NASA Astrophysics Data System

With the growing demand for energy efficient HVAC systems and integration of renewable energy sources, existing energy transformers are being improved and new solutions are being sought. Various energy storage technologies are applied to solve unpredictable renewable energy flows. This paper investigates an innovative ventilation system with roof turbine ventilator and variable volume isobaric air tank, which is used to store an excessive wind energy. The study focuses mainly on isobaric air storage tank operation. The experimental results of the tank charging and discharging processes under different operation conditions are presented. These conditions include different weights placed on the top of the storage and air flow rates in the wind tunnel. The operation of the tank during one windy day in chosen location is studied. The obtained data showed the initial results of the operation of the developed ventilation system and possible modifications in order to improve its functionality.

The electric and magnetic properties of microbridges made from 440 nm thick Y–Ba–Cu–O films prepared on Al2O3 substrates with CeO2 sublayers by the pulsed injection metal organic chemical vapor deposition (MOCVD) method were studied in temperatures ranging from 20 K to 300 K in order to investigate the possibilities of using these microbridges as fast fault current limiters. The application of an external magnetic field (Bex) causes the flux flow induced magneto-resistive effect, which is proportional to Bex and to Bex1/2 at low and high temperatures, respectively. The experimentally obtained S-shaped I–V characteristic and the change of the microbridge resistance over time when affected by a step-like magnetic field can be well explained using a thermo-electrical model based on Joule heating and the flux flow nature of the resistive state (RS). The transition from the superconducting to the RS, studied using ns duration rectangular waveform electrical pulses able to create up to ≈2106 A cm−2 current densities without rises of microbridge temperatures, demonstrated that the I–V characteristics of the RSs consisting of an assembly of straight lines corresponding to the different pinning centers. At temperatures close to the critical temperature of superconductivity, the number of these lines increases, and the I–V characteristics can be described with high accuracy by a power law. It was concluded that microbridges made from these films can be used as protectors against complicated waveform electromagnetic pulses having short rise times, high voltage peak amplitudes and long low-value over-current 'tails'.

Using remote sensing methods to capture environmental contamination is very relevant not only to Lithuania, but also to the whole of Europe. The article examines the Remotely Piloted Aircraft System (RPAS) and its components, in particular aircraft (UAV) mounted camera sensors. From the type of sensor depends what can be identified in the photo. The article presents the geographic informational (GIS) modeling system CALMIM with which the experimental modeling of the landfill territory has been performed. UAV aerial photos captured, modeling described and data analysis carried out.

We present comprehensive experimental study of p-type (Be) and n-type (Si) -doped GaAs/AlAs multiple quantum wells (QWs) intended to be used as selective sensors/emitters in terahertz (THz) range. The structures of various designs and doping levels were studied via different optical-photoreflectance-, surface photovoltage- and differential surface photovoltage. spectroscopies and a THz photocurrent technique using as THz emission source either free electron- or optically-pumped molecular THz laser within 4.300 K range of temperatures. Analysis of Franz-Keldysh oscillations in photoreflectance spectra and line shapes of the differential surface photovoltage spectra enabled to estimate built-in electric fields and excitonic parameters for a large number of QW subbands. The experimental interband transition energies were compared with calculations performed within the envelope function approximation taking into account non-parabolicity of the energy bands. The dominant exciton line broadening mechanisms were revealed, and the interface roughness was evaluated from analysis of the dependence of exciton linewidth broadening on the QW width. Terahertz spectroscopic measurements in p-type structures have indicated strong absorption around 55 µm wavelength due to intraband absorption of the bound holes, while increase in photocurrent in the structures below 80 µm wavelength is caused by photothermal ionization of Be acceptors.

