National Institute Of Materials Physics - Romania

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or gamma-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial [001] symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl- ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl- ions, compressed along an[001] direction.

Structure of polyaniline (PAN)-C-60 nanocomposites was investigated by X-ray diffraction. Raman Spectroscopy was also used in order to find evidence for the C-60 - polymer interface interaction.

Using the new ion beam-induced luminescence (IBIL) apparatus in National Legnaro Laboratories, Italy, a series of measurements concerning both wide-area luminescence spectra and monochromatic luminescence maps with a space resolution of a few mum has been carried out on several CVD diamond and c-BN samples. Protons of 2 MeV with a penetration depth of approximately 25 mum have been used in order to investigate the materials in the bulk. These measurements have been correlated with particle-induced X-ray emission (PIXE) and EPR data. The measurements have been performed at increasing proton doses in order to also investigate the radiation hardness of luminescence peaks. The results indicate that ionoluminescence of CVD diamond is dominated by three bands at approximately 2, 2.4 and 2.9 eV, with the intermediate band being very radiation-hard, and the other two radiation-weak. The band at 2 eV is correlated with N content, and is particularly high in samples with poor electronic properties. IBIL in c-BN is also dominated by three bands, one at approximately 2 eV, and the other two at higher energies with respect to CVD diamond. All these three bands seem to be relatively radiation-hard with respect to CVD diamond, and to be related to defects induced by doping. (C) 2001 Elsevier Science B.V. All rights reserved.

Traps in Multi-Quantum Wells (nc-Si/CaF2)(50) nanostructures using Optical Charging Spectroscopy method were investigated. Three maxima and a final abrupt increase were found in the discharge current versus temperature characteristics. The relative contributions of the tunneling and displacement currents through the insulating calcium fluoride were analyzed.

This paper presents the results of the theoretical and experimental investigations on frequency-temperature characteristics of plan-parallel Y-cut langasite resonatons for various mass-loading conditions. Circular Sawyer Y-cut langasite blanks with 14mm diameter, Au electrodes with 7mm diameter and various thicknesses were used in experiments. The resonance frequencies were measured in the temperature range from -35degreesC to +100degreesC. First, second and third order of the frequency-temperature coefficients were computed for the fundamental third and fifth overtones. The electromechanical coupling factors were computed from measured fundamental and third harmonic resonance frequencies. The dependence of the turnover temperatures at the fundamental third and fifth overtones of the thickness-shear vibrations with the orientation angle of the cut YXI(alpha) was also calculated. The results allow to adjust a desired turnover temperature in accordance with a specific application, and reveal that the influence of the mass-loading on frequency-temperature characteristics of Y-cut langasite resonators is lower than in the case of AT-cut quartz resonators.

Coated quartz microbalances have been produced by spraying a dichloremethan solution of poly [tetradecylmethylsiloxane] on usual, 10 MHz, AT-cut quartz resonators. Standard sensitivity tests have been performed in a computer controlled system for gases and vapours mixing. The influence of the driving oscillator type working temperature, and humidity is observed and discussed.

High temperature superconducting (HTS) thin films were used to design and manufacture microstrip antennas. The high temperature superconducting thin dims of YBa2Cu3O7+delta type were deposed on lanthanum aluminate LaAlO3 substrates by using the RF magnetron sputtering technique. The transition temperature T-c and current density J(c) values proved the high quality of our films. Antenna structure consisted of three low losses dielectric layers. The radiating disk and ground plane with feeding aperture were deposed on one layer and the microstrip feeding line on another layer. The third dielectric layer separates the feeding slot from the feeding line. The measured resonant frequencies and antenna patterns validate the design technique.

A new enamel has been developed to be used as a protective and thermal barrier coating of aircraft engine components subjected to thermal shocks and corrosion. This enamel is based upon the oxide system SiO2-BaO CaO-Cr2O3 and can stand about 1100 degreesC without losing its adherence and main protective characteristics. The microstructural element concentration and crystalline phases have been studied using scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and wide angle X-ray diffraction (WAXD) in order to offer a better understanding of the enamel characteristics.

We succeeded in growing highly Fe-doped GaN films by solid-source molecular beam epitaxy using an electron-cyclotron-resonance microwave nitrogen plasma. The substrate temperature was in the range of 380-520 degreesC. The samples were analyzed by x-ray diffraction and transmission electron microscopy, and showed hexagonal (wurtzite) or cubic (zincblende) structure or a mixture of both phases. The Fe concentration was on the order of 10(19) cm(-3) and extended x-ray absorption fine structure data show that the Fe is substituting the Ga in GaN. The magnetization measurements as a function of temperature reveal ferromagnetic properties below 100 K for the sample grown at the lowest temperature. (C) 2000 American Institute of Physics. [S0003- 6951(01)00601-5].

High-quality epitaxial strontium-hexaferrite (SrFe12O19) thin films were grown by pulsed laser deposition (PLD) on c-cut sapphire using KrF* excimer laser at a fluency of 2 J/cm(2) and substrate temperature of 800 degreesC in 100 mTorr oxygen environment. The X-ray diffraction (XRD) and morphology analyzes showed films with excellent crystalline structure and Aat surface. The thickness was found to influence considerably the surface morphology and magnetic properties of the as-deposited films. The highest orientation and the best morphology with smooth surface and fine grain structure was obtained for the him having a thickness of 750 nm. The highest coercive force of 1453 Oe was measured for this film in perpendicular to the plane direction. (C) 2000 Elsevier Science B.V. All rights reserved.