PhD, EngD, DM and MSc by research theses (Silsoe)

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Browsing PhD, EngD, DM and MSc by research theses (Silsoe) by Author "Allen, David"

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    Electrochemical micromachining of microdevices from NiTi shape memory alloys
    (Cranfield University, 1999-05) Chen, Ta-Tung; Allen, David
    This thesis aimed to develop a reproducible process for batch-fabricating microdevices required for MEMS and medical applications, such as micro actuators and stents, from heat-sensitive NiTi shape memory materials. Electrochemical micromachining was chosen to carry out this work. This is a non-traditional machining process involving photoresist processing and electrolytic etching which has received much attention recently for the processing of thin films. The electrolyte used was a non-aqueous solution of 5% sulphuric acid in methanol. The optimum parameters for the photoresist processing were obtained by evaluation of the thickness and exposure time of the KTFR photoresist coating. A quantitative investigation of the electrolytic etching of NiTi was carried out to study the influence of applied voltage, etch time and line width of the test pattern on the etching behaviour, e.g. etch rate, undercut, depth of etch and etch factor. The anodic polarisation behaviour of NiTi in 5% sulphuric acid in methanol was investigated under a potentiostatic control system to establish the optimum etching parameters. The materials used for the fabrication of micro actuators (required by Forschungszentrum Karlsruhe, Germany to make a prototype microvalve) were NiTi alloy thin film materials (sputtered or cold-rolled) with thicknesses ranging from 5 to 46J...lm displaying a one-way or two-way shape m:emory effect. A variety of optimised designs of micro actuator were successfully etched electrolytically at 8V. The etch rate was found to depend directly on the anodic current density. The addition of a third alloying element such as Pd or eu reduced the anodic current density and maintained a similar etch rate. However it resulted in the breaking of the films during etching due to the reduction in the ductility of the material. The materials for the micro fabrication of stents were 100J...lm thick NiTi sheets. The problem of non-uniform metal dissolution was observed. However, by adding a sacrificial etch band as a current 'robber', periodic rotation of the anode and properly adjusting the electrochemical and geometric parameters, the stents were etched successfully with improved yield and dimensional accuracy.
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    ItemOpen Access
    Photoetching of nickel alloys and reclamation of waste products
    (Cranfield University, 1995-06) White, H. J. A.; Allen, David
    This thesis details a study into the photoetching of nickel alloys with ferric chloride and the reclamation of waste products. Reclamation occurs through the regeneration of the spent etchant and the recovery of the dissolved nickel content. Etchant regeneration is a means by which a constant etching quality can be maintained. The alternative is its replacement with fresh solution and this option entails the disposal of the exhausted solution according to correct legislative procedures. A cost comparison of both processes, was made in the early stage of this work and a model established to represent the economics of regeneration and conventional disposal (ie. not regenerating) based on the amount of nickel etched per annum. Although the dissolved nickel content will not hinder etchant regeneration, it will affect the quality of subsequent etching and the etchant will remain "contaminated". Consequently, its removal must be considered if etchant regeneration is to be implemented. Therefore, in order to establish the economical viability of nickel removal (and etchant regeneration) it was necessary to investigate potential techniques at an experimental level and to determine their technical feasibility first. Cementation and electrodialysis were investigated, the latter proving to be the more successful technique for this application. Through experimentation, it was possible to determine nickel yield data and the associated chemical and power requirements. This information was then used to calculate the 'costs associated with the process in order to extend the original model.
**Copy of record** Please note that the electronic theses currently available on this server have been originally submitted in a paper format. In all cases the bound paper thesis is the copy of record.