Eanna McCarthy

Field emission in ordered arrays of ZnO nanowires prepared by nanosphere lithography and extended Fowler-Nordheim analyses

A multistage chemical method based on nanosphere lithography was used to produce hexagonally patterned arrays of ZnO vertical nanowires, with 1 μm interspacing and aspect ratio ∼20, with a view to study the effects of emitter uniformity on the current emitted upon application of a dc voltage across a 250 μm vacuum gap. A new treatment, based on the use of analytical expressions for the image-potential correction functions, was applied to the linear region below 2000 V of the Fowler-Nordheim (FN) plot and showed the most suitable value of the work function φ in the range 3.3–4.5 eV (conduction band emission) with a Schottky lowering parameter yu2009∼u20090.72 and a field enhancement factor γ in the 700–1100 range. A modeled γ value of ∼200 was calculated for an emitter shape of a prolate ellipsoid of revolution and also including the effect of nanowire screening, in fair agreement with the experimental value. The Fowler-Nordheim current densities and effective emission areas were derived as 1011 Am−2 and 10−17 m2,...

Influence of ZnO nanowire array morphology on field emission characteristics.

In this work the growth and field emission properties of vertically aligned and spatially ordered and unordered ZnO nanowires are studied. Spatially ordered nanowire arrays of controlled array density are synthesized by both chemical bath deposition and vapour phase transport using an inverse nanosphere lithography technique, while spatially unordered arrays are synthesized by vapour phase transport without lithography. The field emission characteristics of arrays with 0.5, 1.0, and 1.5 μm inter-wire distances, as well as unordered arrays, are examined, revealing that, within the range of values examined, field emission properties are mainly determined by variations in nanowire height, and show no correlation with nanowire array density. Related to this, we find that a significant variation in nanowire height in an array also leads to a reduction in catastrophic damage observed on samples during field emission because arrays with highly uniform heights are found to suffer significant arcing damage. We discuss these results in light of recent computational studies of comparable nanostructure arrays and find strong qualitative agreement between our results and the computational predictions. Hence the results presented in this work should be useful in informing the design of ZnO nanowire arrays in order to optimize their field emission characteristics generally.

Effects of the crystallite mosaic spread on integrated peak intensities in 2θ–ω measurements of highly crystallographically textured ZnO thin films

We report x-ray diffraction (XRD) (2θ–ω and rocking curve) and transmission electron microscopy (TEM) measurements on crystallographically textured ZnO thin films of varying thicknesses and crystallite mosaic spread deposited by pulsed-laser deposition on Si. The integrated areas of the (0u20090u20090u20092) ZnO reflections in the 2θ–ω mode do not scale with film thickness and in some cases show discrepancies of two orders of magnitude compared with expectations based solely on sample thicknesses. Intensity differences of this type are regularly used in the literature as indications of differences in sample crystallinity or crystal quality. However TEM data of our samples show no evidence of amorphous deposits or significantly varying crystal quality in different films. X-ray rocking curves of these samples do show substantial variations in the mosaic spread of crystallites in the ZnO films which are the origin of the differences in integrated areas of the (0u20090u20090u20092) ZnO reflections in 2θ–ω measurements. We outline a generally applicable model to treat the 2θ–ω mode peak intensities which shows good agreement with the experimental data (to within an order of magnitude) and which is much simpler than utilizing a full reciprocal space map approach to understand the XRD data. We conclude that the normalized integrated intensity of the (0u20090u20090u20092) ZnO reflection in highly crystallographically textured ZnO thin films is strongly dependent on the rocking curve width in addition to the film thickness and the use of such intensities in isolation as measures of the thin film crystallinity or crystal quality, without reference to the rocking curve width, is likely to be misleading when making judgments of such aspects of the thin film structure.

Laser surface texturing for high control of interference fit joint load bearing

Laser beams attract the attention of researchers, engineers and manufacturer as they can deliver high energy with finite controlled processing parameters and heat affected zone (HAZ) on almost all kind of materials [1–3]. Laser beams can be generated in the broad range of wavelengths, energies and beam modes in addition to the unique property of propagation in straight lines with less or negligible divergence [3]. These features made lasers preferential for metal treatment and surface modification over the conventional machining and heat treatment methods. Laser material forming and processing is prosperous and competitive because of its flexibility and the creation of new solutions and techniques [3–5].This study is focused on the laser surface texture of 316L stainless steel pins for the application of interference fit, widely used in automotive and aerospace industry. The main laser processing parameters applied are the power, frequency and the overlapping laser beam scans. The produced samples were characterized by measuring the increase in the insertion diameter, insertion and removal force, surface morphology and cross section alteration and the modified layer chemical composition and residual stresses.Laser beams attract the attention of researchers, engineers and manufacturer as they can deliver high energy with finite controlled processing parameters and heat affected zone (HAZ) on almost all kind of materials [1–3]. Laser beams can be generated in the broad range of wavelengths, energies and beam modes in addition to the unique property of propagation in straight lines with less or negligible divergence [3]. These features made lasers preferential for metal treatment and surface modification over the conventional machining and heat treatment methods. Laser material forming and processing is prosperous and competitive because of its flexibility and the creation of new solutions and techniques [3–5].This study is focused on the laser surface texture of 316L stainless steel pins for the application of interference fit, widely used in automotive and aerospace industry. The main laser processing parameters applied are the power, frequency and the overlapping laser beam scans. The produced samples were ch...

Laser welding of polypropylene using two different sources

In this paper, laser weldability of neutral polypropylene has been investigated using fibre and carbon dioxide lasers. A design of experiment (DoE) was conducted in order to establish the influence of the main working parameters on the welding strength of the two types of laser. The welded samples were characterized by carrying out visual and microscopic inspection for the welding morphology and cross-section, and by distinguishing the tensile strength.The resulting weld quality was investigated by means of optical microscopy at weld cross-sections. The tensile strength of butt-welded materials was measured and compared to that of a corresponding bulk material.

A review of semi-solid aluminium-steel joining processes

The semi-solid metal (SSM) forming process can be applied to achieve near net shape forming of metal alloys, and provides superior component properties compared to those achievable with conventional casting methods. The technique, also commonly called thixoforming, relies on achieving a spheroidal microstructure within the metal alloy so that its fluidity can be adjusted to achieve a controlled laminar filling of the die. Despite the better quality and the higher mechanical properties of an SSM product, thixoforming still represents only 1% of the total aluminium production, which can be explained by the higher premium cost of the processing equipment compared to conventional die casting. The method has also proven successful as a joining method, for joining similar and dissimilar materials. This paper reviews semisolid forming as a forming method and as a joining method, in particular the joining of dissimilar materials such as stainless steel to aluminium.