Structural study of the oxidation process and stability of NbOxNy coatings

In the present work, we study the oxidation behaviour of NbON multilayer films. The films were deposited by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayered coatings with a period of λ = 10 nm. Three different films with increasing duty cycles have been deposited.Rutherford backscattering spectroscopy (RBS) was used to study the chemical composition variations at different annealing temperatures (as-deposited, 400 °C, 500 °C and 600 °C) combined with X-ray diffraction (XRD) to identify the crystalline phases formed. At 400 °C, for all films a very thin layer starts to form at the surface with enhanced O concentration. The composition of the deeper part of the samples remains unchanged. At 500 °C, the oxide scale grows, encompassing about half the film thickness. At 600 °C, the process is finished and a single layer is formed with reduced Nb and increased O concentration. Fourier-transformation infrared spectroscopy (FTIR) results confirmed the increase of this surface oxidation, while XRD revealed that crystallization of Nb₂O₅ occurs at 600 °C.     

Structures of Au nanowires encapsulated in carbon nanotubes

Classical molecular dynamics simulations have been used to investigate the structure of Au nanowires encapsulated in single-walled carbon nanotubes (SWCNT). It was found that Au nanowires with helical multishell structures can be formed within the SWCNTs. Each shell is composed of helical rows of atoms. The distance between the tube wall and the outermost shell of the Au nanowire is about 3 Å, and the spacing between the Au intershells varies from 2.08 to 2.33 Å. The radii of the SWCNTs and the numbers of filled Au atoms play dominant roles in deciding the final structures of the Au nanowires formed within the SWCNTs over the range of diameters considered. In SWCNTs with a given diameter, the Au nanowires with helical multishell structure will be formed when enough Au atoms have filled the nanotube. Otherwise, Au nanowires with a bulky fcc structure will be formed.     

Dissolved oxygen control and monitoring implementation in the liquid lead-bismuth eutectic loop: HELIOS

A 12 m tall LBE coolant loop, named as HELIOS, has been developed by thermal-hydraulic scaling of the PEACER-300MWe. Thermo-hydraulic experiment and materials test are the principal purposes of HELIOS operation. In this study, an yttria stabilized zirconia (YSZ) based oxygen sensor that was hermetically sealed for long-term applications using the electromagnetically swaged metal-ceramic joining method, have been developed for high temperature oxygen control application over a long period of time. The rugged electrode design has been calibrated to absolute metal-oxide equilibrium by using a first principle of detecting pure metal-oxide transition using electrochemical impedance spectroscopy (EIS). During the materials tests in HELIOS, dissolved oxygen concentration was administered at the intended condition of 10⁻⁶ wt% by direct gas bubbling with Ar + 4%H₂, Ar + 5%O₂ and/or pure Ar while corrosion tests were conducted for up to 1000 h with inspection after each 333 h. During the total 1000 h corrosion test, oxygen concentration was measured by oxygen sensor. The result confirmed that the direct gas bubbling method is a viable and practical option for controlling oxygen concentration in large loops including HELIOS.     

Z2 structure and gas-solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas-solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ? v₀ is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z₂ structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas-solid difference in the velocity range under consideration. The possibility of such an effect due to a gas-solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule.We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe’s assumption of a stopping ratio independent of Z₂ and common for gases and solids in the considered velocity range, although the absolute value appears too high.     

RBS study of Ti/ZnO interface

We have studied the interface stability of the Ti(overlayer)/ZnO(substrate) system. Ti thin film was grown on the Zn face of single crystal ZnO(0 0 0 1) substrate by the vacuum deposition technique. The Ti film thickness was typically 16 nm. Then the samples were annealed in air at 300 and 400 °C for 15 min, respectively. The deposition and annealing effects on the interface structure were investigated with Rutherford backscattering and channeling spectroscopy using 2 MeV He⁺ ion beam. After Ti deposition the minimum yield from the ZnO substrate increased from 2% to 7%. This suggests severe damage caused by deposition, i.e. the interface reaction between Ti and ZnO (even at room temperature). A significant amount of Zn (approximately 6.4 × 10¹⁶ atoms/cm²) moved onto the surface after post-annealing at 400 °C. Since Ti has a stronger tendency to react with O than Zn, it is expected that Ti reacts with substrate oxygen leaving behind free Zn atoms, which can easily migrate onto the surface. We discuss how the Ti/ZnO interface reaction in detail, and seek to find another good metallic contact for ZnO devices, which are attracting much attention recently for practical applications as well as scientific aspects.     

