P. Dey
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1
Two-dimensional (2D) transition metal dichalcogenides (TMDs) offer a platform for nanoscale electronic devices due to their atomically thin geometry and semiconducting band structure. This thesis investigates the fabrication, assembly, and electrical characterization of TMD-based field-effect transistor (FET) devices using MoS2, MoSe2, and WS2 flakes. The primary objective is to evaluate how different transfer method and electrode architecture influence the interface quality of the device inturn having a significant impact on the electrical transport behaviour in 2D semiconductor devices.
In this work, the TMD material was prepared using both bottom-up and top-down approaches. For the transfer of the flakes, two different dry transfer techniques were explored involving the use of either only a PDMS stamp or a PC/PDMS stamps. The Device fabrication itself was performed in Kavli Nanolabs, which provided the cleanroom environment for the process. The whole process of the device fabrication process included several steps including substrate cleaning using fuming nitric acid, organic solvent cleaning, resist coating, photolithography or electron-beam lithography, metal deposition, lift-off, and oxygen plasma cleaning. Three main electrode geometries were fabricated in this work, including two-terminal, four-terminal, and interdigitated structures. The electrical contacts consisted of Ti/Au stacks with thicknesses of 5 nm and 30 nm, respectively, deposited by electron-beam evaporation.
Electrical characterization was conducted at room temperature (∼ 300 K) under vacuum conditions, and these measurements included mainly current-voltage (I-V) and gate-sweep measurements, and four probe measurements (V-I). Across the measured devices, the absolute drain currents ranged from approximately 10−12 A to 10−9 A. For the prepatterned interdigitated device incorporating a 2D MoSe2 flake, the total resistance was calculated to be 2.33 × 1011 Ωand the same device architecture with a nanoscroll device exhibited a resistance of 6.91 × 1011 Ω. For the pre-patterned MoSe2 device, the resistance was measured to be 1.62 GΩ, and it displayed p-type semiconducting behaviour; while the pre-patterned WS2 device exhibited a significantly higher resistance of 1.5 × 1012 Ω and it showed n-type behaviour. The subthreshold swing (SS) and field-effect mobility were also extracted for both these two-electrode prepatterned devices. For the MoSe2 device, the SS was calculated to be 6877.7 mV/dec, with a field-effect mobility of 0.0067 cm2 V−1 s−1. For the WS2 device, the SS was 488.1 mV/dec and the extracted field-effect mobility was 4.1 × 10−4 cm2 V−1 s−1. Finally, resistance measurements were performed on a post-patterned WS2 device, yielding a resistance of 1.65 × 109 Ω. This device exhibited ambipolar semiconducting behaviour, with n-type conduction being dominant. The subthreshold swing for electron transport was calculated to be 10726 mV/dec, and the corresponding field-effect mobility was 0.04 cm2 V−1 s−1. Overall, the results indicated a lower resistance for the post-patterned WS2 device compared to its pre-patterned counterpart, but the results are not comparable since the flake geometry, thickness and quality varied between the pre-patterned and post-patterned devices. However, all gate-sweep measurements demonstrated limited electrostatic modulation, characterized by weak on-off ratios and large subthreshold swing values, which is consistent with suppressed carrier injection. The results indicate that electrical transport in the fabricated devices is dominated by several extrinsic factors, including contact resistance, ambient conditions, interfacial contamination, and measurement constraints, rather than intrinsic TMD channel properties. These require future work and optimization.
Overall this work highlights the challenges associated with fabricating and measuring electrical properties of 2D semiconductor devices and provides practical guidance for improving fabrication workflows, interface engineering, and measurement strategies for future exploration of 1-D TMD-based
electronics.
...
In this work, the TMD material was prepared using both bottom-up and top-down approaches. For the transfer of the flakes, two different dry transfer techniques were explored involving the use of either only a PDMS stamp or a PC/PDMS stamps. The Device fabrication itself was performed in Kavli Nanolabs, which provided the cleanroom environment for the process. The whole process of the device fabrication process included several steps including substrate cleaning using fuming nitric acid, organic solvent cleaning, resist coating, photolithography or electron-beam lithography, metal deposition, lift-off, and oxygen plasma cleaning. Three main electrode geometries were fabricated in this work, including two-terminal, four-terminal, and interdigitated structures. The electrical contacts consisted of Ti/Au stacks with thicknesses of 5 nm and 30 nm, respectively, deposited by electron-beam evaporation.
Electrical characterization was conducted at room temperature (∼ 300 K) under vacuum conditions, and these measurements included mainly current-voltage (I-V) and gate-sweep measurements, and four probe measurements (V-I). Across the measured devices, the absolute drain currents ranged from approximately 10−12 A to 10−9 A. For the prepatterned interdigitated device incorporating a 2D MoSe2 flake, the total resistance was calculated to be 2.33 × 1011 Ωand the same device architecture with a nanoscroll device exhibited a resistance of 6.91 × 1011 Ω. For the pre-patterned MoSe2 device, the resistance was measured to be 1.62 GΩ, and it displayed p-type semiconducting behaviour; while the pre-patterned WS2 device exhibited a significantly higher resistance of 1.5 × 1012 Ω and it showed n-type behaviour. The subthreshold swing (SS) and field-effect mobility were also extracted for both these two-electrode prepatterned devices. For the MoSe2 device, the SS was calculated to be 6877.7 mV/dec, with a field-effect mobility of 0.0067 cm2 V−1 s−1. For the WS2 device, the SS was 488.1 mV/dec and the extracted field-effect mobility was 4.1 × 10−4 cm2 V−1 s−1. Finally, resistance measurements were performed on a post-patterned WS2 device, yielding a resistance of 1.65 × 109 Ω. This device exhibited ambipolar semiconducting behaviour, with n-type conduction being dominant. The subthreshold swing for electron transport was calculated to be 10726 mV/dec, and the corresponding field-effect mobility was 0.04 cm2 V−1 s−1. Overall, the results indicated a lower resistance for the post-patterned WS2 device compared to its pre-patterned counterpart, but the results are not comparable since the flake geometry, thickness and quality varied between the pre-patterned and post-patterned devices. However, all gate-sweep measurements demonstrated limited electrostatic modulation, characterized by weak on-off ratios and large subthreshold swing values, which is consistent with suppressed carrier injection. The results indicate that electrical transport in the fabricated devices is dominated by several extrinsic factors, including contact resistance, ambient conditions, interfacial contamination, and measurement constraints, rather than intrinsic TMD channel properties. These require future work and optimization.
Overall this work highlights the challenges associated with fabricating and measuring electrical properties of 2D semiconductor devices and provides practical guidance for improving fabrication workflows, interface engineering, and measurement strategies for future exploration of 1-D TMD-based
electronics.
...
