Publications

The role of Ag addition on the structural, dielectric, and mechanical harvesting response of 20%(xAg - (1 - x)BaTiO3) - 80%PVDF (x = 0, 2, 5, 7 and 27 vol.%) flexible composites is investigated. The inorganic fillers were realized by precipitating fine (similar to 3 nm) silver nanoparticles onto BaTiO3 nanoparticles (similar to 60 nm average size). The hybrid admixtures with a total filling factor of 20 vol.% were embedded into the PVDF matrix. The presence of filler enhances the amount of beta-PVDF polar phase and the BaTiO3 filler induces an increase of the permittivity from 11 to 18 (1 kHz) in the flexible composites. The addition of increasing amounts of Ag is further beneficial for permittivity increase; with the maximum amount (x = 27 vol.%), permittivity is three times larger than in pure PVDF (epsilon(r) similar to 33 at 1 kHz) with a similar level of tangent losses. This result is due to the local field enhancement in the regions close to the filler-PVDF interfaces which are additionally intensified by the presence of silver nanoparticles. The metallic addition is also beneficial for the mechanical harvesting ability of such composites: the amplitude of the maximum piezoelectric-triboelectric combined output collected in open circuit conditions increases from 0.2 V/cm(2) (PVDF) to 30 V/cm(2) for x = 27 vol.% Ag in a capacitive configuration. The role of ferroelectric and metallic nanoparticles on the increasing mechanical-electric conversion response is also been explained.

Owing to its unique biological and physicochemical properties, hydroxyapatite (HAp) represents one of the most extensively studied biomaterials for biomedical applications. It is well known that Candida is currently one of the fungi frequently involved in the onset and development of post-implant infections and, owing to the appearance of antifungal resistance, it is quite difficult to treat despite all the tremendous efforts made in this regard by the scientific world. Therefore, in this context, we report for the first time in this paper, the development and characterization of europium-doped thin films (5EuHAp, x(Eu) = 0.05) on a Si substrate by a spin-coating method. The results of ultrasound (US), zeta (zeta) potential, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) studies are presented. The XRD studies conducted on 5EuHAp suspension revealed the nanometric dimensions of the particles and sample purity. In addition, a moderate stability of the 5EuHAp suspension was observed. XPS measurements revealed the presence of Eu 3d in the 5EuHAp thin films. In the SEM micrographs, the surface uniformity and the absence of the surface defects could be observed. Moreover, the results of the FTIR studies showed the presence of the vibrational bands specific to the HAp structure in the studied sample. The antifungal activity of the HAp and 5EuHAp suspensions and coatings was evaluated using the Candida albicans ATCC 10231 (C. albicans) fungal strain. The qualitative assays of the antifungal properties of HAp and 5EuHAp coatings were also visualized by SEM and CLSM. The antifungal studies revealed that both 5EuHAp suspensions and coatings exhibited noticeable antifungal activity against C. albicans cells.

Phosphate and tellurite glasses can be used in optics, optoelectronics, magneto-optics, and nuclear and medical fields. Two series of phosphate-tellurite glasses, (50-x)ZnO-10Al(2)O(3)-40P(2)O(5)-xTeO(2) and (40-x)Li2O-10Al(2)O(3)-5TiO(2)-45P(2)O(5)-xTeO(2) (x = 5, 10), were synthesized by a non-conventional wet-route, and the mechanical properties as key performance measures for their application in optoelectronics were investigated. X-ray Diffraction (XRD) measurements revealed the vitreous nature of the investigated materials. Instrumented indentation tests allowed the calculation of hardness (H) and Young's modulus (E) using the Oliver and Pharr model. The influence of increasing the TeO2 content, as well as the substitution of ZnO by Li2O-TiO2, on the variation of hardness, Young's modulus, penetration depth (PD), and fracture toughness (FT) was evaluated in both series. As a general trend, there is a decrease in the hardness and Young's modulus with increasing penetration depth. The addition of Li2O and TiO2 instead of ZnO leads to improved hardness and elastic modulus values. Regarding the H/E ratio, it was found that the samples with lower TeO2 content should be significantly more crack-resistant compared to the higher TeO2 content samples. The H-3/E-2 ratio, being lower than 0.01, revealed a poor resistance of these glasses to plastic deformation. At the same time, a decrease of the fracture toughness with increasing TeO2 content was noticed for each glass series. Based on dilatometry measurements, the thermal expansion coefficient as well as the characteristic temperatures of the glasses were measured. Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray analysis (FESEM-EDX) revealed a uniform distribution of the elements in the bulk samples. The mechanical properties of these vitreous materials are important in relation to their application as magneto-optical Faraday rotators in laser cavities.

