Increasing photosynthetically active radiation (PAR) transmission in greenhouses enhances crop yields. A commercially established strategy to achieve this involves applying anti-reflection (AR) coatings to greenhouse glass, which reduce surface reflectance and can yield up to ∼4% additional PAR transmission.

In this study, we investigate a novel approach that builds upon this existing AR strategy by integrating an additional mechanism: luminescent spectral conversion. This technique can convert ultraviolet (UV) photons, which are poorly utilized by plants, into photosynthetically active wavelengths, thereby potentially further enhancing PAR transmission. Under idealized conditions, assuming complete UV absorption, a photoluminescent quantum yield (QY) of 40% for Sr₂CeO₄, and appropriate corrections for Stokes shift losses, this luminescent mechanism could theoretically contribute an additional ∼3% PAR gain. This thesis was initially motivated by the idea of combining luminescent UV-to-PAR conversion with anti-reflection (AR) coatings to achieve a net PAR improvement exceeding 5%.

Sr₂CeO₄ was selected as the luminescent converter due to its strong UV absorption, broad emission within the photosynthetically active radiation (PAR) range of 400–700 nm, peaking at 465 nm, and a photoluminescent quantum yield of approximately 40% at room temperature in powder form. Sr₂CeO₄ thin films were deposited via reactive magnetron sputtering, and their optical absorption and emission were characterized against key criteria for efficient UV-to-PAR conversion. Conceptual AR multilayer designs that incorporate the luminescent layer were considered at a theoretical level, and spectroscopic measurements were conducted to determine whether the material’s blue–white emission originates from the expected O²⁻ → Ce⁴⁺ charge-transfer transition or an alternative mechanism.

Spectroscopic analysis indicated that Sr₂CeO₄’s luminescence does not stem from a ligand-to-metal charge-transfer event as traditionally thought, but is better explained by a self-trapped exciton mechanism. Other experiments revealed that a Sr₂CeO₄-based luminescent AR coating could not achieve the targeted PAR enhancement. The sputtered Sr₂CeO₄ films did exhibit broad blue–white luminescence (peak 485 nm emission). However, they showed insufficient UV absorption (allowing ∼40% of 400 nm light to transmit) and exhibited insufficient luminescent output, with no visible emission detected, indicating a low photoluminescent quantum yield in thin-film form. These limitations, further exacerbated by significant fabrication challenges—namely, the hygroscopic nature of the Sr target, inconsistent target behavior, and atomic substrate diffusion into the thin films—ultimately precluded any meaningful net enhancement in PAR. It is concluded that Sr₂CeO₄ thin films are not suitable for effective UV-to-PAR conversion in greenhouse coatings.

Consequently, future research should explore alternative luminescent materials that offer stronger UV absorption and less complex thin-film deposition processes. Emphasis should also be placed on developing spectral converters tailored to the specific light requirements of target crops. For example, in the context of crops predominantly cultivated in greenhouses in the Netherlands, it may be advantageous to focus on converting UV or green light toward red and far-red wavelengths, which are particularly beneficial for those crops. ... Read more

