Tieshan Wang

Hydrogen release from diamondlike carbon films due to thermal annealing in vacuum

Abstract Depth profiles of absolute hydrogen concentration in diamondlike carbon (DLC) films deposited on glass substrate by a dual-ion beam sputtering system and in thermally annealed DLC films (pressure: 6.65 × 10−3 Pa) at 300, 400, 500 and 600°C were measured by the Elastic Recoil Detection Analysis (ERDA) using a 2.4 MeV 4He2+ ion beam. The thermal annealing induced Cue5f8H bond changes in films were investigated by the infrared transmittance spectra of film layers. It showed that the thermal annealing led to the release of hydrogen from the films and the breaking of Cue5f8H bonds in the films. The threshold temperature for hydrogen release was about 400°C, but the breaking of Cue5f8H bonds occurred mainly at temperature higher than 400°C. At 500 and 600°C, most Cue5f8H bonds were broken while the annealed films still contained a certain amount of hydrogen atoms which were not in the form of Cue5f8H bonds. It also showed that hydrogen release occurred at both the films surface and the film/substrate interfaces. At temperature higher than 400°C, the release speed of hydrogen through the interfaces was much higher than that through the films surface. This then led to un-uniform distribution of retained hydrogen atoms in the films. The present results were compared with the observations reported before and good agreement was obtained.

Structural changes and tribological behaviors of nitrogen ion-implanted tantalum

Abstract Single-crystal tantalum sheets were implanted by 110 keV nitrogen ions to a dose of 5 × 10 17 ions/cm 2 at a temperature less than 100°C. The structural changes and the concentration depth profiles of the implanted layers were characterized by glancing-angle X-ray diffraction (GXRD), selected area diffraction (SAD) and Auger electron spectroscopy (AES), respectively. The microhardness, the friction coefficient and the wear rate of the specimens against hardened GCri5 steel balls were also determined before and after the implantation. Scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA) were used to analyze the wear tracks. The results showed that there exist deviations between the characterization results, of AES and GXRD or SAD. The AES measurement suggests the formation of the tantalum nitrides with a maximun N/Ta ratio of 1/2, while the GXRD and SAD reveal the formation of bcc Ta(N). fcc TaN and a trace amount of hcp Ta 2 N. This can be explained by considering the inhomogeneous distribution of nitrogen atoms in micio-regions: the enrichment of nitrogen atoms in local micro-regions leads to the formation of fcc TaN; however, the unfavorable structural compatibility between bec Ta and hcp Ta 2 N hinders the formation of hcp Ta 2 N in the regions where the N/Ta ratio reaches 1/2. The detected trace amount of the hcp Ta 2 N phase in the implanted layers can be considered as an afier effect of nitrogen; loss from the originally formed nitrides. The results also showed that the tribological properties of the Ta surfaces were improved due to the implantation. It is believed that the implantation-induced Ta(N), fcc TaN, and hcp Ta 2 N phases are responsible for this improvement.

Grain growth of nanocrystalline 3C-SiC under Au ion irradiation at elevated temperatures

Nanocrystalline silicon carbide (SiC) represents an excellent model system for a fundamental study of interfacial (grain boundary) processes under nuclear radiation, which are critical to the understanding of the response of nanostructured materials to high-dose irradiation. This study reports on a comparison of irradiation effects in cubic phase SiC (3C-SiC) grains of a few nanometres in size and single-crystal 3C-SiC films under identical Au ion irradiation to a range of doses at 700 K. In contrast to the latter, in which the lattice disorder is accumulated to a saturation level without full amorphization, the average grain size of the former increases with dose following a power-law trend. In addition to coalescence, the grain grows through atomic jumps and mass transport, where irradiation-induced vacancies at grain boundaries assist the processes. It is found that a higher irradiation temperature leads to slower grain growth and a faster approach to a saturation size of SiC nanograins. This unusual behaviour could be associated with irradiation-induced grain nucleation and growth in amorphous SiC matrix in which the 3C-SiC grains are embedded. The results could potentially have a positive impact on structural components of advanced nuclear energy systems.

Visible to deep ultraviolet range optical absorption of electron irradiated borosilicate glass

To study the room-temperature stable defects induced by electron irradiation, commercial borosilicate glasses were irradiated by 1.2 MeV electrons and then ultraviolet (UV) optical absorption (OA) spectra were measured. Two characteristic bands were revealed before irradiation, and they were attributed to silicon dangling bond (E-center) and Fe3+ species, respectively. The existence of Fe3+ was confirmed by electron paramagnetic resonance (EPR) measurements. After irradiation, the absorption spectra revealed irradiation-induced changes, while the content of E-center did not change in the deep ultraviolet (DUV) region. The slightly reduced OA spectra at 4.9 eV was supposed to transform Fe3+ species to Fe2+ species and this transformation leads to the appearance of 4.3 eV OA band. By calculating intensity variation, the transformation of Fe was estimated to be about 5% and the optical absorption cross section of Fe2+ species is calculated to be 2.2 times larger than that of Fe3+ species. Peroxy linkage (POL, ≡Si–O–O–Si≡), which results in a 3.7 eV OA band, is speculated not to be from Si–O bond break but from Si–O–B bond, Si–O–Al bond, or Si–O–Na bond break. The co-presence defect with POL is probably responsible for 2.9-eV OA band.

