A laser-machined MEMS axial flow turbine: design, fabrication, testing and materials analysis
File(s)
PhD thesis
Author(s)
Heaton, Mark Edwin
Type
Thesis
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.
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.
Date Issued
2013
Date Awarded
2013-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Holmes, Andrew S.
Sponsor
EPSRC
Grant Number
GR/N18895
Publisher Department
Department of Electrical and Electronic Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)