Numerical Modeling and Experimental Validation by Calorimetric Detection of Energetic Materials Using Thermal Bimorph Microcantilever Array: A Case Study on Sensing Vapors of Volatile Organic Compounds (VOCs)

Abstract

Bi-layer (Au-Si3N4) microcantilevers fabricated in an array were used to detect vapors of energetic materials such as explosives under ambient conditions. The changes in the bending response of each thermal bimorph (i.e., microcantilever) with changes in actuation currents were experimentally monitored by measuring the angle of the reflected ray from a laser source used to illuminate the gold nanocoating on the surface of silicon nitride microcantilevers in the absence and presence of a designated combustible species. Experiments were performed to determine the signature response of this nano-calorimeter platform for each explosive material considered for this study. Numerical modeling was performed to predict the bending response of the microcantilevers for various explosive materials, species concentrations, and actuation currents. The experimental validation of the numerical predictions demonstrated that in the presence of different explosive or combustible materials, the microcantilevers exhibited unique trends in their bending responses with increasing values of the actuation current.

Design and Electro-Thermo-Mechanical Behavior Analysis of Au/Si3N4 Bimorph Microcantilevers for Static Mode Sensing

“… where g is the acceleration due to gravity (9.81 m/s 2 ), β is the thermal expansion coefficient [K −1 ], T s is the wall temperature [K], T ∞ is the ambient temperature [K], ν is the kinematic viscosity [m 2 /s], α is the thermal diffusivity, and L (= Area/Perimeter) is the characteristic length [m]. For the size of the simulation volume determined from the calculation using Equation (6), the convective heat transfer coefficient ( h ) calculated based on the pure conduction correlation by the Nusselt number (i.e., Nu = 1) was consistent with the value obtained from the experiments ( h = 700 W/m 2 ·K) [ 2 , 27 ]. Because the actuation slew rate was less than 10 μs and the actuation latency was less than 5 ms, we selected 4 ms as the total simulation time for a steady-state simulation.…”

“…The microcantilevers used in this study were fabricated based on the commercial foundry process (courtesy of NanoINK, Inc., Campbell, CA, USA) [ 2 ]. Figure 3 presents a schematic of a typical bimorph microcantilever and defines the geometry.…”

“…Generally, the change in vertical position of microcantilevers can be monitored by tracking the reflected light spot on a projection screen or by using position sensitive detectors (PSD) [ 2 , 3 , 4 , 5 , 24 ]. This method requires a high-resolution PSD and the precise value of the distance between the end of the cantilever and the detector, to determine the actual vertical movement of the microcantilever tip from the variations in positions of the reflected light spot.…”

“…Microcantilevers have recently attracted attention as detectors in nanocalorimeters due to their high sensitivity, low analyte requirement, quick response, and so on [ 2 , 3 , 4 , 5 ]. This concept was pioneered based on an idea for their extensive use as the sensing platform of a probe (i.e., microcantilever) for an atomic force microscopy (AFM) [ 6 ].…”

“…In microcantilever-based MEMS sensors, monitoring mechanical deflections (i.e., static mode sensing) in thermal response to changes in temperature has frequently been adopted as the sensing mechanism. For instance, the changes in surface temperature of the microcantilever can be induced by surface catalytic reactions [ 2 , 7 ] or infrared (IR) absorption [ 8 ]. On the other hand, changes in the resonant frequency upon mass uptake are monitored in the dynamic mode [ 9 ].…”