Crystallographyof Mg17Al12 precipitates in AZ91D alloy

Kikuchi diffraction was used to accurately determine the orientation relationship (OR) between Mg17Al12 precipitates and matrix in an AZ91D alloy. For both continuous and discontinuous precipitations, the Burgers OR and the Potter OR were equally observed. The lattice parameter of Mg17Al12 associated with the former is bigger than that of the latter. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Morphology and crystallography of Mg24Y5 precipitate in Mg-Y alloy

Orientation relationships between Mg24Y5 precipitates and matrix in a Mg-Y alloy were accurately determined using Kikuchi line diffraction. The Burgers relationship with habit planes of {10 (1) over bar0}(H) and {31 (4) over bar0}(H) were observed for all precipitates. Compared with the Mg17Al12 precipitate in AZ91, the precipitation hardening effect in this alloy was significantly increased. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The effect of resin type on the sintering of freeformed maraging steel

The role of the resin type on the sintering of maraging steel with boron additions has been investigated. Two different resins were added to the steel mixture and their subsequent debinding was evaluated and sintering responses compared with that of a resin-free alloy. The two resins used, nylon and a mixture of phenolic resin and synthetic wax, possessed different debinding behaviour, with the latter causing significant carbon contamination of the parts. This caused the formation of a Ti-Mo carbide, depleting the matrix of these elements. Consequently, the microstructure consisted of the equilibrium Fe-Fe2B eutectic, as well as a Mo-rich boride. The liquid phase also appeared to contain significant amounts of carbon, which lowered the temperature at which the liquid formed, resulting in high density occurring at a much lower temperature. When nylon was used as the binder, a similar sintering response to the resin-free alloy was observed. (C) 2002 Published by Elsevier Science B.V.

On the sintering of uncompacted, pre-alloyed Al powder alloys

The effect of the addition of elemental Mg, Sri and Pb on the sintering of impressed prealloyed 2124 and 6061 powder has been investigated. Despite being sintered at a temperature that resulted in similar to20 vol.% liquid phase, high density was possible only upon the addition of similar to1% elemental Mg as well as trace amounts of Sri or Pb. Additions of Mg facilitated oxide disruption, while Sri and Pb segregated to the liquid phase, where they reduced the liquid's surface tension, thus improving sintering. (C) 2002 Elsevier Science B.V. All rights reserved.

The R8-BC8 phases and crystal growth in monocrystalline silicon under microindentation with a spherical indenter

The morphology and distribution of high-pressure metastable phases BC8 and R8, formed in monocrystalline silicon under microindentation, were identified and assessed using transmission electron microscopy nanodiffraction analysis. It was discovered that the crystal growth inside the transformation zone was stress-dependent with large crystals in its central region. The crystal size could also be increased using higher maximum indentation loads. The BC8 and R8 phases distributed unevenly across the transformation zone, with BC8 crystals mainly in the center of the zone and smaller R8 fragments in the peripheral regions. Such phase distribution was in agreement with the theoretical residual stress analysis.

On the role of tin in the infiltration of aluminium by aluminium for rapid prototyping applications

The use of tin as an alloying element in the production of freeformed infiltrated aluminium components is explored. Tin slows the growth of the aluminium nitride skeleton which provides dimensional stability, as well as increasing the rate of infiltration of the aluminium liquid into the aluminium nitride skeleton. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of iron on grain refinement of high-purity MG-Al alloys

The effect of iron on the grain refinement of high-purity Mg-3%Al and Mg-91%Al alloys has been investigated using anhydrous FeCl3 as an iron additive at 750degreesC in carbon-free aluminium titanite crucibles. It was shown that grain refinement was readily achievable for both alloys. Fe- and Al-rich intermetallic particles were observed in many magnesium grains. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Long-term in vivo biostability of poly(dimethylsiloxane) / poly(hexamethylene oxide) mixed macrodiol-based polyurethane elastomers

The long-term biostability of a novel thermoplastic polyurethane elastomer (Elast-Eon(TM) 2 80A) synthesized using poly(hexamethylene oxide) (PHMO) and poly(dimethylsiloxane) (PDMS) macrodiols has been studied using an in vivo ovine model. The material's biostability was compared with that of three commercially available control materials, Pellethane(R) 2363-80A, Pellethane(R) 2363-55D and Bionate(R) 55D, after subcutaneous implantation of strained compression moulded flat sheet dumbbells in sheep for periods ranging from 3 to 24 months. Scanning electron microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to assess changes in the surface chemical structure and morphology of the materials. Gel permeation chromatography, differential scanning calorimetry and tensile testing were used to examine changes in bulk characteristics of the materials. The results showed that the biostability of the soft flexible PDMS-based test polyurethane was significantly better than the control material of similar softness, Pellethane(R) 80A, and as good as or better than both of the harder commercially available negative control polyurethanes. Pellethane(R) 55D and Bionate(R) 55D. Changes observed in the surface of the Pellethane(R) materials were consistent with oxidation of the aliphatic polyether soft segment and hydrolysis of the urethane bonds joining hard to soft segment with degradation in Pellethane(R) 80A significantly more severe than that observed in Pellethane(R) 55D. Very minor changes were seen on the surfaces of the Elast-Eon(TM) 2 80A and Bionate(R) 55D materials. There was a general trend of molecular weight decreasing with time across all polymers and the molecular weights of all materials decreased at a similar relative rate. The polydispersity ratio, M-w/M-n, increased with time for all materials. Tensile tests indicated that UTS increased in Elast-Eon(TM) 2 80A and Bionate(R) 55D following implantation under strained conditions. However, ultimate strain decreased and elastic modulus increased in the explanted specimens of all three materials when compared with their unimplanted unstrained counterparts. The results indicate that a soft, flexible PDMS-based polyurethane synthesized using 20% PHMO and 80% PDMS macrodiols has excellent long-term biostability compared with commercially available polyurethanes. (C) 2004 Elsevier Ltd. All rights reserved.

