The use of rutile copper titanium oxide (CuxTi1-xO2) as a buffer layer for the low-temperature growth of thermochromic vanadium dioxide is investigated. Specifically, this study examines how incorporating copper influences the structural and optical properties of titanium dioxide. On the one hand, its incorporation yields alloy formation and lowers the transition temperature of the anatase-to-rutile phase transition, enabling rutile CuxTi1-xO2 to be formed in the sputtering deposition process at temperatures as low as 200 degrees C, compared to the minimum of 600 degrees C required for rutile TiO2. However, on the other hand, the optical transparency of CuxTi1-xO2 in the visible range of the electromagnetic spectrum decreases with increasing Cu content. A tri-layer structure consisting of a CuxTi1-xO2 buffer layer, a thermochromic VO2 layer, and an anatase TiO2 antireflection coating is designed and grown, and its thermochromic key parameters are studied. The performance is almost as good as that of a tri-layer structure where rutile TiO2 is used as the buffer layer. Therefore, using a rutile CuxTi1-xO2 buffer layer allows VO2-based multilayer structures for advanced thermochromic applications to be grown at deposition temperatures approaching those compatible with industrial sputtering apparatuses.

We investigate how probe positioning and pointing schemes affect ion current density measurements, specifically total beam current and divergence efficiency. Experiments were conducted using a five-axis gantry and a nude Faraday probe in the plume of an Advanced Cusp Field Thruster (ACFT). By mimicking Cartesian and spherical diagnostic geometries, we compare typical measurement strategies within a single setup. Additionally, maximizing local current via probe rotation reconstructs the ion plume origin, revealing a volumetric source shaped by the discharge chamber. While mathematical corrections for rigid probe misalignment exist, uncertainties from collector angle and secondary electron emission grow significantly with incidence angle. Results show that optimizing probe alignment to within ±2^∘ reduces divergence and beam current measurement uncertainty to below 2.5

Within the plethora of different vanadium oxide phases, vanadium dioxide (VO2) is a well-studied material for thermochromic applications such as smart windows. To obtain crystalline VO2 thin films, usually high in situ substrate temperatures are necessary. This, in turn, is detrimental on industrial scales, hence, alternatively postgrowth temperature treatment is a viable option. Even so, the surface morphology and roughness of the thin films exhibit strong temperature dependence. As these characteristics directly govern the physical, optical, and electronic responses, precise control of surface morphology-while maintaining phase stability-is vital. This is even more critical in multilayer architectures, where surface morphology influences the multilayer's interfaces. Here, we use ion-beam sputter deposition for the reproducible growth of vanadium oxide and assess a variety of annealing procedures to manipulate both the surface and phase of the material. We show that the surface morphology as well as the phase transition depend on the annealing parameters, such as atmosphere, pressure, temperature, and duration of the chosen treatment.

Mayenite (Ca12Al14O33) is a functional oxide with potential in high-temperature and photonic applications, whose properties can be tuned via cation substitution. We report the successful substitution of niobium for aluminium in phase-pure mayenite (Ca12Al14-xNbxO33+x) using conventional solid-state synthesis (x <= 0.35). The successful incorporation of Nb was verified by Rietveld refinement, X-ray fluorescence, and energy-dispersive Xray spectroscopy. Increasing Nb content leads to a linear increase in the melting point up to 1406 degrees C (x = 0.35), the emergence of pronounced photoluminescence in Nb-substituted mayenite, as well as distinct and systematic shifts in the Raman spectra, all depending on the substitution level. Notably, we reveal that both photoluminescence intensity and band gap energies are directly correlated with lattice strain, which is strongly influenced by the synthesis temperature. These findings provide new insight into the interplay between transition metal substitution, defects, and processing conditions, and demonstrate accessible routes to tailor the structure and optical properties of mayenite for photonic and high-temperature ceramic applications.

