Wang Guanghou

Proton beam modification of isotactic polypropylene

Abstract A systematic investigation of 100 keV proton-irradiated polypropylene has been carried out using mechanical tests and infrared spectroscopy. Some other polymers such as polyethylene, polyvinyl chloride, polystyrene, polyacrylic nitride and natural resin are also studied by proton, γ-ray and neutron irradiation for comparison. The experiments have shown that the mechanical properties of proton-irradiated polypropylene have dramatically changed, and both its elongation and tensile strength increase significantly at certain irradiation doses, while the other polymers undergo a rapid degradiation after 100 keV proton irradiation. The infrared spectra indicate that conformational changes have taken place in the polypropylene under proton bombardment, such as the transition from an ordered to a disordered state in the crystalline region of polypropylene, with the formation of double bonds as well as trans-conformations. This leads to the cross-linking between macromolecules of polypropylene at proper irradiation doses, thus enhancing its mechanical properties; such changes may have some potential applications.

Radiation effects on polyethylene and polypropylene by electrons and protons

Abstract The irradiation of low-density polyethylene and polypropylene films has been carried out by using 20-keV proton and 8-MeV electron beams. Measurement of mechanical properties and X-ray diffraction have shown that the tensile strength and elongation of electron-irradiated polyethylene have improved as a function of electron dose, suggesting the occurrence of cross-linking with proper irradiation dosage. On the contrary, rapid degradation of electron-irradiated polypropylene has been noticed. Protons have quite different effects on these two polymers: they caused a small decrease in the mechanical properties of polyethylene on the one hand, and on the other the mechanical properties of polypropylene were greatly improved (increase in its tensile strength σ ( kg cm 2 ) by 51% and that of its elongation ϵt by 36%. This effect may be attributed to the collision of the incident proton beam with protons in the polypropylene chains, forming cross-links between macromolecules of propylene. This is probably the first time that the cross-linking of polypropylene by proton bombardment has ever been observed.

Cluster structure and dynamics of liquid aluminum under cooling conditions

The thermal evolution and dynamics properties of small clusters in melt are investigated by molecular dynamics simulation with 500 particles. The interatomic interaction is described by a well-fitted semiempirical tight binding potential. We have studied some solidlike clusters and structural properties of liquid aluminum. The atomic volume, internal energy, pair correlation function, and bonded pairs in liquid are obtained from quick cooling and slow cooling simulation. A comparison between the slow and quick cooling process reveals that the cooling rate significantly affects the evolution route of various clusters in liquid aluminum. Our results show that glass contains a lot of 1551 and 1541 five-fold symmetry clusters. On the contrary, the crystal has some 1421 and 1422 clusters corresponding to a face-centered cubic structure and hexagonal closing-packed structure. This work gives clear physical insight of the structure evolution from liquid to solid.

Structural studies of clusters in melt of FeAl compound

By means of constant pressure molecular dynamics simulation technique, a series of simulations of the Fe50Al50 alloy have been carried out. The atoms interact via semi-empirical n-body noncentral potential. The pair correlation functions and the pair analysis technique is applied to reveal the cluster evolution in the process of quick solidification. By using the bond orientation order parameters, we have measured both local and extended orientation symmetries for computer-generated models of dense liquid and glass. A lot of polyhedra in liquid systems, e.g., icosahedra, are also obtained. In order to test the reliance of the computational results, corresponding x-ray diffraction experiments have been performed on the material.

Morphologies of rutile form TiO2 twins crystals

nanjing univ, natl lab solid state microstruct, ctr adv studies sci & technol, nanjing 210093, peoples r china. acad sinica, inst met res, atom imaging solids lab, shenyang 110015, peoples r china. nanjing univ, modern anal ctr, nanjing 210093, peoples r china.;guang-lai, l (reprint author), nanjing univ, natl lab solid state microstruct, ctr adv studies sci & technol, nanjing 210093, peoples r china

Molecular dynamics study of icosahedral ordering and defect in the Ni3Al liquid and glasses

Abstract The amorphous transition in Ni 3 Al system is investigated in detail at a temperature from 2000 K down to 300 K with F–S potential by MD simulation. The pair correlation functions and the pair analysis technique are used to study the cluster evolution in liquid and glass. In addition to a structural change, a dramatic split of second peak of g ( r ) is in association with the glass transition. Especially, the number of the type of defective icosahedron defined by eight 1551 bonds, two 1661 bonds, and two 1441 bonds is much greater than the number of various Frank and Bernal polyhedra.

Helical multishell structures of magnesium nanowires

The structures of free-standing magnesium nanowires are systematically studied by using genetic algorithm simulations based on molecular dynamics with a tight-binding many body potential. Several helical multishell cylindrical growth sequences are observed for magnesium nanowires. The numbers of atomic strands show the even-odd or odd-even coupling between the inner and outershell with the differences of five strands, These multishell structures are made up of coaxial tubes in the three- and four-strands helical, centered pentagonal and hexagonal, and double-chain-core parallel curved surface epitaxy. The average binding energy and coordination numbers are calculated. The angular correlation functions and vibrational properties of magnesium nanowire have also been studied.

Low-Frequency and Abnormal Raman Spectrum in SnO2Nanorods

Raman-scattering spectra in SnO2 nanorods with different diameters were obtained at room temperature and the low-frequency Raman peaks have been observed for the first time. It was found that the low-frequency peaks shifted to high frequencies as the nanorod diameter decreased. The size dependence of the low-frequency peaks in SnO2 nanorods can be identified by the surface modes among the confined acoustic modes of SnO2 nanorods, given by solving Lamb theory. In addition, the Raman peaks of SnO2 nanorods not only vary with the excitation wavelength (514.5 nm and 785 nm), but have their line-width broadening and the line shapes asymmetric as well. Moreover, some IR active modes became Raman-active induced by disorders such as local lattice imperfections and oxygen deficiencies in the thinner nanorods when the diameter of the nanorod decreased down to 15 nm or smaller.

Molecular dynamics computation of the liquid structure of Fe50Al50 alloy

Abstract In this paper, a series of simulations of the Fe 50 Al 50 alloy have been carried out on the basis of the molecular dynamics simulation technique under constant-temperature, constant-pressure. The semi-empirical n-body non-central potential is used to describe the interaction between atoms. The three kinds of pair correlation functions and total pair correlation function are calculated to reveal the cluster evolution in the process of quick solidification. The pair analysis technique is adopted to check how the clusters in liquid metal change in the rapid cooling conditions. The four index parameters, i , j , l , m are used to distinguish the type of clusters in liquid. We measured both local and extended orientation symmetries for computer-generated models of dense liquid and glass by using the bond orientation order parameters. A lot of polyhedra in liquid systems such as icosahedron are obtained. In order to test reliance of the computation results, the corresponding experiments have also been performed on the material by the X-ray diffraction experiment.

Structures of Liquid Aluminium under High Pressure

Molecular dynamics simulation has been carried out for melt Al under constant temperature and constant pressure. The interaction between atoms is described by tight-binding many-body potentials based on the second-moment approximation to the electronic density of states. The pair correlation function and the pair analysis technique are used to reveal the structural features of liquid Al under normal and high pressure. High pressure is favourable to the existence of bcc clusters 1661 and 1441, but has no effect on the fcc cluster 1421. The bond pair 1551 and 1541 with fivefold symmetry exists at high pressure. The microstructure of liquid is more similar to the non-crystalline structure than to the crystalline structure. The simulation results are supported by the x-ray experimental results.