| Περίληψη: | In this thesis, composite systems of epoxy resin and different ceramic nanoparticles (BaTiO3, SrTiO3 and Barium Strontium Titanate (BST) mixture) have been prepared and studied varying the filler content. The main goal of this study was the examination of the nanocomposites capability to store and harvest energy under Direct Current (DC) conditions. The improvement of energy efficiency of such material device constitutes an important task to be investigated and its applicability is crucial in the field of electronics. The presence of ceramic filler enhances the energy efficiency of the manufactured systems, reaching the highest value of 69.41% for the 10phr (parts per hundred resin) SrTiO3 nanocomposite at 50V. It was found that, in all cases, the integration of the semiconductive nanofiller improves the energy efficiency of the nanocomposite systems. However, the optimal performance does not correspond to the highest filler loading. There are several parameters which influence the energy performance of every system, these include the applied DC voltage level, temperature, the type of filler and the reinforcing phase content. The DC field was varied from 10 to 240V and the energy storage and harvesting measurements were performed in the temperature range from 30 to 160oC. The determination of storing and harvesting energy was conducted via integration of the time-dependent charging and discharging current functions.
For the evaluation of the systems’ energy performance the coefficient of energy efficiency (n_eff) was introduced, being the ratio of the retrieved energy upon the stored one. With the increase of temperature, the coefficient of energy efficiency (n_eff) decreases exponentially in the case of BaTiO3 and BST reinforced nanocomposites, indicating leakage currents increase. n_eff in SrTiO3 reinforced nanocomposites follows a sigmoidal function upon temperature, therefore the type of nanofiller appears to play a major role in the nanocomposite systems’ energy performance. Nonetheless, at temperatures higher than 60oC the leakage currents increment is dominant and n_eff decreases dramatically. Finally, an energy comparison was performed between the different types of the studied nanodielectric systems in order to identify which one exhibit the optimal energy performance. Power studies were also conducted, concluding that the output power density takes relatively higher values at high temperatures.
Furthermore, Alternating Current (AC) and DC conductivity have been determined as a function of temperature in all studied nanodielectric systems. The temperature dependence in both cases follows an Arrhenius form. The required activation energy (E_A) by the charges in order to overcome the potential barriers has been calculated in all cases. The activation energies do not follow a systematic pattern, however the E_A under DC conditions acquires in general higher values in comparison with AC ones. In DC conditions the charge carriers are forced to migrate over longer distances, overcoming enhanced potential barrier heights because of the insulating nature of the matrix. In contrast, AC conditions reflect the forward and backward jumps of charge carriers among adjacent localized states. Activation energy values are related to the prevailing type of interaction between the systems’ constituents (i.e. macromolecules-nanoparticles or nanoparticles-nanoparticles). The type and the amount of nanofiller along with several other parameters determine the predominant type of interaction each type. Hopping conduction was found in all cases to be the predominant conduction mechanism, since experimental data in all three different types of nanodielectric systems are in accordance with Variable Range Hopping model.
Prior to energy and conductivity studies, structural characterization of both nano and micro particles was conducted by several experimental techniques (Laser Raman Spectroscopy (LRS), Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), Broadband Dielectric Spectroscopy (BDS)). In case of BaTiO3 the ferroelectric to paraelectric phase in both micro and nano particles was studied. In micro BaTiO3 particles critical temperature (T_C) arises about 130oC. In nanoparticles, the structural transition is complicated due to the nanoscaled particle size. It was found that below T_C temperature both phases co-exist and the ferroelectric to paraelectric transition was defined via the variation of Full Width at Half Maximum (FWHM) upon temperature in the XRD spectra. The formed peak at 115oC indicates the transition. Moreover, nano SrTiO3 and BST particles were all found to exist in the cubic phase. Moreover, structural/morphological characterization (XRD and Scanning Electron Microscopy (SEM)) was also performed in all nanocomposite systems. SEM image and XRD spectra revealed the satisfactory dispersion of nanoparticles (fine nanodispersions co-exist with limited small clusters) and the successful filler integration in the polymer matrix. The structural transition from the tetragonal to the cubic crystal phase in BaTiO3 nanoparticles could be detected in both BaTiO3 and BST nanocomposites.
After the extensive structural analysis in all nanodielectric systems dielectric response was examined in all manufactured systems via Broadband Dielectric Spectroscopy. Dielectric results divulge the presence of three relaxation processes which are referred to: (a) glass to rubber transition of the polymer matrix (α-mode), (b) the re-arrangement of polar side groups (β-mode) and (c) interfacial polarization between systems’ components. AC conductivity found to follow the so-called “AC universality law”. All nanocomposite systems exhibit improved dielectric properties which is mainly attributed to the semiconductive nanoinclusions and to the extended interface between the polymer matrix and the filler.
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