Many challenges have to be faced in order to develop an air-breathing electric propulsion system using a radio frequency ion thruster (RIT). The underlying assumptions of gas-surface-interactions for DSMC simulations of the intake give a wide spread of results in operating points available for the thruster. Thus, it is necessary to replicate the VLEO conditions experimentally in the future. Furthermore, precise experimental characterization of the propulsion system using atmospheric propellants is essential for mission success, although this also presents significant challenges. The background pressure of the test facility can cause an overestimation of the measured atomic-to-molecular ratio by a factor of 10. The sputtering of the beam dump can cause a back-streaming of carbon particles to the thruster of approx. 3.4 × 10^13 particles/(s mA). Moreover, the reactive propellant causes significant material degradation of all materials in direct contact with the plasma.

Accurate characterization of plasmas within the discharge chambers of gridded ion engines is essential for their advancement. This study showcases the effectivity of terahertz time-domain spectroscopy (THz-TDS) as a non-invasive technique for profiling low-pressure inductively coupled plasmas, mimicking the conditions of the discharge chamber in a radio-frequency ion thruster. Operating at pressures of 3 - 7· 10^-3 mbar inside the discharge chamber and a supply power of the radio frequency generator ranging from 10 - 80 W, THz-TDS reveals electron densities in the range of 2· 10^16 - 2· 10^17 m ^-3 for xenon, krypton, and argon plasmas. Our results show good agreement with Langmuir probe measurements and global plasma modeling, highlighting the accuracy and reliability of THz-TDS. This validation, conducted under conditions representative of gridded ion engines, demonstrates that THz-TDS is, in principle, suitable for characterizing the electron system of the plasma ignited in such thrusters in operation. This offers great potential of developing a promising additional tool for plasma diagnostics in electric propulsion systems.

Global energy consumption and the imperative to mitigate climate change have driven the exploration of innovative technologies to enhance energy efficiency in buildings. Among these, smart windows utilizing thermochromic vanadium dioxide-based materials as active materials have emerged as a promising avenue. These windows dynamically modulate their optical properties in response to environmental conditions, helping to reduce the energy consumption for heating/cooling of the building. However, the practical implementation of VO2-based smart windows faces challenges related to optimizing their performance and durability. In this context, this review advocates for further research into VO2-based smart windows incorporating titanium dioxide (TiO2) buffer layers and TiO2 antireflection (AR) coatings. Rutile TiO2 buffer layers are used to promote growth of the VO2 functional films. Additionally, TiO2 antireflection coatings can improve the optical performance of smart windows by minimizing reflections and maximizing light transmission. This work highlights how TiO2-based layers may be used to form a multilayer system surrounding the active VO2 layer in order to enhance the efficiency of VO2-based smart windows. The proposed application of rutile TiO2 buffer layers and anatase TiO2 AR layers holds promise for advancing the development of energy-efficient building technologies.

Gridded ion engines, such as the radio-frequency ion thruster, are highly efficient designs for generating thrust on satellites or spacecrafts for both commercial and scientific missions, due to the very high exhaust velocities achieved. A thorough understanding of the low-temperature and low-pressure plasma parameters is essential in order to characterize, design, and optimize such a thruster. Corresponding plasma parameters can be obtained non-invasively from empirical correlations between the results of Langmuir double probe measurements and optical emission spectroscopy. The plasma parameters can be extracted solely from the recorded optical emission spectra once such an empirical correlation is established for a specific experimental setup and various operation conditions. Light from an object plane at a specified depth within the plasma is focussed onto the projection plane, using a telescopic arrangement of lenses and an aperture. Despite being out of focus, the grid structure is still identifiable though slightly blurred. By carefully scanning across this image, taking spectra at every light spot, we can record a series of optical emission spectra where each spectrum corresponds to a plasma volume located in the object plane behind a grid hole. The object plane is a 2D cross section within the bulk of the plasma at a distance of 5 cm behind the grid. Our approach therefore allows us to monitor the spatial profiles of plasma parameters in this cross section of the RIT 10 for different operating points of the thruster. Such spatial profiles are essential for characterizing thruster performance and improving global modeling of such thrusters. We believe that this method is also applicable for RITs of other sizes in the context of space qualification.

