Accurate aerodynamic modeling of satellites in very low Earth orbit (VLEO) requires gas-surface interaction (GSI) models that capture the full velocity spectrum from thermal to orbital speeds. Atmospheric particles initially strike spacecraft surfaces at hypersonic velocities of 6 000 - 10 000 m/s. Due to surface roughness and complex geometries, especially within air-breathing electric propulsion (ABEP) intake systems, multiple collisions occur, progressively reducing the particle velocities. A recent machine learning framework for deriving scattering kernels from molecular dynamics (MD) simulations has shown promise, but remains limited to high-velocity single impacts and possibly violates fundamental equilibrium principles such as detailed balance. This work extends this machine learning based scattering kernel to cover the complete velocity range using conditional normalizing flows trained with physics-informed constraints, enabling accurate modeling of multi-bounce scenarios in realistic VLEO applications. We train a conditional Real-valued Non-Volume Preserving (cRealNVP) model on expanded molecular dynamics simulations covering velocities from thermal to hypersonic speeds, incorporating a detailed balance loss term. The resulting model demonstrates improved accuracy compared to previous approaches even in the original high-velocity regime, while successfully capturing thermal-velocity scattering. Quantitative assessment shows that thermalization is approximated within acceptable tolerances. This framework provides essential capabilities for accurate ABEP intake optimization and VLEO mission planning while offering a general methodology applicable to broader rarefied gas dynamics problems requiring thermodynamic consistency.

Atomic oxygen (AO) impinging on satellite surfaces in very low Earth orbit (VLEO) transfers momentum and energy, leading to material degradation and drag forces. To be able to counteract these effects, we investigate the impact mechanisms of AO on both crystalline and amorphous alumina (Al _2 O _3 ) surfaces, which occur as oxidized surface layers of bulk aluminum, commonly used in satellite design. Al _2 O _3 also serves as a protective coating for solar panels, often covering extensive satellite surfaces. Using Molecular Dynamics (MD) simulations with classical and machine-learned (ML) force fields, we gain insights into material stability, angular distributions of reflected particles, and adsorption rates. Our findings indicate that bare Al _2 O _3 does not degrade under AO impacts but accumulates oxygen due to high adsorption rates. The angular distribution of reflected particles is highly dependent on the surface structure and the angle of incidence, with a higher ratio of specular reflection observed on smoother surfaces and at larger incidence angles measured to the surface normal.

The free molecular flow regime in very low Earth orbit makes gas-surface interactions (GSIs) crucial for satellite aerodynamic modeling. The Direct Simulation Monte Carlo (DSMC) method is required to estimate aerodynamic forces due to the breakdown of the continuum assumption. DSMC typically uses the Maxwell model for GSI, treating it as a superposition of diffuse and specular reflections with a constant accommodation coefficient. In reality, this coefficient varies with several factors, including the angle and magnitude of incident velocity. A high-precision GSI model could significantly improve aerodynamic optimization and the design of intakes for atmospheric breathing propulsion systems, refining mission planning, reducing fuel needs, and extending operational lifetimes. To gain a deep understanding of the GSI at the microscopic level, molecular dynamics (MD) simulations provide valuable insight into the physical processes involved. However, due to computational limitations, simulating an entire satellite is impractical. Instead, we use MD to analyze the impact of selected velocity vectors on a amorphous Al2O3 surface. The obtained scattering kernels for the respective velocity vectors are then used to train a conditional variational autoencoder (cVAE). This model is able to generate scattering kernels for any incident velocity vector and can be integrated into DSMC simulations, significantly enhancing their accuracy. Applications of this model on a flat plate have shown that the cVAE is able to predict the shift from diffuse to quasi-specular reflection with increasing polar angle. Additionally, the aerodynamic coefficients and molecular fluxes are considerably different from those obtained with the Maxwell model.

Influence of B and N substitutional impurities on the adhesion properties, electronic and magnetic characteristics of the external and internal graphene/Fe(110) semi-coherent interface was investigated by the projector augmented-wave method within the density functional theory. The analysis of interatomic distances, charge densities, charge transfer and magnetic states of atoms was performed. It was shown that B -doped graphene has a higher adhesion energy on the Fe(110) surface and in the iron bulk in comparison with the undoped systems; whereas, N -doped graphene in the Fe bulk demonstrates the opposite trend and on the surface it has no effect. Boron acts as electron donor that increases charges of the nearest C atoms by 0.5-0.6e. Nitrogen due to its high electronegativity is an acceptor and as a result the nearest C atoms lose 0.2-0.3e. Both impurities, depending on the position, influence differently on the magnetic moment of the nearest iron atoms.

In order to contribute to the understanding of advanced materials with improved mechanical properties we investigate strengthening and failure processes in iron-graphene composites using molecular dynamics simulations. Graphene sheets are embedded in {110} planes of bulk iron. Tensile tests and shear tests are conducted and evaluated with regard to the orientation of the graphene sheet. It is found that two different failure mechanisms involving interfacial fracture and dislocation nucleation can occur due to tensile load. The shear tests revealed three mechanisms of dislocation-obstacle interaction. The highest strengthening was found for graphene sheet orientations which normal vectors are not orthogonal to the Burgers vector of the interacting dislocation.

