Diamagnetic levitation offers a contactless, room-temperature platform for creating high-quality mechanical resonators. This study investigates the dynamic behavior of levitated diamond particles as potential high Q resonators. Using a cone shaped pole-piece magnet assembly, we achieve stable levitation of single crystalline diamond particles and clusters up to 400 μm in diameter. By leveraging diamond's extremely low electrical conductivity (∼ 10^-13 S/m), eddy current damping is nearly eliminated, leaving air resistance as the dominant dissipation mechanism. Resonant modes—including transverse, radial, and rotational—are characterized using laser Doppler vibrometry, with actuation enabled via electrostatic forces. Frequency response and Q factor measurements across a range of pressures reveal that while a Q factor up to 4250 is achieved, it remains below theoretical limits due to additional damping, likely caused by inter-particle friction and mode coupling in clusters.  Furthermore, frequency shifts with decreasing pressure suggest that ambient air contributes to effective stiffness, in addition to damping. These findings demonstrate the potential of diamond for next-generation, high Q, room-temperature levitated resonators, while highlighting the challenges posed by cluster dynamics and external perturbations. ... Read more

Within ASML’s extreme ultraviolet (EUV) lithography machines, a photomask defines the pattern on the silicon wafer. To protect the photomask from particles that are shed from inside the machine and to ensure the quality of the printed patterns, a highly EUV-transparent pellicle is suspended in front of the photomask. Various materials have been explored to meet the stringent requirements for such pellicles. One potential material is a random carbon nanotube (CNT) network, due to its favorable mechanical and thermodynamical properties. Despite the high EUV transmission rate of around 90%, the 10% of absorbed EUV radiation can lead to a temperature increase of the pellicle on the order of several hundred kelvin. It is essential to remove this heat as quickly as possible to prevent the pellicle from becoming thermally instable. Therefore, a thorough understanding of the thermodynamics of CNT pellicles at elevated temperatures - up to around 900 K - is required. In this thesis, the optomechanical method is successfully applied to extract key thermal properties such as the thermal expansion coefficient, specific heat capacity, and the thermal conductivty of 2D CNT networks up to 1300 K. ... Read more

Two-dimensional materials have attracted scientific interest due to their exceptional chemical, optical, electronic, and mechanical properties. In particular, their Young’s modulus plays a crucial role in applications such as sensors, flexible electronics, and composite reinforcement. However, material defects, which are inevitable during fabrication and device operation, can significantly impact mechanical properties. The influence of defects on the Young’s modulus remains a topic of debate, with contradictory experimental and theoretical findings presented in literature. The most widely used method for measuring Young’s modulus is atomic force microscopy (AFM) nanoindentation. However, it suffers from tip-sample interactions, high stress concentrations, and significant variability in reported values. To address these limitations, this thesis presents a novel, all-optical method for measuring the Young’s modulus of monolayer membranes, eliminating physical contact with the sample and requiring no preliminary material assumptions. The proposed method provides Young’s modulus values by measuring nonlinear membrane dynamics together with higher harmonics and Brownian motion. Although the results exceed values reported in literature, it is noteworthy that this method yielded results for the modulus, despite inherent challenges associated with the measurement of 2D material monolayers. Various potential error sources have been identified and discussed, including measurement sensitivity, mode shape variations, and contamination effects. Recommendations for future research are provided to refine the approach and improve accuracy. With the right experimental advancements, this contactless optical technique could serve as a viable alternative to AFM nanoindentation, offering a non-invasive way to study the mechanical properties of 2D materials with greater precision and reproducibility. ... Read more

