Comparative evaluation of three-phase high-power-factor AC-DC converter concepts for application in future More Electric Aircraft

A passive 12-pulse rectifier system, a two-level, and a three-level active three-phase pulsewidth-modulation (PWM) rectifier system are analyzed for supplying the dc-voltage link of a 5-kW variable-speed hydraulic pump drive of an electro-hydrostatic actuator to be employed in future More Electric Aircraft. Weight, volume, and efficiency of the concepts are compared for an input phase voltage range of 98-132 V and an input frequency range of 400-800 Hz. The 12-pulse system shows advantages concerning volume, efficiency, and complexity but is characterized by a high system weight. Accordingly, the three-level PWM rectifier is identified as the most advantageous solution. Finally, a novel extension of the 12-pulse rectifier system by turn-off power semiconductors is proposed which allows a control of the output voltage and, therefore, eliminates the dependency on the mains and load condition which constitutes a main drawback of the passive concept.     

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La³⁺) doped BaTiO₃ samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.     

A monolithic CMOS 20-b analog-to-digital converter

The authors present a monolithic 20-b analog-to-digital converter (ADC) based on an oversampling feedback architecture. The converter consists of a time-continuous integrator at the input, a pulsewidth modulator in the forward branch of the loop (corresponding to a 10-b ADC), and a 1-b DAC (digital-to-analog converter) to generate the feedback voltage. The digital evaluation is carried out with a uniformly weighted rectangular window filter. The circuit is implemented in a standard 2-μm CMOS n-well process and requires 14 mm² of silicon, including the pads. Measurement results are presented that demonstrate the feasibility of this architecture for 20-b accuracy. The complete circuit has a power consumption of 6.7 mW     

Study of LaFe12O19 from Mössbauer spectra

A study of lanthanum ferrite, LaFe₁₂O₁₉, by use of the Mössbauer effect has contributed experimental data on the temperature dependence of the internal magnetic field on the sites of the⁵⁷Fe nuclei, on isomer shift and on quadrupole splitting in the region from 90 to 820K. The Curie temperature was determined at 697K. The Mössbauer spectra of lanthanum and thallium lanthanum ferrite suggest that a special Fe²⁺ sublattice is not present.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

A study of the dynamics of a clapper-to-bell impact with the application of a simplified finite-element model

From an engineering point of view, church bells are structures that, during ringing, are exposed to severe loading conditions. They are damaged due to material wear, fatigue loading, material deficiencies, different clapper-to-bell layouts, ringing conditions, etc. To get an insight into the wear-related damage of bells, experimental investigations and numerical simulations of the local contact between the clapper and the bell were carried out as part of the activities of an EU-funded project called Maintenance and Protection of Bells. In order to make a full-scale comparison between the measured and simulated results a simplified model was set up. In this model the clapper was replaced by a cylinder with a rounded tip that was dropped against a block representing the bell wall. The aim of the simplified model was to study the impact phenomenon in a controlled way and to adapt the numerical model for simulating the local contact. In the article the synthesis of a finite-element model for simulating the cylinder-drop test is presented. The results of the finite-element simulations of repetitive cylinder drops are compared to the data that were measured in the laboratory. The effects of the cylinder material, the cylinder radius and the drop height of the cylinder on the local elastic–plastic behaviour of the cylinder and the block are also presented and discussed.     

Low-temperature synthesis of magnetically recoverable,superparamagnetic, photocatalytic,nanocomposite particles

A new, simple, low-temperature method for the synthesis of superparamagnetic, photocatalytic, nanocomposite particles for applications in the decomposition of pollutants in water is presented. The method is based on the coating of clusters of superparamagnetic maghemite (γ-Fe₂O₃) nanoparticles with a photocatalytic anatase layer using the hydrolysis of aqueous TiOSO₄. The clusters of an appropriate size between 100 and 200 nm form by the simultaneous agglomeration of the aminopropyl-triethoxy-silane-grafted maghemite nanoparticles with a size of approximately 15 nm in a suspension of diluted TiOSO₄. During a sudden increase of pH with the addition of NaOH the titania is heterogeneously nucleated at the cluster surfaces. If the hydrolysis was conducted at an elevated temperature of 90 °C, the titania layer was nanocrystalline anatase. The composition of the nanocomposite particles, i.e., the thickness of the anatase layer, can be controlled simply by changing the starting TiOSO₄/Fe₂O₃ ratio for low titania contents, and by multiple coatings to get high titania contents. The photocatalytic activity of the nanocomposites was evaluated in the photocatalytic decomposition of formic acid. The activity seems to increase with an increase in the thickness and the crystallinity of the anatase coating, whereas it decreased after the calcination of the as-synthesized nanocomposite. The coating of the maghemite nanoparticles with a thin layer of insulating silica also slightly improves the photocatalytic activity.     

Hexaferrite/polyethylene composite coatings prepared with flame spraying

Composite coatings from different volume ratios of hexaferrite (BaFe₁₂O₁₉ or SrFe₁₂O₁₉) and polyethylene were prepared, for the first time, with flame spraying. The hexaferrite phase retained its crystal structure and microstructure during the process, while the polyethylene melted and resolidified. The coatings showed magnetic hysteresis loops with high coercivities. The measured electromagnetic behaviour proved that the studied composite coatings would be suitable for electromagnetic wave absorbers in the U-band.     

The hydrothermal synthesis of super-paramagnetic barium hexaferrite particles

Superparamagnetic Ba-hexaferrite nanoparticles were prepared using modified hydrothermal synthesis. The precursor and hydroxide [OH⁻] concentrations were optimized and the synthesis temperature and time were drastically reduced. The size and the morphology of synthesized nanoparticles was confirmed by transmission electron microscopy (TEM) images. The crystal structure of the nanoparticles was characterized by X-ray diffraction data. Powders synthesized at 160 °C exhibit a bimodal particle size distribution while those synthesized at T_S = 150 °C show a monomodal particle size distribution. Zero-field-cooling (ZFC) and field-cooling (FC) magnetization measurements were performed using a superconducting quantum interference device magnetometer from 2 to 300 K to investigate the magnetic properties of nanoparticles. The FC/ZFC magnetization measurements showed a typical superparamagnetic behavior. The synthesized superparamagnetic particles exhibit a disc-like shape, in average 11 nm wide and 3 nm thick with a room temperature magnetization of approximately 10 Am²/kg at 5 T.     

Numerical solving of the vibrational time-independent Schrödinger equation in one and two dimensions using the variational method

A program package for variational solving of the time-independent Schrödinger equation (SE) in one and two dimensions is described. The first part of the the program package includes the fitting program (FIT) with which the ab initio or DFT calculated points are fitted to a computationally inexpensive functional form. Proper fitting of the potential energy surface is crucial for the quality of the results. The second part of the package consists of a program for variational solving of the SE (2DSCHRODINGER) using either a shifted Gaussian basis set or the rectangular basis set proposed by Balint-Kurti and coworkers [J. Chem. Phys. 91 (1989) 3571]. The third part of the program package consists of the calculation of the expectation values, IR and Raman spectra XPECT), and the visualization of results (PLOT). The program package is applied to study a quantum harmonic oscillator and an intramolecular, strong hydrogen bond in picolinic acid N-oxide. For the former system analytical solutions exist, while for the latter system a comparison with the experimental data is made. The advantages and disadvantages of the applied methods are discussed.