The atomic structure and surface energies of several low-index surfaces (0001), (11¯00) and (112¯0) of Ti5Si3 in dependence on their termination were calculated by the projector augmented-wave method within the density functional theory. It was revealed that the mixed TiSi-terminated (0001) surface is stable within the wide range of change in the Ti chemical potential. However, the Ti-terminated Ti5Si3(0001) surface is slightly lower in energy in the Ti-rich limit. The oxygen adsorption on the stable Ti5Si3(0001) surface with TiSi termination was also studied. It was shown that the three-fold coordinated F1 position in the center of the triangle formed by surface titanium atoms is the most preferred for oxygen adsorption on the surface. The appearance of silicon as neighbors of oxygen in other considered F-positions leads to a decrease in the adsorption energy. The factors responsible for the increase/decrease in the oxygen adsorption energy in the considered positions on the titanium silicide surface are discussed.

Abstract A simplified two length-scale model is applied in direct flexoelectricity for two-dimensional problems. Numerical experiments are performed to obtain the two unknown flexoelectric parameters that appear in gradient theory in elasticity. They are determined by fitting the results from an atomistic model with an analytical solution for a simple problem employing gradient theory, namely, a square domain with a simple polynomial variation of displacements. This is demonstrated by a numerical example for alpha alumina α-Al2O3.

Co(0001)/TaC(012¯) interfaces with Re impurities are investigated using the plane-wave pseudopotential method within density functional theory. Two interface configurations and several interfacial sites for Re impurities are considered. The calculations reveal that Re doping can lead to more favourable interfaces with lower interface energies. It is shown that the decrease of interface energy can be understood from the electronic structure. Analyses of bond lengths, overlap populations, charge density distributions and densities of state demonstrate the importance of C-Co and C-Re bonds across the interface.

The adhesion properties of the TiAl/TiO2 interface are estimated in dependence on interfacial layer composition and contact configuration using the projector augmented wave method. It is shown that a higher value of the work of separation is obtained at the interface between the Ti-terminated TiAl(110) surface and the TiO2(110)O one than at that with the Al-terminated alloy. An analysis of structural and electronic factors dominating the chemical bonding at the interfaces is carried out. It is shown that low bond densities are responsible for low adhesion at both considered interfaces, which may affect the spallation of oxide scale from the TiAl matrix.

Copper alloys show a structural variability leading to a range of diverse electronic, transport, and mechanical properties. Here, we investigate the influence of the type and amount of alloying on the electronic transport properties across copper alloys. Specifically, we investigate the electronic transmission along copper in its fcc crystal structure. The characteristic change when adding impurities, such as nickel, aluminum, and silicon in the structure and transport properties is assessed through density functional theory calculations together with the non-equilibrium Greens functions approach. The results are analyzed with respect to the structural characteristics, the electronic and transport properties in these alloys for different concentrations of the impurities. A clear trend in the electronic transmission, thus the conductance, along these materials was found with the addition of certain impurities and their clusters. Coherent with experimental observations, we conclude that the higher the concentration of the impurities, the lower the electronic transmission along the alloys as compared to a pure copper crystal. Furthermore, we show that the bonding environment in the impurity clusters can be associated to additional valence states and significantly influences the conductance. In the end, we discuss the relevance of our results for practical applications.

We present simulations of the formation of Ni3Si precipitates using a combination of molecular dynamics (MD) and the Metropolis Monte Carlo (MMC) method. Applying this technique to a Ni-Si solid solution in Cu matrix leads to Ni3Si precipitates with L1(2) structure as observed in experiments. Since L1(2) structured precipitates are most relevant for precipitation strengthening of several alloys, we focus on planar defects and dislocations in Ni3Si and Ni3Al. Ab initio calculations of the generalised stacking fault energies of Ni3Si presented in our previous work [S. Hocker, H. Lipp, E. Eisfeld, S. Schmauder, J. Roth, J. Chem. Phys. 149, 024701 (2018)] revealed that the complex stacking fault is not stable and the inflection point as well as the minimum corresponding to the antiphase boundary is shifted. In this study it is shown that this behaviour can be understood from the analysis of charge densities. Furthermore, the consequences on dislocations in Ni3Si and Ni3Al are discussed and interactions of edge dislocations with Ni3Si and Ni3Al precipitates are simulated.

The microscopic mechanisms of oxygen adsorption on the doped Ti3Al(0001) surface are investigated by the projector augmented-wave method within the density functional theory. Changes of the adsorption energy due to 4d-metals and p-elements of IIIA and IVA groups being in the first and second neighbors to the oxygen atom on the surface are estimated. It is demonstrated that mechanisms being responsible for the change of the adsorption energy of oxygen in the first neighbors to the impurity atom are the same irrespective of both adsorption position and occupied Al or Ti site. The increase/decrease of the adsorption energy in the specific position is determined by competition of ionic and covalent contributions to oxygen binding energy as follows from analysis of electronic properties of systems under question. The impurity being in the second neighbors of oxygen affects the adsorption energy indirectly via hybridization with alloy surface atoms involved in the direct interaction with oxygen.

