Low Temperature Dielectric Constant Research Articles

Low temperature properties of multiferroic [formula omitted] compound

Single-phase BiFe 0.9Cr 0.1O 3 compound with a grain size of ∼ 63 nm was prepared by the sol–gel technique. A well-defined nonlinearity in the field dependences of magnetization as a result of the collapse of the space-modulated spin structure and the strong coupling between Fe 3+ and Cr 3+ superexchange interaction indicated a weak ferromagnetic character, and the observed significant deviation between the ZFC and FC curves suggested a spin-glass-like behavior at low temperatures. The low temperature dielectric constant showed two subtle anomalies at 50 and 230 K, which are related to the spin-glass behavior and magnetoelectric coupling. Moreover, the magnetoelectric coupling was estimated by determining the variation of the dielectric constant with applied magnetic field.

The Influence of DC Bias on the Dielectric Response of Sr<sub>1-x</sub>Ca<sub>x</sub>TiO<sub>3</sub> Ceramics

The effect of the electric field on the dielectric response of the Sr1-xCaxTiO3 ceramic system was analysed and complemented by structural and microstructural studies. A monotonic decrease of the lattice parameter and an increase of the average grain size from 11 to 23 μm with increasing x were observed by X-ray diffraction and scanning electron microscopy analysis respectively. Samples with x = 0.003 and 0.01 reveal a monotonous increase of low-temperature dielectric constant, corresponding to an enhancement of the tunability and a reduction of the driving electric field at 10-30 K for x = 0.01. For Sr1-xCaxTiO3 ceramic samples with x = 0.04 and 0.10 the ferroelectric phase transition of around 20 K was observed together with the enhanced tunability of the dielectric constant at 30-85 K for x = 0.10.

Effect of high-energy heavy ion irradiation on dielectric relaxation behaviour of kapton-H polyimide

High-energy ion irradiation effects on the thermally stimulated depolarization current (TSDC) behaviour and dielectric constant (ε′)/loss factor have been investigated. Kapton-H samples of 25 μm thickness were irradiated with 50 MeV Si + beam (fluence 2.3×10 12 and 1.38×10 13 ions/cm 2). The TSD currents and dielectric constant/loss factor (frequency 1 and 100 kHz) have been investigated from room temperature to 250 °C, following the usual method. The TSDC spectrum of the irradiated samples reveal that the β-peak (appearing at ≈80 °C) associated with dipolar relaxation has been largely affected due to irradiation. At higher fluence, β-peak almost vanishes. The peak appearing at approximately 180 °C (α-peak) is due to space charge relaxation. The peak magnitude increases in samples irradiated with higher fluence. This is mainly due to the creation of new energy traps by irradiation. The ion irradiation results in a large increase in low temperature dielectric constant (30–70 °C) ascribed to the increase in water absorption capacity. The dielectric constant in temperature range 70–180 °C is mainly governed by dipolar relaxation and space charge relaxation. It is suggested that though the space charge relaxation tends to increase ε′, a major loss in carbonyl groups due to high-energy ion irradiation results in a decrease in ε′. In high temperature region (180–250 °C) the dominance of interfacial polarization increases the ε′ value. The loss peaks appearing at approximately 50, 180 and 250 °C are representative of the various relaxation processes. The findings of the TSDC data are in conformity with the dielectric constant data.

Modified Poole–Frenkel mechanisms in Ge 25Bi xSb 15− xS 60 thin films

The current–voltage ( I– V) characteristics of amorphous Ge 25Bi x Sb 15− x S 60 ( x=0, 5, 10 and 15) thin film samples sandwiched between metal electrodes have been investigated as a function of composition, deposition rate (0.61–2.65 nm s −1), film thickness (128.6–598.2 nm) and temperature (130–373 K). The current–voltage characteristics are ohmic in the lower field regime followed by non-ohmic behavior in the higher voltage regime, which has been satisfactorily explained by the modified Poole–Frenkel effect. The experimentally determined values of Poole–Frenkel factor ( β PF) for different film composition, deposition rate and thicknesses are in good agreement with the theoretically calculated values. Fitting the room temperature data with Jonscher’s field-independent re-trapping model is an indication that a modified Poole–Frenkel process is operating. The Hall model can account for the low-temperature dielectric constant, and gives an indication of modified Poole–Frenkel emission.