Two-dimensional (2D) transition metal dichalcogenides (TMDs) offer a platform for nanoscale electronic devices due to their atomically thin geometry and semiconducting band structure. This thesis investigates the fabrication, assembly, and electrical characterization of TMD-based field-effect transistor (FET) devices using MoS2, MoSe2, and WS2 flakes. The primary objective is to evaluate how different transfer method and electrode architecture influence the interface quality of the device inturn having a significant impact on the electrical transport behaviour in 2D semiconductor devices.
In this work, the TMD material was prepared using both bottom-up and top-down approaches. For the transfer of the flakes, two different dry transfer techniques were explored involving the use of either only a PDMS stamp or a PC/PDMS stamps. The Device fabrication itself was performed in Kavli Nanolabs, which provided the cleanroom environment for the process. The whole process of the device fabrication process included several steps including substrate cleaning using fuming nitric acid, organic solvent cleaning, resist coating, photolithography or electron-beam lithography, metal deposition, lift-off, and oxygen plasma cleaning. Three main electrode geometries were fabricated in this work, including two-terminal, four-terminal, and interdigitated structures. The electrical contacts consisted of Ti/Au stacks with thicknesses of 5 nm and 30 nm, respectively, deposited by electron-beam evaporation.
Electrical characterization was conducted at room temperature (∼ 300 K) under vacuum conditions, and these measurements included mainly current-voltage (I-V) and gate-sweep measurements, and four probe measurements (V-I). Across the measured devices, the absolute drain currents ranged from approximately 10−12 A to 10−9 A. For the prepatterned interdigitated device incorporating a 2D MoSe2 flake, the total resistance was calculated to be 2.33 × 1011 Ωand the same device architecture with a nanoscroll device exhibited a resistance of 6.91 × 1011 Ω. For the pre-patterned MoSe2 device, the resistance was measured to be 1.62 GΩ, and it displayed p-type semiconducting behaviour; while the pre-patterned WS2 device exhibited a significantly higher resistance of 1.5 × 1012 Ω and it showed n-type behaviour. The subthreshold swing (SS) and field-effect mobility were also extracted for both these two-electrode prepatterned devices. For the MoSe2 device, the SS was calculated to be 6877.7 mV/dec, with a field-effect mobility of 0.0067 cm2 V−1 s−1. For the WS2 device, the SS was 488.1 mV/dec and the extracted field-effect mobility was 4.1 × 10−4 cm2 V−1 s−1. Finally, resistance measurements were performed on a post-patterned WS2 device, yielding a resistance of 1.65 × 109 Ω. This device exhibited ambipolar semiconducting behaviour, with n-type conduction being dominant. The subthreshold swing for electron transport was calculated to be 10726 mV/dec, and the corresponding field-effect mobility was 0.04 cm2 V−1 s−1. Overall, the results indicated a lower resistance for the post-patterned WS2 device compared to its pre-patterned counterpart, but the results are not comparable since the flake geometry, thickness and quality varied between the pre-patterned and post-patterned devices. However, all gate-sweep measurements demonstrated limited electrostatic modulation, characterized by weak on-off ratios and large subthreshold swing values, which is consistent with suppressed carrier injection. The results indicate that electrical transport in the fabricated devices is dominated by several extrinsic factors, including contact resistance, ambient conditions, interfacial contamination, and measurement constraints, rather than intrinsic TMD channel properties. These require future work and optimization.
Overall this work highlights the challenges associated with fabricating and measuring electrical properties of 2D semiconductor devices and provides practical guidance for improving fabrication workflows, interface engineering, and measurement strategies for future exploration of 1-D TMD-based
electronics.
In this work, the TMD material was prepared using both bottom-up and top-down approaches. For the transfer of the flakes, two different dry transfer techniques were explored involving the use of either only a PDMS stamp or a PC/PDMS stamps. The Device fabrication itself was performed in Kavli Nanolabs, which provided the cleanroom environment for the process. The whole process of the device fabrication process included several steps including substrate cleaning using fuming nitric acid, organic solvent cleaning, resist coating, photolithography or electron-beam lithography, metal deposition, lift-off, and oxygen plasma cleaning. Three main electrode geometries were fabricated in this work, including two-terminal, four-terminal, and interdigitated structures. The electrical contacts consisted of Ti/Au stacks with thicknesses of 5 nm and 30 nm, respectively, deposited by electron-beam evaporation.
Electrical characterization was conducted at room temperature (∼ 300 K) under vacuum conditions, and these measurements included mainly current-voltage (I-V) and gate-sweep measurements, and four probe measurements (V-I). Across the measured devices, the absolute drain currents ranged from approximately 10−12 A to 10−9 A. For the prepatterned interdigitated device incorporating a 2D MoSe2 flake, the total resistance was calculated to be 2.33 × 1011 Ωand the same device architecture with a nanoscroll device exhibited a resistance of 6.91 × 1011 Ω. For the pre-patterned MoSe2 device, the resistance was measured to be 1.62 GΩ, and it displayed p-type semiconducting behaviour; while the pre-patterned WS2 device exhibited a significantly higher resistance of 1.5 × 1012 Ω and it showed n-type behaviour. The subthreshold swing (SS) and field-effect mobility were also extracted for both these two-electrode prepatterned devices. For the MoSe2 device, the SS was calculated to be 6877.7 mV/dec, with a field-effect mobility of 0.0067 cm2 V−1 s−1. For the WS2 device, the SS was 488.1 mV/dec and the extracted field-effect mobility was 4.1 × 10−4 cm2 V−1 s−1. Finally, resistance measurements were performed on a post-patterned WS2 device, yielding a resistance of 1.65 × 109 Ω. This device exhibited ambipolar semiconducting behaviour, with n-type conduction being dominant. The subthreshold swing for electron transport was calculated to be 10726 mV/dec, and the corresponding field-effect mobility was 0.04 cm2 V−1 s−1. Overall, the results indicated a lower resistance for the post-patterned WS2 device compared to its pre-patterned counterpart, but the results are not comparable since the flake geometry, thickness and quality varied between the pre-patterned and post-patterned devices. However, all gate-sweep measurements demonstrated limited electrostatic modulation, characterized by weak on-off ratios and large subthreshold swing values, which is consistent with suppressed carrier injection. The results indicate that electrical transport in the fabricated devices is dominated by several extrinsic factors, including contact resistance, ambient conditions, interfacial contamination, and measurement constraints, rather than intrinsic TMD channel properties. These require future work and optimization.
Overall this work highlights the challenges associated with fabricating and measuring electrical properties of 2D semiconductor devices and provides practical guidance for improving fabrication workflows, interface engineering, and measurement strategies for future exploration of 1-D TMD-based
electronics.
