Boron carbide (B4C) behavior under irradiation is widely studied in order to predict the lifetime of this material in future (Generation IV) nuclear fission reactors. This paper is focused on the ...effects of high electronic stopping powers (Se) on B4C structure modifications. Sintered B4C samples were irradiated at room temperature with swift heavy ions (between 0.5 and 3 MeV·u − 1) corresponding to Se values in the 4.1 to 15.4 keV·nm−1 range at the sample surface. In order to investigate the structural changes as a function of depth, transmission electron microscopy and Raman mapping were performed on the irradiated samples along the path of the incident ions. For the highest Se values, damage results in the creation of large hillocks at the sample surface along with the amorphization of the bulk. These results are explained, in the frame of the inelastic thermal spike model, by local melting in latent tracks that are created only when irradiations are performed above a Se threshold evaluated at around 9 keV nm−1.
The TiC1-x–TiO2 reactive interface obtained at 1100 °C has been studied by local structural investigations using Transmission Electron Microscopy (TEM) and X-rays microdiffraction. These results were ...completed by coupling complementary chemical characterizations by Nuclear Reaction Analysis (NRA), Rutherford Backscattering (RBS) and STEM (Scanning Transmission Electron Microscopy). A complex pattern of reactive interlayers of various thicknesses was formed as a result of the crossed redox reactions between TiO2 and TiC1-x. Our results show that the TiC1-x carbide is not oxidized by forming TiCxOy oxycarbides in spite of the complete solid solution existing between TiC1-x and TiO. TiC1-x is in fact destabilized by the oxygen liberated during the transformation of TiO2 into Magnéli type compounds (TinO2n-1) to form the most reduced Magnéli sub-oxides following the reaction: 3 TiCxOy + (5-3y)/2 O2 = Ti3O5 + 3x C. Starting from TiO2, a progressive reduction is achieved showing spatially distributed TinO2n-1 Magnéli phases characterized by high values of n close to TiO2 and lower values of n as reaching TiC1-x. The most reduced term is shown to be Ti3O5. Ti3O5 thus represents the final product of the reaction at 1100 °C both for the reaction of reduction of TiO2 and for the reaction of oxidation of TiC1-x.
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The ZrC x O y oxycarbides are well-known relevant ceramic materials for ultra-high temperature applications. The intrinsic macroscopic properties of ZrC x O y being closely related to the C/O ratio, ...a detailed analysis of the C–O–Zr system has been undertaken experimentally in order to accurately determine the extent of the solid solution of oxygen within the oxycarbide phase at different synthesis temperatures. The obtained results were then used as diagrammatic data to extrapolate the ternary C–O–Zr phase equilibria diagram by the CALPHAD method, providing a predictive tool for the oxycarbide synthesis. The model proposed in the temperature range 1650–2000 °C is in fair agreement with results obtained in the literature. The chemical determination of the relative ratio between light elements (oxygen (O) and carbon (C)) being a difficult issue for most of the general applications, an accurate determination of the cell parameters of the different oxycarbide compositions has been performed to propose an abacus reporting the evolution of the cell parameter against the C/O amount. The chemical composition of the oxycarbide is shown to be determined with an accuracy better that a few percent. It is also shown that the evolution of the cell parameter is not linear, indicative of a possible change of the ionocovalent character of the chemical bonds with the composition of ZrC x O y .
Pure powders of TiCxO(1-x) solid solution were synthesized through the carbothermal route. The chemical analysis of the light elements in the as-obtained TiCxO(1-x) oxycarbides powders were performed ...by Instrumented Gas Analysis (IGA). The cell parameters of the samples were determined with an accuracy of about 2% by means of X-ray powder diffraction and the internal standard method. As a result, a model correlating the cell parameters to the chemical composition was established. These reference TiCxO(1-x) oxycarbides powders were then sintered in order to obtain pellets of dense ceramics. After having determined that the sintering process does not change the chemical composition of the starting powder, chemical analysis of the different samples of the solid solution were successfully undertaken by Ion Beam Analysis techniques (IBA). The Nuclear Reaction Analysis (NRA) method - that was used to analyse light elements with very high sensitivity - was coupled with Rutherford Back Scattering (RBS) analysis in order to accurately determine the metallic over light elements ratio and to determine the stoichiometry of the phase on massive samples. Exhaustive simulations of the NRA spectra were performed and demonstrated that discrete compositions of the TiCxO(1-x) can be efficiently measured locally for bulk samples. Compared to IGA results, the relative amounts of carbon and oxygen of bulk materials were determined with a bias lower than 5%. This protocol being implemented for the TiCxO(1-x) system was then tested on HfCxO(1-x)with the same success.
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Nanoscale multilayer CrN/ZrN coatings with bilayer thicknesses ranging from 11.7 to 66.7
nm were prepared by reactive magnetron sputtering techniques. The structure of the thin films was ...characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction results showed that CrN individual layers presented a <1
1
1> preferred orientation in the multilayer coatings. The diffraction peaks of CrN shifted continuously to low diffraction angle with decreasing bilayer thickness. TEM observations showed that the multilayer did not form a superlattice structure instead of the coexistence of nanocrystalline CrN and ZrN layers. Columnar growth for all the coatings was observed by cross-sectional SEM. Nanoindentation tests showed that the multilayer coatings had almost a constant nanohardness of 29
GPa in spite of the variations of bilayer thickness. Pin-on-disk tests indicated that both the friction coefficients and wear rates increased when decreasing bilayer thickness. However, in comparison with the monolayer coating, the multilayer coatings exhibited excellent wear resistance.
