In the transition toward a low-carbon energy system, hydrogen is increasingly recognised as a key energy carrier due to its versatility, environmental neutrality, and capacity to integrate renewable sources across industrial, transport, and residential sectors. Repurposing existing pipeline infrastructure for hydrogen transport offers immediate availability and cost advantages, yet hydrogen exposure can impair the fatigue performance of pipeline steels, potentially leading to brittle failures. Moreover, internal oxide layers that naturally form during long-term service may further influence fatigue behaviour, raising questions about the suitability of vintage steels for hydrogen applications.

This thesis examines the hydrogen-assisted fatigue behaviour of a vintage API 5L X52 pipeline steel, with particular focus on internal oxides formed after 16 years of natural-gas service. Optical Microscopy, surface-roughness measurements, and Raman Spectroscopy revealed a thick (36 ± 17 to 100 ± 27 μm), adherent, multi-layered oxide composed of magnetite (Fe3O4) and goethite (α-FeOOH), with local hematite (Fe2O3). Based on these findings, near-representative artificial oxide layers (AOLs) were produced with galvanostatic anodisation and validated for controlled in-situ gaseous fatigue testing.

Fatigue experiments in 100 bar H2 and N2 environments showed that hydrogen reduced fatigue life by ∼40% in polished specimens and ∼43% in AOL-coated specimens. The comparable magnitude of this reduction indicates that the presence of oxide layers does not impose an additional hydrogen-related penalty on fatigue life. AOLs decreased fatigue life by 52-54% relative to the polished surface condition, for nitrogen and hydrogen environments, respectively. This reduction highlights the accelerated effect of oxides on (hydrogen-assisted) fatigue degradation. Crack initiation was estimated using displacement-signal derivatives, ex-situ Alternating Current Potential Drop (ACPD), and Paris-law propagation calculations. While each method introduced uncertainties, results consistently indicated that hydrogen shortened the initiation phase, with oxide layers amplifying this effect by facilitating local hydrogen ingress through microcracks and pitting. Paris-law estimates a similar large reduction in crack-propagation cycles under hydrogen compared with nitrogen. At the same time, ex-situ ACPD measurements confirmed that most fatigue life was consumed during initiation. SEM fractography revealed a transition from ductile fracture in nitrogen to brittle transgranular fracture in hydrogen, independent of surface condition.

The novelty of this work lies in its combined, experimentally isolated assessment of hydrogen exposure and internal oxide layers, which are typically studied separately. The demonstrated hydrogen/oxide synergy represents a critical, previously overlooked factor in fitness-for-service evaluations of repurposed pipelines. Overall, the findings indicate that the combined presence of hydrogen and oxide layers markedly reduces fatigue life, underscoring the importance of surface condition and oxide integrity for the safe reuse of aged steel infrastructure for hydrogen transport. ... Read more

Studies on the effect of the prior austenite grain size on the phase transformation kinetics of bainite are contradictory. Literature explains these contradictions by the presence of two different types of nucleation sites, the prior austenite grain boundaries and the tips of previously formed bainitic ferrite sub-units. The difference in their activation energies for nucleation, ΔQ, is known to determine whether the phase transformation kinetics of bainite are accelerated by prior austenite grain refinement or coarsening. However, the factors that influence ΔQ are not entirely understood, which is the reason why the contradictory results regarding the effect of the prior austenite grain size on the phase transformation kinetics of bainite observed in the different studies cannot be explained yet.

This master thesis investigates the effect of the prior austenite grain size on the phase transformation kinetics of bainite at different transformation temperatures in a low-carbon high-silicon steel. The experiments were divided into two groups, with one group consisting of specimens with finer prior austenite grains and one with coarser prior austenite grains. Specimens were transformed to bainite at three different isothermal transformation temperatures, 440 °C, 410 °C and 380 °C. Bainite formation was investigated by in-situ synchrotron XRD experiments performed at DESY to study the evolution of the phase fractions, lattice parameters and microstrains of bainitic ferrite and austenite. Furthermore, microstructure investigations on the specimens transformed at DESY and additional interrupted quenching experiments were conducted to understand the effect of the prior austenite grain size and the isothermal transformation temperature on the microstructure and the sheaf morphology formed in the early stages of the phase transformation. Finally, simulations were performed to determine the effect of the prior austenite grain size and the transformation temperature on ΔQ.

