Photodissociation of Cl_2O at 248 and 308 nm

Molecular beam studies of Cl_2O photolysis at 248 and 308 nm have been repeated and the analysis refined. At 248 nm, three distinct dissociation pathways that led to Cl+ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic than previously reported, leaving open the possibility that Cl_2O excited at 308 nm lives longer than a rotational period

Friction stir welding of commercially available superplastic aluminium

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A series of Friction Stir Welds (FSW) has been produced in order to optimise tool designs and weld/process variables to minimise flaws in the weld and obtain the best possible microstructure for superplastic Forming (SPF). Therefore the main goal is to produce friction stir welds which do not fail during subsequent SPF processes. The friction stir welds have been created using novel tools which are oversized for the material thickness used; this creates a wider weld region of fine equiaxed grains which are suitable for SPF. These original tools have been compared to tools which are already in mainstream FSW production. The welds created for this investigation also represent an evolution of friction stir welding by starting with a milling machine; a very basic piece of engineering workshop equipment. This was then replaced by a modified milling machine with force monitoring capabilities and finally using a state of the art dedicated FSW machine for the final welds. Room temperature properties are not usually a good indicator of high temperature response; however in this thesis the room temperature properties are closely linked to the FSW microstructure and have been used to assess the suitability of the weld structure for subsequent superplastic forming operations. The welds created for this thesis have been completed using hot and cold welding conditions, evaluated for room temperature properties and microstructural stability. The results have then been used to assess the welds and select the most suitable structure for cone testing, which is used to test the welds‘ performance during SPF. Friction stir welds were then recreated and cone tested which reveals the different levels of deformation occurring across the entire weld section and the unaffected parent material. Specimens in the as-welded, post-weld annealed and post-SPF have been analysed using standard microscopy techniques and Electron Back-Scattered Diffraction (EBSD). Welds in Aluminium Alloy (AA) 2004 with excellent room temperature properties have been created and shown to be capable of superplastic deformation achieving strain greater than 200%. Welds in AA5083, although producing excellent room temperature properties are unable to deform superplastically due to the difference in strengthening mechanisms employed by the different alloys. AA2004 contains Al3Zr which effectively pins the microstructure allowing grain boundary sliding to occur, AA5083 lacks this grain refinement element and so suffers from abnormal grain growth leading to early failure.Brunel University and EPSR

Hyperthermal neutral beam etching

A pulsed beam of hyperthermal fluorine atoms with an average translational energy of 4.8 eV has been used to demonstrate anisotropic etching of Si. For 1.4 Hz operation, a room-temperature etch rate of 300 Å/min for Si(100) has been measured at a distance of 30 cm from the source. A 14% undercutting for room-temperature etching of Novolac-masked Si features was achieved under single-collision conditions, with no detectable mask erosion. Translational energy and angular distributions of scattered fluorine atoms during steady-state etching of Si by a normal-incidence, collimated beam demonstrate that unreacted F atoms can scatter inelastically, retaining a significant fraction of their initial kinetic energies. The observed undercutting can be explained by secondary impingement of these high-energy F atoms, which are more reactive upon interaction with the sidewalls than would be expected if they desorbed from the surface at thermal energies after full accommodation. Time-of-flight distributions of volatile reaction products were also collected, and they show evidence for a dominant nonthermal reaction mechanism of the incident atoms with the surface in addition to a thermal reaction channel

Protocol for Atomic Oxygen Testing of Materials in Ground-Based Facilities

A second version of standard guidelines is proposed for improving materials testing in ground-based atomic oxygen environments for the purpose of predicting the durability of the tested materials in low Earth orbit (LEO). Accompanying these guidelines are background information and notes about testing. Both the guidelines and the additional information are intended to aid users who wish to evaluate the potential hazard of atomic oxygen in LEO to a candidate space component without actually flying the component in space, and to provide a framework for more consistent atomic oxygen testing in the future

Dissociation Dynamics of CIONO_2 and Relative Cl and ClO Product Yields following Photoexcitation at 308 nm

Chlorine nitrate photolysis at 308 nm has been investigated with a molecular beam technique. Two primary decomposition pathways, leading to Cl + NO_3 and ClO + NO_2, were observed. The branching ratio between these two respective channels was determined to be 0.67 ± 0.06 : 0.33 ± 0.06. This ratio is an upper limit because some of the ClO photoproducts may have undergone secondary photodissociation. The angular distributions of the photoproducts with respect to the direction of polarization of the exciting light were anisotropic. The anisotropy parameters were β= 0.5 ± 0.2 for the Cl + NO_3 channel and β= 1.1 ± 0.2 for the ClO + NO_2 channel, indicating that dissociation of ClONO_2 by either pathway occurs within a rotational period. Weak signal at mass-to-charge ratios of 35 and 51, arising from products with laboratory velocities close to the beam velocity, was observed. While this signal could result from statistical dissociation channels with a total relative yield of 0.07 or less, it is more likely attributable to products from ClO secondary photodissociation or from dissociation of clusters