This thesis studies how to estimate cubic non-linear stiffness in dynamic systems using two-tone excitation. When a structure with cubic stiffness is driven at two frequencies that lie close to each other, it produces inter modulation peaks at sum and difference frequencies. By relating the amplitudes at these frequencies to the system parameters, the cubic stiffness can in principle be extracted without full-scale numerical modelling or fits with convoluted functions. An analytic relation between the amplitude at an excitation tone (A1) and the first-order sum frequency amplitude (A3) is derived using the harmonic balancing method, expressed in terms of the resonance frequency ω0, detuning δ, and cubic stiffness γ. The derivation uses a Ansatz with the corresponding sum frequencies and orders terms to obtain a analytic relation. This analytic curve is then validated in two ways: (i) by numerically solving the harmonic-balance equations, and (ii) by direct time-domain integration (RK45) with amplitude ex traction at the relevant frequencies. Symbolic regression is also used to fit compact additional formulas where the analytic relation loses accuracy. Experiments are carried out on Silicon-Nitride beams using a Polytec MSA-500 LDV and a Moku: Lab for signal generation and spectrum analysis. The main result is that the analytic A1–A3 relation agrees well with harmonic-balance and numeric integration simulations, but only within a certain region: low to moderate drive and sufficiently separated tones so that higher-order products remain small. Outside this region, solution branching and neglected terms reduce accuracy. In measurements, the expected near-linear relation between the two drive amplitudes (A1, A2) is observed, but the measured A1–A3 curves rise more steeply than predicted. This could be caused by additional effects such as higher-order non-linearities, damping, or resonance shifts under strong drive. The work delivers: (1) an analytic and numerical framework that maps where two-tone inter modulation can identify cubic stiffness, and (2) a robust experimental workflow. It also outlines future improvements: adding (non-linear) damping and quadratic stiffness to the model, lock-in based detection, better input conditioning, and resonance tracking to improve the differences between theory and experiments. ... Read more

When a free-standing membrane is actuated with photothermal method, the heat flux requires a certain time to diffuse through this membrane. This duration of time, called thermal time constant, is important for its application in sensors, nano-electromechanical systems, filters, etc. This report is devoted to exploring what plays a major role in the variation of experimentally measured thermal time constants, and to investigating the relationship between dumbbell dimensions (namely, two drum radius R1 for drum 1 and R2 for drum 2, half bridge width y0, bridge length x0) and thermal time constants.

First, dumbbell resonators of various dimensions were fabricated using exfoliated molybdenum disulfide flakes. Optomechanical experiments were conducted on these devices, involving two collocated (actuation and measurement located in the same drum) and two non-collocated (actuated at one drum and measured at the other) measurements for each device. Accordingly, four thermal time constants were extracted for each resonator through curve fitting. To understand temperature distribution and experimental variation, a COMSOL model and an analytical model were established, solving the heat equation with a harmonic laser actuation.

As a result, four thermal time constants for 16 devices were extracted. These experimental data were verified with a synergy of the two models. The primary contributors for large experimental data variation were the 2D material irregularities and laser locations. For collocated τ, it was almost unaffected by dumbbell dimensions, except for R1 which gave a parabolic curve. For non-collocated τ, it increased monotonously with x0, but a minimum was always observed when sweeping the other three parameters.

Such minimum occurred when x0 is around 25% of the drum radius. This minimum was attributed to a balance between the efficiency of heat transport across the bridge and the acceptable duration required to heat the bridge itself. Meanwhile, the COMSOL model and the analytical model disagreed on the relationship between non-collocated τ and y0, R1, R2. Moreover, the analytical model’s deviation from the COMSOL model increased with larger bridge width or thermal conductivity. This stemmed from errors in the assumed boundary conditions: the existence of the bridge altered the temperature distribution at the boundaries of drum 1, and in the analytical model the boundaries of the dumbbell are fixed while in the COMSOL model they are controlled by the substrate. ... Read more