Deviations from the ideal magnetic configuration in a modified Advanced Cusp Field Thruster (ACFT) without cylindrical symmetry are examined to understand their effect on electron behavior and thruster performance. Spatially resolved measurements of the magnetic field (B-field) and temperature at the central magnetic cusp during operation are compared with full-sized 3D particle-tracing simulations of electrons. The simulations reveal that, in addition to the well-known E × B drift, electrons also undergo circumferential motion due to magnetic field gradients and curvature, effects not previously associated with Cusp Field Thrusters (CFTs). Even small imperfections in the magnet configuration are found to measurably affect both the B-field and local heating at the thruster’s pole shoes. These results demonstrate that local deviations from ideal magnetic symmetry reduce electron confinement, increase wall losses, and ultimately affect thruster efficiency.

This study examines the lateral diffusion of hydrogen in tungsten trioxide (WO3) thin films with a nickel oxide (NiO) top layer. It focuses on the impact of the depletion region formed at the NiO/WO3 p-n heterojunction on the diffusion process. This depletion region influences diffusion by acting as a barrier to hydrogen movement. It effectively reduces the thickness in WO3 available for diffusion and increases the diffusion velocity due to the interplay with the concentration-dependent diffusion coefficient in polycrystalline WO3. Our in situ measurement technique allows for the detailed study of lateral hydrogen diffusion by inducing a concentration gradient in the layer plane. This method demonstrates by a direct comparison that diffusion is faster in the WO3/NiO layer structure compared to the pristine WO3 structure. This research demonstrates the technological potential of manipulating and tuning diffusion processes in electrochromic materials by incorporating them in layered structures and paves the way for more advanced applications.

We investigated a RIT-10 ion thruster and a radio-frequency neutralizer using molecular oxygen, molecular nitrogen and mixtures of both gases as propellants to assess their suitability as components for an air-breathing electric propulsion (ABEP) system. A combination of diagnostics - including Langmuir probes, optical emission spectroscopy (OES), Faraday cups, retarding potential analyzers (RPA), mass spectrometry and Raman spectroscopy - was used to characterize the plasma parameters, plume properties and material contamination. A comparison was made between the experimental results for the single gases and a corresponding global model. Our results show that the dissociation of the molecules and chemical reactions taking place in the plasma have a significant impact on the performance of both tested devices and thus need to be assessed for all operating points. Chemical reactions between oxygen and different material surfaces are identified and safe operating points established. A general optimization strategy to improve the performance of both devices is suggested - an essential requirement for successful missions using ABEP systems. The findings provide critical insight into material suitability and operational regimes for ABEP systems. An optimization based on an in-depth understanding of all parameters involved is only possible by combining multiple diagnostic tools and global modeling.

Thermochromic (TC) smart windows regulate building temperature by adjusting their transmittance depending on ambient conditions. Vanadium dioxide (VO2) is a leading TC material due to its insulator-to-metal transition close to, but above, room temperature, which modulates its transmittance in the near-infrared (NIR) region. Implementing TC windows faces challenges such as lowering the phase transition temperature of the TC material, ensuring high visible-light transmittance, or enabling scalable fabrication. Multilayer structures with buffer and antireflective (AR) layers address these challenges by improving crystallinity and enhancing the transmittance properties. We use ion-beam sputtering to deposit multilayer architectures of the type AlN||VO2||AlN on quartz glass substrates. AlN is chosen for its refractive index and protective qualities. The TC properties of corresponding multilayer films with AlN as the buffer and AR layer are explored, and structure-property relationships are established. The results demonstrate promising advancements in TC smart window technologies.

For all technologies, the energy-payback time (EPBT) serves as a critical metric. As an example, we study ion beam sputter deposition (IBSD), a sputter deposition approach where plasma, target, and substrate are decoupled. We compare three different configurations for reactive gas injection in order to demonstrate how the corresponding thin-film deposition processes can be improved, i.e., via the ion source, close to the target, or close to the substrate. The latter two decouple the introduction of inert and reactive gases, thus enabling substantial additional control in the deposition process. We investigate nickel oxide (NiOx) thin films as a versatile model system which is of interest for a wide range of applications. In the growth process, we vary growth times between 5 and 205 min and examine O2/Ar flow ratios between 0.13 and 5.82 for the different gas inlet configurations. Based on detailed structural and compositional analyses of the deposited thin films, we show that the deposition mode significantly influences crystal quality, growth rate, and surface roughness. Notably, the configuration where the reactive gas is injected close to the Ni target leads to significant improvement of the crystalline quality of the deposited NiOx layers for thicknesses of 30-200 nm. Furthermore, reactive gas injection close to the substrate yields films of comparable quality for thicknesses of 800 nm and above, but at almost twice the growth rate. These findings present a promising avenue for optimizing EPBT of IBSD by yielding better films in shorter process times and at less energy consumption. Yet, for low O2/Ar ratios the formation of a secondary phase of NiAl2O4 spinel is observed.

