This report investigates the damage behavior of cold spray repairs subjected to low velocity impact (LVI) loading, with a focus on their potential application for aircraft cowling inlet lip repairs. Cold spray technology is increasingly considered as a promising alternative to traditional thermal spray and Laser- clad technologies due to its ability to significantly reduce repair costs, decrease residual tensile stresses in parts and decrease turn-around time during maintenance and overhaul. Unlike conventional thermal spraying methods, cold spray operates at temperatures below the melting point of the feedstock powder, allowing particles to remain in the solid state throughout the deposition process. As a result, it avoids the formation of heat-affected zones, preserves material properties, and induces favorable compressive residual stresses. These advantages make cold spray particularly suited for both structural and non- structural aerospace repairs. However, insufficient research exists to verify the quality of cold spray repairs on aircraft components to obtain the technology certified.
The primary objective of this research was to identify and characterize the dominant damage mecha- nisms that occur in cold spray coatings and repairs under LVI conditions. Six types of cold sprayed aluminum alloy coupons were evaluated, including both flat coatings and double curved blend-out re- pairs and cold spray depositions of AA1XXX and AA2024. The depositions were applied using either a robotic system or a manual gun. All specimens were subjected to controlled impact loading via drop weight and/or quasi static indentation tests. The results were analyzed using high-speed camera imaging, optical microscopy, and scanning electron microscopy of both surface and cross-sectional features.
Key output parameters such as dent depth, dent width, absorbed energy, and crack formation were analyzed and compared across different coupon types and impact energies. Results show that:
• Deposition geometry plays a critical role in how the material deforms under impact.
• The ductility of the deposition and substrate materials (e.g., AA1XXX vs. AA2024) influences both energy absorption and crack resistance for varying impact location.
• Coarse-gritsurfacepreparationenhancesbondingandincreasesresistancetodelaminationcom- pared to finer surface treatments.
• Barely visible impact damage does not apply to cold spray depositions as a dent is always visible. However, the substrate (non-impact side) showed different crack thresholds and damage mech- anisms compared to the cold spray deposition. The substrate cracked more severely compared to the cold spray deposition.
• Manual gun-sprayed depositions exhibit non-uniform properties, leading to variable crack thresh- olds across the repair area.
To compare performance independently of thickness, the impact data was normalized by the deposi- tion thickness. This revealed that thin coatings (like AA1XXX on top of AA2024) show lower energy absorption per millimeter compared to blend-out repairs.
In conclusion, the study highlights the critical influence of deposition material, geometry, thickness, and surface treatment on the performance of cold spray repairs under impact. It demonstrates key differences between robotic and manual spray systems, particularly in terms of uniformity and damage treshodls. The results show that cold spray depositions are able to withstand low velocity impact and can thus be used to repair dents in cowling inlet lips. The insights contribute to the ongoing research to integrate manual cold spray gun systems into aerospace repairs. ... Read more

The continuous demand for smaller, faster, and more efficient electronic devices has driven research into two-dimensional (2D) materials that can overcome the physical limitations of Si brought on by quantum effects. Among these, transition metal dichalcogenides (TMDs) stand out due to their inherent bandgaps and highly tunable properties. In particular, the one-dimensional (1D) nanostructure formed by rolling up sheets of 2D materials, known as a nanoscroll, has immense potential for emergent optoelectronic properties brought on by its uniquely non-uniform strain field. Stacked and scrolled TMD heterostructures offer a promising route toward realizing novel optoelectronic phenomena driven by broken centrosymmetry and interlayer coupling. However, it is not well reported how the morphology of the initial 2D TMD sheet affects the final scrolled structure, which is a significant barrier to the deterministic control of nanoscrolls. This thesis explores the synthesis and morphological control of molybdenum-based TMDs, specifically MoS₂ and MoSe₂, using chemical vapor deposition (CVD), with an emphasis on understanding how 2D flake morphology governs the formation and properties of 1D nanoscrolls. Through systematic modification of CVD parameters, it was found that synchronizing temperature ramps between precursor zones greatly improved MoS₂ flake uniformity, yielding smaller, triangular monolayers with consistent morphology. Subsequent scrolling experiments demonstrated that flake shape and substrate adhesion critically influence scrolling yield and integrity, establishing a clear relationship between 2D precursor structure and final scroll geometry. In parallel, attempts to extend hydrogen-free CVD growth to MoSe₂ revealed significant challenges associated with selenium’s low reactivity, resulting instead in dominant Mo oxidation processes. MoO_X phase evolution was investigated via these results, with a detailed structural study being carried out on novelly synthesized 2D α-MoO_X nanobelts. Altogether, the findings advance the understanding of how CVD growth parameters dictate morphology and transformation pathways in molybdenum-based 2D materials, highlighting both the opportunities and challenges of fabricating non-hydrogen TMD heterostructures and strain-engineered nanoscrolls for future optoelectronic applications. ... Read more

