G. C. Psarras

Electric modulus and interfacial polarization in composite polymeric systems

The applicability of the electric modulus formalism is investigated on a Debye-type relaxation process, the interfacial polarization or Maxwell–Wagner–Sillars effect. Electric modulus, which has been proposed for the description of systems with ionic conductivity and related relaxation processes, presents advantages in comparison to the classical approach of the real and imaginary part of dielectric permittivity. In composite polymeric materials, relaxation phenomena in the low-frequency region are attributed to the heterogeneity of the systems. For the investigation of these processes through electric modulus formalism, hybrid composite systems consisting of epoxy resin–metal powder–aramid fibres were prepared with various filler contents and their dielectric spectra were recorded in the frequency range 10 Hz–10 MHz in the temperature interval 30–150°C. The Debye, Cole–Cole, Davidson–Cole and Havriliak–Negami equations of dielectric relaxation are expressed in the electric modulus form. Correlation between experimental data and the various expressions produced, shows that interfacial polarization in the systems examined is, mostly, better described by the Davidson–Cole approach and only in the system with the higher heterogeneity must the Havriliak–Negami approach be used.

The dielectric response of a polymeric three-component composite

The dielectric behaviour of a composite system of epoxy resin filled with Kevlar fibres and Aluminum powder is investigated in the frequency range 20 Hz to 10 MHz and the temperature interval 10 to 150°C. Dielectric permittivity is increasing with filler content and temperature, being always higher in the low than in the high frequency range. Dielectric permittivity and loss of the composites is mostly affected by interfacial polarization arising from inhomogeneites at interfaces introduced by the fillers. Equations based on the extension of the logarithmic law of mixtures are formulated and their applicability tested with the experimental data obtained.

Dielectric permittivity and loss of an aluminum-filled epoxy resin

Abstract Dielectric behaviour of an epoxy resin filled with 0–30 wt% aluminum powder is reported. Permittivity, loss index and dissipation factor are characterized as a function of temperature in the range 20–150°C and frequency in the range 20 Hz–20 kHz. The filled resin shows a higher permittivity and higher dielectric loss. An interfacial or Maxwell-Wagner-Sillars polarization effect is clearly evident and glass transition temperature is unaffected by the filler.

Aramid fibers; a multifunctional sensor for monitoring stress/strain fields and damage development in composite materials

Abstract A multifunctional non-destructive technique, based on the Raman response of aramid fibers, is employed for (a) assessing the interface integrity and the overall stress distribution in a unidirectional Kevlar 29®/epoxy composite (b) measuring the stress concentration and its development with applied load in Kevlar 49®/epoxy composites incorporating a circular notch and finally, (c) determining the strain arising in 0° plies due to partial and full crack growth within the θ° plies in multidirectional 0°/θ°/0° composites. It is shown that this technique can be used effectively to study the damage development in composite materials that arise from the presence of different length scale discontinuities.

Adaptive composites incorporating shape memory alloy wires Part I Probing the internal stress and temperature distributions with a laser Raman sensor

Hybrid composite laminates consisting of an epoxy resin reinforced by aramid fibres and incorporating SMA wire actuators have been produced. The residual thermal stresses of the composites were determined with the technique of laser Raman spectroscopy and the generated compressive loads during SMA activation were quantified. The results obtained indicate that the SMA wires can be effectively used as actuating elements whereas the aramid fibres can be exploited as independent thermal and mechanical sensors.

Adaptive composites incorporating shape memory alloy wires. Part 2: development of internal recovery stresses as a function of activation temperature

Shape memory alloy (SMA) wires have been integrated in composite laminates consisting of an epoxy resin reinforced by aramid fibres. In the first part of this series, a new methodology was proposed for the simultaneous measurement of stress and temperature in the reinforcing fibres during shape memory wire activation. Those tests were conducted at three activation temperatures (40, 60 and 100°C) and the internal compressive stress distribution was extracted for low wire volume fraction composites. It was observed that for higher wire volume fractions the high compressive recovery stresses generated during electrical resistive heating of the wires led to the geometric failure of the composite coupons. In this work, measurements are conducted on hybrid laminate coupons of dimensions that prevent geometric (global) specimen failure. The internal stress distribution is measured at relatively high activation temperatures of 80 and 100°C and a filtering technique based on fast Fourier transform (FFT) is employed to simulate the stress and temperature variability and to filter the associated experimental noise. The results show that the higher stresses appear in the middle of the mid-wire distance and that the values at 100°C activation are not significantly different than those at 80°C indicating the presence of an upper limit in the transmission of wire recovery stresses in these systems.