The speed and precision required in the precision motion industry is an ever-growing challenge. Linear control offers intuitive frequency-domain controller design methods that are based on a frequency response function (FRF) of the plant, which is obtained solely from measurement data. Unsurprisingly, linear controllers account for over 90% of the controllers currently used in the industry. However, as linear controllers are subject to inherent performance limitations such as Bode's gain-phase relationship and the waterbed effect, research on nonlinear control solutions to overcome these limitations is ubiquitous. Reset control first showed up in 1958 with the Clegg integrator (CI). According to a sinusoidal-input-describing-function analysis, the CI provides reduced phase lag compared to a linear integrator, suggesting Bode's gain-phase relationship can be overcome and consequently allowing for performance surpassing that of linear control systems. The drawbacks of the CI are the possible emergence of limit cycles and the excitation of high-frequent modes of the system originating from higher-order harmonics of the CI's input caused by discontinuities in the control signal. The generalised-first-order-reset-element-based integrator (GFbI) is the only reset element that can prevent the emergence of limit cycles and can reduce the generation of higher-order harmonics, whilst retaining the advantageous reduced phase lag the CI provides and allowing for closed-loop controller design based on a plant FRF while taking the effects of higher-order harmonics into account, matching design methods of linear controllers. Optimally tuning a reset controller is a complex and time-consuming task. Moreover, established tuning rules are still lacking. This work facilitates designing a reset controller containing a GFbI element by providing two contributions to utilise the potential of the GFbI element to improve upon linear control. The first is a comparative study on the effect of the controller element sequence, aimed at reducing the negative consequences of the higher-order harmonics generated by the reset element. The second is the proposal of an FRF-based optimisation algorithm utilising frequency-domain performance prediction methods to automatically tune a reset controller containing a GFbI element to adhere to the imposed constraints and maximise the benefit gained from the nonlinear element. Validation using a simulated and physical wire bonder showed the algorithm successfully tuned four different reset controllers using two different sequences. The performance of the tuned reset controllers was compared to that of equally-well-tuned linear controllers. In the first use case with the goal of suppressing a dominant vibration in the error signal, the median root-mean-square error was reduced by 16.245%. In the second use case, the goal was to improve the settling time, which was achieved by a median of 9.791%. The best results were achieved using a sequence in which the higher-order harmonics were avoided from passing through the lead filter in open loop, mitigating amplification thereof. The proposed tuning algorithm proved able to tune reset controllers containing a GFbI element such that the performance of linear control based on frequency-domain performance prediction metrics was surpassed, where two use cases confirmed the predicted performance increase through time-domain simulations and experiments on a physical setup. ... Read more

The most problematic eigenmode in machines involving flexure mechanisms is represented by flexural modes. However, it can be noticed that also out-of-plane modes, that occur in the space outside the nominal plane of movement of the mechanism, can be very important, especially in machines with several and coupled degrees of freedom, like 3D positioning stages. In this thesis a new design based on piezoelectric shunt damping is proposed in order to tackle out-of-plane modes of flexure mechanisms, without affecting the capability of attenuating the more common flexural modes. This new concept is validated both analytically and using the FEM software COMSOL and it is shown that attenuations of the order of 15 dB can be achieved. Furthermore, the possibility to add active control to the shunted piezoelectric materials, giving rise to a hybrid control strategy, is explored and it is shown that it results in further increase of the added damping. Finally, a completely new technique based on Eddy current dampers is discussed and shown to be inadequate for out-of-plane mode attenuation. ... Read more

In this thesis, a proof of concept was established for the use of a novel coupled QM-MD approach to modelling metallic (copper) electrode-electrolyte interfaces. SCC-DFTB calculations of the instantaneous electronic structure of a copper electrode were coupled to a classical MD simulation of an electrode-electrolyte interface. The applied QM-MD method was described rigorously, and used to investigate the compound distribution and dynamics at the interface, relative to a fully classical MD simulation. Polarisation effects were observed to bring about a significant increase in the attraction between cations and the cathode. Moreover, local polarisation of the cathode was found to immobilise adsorbed cations, and induce an increased orientational preference of the nearby water dipoles. The secondary goal of this thesis was to explore to what extent neural networks are able to replicate SCC-DFTB calculations of the electronic charge density on a metallic electrode. Using a computer vision approach, qualitative evidence was obtained indicating that neural networks can be used to replicate SCC-DFTB predictions on periodic metallic surfaces. ... Read more