Wire and Arc Additive Manufacturing (WAAM) enables large-scale steel fabrication but often produces rough surfaces that are detrimental under cyclic loading. This thesis investigates how surface finish affects the fatigue performance of WAAM AM70, a high-strength steel comparable to S690, through strain-controlled low-cycle fatigue (LCF) and stress-controlled high-cycle fatigue (HCF) tests. As-built and machined specimens were compared, revealing surface finish as the dominant fatigue driver. Machining increased fatigue strength by factors of 3–9 and reduced scatter. The study provides EN 1990:2023-compliant design curves and mechanistic insight, showing that machined WAAM AM70 achieves fatigue performance approaching conventional S690. ... Read more

Printing patterns in additive manufacturing have been extensively studied regarding their effects on the microstructure and mechanical properties of the materials. However, weaving, a welding technique that produces good quality welds in large areas and ensures good weld penetration has not been applied in additive manufacturing. This study focuses on comparing four weaving patterns to two traditional line printing patterns in Wire arc additive manufacturing of austenitic stainless steel 316L. This thesis covers the groundwork of weaving pattern characterization, starting from optimization of printing parameters and printing path, followed by pattern characterization through measurements of voltage, current, thermal history, and thermal profile, and investigation of surface quality of the samples. Microstructure characterization was then conducted by optical microscopy, electron backscatter diffraction, and X-ray diffraction; and mechanical properties were obtained through microhardness and tensile testing. The weaving patterns showed an improved deposition rate and avoided lack-of-fusion defects compared to the line printing patterns. Two out of four weaving patterns achieved superior surface quality over the line printing patterns and are free of macro-defects such as side wall collapse and spattering. The weaving patterns have excessive heat accumulation and lower cooling rates compared to line printing patterns, which led to coarser microstructures and inferior microhardness and tensile properties. The thermal gradient is also more uniformed and aligned to the build direction, increasing the degree of grain alignment and texture of the weaving patterns across the samples. ... Read more

As sustainable energy technologies continue to attract growing interest worldwide, comprehending their environmental implications becomes essential. Along with cost optimisation and enhancing efficiencies, it is equally important to reduce a wide range of environmental impacts, which is crucial for attaining global sustainability goals. Silicon Heterojunction (SHJ) solar panels are one such example of a growing sustainable energy technology that are anticipated to take up a considerable share of the global PV market in the coming years, owing to its high achievable efficiency. The goal of this study was to conduct a Life Cycle Assessment on a PV system consisting of Silicon Heterojunction solar cells and modules in order to gain insights for the environmental impacts of such a PV system based on manufacturing in the Netherlands. The study included the production steps of SHJ cells and modules, from raw material to final product and use phase until end of life time. Recyling processes were not included. Inverters and mounting structures were also used to complete the PV system. 4 impact categories were analysed in this study for a rooftop PV system with SHJ cells in 2024. The results for these impact categories were: 22 g CO2-Eq/kWh for Climate Change; 14 g 1.4-DCB-eq/kWh for Ecotoxicity Freshwater; 17.5 g 1.4-DCB-eq/kWh for Ecotoxicity Marine and 0.0016 m2 crop − eq/kWh for Land Use. Similarly, the results for the future scenarios were also reported: Climate change impacts will reduce by more than 10 g CO2-Eq/kWh ; Ecotoxicity impacts will reduce by around 0.6 g 1.4-DCB eq/kWh and land use by 0.0006 m2 crop−eq/kWh over the course of a decade. Then, the contribution analyses were presented for these categories, representing the components and process steps that were major contributors to each of these categories. Finally, two sensitivity analyses were conducted, to see how the environmental impacts change by changing certain parameters. The results gathered in this study, and upon comparing them with the LCA results from earlier published studies showed that the SHJ cell and module manufacturing was more environment friendly than some of the other technologies, along with certain room for improvement. Improving the manufacturing processes and with a change in Dutch electricity mix, in the future scenarios, showed that the environmental impacts will further reduce, making this PV technology highly acceptable and implementable. ... Read more

The growing amount of plastic waste generated by the consumer goods industry has led to excessive landfilling and increased greenhouse gas emissions, contributing significantly to the greenhouse effect. Government initiatives have identified the potential for a more sustainable approach to managing this homogeneous plastic waste stream. The Netherlands Circular Economy Program 2023-2030 (NCEP) envisions the MEGA project as a solution to repurpose plastic waste by replacing aging pedestrian infrastructure. Polypropylene (PP), known for its chemical stability and weather resistance, can be reinforced with glass fibers (rPP-G) to meet the requirements of such applications. Additionally, 3D printing is recognized as a promising method to reduce plastic waste during production, making it a valuable component of this initiative.

Nevertheless, outdoor infrastructural applications are subjected to load and temperature cycling due to their exposure to varying weather conditions, static loads, and pedestrians walking at different frequencies. This study aimed to assess and evaluate the effects of hygrothermal aging on the mechanical behavior of rPP-G. Three conditions were studied: a control condition not subjected to the aging environment, an extreme condition subjected to 95 % relative humidity (R.H.) and 80 ◦C, and a condition that approximated real application humidity levels of 77 % R.H. at the same temperature.

