The main objective of the research as described in this thesis was to enhance the piezo- and electrical properties of bismuth ferrite-based (BiFeO3 or BFO) piezoceramics at room and elevated temperatures, using both chemical and physical approaches.

Self-healing of creep-induced damage in newly designed 12Cr self-healing ferritic steels is studied. The damage healing is achieved by the segregation of supersaturated solute atoms at the free surface of the creep-induced cavities, and in this research, the healing phase is the W-riched Laves phase. Two kinds of self-healing steel with different precipitation driving forces of Laves phase are investigated, namely fast self-healing steel (FSHS) and slow self-healing steel (SSHS), in which the FSHS is designed to show the healing effect after 280 hours, while SSHS is designed to show healing effect after 27000 hours. Although both systems are self-healing systems, only FSHS is expected to show the self-healing phenomenon under the testing condition of this study, while SSHS not. Creep tests were performed on bone-shaped samples with constant stresses ranging from 100 to 260 MPa at 550 °C. The creep behavior is compared with traditional 9-12Cr ferritic steels, the relationship between the minimum creep rate and stress obeys Norton's power law, and the relationship between the lifetime and stress also obeys a power law. The total strain and stress exponents in power law of both SSHS and FSHS are relatively high compared to traditional 12Cr ferritic steel with similar grain size. The microstructure of the creep-failed samples, including the fracture surfaces, the uniformly-strained region (i.e., the stress-affected region), and the stress-free region are investigated with scanning electron microscopy (SEM). By comparing the feature of the stress-affected region and stress-free region exposed to the same thermal history, the effect of high-temperature exposure could be isolated from this. The effect of stress on the Laves phase precipitation behavior and the damage evolution could be understood more clearly. For both the SSHS and FSHS, a ductile and transgranular fracture mode is confirmed by analyzing the fracture surfaces. In the stress-affected region and stress-free region of the creep-failed samples, M₂₃C₆ precipitates, Si/C-rich impurities, and pores were found in all observed samples. The Laves phase was only found in the samples with a longer lifetime (> 300 h). A TEM observation and EDX scanning was conducted on the FSHS sample with a lifetime of 2487.2 hours. Laves phase precipitates were found on the grain boundaries and in the matrix, while no grain boundary cavities were found. Statistics based on pores observed on samples tested under different stress showed that the pores were present prior to creep tests and are generally not due to creep. A combined analysis of creep behavior and microstructure showed that dislocation climbing is likely the dominant creep mechanism. The absence of typical creep grain-boundary cavities was allocated to the relatively ductile matrix of 12Cr self-healing steel or the unfavorable dominating creep mechanism for creep cavity forming. With the prerequisite of self-healing not found in all tested samples, self-healing was considered not found under current testing conditions. The statistics based on the Laves phase showed that the precipitation depends on creep time, and the stress is indeed lowering the nucleation barrier, thus promoting the Laves phase to precipitate in the stress-affected areas.

In order to establish a sustained extra-terrestrial presence, habitats need to be built on the Moon and Mars using novel materials made from local resources. Several material processing methods can be applied to transform the locally abundant regolith into materials suitable for structural purposes. One promising method is sintering. This process can be used to create strong material using little to no Earth-imported additives. However, sintering still requires a large amount of energy to heat the material. A possible method to lower the energy requirement is by introducing small amounts of sintering aids. However, little is known about the effect of aids on the properties of sintered regolith. This research aims to investigate the effect of sintering aids on the densification and mechanical properties of sintered Martian regolith simulant.

In this study, the chosen Martian regolith simulant, Martian Global Simulant - 1 (MGS-1), was first investigated using a variety of powder characterisation techniques to assess the similarity with actual Martian material. Using the Spark Plasma Sintering (SPS) technique, disk shaped samples were sintered at temperatures between 700 °C and 1060 °C and pressures of 30 MPa to 50 MPa. Several powder mixtures were used. Two different additives, aluminium and bismuth oxide, were used in two weight percentages, 2.5wt% and 5wt%, and mixed with baseline material. Samples sintered from this enriched material were compared to those made using baseline MGS-1 material. In order to assess the mechanical properties, the Ball-on-Ring (BoR) compression test was used to determine the biaxial flexure strength of the samples. Since the BoR compression test is a semi-standardised method, an effort was made to validate the testing procedure and obtained results using soda-lime glass samples. Additionally, mortar disks were created and tested to provide a reference for terrestrial material properties. After compression testing, some samples were ground back into a powder and examined using X-ray Diffraction (XRD) to assess any bulk composition changes induced by the additive and/or sintering process.

