Publications

Polymer Dispersed Liquid Crystal (PDLC) films are composite materials consisting of LC droplets embedded in polymers. In order to decrease the switching voltage of these devices, we prepared PDLC films doped with single wall carbon nanotubes (SWCNTs). The mesomorphic properties of the films were investigated by polarizing optical microscopy (POM). The electrical characteristics have been investigated using a Broadband Dielectric Spectrometer in a frequency domain (0.01-10(7)) Hz and a temperature range (280-350) K. Some key aspects of an optical model for the analysis of the coherent transmittance of PDLC - NP films are presented.

The latest decades presented a large development of various size and types of Unmanned Aerial Vehicles (UAVs). Nowadays, there are over 200 million UAVs used in various applications and there it is a large effort to adopt national and international regulation for using them. There it is a real need for the development of a reliable system for UAVs identity verification that can be implemented and used at global level. To this moment the software-based methods such as MAC address, IDs memorized on devices are prone to a large number of cyber-security vulnerabilities and attacks. This paper presents an innovative hardware-based identification solution which could be adapted to UAVs in a non-intrusive manner. The proposed solution is based on physical unclonable functions (PUF), which are implemented by using memristors. The proposed solution is very low cost and high performance. By using memristors, it is possible to generate a very large number of unclonable IDs, using only a few electrical components. This solution could be incorporated in any UAVs systems without major modifications and not affecting their flight parameters.

In this paper an IGZO memristor with multiple states and analog tuning extended capability is reported. The device has a planar structure and is fabricated by magnetron sputtering on glass substrate. The device resistance could be gradually increased and decreased within the range of one order of magnitude. Larger resistance changes are also possible but they are mostly irreversible. The obtained memristor looks promising to be used as electronic synapse in hardware implemented artificial neural networks or for applications in analog computing and cryptography.

Carbon-Titanium multilayer thin films were obtained by Thermionic Vacuum Arc (TVA) method. The nanostructured films consisting by 100nm Carbon base layer and seven 40nm alternatively Titanium and Carbon layers were deposed on Silicon substrate. As well, to give C-Ti multilayer films with different percentages in Ti and C of layers, a 100nm thick Carbon base layer was deposed on Si substrate, and then seven Ti-C layers, each of these having thickness of 40nm. In order to achieve the successively layers with C, and Ti different percentages, were adjusted the discharge parameters of C and Ti plasma sources to obtain the desired composition of layers. Also, were obtained composite films having a variable C: Ti atomic ratio 9:1 at interface to 1:9 at the surface. By changing of substrate temperature from room temperature to 100 degrees C, 200 degrees C, 300 degrees C, 400 degrees C respectively, and on the other hand the bias potential up to -700V, different batches of samples were obtained. Characterization of structural properties of films was achieved by Electron Microscopy technique (TEM, STEM) and GIXRD techniques. The measurements show that increase of the substrate temperature reveal changes in TixCy lattice parameters. Thus, according to GIXRD analysis it was found out that the Ti:C atomic ratio changes with increase of synthesis temperature. Also, in the case of composite films an increase of amount and sizes of TiC nanocrystals with the increase of energy of Ti ions determined by increase of bias voltage was observed. The tribological measurements were performed using a ball-on-disk system with normal forces of 0.5, 1, 2, 3N respectively. Was found that the coefficient of friction depends on the synthesis temperature and on the bias voltage. It is also noted that the friction coefficient depends on the pure C content, Ti content and amount of TiC nanocrystallites. These results are due to atomic diffusion at Ti/C interfaces and also are associated with amount of TiC nanocrystallites. To characterize the electrical conductive properties, the electrical surface resistance versus temperature have been measured, and then the electrical conductivity is calculated. Using the Wiedemann-Frantz law was obtained the thermal conductivity.

Mixtures of B4C, alpha-AlB12 and B powders were reactively spark plasma sintered at 1800 degrees C. Crystalline and amorphous boron powders were used. Samples were tested for their impact behavior by the Split Hopkinson Pressure Bar method. When the ratio R = B4C/alpha-AlB12 >= 1.3 for a constant B-amount, the major phase in the samples was the orthorhombic AlB24C4, and when R < 1 the amount of AlB24C4 significantly decreased. Predictions that AlB24C4 has the best mechanical impact properties since it is the most compact and close to the ideal cubic packing among the Al-B-C phases containing B-12-type icosahedra were partially confirmed. Namely, the highest values of the Vickers hardness (32.4 GPa), dynamic strength (1323 MPa), strain and toughness were determined for the samples with R = 1.3, i.e., for the samples with a high amount of AlB24C4. However, the existence of a maximum, detectable especially in the dynamic strength vs. R, indicated the additional influence of the phases and the composite's microstructure in the samples. The type of boron does not influence the dependencies of the indicated mechanical parameters with R, but the curves are shifted to slightly higher values for the samples in which amorphous boron was used.

