Interface stress between structural materials and thin film coatings has a significant influence on the resonant frequency of microelectromechanical system (MEMS) resonators. In this work, the axial stress on different types of buckled bridge MEMS resonator structures is controlled through the solid‐to‐solid phase transition of a VO2thin film coating. The devices have identical dimensions, but different buckling orientations and profiles due to the combined effect of overetching and residual thermal stress mismatch. Thermal actuation is used to tune the resonant frequency of the device, but the changes in frequency are found to be dependent on the type of buckling for the device. Thermal actuation is achieved by applying an electrical current to integrated heaters, or by uniform substrate heating. Bidirectional tunability is found when substrate heating is used, while Joule heating shows a monotonic change in frequency. This phenomenon can be attributed to the transition in boundary conditions, where the turning points are indicated by the prominent changes in buckling amplitude. In addition, devices with opposite buckling orientations exhibit different tuning behaviors which can be explained by different bending moments induced by beam stress interface modification.
A near-field multistage radiative thermal rectifier is proposed based on two different phase-change materials, which can achieve multistage thermal rectification with different rectification ratios. The phase-change materials vanadium dioxide (VO2) and Ge2Sb2Te5(GST), with different metal-insulator transition temperatures, are utilized within the active terminal of thermal rectifier. Four types of active terminal structures, including multi-film and composite nanograting structures, are introduced to explore to multistage thermal rectification. Our calculations find that the active terminal composed of a one-dimensional VO2grating atop a GST thin film is the most suitable for multistage thermal rectification due to its realization of well-distributed and flexible thermal rectification. Furthermore, it is found that the passive terminal temperature of thermal rectifier can significantly affect the multistage radiative thermal rectification by modifying the rectification ratio and adjusting the stage number of multistage thermal rectification. This work sheds light on the role of different phase-change materials within the design of promising radiative thermal rectifiers boasting multistage thermal rectification.
more » « less- NSF-PAR ID:
- 10369334
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optical Materials Express
- Volume:
- 12
- Issue:
- 6
- ISSN:
- 2159-3930
- Page Range / eLocation ID:
- Article No. 2135
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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