Predicting near-surface temperature profiles is an essential, yet often challenging aspect of modeling boundary layer meteorology. The surface temperature is commonly inferred from similarity relationships. These predict the vertical profiles of both wind and temperature at some height above a surface with roughness elements, such as grass. These profiles have a logarithmic shape along the vertical. Due to experimental limitations, there are very few observations in the region close to the surface where roughness elements are present. As a consequence, the logarithmic profiles are commonly extrapolated down to the surface. However, this approach is physically inconsistent at its core. Temperature gradients become infinite as the surface is approached, as a result of the logarithmic properties of the similarity profiles. One consequence is that these profiles are extremely sensitive to small perturbations close to the surface, which is a major source of uncertainty when extrapolating temperatures. To combat this, new physical models are being investigated in an attempt to describe these profiles in a more accurate and physically rigorous manner.
A broader goal in the field of atmospheric science is to study a way to unify internal canopy dynamics with the dynamics above the canopy to yield temperature profiles that are valid from the surface, through the canopy, up into the atmosphere. In working towards these goals, a key element still lacks; precise, high resolution temperature measurements through the canopy-atmosphere interface. Novel measurement techniques such as distributed temperature sensing (DTS) have advanced the quality of datasets significantly, yielding temperature profiles with a resolution and accuracy on the order of centimeters. However, it has thus far not yielded sufficient accuracy for conclusive model comparison and for studying internal canopy temperature profiles. Therefore, there is a need for a more accurate, high-resolution dataset.
To this end, an experiment was designed to gain detailed insight into these regimes. A helical frame structure was designed, built and combined with the method of distributed temperature sensing (DTS) to attain a high resolution temperature profile along the vertical. The setup was installed at Cabauw where several weeks of data were acquired. Preliminary data analysis shows that the resulting data is well suited for the aforementioned purposes. Strong gradients near the surface can be identified as a result of the high measurement resolution on the millimeter scale. Furthermore, close to 80 data points are located within the 10cm canopy, allowing for the investigation of internal canopy transport dynamics. Finally, the data may be used in the future to validate any future models that aim to combine the canopy and atmospheric regimes. ... Read more

An all-solid-state battery represents a promising solution for overcoming current lithium-ion batteries ’technological and safety limitations. However, the individual limitations of both inorganic and organic solid electrolytes hinder technological progression. Hybrid solid electrolytes hold the potential to surpass these limitations by integrating both the inorganic and organic phases. A comparative assessment was conducted between hybrid solid electrolytes produced via solvent and dry synthesis, to address potential solvent interactions during hybrid solid electrolyte production and prioritise sustainability.

At 30°C, the comparative analysis demonstrates that the dry-processed PEO13LPSC10 hybrid solid electrolyte achieves a higher ionic conductivity of 1.61×10−5 S/cm, exceeding that of its solvent pro-cessed counterpart, which exhibits a conductivity of 1.51×10−5 S/cm. Conversely, for the PEO18LPSC10 hybrid solid electrolytes, the solvent processing method leads to a higher ionic conductivity, measured at 8.37×10−6 S/cm, in contrast to 7.61×10−6 S/cm observed for the dry-processed method. Thermal analysis indicates that heating above the polymer’s melting transition temperature leads to slow crystallisation in hybrid solid electrolytes using the dry method, resulting in two crystalline phases, as opposed to the single crystalline phase, which was observed using the solvent method. Both processing methods demonstrate homogeneity when comparing the top and bottom surfaces; however, an analysis of surface compositions between the two synthesis methods reveals distinct differences, as identified through. X-ray photoelectron spectroscopy. Moreover, decomposition is observed in both synthesis approaches but is more significant in solvent synthesis. The chemical stability of hybrid solid electrolytes produced by dry synthesis surpasses the solvent-based method.

Further analysis through the dry method investigation reveals that an ethylene oxide to Li+ ratio of 10:1, and a Li6PS5Cl ratio of 10 wt%, yield the highest ionic conductivity among all studied hybrid solid electrolytes. This combination achieves an ionic conductivity of 3.35×10−5 S/cm at 30° C. Additionally, adding Li6PS5Cl and the alkali salt lithium bis(trifluoromethanesulfonyl)imide enhances the amorphous nature and mobility of the polymer, due to a plasticising effect on the organic matrix.
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Janus particles are colloidal particles for which one half of the surface has different attributes than the other half. One property of a spherical dielectric particle with half of its surface covered by a layer of another dielectric or metal is that it has a non-uniform scattering pattern when exposed to light. However, the angle with which the light is shone on the particle has a large effect on the scattering pattern produced. Thus it is important that we are able to orient these Janus particles. The orientation can be controlled if we apply an electric field to the particle for example. The movement of colloidal particles with an electric field is widely studied and this field is called dielectrophoresis. For a Janus particle, the calculations for the force and torque become complicated. The movement and rotation of these particles have been studied, however, no analytic solution has been found. In this report, we derive a semi-analytic description of the force and torque due to an external electric field on a spherical Janus particle. For this, first the potential due to an external electric field is determined and then the force and torque are calculated with two methods: the dipole approximation and the Maxwell Stress Tensor method. In the dipole approximation, there is no force on the Janus particle. But, there is a torque on the particle in the dipole approximation. Due to this torque, the Janus particle will orient itself such that its cap points in the direction perpendicular to the applied field. For the torque calculated with the Maxwell Stress Tensor, we get a similar result as in the dipole approximation. On the other hand, according to the calculations with the stress tensor, there is a relatively small force on the particle. ... Read more