Hydrogen Diffusion in Multi-Principal Element Alloys
A Kinetic Monte Carlo and Machine Learning Framework for Hydrogen Diffusion in Chemically Complex BCC Alloys
Hydrogen is a promising energy carrier for sustainable energy systems, but its interaction with metallic structures poses significant challenges, particularly hydrogen embrittlement. Multi-principal element alloys (MPEAs), including high- and medium-entropy alloys, offer resistance to hydrogen embrittlement and potential for hydrogen storage due to their disordered atomic lattices, which create effective trapping sites for hydrogen. However, the vast compositional space of MPEAs limits experimental exploration, and conventional simulation approaches are often too computationally intensive for high-throughput screening. This thesis introduces an efficient and comprehensive computational framework for predicting hydrogen diffusivity in body-centred cubic (BCC) MPEAs. The diffusion energy landscape is characterised by statistical parameters that describe the distribution of saddle point and well-energies. Through kinetic Monte Carlo (KMC) simulations, a large dataset of hydrogen diffusivity was generated using synthetic energy landscapes defined by these statistical parameters. Machine learning symbolic regression (MLSR) was then employed to derive analytical expressions that relate the statistical descriptors to macroscopic diffusivity. To apply the model to real alloys, hydrogen diffusivity is obtained through the MLSR expressions based on energy landscape statistics that were determined using climbing-image nudged elastic band (CI-NEB) calculations with universal machine learning interatomic potentials (uMLIPs). The predictions were validated against molecular dynamics (MD) simulations, showing reasonable agreement. This framework enables fast, scalable prediction of hydrogen diffusion in complex alloys, supporting accelerated materials discovery for hydrogen-related applications.
...
Hydrogen is a promising energy carrier for sustainable energy systems, but its interaction with metallic structures poses significant challenges, particularly hydrogen embrittlement. Multi-principal element alloys (MPEAs), including high- and medium-entropy alloys, offer resistance to hydrogen embrittlement and potential for hydrogen storage due to their disordered atomic lattices, which create effective trapping sites for hydrogen. However, the vast compositional space of MPEAs limits experimental exploration, and conventional simulation approaches are often too computationally intensive for high-throughput screening. This thesis introduces an efficient and comprehensive computational framework for predicting hydrogen diffusivity in body-centred cubic (BCC) MPEAs. The diffusion energy landscape is characterised by statistical parameters that describe the distribution of saddle point and well-energies. Through kinetic Monte Carlo (KMC) simulations, a large dataset of hydrogen diffusivity was generated using synthetic energy landscapes defined by these statistical parameters. Machine learning symbolic regression (MLSR) was then employed to derive analytical expressions that relate the statistical descriptors to macroscopic diffusivity. To apply the model to real alloys, hydrogen diffusivity is obtained through the MLSR expressions based on energy landscape statistics that were determined using climbing-image nudged elastic band (CI-NEB) calculations with universal machine learning interatomic potentials (uMLIPs). The predictions were validated against molecular dynamics (MD) simulations, showing reasonable agreement. This framework enables fast, scalable prediction of hydrogen diffusion in complex alloys, supporting accelerated materials discovery for hydrogen-related applications.
DFT study of Functionalised Graphene as an Electrode Material for Sodium-Ion Batteries
Testing the Viability of NHCH3 - Benzene, Aminobenzene and Phenol as Spacers Between Graphene Layers
Sodium-ion batteries as an alternative to lithium-ion batteries are a promising candidate due to the cost-effectiveness, abundance and safety of sodium when compared to lithium. A major drawback is that sodium is not capable of intercalation in graphite. This study examines the viability of NHCH3-Benzene, Aminobenzene and Phenol as spacers between graphene layers for use as a battery electrode. These spacers were chosen for their ability to create space between graphene layers for sodium, as well as activating the host structure for a stronger attraction to the sodium ions. Using Density Functional Theory (DFT), the structures were relaxed to their most stable configuration and loaded with sodium atoms to observe the effect of spacer material on atom behaviour. The aminobenzene spacer was identified to be the most attractive option from the tried materials in this study, both in formation energy and atom behaviour, as well as being an easily available material. Additionally, the effect of the starting positions of the inserted atoms before structure relaxation was tested. The starting position was found to directly influence the configuration of the sodium atoms in the host structure after relaxation, but the most stable positions close to the spacer were always filled, regardless of starting positions.
...
Sodium-ion batteries as an alternative to lithium-ion batteries are a promising candidate due to the cost-effectiveness, abundance and safety of sodium when compared to lithium. A major drawback is that sodium is not capable of intercalation in graphite. This study examines the viability of NHCH3-Benzene, Aminobenzene and Phenol as spacers between graphene layers for use as a battery electrode. These spacers were chosen for their ability to create space between graphene layers for sodium, as well as activating the host structure for a stronger attraction to the sodium ions. Using Density Functional Theory (DFT), the structures were relaxed to their most stable configuration and loaded with sodium atoms to observe the effect of spacer material on atom behaviour. The aminobenzene spacer was identified to be the most attractive option from the tried materials in this study, both in formation energy and atom behaviour, as well as being an easily available material. Additionally, the effect of the starting positions of the inserted atoms before structure relaxation was tested. The starting position was found to directly influence the configuration of the sodium atoms in the host structure after relaxation, but the most stable positions close to the spacer were always filled, regardless of starting positions.
Accurate prediction of thermodynamic properties of hydrocarbons is essential for chemical process modeling. Conventional group contribution methods often used to predict these properties. However, these methods often require extensive parameter set to handle structural complexities. A refined group contribution method for predicting thermodynamic properties of hydrocarbon isomers with reduced complexity and improved accuracy is presented and discussed. By combining the structural framework of Constantinou and Gani (CG94) with a sensitivity-based selection of second-order groups, a reduced yet highly effective set of twelve second-order groups is identified. This reduced set retains the predictive power comparable to more complex models while significantly reducing the number of parameters. Linear regression is applied to model standard enthalpies and Gibbs free energies of formation for a wide temperature range. To test broader applicability, the model is further extended to properties that require nonlinear regression, including critical temperatures, critical pressures, acentric factors, and liquid densities. For all cases, the proposed model achieves high predictive accuracy, demonstrating its robustness and generalizability. This methodology balances interpretability, efficiency, and performance, making it suitable for both research and industrial thermodynamic modeling.
...
Accurate prediction of thermodynamic properties of hydrocarbons is essential for chemical process modeling. Conventional group contribution methods often used to predict these properties. However, these methods often require extensive parameter set to handle structural complexities. A refined group contribution method for predicting thermodynamic properties of hydrocarbon isomers with reduced complexity and improved accuracy is presented and discussed. By combining the structural framework of Constantinou and Gani (CG94) with a sensitivity-based selection of second-order groups, a reduced yet highly effective set of twelve second-order groups is identified. This reduced set retains the predictive power comparable to more complex models while significantly reducing the number of parameters. Linear regression is applied to model standard enthalpies and Gibbs free energies of formation for a wide temperature range. To test broader applicability, the model is further extended to properties that require nonlinear regression, including critical temperatures, critical pressures, acentric factors, and liquid densities. For all cases, the proposed model achieves high predictive accuracy, demonstrating its robustness and generalizability. This methodology balances interpretability, efficiency, and performance, making it suitable for both research and industrial thermodynamic modeling.
