Materials Science Forum Vol. 820

Abstract: Strontium titanate ceramic powders (SrTiO₃), pure and doped with lanthanum (La³⁺) and neodymium (Nd³⁺), were synthesized by solid state reaction (SSR) and microwave assisted hydrothermal technique (MHT). For SSR, a mixture of SrCO₃, TiO₂, La₂O₃ and Nd₂O₃ oxides was performed in stoichiometric ratio, to produce SrTiO₃ (STO), Sr_0.96La_0.04TiO₃ (STO-04La), Sr_0.96Nd_0.04TiO₃ (STO - 04Nd) and Sr_0.96La_0.02Nd_0.02TiO₃ (STO-02La02Nd), in a ball mill, for 3.5 h. This mixture was dried at 70°C for 24h. This powder was calcined at 1150°C for 2h in a conventional oven. For MHT synthesis, Ti (C₄H₉O)₄, SrCO₃, La₂O₃ and Nd₂O₃ precursors were solubilized in nitric acid (10M), in stoichiometric proportions to form STO , STO-04La, STO-04Nd and STO-02La02Nd. This solution was precipitated adding NH₄OH (10M). Quota of 2g of precipitated powder was then dried in an air oven at 70°C for 24 hours, and then added to 40 ml of a KOH solution (10M). These suspensions were subjected to MHT for 1h, at 120°C. The ceramic powders obtained by the two routes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystallite size was calculated by Scherrer method and from SEM image, linear intercept method for both the SSR and MHT powders was used to measure the particles size, which show the increase of particles size related to the cation substitution.

Abstract: Tape casting process was used to produce Al₂O₃ substrates in an aqueous system with acrylic latex emulsion as binder. The present work studied the slurry formulations in aqueous medium of Al₂O₃ powders with different particle size distribution and made correlation to the green and sintered tapes. Two commercial alumina powders, one sub-micrometric and other micrometric were used. Compositions of Al₂O₃ slurries with 80 and 83 wt% of solids were prepared by dispersing the powders in water with a dispersant with subsequently additions of 7 and 10 wt% of binder. Sub-micrometric Al₂O₃ resulted in a high densification tapes regardless solid concentration and binder amount in the slurry, though green density was affected. For micrometric alumina, increasing the solid concentration resulted in a little higher final density.

Abstract: This paper proposes evaluate the effect on doping of 0.4mol of Co in ZnO, Aiming to use in diluted magnetic semiconductor (DMSs), so for this purpose, an evaluation of the characteristics: structural, morphological and magnetic was investigated. The samples were synthesized by combustion reaction using urea as fuel and an electric heater as source of heating. During the synthesis was performed measurements of temperature and time of combustion flame. The samples were characterized by X-ray diffraction (DRX), adsorption of nitrogen (BET), scanning electron microscopy (SEM), particle size distribution and magnetic measurement. The maximum temperature, and reaction time were, 520 oC and 240 s. The data of the XRD showed how phase majority the ZnO, with crystallite size of 27 nm and trace of second phase CoCo₂O₄. Presented superficial area of 33.5 m²/g, with morphology consisting of agglomerates in the form of balls smaller than 5 μm. The sample presented behavior ferrimagnetic with saturation magnetization of 0.32 emu/g, type of a magnetic semiconductor.

Abstract: Tetragonal Tungsten Bronze structure TTB-type structure has attracted interest by the high anisotropy of the crystal structure. The dielectric characterization of iron-doped niobate of TTB-type structure, with stoichiometry KSr₂(Fe_0.25Nb_4.75)O_15-δ, prepared by Modified Polyol Method was investigated. Nanocrystalline single phase powders were obtained after calcination of the precursor powder at 1150 °C for 10 hours in an oxygen atmosphere. The dielectric characterization was performed by impedance spectroscopy, from room temperature to 600 °C, in the frequency range of 5 Hz to 13 MHz. The permittivity values obtained for KSr₂(Fe_0.25Nb_4.75)O_15-δ showed superior to the permittivity values of the KSr₂Nb₅O₁₅ host structure in all temperature range investigated. At room temperature, the permittivity values (2100) of KSr₂(Fe_0.25Nb_4.75)O_15-δ is two times the permittivity values of KSr₂Nb₅O₁₅. The substitution of niobium cation by iron cation in the KSr₂Nb₅O₁₅ host structure showed a suppression of the ferroelectric (P4bm) → paraelectric (P4/mbm) phase transition.

