Crystallization of Gels in the Apatite-Mullite System

Crystallization processes in gels of the apatite-mullite system were studied to obtain information for the synthesis of bioglass-ceramics and composite materials. SiO2-sol, Al(NO3)3·9H2O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O and CaF2 were used as precursors. CaF2 was added before and after gelation. Mixtures of mullite gel-glass and fluorapatite in the range 10 to 90 mol% were investigated for synthesis of composites. All the samples were heat treated at different temperatures in the range 950–1250°C and the structural changes were established using X-ray diffraction and IR-spectroscopy. When the gels were treated at 1050°C and at 1150°C, the main crystalline phases found were fluorapatite and mullite independent of the CaF2 content and the manner of its addition. At 1250°C the relative amounts of fluorapatite and mullite decrease and gehlenite appears. Composite materials containing fluorapatite and mullite as main crystalline phases can be obtained only when the content of mullite gel-glass in the initial mixture is more than 60 mol%.     

Photochemical synthesis of ZnO/Ag nanocomposites

Composite ZnO/Ag nanoparticles have been formed via the photocatalytic reduction of silver nitrate over the ZnO nanocrystals, their optical, electrophysical and photochemical properties have been investigated. Mie theory has been applied to analyze the structure of the absorption spectra of ZnO/Ag nanocomposite. The irradiation effects upon the optical properties of ZnO/Ag nanostructure have been investigated. It has been found that the irradiation of ZnO/Ag nanoparticles results in electrons accumulation by both the semiconductor and the metallic components of the nanocomposite. It has been found that silver nitrate can be photochemically deposited onto the surface of ZnO nanoparticles under the illumination with the visible light in the presence of the sensitizer – methylene blue. Kinetics of the sensitized Ag(I) photoredution has been studied. It has been concluded that the key stage of this process is the electron injection from singlet-excited methylene blue molecule into ZnO nanoparticle.     

Synthesis of transparent BaTiO<Subscript>3</Subscript> nanoparticle/polymer hybrid

Transparent BaTiO₃ nanoparticle/polymer hybrid was synthesized by polymerization and hydrolysis of barium titanium alkoxide modified with 2-vinyloxyethoxy ligand. Barium alkoxide, titanium alkoxide and 2-vinyloxyethanol were reacted affording a BaTiO₃ precursor, which was then hydrolyzed and polymerized to form BaTiO₃ particle/polymer hybrids below 100°C. BaTiO₃ particles increased in crystallinity with increasing water amount for hydrolysis. The absorption edge of the hybrid film on silica plates shifted to shorter wavelength with decreasing crystallite size. Nano-sized BaTiO₃ particle/polymer hybrid polymerized with methyl methacrylate (MMA) was shaped into a transparent and self-standing film with a refractive index of 1.595 at 589 nm.     

Enhanced optical nonlinear absorption of graded Au--TiO2 composite films

This paper reports that single-layer and graded Au-TiO2 granular composite films with Au atom content 15%- 66% were prepared by using reactive co-sputtering technique. The third-order optical nonlinearity of single-layer and graded composite films was investigated by using s- and p-polarized Z-scans in femtosecond time scale. The nonlinear absorption coefficient βeff of single-layer Au-TiO2 films is measured to be -2.3×10^3-0.76×10^3 cm/GW with Au atom content 15%-66%. The βeff value of the 10-layer Au-TiO2 graded film is enhanced to be -2.1×10^4cm/GW calculated from p-polarized Z-scans, which is about ten times the maximum βeff of single-layer films. Broadened response in the wavelength region 730-860 nm of the enhanced optical nonlinearity of graded Au-TiO2 composite films was also investigated.     

