Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer.

The transition from old space to new space along with increasing commercialization has a major impact on space flight, in general, and on electric propulsion (EP) by ion thrusters, in particular. Ion thrusters are nowadays used as primary propulsion systems in space. This article describes how these changes related to new space affect various aspects that are important for the development of EP systems. Starting with a historical overview of the development of space flight and of the technology of EP systems, a number of important missions with EP and the underlying technologies are presented. The focus of our discussion is the technology of the radio frequency ion thruster as a prominent member of the gridded ion engine family. Based on this discussion, we give an overview of important research topics such as the search for alternative propellants, the development of reliable neutralizer concepts based on novel insert materials, as well as promising neutralizer-free propulsion concepts. In addition, aspects of thruster modeling and requirements for test facilities are discussed. Furthermore, we address aspects of space electronics with regard to the development of highly efficient electronic components as well as aspects of electromagnetic compatibility and radiation hardness. This article concludes with a presentation of the interaction of EP systems with the spacecraft.

Materials processing using radio-frequency ion-sources: Ion-beam sputter-deposition and surface treatment.

The capabilities of ion-beam techniques for thin-film processing, i.e., for materials deposition by ion-beam sputtering and surface treatment, are reviewed. The basic interaction mechanisms between ions and solids are summarized and related to materials processing by ion sources. Typical geometries of ion sources, targets, and samples are discussed for corresponding experimental apparatus. The versatility of ion-beam techniques in the preparation of thin films and multilayer structures is illustrated by several examples: ion-beam sputter-deposition of various binary oxide materials (including crystalline MgO, NiO, ZnO, SnxOy, and CuxOy) as well as combinatorial growth of materials libraries of amorphous ternary oxides. Furthermore, controlled ion-beam etching of surfaces is discussed.

Correlation of electrochromic properties and oxidation states in nanocrystalline tungsten trioxide.

Although tungsten trioxide (WO3) has been extensively studied since its electrochromic properties were first discovered, the mechanism responsible for the coloration or bleaching effect is still disputed. New insights into the coloration mechanism of electrochromic, nanocrystalline WO3 are provided in this paper by studying thin WO3 films combining the electrochemical and spectroscopic techniques. By employing in situ UV-Vis transmission spectroscopy at a fixed spectral band pass during electrochemical experiments, such as cyclic voltammetry, a two-step insertion process for both protons and lithium ions is identified, of which one step exhibits a significantly higher coloration efficiency than the other. To obtain a better understanding of the insertion process AxWO3 (A = H, Li,…) thin films were studied at different stages of intercalation using UV-Vis and X-ray photoelectron spectroscopy. The results show that the first step of the intercalation process represents the reduction from initial W(6+) to W(5+) and the second step the reduction of W(5+) to W(4+). We found that the blue coloration of this nanocrystalline tungsten trioxide is mainly due to the presence of W(4+) rather than that of W(5+).

Electrical transport properties of Ge-doped GaN nanowires.

The conductivity and charge carrier concentration of single GaN nanowires (NWs) doped with different concentrations of Ge were determined by four-point resistivity and temperature-dependent Seebeck coefficient measurements. We observed high carrier concentrations ranging from 9.1 × 10(18) to 5.5 × 10(19) cm(-3), well above the Mott density of 1.6 × 10(18) cm(-3), and conductivities up to 625 S cm(-1) almost independent of the NW diameter. The weak temperature dependence of the conductivity between 2 and 10 K could be assigned to the formation of an impurity band. For the sample with the highest conductivity metallic behaviour was found, indicated by a positive temperature coefficient of the resistivity. The near band edge emission analyzed by micro-photoluminescence spectroscopy showed only a small increase of the peak width up to 70 meV and no spectral shift for carrier concentrations up to 5.5 × 10(19) cm(-3). The latter was attributed to the simultaneous influence of band filling, band gap renormalization, and strain.

Effects of silver nanoparticles on microbial growth dynamics.