The tensile properties were evaluated by applying the load at a speed of 1 mm/min. The dynamic mechanical response was investigated at frequencies of 1 Hz and 2 Hz to simulate a crowd walking and a crowd brisk-walking on the material. The creep response was examined through short-term creep tests at elevated temperatures, and a methodology was derived based on the Time-Temperature Superposition (TTS) principle to extrapolate the behavior at ambient temperatures for stress levels of 8 and 10 MPa. For further analysis of the results, Scanning electron microscopy (SEM) captures were acquired, and the IR spectrum and diffraction pattern of rPP-G under different aging conditions was measured. The tensile properties suggested increasing degradation with hygrothermal effects, leading to a 14.3 % reduction in ultimate tensile strength (UTS), a 16 % reduction in Young’s modulus, and a 93.3 % increase in strain at break. This indicated weakening of the fiber/matrix interface, verified by increased fiber pull-out. However, no hydrolytic degradation was observed, as no differences in chemical compounds and crystallinity were detected. The dynamic mechanical response suggested the existence of two mechanisms: water acting as a stiffener at cryogenic temperatures, and induced plasticization and fiber/matrix weakening from the hygrothermal effects. However, fiber/matrix weakening dominated under extreme aging conditions, hindering the stiffening effects of water on the molecular structure of rPP-G. This effect was more pronounced under median aging conditions for the 1 Hz loading, as more time allowed for molecular relaxation. Increasing frequencies appeared to shift the glass transition temperature (Tg) to higher temperatures, highlighting the material’s elastic character. The creep response of rPP-G revealed an exponential relationship between temperature and time to fracture, allowing the use of Time-Temperature Superposition (TTS) with the Arrhenius equation to predict its behavior at ambient temperatures. The derived methodology suggested that hygrothermal effects leading to the degradation of tensile properties are significantly more apparent under long-term load applications, revealing a 282.6% reduction of time to fracture at 8 MPa under extreme conditions.

This thesis demonstrated the significance of dynamic mechanical analysis in understanding the effects of degradation on reinforced thermoplastic polymers. Additionally, it highlighted the importance of creep testing for such applications, revealing crucial insights into the long-term load application effects on the service life of thermoplastic polymers.
... Read more

The exposure of certain carbon steels to sour environments can result in severe hydrogen induced cracking (HIC) damage in the oil and gas industry. Current mitigation techniques in this field have low reliability or are not able to provide long-term protection against such damage. Recent advancements in thermal spray technology have resulted in a promising and cost-effective solution. Improvements in particle velocity and deposition efficiency have enabled coatings to achieve higher density and uniformity. High-velocity air fuel (HVAF) thermal sprayed NiCrMoW coatings are particularly interesting due to their outstanding corrosion resistance and mechanical properties. To ensure that equipment is sufficiently protected against the harsh environment of this industry, a thick coating is desired. However, as coating thickness increases, the performance of thermal sprayed coatings is frequently affected by residual stresses and unfavourable microstructural features.

To identify this effect, three NiCrMoW coatings with thicknesses of 250, 375, and 500 \textmu m were applied with HVAF thermal spray technology on S235JR carbon steel. Samples were analyzed in order to evaluate differences in terms of microstructure, mechanical behaviour, HIC resistance, and corrosion resistance. An AK07 HVAF instrument in a controlled setting at the IOT research centre of the University of Aachen was used to ensure consistency among the coatings during the spraying process. Experiments to evaluate HIC resistance and corrosion resistance involved prolonged immersion in a sour environment, cathodic charging, open circuit potential measurements, and potentiodynamic polarization tests. Microstructural variation was examined with the use of SEM-EDS and optical microscopy. Additionally, subsurface microhardness measurements of the coating and underlying substrate were used to evaluate hardness and give an indication of the presence of residual stresses.

Findings indicate that the coatings exhibit excellent corrosion resistance. A small but noticeable decrease in resistance was however observed with increasing coating thickness. This decline can be attributed to two factors: an increase in the degree of oxidation and accumulation of residual stresses within the thicker coatings. Additionally, it is noteworthy that while the degree of oxidation and residual stresses increased with coating thickness, the porosity fraction decreased. Microstructural features in the coatings varied as a result of differences in thermal input, cooling passes and the influence of shot peening effects. Resistance to HIC of carbon steel in a sour environment was significantly improved by the application of the coatings in comparison with uncoated samples. This can be attributed to the excellent corrosion resistance, uniformity and absence of through-coating porosity in the coatings, the thickness did not have an influence. Furthermore, it was found that the galvanic interaction between the NiCrMoW coating and the S235JR carbon steel significantly accelerates the corrosion of the underlying substrate. Thicker coatings might be able to provide a greater physical defect-free barrier which can resist breaking, damage and erosion to prevent this galvanic effect. ... Read more

In this research, a thermal analysis of the wire and arc additive manufacturing process is presented based on the F.E. method.

An important quality of the F.E. model that is presented in this research is to describe the temperature field that is experienced by the deposited material in the wire and arc additive manufacturing process. In contrast to the traditional method of monitoring the substrate temperature, the F.E. model allows to describe the temperature field that is experienced by the deposited material constituting both the multilayer weld deposit and the component. Accordingly, the F.E. model is capable to describe the effect of the locally attained temperature field on the microstructure of the deposited material constituting the multilayer weld deposit in terms of the thermal characteristics including the temperature distribution, the locally attained temperature values, the cooling rates and the temperature gradients throughout the component.