Dipole moment differences of polar dyes in polymeric matrices: Stark effect and photochemical hole burning. II

Abstract As an extension of our first article of this series (J. Chem. Phys. 97 (1992) 5316), in which we reported data of the induced dipole moment differences Δμind of nonpolar dyes in polymeric matrices, we present results which were obtained with polar optical probe molecules, namely, chlorin and naphthazarin. We used the technique of persistent spectral hole burning to investigate both the frequency dependence of the permanent dipole moment differences Δμper of polar dyes, and the correlation of the dipole moment differences measured in the absorption maximum, 〈Δμabsmaxper〉 with the low-temperature dielectric constant ϵHe. In contrast to the difference Δμind of the induced dipole moments of centrosymmetric molecules the difference Δμper of the permanent dipole moments of polar dyes does not increase from higher to lower optical frequencies but is constant or decreases with decreasing frequency in matrices which are nonpolar or only moderately polar. The 〈Δμabsmaxper〉 values exhibit a linear dependence on the low-temperature dielectric constant, which is similar to the behavior found for centrosymmetric molecules. This linear dependence allows the separation of the matrix-induced contribution to the Δμper values and the “molecular” Δμmol values. Implications of the dependences of Δμ on the absorption frequency and on the dielectric constant of the matrix are discussed.

The Metallic State of Conducting Polymers: Microwave Dielectric Response and Optical Conductivity

ABSTRACTRecent advances in processing of polyaniline and polyacetylene have resulted in a new generation of conducting polymers with higher dc conductivities. We present the temperature (T) dependent microwave frequency dielectric constant, dc conductivity, and Kramers-Kronig analysis of conducting polyaniline and polyacetylene. The low temperature dielectric constant, ε, increases with the square of the x-ray crystalline domain length for preparations of HCl protonated polyaniline. For the highest crystalline polyaniline samples, ε increases dramatically with increasing T, supporting formation of three-dimensional (3-D) coupled “mesoscopic” Metallic regions. A “metallic” negative ε is observed for d,1-camphor sulfonie acid doped polyaniline prepared in m-cresol. Optical studies show a linear increase in reflectivity below 7000 cm-1. Below 600 cm-1 the reflectance increases rapidly. Kramers-Kronig analysis of the ir-visible results are presented. Highly conducting polyaniline is shown to have two plasma frequencies, one at ∼ 1.1 eV involving all the conduction band electrons, and one at ∼0.015 eV (120 cm-1) that is suggested to arise from the most delocalized electrons. The concept of inhomogeneous disorder is introduced. The results for polyaniline are compared to those of highly doped polyacetylene which also show metallic negative ε demonstrating the intrinsic metallic nature of the new generation of conducting polymers.

Low-temperature dielectric constant of near-stoichiometric Naβ-alumina

Measurements of the dielectric constant of Na $\ensuremath{\beta}$-alumina at temperatures below 2 K reveal that the density of low-energy excitations is not a function of the Na stoichiometric excess for the range 7-35% excess Na.

Transport properties and dielectric constant of DSeDTF-TCNQ

The d.c. conductivity and microwave conductivity of the di-substituted Se derivative of TTF-TCNQ are reported at temperatures from 4.2 to 300 K. The low temperature dielectric constant is unusually large, ϵ 1=3500. The results are similar to earlier observations in TTF-TCNQ and imply that a similar mechanism is involved in this class of one-dimensional conductors.

Electric susceptibility and d.c. conductivity of crystalline TTFTCNQ

An experimental study has been made of the intrinsic d.c. and microwave conductivity of “high purity” crystalline TTFTCNQ. The maximum normalized peak conductivity in each case was found to be about 50 times the room temperature value. The cavity perturbation technique, used to determine the microwave conductivity and low temperature dielectric constant, is also shown to be a useful contactless method for measuring the thermal expansion.