Multifunctional Magnetoelectric Materials for Device Applications

Mutiferroics are a novel class of next generation multifunctional materials, which display simultaneous magnetic spin, electric dipole, and ferroelastic ordering, and have drawn increasing interest due to their multi-functionality for a variety of device applications. Since single-phase materials exist rarely in nature with such cross-coupling properties, an intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric (ME) coupling. This review article summarizes the development of different kinds of multiferroic material: single-phase and composite ceramic, laminated composite, and nanostructured thin films. Thin-film nanostructures have higher magnitude direct ME coupling values and clear evidence of indirect ME coupling compared with bulk materials. Promising ME coupling coefficients have been reported in laminated composite materials in which signal to noise ratio is good for device fabrication. We describe the possible applications of these materials

Electrical coupling analysis of 2D time-multiplexing memory actuators exhibiting asymmetric butterfly hysteresis

We present the modeling and analysis of electrical coupling in a hysteretic deformable mirror with 2D memory piezoac- tuators, which are made of a purposely-designed piezomaterial sandwiched between electrodes arranged crosswise and actuated by a multiplexing approach. Using a modified Miller model to describe the memory effect which is based on the ferroelectric domain switching processes, the proposed framework is used to simulate the electric field dependence of the strain in the piezoelectric material that exhibits asymmetric butterfly loops with remnant deformation through the finite element method. The desired butterfly memory effect in the material is obtained by modifying the saturated dipole polarization curve in the Miller model. The proposed method allows us to numerically investigate the electrical coupling between actuators in more detail and correspondingly understand their influence to the mirror facesheet

Nonlinear characterisation of power ultrasonic devices used in bone surgery

Ultrasonic cutting has existed in surgery since the 1950s. However, it was not until the end of the 20th century that advances in ultrasonic tool design, transduction and control allowed commercially viable ultrasonic cutting devices to enter the market. Ultrasonic surgical devices, like those in other power ultrasonic applications such as drilling and welding, require devices to be driven at high power to ensure sufficient output motion is produced to fulfil the application it is designed to perform. With the advent of novel surgical techniques surgeons require tuned ultrasonic tools which can reduce invasiveness while giving access to increasingly difficult to reach surgical sites. To fulfil the requirements of novel surgical procedures new tuned tools need to be designed. Meanwhile, it is well documented that power ultrasonic devices, whilst driven at high power, are inherently nonlinear and, if no attempt is made to understand and subsequently control these behaviours, it is likely that these devices will suffer from poor performance or even failure. The behaviour of the commercial ultrasonic transducer used in bone surgery (Piezosurgery® Device) is dynamically characterised through finite element and experimental methods whilst operating in conjunction with a variety of tuned inserts. Finite element analysis was used to predict modal parameters as well as stress levels within the tuned devices whilst operating at elevated amplitudes of vibration, while experimental modal analysis validated predicted resonant frequencies and mode shapes between 0-80kHz. To investigate the behaviour of tuned devices at elevated vibrational amplitudes near resonance, responses were measured whilst the device was excited via the burst sine sweep method. In an attempt to provide an understanding of the effects that geometry, material selection and wavelength of tuned assemblies have on the behaviour of an ultrasonic device, tuned inserts consisting of a simple rod horn design were characterised alongside more complex cutting inserts which are used in maxillofacial and craniofacial surgery. From these results the aim will be to develop guidelines for design of tuned inserts. Meanwhile, Langevin transducers, commonly known as sandwich or stack transducers, in their most basic form generally consist of four parts; a front mass, a back mass, a piezoceramic stack and a stud or bolt holding the parts together under a compressive pre-load. It is traditionally proposed that the piezoceramic stack is positioned at or close to the vibrational nodal point of the longitudinal mode, however, this also corresponds with the position of highest dynamic stress. It is also well documented that piezoceramic materials possess a low linear stress threshold, therefore this research, in part, investigates whether locating the piezoceramic stack away from a position of intrinsic high stress will affect the behaviour of the device. Through experimental characterisation it has been observed that the tuned devices under investigation exhibited; resonant frequency shifts, jump amplitudes, hysteretic behaviour as well as autoparametric vibration. The source of these behaviours have been found to stem from device geometry, but also from heating within the piezoceramic elements as well as joints with different joining torques