Two cooling methods are proposed to control the heat dissipation from the component to the environment based on the application of an interlayer waiting time and immersing the component into a cooling medium. The results show a significant effect of the cooling methods on the temperature field that is experienced by the material constituting both the multilayer weld deposit and the component. Indicating that the cooling methods that are proposed in this research are effective to control the microstructure and the temperature field that is experienced by the component in the wire and arc additive manufacturing process. The resulting microstructure is characterised in terms of the microstructural morphology and the microstructural constituents, using optical microscopy based on the average grain size and the distribution of alloying elements throughout the material constituting the multilayer weld deposit. In addition, the average grain size and the distribution of the alloying elements throughout the material are evaluated in terms of the hardness values. ... Read more

Aluminium alloy AA2024-T3 is a lightweight and damage tolerant material, and is therefore often used in aerospace applications. However, this alloy is difficult to weld using conventional fusion welding techniques due to defects caused by the meld pool.

Friction stir welding (FSW) was developed by The Welding Institute (TWI) in 1991 in order to overcome welding difficulties associated with the conventional fusion welding of difficult to fusion weld materials such as AA2024-T3. Since then, efforts have been made to improve the process parameters. One such improvement was the development of a stationary shoulder tool, which reduces heat input by 30% and enables stronger welds with smaller heat affected zones (HAZ). However, since AA2024-T3 is susceptible to pitting corrosion, intergranular corrosion (IGC) and stress corrosion cracking (SCC), the welds need to be protected in order for any product to function in the long term. AA1050 is an often applied clad layer, which acts as a sacrificial anode with respect to the cathodic substrate. The microstructure and corresponding corrosion mechanisms are known for unwelded and FSW’d AA2024-T3, however, little research exists on the corrosion behaviour of stationary shoulder friction stir welded (SSFSW’d) bare and Alclad AA2024-T3 butt welds. Therefore, the main focus of this project is to investigate the mechanical and corrosion properties of SSFSW’d bare and Alclad AA2024-T3. This was done by performing optical microscopy, microhardness tests, open circuit potential measurements, linear polarisation resistance tests and potentiodynamic polarisation tests on the cross-sections of three weld configurations and thicknesses: 1.6 mm bare, 1.6 mm Alclad and 3.2 mm Alclad AA2024-T3 sheets. Simultaneously, tensile tests and immersion tests were also performed using dedicated tensile test and immersion specimens.

Analysis shows that the 1.6 mm bare weld is the strongest weld but also the most susceptible to pitting corrosion, compared to the Alclad welds. Cladding provides sufficient corrosion protection, even to an exposed section of the weld and when mixed into the weld. However, cladding lowers the tensile strength overall, and cladding mixed into the weld reduces the ductility of the weld compared to the bare weld. Furthermore, possible material flow issues at the root of the Alclad welds may cause voids, which lowers ductility and enables pitting at these locations. Nevertheless, even though specimens were immersed in 3.5% NaCl solution for 24 hours and small to severe pitting was visible on the specimens, the ultimate tensile strength was not affected compared to uncorroded weld specimens. However, a reduction of the maximum elongation of the bare specimen was observed after immersion. Similar to conventional FSW, the HAZ/TMAZ was the most susceptible to corrosion, due to the most active corrosion potential at this zone. Based on literature, this was deemed to be due the formation of S-phase precipitates along the grain boundaries in this zone. In all, stationary shoulder friction stir welded bare AA2024-T3 provide the best welds regarding mechanical properties, and should be protected against corrosion after welding instead of using preclad sheets, to avoid issues with the macrostructure and corresponding mechanical properties. ... Read more

Large-scale hydrogen storage is a crucial part of the energy transition. The usage of salt caverns has a great potential in this process, but there are open questions regarding the construction’s lifetime which need to be investigated prior to their implementation. In this work, potential construction steels were studied. The conditions in a salt cavern were imitated on laboratory scale with an experimental high-pressure setup. Two steels, J55 and H2-ready X56, were systematically exposed to pressure/temperature cycles, gas (H2 and N2), water and brine. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques were used for the characterisation of the steels’ surface, focussing on corrosion effects and crack formation. For both steels, a significant impact of moisture and salt ions could be shown. However, only for J55, intensification of corrosion and cracking on the surface due to hydrogen gas exposure was found. Pronounced crack formation over the entire surface of J55 was revealed. For X56 significantly less crack formation could be observed. Overall, the results strongly indicate better resistance of X56 than J55 against the conditions in a salt cavern, used for hydrogen storage. ... Read more

Studies on the impact of Hydrogen on the electrochemical and mechanical behaviour of Iron-based materials have been increasingly conducted in the past few years. This is mainly due to the ever-growing demand for sustainable energy sources, which involve Hydrogen and to meet the high structural and economic demands from the automobile sector and other industrial demands for which high strength steels like dual-phase steels have been developed. Steels may absorb hydrogen both throughout the production process and during different phases of use. High-strength steels (particularly dual phase steels) and martensitic stainless steels are highly susceptible to hydrogen embrittlement. Moreover, hydrogen embrittlement is already possible at quantities as low as 0.1 ppm[1]. Therefore, understanding Metal-hydrogen interactions is essential.