New photoelectric seven-color observations in the Vilnius system are presented for 65 stars in the region of the open cluster NGC752. Based on individual stars with accurate photometric classifications, we determine the apparent distance modulus (m-M)(V) = 8.38 +/- 0.14 and the mean reddening to the cluster EY-V= 0.027 +/- 0.010, or EB-V= 0.034 +/- 0.013 (the errors given are the standard deviations for one star). The mean photometric metallicity for the main-sequence stars, [Fe/H] = -0.14 +/- 0.03, is found to be slightly lower than that derived for the red clump giants, [Fe/H] = -0.08 +/- 0.09. This difference suggests that red giants in later evolutionary, phases may not have zero-age surface values of [Fe/H]. We made use of the least-squares minimization techniques to fit the Padova theoretical isochrones to the CMD, when the binary star population is taken into account. By varying the distance modulus, metallicity and age, the best match has been found between the seven magnitudes and colors of the observed stars and those of model binaries, which gives the distance modulus by 0.2 mag smaller than that derived from individual stars, i.e., (m-M)(V) = 8.18, a closely similar metallicity ([Fe/H] = -0.12), and age of 1.6 Gyr. With these results, the fraction of photometric binaries among the main-sequence stars is >= 40%.

We present a study of phonon sidebands in the photoluminescence spectra of Be acceptor-doped GaAs/AlAs multiple quantum wells at liquid nitrogen temperature. Up to two phonon satellites with a separation close to the GaAs longitudinal optical phonon energy are observed in the sideband of the photoluminescence. A theoretical analysis of the satellite-related photoluminescence lineshapes and their energetic position and impurity induced spectra is presented. The results show that the phonon satellites can be attributed to free-electron-Be acceptor transitions involving longitudinal optical phonon of GaAs - the host material of the studied quantum wells.

Two-terminal diode-like porous silicon structures have been investigated under the impact of strong electric field. Strong electric field I-V current-voltage characteristics have been measured in pulse regime by applying electric pulses of 15 ns duration, at repetition rate of (100-150) Hz, creating average electric field in the structure up to (103-104) V/cm. Modification of structured state of the structures have been revealed at strong electric field influence, resulting in change and stabilizing of their series resistance.

Composite system of iron porphyrin (FeTPPS) on aminosilanized Si substrates was investigated by studies of surface morphology and optical response. Aminofunctionalized siloxane films on silicon substrates were produced from different sol-gel precursors prepared from 3-aminopropyltriethoxysilane (APTES) dissolved in water, ethanol and acetone. Immersion technique was used for the formation of APTES films. Aqueous solutions of FeTPPS were deposited on aminosilanized Si substrates. Topographic features of nanostructures in composite FeTPPS/APTES/Si system were investigated by atomic force microscopy (AFM) technique. High-resolution phase contrast imaging revealed the substructure of siloxane films on silicon substrates and nanoaggregates of FeTPPS immobilized on aminosilanized Si surface. Spectroscopic ellipsometry measurements were carried out in the region 1-5 eV in order to characterize the siloxane films and composite structures. The analysis of obtained results allowed one to conclude that FeTPPS molecules were attached to the surface by covalent bonds between the functional group of sulfonic acid SO3 of iron porphyrin and (-NH2H+)-group of APTES.

Magnetic circular dichroism of iron porphyrin aqueous solutions was studied using a photometric ellipsometer with photoelastic modulator of light polarization in the spectral range 350-750 nm in magnetic fields up to 350 mT. Buffered aqueous solutions of iron porphyrin with pH-values equal to 1.7, 3.6 and 6.8 and concentration 0.1-1.0 mM were prepared by dissolving Fe(III) meso-tetra(4- sulfonatophenyl)porphine chloride. The spectra of magnetic circular dichroism were observed in the region of Soret band (390-420 nm) and Q-bands (500-700 nm). The lineshape analysis of magnetic circular dichroism spectra has shown that A-terms were dominant though a significant overlap masked the fine structure of the Q-bands. The magnetic circular dichroism signal was of order 1x10-2 cm-1/T in a linear approximation with respect to magnetic field. The data indicated that complete lifting of the degeneracy of the states involved in the corresponding optical transitions did not occurred in magnetic fields used. The model of the ground and excited states in iron porphyrin was discussed.

We report the realization of microdisk and microring quantum-cascade lasers (QCLs) emitting in the terahertz (THz) region between 3.0 THz and 3.8 THz. The GaAs/Al0.15Ga0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900 A/cm2 at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K. Finite-Difference Time-Domaine (FDTD) simulations were performed in a strong field limit to obtain the field distribution within a microdisk THz QCL resonator.