Wu Zhong-Hua

Small-Angle X-Ray Scattering Study on Nanostructures of Polyimide Films

Inorganic nanohybrid polyimide (PI) is widely applied in electrical and electronic devices for its outstanding insulating properties. Samples 100CR and 100NH are made in Dupont. Among them, 100NH is a kind of pure PI films; however, 100CR is a kind of inorganic nanohybrid PI films with excellent corona-resistance. The nanostructure of PI films is investigated with small-angle x-ray scattering technique and transmission electron microscopy (TEM). The normalized volume fractions of the scatterers in the specimens are obtained with a tangent-by-tangent data analysis for the small-angle x-ray scattering data. The multi-hierarchical scatterers of 100NH can be divided into two dominant components, i.e., the sharp component and the wide component. The sharp component is corresponding to the contribution of PI molecular chains, and the wide component includes the aggregates formed by PI molecular chains and the film has nested dual-fractal characteristics, nevertheless the 100CR film possesses three types of scale scattering made up of inorganic nanoparticles, molecular chains and aggregates. The present films have multi-fractal structures. The distribution and structure of scattering body of two kinds of PI films are analyzed. The results of SAXS agree well with those of TEM methods.

Surface atomic structures of Fe2O3 nanoparticles coated with cetyltrimethyl ammonium bromide and sodium dodecyl benzene sulphonate: an extended x-ray absorption fine-structure study

Fe2O3 nanoparticles coated with sodium dodecyl benzene sulphonate (DBS) or cetyltrimethyl ammonium bromide (CTAB) were prepared by using a microemulsion method in the system water/toluene. The nanoparticles were characterized by means of transmission electron microscopy and average particle sizes of 5.0 nm and 6.0 nm were found for DBS-modified and CTAB-modified nanoparticles respectively. The local atomic structures of these iron(III) oxide nanoparticles were probed by using the extended x-ray absorption fine-structure technique. Fe K absorption spectra were collected at beam line 4W1B of Beijing Synchrotron Radiation Facility. A structural model was proposed for describing their atomic structures. The Fe-O bond length at the surface of DBS-coated Fe2O3 nanoparticles was found to be similar to that in bulk Fe2O3, but there was about 0.04 A expansion for the CTAB-coated Fe2O3 nanoparticles. On the basis of the model proposed in this paper, the thicknesses of the surface layers were estimated to be 0.5 nm and 0.7 nm, respectively, for the DBS-coated and CTAB-coated Fe2O3 nanoparticles. The anharmonicity of the atomic vibration and the asymmetry of atom-pair distribution were found to be larger at the surface of the nanoparticles than in the bulk material, while the Debye-Waller factors are almost the same for the surface and the core parts of the nanoparticles. It can be concluded that the atomic structure of the nanoparticle surface is ordered, but the atom-pair distribution is asymmetric.

Characterization of the nanopore structures of PAN-based carbon fiber precursors by small angle X-ray scattering

The nanopore structures in precursors are crucial to the performance of PAN-based carbon fibers. Four carbon-fiber precursors are prepared. They are bath-fed filaments (A), water-washing filaments (B), hot-stretching filaments (C) and drying-densification filaments (D). Synchrotron radiation small angle X-ray scattering is used to probe and compare the nanopore structures of the four fibers. The nanopore size, discrete volume distribution, nanopore orientation degree along the fiber axis and the porosity are obtained. The results demonstrate that the nanopores are mainly formed in the water-washing stage. During the processes of the subsequent production technologies, the slenderness ratio of nanopores and their orientation degree along the fiber axis increase further and simultaneously, the porosity decreases. These results are helpful for improving the performance of the final carbon fibers.

Local structures in zincblende-type random solid solutions

The local structures of zincblende-type solid solutions A1-xBxC can be described with five distorted unit cells based on a phenomenological valence force-field approach. Bond lengths and strain energies in these solid solutions can be obtained. The distorted feature of the local structure is well described by the five distorted unit cells. This structural model has been applied to 18 A1-xBxC systems. Results are compared with those from extended X-ray absorption fine structure and demonstrate that all the first- and second-neighbour bond lengths are in agreement with experimental values, with the largest divergence being about 0.01 AA. In addition, this model is also compared with the five special tetrahedra model.

GISAXS and SAXS studies on the spatial structures of Co nanowire arrays

The spatial structures of magnetic Co nanowire array embedded in anodic aluminium membranes were investigated by grazing incidence small angle X-ray scattering (GISAXS) and conventional small angle X-ray scattering (SAXS) techniques. Compared with SEM observation, the GISAXS and SAXS measurements can get more overall structural information in a large-area scale. In this study, the two-dimensional GISAXS pattern was well reconstructed by using the IsGISAXS program. The results demonstrate that the hexagonal lattice formed by the Co nanowires is distorted (a≈105 nm, b≈95 nm). These Co nanowires are isolated into many structure domains with different orientations with a size of about 2 μm. The SAXS results have also confirmed that the nanopore structures in the AAM can be retained after depositing Co nanowires although the Co nanowires can not completely but only just fill up the nanopores. These results are helpful for understanding the global structure of the Co nanowire array.