High fluence nitrogen implantation in Al/Ti multilayers

We have studied the effects of high fluence nitrogen ion implantation on the structural changes in Al/Ti multilayers, with the aim of achieving multilayered metal-nitrides. The starting structures consisted of 10 alternate sputter-deposited Al and Ti films, with a total thickness of 270 nm, on (1 0 0) Si substrates. They were implanted with 200 keV , to 1 × 10¹⁷ and 2 × 10¹⁷ at/cm², the projected range being around half-depth of the multilayers. Structural characterization was performed by Rutherford backscattering, Auger electron spectroscopy and transmission electron microscopy. It was found that ion implantation to the higher fluence induces a full intermixing of Al/Ti layers, resulting in a multilayered structure with different content of Al, Ti and N. The applied method can be interesting for preparation of graded (Al,Ti)N multilayers, with a controlled content of nitrogen and a controlled level of Al-Ti intermixing within the structures.     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Degradation of elastomer by heat and/or radiation

This article studied various problems on the degradation of elastomers by heat and/or radiation. Three kinds of elastomers were irradiated and evaluated by the radiation resistant property using the measurement of tensile test. The fluorine containing elastomer, which has excellent heat resistant properties, was found to be less durable for irradiation than ethylene-propylene-diene (EPDM) elastomer. Ten kinds of different compounding formulas of EPDM were prepared to investigate whether the compounding for heat resistant has durability for irradiation. The thermal exposure was performed in an air oven. The duration of thermal exposure at 140 °C was 384 h. The irradiation condition was 5.0 kGy/h at 70 °C, and the total dose was 0.9 MGy. Elongation retained was taken for the evaluation of the stability. It was found that the formulas for improving the thermal stability did not bring radiation resistant of samples in the experiment.The rate constant of the increase in CO concentration by heat and radiation was measured and defined as k_c(h) and k_c(r), respectively. The rate constant of that under the combined addition of the heat and the radiation is expressed as k_c(h + r). Eq. (1) was obtained by the experiment and it was found that there is a synergistic relationship between heat and radiation on the increase in CO concentration

(1)     

Bilinear CTOD/CTOA scheme for characterisation of large range mode I and mixed mode stable crack growth through AISI 4340 steel

An investigation has been reported concerning characterisation of large span stable crack growth (SCG) through AISI 4340 steel in terms of CTOD/CTOA under both mode I and mixed (I and II) mode loadings. The characterisation has been possible through finite element analysis of published experimental results with compact tension type of specimens. As against the earlier observations by many investigators of a bilinear CTOD/CTOA variation, decreasing initially and constant later, characterising the crack growth through other materials, an increasing initially and constant later type of variation is found to be suitable for AISI 4340 steel. The same variation is found to characterise both mode I and mixed mode SCG. The starting value of CTOD/CTOA is 0.035 mm/0.0875 rad; the value at the later stages is 0.08 mm/0.20 rad. The same variation is found to predict accurate enough for engineering applications the initiation (P_i) and maximum (P_max) loads and the variation of load-displacement diagrams over a span of crack growth up to 10 mm.     

Corrosion of UO2 and ThO2: A quantum-mechanical investigation

The addition of Th to U-based fuels increases resistance to corrosion due to differences in redox-chemistry and electronic properties between UO₂ and ThO₂. Quantum-mechanical techniques were used to calculate surface energy trends for ThO₂, resulting in (1 1 1) < (1 1 0) < (1 0 0). Adsorption energy trends were calculated for water and oxygen on the stable (1 1 1) surface of UO₂ and ThO₂, and the effect of model set-up on these trends was evaluated. Molecular water is more stable than dissociated water on both binary oxides. Oxidation rates for atomic oxygen interacting with defect-free UO₂(1 1 1) were calculated to be extremely slow if no water is present, but nearly instantaneous if water is present. The semi-conducting nature of UO₂ is found to enhance the adsorption of oxygen in the presence of water through changes in near-surface electronic structure; the same effect is not observed on the insulating surface of ThO₂.     

