Publications

Pb(Zr-x Ti1-x)O-3 (x = 55, 65, 92), PZT step graded-structure have been deposited by sol-gel technique on Pt-coated Si. Up-graded and down-graded structures has been obtained by changing the Zr/Ti ratio in steps a long to the film thickness. Up means that the ratio was increased from the bottom to the top while down means it was decreased. The properties of these films were investigated and compared with those of conventional films with homogeneous composition. It was found that the properties of up and down graded structures are completely different. Up-graded films have better ferroelectric properties than the downgraded ones. Pyroelectric signal could be detected also on as-grown up-graded films, in accordance with the structural results that show a strong (100) orientation in this case, The coercive field of the up-graded film is lower than the values obtained in case of homogeneous films with the same Zr/Ti ratios like those used in the graded structures.

We have proposed a lattice matching mechanism for the selection of (111) PZT orientation, based on the! experimental evidence of the formation of cubic TiO on the Pt electrode surface in the earlier stages of thermal processing of PZT layers Thus, a fitting of TiO with Pt and, consequently of PZT (Ti-rich) with TiO (orientation relationship:TiO (111)//Pt(111) and respectively PZT(111)//TiO(111)) has been evaluated. Similar mismatch degree values were obtained comparatively with the classical cases of intermetallic layer "templates" (for example PtxPb, Pt3Ti, etc). The very good lattice matching supports the proposed mechanism, consisting in the formation of cubic TiO on Pt and, later, of the nucleation and growth of PZT (tetragonal) of cubic TiO, where is formed. In comparison with the case of TiO2(rutile) matching, previously proposed by other groups,having larger mismatch values,the cube-to-cube orientation relationship could explain the selection of (111) orientation as a major one in some specific conditions affected by some parameters especially related to solution chemistry and thermal processing steps.

Hydroxyapatite (HA) thin films for applications in bone prosthesis fabrication were obtained by the radio-frequency magnetron sputtering technique. The depositions were performed from pure HA targets on Si and Ti-5AI-2.5Fe alloy substrates. In some experiments a buffer layer of TiN was introduced by pulsed laser deposition before HA coating. The films were deposited in low-pressure argon or Ar-O-2 mixtures at substrate temperatures ranging from 70 to 550 degreesC. We observed that the films grown at temperatures below 300 degreesC were prevalently amorphous and contain a small amount of crystalline material. The initial crystalline structure of the target is reconstructed after 1 h films annealing at 550 degreesC in ambient air. Both the films directly deposited at 550 degreesC and the ones obtained at room temperature and after that annealed at this temperature-mostly contain the HA phase and exhibit good mechanical characteristics. Weaker peaks of CaO secondary phase are visible in the X-ray diffraction patterns of the films directly grown at 550 degreesC. Traces of TiO2 were detected at the interface with the metallic substrate in case of structures grown without the TiN interlayer. All films were smooth, with an average surface roughness of 50 nm. The films grown on TiN interlayer are harder, have higher elasticity modulus values and an increased mechanical resistance at the indenter penetration. (C) 2003 Elsevier Science B.V. All rights reserved.

The alkoxidic route and the spinning deposition were used to prepare monolayer sol-gel indium tin oxide (ITO) films. The morphology and crystalline structure were investigated by cross-section transmission electron microscopy (XTEM) and atomic force microscopy (AFM). The ITO sol-gel mono-layer contains three regions of different porosities. The basic crystalline structure is that of the In2O3 lattice. The optical properties have been studied by optical transmission and spectroscopic ellipsometry. (C) 2003 Elsevier Science B.V. All rights reserved.

The present work is focused on several 2:17 intermetallic compounds, namely Nd2Fe15Si2, Nd2Fe15Al2, Nd2Fe15SiAl and Nd2Fe15SiMn as well as several 1:12 intermetallic compounds, such as NdFe10Si2, NdFe10Al2, NdFe10SiAl and NdFe10MnAl. The substitutional effect on their magnetic properties is studied by Mossbauer spectroscopy using hyperfine magnetic field distributions. The single-phase structure of both classes of intermetallics was supported by X-ray diffraction (XRD) measurements. (C) 2003 Elsevier Ltd. All rights reserved.

The importance of vertex corrections in the description of the absorption spectra in semiconductors is discussed. These corrections are responsible for the excitonic correlation of the phonon-scattered carriers and it is shown that these correlations are essential for the description of excitonic sidebands in the case of intermediate coupling. Different approximation schemes are compared in numerical examples.

The fundamental and third-harmonic susceptibility response of YBa2Cu3O7-delta (YBCO) and TlBa2Ca2Cu3O10-y (Tl:1223) high-critical temperature (T-c) superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-T-c superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high T-c of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach. (C) 2003 American Institute of Physics.

A model for trapping phenomena in nanocrystalline silicon investigated by the optical charging spectroscopy method is proposed. The model takes into account all the possible contributions to the discharge current. The results previously obtained on fresh and passivated porous silicon samples are interpreted within the framework of the model, which provides a good fit to the experimental data. The role played by the different kinds of trapping centers (donors and acceptors, surface and bulk) and the different trap parameters is analyzed. (C) 2003 American Institute of Physics.

Mossbauer spectrometry and magnetic measurements are employed to experimentally investigate the magnetic behavior of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons obtained by appropriate annealing of the amorphous precursor. A detailed analysis of the correlation between the microstructure of annealed samples and their magnetic properties is provided. Thermomagnetic data allow the Curie temperatures of both amorphous residual matrix and nanocrystalline phase to be estimated. The differences between Curie temperatures of amorphous residual matrix and amorphous precursor are investigated and explained in terms of magnetic polarization of the matrix by exchange fields arising from the nanocrystalline grains. Theoretical systems of spins consisting of a single ferromagnetic nanocrystalline grain immersed in weakly ferromagnetic environment, quite similar to our real samples, are considered and their magnetic behavior is investigated by Monte Carlo simulation of low temperature spin ordering, with emphasize on the matrix-nanocrystalline grain interface which is shown to exhibit peculiar magnetic behavior. The magnetic features of the matrix-nanocrystalline grain interface are studied, as depending on matrix-nanocrystalline grain exchange coupling as well as crystalline fraction of the nanocrystalline systems.