Publications

Magnesium (Mg) alloys are very promising biocompatible materials for biodegradable biomedical implants, however, the main problem in using them is their fast degradation in the conditions of human body. In this work, we coated Mg-Ca (calcium) alloy substrate with hydroxyapatite (HA) to improve its resistance to corrosion and to control the in vitro degradation. Pulsed Laser Deposition technique was applied to deposit HA coatings. Their properties were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, high resolution transmission electron microscopy, Vickers microhardness, and Tafel plot electrochemical technique. The substrate temperature was ranged in the interval from room up to 400 degrees C. According to the obtained results, the recommended substrate temperatures are 200-300 degrees C, since higher ones lead to the HA decomposition. Furthermore, the HA films deposited at 200 and 300 degrees C show good corrosion resistance in Simulated Body Fluid. The obtained coatings are promising for a perspective use in biomedical implant applications.

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field H-dc lower than the matching field H-Phi. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential U-c from the characteristics of electric field E versus J. We show that the dynamic critical current J(d) induced at the depinning frequency f(d) can be experimentally obtained by measuring m'(T) and m ''(T) at different frequencies and amplitudes. It was found that the value of f(d) obtained by extrapolation of J to J(d) in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements. AC susceptibility, vortex dynamics, pinning energy, depinning frequency, dynamic critical current, microwave impedance measurements

The dynamic effects observed in the J-V measurements represent one important hallmark in the behavior of the perovskite solar cells. Proper measurement protocols (MPs) should be employed for the experimental data reproducibility, in particular for a reliable evaluation of the power conversion efficiency (PCE), as well as for a meaningful characterization of the type and magnitude of the hysteresis. We discuss here several MPs by comparing the experimental J-V characteristics with simulated ones using the dynamic electrical model (DEM). Pre-poling conditions and bias scan rate can have a dramatic influence not only on the apparent solar cell performance, but also on the hysteretic phenomena. Under certain measurement conditions, a hysteresis-free behavior with relatively high PCEs may be observed, although the J-V characteristics may be far away from the stationary case. Furthermore, forward-reverse and reverse-forward bias scans show qualitatively different behaviors regarding the type of the hysteresis, normal and inverted, depending on the bias pre-poling. We emphasize here that correlated double-scans, forward-reverse or reverse-forward, where the second scan is conducted in the opposite sweep direction and begins immediately after the first scan is complete, are essential for a correct assessment of the dynamic hysteresis. In this context, we define a hysteresis index which consistently assigns the hysteresis type and magnitude. Our DEM simulations, supported by experimental data, provide further guidance for an efficient and accurate determination of the stationary J-V characteristics, showing that the type and magnitude of the dynamic hysteresis may be affected by unintentional pre-conditioning in typical experiments.

In this study, we compared the structural, morphological, and optical properties of a series of Zn1-xFexO (x = 0.00, 0.01, and 0.03) powders synthesized via a hydrothermal route with and without cetyltrimethylammonium bromide as a cationic surfactant. Our results highlighted the critical effects of the surfactant and the iron concentration on the structure, morphology, and optical properties of ZnO. X-ray diffraction, Mossbauer spectroscopy, and X-ray photoelectron spectroscopy analyses indicated the presence of a single phase comprising ZnO with a hexagonal wurtzite structure in all samples and a single oxidation state (+3) for the iron (Fe3+) that replaced Zn2+ in the ZnO structure. Morphological investigations by scanning electron microscopy showed that the surfactant and Fe3+ dopant greatly affected the shape of the ZnO grains, which varied between sheets and rod-like flowers. We found that the morphological and photocatalytic properties of the two series of samples comprising iron-doped ZnO with and without the surfactant were in opposition. We propose a possible growth mechanism for the ZnO polycrystalline grains in the presence of the Fe dopant and/or surfactant. The photocatalytic properties increased for the samples prepared with surfactant as the iron content increased, which was confirmed by ultraviolet-visible reflection measurements. The photocatalytic activities of the samples prepared without surfactant decreased in both the ultraviolet and visible spectral regions as the iron content increased in the samples.

