The dielectric properties of epoxy nanocomposites with insulating nano-fillers, viz., TiO2, ZnO and AI2O3 were investigated at low filler concentrations by weight. Epoxy nanocomposite samples with a good dispersion of nanoparticles in the epoxy matrix were prepared and experiments were performed to measure the dielectric permittivity and tan delta (400 Hz-1 MHz), dc volume resistivity and ac dielectric strength. At very low nanoparticle loadings, results demonstrate some interesting dielectric behaviors for nanocomposites and some of the electrical properties are found to be unique and advantageous for use in several existing and potential electrical systems. The nanocomposite dielectric properties are analyzed in detail with respect to different experimental parameters like frequency (for permittivity/tan delta), filler size, filler concentration and filler permittivity. In addition, epoxy microcomposites for the same systems were synthesized and their dielectric properties were compared to the results already obtained for nanocomposites. The interesting dielectric characteristics for epoxy based nanodielectric systems are attributed to the large volume fraction of interfaces in the bulk of the material and the ensuing interactions between the charged nanoparticle surface and the epoxy chains.

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Dielectric properties of epoxy nanocomposites

High-quality brookite flowers were fabricated via a facile solution chemistry technique. The synthetic conditions to the flower-like brookite were monitored by a series of time-resolved experiments and further optimized by adjusting the concentrations of the Na+ and OH− species involved in the reaction system. Careful sample characterizations by the combined techniques of X-ray diffraction, Raman, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectra indicate the formation of highly phase-pure and well-crystallized brookite with an extremely low defect concentration. Different from the natural brookite mineral with an indirect transition (Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154), the present high-quality brookite flowers showed a direct transition with a bandgap energy of 3.4 ± 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ± 0.1 eV for rutile and indirect band g...

High-Quality Brookite TiO2 Flowers: Synthesis, Characterization, and Dielectric Performance

To achieve a compact and reliable design of electrical equipment for the present day requirements, there is an urgent need for better and smart insulating materials and in this respect, the reported enhancements in dielectric properties obtained for polymer nanocomposites seems to be very encouraging. To further understand the dielectric behavior of polymer nanocomposites, this experimental work reports the trends of dielectric permittivities and tan delta (loss tangent) of epoxy nanocomposites with single nano-fillers of Al2O3 and TiO2at low filler concentrations (0.1%, 0.5%, 1% & 5%) over a frequency range of 1 MHz-1 GHz. Results show that the nanocomposites demonstrate some very different dielectric characteristics when compared to those for polymer microcomposites. Unlike the usual expectations of increasing permittivity with increasing filler concentration in polymer microcomposites, it has been seen that up to a certain nano-filler concentration and depending on the permittivity of the nano-filler, the permittivities of the epoxy nanocomposites are less than that of the unfilled epoxy at all the measured frequencies. This suggests that there is a very strong dependence of the filler concentration and nano-filler permittivity on the final permittivity of the nanocomposites at all these frequencies. But, in the case of tan delta behavior in nanocomposites, significant effects of filler concentrations were not observed with both Al2O3 and TiO2 fillers. Tan delta values in nanocomposites with Al2O3 fillers are found to be marginally lower at all filler concentrations when compared with the value for unfilled epoxy. But, in TiO2Oepoxy nanocomposites, although the variations in tan delta are not significant with respect to unfilled epoxy, some interesting trends are observed with respect to the frequencies of measurement.

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Permittivity and tan delta characteristics of epoxy nanocomposites in the frequency range of 1 MHz-1 GHz

Polar molecules dominated electrorheological fluids mainly comprising a mixture of dispersed phase of solid particles and/or dispersing liquid medium. The dispersed phase solid particles, on the surface, or the liquid dispersing medium contain polar molecules or polar groups, the dipole moment of which is 0.5-10 deb and the size is between 0.1 nm and 0.8 nm. Dispersed phase solid particles are spherical or nearly spherical, of which the size is 10-300 nm and dielectric constant is higher than 50. The conductance rate of the liquid dispersing medium is lower than 10 −8 S/m, and the dielectric constant is lower than 10. The PM-ER fluids possess the characteristics of high yield strength, high dynamic shear strength, low leakage current, the linear dependence of yield strength on electric field, and high yield strength at low electric field, etc. The yield strength improves to almost 100 times of that of ordinary ER fluids and reaches to more than 200 Kpa.

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Polar molecule dominated electrorheological fluid

Low-density polyethylene (LDPE) filled with a small amount of titanium dioxide proves to be very suitable for studying the molecular motion of polyethylene by means of dielectric spectroscopy. The titanium dioxide acts as an oxidizing agent that introduces a small concentration of polar carbonyl groups, which does not alter molecular motion, but couples it to the alternating electric field. The frequency and temperature dependences of molecular-motion processes in LDPE are determined from both isochronal and isothermal loss plots. In particular, the temperature dependence of the β process connected with the glass-rubber transition in polyethylene obeys the Vogel-Fulcher-Tammann-Hesse law, which indicates co-operative behaviour. For pure LDPE, a glass-transition temperature of -31 °C is determined dielectrically. It increases with increasing titanium dioxide content.

https://iopscience.iop.org/article/10.1088/0022-3727/34/20/308/pdf

Complete relaxation map of polyethylene: filler-induced chemical modifications as dielectric probes

Two kinds of nano-TiO 2 samples, including nano-anatase and nano-rutile, were prepared by the hydrothermal method and the gas condensation method, respectively. Results of dielectric measurements are as follows: 1. Nano-TiO 2 bulk samples (anatase and rutile) exhibit much higher dielectric constant than coarse-grain samples in the low measuring frequency range. 2. For both nano-anatase and nano-rutile, one peak appears in the spectrum of dielectric constant versus temperature, and it is located in the measuring temperature range of -150 to 150 °C for nano-anatase and in the measuring temperature range of 25 to 450 °C for nano-rutile. 3. For both nano-anatase and nano-rutile, in the spectrum of dielectric loss versus temperature there exists one peak. The mechanisms of the above-mentioned phenomena were discussed in detail.

Dielectric behaviour of nano‐TiO2 bulks

The isochronal annealing effects on the phase and microstructure of an amorphous Bi1.6Pb0.4Sr1.9Ca2Cu3.1Oy (BPSCCO) quenched from the liquid state have been studied using positron lifetime spectroscopy (PAS) in conjunction with X-ray diffraction, scanning electron microscopy and ac susceptibility measurements. It is shown that the bulk lifetime of positron annihilation in the amorphous state (255–260 ps) is longer than that in the crystalline one (223–228 ps). The crystalline transformation from the amorphous BPSCCO is a multiphase eutectic process that occurs around 450–500°C. The n = 3 phase also could be produced by the appropriate heat treatment and a Tc near 110 K could be obtained.

An investigation of post-annealing effects on amorphous (BiPb)SrCaCuO

Positron annihilation in thermally quenched YBa2Cu3O7‐x

The positron annihilation Doppler energy spectroscopy is used to monitor the movement of oxygen vacancy of YBa2Cu3O7−x and an activation energy of (0.80 ± 0.10) eV for migration of oxygen vacancy in orthorhombic phase is obtained.



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Hong Zhang

Papers

Dielectric behaviour of nano‐TiO2 bulks

An investigation of post-annealing effects on amorphous (BiPb)SrCaCuO

Positron annihilation in thermally quenched YBa2Cu3O7‐x

Study of the movement of oxygen vacancies in the orthorhombic phase of yba2cu3o7-x by positron doppler broadening spectroscopy

A vacancy complex in the dilute fe-v-n alloy identified by positron-annihilation spectroscopy