The results from the powder characterisation techniques show that the chemical composition of MGS-1 is close to that of actual Martian material. In order to achieve strengths comparable to terrestrial mortar, a relative density of at least 70 % needs to be achieved. For the baseline material, the sintering temperature needs to exceed 1000 °C in order to obtain these results. This value is 950 °C for 2.5wt% aluminium additive material, and 900 °C for 5wt% aluminium, 2.5wt% and 5wt% bismuth oxide additive material. For equal sintering temperatures, the biaxial strength of enriched powders exceeds that of baseline material. Hence, the sintering temperature can be lowered for enriched materials to achieve similar strengths. Samples made from bismuth oxide enriched material exhibited superior properties compared to aluminium enriched material. For aluminium enriched material, no clear increase in properties is observed with increasing additive fraction. For bismuth oxide enriched material, there appears to be an increase in properties with increasing additive fraction. A material behaviour transition from brittle to tough appears to be linked to biaxial strengths exceeding 12 MPa. Compared to literature on Martian regolith-based materials, the results for sintered enriched MGS-1 perform well in terms of required additive fraction and mechanical properties. Using additives could potentially be a way to lower the energy requirement for regolith sintering on Mars. This work opens up areas of further research into the optimal additive, additive amount and sintering parameters for on-site application.

It has been over 50 years since man first set foot on the Moon and proved that mankind could extend its borders beyond Earth, and yet no permanent lunar outpost has been set yet. Similar to how the construction of larger ports allowed for the exploration of the New World in the Renaissance, human exploration of space requires infrastructure beyond that required for spaceships and launchers. As our only natural satellite, the Moon is the best choice for a permanent base, one that would be capable of refuelling interplanetary missions, providing plentiful resources for the needs of people on Earth, and giving opportunities for fundamental research...

Piezoelectric materials have the ability to convert mechanical to electrical energy (direct effect) and vice versa. They are readily used in the aerospace, automobile, telecommunication industry etc. as both sensors and actuators. For this work the focus is on the sensor application, which utilizes the direct piezoelectric effect. With the rapidly growing technological demands, sensors should be flexible enough to adapt to different applications while also having adequate sensing capabilities. Currently, lead- and lead-based piezoelectrics are used in the industry due to their excellent piezoelectric properties. However, due to their toxic nature, research has been ongoing into more lead-free systems which are capable of replicating the performance of these lead-based systems. In this work, we aim to improve the sensing capabilities of lead-free piezoelectric composites. To further improve their performance, reducing the dielectric constant (ε) of the composite is the main strategy of this work. The reduction is achieved by fabricating a porous composite structure. The reduction in permittivity leads to an increase in the piezoelectric voltage constant (g), which defines the sensitivity of the piezoelectric composite.

The main focus of this work is to optimize a polymer and polymer foaming technique to obtain a high level of porosity, while also retaining adequate mechanical properties. The next step is to achieve a high poling efficiency for the composite in order to obtain good piezoelectric properties. For the polymer system, polyvinyl alcohol (PVA) is selected as the matrix due to its excellent film forming ability as well as its relatively high dielectric properties (compared to polymers). The direct foaming technique is used for this work, due to its simplicity and its reproducibility. For the lead-free ceramic system, Barium Titanate (BaTiO3) and Sodium Potassium Niobate doped with Lithium (KNLN3) is selected as they have good piezoelectric properties, and have been used in piezoelectric composites extensively. As a porous piezoelectric composite is used in this work, the contact poling is replaced by the corona poling method to prevent localized dielectric breakdowns and non-uniform poling.

With the direct foaming technique, foams with porosites in the range of 90-95 % are obtained, resulting in a drastic reduction in the permittivity of the composite. Such a high porosity level also results in a much softer composite. The optimization of the corona poling process is done by selecting the adequate poling temperature and the grid voltage, which is found to be 110 °C and 6 kV respectively. The effective piezoelectric charge coefficient is measured using Al plates as electrodes, to prevent the soft composites from compressing locally. The foam composites exhibit remarkably high g33 values exceeding the 1000 mV.m/N mark, almost double the best sensor used in the industry currently (PVDF). This is attributed to the high poling efficiency and the reduced dielectric permittivity of the composite. This opens up the vast number of possibilities for future systems based on porous structures to be used as sensors which can showcase good piezoelectric properties as well as being more flexible/conformable.

