Phononic Metamaterials for Surface Acoustic Wave Sensing

This thesis investigates the sensitivity of phononic metamaterials to the presence of materials and changes in their environment. The behaviour of surface acoustic waves (SAWs) in periodic arrays of holes was investigated with finite element modelling and experimentally. SAW bandstructures and bandgap attenuation were obtained from simulations of arrays of cylindrical and annular holes which were filled with materials with different SAW velocities. Each type of hole array exhibited two distinct scattering regimes (Mie and Bragg scattering). The dependence of the bandgap frequency on the velocity was found to be stronger for annular holes than for cylindrical holes, suggesting that annular holes are potentially a better route to create tuneable phononic metamaterials. Annular holes also displayed a higher bandgap attenuation than cylindrical holes, meaning that annular hole arrays might be exploited for greater sensitivity in applications such as mass loading sensing. SAW attenuation due to mass loading of air was calculated by measuring SAW amplitude on a SAW device using an oscilloscope system and by laser Doppler vibrometry (LDV). An extraordinary increase of 2 to 3 orders of magnitude in mass loading attenuation was observed at the bandgap frequency when a phononic metamaterial was present, with only 4 resonator elements needed to produce this result. The measurements obtained by both experimental systems displayed similar frequency dependencies of mass loading attenuation coefficients. Some mass loading effects were also reproduced using finite element modelling. These approaches show great promise for improving the sensitivity of SAW pressure sensors. Finally, bandstructures were obtained from finite element simulations for an array of annular holes filled with a small sphere comprised of materials with different SAW velocities. The array exhibited similar scattering regimes as before, with an overlapping region. The dependence of the bandgap frequency on the velocity was found to be stronger when the annular holes contained the sphere than when they are fully-filled, suggesting that annular holes are potentially a good candidate for probing biological cells. Higher bandgap attenuation by up to a factor of 2 was exhibited by the single spherical inclusion compared to fully-filled holes. Since annular holes have more degrees of geometrical freedom than conventional phononic crystals, devices with greater sensitivity might be realised for applications such as biological sensing and lab-on-a-chip diagnostics.Engineering and Physical Sciences Research Council (EPSRC

A Novel Drive Option for Piezoelectric Ultrasonic Transducers

This paper concentrates on ultrasonic transducers, which are driven by piezoelectric ceramic rings that are arranged in a stack. A novel drive option, where the stack contains a new type of divided piezoelectric rings, is analyzed using the finite element method, prototyped, and tested. To gain a better sense of the vibration behavior, the studies focus initially on one ring and subsequently on the different possibilities to assemble the transducer. The investigations point out that natural bending frequencies can be excited at the transducer. Thus, multiple vibration directions of the tip can be controlled, what can be advantageous for instance in dental applications

Zero group velocity Lamb waves in diamond/AlN-based layered structures

The propagation of the Lamb-like modes along a diamond/AlN thin supported structure was simulated in order to exploit the intrinsic zero group velocity (ZGV) features to design high frequency electroacoustic resonators. As the ZGV points are associated with an intrinsic energy localization under the metal electrodes, acoustic micro-resonators can be designed that employ only one interdigital transducer (IDT) and no reflectors, thus reducing both the device size and technological complexity. The ZGV resonant conditions in the diamond/AlN composite plate, i.e., the frequencies where the mode group velocity vanishes while the phase velocity remains finite, were investigated in the frequency range from few hundreds of MHz up to 3500 MHz. Thin film bulk acoustic resonators (TFBARs) based on c-AlN and on 45° c-axis tilted AlN film on diamond suspended membrane were simulated that operate in longitudinal and shear mode: the former is a thickness-extensional mode, while the latter is a thickness-in plane-shear mode that is suitable for liquid sensing applications. A smart structure based on diamond/AlN composite suspended membrane was modelled that provides several integrated functions including sensing in gaseous and liquid environment, and stable frequency source

Design and modelling of solidly mounted resonators for low-cost particle sensing

This work presents the design and fabrication of Solidly Mounted Resonator (SMR) devices for the detection of particulate matter (PM2.5 and PM10) in order to develop a smart low-cost particle sensor for air quality. These devices were designed to operate at a resonant frequency of either 870 MHz or 1.5 GHz, employing zinc oxide as the piezoelectric layer and an acoustic mirror made from molybdenum and silicon dioxide layers. Finite element analysis of the acoustic resonators was performed using COMSOL Multiphysics software in order to evaluate the frequency response of the devices and the performance of the acoustic mirror. The zinc oxide based acoustic resonators were fabricated on a silicon substrate using a five mask process. The mass sensitivity of the acoustic resonators was estimated using a 3-D finite element model and preliminary testing has been performed. The theoretical and observed mass sensitivity were similar at ca. 145 kHz/ng for the 870 MHz resonator when detecting PM2.5 suggesting that SMR devices have potential to be used as part of a miniature smart sensor system for airborne particle detection.This work was funded under the European Commission 7th Framework Programme, Project No. 611887, “Multi-Sensor-Platform for Smart Building Management: MSP”. F.H.Villa-Lopez thanks the financial support from the National Mexican Council of Science and Technology (CONACYT). G. Rughoobur wishes to acknowledge financial support from the Cambridge Trusts.This is the author accepted manuscript. The final version is available from IOP via http://dx.doi.org/10.1088/0957-0233/27/2/02510

