Zélia Soares Macedo

Laser sintering of Bi4Ti3O12 ferroelectric ceramics

Abstract Bi 4 Ti 3 O 12 (BIT) ceramics were synthesized by solid-state reaction and sintered by CO 2 laser heating. In the sintering process, the laser beam with a diameter of 6±1 mm was fixed on the center of the sample, which was pre-heated at 350 °C. The laser power was raised at a linear rate, according to an irradiation program. The microstructure of laser-sintered ceramics was compared to a reference, which was sintered in electrical furnace. Samples sintered by the conventional process presented grain size of 14±3 μm, measured along the larger dimension, and relative density of 94±2%, whereas BIT ceramics sintered under P max =1.06 W/mm 2 for 5 min had grain sizes of 7±4 μm and relative density of 97±1%.

Particle size control of Y 2 O 3 : Eu 3+ prepared via a coconut water-assisted sol-gel method

Eu3+-doped Y2O3 nanoparticles were produced through proteic sol-gel technique, and the adjustment of pH was tested in order to control the particle size of the powders. A strong correlation between the initial pH and the temperature of crystallization was observed, allowing the production of particles with controlled diameter from 4nm to 50 nm. The samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy, optical microscopy and photoluminescence spectroscopy in both absorption and emission modes. A blue shift of the excitation peak corresponding to energy transfer from Y2O3:Eu3+ host to Eu3+ ions was observed, as the particle size was reduced from 50 to 4 nm. The suppression of a charge transfer band also resulted from the reduction of the particle size. The emission spectrum of the Y2O3: with particles of 50nm was found to be similar to that of bulk material whereas 4 nm particles presented broadened emission peaks with lower intensities.

Recycling of chromium wastes from the tanning industry to produce ceramic nanopigments

Growth in the use of colored ceramics and the demand for better quality have stimulated research into the development of new classes of pigments with superior durability and color reproducibility, which can be produced using inexpensive, straightforward, and eco-friendly synthesis procedures. The present work proposes a modified sol–gel route for the production of green and pink Cr–Al2O3 nanoparticles, employing solid and liquid tannery wastes together with NOM-rich water. The synthesis involved preparing a solution of Al2Cl3 and the wastes in NOM-rich water. The solution was kept under stirring to form a gel that was then heated to 100 °C in order to eliminate water. This material was subsequently homogenized and calcined. The results indicated that single phase chromium-doped α-alumina could be obtained using an initial solution pH of 4.0 and calcination at 1100 °C for 4 h. Colorimetric analysis of the pigments was performed according to the CIE 1976 L*a*b* norm. The results revealed a strong dependence of pigment color on the dopant concentration and pH used in the synthesis. The presence of the Cr3+ oxidation state in all the samples studied was confirmed using X-ray absorption near-edge structure (XANES) spectroscopy. The particle size of the samples was 39 ± 1 nm, as determined by FEG-SEM measurements. The catalytic tests demonstrated the potential of tannery wastes for use in the catalytic reduction of nitrophenol, with conversion rates between 49 and 98%. It could be concluded that the proposed route was effective for the production of α-alumina pigments and catalysts using tannery wastes, offering important technological and environmental advantages.

Dielectric relaxation mechanism of single crystal and polycrystal bismuth germanate

We report the results of investigation on the mechanism of dark conductivity and dielectric relaxation of Bi12GeO20 single crystals and dense ceramics. Alternating current electric characterization was performed in the temperature interval from 30 °C up to 730 °C and frequencies from 1 Hz to 13 MHz. The samples presented ohmic behavior even at high temperatures, for applied potentials up to 2.2 V. The electric conductivity of single crystals was thermally activated, with potential barriers changing from 0.75±0.05 eV to 1.4±0.2 eV at 500 °C. The same conduction mechanisms were observed for ceramic samples, with activation energies of 0.9±0.1 eV and 1.5±0.1 eV for charge transport through grains and grain boundaries, respectively. These energy values were related to defects arising from substitutional Bi3++h+ in Ge4+ sites. In the ceramic samples, the higher resistivity of the grain interfaces leads to a Maxwell–Wagner polarization of the grain boundaries due to local charge rearrangement. As a result, the ...

A Comparative Study of Er3+, Er3+-Eu3+, Er3+-Tb3+, and Er3+-Eu3+-Tb3+Codoped Y2O3Nanoparticles as Optical Heaters

Fluorescence intensity ratio (FIR) technique, based on the thermal coupling of and energy levels of erbium ions, was used to study the optical heating behavior of rare earth doped yttrium oxide nanophosphors (Y2O3:Er3Fluorescence intensity ratio (FIR) technique, based on the thermal coupling of 2H11/2 and 4S3/2 energy levels of erbium ions, was used to study the optical heating behavior of rare earth doped yttrium oxide nanophosphors (Y2O3:Er3+, Y2O3:Er3+-Eu3+, Y2O3:Er3+-Tb3+, and Y2O3:Er3+-Eu3+-Tb3+) synthesized via PVA-assisted sol-gel route. The samples were optically heated by an 800 nm CW diode laser, while the upconverted green emissions were used to measure their temperatures in real time. The experimental results indicate that the studied nanoparticles are promising candidates to applications such as photothermal treatments and hyperthermia.

