Conventional and microwave sintering of CaCu(3)Ti(4)O(12)/CaTiO(3) ceramic composites: non-ohmic and dielectric properties

The non-ohmic and dielectric properties as well as the dependence on the microstructural features of CaCu(3)Ti(4)O(12)/CaTiO(3) ceramic composites obtained by conventional and microwave sintering were investigated. It was demonstrated that the non-ohmic and dielectric properties depend strongly on the sintering conditions. It was found that the non-linear coefficient reaches values of 65 for microwave-sintered samples and 42 for samples sintered in a conventional furnace when a current density interval of 1-10 mA cm(-2) is considered. The non-linear coefficient value of 65 is equivalent to 1500 for samples sintered in the microwave if a current interval of 5-30 mA is considered as is shortly discussed by Chung et al (2004 Nature Mater. 3 774). Due to a high non-linear coefficient and a low leakage current (90 mu A) under both processing conditions, these samples are promising for varistor applications. The conventionally sintered samples exhibit a higher relative dielectric constant at 1 kHz (2960) compared with the samples sintered in the microwave furnace (2100). At high frequencies, the dielectric constant is also larger in the samples sintered in the conventional furnace. Depending on the application, one or another synthesis methodology is recommended, that is, for varistor applications sintered in a microwave furnace and for dielectric application sintered in a conventional furnace.

DC Electric Field Dependence of the Dielectric Constant of Pzt Thin Films Prepared by Polymeric Precursor Method

This work reports dielectric measurements performed on Pb(Zr0.53Ti0.47)O3 (PZT) thin films prepared by a polymeric precursor method. The -E curves obtained for the PZT film measured at 100 kHz, under a small ac 0.2 kV/cm signal-test and a dc scan featured a typical butterfly curve. However, the -E curves obtained for PZT film under a dc scan, with a scan rate of 0.003 V/s, shows a pronounced asymmetry. The absence of a symmetric secondary peak in -E curves could be an indication of essentially 180 domain switching.

Piezo and Dielectric Properties of PHB-PZT Composite

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

The effect of changes in the bulk dielectric constant on the DNA torsional properties

The effect of changes in the bulk dielectric constant on the DNA torsional properties was evaluated from plasmid circularization reactions. In these reactions, pUC18 previously linearized by EcoRI digestion was recircularized with T4 DNA ligase. The bulk dielectric constant of the reaction medium was decreased by the addition of different concentrations of neutral solutes: ethylene glycol, glycerol, sorbitol, and sucrose, or increased by the addition of glycine. The topoisomers generated by the ligase reaction were resolved by agarose-gel electrophoresis. The DNA twist energy parameter (K), which is an apparent torsional constant, was determined by linearization of the Gaussian topoisomers' distribution. It was observed that the twist energy parameter for the given solutes is almost linearly dependent on the bulk dielectric constant. In the reaction buffer, the twist energy parameter was determined to be 1100 +/- 100. By decreasing the dielectric constant to 74 with the addition of sorbitol, the value of the parameter reaches K = 900 +/- 100, whereas the addition of ethylene glycol leads to kappa = 400 +/- 50. Upon addition of glycine, which resulted in a dielectric constant equal to 91, the value of the twist energy parameter increased to K 1750 +/- 100. (c) 2007 Wiley Periodicals.

A polaronic stacking fault defect model for CaCu3Ti4O12 material: an approach for the origin of the huge dielectric constant and semiconducting coexistent features

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

High dielectric constant of SrTiO3 thin films prepared by chemical process

SrTiO3 thin films were prepared by the polymeric precursor method and deposited by spin-coating onto Pt/Ti/SiO2/Si(100) substrates. The spin-coated films heat treated at 700 degrees C were crack-free, dense, and homogeneous. Microstructural and morphological evaluations were followed by grazing incident X-ray, scanning electron microscopy and atomic force microscopy. Dielectric studies indicated a dielectric constant of about 475, which is higher than that of ceramic SrTiO3, and a factor dissipation of about 0.050 at 100 kHz. SrTiO3 thin films were found to have paraelectric properties with C-V characteristics. (C) 2000 Kluwer Academic Publishers.

Role of oxygen vacancies in the magnetic and dielectric properties of the high-dielectric-constant system CaCu3Ti4O12: An electron-spin resonance study

We report experiments of electron spin resonance (ESR) of Cu2+ in polycrystalline samples of CaCu3Ti4O12 post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of CaCu3Ti4O12 were annealed for 24 h at 1000 degrees C under air, Ar or O-2. Our temperature dependent ESR data revealed for all samples nearly temperature independent g value (2.15(1)) and linewidth for T > T-N approximate to 25 K. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the O-2 or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the Cu2+ ESR lines due to the presence of oxygen vacancies which induce an S=1/2 spin inside the TiO6 octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of CaCu3Ti4O12 and for polycrystals of CdCu3Ti4O12 are reported.

