Electron-density distribution and physical properties of plutonium-gallium alloys: Ab initio cluster calculations

A cluster model based on ab initio density-functional theory was used to model gallium-stabilized δ-plutonium alloys, and to calculate the electron-density distribution, its pressure dependence, bond lengths, elastic properties (second order and third order), and inelastic properties for Pu₁₂Ga (7.7 at% Ga) and Pu₁₈Ga (5.3 at% Ga). The electron distribution was found to contain localized, semi localized, and delocalized contributions, with the second possessing covalent character. Two of plutonium’s 8 valence electrons were found to be itinerant, consistent with a recent prediction based on an electrostatic model, with the electron configuration for plutonium being 7s^0.57p^0.55f¹ (itinerant) and 6d¹5f⁵ (localized), and that for gallium being 4s¹4p². Applied hydrostatic pressure shifts the charge density toward a more localized Pu(d)-based distribution. The onset of the pressure-induced δ-Pu to α-Pu phase change is accompanied by a ∼0.2 electron increase in the localized population that may serve as a driving force for the phase change. Interior bonding within the Pu₁₂Ga subunits is stronger than that of the surrounding plutonium lattice, and the Pu-Ga bonds therein relax in a direction opposite to lattice strain. This study predicts covalency in metallic plutonium, both in the Pu-Ga bonding and in the Pu-Pu bonding.     

Improved mechanical properties of hydroxyapatite/poly(?-caprolactone) scaffolds by surface modification of hydroxyapatite

Scaffolds comprising hydroxyapatite (HAP) or poly(?-caprolactone)-grafted hydroxyapatite (g-HAP) and poly(?-caprolactone) (PCL) were prepared using the thermally induced phase separation/salt leaching technique. The g-HAP nanoparticles were evaluated by Fourier Transformation Infrared Spectroscopy (FTIR) and thermal gravimetric analysis (TGA). Power X-ray Diffraction (XRD) patterns confirmed the successful grafting on the surface of HAP. The effects on mechanical strength, porosity and thermal property of scaffolds by the introduction of nanoparticles were extensively investigated. The compressive modulus of the scaffold was greatly improved by the addition of g-HAP nanoparticles. Especially the compressive modulus of the g-HAP/PCL scaffold containing 20 wt% of g-HAP was 59.4% higher than that of the corresponding HAP/PCL scaffold.     

Functional and optical properties of Au:TiO2 nanocomposite films: The influence of thermal annealing

A set of nanocomposite thin films consisting of Au nanoclusters dispersed in a TiO₂ dielectric matrix was deposited by reactive magnetron sputtering, and subjected to thermal annealing in vacuum, at temperatures ranging from 200 to 800 °C. The obtained results show that the structure and the size of Au clusters, together with the matrix crystallinity, changed as a result of the annealing, and were shown to be able to change the optical properties of the films and keeping good mechanical properties, opening thus a wide number of possible applications. The crystallization of the gold nanoclusters induced by the annealing was followed by a systematic change in the overall coating behaviour, namely the appearance of surface plasmon resonance (SPR) behaviour. This effect enables to tailor the thin films reflectivity, absorbance and colour coordinates, contributing to the importance of this thin film system. The different attained optical characteristics (reflectance values ranging from interference to metallic-like behaviours and colour varying for interference rainbow-like to several tones of red-brownish), associated with a reasonable mechanical resistance of the coatings (good adhesion to different substrates and hardness values ranging from 5 to 7.5 GPa), induce the possibility to use this film system in a wide range of decorative applications.     

Properties and hydrolysis of PLGA and PLLA cross-linked with electron beam radiation

Radiation has been used as a processing tool to modify the properties of polymers. The aim of this study is to understand how electron beam radiation, together with pentaerythritol tetraacrylate (PTTA) as a tetra-functional monomer, can alter the properties (i.e. thermal and mechanical) and hydrolysis rates of PLGA and PLLA. The effects of radiation dose and PFM concentration on the physical properties of the polymers were investigated. The results showed that upon irradiation PLGA and PLLA cross-linked, and an increased in gel content was observed. Glass transition temperature (T_g) and mechanical properties of the polymers also increased. Cross-linked PLGA and PLLA samples were found to retard hydrolytic degradation. The mechanical properties of these polymers were also unaffected by hydrolysis. In summary, PLGA and PLLA cross-linked with PTTA were found to have enhanced mechanical properties and were able to retard hydrolytic degradation.     