Vibration energy harvesting is a growing field of research. Harvesting energy from vibrations can be of use in devices in hard to reach places, such as sensors on a train track or a pacemaker. These devices makes use of batteries, which need to be replaced. Using energy harvesting this battery life could be improved.
The difficulty lies in harvesting low frequency vibrations. These vibrations are hard to harvest using a transducer such as a piezo. A solution could be using a frequency up converter, which can raise a vibrations frequency. This thesis shows the design of a Bi-stable Impact-driven Snap-through frequency up converter (BISup). Enhancing the behaviour of this bi-stable design could improve the energy harvesting capabilities. Different designs are made using ortho planar spring design, these designs are simulated and compared using ANSYS. These simulations are experimentally verified.
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Energy harvesting from renewable energy sources has become more popular in the last decades than ever before. New energy sources consisting of human input vibrations are hopeful alternatives for powering wearable low power electronics. These sensors currently rely on the lifetime of the battery and come along with high maintenance costs in case of a replacement. Many designs found within literature are based on the working principles of linear resonant energy harvesters, consisting of high output generation when they are exited on their resonance frequency. However, if the vibration energy harvester is not accurately tuned to the input signal of the real world, poor output performance can be expected. Bistability, consisting of a unique double well potential energy curve is an interesting alternative. The oscillation between the two potential wells contribute to higher output performance in comparison with resonant configurations. However, these potential wells are segregated by an energy barrier and the motion between the two wells will not always occur during excitation. A solution is found to reduce the energy barrier by means of mechanical end-stops. A mechanical model based upon beam theory is created in ANSYS and their stiffness characteristics are used as an input parameter for the dynamical model. To confirm this model a prototype is constructed and investigated. A mechanical analysis is carried out using a quasistatic forcedeflection measurement and the dynamical analysis is performed on a linear air bearing stage, consisting of a maximum stroke of half a meter being able to reproduce low frequency input excitations. It could be observed that participation of the desired trajectory between the two potential wells is enhanced and occur at lower input accelerations, as the oscillators motion is confined by means of hard mechanical end-stops. Therefore, the integration of mechanical end-stops as a design parameter for bistable energy harvesters can be considered as a viable solution to capture the kinetic energy induced by human input motion with the use of bistable mechanisms. ... Read more

Vibration energy harvesters have been proposed as a solution to increase the lifetime of wireless and portable medical devices. One example of implantable medical devices for which energy harvesters could be interesting, are pacemakers. With a lifespan of about 6 to 12 years, the battery must be replaced after this period of time. Using an energy harvester instead of a battery is therefore seen as an interesting alternative. However, the human heart rate is usually between 0.6-2Hz and consists of low acceleration peaks (<1g). The use of resonance at low frequency is extremely difficult, especially when the motion amplitude is larger than the device itself. A solution is sought in the non-resonant bistable energy harvesters. When enough force is applied to overcome the potential energy barrier, snap through motion is induced, resulting in a significant increase in power output. However, large threshold accelerations are limiting the usability of these systems. Therefore, stiffness compensation is required. A prototype was fabricated in which buckled flexures were used to add negative stiffness to a piezoelectric cantilever, resulting in a stiffness compensated bistable energy harvester suitable for energy harvesting from low frequency and low force excitations. The dynamical behaviour and practical performance of the prototype was studied in relation to a heartbeat, sawtooth wave and sine waves. The output power of the non-resonant prototype was compared to a resonant device, which in all cases showed that the non-resonant prototype outperformed the resonant device. This shows that stiffness compensated bistable energy harvesters can be used in order to make energy harvesting for low force and low frequency excitations, such as a heartbeat, possible. ... Read more

