Papers by Author: Walter Kenji Yoshito

Abstract: In this work it was investigated the influence of CTAB surfactant concentration on the synthesis of the compound Mn_0.75Zn_0.25Fe₂O₄ by the coprecipitation method. It was also compared the influence of hydrothermal treatment on the synthesized materials. The magnetic properties were characterized by AC susceptometry for the determination of the magnetic susceptibility and magnetic density energy. The phases, crystal structure and morphology of the nanoferrites were determined by Rietveld analysis of X-ray diffraction data. It was found the presence of two phases: Franklinite and Akaganeite and it was shown that the samples synthesized only by coprecipitation presented the tendency to increasing the crystallite sizes of the akaganeite phase and decreasing of crystallite sizes of the Franklinite phase as a function of CTAB concentration. The samples submitted to subsequent hydrothermal treatment presented a tendency to decreasing the crystallite sizes of both phases and increasing in Franklinite phase fraction, compared to the samples synthesized only by coprecipitation, suggesting that the hydrothermal treatment was effective in obtaining nanostructured materials of smaller particles.

Abstract: A nanosized magnesium substituted beta-tricalcium phosphate (Mg:β-TCP) was synthesized by an aqueous precipitation method, at room temperature, in one single step. In the present study, the novel and stable Mg:β-TCP resulted in a crystalline and spherical nanoparticles (diameter of approximately 20 nm) with mesoporous structures and a high specific surface area (about 574 m²/g). These special characteristics make this novel crystalline mesoporous Mg: β-TCP nanoparticles ideal candidates for drug delivery system and a promising non-viral vector for gene therapy.

Abstract: Nanostructured Mn-Zn ferrites were synthesized using co-precipitation in alkaline solution with different pH. The samples were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), thermal analysis (TG-DTA), dynamic light scattering (DLS) and scanning electron microscopy (SEM) techniques. Monophasic nanoparticles were formed when synthesized with pH 10.5. This sample was heat-treated and its XRD data was refined by the Rietveld method. Mean crystallite sizes and microstrains were determined from X-ray line profile analysis using Single-Line and Warren-Averbach methods, which revealed a mean crystallite size of approximately 10 nm and negligible microstrains. Zn content was estimated using refined cell parameters, giving a value of 33 at %, in accordance with XRF result. TG-DTA revealed that the incorporation of α-Fe₂O₃ occurs around 1130 °C and 1200 °C with recrystallization of the Mn-Zn ferrite spinel phase. DLS showed that mean particle size increase with temperature up to 1159 nm at 800 °C. SEM analysis showed the samples agglomerate and present similar morphology with negligible size changing when calcined between 280 °C and 800 °C. However, the sample calcined at 1200 °C presents larger agglomerates due to the sintering process.

Abstract: Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is used for dental prosthodontics, however, it can present accelerated tetragonal to monoclinic phase transformation in oral environment. The aim of this study was to compare the behavior of a Y-TZP synthesized in laboratory by the coprecipitation method to a commercial Y-TZP, after hydrothermal aging in pressurized reactor (150°C/ 35 hours). The discs were sintered at 1520°C for two hours. The kinetics curve of phase transformation was determined through the data collect by XRD diffractograms treated by the Rietveld method. The experimental and commercial control groups did not present monoclinic phase. After 35 hours of aging, the experimental group presented 69% of monoclinic phase compared to 67% for the commercial group. Scanning electron microscopy and atomic force microscopy images suggested that the commercial group presented heterogeneity of grain size and that the experimental group was more homogeneous. All groups presented superficial degradation process.

Abstract: Improvements of the catalytic efficiency of nickel–aluminum oxides in reforming reactions for hydrogen production have been achieved by the development of synthesis processes that provide powders with high surface area. The reduction of the crystallization temperature is one of the procedures in this direction. In this work, the effect of solvothermal treatment on coprecipitated gels with 15 wt% nickel content in alumina matrix was evaluated. Powders were obtained by coprecipitation with and without treatment of gels under n-butanol vapor pressure at 150oC. Products were characterized by TG/DTA, X-ray diffraction, specific surface area measurements, scanning electron microscopy and laser beam scattering for granulometric distribution determination. The results showed that calcined powders have high specific surface area (ranging from 170 to 260 m².g^-1) and are composed by gamma alumina and nickel aluminate phases. Aging treatment did not promote hydroxides decomposition under solvothermal conditions, indicating the need of calcination step.