The experiments showed that, by decreasing the transformation temperature, the phase transformation kinetics of the group with fine prior austenite grains were decelerated, whereas the phase transformation kinetics of the group with coarse prior austenite grains were accelerated. The simulations exhibited an increase in ΔQ as the isothermal transformation temperature was decreased, indicating that sheaf growth by successive nucleation events at the tips of previously formed sub-units becomes increasingly prevalent. While the specimens with coarse prior austenite grains provide more potential nucleation sites at the tips of previously formed sub-units, the specimens with fine prior austenite grains provide more nucleation sites for grain boundary nucleation, which explains the reverse effect of the transformation temperature on the phase transformation kinetics of the two experiment groups. Microstructure observations have shown that the effect of the transformation temperature on processes, such as carbon partitioning and the transition from upper to lower bainite, could play an important role in explaining the observed effect of the prior austenite grain size on the phase transformation kinetics of bainite. ... Read more

The global transition to sustainable energy has accelerated the demand for hydrogen as a
clean fuel source. Carbon steel pipelines play a vital role in hydrogen transport and storage infrastructure, but their potential susceptibility to hydrogen embrittlement poses a significant challenge.
Hydrogen ingress, facilitated by environmental and operational factors, undermines the
structural integrity of these steel pipelines, making it critical to develop strategies to mitigate
hydrogen-induced degradation. The formation of internal surface oxide layers on these steels
significantly influences hydrogen-material interaction processes, underscoring the need for
further investigation and understanding, a focus of this study.
This study focuses on a naturally formed oxide layer on pipeline steel API 5L X65 and its role in
influencing hydrogen permeation behaviour. Using characterization techniques including optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy, the composition and thickness of the oxide layer were studied.
Electrochemical hydrogen permeation experiments using the Devanathan–Stachurski (D-S)
cell were performed on bare steel samples (with oxide removed from the surface) and oxide-covered steel samples. The results showed a significant delay in hydrogen permeation in the
case of oxide-covered steel. An estimation of the hydrogen diffusion coefficient (D_eff) was
carried out, showing significantly lower values for oxide-covered steel (5.57×10⁻¹⁰ cm²/s and
7.13 × 10⁻¹¹ cm²/s) compared to bare steel (3.46 × 10⁻⁶ cm²/s).
Electrochemical analysis by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the surface before and after hydrogen charging revealed a significant degradation of the barrier properties of the oxide after charging. Further optimization of the experimental conditions during the D-S method will be necessary to verify that the oxide integrity is solely affected by hydrogen ingress and not by the potential or current applied during charging.
This research provides a better understanding of the properties of naturally formed oxide layers on pipeline steel and their role in mitigating hydrogen permeation. These insights contribute to a deeper understanding of the role of oxide layers in the hydrogen transportation steel infrastructure.
Recognizing the significance of these interactions is crucial for developing accurate
testing and qualification protocols to ensure the reliability and performance of these materials in hydrogen transport applications. ... Read more

In response to climate change mitigation and the pursuit of circularity, Tata Steel Netherlands plans to construct an Electric Arc Furnace (EAF) to replace the carbon-based blast furnace. Initially, the EAF will operate with a ratio of 70% Direct Reduced Iron (DRI) and 30% steel scrap, with future intentions to increase scrap input for enhanced circularity. However, the capacity of an EAF to effectively handle significant changes in the scrap ratio remains uncertain. This thesis includes a detailed literature review addressing this gap by exploring the implications of Scrap and DRI, in the EAF operations, covering various fundamentals of the EAF process - raw materials, operating regimes, melting practices, metallurgy, slag engineering, and furnace constructions, with a specific focus on Consteel technology. For optimal EAF operation, fine-tuning slag chemistry is essential to achieve the desired balance of slag basicity, foaming, volume, and ensuring long refractory life of the furnace. Slag composition can vary widely due to multiple process variables including the type of charge mix, charge composition, charge feeding rate, oxygen lancing, carbon injection/addition, fluxing agents, and refractory erosion. These variables cause slag composition and rates to be transient parameters that evolve throughout the heating process, leading to dynamic changes in all related phenomena and outcomes. To accurately capture these effects and realistically simulate EAF operations, especially in the absence of real plant data, a dynamic process model is essential. This thesis investigates the operational implications of varying the Scrap:DRI ratio in an EAF process with continuous raw material charging using the dynamic Effective Equilibrium Reaction Zone (EERZ) model. This model allows for a comprehensive analysis of the transient behavior of slag composition and its impact on EAF performance. This thesis examines three cases of charge mix: 70% DRI with 30% scrap, 50% DRI with 50% scrap, and 30% DRI with 70% scrap. Pilot EAF trial data will be used for the validation of the model. Overall, the thesis aims to provide insights into optimizing Scrap:DRI ratios for sustainable and efficient steelmaking. ... Read more

To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern.

This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens.

The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C.

TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase.

Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement.

Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms.