FTIR and RBS study of ion-beam synthesized buried silicon oxide layers

Single crystal silicon samples were implanted at 140 keV by oxygen (¹⁶O⁺) ion beam to fluence levels of 1.0 × 10¹⁷, 2.5 × 10¹⁷ and 5.0 × 10¹⁷ cm⁻² to synthesize buried silicon oxide insulating layers by SIMOX (separation by implanted oxygen) process at room temperature and at high temperature (325 °C). The structure and composition of the ion-beam synthesized buried silicon oxide layers were investigated by Fourier transform infrared (FTIR) and Rutherford backscattering spectroscopy (RBS) techniques. The FTIR spectra of implanted samples reveal absorption in the wavenumber range 1250-750 cm⁻¹ corresponding to the stretching vibration of Si-O bonds indicating the formation of silicon oxide. The integrated absorption band intensity is found to increase with increase in the ion fluence. The absorption peak was rather board for 325 °C implanted sample. The FTIR studies show that the structures of ion-beam synthesized buried oxide layers are strongly dependent on total ion fluence. The RBS measurements show that the thickness of the buried oxide layer increases with increase in the oxygen fluence. However, the thickness of the top silicon layer was found to decrease with increase in the ion fluence. The total oxygen fluence estimated from the RBS data is found to be in good agreement with the implanted oxygen fluence. The high temperature implantation leads to increase in the concentration of the oxide formation compared to room temperature implantation.     

Radiation effects on ohmic and Schottky contacts based on 4H and 6H-SiC

A systematic study of Ni based ohmic and Schottky contacts (SCs) onto the n-4H-SiC and n-6H-SiC under relatively low-dose (1 × 10¹² e⁻ cm⁻²) and high-energy (6, 12, 15 MeV) electron irradiation (HEEI) has been introduced. Lower specific contact resistivity has been reached for Ni based ohmic contact structures on both 4H and 6H-SiC after each electron irradiation. This finding has been explained by the displacement damage produced by the collision of electrons with atoms of Ni contact material. It has been observed that the HEEI caused to increase in the ideality factors of both SCs indicating deviation from thermionic emission theory in current transport mechanism. While the Schottky barrier height (SBH) for Ni/4H-SiC SC remains nearly constant, an increase has been observed for the Ni/6H-SiC SC. Donor concentrations for both diodes have decreased with increasing electron energy probably due to the trapping effect of the irradiation induced defect(s).     

Influence of the residual oxygen in the plasma immersion ion implantation (PI3) processing of materials

In this work, we investigated the effects of the contaminants present in the vacuum chamber of the PI3 system, in particular, the residual oxygen, which results in the formation of the oxide compounds on the surface and hence is responsible for the high implantation energies required to achieve reasonably thick treated layers. We used a mass spectrometer (RGA) with a quadruple filter to verify the composition of the residual vacuum and pressure of the elements present in the chamber. Initially we found a high proportion of residual oxygen in a vacuum with a pressure of 1 × 10⁻³ Pa. Minimizing the residual oxygen percentage in about 80%, by efficient cleaning of the chamber walls and by improving the gas feeding process, we mitigated the formation of oxides during the PI3 process. Therefore we achieved a highly efficient PI3 processing obtaining implanted layers reaching about 50 nm, even in cases such as an aluminum alloy, where is very difficult to nitrogen implant at low energies. We performed nitrogen PI3 treatment of SS304 and Al7075 using pulses of only 3 kV and 15 × 10⁻⁶ s at 1 kHz with an operating pressure of 1 Pa.     

Behaviour of chromium steels in liquid Pb-55.5Bi with changing oxygen content and temperature

The behaviour of protective oxide layers on P122 steel and its welds and of ODS steel in liquid Pb_44.5Bi_55.5 (LBE) is examined under conditions of changing temperatures and oxygen concentrations. P122 (12Cr) and its welded joints are exposed to LBE at 550 °C for 4000 h with oxygen concentrations of 10⁻⁶ and 10⁻⁸ wt% (p(O₂) = 8.1 × 10⁻²³ bar and 5.2 × 10⁻²⁷ bar) which change every 800 h. It is found that like in case of constant oxygen concentration of 10⁻⁶ wt% a protective spinel layer (Fe(Fe_1−xCr_x)₂O₄) was maintained on P122 and also on its welded joint. Two experiments with exposure times of 4800 h are conducted on ODS steel, both with temperatures changing from 550 to 650 °C and back every 800 h, one experiment with 10⁻⁶ the other with 10⁻⁸ wt% oxygen in LBE. Both experiments show strong local dissolution attack after 4800 h which is in agreement with the behaviour of ODS in LBE at a constant temperature of 650 °C. However, dissolution attack is less in LBE with 10⁻⁸ wt% oxygen (p(O₂) = 3.0 × 10⁻²⁵ bar).     

Oxygen removal with D2-ICR cleanings after oxidation experiment in HT-7

The effectiveness of D₂-ICR cleaning with a pressure up to 0.18 Pa was surveyed on hot walls of 400-470 K for oxygen removal after oxidation experiment in the HT-7 superconducting tokamak. The oxygen removal rate in D₂-ICR cleaning was about (7-9) × 10²¹ O-atoms/h, about 5-10 times higher that that He-ICR cleanings before and after the D₂-ICR cleaning. In about 130 min He-ICR and D₂-ICR cleanings, about 8 × 10²¹ O-atoms were removed. After the cleanup, a lot of water still retained in HT-7 vessel and 67 disruptive plasmas were required before obtaining normal plasma. In present experiment, the recovered plasmas were easily controlled and much better than previous report in HT-7 oxidation experiments.     

