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

  • Microstrings
  • MIT
  • Microcantilevers
  • MEMS
  • thin films deposition
  • Vanadium dioxide
  • EMIT

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-03g9-9j84

• License

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