It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect. ... Read more

Steel manufacturing is a carbon intensive process, that is responsible for approximately 7% of the total global CO2 emissions. Therefore, TATA Steel IJmuiden aims to lower its carbon emissions. One way to bring down emissions, is to replace the existing blast furnace (BF) CO reduction process with the H2-based direct reduction of iron ore (DRI). The two most widely applied H2-DRI processes around the world are the low pressure MIDREX, abbreviated as MLP, and medium pressure HYL-Energiron, abbreviated as EMP. Since TATA Steel IJmuiden wants to study the switch from BF to H2-DRI steelmaking, it is relevant to gain insight into which gas phase reactions are dominant for both processes, into the direct reduction process itself and into the behaviour of the carburization reactions that improve the steel quality. In the gas phase reactions, it was seen that for the MLP process in situ reforming of natural gas can be a viable option before switching to a 100% H2 process. This may prove to be worthwhile in the early stages of H2-DRI steel production, when green H2 is still scarce and expensive. For EMP, internal reforming seems less of a possibility due to the high reaction rate for the reverse water gas-shift. When comparing both MLP and EMP, reaction rates are generally higher for EMP than for MLP and hence smaller reactor volumes are required for the EMP process to acquire the same amount of output. Direct reduction with H2 has a higher reaction rate than reduction with CO, while for the carburization reactions methane cracking was found to be the dominant reaction. Techno-economic scenarios for 100% H2-based DRI in which green H2 is imported are only feasible when H2-prices fall below €1.80/kg. Meanwhile, a scenario with an on-site electrolyzer powered by grey grid electricity only proves to be worthwhile for electricity prices lower than €20/MWh. The most promising techno-economic scenario, which includes an on-site electrolyzer and the construction of a wind farm just off the coast from the TATA Steel IJmuiden site, assumes an electricity price of €40/MWh. ... Read more

Space exploration depends on materials that can withstand extreme conditions, particularly for rocket thrusters. Ceramic matrix composites (CMCs) like carbon-carbon (C/C) and carbon-silicon carbide (C/SiC) have been widely used for rocket nozzle applications ,due to their thermal and mechanical properties. However, the demand for materials capable of higher temperature tolerance and reusability has shifted focus to ultra-high temperature ceramics (UHTCs) and UHTC matrix composites (UHTCMCs). Among UHTCs, ZrB₂ stands out for its excellent mechanical properties, oxidation resistance, and lower cost compared to HfB₂. To overcome ZrB₂'s inherent brittleness and enhance properties like densification and oxidation resistance, additives such as SiC and carbon fibers are introduced. SiC enhances densification, controls grain growth, and improves oxidation resistance via the formation of a SiO₂ protective layer, while carbon fibers improve mechanical strength, oxidation resistance, and reduce density. Spark Plasma Sintering (SPS) is a preferred fabrication method for its ability to rapidly densify materials while maintaining fine microstructures.
Although ZrB₂-SiC composites are extensively studied for high-temperature applications, the relationships between sintering parameters (e.g., temperature, pressure, and dwell time) and densification, microstructure, and mechanical properties remain underexplored. This study investigates these correlations using SPS and examines the impact of milling methods—high-energy milling with WC balls versus regular milling with ZrO₂ balls—on final material properties. The feasibility of incorporating short carbon fibers into the ZrB₂-SiC matrix is also assessed, focusing on the effects of preparation techniques and fiber length.
Fabrication insights revealed that increased sintering temperature generally improved densification due to enhanced atomic diffusion, grain boundary migration, and mass transport. High-energy WC milling achieved superior densification compared to ZrO₂ milling, with ZSW samples reaching a maximum relative density of 99.2%, versus 96.5% for ZSZ samples under similar conditions. ZSW samples, however, developed a secondary ZrO₂ phase due to more intense abrasion and oxygen diffusion during milling, while ZSZ samples maintained a finer microstructure, with an average grain size of 2.65 μm compared to 2.91 μm for ZSW. Attempts to incorporate 35 vol% short carbon fibers were unsuccessful under current sintering conditions, but improvements in fiber distribution were achieved with a rotary evaporator. Shorter fibers showed better structural integrity by reducing stress concentrations.
Mechanical properties were strongly influenced by sintering temperature. Higher temperatures caused grain coarsening, leading to reductions in hardness, flexural strength, and fracture toughness. For instance, ZSW hardness decreased from 14.33 GPa at 1950°C to 13.92 GPa at 2050°C, flexural strength declined from 407 MPa to 384 MPa, and fracture toughness dropped from 3.71 MPa·m¹/² to 3.58 MPa·m¹/². Milling methods also played a critical role; ZSW samples showed lower hardness and toughness due to the softer ZrO₂ phase and coarser grain sizes, with a maximum fracture toughness of 3.76 MPa·m¹/² compared to 3.97 MPa·m¹/² for ZSZ samples. However, ZSW samples exhibited comparable or higher flexural strength (384–516 MPa) due to ZrO₂’s transformation toughening effect, while ZSZ samples ranged from 317 to 476 MPa.
... Read more

