Main results
The project aims to obtain novel polymer composites with one or several fillers with the purpose of enhancing their permittivity and dielectric breakdown, and through them the storage characteristics.
Results of Stage 1/2022
The first step of the project led to preparation of several types of bio-fillers, which are derived from regenerable sources (leaves, clays, burned plants ash, cereal powder). These materials were introduced in two cellulose derivatives, which were selected as matrices for composites obtaining. The samples in solution phase were examined via rheology tests. By means of these anlyses, it was proved that the shear sensitivity of the viscosity dictates the conditions to achieve uniform films. At casting velocities beyond 6 mm/s the films thickness is constant, as desired for the targeted capacitor application. Also, Fourier Transformed Infrared Spectroscopy (FTIR) experiments were conducted to confirm the structural characteristics of the prepared composites.
Results of Stage 2/2023
The second step of the project involved continuation of the rheological investigations on multicomponent dispersions composed of polymer/bio-additive/inorganic filler. By polymer loading with the filler mixture the viscosity is less affected by the imposed shear. Hence, when processing the obtained dispersion by spin coating, the film thickness profile becomes more uniform. By heating the sample, the registered viscosity curves are more resemblat to pseudoplastic fluids, thus the thickness profile presents undesirable variations. So, lower processing temperatures (under 30oC) are desirable to obtain a constant film thickness of the composites.
The sample morphology was checked at macro/micro-scale by means of optical microscopy (OM) and scanning electron microscopy (SEM). The recorded OM pictures indicate good homogeneity of inspected films, regardless the used filler. The SEM images collected on the surface and cross-section of the samples highlight a good distribution of both filler and bio-additive/inorganic powder mixtures incorporated in the cellulose derivative matrices. Furthermore, atomic force microscopy (AFM) technique was employed to study the topography aspects at the nano-scale for a leaf-based bio-additive added in a cellulose derivative. The scans demonstrated that polymer loading increases the magnitude of the surface heights. Metal sputtering of the samples was undertaken to check the interfacial compatibility among the dielectric and metallic electrode. AFM scans evidenced proper metal covering on the loaded sample surface and slight increase in the surface roughness, which is adequate for capacitor applications. Local mechanical forces estimated from approach/retract curves revealed higher adhesion forces especially upon doping with the leaf-based bio-additive.
Surface energy experiments were done to estimate the adhesion features when the sample is interfaced with metal electrodes (Ag, Pt, Au). It was found that the hydroxyl groups present in the bio-filler structure enhanced the material's polarity and thus the sample/metal adhesion was improved especially with silver or platinum. The composite exposure to plasma is further contributing to better interfacial dielectric/electrode compatibility.
Thermal characterization (made by differential scanning calorimetry) of the samples revealed that the presence of the filler does not significantly affect the glass transition temperature, hgihlighting the good thermal resistance of the obtained materials. Also, the thermal expansion coefficient is slightly influenced by the sample composition and polymer type.
Tensile mechanical testing was performed revealing a moderate decrease of the Young Modulus upon cellulosic matrix reinforcement, while the ellongation at break has distinct rates of decreasing (upon filler adition), which are dependent on the type of the used cellulose derivative.
Electrical behavior of the samples was studied in terms of dielectric breakdown and permittivity. The introduction of the bio-additive produces the reduction of the dielectric breakdown strength, but the rate of decrease is noted to be smaller for the polymer doped with cereal powder. On the other hand, the inserted polar bio-powders led to ehnacement of the dielectric constant at optical and radio frequencies. These results are favorable for the pursued applications in capacitor manufacturing.
Results of Stage 3/2024
In the final stage of the project additional electrical characterization of the eco-composites was undertaken. The project has developed two approaches of increasing the dielectric breakdown - which is an crucial factor that affects the energy storage performance. The first method involves a multi-stage mechanical deformation prior and upon sample solidification and the second one consisted in shearing a liquid crystal polymer containing polarizable filler. The recorded data indicated that the expected increase in the breakdown strength was attainable by employing these methods. Dielectric broadband spectroscopy experiments (conducted in the frequency range of 1 kHz - 1 MHz) revealed that upon gradual loading (0-10 wt% filler) of the biodegradable matrix a permittivity increase of about 1.52 (at 1 kHz) the takes place. Moreover, dielectric constant data at variable temperatures (20-70 oC) reveal a remarkable increase of this parameter due to enhanced dipole mobility in the samples, which is diminished at high particle loading in the biopolymer matrix. Also, it was found that Coulomb blockade effect is in close relation to the composite microstructural changes produced by conductive particle incorporation and this is a paramount factor in controlling the dielectric performance of the material. Further analyses were done concerning the energy density, which was found to increase for the prepared eco-composites, especially for the samples based on hydroxypropylmethyl cellulose. The dielectric spectroscopy data and breakdown strength results for these systems are recommending them for energy storage applications. Project outcome represents a significant step for modernization electrical energy storage technologies, opening perspectives on new design of the dielectric materials that have a positive impact on the environment after recycling the capacitor devices. | 