Produção de nanopartículas de Al2O3 utilizando água de coco maduro (coco seco)

The present work proposes a new preparation route of nanoparticles to be used in the ceramic industry. This new route, known as proteic sol-gel, usually employs green coconut water to dissolve the precursor salts, instead of conventional metal alcoxides. However there are no reports about the employment of water from mature coconut in the preparation of nanoparticles, which is usually disposed in the coconut processing industry. Al2O3 nanoparticles were produced via proteic sol-gel route using mature coconut water and were characterized through DTA/TG and XRD measurements and SEM and AFM images. The γ-Al2O3 phase was obtained after calcination at 700 °C, and was stable even after calcination at 1200 °C. An increase in particle size while the temperature of calcination was raised was observed, and the particle size was about 400 nm in sample treated at 1200 °C.

X-ray-excited optical luminescence and X-ray absorption fine-structures studies of CdWO4 scintillator

X-ray-excited optical luminescence (XEOL) emission and excitation spectra as well as the EXAFS signal of CdWO(4) were measured in the energy region of the Cd and W absorption edges. From EXAFS refinement, structural parameters such as number of atoms, distance from the absorbing atom and width of coordination shells in the W neighborhood were determined. The role of W-O interactions on the intrinsic luminescence of CdWO(4) is discussed. The efficiencies of conversion, transfer and emission processes involved in the scintillation mechanism showed to be high when self-trapped excitons are formed locally by direct excitation of W ions. Annihilation of these excitons provides the characteristic scintillation of CdWO(4), a broad band emission with maximum at 500 nm. The presence of two energetically different O positions in the lattice gives rise to the composite structure of the luminescence band, and no influence of extrinsic defects was noticed. A mismatch between the X-ray absorption coefficient and the zero-order luminescence curves corroborates that the direct excitation of Cd ions induces secondary electronic excitations not very effective in transferring energy to the luminescent group, WO(6).

Avaliação do potencial de argilas de Sergipe e Alagoas na produção de agregados para uso em concreto

This study aims to characterize technologically three clays employed by ceramic industries in Sergipe state, Brazil. Its potential use for the production of calcined synthetic aggregate to substitute gravel in concrete production was evaluated. The characterization of the clays included particle size and plasticity analysis, X-ray diffraction, differential and thermogravimetric analyses, dilatometry, X-ray fluorescence, organic matter content, cation exchange capacity, specific area, and scanning electron microscopy. Ceramic bodies were uniaxially pressed under 30 MPa, into rectangular and cylindrical shapes. Ceramic bodies of individual clays and also ceramic formulations were fired and subsequently characterized to determine their water absorption, apparent density, compressive strength, and grain morphology. It was observed that, after firing at 1120 oC, two formulations presented strength, water absorption and specific mass comparable to those of gravel used in concrete.

New bismuth germanate oxide nanoparticle material for biolabel applications in medicine

Bismuth germanate (Bi4Ge3O12, BGO) has been the focus of several studies due to its scintillation properties. It has been employed as detector in scientific research and medicine, and herein we studied its possible biomedical applications. The photoluminescence properties of the uncoated and protein-coated nanoparticles were analyzed in different body fluids, at different pH. The nanoparticles yielded blueish-white luminescence with a maximum emission peak at 485 nm corresponding to the 3P1 → 1S0 electron transition of Bi3+. They showed luminescence properties at different pH values and in human fluids, such as urine and blood serum. Finally, the BGO nanoparticles were functionalized with the anti-HLA I W6/32 monoclonal antibody and the capacity of the antibody-loaded nanoparticles to recognize the cognate antigen (HLA I) of the W6/32 mAb was assessed on the human promyelocytic leukemia cell line THP-1. The possibility of functionalizing BGO nanoparticles with W6/32 antibodies and their specificity to identify THP-1 cells make them promising candidates for biomedical applications as biolabels.

Influence of the Uniaxial Hot-Pressing Sintering Condition on Bi4Ge3O12 Ceramic Scintillators

The production of high-density bismuth germanate (Bi3Ge4O12) ceramic scintillators by uniaxial hot pressing was investigated as a function of different applied pressure conditions. The X-ray diffraction showed that the sintering process was able to eliminate the undesirable secondary phase present in the nonsintered samples. The height changes from samples with higher applied pressure rate and applied pressure duration lead to a better relative density value, >95% for samples sintered under a pressure of 0.14 and 0.18 MPa. The radioluminescence results showed that all samples have the characteristic emission spectra of Bi3Ge4O12 and that the hot-pressed samples have higher radioluminescence emission efficiency.