Energy harvesting has become a popular topic for battery substitution in recent years. In particular, mechanical vibrational energy, one of the most common energy sources, has been intensely studied. Piezoelectric materials play a crucial role in vibrational energy harvesting as they can directly convert the mechanical energy to the desired electric energy. In particular these are attractive when they are made into a cantilever form allowing the harvester to harness larger energy. This project aimed at optimizing the performance of single-layered piezoelectric cantilevers, also known as the unimorphs, from the point of view of output power and operation lifetime with the use of a stroke limit stopper. \\
In this study, a cantilever beam consisting of PZT (PZT5A4) bonded to an elastic substrate (Pernifer 45) with a tip mass attached to the free end is used as the harvester. To reduce the cracks generated in the piezoelectric material and subsequently increase the lifetime, a limit stopper is introduced to constrain the deflection of the unimorph.The influence of the stroke on the lifetime and power output is shown by a series of vibration tests under different operation conditions with varied unimorph lengths, configurations, attached tip masses and stroke distances. The result confirms the possibility of the limit stopper to improve output energy of longer unimorphs in certain distance ranges. It also points out that current designs do not allow performance optimization in both output and lifetime simultaneously. One ought to choose one of the two, as the methods which increased the instant power output also resulted in a sharp reduction in lifetime as well as total power capacity.

Unlike traditional wet-adhesives, dry adhesives are bioinspired solid polymers that owe their adhesion to hierarchical structures. Gecko’s are the most recognizable living creatures relying on this concept to climb walls. A close look at the gecko’s fingers reveals a hierarchical system of micro-and nanoscale filaments. These ensure excellent adaptability and contact to (rough) surfaces and the formation of a large amount of close-range van der Waals surface interactions. The reversible nature of these bonds allows for reversible and repeatable adhesion. This hierarchical system has been identified as the main reason for dry adhesion and has been object of intense research to manufacture man-made solid adhesives. Despite the many efforts, there are some unclear aspects resulting from contradictory scientific reports and a strong focus on a particular polymer chemistry used to make the hierarchical structures. This has left the effects of the material choice on dry adhesion largely neglected. In this research project, we aimed to shed some light on the role of different polymer architecture features on dry adhesion. Most available research has focused on the use of commercial siloxane elastomers that offer almost no control on the polymer synthesis. Instead, we opted to use thermoplastic polyurethane chemistry due to the large versatility this chemistry offers in terms of modification of relevant polymer architecture features. In the absence of available works on thermoplastics a new manufacturing process of such hierarchical structures had to be developed relying on heating, pressure and vacuum to respectively melt the polymer, overcome the high viscosity and impede oxidation. We studied the effects of the polymer architecture and properties on adhesion using a single micropillar architecture and varying the chemical composition (polyol length, aromatic content, hard/soft block ratio), the testing temperature and the pull-off speed. As to the polymer architecture, the best results were found with a short polyol and the lowest hard block fraction that still guaranteed structural integrity. Next to that, it was found that having aromatic rings in the hard segments was crucial, likely due to its beneficial effect on nanophase separation. With those compositions, values exceeding those of state-of-the-art dry adhesives were found, with a maximum of 440 kPa at the Tg and high retraction speeds. Furthermore, we show that while existing models are valid for thermoplastic polyurethanes well below their Tg, once they exhibit viscoelastic behaviour, the loss factor is a much more reliable indicator of performance than the reported stiffness. The effect of the surface energy was also evident but minimal compared to the mechanical properties of the polymers.