Fissile core and Tritium-Breeding Blanket: structural materials and their requirements

High radiation resistant structural materials for fusion and fission nuclear power plants are a key issue for the development of both types of reactors. Selection criteria, elements of metallurgy of the selected materials, and the major issues as they are revealed by the results of the present development programmes, are presented. At low temperature (300 °C) ferritic/martensitic steels are suffering from He-embrittlement, associated with possible hardening due to α/α^′ unmixing. The kinetics of hardening and embrittlement versus dose, especially saturation with dose, are still open key issues, difficult to settle on the basis of a purely experimental programme. Important progress is still to be made in mastering the initial microstructure, inclusion cleanness and joining techniques of oxide dispersion strengthened steels for higher heat resistance. Physics modeling as presented in this issue should promote guidance to the understanding of the mechanisms involved, provide solutions to master the initial microstructure and phase stability, and mitigate the in-service property degradation. To cite this article: J.-L. Boutard et al., C. R. Physique 9 (2008).     

Size and Interface Control of Novel Nanocrystalline Materials Using Pulsed Laser Deposition

We have developed a novel method based upon pulsed laser deposition to produce nanocrystalline materials with an accurate grain size and interface control. Using this method, the grain size in the case of Cu thin films was controlled by introducing a few monolayers of insoluble elements having high surface energy such as W, which increases interfacial energy and provides more nucleation sites. The grain size is determined by the thickness of Cu layer and the substrate temperature at which it transforms into islands (nanocrystalline grains) of fairly uniform size which we desgnate as self-assembling approach. Using this approach, the grain size was reduced from 160nm (Cu or Si (100) substrate) to 70–80nm for a simple W layer (Cu/W/Si (100)) to 4nm for a multilayer (Cu/W/Cu/W/Si (100)) thin film. The hardness of these films was evaluated using a nanoindentation technique, a significant increase in hardness from 2.0GPa for coarse-grained 180nm to 12.5GPa for 7nm films was observed. However, there is decrease in hardness below 7nm for copper nanocrystals. The increase in hardness with the decrease in grain size can be rationalized by Hall–Petch model. However, the decrease in slope and eventually the decrease in hardness below a certain grain size can be explained by a new model based upon grain-boundary deformation (sliding). We also used a similar materials processing approach to produce quantum dots in semiconductor heterostructures consisting of Ge and ZnO dots or nanocrystals in AlN or Al₂O₃ matrix. The latter composites exhibit novel optoelectronic properties with quantum confinement of phonons, electrons, holes and excitons. Similarly, we incorporated metal nanocrystals in ceramics to produce improved mechanical and optical properties.     

Effective Conductivity of Nonlinear Two-Phase Media: Homogenization and Two-Point Padé Approximants

The aim of this paper is a study of the quasilinear transport equation, for instance the stationary heat equation. For periodically microheterogeneous media asymptotic homogenization has been performed with the local problem formulated as a minimization problem. The Golden–Papanicolaou integral representation theorem and some bounds developed for the linear equation have been extended. Two-point Padé approximants have been used to calculate bounds. Examples are also provided.     

Development of the morphology and crystalline state due to hybridization of reinforced toughened nylon containing a liquid‐crystalline polymer

A hybrid composite consisting of rubber‐toughened nylon‐6,6, short glass fibers, and a thermotropic liquid‐crystalline polymers (LCP) was investigated by the LCP content being varied. The thermal behavior, morphology, and crystallization behavior due to hybridization were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and wide‐angle X‐ray scattering (WAXS). DSC results indicated that the crystallinity of the glass‐fiber‐reinforced toughened nylon‐6,6 was reduced by LCP addition, particularly 5–10 wt % LCP. DMA data showed that the miscibility between the blended components was maximum at the 5 wt % LCP composition, and the miscibility decreased with increasing LCP content. SEM photomicrographs revealed information consistent with the thermal behavior on miscibility. It was also observed that the 10 wt % LCP composition showed predominantly an amorphous character with FTIR and WAXS. WAXS results indicated that LCP hybridization increased the interplanar spacing of the hydrogen‐bonded sheets of the nylon crystals rather than the spacing between the hydrogen‐bonded chains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 549–559, 2003