Four new Ni(II) complexes with acrylate and imidazole (Him) or imidazole derivatives [2-methylimidazole(2-MeIm)/5-methylimidazole(5-MeIm)/2-ethylimidazole(2-EtIm)] as ligands were prepared and characterized. All coordination compounds were characterized by elemental analysis, infrared (FTIR) and ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, mass spectroscopy, magnetic moments measurements and thermal analysis (TG). The resulted complexes were formulated as follows: [Ni(HIm)(2)(acr)(2)] (1), [Ni(2-MeIm)(2)(acr)(2)(H2O)] center dot H2O (2), [Ni(5-MeIm)(2)(acr)(2)]center dot H2O. (3), [Ni(2-EtIm)(2)(acr)(2)(H2O)] center dot H2O (4). On the basis of magnetic moments measurements and on UV-Vis-NIR spectra, for all Ni(II) complexes was proposed an octahedral stereochemistry. Acrylate ions act as bidentate in complexes (1) and (3); meanwhile, in (2) and (4) they behave as bidentate and unidentate. Antimicrobial activity of complexes was investigated on ATCC reference and clinical microbial strains. The MIC (minimum inhibitory concentration) values revealed moderate antimicrobial activity of complex (1) against Enterococcus faecium and of complex (2) against Pseudomonas aeruginosa and Bacillus subtilis.

Sm3+, Pr3+ and Dy3+-doped Ca3Nb1.6875Ga3.1875O12 - CNGG and Ca3Li0.275Nb1.775Ga2.95O12 - CLNGG crystals are of interest as solid-state laser materials emitting in the yellow-orange spectral range, especially for diode pumping due to their partially disordered nature. In this work, for the first time to our knowledge, single crystals of Sm, Pr and Dy-doped CNGG and CLNGG crystals were grown by the Czochralski method. For the doping of CNGG and CLNGG crystals, five new stoichiometric garnets were synthesised by solid-state reaction method: praseodymium-gallium-garnet (PrGG), dysprosium-gallium-garnet (DyGG) and praseodymium-lithium-niobium gallium-garnet (PrLNGG), samarium-lithium-niobium-gallium-garnet (SmLNGG), and dysprosium-lithium-niobium-gallium-garnet (DyLNGG). Samarium-gallium-garnet (SmGG) was also sintered and used for the doping of Sm:CNGG crystal. Structural properties of sintered materials and grown crystals were investigated by X-ray diffraction and the chemical compositions of the grown crystals were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) method. The spectroscopic investigations of the grown crystals have shown that they are promising laser materials in the yellow-orange spectral range.

Interfacial properties of organic adsorbates featuring aromatic -orbitals on metal surfaces play an important role for organic electronics and spintronics. Pyrene is a flat aromatic molecule with a size between ultimately small benzene and extended graphene segments. The deposition of pyrene molecules onto clean and reactive surfaces with a sub-monolayer coverage under ultra-high vacuum (UHV) conditions is challenging, since pyrene is a solid with a high vapor pressure. Here, a sublimation procedure under UHV and image pyrene adlayers on in situ prepared Au(111) and Fe/W(110) substrates by means of low-temperature scanning tunneling microscopy is presented. For Au(111), the molecule-surface interaction is weak as indicated by the specific herringbone reconstruction of the Au(111) surface that is visible through the self-assembled pyrene adlayer. Pyrene desorption due to weak intermolecular interaction self-limits the growth to one monolayer (ML). On the more reactive 2-4 ML thick Fe films on W(110), the molecular order of the pyrene adlayer sensitively depends on the Fe thickness-dependent dislocation pattern at the substrate surface. Irregular arrangements occur for 1 ML Fe and near substrate dislocations for 2-4 ML Fe. Self-assembled ordered arrays form predominantly for 2 ML Fe, where the dislocation pattern leaves sufficiently large unperturbed areas between the dislocation lines.

