Francisco Cristóvão Lourenço de Melo

Sol–gel silica film preparation from aqueous solutions for corrosion protection

Abstract SiO2 coatings were deposited on aluminum (98% Al) surfaces by dip-coating in silicic acid aqueous solutions containing urea as dry chemical control agent (DCCA). These films are of special interest for protection of metals against oxidation and acid corrosion at elevated temperature. In this work, we investigate the effect of two different treatments of aluminum surfaces on the adhesivity of the silica films. The optimum urea/silanol ratio in the precursor gel solution and the number of dips were also investigated. The corrosion protection afforded by the silica film was determined by anodic polarization of the coated surfaces in 0.5 mol/l sodium chloride aqueous solution exposed to air at room temperature. The best performance was obtained for surfaces anodized prior to dip coating, and for urea/silanol ratio and number of dips equal to, respectively, 7 and 5.

Influence of NiO on the crystallization kinetics of near stoichiometric cordierite glasses nucleated with TiO2

This work presents the effect of NiO on the thermal behavior and the crystallization kinetics of glasses lying near the stoichiometric cordierite composition nucleated with TiO2. Three glasses with NiO content varying between 1 and 5 mol% have been synthesized in Pt crucibles. Activation energies for structural relaxation and viscous flow have been calculated using the data obtained from differential thermal analysis (DTA). Kinetic fragility of the glasses along with other thermal parameters has been calculated. Non-isothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all three glasses. The crystallization sequence in the glasses has been followed by x-ray diffraction analysis of the heat treated glass samples in the temperature range of 800–1200 °C. μ-cordierite has been observed to be the first crystalline phase in all the glass samples after heat treatment at 850 °C, while NiO plays an important role in determining the crystallization sequence at higher temperatures, leading to the formation of α-cordierite.

Superficial modifications in TiO2 and Al2O3 ceramics

The properties of hydrophilicity or hydrophobicity of materials are defined mainly, though not exclusively, by their composition, morphology and surface energy. In this work, titanium dioxide (TiO2) and aluminum oxide-alumina (Al2O3) ceramics prepared by uniaxial pressing were studied in terms of surface energy. The surfaces of these ceramics were treated with nitrogen plasma, using a stainless steel reactor excited by a 13,6 MHz radio frequency operating at 50 W input power and 13 Pa nitrogen pressure. The surface morphology was investigated by scanning electron microscopy (SEM) analysis. Surface energy and contact angle measurements were taken using a RAME-HART goniometer. These measurements were taken as function of time, over a 21-day period. The contact angle and surface energy values were found to change by almost 34% in comparison to their initial values immediately following plasma treatment. Nonetheless, the hydrophilic character of the Al2O3 and TiO2 remained constant throughout the test period.

CERAMIC PROCESSING ROUTE AND CHARACTERIZATION OF A NI-ZN FERRITE FOR APPLICATION IN A PULSED-CURRENT MONITOR

Pulsed-current sensors require transducers constituted of magnetic materials with high magnetic permeability in a frequency range compatible with the period and the frequency of the current pulse. The use of ferrites in this application has the advantage of low cost and low losses in high frequencies. The aim of this work is to present a procedure for selection of the ceramic processing route of Ni-Zn ferrite for application in a pulsed-current sensor. The ferrite samples were prepared under different processing parameters and characterized in terms of microstructure, chemical analysis, complex magnetic permeability, and magnetic hysteresis. The chosen processing route included high energy milling of the pre-sintered powder, its disaggregation before sample forming, and sintering of the samples

Mechanical properties of sialon

Abstract Silicon nitride based ceramics have several important applications in the fabrication of hard materials, such as ceramic cutting tools. In this work, aluminum nitride, yttrium oxide and cerium oxide were used as sintering aids on pressureless sintered Si 3 N 4 . The relationships between the type and quantities of these additives with micro hardness (HV), fracture toughness and formed crystalline phases, such as Sialon, have been evaluated. The correlation between microstructure and the mechanical properties has been studied, using the scanning electron microscopy.

Pressure study of monoclinic ReO2 up to 1.2 GPa using X-ray absorption spectroscopy and X-ray diffraction.

The crystal and local atomic structure of monoclinic ReO2 (alpha-ReO2) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X-ray absorption spectroscopy and high-resolution synchrotron X-ray powder diffraction and a home-built B4C anvil pressure cell developed for this purpose. Extended X-ray absorption fine-structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re-Re distances and a distorted ReO6 octahedron in the alpha-ReO2 structure. X-ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the alpha-phase, demonstrating a modulation of the Re-Re distances. The relatively small portion of the diffraction pattern accessed in the pressure-dependent measurements does not allow for a detailed study of the crystal structure of alpha-ReO2 under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re-Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that alpha-ReO2 is a possible inner pressure gauge for future experiments up to 1.2 GPa.

