Papers by Author: José Maria F. Ferreira

Abstract: TiO2 films were prepared by infiltrating a titanium (IV) butoxide solution to the porous structures of glass and rutile TiO2, respectively, then dried and calcined at a moderate temperature of 550°C. Well-characterized porous nanometer TiO2 films were formed. Nanometer TiO2 film on glass matrix has an anatase phase, while that on rutile TiO2 has a rutile phase. These indicate that the phase formation of nanometer TiO2 could be tailored by the different matrices.

Abstract: This paper reports preliminary studies regarding a new fabrication process for aluminium alloy matrix particulate reinforced composites, which uses ceramic preforms with alumina and tailored amounts of reactive copper oxide, CuO. An Al2O3-CuO mixture with 75 mol% CuO was selected, aiming at a 10-40vol% reinforcement phase fraction in the final composite, after aluminium infiltration. Molten aluminium infiltration progress was studied as a function of ceramic’s composition, doping, and infiltration time. The resulting microstructures were investigated by OM, SEM, FESEM and EDS in order to establish the liquid aluminium infiltration profile at the metal/ceramic interface. Infiltration experiments showed that the 3CuO (s) + 2Al (l) → 3Cu (l) + Al2O3 (s) redox reaction is triggered at the experimental conditions used, but the infiltration process is slow and does not go to completion. The use of NaOH as a doping agent promotes effective infiltration of molten aluminium upon the ceramic green mixture.

Abstract: The present work aims to develop porous ceramic plates based on cordierite, using the polymer foam replication method, to be used for gas burners. Ceramic foams were produced by coating polyurethane foams of different pore sizes with ceramic powders dispersed in aqueous slurries. The final porous structures were shown to depend on the structure of the original polymer sponge and on the details of the fabrication procedure. However, a suitable process control enabled to obtain a close relation between the final properties and the template polymer characteristics. The performance of the porous cordierite plates, with different pore size distributions, as medium burners was tested and compared by measuring the pressure drop in the burner against the air flow rate. It could be concluded that good performance is achieved with the higher porous materials due to its higher porosity fraction, and more suitable pore size range.

Abstract: β-Si3N4 and β-SiAlON powders were prepared by combustion synthesis with SrCO3 and NH4F used as additives. The resultant β-Si3N4 and β-SiAlON powders consisted of elongated prismatic microcrystals. By adding SrCO3, the anisotropic growth of β-Si3N4 and β-SiAlON crystals is improved and their aspect ratios increase. The addition of NH4F enhanced nitridation reactions and reduced the residual Si in combustion products. It was proposed that the elongated prismatic β-Si3N4 and β-SiAlON crystals grew from liquid phase and the composition and property of this liquid was affected by the addition of SrCO3.

Abstract: We describe the production of complex shaped ceramic green bodies with high strength and reliability using a novel forming method: temperature-induced gelation. Gelation is performed by moderately decreasing the temperature of the suspension, which induces in situ gelation and forms a network to bridge the suspended particles, leading to a stiff green body. The gelation mechanism is based on the separation of dispersant KD1 from solvent or the collapse of adsorbed layer on particle surface, which depends on the stability of starting suspensions.

Abstract: In this paper, α-SiAlON powders co-doped with (Ca+M) (M=Mg, Yb, Sr) were prepared by combustion synthesis. The effect of Ca incorporation on the phase composition and grain morphology of reaction products was discussed. The experimental results showed that Ca incorporation promoted the growth of rod-like α-SiAlON crystals in (Ca+Mg) and (Ca+Yb) systems. For (Ca+Sr) system, the addition of Ca assisted Sr into α-SiAlON crystal lattice and increased the relative content of α-SiAlON in reaction product.

Abstract: In this paper, b-SiAlON whiskers were synthesized by combustion of silicon and aluminium powders with SiO2 and Y2O3 as additives and a-Si3N4 or b-SiAlON powders as diluents. SEM observation of the products showed that the crystals with an aspect ratio higher that 5, which could be assigned to almost pure and well crystallized b-SiAlON. The influence of the amount of NH4F additive, the type of diluent on the phase formation and on the morphology of products was investigated.

Abstract: In this paper, Yb a-sialon powders with elongated grains have been prepared by combustion synthesis. The effects of diluents, nitrogen pressure, adding a-sialon seeds and NH4F on the phase transformation and microstructure development of the samples are discussed systematically. The experimental results show that the addition of diluents, proper nitrogen pressure and additive NH4F can facilitate the nitridation of Si. It was also found that the added a-sialon seeds and NH4F are beneficial to the nucleation and growth of elongated grains.

Abstract: A universal model for describing the wetting kinetics at solid/liquid interfaces, where interfacial chemical reaction occurs, is proposed, whereby four distinct stages separated from each other by transition points are anticipated. The stages are described by means of comparing the dimensions of the base of the liquid sessile drop with the evolution of the reaction product forming on the solid/liquid interface, over time.

Abstract: AlN whiskers have been successfully synthesized by carbothermal reduction. The thermodynamics and growth kinetics of AlN whiskers were studied at 1600°C using CaCO3 as a catalyst. The research indicated that AlN whiskers are more easily nucleated from the liquid phase than at the surface of solid phase. AlN whiskers are nucleated by VLS mechanism and the liquid, which plays a dominant role in the VLS mechanism, is formed by Al-Ca interphases, such as CaO×2Al2O3 and CaO×6Al2O3. Kinetic studies suggest that the catalyst reacts with Al2O3 to form a low melting point eutectic (1390°C). The liquid phase formed at this low melting point eutectic provides good conditions for nucleation of AlN whiskers. At the synthesis temperature, the liquid phase vaporizes, thus creating suitable conditions for the subsequent growing of whiskers by the VLS mechanism. This growing mechanism conforms to thermodynamics and a lot of proof indicates that screw dislocations play an important role in the process of the whiskers' formation.