Materials Science Forum Vols. 480-481

Abstract: The apparent activation energy W, the temperature Ta at which the molecular movement is assumed frozen (the relaxation time is infinitely large) and the preexponential factor t0 were determined for Nylon 11 using the data obtained by fractional polarization thermally stimulated discharge current technique. The aim was to investigate the temperature dependence of the relaxation time in the range of the glass transition temperature. An experimental thermogram can be analyzed equally well using the Vogel-Fulcher-Tammann-Hess relationship or the Arrhenius equation. Experimental results and simulations, for nearby peaks, demonstrate that the relaxation time at peak temperature changes from about 100 to 500 s revealing a very narrow distribution in relaxation times. On contrary, W and especially t0 varies in large limits and the values for t0 are in many situations physically meaningless. It was found that the temperature at which the molecular movement can be assumed frozen for electric effects is about 150 K below the glass transition temperature in Nylon 11. A small change in W (in the limit ± kTm which represents the minimum uncertainty interval in W) induces a change in t0 of about two decades, indicating (i) that any analysis assuming t0 constant should be avoided and (ii) the selectivity limit of the method.

Abstract: Multialkali photocathodes are used from 200 to 850 nm, but high transparency at longer wavelengths reduces the quantum efficiencies. Theoretically, routes to enhance absorption could achieve high efficiency over most of this spectral range. Realisations of some of the concepts have been successfully attempted leading to quantum efficiencies of ~50%, with examples from 200 to 750 nm. Improvement factors of up to 50 times occur by ~900 nm, giving an extension of the useful wavelength operating range.

Abstract: Germanium containing heterofullerenes are produced by arc-discharge technique under ~100 torr He atmosphere and were extracted from the soot using CS2. Production and extraction procedures are detailed. FTIR spectrum of extract was presented. The mass distribution of the extract analyzed by FAB mass spectrum showed the stability of heterofullerene structures.

Abstract: The composition and properties of powders prepared by high-energy milling of copper sulphides with silicon have been studied by methods of X-ray diffractometry and surface area measurement. The combination of several methods was utilized to develop a coherent picture of the process. The overall process of mechano-chemical transformations in copper sulphide-silicon system proceeds via complicated mechanism with several overlapping steps. Phase transformations in copper sulphide phases, the formation of elemetal nanocopper with X-ray determined particle size of 23 nm and creation of ternary sulphide Cu8SiS6 with anomalous surface area values are the main products of the mechanochemical reaction.

Abstract: A new kind of a bound state has been identified at strong external excitation levels in semiconductors with high amphoteric doping, which is formed of a close donor-acceptor molecule and a neighboring second donor and/or acceptor. The recombination behavior of such a bound state is best described by AUGER-type transitions, reason for which this state is called an AUGER molecule. We have determined the existence region of such molecules in silicon-doped Ga1-xAlxAs with respect to excess carrier density and temperature by means of electron-beam excited luminescence spectroscopy at low and medium temperatures. The donor-acceptor pair radiation intensity is affected in a characteristic manner by the existence of the AUGER molecule state at elevated excitation levels and not too low temperatures. A computerized model calculation of corresponding rate coefficients turns out to be in good agreement with experimental results.

Abstract: Nanoscale metallic wires play a pivotal role in future microelectronics. Extremely high current densities, present in silicon-based integrated circuits, cause wire destruction by electron-wind induced atomic migration and void formation. In the present paper we elaborate a theoretical model, which describes the interaction of impurity-vacancy pairs. Criteria are given for an optimum material selection, based on the atomic valence of matrix and alloying metal, which reduce (or enhance) the probability of void formation.

Abstract: We present magnetic and morphological characterization of iron- and iron-carbide- based nanopowder obtained by the laser synthesis from sensitized gas phase mixture containing acetylene and iron pentacarbonyl vapors. The analysis was performed on the as-prepared material and the annealed material. The results of TEM, XRD, Mössbauer and magnetic measurements are reported. Phase transformations taking place during annealing of the nanopowder when core-shell nanoparticles appear are discussed.

Abstract: The aim of this work is to study the new method of direct synthesis of lead and iodine as the input material of PbI2. This method has not been studied for this material till now, and seems to be one of the new methods for preparation of the input material. The photoluminescence measurement and measurement of resistivity has been done and compared with the measurements done by precipitation and zone purification.

Abstract: The preparation and characterization of thick InP layers by Liquid Phase Epitaxy with admixture of Ce, Tb, Dy and Yb in the growth melt is reported. Measurement of temperature dependent Hall affect, C-V characteristics and low-temperature photoluminescence show the change of n®p type conductivity and considerable improvement of structural and electro-optical parameters for all studied rare-earth elements. Mn was identified as dominant acceptor impurity in the case of Tb and Dy addition. In the case of Ce and Yb the dominant acceptor was identified as isoelectronic Ce or Yb on the In site subjected to a strong electron-lattice interaction. The reported high quality thick p-type InP layers could well be used for the preparation of radiation detector structures.

Abstract: Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).