An experimental study is conducted on the design of actively controlled semi-permeable inserts for mitigating turbulent boundary layer noise originating from the trailing edge of an airfoil. Two sets of perforated inserts with hole diameter: 0.8 mm and hole spacing: 1.5 mm & 3 mm, are considered. The inserts are 3-D printed with a middle-disk shift mechanism to change flow parameters across the perforated disks without modifying the hole diameter and hole spacing. The pressure drop experiments across the inserts are carried out at pipe-level Reynolds numbers ranging from 160 to 2380 to characterize the variation in flow permeability, flow resistivity and formfactor of inserts. The tests are carried out in two phases: static and dynamic configuration. A novel static set-up with orientation pins is devised to implement the middle-disk shift mechanism. For the dynamic configuration, an actuation device is designed and constructed to control permeability on-line through an external motor controller. The findings show that the middledisk shift mechanism can achieve a maximum permeability variation of DeltaK_max>4.5x10^-9 m2 at a net effective porosity change of epsilon_Eff >72%, for the current set of perforated inserts. The results also suggest that the mechanism is most potent in the range of 50%closed hole (half shut)- to 100% closed hole (completely shut)- orientations. The flow parameter variations for the static configuration are fairly replicated by the dynamic configuration, thereby, establishing repeatability of the actuation device. Therefore, the dynamic configuration set-up holds good promise for up-scaling to real structures, that is, trailing edge of an airfoil with relevant optimization.

Solid-state phase transformations in steels cover a broad range of aspects. The underlying physics behind these phase transformations usually include nucleation, diffusion, lattice reconstruction and interactions between solutes and grain boundaries and interfaces. These features and the fact that the events take place at high temperatures, in the bulk, at time scales ranging from milliseconds to hours and at length scales ranging from atomic dimensions to millimeters make even the most widely studied phase transformation in steel, the austenite-ferrite phase transformation, only approximately understood and therefore an attractive topic for investigation. Quantitative data obtained by sophisticated physical characterization techniques in combination with supporting physical microstructural models addressing the relevant length and time scale are required to bring the field of ferrous physical metallurgy further.
This thesis focusses on two new approaches to orchestrate phase transformations in steels such that more physical insight is obtained or that new properties can be reached: (1) cyclic partial austenite-ferrite phase transformations that are designed to unravel the grain growth, and more specifically the interface mobility by avoiding concurrent nucleation of new phases. This topic is studied by computational studies and 3D neutron depolarization studies that are capable to in-situ monitor the ferrite grain size and fraction. (2) Self healing of creep damage by site selective precipitation of supersaturated iron-based alloys. A strong preference for precipitation at free creep cavity surfaces compared to that in the bulk can result in a filling of creep cavities and a significant extension of the creep life time. To make this self-healing mechanism applicable for creep-resistant steels, a search for an alternative healing agent for Au in Fe is to be executed and new design recipes need to be extracted on the basis of the experimental input from advanced characterization techniques such as electron microscopy and X-ray nanotomography.

The development in computational simulation techniques has brought significant progress in the realization of computational alloy design. The advantages are most significant when developing novel materials of which the research & development cycles are particularly time and energy (and hence cost) consuming, such as high-temperature alloys. In our former research, a computational alloy design approach coupling thermodynamics, kinetics, metal physics and genetic algorithm has been developed. By applying this new approach, novel heat resistant steels have been successfully designed with different microstructural features, which manage to nicely outperform existing commercial alloys. In this thesis, we follow the same approach while more focus has been given on adjusting the alloying level of different elements to solve specific issues, such as the high cost issue caused by a high Cobalt level, and the high microstructural instability caused by a high Chromium concentration. Extended application has been made to design novel heat resistant steels by introducing the concept of self-healing mechanisms. The creep damage (grain boundary cavities) of newly-developed steels during service are expected to be automatically filled by the special-designed Laves phase of which their formation kinetics was adjusted.

Piezoelectric materials have found application in wide range of devices from accelerometers to energy harvesters. They have proven their potential and versatility in aerospace, automotive, electronics and biomedical industry. Piezoelectric ceramics are not suitable for energy harvesting applications due to its brittleness and low strain levels, although having a high piezoelectric charge constant (d33). The polymer based piezoelectric composites offers high flexibility with low dielectric constant, hence having potential application in energy harvesting systems. Currently quasi-structured and fiber based piezoelectric composites offer the highest piezoelectric properties having the required flexibility for energy harvesting application. The polling processes of these composites though, are not so efficient due to the dielectric mismatch between the ceramic filer and surrounding polymer matrix. This leads to low piezoelectric charge constant than theoretically possible. To avoid this dielectric mismatch, dielectric constant cermic filler is added to get the required chain like particle structuring. The decrease in dielectric mismatch would lead to better poling due to increase in the active electric field experienced by the ceramic filler. The work here majorly focuses on development of a 3 phase piezoelectric composites having two different types of ceramic filers. Barium Titanate (BT) nanoparticles were added into a two phase Lead Zirconate Titanate (PZT)/Epoxy composite and were characterized to evaluate piezoelectric properties and particle structuring. The particle size effects of BT nanoparticles were also investigated.

