National Institute Of Materials Physics - Romania

Dendritic structures were grown inside the quartz crystal by electrodiffusion of sodium ions from a NaCl layer at the anode. The electric field was applied along the optical axis Z and the structures were grown in the xy plane. An experimental arrangement using a pointed cathode and a graphite plate anode was employed. X ray diffraction analysis shows that the structures' chemical composition is a mixture of sodium silicates. To explain the structures' growth a model is proposed. The model is a bidimensional one. The transport of the particles along the Z axis structural channels by the electric field was taken into account by introducing a probability of generating the particles in the growth plane as a function of the distance to the cathode. The computer generated results and the experimental ones were compared and a good correlation was found.

Pd-1wt% Eu/gammaAl(2)O(3) catalysts with different palladium loadings (0, 1, 03 and 0.5 wt%) were prepared by the wetness successive impregnation method. The state of the supported particles and their interaction with the support were evidenced by transmission electron microscopy (TEM) and electron diffraction (SAED) measurements. The deposition of palladium and europium was found to result in a heterogeneous surface. These catalysts were tested in the hydrogenation of the unsaturated aromatic compounds (styrene, divinylbenzene, indene) and the catalytic results were correlated with the TEM and SAED data. On the basis of this. correlation it was concluded that the hydrogenation occurs on both metallic palladium particles and mixed aluminium-europium oxides. Onto metallic particles hydrogenation occurs with radicalic hydrogen atoms while on the oxidic sites with hydrogen spilled over from palladium.

Some Cr, Ni films have been deposited by RF (1.78 MHz) magnetron sputtering onto thermoelectric materials substrates at low temperatures (<100 degreesC). We have found good adherence values (>100 kgf/cm(2)) onto n and p type substrates. Peltier devices prepared from these wafers and tested using the standard procedure (Melcor Corp.) demonstrated good values of various electrical parameters, similar with those obtained using classical contacts. This research proved the possibility of producing good quality Peltier devices using Ni/Cr electrical contacts obtained by a large scale and versatile deposition method.

Structural and piezoelectric properties of PbTiO3 (PT) ceramics materials doped with different rare earth elements were investigated. The basic chemical formula was Pb(0.88)Ln(0.08)Ti(0.98)Mn(0.02)O(3), where Ln stands for rare earth element (Ce, Pr, Gd, Nd). The powders were prepared by conventional ceramic method. All samples showed a tetragonal perovskite structure at room temperature. After sintering, bulk materials with high density were achieved for all compositions. Followed by XRD, the cell evolution and the structural anisotropy depend on the lanthanide ionic radius. The crystallite size was estimated from XRD measurements. Piezoelectric measurements (d(33) and k(p)) were performed using resonance - antiresonance frequencies. Ce-doped PT showed the highest d(33) coefficient (similar to39pC/N), while for the other doped materials d(33) was only of the order 30 pC/N.

Nitrogen doped 4H-SiC epitaxial layers grown by hot-wall chemical vapor deposition were investigated by deep level transient spectroscopy in the as-grown state. Besides the Z(1,2) defect level at E-C - 0.66 eV, that seems to be omnipresent, four other electron traps labeled IL1 to IL4 With ionization energies between 0.87 and 1.31 eV could be detected. The dependence of the deep level concentrations on the incorporated N concentration ranging from few 10(14) to Some 10(15) cm(-3) and the C/Si ratio (1.2divided by3) was examined. The concentration of both Z(1,2) and IL1 increases with increasing N doping. For medium C/Si ratios this dependence is linear for Z(1,2) and quadratic for IL1. High C/Si ratios enhance the formation of Z(1,2) while they suppress that of IL1. These results suggest a complex of interstitial C and N on carbon site (C-i-N-C) for Z(1,2) and a nitrogen pair (N-C-N-i) for IL1. However, other possibilities like Si vacancy related defects cannot be ruled out at present.

MOS-like structures were obtained by chemical deposition of a polycrystalline PbS thin film on top of a silicon dioxide/Si substrate. Gold ohmic electrodes in coplanar configuration were subsequently deposited by vacuum evaporation on PbS surface (drain and source electrodes). The gate aluminum electrode was deposited on the back of the Si substrate. The dependence of the photoconductive signal, generated in the PbS film, on the gate voltage was studied for wavelengths ranging between 800 nm and 3000 nm at room temperature as well as at low temperatures. It was found that the relative variation of the signal could be as high as 50% for gate voltages ranging between -30 V and +30 V. Two possible mechanisms are proposed to explain the signal variation with the gate voltage: 1) Variation of the depleted region's thickness in the PbS film, that leads to a variation of the conduction channel's resistance (the reference resistance called, also, the dark resistance), 2) The possible variation of the majority carriers (holes) life-time due to the electron blocking at the PbS/oxide interface when positive gate voltages are applied on the back electrode. Integrated IR detectors with controlled sensitivity in the 800-3000 nm range can be manufactured at a relatively low cost using the PbS/oxide/Si MOS-like structure.

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.

Nanocrystalline silicon is studied with a view to obtaining a new photonic material. Non-equilibrium electronic processes in such materials play a significant role. We have studied trapping phenomena in nanocrystalline porous silicon and nanocrystalline silicon-based Multi-Quantum Wells structures by means of Optical Charging Spectroscopy method, which is a very good and sensitive method. We have also analyzed the modeling of the processes that occur during our measurements. This modeling allows us to separate the relative contribution of the different types of discharge currents that can appear: ohmic conduction currents of either equilibrium or non-equilibrium carriers, displacement currents, diffusion currents and tunneling currents through insulating layers (in Multi-Quantum Wells structures). It also allows us to increase the accuracy of the determination of the experimental trap parameters and to determine parameters that are not directly measurable. The model can be applied to other nanocrystalline semiconductors and can be easily generalized for other high resistivity materials.

Glassy composition As25Te35Si40 was investigated by microhardness, differential scanning calorimetry, X-ray diffraction, electrical conductivity and Raman spectroscopy measurements. A continuous random network model of the glass structure has been developed.