The long-distance maritime transport sector plays one of the most relevant roles in the transport sector. Therefore, this project designs a modular solid hydrogen solution, studying the assumptions made in the literature and including new simplifications that reduce the computational costs of its simulation. Moreover, this project compares the viability of this solution with the different prototypes found on the market. ... Read more

Ceramics are being explored as an alternative material by ASML to replace their stainless steel 316L (SS316L) reticle masking blades. Ceramics offer distinct advantages over stainless steel in the semiconductor industry. While stainless steel exhibits good mechanical strength, ceramics excel in electrical insulation and thermal conductivity, making them essential for semiconductor manufacturing. Ceramics, with their lower thermal expansion coefficient, enhanced chemical stability, lightweight nature, and dielectric properties, are essential for improving the performance, miniaturization, and reliability of semiconductor devices. This research addresses the challenge of replacing the SS316L blades in ASML’s EUV lithography reticle masking with high-purity alumina (99.7%) through binder jetting. Concurrently, emphasis was placed on optimizing the existing REMA blade design for the binder jetting technique. This research further assesses the thermo-mechanical performance of the new blade suggesting the necessity of cooling channels in the new blades. High-purity alumina was chosen for its superior material properties compared to SS316L. The research also focuses on analyzing the impact of particle size, printing parameters, and sintering conditions on the densification and mechanical properties of binder-jetted alumina samples. Unimodal 20 μm, unimodal 10 μm, and trimodal (equal concentrations of 10, 5, and 2 μm) powder batches were printed using binder jetting. The trimodal samples with 90% binder saturation exhibited the best results in green body density (61.3%) and sintered body density (66.9%). The Young’s modulus and flexural strength achieved by the trimodal sample was also the highest with a value of 77.3 ± 4.9 GPa and flexural strength of 59.5 ± 3.2 MPa. To enhance densifications further, it is recommended to incorporate sintering additives in the powder mixture and/or utilize nanoparticle densifiers in the binder. Additionally, employing the discrete element method using smaller particles with a multimodal powder mixture is also recommended for achieving higher densification.
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Hydrogen as an alternative energy source has risen in popularity due to increased environmental awareness. Existing natural gas infrastructure is considered as a means to transport hydrogen, due to practicality and financial aspects. However, hydrogen can deteriorate the fatigue behaviour and thus induce premature failure. Since fatigue is a common failure mode in pipelines, a more thorough understanding of the effects of hydrogen on fatigue behaviour is required. In this work, the hydrogen fatigue of X60 pipeline steel and its girth welds was investigated through a combined approach of modelling and in-situ fatigue testing. A novel in-situ gaseous hydrogen charging fatigue set-up was developed, which involves a sample geometry that mimics a small-scale pipeline with high internal hydrogen gas pressure. The specimen geometry involved an internal circumferential notch that induces a stress concentration factor (Kt = 3.0) related to the worst case scenario for pipelines. The effect of hydrogen was investigated by measuring the onset of crack initiation and growth using a newly designed direct current potential drop setup which probes the outer surface of the specimen. A FEA modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens, as well as to determine the stress states in the material. Results showed that both materials experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life (37%) manifested solely in the crack growth phase; hydrogen accelerated the crack growth (factor 4). In contrast, the reduction in fatigue life (68%) of the weld metal was due to accelerated crack growth (factor 8) and a decrease in resistance to crack initiation (57%). Varying the hydrogen gas pressure from 70 barg to 150 barg did not cause any differences in the fatigue behaviour. The presence of hydrogen influenced the fracture mechanisms of both materials. The fracture path of the base metal transitioned from transgranular and ductile in nature, to a mixed-mode transgranular and intergranular quasi-cleavage fracture. The weld metal exhibited a similar transition, however in the inert environment some intergranular features were observed at the prior austenite grain boundaries. The presence of hydrogen reduced the crack tortuosity. This is associated with a decrease in roughness- and plasticity-induced crack closure, thereby accelerating the crack growth. It is inferred that hydrogen-enhanced localised plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) were the dominant types of hydrogen embrittlement mechanisms during fatigue of this pipeline material. It was concluded that the weld metal is more susceptible to hydrogen fatigue than the base metal in a gaseous hydrogen environment. The worst-case scenario for pipelines is in the case of weld defects. The weld defects involved in this work were macropores (0.5-1.0 mm) with a spheroid morphology. When these defects were located at the notch surface, the resistance to crack initiation decreased by 92% compared to non-porous specimens in nitrogen. The existing natural gas infrastructure could have accumulated similar flaws during service life, which would make them unreliable for safe hydrogen transport. The costs associated with the repurposing of these pipe segments could raise unexpected economic hurdles, hindering the transition to a hydrogen economy. ... Read more