Since the discovery of the photovoltaic properties of metal halide perovskites in 2009, the material has rapidly gained interest in the scientific community. In less than 10 years, the power conversion efficiency of perovskite solar cells (PSC) has increased from 3.9% to over 25%, reaching levels of conventional silicon cells. PSCs have multiple favorable properties, such as low manufacturing costs, thin film flexibility, and a tunable bandgap, which is promising for tandem solar cells that can surpass the Shockley-Queisser efficiency limit of conventional (single-junction) solar cells. Other applications are color-tunable LEDs and super sensitive (x-ray) photodetectors. However, there are still mysteries surrounding perovskites that need to be solved in order to fully understand the extraordinary properties of the material. For a specific perovskite, CH3NH3PbI3 (MAPI), it is debated whether it has a direct or indirect bandgap. In this work, a new Photothermal Deflection Spectroscopy (PDS) setup is designed and build that can perform sensitive below-bandgap absorption measurements under hydrostatic pressure (up to 400 MPa). Measurements performed with this setup resulted in absorption spectra of MAPI at different pressures, showing a transition from a primarily indirect bandgap at ambient pressure to a more direct bandgap at 375MPa. The data provides new empirical evidence indicating an indirect-to-direct bandgap transition at 325MPa, which opens a novel perspective on unraveling the nature of the bandgap of metal halide perovskites. ... Read more

Viscoelasticity is a material property that is relevant in a variety of nanoscale materials and interfaces in medicine and industry. Therefore, a method of mechanical quantification has become exceedingly desired. In this thesis the Atomic force microscope (AFM) is applied to accurately characterize the mechanical behavior of viscoelastic samples. The goal is to enhance viscoelastic characterization using the so-called Intermodulation AFM (ImAFM) technique by applying, adapting and improving multiple modelling and optimization methods. In ImAFM force reconstruction is performed by extracting intermodulations around resonance in the cantilever response. These intermodulations present new observables that can be used for characterization. This thesis investigates the potential of this technique in combination with an up-and-coming model describing viscoelastic interaction. A toolbox has been developed for numerical simulations of the model to resemble the experiments. The model has been evaluated in a variety of situations using sensitivity analysis in a large feasibility range, encompassing many complex dynamics. Because of the diversity in model dynamics a global optimization has been performed for experimental reconstruction. ... Read more

Graphene is an attractive material to be used for pressure sensors due to its thinness, electrical conductivity, and potential high gauge factor. One of the issues with processing graphene is the scalability, which is largely limited by the transfer process that is required for graphene deposited by chemical vapour deposition (CVD). In this work we employed a novel, transfer-free bulk-micromachining approach to realize graphene-based differential pressure sensors. The devices were successfully fabricated, and the samples were examined under Raman Spectroscopy, and electrically characterized. Further, pressure dependent measurements were performed for a dynamic range of 0 to 80 kPa of differential pressure and the corresponding change in resistance of the membrane was measured. The fabricated devices have a mean Gauge Factor of 2.80. ... Read more

Running robots at insect scale are an upcoming research field, because of their numerous potential applications like exploration of hazardous environments such as collapsed buildings, natural disaster sites and debris. To date no report exists on such a robot that either exploits its resonance, or that is monolithic. These properties, however, could improve the performance of these robotic insects in terms of efficiency, actuation complexity, manufacturing tolerance, suitability for rapid prototyping, large number reproduction and miniaturization. This article presents the monolithic design of FARbot; a Frequency Actuated Resonant robot that uses resonance to increase its stride length and consists of one single piece. To achieve the design of FARbot a novel design methodology is presented and utilized to systematically obtain the necessary compliant mechanism that resonates in a desired motion and at a desired frequency. The final design has been manufactured monolithically from the material HTM140-V2 using digital light processing 3D-printing technology. In terms of production time FARbot outperforms all other robots in this research field. An earlier produced, non-monolithic prototype shows resonance at the desired eigenfrequency at which its stride length is amplified by a maximum factor of 22.74 for constant energy demand. This proves the benefit of using resonance and also validates the proposed design methodology. ... Read more