This thesis aims to study the effect of Hydrogen charging on three different
iron-based materials and the possible effect on the electrochemical/corrosion performance. The Iron-based materials used in the study are Pure Iron, DP1000 and AISI 420. The methodology involves using an electrochemical procedure based on potentiostatic Hydrogen charging and Cyclic Voltammetry (CV), applied on these Iron-based materials having different phases to monitor H-uptake in the materials. The materials are charged with Hydrogen both on the active and passive surfaces. The electrochemical method is capable of measuring the diffusible H concentration (including mobile Hydrogen) for the steel alloys under H-charging conditions. Using X-Ray Diffraction (XRD) Technique, microstructural analysis is carried out on Pure Iron, DP1000 and AISI 420 stainless steel to identify the different phases interacting with Hydrogen. To gain additional insights relating to the electrochemical/corrosion performance of the investigated materials and into the H-related findings, potentiostatic polarization techniques, Electrochemical Impedance Spectroscopy (EIS) and Scanning Kelvin Probe (SKP) are carried out.

As a result, it was discovered that Pure Iron had the maximum amount of diffusible Hydrogen under the charging conditions tested, followed by DP1000 and then AISI 420 Steel with the lowest amount. EIS technique was useful in identifying a trend between the barrier properties of the passive layer and the H desorption values for the three materials. The results also revealed that materials charged with Hydrogen on active and passive surfaces behaved differently with respect to the amount of Hydrogen desorption. The microstructural features of the active surface and the passive oxide film that formed on the materials are further discussed in relation to this. ... Read more

Silicon heterojunction (SHJ) solar cells have exhibited efficiencies well above 25%. To further boost the efficiencies of c-Si-based solar cells, high-bandgap perovskite cells are stacked on top achieving a record efficiency of 29.52%. However, as most of the high-quality perovskite films are solution-processed, the front surface of the bottom device should be flat. Therefore, in this work SHJ bottom c-Si cells featuring front-side-flat and rear-side-textured morphology, which delivers high V_OC together with excellent near-infrared response, have been optimized as bottom cells for tandem configurations.

Firstly, RF-PECVD deposition conditions of a (i)a-Si: H monolayer for symmetric <100> flat c-Si surfaces were optimized. The optimized (i)a-Si:H monolayer ( 10-nm-thick) was obtained using pure SiH₄, which results in rather moderate passivation performances (teff = 1.2ms, i-V_OC = 701 mV).
To improve further the passivation quality of monolayer (i)a-Si:H on flat <100> surface, other passivation approaches aiming at incorporating more H without promoting detrimental epitaxial growth have been investigated.
With a bilayer deposition approach, which features firstly a less H-containing (i)a-Si:H to prevent epitaxial growth and then a second H-rich (i)a-Si:H layer, the passivation properties were slightly enhanced to τeff=1.4 ms and i-V_OC=704 mV. Subsequently, by combining the bilayer approach with a post HPT, τeff of 2.0
ms and an i-V_OC of 714 mV were achieved. Finally, by combining the bilayer approach with an intermediate HPT, the optimal passivation sample was deposited, with τeff of 2.4 ms and an i-V_OC of 720 mV on the flat <100> surface.
To gain a better understanding of the correlation between passivation qualities and the microstructure properties of (i)a-Si:H on flat <100> surface, the layers have been characterized mainly via Fourier-transform infrared spectroscopy (FTIR). From the analysis, it can be concluded that the passivation layer that contains
sufficient H and a higher fraction of monohydrides is beneficial for achieving a better passivation quality.

For the two-terminal tandem solar cells, bottom cells with (n)-contact on top are preferred due to the optical advantage of the perovskite top cells with the p-i-n configuration. Therefore, a first tandem cell with (n)a-Si:H has been fabricated in collaboration with TU Eindhoven resulting in 22.2% efficiency. Starting from
this first fabricated tandem cell, its main optical limitations have been identified by performing advanced optical simulations using GenPro4, and the main strategies to overcome these optical drawbacks have been defined. By optimizing the front anti-reflection layers (MgF₂ and ITO) thicknesses (at 100 nm and 20 nm, respectively), and reducing C₆₀ thickness from 20 to 10 nm, front reflections, and parasitic absorption can be minimized. Thus a gain of implied photocurrent density of 1.8 mA/cm² for the tandem cell was obtained.
Further, by implementing (n)nc-SiOx:H doped layer in the SHJ bottom cell, instead of standard (n)a-Si:H layer the reflection between the top and bottom cell is also reduced, and enhanced light incorporation into the bottom cell is obtained. By adopting all the above optimizations and also adjusting the perovskite
layer from 473 nm to 530 nm, a total improvement of 2.7 mA/cm² in implied photocurrent density with respect to the initial 22.2% tandem cell can be achieved.
After having identified different optically optimized SHJ bottom cells for tandem applications, both rear junction and front junction single-side-textured SHJ solar cells were fabricated. Firstly, the passivation quality of (i)a-Si:H/(n)-layer and (i)a-Si:H/(p)-layer on different (i)a-Si:H were investigated. Then RJ solar cells
with three different (n)-type layers [(n)nc-SiOx:H;(n)nc-Si:H;(n)a-Si:H)] have been fabricated with optimal thicknesses individuated from the tandem optical simulations. Furthermore, a tunnel recombination junction SHJ solar cell with a layer stack of (n)nc-Si:H/(p)nc-SiOx:H/(p)nc-Si:H has been fabricated and measured as well.

