Design and analysis of magnetostrictive sensors for wireless temperature sensing

Magnetostrictive transducers are commonly used as actuators, sonar transducers, and in remote non-destructive evaluation. Their use in wireless thermometry is relatively unexplored. Since magnetostriction-based sensors are passive, they could potentially enable long-term near-field thermometry. While the temperature sensitivity of resonance frequency in magnetostrictive transducers has been reported in previous studies, the origin of the temperature sensitivity has however not been elucidated. Here, we identify material properties that determine temperature sensitivity, and identify ways to improve sensitivity as well as the detection technique. Using a combination of analytical and computational methods, we systematically identify the material properties that directly influence the temperature coefficient of resonance frequency (TCF). We first experimentally measure the shift in resonance frequency due to temperature changes in a Metglas strip to be 0.03%K-1. Using insights from theory, we then experimentally demonstrate a 5-fold improvement to the TCF by using Terfenol in place of Metglas as the magnetostrictive sensor material. We further demonstrate an alternate temperature sensing technique that does not require measuring the resonance frequency, consequently reducing instrument complexity. This work provides a general framework to analyze magnetostrictive materials and the sensing scheme for near-field wireless thermometry.NSF-CBET-17-06854Ope

Lensing in the Ultrasonic Domain using Negative Refraction Induced by Material Contrast

Abstract The focusing of ultrasound using topographic lenses, typically made of plates with step changes that cause an interaction between forward- and backward-propagating guided waves, has been widely studied in recent years. However, such ‘step-change’ lenses require precise machining and moreover, the thick-thin structure can be unstable during deployment in practical inspection applications. The work reported here follows from the insight that perhaps any approach to induce a mismatch in acoustical impedance as achieved by the step-change can also lead to focusing of ultrasonic guided waves. By carefully choosing the impedance pairing, a novel material contrast lens stacking Aluminium and Molybdenum plates in series is shown to achieve focusing of ultrasound through negative refraction. The interface between the two metals causes the interaction of the forward-propagating second symmetric Lamb mode S2 into the backward- propagating first symmetric S2b. The focusing of Lamb waves is demonstrated using numerical simulations validated by experiments. Comparison with a simple Aluminium-Aluminium plate combination brings out the underlying physics of focusing using the proposed material contrast lens. Simulation results showing super-resolution imaging using the proposed material contrast lens  are also presented, demonstrating the power of the proposed approach. This report opens up the possibilities of developing new lensing devices for use in medical imaging and nondestructive evaluation, among other possible applications

Specularity of Longitudinal Acoustic Phonons at Rough Surfaces

The specularity of phonons at crystal surfaces is of direct importance to thermal transport in nanostructures and to dissipation in nanomechanical resonators. Wave scattering theory provides a framework for estimating wavelength-dependent specularity, but experimental validation remains elusive. Widely available thermal conductivity data presents poor validation since the involvement of the infinitude of phonon wavelengths in thermal transport presents an underconstrained test for specularity theory. Here, we report phonon specularity by measuring the lifetimes of individual coherent longitudinal acoustic phonon modes excited in ultrathin (36–205 nm) suspended silicon membranes at room temperature over the frequency range~20–118 GHz. Phonon surface scattering dominates intrinsic Akhiezer damping at frequencies >~ 60 GHz, enabling measurements of phonon boundary scattering time over wavelengths ~72–140 nm. We obtain detailed statistics of the surface roughness at the top and bottom surfaces of membranes using HRTEM imaging. We find that the specularity of the excited modes are in good agreement with solutions of wave scattering only when the TEM statistics are corrected for projection errors. The often-cited Ziman formula for phonon specularity also appears in good agreement with the data, contradicting previous results. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.NSF-CBET-12-50192NSF-CBET-17-06854Ope