National Institute Of Materials Physics - Romania

Abstract: Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 <= x <= 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 degrees C the BST ceramics are ferroelectric for 0.3 <= x <= 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7-0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

Abstract: Hybrid organic-inorganic thin films based on zinc phthalocyanine (ZnPc) and ZnO nanoparticles were deposited by Matrix Assisted Pulsed Laser Evaporation (MAPLE). Synthesized by a simple wet chemical precipitation method, the ZnO nanoparticles were featured by a hexagonal wurtzite structure, a band-gap value of similar to 3.3 eV and emission bands typical for this semiconductor. The hybrid films containing ZnPc and various amounts of ZnO nanoparticles were evaluated from morphological, compositional, structural, optical and electrical point of view. No chemical decomposition of the organic compound was observed in the FTIR spectra of the deposited layers. The transmittance and photoluminescence spectra recorded on hybrid films disclose the optical signature of both organic (ZnPc) and inorganic (ZnO) components. The electrical measurements carried out under illumination emphasized the importance of the quantity of the inorganic component on the performance parameters of the structures prepared with the hybrid films. Our study provides new insight in the MAPLE deposition of the organic-inorganic hybrid films with potential applications in the photovoltaic cells area.

Abstract: Structural analysis of the Eu2+-doped BaCl2 nanocrystals and the doping process was monitored and characterized via electron paramagnetic resonance spectroscopy. Structural analysis has shown a slight distortion of the cell which is reflected in the low value of microstrain and the Eu2+-doping effect is limited to the first order chlorine ions neighbors of Ba2+. Electron paramagnetic resonance measurements have indicated the presence of the Eu2+-dopant ions in the BaCl2 host matrix with a solubility limit of about 2%. From the spectra simulation, the isotropic g-value g(iso) = 1.9951(7), the isotropic hyperfine coupling constant A(iso)= 42.4 MHz and the high-order zero-field splitting parameters from the crystal field B-2(0) = 21 MHz and B-2(2)= -493 MHz were obtained. During X-ray irradiation, defects are produced and stabilized by the Eu2+ dopant ions. The single dominant thermoluminescence peak at 132 degrees C (activation energy E = 1.1 eV) was assigned to the recombination of the F(Cl)-center with Eu2+ related hole centers. (C) 2019 Elsevier B.V. All rights reserved.

Abstract: We developed a new type of chalcohalide glasses with physicochemical and nonlinear optical properties that are tunable by composition. It is found that more than 60 mol.% CdCl2 heavy metal halide can be dissolved into the ternary Ge Sb S system and forming stable glasses. The visible-light transparency range is extended to shorter wavelengths with the addition of CdCl2, which is beneficial for the optical quality control and infra-red (IR) system alignment. The third-order optical nonlinearity (TONL) is studied using the femtosecond Z-scan method. The results show that both the nonlinear refractive index and two photon absorption co-efficient decrease with CdCl2. Benefiting from the favorable property-tailoring effects of CdCl2, the TONL figure of merit (FOM) can be improved to meet the requirement (FOM < 1) for all-optical switching and IR photonic applications.

Abstract: A study on clay mineral polymer nanocomposites (CPN), namely polyaniline/montmorillonite-cetyltrimethylammonium bromide (PANI/Mt-CTAB), poly o-anisidine/montmorillonite-cetyltrimethylammonium bromide (poly(o-ANIS)/Mt-CTAB) and poly o-anisidine-co-aniline/montmorillonite-cetyltrimethylammonium bromide (poly(o-ANIS-co-ANI)/Mt-CTAB), synthesized by oxidative chemical polymerization method is presented. The nanocomposites have been characterized by Fourier transform infrared spectroscopy, UV-vis spectroscopy, and cyclic voltammetry, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and thermogravimetry and differential scanning calorimetry analysis. By UV-vis measurements different electronic transitions for the CPNs were pointed out. The voltammograms indicate that the synthetized materials are electroactive. The FTIR analysis reveals the characteristic bands of the polymers and of the Mt-CTAB. The shift of the bands to higher/lower wavenumbers demonstrate the interaction between the pristine polymers macromolecular chains and the montmorillonite (Mt). The intercalation of the polymers inside the mineral clay was confirmed by the increased interlayer distance connected with the position of the 011 diffraction plane of the Mt., and the intercalation and exfoliation states were highlighted in the scanning and transmission electron microscopy images. The obtained results are encouraging in respect with the purpose to use them in the field of photovoltaic applications.

Abstract: Hematite-titania thin films with various ratios of Ti/Fe have been prepared on FTO substrate via cathodic and anodic deposition of amorphous Ti oxyhydroxide gel (TiOx) and iron oxyhydroxide (FeOOH) films, respectively followed by annealing processes. TiOx underlayer behaved as a template for FeOOH electrodeposition. The photocatalytic behavior of the photoanodes based on the prepared structures was studied. An increased photocurrent density was obtained without anodic shifting of the photocurrent onset potential. Mott-Schottky studies showed that Ti contributes little to donor density in hematite-titania photoanodes prepared by this technique. The impedance measurements suggest that both separation and transfer of charge carriers are promoted in these hematite-titania photoanodes. For comparative studies, hematite-titania photoanodes have been prepared from TiOx and FeOOH precursors formed by spin-coating technique and anodic deposition method, respectively. The photoelectrochemical performance of these photoanodes in the presence of cobalt-phosphate catalyst was also evaluated.

Abstract: 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.

Abstract: 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,

Abstract: Monolayers of transition metal (from the group VI B) dichalcogenides (MoS2, MoSe2, WS2 and WSe2) show nonvolatile resistance switching: a transition from a high to a low resistance state. Here we propose two explanations for this behaviour. The first one is that the transition metals swaps from a trigonal prismatic to an octahedral coordination (due to a high applied electric field and pressure) and thus the monolayer switches from a semiconducting to a metallic phase. The second one is a two-step process where the high electric field and pressure break the M-X bonds and the transition metal atoms become firstly tetrahedrally coordinated and afterwards square-planar coordinated. Thus, all transition metal and chalcogen atoms are in the same plane, and the transition metal atoms are in contact with the atoms of the top and bottom electrodes.

Abstract: Molecular mobility of cyanophenyl alkylbenzoates (CPnBs) (n = 2, 3, 7 - number of carbon atoms in the alkyl chain) in the bulk and in composites with aerosil A380 is investigated by broadband dielectric spectroscopy, while thermal analysis and infrared spectroscopy were applied to characterise the molecular species. The work completes preliminary results obtained for the members with n = 4 ... 6. An interaction by hydrogen bonding, between aerosil surface - OH groups and - CN or ester groups of the CPnB molecules takes place. It slows down the relaxation process as observed for related composites in comparison to the pure materials. The existence of two types of bonding might be the reason that Vogel temperature for the relaxation process in the surface layer does not show the odd-even effect. Temperature dependence of the relaxation rates for composites shows a crossover behaviour from a high to a low temperature regime. Moreover, the temperature dependence of the dielectric strength is unusual. As the loading degree is similar, comparison of the dielectric, spectroscopic and thermal data obtained here and with the results obtained for the composites with n = 4 ... 6 can be directly done. Increasing the number of the members of the homologous series confirms and hardens the preliminary conclusions.