One of the most promising ways for the realization of multi-functional materials is the integration of oxides with different properties in artificial heterostructures. In this paper, a novel piezoelectric-ferromagnetic heterostructure consisting of 0.92Na(0.5)Bi(0.5)TiO(3)-0.08BaTiO(3) (abbreviated as BNT-BT0.08) and CoFe2O4 layers is fabricated on Si-Pt substrate, by sol-gel method coupled with spin-coating technique. The composite thin film shows only perovskite Bi0.5Na0.5TiO3-like rhombohedral phase and CoFe2O4 cubic phase. The thickness of CoFe2O4 and BNT-BT0.08 layers is similar to 280 and similar to 400 nm, respectively. BNT-BT0.08/CoFe2O4 heterostructure thin film shows a saturation magnetization of 0.11 emu/g at 5 K and 0.07 emu/g at 295 K, dielectric constant of 235 at 1 kHz and tunability of 70% at 1 kHz and an electric field E = 110 kV/cm. The results reveal that the investigated hybrid piezoelectric/ferromagnetic structure shows piezoelectric behavior, good ferroelectric and ferromagnetic properties. This bilayer composite can be used in miniature low-frequency magnetic sensor and piezoelectric sensor for biomedical domain.

Refined structure of the ferritic phase induced by mechanical milling (under reducing atmosphere) and its thermal stability are required in various applications of nanostructured ferritic alloys. The impurification with nitrogen and oxygen uptaken from the air is very probable during ball-milling, especially at the long-time high-energy milling conditions. As a rule, these interstitial impurities in as-milled powders are in quantities under the sensibility limit of conventional measurement techniques, such as XRD and SEM/ EDS. To evidence the tendency for microstructure modification by impurities introduced during milling, the Fe-14Cr-3W-0.4Ti-0.25Y(2)O(3) (Fe14Cr) ferritic steel powders (re)loaded in air and milled up to 170 h with interruption of the milling process, and heated up to 1373 K were investigated by thermal analysis in correlation with X-ray diffraction and scanning electron microscopy. XRD failed to detect the impurities in powders milled up to 38 h in air although a consistent mass loss related to the degassing of N-2 was registered in thermogravimetric, TG, curves. (Fe,Cr)(4)N, fcc-c, (Fe, Cr)(2)O-3 impurity phases in powders milled over 38 h in air and (Fe,Cr)(2)O-4 formed upon heating were readily detected by XRD. The analysis of these results allowed to better understand the impurification process and to generalise it for any as-milled Fe-Cr-based alloy powder processed in any milling conditions irrespective of the milling atmosphere, duration and thus, of amounts of contaminants. The quality of three powders milled for 170 h in three different conditions was compared: in air, under an argon atmosphere with interruptions of the milling process and under an argon atmosphere without interruption of the milling process. The contamination of powder milled for 170 h under an argon atmosphere without interruption of the milling process is insignificant (corresponding to less than 0.5 mass% mass loss in TG) as compared to powders obtained in the other two milling conditions. New approaches for minimising the contamination from air are suggested.

Arrays of Ni submicron wires surrounded by poly (vinylidene fluoride) (PVDF) submicron tubes were prepared via solution-processed polymer and Ni electrodeposition into anodic aluminum oxide template. The PVDF solution was filtered in vacuum through the template and the resulting dried structure was used for the electrodeposition of Ni wires. The obtained core-shell submicron wire structure consists of a metallic magnetic nanowire core of about 50 mu m in length and about 300 nm diameter surrounded by a polymer tube shell with thickness less than 10 nm. The specific ferroelectric beta-phase of the polymer was obtained whereas the magnetic behavior of the Ni-cores was proven to be specific to an array of ferromagnetic Ni cylindrical wires (about 0.62 mu(B)/Ni atom) with magnetization reversal mechanisms dominated by dipolar interactions and domain wall displacements. No significant differences of the magnetization reversal mechanism were observed in case of Ni submicron wires surrounded by PVDF tubes and similar Ni wires without PVDF shell, suggesting that magneto-coupling effects in such systems might be observed only by measuring the perturbation of the electric state of the polymer shell under a magnetic excitation of the Ni cores.