Morphology and stress study of nanostructured porous silicon as a substrate for PbTe thin films growth by electrochemical process

Porous silicon layers (PSL) were produced by stain etching from a HF:HNO3 500:1 mixture with etching time varying in the range of 1 up to 10 min. The samples have presented nanometric porosity as a function of etching time, characteristic of heavily doped p type silicon. The residual stress and the correlation length of the layers were obtained through the analysis of the micro-Raman spectra using a phonon confinement model including a term to account for the amorphous phase. The residual compressive stress tends to increase as expected due to the contribution of smaller crystallites to be more representative as the etching time increases. PbTe thin films were electrodeposited on PSL from aqueous alkaline solutions of Pb(CH3COO)2, disodium salt of ethylendiaminetetraacetic acid (EDTA) and TeO2 by galvanostatic and potentiostatic method. It was also obtained nanostructured PbTe thin films with polycrystalline morphology evidenced by X-ray Diffraction (XRD) spectra. Scanning Electron Microscopy (SEM) analysis has demonstrated good films reproducibility with an average grain size of 100 nm.

Microstructure and Complex Magnetic Permeability of Ni0.3Zn0.7Fe2O4 Ferrite

Ni-Zn ferrites are magnetic ceramics that are widely used in electric and electronic components. Among Ni1-xZnxFe2O4 ferrites, Ni0.3Zn0.7Fe2O4 is known to be the one of highest magnetic permeability in lower frequencies. This work is divided into two parts: the first part deals with the influence of sintering temperature and forming conditions on the microstructure of Ni0.3Zn0.7Fe2O4 ferrite. The second part deals with the influence of sintering temperature and environmental variations of temperature on the complex magnetic permeability of such ferrite. The ferrite studied in this work was fabricated by means of the conventional ceramic method. Complex magnetic permeability between 100 kHz – 100 MHz is discussed in relation to sintering temperature. The influence of environmental variations in temperature (-40 oC to +50 oC) on the complex magnetic permeability of the ferrite sintered at 1300 oC is discussed.

Properties of Glass-Ceramics Obtained from Crystallization of the SiO2-Al2O3-MgO-Li2O System with Addition of ZrO2

This paper presents the study results with glass-ceramics obtained from base glass (MgO-Al2O3-SiO2-Li2O system) with addition of ZrO2 as nucleating agent. The glass was melted at 1650 oC for 3 h and at a heating rate of 10 °C/min. The molten glass was poured into a graphite mold to obtain monolithic samples and also in water in order to obtain particulate material. Such material was grinded and then pressed by both uniaxial and isostatic pressing methods before being sintered. Both the monolithic and pressed samples were performed under two different conditions of heat treatment so that their nucleation and crystallization occurred. In the first one, the samples were heated to 1100 oC with a heating rate of 10 oC/min. In the second one, there was an initial heating rate of 10 oC/min up to 780 oC, which was kept for 5 minutes. After that, the samples were heated to 1100 oC at a heating rate of 1 oC/min. Microhardness analyses showed that base glass presented values around 7.0 GPa. The glass-ceramics obtained from the powder sintering showed microhardness values lower than those obtained from monolithic samples. The highest hardness values were observed in the samples which were treated with two heating rates, whose values were around 9.2 ± 0.5 GPa. Moreover, the glass-ceramics which were produced with an only heating rate, presented values around 7.1 ± 0.2 GPa, very close to those observed in the base glass.

Effect of TiO2 and TiB2 on Pressureless Sintering of B4C

This paper presents results of experiments on pressureless sintering of boron carbide (B4C) with addition of titânia (TiO2) and titanium diboride (TiB2). The TiB2 powder was added as a second phase and the TiO2 powder for reactive sintering and in-situ formation of TiB2. The final concentrations of TiB2 in the composites were 0 to 10 vol%. Sintering was performed at 2050 °C/30min in argon atmosphere. TiO2 was completely transformed into TiB2 with fine equiaxed grains distributed homogeneously. Composites obtained by in-situ reaction showed a densification increase with the concentration increase, while the composites with TiB2 powder mixture showed low densification in all compositions. Relative Density of the composite with 10 vol% of TiB2 obtained in-situ was 91% (TD) compared to 86 % for B4C only. Vickers hardness was about 29 GPa.