The demand for more sustainable and economic industrial activities have imposed great challenges in the energy storage and transportation sectors as well as steelmaking. In this context, hydrogen and recycled steel are expected to play an increasingly important role as an energy carrier and infrastructure components respectively. However, both scenarios involve the introduction of hydrogen, and tramp elements such as copper and phosphorus which ultimately lead to the deterioration of mechanical properties of steels. On the one hand, hydrogen atoms (H) can be introduced into the lattice during service conditions and cause hydrogen embrittlement (HE), while on the other, current recycling processes can lead to accumulation of tramp elements at grain boundaries that can also negatively influence the mechanical integrity of steel alloys. It is known that an important aspect of HE is hydrogen trapping at susceptible defects such as grain boundaries, rendering the interaction with copper (Cu) and phosphorus (P) highly probable. In order to produce recycled steel components with improved HE properties, the co-segregation effects of H with P and Cu have to be understood in a systematic way. However, accomplishing this goal requires the direct experimental detection of H at these defects which has comprised a towering challenge.
With the aim of overcoming these difficulties, this study adopts a first principles calculations approach based on density functional theory (DFT) to investigate the influence of Cu and P on the underlying features of the H dissolution processes into bulk and grain boundary (GB) structures of α- and γ-Fe. The findings revealed that H prefers interstitial sites with higher electron pair density as described by the electron localization function (ELF) and more extensive charge transfer in bulk ferrite while the opposite behavior was observed for hydrogen accommodation in pure austenite. The addition of Cu was found to facilitate H dissolution in both phases of Fe while P was found to hinder this phenomenon. These effects were related to H’s positive and negative impact on the stability of the Fe-Cu and Fe-P bonds. Regardless of the interfacial character of the GBs investigated in this study, favorable H segregation takes place at sites with lower ELF and are accompanied by less charge accumulation. The hydrogen co-segregation effects with Cu and P exhibited different features between the closed-packed and open GB structures. In the former, the presence of the substitutional elements created an overall unfavorable dissolution environment for H regardless of the crystal structure. On the other hand, it was found that in the more disordered GBs the presence of Cu and P exhibited varying influence on the H dissolution processes. While for the ferritic GB Cu can enhance the H segregation at the interface by means of increasing the interstitial volume, it was found to alter the site preferability of H towards the open γ-Fe GB structure. In contrast, the result for P revealed a slightly favorable tendency for H dissolution suggesting the formation of bonds with the local Fe atoms, while in the α-Fe P mainly repels H from the interface. In both cases, the results revealed that simultaneous presence of both Cu-H and P-H cannot be excluded at more disordered GBs.
...
With the aim of overcoming these difficulties, this study adopts a first principles calculations approach based on density functional theory (DFT) to investigate the influence of Cu and P on the underlying features of the H dissolution processes into bulk and grain boundary (GB) structures of α- and γ-Fe. The findings revealed that H prefers interstitial sites with higher electron pair density as described by the electron localization function (ELF) and more extensive charge transfer in bulk ferrite while the opposite behavior was observed for hydrogen accommodation in pure austenite. The addition of Cu was found to facilitate H dissolution in both phases of Fe while P was found to hinder this phenomenon. These effects were related to H’s positive and negative impact on the stability of the Fe-Cu and Fe-P bonds. Regardless of the interfacial character of the GBs investigated in this study, favorable H segregation takes place at sites with lower ELF and are accompanied by less charge accumulation. The hydrogen co-segregation effects with Cu and P exhibited different features between the closed-packed and open GB structures. In the former, the presence of the substitutional elements created an overall unfavorable dissolution environment for H regardless of the crystal structure. On the other hand, it was found that in the more disordered GBs the presence of Cu and P exhibited varying influence on the H dissolution processes. While for the ferritic GB Cu can enhance the H segregation at the interface by means of increasing the interstitial volume, it was found to alter the site preferability of H towards the open γ-Fe GB structure. In contrast, the result for P revealed a slightly favorable tendency for H dissolution suggesting the formation of bonds with the local Fe atoms, while in the α-Fe P mainly repels H from the interface. In both cases, the results revealed that simultaneous presence of both Cu-H and P-H cannot be excluded at more disordered GBs.
...
The demand for more sustainable and economic industrial activities have imposed great challenges in the energy storage and transportation sectors as well as steelmaking. In this context, hydrogen and recycled steel are expected to play an increasingly important role as an energy carrier and infrastructure components respectively. However, both scenarios involve the introduction of hydrogen, and tramp elements such as copper and phosphorus which ultimately lead to the deterioration of mechanical properties of steels. On the one hand, hydrogen atoms (H) can be introduced into the lattice during service conditions and cause hydrogen embrittlement (HE), while on the other, current recycling processes can lead to accumulation of tramp elements at grain boundaries that can also negatively influence the mechanical integrity of steel alloys. It is known that an important aspect of HE is hydrogen trapping at susceptible defects such as grain boundaries, rendering the interaction with copper (Cu) and phosphorus (P) highly probable. In order to produce recycled steel components with improved HE properties, the co-segregation effects of H with P and Cu have to be understood in a systematic way. However, accomplishing this goal requires the direct experimental detection of H at these defects which has comprised a towering challenge.
With the aim of overcoming these difficulties, this study adopts a first principles calculations approach based on density functional theory (DFT) to investigate the influence of Cu and P on the underlying features of the H dissolution processes into bulk and grain boundary (GB) structures of α- and γ-Fe. The findings revealed that H prefers interstitial sites with higher electron pair density as described by the electron localization function (ELF) and more extensive charge transfer in bulk ferrite while the opposite behavior was observed for hydrogen accommodation in pure austenite. The addition of Cu was found to facilitate H dissolution in both phases of Fe while P was found to hinder this phenomenon. These effects were related to H’s positive and negative impact on the stability of the Fe-Cu and Fe-P bonds. Regardless of the interfacial character of the GBs investigated in this study, favorable H segregation takes place at sites with lower ELF and are accompanied by less charge accumulation. The hydrogen co-segregation effects with Cu and P exhibited different features between the closed-packed and open GB structures. In the former, the presence of the substitutional elements created an overall unfavorable dissolution environment for H regardless of the crystal structure. On the other hand, it was found that in the more disordered GBs the presence of Cu and P exhibited varying influence on the H dissolution processes. While for the ferritic GB Cu can enhance the H segregation at the interface by means of increasing the interstitial volume, it was found to alter the site preferability of H towards the open γ-Fe GB structure. In contrast, the result for P revealed a slightly favorable tendency for H dissolution suggesting the formation of bonds with the local Fe atoms, while in the α-Fe P mainly repels H from the interface. In both cases, the results revealed that simultaneous presence of both Cu-H and P-H cannot be excluded at more disordered GBs.
