Biagio De Vivo

Development of epoxy mixtures for application in aeronautics and aerospace

This work describes a successful attempt toward the development of composite materials based on nanofilled epoxy resins for the realization of structural aeronautic components providing efficient lightning strike protection. The epoxy matrix is prepared by mixing a tetrafunctional epoxy precursor with a reactive diluent which allows the moisture content to be reduced and facilitates the nanofiller dispersion step. The reactive diluent also proves to be beneficial for improving the curing degree of nanofilled epoxy mixtures. It increases the mobility of reactive groups resulting in a higher cure degree than the epoxy precursor alone. This effect is particularly advantageous for nanofilled resins where higher temperature treatments are needed, compared to the unfilled resin, to reach the same cure degree. As nanofiller, different carbon nanostructured fiber-shaped fillers are embedded in the epoxy matrix with the aim of improving the electrical properties of the resin. The results highlight a strong influence of the nanofiller nature on the electrical properties especially in terms of electrical percolation threshold (EPT) and electrical conductivity beyond the EPT. Among the analyzed nanofillers, the highest electrical conductivity is obtained by using multiwalled carbon nanotubes (MWCNTs) and heat-treated carbon nanofibers (CNFs). The achieved results are analyzed by considering the nanofiller morphological parameters and characteristics with respect to the impact on their dispersion effectiveness.

The role of carbon nanofiber defects on the electrical and mechanical properties of CNF-based resins

Heat treatment of carbon nanofibers has proven to be an effective method in removing defects from carbon nanofibers, causing a strong increase in their structural perfection and thermal stability. It affects the bonding states of carbon atoms in the nanofiber structure and causes a significant transformation in the hybridization state of the bonded carbon atoms.Nanofilled resins made of heat-treated CNF show significant increases in their electrical conductivity even at low concentrations. This confirms that enhancement in the perfection of the fiber structure with consequent change in the morphological features plays a prominent role in affecting the electrical properties. Indeed heat-treated CNFs display a stiff structure and a smooth surface which tends to lower the thickness of the unavoidable insulating epoxy layer formed around the CNF which, in turn, plays a fundamental role in the electrical transport properties along the conducting clusters. This might be very beneficial in terms of electrical conductivity but might have negligible effect on the mechanical properties.

Carbon nanotube induced structural and physical property transitions of syndiotactic polypropylene

In this paper we have studied the effect of increasing carbon multi-walled nanotube (CNT) concentration in composites of syndiotactic polypropylene (sPP) having the same crystalline form but different morphologies. The attention was focused on the form I of sPP with different degrees of perfection (in terms of percentages of chains in helical conformation, crystal dimensions and crystallinity) obtained using two different quenching temperatures from the melt, i.e. 25 and 100 °C. We observed a decreasing effect of the crystallization temperature on increasing the nanotube content up to the samples with 10% of CNT, that show a very similar structural organization independent of the undercooling. Only the amorphous phase turns out more relaxed in the samples crystallized at the highest temperature. Either the thermal or the mechanical properties are improved on increasing the CNT content in both series of samples. The electrical conductivity increases in a similar manner in both series of samples and between 1 and 3 wt% it shows a sizable step of about eight orders of magnitude, a phenomenon that can be regarded as the onset of a percolating structure for which charge transport may take place.

Equivalent Electric Circuits for the Simulation of Carbon Nanotube-Epoxy Composites

Equivalent electric circuits allowing the simulation of the behavior of nanocomposites based on thermosetting resin and nanocarbon filler are presented. The electric circuits are constructed by employing a multistep simple procedure in which the values and number of the parameters are adjusted until a suitable criterion, based on the comparison between simulated and experimental frequency spectra is satisfied. A resistance-capacitance (RC) simple parallel branch in parallel with a variable number of RC series branches is shown to be capable to reproduce the frequency response of a prepercolative carbon black and two carbon nanotube (CNT) nanocomposites with CNT concentration close to the percolation threshold. The obtained equivalent circuits may be employed for the interpretation of the physical mechanisms underlying the electromagnetic (EM) behavior. Moreover, they can be used in circuit simulators for first-approximation design of EM devices based on such composites.

Simulation of the Bearing Voltage in an Inverter-Fed Induction Motor by a Full Three Phase Multi Conductor Transmission Line Model

An accurate numerical model, based on multiconductor transmission lines (MTL) able to evaluate the voltage dynamics across the motor bearings and associated currents of an inverter-fed motor is presented. A full three phase stator winding of the wound type of a high power traction motor is considered in the proposed analysis. The difierent regions of the motor are modeled as suitable connections of lossy MTL which are then studied in the time domain. The per unit length characteristic matrices describing the MTL are accurately calculated by a FEM based software. The efiects of the rise time of the input voltage and the length of the feeder cables are discussed. The reliability of the numerical results achieved by means of the MTL model is checked by performing a comparison with those obtained by considering a lumped parameter equivalent circuit.

