A large-stroke thermomechanical MEMS actuator with lever amplification for helium detection

Abstract

Helium plays a critical role as a fill gas in dry cask storage systems for spent nuclear fuel, where its leakage can compromise thermal performance and safety. This study presents the design, fabrication, modeling, and experimental validation of a novel MEMS-based thermomechanical actuator for helium detection, leveraging the gas's distinct thermal conductivity relative to air. The device features a U-shaped electrothermal actuator composed of a joule-heated fixed hot arm and a conduction-heated flexible cold arm. The cold arm acts as a mechanical lever, amplifying displacement differences caused by gas-specific thermal gradients. Fabricated from silicon, the actuator exhibits a large-stroke out-of-plane displacement, with a measured difference of 7.75 µm between helium and air environments. This differential enables the actuator to function as a normally closed mechanical switch that opens in the presence of helium. Finite element simulations using COMSOL Multiphysics closely match experimental results. Furthermore, uncertainty quantification reveals minimal variation in maximum temperature but notable sensitivity in displacement, underscoring the importance of fabrication precision. These results establish a foundation for developing compact, low-power, high-sensitivity helium detection switches for nuclear and other safety-critical applications.