With the aim of overcoming these difficulties, this study adopts a first principles calculations approach based on density functional theory (DFT) to investigate the influence of Cu and P on the underlying features of the H dissolution processes into bulk and grain boundary (GB) structures of α- and γ-Fe. The findings revealed that H prefers interstitial sites with higher electron pair density as described by the electron localization function (ELF) and more extensive charge transfer in bulk ferrite while the opposite behavior was observed for hydrogen accommodation in pure austenite. The addition of Cu was found to facilitate H dissolution in both phases of Fe while P was found to hinder this phenomenon. These effects were related to H’s positive and negative impact on the stability of the Fe-Cu and Fe-P bonds. Regardless of the interfacial character of the GBs investigated in this study, favorable H segregation takes place at sites with lower ELF and are accompanied by less charge accumulation. The hydrogen co-segregation effects with Cu and P exhibited different features between the closed-packed and open GB structures. In the former, the presence of the substitutional elements created an overall unfavorable dissolution environment for H regardless of the crystal structure. On the other hand, it was found that in the more disordered GBs the presence of Cu and P exhibited varying influence on the H dissolution processes. While for the ferritic GB Cu can enhance the H segregation at the interface by means of increasing the interstitial volume, it was found to alter the site preferability of H towards the open γ-Fe GB structure. In contrast, the result for P revealed a slightly favorable tendency for H dissolution suggesting the formation of bonds with the local Fe atoms, while in the α-Fe P mainly repels H from the interface. In both cases, the results revealed that simultaneous presence of both Cu-H and P-H cannot be excluded at more disordered GBs.
Optimization of thermoelectric Si-Ge
Doping optimization of nanostructured Si80Ge20Bx for radioisotope thermoelectric generators
Radioisotope thermoelectric generators produce electricity for space exploration. These generators use thermoelectric material, usually a silicon-germanium alloy, to turn heat from radioactive decay into electricity. Recently, the thermoelectric performance (ZT) of Si80Ge20 was improved by reducing the grain size to the nanometer range, significantly lowering the thermal conductivity.
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties. ...
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties. ...
Radioisotope thermoelectric generators produce electricity for space exploration. These generators use thermoelectric material, usually a silicon-germanium alloy, to turn heat from radioactive decay into electricity. Recently, the thermoelectric performance (ZT) of Si80Ge20 was improved by reducing the grain size to the nanometer range, significantly lowering the thermal conductivity.
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties.
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties.
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. MoOX phase evolution was investigated via these results, with a detailed structural study being carried out on novelly synthesized 2D α-MoOX 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.
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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. MoOX phase evolution was investigated via these results, with a detailed structural study being carried out on novelly synthesized 2D α-MoOX 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.
Molecular Simulations for Hydrogen Storage and Production
From quantum to force field-based methods
In this thesis, molecular simulations are performed to design and assess novel 2D materials for H2 storage applications (chapters 2-3) and to predict thermodynamic and transport properties of H2 in aqueous electrolyte solutions for storage and production of H2 (chapters 4-8). Both ab-initio and force field-based methods are used in this thesis.....
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In this thesis, molecular simulations are performed to design and assess novel 2D materials for H2 storage applications (chapters 2-3) and to predict thermodynamic and transport properties of H2 in aqueous electrolyte solutions for storage and production of H2 (chapters 4-8). Both ab-initio and force field-based methods are used in this thesis.....
Anisotropic Stress, Plasticity, and Microstructural Evolution in Crystalline Materials
From Grain Boundaries to Nanostructures
Metallic materials exhibit structural and performance anisotropy at various scales, including the crystal structure, microstructure, and bulk levels. The anisotropy influences how stress and strain distribute within the material. The localized stress concentration is closely related to deformation and several failure mechanisms. A systematic exploration of the anisotropy can deepen our understanding of stress concentration and material failure, and guide the design of high-performance materials tailored for specific applications. Additionally, this work supports sustainable development by enhancing the service life and recyclability of metallic materials—an increasingly critical priority in materials science.....
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Metallic materials exhibit structural and performance anisotropy at various scales, including the crystal structure, microstructure, and bulk levels. The anisotropy influences how stress and strain distribute within the material. The localized stress concentration is closely related to deformation and several failure mechanisms. A systematic exploration of the anisotropy can deepen our understanding of stress concentration and material failure, and guide the design of high-performance materials tailored for specific applications. Additionally, this work supports sustainable development by enhancing the service life and recyclability of metallic materials—an increasingly critical priority in materials science.....
Master thesis
(2024)
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Z. Li, P. Dey, F.S. Shuang, S. Echeverri Restrepo, M.J. Santofimia Navarro, M.H.F. Sluiter
The detrimental effects of Cu contamination during steel recycling and production are primarily due to the segregation of Cu at Fe grain boundaries (GBs). A promising approach to mitigate these effects is the introduction of alloying elements that inhibit Cu segregation at Fe GBs, which can be investigated through atomistic simulations. Currently, popular simulation methods include density functional theory (DFT)-based simulations and empirical interatomic potentials (EIPs)-based simulations. DFT calculations can provide simulations with high accuracy, while the high computational cost limits their application to simpler GB structures. Moreover, simulations utilizing EIPs may lack reliable potentials, especially when predicting GB segregation tendencies across a broader range of alloy systems. To address these challenges, universal machine learning interatomic potentials (uMLIPs), which are trained on DFT data and applicable for most of elements, have emerged as a promising alternative. Although uMLIPs have shown potential in various materials simulation tasks, their reliability for out-of-distribution tasks, such as simulating GB segregation behavior, remains unproven.
This thesis project evaluates the performance of the best available uMLIPs, specifically MACE-MP-0, CHGNet, M3GNet, and SevenNet-0, in predicting single-solute GB segregation energies, GB energies for both body-centered cubic (BCC) Fe and face-centered cubic (FCC) Fe systems, and solution enthalpies for BCC Fe and cementite. The results were compared against existing studies conducted via DFT calculations to assess the accuracy and applicability of each uMLIP. Additionally, some EIPs were tested for comparison, serving as an extra reference. The findings reveal that MACE-MP-0 generally outperforms the other uMLIPs in both accuracy and stability of convergence. While all tested uMLIPs perform well in BCC Fe systems, CHGNet(v0.2.0) and SevenNet-0 show reduced accuracy in FCC Fe simulations. Although most of the simulations using uMLIPs converged well in BCC Fe GBs, many unconverged cases were reported in FCC Fe systems, particularly for uMLIPs other than MACE-MP-0 and CHGNet(v0.3.0). Furthermore, a consistent underprediction of segregation tendencies for highly segregating solute elements, such as Cu, is observed in the results of MACE-MP-0 and CHGNet. This suggests that while uMLIPs hold significant potential for atomistic simulations, fine-tuning pre-trained uMLIP models for out-of-distribution tasks, such as calculating GB segregation energy, is recommended to improve accuracy and convergence behavior. This work offers a valuable benchmark for using uMLIPs in future GB segregation studies. ...
