Publications

Bulk nanocrystalline La0.67Ba0.33_xKxMnO3 (with x = 0, 0.05, 0.1, and 0.2) manganites have been prepared by the modified sol-gel method (Pechini). The single-phase rhombohedral crystal structure with the R-3c (no. 167) space group was verified by X-ray diffraction (XRD) and sustained by Rietveld refinement. As follows from the results of XRD structural analyses, the increase in K-doping triggers an increase in the distortion of the MnO6 octahedra, which eventually causes the narrowing of the eg bandwidth. Mn is in a mixed valence state of Mn4+/ Mn3+ as inferred by X-ray photoelectron spectroscopy. Magnetic measurements confirm that the Curie tem-perature decreases from 348 K for La0.67Ba0.33MnO3 to 316 K for La0.67Ba0.13K0.20MnO3. The increasing of the Mn4+ ion concentration at the B-site sublattice and A-site ionic disorder (sigma 2) breaks up the double exchange interaction between the Mn3+ and Mn4+ ions. The ferromagnetic to paramagnetic second-order magnetic phase transition at TC is also confirmed by electron paramagnetic resonance. According to magnetic field-dependent magnetization isotherms at different temperatures, La0.67Ba0.13K0.20MnO3 shows a relatively large magneto -caloric effect (1400 mJ cm_ 3 K_ 1 at 316 K under 5 T applied magnetic field), which raises the possibility of using this material for room-temperature micro-scale magnetic cooling.

In this work, a ZnO nanowires/graphene nanohybrid was synthesized by a three steps approach. Copper substrates were covered with graphene by chemical vapor deposition, further ZnO nanowires were electrochemically deposited on the as grown graphene on copper and finally a transfer process was employed for moving the heterostructure onto a different substrate. A comprehensive structural analysis which included scanning electron microscopy, X-ray diffraction and Raman measurements revealed that the ZnO nanowires crystallize in wurtzite structure perpendicular to graphene, the process leading to the formation of a nanohybrid heterostructure. The band gap energy of the ZnO nanowires deposited on graphene was estimated to be 3.11 eV, as calculated from the reflectance spectrum analysis. The GGA-PBE+U within Grimme (DFT-D) approach was used to provide an accurate description of the interface structure in terms of electronic and optical properties, confirming that the decrease in the band gap energy of ZnO nanowires is caused by the interaction with the graphene surface. The findings of this study could serve as an experimental and theoretical reference for upcoming studies on ZnO NWs/Graphene nanohybrid-based optoelectronic applications.

The development of new materials for short-wavelength infrared (SWIR) optical sensors is of high importance for the fast development of different applications, as, for example, Internet of Things, road safety, and pollution monitoring. Group IV SiGe provides more sustainable As-, Cd-, and Pb-free nanomaterials that are cheaper and ecologic and offer easy integration with CMOS technology. This Review is on Ge and SiGe quantum dots/nanocrystals (QDs/NCs) embedded in dielectrics for VIS-SWIR photodetection, in which we highlight and discuss photocurrent mechanisms, correlation of photodetection parameters and characteristics with crystalline structure, morphology and energy bandgap, and applications as photodetectors, optical sensors, phototransistors, and solar cells. The embedding matrix induces NC surface passivation, stress field, and nanocrystallization effects and brings specific advantages depending on the matrix material. SiGe NCs in oxides for VIS-SWIR sensing represents a niche domain, showing high photosensitivity (photocurrent) in SWIR up to 1.8 mu m at room temperature and 2 mu m at 100 K, deeper in SWIR than Ge. By alloying Ge with a small content of Si, NC thermal stability is much improved as the detrimental Ge fast diffusion in oxides is hindered and SWIR photosensing is enhanced due to light absorption in Ge-rich SiGe NCs.

