National Institute Of Materials Physics - Romania

The chemical interaction of chalcogenide vitreous semiconductors (ChVS) with absorbed impurities induced by high-energetic (E>1 MeV) gamma-irradiation has been investigated at the example of vitreous v-As2S3. Radiation-induced processes of oxidation, hydrogenization, hydratation, carbonization and hydrocarbonization have been observed in this material after prolonged gamma-irradiation and studied using IR spectroscopy (4000-400 cm(-1)), laser mass-spectroscopy methods and electron microprobe analysis.

(Zr0.8Sn0.2)TiO4 ceramic (ZST) has been prepared and characterized. The effects of sintering parameters such as sintering temperature, sintering time and NiO addition on structural and dielectric properties were investigated. The material has a dielectric constant epsilon(r) similar to 36.0 and high values of the Q . f product from 32,170 to 50,000 at microwave frequencies. The tan delta values are decreased by low level doping, of NiO, while the temperature coefficient of the resonance frequency tau(f) takes values in the range (-2divided by+4) ppm/degreesC. Investigations on whispering gallery modes revealed low dielectric losses in millimeter wave domain. An intrinsic quality factor of 480 was measured at 136.3 GHz. 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 filters, hybrid microwave integrated circuits, etc.

In this paper the results of electrical measurements of the mass-loading influence on Y-cut langasite resonator parameters are compared with those obtained by X-ray topography analysis of the same resonators. Based on the Ballato's transmission-line analogs of the trapped-energy ;resonators vibrating in thickness-shear mode [1], the mass-loading effect on resonator characteristics was studied. The effective mass-loading, motional inductance and quality factor of langasite resonators were computed. Sawyer plan-parallel polished Y-cut langasite resonators with 14mm diameter, 5 MHz resonant frequency, Au electrodes of 7 mm diameters and various thickness were used in experiments. X-ray topography measurements were performed by conventional transmission Laue setting using the white beam synchrotron radiation on fundamental, third and fifth overtones. The results are in agreement with those obtained by electrical measurements. The comparison of X-ray diffraction topography images previously performed on AT-cut quartz resonators with X-ray topographs on langasite resonators pointed out that the Y-cut langasite resonators are less influenced by the mass-loading than the AT-cut quartz resonators.

Raman spectroscopy has been employed to study spectra of the vibrational frequencies of carbon in the range 1000-1800 cm(-1) in amorphous carbon films modified with iron. The content of iron in a film has been set technologically by varying between the samples the area ratio of the graphite and iron parts of the target and monitored by Rutherford backscattering. Amorphous carbon layers containing 3, 26, 38, and 54 at. % iron have been obtained. The Raman spectra have the form of a broad band with features that are characteristic of amorphous carbon and lie at 1332 cm(-1) (D-band) and 1552 cm(-1) (G-band). At iron content of 38 and 54 at. %, the spectrum breaks up into a number of bands, which points to fragmentation of the matrix of amorphous carbon with increasing iron content. It is shown that the integral amplitude of scattering decreases nonlinearly with increasing iron content in a sample. The effect is analyzed in terms of a model that accounts for the linear dependence of the rate at which the number of atoms contributing to the scattering changes with the atomic fraction of carbon relative to the total number of carbon atoms in the film. (C) 2003 MAIK "Nauka/Interperiodica".

The thermal behaviour of three coordination compounds, potential precursors of nickel ferrite [Fe2Ni(C4H4O5)(2.5)(OH)(2)]NO3.5H(2)O,[Fe2Ni(C4H8O3N2)(4)](NO3)(8).24H(2)O and (NH4)[Fe2Ni(C4H4O5)(3) (OH)(3)].3H(2)O has been investigated to evaluate their suitability as precursors for nickel ferrite. For a complete and reliable assignment of the thermal transformations, the isolable solid intermediates and end products were characterized by IR, X-ray diffraction and Mossbauer investigations. A decomposition scheme is proposed.

Designs of new compact filters using cross-coupled resonators are presented. The square shape resonators require only 40% of the area of folded open loop half-wavelength resonators. The filters were designed for the 900 MHz band of the mobile communications systems (GSM, GPRS, etc.). The cross coupling provides a better filter skirt for a higher selectivity. The effect of the resonator finite quality factors was confirmed for the cross-coupled filters.

We study the transport properties of metal-insulator-semiconductor-type quantum devices with a back gate located close to the field induced two-dimensional electron gas (2DEG). These devices allow for a detailed analysis of the coupling between the quantum system and the contacts (top- and back gate) which we consider as classical particle reservoirs with infinite conductivity. The transport properties are described in the Landauer-Buttiker formalism and the Hartree approximation is used for the self-consistent calculations of the ground state. Decreasing the width of the insulator barrier, at low temperatures a leakage tunneling current starts to flow. The states of the 2DEG provide a resonant tunneling level for the vertical transport between back- and top gate. We analyze the effect of the resonance on the current and on the capacitance of the system. Analytical calculations based on our previous theory on resonant transport allow for a better numerical implementation of the model and for a detailed understanding of the electronic states. (C) 2003 Elsevier Ltd. All rights reserved.

Two series of PT-type materials doped with Eu+3 with the formula PbxEuyTi0.98Mn0.02O3 (x=0.98, 1.00 and y=0.00, 0.02, 0.04, 0.08, 0.12) were prepared by conventional ceramic technique. The compounds with compensated and uncompensated valences were sintered between 1150 and 1220degreesC. The two types of compounds and the content of the donor ion, Eu+3 , influence the mechanical and dielectric properties of sintered samples. The spectra and data obtained on Eu-151 Mossbauer measurements are discussed.

It is shown that self-organization can be revealed in non-crystalline materials. Experimental and modelling studies are discussed. The importance of the Boolchand intermediary phase for the chalcogenide network glasses with self-organization is pointed out. The basical principles of self-organization are the key for the production of nano-materials with new properties.

Hybrid polymer based on selenium-sulphur doped by paraffins has been deposited by pulsed laser deposition. The properties of the thin films were investigated. Sample stabilization was carried out by annealing. A nano-crystalline fraction was obtained in the films annealed at 100degreesC. The structure of the nanocrystals corresponds to the Se3S5 crystallographic form. The films exhibit optical memory properties and are suggested for use in optical phase change memories.