This thesis project evaluates the performance of the best available uMLIPs, specifically MACE-MP-0, CHGNet, M3GNet, and SevenNet-0, in predicting single-solute GB segregation energies, GB energies for both body-centered cubic (BCC) Fe and face-centered cubic (FCC) Fe systems, and solution enthalpies for BCC Fe and cementite. The results were compared against existing studies conducted via DFT calculations to assess the accuracy and applicability of each uMLIP. Additionally, some EIPs were tested for comparison, serving as an extra reference. The findings reveal that MACE-MP-0 generally outperforms the other uMLIPs in both accuracy and stability of convergence. While all tested uMLIPs perform well in BCC Fe systems, CHGNet(v0.2.0) and SevenNet-0 show reduced accuracy in FCC Fe simulations. Although most of the simulations using uMLIPs converged well in BCC Fe GBs, many unconverged cases were reported in FCC Fe systems, particularly for uMLIPs other than MACE-MP-0 and CHGNet(v0.3.0). Furthermore, a consistent underprediction of segregation tendencies for highly segregating solute elements, such as Cu, is observed in the results of MACE-MP-0 and CHGNet. This suggests that while uMLIPs hold significant potential for atomistic simulations, fine-tuning pre-trained uMLIP models for out-of-distribution tasks, such as calculating GB segregation energy, is recommended to improve accuracy and convergence behavior. This work offers a valuable benchmark for using uMLIPs in future GB segregation studies. ...
The detrimental effects of Cu contamination during steel recycling and production are primarily due to the segregation of Cu at Fe grain boundaries (GBs). A promising approach to mitigate these effects is the introduction of alloying elements that inhibit Cu segregation at Fe GBs, which can be investigated through atomistic simulations. Currently, popular simulation methods include density functional theory (DFT)-based simulations and empirical interatomic potentials (EIPs)-based simulations. DFT calculations can provide simulations with high accuracy, while the high computational cost limits their application to simpler GB structures. Moreover, simulations utilizing EIPs may lack reliable potentials, especially when predicting GB segregation tendencies across a broader range of alloy systems. To address these challenges, universal machine learning interatomic potentials (uMLIPs), which are trained on DFT data and applicable for most of elements, have emerged as a promising alternative. Although uMLIPs have shown potential in various materials simulation tasks, their reliability for out-of-distribution tasks, such as simulating GB segregation behavior, remains unproven.
This thesis project evaluates the performance of the best available uMLIPs, specifically MACE-MP-0, CHGNet, M3GNet, and SevenNet-0, in predicting single-solute GB segregation energies, GB energies for both body-centered cubic (BCC) Fe and face-centered cubic (FCC) Fe systems, and solution enthalpies for BCC Fe and cementite. The results were compared against existing studies conducted via DFT calculations to assess the accuracy and applicability of each uMLIP. Additionally, some EIPs were tested for comparison, serving as an extra reference. The findings reveal that MACE-MP-0 generally outperforms the other uMLIPs in both accuracy and stability of convergence. While all tested uMLIPs perform well in BCC Fe systems, CHGNet(v0.2.0) and SevenNet-0 show reduced accuracy in FCC Fe simulations. Although most of the simulations using uMLIPs converged well in BCC Fe GBs, many unconverged cases were reported in FCC Fe systems, particularly for uMLIPs other than MACE-MP-0 and CHGNet(v0.3.0). Furthermore, a consistent underprediction of segregation tendencies for highly segregating solute elements, such as Cu, is observed in the results of MACE-MP-0 and CHGNet. This suggests that while uMLIPs hold significant potential for atomistic simulations, fine-tuning pre-trained uMLIP models for out-of-distribution tasks, such as calculating GB segregation energy, is recommended to improve accuracy and convergence behavior. This work offers a valuable benchmark for using uMLIPs in future GB segregation studies.
This thesis project evaluates the performance of the best available uMLIPs, specifically MACE-MP-0, CHGNet, M3GNet, and SevenNet-0, in predicting single-solute GB segregation energies, GB energies for both body-centered cubic (BCC) Fe and face-centered cubic (FCC) Fe systems, and solution enthalpies for BCC Fe and cementite. The results were compared against existing studies conducted via DFT calculations to assess the accuracy and applicability of each uMLIP. Additionally, some EIPs were tested for comparison, serving as an extra reference. The findings reveal that MACE-MP-0 generally outperforms the other uMLIPs in both accuracy and stability of convergence. While all tested uMLIPs perform well in BCC Fe systems, CHGNet(v0.2.0) and SevenNet-0 show reduced accuracy in FCC Fe simulations. Although most of the simulations using uMLIPs converged well in BCC Fe GBs, many unconverged cases were reported in FCC Fe systems, particularly for uMLIPs other than MACE-MP-0 and CHGNet(v0.3.0). Furthermore, a consistent underprediction of segregation tendencies for highly segregating solute elements, such as Cu, is observed in the results of MACE-MP-0 and CHGNet. This suggests that while uMLIPs hold significant potential for atomistic simulations, fine-tuning pre-trained uMLIP models for out-of-distribution tasks, such as calculating GB segregation energy, is recommended to improve accuracy and convergence behavior. This work offers a valuable benchmark for using uMLIPs in future GB segregation studies.
For the first time, we present data on the actual growth rate, supersaturation, viscosity, and growth mechanism that lead to spherulitic growth. So far, fundamental questions regarding the growth conditions, mechanisms, and the necessity of a viscous growth medium have remained unresolved. Surprisingly, we observed a sol-gel transition in evaporating silica-free sulfate mixtures, followed by the growth
of sodium sulfate spherulites. We characterized the spherulites’ morphological evolution and chemical/structural composition using various microscopy techniques and Raman Spectroscopy. The study reveals that faceted crystals, during their morphological evolution, can transiently exhibit a spherulitic morphology before attaining their final shape. We demonstrate that adding bivalent ions to sulfate solutions can create the conditions required for the spherulitic growth of the crystal phase. We show how to obtain perfectly developed spherulites through an in-depth experimental investigation of ion concentrations, evaporation rate, and geometric constraints. Moreover, quantifying the growth conditions enables a precise understanding and facilitates a comprehensive discussion on a general approach for
cultivating spherulites through solvent evaporation that is imperative for innovative purposes.
...
of sodium sulfate spherulites. We characterized the spherulites’ morphological evolution and chemical/structural composition using various microscopy techniques and Raman Spectroscopy. The study reveals that faceted crystals, during their morphological evolution, can transiently exhibit a spherulitic morphology before attaining their final shape. We demonstrate that adding bivalent ions to sulfate solutions can create the conditions required for the spherulitic growth of the crystal phase. We show how to obtain perfectly developed spherulites through an in-depth experimental investigation of ion concentrations, evaporation rate, and geometric constraints. Moreover, quantifying the growth conditions enables a precise understanding and facilitates a comprehensive discussion on a general approach for
cultivating spherulites through solvent evaporation that is imperative for innovative purposes.
