Chengjian Xiao

Constructing three-dimensional (3D) nanocrystalline models of Li4SiO4 for numerical modeling and simulation

The three-dimensional (3D) nanocrystalline models of lithium silicates with the log-normal grain size distribution are constructed by constrained Voronoi tessellation. During evolution process, the algorithm is improved. We proposed a new algorithm idea by combining Genetic Algorithm (GA) with Least Square (LS) method to make up for the disadvantages of traditional genetic algorithm which may be easily trapped in local optimal solution. In the process of modeling, it is the first time, to the best of our knowledge, that we keep the whole sample showing the charge neutrality by deleting the excess atoms on the polyhedron boundary during the modeling. By using the molecular-dynamics method, the relaxation procedure of nanostructured Li4SiO4 is carried out. The results show that the average mass density of the sample is slightly lower than the experimental data of the perfect crystal after relaxation process. In addition, boundary component proportion (BCP) and density reduction proportion (DRP) of the sample is obtained, respectively. The present results display a significantly reduced BCP but an increased DRP when increasing the mean grain size of the sample.

Analysis of hydrogen isotopes with quadrupole mass spectrometry

Hydrogen isotope separation is one of the most critical technological problems in nuclear fusion research, and, in order to assess accurately the performance of hydrogen isotope separation, quantitative analysis of hydrogen isotopes takes priority and becomes the first essential problem to be addressed. However, since hydrogen isotopes have almost identical shape, size, and chemical properties, separation and analysis of hydrogen isotopes is really not an easy task. By using the thermal-desorption spectroscopy (TDS) method, a quadrupole mass spectrometer (MS) was calibrated for the quantitative analysis of hydrogen isotopes in this paper with a methodic error less than ±3% using titanium hydride and titanium deuteride as the calibration standards. The linear response range of MS was extracted. Deviations that originated from the H+/D+/HD+ species revealing a negligible influence on real H2/D2 mixture analysis were also discussed. Due to the mass discrimination of the ion source and the isotopic fractionation effect of the molecular pump, the actual sensitivity of MS towards H2 and D2 is not the same, revealing some deviation from theoretical results.

Competition of surface reactions of tritium release from irradiated Li 4 SiO 4 pebbles

Out-of-pile tritium release experiments were performed on Li4SiO4 pebbles produced from lithium hydroxide under various compositions of sweep gas (He, 1.1% H2/He) and environmental moisture conditions. The experimental results indicate that tritium gas can be released directly from “dry” Li4SiO4 under pure He gas. This phenomenon did not happen on “wet” samples, which means that the chemical form of released tritium is sensitive to moisture. Adding H2 to sweep gas may increase the overall desorption rate of tritium gas through H2 isotope exchange reaction, which occurs at a lower temperature than those of directly released tritium gas. Yet, the threshold of the H2 isotope exchange reaction is higher than the desorption reaction of tritiated water. Consequently, the effect of H2 isotope exchange reaction would reduce significantly on water adsorbed Li4SiO4 samples.

First-principles study of electronic, dynamical and thermodynamic properties of γ-Li4SiO4

We have studied the structural, electronic and dynamic properties of γ-Li4SiO4 (lithium orthosilicate) using density functional theory (DFT) with the generalized gradient approximation (GGA). The crystal structure is fully relaxed. The electronic band structure and Density of States (DOS) calculations indicate that γ-Li4SiO4 is an insulator with an indirect band gap of 5.19 eV and it has a conduction band with the width of 5.92 eV and two valance bands with the width of 4.45 eV and 0.57 eV, respectively. In the partial DOS, Li and Si electronic densities increase more sharply than O atoms. Comparing with previous works, the phonon dispersion curves without negative frequencies are calculated along high symmetry points. By adding the Born effective charges in the phonon calculation, the LO–TO splittings are also calculated which indicate that γ-Li4SiO4 is polar and anisotropic. The optical modes of phonon frequencies at Γ point are assigned as Raman and Infrared-active modes. Additionally, the thermodynamic functions (entropy, internal energy, Helmholtz free energies and constant-volume specific heats) were determined by using the phonon DOS. The calculated results may provide useful guidance of γ-Li4SiO4 for future experimental studies in some degree.

Synthesis of Platinum Nanocrystals within Iodine Ions Mediated

A liquid-phase reducing method of synthesizing Pt nanocrystals was demonstrated, and dendrite-, cube-, and cuboctahedron-shaped Pt nanocrystals (NCs) with well-defined monomorphic were successfully synthesized through iodine ions mediated with the CTAB agent. When the KI concentration was increased to thirty times of K2PtCl4 at the nucleation stage, the high-quality Pt nanodendrites could be obtained. However, no matter how many KI were added at the growth age, only cube- and cuboctahedron-shaped Pt nanocrystals formed. The results of high-resolution TEM, EDX, and XRD indicated that the size and shape of Pt NCs could be turned by changing the concentration and time of KI. In the nucleation stage, it might be due to that some iodine ions adsorb on the surfaces of Pt NCs, which probably cause the rapid growth process resulting in the formation of Pt nanodendrites. In the growth stage, although high concentrations of I− ions could contribute to the shape control and generate bigger particles of Pt NCs, small Pt particles do not grow into dendrites. The insight into the role of I− ions in synthesis of Pt NCs reported here provided a viewpoint for clearly understanding the formation mechanism of anisotropic platinum nanostructures.

Ultrahigh Effective H2/D2 Separation in an Ultramicroporous Metal-organic Framework Material through Quantum Sieving

An ultramicroporous MOF material, {[Fe(OH)(H2bta)](H2O)}n, was diligently selected for the experimental investigations of ultralow-temperature separation of H2/D2 through quantum sieving. With an extreme two-dimensional confinement experienced by hydrogen molecules, an extraordinary separation factor as high as 41.4 ± 0.4 at 20 K was for the first time obtained from experiment.

Dissociation mechanism of H2 molecule on the Li2O/hydrogenated-Li2O (111) surface from first principles calculations

Hydrogen molecules in a purge gas are known to enhance the release of tritium from lithium ceramic materials, which has been demonstrated in numerous in-pile experiments. The static computational results suggest that the molecular adsorption of H2 on the “ideal” Li2O/hydrogenated-Li2O (111) surface encounters high dissociation barriers in various entrance channels. The surface chemical inertness of the plane can be broken by introducing vacancy defects. In the present work, a combination of static DFT calculations and ab initio molecular dynamics has been performed to investigate the H2 dissociative mechanism. Our theoretical results, that the end-on oriented H2 could dissociate on the hydrogen monomer vacancy surface with one hydrogen atom ejected into the gas phase by the abstraction channel and the parallel H2 molecule dissociates on the hydrogen dimer vacancy surface with two hydroxyls forming, suggest that hydrogen vacancy defects facilitate the adsorption and dissociation of H2 molecule. The presence of the O2− ion induced by the hydrogen vacancy provides some low energy states in which the H2 electrons can be accommodated. This is very instructive for the comprehension of phenomena that occur during the operation of a thermonuclear reactor.