Very large dielectric constant of highly oriented Pb1-xBaxTiO3 thin films prepared by chemical deposition

Highly (100) oriented Pb0.8Ba0.2TiO3/LaNiO3 structures were grown on LaAlO3(100) substrates by using a wet, soft chemical method and crystallized by the microwave oven technique. The Au/PBT/LaNiO3/LaAlO3 capacitor shows a hysteresis loop with remnant polarization, P-r, of 15 muC/cm(2), and coercive field, E-c, of 47 kV/cm at an applied voltage of 3 V, along with a dielectric constant over 1800. Atomic force microscopy showed that Pb0.8Ba0.2TiO3 is composed of large grains about 300 nm. The experimental results demonstrated that the microwave preparation is rapid, clean, and energy efficient. Therefore, we demonstrated that the combination of the soft chemical method with the microwave process is a promising technique to grow highly oriented thin films with excellent dielectric and ferroelectric properties, which can be used in various integrated device applications. (C) 2004 American Institute of Physics.

Optical and electrical diagnostics measurements of plasmas generated in aromatic compounds

Plasmas generated in de discharges in aromatic compounds have been used for several years in polymerization processes. The chemical kinetics developed in such a plasma environment are extremely complicated. Therefore it is extremely important to set up optical and electrical diagnostics in order to establish the kinetics of the film growth, In this work we studied de plasmas generated ill low-pressure atmospheres of benzene for different values of gas pressure and power coupled to the discharge. The pressure range varied from 0.2 to 1.0 mbar for electric power running from 4 to 25 W, the main chemical species observed within the discharge were CH, H and C. It was observed that the CH relative concentration increases continuously with the power in the range investigated. The electron temperature varied from 0.5 to 2.0 eV with the increase of the power, for a fixed value of gas pressure. The relative dielectric constant of the plasma polymerized benzene was kept around 4.8 from 100 Hz to 10 kHz, presenting a resonance near 25 kHz. This electric behaviour of the film was the same fur different conditions of polymeric film deposition, (C) 1997 Elsevier B.V. S.A.

Emprego da reflectometria por domínio do tempo na determinação da umidade de madeiras comerciais

Pós-graduação em Agronomia (Energia na Agricultura) - FCA

Síntese e caracterização de compósitos poli(fluoreto de vinilideno-trifluoretileno)/titanato de bário para uso em regeneração tecidual guiada

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Calibração de um equipamento TDR (Reflectometria por Dominio do Tempo) para a Determinação da Umidade da Madeira de Grevillea robusta (Cunn.)

This study aimed to establish a calibration model of a TDR equipment to determinate thewood moisture of Grevillea robusta (Cunn.). It was selected alleatory, 11 logs of Grevillea robusta (Cunn.), it was dried in a regulate electric greenhouse at 40o C. During the dry process, the moisture was determinate, periodically and systematically by gravimetric and TDR methods. Through the gravimetric method, moisture vary from 125,88% to 6,62% while the TDR method, the variation was from 119,83% to 5,79%. The best model adjusted to the wood moisture in the determination of the function of relative dielectric constant, using this technique, was the exponential 50,20 132,72 140,27 K U e − = − ⋅ , with a R2 = 0,96, with indicator of trust or performance 0,97 they were classified as very good, according to Camargo and Sentelhas (1997). The correlation between the gravimetric method (reference) and TDR, showed to be equivalent for both methodologies. It was concluded that the TDR can be used to the determination of wood moisture of Grevillea robusta (Cunn.), in a precise and fast way, once it was established it calibration.

Structural and dielectric properties of relaxor Sr0.5Ba0.5Bi2Nb2O9 ceramic

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

Electric measurements with constant current: A practical method for characterizing dielectric films

This article assesses the use of the constant current (CC) method for characterizing dielectric films. The method is based on charging the sample with a constant current (current stress) and measuring the corresponding voltage rise under the closed circuit condition. Our article shows that the CC method is an alternative to the constant voltage stressing method to study the electric properties of nonpolar, ferroelectric, and polar polymers. The method was tested by determining the dielectric constant of polytetrafluoroethylene, and investigating the electric conduction in poly(ethylene terephthalate). For the ferroelectric polymer poly(vinylidene fluoride), it is shown that hysteresis loops and the dependence of the ferroelectric polarization on the electric field can be obtained. (C) 2001 American Institute of Physics.

Structural and dielectric properties of relaxor Sr0.5Ba 0.5Bi2Nb2O9 ceramic

Sr0.5Ba0.5Bi2Nb2O 9 ceramic was prepared by a conventional solid state reaction method and studied using X-ray powder diffraction and dielectric measurements. At room temperature, an orthorhombic structure was confirmed and their parameters were obtained using the Rietveld method. Dielectric properties were studied in a broad range of temperatures and frequencies. Typical relaxor behaviour was observed with strong dispersion of the complex relative dielectric permittivity. The temperature of the maximum dielectric constant Tm decreases with increasing frequency, and shifts towards higher temperature side. The activation energy Ea≈0·194±0·03 eV and freezing temperature Ta≈371±2 K values were found using the Vogel-Fulcher relationship. Conduction process in the material may be due to the hopping of charge carriers at low temperatures and small polarons and/or singly ionised oxygen vacancies at higher temperatures. © 2010 Maney Publishing.