...
For the first time, we present data on the actual growth rate, supersaturation, viscosity, and growth mechanism that lead to spherulitic growth. So far, fundamental questions regarding the growth conditions, mechanisms, and the necessity of a viscous growth medium have remained unresolved. Surprisingly, we observed a sol-gel transition in evaporating silica-free sulfate mixtures, followed by the growth
of sodium sulfate spherulites. We characterized the spherulites’ morphological evolution and chemical/structural composition using various microscopy techniques and Raman Spectroscopy. The study reveals that faceted crystals, during their morphological evolution, can transiently exhibit a spherulitic morphology before attaining their final shape. We demonstrate that adding bivalent ions to sulfate solutions can create the conditions required for the spherulitic growth of the crystal phase. We show how to obtain perfectly developed spherulites through an in-depth experimental investigation of ion concentrations, evaporation rate, and geometric constraints. Moreover, quantifying the growth conditions enables a precise understanding and facilitates a comprehensive discussion on a general approach for
cultivating spherulites through solvent evaporation that is imperative for innovative purposes.
of sodium sulfate spherulites. We characterized the spherulites’ morphological evolution and chemical/structural composition using various microscopy techniques and Raman Spectroscopy. The study reveals that faceted crystals, during their morphological evolution, can transiently exhibit a spherulitic morphology before attaining their final shape. We demonstrate that adding bivalent ions to sulfate solutions can create the conditions required for the spherulitic growth of the crystal phase. We show how to obtain perfectly developed spherulites through an in-depth experimental investigation of ion concentrations, evaporation rate, and geometric constraints. Moreover, quantifying the growth conditions enables a precise understanding and facilitates a comprehensive discussion on a general approach for
cultivating spherulites through solvent evaporation that is imperative for innovative purposes.
Quantum to Transport
Modeling Transport Properties of Aqueous Potassium Hydroxide by Machine Learning Molecular Force Fields from Quantum Mechanics
In this work, the added value of machine learning (ML) molecular force fields (FF) for the community of molecular simulations is showcased by successfully calculating transport properties of aqueous potassium hydroxide (KOH (aq)). Classical FFs use relatively simple interatomic potentials to simulate the nano scale. These simulations can predict macroscopic properties, such as density, heat of evaporation, viscosity, and self-diffusivity of the modeled materials. However, these FFs struggle to model materials in which more complicated interactions are relevant for the macroscopic behavior. Examples of such interactions are three-body interactions and chemical reactions. Quantum scale simulation methods are able to compute properties of materials in which these challenging interactions occur, although these methods are limited in length and time scales that can be modeled with realistic computational costs. Transport properties, such as viscosity, self-diffusivity and electric conductivity need these larger length and time scales to be determined accurately. ML can be used for a multi scale approach, bridging the gap between the quantum and the nano scale by training coefficients of general interatomic potentials. This provides the possibility of reaching the time and length scales of traditional molecular simulations with the accuracy of quantum mechanic models. KOH (aq) is selected to highlight the prospects of these multi scale techniques, as the self-diffusion of OH- in this electrolyte is dominated by proton transfer reactions, which has not been modeled successfully with classical FFs.
Results of structure properties produced with ab initio molecular dynamics (AIMD, at quantum scale) simulations are compared with machine learning molecular dynamics (MLMD, at multi scale) simulations. There are no significant differences in the calculated shortest typical atomic distances and coordination numbers for both KOH (aq) and pure water systems. The determined transport properties are in the same order of magnitude as experimental results, although the calculated viscosity is overestimated and the self-diffusion of H2O and K+ are underestimated. This is because the system is simulated at a higher than experimental density and hydrogen bonding is overestimated with the selected quantum mechanics model. The proton transfer reactions are captured in the MLMD simulations, calculating the enhanced self-diffusion of OH- to be (6±2)e-9 m squared per second, which matches experimental results at infinite dilution. ...
Results of structure properties produced with ab initio molecular dynamics (AIMD, at quantum scale) simulations are compared with machine learning molecular dynamics (MLMD, at multi scale) simulations. There are no significant differences in the calculated shortest typical atomic distances and coordination numbers for both KOH (aq) and pure water systems. The determined transport properties are in the same order of magnitude as experimental results, although the calculated viscosity is overestimated and the self-diffusion of H2O and K+ are underestimated. This is because the system is simulated at a higher than experimental density and hydrogen bonding is overestimated with the selected quantum mechanics model. The proton transfer reactions are captured in the MLMD simulations, calculating the enhanced self-diffusion of OH- to be (6±2)e-9 m squared per second, which matches experimental results at infinite dilution. ...
In this work, the added value of machine learning (ML) molecular force fields (FF) for the community of molecular simulations is showcased by successfully calculating transport properties of aqueous potassium hydroxide (KOH (aq)). Classical FFs use relatively simple interatomic potentials to simulate the nano scale. These simulations can predict macroscopic properties, such as density, heat of evaporation, viscosity, and self-diffusivity of the modeled materials. However, these FFs struggle to model materials in which more complicated interactions are relevant for the macroscopic behavior. Examples of such interactions are three-body interactions and chemical reactions. Quantum scale simulation methods are able to compute properties of materials in which these challenging interactions occur, although these methods are limited in length and time scales that can be modeled with realistic computational costs. Transport properties, such as viscosity, self-diffusivity and electric conductivity need these larger length and time scales to be determined accurately. ML can be used for a multi scale approach, bridging the gap between the quantum and the nano scale by training coefficients of general interatomic potentials. This provides the possibility of reaching the time and length scales of traditional molecular simulations with the accuracy of quantum mechanic models. KOH (aq) is selected to highlight the prospects of these multi scale techniques, as the self-diffusion of OH- in this electrolyte is dominated by proton transfer reactions, which has not been modeled successfully with classical FFs.
Results of structure properties produced with ab initio molecular dynamics (AIMD, at quantum scale) simulations are compared with machine learning molecular dynamics (MLMD, at multi scale) simulations. There are no significant differences in the calculated shortest typical atomic distances and coordination numbers for both KOH (aq) and pure water systems. The determined transport properties are in the same order of magnitude as experimental results, although the calculated viscosity is overestimated and the self-diffusion of H2O and K+ are underestimated. This is because the system is simulated at a higher than experimental density and hydrogen bonding is overestimated with the selected quantum mechanics model. The proton transfer reactions are captured in the MLMD simulations, calculating the enhanced self-diffusion of OH- to be (6±2)e-9 m squared per second, which matches experimental results at infinite dilution.