In recent decades, high strength steels in thick sections have been increasingly used in offshore structures where they are subjected to harsh service conditions such as freezing temperatures and high static/dynamic loading. At these conditions they are susceptible to a transition from ductile failure to a dangerous brittle (cleavage) predominant type of failure, which occurs well before yielding. One of the main challenges in employing thick sections is the through-thickness heterogeneous variance of microstructures as a result of the processing route owing to a gradient of cooling rates from the surface to the bulk. As the material’s mechanical and fracture behaviour strongly depend on the microstructure, the through-thickness microstructural heterogeneity leads to a significant scatter in mechanical and fracture properties, which makes it difficult to predict and control cleavage fracture.

From the body of literature establishing microstructural dependence on cleavage failure, several features contributing to this type failure can be identified. Phases, grain size, grain boundary misorientations and the presence of secondary phase constituents can play a major role in failure through cleavage. Additionally, cleavage failure is also sensitive to the crack depth to width ratio (a/W). This study investigates the microstructural features contributing to cleavage failure in a 100 mm thick S690QT high strength steel plate by performing mechanical and fracture toughness tests. In order to improve mechanical properties and cleavage fracture toughness, an isoparametric study employing rapid cyclic heating with the objective of grain refinement was performed.

The steel plate has coarser prior austenite grain (PAG) sizes, and a larger area and number fraction of inclusions in the middle section. Additionally, segregation bands as a result of solute segregation during the solidification process were observed to be dispersed throughout the middle section. The middle section was also characterized by lower hardness compared to the top section. The detrimental effects of the middle section were evidenced by inferior low temperature tensile properties and cleavage fracture toughness. This was attributed to the larger PAG sizes, larger area and number fractions of inclusions, and segregation bands in the middle section. The specimen orientation with respect to the rolling direction was found to have no effects on the tensile properties. Additionally, different a/W geometries and notch orientations with respect to the rolling direction were used to investigate the role of constraint effect and rolling orientation, respectively, in the fracture behaviour. Shallow-notched specimens representative of the defects found in offshore structures demonstrated a higher fracture toughness than the deep-notched specimens. This was attributed to lower hydrostatic stresses at the crack tip, which reduces stress triaxiality. The isoparametric study resulted in average grain size reduction by 41% and proved to improve micro-hardness, low temperature tensile properties and cleavage fracture toughness by 5%, 13% and 41% respectively. Fractographic analysis on the fracture toughness specimens revealed the presence of O, C-rich regions which are known to promote brittle behaviour.
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Climate change is one of the top global issues that the United Nations has identified that can adversely impact people all around the globe. Moving towards a hydrogen economy can reduce greenhouse emissions produced from burning fossil fuels which is one of the biggest contributors to global warming and climate change. Hydrogen fuel has a high gravimetric density and being a clean fuel it has the potential to become a sustainable energy source for the growing market. However, its low volumetric density makes it a difficult fuel to store, thus making storage technologies in the hydrogen supply chain an important part. The current storage technologies, however, are impeded by shortcomings such as low hydrogen densities, extreme pressure and temperature operating conditions and inefficiencies during the storage process. Combining existing hydrogen storage technologies such as compressed hydrogen gas and metal hydrides with 2D materials comes across as an excellent option as they can complement each other in their functioning. In this regard, borophene is considered a viable material for hydrogen storage due to its lightweight, good thermal, mechanical and electrical properties. Most of the studies performed so far on hydrogen storage in borophene were on hydrogen physisorption via weak van der Waals forces. In this thesis work, the chemisorption of hydrogen on borophene via strong covalent bonds is studied. This is because borophene with chemisorbed hydrogen is more energetically stable than with physisorbed hydrogen. Also, chemisorbed hydrogen on borophene performs better than physisorbed hydrogen in terms of safety and long-term storage. Hydrogen chemisorption on pristine, defective (i.e. with single and double vacancies) and metal decorated borophene are studied in this work, using density functional theory (DFT) and nudged elastic band (NEB) calculations to compute quantities such as enthalpy and activation energy barriers of chemisorption. Using Bader charge analysis and partial density of states analysis, it was observed that the addition of charge on the hydrogen bond weakens the bond, expediting the chemisorption reaction. The charge transfer from borophene to the chemisorbed H atoms was found to stabilize the final chemisorbed state. In the case of metal decorations, there is an additional steric factor that influences the activation barrier height for chemisorption. In general, metal decorations performed better than pristine and defective borophene systems in terms of the desorption barrier height, among which double decorated K atoms on borophene substrate had the best performance, improving the barrier height by 20% compared to the corresponding value for the pristine system. ... Read more