Understanding nonlinear vibration behaviours in high-power ultrasonic surgical devices

Ultrasonic surgical devices are increasingly used in oral, craniofacial and maxillofacial surgery to cut mineralized tissue, offering the surgeon high accuracy with minimal risk to nerve and vessel tissue. Power ultrasonic devices operate in resonance, requiring their length to be a half-wavelength or multiple-half-wavelength. For bone surgery, devices based on a half-wavelength have seen considerable success, but longer multiple-half-wavelength endoscopic devices have recently been proposed to widen the range of surgeries. To provide context for these developments, some examples of surgical procedures and the associated designs of ultrasonic cutting tips are presented. However, multiple-half-wavelength components, typical of endoscopic devices, have greater potential to exhibit nonlinear dynamic behaviours that have a highly detrimental effect on device performance. Through experimental characterization of the dynamic behaviour of endoscopic devices, it is demonstrated how geometrical features influence nonlinear dynamic responses. Period doubling, a known route to chaotic behaviour, is shown to be significantly influenced by the cutting tip shape, whereas the cutting tip has only a limited effect on Duffing-like responses, particularly the shape of the hysteresis curve, which is important for device stability. These findings underpin design, aiming to pave the way for a new generation of ultrasonic endoscopic surgical devices

Distributed actuation systems for adaptive optics: from structural modeling to design optimization

Deformable mirrors are active optical systems used for wavefront control and correction of optical aberrations in many imaging and non-imaging applications such as microscopy, three-dimensional imaging, and medical or industrial applications. Their surface can be deformed by actuator arrays that are mounted below the reflective top-layer. While the performance increases as the density of actuators increases, scaling of the current mirror designs to the needed capabilities remains challenging. Well-engineered mirrors have actuator numbers ranging from 100 to 6000, but new concepts aim to achieve much denser arrays. A recently presented concept of a hysteretic deformable mirror features an actuator array of more than 16000 individually addressable locations, realized by 2D memory actuators utilizing piezoelectric hysteresis for stable shape configurations. A mechanical model has been proposed for describing the deformation of the reflective surface and predicting the accuracy of the mirror, while detailed multiphysics simulations were performed to investigate the behavior of the actuators in an array. Although this concept presents one possibility to achieve a high number of actuators with a design that relies on long-lasting technical components, we have also researched novel ways to realize new approaches for actuation systems. Kirigami actuators, whose mechanisms are grounded on geometrical cut patterns, can also open up the way for modular arrays used for deformable mirrors that will require a lightweight design

Nonlinear Thermo‐Electro‐Mechanical Behaviors of Ag/BaTiO3 Composites

The focus of this study is to understand the influences of blending silver (Ag) phase into barium titanate (BaTiO3) ceramic on its thermal, mechanical and dielectric properties. Silver-barium titanate (Ag/BaTiO3) active composites with varying silver composition were fabricated using powder metallurgy method. Coefficient of thermal expansion (CTE) and heat capacity were measured by thermal mechanical analyzer (TMA) and differential scanning calorimetry (DSC), respectively. Hot disk technique was employed to determine the thermal conductivity. Addition of silver did not change the phase transformation temperatures. CTE stays constant at each crystalline phase, but increases as BaTiO3’s crystal structure changes from orthorhombic to tetragonal phase and further to cubic. Increase of silver content significantly enhances the thermal conductivity. Elastic and dielectric constants were determined using resonant ultrasound spectroscopy (RUS) and dielectric (impedance) spectroscopy, respectively. Young’s modulus decreases as the increase of silver composition, while the dielectric constant was significantly improved by blending silver. Two peaks were observed on dielectric constant around the transformation temperatures, with a larger magnitude at the Curie point. Micromechanics models based on detailed microstructures, either generated randomly by computer algorithm or created by converting scanning electron microscope (SEM) images, were created to numerically study the effects of microstructures on the effective properties of Ag/BaTiO3 composite. Numerical results showed that microstructure induced anisotropy is negligible and the effective properties are insensitive to loading directions. Effective CTE is insensitive to the yielding of silver particles, porosity, and the elastic modulus of BaTiO3. The predictions of CTE and elastic constants were pretty close to the experiment results, while the effective thermal conductivity and dielectric constant predictions underestimated the measured values. The hysteretic mechanical behavior of Ag/BaTiO3 composite was measured under cyclic uniaxial compressive loading using materials test system (MTS). Specimens with 5 vol% and 13 vol% silver composition were broken before the maximum stress was reached. The fractured specimens showed a fracture angle of approximate 45⁰C. Furthermore, a one-dimensional constitutive model based on the thermodynamics of irreversible process was presented to model the hysteretic response from experiment