Small angle X-ray scattering of the colloidal crystal1;1)

The monodisperse polystyrene spheres are assembled into the colloidal crystal on the glass substrate by vertical deposition method, which is aimed at the so-called photonic crystal applications. The structural information of the bulk colloidal crystal is crucial for understanding the crystal growth mechanism and developing the various applications of colloidal crystal. Small-angle X-ray scattering (SAXS) technique was used to obtain the bulk structure of the colloidal crystal at Beamline 1W2A of BSRF. It is found that the SAXS pattern is sensitive to the relative orientation between the colloidal sample and the incident X-ray direction. The crystal lattice was well distinguished and determined by the SAXS data.

Nanoscale Inhomogeneities in the Glass Transition of Zr41.2Ti13.8Cu12.5Ni10Be22.5 Bulk Metallic Glass

By using small-angle x-ray scattering technique, correlation lengths of the density fluctuation are determined for the bulk metallic glass Zr41.2Ti13.8Cu12.5Ni10Be22.5 around the glass transition temperature Tg. Nanoscale inhomogeneities were found to exist in the supercooled liquid and glass state. The correlation length was found to vary with temperature T in an exponential law δ(T) = δ0+A1exp[−(T−T∞)/T1], where T∞ is the Vogel temperature, and parameters δ0, A1 and T1 were simulated to be 16.45 nm, 1.88×1029 nm, and 9.65 K, respectively, in the temperature region from Tg to Tg+80 K. The correlation length of the density fluctuation obtained by the small-angle x-ray scattering method can be considered as the characteristic length of glass transition.

Hierarchical structure and biomineralization in cricket teeth

Cricket is a truculent insect with stiff and sharp teeth as a fighting weapon. The structure and possible biomineralization of the cricket teeth are always interested. Synchrotron radiation X-ray fluorescence, X-ray diffraction and small angle X-ray scattering techniques were used to probe the element distribution, possible crystalline structures and size distribution of scatterers in cricket teeth. Scanning electron microscope was used to observe the nanoscaled structure. The results demonstrate that Zn is the main heavy element in cricket teeth. The surface of the cricket teeth has a crystalline compound like ZnFe2(AsO4)2(OH)2(H2O)4. While, the interior of the teeth has a crystalline compound like ZnCl2, which is from the biomineralization. The ZnCl2-like biomineral forms nanoscaled microfibrils and their axial direction points at the top of tooth cusp. The microfibrils aggregate random into intermediate filaments, forming a hierarchical structure. A sketch map of the cricket tooth cusp was proposed and a detailed discussion was given in this paper.The cricket is a truculent insect with stiff and sharp teeth as a fighting weapon. The structure and possible biomineralization of cricket teeth are always interesting. Synchrotron radiation X-ray fluorescence, X-ray diffraction, and small angle X-ray scattering techniques were used to probe the element distribution, possible crystalline structures and size distribution of scatterers in cricket teeth. A scanning electron microscope was used to observe the nanoscaled structure. The results demonstrate that Zn is the main heavy element in cricket teeth. The surface of a cricket tooth has a crystalline compound like ZnFe2(AsO4)2(OH)2(H2O)4. The interior of the tooth has a crystalline compound like ZnCl2, which is from the biomineralization. The ZnCl2-like biomineral forms nanoscaled microfibrils and their axial direction points towards the top of the tooth cusp. The microfibrils aggregate randomly into intermediate filaments, forming a hierarchical structure. A sketch map of the cricket tooth cusp is proposed and a detailed discussion is given in this paper.

Positive deviation from Debye's theory in small-angle x-ray scattering

The small-angle x-ray scattering (SAXS) resulting from a non-ideal two-phase system having sharply defined phase boundaries but with micro-fluctuations of electron density within any phase shows a positive deviation from Debyes theory. We have found a simple method suitable for fitting the slit-smeared SAXS data and correcting the positive deviation. The validity of this procedure is tested on porous ZrO2 xerogel prepared using the sol-gel process.

Synchrotron X-Ray Study on Structures of Ni80Fe20/Cu Superlattices

We have shown that, in contrast to the results in the literature, the Bragg peak intensity of Ni80Fe20/Cu superlattices is enhanced at the incident x-ray energy slightly higher than the absorption edge of the heavier element (Cu). The atomic density at Ni80Fe20/Cu interface was analysed by the diffraction anomalous fine structure technology with the incident angle of x-ray fixed at the first Bragg peak. Our results demonstrate the epitaxy growth of Ni80Fe20/Cu superlattices. Upon annealing, the epitaxity of Ni80Fe20/Cu multilayers becomes poor but the local crystallinity in each layer is improved.