144,157 research outputs found
Dielectric behavior of Copper Tantalum Oxide
A thorough investigation of the dielectric properties of Cu2Ta4O12, a
material crystallizing in a pseudo-cubic, perovskite-derived structure is
presented. We measured the dielectric constant and conductivity of single
crystals in an exceptionally broad frequency range up to GHz frequencies and at
temperatures from 25 - 500 K. The detected dielectric constant is unusually
high (reaching values up to 105) and almost constant in a broad frequency and
temperature range. Cu2Ta4O12 possesses a crystal structure similar to
CaCu3Ti4O12, the compound for which such an unusually high dielectric constant
was first observed. An analysis of the results using a simple equivalent
circuit and measurements with different types of contact revealed that
extrinsic interfacial polarization effects, derived from surface barrier
capacitors are the origin of the observed giant dielectric constants. The
intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively
high) dielectric constant in the order of 100 and by charge transport via
hopping conduction of Anderson-localized charge carriers.Comment: 18 pages, 6 figures, submitted to Jouranl of Physical Chemestr
The dielectric properties of soil-water mixtures at microwave frequencies
Recent measurements on the dielectric constants of soil-water mixtures show the existence of two frequency regions in which the dielectric behavior of these mixtures was quite different. At the frequencies of 1.4 GHz to 5 GHz, there were strong evidences that the variations of the dielectric (epsilon) with water content (W) depended on soil type. While the real part of epsilon for sandy soils rose rapidly with the increase in W, epsilon for the high-clay content soils rose only slowly with W. As a consequence, epsilon was generally higher for the sandy soils than for the high-clay content soils at a given W. On the other hand, most of the measurements at frequencies 1 GHz indicated the increase of epsilon with W independent of soil types. At a given W, epsilon' (sandy soil) approximately equals epsilon (high-clay content soil) within the precision of the measurements. These observational features can be satisfactorily interpreted in terms of a simple dielectric relaxation model, with an appropriate choice of the mean relaxation frequency f(m) and the range of the activation energy (beta). It was found that smaller f(m) and larger beta were required for the high-clay content soils than the sandy soils in order to be consistent with the measured data
High-frequency dielectric spectroscopy of batio3 core - silica shell nanocomposites: Problem of interdiffusion
Three types of BaTiO3 core - amorphous nano-shell composite ceramics were
processed from the same core-shell powder by standard sintering, spark-plasma
sintering and two-step sintering techniques and characterized by XRD, HRSEM and
broad-band dielectric spectroscopy in the frequency range 10^3 - 10^13 Hz
including the THz and IR range. The samples differed by porosity and by the
amount of interdiffusion from the cores to shells, in correlation with their
increasing porosity. The dielectric spectra were also calculated using suitable
models based on effective medium approximation. The measurements revealed a
strong dielectric dispersion below the THz range, which cannot be explained by
the modeling, and whose strength was in correlation with the degree of
interdiffusion. We assigned it to an effect of the interdiffusion layers,
giving rise to a strong interfacial polarization. It appears that the
high-frequency dielectric spectroscopy is an extremely sensitive tool for
detection of any gradient layers and sample inhomogeneities even in dielectric
materials with negligible conductivity
Time-temperature superposition in viscous liquids
Dielectric relaxation measurements on supercooled triphenyl phosphite show
that at low temperatures time-temperature superposition (TTS) is accurately
obeyed for the primary (alpha) relaxation process. Measurements on 6 other
molecular liquids close to the calorimetric glass transition indicate that TTS
is linked to an high-frequency decay of the alpha loss, while
the loss peak width is nonuniversal.Comment: 4 page
Near-Field Microwave Microscopy of Materials Properties
Near-field microwave microscopy has created the opportunity for a new class
of electrodynamics experiments of materials. Freed from the constraints of
traditional microwave optics, experiments can be carried out at high spatial
resolution over a broad frequency range. In addition, the measurements can be
done quantitatively so that images of microwave materials properties can be
created. We review the five major types of near-field microwave microscopes and
discuss our own form of microscopy in detail. Quantitative images of microwave
sheet resistance, dielectric constant, and dielectric tunability are presented
and discussed. Future prospects for near-field measurements of microwave
electrodynamic properties are also presented.Comment: 31 pages, 9 figures, lecture given at the 1999 NATO ASI on Microwave
Superconductivity Changes suggested by editor, including full reference
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