In conclusion, various doped contacts (both n- and p-type) were successfully implemented into SHJ solar cells, which delivered V_OCs range from 700 to 714 mV and FFs range from 77.8% to 80.9%. Therefore, different well-functioning SHJ solar cells have been developed and are ready to be implemented as bottom cells for high-efficiency tandem devices. ... Read more

Liquid metal embrittlement (LME) is a problem encountered in the resistance spot welding joining process of advanced high-strength steels in the automotive industry. Its occurrence reduces the mechanical performance of welds. The nature of resistance spot welding prevents in-situ characterisation of thermo-mechanical conditions causing LME. Laser-beam welding (LBW) under tension is proposed as an alternative method to analyse LME cracks growing during the welding process of a DP1000 dual-phase steel grade. The influence of global thermo-mechanical parameters on the degree of embrittlement is investigated through Gleeble hot tensile tests. LBW schedules are explored, and material characterisation used to find and prove LME crack occurrence. Finite element analysis with COMSOL is used to connect results from Gleeble hot tensile tests with results from LBW and relevance to RSW is outlined. Results show that a temperature dependent ductility trough is present between 750 and 900°C. The Fe-Zn system is further found to require specific mechanical conditions (stress and strain rate) to become susceptible to LME. The proposed LBW setup is found to be susceptible to LME, but not in a high enough severity to be detectable through SEM and EDS. Changes should be made to the loading setup of the LBW setup to induce LME crack growth to a sufficient degree to allow for in-situ monitoring of local thermo-mechanical conditions surrounding the crack. ... Read more

The strength-ductility trade-off has been a long standing dilemma in material science. With the use of laser surface treatments, effort has been made in order to obtain a heterogeneous material with a well defined architectured microstructure to optimize this trade-off. In the present study, it is investigated how the deformation behaviour of Martensite/Austenite steel microstructures in a Fe-25Ni-0.2C alloy can be tailored by the creation of patterned microstructures with localized laser treatment. Due to these treatments strong variations in microstructure are observed. Two different patterns are created (i) a dotted diagonal pattern and (ii) a dotted horizontal pattern which are both evaluated using as-quenched and tempered martensite as base material. The deformation behaviour of the patterned microstructures is investigated using both experiments and simulations. In the experimental approach characterization of the laser treated specimens is carried out using Optical Microscopy (OM) and micro-hardness measurements. The local deformation of the patterned microstructure is investigated using Digital Image Correlation (DIC). During deformation, strain partitioning is observed in the austenitic areas. In these areas mechanically induced martensitic transformation takes place, influencing the hardening of the material. The phase strength does not seem to influence the austenite stability in the analysed patterns, however it does show difference in hardness of the freshly formed martensite. For the simulation, Crystal Plasticity Finite Element Modelling (CPFEM) is used to describe the plastic deformation of the patterned material. Both the local and overall behaviour are investigated and validated with the experimentally obtained results. The simulation can serve as a tool to identify patterns which show promising deformation behaviour. ... Read more