Results of structure properties produced with ab initio molecular dynamics (AIMD, at quantum scale) simulations are compared with machine learning molecular dynamics (MLMD, at multi scale) simulations. There are no significant differences in the calculated shortest typical atomic distances and coordination numbers for both KOH (aq) and pure water systems. The determined transport properties are in the same order of magnitude as experimental results, although the calculated viscosity is overestimated and the self-diffusion of H2O and K+ are underestimated. This is because the system is simulated at a higher than experimental density and hydrogen bonding is overestimated with the selected quantum mechanics model. The proton transfer reactions are captured in the MLMD simulations, calculating the enhanced self-diffusion of OH- to be (6±2)e-9 m squared per second, which matches experimental results at infinite dilution.
In this thesis, the first part will be a review of the literature that includes as much of the commercially available information on carbon capture and mainly, Direct Air Capture (DAC). What are the difficulties surrounding capturing CO2 from gas mixtures and what are the possible solutions that have been investigated? What type of negative emission technologies have been proposed, what are the important parameters for the design of carbon removal technologies and what are the biggest obstacles that need to be overcome? Gathering sufficient data and comparing the different finding will be of importance, given that DAC technologies are still in developing stages and information is limited. The focus of this thesis will be on chemically amine based solid sorbents, as this can be argued that this is the most promising technology for a viable DAC system.
The modelling part of the thesis will be focused on the design of and development of a DAC model in the open source software ’Python’. The amine based sorbent that is investigated and used for the creation of the model is Lewatit VP OC 1065. Using experimental data gathered from the literature together with feasible assumptions, a model will be built to recreate the the whole DAC process and analyse the system. The main focus will be on acquiring a flexible model for both the adsorption and desorption parts of the DAC process to make further investigation of the system parameters possible. This model will be used after this thesis for further development and, for instance, analysing different possible sorbents. ...
The modelling part of the thesis will be focused on the design of and development of a DAC model in the open source software ’Python’. The amine based sorbent that is investigated and used for the creation of the model is Lewatit VP OC 1065. Using experimental data gathered from the literature together with feasible assumptions, a model will be built to recreate the the whole DAC process and analyse the system. The main focus will be on acquiring a flexible model for both the adsorption and desorption parts of the DAC process to make further investigation of the system parameters possible. This model will be used after this thesis for further development and, for instance, analysing different possible sorbents. ...
In this thesis, the first part will be a review of the literature that includes as much of the commercially available information on carbon capture and mainly, Direct Air Capture (DAC). What are the difficulties surrounding capturing CO2 from gas mixtures and what are the possible solutions that have been investigated? What type of negative emission technologies have been proposed, what are the important parameters for the design of carbon removal technologies and what are the biggest obstacles that need to be overcome? Gathering sufficient data and comparing the different finding will be of importance, given that DAC technologies are still in developing stages and information is limited. The focus of this thesis will be on chemically amine based solid sorbents, as this can be argued that this is the most promising technology for a viable DAC system.
The modelling part of the thesis will be focused on the design of and development of a DAC model in the open source software ’Python’. The amine based sorbent that is investigated and used for the creation of the model is Lewatit VP OC 1065. Using experimental data gathered from the literature together with feasible assumptions, a model will be built to recreate the the whole DAC process and analyse the system. The main focus will be on acquiring a flexible model for both the adsorption and desorption parts of the DAC process to make further investigation of the system parameters possible. This model will be used after this thesis for further development and, for instance, analysing different possible sorbents.
The modelling part of the thesis will be focused on the design of and development of a DAC model in the open source software ’Python’. The amine based sorbent that is investigated and used for the creation of the model is Lewatit VP OC 1065. Using experimental data gathered from the literature together with feasible assumptions, a model will be built to recreate the the whole DAC process and analyse the system. The main focus will be on acquiring a flexible model for both the adsorption and desorption parts of the DAC process to make further investigation of the system parameters possible. This model will be used after this thesis for further development and, for instance, analysing different possible sorbents.
Boron-based two-dimensional materials showcase promise in variety of fields like hydrogen storage, fabrication of electronic devices and catalytic applications. These materials have garnered interest owing to their unique properties such as high electron mobility, high gravimetric capacity for hydrogen especially after metal decoration, thermal conductivity and tensile strength amongst others. However, for sustained operations of the devices involving these materials, their chemical stability against oxygen is of paramount importance. Especially in the applications involving exposure of the material to air. Abundance of oxygen in the air and its high reactivity increases the likelihood of oxidation of the material. In this work, chemical stability of hydrogen passivated 2D Boron structures; Borophane and 2D Boron Hydride against oxygen were analysed. First-principles calculations reveal that Borophane and 2D Boron Hydride have a less negative binding energy thus indicating that the oxygen binds less strongly to compared to Borophene which does not possess surface passivation by hydrogen. Experimental studies in the literature involving synthesis of 2D Boron Hydride reported presence of vacancies. The effect of vacancy site towards the reaction with oxygen was therefore analysed to maintain consistency with the synthesized materials. The simulations were performed on Borophane and 2D Boron Hydride by introducing a Boron vacancy in the system. The DFT simulation of defect containing Borophane revealed that oxygen binds less strongly in the physisorbed state and more strongly in the chemisorbed state, relative to the values obtained for Borophane without any defects. In case of 2D Boron Hydride it was observed that the vacancies provide a stable site for oxygen, for both physisorbed and chemisorbed state. From these simulations we can conclude that presence of vacancies in Boron based 2D materials generally leads to a stable site for oxygen.
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Boron-based two-dimensional materials showcase promise in variety of fields like hydrogen storage, fabrication of electronic devices and catalytic applications. These materials have garnered interest owing to their unique properties such as high electron mobility, high gravimetric capacity for hydrogen especially after metal decoration, thermal conductivity and tensile strength amongst others. However, for sustained operations of the devices involving these materials, their chemical stability against oxygen is of paramount importance. Especially in the applications involving exposure of the material to air. Abundance of oxygen in the air and its high reactivity increases the likelihood of oxidation of the material. In this work, chemical stability of hydrogen passivated 2D Boron structures; Borophane and 2D Boron Hydride against oxygen were analysed. First-principles calculations reveal that Borophane and 2D Boron Hydride have a less negative binding energy thus indicating that the oxygen binds less strongly to compared to Borophene which does not possess surface passivation by hydrogen. Experimental studies in the literature involving synthesis of 2D Boron Hydride reported presence of vacancies. The effect of vacancy site towards the reaction with oxygen was therefore analysed to maintain consistency with the synthesized materials. The simulations were performed on Borophane and 2D Boron Hydride by introducing a Boron vacancy in the system. The DFT simulation of defect containing Borophane revealed that oxygen binds less strongly in the physisorbed state and more strongly in the chemisorbed state, relative to the values obtained for Borophane without any defects. In case of 2D Boron Hydride it was observed that the vacancies provide a stable site for oxygen, for both physisorbed and chemisorbed state. From these simulations we can conclude that presence of vacancies in Boron based 2D materials generally leads to a stable site for oxygen.
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. ...
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. ...
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.
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.