Abstract: The dielectric resonator (DR) is a ceramic component used in electronic circuits that can operate in microwave frequency range, where it plays the role of resonant element and enables the construction of high selective filters and oscillators. The dielectric properties of a ceramic resonator are influenced by their microstructure characteristics as pores amount. This work shows a study of the influence of pores amount (porosity) on the dielectric constant values of ceramics from ZnO-Nb₂O₅-TiO₂ system. Mixtures of Nb₂O₅, ZnO, and TiO₂ powders were compacted by uniaxial (100 MPa) and isostatic (300 MPa) pressing and sintered at 1100, 1200, and 1250°C. The experimental dielectric constant values of the sintered ceramics were corrected to eliminate the porosity influence. The results showed that the porosity, the type and quantities of crystalline chemical compounds in the ceramics influence the dielectric constant values.

Abstract: The applicability of micromagnetics for phases with high magnetocrystalline anisotropy as barium ferrite Ba₂Fe₁₂O₁₉ and Nd₂Fe₁₄B is discussed. The Stoner-Wohlfarth model is very suitable for such phases, and also for PtFe and PtCo. It was discussed how to take into account the total energy of the system for grain size above the single domain particle size. For this situation of large grain size, the net magnetostatic energy of the system cannot be neglected. From energy considerations, it follows that the coercive force should decrease with the inverse of the square root of the grain size.

Abstract: This work reports the photoluminescence chitosan/ZnAl₂O₄ films in mass ratios 1:1, 1:2, 1:3, 1:4 and 1:5, respectively. The films were presented flexible, opaque, with a thickness of 0.04 mm and were characterized by XRD, FTIR, emission and excitation. The results show the presence of characteristic peaks of chitosan and ZnAl₂O₄, and bands related to the presence of chitosan, silanol and siloxane groups and silane agent used in surface modification ZnAl₂O₄, which acts as a binding agent with the chitosan all movie reviews. The excitation and emission spectra showed the presence of broadband processes associated with charge transfer from Al ⁺³ the O^-2 in all the films, and that the highest photoluminescence intensities were observed for the films 1:1 and 1:2, which showed higher concentrations than the concentrations Qs 1:3, 1:4 and 1:5. Thus, Qs acted as a binder, transferring energy to Al ⁺³ the O^{- 2}.

Abstract: The advancement of the oil sector has generated the need for the use of materials resistant to aggressive environments to oil. Although ceramics have high melting point and high hardness is, on the other hand, more fragile and less tough, which can cause damage to the metal structure. The Al₂O₃ based ceramics reinforced with rare earth oxide can improve tenaciousness and makes the ceramic material more resistent. This article aims to present the production of composite Al₂O₃ - Y₂O₃ stabilized ZrO₂ by uniaxial pressing, following sintering (1200-1350 °C). Structural and microstructural characterizations as XRD (X-Ray Diffraction) and SEM (Scanning Electron Microscopy) and mechanical tests as Vickers hardness, % absorption and % linear shrinkage were conducted to evaluate the feasibility of using the composite and ceramic coating for storage and transportation of oil tanks. The results indicate that the proportions of 5%, 10% and 30% ZrO₂ make it suitable as a good composite suitable coating.

Abstract: Planarity is one of indispensable attribute of a ceramic tile quality. During the firing changes on curvature may be permanent causing damage on quality and productivity. The objective was to evaluate curvature evolution on porcelain tile during heating stage of firing. Effects of engobe and glaze coating were evaluated. The performance of two compositions of porcelain tile was studied on industrial (1000 mm x 500 mm x 10 mm) and laboratorial (100 mm x 50 mm x 6 mm) scale. Industrial test pieces were fired at 950 and 1200 °C. Final curvature was measured by conventional dial indicator. Curvature evolution of laboratorial test pieces was evaluated by thermal fleximetry from 25 to 1200 °C. This characterization was capable to identify very significant positive curvature deflection on temperature range from 930 to 1200 °C, directly associated to permanent planarity defects. Differences on composition affect significantly the curvature evolution.

Abstract: Corrosion is an undesirable action which can occur in metals. To minimize this problem, there are several types of coat to the metal surface being researched in an attempt to retard the oxidation processes. In this work, the deposition of a new ceramic coating based on niobium oxide over aluminum alloy AA 3003 was studied in order to obtaining a corrosion resistant and less polluting film. It was utilized the experimental planning to optimize the deposition of ceramics, and it was performed electrochemical tests, scanning electron microscopy and dispersive energy spectroscopy to evaluate these films. The ceramic coating based on niobium oxide obtained presented itself as an effective corrosion inhibitor for aluminum alloy AA 3003.