Several technological routes are being investigated for improving the energy storage capability and power delivery of electrochemical capacitors. In this work, ternary hybrid electrodes composed of conducting graphene/reduced graphene oxide (rGO), which store charge mainly through electric double-layer mechanisms, covered by NiO nanostructures, for adding pseudocapacitance, were fabricated through a matrix assisted pulsed laser evaporation technique. The incorporation of multiwall carbon nanotubes (MWCNTs) provokes an increase of the porosity and thus, a substantial enhancement of the electrodes' capacitance (from 4 to 20 F cm(-3) at 10 mV s(-1)). Volumetric capacitances of 34 F cm(-3) were also obtained with electrodes containing just carbon nanotubes coated with NiO nanostructures. Moreover, the use of nitrogen containing precursors (ammonia, urea) for laser-induced N-doping of the nanocarbons also provokes a notable increase of the capacitance. Remarkably, N-containing groups in rGO-MWCNTs mainly add electric double layer charge storage, pointing to an increase of electrode porosity, whereas redox reactions contribute with a minor diffusion fraction. It was also observed that the loading of carbon nanotubes leads to an increase of diffusion-controlled charge storage mechanisms versus capacitive ones in rGO-based electrodes, the opposite effect being observed in graphene electrodes.

Reduced activation ferritic and martensitic steel like EUROFER (9Cr-1W) are considered as potential structural materials for the first wall of the future next-generation DEMOnstration Power Station (DEMO) fusion reactor and as a reference material for the International Thermonuclear Experimental Reactor (ITER) test blanket module. The primary motivation of this work is to study the re-deposition of the main constituent materials of EUROFER, namely tungsten (W), iron (Fe), and chromium (Cr), in a DEMO type reactor by producing and analyzing complex WxCryFe1-x-y layers. The composite layers were produced in laboratory using the thermionic vacuum arc (TVA) method, and the morphology, crystalline structure, elemental composition, and mechanical properties were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-X-ray fluorescence (micro-XRF), and glow discharge optical emission spectrometry (GDOES), as well as nanoindentation and tribology measurements. The results show that the layer morphology is textured and is highly dependent on sample positioning during the deposition process. The formation of polycrystalline WxCryFe1-x-y was observed for all samples with the exception of the sample positioned closer to Fe anode during deposition. The crystalline grain size dimension varied between 10 and 20 nm. The composition and thickness of the layers were strongly influenced by the in-situ coating position, and the elemental depth profiles show a non-uniform distribution of Fe and Cr in the layers. The highest hardness was measured for the sample positioned near the Cr anode, 6.84 GPa, and the lowest was 4.84 GPa, measured for the sample positioned near the W anode. The tribology measurements showed an abrasive sliding wear behavior for most of the samples with a reduction of the friction coefficient with the increase of the normal load.

The bio hydroxyapatite (HAp) was used from a long time in different medical and environmental applications. The HAp layers with a uniform surface were used for various medical applications such as orthopedic and dental metal implants. In this work, we reported on the influence of X-ray radiation on the structural and morphological properties of composite layers based on HAp and polydimethylsiloxane (PDMS) deposited on titanium substrates. The HAp:PDMS layers were investigated by different complementary methods such as scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and glow discharge optical emission spectrometry (GDOES). FTIR spectral analysis showed that the molecular structure of the coatings was not changed after their irradiation even though, the depth profile analysis performed by GDOES indicated a depletion of Ca and P elements from the HAp:PDMS irradiated samples. By SEM, we showed that the morphological features of the coatings were also changed, as the irradiated layers are delaminated. The biological assays confirmed that the antibacterial activity of HAp:PDMS composite layers increased after irradiation. The results obtained in this study highlighted that the biological properties of HAp:PDMS layers could be influenced by irradiation. (C) 2019 Society of Plastics Engineers.

In this study, the results about the influence of the surface morphology of layers based on montmorillonite (MMT) and silver (Ag) on antimicrobial properties are reported. The coating depositions were performed in the plasma of a radio frequency (RF) magnetron sputtering discharge. The studied layers were single montmorillonite layers (MMT) and silver/montmorillonite multilayers (MMT-Ag) obtained by magnetron sputtering technique with a different surface thickness. The resultant MMT-Ag biocomposite multilayers exhibited a uniform distribution of constituent elements and enhanced antimicrobial properties against fungal biofilm development. Glow-discharge optical emission spectroscopy (GDOES) analysis revealed the formation of MMT-Ag biocomposite multilayers following the deposit of a silver layer for an MMT layer that was initially deposited on a Si substrate. The surface morphology and thickness evaluation of deposited biocomposite layers were performed by scanning electron microscopy (SEM). A qualitative analysis of the chemical composition of thin layers was performed and the elements O, Ag, Mg, Fe, Al, and Si were identified in the MMT-Ag biocomposite multilayers. The in vitro antifungal assay proved that the inhibitory effect against the growth of Candida albicans ATCC 101231 CFU was more emphasized in the case of MMT-Ag biocomposite multilayers that in the case of the MMT layer. Cytotoxicity studies performed on HeLa cells showed that the tested layers did not show significant toxicity at the time intervals during which the assay was performed. On the other hand, it was observed that the MMT layers exhibited slightly higher biocompatible properties than the MMT-Ag composite layers.

Raman investigations on nanocomposites obtained by loading various amounts of vapor grown carbon nanofibers within an isotactic polypropylene matrix, and gamma irradiated in air, at various integral doses ranging between 0 and 27 kGy, are reported. The analysis is focused on the polymer's answers as revealed by Raman spectroscopy and investigate in detail the effect of ionizing radiation on the position of the Raman line originating from the polymer. The as-obtained data are correlated to the elastic features of the nanocomposites. A competition between gamma irradiation and loading by carbon nanofiber, resulting in the stretching of the polymeric matrix and revealed as a displacement of Raman lines towards smaller wavenumber is reported. It is concluded that side groups (CH3) are less affected by the loading with carbon nanofibers,