We report the mechanical behavior of a bulk boron ceramic prepared by spark plasma sintering of commercially available beta-boron powder. In order to fabricate polycrystalline boron ceramic, we used a protective tantalum foil reacted with carbon from the graphite die or graphite foil forming a thin layer of TaB2 and TaC covering the boron specimen. This is the first study to show the high-temperature flexural strength, toughness, and Young's moduli of boron up to 1400 degrees C. At 1600 degrees C and above, boron will react with testing environment forming an outer shell. The flexural strength and fracture toughness at room temperature reached an average of 340 MPa and 4.1 MPa m (1/2) , respectively. Despite showing clear signs of plastic deformation on the strain-stress curves, the yield strength of the monolithic boron ceramic exceed 1 GPa at 1200 degrees C. It was determined that fracture at elevated temperatures follows a quasi-transgranular mechanism, where the sub-grains of the boron fracture as plate-like structures. An interpretation for the observed fracture behavior was proposed.

Silver doped hydroxyapatite [AgHAp, Ca10-xAg(PO4)(6)(OH)(2)], due to its antimicrobial properties, is an advantageous material to be used for various coatings. The AgHAp thin films with x(Ag) = 0.05 and x(Ag) = 0.1 were achieved using the spin-coating method. The resulting samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). XRD analysis revealed that the particles of both samples are ellipsoidal. Also, in agreement with the results obtained by XRD measurements, the results of the SEM studies have shown that the particles shape is ellipsoidal. Optical properties of silver doped hydroxyapatite thin films deposited on Si substrate were investigated through Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The results obtained by the two complementary techniques highlighted that the molecular structure of the studied samples is not influenced by the increase of the silver concentration in the samples. Our studies revealed that the surface morphology of the obtained samples consist of uniform and continuous layers. The biocompatibility of the obtained thin films was also evaluated with the aid of human osteosarcoma MG63 (ATCC CRL 1427) cell line. Moreover, the in vitro antifungal activity against Candida albicans fungal strain of the AgHAp thin films was studied and the obtained results revealed their antifungal effect. The results of the biological assays showed that the AgHAp thin films are a very promising material for biomedical applications.

MoO2-Fe2O3 nanoparticle systems were successfully synthesized by mechanochemical activation of MoO2 and alpha-Fe2O3 equimolar mixtures throughout 0-12 h of ball-milling. The role of the long-range ferromagnetism of MoO2 on a fraction of more defect hematite nanoparticles supporting a defect antiferromagnetic phase down to the lowest temperatures was investigated in this work. The structure and the size evolution of the nanoparticles were investigated by X-ray diffraction, whereas the magnetic properties were investigated by SQUID magnetometry. The local electronic structure and the specific phase evolution in the analyzed system versus the milling time were investigated by temperature-dependent Mossbauer spectroscopy. The substantially shifted magnetic hysteresis loops were interpreted in terms of the unidirectional anisotropy induced by pinning the long-range ferromagnetic order of the local net magnetic moments in the defect antiferromagnetic phase, as mediated by the diluted magnetic oxide phase of MoO2, to those less defect hematite nanoparticles supporting Morin transition. The specific evolutions of the exchange bias and of the coercive field versus temperature in the samples were interpreted in the frame of the specific phase evolution pointed out by Mossbauer spectroscopy. Depending on the milling time, a different fraction of defect hematite nanoparticles is formed. Less nanoparticles supporting the Morin transition are formed for samples exposed to a longer milling time, with a direct influence on the induced unidirectional anisotropy and related effects.

Active semiconductor layers of TiO2 were synthesized via pulsed laser deposition in He, N-2, O-2, or Ar to manufacture DSSC structures. As-prepared nanostructured TiO2 coatings grown on FTO were photosensitized by the natural absorption of the N719 (Ruthenium 535-bis TBA) dye to fabricate photovoltaic structures. TiO2 photoanode nanostructures with increased adsorption areas of the photosensitizer (a combination with voluminous media) were grown under different deposition conditions. Systematic SEM, AFM, and XRD investigations were carried out to study the morphological and structural characteristics of the TiO2 nanostructures. It was shown that the gas nature acts as a key parameter of the architecture and the overall performance of the deposited films. The best electro-optical performance was reached for photovoltaic structures based on TiO2 coatings grown in He, as was demonstrated by the short-circuit current (Isc) of 5.40 mA, which corresponds to the higher recorded roughness (of 44 +/- 2.9 nm RMS). The higher roughness is thus reflected in a more efficient and deeper penetration of the dye inside the nanostructured TiO2 coatings. The photovoltaic conversion efficiency (eta) was 1.18 and 2.32% for the DSSCs when the TiO2 coatings were deposited in O-2 and He, respectively. The results point to a direct correlation between the electro-optical performance of the prepared PV cells, the morphology of the TiO2 deposited layers, and the crystallinity features, respectively.