AIMS: Engineered metal nanoparticles are increasingly used in consumer products, in part as additives that exhibit advantageous antimicrobial properties. Conventional nanoparticle susceptibility testing is based largely on determination of nontemporal growth profiles such as measurements of inhibition zones in common agar diffusion tests, counting of colony-forming units, or endpoint or regular-interval growth determination via optical density measurements. For better evaluation of the dynamic effects from exposure to nanoparticles, a cultivation-based assay was established in a 96-well format and adapted for time-resolved testing of the effects of nanoparticles on micro-organisms. METHODS AND RESULTS: The modified assay allowed simultaneous cultivation and on-line analysis of microbial growth inhibition. The automated high-throughput assay combined continuous monitoring of microbial growth with the analysis of many replicates and was applied to Cupriavidus necator H16 test organisms to study the antimicrobial effects of spherical silver [Ag(0)] nanoparticles (primary particle size distribution D90 < 15 nm). Ag(0) concentrations above 80 µg ml(-1) resulted in complete and irreversible inhibition of microbial growth, whereas extended lag phases and partial growth inhibition were observed at Ag(0) concentrations between 20 and 80 µg ml(-1) . Addition of Ag(0) nanoparticles at different growth stages led to either complete inhibition (addition of 40 µg ml(-1) Ag(0) from 0 h to 6 h) or resulted in full recovery (40 µg ml(-1) Ag(0) addition ≥9 h). CONCLUSIONS: Contrary to the expected results, our data indicate growth stimulation of C. necator at certain Ag(0) nanoparticle concentrations, as well as varying susceptibility to nanoparticles at different growth stages. SIGNIFICANCE AND IMPACT OF THE STUDY: These results underscore the need for time-resolved analyses of microbial growth inhibition by Ag(0) nanoparticles. Due to the versatility of the technique, the assay will likely complement existing microbiological methods for cultivation and diagnostics of microbes, in addition to tests of other antimicrobial nanoparticles.

Modeling of interface roughness in thermoelectric composite materials.

We use a network model to calculate the influence of the mesoscopic interface structure on the thermoelectric properties of superlattice structures consisting of alternating layers of materials A and B. The thermoelectric figure of merit of such a composite material depends on the layer thickness, if interface resistances are accounted for, and can be increased by proper interface design. In general, interface roughness reduces the figure of merit, again compared to the case of ideal interfaces. However, the strength of this reduction depends strongly on the type of interface roughness. Smooth atomic surface diffusion leading to alloying of materials A and B causes the largest reduction of the figure of merit. Consequently, in real structures, it is important not only to minimize interface roughness, but also to control the type of roughness. Although the microscopic effects of interfaces are only empirically accounted for, using a network model can yield useful information about the dependence of the macroscopic transport coefficients on the mesoscopic disorder in structured thermoelectric materials.

Influence of magnetic dopants on the metal-insulator transition in semiconductors.

InSb:Mn and InSb:Ge reveal differences in their resistivity near the metal-insulator transition although both are acceptors of comparable depth. InSb:Ge shows the commonly observed behavior whereas InSb:Mn exhibits a strong enhancement of the resistivity below 10 K and pronounced negative magnetoresistance effects at 1.6 K. Both effects increase by applying hydrostatic pressure. The different behavior arises from the differences in the filling of the 3d shell, half filled 3d;{5} for Mn with a total spin of S=5/2 and entirely filled 3d;{10} for Ge with total angular momentum of J=0. The exchange interaction between the hole spin of the Mn acceptor and the S=5/2 spin of its 3d;{5} shell is the dominant correlation effect leading to the formation of an antiferromagnetic alignment of the Mn 3d;{5} spins along the percolation path which inhibits hopping of holes between neighboring Mn sites.

Electron mass in dilute nitrides and its anomalous dependence on hydrostatic pressure.

The dependence of the electron mass on hydrostatic pressure P in N-diluted GaAs1-xNx (x=0.10% and 0.21%) is investigated by magnetophotoluminescence. Exceedingly large fluctuations (up to 60%/kbar) in the electron mass with increasing P are found. These originate from a pressure-driven tuning of the hybridization degree between the conduction band minimum and specific nitrogen-related states. Present results suggest a hierarchy between different nitrogen complexes as regards the extent of the perturbation these complexes exert on the electronic properties of the GaAs host.

Tuning the magnetic properties of GaAs:Mn/MnAs hybrids via the MnAs cluster shape.

We report a systematic study of ferromagnetic resonance in granular GaAs:Mn/MnAs hybrids grown on GaAs(001) substrates by metal-organic vapour-phase epitaxy. The ferromagnetic resonance of the MnAs clusters can be resolved at all temperatures below T(c). An additional broad absorption is observed below 60 K and is ascribed to localized charge carriers of the GaAs:Mn matrix. The anisotropy of the MnAs ferromagnetic resonance field originates from the magneto-crystalline field and demagnetization effects of the ferromagnetic MnAs clusters embedded in the GaAs:Mn matrix. Its temperature dependence basically scales with magnetization. Comparison of the observed angular dependence of the resonance field with model calculations yields the preferential orientation and shape of the clusters formed in hybrid layers of different thickness (150-1000 nm) grown otherwise at the same growth conditions. The hexagonal axes of the MnAs clusters are oriented along the four cubic GaAs space diagonals. Thin layers contain lens-shaped MnAs clusters close to the surface, whereas thick layers also contain spherical clusters in the bulk of the layer. The magnetic properties of the hexagonal MnAs clusters can be tuned by a controlled variation of the cluster shape.