Magnesium ions, MgO nanoparticles and thin films, magnesium alloys and cerium compounds are materials intensively studied due to their corrosion protection, antibacterial and pharmacological properties. In this work, we have designed, prepared and investigated, novel thin films of MgO doped with cerium, deposited on Mg alloy (AZ31) for temporary implants, in order to enhance their life time. More precisely, we report on microstructure and corrosion behavior of MgO pure and doped with 0.1 at % Ce films, fabricated by sol-gel route coupled with spin-coating technique, on AZ31 alloy substrate. A modified sol-gel method that start from magnesium acetylacetonate, cerium nitrate and 2-methoxyethanol (as a stabilizer for the sol) was been used successfully for cerium doped MgO sol precursor preparation. The structure and morphology of the surface of the coatings, before and after immersion for 7-30 d in Hank's solution at 37 degrees C, were characterized by x-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscope, x-ray photoelectron spectroscopy and Fourier infrared transmittance spectrum (FT-IR). A comparison between the corrosion protection of undoped MgO and MgO doped with 0.1 at % Ce coatings on the AZ31 alloy substrate is performed by electrochemical tests and immersion tests using open circuit potential and electrochemical impedance spectroscopy in Hank's solution, at 37 degrees C. The electrochemical results showed that the protection of the AZ31 alloy substrate against corrosion was better with the doped with 0.1 at % Ce MgO film deposited than with pure MgO coting. The investigations of the films after immersion in Hank's solution, at 37 degrees C, for 7, 21 and 30 d indicated that the grown layer on the film is bone like apatite that suggests a good bioactivity of 0.1 at % Ce-doped MgO coating. Our work demonstrates that the performance corrosion protection of the biodegradable magnesium alloys used for orthopedic applications, in simulated physiological environments (Hank and Ringer) can be enhanced through coating with Ce3+ doped MgO sol-gel thin film.

When a severe plastic deformation (SPD) process is performed at high temperatures, it becomes more versatile. Designed originally for the bulk nanoconstruction of hard-to-deform alloys, high-speed high-pressure torsion (HSHPT) is an SPD method used in this research for assembling multiple layers of shape memory nanocomposites. Three hard-to-deform magnetic alloys in the cast state were used. Soft magnetic shape memory alloys, NiFeGa and FePdMn, and a potentially hard magnetic alloy, CoZr, were assembled in various composites. Both grain refinement and strong layer bonding were achieved in ZrCo/FePdMn and ZrCo/NiFeGa composites in seconds. The very short SPD time is specific to HSHPT because of the intense friction that occurs under high pressures, which generates huge amounts of heat. After SPD, the temperature rises in bulk material like a pulse, being dissipated mostly through heat conduction. The SPD parameters were carefully controlled with an advanced automation system using a programmable logic controller. Nevertheless, the major drawbacks of high-pressure torsion were overcome, and large SPD discs were obtained. Various investigation techniques (optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and atomic force microscopy) show well-defined interfaces as well as a fine and ultrafine structure.

The CuFeTaTiW multicomponent alloy has been devised as an interlayer thermal barrier in nuclear fusion re-actors. In order to predict the phase constitution of this alloy, two different lines of work were performed: (a) simulation using Molecular dynamics and Monte Carlo and (b) study of the influence of mechanical alloying parameters on the structures formed. The simulation results show that the most stable structure is achieved starting from a bcc type-structure and using Monte Carlo simulation. In fact, in these conditions the separation into two bcc phases Fe-Ta-W and Cu-Ti is predicted at room temperature. However, the experimental preparation of the materials with mechanical alloying revealed that from 2 h of milling a single bcc phase is formed. The structure of the milled powder was not much influenced by the amount of the process control agent and the by the size of the W starting particles, but generally there was formation of Ta2H from the reaction between the powders and the process control agent.

Preparation under non-identical conditions involving the reduction of sol-gel-derived precursors of various Zn:Fe atomic ratios (0.97:0.03, 0.8:0.2 and 0.4:0.6) in a hydrogen-containing atmosphere was used to obtain composites of nanosized ferromagnetic (FM) alpha-Fe and antiferromagnetic (AFM) FeO in different concentrations and of the desired magnetic responses. ZnO nanoparticles, developed in all preparation stages, served primarily as a matrix for the separation of Fe (and FeO) nanoparticles, preventing their agglomeration and coarsening. The average crystallite/particle size is about 5-32 nm, 10-75 nm and 21-31 nm for FeO, Fe and ZnO, respectively. Magnetisation investigations of FeO-Fe-ZnO nanocomposites revealed a coexistence of ferromagnetic and superparamagnetic behaviours ascribed to the Fe nanoparticles in different magnetic states. All samples exhibit the exchange bias effect, EB. Values up to 375 Oe for coercivity at 300 K, 600 Oe for coercivity, 223 for coercivity enhancement and 243 Oe for EB field at 5 K were measured. The magnitude of EB depends on the processing conditions - the EB field and coercivity enhancement are larger for samples processed under conditions of a higher degree of non-equilibrium. The presence of FeO appears crucial for the occurrence of EB, irrespective of its quantity. The EB is ascribed primarily to the exchange coupling between the AFM and FM spins at the FeO/Fe interfaces of nanostructures after field-cooling from above the TN of FeO and below the Curie temperature of Fe. An approach based on magnetically disordered AFM/FM interfaces featuring like spin-glass systems was adopted to explain the EB effects. An aspect of practical relevance is the suppression of room temperature coercivity in apparent correlation with the training of the EB; an attempt has been made to understand the origin of this suppression.