In this work, the thermally stimulated current (TSC) technique has been used to investigate the properties of the radiation-induced interstitial boron and interstitial oxygen defect complex by 23-GeV (E-kin) protons, including activation energy, defect concentration, as well as the annealing behavior. At first isothermal annealing (at 80 degrees C for 0-180 min) followed by isochronal annealing (for 15 min between 100 degrees C and 190 degrees C in steps of 10 degrees C), studies had been performed in order to get information about the thermal stability of the interstitial boron and interstitial oxygen defect in 50-Omega cm material after irradiation with 23-GeV protons to a fluence of 6.91 x 10(13) p/cm(2). The results are presented and discussed. Furthermore, the extracted data from TSC measurements are compared with the macroscopic properties derived from current-voltage and capacitance-voltage characteristics. In addition, the introduction rate of interstitial boron and interstitial oxygen defect as a function of the initial doping concentration was determined by exposing diodes with different resistivities (10, 50, 250, and 2 k Omega cm) to 23-GeV protons. These results are compared with data from TSC and deep-level transient spectroscopy measurements achieved by the team of the CERN-RD50 "Acceptor removal project."

This paper presents the effect of Mn2+ substitution for Co2+, in CoFe2O4 embedded in SiO2 matrix, on the structural, surface, morphological and magnetic properties. X-ray diffraction (XRD) and Mossbauer spectroscopy indicate the presence of a nanocrystalline mixed cubic spinel. In all cases, for the nanocomposites (NCs) heat-treated at 200 degrees C, a single, low crystalline ferrite phase was remarked, while for the other heat-treatment temperatures up to 1200 degrees C and with increasing Mn content, the secondary phase of alpha-Fe2O3 appears, accompanied also by the secondary phase of SiO2 at 1200 degrees C. The Fourier transform infrared (FT-IR) spectroscopy confirms the consumption of starting metallic nitrates, the formation of Co-O, Mn-O, Fe-O bonds in ferrites@SiO2 matrix. The Mossbauer spectra show the characteristic magnetic patterns of Co and Mn spinels. According to the atomic force microscopy (AFM) analysis, the particle size increases from 15 to 80 nm with the increase of Mn content. The specific surface area varies in the range 150-450 m(2)/g due to the substitution of Co2+ ion with Mn2+ ion and decreases with increasing heat treatment temperature, reaching values below 1 m(2)/g at 1200 degrees C. All NCs have pores within the mesoporous range, with high dispersion of pores' sizes. Furthermore, the release of fine nanoparticles in aqueous environment is facilitated by the powders' mesoporous structure preserved at 200, 500 and 800 degrees C heat treatment temperatures. The porous network collapse after heat treatment at 1200 degrees C leads to releasing of bigger nanoparticles, in good agreement with AFM observation. Magnetization, coercivity and anisotropy evolve proportionally with the particle size for the NCs heat-treated at 800 degrees C (M-s = 18.9-36.3 emu/g; M-R = 3.05-14.1 emu/g, H-c = 31.83-53.2 kA/m, K= 0.378.10(-3) -1.21.10(-3) erg/cm(-1)) and inverse proportionally for those heat-treated at 1200 degrees C (M-s = 30.7-19.4 emu/g; M-R = 11.60 7.20 emu/g, H-c= 127.3-15.9 kA/m, K= 2.45.10(-3)-0.19.10(-3) erg/cm(-1)). The NCs with high Mn content heat-treated at 1200 degrees C show superparamagnetic behavior, while those with low Mn content display ferrimagnetic behavior. (C) 2021 Elsevier B.V. All rights reserved.

Anodization was used to obtain a nanotubular TiO2 photoanode on F-SnO2 glass. Subsequent annealing in the CH4 atmosphere promoted the C-doping and improved the crystallinity of the TiO2 nanotubes. The pulsed laser deposition was applied to cover the nanotubes with Bi2O3, serving as a hole transport material. X-ray photoelectron spectroscopy analyses of the doped samples reveal a shift in the valence band's maximum position towards lower binding energy as compared to those observed for the undoped samples (annealed in the air). The doping positively affects the absorption by shifting the absorption edge to 567 nm. I-V measurements under illumination show that the C-doping of TiO2 increases the current density following the absorbance results. The highest open circuit voltage was reached for the samples with the 300 degrees C-deposited Bi2O3 layer, pointing to better quality of the p-n junction, hence of the contact between Bi2O3 and TiO2. This in situ annealing provided the formation of close contact between Bi2O3 and TiO2, which enabled a faster charge transport as compared to the contact obtained with no annealing or even with post annealing.