Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

Enhanced carrier-carrier interaction in optically pumped hydrogenated nanocrystalline silicon

A femtosecond pump-probe setup was used to measure the time resolved reflectivity of hydrogenated amorphous silicon containing crystalline silicon nanoparticles at eight different incidence angles. Results fitted with the Drude model found a scattering rate of G = 2-1+1.2×1015?s-1 at a corresponding carrier concentration of ~ 1020?cm-3. The observed scattering rate is attributed to enhanced carrier-carrier interaction in optically pumped nanocrystals.

An efficient route to highly organized, tunable macroporous-mesoporous alumina

(Figure Presented) Organized macroporous-mesoporous alumina can be obtained via a dual-templating approach. Monodispersed polystyrene beads promote macropore formation, while a P123 surfactant templating agent drives the formation of ordered hexagonal mesopores throughout the alumina framework. These well-defined pore networks coexist over a wide range of temperatures and macropore sizes. © 2009 American Chemical Society.

Fabrication and characterization of macroporous poly(ethylene glycol) hydrogels generated by several types of porogens

Polymer scaffolds play an important role in tissue engineering applications. Poly(ethylene glycol) based hydrogels have received a lot of attention in this field because of their high biocompatibility and ease of processing. However, in many cases they do not exhibit proper tissue invasion and nutrient transport because of their dense structure. In the present work, several approaches were developed and compared to each other to produce interconnected macroporous poly(ethylene glycol) hydrogels by including different types of porogens in the photocrosslinking reaction. The swelling capacity of the resulting hydrogels was analyzed and compared to non-porous hydrogel samples. Moreover, the obtained materials were characterized by means of mechanical properties and porosity using rheometry, scanning electron microscopy, and mercury intrusion porosimetry. Results showed that interconnected and uniform pores were obtained when a porogen template was used during hydrogel fabrication by photocrosslinking. On the other side, when the porogen particles were dispersed into the macromer solution before matrix photocrosslinking the interconnexion was negligible. The templates must be dissolved before the hydrogel's cell-seeding in vitro, while the dispersed porogen can be used in situ in the in vitro seeding tests. Copyright © 2013 Taylor & Francis Group, LLC.

The formation and properties of some cyclic compounds of silicon an dtin

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Study of epitaxial silicon layers with reference to the technology of semi-conductor devices

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The removal of temporary organic binder from vitreous silicon carbide grinding wheels during firing

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Efficient THz radiation from a nanocrystalline silicon-based multi-layer photomixer

In this paper we propose a novel type of multiple-layer photomixer based on amorphous/nano-crystalline-Si. Such a device implies that it could be possible to enhance the conversion efficiency from optical power to THz emission by increasing the absorption length and by reducing the device overheating through the use of substrates with higher thermal conductivity compared to GaAs. Our calculations show that the output power from a two-layer Si-based photomixer is at least ten times higher than that from conventional LT-GaAs photomixers at 1 THz.

Controlled growth of poly(2-(diethylamino)ethyl methacrylate) brushes via atom transfer radical polymerisation on planar silicon surfaces

Progress in making pH-responsive polyelectrolyte brushes with a range of different grafting densities is reported. Polymer brushes of poly(2-(diethylamino)ethyl methacrylate) were synthesised via atom transfer radical polymerisation on silicon wafers using a 'grafted from' approach. The [11-(2-bromo-2-methyl) propionyloxy]undecyl trichlorosilane initiator was covalently attached to the silicon via silylation, from which the brushes were grown using a catalytic system of copper(I) chloride and pentamethyldiethylenetriamine in tetrahydrofuran at 80°C. X-ray reflectivity was used to assess the initiator surfaces and an upper limit on the grafting density of the polymer was determined. The quality of the brushes produced was analysed using ellipsometry and atomic force microscopy, which is also discussed.

Investigation of an optically reconfigurable plasma for silicon based microwave applications

This paper investigates the use of photoconductive plasmas for controlling microwave circuits and antennas on semiconductor substrates. Initial experiments show that significant changes in the reflection coefficient characteristics can be obtained by varying the length of a photo-illuminated plasma region from 0 to 2mm. The resulting structure forms the basis for further experiments involving tuneable microwave devices. © 2013 European Microwave Association.