The corrosion protection property of three Brij-type surfactants, namely, Brij 35, Brij 56 and Brij 58P, was considered on OLC 45 carbon steel in a 0.5 M H2SO4 medium. The efficacy for these organic compounds was examined using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods, scanning electron microscopy (SEM) procedures, and Fourier transform infrared (FT-IR) spectroscopy. We hypothesized that these surfactants hinder the corrosion for OLC 45 samples through a protecting mechanism owing to the adsorption of organic molecules that form an inhibitive film or through the formation of complex oxides. These surfactants exhibited an appreciable protective effect against OLC 45 corrosion, operating as mixed inhibitors, as could be demonstrated by their influence on the electrochemical characteristics of the metallic substrates. The adsorption of surfactants over the substrates zone conformed to the representation of the Langmuir isotherm. The effect of temperature on the electrochemical comportment of the OLC 45 specimens in H2SO4 without and with Brij at 800 ppm was examined in the temperature interval of 293 to 333 K. The negative estimate of thermodynamic attributed as Gibbs free energy of adsorption presented the spontaneity of the adsorption activity. The investigation with FT-IR and SEM established the adsorption of Brij and the constitution of the corrosive components on the OLC 45 surface. Electrochemical determinations of these surfactants indicated its anticorrosion inhibition performance and the highest inhibition of 96% was reached when the Brij 35 concentration was at 800 or 1000 ppm, while for Brij 56 and Brij 58P, the highest inhibition was obtained when their concentrations were 500, 800, or 1000 ppm.

Novel biocompatible composite hydrogels with good elastic and antimicrobial properties have been fabricated by e-beam cross-linking using chitosan and water-soluble polymers mixed with commercial silver nanoparticles (AgNP). Hydrogels having different formulations were characterized by rheological, swelling, FTIR, SEM, and biodegradation measurements. The network structure, biocompatibility, and antimicrobial properties were evaluated as well. The composite hydrogels showed higher stability and absorb large amounts of fluids specific to an infected wound without losing structural integrity. The rheological, SEM, and network structure results demonstrate that at above 0.1 mg/mL AgNP, hydrogels with high cross-linking density are obtained. The in vitro response of fibroblasts proved the high biocompatibility of the hydrogel's composites. The antimicrobial activity is directly influenced by the amount of AgNP and cross-linking degree of the hydrogel. Significant antimicrobial activity was recorded against Gram-negative bacteria, while for the Gram-positive ones, the growth inhibition seems to require a decrease of the cross-linking degree.

This work investigates the conduction mechanism of hydrothermally grown Gd2O3-sensitive material in order to explain its electrical resistance behaviour when exposed to increasing concentrations of CO2 under in-field conditions. To achieve this, the experimental investigation began with X-ray photoelectron spectroscopy of the Gd2O3 microstructure to verify the oxidation states of the surface. Subsequently, the impact of constant atmospheric factors such as oxygen and relative humidity on the electrical resistance of the Gd2O3 layer was examined. Finally, a progressive dosing of CO2 concentrations ranging from 400 to 3000 ppm was conducted. The DC electrical resistance measurements were performed using a computer-controlled Gas Mixing System operated under a dynamic gas flow regime. Experimental data was validated using the Boltzmann distribution statistics and the grain-to-grain Schottky barrier model. The results highlight the preservation of the n-type semiconductor behaviour of Gd2O3 irrespective of the background relative humidity and bring the oxidising character of CO2 to the fore.