Peng Shuming

Ab initio Study of He Stability in hcp-Ti

The stability of He in hcp-Ti is studied using the ab initio method based on the density functional theory. The results indicate that a single He atom prefers to occupy the tetrahedral site rather than the octahedral site. The interaction of He defects with Ti atoms is employed to explain the relative stabilities of He point defects in hcp-Ti. The relative stability of He defects in hcp-Ti is useful for He clustering and bubble nucleation in metal tritides, which provides the basis for development of improved atomistic models.

A pressure calibration method for a portable wide-access “panoramic” cell

A simple and convenient pressure calibration method is developed for a newly designed portable wide-access ‘panoramic’ cell. This cell is adapted to angle-dispersive-mode high-pressure in situ neutron diffraction of reactor neutron sources. This pressure calibration method has established a relationship between the cell pressure and the anvil displacement (gasket compression) based on the fixed-point calibration technique. By employing TiZr gasket with a thickness of 3 mm and WC anvil with a culet of 4 mm diameter, the average anvil displacements are 1.31 mm and 2.22 mm for Bi phase transitions (2.55 GPa and 7.7 GPa), and 1.85 mm for Ba phase transitions (5.5 GPa), respectively. In this pressure range, the pressure increases quickly with decreasing gasket thickness, and undergoes a linear increase with the anvil displacement. By extrapolating the calibration curve, the cell pressure will achieve 10 GPa when the anvil displacement is around 2.5 mm.

Formation of diamond powders from melamine under high pressure and high temperature

High pressure pyrolysis of melamine has been attracting great interest recently, due to it being considered as a suitable precursor to realize the g-C3N4 and even superhard C3N4. In this work, we studied the detailed pyrolysis behavior of melamine at 22 GPa. Melamine was stable at 800 °C, and decomposed to diamond in the form of powder at 1500–2000 °C under this pressure condition. At 2000 °C, the pure cubic diamond powders with 0.1–0.5 μm grain size were obtained. The diamond particles exhibited euhedral forms and dispersed to each other, we proposed that these novel features were caused by the presence of liquid N2 and NH3 during diamond formation. The high pressure pyrolysis of melamine may provide a new means of producing micrometer-sized diamond powders.

氢分子在Cu (111) 表面吸附与解离的第一性原理研究

采用基于密度泛函理论的第一性原理对氢分子与Cu (111) 表面的相互作用进行了研究。计算结果表明氢分子是否解离取决于氢分子距表面的初始距离和其初始构型。当氢分子垂直于Cu (111) 表面放置时，在距离表面0.37~4.0 A范围内，氢分子与Cu (111) 表面相互作用后均不会解离，物理吸附在Cu (111) 表面；当氢分子平行于Cu (111) 表面放置时，有的氢分子解离成氢原子后化学吸附于表面六角 (hcp) 或面心 (fcc) 位。氢分子在桥位 (bri) 并沿[211]方向平行靠近Cu (111) 表面时，氢分子解离的临界距离约为1.35 A，其他情况下在0.65~0.85 A之间。

Microstructure Evolution of Ti Tritides During Aging

Metal tritides are used for stable, high-density storage of tritium. The behavior of helium in metal tritides is of interst because He is produced by the radioactive decay of tritium. Helium accumulates in metal tends to precipitate into bubbles. During the first few years of storage, He is released at less than about 1% of the generation rate from many tritides. When the helium concentration reaches a critical value, helium is released at rates equal to and sometimes exceeding the generation rate. The size, density and distribution of helium bubbles is the main parameters controlling the helium bubble evolution during the aging of metal tritides. The object of this paper is to study the microstructure of helium bubbles in the Ti tritides by TEM.

Thermal Simulations of the Hohlraum Cryogenic Target: Low-Mode Control and Parameter Optimization

Abstract The inertial confinement fusion design requires smooth and uniform deuterium-tritium (D-T) ice layers in a spherical shell. One of the most important factors affecting the ice layer quality is the thermal environment around the capsule, which determines the low-mode thickness asymmetries of the D-T ice layer. In this paper, we report the thermal simulation results for the hohlraum cryogenic target in the China Academy of Engineering Physics, focusing on the ice layer roughness analysis and low-mode control. The simulations were carried out with a computational fluid dynamics code. A calculation procedure based on the dynamic mesh method was applied to predict the D-T ice layer profile. The sensitivity of ice layer low-mode roughness to some operating and thermal condition has been investigated. The conclusions in this paper provide better direction for our cryogenic target’s further design and improvement.

Trapping Behavior of He in Ti Revisited by ab initio Calculations

We report a detailed ab initio study of the trapping behavior of interstitial helium atoms (IHAs) in hcp Ti. The tetrahedral interstitial site for one He is confirmed to be the most stable IHA configuration, but the most favorable interstitial site comprises two adjacent octahedral sites for two helium atoms. The octahedral IHA can trap another IHA regardless of the site where it is initially located, whereas the tetrahedral IHA cannot. Hybridization among the different states is responsible for the stable order, which has significant implications for He clustering and bubble nucleation that can affect material performance in future fusion reactors. These results provide the basis for the development of improved atomistic models.

Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 300K. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium–titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.

Hydrogenation Properties of Zr Films Under Various Conditions of Hydrogen Plasma

The hydrogenation properties of Zr samples with and without an Ni overlayer under various plasma conditions were investigated by means of non-Rutherford backscattering and elastic recoil detection analysis. The theoretical maximum hydrogen capacity, 66.7 at%, could be achieved at a hydrogen absolute pressure of ~2 Pa and a substrate temperature of ~393 K for a plasma irradiation of only 10 min; this was significantly greater than that for gas hydrogenation under the same hydrogen pressure and substrate temperature. It was also found that the C and O contamination on the sample surface strongly influences the hydrogenation, and that the maximum equilibrium hydrogen content drops dramatically with the increasing total contamination. In addition, the influence of the Ni overlayer on the plasma hydrogenation is discussed.

Calculation of diffusion barriers for helium atom in metals

The diffusion barriers for the single helium atom in 3d transition metals are systematically studied by effective medium theory without any adjustable parameters. In the calculation, the relaxiation effects of lattice are taken into account. The comparison of our calculated results with the available experimental data and other theoretical values shows good agreement.