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Title:Development of predictable palladium based optomechanical hydrogen sensors
Author(s):McKeown, Steven James
Director of Research:Goddard, Lynford
Doctoral Committee Chair(s):Goddard, Lynford
Doctoral Committee Member(s):Boppart, Stephen; Allain, Jean Paul; Dragic, Peter
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):optical fiber sensors
hydrogen sensors
optomechanical sensors
hydrogen induced lattice expansion
Abstract:In addition to its use as an energy storage medium or fuel, hydrogen gas has a variety of commercial applications such as methanol and ammonia production. Given the volatility and flammability of hydrogen, as well as its small molecular size, fast and accurate sensors capable of operating in a variety of environments are necessary. A large subset of hydrogen gas sensors rely on palladium metal, which is known to reversibly react with hydrogen to form palladium hydride. This results in a change in the optical, electrical and mechanical properties of the film. These changes are a result of a change the Fermi level and band structure of the metal, as well as an increase in lattice constant in the presence of hydrogen. The change in complex refractive index plays a role in both reflection/transmission, and for determining resonances or guided modes in waveguides and other sub-wavelength features. However, the increase in the lattice constant of the metal, a process called hydrogen induced lattice expansion, was found to be equally important in modeling the response of the sensors, both from an optical and a mechanical perspective. This dissertation is concerned with the simulation, fabrication, and testing of palladium based optomechanical sensors, particularly to elucidate the role of hydrogen induced lattice expansion in their design and functionality. Two specific sensor designs: a nano-aperture based sensor and a cantilever based sensor were designed, fabricated, characterized, and modeled. The first sensor developed was based on a single nano-aperture etched into a palladium coated fiber facet. Designed to operate based on the principle of extraordinary transmission and the change in optical constants of the palladium, this sensor showed experimental sensitivity down to 150ppm in transmission and 50 ppm reflection. However, without inclusion of the mechanical effects, the device behavior was unpredictable. Separate work was thus carried out to characterize lattice expansion in thin palladium films using quantitative phase imaging techniques, so that a new sensor could be designed and accurately modelled. This second fabricated sensor consisted of a Pd coated cantilever which operated based on optical probing of mechanical deflections. For more thorough characterization, the cantilever was measured using the same phase imaging techniques. The results of this analysis further improved the understanding of thin film expansion and the capabilities of diffraction phase microscopy for material analysis. Furthermore, this culminated in the fabrication of a sensitive and reliable optomechanical hydrogen sensor whose response matched theory.
Issue Date:2016-04-20
Rights Information:Copyright 2016 Steven J. McKeown
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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