National Institute Of Materials Physics - Romania

Short powder-in-tube tapes of MgB2 in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB2 core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB2 crystallite size, a weak contact between Fe and MgB2 core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB2 cores, the detected impurity phase is MgO, while at the interface with Fe, MgB4 also occurs. Impurity phases found at interface, MgO and MgB4, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB2 powder has an important contribution to observed differences, and its removal or control is recommended.

We report significant photoelectrochemical activity of Y-doped BiFeO3 (Y-BFO) epitaxial thin films deposited on Nb: SrTiO3 substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2 theta-omega scans of the symmetric (00l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L-parallel to) is preserved on larger nanoregions/ nanodomains. For higher thickness values L-parallel to is decreasing while domains tilt angles (alpha(tilt)) is increasing. The photocurrent value obtained for the thinnest sample was as high as J(ph) = 0.72 mA/cm(2), at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film's thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.

To avoid the deleterious effects of dopant segregation, synthesis methods that facilitate a homogenous dopant distribution in the ceria lattice were employed. Though doping ceria by wet impregnation was also credited to induce a homogeneous solid solution even in the heavy regime (concentration >= 20%, A. Corma, P. Atienzar, H. Garcia and J. Chane-Ching, Nat. Mater., 2004, 3, 394-397), no follow up investigation has been reported. Herein, we investigated ceria nanoparticles (1% Tm-CeO2 and 1% Eu-CeO2) wetimpregnated with trivalent rare-earth (Yb, 20%), bivalent (Ca, 20%) and isovalent (Zr, 30%) metals, followed by annealing in air. Homogeneity of the solid solutions of Yb-impregnated ceria was confirmed by a two-feature characterization toolbox that included X-ray diffraction, Raman spectroscopy, transmission electron microscopy, as well as up-conversion emission as a probe tool. Since the up-conversion emission of Tm was not detectable in the absence of Yb while its efficiency depends on the average distance between Yb and Tm ions, the Yb incorporation and its migration from the surface to the lattice bulk sites in wet-impregnated ceria can be "visualized" and compared with that of the Yb bulk-doped counterpart. The use of Eu luminescence as a local probe confirmed the homogeneity of solid solutions of Ca and Zr-impregnated ceria and also sustained the opposite roles of Ca and Zr as the repeller and the scavenger of oxygen vacancies, respectively. All these results suggested that heavy doping of ceria by wet impregnation with metals with +2, +3 and +4 valencies represent a facile alternative to conventional doping approaches. Therefore, the effects of the amount and the type of metal dopant on the structural properties of CeO2 could be investigated in a more systematic and probably a more reproducible manner, which would significantly increase the potential of ceria in catalysis and other applications.

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.

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.

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.

Pd nanoparticles are stabilized onto the surface of Fe3O4-entrapped poly(aminoethyl acrylamide) via functionalization of polymer with a new class of NNN-pincer ligand providing a high loading and stable catalyst for aerobic oxidation of alcohols to aldehydes. Varieties of alcohols are oxidized to the corresponding aldehydes with good selectivity using 0.95 mol% Pd catalyst. The catalyst is reusable (5 runs) without loss of activity and leaching or agglomeration of Pd nanoparticles while structural regularity of the catalyst is preserved. Oxidation of benzyl alcohol is also scaled up to a few grams. (C) 2018 Elsevier B.V. All rights reserved.

Arrays of magnetic Ni-Cu alloy nanowires with different compositions were prepared by a template-replication technique using electrochemical deposition into polycarbonate nanoporous membranes. Photolithography was employed for obtaining interdigitated metallic electrode systems of Ti/Au onto SiO2/Si substrates and subsequent electron beam lithography was used for contacting single nanowires in order to investigate their galvano-magnetic properties. The results of the magnetoresistance measurements made on single Ni-Cu alloy nanowires of different compositions have been reported and discussed in detail. A direct methodology for transforming the magnetoresistance data into the corresponding magnetic hysteresis loops was proposed, opening new possibilities for an easy magnetic investigation of single magnetic nanowires in the peculiar cases of Stoner-Wohlfarth-like magnetization reversal mechanisms. The magnetic parameters of single Ni-Cu nanowires of different Ni content have been estimated and discussed by the interpretation of the as derived magnetic hysteresis loops via micromagnetic modeling. It has been theoretically proven that the proposed methodology can be applied over a large range of nanowire diameters if the measurement geometry is suitably chosen.

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) thin films grown on 0.7 wt. % Nb-doped (001)-SrTiO3 (Nb:STO) single-crystal have been investigated. High-resolution transmission electron microscopy and electron energy loss spectroscopy revealed a very sharp Nb:STO/BCZT interface, while selected area electron diffraction revealed the epitaxial growth of the BCZT layer on the Nb:STO substrate. The ferroelectric nature of the BCZT films have been investigated by piezoresponse force microscopy and hysteresis loops. The effect of electric field on polarization switching kinetics has been investigated and has been analyzed by the nucleation limited switching model with a Lorentzian distribution function. The local field variation was found to decrease with the increase in the electric field, and thus, the switching process becomes faster. The peak value of the polarization current and the logarithmic characteristic switching time exhibited an exponential dependence on the inverse of electric field. This model gave an excellent agreement with the experimental polarization reversal transients throughout the whole time range. Published by AIP Publishing.

The production and thermal stability of the irradiation paramagnetic point defects (IPPDs) in the as-grown, standard float-zone silicon (STFZ) and oxygen doped float-zone silicon (DOFZ) irradiated at room temperature with high fluence 3.5 MeV (1 x 10(17) cm(-2)) low energy and 27 MeV (2 x 10(16) cm(-2)) high energy electrons were investigated by Electron Spin Resonance (ESR). The nature and concentration of the IPPDs identified in the as-irradiated and isochronally annealed up to 300 degrees C samples by ESR measurements under intense above the gap 1.06 mu m in-situ laser illumination, were found to depend on oxygen concentration and electrons energy. While irradiation of STFZ with electrons produced as main IPPDs, besides divacancies, larger tetravacancy and pentavacancy cluster defects, irradiation of DOFZ resulted mainly in divacancies, vacancy-oxygen and interstitial oxygen-carbon impurity pairs. The observed variation in the nature of the main resulting IPPDs with the concentration of incorporated oxygen is explained by differences in the dominant defects production mechanisms. Thus in STFZ with lower oxygen concentration (1 x 10 16 cm(-3)) the dominant production mechanisms are the direct defects formation from a chain of neighboring vacancies by a cascade of secondary recoils across the path of the high energy irradiating particle and the divacancies diffusion and trapping/aggregation. Meanwhile, in DOFZ with larger oxygen content (1.2 x 10(17) cm(-3)) the primary vacancy and interstitial trapping by the oxygen and carbon impurities is the dominant defect production mechanism. Variations in the concentration and nature of the IPPDs observed during isochronal annealing are discussed in terms of defects thermal activated diffusion and recombination processes.