Publications

Fresh silica gels prepared by the sol-gel route with acid catalysis display typical alterations of the X-ray diffraction (XRD) pattern, in comparison with vitreous (v) silica. A positive shift of the first peak at k(1) is noticed, which also shows excess broadening. The changes can be accounted for by the alteration of the intermediate-range order (IRO) in gels; in comparison with v-SiO2 Thermal annealing causes a gradual decrease in the shift and broadening of the first peak, suggesting that the evolution of the vitrification can be followed on the basis of XRD data. Two types of IRO alteration were modelled, which account quantitatively for the experimental observations using a phe nomenological disorder parameter b. The evolution of the b values with annealing temperature reveals well-defined stages in the vitrification process.

Metastable phases can appear in metallic alloys if their surface response function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. in this case, rapid solidification procedures can result in their formation. On the other hand, metastable alloys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decomposition. These microstructures depend on the thermal history of the sample (e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag1-xGex (x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase is the Hume-Rothery type. Besides, an alpha-Ag-Ge solid solution was present. Amounts of non-reacted Ge were also found, with a lattice constant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatures of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks appeared with relatively high heat release (46.6 and 48.8 J g(-1), respectively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidification process and are expected to affect their stability. (C) 1997 Elsevier Science S.A.

The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.

The effect of LiOH addition to YBa2Cu3O7-x is presented. The crystal structure remains orthorhombic, but the orthorhombic splitting decreases from 0.0171 at 2 at. % Li to 0.0158 at 50 at. % Li. The critical temperature exhibits a decrease from 91-.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law, j(c) proportional to B-alpha, with alpha approximate to 1/2. V-I characteristics are typical for grained superconducting materials: V proportional to(I-I-c)(n(B,T)). The transport critical current dependence on temperature and magnetic field is also presented.

The authors obtained the Fe-57 Mossbauer spectra of two pumpellyite samples obtained from two different mineral sources in the Romanian Carpathians. Using a specialized computation program of the quadrupole splitting spectral parameter, QSCOMP, the observed quadrupole doublets were attributed to the ferric and ferrous ions in octahedral sites. The refined analysis of spectral parameters permitted us to characterize both the oxidation state, the electronic configuration and the coordination symmetry with the distortions and occupancies of pumpellyite constituting iron ions. The differences of the spectral parameters giving the data of the iron abundance, the chemical iron-oxygen bonds, the deformation and occupancies of iron octahedra, observed at the two studied samples, are discussed and correlated by the different genesis conditions of two pumpellyite-bearing occurrences.

An extended Williams-Landel-Ferry equation is derived within the free volume approximation. This equation relates the jump frequency of an arbitrary molecule (around the glass transition temperature) to the segmental jump frequency at the glass transition temperature. The connection with the usual WLF equation is discussed.

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

An attempt has been made to put in evidence the modified V-K centres in low temperature UV irradiated Kl:Cl crystals. The interpretation of the results seems to favour a new type of V-K centre localised near an impurity halogen instead of a heteronuclear V-K centre.

Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration field H* was determined, at different temperatures, from the field dependence of chi '' in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T-c, which are consistent with the usually reported data for this material.