A laser-machined MEMS axial flow turbine: design, fabrication, testing and materials analysis

Abstract

This thesis details the design, fabrication and characterisation of a 13 mm-diameter axial flow microturbine with an integrated electromagnetic generator. Axial turbine blades are not amenable to fabrication by traditional MEMS (micro-electromechanical systems) processes because they cannot be produced by machining prismatic shapes into the rotor disc; the direction of machining has to change as material is removed to produce the required blade curvature. This challenge was met by laser machining the blades with a novel moving-mask process so as to produce a step-wise approximation to the desired profile. The chosen material for making the microturbine rotor was the negative photo-resist SU-8. Once properly cured into its solid state, this polymer becomes very durable and dimensionally stable. The SU-8 was readily preformed using lithography and RIE (reactive ion etching), and was also responsive to excimer laser ablation as necessary for finishing the blade profiles. The microturbine was designed to be assembled into a stacked MEMS device comprising a rotor embedded with ten rare earth magnets sandwiched between upper and lower silicon stators carrying electroplated generator coils. Characterisation of the turbine showed that mechanical losses, mainly in the bearings, were significantly reducing the efficiency. A laser scanning vibrometer (Polytec MSA-400) was used to measure the turbine rundown time which was found to be only ~150 ms due to high bearing friction. The in-plane and out-of-plane vibration (wobble) of the rotor as it spun around on its micro roller bearings were also mapped to determine if bearing alignment was reducing power output. The out-of-plane vibration was found to be the main problem, so a new one-piece rotational support holder was constructed for the device. Some microturbine rotors were found to shatter above 100,000 rpm, and this led to interest in the mechanical properties of the cured SU-8. Firstly, PGAA (prompt gamma activation analysis) was used to measure the constituent element percentages and contaminants in a range of SU-8 samples subjected to different heat curing temperatures and UV cross-linking times. It was of interest to see how the O and H percentages changed as these are normally expected to vary depending on temperature and humidity. SANS (small angle neutron scattering) tests were also performed using a 10 MW reactor which measured sub-surface scattering for the same samples to reveal material defects.