Publications

In this work, the thermal degradation of PP blends with previously treated sisal fiber (10mm), and gamma irradiated in air at different doses (10, 25, 50, 60, 70 kGy), is studied. The treatment to which sisal fibers were previously submitted were: (a) the addition of a coupling agent (Silane A-172), 1% by weight, (b) a chemical treatment at different times (1/2 and 1 h) with NaOH, and (c) the addition of a functionalized polypropylene known as Polybond 3150 and 3200, 5% by weight. After the treatment, mixtures of PP with 20% of sisal fibers were prepared. Their thermograms were analyzed using different integral methods in order to evaluate the activation energy. For the blend of PP with sisal fibers, previously treated for 1 h with NaOH, the largest value amounted to 60 kJ/mol (approximately) for irradiation doses under 40 kGy. At higher irradiation doses a decay on the activation energy was observed. When the fibers were treated with NaOH for 30 min, the activation energy decreased from 55 kJ/mol to values under 45 kJ/mol, when subjected to high irradiation doses (> 50 kGy). This can be explained by the fact that, when sisal fibers are treated with NaOH solutions over a longer time, impurities and lignin are better removed resulting in a higher surface roughness; as a consequence a higher polymer to fiber interaction is possible, which, in turn, makes the blend more stable at low irradiation doses. Instead, when the sisal fibers were treated with silane, the activation energy for thermal degradation decreased at higher irradiation doses until an inversion of the tendency was observed for the 50 kGy dose; this may imply that this irradiation induces the production of PP-filler bridges. With composites containing compatibilizers, a similar behavior as with silane was observed but with the inversion on the tendency at 70 kGy. From these results, it can be concluded that a l-h treatment with NaOH produces a higher thermal stability than the others. Moreover, this treatment has the best profit/cost ratio, which makes it commercially profitable when compared with others. The mathematical analysis shows that the blend of PP with NaOH-treated sisal fiber at the two treatment times (1 h and 1/2 h), showed competing degradation and single decomposition processes for the irradiation intervals between 0-50 kGy and 50-70 kGy, respectively. Regarding the blends of PP with silane-treated sisal fiber or Polybond addition showed bimolecular decomposition behavior for both irradiation intervals. (C) 2001 Elsevier Science Ltd. All rights reserved.

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI <5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-EN ponders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp(3), as in the case of c-BN, (C) 2001 Elsevier Science B.V. All rights reserved.

The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S = 1/2, local symmetry axis along one of the crystal [111] axes and principal g values g(parallel to) = 2.0032 +/- 0.0009 and g(perpendicular to) = 2.0094 +/- 0.0005 at T = 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.

A method to produce composition modulated Bi2-xSbxTe3 alloys by electrodeposition has been developed. The electrolyte, which consists in a HCl 6M (pH = 0) aqueous solution, allows the codeposition of bismuth, antimony and tellurium to be accomplished at room temperature on glassy carbon, platinum or nickel electrode. The composition of the films. their crystal structures and morphology were studied as function of electrochemical parameters and bath composition. It is shown that the electrodeposits are monophasic and exhibit a polycrystalline state. The electrical resistivity was of the order of 7-10 mOmegacm, whereas the electrodeposition rate value was 30-50 mum/h for the films with good thermoelectric properties.

Several Tl-0 (6s(2)6p(1))-type paramagnetic centers, produced by low temperature X-ray irradiation, were observed and studied by electron spin resonance (ESR) in the orthorhombic ferroelectric phase of thallium doped Rb2ZnCl4 crystals. The centers were formed by electron trapping at Tl+ ions localized substitutionally at Rb+ sites. The number and properties of the observed centers account for the tripling of the unit cell in the ferroelectric phase.

The alkoxide route and the spinning deposition were used to prepare sol gel ITO films. The morphology and crystalline structure were investigated by Cross-section Transmission Electron Microscopy and Atomic Force Microscopy. The inside of the ITO layer has three regions of different porosity and the basic crystalline structure is that of the In2O3 lattice. Three types of surface morphological structures were found from AFM images.

Dark-brown crystalline powders and selected single crystals of cubic boron nitride grown by the HP-HT method, with boron excess, have been studied in a broad temperature range by electron spin resonance (ESR) in the X and W microwave frequency bands, respectively. The X-band spectra consist of two superimposed lorentzian components attributed to two types of related paramagnetic defects called D1 and D2, respectively. According to the W-band data, the D I centre exhibits local axial symmetry and ground spin state S = 1/2, with g(parallel to) = 2,0033 and g(perpendicular to) = 2.0094 at T = 10 K. The broad D2 line, observed only in the X-band, at g = 2.0084, at higher temperatures, seems to result from transitions inside the excited levels of another boron related defect.

Conventional and mechanical activation techniques were used to prepare a soft PZT type piezoceramic material. The resulting powders exhibit micro and nanostructures, respectively as indicated by X-ray and electron microscopy. Average crystallite sizes of 1 mu and 50 nm respectively were estimated for the powders. The sintered behavior of pressed samples from the two types of powders was investigated within the 8001300 degreesC temperature range and the densities, structural morphology and dielectric and piezoelectric properties were investigated. Optimum sintering temperature were about 100 degreesC lower for samples made from mechanical activated powder compared to the conventionally ones. Sintered samples from this powder exhibited also enhanced properties compared to those made from the conventionaly prepared powder. It is concluded that the mechanical activation technique is a reliable and simple method to prepare high quality PZT type materials.

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or gamma-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial [001] symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl- ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl- ions, compressed along an[001] direction.

Structure of polyaniline (PAN)-C-60 nanocomposites was investigated by X-ray diffraction. Raman Spectroscopy was also used in order to find evidence for the C-60 - polymer interface interaction.