Radiation cross-linking of small electrical wire insulator fabricated from NR/LDPE blends

A low voltage, radiation-crosslinked wire insulator has been fabricated from blends of natural rubber block (STR-5L) and LDPE with phthalic anhydride (PA) as a compatibilizer. Physical properties of the NR/LDPE blend ratios of 50/50 and 60/40 with 0.5, 1.0, and 1.5 wt% PA were evaluated. The gel content increased as the radiation dose increased. Tensile at break exhibited a maximum value of 12 MPa at 120 kGy for 1.0 and 1.5 wt% PA of both blend ratios. A higher PA content yielded a higher modulus for the same blend ratio. Blends of 60/40 ratio with 1.0 wt% PA and 0.8 wt% antimony oxide flame retardant gave the highest limiting oxygen index (LOI) of >30% at above 150 kGy. Other electrical properties of the wire insulator were investigated. It was found that an insulator fabricated from a PA content of 1.0 wt% in the NR/LDPE blend ratio of 50/50, after gamma ray cross-linked at a dose of 180 kGy in low vacuum (1 mm Hg), met the Thai Industrial Standard 11-2531 for low voltage wire below 1.0 kV. To comply with the standard for vertical flame test, a more suitable flame retardant was needed for the insulator.     

Effect of 50 MeV Li ion irradiation on electrical, optical and mechanical properties of 4,4′-dimethylbenzophenone

4,4′-Dimethylbenzophenone (DMBP) single crystals were irradiated at room temperature and at liquid nitrogen temperature with 50 MeV Li³⁺ ions at fluences 1 × 10¹² and 1 × 10¹³ ions/cm². The dielectric constant and dielectric loss as a function of frequency of the applied ac field in the range from 20 Hz to 1 MHz and at temperatures ranging from 313 to 353 K were analyzed. The dielectric constant decreases with increase in frequency for all the temperatures. The dielectric constant and dielectric loss increase with fluence. Optical absorption was measured at different conditions. UV-Vis studies reveal the decrease in bandgap. The unirradiated as well as irradiated crystals were characterized by photoluminescence. Ion-induced changes were also studied with respect to their mechanical response using the Vicker’s microhardness technique and parameters including fracture toughness, brittleness index and yield strength are calculated.     

M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.     

Growth, characterization and effect of substrate bias on ZnO prepared by reactive capillaritron ion beam sputtering deposition

ZnO thin films have been deposited on Si substrates by reactive ion beam sputtering deposition utilizing a capillaritron ion source. All the as-deposited ZnO films exhibit a preferred (0 0 2) growth direction while the grain size increases as oxygen partial flow rate increases. An optimum photoluminescence result was achieved using oxygen partial flow rate of ∼40% that the ratio of integrated deep level emission to that of integrated near-band-edge emission is less than 1.5. The atomic percentage ratio of Zn:O remains at 55/45, regardless of oxygen partial flow rates. With the applied substrate bias, the atomic percentage ratio of Zn/O changes to 40/60, indicating that stoichiometric ZnO may be achieved by selecting appropriate substrate bias. With the substrate bias applied, increased amount of oxygen atoms were found located in oxygen deficient matrixes, likely due to the bombardment of secondary ions.     

Surface modification of SUS304 stainless steel by atmospheric pressure Ar/N2/O2 plasma

The surface characteristics of SUS304 stainless steel are investigated before and after surface modification by Ar/N₂/O₂ plasma under atmospheric pressure conditions. It was found that plasma treatment of a stainless steel plate has a significant effect on the wettability, contact angle, and free energy of the SUS304 surface. The contact angle and surface free energy were analyzed. The optimal surface modification parameters are a power of 1000 W, a torch-to-sample distance of 80 mm, a treatment time of 300 s, and an oxygen content of 1.5 wt%. Under these processing conditions, a contact angle of just 1.60° was obtained. The surface morphology, surface element composition, and surface roughness of the treated SUS304 specimens were examined using scanning electron microscopy (SEM), and atomic force microscopy (AFM), respectively. The results show that the optimal surface modification conditions lead to the formation of fine, uniformly distributed crystallites in the SUS304 microstructure. Moreover, compared to the untreated surface, the treated surface had a significantly lower carbon content and a more uniform distribution of surface peaks.