Back pain is a major economic problem that can dramatically affect the quality of life. There are many causes that can lead to back pain, but intervertebral disc (IVD) degeneration is a recognised cause. The harmful effect of whole-body vibration (WBV) environment on the human intervertebral disc has been demonstrated to have an increased relative risk of failure. One of the treatments for a degenerative disc is spinal fusion procedure, in which the human IVD is replaced by a spinal fusion cage. In this thesis, the influence of WBV on custom-made spinal fusion cages was investigated.
With the advent of additive manufacturing lattice-based scaffolds were designed as spinal fusion implants and manufactured using a Polyet 3D-printer. The influence of the unit cell design on the build quality and the quasi-static mechanical properties as well as the dynamic WBV reaction was observed. Therefore, four different unit cell types were designed: body-centered cube (BC-cube), face-centered cube (FX-cube), truncated octahedron (T-octa) and negative Poisson cube (NP-cube). The influence of 3D-printing direction and scaffold porosity on print quality and mechanical properties were also investigated.
The build quality was determined by measuring the nominal size, determining the beam diameter and using μCT. The quasi-static mechanical properties were determined with the compression test. The dynamic response to WBV was determined with a mechanical test setup that imitates a base excitation model. Two cylindrical bases were printed as one sample together with the scaffold. The bases were embedded and mounted in the testing cups. A constant sinusoidal displacement to the lower embedding cup was applied and the accelerations of the input and response were measured. The difference between the response and input was compared and observed at which frequency the sample broke or reached the highest gain. This was done for all samples and the influence of unit cell type, 3D-printing direction and porosity on the dynamic response was observed.
The direction of 3D-printing had the greatest influence on the build quality, while the type of unit cell and porosity had a less obvious influence.
The compression test provided insight into which unit cell types have a higher elastic modulus, that the 3D-printing direction perpendicular to the test direction achieved higher E values, and that higher porosity, which is desirable for implants, resulted in a lower Young's modulus.
In terms of the dynamic response, the results of this work indicate that the FX cube design is the most robust and would improve the safety of the implant when exposed to WBV.
For the 3D-printing direction, it may be concluded that the perpendicular direction, i.e. the layers are perpendicular to the test direction, can best withstand the dynamic forces. The increase in porosity from 50% to 70% showed a decrease in resistance to the dynamic loading, with scaffolds failing more often and at lower frequencies. Understanding how different designs and porosity of the spinal fusion scaffolds influence the resistance to WBV conditions will help to develop safe and resistant implants which replace degenerated natural IVD.
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The formability of Advanced High Strength Steels is critical for their usability in automotive applications. It has been observed that the presence of hydrogen, even in concentrations of the order of 1 ppm, leads to a considerable drop in formability. Hydrogen atoms may get absorbed during steel-making and are known to get trapped at various sites in the lattice. When sufficient activation energy is made available, hydrogen atoms that are weakly trapped can diffuse towards critical regions in the microstructure, such as crack tips and voids, where one or more embrittlement mechanisms might be activated. On the other hand, a strongly trapped hydrogen atom remains immobile and plays no part in the embrittlement process. Precipitates of transition metals are known to be strong traps for hydrogen. It is speculated that by promoting the formation of strong traps in the microstructure, the amount of freely diffusible hydrogen can be limited, which would lead to an improvement in mechanical performance.

In this work, a combined ab-initio - experimental approach was used to study the absorption of hydrogen in dual-phase steel. Density Functional Theory (DFT) calculations were employed to study and compare the trapping of hydrogen by carbide and nitride of titanium and vanadium. A carbon or nitrogen vacancy in the bulk of the precipitate was found to be the most efficient trap site. When coupled with the vacancy formation energy, trapping was found to be more efficient in off-stoichiometric vanadium carbide and nitride than that in titanium carbide and nitride. To validate the theoretical findings, cyclic voltammetry experiments were conducted on two grades of DP800 steel with different concentrations of vanadium and titanium. The amount of diffusible hydrogen in the vanadium grade was found to be approximately 25 \% higher than that in the titanium grade. This was in contradiction to the theoretical results. Characterisation of the specimen post testing revealed that an oxide film had formed on the sample surface and while the film on vanadium grade was uniform and dense, that on titanium grade was sparse and irregular. It was evident that the oxide layer contributed to trapping of hydrogen, however the amount of hydrogen trapped by the oxide could not be specified. Overall, designing steels resistant to hydrogen embrittlement by promoting the formation of precipitates of a particular element is theoretically attainable, however, it was not possible to obtain experimental validation with the method employed. ... Read more

The urge for more sustainable and cost-efficient repair methods for damaged parts is growing every day. On the other hand, laser metal deposition (LMD) is gaining momentum. This flexible technique, where material is added in a layer-by-layer fashion with a minimal heat input offers solutions for a wide range of production and repair purposes. Especially restoration or replacement of large and complex high-performance parts with long production times could profit from these developments. Industrial gears are certainly such type of parts. Occasionally, unforeseen tooth breakage of gears occurs, often leading to the replacement of the parts. No reliable repair techniques have emerged yet, due to the desired material properties and the uneven distributed hardness seen in carburized gears. This study was initiated to examine the feasibility of LMD for the repair of industrial gears with the goal to reduce both material waste and down time while retaining or exceeding the required mechanical performance. The initial part of the research is focused on finding the best suitable material and deposition parameters to ensure the desired mechanical properties of the core and a good bonding of the structure to the gear. Multiple deposition parameters are varied followed by destructive and non-destructive testing. It was found that the mechanical properties of deposited structures Inconel 718 and its bonding to a carburized substrate can be adjusted by accurately selecting the deposition parameters. Large differences in outcome were obtained by changing the deposition direction between each layer and tuning of the shielding gas flow rate. In further investigation the effects on the mechanical behavior of a deposited tooth structure were studied by altering the thermal cycle during and after deposition. The tests are performed on a specially developed static load testing setup. The results show a negative correlation between the interpass temperature and both the yield and ultimate tensile strength. This effect is attributed to a lower cooling rate during solidification at higher interpass temperatures. Ultimately, the results of a deposited tooth with hard flanks and built on a case hardened base were compared to a case hardened gear tooth that was manufactured according to the regular production method. It is found that the deposited structure exhibits significantly lower yield strength. Nonetheless, the superior ductility of the newly deposited tooth structure in the presented work is considered as a promising method for future gear repair. ... Read more