Ferrite-filled cavities for compact planar resonators

Copyright © 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters, Volume 104 (2), article 022405, and may be found at http://dx.doi.org/10.1063/1.4811521Sub-wavelength metallic planar cavities, closed at one end, have been constructed by wrapping aluminium foil around teflon or ferrite slabs. Finite cavity width perturbs the fundamental cavity mode frequency of ferrite-filled cavities due to different permeability inside and outside of the cavity, in contrast to teflon-filled cavities, while the cavity length required to achieve a specific resonance frequency is significantly reduced for a ferrite-filled cavity. Ferrite-filled cavities may be excited by an in-plane alternating magnetic field and may be advantageous for high-frequency (HF) and ultra HF tagging and radio frequency identification of metallic objects within security, manufacturing, and shipping environments.Engineering and Physical Sciences Research Council (EPSRC)University of Exeter Open Innovation FundCrown Packaging UK PL

Multiscale Mathematical Modelling of Nonlinear Nanowire Resonators for Biological Applications

Nanoscale systems fabricated with low-dimensional nanostructures such as carbon nanotubes, nanowires, quantum dots, and more recently graphene sheets, have fascinated researchers from different fields due to their extraordinary and unique physical properties. For example, the remarkable mechanical properties of nanoresonators empower them to have a very high resonant frequency up to the order of giga to terahertz. The ultra-high frequency of these systems attracted the attention of researchers in the area of bio-sensing with the idea to implement them for detection of tiny bio-objects. In this thesis, we originally propose and analyze a mathematical model for nonlinear vibrations of nanowire resonators with their applications to tiny mass sensing, taking into account thermal, piezoelectric, electromagnetic, surface, and external excitations.~The mathematical models for such nanowires are formulated using the Euler-Bernoulli beam theory in conjunction with the nonlocal differential constitutive relations of Eringen type. In order to analyze the obtained nonlinear partial differential equation (PDE), we first use the Galerkin method in combination with a perturbation technique to obtain the primary resonance.~After finding the primary resonance, a parametric sensitivity analysis is carried out to investigate the effects of key parameters on the sensitivity of the nanowire resonators in mass sensing.~Our main hypothesis is that bio-particles attached to the surface of the nanowire resonator would result in a detectable shift in the value of the jump frequency.~Therefore, a mathematical formula is developed based on the jump frequency to scrutinize the sensitivity of the considered nanowire resonators. Our mass sensitivity analysis aims at the improved capability of the nanowire resonators in detection of tiny bio-particles such as DNA, RNA, proteins, viruses, and bacteria.~Numerical solutions, obtained for the general nonlinear mathematical model of nanowire resonators, using the Finite Difference Method, are compared with the results obtained with a simplified approach described above. Finally, we investigate the sensitivity of the nanowire resonator for mass sensing using molecular dynamics simulations to provide a validation for our results from the obtained continuum models. It is expected that the results of this research may assist in our better understanding of key characteristics of nanowire resonators for their applications in detection of bio-particles, ultimately impacting the development of advanced approaches to disease diagnostics and treatments

Modelling of layered surface acoustic wave resonators for liquid media sensing applications

In this thesis a model is developed to characterise the behaviour of layered SAW 2- port resonator sensors operating in liquid media. In the critical review of literature, it is found that methods based on the periodic Green's function combined with the COM model are best suited to this task. However, an important deficiency of this approach is the lack of a good model for electrodes buried within layered media. This deficiency is resolved in this thesis by the formulation of a periodic matrix eigen-operator, using a phase-shifted Fourier series representation. This model is then utilised in the study of resonator behaviour as a function of guiding layer thickness, including the mass sensitivity. Based on this modelling work, a SAW resonator structure is designed, and its frequency response is found to be in generally good agreement with theoretical predictions. The mass sensitivity of this device is then analysed using both theoretical and experimental means. In contrast to the sensitivity analyses found in the literature, sensitivity variation across the device surface is considered in this work. For the resonator structure it is found that sensitivity is greatest at the device centre, with the ends of the device making negligible contribution to the complete device response. The result is that the sensitive material may be deposited only in a small region in the centre of the device, with minimal reduction in device response

ΔE-Effect Magnetic Field Sensors

Many conceivable biomedical and diagnostic applications require the detection of small-amplitude and low-frequency magnetic fields. Against this background, a magnetometer concept is investigated in this work based on the magnetoelastic ΔE effect. The ΔE effect causes the resonance frequency of a magnetoelastic resonator to detune in the presence of a magnetic field, which can be read-out electrically with an additional piezoelectric phase. Various microelectromechanical resonators are experimentally analyzed in terms of the ΔE effect and signal-and-noise response. This response is highly complex because of the anisotropic and nonlinear coupled magnetic, mechanical, and electrical properties. Models are developed and extended where necessary to gain insights into the potentials and limits accompanying sensor design and operating parameters. Beyond the material and geometry parameters, we analyze the effect of different resonance modes, spatial property variations, and operating frequencies on sensitivity. Although a large ΔE effect is confirmed in the shear modulus, the sensitivity of classical cantilever resonators does not benefit from this effect. An approach utilizing surface acoustic shear-waves provides a solution and can detect small signals over a large bandwidth. Comprehensive analyses of the quality factor and piezoelectric material parameters indicate methods to increase sensitivity and signal-to-noise ratio significantly. First exchange-biased ΔE-effect sensors pave the way for compact setups and arrays with a large number of sensor elements. With an extended signal-and-noise model, specific requirements are identified that could improve the signal-to-noise ratio. The insights gained lead to a new concept that can circumvent previous limitations. With the results and models, important contributions are made to the understanding and development of ΔE-effect sensors with prospects for improvements in the future