This thesis contains the development of continuous Kepler orbit- and a discrete numerical integration-based collision detection algorithms in a system of LEO satellites, which in combination with collision algorithm form a simplified space debris evolution model. This model is then used to study the Kessler syndrome. The continuous and discrete algorithms get their names from the solutions of the Two Body Problem (TBP) and the methods for collision detection that they are based on; the analytical and continuous time solution of TBP resulting in the Kepler orbits and the numerical, discrete time Velocity Verlet integration of the TBP. The collision model consists of an algorithm for fragmentation collisions largely based on the NASA Standard Breakup Model and a method for elastic, random scattering collisions. Comparison between the continuous and discrete algorithms shows that on average both predict the same time to the first collision in a system of homogeneously distributed satellites. The algorithms differ in their efficiency depending on the number and the radius of the satellites in and the geometry of the system. For relatively small satellite numbers in large systems, the continuous algorithm is computationally more efficient. However, as more satellites or fragments result from previous collision, the continuous algorithm is outperformed by the discrete algorithm. Consequentially, its time complexity appears to be O(N²). Armed with this knowledge, the continuous algorithm is used to show that an initially small system of satellites is able to evolve into a large population of debris particles within several decades. Similarly, the discrete algorithm is used to show that an ordered collection of satellites in an homogeneously distributed system of debris-like particles exhibits the effect that a collision early on in the simulation can cause a cascade of collisions at a later stage. Hence Both the discrete and continuous algorithms predict a Kessler Syndrome and mimic predictions made by more advanced models from leading space agencies like NASA’s LEGEND, ESA’s DELTA and JAXA’s LEODEEM [Lio+13].Future research could focus on including atmospheric drag and gravitational perturbations to the continuous algorithm, thereby lengthening the time frame during which it can realistically simulate a system of satellites in LEO. To achieve this, it is suggested that one execute the calculations inherent to the algorithm in parallel on a GPU, as these are independent of each other.  ... Read more

A new intense positron beam lifetime spectrometer is being developed at the Reactor Institute Delft that should be able to perform positron lifetime measurements with depth profiling. The positrons are generated by pair formation using high energy gamma-photons from the Hoger Onderwijs Reactor. This new positron lifetime spectrometer has a unique way to determine the point in time when a positron is injected into the sample material. Before the positrons are injected, they travel through a carbon foil which will then release secondary electrons. These secondary electrons are detected by a microchannel plate detector which starts a timer. Once the positron annihilates within the sample it releases two 511 keV photons. If one of these photons is detected, the timer is stopped, and the elapsed time is stored. Many measurements of the elapsed time are needed to create a positron lifetime spectrum.

The main research goal is to further develop this new positron lifetime spectrometer such that it is able to measure a positron lifetime spectrum. Multiple adaptations and tests were performed in order to achieve the research goal.

At the start of the project, the positron lifetime spectrometer could not deliver a thermalized positron beam from its 50 nm thick tungsten moderator. This problem was solved by reannealing this moderator. Additionally, a new moderator was prepared and annealed. The Variable Energy Positron (VEP) facility was used to perform characterization measurements on the moderators. This facility was modified to allow the transmission of positrons through the moderator and measure emitted thermalized positrons by the moderator. At the same time, a model based on the positron transport equation was developed to simulate and verify the characterization experiments. Both the old and new moderator have shown that they can emit thermalized positrons. The old moderator has a measured efficiency of 0.073 ± 0.002 when the positron implantation energy equals 3.7 ± 0.1 keV. The new moderator has a measured efficiency of 0.123 ± 0.002 when the positron implantation energy equals 3.2 ± 0.1 keV.