For in-­situ resource utilisation start-up Maana Electric, an investigation was undertaken to determine whether a cover glass for solar panels can be produced using only desert sand as the raw material. During this investigation, the composition of desert sand, melt formation, processing temperatures, and mechanical and optical properties were considered. The composition of 18 desert sands was analysed by means of X­ray fluorescence and estimations of mineralogical composition were made, after which an attempt was made to melt the unmodified sand samples in a microwave furnace built for the purpose. Melt formation was further observed by melting binary combinations of store bought minerals that were found in the desert sands. The composition data and modelling of temperature-viscosity curves were employed to explore lowering the practical melting of the sand point by modification of the composition through benificiation. `Synthetic benificiated desert sand' was produced and melted based on the results. Glass samples produced were characterized using X­ray fluorescence, visual inspection, optical spectrometry, and fracture mirror analysis. It was found that about half of the desert sand samples assessed contain over 90 wt% silica, making it less feasible for use as raw material for glass due to high melting temperatures and/or large waste streams from benificiation, while sands containing larger fractions of carbonates and/or feldspars will form a melt at less than 1650 degrees Celsius if the SiO2 content is less than 55 wt%. Transmission of 85 % of ~550 nm wavelength light was shown to be possible for desert sand glass of 3 mm thickness if Fe2O3 content is lower than 0.1 wt%, while for the same transmission in the complete effective spectrum of silicon based solar cells the iron content needs to be lowered further. Known absorbing species such as Cr2O3, NiO and CuO were detected in desert sand in trace amounts, but were not present in the synthetic mixtures, the influence of these contaminants on transmission requires further research. Mechanical analysis was inconclusive due to a limited number and low quality of the samples produced, but a review of the literature implies that a Young's modulus of >70 GPa and flexural strength of >45 MPa are attainable in a glass produced from desert sand components. ... Read more

Brittle fracture in ferritic steels is a field in which a lot of research has been performed over the years. Most of this research has been done on uniaxially loaded specimens, such as single edge notched bend specimens. From these experiments, the maximum principal stress criterion is used in the Weibull distribution method as proposed by Beremin, and is observed to accurately predict the fracture toughness distribution. However, when multiaxial specimens are considered, the maximum principal stress criterion no longer accurately predicts the fracture toughness distribution. In this thesis, other failure criteria than the standard maximum principal stress are considered as a solution to this problem. Using the data provided in the 2006 paper "An Experimental Investigation of the Effect of Biaxial Loading on the Master Curve Transition Temperature in RPV Steels" by R. Link, A Joyce and C. Roe, the properties of Shoreham pressure vessel steel are obtained. Furthermore, the fracture toughness from the tested cruciform specimens allows for the reconstruction of the stress state around the crack tip during fracture. This is done by creating various cruciform specimens in the Abaqus finite element analysis program, which are loaded in accordance with the paper. The resulting stress state is used in the calibration of the Weibull parameters in the Weibull distribution. It was found that uniaxially loaded specimens show good agreement with the predicted failure probabilities. Additionally, it was confirmed that the biaxially loaded specimens do not show good agreement when the maximum principal stress is the failure criterion. When the failure criterion is altered so that only microcracks that do not experience large triaxiality contribute to fracture, good agreement is obtained for both the uniaxially and biaxially loaded specimens. Hence, it is found that triaxiality is very important for cleavage fracture, with high levels of triaxiality preventing microcracks from propagating. This leads to a proposed failure criterion where the maximum principal stress criterion is applied, and only elements that do not experience high levels of triaxiality contribute.
It is suggested to further test these conclusions under a variety of loading conditions, for which an alternate specimen is proposed. ... Read more

Gecko-inspired adhesives mimic the external structure of geckos with micropatterned surfaces and the internal structure by fabric reinforcement in soft elastomer adhesive pads. Previous research measured the friction forces of synthetic adhesives, with either an external or internal structure, mainly on hard substrates. Much less is known about the effects on static friction forces on soft substrates of adhesives with a combined external and internal structure and with a contact area beyond a centimetre square. We fabricated 40 by 40 mm adhesive pads (Epad = 2.1 +/- 0.1 MPa) from polydimethylsiloxane (PDMS) elastomer and tested them on two soft PDMS substrates (Esub = 2.6 +/- 0.2 MPa and 1.0 +/- 0.1 MPa). A colloidal lithographic approach was used to fabricate the external structures of the adhesive pads with microscale dimples with and without a terminal layer (TL). The internal structures were fabricated by reinforcement of the adhesive pads with carbon fibre fabric (CFF), which varied in the types of CFF weave and its orientation with respect to the substrate. We found that samples without an external structure generated lower friction on the softer substrates, whereas samples with micropatterned surface generated similar friction between the substrates, presumably due to mechanical interlocking between the external structure and soft substrates. Samples with microscale dimples without TL generated the lowest friction forces among all samples, likely due to limited initial contact with the substrates. Samples with microscale dimples with TL generated similar friction forces as samples without an external structure. Those samples with TL were not able to generate higher friction, due to their fabrication method which restricted the movement of the fibre bundles, hindering the stress redistribution along the sample during the measurements. Samples with an internal structure showed significant higher friction compared to samples without reinforcement, due to a better stress distribution along the samples, but generated similar friction forces among the types of CFF weave. ... Read more