Tunable Permittivity Sensors

This thesis presents a novel electric permittivity sensor based on Bleustein- Gulyaev (BG) waves; waves that propagate along the surface of shear-poled piezoelectric materials. BG waves couple electromagnetic and acoustic waves, thereby reducing the speed of electromagnetic propagation to near acoustic speeds. Exploiting this property allows the development of permittivity sensors that feature several orders of magnitude reduction in size and operating frequency. This releases the limitations of RF complexity while reducing cost considerably. It also makes the sensor attractive for biological applications, as opposed to RF sensors that are limited by the water relaxation phenomenon at frequencies beyond 4 GHz. To date, sensors that used BG waves were limited to sensing mechanical properties, such as viscosity and density, which exploited the acoustic component of the wave only. To our best knowledge, this is the first attempt to probe and sense an electrical property acoustically using BG-waves. Towards that end, the nonlinear partial differential equations governing an electromechanical BG wave resonator are formulated. The permittivity of the medium-under-test was found to influence the sensor eigenvalues, enabling the implementation of a frequency-shift permittivity sensor. We also find that the sensor sensitivity is enhanced by increasing bias voltage to drive the sensor into the nonlinear regime, but this is limited by electrical breakdown. Sensor prototypes were fabricated on PZT4 and LiNbO3 shear-poled substrates. A novel method to characterize shear-horizontal surface acoustic waves, SHSAW, using a 1D Laser Doppler Vibrometer was developed to test the sensors. The method was also shown to be able to estimate the in-plane displacement field decay rate into the substrate. This technique provides researchers with a quick and effective method for the characterization of SH-SAW. The resonator model was validated using this experimental method. A Vector Network Analyzer was employed to observe the shift in the fundamental natural frequency of the fabricated permittivity sensors in the presence of various media-under-test. Measurements show deterministic and repeatable frequency shifts in the natural frequency in the presence of ethanol and deionized water compared to that of the bare surface, thereby demonstrating the permittivity sensor

Modeling and Compensation of Hysteresis In Piezoelectric Actuators: A Physical Approach

A study in the polarization domain is conducted by probing the impedance of the piezoelectric actuator as it moves along its trajectory. A sensing signal is overlaid over a driving signal that is used to vary the position of the device. The electric polarisation is extracted from the capacitance measurement calculated from the impedance. These polarisation curves are then modelled using the Jiles-Atherton model and compensated for using the inverse model. These measurements give insight into the ferroelectric processes within the piezoelectric actuator, which operate on the polarisation state. In addition, research has been conducted on the topic of parameter estimation of hysteresis models. This dissertation proposes a Monte Carlo study on a novel normalised Jiles-Atherton model to generate a statistical set of model solutions to compare area and remnant displacement characteristics for different parameter selections. Two parameters were found to be the most responsible for changes in these characteristics, and solutions near the desired values of the measured hysteresis curves were found to be densely distributed in certain areas of the parameter space. Different parameter estimation techniques are proposed for the Prandtl-Ishlinskii model. For this model, the parameters have geometrical significance in the slope of certain points of the hysteresis curve. A novel rescaling procedure is developed to scale a Prandtl-Ishlinskii model hysteresis curve area to a new value without requiring a refitting of the coefficients and a frequency-dependent Prandtl-Ishlinskii model is developed. Finally, a temperature-dependent, asymmetric Prandtl-Ishlinskii (TAPI) model is developed to account for the changes in hysteresis due to the external temperature. These effects are modelled in the charge domain as an extra bound charge that appears as a result of domain reorientation effects. The temperature effectively changes the amount of energy required to break pinning sites in the actuator which changes the shape of the curve. The TAPI model is then implemented on a Fabry-Perot interferometer system consisting of three piezoelectric actuators controlling the placement of a mirror forming the etalon. A decoupled inverse TAPI model is shown to effectively linearise the output of this system at different temperatures

Ultrasonic transducer-guided electrochemical impedance spectroscopy to assess lipid-laden plaques

Plaque rupture causes acute coronary syndromes and stroke. Intraplaque oxidized low density lipoprotein (oxLDL) is metabolically unstable and prone to induce rupture. We designed an intravascular ultrasound (IVUS)-guided electrochemical impedance spectroscopy (EIS) sensor to enhance the detection reproducibility of oxLDL-laden plaques. The flexible 2-point micro-electrode array for EIS was affixed to an inflatable balloon anchored onto a co-axial double layer catheter (outer diameter = 2 mm). The mechanically scanning-driven IVUS transducer (45 MHz) was deployed through the inner catheter (diameter = 1.3 mm) to the acoustic impedance matched-imaging window. Water filled the inner catheter to match acoustic impedance and air was pumped between the inner and outer catheters to inflate the balloon. The integrated EIS and IVUS sensor was deployed into the ex vivo aortas dissected from the fat-fed New Zealand White (NZW) rabbits (n = 3 for fat-fed, n = 5 normal diet). IVUS imaging was able to guide the 2-point electrode to align with the plaque for EIS measurement upon balloon inflation. IVUS-guided EIS signal demonstrated reduced variability and increased reproducibility (p < 0.0001 for magnitude, p < 0.05 for phase at <15 kHz) as compared to EIS sensor alone (p < 0.07 for impedance, p < 0.4 for phase at <15 kHz). Thus, we enhanced topographic and EIS detection of oxLDL-laden plaques via a catheter-based integrated sensor design to enhance clinical assessment for unstable plaque

