National Institute Of Materials Physics - Romania

We studied by means of optical absorption and transmission electron microscopy the formation of Ag nanoclusters from Ag--ions in three different potassium halides (KCl, KI and KBr). By stepwise UV illumination at a suitable temperature, the electrons of Ag--ions were detached and the controlled formation and growth of Ag nanoclusters from Ago-atoms take place. At the same time, the detached electrons are captured by K+-ions, proved by the appearance of K-colloid bands. The kinetics of the conversion Ag- + hv + U --> Ag nanoclusters was followed by monitoring the Ag--ions and Ag nanoclusters absorption. The temperatures of illumination are different, depending on the matrix in which Ag nanoclusters are formed (220 degreesC for KCl, 160 degreesC for KBr and 140 degreesC for KI). By optical absorption and TEM we proved that the nanoclusters of Ag obtained by our method are separated, spherical and ovoidal, with average diameters between 7.5 and 50 nm. (C) 2002 Elsevier Science B.V. All rights reserved.

We found Oxygen-doped GaAs crystals to be suitable materials for CO2 laser optical component preparation, with application at 10.6 mum. An optical transmission of 55% in the IR spectrum range, between 2 and 15 mum has been reached for such a GaAs type material. The GaAs crystals that we have analysed were grown by two procedures: Horizontal Bridgman (HB) and Liquid Encapsulated Czochralski (LEC). The FIB method has been used for obtaining pure (undoped) crystals, while the oxygen-doped GaAs ingots were grown by LEC technique. The two types of samples processed in the same manner as regards mechanical polishing and chemical etching, which were investigated by Hall measurements, optical transmission spectrometry and elastic recoil detection analysis (ERDA) technique. The GaAs:O (LEC) has near semi-insulating properties as can be observed from the results of the electrical resistivity and Hall effect measurements. The ERDA spectrum shows an intense signal of oxygen in the bulk of GaAs:O (LEC) crystals, while the oxygen signal is not present in the ERDA spectrum of the undoped GaAs (HB). We consider that these results could recommend the ERDA technique as a possible qualitative and quantitative analysis in an ion-beam accelerator for oxygen content in oxygen-doped GaAs crystals. The analysis is not sensitive to the native oxide, as could be seen by measuring GaAs (HB) undoped crystals. (C) 2002 Elsevier Science B.V. All rights reserved.

In the case of AsSe:Sn and Ge27Sb13Se60 amorphous thin films deposited on Si(100) wafer, it was observed that the diffraction pattern is distorted by the escape-peak, which is very sensitive in intensity to the wafer's position in its own plane, This peak was found to be superimposed on the first sharp diffraction peak, which characterises the medium range order in the amorphous materials. A method for the minimisation of the escape-peak effect is presented. A different method is suggested to avoid this peak. (C) 2002 Elsevier Science B.V. All rights reserved.

Photodarkening relaxation under light exposure of a-As2Se3 amorphous films doped with 0.5 at.% of metals Sn, Mn, Sm or Dy and a-AsSe films doped up to 10.0 at.% Sn was studied to determine its dependence on the type and concentration of impurities and thermal treatments. Both factors reduce photodarkening with the degree of reduction dependent on the type of impurity. The relaxation process may be described by a stretched exponential with the dispersion parameter 0.4 less than or equal to alpha < 0.9 and the time constant increasing with Impurity concentration or thermal annealing. The results are discussed in the context of the recently formulated 'slip-motion' model of photodarkening in chalecgenide glasses. (C) 2002 Elsevier Science B.V. All rights reserved.

In-plane and out-of-plane magnetoresistivity (MR) measurements were performed on the same antiferromagnetic (AF)YBa2Cu3O6.25 single crystal using a six-lead configuration, in magnetic fields H applied along the ab plane. We identified three terms contributing to both MRs. The first term is anisotropic with respect to the in-plane field orientation, with a twofold symmetry for the in-plane MR and a fourfold symmetry for the out-of-plane MR. We show that these anisotropic features of the magnetoresistivity tensor can be understood in terms of the coupling between the spin and the elastic degrees of freedom. The second term in both MRs is negative and isotropic upon in-plane H rotation. We ascribe this term to charge-carrier scattering on the AF domain walls. Finally, the third term is positive, quadratic in H and seems to correlate with the AF phase transition.