Lattice damage and compositional changes in Xe ion irradiated InxGa1-xN (x = 0.32−1.0) single crystals

Lattice disorder and compositional changes in InxGa1-xN (xu2009=u20090.32, 0.47, 0.7, 0.8, and 1.0) films on GaN/Al2O3 substrates, induced by room-temperature irradiation of 5u2009MeV Xe ions, have been investigated using both Rutherford backscattering spectrometry under ion-channeling conditions and time-of-flight secondary ion mass spectrometry. The results show that for a fluence of 3u2009×u20091013u2009cm−2, the relative level of lattice disorder in InxGa1-xN increases monotonically from 59% to 90% with increasing indium concentration x from 0.32 to 0.7; a further increase in x up to 1.0 leads to little increase in the disorder level. In contrast to Ga-rich InxGa1-xN (xu2009=u20090.32 and 0.47), significant volume swelling of up to ∼25% accompanied with oxidation in In-rich InxGa1-xN (xu2009=u20090.7, 0.8, and 1.0) is observed. In addition, irradiation-induced atomic mixing occurs at the interface of In-rich InxGa1-xN and GaN. The results from this study indicate an extreme susceptibility of the high In-content InxGa1-xN to heavy-ion irradi...

Nitrogen ion implantation for the surface modification of 9Cr18Mo steel

Abstract The effect of nitrogen ion implantation on the electrochemical and fretting behavior of 9Cr18Mo steel has been studied. Anodic polarization measurement in a 0.1N sulphuric acid solution was used to evaluate the corrosion resistance of the steel before and after nitrogen ion implantation. Lower passive current density and higher corrosion potential were obtained on the implanted specimens. The best effect was observed on the fourth polarization scan in multipolarization tests, which correlated well with the nitrogen concentration depth profile determined by AES. The variation of corrosion current density vs corrosion time was also tested. The corrosion current density of the implanted specimen is two orders of magnitude lower than that of the unimplanted specimen, indicating that the corrosion rate of the implanted surfaces is much lower. Fretting tests were also used to evaluate the effect of nitrogen ion implantation. The fretting resistance was clearly improved after implantation.

Structural origin of hardness decrease in irradiated sodium borosilicate glass

Mechanical properties such as hardness and modulus of sodium borosilicate (NBS) glasses in irradiation conditions were studied extensively in recent years. With irradiation of heavy ions, a trend that the hardness of NBS glasses decreased and then stabilized with increase of dose has been reported. Variations in network structures were suggested for the decrease of hardness after irradiation. However, details of these variations in a network of glass are not clear yet. In this paper, molecular dynamics was applied to simulate the network variations in a type of NBS glass and the changes in hardness after xenon irradiation. The simulation results indicated that hardness variation decreased with fluence in an exponential law, which was consistent with experimental results. The origin of hardness decrease after irradiation might be attributed to the break of Biv-O links that could be derived from the (1) decrease of average coordinate number of boron, (2) decrease of Si-O-Biv bonds, and (3) increase of non-bridging oxygen.

High temperature and full-in-plane-direction workable high-frequency soft magnetic epitaxial FeSi thin films on MgO(0 0 1)

The epitaxial FeSi(0 0 1)[1 1 0]//MgO(0 0 1)[1 0 0] films were fabricated by sputtering and post annealing at 800 degrees C. A four-fold symmetric angular dependence of remanence ratios and coercivities of FeSi films were observed and well fitted by theoretical models considering the cubic anisotropy. The experimental ferromagnetic resonance frequency (f(r)) of epitaxial FeSi films reaches to 8.0 GHz, which is in agreement with the theoretical value derived from Landau-Lifshitz-Gilbert equation at room temperature. Moreover, the resonance phenomenon can be observed in any in-plane directions in contrast with the absence of resonance phenomenon in some specific directions for in-plane uniaxial soft magnetic Fe2Co films. Although the saturation magnetization, cubic anisotropy constant and fr all decrease with increasing temperature, fr still can keep as high as 3.2 GHz at 800 K, indicating that the epitaxial FeSi films with high Curie temperature have potential application in full angle workable microwave devices at relatively high temperature.

Morphology and elemental behaviour in the alteration layer of borosilicate glass in simulated groundwater

ABSTRACT To understand the behaviour of nuclear waste glass in groundwater, borosilicate glasses were placed in simulated groundwater for more than 200 days. The composition of the simulated groundwater was similar to that of the groundwater in Beishan (a potential nuclear waste site). The pH value of groundwater was adjusted to 7.5, and the ratio of the surface area of glass to the volume of the solution (SA/V) was set to 10u2009m−1. Solutions and bulk glasses were characterised to obtain the elemental behaviour and surface morphology of the glass/solution interface, which was named the alteration layer. The mean thicknesses of the alteration layer were 5.16u2009±u20090.11u2009µm and 11.67u2009±u20090.28u2009µm at 70°C and 90°C, respectively. A thicker alteration layer was attributed to the lower surface activation energy of the glass and a high ion exchange between K+ and Na+ in the interface between the glass surface and the solution. For the elemental behaviour, mobile species B and Na were depleted, while K and Ca from the solution were enriched in the alteration layer due to ion exchange. Network species Si decreased in the layer, leading to the corrosion of the backbone of the glass; however, species Al increased, which implied that some [SiO4] units were partially replaced by [AlO4] units. In this work, glass in groundwater suffered much more intense corrosion than that in de-ionised water.