Publications

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.

The optical and morphological properties of hydrogenated carbon nitride thin films obtained by ArF excimer laser photodeposition (lambda = 193 nm) from different ammonia/acetylene mixtures were investigated. The optical properties of the films depend on the nitrogen content. The band gap deduced from measurements of the optical absorption coefficient in the UV-VIS spectrum is found to be superior to 2.3 eV. Film thicknesses and refraction indexes were investigated by m-lines spectroscopy The refraction index varies from 1.66 to 1.72. The microstructure of the deposited films as revealed by scanning and transmission electron microscopy is discussed as a function of the deposition parameters. 2000 Elsevier Science B.V. All rights reserved.

Quaternary Ti53Zr21Ni13Fe13 alloys were prepared by melt-spinning using peripheral wheel speeds omega =19.5, 24.2 and 30m/s. The icosahedral (i-) phase formation was studied using high resolution synchrotron radiation powder diffraction. An amorphous phase component is present irrespective of the rapid solidification conditions, its volume fraction decreasing towards lower quenching speed values. The amorphous phase coexists with a b.c.c. rational approximant phase (W-phase). For omega below 20 m/s the formation of an i-phase was also noticed. Transmission Fe-57 Mossbauer spectroscopy measurements indicate that the number of asymmetrical Fe positions decreases with decreasing w in agreement with the improved structural perfection of the W-phase and with the formation of an additional i-phase. (C) 2000 Elsevier Science B.V. All rights reserved.

We present a method for processing hydroxyapatite (HA) thin films on Ti-5Al-2.5Fe substrates. The films were grown by pulsed laser deposition (PLD) in vacuum at room temperature, using a KrFdouble dagger excimer laser. The amorphous as-deposited HA films were recrystallized in ambient air by a thermal treatment at 550 degreesC. The best results have been obtained when inserting a buffer layer of ceramic materials (TiN, ZrO2 or Al2O3) The films were characterized by complementary techniques: grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), cross-section transmission electron microscopy (XTEM), SAED, energy dispersive X-ray spectroscopy (EDS) and nanoindentation. The samples with buffer interlayer preserve the stoichiometry are completely recrystallized and present better mechanical characteristics as compared with that without buffer interlayer. (C) 2000 Published by Elsevier Science B.V.

The laser ablation (LA) synthesis of ultra-fine-grained powders with dominant icosahedral structure has been attempted using Al-Cu-Fe and Al-Pd-Mn targets. The structure of the Al-Cu-Fe LA powders was examined using synchrotron radiation diffraction experiments which revealed the presence of the icosahedral Al-Cu-Fe phase. The Al-Pd-Mn powders were studied using scanning electron microscopy (SEM) and energy dispersive synchrotron radiation diffraction. For Al-Pd-Mn, the icosahedral phase is also a constituent phase of the LA alloy powders. Our results indicate that LA may be successfully used for obtaining quasicrystalline-based ultra-fine grained powders and/or surface coatings, while also providing a basis for the powder metallurgy processing of these materials. (C) 2000 Elsevier Science B.V, All rights reserved.

A detailed formula for the equivalent pyroelectric coefficient of a bimorph structure acting as a pyroelectric detector was deduced. It is shown that electric fields occur in the structure when it is subjected to a temperature variation, due to the unequal charges generated by the two components. The field dependence of the ferroelectric polarization has to be considered in this case and the (partial derivativeP/partial derivativeE) derivative appears explicitly in the deduced formula. The effect is that the equivalent pyroelectric coefficient has different values for heating and cooling. This could lead to a redressing effect on the pyroelectric signal if the temperature difference varies periodically. The equivalent pyroelectric coefficient is, also, thickness dependent. This fact offers the possibility of designing structures with high values of pyroelectric coefficient compared with the values of the component phases. (C) 2000 American Institute of Physics. [S0021- 8979(00)04701-6].

The formation of icosahedral phases in Ag-substituted Ti-Zr-Ni alloys was investigated by in situ annealing diffraction experiments using synchrotron radiation. The as-quenched (Ti1-xAgx)-Zr-Ni alloys (x=2, 5, 7 and 10 at.%) are amorphous and/or in a mixed nanostructured state. Upon heating, the Ti-Zr-Ni-Ag alloys transform to either a crystalline or a quasicrystalline state, depending on the Ag content. Differential scanning calorimetry (DSC) studies were performed to identify the nature and sequence of the phase transitions. (C) 2000 Elsevier Science B.V. All rights reserved.