Trivalent chromium (Cr(III)) is one of the most promising non-toxic replacements for hexavalent chromium (Cr(VI)) coatings in the steel packaging industry. The application of a chromium layer to packaging steel is essential for providing a protective layer on the steel packaging’s external surface area, which prevents corrosion during its use. However, the deposition process of Cr-metal from Cr(III) solution cannot produce a deposit thicker than 10 μm with sufficient corrosion and wear resistance without the application of a complexing agent. This study provides both an experimental research as an comparative computational model. Experiments are set up to analyse the effect of the formate ion HCOO(-) concentration as a complexing agent in Cr_2(SO_4)_3 electrolytes, which results are compared to the output of the used computational model. The results provide insights into the initial composition of species in plating electrolytes, which is essential for the determination of the deposition mechanism from Cr(III) electrolytes to improve the electroplating process. Ligand exchange and bonding with HCOO(-) in chromium complexes is studied with Ultraviolet-visible light (UV-VIS) spectroscopy and Attenuated Total Reflectance Fourier InfraRed spectroscopy (ATR-FTIR) at various concentrations of HCOO(-). Additionally, computational modelling is performed with Density functional theory (DFT) to predict the spontaneous character of various ligand substitutions, and simulations of spectroscopic spectra are performed as a reference to mimic the experimentally observed data. It was found that ligand exchange with HCOO(-) in Cr_2(SO_4)_3 solutions occurs spontaneously within two days at elevated temperatures at the analysed concentrations. The experimental data shows, there is a transition point in the complex formation between the ratio of [Cr(3+)]:[ HCOO(-)] is respectively [1]:[1.61] and [1]:[3.32]. This indicates that the complex formation decreases past the ratio of [Cr(3+)]:[ HCOO(-) ] = c.a.[1]:[3] and that the concentration at which the most chromium-formate complexes can be observed is within the range of [Cr(3+)]:[ HCOO(-) ] = [1]:[1.61] to [Cr(3+)]:[ HCOO(-) ]= [1]:[3.32]. The formation of chromium-formate complexes is beneficial for the deposition process, as the addition of formate increases the amount of chromium deposited on the steel surface. Furthermore, it is found that the presence of SO_4(-2), originating from the chromium salt, is beneficial for the ligand exchange of HCOO(-). Also, the bonding of formate to chromium is found to be as monodentate binding. ... Read more

The distribution of multiple materials within a single structure is a strategy that various biological systems rely on to achieve outstanding mechanical performances. These biological examples illustrate the effective utilization of hard rigid and soft flexible materials in particular. The proper composition of such hard-soft materials exceeds the structural limitations found in their individual material counterparts. The manufacturing of hard-soft material structures is especially relevant today due to recent developments in additive manufacturing that certify the technology with local material-specific functionalities and enlarged design spaces. However, to unravel the next generation of unprecedented structural performance, today’s state of engineering and research has yet to overcome the challenges encountered in multi-material design. The dissimilar material junctions within multi-material structures are prone to load transmissions, so they carry a crucial structural responsibility. Therefore, the interface design process must be subjected to representative interface characteristics which are often overlooked in the literature. In this work, we present a method to characterize and model multi-material structures to provide an optimal interface design in terms of the multi-material’s joining strength.

We consider the joining strength of 3D printed hard Verocyan and soft Agilus30 by its fundamental joining principles of material bonding and mechanical interlocking. Material bonding is characterized by the extent of allowable traction between the two materials. We experimentally quantify the loading-dependent critical stress at which interface debonding initiates through mapping of digital image correlation deformations on a finite element model. We numerically define the extent of mechanical interlocking by the force required to achieve an unlocked multi-material state. The finite element models contain experimentally calibrated elastoplastic and hyperelastic material models to represent the hard and soft material behaviors, respectively. Subsequently, a structural optimization based on a genetic algorithm iteratively updates a constrained parametrized interface design according to material bonding and mechanical interlocking objectives.

The numerical evaluations of calibrated hard and soft material characteristics show good agreement in structural response with their real-world equivalents. The digital image correlation deformation method successfully acquires the loading-dependent critical stresses at which the two materials debond from one another. The finite element analyses of individual joining principles adequately determine a design’s material bonding and mechanical interlocking performances. The optimization’s objective function value evolution suggests a trade-off in joining contributions where mechanical interlocking maximizes performance in more shallow, wider interface designs, whereas material bonding performs better in narrow, deeper ones. Validation experiments illustrate the dominating contribution of material bonding in A30-VC structures. Optimizing for two distinct hypotheses of interface failure equations shows no significant difference in physical joining strength. However, they do support the concept that the interface characteristics affect the optimal joining shape. Despite adequate estimation of the individual joining principle performances, a more accurate approximation of the multi-material physical joining strength necessitates the consideration of the effects induced by the interaction of material bonding and mechanical interlocking. Nonetheless, this work underlines the emphasis regarding interface characteristics in the promising structures of multi-material. ... Read more