IMD is a computer simulation package designed for large-scale simulation studies in materials sciences. IMD can be run with a large number of effective many-body interactions which can be produced from ab-initio calculations. This report is intended to give an overview of the following topics: how to obtain IMD, the design of IMD, capabilities of IMD with special focus on laser ablation, and interaction of IMD with other codes.

Effective interaction potentials suitable for Cu/δ-Ni2Si and Cu/β-Ni3Si are developed. We optimise the potential parameters of an embedded atom method potential to reproduce forces, energies, and stresses obtained from ab initio calculations. Details of the potential generation are given, and its validation is demonstrated. The potentials are used in molecular dynamics simulations of shear tests to study the interactions of edge dislocations with coherent δ-Ni2Si and β-Ni3Si precipitates embedded in a copper matrix. In spite of significantly different crystallographic structures of copper and δ-Ni2Si which usually result in circumvention of dislocations, we also observed cutting processes in our simulations. Dislocations cut for a specific orientation of the δ-Ni2Si precipitate and in some cases where dislocation loops originating from previous circumvention processes are present in the glide plane. It is found that β-Ni3Si precipitates have a similar effect on precipitation strengthening as δ-Ni2Si. Dislocations usually cut β-Ni3Si but increased coherency strain can lead to circumvention processes.

Molecular dynamics simulations of edge dislocation interactions with coherent and incoherent silver precipitates in the copper matrix are applied to investigate precipitation strengthening. Simulated shear tests with spherical and octahedral precipitates revealed that dislocations can cut a precipitate or circumvent it by the Orowan mechanism. Precipitates with radii below 3nm are cut whereas both processes were observed for radii in the range of 3-9nm. The reason for the occurrence of the Orowan mechanism is that dislocation reactions at the interface can lead to sessile dislocations. Orowan circumvention is more likely for spheres than for octahedra which is due to different dislocation types existing at the matrix/precipitate interfaces. On average, the critical resolved shear stress is found to be slightly higher for Orowan processes. In case of small precipitates, the critical resolved shear stress depends strongly on the coherency, whereas for larger precipitates, it is mainly influenced by dislocation reactions at the interface. In some cases, the formation of a jog was observed which can reduce the critical resolved shear stress whereas it was increased significantly in the cases of pronounced cross-slip without jog formation.

A kinetic Monte Carlo approach is used to study the influence of Cr, Fe, Al, or Mg addition on the precipitation in a Cu-Ni-Si alloy. The simulation method is based on a vacancy diffusion model. The crucial parameters of this method are the pairwise mixing energies of all contained elements which are determined by ab initio calculations. The number of dissolved atoms in equilibrium state is used to estimate the influence of Cr, Fe, Al, or Mg on the electrical conductivity. In order to estimate the influence of the alloying elements on strength ab initio calculations of misfit strain at the Cu/Ni3Si interface are performed. The atomistic kinetic Monte Carlo (AKMC) simulations reveal that Cr, Fe, Al, and Mg atoms are located at different positions: Mg and Al atoms are preferably located at the interface of Ni-Si precipitates and Cu matrix, Cr atoms diffuse into Ni-Si precipitates, and Fe atoms form Fe clusters surrounded by Ni-Si shells. Cr addition leads to a significantly reduced fraction of atoms dissolved in the matrix which indicates an increase of electrical conductivity, whereas Mg addition results in a high misfit strain at the Cu/Ni3Si interface which can contribute to increased strength.

Molecular dynamics simulations are used to investigate tensile strengths and failure mechanisms of aluminium nanopolycrystals with an average grain diameter of 8nm containing dissolved Ni, Cu, Mg or Ti atoms. It is shown that tensile strengths are influenced by several factors such as stacking fault energies as well as types, concentrations and positions of the dissolved atoms. A strong strengthening was found in case of Cu whereas Ni or Ti lead to a moderate increase of strength, while Mg even lowers the tensile strength.It was found that both dislocation processes as well as grain boundary dominated effects contribute to plastic failure mechanisms. Type and concentration of dissolved atoms possess a significant influence on these failure mechanisms. Whereas alloying with Ni, Cu or Mg supports grain boundary dominated failure processes, dissolved Ti atoms lead to a significant increase of stacking faults and dislocations.

Ab initio calculations using plane wave pseudopotential method within density funtional theory are applied to investigate mechanical and electronic properties of Al-terminated Me(111)/Al2O3(0001) (Me = Al, Ag, Cu, Nb) interfaces. Stress-displacement relationships of separation perpendicular to the interface are calculated. It is shown that obtained results such as work of separation and tensile strength can be understood from electronic structure.