Results 1 - 10 of 293995
Results 1 - 10 of 293995. Search took: 0.104 seconds
|Sort by: date | relevance|
[en] The out of core structural material in the primary heat transport system of Indian Pressurized Heavy Water Reactor is Carbon Steel (CS). Prolonged operation of these reactors at high temperature and high pressure leads to the formation of activated corrosion products which get deposited on out-of-core surfaces causing radiation field build up and man-rem problems. Though the corrosion rate of CS is reduced by passive magnetite (Fe3O4) film, the radioactivity transport problem necessitates further reduction in the metal ion release. Metal ion passivation method is used to modify the oxide films to control the base metal corrosion. In this context, experiments were carried out to know the extent of improvement in corrosion resistance of CS with the addition of external metal ions such as Ni2+, Zn2+ and Mg2+ compared to Fe3O4. In addition, the effect of Mg2+ on the surface oxide films formed on Zircaloy-2 and Incoloy-800 were also studied. For CS, all the experiments were carried out in a static autoclave with lithium hydroxide in presence of Ni2+, Zn2+ and Mg2+ ions respectively at 250°C exposed for 96 and 240 hours by hydrothermal method. For Incoloy-800 and Zircaloy-2, experiments were done in a dynamic loop for duration of 7, 21 and 42 days only with magnesium ion. The films were characterized both by surface analytical techniques like Raman and GI-XRD for compositional analysis and also by electrochemical techniques for evaluating corrosion resistance properties. On CS, the films grown in presence of metal ions exhibited a higher polarization resistance compared to its absence by impedance studies. Potentiodynamic anodic polarization studies showed a similarity in corrosion current densities for the films obtained in the absence and presence of metal ions. The calculated thickness of these modified films using Clarke’s method was lower in presence of Zn2+ and Mg2+ ions compared to Ni2+ ions and Fe3O4. From the results, it was inferred that the zinc and magnesium ferrites form a more adherent and compact film compared to Fe3O4 and nickel ferrite. On both Incoloy and Zircaloy, a restructuring of oxide was indicated as a function of time and the corrosion rates and defect densities were found to be significantly lower on Mg2+ incorporated oxides. (author)
[en] GaN/AlN resonant tunneling diodes (RTDs) are studied to realize a high-speed nonvolatile memory based on the intersubband transactions and electron accumulation in the quantum well, which has the potential to operate at picosecond time scales. The crystal quality of GaN/AlN RTDs is improved by changing the growth conditions and structure of the buffer layer. The surface roughness and dislocation density of the GaN/AlN RTDs are successfully suppressed, and clear ON/OFF switching due to intersubband transitions is observed by inputting pulse voltage sequences. However, the voltages for write and erase operations are changed by improving the crystal quality of GaN/AlN RTDs. The theoretical analysis of resonant levels in the GaN/AlN RTDs indicates that the voltages for write and erase operations are very sensitive to the well and barrier widths and the density of electrons accumulating in the quantum well. Based on the results, the design of GaN/AlN RTDs for higher-performance nonvolatile memory operations is investigated. (© 2020 Wiley‐VCH GmbH)
[en] IN-738 turbine blade samples, deteriorated after long term service at high temperatures, were solution heat-treated at two temperatures, 1398 K and 1473 K, for 7.2 ks. Subsequently, the samples were cooled down in different atmospheres, in air and in furnace, for the purpose of studying the effects of different cooling media (rates) on the restored microstructures. Following this, the samples were aged at 1118 K for 43.2 ks and 86.4 ks in order to determine the characteristic of re-precipitated gamma prime particles. A scanning electron microscope (SEM) and ImageJ analysis software were used. The results show that the cooling in air provided gamma prime particles re-precipitating in spherical shape while the cooling in a furnace resulted in coarse gamma prime particles re-precipitating in irregular shape. The samples solutionized at 1398 K for 7.2 ks cooled down in air and then aging at 1118 K provided bimodal microstructure, while the sample solutionized at 1473 K for 7.2 ks, followed by air cooling and aging at 1118 K generated unimodal γ’ precipitation in spherical shape. Cooling in a furnace provides coarse γ’ recipitated particles in more irregular shape for the both solutionizing temperatures studied here. Cooling in a furnace provides coarse γ’ precipitated particles in more irregular shape for the both solutionizing temperatures studied here.
[en] This article presents results of the study on the dependence of the structural-phase state of alloys based on Ti-12.52Al-43.08Nb system (wt.-%) on the temperature of spark plasma sintering. It has been established that spark plasma sintering of Ti-Al-Nb alloys under the temperature of 1500 °C resulted in melting the aluminum component of the mixture that, in turn, negatively affects the quality of ready products. It has also been shown that stepping up the sintering temperature from 1000 °C to 1300 °C leads to increasing volume fraction of O-phase up to 49.63 % due to rapid precipitation of O-phase from B2-phase and TiAl-phase. It has been revealed that intermetallic composites obtained under the temperature of 1300 °C are characterized by a dominant two-phase В2+О structure which is more suitable for strengthening sorption properties of hydrogen-storing materials based on Ti-Al-Nb.
