Wenguan Liu

Vacuum electron field emission from SnO2 nanowhiskers synthesized by thermal evaporation

Rod-shaped and wire-shaped SnO2 nanowhiskers were synthesized by thermal evaporating of tin powders at 900 °C. Three Raman peaks (474, 632, 774 cm−1) showed the typical feature of the rutile phase of as-synthesized SnO2 nanowhiskers, which was consistent with the result of x-ray diffraction. A relatively low turn-on field of 1.37 V µm−1 at a current density of 0.1 µA cm−2 was obtained. The dependence of emission current density on the electric field followed a Fowler–Nordheim relationship. Our results indicated that SnO2 nanowhiskers had an interesting FE property as a wide band gap semiconductor.

Photoluminescence and photosensitive properties of ZnO strands self-twined by nanowires

Using argon as the transporting gas, freestanding macroscopic ZnO strands have been synthesized by evaporating metal zinc pellets at 900 °C in a quartz tube. Scanning electron microscopy images show that the macroscopic ZnO strands are self-twined by ZnO nanowires with a diameter in the range of 20–30 nm. A ZnO strand synthesized under larger argon flux contains more oxygen vacancy, which will result in a stronger green emission at 2.43 eV. Under illumination with a power of 10 W, the photosensitivity of the ZnO strand is found to be about 800 at the bias voltage of 20 V. It is also experimentally demonstrated that the recovery time of the ZnO strand is relatively long due to the dominating surface process.

A comparative first-principles study of the electronic, mechanical, defect and acoustic properties of Ti2AlC and Ti3AlC

Ti2AlC and Ti3AlC are both considered as candidate materials for structural applications in harsh environments. In this paper, we adopt density functional theory to study the electronic, elastic, acoustic and defect properties of Ti2AlC and Ti3AlC. It was found that the mechanical properties of Ti2AlC and Ti3AlC vary little; the bulk modulus at equilibrium of Ti3AlC is 10% higher than that of Ti2AlC. However, we found that their defect properties were very different. The migration barrier of an Al vacancy in Ti2AlC is 0.834 eV compared with 4.518 eV in Ti3AlC. The difference by a factor of six in the migration barriers is explained by the bond angle variance of the C-centred octahedral. Using the calculated elastic constants, the slowness surface of the acoustic waves is obtained using the Christoffel equation. Since Ti2AlC and Ti3AlC are often presented as a second Ti–Al–C phase in each other, the individual properties calculated can be used to assess their characterizations in experiments.

First-principles study of the effect of phosphorus on nickel grain boundary

Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.

Buried tungsten silicide layer in silicon on insulator substrate by Smart-Cut®

A single-crystalline Si/SiO2/poly-WSix/Sub-Si structure has been successfully fabricated by a new method incorporating a standard smart-cut® technique and a high temperature reaction between tungsten and silicon. Annealing at 800–1100 °C does not only strengthen the bonding of the wafers but also induces solid phase reaction of deposited tungsten and silicon. A poly-crystalline WSix (1 < x < 2) layer with a tetragonal structure is formed below the buried oxide layer. Cross section images of TEM show three steep interfaces of the four layers. It is found that increasing the annealing temperature is in favour of decreasing the sheet resistance of tungsten silicide and improving the crystal quality of the top silicon layer. However, a spreading resistance profile measurement shows that annealing under high temperature (≥1000 °C) will induce diffusion of tungsten into the Si substrate which is confirmed by the EDX results and the reason is presented.

Development of the Tritium Transport Analysis Code for the Thorium-Based Molten Salt Reactor

Abstract The Thorium-Based Molten Salt Reactor (TMSR) has been highlighted for its safety, economy, and nuclear nonproliferation. A program for developing the TMSR system has been launched in Shanghai Institute of Applied Physics, Chinese Academy of Sciences. In the TMSR system, mixtures of LiF and BeF2, termed FLiBe, are proposed and used as the primary coolant salt, in which tritium is produced mainly by the neutron reactions of lithium. In the TMSR system, at high temperatures, tritium can permeate through metal walls to the surroundings, leading to a potential radiological hazard. Thus, tritium control becomes a major problem hindering the development of the TMSR system. Evaluation of the tritium distribution is necessary for tritium control in the TMSR system. In this study, the Tritium Transport Analysis Code (TTAC) has been developed for simulating the tritium behaviors in the TMSR system (hence, the code TMSR-TTAC), such as tritium chemical forms in coolant salts, tritium transport behaviors, and tritium distribution in the system. The model code is developed by the MATLAB/SIMULINK package, and it is based on the mass balance equations of the tritium-containing species and hydrogen. TMSR-TTAC is benchmarked with the molten salt reactor model, which is based on Molten Salt Reactor Experiment designs. The results show that TMSR-TTAC has the ability to calculate the tritium distribution in the TMSR system.

Tritium Transport Analysis in a 2-MW Liquid-Fueled Molten Salt Experimental Reactor with the Code TMSR-TTAC

Abstract In the Thorium-Based Molten Salt Reactor (TMSR), tritium is produced at a high rate, which results in huge difficulties regarding tritium control. Tritium distributions in a 2-MW liquid-fueled molten salt experimental reactor (TMSR-LF1) were simulated with the TMSR–Tritium Transport Analysis Code (TTAC) (TMSR-TTAC) that was developed for analysis of tritium behaviors in the TMSR. The simulation for normal operation showed that about 60% of the tritium would permeate through the metal walls of the system, 25% of the tritium was removed by the purge gas system, and 15% of the tritium was absorbed on the core graphite. In addition, the effects on tritium distribution of the chemical-redox potential in fuel salt, the tritium permeation behavior through the metal walls, and various tritium removal methods in the TMSR-LF1 have also been simulated. The simulation results based on those conditions are analyzed in this paper to improve the knowledge of tritium behavior in the TMSR-LF1 and to provide reliable methods and strategies for tritium control in the TMSR system.

Strain-induced tunable negative differential resistance in triangle graphene spirals

Using non-equilibrium Greens function formalism combined with density functional theory calculations, we investigate the significant changes in electronic and transport properties of triangle graphene spirals (TGSs) in response to external strain. Tunable negative differential resistance (NDR) behavior is predicted. The NDR bias region, NDR width, and peak-to-valley ratio can be well tuned by external strain. Further analysis shows that these peculiar properties can be attributed to the dispersion widths of the p z orbitals. Moreover, the conductance of TGSs is very sensitive to the applied stress, which is promising for applications in nanosensor devices. Our findings reveal a novel approach to produce tunable electronic devices based on graphene spirals.