National Institute Of Materials Physics - Romania

A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC2O4.0.5H(2)O and TiO(OH)(2).1.5H(2)O in a molar ratio of 1:4. Crystallized BaTi4O9 was obtained by the thermal decomposition of a precipitate precursor at 1300degreesC for 2 h in air. The dimensions of the powder calcined at 1000degreesC are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300degreesC are of the order of 10 to 30 mum. Dielectric properties of disk-shaped sintered specimens in the microwave frequency region were measured using the TE011 mode. Excellent microwave characteristics for BaTi4O9-epsilon = 38 +/- 0.5, Q = 3800-4000 at 6-7 GHz and tau(f) = 11 +/- 0.7 ppm/degreesC-were found.

We propose an improved method for the analysis of Thermally Stimulated Currents (TSC) measured on highly irradiated silicon diodes. The proposed TSC formula for the evaluation of a set of TSC spectra obtained with different reverse biases leads not only to the concentration of electron and hole traps visible in the spectra but also gives an estimation for the concentration of defects which not give rise to a peak in the 30-220 K TSC temperature range (very shallow or very deep levels). The method is applied to a diode irradiated with a neutron fluence of phi(n) = 1.82 x 10(13) n/cm(2). (C) 2002 Published by Elsevier Science B.V.

ZnO: Au films have been obtained by a reactive rf magnetron sputtering method using pure Zn and Au targets. Layers with a Au- to- Zn ratio of up to 0. 71 have been deposited. They were analysed by Rutherford back- scattering spectroscopy, elastic recoil detection analysis, transmission electron microscopy, high- resolution electron microscopy, selected- area electron diffraction, X- ray diffraction and Mossbauer spectroscopy. The only crystalline phase found for all investigated Au concentrations was that of ZnO. We conclude that some of the Au enters as interstitial atoms in the compact hexagonal lattice of ZnO being bonded with any three O atoms of the structure and the other part forms an amorphous hydrated gold oxide that coexists with the ZnO crystallites.

The reactive-field-assisted-sintering technique (RFAST) has been applied to freeze-dried decomposed precursor powders with a starting cation composition of Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5. A short time (< 15 min) RFAST treatment at 800-900degreesC did not lead to the formation of Bi2Sr2Ca2Cu3O10+x, (2223-phase). But RFAST was favourable to Bi2Sr2CaCu2O8+y, (2212-phase) formation particularly in samples with a low content of the 2212-phase in the precursor powder. Heat treatment (HT) after RFAST showed that the 2223 phase could form more rapidly than in freeze-dried pressed and sintered pellets produced by the traditional technique. The resultant ceramics after 70 h of annealing contain 75% 2223-phase and have T-c(R=0) = 100.7 K, T-c = 109.5 K and DeltaT(c) = 22.4 K.

We study the effect of magnetoplasmons on the drag resistance in a strong magnetic field, at finite temperatures, The typical magnetic field is about 1 T, and the temperature is up to 10 K. The Landau levels are broadened by disorder, but well separated in energy. We discuss intra-Landau level magnetoplasmons, with low frequencies, below omega(c), and inter-Landau level magnetoplasmons (also called Bernstein modes), with high frequencies, close to multiples of omega(c). We compare the temperature dependence of the minima and maxima of the Shubnikov-de Haas oscillations of the transresistance. (C) 2002 Published by Elsevier Science B.V.

We have investigated Ga0.47In0.53As/Al0.48In0.52As quantum well structures at magnetic fields close to the filling factor 1. At the critical current. a bistable and abrupt switching (transition width less than 4 ppm with respect to the filling factor) between quantum Hall conduction and dissipative conduction was observed at filling factors 1 < v < 1.5. As confirmed by Hartree-Fock calculations. we attribute this switching to a feedback effect between the current-dependent population of different spin levels and the exchange-enhanced spin gap, The effect is accompanied by a ferromagnetic memory of the spin polarization when changing the external magnetic field. (C) 2002 Elsevier Science B.V. All rights reserved.

Pyroelectric properties of triglycine sulfate (TGS) thick films, separately doped with L and D alanine were investigated. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilize the polarization in the opposite direction on the ferroelectric axis. Pyroelectric current (under constant stress) was recorded with a computer controlled Keithley 6517 electrometer, crossing up and down the Curie point. A reverse external electric field was applied on doped materials during heating, crossing up the Curie point. It is shown that the pyroelectric coefficient can be increased about four times at room temperature under un optimized DC electric field applied on the pyroelectric wafer.

The possibility of obtaining copper ferrite through the thermal decomposition of the two polynuclear coordination compounds: (NH4)(8) [Fe2Cu(C2O4)(8)](I) and [Fe2Cu(C2O4)(2) (OH)(4)].4H(2)O (II) was considered. The polynuclear compounds were characterized by various physical chemical techniques, e. g., IR, UV-VIS, EPR, Mossbauer spectra, thermal analysis and magnetic measurements. The final products obtained after thermal decomposition of the complex compounds were analysed by X-ray diffraction. A mixture of tetragonal CuFe2O4, alpha-Fe2O3 and CuO is generated from the thermolysis of compound I, while a clean tetragonal CuFe2O4 with saturation magnetization of 26.89 emu g(-1) is obtained from compound II.

In high temperature superconductors (HTS) the coherence length along non-c axis directions is longer. This feature can be useful when designing electronics devices based on HTS. Therefore growth and characterization of non-c axis oriented thin HTS films is of great interest. In this paper we present a short review of our data regarding (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO3 and (110) SrTiO3. The emphasis is made on improvement and control of the quality of the films by the "two-temperature" technological approach and/or use of the vicinal substrates. Phase and morphology evolution for different processing conditions, substrate's type and off-angle are presented. The highest critical temperatures of T-c0=67.2 K and T-c0=74 K for the "single-" and "two-" temperature routes were obtained on vicinal SrTiO3 with the off-angle of 20degrees. A higher off-angle promoted the formation of a specific step-like morphology with lower roughness. For the films grown on flat substrates the morphology was of mountain-range shape. Surface morphology as a result of two types of growth mechanisms (two-dimensional (2D), assisted by a so-called "twin"-growth and step-flow growth) for the (119) Bi-2223 films are discussed.

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a Subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm(-1), observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm(-1) up-shifted replicas i.e. the bands at 178 and 190 cm(-1). A different behavior of these bands in the anti-Stokes side was observed. (C) 2002 Elsevier Science Ltd. All rights reserved.