[en] Herein, the optically induced operation of ZnO-based laser structures is reported, fabricated with plasma-assisted molecular beam epitaxy (PA-MBE) on native ZnO substrate. ZnMgO is used both to confine the optical mode within ZnO waveguide and to form quantum barriers of ZnO quantum wells. The resonator of these devices is defined by reactive ion etching (RIE) with a chlorine/argon plasma. The lowest laser threshold is measured to be approximately 0.4 MW cm at room temperature when excited via the third harmonic of a YAG:Nd (355 nm). It is observed that the mode spacing depends on both the resonator length and the excitation power density, which is explained by introducing plasmonic corrections to the waveguide refractive index. (© 2020 Wiley‐VCH GmbH)
[en] The quality of welded joints depends on the most optimal welding parameters and the selection of shielding gas type. The shielding gas was selected for joining stainless steels through gas metal arc welding methods by considering properties such as chemical-metallurgical interaction of shielding gas and the molten weld metal during the welding process, heat transmission capability of the gas and cost. In this study, the effect of different shielding gas combinations on the mechanical and microstructural properties of 316 austenitic stainless steel joined by the metal inert gas (MIG) welding method was investigated. In the welding process, pure argon (100 % Ar), 98.5 % Ar + 1.5 % H and 95 % Ar + 5 % H were used as shielding gases. Tensile, hardness, and bending tests were conducted to determine mechanical properties of the welded samples. In addition, metallographic examinations were carried out to detect the macrostructural and microstructural properties of weld zones. According to the results obtained from the study, the highest tensile strength was obtained from the joints welded using 100 % Ar shielding gas. When the addition of H into the Ar gas increased, the tensile strength of the welded samples decreased. As a result of the tensile test, fractures occurred in the base metal in all welded samples. In all welding parameters, the hardness of the weld metal was lower as compared to the heat affected zone (HAZ) and the base metal. As a result of the bending test, crack and tearing defects were found in the weld zone.
[en] Optical absorption and emission spectra are the important quantifiable properties for CuI as a promising optoelectronic material. Previous research on the sputter deposition of CuI focuses on room-temperature growth. Herein, the effect of growth temperature on the selected optical features of sputtered CuI thin films is investigated. An enhanced visible light transparency and a steeper absorption edge are achieved for CuI thin films by optimizing the growth temperature. The PL intensity ratio of free exciton to defect-related emission increases with increasing substrate temperature. These results suggest a strategy of growth temperature optimization for the enhanced absorption and emission of CuI for advanced optoelectronic applications. (© 2020 Wiley‐VCH GmbH)
[en] A nano-sized two-terminal memristor exhibiting volatile threshold switching (TS) is a promising candidate for the emulation of biological synaptic functions to realize efficient neuromorphic computing systems. The Ca dynamics play a vital role in generating a temporal response for neural functions by changing the synaptic weight of biological synapses. Herein, a thinnest synaptic device is fabricated demonstrating drift dynamics of Ag migration through the exfoliated h-BN sheets, which emulates neuromorphic computing operations. The TS characteristics with a large I up to ≈10 lead to bio-synaptic applications, including short-term and long-term memory. The experimental realization of the synaptic behavior is demonstrated with paired-pulse facilitation (PPF), spike-rate-dependent plasticity (SRDP), and transition from short-term plasticity (STP) to long-term plasticity (LTP). The transition from STP to LTP in this synaptic device verifies the Atkinson and Shiffrin psychological model of human brain learning experimentally. The input pulses with different spike-times are used to replicate the synaptic functionalities. The two-terminal diffusive memristors constructed with thin sheets of 2D-flexible h-BN resistive materials may lead to flexible neuromorphic devices for biological applications. (© 2020 Wiley‐VCH GmbH)
[en] Herein, new spin correlated-plasmons in PrSrCoMnO (x = 0-0.4) that are tunable and stable at room temperature are demonstrated. Using advanced spectroscopic techniques, the charge, spin, and orbital interaction that triggers the spin correlated-plasmons in spin-polarized Co, O, and Mn is identified. As a function of x, Co systematically changes the spin state of the spin correlated-plasmons (from high, intermediate, to low spin), which also sequentially transforms the metallic cobaltite into a Mott insulator. Spin correlated-plasmons in the IR range, being excited from Co, tend to dominate in the metallic phase of the cobaltites, whereas Mn and O are the primary source for the spin correlated-plasmons in the UV-vis ranges in the insulating phase. (© 2020 Wiley‐VCH GmbH)
[en] Single-molecule magnets (SMMs) are at the forefront of new technological advances in quantum information processing and spintronics. Despite the recent impressive breakthroughs in extending the magnetic blocking temperatures beyond liquid-nitrogen temperatures, significant challenges await in terms of integrating and addressing such compounds in devices. With this ultimate goal in mind, the design of multifunctional SMMs not only allows to imbue molecules of interest with specific properties that would allow for in situ monitoring of the SMM operation in real time, but can also provide critical insights into our understanding of the magnetic behaviour. In this Review, we highlight how magnetism and luminescence can be harmoniously combined within single molecules to achieve these objectives. The key design principles to attain the simultaneous combination of photoluminescence and slow relaxation of the magnetization are discussed, along with an outlook on how such molecules could be beneficial for emerging next-generation spintronics devices. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)