National Institute Of Materials Physics - Romania

Estimating the low densities of misfit dislocations in epitaxial layers in the early stages of relaxation is difficult, requiring a very sensitive method. The characteristic patterns of diffuse x-ray scattering are therefore utilized to quantify the linear density of misfit dislocations. This is demonstrated for ZnSe/GaAs(001) heterostructures with ZnSe layer thickness slightly above the critical thickness. The densities that resulted from calculated patterns of x-ray scattering perpendicular to the diffraction vector are found to be in good agreement with the results of cathodoluminescence and transmission electron microscopy. Moreover, the diffuse x-ray scattering is used to quantitatively study the thermally induced increase in the linear densities of misfit dislocations. Samples are annealed at various temperatures for increasing periods successively and are analyzed between the annealing steps by high-resolution x-ray diffraction. The density of dislocations is found to saturate for longer annealing periods. The saturation value and the rate of increase of the dislocation density depend both on the annealing temperature and on the crystallographic direction. (c) 2005 American Institute of Physics.

The rotational molecular dynamics of water confined to nanoporous molecular sieves of a regular hexagonal (SBA-15) and of a foamlike pore structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(9) Hz and in a broad temperature interval. Two relaxation processes were observed: the process at lower frequencies is related to water molecules forming a layer, which is strongly adsorbed at the pore surface, whereas the relaxation process at higher frequencies is assigned to fluctuations of water molecules situated close to the center of the pore. The relaxation times of the low-frequency process for both materials and of the high-frequency process for the SBA-15 material have an unusual saddlelike temperature dependence, reported here for the first time. To describe this temperature dependence, a model developed for water confined to nanoporous glasses by Ryabov et al. [J. Phys. Chem. B 2001, 105, 1845] was applied, which considers two competing effects. The characteristic features of these two competing processes were compared with those reported for other porous systems.

Capacitance-voltage (C-V) measurements were performed on epitaxial layers of PbsZr(0.2)Ti(0.8)dO(3) (PZT) with top and bottom SrRuO3 (SRO) electrodes. It is shown that the sharp capacitance peak/discontinuity which is present in the C-V characteristics at different frequencies is directly associated with the polarization reversal. The ferroelectric film is assumed as a large band-gap semiconductor with Schottky contacts at the metal-ferroelectric interfaces. The capacitance discontinuity at the reversal scoercived voltage is associated with a discontinuity in the built-in potential at the PZT/ SRO interfaces. The C-V characteristics for voltage ranges outside the coercive values can be used to extract the free carrier concentrations as in the case of Schottky metal-semiconductor contacts. The carrier concentration was found to be (2-4) x 10(18) cm(-3), independent of measuring frequency and temperature up to 1 MHz and 170 degrees C, respectively, suggesting completely ionized shallow impurities. (c) 2005 American Institute of Physics.

The inhibiting effect of N-ciclohexil-benzothiazole-sulphenamida (NCBSA) on the corrosion of carbon-steel in diluted ammonia solution at room temperature has been studied by Mossbauer spectroscopy, electrochemical measurements, weight loss and surface microscopic analysis. A decrease in the corrosion spot concentration with the increase of the NCBSA concentration indicates a good adsorbability of NCBSA on the metal surface. Mossbauer spectrometry shows the formation of a layer at the surface of the corroded sample, which is interpreted in terms of the formation of complexes between NCBSA and the metal cations present in the carbon-steel structure. (c) 2004 Elsevier Ltd. All rights reserved.

(Zr-0.8, Sn-0.2)TiO4 ternary compounds (ZST) have been prepared by conventional solid-state reaction from raw materials. The effects of such sintering parameters as sintering temperature, sintering time, and NiO addition on structural and dielectric properties were investigated. The material exhibits a dielectric constant epsilon(r), similar to 36.0 and high values of the product Qf of the intrinsic quality factor Q and the frequency f from 32,170 to 50,000 at microwave frequencies. The dielectric loss tan delta values of ZST ceramics are decreased by low-level doping of NiO, while the temperature coefficient of the resonance frequency tau(f) takes values in the range -2 to +4ppm/degrees C. Investigations on whispering gallery modes revealed low dielectric loss in millimetre-wave domain. An intrinsic quality factor of 480 was measured at 115.6GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter-wave applications, such as dielectric resonators, filters, planar antennas, hybrid microwave integrated circuits, etc. (c) 2005 Elsevier B.V. All rights reserved.

