Very Low Earth Orbit (VLEO) improves the geometric performance of optical payloads and increases communication link margins relative to conventional Low Earth Orbit (LEO), but exposes spacecraft to aerothermal heating, aerodynamic disturbances, and large environmental uncertainty. These effects are particularly challenging for CubeSat-class platforms, where mass, volume, and available thermal-control hardware are limited. This thesis assesses the thermal feasibility of a predominantly commercial-off-the-shelf 16U CubeSat platform operating down to a minimum perigee altitude of 130 km.
A simulation-based thermal-analysis framework is developed to construct conservative VLEO thermal cases and propagate them into spacecraft temperatures. Atmospheric, aerothermal, aerodynamic, and Earth-radiation inputs are defined for bounding hot and cold conditions. Analytical free-molecular aerothermal and aerodynamic models are benchmarked against DSMC reference data and used for rapid altitude- and attitude-dependent load evaluation. Earth albedo and infrared radiation are represented with a CERES-derived zonal-harmonics model after comparison with constant and ECSS-based alternatives. The resulting cases are analysed with a reduced-order LTspice thermal network and a spatially resolved COMSOL finite-element model. Morris screening, deterministic design sweeps, and surrogate-based uncertainty propagation are then used to size and verify the passive thermal configuration.
The baseline configuration is non-compliant in the hot case even without internal dissipation, identifying insufficient bus heat rejection as the limiting mechanism. A passive design combining a ram-facing heat shield, high-emissivity side panels, and thermally controlled external interfaces permits a deterministic payload duty cycle of 20 % at 130 km. The propagated maximum structure temperature is centred at 39.7 °C with a standard deviation of 0.85 °C and remains more than 7 °C below the 50 °C structure-to-payload interface requirement. No requirement exceedance is observed within the propagated uncertainty set. The results indicate that passive thermal control of a 16U COTS-based CubeSat is feasible for short-duration operation down to 130 km perigee, provided that interface conductances and subsystem-level thermal behaviour are verified by further modelling and test correlation. ... Read more

The Cauchy problem on a lightcone differs from the one on a smooth spacelike surface in several ways; the initial surface cannot be smooth everywhere, and the initial surface being characteristic also has implications for the values the initial data may take. We show well-posedness of the vacuum Cauchy problem for a smooth spacelike initial surface in arbitrary dimension. The initial data for this problem is related to initial data for the characteristic Cauchy problem. We derive the Raychaudhuri equation for lightlike geodesics and use it to analyze initial data for the Cauchy problem on a lightcone.
We show how to derive the extrinsic curvature of a lightcone using a field of connection coefficients and the metric. We discuss the Raychaudhuri equation as a constraint equation if one uses the metric as initial data. Alternatively, we show how to use the Raychaudhuri equation to complete initial data when a conformal class of metrics is used as initial data instead. In this case, we give an exact expression for the conformal factor associated with a representative of the conformal class. ... Read more

This work aims to clarify the fundamental precipitation mechanisms in the Fe–Al–Mn–C system, spanning from atomic-level processes to the overall mechanical response. A multiscale characterization approach, including high-resolution transmission electron microscopy, atom probe tomography, and synchrotron X-ray diffraction, was combined to characterize the evolution of κ-carbides during aging, providing atomic-level insights into their formation. This enabled the analysis of carbide size, morphology, interparticle spacing, and volume fraction at different stages of aging. Tensile and hardness tests on samples aged at 550 ℃ for different times were performed to correlate macroscale mechanical properties with the nanoscale κ-carbides. The results show that spinodal decomposition occurs throughout the 8 h of aging, forming solute-rich regions. In contrast, the formation of κ-carbides begins in the solute-rich areas through short-range ordering in the early stages of aging, i.e., within 30 min at 550 ℃, and both the fraction and size increase with longer aging times. After 2.5 h of aging, the fraction and lattice parameter of the κ-carbides saturate and stop increasing. The yield strength and hardness follow the same trend as the κ-carbide fraction, with a steep increase in the initial stages of aging, up to 2.5 h, reaching 925 MPa and 360 HV, and then saturating after 2.5 h. This work provides a clear mechanistic understanding of the underlying strengthening mechanisms and the resulting mechanical behavior. ... Read more

Perovskite–silicon tandem solar cells can exceed the single-junction silicon efficiency limit by combining a wide-bandgap perovskite top cell with a crystalline-silicon bottom cell, but the silicon bottom cell must absorb transmitted long-wavelength light efficiently while remaining compatible with the thin layers deposited above it. Since surface texturing reduces reflection and improves light trapping but conventional micrometre-scale pyramids can be too large for conformal perovskite deposition, this thesis investigates wet-chemical nanopyramidal texturing of double-side-textured c-Si wafers for silicon heterojunction (SHJ) bottom-cell test structures.