Modeling and Control of Piezoactive Micro and Nano Systems

Piezoelectrically-driven (piezoactive) systems such as nanopositioning platforms, scanning probe microscopes, and nanomechanical cantilever probes are advantageous devices enabling molecular-level imaging, manipulation, and characterization in disciplines ranging from materials science to physics and biology. Such emerging applications require precise modeling, control and manipulation of objects, components and subsystems ranging in sizes from few nanometers to micrometers. This dissertation presents a comprehensive modeling and control framework for piezoactive micro and nano systems utilized in various applications. The development of a precise memory-based hysteresis model for feedforward tracking as well as a Lyapunov-based robust-adaptive controller for feedback tracking control of nanopositioning stages are presented first. Although hysteresis is the most degrading factor in feedforward control, it can be effectively compensated through a robust feedback control design. Moreover, an adaptive controller can enhance the performance of closed-loop system that suffers from parametric uncertainties at high-frequency operations. Comparisons with the widely-used PID controller demonstrate the effectiveness of the proposed controller in tracking of high-frequency trajectories. The proposed controller is then implemented in a laser-free Atomic Force Microscopy (AFM) setup for high-speed and low-cost imaging of surfaces with micrometer and nanometer scale variations. It is demonstrated that the developed AFM is able to produce high-quality images at scanning frequencies up to 30 Hz, where a PID controller is unable to present acceptable results. To improve the control performance of piezoactive nanopositioning stages in tracking of time-varying trajectories with frequent stepped discontinuities, which is a common problem in SPM systems, a supervisory switching controller is designed and integrated with the proposed robust adaptive controller. The controller switches between two control modes, one mode tuned for stepped trajectory tracking and the other one tuned for continuous trajectory tracking. Switching conditions and compatibility conditions of the control inputs in switching instances are derived and analyzed. Experimental implementation of the proposed switching controller indicates significant improvements of control performance in tracking of time-varying discontinuous trajectories for which single-mode controllers yield undesirable results. Distributed-parameters modeling and control of rod-type solid-state actuators are then studied to enable accurate tracking control of piezoactive positioning systems in a wide frequency range including several resonant frequencies of system. Using the extended Hamilton\u27s principle, system partial differential equation of motion and its boundary conditions are derived. Standard vibration analysis techniques are utilized to formulate the truncated finite-mode state-space representation of the system. A new state-space controller is then proposed for asymptotic output tracking control of system. Integration of an optimal state-observer and a Lyapunov-based robust controller are presented and discussed to improve the practicability of the proposed framework. Simulation results demonstrate that distributed-parameters modeling and control is inevitable if ultra-high bandwidth tracking is desired. The last part of the dissertation, discusses new developments in modeling and system identification of piezoelectrically-driven Active Probes as advantageous nanomechanical cantilevers in various applications including tapping mode AFM and biomass sensors. Due to the discontinuous cross-section of Active Probes, a general framework is developed and presented for multiple-mode vibration analysis of system. Application in the precise pico-gram scale mass detection is then presented using frequency-shift method. This approach can benefit the characterization of DNA solutions or other biological species for medical applications