Already used as neutron absorber in the current French nuclear reactors, boron carbide (B4C) is also considered in the future Sodium Fast Reactors of the next generation (Gen IV). Due to severe ...irradiation conditions occurring in these reactors, it is of primary importance that this material presents a high structural resistance under irradiation, both in the ballistic and electronic damage regimes. Previous works have shown an important structural resistance of boron carbide even at high neutron fluences. Nevertheless, the structural modification mechanisms due to irradiation are not well understood. Therefore the aim of this paper is to study structural modifications induced in B4C samples in different damage regimes. The boron carbide pellets were shaped and sintered by using spark plasma sintering method. They were then irradiated in several conditions at room temperature or 800 degree C, either by favoring the creation of ballistic damage (between 1 and 3dpa), or by favoring the electronic excitations using 100MeV swift iodine ions (S e approximately 15keV/nm). Ex situ micro-Raman spectroscopy and Doppler broadening of annihilation radiation technique with variable energy slow positrons were coupled to follow the evolution of the B4C structure under irradiation.
Thermodynamic study of the U–Si system Berche, A.; Rado, C.; Rapaud, O. ...
Journal of nuclear materials,
05/2009, Letnik:
389, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
The uranium–silicon phase diagram is a key system to predict the possible interaction between the fuel kernel (U, Pu)C and the inert matrix SiC considered for the gas-cooled fast reactor systems. The ...experimental data from the literature on the uranium–silicon system are critically reviewed. Differential Thermal Analysis experiments are carried out to measure the temperatures of the phase transitions in the composition range 6–46% at Si. The experimental results are compared to the available data of the literature. The microstructure of the samples has been analysed using scanning electron microscopy. In view of the analyses, some solidification paths are proposed. Finally, the present experimental results and the available data of the literature have been used to perform a thermodynamic modelling of the uranium–silicon system using the CALPHAD method.
Binary 50Ni–50Ti mixture was prepared by mechanical alloying from elemental powders. After 48
h of milling, the nanocrystalline B2-NiTi powder was produced. Then, this as-milled powder was deposited ...by cold spraying in order to produce a target which can be used to create thin films by magnetron sputtering technique. The objective is to improve the electrical characterizations of the NiTi/SiO
2/Si M.O.S structures. The morphology evolution of the powder particles, the phase identification and the alloying evolution process as function of milling time were studied using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. In addition, the target was also characterized using X-ray diffraction, Scanning electron microscopy, and microhardness measurements. The milling of powders leads to the formation of disordered nanocrystalline B2-NiTi with a crystallite size of about 12
nm and a microstrain of level 2.10%, after 48
h of milling. The microstructure, the composition and grain size of the milled powders for 24
h and 48
h characterized by TEM are heterogeneous. The as-deposited intermetallic NiTi can be retained in the coating with a lattice parameter of 0.3
nm, crystallite size of 14
nm, microstrain and high microhardness of 2% and 694 Hv
0.25, respectively.
The structural parameters of nanocrystalline NiTi obtained by mechanical alloying process was reported. The structural evolution as well as the phase identification and microstructural parameters such as lattice parameter a, crystallite size and microstrain have been investigated. After 48
h of milling, the nanocrystalline B2-NiTi powder was produced. Then, this as-milled powder was deposited by cold spraying in order to produce a target which can be used to create thin films by magnetron sputtering technique.
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► B2-NiTi phase formation after 48
h of milling. ► Deposited of the nanocrystalline B2-NiTi by cold spray. ► Structural, microstructural and microhardness of the coating were compared with the milled powder for 48
h.
Cr–Ni–N coatings were deposited on 304 stainless steel substrates using a conventional direct current magnetron reactive sputtering system in nitrogen–argon reactive gas mixtures. The influence of Ni ...content (0
≦
x
≦
20 at.%) on the coating composition, microstructure, and tribological properties was investigated by glow discharge optical spectroscopy, X-ray diffraction and transmission electron microscopy, scanning electron microscopy (SEM), nano-indentation, and pin-on-disk tests. The results showed that microstructure and properties of coatings changed due to the introduction of Ni. The ternary Cr–Ni–N coatings exhibited solid solution structures in spite of the different compositions. The addition of Ni strongly favoured preferred orientation growth of . This preferred orientation resulted from the formed nano-columns being composed of grains with the same crystallographic orientation, as confirmed by SEM cross-sectional observations. The mechanical properties including the nano-hardness and reduced Young's modulus decreased with increasing Ni content. Pin-on-disk tests showed that low Ni content coatings presented higher abrasion resistance than high Ni content coatings.
Zirconium modified chromium nitride coatings with various Zr contents have been prepared by a DC reactive magnetron sputtering technique. The detailed investigations in terms of composition, phase ...structure, morphology and corrosion properties have been performed by GDOES, XRD, SEM and electrochemical measurements, respectively. The as-deposited coatings with Zr contents ranging from 0 to 3.2
at. % form nanocrystalline solid solutions, where Zr substitute Cr in the CrN lattice. With increasing Zr contents, the lattice parameters increase but the grain sizes show little effects. All the coatings exhibit dense compact columnar structures in SEM cross-sectional observations. Electrochemical measurements in 3.5% NaCl solutions revealed that the additions of Zr into CrN coatings improved their chemical inertness. The coated samples with much low corrosion current densities in nA/cm
2 range show their excellent protective characteristics to the stainless steel substrates. The corrosion mechanism, however, was due to the slight pitting corrosions, which were mainly localized in the growth defects.
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