Results of x-ray powder diffraction and magnetic measurements performed on RFe11.3W0.7 compounds, where R=Dy, Ho, Er, and Lu, are presented. The Curie temperature, saturation magnetization, and spin reorientation transitions are discussed in relation to several RFe12-xTx systems, where T=Ta, Nb, Mo, Ti, and the x-substituent concentration is within a narrow range x=0.5-1.0. In these systems, the dependence of the Curie temperature versus the unit cell volume, for a given R partner, suggests combined size and electronic effects produced by the T atom. The strengths of the rare-earth-iron and iron-iron exchange interactions are evaluated in the mean-field framework, and analyzed with reference to RFe12-xTx compounds, with T=Ta, Re, Nb, Mo, Ti, Cr, and V. Based on (i) the phenomenological mean-field model accounting for the sublattice of conduction band electrons, (ii) Brooks' reinterpretation of Campbell's formulation of the intersublattice exchange, and (iii) Wigner-Seitz cell analysis, we explore the role played by the conduction band electrons, particularly the nd-type electrons (n=3, 4, 5), on the intersublattice exchange coupling in the iron-rich RFe12-xTx class of compounds. It is proposed that the variation in the conduction-electron density, by varying the T element type and concentration, leads to a modification of the 5d band occupancy number and consequently, of the R-Fe intersublattice exchange coupling.

The crystallization behaviour of amorphous melt-spun SmxFe80-xB20 ribbons is investigated by differential scanning calorimetry, X-ray diffraction and Mossbauer spectrometry. On increasing the Sm content. the crystallization temperature reaches a maximum for a critical Sm content (x(c) approximate to 8) and the main crystallization products are different below and above x,. This is attributed to a change in the short-range local order in the amorphous alloy. (C) 2002 Elsevier Science B.V. All rights reserved.

The continuity between the thermally driven first-order vortex lattice melting observed at high temperatures and the disorder-driven transition appearing in the low-temperature region of the vortex phase diagram of YBa2Cu3O7-delta (YBCO) is still controversial. Pal et al. have reported a rich vortex phase diagram for relatively clean YBCO crystals, in which, in qualitative agreement with a recent theoretical proposal, the end point of the vortex lattice melting is located well above the disorder-driven transition line. However, the results presented by Pal et al. suggest the existence of domains of different quenched disorder degree in the investigated crystal, and in these conditions the global magnetic measurements cannot lead to definite conclusions about the vortex phase diagram of high-temperature superconductors.

The fuzzy-logic process control system has been adapted to the unbalanced magnetron reactive sputter deposition of Ti1-xAlxN coatings. This made possible the dynamic control of the deposition rate, chemical composition and structure of the coatings and provided stable working conditions inside the hysteresis loop of the reactive sputtering also. By applying this control system, multilayers of micro- and nanocrystalline composite structures were prepared with compositions of Ti39Al61N and Ti56Al44N of TiN-AlN cubic solid solution phases. (C) 2002 Elsevier Science B.V. All rights reserved.

Advanced electroceramics have played a critical role in the development of new and modem technologies such as computers, telecomunications and aerospace and they will continue to play the leading role in the technology of the future. The present report presents and discusses a number of advanced electroceramic materials with special references to their specific properties that make them indispensable in a large area of applications. The materials discussed include cobalt-mangan oxides used as negative temperature coefficient thermistors, semiconducting doped barium titanate used for positive temperature coefficient thermistors with large applications in thermal protections and piezoelectric electroceramics of PZT type, which find new application as ceramic transducers and sensors in biomedical and aerospace industries or as simply buzzers, filters, igniters, ultrasonic cleaners, towed array sonars, medical imaging system, ink-jet printing heads, camera shuttlers, positioners for optical systems, micromotors and actuators. Such advanced electroceramic materials are extremely useful for the future sophisticated technologies such as heads for magnetic recording, scanning tunneling microscope, sensors for antilock braking systems, ultrasonic bone-healing devices and some other biomedical applications.