Development of Bulk-scale and Thin Film Magnetostrictive Sensor

Abstract

Three key areas were investigated in this research. These are: (1) finite element modeling using modal analysis to better understand the mechanics of longitudinal vibration system, (2) thin film material Young’s modulus measurement in a nondestructive manner by a magnetostrictive sensor, and (3) optimization of a deposition process for sputtering magnetostrictive thin films from Metglas 2826 MB ribbon and machining them into useful sensor platforms. We have verified the principle of operation for the longitudinal vibrating system through experimentation and comparison with numerical simulations of cantilevers, bridges, and beams. The results indicated that the governing vibration equation should use the plane-stress or biaxial modulus. Furthermore, the Poisson’s ratio for Metglas 2826 MB was found to be 0.33. A resonating mechanical sensor was constructed from commercially available Metglas 2826 MB strip material and was used to measure Young’s modulus of sputter deposited thin film material, e.g. Cu, Au, Al, Cr, Sn, In, SnAu (20/80 eutectic), and SiC, with a proposed measurement methodology. The determined Young’s modulus values were comparable to those found in the literature. In addition, a finite element modeling analysis was employed to verify the Young’s modulus determined by experimentation. Glass beads (size of ~425 µm) were attached to freestanding (free-free ended) magnetostrictive sensors in order to simulate the attachment of target species. These mass-loading results indicated that the frequency shifts are sensitive to the location of the mass on the sensor’s surface. Finite element analysis was conducted and ascertained that when a particle comparable in size to E. Coli O157 cell (mass in pico-gram range) attaches to sensor of 250 x 50 x 1.5 microns in size, a significant resonant frequency shift results, indicating that the sensor has the potential to detect the attachment of a single bacterium. These simulations also confirm that the resonant frequency shift is dependent on the location of the mass attachment along the longitudinal axis of the sensor. Finally, a process for depositing magnetostrictive thin film material from directly sputtering of Metglas 2826 MB ribbon was developed. Microscale sensors were fabricated with this film material. Dynamic testing of these microscale sensors was carried out on freestanding particles of the size 500 x 100 x 3 microns. The resonant frequency of these microfabricated particles was found to increase significantly in both magnitude and amplitude after the particle was annealed. A model was employed to explain why the magnetoelastic sensor behavior changed after annealing.