This project investigates the combination of PPS thermoplastic UD and fabric composite materials with novel mass-produced vertically aligned carbon nanotube (VACNT) materials at the interlaminar region. There are two goals for this project, the first is to understand how the VACNT materials can be embedded and consolidated with the use of elevated temperatures and pressures and how that impacts the resulting morphology of the VACNTs. The second is to understand if VACNTs can increase the interlaminar related mechanical properties of the PPS composite materials similar to that seen with thermoset materials. Processes for embedding of VACNTs to a single ply was found to be possible and repeatable, though secondary multi-ply consolidation lead to matting of the VACNTs at the interlaminar layer. Subsequent mechanical testing showed that this matting of VACNTs led to a decrease in shear and compression strengths by 10.6% and 8.5% respectively against control specimens.

Metamaterials are a new class of materials that have properties that cannot be found in nature. Though these artificially engineered structures have not been prominently used in many applications, they have been theoretically studied to have a variety of applications in many sectors. Acoustic metamaterials have the capability of manipulating sound waves in order to achieve the required unique properties.
The aim of the project is to design an active acoustic metamaterial consisting of an array of Helmholtz resonators to assist in reduction of noise levels in aerospace applications using the method of 3D printing.
This project presents an active Helmholtz resonator based design that can attenuate a broadband range of targeted frequencies in the low-frequency regime. To this end, a passive metamaterial consisting of an array of Helmholtz resonator unit cells, with a single varying design variable, is designed and tested to establish the effectiveness and region of performance. The selected design variable for change is identified through the frequency response for each parameter of the Helmholtz resonance equation, to achieve a broadband frequency range of the metamaterial. An active model of this design is then fabricated and tested. Two actuation mechanisms are presented for this design. The resulting acoustic systems are capable of providing an attenuation of up to 20 dB, for an open system, and up to 35 dB, for a closed system, at frequencies between 150 Hz and 500 Hz. Unlike most other studies conducted, this active acoustic design is capable of attenuating isolated frequencies as well as multiple frequencies simultaneously. The added control that is achieved through the incorporation of the electric linear motor based actuation allows for the advantage of accurate frequency targeting along with the base attenuation levels of the passive resonant acoustic metamaterial.

Alumina (Al2O3) is an attractive ceramic for engineering applications operating at elevated or high temperatures because of its good thermal and chemical resistance. It also maintains high strength and hardness at high temperatures. These desirable properties are due to the strong covalent and ionic bonds existing between its atoms.
However, these same strong and directional bonds are the origins of its inherent brittleness. Over the last decade, material scientists have adopted self-healing as a means of restoring the load bearing capability of such materials after damage from micro-sized surface cracks. In this methodology, the material is restored to a status comparable to the original one by the ‘healing’ of such surface cracks at high temperatures. Healing is achieved by the addition of ‘healing agents’ to the base ceramic material which upon the occurrence of a crack oxidise into a healing oxide which fills and seals of the crack. There are some gaps in the build-up of the knowledge ladder of self-healing ceramics to an application ready level. This thesis addresses some design questions and tests the capability of newly identified healing particles under laboratory and application conditions.

The scope of this thesis is to develop new tooling concepts and production techniques to improve the quality of composite skin panels. The research focuses on closed mould manufacturing techniques using solid-solid and solid-flexible tool sets. Subsequently the project introduces prepreg/resin infusion concept as an additional way to reduce manufacturing defects like voids and porosities in the composite panels. The closed mould tool setup was optimized with repeatable tests for both tooling sets. The composite laminates produced had no visible defects like fiber flow-out and wrinkles on the surface. The panels produced with both tooling sets were mechanically tested for inter laminar shear strength and the cross-sections were analyzed under optical microscope to determine the void content. The prepreg/resin infusion technique was used with the developed mould setup as an attempt to eliminate voids in the produced laminates. However, the attempt was not successful due to different processing temperatures of liquid resin and prepreg material used in the project. Therefore, it is recommended to use both prepreg and liquid resin with similar chemical and processing conditions. Furthermore, it is advised more research has to be conducted for improving infusion concepts with the closed mould tool setup used in this project.