Applied Materials
Nelson Sepúlveda | sepulve6@egr.msu.edu | www.egr.msu.edu/~nelsons/
The Applied Materials Group (AMG) is focused on the integration of novel smart materials into micrometer sized devices.
Our main emphasis has been on vanadium dioxide (VO2) thin films. VO2 exhibits a solid-solid phase change, which can be thermally induced at a temperature of ~ 68 °C. During this phase transition, the material goes through an electronic/structural transformation (Mott-Peierls type) during which many of the material properties (e.g., electrical, optical, mechanical) change abruptly. The multifunctional properties of VO2, the relative proximity of its transition temperature to room temperature, and its intrinsic hysteretic behavior—which gives the material memory capabilities—make this material very attractive for practical applications. In 2010, our group demonstrated that VO2-coated micromechanical structures experience very large deflections during the coating’s phase transition; deflections much larger than what could be accomplished by solely the thermal expansion difference between the microstructure and the coating. It was found that the mechanism responsible for the observed deflections was associated with the crystallographic changes that the coating experiences during its structural phase change.
VO2 Micromechanical Actuators
Given the repeatability, large strain energy density, robustness, and inherent multi-functionality that VO2 brings to MEMS actuators, these devices are expected to replace currently used actuation technologies. Our group has recently begun to combine VO2 thin films with other materials, such as carbon nanotubes (CNTs) and integrate both materials into monolithic energy-efficient MEMS actuators. Our uncoated VO2-based micromechanical actuators can oscillate at frequencies around 1 kHz —which is considerably fast for a thermal-based actuation process—while sustaining their large vibration amplitudes. Also, due to the intrinsic hysteretic behavior of the VO2 film, the electrical, mechanical, and optical properties in VO2-based devices can be programmed, thus allowing for multifunctional MEMS memories. Perhaps the most interesting aspect of these CNT/VO2-based actuators is the fact that the energy consumption is significantly less without sacrificing performance in terms of speed or total actuation range. This is due to the low elastic modulus and density of CNT films.
Variable Optical Attenuators
The abrupt changes in VO2’s optical properties, and its inherently hysteretic behavior has also been used to program and project near infrared (NIR) images as well as variable optical attenuators (VOAs). These devices attenuate a propagating laser beam, while simultaneously monitor the film’s resistance to track its phase transition and control the intensity of the transmitted light. The strong correlation between optical and electrical properties across the phase change allow for the implementation of self-sensing techniques, where the resistance of the material is used to estimate its optical transmittance.
VO2 Antenna
In addition, our group has demonstrated a new method to reconfigure, tune or program an antenna, using the phase change of VO2. Self-sensing techniques have also been implemented for the electrical and mechanical properties of the VO2-based device across the material’s phase transition.
The multifunctionality of VO2 and the proximity of its phase transition to room temperature has awaken the interest of multiple applied research groups in recent years. Our group is dedicated to the strategic integration of multiple smart multifunctional materials into monolithic micrometer-sized devices in order to shed light on new approaches and overcome theoretical boundaries imposed by the use of single materials. Currently, our emphasis is on VO2, but we try to keep our vision always one step ahead of the bottlenecks imposed by material limitations.