Stress-induced platelet formation in silicon:a molecular dynamics study

The effect of stress on vacancy cluster configurations in silicon is examined using molecular dynamics. At zero pressure, the shape and stability of the vacancy clusters agrees with previous atomistic results. When stress is applied the orientation of small planar clusters changes to reduce the strain energy. The preferred orientation for the vacancy clusters under stress agrees with the experimentally observed orientations of hydrogen platelets in the high stress regions of hydrogen implanted silicon. These results suggest a theory for hydrogen platelet formation. © 2005 The American Physical Society.

High-frequency conductivity of optically excited charge carriers in hydrogenated nanocrystalline silicon investigated by spectroscopic femtosecond pump-probe reflectivity measurements

We report an investigation into the high-frequency conductivity of optically excited charge carriers far from equilibrium with the lattice. The investigated samples consist of hydrogenated nanocrystalline silicon films grown on a thin film of silicon oxide on top of a silicon substrate. For the investigation, we used an optical femtosecond pump-probe setup to measure the reflectance change of a probe beam. The pump beam ranged between 580 and 820nm, whereas the probe wavelength spanned 770 to 810nm. The pump fluence was fixed at 0.6mJ/cm2. We show that at a fixed delay time of 300fs, the conductivity of the excited electron-hole plasma is described well by a classical conductivity model of a hot charge carrier gas found at Maxwell-Boltzmann distribution, while Fermi-Dirac statics is not suitable. This is corroborated by values retrieved from pump-probe reflectance measurements of the conductivity and its dependence on the excitation wavelength and carrier temperature. The conductivity decreases monotonically as a function of the excitation wavelength, as expected for a nondegenerate charge carrier gas.

Hierarchical macroporous-mesoporous SBA-15 sulfonic acid catalysts for biodiesel synthesis

Hierarchical macroporous-mesoporous SBA-15 silicas have been synthesised via dual-templating routes employing liquid crystalline surfactants and polystyrene beads. These offer high surface areas and well-defined, interconnecting macro- and mesopore networks with respective narrow size distributions around 300 nm and 3-5 nm for polystyrene:tetraethoxysilane ratios ≥2:1. Subsequent functionalisation with propylsulfonic acid yields the first organized, macro-mesoporous solid acid catalyst. The enhanced mass transport properties of these new bi-modal solid acid architectures confer significant rate enhancements in the transesterification of bulky glyceryl trioctanoate, and esterification of long chain palmitic acid, over pure mesoporous analogues. This paves the way to the wider application of hierarchical catalysts in biofuel synthesis and biomass conversion. © 2010 The Royal Society of Chemistry.

Corrosion of silicon carbide ceramics using conventional and electrochemical methods

Silicon carbide ceramics are candidate materials for use in aggressive environments, including those where aqueous acids are present. Standard corrosion testing methods such as immersion testing are not always sufficiently sensitive for these ceramics owing to the very low, almost unobservable, corrosion rates encountered. Using electrochemical methods the corrosion processes can be assisted, leading to higher rates and thus the elucidation of reaction mechanisms. The behaviour of a sintered and a reaction bonded silicon carbide has been investigated in aqueous HCl, HF, HNO3, and H2SO4, using standard immersion and new electrochemical methods. Both materials were passive in HCl, HNO3, and H2SO4 because of the formation of a surface silica film, and were active in HF. In HF, corrosion of sintered silicon carbide was slight and the residual silicon was removed from reaction bonded specimens.

Spin dependent photoconductivity in silicon-on-sapphire

Resonant and non resonant spin dependent photoconductivity is observed in(100) silicon films grown on sapphire by CVD and MBE techniques. The CVD films are either in their as-grown state or have undergone single or double solid phase epitaxial regrowth. For all samples, a resonant decrease in photoconductivity is observed at a field of about 0.34 T for a microwave frequency of about 9.7 GHz and at about 3.3 mT when the frequency is about 92 MHz. For all samples the maximum fractional change in photoconductivity is approximately 10-4 independent of magnetic field strength.