Wearable technologies are attracting increasing attention in the materials science field, prompting a quest for active components with beneficial functional attributes whilst ensuring human and environmental safety. Hydrogels are highly biocompatible platforms with interesting mechanical properties, which can be exploited for the construction of strain sensors. In order to improve the directionality of their strain response and combine it with electrical properties to fabricate piezoresistive devices, it is possible to incorporate various types of nanofillers within the polymeric network of the hydrogels. 2D materials are ideal nanofillers thanks to their intrinsic two‐dimensional anisotropy and unique electronic properties. Herein, the covalent functionalization of 2D 1T‐MoS2 is exploited to build robust hybrid cross‐linked networks with a polyethylene glycol diacrylate gel (PEGDA). The conductivity of this nanocomposite is also further improved by inducing the interfacial polymerization of aniline. The resulting free‐standing samples demonstrate a linear and highly reversible piezoresistive response in a pressure range compatible with that of peripheral blood, while also featuring good compatibility with human skin cells, thereby making them interesting options for incorporation into wearable strain sensors.

In this work, the fabrication and characterization of a fully functional field‐effect transistor (FET) are addressed based on a non‐intentionally doped GaN‐nanowire FET (NW–FET). Universal conductance fluctuations (UCFs) are observed at temperatures below 140 K. In contrast to other reports in literature, UCFs appear in the analyzed NW–FET only under the influence of an electrical field when applying a gate voltage, while no UCF signatures are observed when performing magnetic‐field‐dependent measurements. The reason is the considerable impact of the applied voltage on the narrow conductive channel of the non‐intentionally doped NW. The electrical field influences the Fermi level as well as the width of the depletion region, both changing the effective impurity distribution which determines the set of possible electron paths. The electric‐field‐induced variation of the set of electron paths correlates with a conductance variation, which leads to the occurrence of UCFs. Furthermore, the reliability of determining the phase coherence length from the NW–FET transfer characteristics is analyzed. It is shown that the value of is significantly affected by the choice of the gate voltage range due to the current dependence of the magnitude of the UCFs.

Conventional ion thruster technologies face challenges such as electrode and grid erosion and the need for additional neutralizer devices. In this article, we discuss two thruster concepts that achieve plasma acceleration by means of a magnetic nozzle, and thus avoid the need of a neutralizer device. The concept of a magnetic nozzle converting the thermal energy available in the electrons movement to ion kinetic energy is a well accepted model in the community. We discuss a novel thruster concept based on electrode-less electron cyclotron resonance plasma generation via a slot antenna design. The geometry of this thruster concept results in a converging-diverging character of the magnetic field topology along the plume direction. To this date it is not known in which way this new thruster design influences the electron dynamics and thus the ion energy. To understand the correlation between ion energy and electron temperature of this thruster system, it is compared with a well-known thruster prototype operating on similar principles, however, realizing microwave coupling and magnetic field topology in a different way. Both thruster designs operate within comparable power, frequency, and volume flow ranges. The ion energy with maximum probability is measured for both thrusters using a retarding potential analyzer in the same vacuum environment. Additionally, the electron temperature is measured with a Langmuir probe for various operation points of the thrusters, differing in input power level, volume flow, set excitation frequency, and argon or xenon as propellant.

