lowest of all for W2B5. While isotopes from tungsten largely decay within 3 years of shut-down, those from boron have a much longer decay life. An acceptable 70% of the absorbing 10B isotope will remain after 30 years of operations behind the first wall for a 1400 mm radius tokamak. Gaseous production is problematic in boride shields, where 4He in particular is produced in quantities 3 orders of magnitude higher than in W or WC shields. The FISPACT-II displacements per atom (dpa) tend to increase with boron content, although they decrease with increased 10B isotopic content. The dpa ranges of boride shields tend to lie between those of W and WC. Overall, the results confirm that the favourable fusion reaction shielding properties of W2B5 are not seriously challenged by its irradiation and transmutation properties, although helium gas production could be a challenge to its thermal and mechanical properties.

Electron-Beam Surface Modification of Boron-Based Diffusion Layers Obtained of the Surface of Steel H21

In recent years the interest in the development of new protective coatings with improved functional properties for machine parts’ surface have been of great fundamental and applied importance. The current study is devoted to the creation of coatings based on boron and aluminium on the surface of alloy steel using a cutting-edge method, combining thermal-chemical treatment (TCT) and subsequent electron beam processing (EBP). TCT was carried out in treatment pastes based on boron carbide and aluminum at 950°C and 1050°C for 2 hours. As a result of processing, diffusion layers with a thickness of up to 120 μm and 580 μm were formed on the steel surface after TCT at 950°C and 1050°C respectively. The subsequent EBP led to a complete transformation of the primary diffusion layer and an increase in its thickness to 1.6 mm. XRD analysis showed significant differences in composition before and after EBP: new compound, such as tungsten borides (WB, W2B9) and iron boride (Fe2B) were detected. In addition, it was determined that the distribution of microhardness and elemental composition (B, Al, W) over the layer thickness after EBP had a more favorable profile without significant fluctuations compared to the sample after TCT. The concentration of Al decreased significantly after EBP. It dropped from 18% after TCT to a low of 1%.

Zirconium alloyed tungsten borides synthesized by spark plasma sintering

Tungsten borides (WBx; x = 2.5 or 4.5) with an increasing substitution of tungsten by zirconium from 0 to 24 at.% were synthesized by spark plasma sintering (SPS) for the first time. The influence of the holding time (2.5–30 min) on the densification behavior, microstructure evolution and development of the properties of W–Zr–B compounds were studied. The samples were characterized using scanning electron microscopy (SEM) for microstructure analysis, X-ray diffraction (XRD) for phase identification, Vickers micro-indentation for microhardness measurements, tribological tests to determine the coefficient of friction and specific wear rate, as well as measurements of electrical conductivity. The XRD results confirm the presence of the WB4 phase in the microstructure, despite the high sintering temperature (1800 °C) and small overstoichiometric excess of boron (4.5) addition in the sintered samples. This is caused by the high heating rate (400 °C/min), short holding time (2.5 min) and addition of zirconium. The Vickers hardness (HV) values measured at 1 N are 24.8 ± 2.0 and 26.6 ± 1.8 GPa for 24 at.% zirconium in WB2.5 and for 0 at.% zirconium in WB4.5, respectively. In addition, the hardest sample (W0.76Zr0.24B2.5) showed electrical conductivity up to 3.961·106 S/m, which is similar to WC–Co cemented carbides. The friction and wear test results reveal the formation of a boron-based film which seems to play the role of a solid lubricant.

Multiple crystal phases of intermetallic tungsten borides and phase-dependent electrocatalytic property for hydrogen evolution

Four crystal phases of intermetallic tungsten borides, including W2B, WB, WB2 and WB3, were selectively synthesized, and their crystal and electronic structures, as well as activity trend to HER, were studied.