Papers by brendan hanrahan
IEEE Transactions on Components, Packaging and Manufacturing Technology, 2015
This dissertation explores the fundamental tribology of microfabricated rolling bearings for futu... more This dissertation explores the fundamental tribology of microfabricated rolling bearings for future micro-machines. It is hypothesized that adhesion, rather than elastic hysteresis, dominates the rolling friction and wear for these systems, a feature that is unique to the micro-scale. To test this hypothesis, specific studies in contact area and surface energy have been performed. Silicon microturbines supported on thrust bearings packed with 285 µm and 500 µm diameter stainless steel balls have undergone spin-down friction testing over a load and speed range of 10-100mN and 500-10,000 rpm, respectively. A positive correlation between calculated contact area and measured friction torque was observed, supporting the adhesion-dominated hysteresis hypothesis. Vapor phase lubrication has been integrated within the microturbine testing scheme in a controlled and characterized manner.
Journal of Microelectromechanical Systems, 2015
Journal of Micromechanics and Microengineering, 2013
ABSTRACT This work explores the performance of different silicon retainer ring designs when integ... more ABSTRACT This work explores the performance of different silicon retainer ring designs when integrated into silicon micro-turbines (SMTs) incorporating thrust style bearings supported on 500 µm diameter steel balls. Experimental performance curves are presented for SMTs with rotor diameters of 5 mm and 10 mm, each with five different retainer designs varying in mechanical rigidity, ball pocket shape and ball complement. It was found that the different retainer designs yielded different performance curves, with the closed pocket designs consistently requiring lower input power for a given rotation speed, and the most rigid retainers giving the best performance overall. Both 5 mm and 10 mm diameter devices have shown repeatable performance at rotation speeds up to and exceeding 20 000 RPM with input power levels below 2 W, and devices were tested for over 2.5 million revolutions without failure. Retainer rings are commonly used in macro-scale bearings to ensure uniform spacing between the rolling elements. The integration of retainers into micro-bearings could lower costs by reducing the number of balls required for stable operation, and also open up the possibility of 'smart' bearings with integrated sensors to monitor the bearing status.
Journal of Micromechanics and Microengineering, 2014
ABSTRACT A multi-step plasma etching technique is developed to obtain deep-grooved micro-scale ba... more ABSTRACT A multi-step plasma etching technique is developed to obtain deep-grooved micro-scale ball-bearing raceways and employed in the fabrication of multiple ball-bearing supported microturbines. Deep-groove geometry has been chosen for the microball-bearing systems because of the ability to handle mixed axial and radial loads, allowing for stable, high-speed operation compared to previous iterations of the microball-bearing raceways. The multi-step inductively coupled plasma-based process is optimized to obtain <2% deviation amongst intended raceway depth, width and curvature. Etching non-uniformity is measured to be 0.15% within the raceway of a single device. The bearing dynamics with the new deep-groove geometry have been simulated. The deep-groove raceway packed with off-the-shelf precision ball-bearings provided a stability improvement over previous demonstrations of high-performance rotary micromachines operating at high speeds.
IEEE Sensors Journal, 2000
Microelectromechanical Systems, Journal of, 2013
Electrochimica Acta, 2012
Carbon nanotubes (CNTs) are being widely investigated as a replacement for activated carbon in su... more Carbon nanotubes (CNTs) are being widely investigated as a replacement for activated carbon in supercapacitors. A wide range of CNT specific capacitances have been reported in the literature based on experiments using different CNT materials, fabrication methods, and characterization routines; making it difficult to draw conclusions about the relative merits of the different fabrication methods. This work systematically compares four solution-based electrode fabrication methods (drop casting, air brushing, filtration, and electrospraying) and, to a lesser extent, some solution preparation techniques to determine if there is an optimum method for fabricating electrochemical capacitor electrodes out of single-wall CNTs (SWCNTs). We have found that it is best to use CNT solutions free from additives that may be difficult to remove from the fabricated electrode. In addition, the CNT solution preparation (e.g., dilution and sonication) had little effect on the resulting specific capacitance. Large differences in performance due to the fabrication methods were not seen, and the differences that were seen could be ascribed to material loss or contamination during the deposition. A single-layer graphene electrode was also fabricated and tested to obtain an estimate of the specific capacitance potentially achievable with SWCNTs, with 550 F/g demonstrated using 1 molar (M) sulfuric acid.
Electrochimica Acta, 2012
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Papers by brendan hanrahan