Papers by Author: Valmir José da Silva

Abstract: Aluminum and zirconia oxides are ceramic materials widely used in the manufacturing of dental products, however, the thermal treatment stages demand high temperatures. Aiming at the reduction of the energy consumption and of the time necessary for the thermal treatments, this work intends to evaluate the influence of the microwave energy over the thermal processes of alumina substrate and of the alumina/zirconia system. The samples were made by slip-casting and then submitted to different thermal treatments. For the microwave oven, we used powers of 1.44 and 1.66 kW and heating times of 10, 15 and 20 minutes. The samples were submitted to the determination of physico-mechanical properties: flexural strength at three points, porosity and Vickers microhardness. Based on the analysis, we conclude that the microwaves energy is promising for the processing of dental ceramic, causing a significant reduction in the firing time.

Abstract: Front of the large number of applications of the mullite mineral, many researchers seek to synthesize it from the mixture of Al₂O₃ and SiO₂ powders, or from aluminossilicates, such as smectite. The synthesis of mullite from clays via rapid heating by microwaves emerges as an alternative process. This process accelerates the kinetics of nucleation and the development of mullite phase. Thus, the aim of this work is to study the effect of power and timeon synthesis of mullite from clay, via rapid heating in microwave. The synthesis times were 15 and 20 minutes, with powers of 1.26 and 1.44kW. The obtained powderswere characterized by X-ray diffraction and scanning electron microscopy. The best results were obtained with a power of 1.44kW for 20 minutes.

Abstract: Among the ceramic raw materials, mullite is highlighted for presenting features which are of great interest for industry. However, the process through which this material is obtained requires pure reagents, raising the production costs. Therefore, this work is intended to obtain mullite from bentonite clays (source of silicon and aluminum oxides) and alumina residue, resulting from the Bayer process, thus becoming an interesting process from the economic and environmental standpoint. For this purpose, we made use of the thermal treatment in conventional furnace, temperature of 1500°C, heating rate of 5°C/min and dwell time of 60 min at maximum temperature. The synthesized samples were characterized by X-ray fluorescence (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled to an EDS, besides the quantification of the phases present in the samples. The results revealed that mullite was the major phase produced, but corundum appeared as secondary phase.

Abstract: The objective of this work is to obtain synthesis of ZSM-5 zeolite using kaolin as Si and Al alternative source. Kaolin powder was dispersed under constant stirring into the sílica sol where the mass ratio of kaolin to silica was 1:2. After stirring for 2 hours, the dispersion was dried at 160 °C and calcined at 700 °C/2 hours. The synthesis of ZSM-5 zeolite was by hydrothermal treatment at 170 °C/48 hours under autogenous pressure. The materials were characterized by XRD, SEM, EDX, BET. According to the XRD, was possible to observe the formation of ZSM-5 zeolite with intense and well defined peaks set located between 2θ = 7-9° and 23-25°, typical of a crystalline material. The SEM showed the formation of particles clusters with orthorhombic shape characteristic of MFI structure. The results show that the hydrothermal treatment was efficient to produce ZSM-5 using calcined kaolin as alternative source.

Abstract: The general aim of this study was to develop materials of the type: Y zeolite (hydrothermal synthesis), ceramic support (forming of powder) and zeolite membrane (rubbing). The preparation of the Y zeolite was conducted in accordance with the hydrothermal synthesis method, the time of crystallization was one day. The ceramic support was prepared by means of the forming of powder technique and subsequently subjected to sintering at a temperature of 1400°C/1h. The zeolite membrane (Y/α-alumina) was prepared by secondary growth method (rubbing). These materials were characterized by XRD and SEM. Obtaining Y zeolite could be confirmed by X ray diffractograms. From the images obtained by SEM, it was possible to derive from analysis that the Y zeolite is composed of a homogeneous morphology, where the particles are crowded, with uniform size. The results obtained for the ceramic support (α-alumina) showed that it displays characteristics peaks of aluminum oxide. By using micrographs it was possible to observe a heterogeneous microstructure with a compact form, without cracks upon the layers. According to the XRD, for the methodof secondary growth (rubbing), it was observed that the Y zeolite wich had been synthesized on the ceramic support displayed a crystalline structure. The micrography of the zeolite membrane (Y/α-alumina) showed the formation of a layer of zeolite on the ceramic support.

Abstract: Zeolite ZSM-5 have been synthesized by hydrothermal method, using tetrapropylammonium bromide as template. This work aims to evaluate the preparation and characterization structural and morphology of zeolite ZSM-5 resulting in a pure and crystalline material. The ZSM-5 zeolite was prepared by hydrothermal synthesis at 170 °C, using silica, deionized water and structure-directing salt (tetrapropylammonium bromide-TPABr). Several techniques like X ray diffraction (XRD), scanning electron microscopy (SEM) and X ray energy dispersion spectrophotometer (EDX) have been utilized to characterize the framework structure and morphology of the sample. Obtaining ZSM-5 zeolite could be confirmed by X ray diffratograms. The result X ray diffraction showed that the synthesized sample presented typical peaks of the ZSM-5 zeolite. According to the XRD, it is possible to observe intense and well defined peaks. The acquired X ray patterns identified the product as highly-crystalline materials, without the presence of impurities. From the images obtained by SEM, it was possible are composed of showed that the crystal accumulations of ZSM-5 zeolite, without the presence secondary phases on the crystals.

Abstract: This work involved a morphological, microstructural and magnetic characterization of nanosized powders and sintered samples of Ni-Zn ferrite doped with chromium. The effect of substituting Fe3+ for Cr3+ on the final characteristics of the powders and sintered samples was investigated. The Ni-Zn ferrite powders were prepared by combustion reaction using nitrates and urea as fuel, based on the concepts of propellant chemistry. The samples were uniaxially compacted by dry pressing (385 MPa) and sintered at 1200oC/2h, using a heating rate of 5°C/min. The Ni-Zn powders and compacted samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the measures of their magnetic properties. The results revealed the formation of the cubic crystalline phase of the inverse spinel of Ni-Zn-Cr ferrite. The average crystallite size was 21 nm and 57 nm while the saturation magnetization was 47.0 and 73.1 emu/g for the powder and the sintered sample, respectively.