A. J. Papworth

Direct synthesis of hydrogen peroxide from H2 and O2 using Pd and Au catalysts

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of supported metal catalysts is described and discussed. A detailed study of the factors influencing the formation and decomposition of hydrogen peroxide is presented for a Pd/sulfonated carbon catalyst in a methanol/water solvent. The use of low temperatures (1–2 °C) and short reaction (residence) time are identified as the key factors that favour high selectivity to hydrogen peroxide. Decomposition of hydrogen peroxide, mainly via further hydrogenation, prevents the formation of high concentrations of hydrogen peroxide. Combustion of hydrogen to water is a competing reaction that becomes significant at higher temperatures, but this can be partially inhibited by the addition of HBr. A second set of supported Pd and Au catalysts are evaluated for the direct synthesis of hydrogen peroxide using supercritical CO2 as a solvent. The use of supercritical CO2 is shown to be beneficial when compared with hydrogen peroxide formation at a temperature just below the critical temperature for CO2. However, at the critical temperature of CO2 (31.1 °C), the decomposition of hydrogen peroxide is rapid and only low rates of hydrogen peroxide formation are observed. At low temperature (2 °C) supported Au catalysts are shown to be very selective for the synthesis of hydrogen peroxide. The rate of hydrogen peroxide synthesis is enhanced markedly when Pd is present with Au and a detailed scanning transmission electron microscopy study shows that the 2–9 nm metal nanoparticles present in this supported catalyst are a Au∶Pd alloy.

Carbon films with ansp2network structure

Laser-Arc evaporation of a graphite target has been used to deposit carbon films that exhibit high hardness (45 GPa) and elastic recovery (85%). High Resolution Electron Microscopy (HREM) and Electron Energy Loss Spectroscopy (EELS) were subsequently used to study the microstructure and bonding of the resultant layers. The structure of the films from HREM is seen to consist of a dense array of parallel curved graphene sheet segments packed in various orientations. EELS reveals that the films are comprised of mainly sp2 bonded carbon. The results suggest that a new form of carbon thin film with fullerene-like structure can be realised. In order to explain how a predominantly sp2 bonded material can exhibit such a high hardness, a simple model is proposed to correlate the excellent mechanical properties with the observed structure.

Direct formation of hydrogen peroxide from H2/O2 using a gold catalyst.

Supported Au catalysts are very selective for the direct formation of hydrogen peroxide from H2/O2 mixtures at 2 degrees C; the rate of H2O2 synthesis is markedly increased if Au-Pd alloy nanoparticles are generated by the addition of Pd.

Fabrication of GaN cantilevers on silicon substrates for microelectromechanical devices

The fabrication of free-standing GaN cantilevers on Si(111) is demonstrated, and the growth of III-nitride epilayers on silicon (111) using an AlN buffer layer is characterized. Mechanically releasing GaN structures from Si(111) required a combination of two dry inductively coupled plasma etch processes using Cl2/Ar and CF4/Ar/O2 chemistries, and a potassium hydroxide (KOH) aqueous etch. Scanning transmission electron microscopy reveals a columnar growth habit for the nitrides. Electron energy loss spectroscopy imaging of an AlGaN/GaN interface indicates columnar growth may strongly influence the potential piezoelectric properties of III-nitride cantilever microelectromechanical devices.

Incorporation of Sb in InAs∕GaAs quantum dots

The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs∕GaAs quantum dots emitting at 1.3μm. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmission electron microscopy. The intermixing process giving rise to the formation of this quaternary alloy takes place despite the large miscibility gap between InAs and GaSb binary compounds, and is probably driven by the existence of strain in the quantum dots.

The disruption of oxide defects within aluminium alloy castings by the addition of bismuth

Oxide defects are commonplace in most aluminium castings, if insufficient care has been taken during the casting process. Castings that are made by the squeeze-casting technique and incorporate a region or regions that have been reinforced using MMCs, are even more prone to have oxide defects within the casting, especially at the reinforced/un-reinforced boundaries within the casting. This paper proposes that the addition of small amounts of bismuth to the aluminium alloy will stop the formation of this type of defect within the casting.

Oxide film casting defects in squeeze cast metal matrix composites

Squeeze casting is a commonly used method of producing metal matrix composites (MMCs). It is especially popular in the automotive industry for the production of pistons for diesel engines. This paper describes a casting defect associated with the production of metal matrix composites by this method. The mechanism of formation of the defect is considered and its effect on the future of squeeze casting considered.

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl2/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 μm and 0.7 μm thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl2/Ar and CF4/Ar/O2 chemistries, and a potassium hydroxide (KOH) aqueous wet etch.

Enhancement of the properties of pulsed laser-deposited carbon nitride by the synchronisation of laser and N2 gas jet pulses

In surface coatings technology, especially in carbon-based materials, the deposition of energetic species is acknowledged as one of the most important factors in producing hard coatings. Pulsed laser deposition (PLD) of carbon under vacuum creates such energetic species and so carbon films with very high hardness values have been reported. However, when PLD takes place in a gas ambient, the ablated carbon species are decelerated to an extent that depends on the background pressure. As a result, during CN x deposition, although the carbon species react eVectively with N 2 at the beginning of their trajectories, when they reach the substrate they usually do not have suYcient kinetic energy to form a hard coating. In this paper we describe a new technique that combines the intense environment of an expanding N 2 jet with low-pressure PLD to produce CN x species that travel almost free of collisions and reach the substrate with high kinetic energies. This new configuration is based on the synchronised pulsing of a N 2 jet with the laser pulse. The CN films produced are shown to have an increased film hardness without suppressing the nitrogen content. Furthermore, electron energy-loss spectroscopy shows the layers to have a very high proportion of p bonding, which can be correlated to the existence of sp-hybridised carbon in the form of ‐CON bonds.

Nanometer-scale strain measurements in semiconductors: An innovative approach using the plasmon peak in electron energy loss spectra

We present an innovative technique for quantitative measurement of strain in semiconductor materials with high spatial resolution. The plasmon loss peak, seen in electron energy-loss spectra, has been considered following the Drude-Lorentz model, and we find that plasmon energy is extremely sensitive to lattice parameter. We have tested this model using a heterostructure of In0.2Ga0.8As and AlAs layers in GaAs. The experimental data are in excellent agreement with the model. We estimate that strains smaller than 0.036% can be detected, corresponding to a change of x=0.005 in InxGa1−xAs, at a spatial resolution better than 2.8 nm.