Vanadium dioxide (VO2) undergoes a reversible first-order metal-to-insulator transition (MIT) from a high-temperature metallic phase to a low-temperature insulating phase at a critical temperature T-c of 68 degrees C. The MIT is accompanied by a structural phase transition. In addition to the metallic high-temperature rutile phase, several insulating phases may be involved depending on doping, interfacial stress, or external stimuli. Unambiguously identifying the crystal phases involved in the phase transition is of key interest from the point of view of application as well as fundamental science. We study the impact of Ti doping of VO2 thin films on (110) rutile TiO2 substrates. We conduct a careful analysis of structural properties by combining results of x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The transition temperature T-c of the deposited thin films decreases with increasing Ti-content. All our thin film samples undergo a structural phase transition from the monoclinic M-1-phase to the rutile R-phase with increasing temperature without passing the intermediate monoclinic M-2-phase. A careful analysis of polarization and angle-dependent Raman data reveals that, above T-c, the unit cell of the high-temperature rutile TixV1-xO2 phase is aligned with that of the rutile TiO2 substrate whereas, below T-c, 180 degrees-domains of the M-1-phase of TixV1-xO2 are observed. The structural relationship between TiO2 substrate and the high respective low-temperature phase of the TixV1-xO2 determined by Raman spectroscopy is in excellent agreement with TEM results on these samples. Raman spectroscopy is a powerful tool for studying structural changes of VO2-based samples in the vicinity of MIT.

AbstractDiffusion in polycrystalline tungsten trioxide (WO3) thin films is studied in a lateral geometry to better understand the impact of hydrogen‐induced structural phase transitions on the diffusion. WO3 thin films are coated with polymethylmethacrylate layer (PMMA). The latter is microstructured in such a way that a narrow stripe‐like gap occurs in the PMMA layer exposing the surface of the WO3 thin film. This stripe serves as the contact to the electrolyte in the intercalation experiment with hydrogen. After intercalation, the lateral diffusion of hydrogen inside WO3 below the PMMA layer can be observed, increasing the analyzable path and time scale by several orders of magnitude compared to the film thickness, thus, significantly improving spatial and temporal resolution of in situ transmission and Raman measurements. Spatially resolved transmission measurements in the wavelength range of 633±55 nm show that the diffusion process is dependent on hydrogen concentration and exhibits two regimes describable by different diffusion coefficients. Time‐resolved Raman spectroscopic measurements at different distances from the electrolyte contact area show that the switching between the two diffusion coefficients occurs at the phase transition from the orthorhombic to the tetragonal phase. The results are further supported by a simulation. The measurement approach is universally applicable for electrochromic films or multilayers.

An internally wetted capillary-type electrospray thruster design is presented. The capillary emitters are optimized for fabrication using 3D micro lithography and can achieve sub 10-micrometer capillary diameters with an aspect ratio of over 20. Also provided is a design for a completely modular integrated extraction electrode that comprises an electrode carrier produced by 3D micro lithography and a thin metal film. The electrode orifices, distance to emitters, and size are all customizable thanks to the modularity of the design, which is compatible with any electrospray thruster type. The design provides alignment precision within 5 micrometers of the emitter tip and electrode orifice. While our new electrode achieved reproducible extraction, instability is still present. The data on emission from these emitter-electrode stacks is presented, as well as in situ microscopic optical observation of individual emitters. The images demonstrate emission in multiple extraction modes, microfluidic behaviour of the capillaries in space-like conditions, and interactions of the emission modes with the integrated electrode.

Ensuring reliability in the measurement and interpretation of plasma diagnostics is essential for characterizing thruster concepts in the growing field of electric propulsion. Langmuir probes are often used to determine the properties of low temperature plasmas. However, not much research is found on Langmuir probe measurements on electron cyclotron resonance thrusters with magnetic nozzle, especially no detailed studies on variable probe orientations with respect to magnetic field lines. The question arises which differences can be shown in the electron energy distribution regarding the probes orientation and in what way it is possible to interpret probe alignment consequences on determined plasma characteristics. We perform our study as follows: We measure in two orientations, parallel and orthogonal to magnetic field lines, at different operating states of the thruster (variable power and volume flow settings). The Langmuir probe data shows clearly a dependency on the probes orientation. We use the measured ion energy distribution of the retarding potential analyzer as a comparison tool to the determined electron energy distribution of the single Langmuir probe measurements. Due to the determined anisotropic non-Maxwellian distribution mainly visible in parallel orientation we conclude a parallel orientation of Langmuir probes with respect to magnetic field lines is preferable for ECRT with MN.