A single photon interacting with a single atom is the most fundamental form of light interacting with matter and has been extensively studied in the field of Cavity Quantum Electrodynamics (cavity QED). Here, a non-linearity like an atom is coupled to a single mode of the electromagnetic field in a cavity. Another field which explores the quantum mechanical nature of photons is Circuit Quantum Electrodynamics (cQED) where photons are the quantized excitations of a superconducting microwave resonator and non-linearity is introduced by the Josephson junction. Like this, setups analog to that of in cavity QED can be copied to cQED, with a number of differences. For example, the photons propagating in a transmission line are more confined and the circuits are made with conventional lithography techniques, allowing for more freedom in engineering the system parameters.
In the first part of the thesis, we build a numerical model in order to examine the feasibility of quenching the ground state of a coplanar waveguide (CPW) interrupted by a tunable coupling element. Next, by means of experiments and simulations we considered the feasibility of observing experimentally a synchronization effect in a driven CPW with its central conductor interrupted by equally spaced capacitively shunted Josephson junctions (Josephson crystal) based on a recent proposal. Finally, we made a first step in understanding the
synchronization from a classical perspective by modelling a Josephson crystal of two junctions as two degenerate non-linearly coupled Duffng oscillators.
Concerning the quenching experiment, we found that the plasma frequency must be tuned faster than 1/f with f the resonance frequency of the CPW, which is in the sub-nanosecond regime and therefore unfeasible with current state the art electronics. We also found that, in contrast to what was claimed in the proposal, the synchronization effect cannot be observed for the parameters common in cQED. One way would be to push the limits of the capacitances to several picofarads. Finally, we found that the non-linear coupling causes the two degenerate non-linearly coupled Duffng oscillator to synchronize, which is a first step in understanding the proposed synchronization effect in a fully classical way. ... Read more

The outstanding mechanical properties of graphene have made it a suitable candidate for awide range of sensor and actuator applications in modern technology. However, before the full potential of future applications can be achieved, a proper characterisation of the fundamental properties of graphene is crucial. The aim of this project was to contribute to the understanding of the mechanics of graphene membranes in presence of surface imperfections. To this end two configurations are investigated: ribbons and cantilevers, respectively. Wrinkled graphene nanoribbons are used to investigate the mechanical behaviour during the transition from the wrinkled state to the flat state. A molecular dynamics model has been developed of a single layer graphene ribbon to describe both the formation of wrinkles as well as the transition from the wrinkled state to the flat state. Also, a continuum model was developed to investigate the formation of wrinkles in graphene nanoribbons. Different constitutive laws have been investigated to describe the mechanical response of wrinkled membranes during the transition from the wrinkled state to the flat state. It was concluded that an exponential version of Hooke’s law fails to describe this transition correctly. The transition is however well described by the first order compressible Ogden’s law. Ogden’s law provided further insights into different mechanical properties of the wrinkled layer. Ogden’s law predicted that wrinkledmembranes exhibit a negative Poisson’s ratio at small strains, which is in agreement with previous research. Also, Ogden’s law predicted a decreasing shearmodulus and an increasing Poisson’s ratio after flattening of the membrane. Single layer graphene cantilevers show great potential, however, due to the difficult manufacturing of these fragile structures they remain virtually unstudied. Herein, a molecular dynamics model has been developed to investigate if nanocantilevers could be stabilised by implying a curvature. We found that, depending on the aspect ratio of the membrane and the applied rate of curvature, single layer graphene cantilevers could be (partly) stabilised by implying a curvature. In conclusion, with this research we provided new insights for designing and investigating the next generation of graphene nanoelectromechanical devices. ... Read more

