Papers by Keyword: Powder Synthesis

Abstract: The work is devoted to the problem of synthesis of highly dispersed spherical powder granules by laser ablation. The formation of microspheres is carried out by the action of femtosecond laser radiation on the surface of a titanium target in a two-component reaction medium. The process of titanium treatment in n-hexane by ultrashort laser pulses is investigated. It is shown that the use of n-hexane as a working medium component allows the synthesis of micro-and nanopowders containing titanium carbide, which are resistant to atmospheric environment during prolonged exposure. The presented method provides the possibility of synthesis of high-purity powder materials with typical sizes from 1 to 3 microns.

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: The perovskite CaCu₃Ti₄O₁₂, with giant electric permittivity, was prepared by a soft chemistry route and by the solid state synthesis technique. The main purpose of this work is to verify the effects of the synthesis techniques on the powder characteristics, and the microstructure and dielectric properties of the sintered materials. The synthesized nanopowders were calcined, pressed into pellets and sintered using the same procedures as for the materials prepared by the conventional method. Phase analysis was performed by X-ray diffraction, microstructure observations were carried out by scanning electron microscopy, and the dielectric properties were studied by impedance spectroscopy. The electric permittivity ranged from 11,600 to 27,000 and from 5,000 to 9,000 for specimens prepared by the conventional and the soft chemistry techniques, respectively, and the dielectric losses varied between 0.1 and 1.3 (conventional) and 0.095 and 0.3 (soft chemistry) depending on the sintering temperature.

Abstract: Zinc oxide (ZnO) powders were synthesized by a simple precipitation and a hydrothermal process at the temperature range RT-100°C. In precipitation process, the powders were formed by mixing aqueous solutions of zinc nitrate with NaOH aqueous solution under controlled process conditions such as precursor concentration, reaction pH and temperature. Single phase ZnO particles can be easily synthesized in lower precursor concentration, higher reaction pH and temperature. The powders synthesized at room temperature exhibited plates, rods or pointed multipod morphologies depending on the concentration and pH. ZnO crystallites synthesized by hydrothermal process consisted mostly of well developed large or elongated crystallites of plates or rods in shape. The results reveal that the ZnO crystallite sizes and shapes would be efficiently controllable by changing the processing parameters of the preparation processes.

Abstract: Ultrafine Cr3C2 powders with globular-like particle of ~0.3-0.8 μm were synthesized by a new precursor method, namely carbothermal reduction of complex chromium oxide-carbon mixture, which was made initially from salt solution containing chromium and carbon elements by air drying and subsequent calcining at 400 °C for 1 h. Phase composition of reaction products was discussed by X-ray diffraction (XRD), and microstructure of the calcined powders and final products was studied by scanning electron microscopy (SEM). The results show that the synthesizing temperature of Cr₃C₂ powders was reduced greatly by the new precursor method. Thus, the preparation of ultrafine Cr₃C₂ powders is at only 1000 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the complex oxide-carbon mixture. There is a coarsening tendency of Cr₃C₂ powders with the increase of reaction temperature and time.

Abstract: The corundum-type magnesium niobate, Mg₄Nb₂O₉, powders were synthesized by the conventional solid-state process. The mixtures of MgO and Nb₂O₅ were calcined in the temperature ranging from 500°C to 1200°C for crystallization in air atmosphere. The formation of the Mg₄Nb₂O₉ phase was investigated as a function of calcination temperatures by DTA and XRD. The morphology and phase composition were determined via a combination of SEM and EDX techniques. The results showed that the single-phase Mg₄Nb₂O₉ could be obtained at 1050°C for 2h with the size of less than 5μm.

Abstract: Nanosized and highly reactive magnesium niobate Mg₄Nb₂O₉ were successfully synthesized by a sol-gel method as potential candidates for use in microwave applications as a low-temperature co-fired ceramic (LTCC). The Nb-Mg-citric acid solution was evaporated resulting in a dry gel that was calcined in the temperature ranging from 550°C to 850°C for crystallization in air atmosphere. The crystallization process from the gel to the crystalline Mg₄Nb₂O₉ was followed by thermal analysis and during calcinations phase evolution of the powders was studied using X-ray diffraction (XRD) technique. Particle size and morphology were examined by high resolution scanning electron microscopy (HR-SEM). At 850°C the corundum-like phase of magnesium niobate was obtained in air atmosphere with an average particle diameter of 80 nm approximately calculated from the HR-SEM images.

Abstract: Cornstalk fibers were modified with surface modification and the properties of cornstalk fiber/gypsum composites was studied. The results showed that mechanical properties of cornstalk fiber/gypsum composites were enhanced by adding unmodified cornstalk fibers, but composites waterproof performance was decreased greatly. Through surface modification on cornstalk fibers, composites mechanical properties were further enhanced, and composites waterproof performance was improved obviously. The mechanism of surface modification was discussed, and the model of interfacial zone between modified cornstalk fiber and gypsum matrix was proposed.

Abstract: The Aurivillius phases in the Bi-Fe-Ti-O system showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. These compounds were usually prepared by high temperature solid-state reaction method using respective oxide powders mixed in required stoichiometry. An excess of bismuth oxide was often added due to its evaporation during heat treatment. The mixture of the oxide powders were calcined in air between 700 and 900°C for several or even for several dozen hours. In the present paper the Bi6Fe2Ti3O18 Aurivillius phase was prepared by direct solid state reaction between respective oxides and by co-precipitation – calcination method. Mixture of the oxides and co-precipitated gels were calcined at different temperatures and X-ray diffraction analysis was used for identification of phase composition of the products.

Abstract: The focus of this work is to investigate the effect of seeding on the synthesis of B4C powder obtained by carbothermal reduction. B4C particles were added to act as seeds during reaction. Starting materials were boric acid (H3BO3) and two different carbon sources (graphite and carbon black). Mixtures of C, B2O3 (25 wt% B2O3 excess in relation to stoichiometric composition) and commercial B4C powder (0, 2.5 and 5.0 wt%) were prepared. The effect of 1.5 wt% of NaCl was also investigated. The powder mixtures were uniaxially die-pressed to form cylindrical shaped green bodies, then placed in a graphite crucible and heated at 1750°C/1h with a heating rate of 30°C/min, under argon-flowing atmosphere. Samples with seeds addition exhibited a decrease in particle size distribution. The particle morphology was uniform with a narrow size distribution and a medium particle size around 6 m regardless the initial concentration of seeds.