Effect of the heat flux direction on electrical properties of SrBi2Nb2O9 thin films crystallized using a microwave oven

Ferroelectric SrBi2Nb2O9 (SBN) thin films were prepared by the polymeric precursors method and deposited by spin coating onto Pt/Ti/SiO2/Si substrate and crystallized using a domestic microwave oven. It was studied the influence of the heat flux direction and the duration of the thermal treatment on the films crystallization. An element with high dielectric loss, a SiC susceptor, was used to absorb the microwave energy and transfers the heat to the film. Influence of the susceptor position to the sample crystallization was verified, the susceptor was, placed or below the substrate or above the film. The SBN perovskite phase was observed after a thermal treatment at 700 degreesC for 10 min when the susceptor was placed below the substrate and for 30 min when the susceptor was placed above the film. Electrical measurements revealed that the film crystallized at 700 degreesC for 10 min, with the susceptor placed below the film, presented dielectric constant, dielectric loss, remanent polarization and coercive field of, 67, 0.011, 4.2 muC/cm(2) and 27.5 kV/cm, respectively. When the films were crystallized at 700 degreesC for 30 min, with the susceptor placed above the film, the dielectric constant was 115 and the dissipation factor was around of 0.033, remanent polarization and coercive field were 10.8 muC/cm(2) and 170 kV/cm, respectively. (C) 2003 Elsevier B.V. All rights reserved.

Structural Refinement and Photoluminescence Properties of MnWO4 Nanorods Obtained by Microwave-Hydrothermal Synthesis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural refinement and photoluminescence properties of irregular cube-like (Ca-1_Cu-x(x))TiO3 microcrystals synthesized by the microwave-hydrothermal method

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Structural refinement, optical and microwave dielectric properties of BaZrO3

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effect of microwave disinfection on physical and mechanical properties of acrylic resins

This study evaluated the effect of microwave energy on the hardness, impact strength and flexural strength of the Clássico, Onda-Cryl and QC-20 acrylic resins. Aluminum die were embedded in metallic or plastic flasks with type III dental stone, in accordance with the traditional packing technique. A mixing powder/liquid ratio was used according to the manufacturer's instructions. After polymerization in water batch at 74°C for 9 h, boiling water for 20 min or microwave energy at 900 W for 10 min, the specimens were deflasked after flask cooling at room temperature, and submitted to finishing. Specimens non-disinfected and disinfected by microwave irradiation were submitted to hardness, impact and flexural strength tests. Each specimen was immersed in distilled water and disinfected in a microwave oven calibrated to 650 W for 3 min. Knoop hardness test was performed with 25 g load for 10 s, impact test was carried out using the Charpy system with 40 kpcm, and 3-point bending test with a crosshead speed of 0.5 mm/min until fracture. Data were submitted to statistical analysis by ANOVA and Tukey's test (α=0.05). Disinfection by microwave energy decreased the hardness of Clássico and Onda-Cryl acrylic resins, but no effect was observed on the impact and flexural strength of all tested resins.

Sintering temperature influence on microwave dielectric properties of TiO2-ZrO2 ceramics

Dielectric ceramics have been widely investigated and used for microwave applications such as resonators and filters. The present study deals with the influence of sintering temperature on microwave dielectric properties of TiO2 ceramics with 10, 20, and 30 wt% ZrO2. Three compositions have been developed through mixing procedures and then tested for each sintering temperature: 1500 and 1400°C. X-ray diffraction and scanning electron microscopy are carried out aiming to explain the ceramic behavior of each sample. The dielectric constants of different ceramics for both temperatures varied from 85.4 to 62.6, while their quality factor due to dielectric losses varied from 3110 to 1630. The Q decrease is attributed to the non uniform grain growth and to the obtained crystalline phases. The best microwave parameters were obtained for the ceramics sintered at 1400°C, which can be applied in microwave circuits as dielectric resonators. © (2010) Trans Tech Publications.

Structural refinement and photoluminescence properties of cube-like (Ca 1-xCu x)TiO 3 crystals synthesized by the microwave-hydrothermal method

In this work, (Ca 1-xCu x)TiO 3 crystals with (x = 0, 0.01 and 0.02), labeled as CTO, CCTO1 and CCTO2, were synthesized by the microwave-hydrothermal method at 140°C for 32 min. XRD patterns (Fig. 1), Rietveld refinement and FT-Raman spectroscopy indicated that these crystals present orthorhombic structure Pbnm. Micro-Raman and XANES spectra suggested that the substitution of Ca by Cu in A-site promoted a displacement of the [TiO6]-[TiO6] clusters adjacent from its symmetric center, which leads distortions on the [CaO 12] clusters neighboring and consequently cause the strains into the CaTiO3 lattice. FE-SEM images showed that these crystals have an irregular shape as cube like probably indicating an Ostwald-ripening and self-assemble as dominant mechanisms to crystals growth. The powders presented an intense PL blue-emission.

CdMoO 4 synthesized by a microwave-assisted hydrothermal method and their photoluminescence and photocatality properties

In this work CdMoO 4 nanoparticles were obtained under hydrothermal conditions using microwave radiation (2.45 GHz) (MH) at 100°C for different times. These powders were analyzed by X-ray diffraction (XRD), Field-emisson gum scanning electron microscopy (FEG-SEM), Ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD pattern confirmed that the pure CdMoO 4 phases were obtained. FEG-SEM powders present large-scale and homogeneous particles with microspheres-like morphology. UV-vis results were employed to determine the optical band gap these materials. Also, it showed existence of photoluminescence (PL) emission in the green wavelength range of 540-546 nm. Photocatalytic activity of CdMoO 4 nanocrystals was examined by monitoring the degradation of rhodamine B dye.

