Papers by Author: Douglas Gouvêa

Abstract: Zinc oxide is widely used in semiconductors fabrication, pigments and mainly in varistors. The electrical properties of ZnO based varistors are related to the grain boundary characteristics, and it can be modified using additives. In this work, zinc oxide based powders were prepared by chemical route derived from the Pechini’s method, and small amounts of tin ions were added to the system. The XRD measurements showed that the tin oxide not forming solid solution in the bulk, and it was observed that the increasing of tin concentration causes an increasing of the surface area at isothermal conditions of calcination. Once the isoelectric point was dislocated to more acid values, the hypothesis of segregation onto surface was considered.

Abstract: In this work tin dioxide (SnO2) doped with nickel is studied with the aim of develop integrated gas sensors for some important pollution gases into environment, principally sulphur dioxide (SO2). SnO2 powders doped with Nickel were prepared by Pechini`s method [1] and characterized morphologically by X-Ray diffraction, measurement of specific surface area (SBET) and transmission electron microscopy (TEM), which revealed grain sizes around 5 to 10 nm. Gas sensors were fabricated by spin-coating deposition of Ni doped SnO2 films over alumina substrate with gold contacts. Electrical resistance of such device is characterized, at room temperature, as function of SO2 concentration into environment, showing a sensitivity around 0.0058 ppm-1 for relative resistance variation, with good linearity for SO2 concentrations up to 30 ppm. Also, FTIR analysis shows the desorption of SO2 from SnO2 surface, after cleaning into fresh air.

Abstract: It is well known that colloidal powder particles (between 1 mm and 0.001 mm) tend to agglomerate due to electrostatic forces. Then assuring an optimal dispersion condition is essential for good particle-size analysis results, since aggregates or weak agglomerates can be measured as single particles. In this paper the particle size distribution of an alumina powder A1000SG (ALCOA) was measured using distinct dispersion procedures. Distilled water was used as dispersant liquid in the pure state and with additives (citric acid and Duramax D-3005). Dispersion by supersonic vibration was also investigated, but only the application time was varied. Particle size analysis was accomplished by laser scattering technique and the dispersion condition was evaluated through zeta potential. The results showed that the Duramax’s electrosteric impediment is more efficient than citric acid’s electrostatic force, thereby providing better dispersion. Although useful, the supersonic vibration was not good enough to assure an optimal dispersion, at least for the material tested here.