This report investigates the potential reduction of steel weight for offshore wind turbine supporting jacket structures, if conventional welded joints are replaced by innovative wrapped FRP joints. This new type of connection is under development by Dr. Marko Pavlovic at the Delft University of Technology, and shows outstanding fatigue performance compared to welded counterparts. As jacket structures suffer highly cyclic load, member thickness of current jackets is governed by the fatigue performance of welds. Due to the superior fatigue performance of wrapped FRP joints, substantial weight benefit is expected to be made. The study examines a jacket supported 5 MW wind turbine located in 50-meter water depth in the North Sea. The structure and model are based on the UpWind project. The model includes soil-structure interaction by non-linear depth-dependent springs along with foundation piles. Fatigue limit state (FLS) and ultimate limit state (ULS) are simulated by respectively five and three scenarios. The scenarios consider different combinations of wind (speed and direction), waves (height, period and direction) and current (speed and direction). Six 10-minute simulations are performed for each scenario with different wind turbulence and wave irregularity seeds. Wind and waves are applied in a single simulation, and normal force N and bending moments Mip and Mop time series are recorded at a selection of elements. The time series are post-processed in a self-written MATLAB procedure. For FLS, detailed fatigue analyses of welded joints are performed by evaluating crown and saddle hotspot stress, according to DNVGL-RP-C203. For every time step, the hot spot stress is calculated by applying geometry and load-dependent stress concentration factors (SCFs). Rainflow counting is applied, and the resulting stress range is projected on the details’ S-N curve to evaluate the damage. Linear Palmgren-Miner is applied to accumulate damage. A similar procedure, including stress concentration at thickness transition, is applied to calculate fatigue of elements. For ULS, welded joints are checked for chord face and punching shear failure. Members are checked for tension yielding, local buckling and global buckling. ULS calculations are performed for all time steps and according to Eurocode manuals. The unity check of both FLS and ULS is calculated for each individual member. Next, the member thickness is manually optimised to obtain the most optimal use of material. This optimisation is performed for three different cases with both mild S355 steel and high strength S690 steel. The welded steel structure, case 1, acts as a reference. The unwelded structure, case 2, is the lightest structure if joint fatigue does govern design. Case 3 gives the wrapped FRP structure and includes fatigue results obtained from small scale lab tests. Additionally, due to limited production length of steel tubular elements, it includes circumferential welds in the legs. The potential jacket weight reduction if wrapped FRP joints are applied is large, and the governing unity check shifts from fatigue to global buckling. For mild steel, the reduction of steel weight is more than 50%. The additional reduction of mass for high strength steel is low and not economical. The eigenfrequency of the wrapped FRP structure is viable, as it is outside operating frequencies. The results for the wrapped FRP structure are based on two major assumptions. Firstly, satisfactory joint performance can be obtained, and secondly, this can be accomplished by increasing wrapping thickness only. These assumptions should be verified by future experiments to support the weight reduction statement. In conclusion, the potential benefit of wrapped FRP joints to offshore wind turbine supporting jacket structures is large, and future experiments will show if, or to what extent, the full potential can be exploited. ... Read more