Novel rheological features of molten SEBS copolymers: Mechanical relaxation at low frequencies and flow split

Poly(styrene)-poly(ethylene-co-butylene)-poly(styrene) (SEBS) triblock copolymers with a PS volume fraction ? = 0.30 are investigated. The analysis of the loss tangent at low frequencies leads to define a mechanical relaxation associated to the blocking effect of ordered PS cylinders on PEB chains motion. Submitting SEBS to a shear flow at 250 °C an orientation of the cylinders is observed. The effect of the molecular weight and the cylinder alignment on the relaxation is studied. Continuous flow experiments, in torsion and capillary mode, reveal the presence of very severe surface cracks which cause the phenomenon called “flow split”. The correlation between dynamic viscoelastic results and flow split is investigated, concluding that this is related to an entanglement-disentenglement process, rather to an alignment effect of PS cylinders.     

Transverse elasticity of multi-walled carbon nanotubes

A discrete shell model is proposed to describe the radial deformation of carbon nanotubes under a hydrostatic pressure and the radial Young's modulus of (single- or multi-walled) nanotubes is obtained. It is found that the radial modulus decreases with increasing tube diameter while increases with increasing number of layers. The computational results agree well with the previous results of SWNTs and indicate that the radial modulus of carbon nanotubes is independent of the Poisson's ratio.     

Consolidation of blended powders by severe plastic deformation to form amorphous metal matrix composites

Warm equal channel angular extrusion (ECAE) in 90° tooling is used to consolidate Vitreloy 106a plus tungsten powders. A fully dense amorphous matrix composite showing good infiltration of the amorphous phase in between crystalline particles and uniform consolidation is achieved after one ECAE extrusion in the supercooled liquid temperature region of the amorphous phase. The results demonstrate that ECAE appears to be a viable processing method for producing bulk metallic glass matrix-crystalline particle composites.     

Three-dimensional investigation of thick single-lap bolted joints

Stresses in single-lap bolted joints of thick plates are complex and difficult to analyze. Previous studies involving stresses through the thickness of bolted joints have been limited to finite element method (FEM) simulations and have been implemented only for the joining of relatively thin plates. In this paper we report on several experimental and numerical analyses that were conducted to evaluate the stress distribution inside thick bolted plates along the bearing plane normal to the plate surface. Experimental analysis was conducted via embedded-polariscope photoelasticity and embedded resistance strain gages. The FEM analysis was performed with the ABAQUS commercial code using material properties and other data obtained experimentally as input. Experimental and numerical results agreed reasonably well, and are believed to depict the behavior of the joint under load well enough to assist in development of improved joint design.     

Mechanical behaviour and fracture mechanism of flux grown holmium orthoferrite single crystals

Results of indentation induced microhardness and fracture studies on (110) and (001) planes of flux grown HoFeO₃ single crystals using a Vicker's hardness tester, in the load range of 10-100 g are presented. The variation of microhardness with load is found to be non-linear and is explained by using Hays and Kendall's law. The hardness results and the indentation-induced cracking yields the values of the yield strength, fracture toughness and brittleness index for (110) and (001) planes of HoFeO₃ crystals. Transition from Palmqvist to median crack systems takes place at higher loads for (110) plane of the crystal whereas (001) plane is characterized by Palmqvist cracks only.     

Three-dimensional size effects in composite pin joints

Mechanical fastening is commonly used in joining composite laminates. It is especially superior to adhesive bonding in joining thick composite laminates. The significant effects of composite thickness on the efficiency of pin joining have been reported in a previous study. The present study further investigated the three-dimensional (thickness as well as in-plane dimensions) size effects on pin joining. A glass-fabric/phenolic composite material was used as the studied material while double-lap-single-pin joints were used as the experimental models. Based on four different thicknesses and seven different hole diameters, in total nine composite joint configurations were prepared. The nine configurations were organized into three groups with differentH/D ratios (thickness versus hole diameter). Experimental results revealed that the load-displacement curves of the groups withH/D=0.4 and 0.75 were brittle-type while those of the group withH/D=1.5 were ductile-type. The microscopic damage modes of the joints with brittle-type curves were due to outward buckling for 8-ply cases and kink bands for 24-ply and 40-ply cases. The bearing strengths of the pin joints decreased monotonically in the group withH/D=0.4 but increased then decreased in the groups withH/D=0.75 and 1.5 as the joint size increased. It was found that both the material properties and the geometry parameters were responsible for the three-dimensional size effects.