Primary and secondary dissociation pathways in the ultraviolet photolysis of Cl_2O

The photodissociation of dichlorine monoxide (Cl_2O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl–O bond fission with production of ClO+Cl. A fraction of the ClO photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center-of-mass translational energy distribution for the ClO+Cl channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 nm distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to ClO+Cl products. The high product internal energy of the second component raises the possibility that ClO is formed in a previously unobserved spin-excited state a 4∑−. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl_2+O was observed in addition to primary Cl–O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time-of-flight distributions of the Cl_2+O products suggest that these fragments are formed in two different channels Cl_2(3II)+O(3P) and Cl_2(X1∑)+O(1D)

Semiconductor etching by hyperthermal neutral beams

An at-least dual chamber apparatus and method in which high flux beams of fast moving neutral reactive species are created, collimated and used to etch semiconductor or metal materials from the surface of a workpiece. Beams including halogen atoms are preferably used to achieve anisotropic etching with good selectivity at satisfactory etch rates. Surface damage and undercutting are minimized

Inelastic and reactive scattering of hyperthermal atomic oxygen from amorphous carbon

The reaction of hyperthermal oxygen atoms with an amorphous carbon-13 surface was studied using a modified universal crossed molecular beams apparatus. Time-of-flight distributions of inelastically scattered O-atoms and reactively scattered CO-13 and CO2-13 were measured with a rotatable mass spectrometer detector. Two inelastic scattering channels were observed, corresponding to a direct inelastic process in which the scattered O-atoms retain 20 to 30 percent of their initial kinetic energy and to a trapping desorption process whereby O-atoms emerge from the surface at thermal velocities. Reactive scattering data imply the formation of two kinds of CO products, slow products whose translational energies are determined by the surface temperature and hyperthermal (Approx. 3 eV) products with translational energies comprising roughly 30 percent of the total available energy (E sub avl), where E sub avl is the sum of the collision energy and the reaction exothermicity. Angular data show that the hyperthermal CO is scattered preferentially in the specular direction. CO2 product was also observed, but at much lower intensities than CO and with only thermal velocities

Molecular beam scattering from C-13 enriched Kapton and correlation with the EOIM-3 carousel experiment

Mass spectra of products emerging from identical samples of a C-13-enriched polyimide polymer (chemically equivalent to Kapton) under atomic oxygen bombardment in space and in the laboratory were collected. Reaction products unambiguously detected in space were CO-13, NO, (12)CO2, and (13)CO2. These reaction products and two others, H2O and CO-12, were detected in the laboratory, along with inelastically scattered atomic and molecular oxygen. Qualitative agreement was seen in the mass spectra taken in space and in the laboratory; the agreement may be improved by reducing the fraction of O2 in the laboratory molecular beam. Both laboratory and space data indicated that CO and CO2 products come preferentially from reaction with the imide component of the polymer chain, raising the possibility that the either component may degrade in part by the 'evaporation' of higher molecular weight fragments. Laboratory time-of-flight distributions showed: (1) incomplete energy accommodation of impinging O and O2 species that do not react with the surface; and (2) both hyperthermal and thermal CO and CO2 products, suggesting two distinct reaction mechanisms with the surface

Artificial neural networks for controlling the temperature of internally cooled turning tools

Copyright @ 2013 Scientific Research PublishingBy eliminating the need for externally applied coolant, internally cooled turning tools offer potential health, safety and cost benefits in many types of machining operation. As coolant flow is completely controlled, tool temperature mea- surement becomes a practical proposition and can be used to find and maintain the optimum machining conditions. This also requires an intelligent control system in the sense that it must be adaptable to different tool designs, work piece materials and machining conditions. In this paper, artificial neural networks (ANN) are assessed for their suitability to perform such a control function. Experimental data for both conventional tools used for dry machining and internally cooled tools is obtained and used to optimise the design of an ANN. A key finding is that both experimental scatter characteristic of turning and the range of machining conditions for which ANN control is required have a large effect on the optimum ANN design and the amount of data needed for its training. In this investigation, predictions of tool tem- perature with an optimised ANN were found to be within 5°C of measured values for operating temperatures of up to 258°C. It is therefore concluded that ANN’s are a viable option for in-process control of turning processes using inter- nally controlled tools.This study is funded by the European Commission