Increase Of Dielectric Relaxation Time Research Articles

Influence of confined phase transition on dielectric relaxation characteristics of thermochromic epoxy insulating materials

The dielectric relaxation characteristic and mechanism of thermochromic microcapsule-epoxy insulating material are investigated. The results show that thermochromic microcapsule-epoxy insulating material exhibits non-monotonic dielectric relaxation characteristic, namely the dielectric relaxation time gradually increases with the temperature rising in a range of about 58–66 ℃, which cannot be depicted by the conventional Arrhenius equation or Vogel-Fulcher-Tammann equation. It is proposed that the non-monotonic dielectric relaxation characteristic is derived from the free volume variation induced by the confined phase transition in microcapsule. With the increase of temperature, the solid-liquid phase transition occurs in the limited space of microcapsule, reducing the free volume inside the microcapsule, which could restrict the reorientation of dipole with the external electric field and lead to the increase of dielectric relaxation time. The non-monotonic dielectric relaxation characteristic of thermochromic epoxy specimen is fitted based on the confined dielectric relaxation model, obtaining the activation energy of dielectric relaxation. The relaxation activation energy values of thermochromic epoxy insulating materials with different microcapsule content are of the same order of magnitude, indicating that the non-monotonic dielectric relaxations occur inside the thermochromic microcapsule, verifying the role of confined phase transition in the non-monotonic dielectric relaxation characteristic.

Dielectric relaxation study of aqueous 2-ethoxyethanol using time domain reflectometry technique

The complex permittivity spectra of 2-ethoxyethanol in water solutions have been studied at different concentrations and temperatures using a picosecond time domain reflectometry technique. The complex dielectric permittivity spectrum of 2-ethoxyethanol shows Cole-Davidson type behavior. Increase in dielectric relaxation time may be due to increase in hetero molecular interaction strength. Minimum in Excess dielectric constant values provides the information about stable complex adduct. The Kirkwood correlation factor, thermodynamic properties and Bruggeman factor have also been determined and the results are interpreted in terms of hydrogen bonding and interactions among the solute — solvent molecules.

In situ analysis of carrier lifetime and barrier capacitance variations in silicon during 1.5 MeV protons implantation

Results of the in situ measurements of the recombination lifetime and of barrier capacitance variations in Si substrates and pin diodes, respectively, during 1.5 MeV protons implantation are presented. Carrier recombination lifetime has been measured by employing microwave probed photoconductivity method, while parameters of the barrier capacitance changes have been extracted by transient technique of barrier capacitance charging current measurements using linearly increasing voltage pulses. Sub-linear decrease of carrier lifetime as a function of fluence has been revealed and peculiarities of such characteristic are explained in terms of formation of two layered structure within implanted Si material. Carrier recombination processes determine the increase of dielectric relaxation time within electrically neutral region (ENR) of a diode base. Carrier capture/emission processes within space charge (SC) and transition layer (between ENR and SC) regions lead to increase of generation/recombination currents in the irradiated diode.

Dielectric properties of montmorillonite clay filled poly(vinyl alcohol)/poly(ethylene oxide) blend nanocomposites

Organic–inorganic nanocomposites of poly(vinyl alcohol) (PVA)–poly(ethylene oxide) (PEO) blend filled with montmorillonite (MMT) nanoclay up to 10 wt.% concentration were synthesized by aqueous solution-cast technique. The complex dielectric function, electrical conductivity, electric modulus and impedance spectra of the nanocomposites were measured in the frequency range 20 Hz–1 MHz at ambient temperature. A direct correlation was observed between the real part of dielectric function and the mean relaxation time of the polymer chain segmental dynamics, with the exfoliated and intercalated MMT clay structures, and the extent of miscibility between PVA and PEO due to hydrogen bonded bridging through exfoliated MMT clay nanosheets. The large increase of dielectric relaxation time revealed that the dispersed exfoliated nanoscale MMT clay in the polymers blend matrix produces a large hindrance to the polymer chain dynamics. Results confirm that the real part of dielectric function of the nanocomposites can be tailored by varying amount of MMT clay filler for their use as nanodielectric materials in the microelectronic technology.

Hysteresis in acoustic properties of ferroelectric relaxor Pb[(Zn1∕3Nb2∕3)0.955Ti0.045]O3 single crystals studied by Brillouin and dielectric spectroscopies

Acoustic and dielectric properties of Pb[(Zn1∕3Nb2∕3)1−xTix]O3 (PZN-xPT) single crystals with x=0.045 have been studied by the high-resolution micro-Brillouin scattering and dielectric spectroscopy in a wide temperature range. The softening of the Brillouin shift and the increase of dielectric relaxation time upon cooling indicated the formation of polar nanoregions (PNRs) and the slowing down of their dynamics. In contrast to the acoustic properties of typical model relaxors such as lead magnesioniobate, the change in the Brillouin shift near its minimum became sharper on heating compared to the change on cooling, pointing to the clear existence of hysteresis in the dynamics of the diffuse phase transition in PZN-4.5%PT. Since the number of PNRs will increase upon cooling, it may be expected that the kinetics of the phase transition would become slower, the lower the transition temperature resulting in the more sluggish, broad feature of the Brillouin shift observed during cooling. This result may indicate that the number and size of polar nanoregions, which are dependent on temperature, play an important role in the development of the mesoscopic ferroelectric order in PZN-4.5%PT.

A study of the interaction of some neuropeptides and their analogs with bilayer lipid membranes and liposomes

Several biophysical methods have been used to study the interaction of the amphiphilic hormone derivative ACTH t-24, the wasp venom peptide mastoparan and the synthetic neuropeptide galparan with planar bilayer lipid membranes (BLM) and liposomes. Addition of ACTH 1–24, galparan and mastoparan at different concentrations (0.1 μM to 0.1 mM) to BLM from soybean phosphatidylcholine leads to an increase of membrane electrical conductance to an extent that correlates with the activity of the different peptides on mast cells. Addition of ACTH 1–24 to a suspension of DMPC + DMPG liposomes leads to an increase of dielectric relaxation time, however the less charged peptides did not influence the dielectric relaxation properties of liposomes. The velocity number [ u], measured by ultrasonic velocimetry in suspensions of liposomes from DMPC, decreased after addition of the peptides. This reveals an increase of the adiabatic volume compressibility of the liposomes. The presence of ACTH 1–24, galparan and mastoparan in liposomes of DMPC (mole fraction peptide/DMPC=7.7 × 10 −4) resulted in different changes of absorption number Δ[αλ] of ultrasound at temperatures below and above the phase transition of DMPC. The results obtained are consistent with the idea that the lipid bilayer may function as a mediator for the receptor-independent stimulation of G-proteins.