Superplasticity and superplastic forming ability of a Zr-Ti-Ni-Cu-Be bulk metallic glass in the supercooled liquid recgion

The superplastic deformation behavior and superplastic forming ability of the Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass (BMG) in the supercooled liquid region were investigated. The isothermal tensile results indicate (hat the BMG exhibits a Newtonian behavior at low strain rates but a non-Newtonian behavior at hiqh-strain rates in the initial deformation stage. The maximum elongation reaches as high as 1624% at 656 K. and nanocrystallization was found to occur during the deformation process. Based cm the analysis on tensile deformation. a gear-like micropart is successfully die-forged via a superplastic forgings process. demonstrating that the BMG has excellent workability in the supercooled liquid region. (C) 2004 Elsevier B.V. All rights reserved.

Grain coarsening of magnesium alloys by beryllium

A trace of beryllium can lead to dramatic grain coarsening in Mg-Al alloys at normal cooling rates. It is, however, unclear whether this effect applies to aluminium-free magnesium alloys or not. This work shows that a trace of beryllium also causes considerable grain coarsening in Mg-Zn, Mg-Ca, Mg-Ce and Mg-Nd alloys and hinders grain refinement of magnesium alloys by zirconium as well. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Permeation of binary gas mixtures in ultramicroporous membranes

High-quality nanometer thick ultramicroporous membranes were prepared from silica sol-gel processes and tested for the permeation of binary gas mixtures of He, H-2, CO2, and CH4 across different temperature and partial pressure regimens. Pore size distribution by molecular probing showed that the majority of pore sizes had dimensions below 2.9 Angstrom. In 50:50 binary mixtures, the fluxes of gases increased as a function of temperature, indicating an activated transport mechanism. The ultramicroporous membranes showed high selectivities at 150 degreesC for He/CO2 (30), He/CH4 (93), H-2/CO2 (10), and H-2/CH4 (9) with lower selectivities for CO2/CH4 (5). High activation energies (E-a) were observed for the permeance of 50:50 binary mixtures containing He and H-2 of 22.1-27.5 and 17.6-23.1 kJ.mol(-1), respectively. The E-a for the permeance of the total mixture approached the E-a for the permeance of the molecule with the smaller kinetic diameter (He or H-2).

A Two-Step Sol–Gel Method for Synthesis of Nanoporous TiO2

This article reports a study of the effects of synthesis parameters on the preparation and formation of mesoporous titania nanopowders by employing a two-step sol-gel method. These materials displayed crystalline domains characteristic of anatase. The first step of the process involved the hydrolysis of titanium isopropoxide in a basic aqueous solution mediated by neutral surfactant. The solid product obtained from step 1 was then treated in an acidified ethanol solution containing the same titanium precursor to thicken the pore walls. Low pH and higher loading of the Ti precursor in step 2 produced better mesoporosity and crystallinity of titanium dioxide polymorphs. The resultant powder exhibited a high surface area (73.8 m(2)/g) and large pore volume (0.17 cm(3)/g) with uniform mesopores. These materials are envisaged to be used as precursors for mesoporous titania films as a wide band gap semiconductor in dye-sensitized nanocrystalline TiO2 solar cells.

Comparative study of hydrogen storage in Li- and K-doped carbon materials - theoretically revisited

Hydrogen adsorption in alkali-doped carbon materials is investigated theoretically. Our calculations show that hydrogen molecules can be physically adsorbed on alkali-doped graphite at 0 K but such an adsorption is thermodynamically unfavourable. The binding energy of hydrogen adsorption decreases significantly with the increase in temperature and becomes nearly zero at ambient temperature. We suggest that it may be unlikely to observe any hydrogen uptake in alkali-doped carbon materials at or above ambient temperature in the TGA (thermogravimetric) system, the previously reported hydrogen uptake in alkali-doped carbon materials was caused by either uncyclable chemisorbed hydrogen on the defects of carbon (defects were produced by repeated heat treatment) and/or moisture adsorption. (C) 2004 Elsevier Ltd. All rights reserved.

Evidence for energy relaxation via a radiative cascade in surface-passivated PbS quantum dots

Multiple emission peaks have been observed from surface passivated PbS nanocrystals displaying strong quantum confinement. The emission spectra are shown to be strongly dependent on the excited-state parity. We also find that intraband energy relaxation from initial states excited far above the band-edge is nearly three orders of magnitude slower than that found in other nanocrystal quantum dots, providing evidence of inefficient energy relaxation via phonon emission. The initial-state parity dependence of the photoluminescent emission properties suggests that energy relaxation from the higher excited states occurs via a radiative cascade, analogous to energy relaxation in atomic systems. Such radiative cascade emission is possible from ideal zero-dimensional semiconductors, where electronic transitions can be decoupled from phonon modes.

Direct observation of mixed-parity excited states in surface-passivated PbS nanocrystals

Photoluminescent emission is observed from surface-passivated PbS nanocrystals following the two-photon excitation of high-energy excitonic states. The emission appears directly at the excitation energy with no detectable Stokes-shift for a wide range of excitation energies. The observation of direct emission from states excited by two-photon absorption indicates that the parity of the excited states of surface-passivated PbS nanocrystals is partially mixed.