During the project, the microchannel plate detector which measures secondary electrons had to be tested, because no detections were observed in previous experiments. Additionally, the scintillation detector which is used to measure annihilation photons was tested. Eventually, both detectors have showed that they work correctly.

Finally, the positron beam of the lifetime spectrometer was aligned such that the positrons travel through the carbon foil onto the target. Secondary electrons generated by positrons have been measured by the microchannel plate detector. Afterwards, the first positron lifetime spectrum was measured using this new instrument. This lifetime spectrum is still rather crude as it shows multiple peaks and a time resolution function with a full width half maximum of 0.57 ns. The research goal of this project has been achieved, but further development is needed in order to accurately measure positron lifetimes. ... Read more

In this thesis, an algorithm to model the magnetic perturbation field caused by ships is designed and implemented. A systematic description of methods used for modelling the magnetic signature of ships is given. The algorithm fits coefficients of a prolate spheroidal harmonic expansion of the scalar potential of the magnetic field using a least angle regression method (LARS) modified to implement Lasso regularisation. A Monte Carlo method with model selection based on Akaike's information criterion (AIC) is used to select optimal parameters specifying the prolate spheroidal coordinate system centred on the ship. Furthermore, a method to restrict the degree and order of the harmonic expansion is presented and an extension of the scikit-learn module in Python is given. The predictive power of the model was verified using simulated test data, which showed that the designed model is able to make adequate predictions, but improvements are needed. Different analyses on the inputs of the model showed that the model is succesful for low levels of noise, but is susceptible to overfitting for higher levels of noise. Several recommendations for further research are made. ... Read more

The behaviour of ODS 12Cr steel under thermal treatment is studied in this report at micro and macro level. ODS 12Cr steel is always in the ferrite phase and has a melting point at 1500 ºC. Before studying,
the retrieved samples were metallographically prepared in four steps: mounting, grinding, mechanically polishing and electrolytically polishing. A measurement was done to analyse the effect of electrolytically polishing on bulk S and W parameters. This showed that mechanically polishing with 0.04 μm alumina particles has the same effect as electrolytically polishing. Next, the alloy was studied with three different measurement techniques: positron annihilation Doppler broadening spectroscopy, Vickers hardness test and positron lifetime measurements. Before these measurements the samples were annealed at selected temperatures for 10 minutes and naturally cooled.

The Doppler broadening spectroscopy was done with the Variable Energy Positron beam (VEP) at the Reactor Institute Delft (RID). With this set-up the S and W parameter were measured at different im-
plantation energies and converging bulk values of them were determined. The results were fitted with VEPFIT. This measurement was done in the as received state and after annealing from 200 ºC up to
1300 ºC in steps of 100 ºC. The Vickers hardness test was done with a load of 0.3 kgf and in the same temperature range as the VEP measurements but with steps of 200 ºC. A lifetime spectrum was measured in the as received state with two sample sandwiching a 22Na source packed with kapton. The average lifetime was determined by fitting the spectrum with LT 9.2. This measurement was repeated
after annealing for 10 minutes from 200 ºC up to 700 ºC in steps of 100 ºC.

The three different measurement techniques show corresponding results. Up to annealing temperatures of 1000 ºC the results stay constant. The bulk S and W parameters are respectively 0.475 and 0.078.
The Vickers hardness stays at 395 HV and is comparable to literature values. The average lifetime is approximately 208 ps, which corresponds with defects with sizes comparable to or at least larger than
that of divacancies. At this temperatures most thermal vacancies are trapped by the nano-oxide particles and are not able to liberate themselves which causes the constant results. At higher temperatures (larger than 1100 ºC) the thermal vacancy concentration increases and the vacancies gain enough energy to be liberated. This causes, in combination with oxide particles clustering, more defects in the material and explains the increasing bulk S value for temperatures up from 1100 ºC. The high bulk S values, compared with an iron alloy with less chromium, can also originate from clustering of chromium atoms. The surface S parameters are also increased in this temperature domain due to the formation of an oxide layer which can be seen with the naked-eye. Finally, the hardness value decreases to 374 HV after annealing at 1200 ºC which is a significant decrease and indicates that the pinning of the nano-particles decreases, resulting in grains growing easier. ... Read more