Polycrystalline Perovskite Ferroelectrics: Microstructural Origins of the Macroscale Electromechanical Response

Ferroelectrics are a class of electromechanically coupled materials which possess an electric dipole polarization that can be permanently reoriented by applied electric and mechanical stress fields. Their reorientable polarization results in complex, nano- to micrometer scale domain structures whose evolution under electric and mechanical stress fields alters the material's overall time-dependent electrical and viscoelastic properties. To understand domain structure evolution, in-situ microscopy of domain switching processes in ferroelectric thin films, single crystals and nanoparticles have been well-studied in the past. However, domain evolution in bulk polycrystals is less well understood as their local stress and electric field environment differs from thin specimens. This work seeks to understand ferroelectric domain evolution in bulk ferroelectric perovskite polycrystals using a combination of a recently-developed electromechanical characterization technique, Broadband Electromechanical Spectroscopy (BES), and theoretical-computational predictions. A constitutive material model for polycrystalline ferroelectrics is first developed and applied to simulate barium titanate single crystals and polycrystals. Simulated polarization, strain and energy dissipation hysteresis curves show good qualitative agreement to experimental data and demonstrate that macroscale properties can be efficiently predicted from microscale physics to some extent. The microstructural origins of fatigue behavior in bulk polycrystalline lead zirconate titanate (PZT) are investigated using a combination of macroscale electrical and viscoelastic property characterization via BES, and scanning electron microscopy (SEM) imaging of microstructure. The evolution of electrical and viscoelastic properties during bipolar electrical fatigue show differences in the effects of electrical vs. mechanical fatigue processes, and the latter is verified through SEM imaging and measurement of microcracks. Finally, the same electromechanical BES characterizations are performed on specimens of bulk polycrystalline barium titanate (BT). Results reveal stark qualitative differences in electrical and viscoelastic responses from PZT despite both materials being perovskite ferroelectrics. A growth vs. nucleation hypothesis is proposed to explain the observed results, guided by preliminary imaging of domain microstructure. In summary, the BES is a powerful tool to elucidate domain switching processes within bulk ferroelectric specimens, while a computational method which bridges the micro- and macroscale further adds to the diagnostic toolbox of understanding bulk ferroelectric domain switching mechanisms. This opens the pathway to designing future applications which make use of the unique electrical and viscoelastic properties of ferroelectric switching.</p

Development and characterisation of traceable force measurement for nanotechnology

Traceable low force metrology should be an essential tool for nanotechnology. Traceable measurement of micro- and nanonewton forces would allow independent measurement and comparison on material properties, MEMS behaviour and nanodimensional measurement uncertainties. Yet the current traceability infrastructure in the UK is incomplete. This thesis describes the incremental development of the low force facility at the National Physical Laboratory (NPL). The novel contribution of this thesis has three components. First, specific modifications to the NPL Low Force Balance were undertaken. This involved developing novel or highly modified solutions to address key issues, as well as undertaking detailed comparions with external ans internal traceability references. Second, a triskelion force sensor flexure was proposed and mathematically modelled using both analytical and finite element techniques, and compared to experimentally measured spring constant estimates. The models compared satisfactorily, though fabrication defects in developed prototype artefacts limited the experimental confirmation of the models. Third, a piezoelectric sensor approach for quasistatic force measurement was proposed, experimentally evaluated and rejected. Finally, an improved design for a low force transfer artefact system is presented, harnessing the findings of the reported investigations. The proposed design combines proven strain-sensing technology with the advantageous triskelion flexure, incorporating an external stage and packaging aspects to achieve the requirements for a traceable low force transfer artefact

Microwave Impedance Microscopy Of Nanostructured Carbon

Microwave impedance microscopy (MIM) is a scanning probe technique that measures local changes in tip-sample admittance. The imaginary part of the reported change is calibrated with finite element simulations and physical measurements of a standard capacitive sample, and thereafter the output ∆Y is given a reference value in siemens. Simulations also provide a means of extracting sample conductivity and permittivity from admittance, a procedure verified by comparing the estimated permittivity of polytetrafluoroethlyene (PTFE) to the accepted value. Finally, the well-known effective medium approximation of Bruggeman is considered as a means of estimating the volume fractions of the constituents in inhomogeneous two-phase systems. Specifically, we consider the estimation of porosity in nanostructured carbons often used in charge storage devices, such as carbide derived carbon (CDC) and onion-like carbon (OLC)