Electronic structure and magnetic properties of FeWO 4 nanocrystals synthesized by the microwave-hydrothermal method

This communication reports that FeWO 4 nanocrystals were successfully synthesized by the microwave-hydrothermal method at 443 K for 1 h. The structure and shape of these nanocrystals were characterized by X-ray diffraction, Rietveld refinement, and transmission electron microscopy. The experimental results and first principles calculations were combined to explain the electronic structure and magnetic properties. Experimental data were obtained by magnetization measurements for different applied magnetic fields. Theoretical calculations revealed that magnetic properties of FeWO 4 nanocrystals can be assigned to two magnetic orderings with parallel or antiparallel spins in adjacent chains. These factors are crucial to understanding of competition between ferro- and antiferromagnetic behavior. © 2012 Elsevier Inc.

Titanate nanotubes produced from microwave-assisted hydrothermal synthesis: Photocatalytic and structural properties

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Structural, optical, and magnetic properties of NiMoO4 nanorods prepared by microwave sintering

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Morphology and properties of (Ba, Sr, Ca) titanates synthesized by microwave-assisted hydrothermal method

Ba1-xCaxTiO3, Ba1-xSrxTiO3 and Sr1-xCaxTiO3 (x = 0, 0.25, 0.50, 0.75 and 1) nanoparticles were synthesized using the microwave-assisted hydrothermal method. Samples were prepared for 40 minutes at 140°C under a pressure of 3 MPa using an adapted domestic microwave oven. The samples were characterized by X-Ray diffraction (XRD), scanning electron microscopy (FE-SEM), and Raman, photoluminescence (PL) and ultraviolet-visible (UV-Vis) spectroscopies. XRD data show that ceramic powders have crystalline phases associated with a short-range structural disorder. This structural disorder is confirmed by Raman spectral bands indicating multi-phonon processes and the presence of defects or impurities. Such defects account for a broad band in the photoluminescence spectrum in the green light (460 nm) region for all samples. Gap energy variation, obtained from UV-Vis spectra, suggest a non-uniform band structure of these titanates in accordance with the PL results. The morphology of each sample is changed with doping and varies from a spherical to cubic appearance for energy minimization.

Structural refinement, optical and microwave dielectric properties of BaZrO3

In this work, barium zirconate (BaZrO3) ceramics synthesized by solid state reaction method and sintered at 1670 degrees C for 4 h were characterized by X-ray diffraction (XRD), Rietveld refinement, and Fourier transform infrared (FT-IR) spectroscopy. XRD patterns, Rietveld refinement data and FT-IR spectra which confirmed that BaZrO3 ceramics have a perovskite-type cubic structure. Optical properties were investigated by ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) measurements. UV-vis absorption spectra suggested an indirect allowed transition with the existence of intermediary energy levels within the band gap. Intense visible green PL emission was observed in BaZrO3 ceramics upon excitation with a 350 nm wavelength. This behavior is due to a majority of deep defects within the band gap caused by symmetry breaking in octahedral [ZrO6] clusters in the lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki-Coleman. The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite ground plane and Ansoft's high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated by adjusting the dimension of the same material. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Structural Refinement and Photoluminescence Properties of MnWO4 Nanorods Obtained by Microwave-Hydrothermal Synthesis

Manganese tungstate (MnWO4) nanorods were prepared at room temperature by the co-precipitation method and synthesized after processing in a microwave-hydrothermal (MH) system at 140 degrees C for 6-96 min. These nanorods were structurally characterized by X-ray diffraction (XRD), Rietveld refinements and Fourier transform (FT)-Raman spectroscopy. The growth direction, shape and average size distribution of nanorods were observed by means of transmission electron microscopy (TEM) and high resolution TEM (HR-TEM). The optical properties of the nanorods were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinement data and FT-Raman spectroscopy indicate that the MnWO4 precipitate is not a single phase structure while the nanorods synthesized by MH processing have a wolframite-type monoclinic structure without deleterious phases. FT-Raman spectra exhibited the presence of 17 Raman-active modes from 50 to 1,000 cm(-1). TEM and HR-TEM micrographs indicated that the nanorods are aggregated due to surface energy by Van der Waals forces and grow along the [100] direction. UV-vis absorption measurements confirmed non-linear values for the optical band gap (from 3.2 to 2.72 eV), which increased as the MH processing time increased. The structural characterizations indicated that the presence of defects in the MnWO4 precipitate promotes a significant contribution to maximum PL emission, while MnWO4 nanorods obtained by MH processing decrease the PL emission due to the reduction of defects in the lattice.

SnO2 nanocrystals synthesized by microwave-assisted hydrothermal method: towards a relationship between structural and optical properties

The exploration of novel synthetic methodologies that control both size and shape of functional nanostructure opens new avenues for the functional application of nanomaterials. Here, we report a new and versatile approach to synthesize SnO2 nanocrystals (rutile-type structure) using microwave-assisted hydrothermal method. Broad peaks in the X-ray diffraction spectra indicate the nanosized nature of the samples which were indexed as a pure cassiterite tetragonal phase. Chemically and physically adsorbed water was estimated by TGA data and FT-Raman spectra to account for a new broad peak around 560 cm(-1) which is related to defective surface modes. In addition, the spherical-like morphology and low dispersed distribution size around 3-5 nm were investigated by HR-TEM and FE-SEM microscopies. Room temperature PL emission presents two broad bands at 438 and 764 nm, indicating the existence of different recombination centers. When the size of the nanospheres decreases, the relative intensity of 513 nm emission increases and the 393 nm one decreases. UV-Visible spectra show substantial changes in the optical absorbance of crystalline SnO2 nanoparticles while the existence of a small tail points out the presence of localized levels inside the forbidden band gap and supplies the necessary condition for the PL emission.