2,028 research outputs found
Dielectric properties of Li2O-3B2O3 glasses
The frequency and temperature dependence of the dielectric constant and the
electrical conductivity of the transparent glasses in the composition
Li2O-3B2O3 (LBO) were investigated in the 100 Hz- 10 MHz frequency range. The
dielectric constant and the loss in the low frequency regime were electrode
material dependent. Dielectric and electrical relaxations were respectively
analyzed using the Cole-Cole and electric modulus formalisms. The dielectric
relaxation mechanism was discussed in the framework of electrode and charge
carrier (hopping of the ions) related polarization using generalized Cole-Cole
expression. The frequency dependent electrical conductivity was rationalized
using Jonscher's power law. The activation energy associated with the dc
conductivity was 0.80 \pm 0.02 eV, which was ascribed to the motion of Li+ ions
in the glass matrix. The activation energy associated with dielectric
relaxation was almost equal to that of the dc conductivity, indicating that the
same species took part in both the processes. Temperature dependent behavior of
the frequency exponent (n) suggested that the correlated barrier hopping model
was the most apposite to rationalize the electrical transport phenomenon in
Li2O-3B2O3 glasses. These glasses on heating at 933 K/10h resulted in the known
non-linear optical phase LiB3O5.Comment: 32 pages, 13 figure
Charge and momentum transfer in supercooled melts: Why should their relaxation times differ?
The steady state values of the viscosity and the intrinsic ionic-conductivity
of quenched melts are computed, in terms of independently measurable
quantities. The frequency dependence of the ac dielectric response is
estimated. The discrepancy between the corresponding characteristic relaxation
times is only apparent; it does not imply distinct mechanisms, but stems from
the intrinsic barrier distribution for -relaxation in supercooled
fluids and glasses. This type of intrinsic ``decoupling'' is argued not to
exceed four orders in magnitude, for known glassformers. We explain the origin
of the discrepancy between the stretching exponent , as extracted from
and the dielectric modulus data. The actual width of the
barrier distribution always grows with lowering the temperature. The contrary
is an artifact of the large contribution of the dc-conductivity component to
the modulus data. The methodology allows one to single out other contributions
to the conductivity, as in ``superionic'' liquids or when charge carriers are
delocalized, implying that in those systems, charge transfer does not require
structural reconfiguration.Comment: submitted to J Chem Phy
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
We investigated the dynamics of a series of room temperature ionic liquids
based on the same 1-butyl-3-methyl imidazolium cation and different anions by
means of broadband dielectric spectroscopy covering 15 decades in frequency
(10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An
ionic conductivity is observed above the glass transition temperature T_{g}
with a relaxation in the electric modulus representation. Below T_{g}, two
relaxation processes appear, with the same features as the secondary
relaxations typically observed in molecular glasses. The activation energy of
the secondary processes and their dependence on the anion are different. The
slower process shows the characteristics of an intrinsic Johari-Goldstein
relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found,
as observed in molecular glasses.Comment: Major revision, submitted to Phys. Rev. Let
Kinetic description of particle interaction with a gravitational wave
The interaction of charged particles, moving in a uniform magnetic field,
with a plane-polarized gravitational wave is considered using the
Fokker-Planck- Kolmogorov (FPK) approach. By using a stochasticity criterion,
we determine the exact locations in phase space, where resonance overlapping
occurs. We investigate the diffusion of orbits around each primary resonance of
order (m) by deriving general analytical expressions for an effective diffusion
coeficient. A solution to the corresponding diffusion equation (Fokker-Planck
equation) for the static case is found. Numerical integration of the full
equations of motion and subsequent calculation of the diffusion coefficient
verifies the analytical results.Comment: LaTeX file, 15 page
Considerações sobre índices de produtividade da pecuária de corte em Mato Grosso do Sul.
A pecuária de corte, apesar de ser um dos segmentos mais importantes da economia do Estado de Mato Grosso do Sul, desde o inicio da década de 90 vem apresentando queda na produtividade geral. Isso esta ocorrendo como conseqüência da redução da produtividade das pastagens, que por sua vez, e resultado de uma serie de fatores que vão desde a escolha da espécie forrageira e o manejo de formação ate a escolha do animal a ser utilizado. Pode-se ressaltar, no entanto, que uma das principais causas e o manejo inadequado das pastagens, em especial, no que diz respeito a pratica generalizada do excesso de lotação. O reflexo primário de tal pratica faz-se sentir na degradação das pastagens com serias conseqüências ambientais, em especial em solos mas arenosos. Do ponto de vista econômico, este cenário resulta em diminuição da rentabilidade do sistema de produção. Em ultima instancia, o que se verifica e sua inviabilidade do sistema de produção. Em ultima instancia, o que se verifica e sua inviabilidade bioeconômica. Apesar de a capacidade de suporte media das pastagens, no Estado, ser de 0,9 cabeça/hectare, ela apresenta variações acentuadas em função das condições de solo, clima e sistema de produção. Todavia, e possível, utilizando-se de tecnologias já disponíveis, reduzir tais variáveis e/ou implementar alternativas que possibilitem, em algumas regiões, a sua melhoria. Para isso, fazem-se necessárias, na maioria dos casos, a recuperação e a manutenção das pastagens. Torna-se, portanto, indispensável o uso de fertilizantes, o que por sua vez, em muitas situações, e impraticável, quer seja pela inviabilidade econômica, quer seja pela baixa disponibilidade de adubos. Assim, para se aumentar a produtividade da pecuária de corte em MS, e imprescindível que diferentes setores se unam para viabilizar a geração, a transferência e a validação de tecnologias. Aliado a isso e importante criar condições para que os sistemas tenham gerenciamento mais eficientes, e para que os créditos sejam mais compatíveis com a atividade.bitstream/item/135978/1/DOC-70.pd
Cianamida hidrogenada na quebra de dormência da amoreira-preta e atividade da enzima peroxidase.
Adaptação de populações de Spodoptera frugiperda de diferentes regiões de Minas Gerais em hospedeiros alternativos.
Aspectos biológicos de Orius insidiosus (Say, 1832) (Hemiptera: Anthocoridae) alimentados com ovos de Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae).
Desenvolvimento do predador Podisus nigrispinus (Dallas, 1851) alimentado em Spodoptera frugiperda (J. E. Smith, 1797) contaminada por Bacillus thuringiensis (BT.)
Universal Parametric Correlations of Eigenvalues of Random Matrix Ensemble
Eigenvalue correlations of random matrix ensembles as a function of an
external perturbation are investigated vis the Dyson Brownian Motion Model in
the situation where the level density has a hard edge singularity. By solving a
linearized hydrodynamical equation, a universal dependence of the
density-density correlator on the external field is found. As an application we
obtain a formula for the variance of linear statistics with the parametric
dependence exhibited as a Laplace transform.Comment: 23 pages, late
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