National Institute Of Materials Physics - Romania

A new model is proposed for conduction in thick-film SnO2 gas sensors, which offers a good explanation for the ageing effect. in this model, electrical resistance is governed by necks and it results from three interdependent resistances of three different regions of the sample, connected in parallel: a surface resistance which is that of a gas-sensitive, predominantly n-type semiconductor, a resistance, belonging to a different region of the sample, which, due to its interaction with water vapour, behaves like a mixed n-type and p-type semiconductor, and the n-type bulk resistance of the neck, slightly affected by gases and/or water vapour. (C) 1999 Elsevier Science S.A. All rights reserved.

A microstructural investigation of porous silicon films, related with a study of carriers transport. is presented. Scanning electron microscopy and conventional transmission electron microscopy with selected area electron diffraction were performed. The films show a double scale porosity: the macroporosity forms an alveolar structure at a micrometric scale and the nanoporosity forms a silicon nanowires network in the alveolar walls and at their surface. Current-voltage characteristics in the -5 to +5 V range and the temperature dependence of dark current in the 150-330 K interval at low voltage were measured. The transport properties are explained by means of a simplified quantum confinement model. Two symmetries of the nanowires, cylindrical and square, are investigated and the differences between them are shown to be insignificant. (C) 1999 The Electrochemical Society. S0013-4651(98)12-109-2. All rights reserved.

gamma-Fe2O3 nanoparticles dispersed in a polymer matrix have been characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The electron paramagnetic resonance (EPR) in 64 Angstrom particles of gamma-Fe2O3 dispersed in a polymer matrix has been investigated in the range 136-286 K.

Iron carbides and iron composites preseal a particular interest due to their magnetic and catalytic properties which could be highly enhanced if particles in the nanometric size range were used. We present results on the synthesis and characterization of iron-carbide nanoparticles by sensitized CO2 laser pyrolysis or gas mixtures containing iron pentacarbonyl (vapors) and ethylene/acetylene precursors. The influence of the nature of hydrocarbon on the structural properties of the nanopowders are revealed by different analytical techniques (IR spectrophotometry, X-ray diffraction, transmission electron microscopy and Mossbauer spectroscopy).

Carbon nitride (CNx) this films were produced by CO2 laser (lambda=10.6 mu m) irradiation of mixtures containing C2H2/N2O/NH3, in a flow reactor, on Si substrates. The experimental parameters (partial concentrations of the reactants, gas flows. total pressure) were chosen in order to maximize the nitrogen incorporation in films. Chemical composition and bonding structure of the deposited films were investigated by; X-ray photoelectron spectroscopy (XPS). Thus, it was found that nitrogen is chemically bonded to C in sp(2) or sp(3) configurations, the NIC ratio (considering only the N atoms bonded to carbon) being similar to 20%. Scanning electron microscopy (SEM) shows a specific growth morphology, while the transmission electron diffraction (TED) and X-ray diffraction (XRD) analysis revealed that the CN, films were crystalline, with diffraction lines that marches rather well with those of the predicted beta-C3N4 form.

We compare the quantum and the classical description of the two-dimensional motion of electrons subjected to a perpendicular magnetic field and a one-dimensional lateral superlattice defined by spatially periodic magnetic and electric fields of large amplitudes. We explain in detail the complicated energy spectra, consisting of superimposed branches of strong and of weak dispersion, by the correspondence between the respective eigenstates and the ''channeled'' and ''drifting'' orbits of the classical description. [S0163-1829(99)04732-3].

Hall effect, magnetoresistance, and electrical conductivity measurements, carried out on ZnO surface wells created by a large variety of methods, are analyzed in the frame of the weak-localization theory. The ZnO surface wells have some unique features that allow the investigation of the weak-localization effects: ZnO has a single valley conduction band; the Thouless length is much larger than the elastic mean-free path even at room temperature; the well accumulates the largest surface electron concentration obtained up to now in a surface quantum well; there are a large variety of preparation methods, some of them making it possible to modify independently both the width and the depth of the surface wells. These features allowed us to investigate: the presence of the weak-localization effect in the largest range of temperatures(1.6-300 K) reported up to now for a quantum well; the influence on the transport properties of the increase in the number of subbands in the well; the effect of the presence of more inelastic scattering mechanisms and their weights in the entire scattering process; and the passage from a quasi-two-dimensional system to a three-dimensional one. [S0163-1829(99)01932-3].

A comparative theoretical study of the damages produced by protons and pions, in the energy range 50 MeV-50 GeV, in diamond, is presented. The concentration of primary defects (CPD) induced by hadron irradiation is used to describe material degradation. The CPD has very different behaviours for protons and pions: the proton degradation is important at low energies and is higher than the pion one in the whole energy range investigated, with the exception of the Delta(33) resonance region, where a large maximum of the degradation exists for pions. In comparison with silicon, the most investigated and the most utilised semiconductor material for detectors, diamond theoretically proves to be one order of magnitude more resistant both to proton and pion irradiation. (C) 1999 Elsevier Science B.V. All rights reserved.

A unique resolution function of angular-dispersive synchrotron radiation powder diffractometers, properly parameterized to be valid for any multicrystal variant of this instrument, has been derived paying special attention to the absolute correctness of the formulae by avoiding any unknown proportionality factors. Moreover, the concept of experimental performance criterion has been proposed in an attempt to introduce a kind of 'worth-scale' for the results of measurements performed with a certain instrumental configuration. Both entities were used in computer simulations to determine the most convenient configuration of instrumental parameters that, apart from the fulfilment of the experiment seal, ensure an adequate counting rate. Several case studies using different instrumental layouts, different single-crystal monochromators and samples of different low symmetries and unit-cell volumes were considered. The results offer reliable information that may be useful for designing diffractometers as well as for conducting experiments.

Single-phase icosahedral phases (i-phases) of TiZrNi and AlCuFe alloys were obtained by single roller melt-spinning in protective argon atmosphere. The high-pressure (up to 25 GPa) compression properties of i-TiZrNi and i-AlCuFe were investigated by in situ energy-dispersive synchrotron radiation (SR) diffraction experiments using a gas-membrane diamond-anvil cell. In contrast to i-AlCuFe, reproducible deviations from the Birch-Murnaghan equation of state were noticed in the form of hysterezis loops during the first compression cycles of i-TiZrNi alloys. This response of quasicrystalline matter to hydrostatic pressure conditions is here reported for the first time. Plastic deformation processes and effects related to the small grain size of the rapidly-quenched TiZrNi alloys are tentatively considered as possible explanations. (C) 1999 Elsevier Science B.V. All rights reserved.