A standard KOH–MonoTEX H2.6 texturing process and a silicate-assisted KOH–K2SiO3–MonoTEX H2.6 nanotexturing process were developed and compared, with the resulting surface morphology quantified using an improved semi-automated Pyramid Height Estimation tool applied to top-view scanning electron microscopy images. The tool extracts pyramid-height distributions, including the mean height and the 95th-percentile height, h95, defined as the height below which 95% of the detected pyramids fall. These morphology metrics were combined with average reflectance measurements over 300–1200 nm. Selected textures were then processed into symmetric SHJ bottom-cell test structures with (i )/(n) and (i )/(p) Si-based thin-film stacks. These symmetric structures do not represent complete solar cells, but isolate whether each
texture can be passivated and contacted under the selected SHJ process flow. Their electrical quality was evaluated by measuring the effective minority-carrier lifetime after PECVD deposition of these stacks, after ITO sputtering and after annealing, followed by effective contact-resistivity extraction after metallisation.

The pre-texturing nitric acid oxidation cycle (NAOC) was essential for obtaining uniformwet-etched surfaces and reducing untextured regions. For standard KOH–MonoTEX texturing, increasing the bath temperature from 70 to 80 ◦C improved the average reflectance, whereas increasing theMonoTEX H2.6 volume from 30 to 50mL did not further improve the optical response. The selected standard texture, obtained after 8 min at 80 ◦C with 30 mLMonoTEX H2.6, achieved the lowest standard-texture average reflectance of 14.956% over
300–1200 nm and served as the optical benchmark. For silicate-assisted nanotexturing, K2SiO3 moderated pyramid growth and enabled sub-micrometre mean heights, with the 10 min process at 70 ◦C using 50 mL MonoTEX H2.6 and 55/110 g KOH/K2SiO3 providing the best balance between morphology and optics: side-averaged mean heights of 0.682 and 0.702 μm on the front and rear sides, h95 = 1.327 μm on both sides, and an average reflectance of 15.826%.

The electrical characterisation showed that the selected textures could be processed into SHJ bottom-cell test structures withmeasurable passivation and contact-resistivity performance. Valid n-type structures reached effective minority-carrier lifetimes above 20 ms after PECVD, while p-type structures showed lower recovered lifetimes of approximately 2–4 ms, consistent with the stronger process sensitivity of the investigated p-type contact stack. ITO sputtering caused a strong temporary lifetime loss, but annealing recovered a substantial part of the lost passivation quality. The extracted effective contact resistivities did not show a simple dependence on nanotexturing time or pyramid height, but were more strongly influenced by contact-stack and run-to-run variation, with valid n-type values mainly between 80 and 120mΩ·cm^2 and valid p-type values spanning approximately 64–141mΩ·cm^2.

Overall, the selected standard texture remains the lowest-reflectance benchmark, whereas the 10 min silicate-assisted nanotexture provides the best tandem-oriented compromise between reduced pyramid height, controlled upper-tail morphology, optical response, recovered passivation quality and effective contact resistivity. Further work should increase the sample size, validate SEM-based height estimates with direct three-dimensional measurements, and improve bath-conditioning and PECVD reproducibility.

Beyond the symmetric test structures studied here, controlled nanotextured c-Si surfaces have also been reported in approximately 31% perovskite–silicon tandem devices and, outside photovoltaics, as nanotextured Si/SiO2 templates for low-impedance multilayer graphene neural electrodes [1], [2]. ... Read more

The neighbour-sum problem asks whether a graph admits a non-trivial assignment of real numbers to its vertices such that the value at each vertex equals the sum of the values assigned to its neighbours. This problem can be reformulated as the eigenvalue equation Ax=x, where A is the adjacency matrix of the graph, reducing the question to determining whether 1 is an eigenvalue of A.

This thesis investigates the neighbour-sum problem for several classes of graphs using techniques from spectral graph theory, linear algebra, and Fourier analysis on finite groups. Complete characterizations are obtained for path graphs and cycle graphs through explicit formulas for their spectra. These results are extended to Cayley graphs over finite abelian groups, yielding explicit eigenvalue conditions for circulant graphs in terms of group characters. For trees, theoretical results are derived for several special families, including stars, double stars, and caterpillar trees, and an asymptotic result shows that the proportion of solvable trees tends to one as the number of vertices increases.

In addition, computational experiments were performed on all non-isomorphic trees with up to twenty vertices and on connected circulant graphs of small order. The experiments reveal how structural properties such as the number of leaves, maximum degree, diameter, and generating set influence solvability, and suggest that larger trees are increasingly likely to admit non-trivial solutions. Together, the theoretical and computational results provide insight into the relationship between graph structure and the existence of neighbour-sum assignments. ... Read more