The current industrial application of carbon capture utilization and storage (CCUS) is limited due to technological drawbacks such as high energy demand and environmental pollution. Ionic liquids (ILs) and deep eutectic solvents (DESs) are considered promising alternative solvents for the capture of carbon dioxide (CO2). DESs are often characterized by high viscosities, which hinders industrial application. This problem might be solved by mixing the DES with an organic solvent. This study aims to assess the DESs choline chloride-ethylene glycol (ethaline) and choline chloride-urea (reline) mixed with methanol and propylene carbonate (PC) for their suitability as a medium for the combined capture and electrochemical conversion of CO2. Molecular dynamics (MD) simulations are performed to obtain the densities, the viscosities, the self-diffusivities, the ionic conductivities and insight into the molecular interactions of these mixtures. Independent MD simulations are performed of these mixtures with low concentrations of the solutes CO2, oxalic acid and formic acid. Complementary studies within the Bio-cel project are conducted to characterize the solubility and electrochemical reaction of CO2 and the techno-economics.
The viscosities of the mixtures monotonically decrease for an increase of mole fraction of organic solvent, which is benign for the application of CCUS. The self-diffusivities of all constituents increase monotonically for an increase of mole fraction of organic solvent. The ionic conductivity is calculated based on the ion self-diffusivities. Ionic conductivity optima are found at a mole fraction of DES of approximately 0.6 for ethaline-PC and approximately 0.2 for ethaline-methanol and reline-methanol. For higher mole fractions of organic solvent, the ionic conductivity decreases due to a depletion of ions. Radial distribution functions (RDFs) are used to analyse the intermolecular interactions. RDF peaks between chloride-choline and chloride-ethylene glycol show an increase for an increasing mole fraction of organic solvent, which was unexpected. The numbers of hydrogen bonds decrease for addition of methanol to pure deep eutectic solvent. For addition of propylene carbonate, this decrease is less pronounced. The depletion of hydrogen bonds at low mole fractions of deep eutectic solvent is in correspondence with the decrease in viscosity and increase in self-diffusivities. The results indicate that, for the studied properties, deep eutectic solvents mixed with organic solvents are more favourable than pure deep eutectic solvents for the absorption and electrochemical conversion of CO2. ... Read more

With fossil fuels being phased out and growing global interest in a hydrogen economy, there is demand for re-purposing existing pipelines for transportation of hydrogen gas. However, hydrogen is known to have adverse affects on the properties of steels. Hydrogen will dissolve in the steel matrix and contribute to a reduction of mechanical properties, causing Hydrogen Embrittlement (HE). There is currently a knowledge gap about the behaviour of pipeline steels and their welds in a gaseous hydrogen environment that prevents re-purposing of pipelines for hydrogen transport. In this work, a combined approach of modelling and in-situ mechanical testing was used to assess the HE susceptibility of X60 pipeline steel and its girth welds. To this end, a novel tensile setup featuring in-situ charging with high pressure H2 gas and a sample geometry representing miniature pipelines was developed and validated. To our best knowledge, this setup design has not been reported elsewhere, making this a breakthrough design. An FEA modelling approach was used to estimate the pre-charging duration as well as to gain an insight into the stress states inside the notched samples. It was found that both the base and weld metals lose ductility when subjected to gaseous H2. The base metal showed 27% loss of ductility when subjected to 100 bar H2, which further increased to 40% in notched samples. A trend of increasing loss of ductility was found with increasing pressure for the weld metal, which showed up to 14% loss of ductility at 100 bar H2. The weld metal also retained more of its reduction in cross-sectional area after fracture in H2 as compared to N2 than the base metal. Other characteristics like yield strength and UTS were not affected by the hydrogen gas. The fracture mechanism in the both metals was found to change from microvoid coalescence (MVC) fracture to quasi-cleavage (QC) fracture. The base metal fracture mechanism changed to QC completely, while the weld metal only showed partial QC fracture. In the base metal, ductile fracture mechanisms like HELP and possibly AIDE were found to be dominant even in the QC fracture mode. It was concluded that the weld metal is less susceptible to HE than the base metal in a gaseous hydrogen environment. For both metals, HE effects were only observed at high amounts of plastic strain, which is outside of the operating conditions of a pipeline. However, before pipelines can be repurposed for hydrogen transport, fatigue testing should be performed to assess the influence of existing defects and cyclic loading conditions on the HE performance of both steels. ... Read more