There is an increasing demand of high permittivity ferroelectric materials in microwave devices. Particularly, the Ba1-xSrxTiO3 system (BST), presents a major interest for electrically controlled devices, such as tunable filters, steerable antennas, phase shifters, varactors, etc. [F. De Flaviis, N.G. Alexopoulos, O.M. Stafsudd, IEEE Trans. Microw. Theory Tech., MTT-45 (6) (1997) 963-969]. Solid solutions with X=25, 50, 75, 90% were prepared by standard ceramic technology (solid state reaction) and sintered at 1230 and 1260 degrees C. The temperature dependence of permittivity and of losses at low frequency (1 kHz) has shown interesting results [A. loachim, M.I. Toacsen, M.G. Banciu, L. Nedelcu, D. Ghetu, H.V. Alexandru, C. Berbecaru, G. Stoica, Proceedings on the XVII International Conference on Electromagnetic Fields and Materials, Warszawa, May 2004, pp. 78-82]. The unit cell volume depends linearly on the Sr content ohm (angstrom(3)) = 61.1 + 3.6X (estimated from Ref. [D. Roy, S. D. Krupandihi, Appl. Phys. Lett. 62 (1993) 1056-1058]). The measured densities of our samples p (g/cm(3)); approximate to 5.50 - 1.00X, represent about 92% of the X-ray estimated density p (g/cm(3)) approximate to 5.99 - 0.99X. Microwave measurements around 1 GHz show substantial decrease of the permittivity from about 1600 to 200 and also of the losses from 0.8% to less than 0.15% with the Sr concentration increase, X = 50-90%. It is concluded that sintering temperature has to be increased for samples of high Sr concentration. Moreover, the addition of MgO and MnO2, 1 wt.% each, improve sintering conditions and decreases the microwave losses. Experimental data show an approximate linear decrease of the Curie point, versus X strontium concentration, according to the formula: T-C (degrees C) approximate to 120-360X. Room temperature permittivity and dielectric loss have been measured in microwaves range using the Hakki-Coleman method. (c) 2005 Elsevier B.V All rights reserved.

Pure and L- or D-alanime doped triglycine sulphate crystals (TGS) where grown in paraelectric phase around 52 degrees C. Doped crystals show lower and reproducible permittivities (crossing down the Curie point) and much lower losses versus pure crystals. Higher figures of merit have to be expected for highly doped crystals, suitable for IR detection. A remarkable difference of the experimental curves was noticed in the temperature dependence of the pyroelectric coefficients (p = dP(S)/dT) for L- and D-alanine doped samples. This fact suggests a non-equivalent L/D-alanine dopant substitution of the glycine group GI in the host lattice. Spontaneous polarization was calculated by integration of pyroelectric current data, and separately from the hysteresis loops recorded on a large temperature range, using Sawyer-Tower (phase compensated) circuit. Polarization values estimated from the two method compare reasonable well, except the temperature range in the proximity of the Curie point. lnternal bias field of similar to 1 kV/cm, induced by L-alanine dopant, stabilizes the polarization components P+ and P- opposite directions, with a peculiar dependence on the ac electric field. (c) 2005 Elsevier B.V. All rights reserved.

Surface enhanced Raman scattering with 676.4 and 1064 nm excitations was used to investigate single-walled carbon nanotubes (SWNTs) compressed non-hydro statically at 0.58 GPa, alone or dispersed into chemical reactive and non-reactive host matrices. By compression, SWNTs break in fragments of different size. Between SWNTs fragments and the host matrix various mechanico-chemical reactions take place. In matrices such KI and Ag, donor-acceptor complexes are formed. In aromatic hydrocarbons like biphenyl or p-terphenyl, an ionic and covalent functionalization of SWNTs fragments is demonstrated by new bands at 1160 and 1458 cm(-1), the latter being associated to an A(g)(2) pentagonal pinch mode. (c) 2005 Elsevier B.V. All rights reserved.

The investigation of the structure, magnetic and Mossbauer properties for the series of R-3(Fe,MO)(29) compounds has been performed, where R = Y, Nd, Sm, Gd, Tb, and Dy. The crystallographic structure of the ternary phase compounds has been investigated by Rietveld refinement of the X-ray diffraction patterns obtained at room temperature. The quality of the single-phase compounds was also checked by thermomagnetic measurements, from room temperature to above the Curie temperatures. From the magnetic isotherms for the free powder samples, measured at 4.2 K, the saturation magnetizations and the iron average magnetic moments have been derived. Fe-57 Mossbauer spectra of the R3 (Fe,MO)29 compounds have been measured at 15 K. The analysis of spectra, in a model which takes into account both the Fe atom nearest neighbor numbers and the Fe-Fe nearest neighbor bond lengths, indicates that the transferred contribution to the hyperfine field at the iron sites, due to rare earth moments, can be correlated with the rare earth effective spin. (c) 2004 Elsevier B.V. All rights reserved.

Nd-doped strontium lanthanum aluminate crystals, Nd:ASL (Sr1-xNdyLax-yMgxAl12-xO19), with an extended composition parameter x (0.05 and 0.5) have been grown by the Czochralski pulling technique. Structural and compositional properties of the as-grown crystals have been studied using X-ray diffraction, chemical microanalysis and optical spectroscopy. The results show that high crystalline perfection and large size crystals of both compositions can be grown. Low-temperature optical absorption spectra of crystals with compositions corresponding to x = 0.5 and 0.05 reveal that such crystals predominantly contain either C-1- or C-2-type Nd3+-centers, respectively. The possibility to thus differentiate between the two types of the dopant structural centers is of utmost importance for laser applications. (c) 2005 Elsevier B.V. All rights reserved.