The spinal cord, considered to be the most important path of the human body, when injured induces severe motor dysfunction. Therefore, patients affected by lesions on the spinal cord, are most of the time unable to walk, stand or perform motor activities that are trivial for healthy people. To provide a better quality of life for these patients, extensive research and effort have been put by both neuroscientists and engineers to provide clinical therapies for pain relief and locomotion restoration together with dedicated platforms that could deliver these therapies. Currently, for these purposes, epidural spinal cord stimulation is widely used. Apart from being used as a method to reduce pain, it has also been proven to promote locomotion recovery. Apart from clinical trials, it is of great importance to understand the mechanisms that occur while delivering specific therapies. To this end, more exploratory research is mostly conducted in rodents. However, the availability of tailored neurotechnologies, for experiments conducted in small animals, is limited mostly due to size constraints. Moreover, when developing implantable devices that would target the spinal cord, careful selection of the materials used is equally important. However, understanding the underlying mechanism leading to a specific behaviour or motor outputs requires exploring and quantifying new methods of stimulation. For instance, optogenetics has been gaining a lot of popularity in the field of neural stimulation as it is a more specific technique that could help neuroscientists map the neuronal circuitry within the human body. Thus, apart from developing spinal cord implants that resemble best the anatomy of the body, while inducing as little stress as possible on the spinal cord, for exploratory reasons the developed implants must provide optogenetic compatibility. Therefore, this thesis reports the development as well as the characterization of both passive and active spinal cord implants with optogenetic compatibility.
To achieve the desired goal of having a fully implantable, flexible spinal cord implant with optogenetic compatibility, a scalable and reproducible microfabrication process has been developed. Materials such as graphene, for transparency, flexibility and conductivity were used to develop the microelectrode arrays. Moreover, soft, polymeric encapsulation was employed to sustain the high flexibility and transparency of the implant. The end result of the microfabrication process would lead to a device consisting of a multi-layered graphene structure between two polymeric-based encapsulation layers and metal test pads for interconnection to the outside world. However, towards achieving this final structure, several challenges were encountered. Suspension of the implants after developing them on a rigid substrate, yet ensuring high quality for the graphene layer leads to several iterations of the fabrication process. Despite the challenges encountered, several prototypes were successfully developed. However, having prototypes that can only validate the process flow would not suffice. Therefore, extensive evaluation of the devices has been conducted and reported. Methods such as Raman spectroscopy and optical transmittance to evaluate the graphene layer or cyclic voltammetry and electrochemical impedance spectroscopy to characterize the performance of the fabricated devices were employed. The degree of transparency obtained using the reported microfabrication process was ~78 %, leading to the conclusion that the number of graphene layers for the final device was 10. It has been proven that graphene does not deteriorate over time when soaked in saline solution for several consecutive days and apart from that, the graphene-based implants showed no performance deterioration when bent over rods down to 3 mm in diameter. Moreover, the graphene electrodes provided impedance values of ~8 kΩ at 1 kHz frequencies, values comparable to what literature has previously reported. Apart from developing a passive graphene-based spinal cord implant, the focus of this thesis was also to fabricate and characterize an active implant. However, embedding active components with a flexible, graphene-based array of electrodes is not trivial. Therefore, system integration of small test chips was investigated and after several iterations of flip-chip bonding processes, a complete, active, graphene-based prototype was obtained. The measurements performed after the bonding process have proven that both bonding on graphene-only as well as on graphene and metal substrates is possible and the four-point measurement results indicated resistance values ranging from 10 mΩ up to 16 Ω for individual connections, depending on the substrate used.
Therefore, with this research project, not only the first fully transparent, graphene-based spinal cord implants have been developed but also the results obtained from their characterization illustrate that the process is stable and the performance of the devices is promising. ... Read more

The concept of optical refrigeration dates back to 1929, when Pringsheim recognized that thermal energy associated with the translational degrees of freedom of isolated atoms could be reduced by the process of anti-Stokes fluorescence. Optical refrigeration of a solid was first experimentally demonstrated in 1995 with the Ytterbium-doped fluorozirconate glass by Epstein and his team and since then this invigorating field has garnered much scientific interest for development of an all optical refrigerator. The present works discusses the recent candidate materials including crystals, semiconductors, and ionically doped glasses. Cooling processes and necessary conditions for cooling are outlined, and general thermodynamic limitations are discussed.

10% wt. Ytterbium doped Yttrium Lithium Fluoride (Yb+3:YLF) is chosen as the candidate active material. The Carnot efficiency for laser and sun-light as a pump source is evaluated using a narrow-band approximation outlined by Stephen and his team. A quantum-mechanical cooling model based on Epstein and his team, is developed. In the proposed system, the candidate material is placed on a magnetically suspended platform inside a vacuum chamber and illuminated with laser light with the appropriate wavelength in the near infrared region. The dynamics of important cooling parameters are simulated and studied. The cooling effects due to radiative relaxation compete with the heating effects due to parasitic absorption and non-radiative relaxation but net cooling is observed confirming validity of light source and material parameter selection.

In addition to laser, the conventional source of pump radiation, sun-light as a pump input to the quantum-mechanical model is simulated and the effects on the cooling power and efficiency are studied. To enhance the energy efficiency of the system, fluorescence recovery schemes using photovoltaics are built and studied. Suggestions for experimental realization are given. The developed model can be base for designing a practical optical refrigeration system for laser and sun-light based optical sources.
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