Fossil fuels have been the primary source of rising energy requirements for humankind. However, the extensive use of fossil fuels has led to an increase in Earth's surface temperature. To tackle rising energy demands and the increase in Earth's surface temperature, various organizations like Inter-governmental Panel for Climate Change and the European Environmental Agency have suggested the use of renewable energy as an alternative energy supply. E.g., the use of hydrogen as an alternative fuel in transportation will reduce greenhouse gas emissions. Besides, converting CO2 and N2 to fuels and industrial feedstock like CO or NH¬3 can curb the Earth's increasing surface temperature. As a result of this, in this thesis, the catalysts for the synthesis of hydrogen from water-splitting (hydrogen evolution reaction- HER), conversion of CO2 to CO via carbon dioxide reduction reaction (CO2RR), and reduction of N2 from air to NH3 (Nitrogen reduction reaction-N2RR) are studied. The conventional catalysts used for these reactions are Pt for HER, Cu for CO2RR Cu, and Ru for N2RR. Although these catalysts are active and exhibit a high yield of products, they have some disadvantages, such as the long-term availability and cost of Pt and Ru. On the other hand, Cu suffers from the low selectivity for the conversion from CO2 to CO. To overcome these disadvantages; scientists have developed a new kind of catalyst with a higher specific activity, known as the Single-Atom Catalysts (SAC). The SACs use fewer precious elements than the conventional bulk catalysts without compromising the activity. The use of binding energy (EB) as a descriptor for the reactions mentioned has been proven in the literature. Therefore, EB is used in this thesis to predict novel SACs through high-throughput DFT calculations using 3-N doped graphene as the substrate. The descriptor for HER is the EB of H atom, for CO2RR is EB of CO, and that of N2RR is EB of N on the respective catalyst surfaces. These calculated binding energies are compared against the descriptor EB on the conventional catalysts to obtain the novel SACs. With EB as the descriptor, the candidate catalysts for HER are B, Cr, Mn, Fe, Co, Ni, Ge, Ru, In, Sb, La, and Pb. The candidate catalysts for N2RR are Ru, Mo, and Cr. The candidate catalysts for CO2RR are Mg, Al, Ca, Zn and Se,. In addition to this, the charge dissipation of the adsorbent species on the SAC and the effect of atomic size on the EB is studied. It was seen that the computational predictions go hand in hand with the predictions of existing experiments for HER and CO2RR. ... Read more

This thesis demonstrates the feasibility of Extreme-ultraviolet (XUV) high-harmonic generation from structured silica, and was performed at the Advanced Research Centre for Nanolithography (ARCNL). The project focuses on High-harmonic generation (HHG) from condensed matter, and further explores the possibility of high-harmonic generation from micro- and nano-structured silica. HHG from solids was discovered less than a decade ago, and it is expected to be a new source for coherent ultrafast pulses, showing potential in many applications such as HHG spectroscopy, imaging and photonic devices. By generating high-harmonics in structured solids, the capability to control HHG properties by engineering the topology of the surface on solids has been demonstrated. Previous research has demonstrated the control of HHG in the visible light regime by generating from structured semiconductors. In this project, we aim to generate high-harmonics in the XUV regime, and control the properties of XUV light. Our work shows the potential of using structured dielectric materials as new XUV optics, and applications on HHG high-resolution lens-less imaging. ... Read more

Metamaterials are a new class of materials where the properties crucially depend on the design of the unit cell that is periodically repeated in space. In this study a new metamaterial unit cell concept has been proposed, inspired by a class of space structures known as deployable masts. The ability of these structures to contract to a fraction of their size made them suitable candidates for energy absorbing applications. One of the main design targets of energy absorbers is the ability to tune the material response to specific applications. Tunability of the mechanical response of the metamaterial concept requires deep understanding of the influence of design parameters. The prime focus of this study was to gain this understanding via data-driven insights. Conventionally, the design of a new material is carried out by making educated guesses about the design parameters and subsequently performing expensive and time-consuming experiments. In this work computational simulations were utilized to create databases of mechanical responses. These databases are later used to model the relationship between the inputs and the output response. This generated the issue of how and what method to use to effectively determine this relationship. This work explored state-of-the-art machine learning methods to enhance a recently proposed data-driven framework with thegoal of designing a new super-compressible metamaterial with large energy absorption. Importantly, the data-driven design process included the influence of manufacturing imperfections on the mechanical response of the metamaterial.The study revealed that by tuning the design parameters, significantly different mechanical response of the structure was achievable. The proposed learning model has enabled mapping of the influence of design parameters in the design space, moreover the sensitivity to those parameters varied across the design space. The increased energy absorption has been attributed to the resistance to bending of the main load carrying components of the design. It was demonstrated that the number of those components and the elastic modulus were scaling factors for the quantities of interest. Based on the insightsgained, a unit cell metamaterial design with significantly improved energy absorbing capability was proposed. ... Read more