Abstract: Zirconium dioxide (zirconia) ceramics are known by its high strength and toughness and titanium dioxide (titania) ceramics has outstanding surface properties. The ceramic composite formed between the two oxides are expected to have advantages of both ceramics, especially when its surface area is increased by pores. In this work, ceramic composites of ZrO₂-Y₂O₃-TiO₂ were synthesized by coprecipitation and rice starch was added as pore former in 10, 20 and 30 wt%. Powders were cold pressed as cylindrical pellets and sintered at 1500 °C for 01 hour and ceramics were characterized by techniques as Archimedes method for density measurements, X-ray diffraction and scanning electron microscopy. Results showed that pores are inhomogeneously distributed through ceramic bodies.

Abstract: The synthesis and ceramic processing of samaria and gadolinia doped ceria - nickel oxide composites, mainly applied as Solid Oxide Fuel Cell anodes, were studied in this work. Powders with composition Ce_0.8(SmGd)_0.2O_1,9/NiO and mass ratio of 40/60%, were synthesized by hydroxide coprecipitation with CTAB surfactant, followed by solvothermal treatment in n-butanol, ethanol and n-propanol at 150 °C for 16 hours, calcination at 600°C for 1 hour and milling. Sintering of compacted samples was performed at 1300°C for 1 hour. The powders were analyzed by X-ray diffraction, scanning electron microscopy, nitrogen adsorption method (BET), laser beam scattering (Cilas) and TG/DTA. The ceramics were analyzed by scanning electron microscopy, EDS, XRD and density measurements by Archimedes method. The results showed that the powders have a high specific surface area (42 - 85 m²/g). The ceramic characterizations showed a high chemical homogeneity and density value of 99% TD for powders treated with ethanol and propanol.

Abstract: Titania ceramics have many applications due to its surface properties and, recently, its nanostructured compounds, prepared by hydrothermal treatments, have been described to improve these properties. In this work, commercial titanium dioxide was treated with 10% sodium hydroxide solution in a pressurized reactor at 150°C for 24 hours under vigorous stirring and then washed following two different procedures. The first one consisted of washing with water and ethanol and the second with water and hydrochloric acid solution (1%). Resulting powders were characterized by X-ray diffraction, N2 gas adsorption and field emission gun scanning and transmission electronic microscopy. Results showed that from an original starting material with mainly rutile phase, both anatase and H2Ti3O7 phase could be identified after the hydrothermal treatment. Surface area of powders presented a notable increase of one order of magnitude and micrographs showed a rearrangement on the microstructure of powders.

Abstract: Zirconia stabilized with 8.5 mol% yttria (YSZ) were synthesized by coprecipitation and resulting gels were hydrothermallly treated at 200°C and 220 PSI for 4, 8 and 16 hours. Products were oven dried at 70°C for 24 hours, isostatically pressed as pellets and sintered at 1500 °C for 1 hour. Powders were characterized for surface area with N2 gas adsorption, X-ray diffraction, laser diffraction granulometric analysis and scanning and transmission electronic microscopy. Density of ceramics was measured by an immersion method based on the Archimedes principle. Results showed that powders dried at 70°C are amorphous and after treatment has tetragonal/cubic symmetry. Surface area of powders presented a significant reduction after hydrothermal treatment. Ceramics prepared from hydrothermally treated powders have higher green density but sintered pellets are less dense when compared to that made with powders calcined at 800°C for 1 hour due to the agglomerate state of powders. Solvothermal treatment is a promising procedure to enhance density.

Abstract: Nickel oxide-yttria stabilized zirconia (NiO-YSZ) for use as solid oxide fuel cell anode were synthesized by coprecipitation to obtain amorphous zirconia and crystallized β-nickel gels of the corresponding metal hydroxides. Hydrothermal treatment at 200°C and 220 psi from 2 up to 16 hours, under stirring, was performed to produce nanocrystalline powder. The as-synthesized powders were uniaxially pressed and sintered in air. Powders were characterized by X-ray diffraction, laser scattering, scanning and transmission electron microscopy (SEM/TEM), gas adsorption technique (BET) and TG-DTA thermal analysis. Ceramic samples were characterized by dilatometric analysis and density measurements by Archimedes method. The characteristics of hydrothermally synthesized powders and compacts were compared to those produced without temperature and pressure application. Crystalline powders were obtained after hydrothermal process, excluding the calcination step from this route. The specific surface area of powders decreases with increasing time of hydrothermal treatment while the agglomerate mean size is not affected by this parameter.