Milton Ferreira de Souza

Solid softwood coated with plasma-polymer for water repellence

Abstract Cold plasmas were produced by electrical glow discharges in a gas medium at reduced pressure and at low frequency. The precursor gases used were ethylene, acetylene, butene-1, and vapor of vinyl acetate. The treatment caused the solid softwood surface to become hydrophobic; 1-butene-plasma produced the best results. Although the surface plasma treatment resulted in water repellence, permeability to water vapor remained. Using a mixture of tetraethyl orthosilicate vapor (TEOS) and oxygen (O2) it was possible to investigate the inability of the glow discharge to coat the surface of wood orifices.

Electrical properties of zirconia–alumina composites

Abstract Sintered zirconia–alumina composites, prepared in a wide range of compositions, are studied in terms of their electrical response. Both grain conductivity and dielectric constant show the typical characteristics expected from the percolation theory, with v c =0.14±0.2 as the critical zirconia volume fraction for the onset of conduction. When the conducting zirconia phase is calcined prior to forming the composite, the whole system still shows a strongly reduced conduction response even for zirconia volume fractions ( v ) in the range of 0.4–0.5, after which it is considerably enhanced for v =0.7. These results are discussed in terms of (i) the influence of the materials microstructure and (ii) the effect of stress resulting from the alumina sintering on the calcined zirconia grains on the overall electrical response of the composite.

In situ characterization of the grain and grain-boundary electrical responses of zirconia ceramics under uniaxial compressive stresses

The electrical properties of ion-conducting tetragonal zirconia ceramics subjected to mechanical stresses were studied using impedance spectroscopy. The material’s overall resistance (grain and grain boundary) was found to increase when stress was applied perpendicularly to the measuring electric field (σ⊥E), while only comparatively discreet variations, involving a decreasing trend of resistance, occurred when the electric field and mechanical stress were parallel (σ∥E). The increment in electrical resistance for σ⊥E was found to be consistent with an increase of the conduction process energy barrier. The mechanical effect reported here is of elastic nature, covering a wide range of applied stresses. The electrical characteristics from cracking and fracturing at higher stresses are also presented.

Ionic transport in polycrystalline zirconia and Frenkel's space-charge layer postulation

Various physical and chemical techniques have been applied elsewhere to get insight into the peculiarities of conduction processes in zirconia-based ionic conductors. Although Frenkel’s space-charge model is popularly accepted to describe well the capability of grain boundaries for limiting ion transport throughout these materials, validating this model has generally resulted to be a difficult task provided substantial restrictions to access the subgrain nanometer-sized regions involved. The present work reports a simple but consistent method to also approach this important question, and consists of studying the bulk and grain-boundary electrical responses of these materials while they are subjected to external mechanical loads.

Chaotropic substances and their effects on the mechanical strength of Portland cement-based materials

Portland cement-based materials are present in our everyday life. Over the last two decades, important developments have been made to improve their mechanical strength, mainly through microstructural design. In addition, another promising parameter still remains in the early stages of understanding: that of adhesion. Recently, interdisciplinary researchers have considered the issue of water confinement by the hydrated cementitious surfaces. It could contribute to increase the adhesion and strength in these materials. On the other hand, ionic and nonionic chaotropic substances might be able to disorder the structure of such special water. The results presented in this paper show important effects of these chaotropic substances on the strength of cementitious materials. They highlight the role of the confined water on the adhesion, when the microstructural parameters are kept constant. More than contributing to the fundamental understanding of adhesion within cement paste, these results provide basic insights on in-situ nanotechnology.

Silicon carbide coating of mullite substrates by the CVD technique

Abstract Silicon carbide films were applied on polished and rough mullite substrates by the APCVD technique at 1100, 1200 and 1300 °C, using methyltrichlorosilane as precursor gas. XRD analysis shows that graphite films were deposited on substrates at 1100 °C, and only above 1200 °C were deposited β-SiC films. The crystallinity was found to improve as the deposition temperature increases, and no preferential alignment of the crystal structure was observed. Rough substrates produced dense films while polished substrates produced porous ones. Tests on films produced at 1300 °C revealed microhardness to be 3813 kgf/mm 2 . Wear tests on films produced under the same conditions showed a mass loss of 0.0029 g/km without film detachment, indicating good adherence of the film to the substrate.

Optical and EPR observation of the F center in KCN

Abstract The production and identification fo the F centers in KCN is reported and the effects of the transition occuring at 83°K on the optical and EPR spectra are shown. A comparison of the F center properties in KCL and KCN is presented and the results are discussed.

High Strength Phosphogypsum and Its Use as a Building Material

Inovamat Ltda. R. Alberto Lanzone, 731 – 13566-590 Sao Carlos, SP Abstract. A new process (patent applied) that works equally well with both plaster of mineral gypsum and phosphogypsum for the preparation of gypsum components, UCOS, has been developed. The process consists of the following steps: humidification of plaster by fine water droplets, uni-axial compression, hydration reaction and drying. Strong hydrogen bonds develop among the crystals together with adhesion provided by confined water that accounts for nearly 70% of the adhesion forces. By reducing the plaster to water ratio to close the minimum necessary, new features are generated. An experimental house has been constructed, in which walls and ceilings have been built of gypsum and phosphogypsum. Since phosphogypsum potentially contain radioactive elements, the application of an activity concentration index to the phosphogypsum employed in the building was carried out. Keywords: Phosphogypsum, Building Materials, Activity Concentration Index. PACS: 89.20.-a

GRAIN BOUNDARY CONDUCTIVITY OF ZIRCONIA YTTRIA AND THE SPACE CHARGE CONCEPT

Abstract Liquid phase sintering of YSZ ceramics containing a small part of Er2O3 displays one transient increase in the grain boundary conductivity after sintering for one hour. This transient increase has been measured for short additional time intervals between one and two hours and found to last for 30 min. The transient effect is attributed to changes in the space charge due to Er+3 ion diffusion to the grains from the glass phase. The driving force for the inversion of the segregation behavior of the stabilizing ions is attributed to a phase separation in the sintering glass phase. The effective thickness of the space charge and its conductivity have been obtained.

Glass phase expelling during liquid phase sintering of YSZ

Expelling of the liquid phase during sintering of ZrO2-6.5 mol% Y2O3- 0.5 mol% Pr2O3 ceramic was observed as a result of grain coarsening. ZrO2- 7.0 mol% Y2O3 samples, without Pr2O3 addition, do not show this effect under the same sintering conditions. The expelling process is caused by surface tension forces and attracting van der Waals forces between the grains, coupled with the existence of two glass phases on the grain boundaries. The amount of expelled glass phase increases with grain growth, but saturates above 16 mm average grain size.