Dependence of electrical properties of polypropylene isomers on morphology and chain conformation

The electrical properties of polypropylene isomers were correlated with the morphology and chain conformation of differently obtained isotactic (iPP) and syndiotactic (sPP) samples. In the case of iPP, a crystallized and a smectic sample were prepared, whereas for sPP two crystalline helical samples and a mesophase with the chains in trans-planar conformation were considered. The phase composition was obtained for all the samples comparing x-ray diffractograms and transport properties of vapours, which give the crystallinity and the amorphous fraction, respectively. The fraction of mesophase was obtained by the difference of the previous values. The study of the morphology evidenced similarities and differences among the samples, which were discussed and correlated with the phase composition. The electrical conductivity was measured for all the samples, and the syndiotactic isomer showed the lowest value as well as a dependence on the structure. In contrast, the isotactic isomer showed the same behaviour for either polymorph. Based on the structural and electrical results, a phenomenological explanation of the conduction mechanisms taking place in the different forms has been proposed. In particular, the current in the iPP seems to be controlled by Schottky emission, i.e. by field-assisted thermo-ionic injection of carriers from the electrode into the polymer, whereas for the sPP more than one mechanism is likely to be effective, although the ionic transport appears as the predominant one. The experimental data confirm a different behaviour of the ionic conduction properties for the different polymorphs, highlighting the greater insulating characteristics of the mesomorphic structure of the syndiotactic isomer.

A Multi Conductor Transmission Line Model for the Evaluation of the Rotor Shaft Voltages in Adjustable Speed Drive Motors

Second University of Naples, ItalyAbstract—The use of switching devices, such as IGBTs, characterised by high switching frequencies and verylow switching times in new generation pulse width modulation (PWM) inverters has increased the efficiencyand performances of Adjustable Speed Drives (ASDs) for industrial and traction applications. However, suchsystems may be affected by disadvantages like over voltages at the motor terminals, when long cables are usedbetween the drive, and the generation of rotor shaft voltage, due to the capacitive couplings in the motor(between the windings and the rotor and between the rotor and the stator). The shaft voltage may cause thebreakdown of the lubricating film in the bearings. The resulting impulsive currents, by damaging the bearingelements shorten the component life, which in turn seriously affects the ASD reliability. For this reason, it is ofgreat importance to develop numerical models able to predict the shaft voltage so as to estimate the currentsflowing through the bearings. Several works, based on either concentrated or distributed circuit models, havebeen proposed for the evaluation of the shaft voltage magnitudes for several motors sizes. However, the resultsobtained by such approaches suffer from approximations and simplifications in the considered circuit model.Therefore, in the present paper, a numerical model able to accurately predict the shaft voltage in high powerinduction motor for traction applications fed by a PWM inverter is presented. The windings of the motorare modelled by a multi conductor transmission line (MTL), whereas the cables between the source and themotor are described by a single transmission line. The effect of wave propagation and reflection and of thefrequency-dependent distributed losses is considered by using a time-domain equivalent circuit to represent theMTL. A semi-analytical method, based on the perturbation theory of the spectrum of symmetric matrices, isadopted. The parameters of the MTL are obtained either analytically or numerically by using a commercialsoftware (Maxwellr by Ansoft). The effects of the rise time of the input voltage together with the length ofthe cables are considered.

Analysis of the Effects of Hydrotalcite Inclusion on the Temperature-Sensing Properties of CNT-Epoxy Nanocomposites

Multiphase nanocomposites based on epoxy resin loaded with 1 wt% of multiwalled carbon nanotubes (MWCNTs) and different amounts of hydrotalcite clay (HT) are experimentally characterized in order to investigate their possible usage as temperature sensors and/or heating elements. In particular, the effects of clay content on the temperature dependence of the electrical and thermal properties of the samples are investigated. Moderate clay content (0.7% or 1% by weight) induces an improvement of the electrical and thermal conductivities, thus allowing to achieve fixed heating power levels with applied voltages lower than those required for MWCNT filled epoxy systems. The operating temperatures can be designed to be lower than that the glass transition reducing the risk of material degradation. Furthermore, the presence of clay increases the sensitivity compared to the (binary) MWCNT-based composites. The proposed multiphase nanocomposites could be used for the deicing of aircraft structural parts, in small boilers, floor pipes, and so on.

Morphological and electrical characterization of epoxy resin filled with exfoliated graphite

An aeronautic epoxy resins with enhanced mechanical properties is morphologically and electrically characterized to study the effect of different percentages of exfoliated graphite (EG) inclusion. In particular, nanocomposites filled with graphite, characterized by 56% of the exfoliation degree and Brunauer-Emmett-Teller (BET) specific surface area of 14.7 m2/g were prepared with loading levels from 0.32% to 6.5% by weight (wt). The morphological characterizations show the peculiarities of the filler dispersion. The DC electrical characterization puts in evidence a percolation threshold (EPT) lower than 3% wt and an electrical conductivity of about 0.66 S/m in the case of samples filled at 6.5% wt. The AC electrical characterization in the frequency range 100Hz-1MHz of the nanocomposites is also reported in order to investigate the material properties around the EPT.

Evaluation of the electrical properties of epoxy-based nanocomposites for motor insulation

Polymeric nanocomposites are increasingly adopted for replacing conventional insulation to provide enhanced performances such us reliability, environmental compatibility and power rating in new generation inverter-fed Adjustable Speed Drives electrical motors. This paper describes an experimental study aimed at evaluating the performances of several candidate nanocomposites obtained by adding different clays to an epoxy matrix already used for manufacturing motor insulation system. The electrical, thermal and mechanical properties of the nanocomposites are investigated in order to evaluate their performances as new impregnation materials in multilayered insulation systems. The effect of the different clays on the dc conductivity and permittivity is investigated as a function of the electric field and temperature applied to the nanocomposites.