National Institute Of Materials Physics - Romania

Pd-Tm/gamma-Al2O3 catalysts containing 0.1% Pd-1% Tm, 0.3% Pd-1% Tm and 0.5% Pd-1.% Tm, respectively have been obtained by successive impregnation of gamma-Al2O3, first with palladium and then with thulium. Introduction of Tm leads to an increase in the conversion of vinylbenzenes compared to that with monometallic Pd/gamma l-Al2O3 catalysts. For monocomponent Pd/gamma-Al2O3 catalysts, turnover frequency (TOF) is unchanged. For bicomponent Pd-Tm/gamma-Al2O3 catalysts, if one considers that the reaction occurs only on the Pd particles, then introduction of Tm seems to increase the TOF. However, by taking Tm into consideration when calculating the TOF, then the bicomponent catalysts are seen to behave as the monocomponent catalysts. This behaviour is highlighted also by using the Koros-Nowak criterion. If Tm is not considered to be an active species, vinylbenzene hydrogenation appears to be controlled by mass transfer processes. However, the texture characteristics of the bicomponent catalysts are not sufficiently modified by Tm introduction to support such a hypothesis. On the contrary, by considering the Tm participation in the reaction, the slope of the Koros-Nowak dependence is included in a range typical for kinetically controlled reactions.

Current-voltage characteristics of multifilamentary Bi2Sr2Ca2Cu3O10/Ag tapes (short samples) produced by the 'powder in tube' technique were measured at different temperatures close to the mean-field critical temperature, and in zero applied magnetic field. After performing the required corrections due to the current flowing in the silver matrix, the I-V curves were interpreted in terms of current-induced unbinding of the thermally created vortex-antivortex pairs. Two possible mechanisms for appearance of a finite critical current in zero applied magnetic field are discussed: the Jensen-Minnhagen quasi-two-dimensional (2D) approach, that takes into account the interlayer Josephson coupling, and a model of size limitation of vortex fluctuations. From our analysis, it seems that the latter model is more suitable for this kind of superconducting material, due probably to an accentuated intrinsic anisotropy.

The dependence of the displacement damage induced by pions in silicon has been calculated as a function of their kinetic energy, between 50 and 1000 MeV, using data on pion-silicon interaction. The region of the delta resonance has been carefully considered. because it corresponds to a sharp maximum in the spectra of simulated pion production in the central cavity of LHC. Results are reported on previous calculations and on experimental damage data measured in silicon detectors irradiated with pions.

Four contact bulk samples and p(+)n junction diodes produced from high resistivity n-type silicon wafers of the same ingot have been irradiated with 1 MeV-neutron fluences between 10(12) and 3 x 10(13) cm(-2). The effective impurity concentration N-eff has been calculated from CV measurements in irradiated diodes, and bulk resistivity has been measured with the Van der Pauw method on four contact samples. The experimental resistivity and N-eff dependence on the irradiation fluence, self-annealing time and storage temperature have been modeled considering the exponential shallow donor decay and the creation of acceptor traps in the irradiated silicon. A numerical fit to the experimental data, based on the solution of the neutrality and Poisson equations, has been carried out considering an equivalent deep acceptor defect created during and after irradiation in the silicon lattice. The energy level and the introduction rate of this defect, evaluated as best-fit parameters in the numerical procedure, are E-t = 0.60-0.65 eV above the valence band edge and b similar to 0.06 cm(-1).

The UV/Vis diffuse reflectance (DR) spectra of NiO-Al2O3 systems obtained by mechanical mixing of oxides followed by calcination between 350 and 1400 degrees C are used for a detailed discussion and the reactualisation of an earlier band assignment. From temperature-programmed reduction (TPR) curves, a percent distribution of the total nickel content among various oxidic species, identified by hydrogen consumption peaks, was estimated. An attempt was made to estimate the fraction of octahedral Ni2+ ions in the spinel forms from the same TPR data. The results were correlated with XRD data and magnetisation measurements on the reduced samples.

When Polushkin and Vasiliev [V.N. Polushkin and B.V. Vasiliev, Investigation of rf SQUID behavior at liquid nitrogen temperature, Modern Phys. Lett. B, 3 (17) (1989) 1327] announced a HTS Zimmerman type SQUID, we had a laboratory technology for making niobium SQUIDs. The easy and less expensive capability of producing temperatures of about 80 K tempted us to follow the suggested method and we succeeded in making some HTS-SQUIDs. A technological research was developed undertaken and, as a result, a pretty good material for making SQUIDs was obtained. In order to characterize the SQUIDs, a PC based testing system was undertaken. A dedicated software was developed. The system structure, the main characteristics and some relevant results are presented. A digital-analog flux meter model using HTS-SQUIDs as a transducer is now in use. Its characteristics are summarised in this paper. (C) 1997 Elsevier Science S.A.

In this work, an InAs classical Hall effect based magnetic field sensor with linear response up to 1.00 T at room temperature is reported. InAs used is homogeneous polycrystalline n-type material. A supplementary simulation circuit and a suitable soft are used to insure a linear dependence of the Hall voltage on the magnetic induction. A 'butterfly' form for the sensor is used to increase the magnitude of the Hall signal. The influence of the magnetoresistive effect and irradiation with thermal neutrons on the response of the sensor is also investigated. (C) 1997 Elsevier Science S.A.

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation, The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-mu m-wide stripes was as high as 6.4 W before catastrophical optical degradation, If scaled to continuous-wave (CW) conditions, this value would be 800-1100 mW, which would mean a factor of 2-2.7 times more than reported for the best devices with normal design for threshold minimization, The absorption coefficient for the symmetrical structure is as low as 1.1 cm(-1), in spite of the low trapping efficiency of carriers in the quantum well (QW), The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions), Threshold current densities were 800 A/cm(2) for 5-mm-long devices in the symmetrical case and 2200 A/cm(2) in the asymmetrical one, The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed.

The active current and brightness pulses have been measured and recorded for good thin film electroluminescent devices. The transferred charge and the total number of emitted photons per pulse have been evaluated by direct numerical integration. Both these quantities depend almost linearly on the amplitude of the applied voltage pulses. In the investigated range of low frequencies (1 to 100 Hz) it was found that the measured efficiency per pulse is independent of voltage, brightness and frequency.

The correlation between the structure, chemical composition, electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals is presented. As revealed by SEM measurements, the small samples (approximately 1 mm x 1 mm x 0.2 mm) are stacks of thin plates (1-2 mu m). Microcompositional measurements (energy dispersive spectroscopy) show the Bi2Sr2CaCu2O8+delta (2212)-phase and a small quantity of residual phases enriched in bismuth and strontium and calcium-deficient. Magnetically modulated microwave absorption measurements (MAMMA) lead to a value for the anisotropy of the upper critical field of about 7, which is more than twice the reported value for YBa2Cu3O7-delta single crystals (also determined from MAMMA responses). The plot of the resistance versus temperature suggests that only the first 10-20 plates from the surface of the sample have a contribution to the sample conductivity. The imaginary part, chi'', of the magnetic susceptibility of our samples shows three peaks.