Conductivity and dielectric characterization of polymer nanocomposites

Abstract: Dielectric relaxations are present in both polymers and polymer matrix composites. In polymer nanocomposites recorded relaxation phenomena include contributions from both the polymeric matrix and the presence of nanofiller. Dielectric data could provide valuable information regarding polymer/nanofiller interactions, distribution of nanoinclusions and nanocomposite’s morphology, improving our knowledge upon their structure–properties relationship. The electrical performance of nanocomposites can be adjusted by selecting the appropriate type and amount of nanofiller. Transition from insulating to conductive behaviour can be studied in terms of the scaling law of percolation theory. Finally, the influence of temperature and frequency on conductivity data can be exploited in revealing the occurring charge transport mechanisms.

Graphite nanoplatelets/polymer nanocomposites: thermomechanical, dielectric, and functional behavior

Exfoliated graphite nanoplatelets (GNP)/epoxy resin nanocomposites were prepared and tested, varying the amount of the filler content. Systems’ morphology was investigated by means of scanning electron microscopy, while their thermal response was examined via differential scanning calorimetry (DSC). Broadband dielectric spectroscopy and dynamic mechanical thermal analysis were employed in order to characterize the produced systems. Static mechanical tests were also conducted at ambient. Reinforced systems exhibit improved performance under mechanical and electrical excitation. In particular, storage modulus increases systematically with GNP content. DSC results imply that glass transition temperature is not affected by the presence of GNP. Flexural modulus and storage modulus, as determined by static and dynamic mechanical tests, respectively, increased with filler content. Dielectric permittivity increases also systematically with GNP content. Recorded relaxation processes arise from the glass to rubber transition of the polymer matrix (α-mode), re-orientation of polar side groups of the polymer chains (β-mode), and interfacial polarization because of the accumulation of charges at the systems’ interface. Finally, the energy storing efficiency of the nanocomposites enhances with reinforcing phase in the examined frequency and temperature range. Optimum performance corresponds to the nanocomposite with maximum GNP loading.

Composite coatings and their performance in corrosive environment

AbstractPretreated steel specimens coated with epoxy resin, either pure or containing iron powder, were exposed to a corrosive environment (aqueous 3·5 wt-%NaCl solution). Data concerning the modification of their properties were recorded via electrochemical impedance spectroscopy measurements at medium and high frequencies. Analysis of time dependent elements of proposed models has enabled the individual contributions to the impedance of the phases created during immersion to be distinguished and changes in the behaviour of the coatings due to water uptake to be monitored. These results, together with water permeability and hardness measurements and visual analysis, have revealed minor differences in the performance of the metal filled coatings compared with those of the pure epoxy.

Electromagnetic wave absorption properties of ternary poly(vinylidene fluoride)/magnetite nanocomposites with carbon nanotubes and graphene

Ternary nanocomposite systems of poly(vinylidene fluoride)/magnetite/carbon nanotube (PVDF/Fe3O4/CNT) and poly(vinylidene fluoride)/magnetite/graphene (PVDF/Fe3O4/GN), were prepared using high shear twin screw compounding followed by compression moulding. The electromagnetic (EM) microwave absorption properties of the nanocomposites were investigated in the frequency range of 3–10 GHz. PVDF/Fe3O4/CNT samples with the thickness d = 0.7 mm present a minimum reflection loss (RL) of −28.8 dB at 5.6 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB. PVDF/Fe3O4/GN with a thickness of 0.9 mm, presents a minimum RL of −22.6 dB at 5.4 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB as well. The excellent microwave absorption properties of both nanocomposites, in terms of minimum RL value and broad absorption bandwidth, are mainly due to the enhanced magnetic losses. The results indicate that the ternary nanocomposites studied here, can be used as an attractive candidate for EM absorption materials in diverse fields of various technological applications, not only in the frequency range 3–10 GHz, but also at frequencies 10 GHz for PVDF/Fe3O4/GN with a realistic thickness of close to 1 mm.