A Study on Phase Change Materials at Low Thermal Mass with Micro-Electro-Mechanical Resonators

  • Author / Creator
    Bukhari, Syed Asad Manzoor
  • Micro-electromechanical systems or MEMS is a modern technology used
    to create tiny, integrated devices or systems that combine mechanical and
    electrical components. These miniaturized devices can sense,
    control and actuate on the micro and nano scales, and generate observable
    effects on the macro scale. In modern times, one of the key aspects is to
    improve the geometry and incorporate functional materials to increase device
    performance. These devices typically require a high vacuum to operate
    with large amplitudes and sensitivity because of the damping provided
    by the fluid. To overcome operational issues in fluids, a suspended microchannel
    cantilever resonator is proposed which can incorporate polymeric or
    liquid samples at the same time operating under vacuum with high-quality
    factor. To achieve multi-functional device performance and control,
    tuning of the mechanical frequency of the resonator is required. Among
    many materials studied in the past, vanadium dioxide VO2 is one of the key
    material which can satisfy these demands to a larger extent because of its
    phase changing nature. It possesses a reversible metal-insulator transition
    (MIT) ideally at 68 °C where monoclinic (M1 phase-insulator) phase transforms
    to tetragonal or rutile (R phase-metal). Both of the aforementioned
    devices have high sensitivity, selectivity and integrated functionalities for
    better performance.
    Suspended microchannel resonators (SMCR) cantilevers can be used for
    the thermomechanical analysis of polymers. Such a device was used to detect
    multiple thermal transitions in picogram amounts of two well-known polymers,
    semicrystalline poly (L-lactide) (PLA) and amorphous poly (methylmethacrylate)
    (PMMA). The polymer samples, when loaded inside the cantilever
    and heated, affect its resonance frequency due to changes in its density
    and stiffness. Continuous monitoring of the resonance frequency provided
    information about b-transition (Tb), glass transition (Tg), crystallization (Tc),
    and melting (Tm) of the confined polymer samples.
    Phase change materials can also be integrated with conventional semiconducting
    MEMS resonators to achieve frequency tuning with various external
    stimuli. Vanadium dioxide (VO2) is a class of quantum materials which has
    a characteristic insulator to metal transition complemented by a four order
    of magnitude change in resistance and structural change from monoclinic to
    rutile phase. This transition can also be triggered via optical and electrical
    input power while keeping the device at room temperature. By exploiting the
    phase transformation, thermal, optical, electrical and electro-optic excitation
    were used for bi-directional mechanical frequency tuning. First, thin-film
    deposition process parameters were optimized to achieve a large
    magnitude change in resistance and lower onset temperature. Thin films of
    VO2 was then deposited on microstrings. Bi-directional frequency tuning
    was achieved with a single pump and probe optical source. Frequency tuning
    was also studied by scanning laser at fixed distances from the anchor
    to the center of the resonator. Electro-thermal excitation of AC and DC signals
    was used to pump the system and bi-directional frequency tuning was
    observed. This study has potential applications in thermal, electronic and
    optical switches, ultra-fast mechanical frequency tuning and gas sensing.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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