Structure of nanoporous zirconia-based powders synthesized by different gel-combustion routes

Zirconia-based ceramics that retain their metastable tetragonal phase at room temperature are widely studied due to their excellent mechanical and electrical properties. When these materials are prepared from precursor nanopowders with high specific surface areas, this phase is retained in dense ceramic bodies. In this work, we present a morphological study of nanocrystalline ZrO2-2.8 mol% Y2O3 powders synthesized by the gel-combustion method, using different organic fuels - alanine, glycine, lysine and citric acid - and calcined at temperatures ranging from 873 to 1173 K. The nanopore structures were investigated by small-angle X-ray scattering. The experimental results indicate that nanopores in samples prepared with alanine, glycine and lysine have an essentially single-mode volume distribution for calcination temperatures up to 1073 K, while those calcined at 1173 K exhibit a more complex and wider volume distribution. The volume-weighted average of the nanopore radii monotonically increases with increasing calcination temperature. The samples prepared with citric acid exhibit a size distribution much wider than the others. The Brunauer-Emmett-Teller technique was used to determine specific surface area and X-ray diffraction, environmental scanning electron microscopy and transmission electron microscopy were also employed for a complete characterization of the samples.

Characterization of lanthanum-doped bismuth titanate thin films prepared by polymeric precursor method

Lanthanum-modified bismuth titanate, Bi4-xLaxTi3O12 (BLT), with x ranging from 0 to 0.75 was grown on Pt/Ti/SiO2/Si substrates using a polymeric precursor solution and spin-coating method. The dielectric constant of highly doped bismuth titanate was equal to 148 while dielectric losses remained low (tan delta = 0.0018), and the films showed well-saturated polarization-electric field curves (2P(r) = 40.6 muC/cm(2) and V-c = 0.99 V). The leakage current densities improve for the lanthanum-doped system. For five-layered BLT films with x = 0.75, a charge storage density of 35 fC/mum(2) and a thickness of 320 nm were found. (C) 2004 Elsevier B.V. All rights reserved.

Formation of TiO2 ceramic foams from the integration of the sol-gel method with surfactants assembly and emulsion

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

NiTiO3 nanoparticles encapsulated with SiO2 prepared by sol-gel method

NiTiO3 (NTO) nanoparticles encapsulated with SiO2 were prepared by the sol-gel method resulting on core-shell structure. Changes on isoelectric point as a function of silica were evaluated by means of zeta potential. The NTO nanoparticles heat treated at 600 degrees C were characterized by X-ray diffraction, transmission electron microscopy (TEM) and energy dispersive X-ray analysis. TEM observations showed that the mean size of NTO is in the range of 2.5-42.5 nm while the thickness of SiO2 shell attained 1.5-3.5 nm approximately.

NiTiO3 powders obtained by polymeric precursor method: Synthesis and characterization

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Synthesis of Fine Micro-sized BaZrO3 Powders Based on a Decaoctahedron Shape by the Microwave-Assisted Hydrothermal Method

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Crystal structure of propylthiouracil determined using high-resolution synchrotron X-ray powder diffraction

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

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

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

ZnO architectures synthesized by a microwave-assisted hydrothermal method and their photoluminescence properties

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Electronic structure, growth mechanism and photoluminescence of CaWO4 crystals

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

Very Intense Distinct Blue and Red Photoluminescence Emission in MgTiO3 Thin Films Prepared by the Polymeric Precursor Method: An Experimental and Theoretical Approach

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

GCHF basis sets and their application in the electronic structure study of PrMnO3

The approach called generator coordinate Hartree-Fock (GCHF) method is used in the selection of Gaussian basis set [25s18p for O ((3)p), 31s21p14d for Mn (S-6), and 33s22p16d9f for Pr ((4)J)] for atoms. The role of the weight functions in the assessment of the numerical integration range of the GCHF equations is shown. These basis sets are contracted to (25s18p/9s5p), (31s21p14d/9s6p4d), and (33s22pl6d9f118sl2p5d3f) by segmented contraction scheme of Dunning and they are utilized in calculations of Restricted-Open-HF (ROHF) Total and Orbital energies of the (MnO+1)-Mn-3 and (PrO+1)-Pr-1 fragments, to evaluate their quality in molecular studies. The addition of one d polarization function in the contracted (9s5p) basis set for O(P-3) atom and their application with the contracted (9s6p4d), (18s21p5d3f) basis sets for Mn (S-6) and Pr-Pr ((4)j) atoms lead to the electronic structure study of PrMnO3. The dipole moment, the total energy, and total atomic charges properties were calculated and were carried out at ROHF level with the [PrMnO3](2) fragment. The calculated values show that PrMnO3 does not present piezoelectric properties. (C) 2004 Elsevier B.V. All rights reserved.

Gaussian basis sets to the theoretical study of the electronic structure of perovskite (LaMnO3)

The Generator Coordinate Hartree-Fock (GCHF) method is applied to generate extended (20s14p), (30s19p13d), and (31s23p18d) Gaussian basis sets for the 0, Mn, and La atoms, respectively. The role of the weight functions (WFs) in the assessment of the numerical integration range of the GCHF equations is shown. These basis sets are then contracted to [5s3p] and [11s6p6d] for 0 and Mn atoms, respectively, and [17s11p7d] for La atom by a standard procedure. For quality evaluation of contracted basis sets in molecular calculations, we have accomplished calculations of total and orbital energies in the Hartree-Fock-Roothaan (HFR) method for (MnO1+)-Mn-5 and (LaO1+)-La-1 fragments. The results obtained with the contracted basis sets are compared with values obtained with the extended basis sets. The addition of one d polarization function in the contracted basis set for 0 atom and its utilization with the contracted basis sets for Mn and La atoms leads to the calculations of dipole moment and total atomic charges of perovskite (LaMnO3). The calculations were performed at the HFR level with the crystal [LaMnO3](2) fragment in space group C-2v the values of dipole moment, total energy, and total atomic charges showed that it is reasonable to believe that LaMnO3 presents behaviour of piezoelectric material. (C) 2003 Elsevier B.V. All rights reserved.

Mechanochemical synthesis of ceramic powders with perovskite structure

The mechanical activation is one of the most effective method for obtaining highly disperse system due to mechanical action stress fields form in solids during milling procedure. This effect results in changes of free energy, leading to release of heat, formation of a new surface, formation of different crystal lattice defects and initiation of solid-state chemical reaction. The accumulated deformation energy determines irreversible changes of crystal structure and consequently microstructure resulting in the change of their properties. Mechanochemical processing route has been developed recently for the production of intermetallic and alloy compounds. The intrinsic advantage of this process is that the solid-state reaction is activated due to mechanical energy instead of the temperature. It was shown that the chemical reactivity of starting materials could be improved significantly after mechanochemical activation and, subsequently, the calcination temperature was reduced. Besides, it was apparent that the mechanochemical treatment could enhance the reactivity of constituent oxides; however, the sintering process could not be avoided to develop the desired ceramics. A novel mechanochemical technique for synthesis of fine-grained perovskite structured powders has shown that it is possible to form perovskite at room temperature. The effect of milling on the formation of perovskite structure of barium titanate (BT), lead titanate (PT), PZT, PZN, magnesium niobate (PMN) and LM ceramic materials was analyzed. The dielectric properties of sintered ceramics are comparable with those prepared by other methods in the literature. (C) 2003 Elsevier B.V. B.V. All rights reserved.