1000 articles trouvés
1. Influence of the synthesis parameters on the transport properties of Mg2Si0.4Sn0.6 solid solutions produced by melting and spark plasma sintering

people Auteurs: Assahsahi, I; Popescu, B; Enculescu, M; Galatanu, M; Galca, AC; El Bouayadi, R; Zejli, D; Galatanu, A
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Publié: APR 2022, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 163, DOI: 10.1016/j.jpcs.2021.110561

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    In this work, the influence of the preparation route on the structural, morphological, and thermoelectric properties of the Mg2Si0.4Sn0.6 solid solutions is investigated. The synthesis based on melting the constituent elements in a closed graphite crucible followed by spark plasma sintering allows mixing elements with a large difference of their melting temperatures and a good control of sample stoichiometry. The optimized synthesis route is validated by the doped V and Sb samples, which yield good thermoelectric performance. The n-type doping leads to two orders of magnitude increase of the carrier concentration, and thus a subsequent increase of the electrical conductivity, which, in turn, augments greatly the power factor of the Mg1.98V0.02Si0.385Sn0.6Sb0.015 to 42.61 x10(-4) Wm(-1)K 2 at 650K. Although doping slightly enlarges the thermal conductivity, a peak value of the figure of merit ZT similar to 1.15 is obtained at 723K, which is 20 times higher than the ZT of un-doped material.
2. C-doped TiO2 nanotubes with pulsed laser deposited Bi2O3 films for photovoltaic application

people Auteurs: Bjelajac, A; Petrovic, R; Stan, GE; Socol, G; Mihailescu, A; Mihailescu, IN; Veltruska, K; Matolin, V; Siketic, Z; Provatas, G; Jaksic, M; Janackovic, D
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Publié: FEB 15 2022, CERAMICS INTERNATIONAL, 48, 4657, DOI: 10.1016/j.ceramint.2021.10.251

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    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.
3. Nanocrystalline rhenium-doped TiO2: an efficient catalyst in the one-pot conversion of carbohydrates into levulinic acid. The synergistic effect between Bronsted and Lewis acid sites

people Auteurs: Avramescu, S; Ene, CD; Ciobanu, M; Schnee, J; Devred, F; Bucur, C; Vasile, E; Colaciello, L; Richards, R; Gaigneaux, EM; Verziu, MN
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Publié: JAN 4 2022, CATALYSIS SCIENCE & TECHNOLOGY, 12, 180, DOI: 10.1039/d1cy01450a

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    Catalytic activity of TiO2, 2%Re-TiO2 and 10%Re-TiO2 in the conversion of carbohydrates into levulinic acid under autoclave conditions was evaluated. These materials were prepared by aerogel method, for the first time to the best of our knowledge, and characterized by XPS, SEM-EDX, DRIFTS, DR UV-vis, Raman, N-2 adsorption/desorption isotherms, TGA and XRD. Further, the surface acidity was probed by NH3-TPD and pyridine-FT-IR where it was observed that increasing the amount of rhenium doped into TiO2 led to an increase in the total number of acid sites (Lewis + Bronsted) but with an overall lower strength. The presence of both Bronsted and Lewis acid sites led to the hypothesis that these materials may be well suited for conversion of carbohydrates into levulinic acid. Indeed a levulinic acid yield of 57% was reached over 10%Re-TiO2 for a low mass ratio catalyst to glucose (1 : 5). Moreover, the 10%Re-TiO2 catalyst was reused in the conversion of glucose for four catalytic cycles without a significant loss of the catalytic activity.
4. Catalytic transformation of the marine polysaccharide ulvan into rare sugars, tartaric and succinic acids

people Auteurs: Podolean, I; Coman, SM; Bucur, C; Teodorescu, C; Kikionis, S; Ioannou, E; Roussis, V; Primo, A; Garcia, H; Parvulescu, VI
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Publié: JAN 1 2022, CATALYSIS TODAY, 383, 357, DOI: 10.1016/j.cattod.2020.06.086

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    The green macroalga Ulva rigida represents a promising feedstock for biorefinary due to its fast growth and cosmopolitan distribution. The main component of the cell walls of U. rigida is a sulfated glucuronorhamnan polysaccharide known as ulvan. Herein it was found that due to the high (hydrogen)sulfate group content of ulvan, hydrothermal autohydrolysis at 130 degrees C renders a high percentage of rhamnose (78-79 % recovery from the initial content in the raw material), a rare sugar of high added value. In addition, acid catalysis by a triflate-based graphene oxide under oxygen-free conditions at 180 degrees C affords moderate amounts of tartaric acid (24-26 %). The same triflate-based graphene oxide catalyst under oxygen pressure yields remarkably high percentages of succinic acid (65 %). The catalyst preserves its activity for at least five consecutive reuses.
5. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

people Auteurs: AitMellal, O; Oufni, L; Messous, MY; Tahri, M; Neatu, S; Florea, M; Neatu, F; Secu, M
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Publié: JAN 2022 , JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, DOI: 10.1007/s10854-021-07615-6

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    To enhance the spectral response of solar cells, an experimental study on LaPO4:0.01Ce(3+)/xNd(3+) (x = 0, 2, 4 mol%) was carried out, where structural and morphological properties of the prepared samples were well characterized by the means of X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electronic microscope. Additionally, the photoluminescence behavior of phosphors in ultraviolet-visible (UV-VIS) and Near-infrared (NIR) regions were investigated to confirm the energy transfer (ET) from Ce3+ to Nd3+. Moreover, the quantum efficiency of Ce3+/Nd3+-co-doped samples was estimated as high as similar to 172% and the possible ET process was described. Accordingly, the LaPO4:Ce3+/Nd3+ phosphors can convert the UV light (275 nm) into NIR photons (approx. 1059 nm) through the possible two-pathway energy transfer processes from Ce3+ sensitizer ions to Nd3+ activators. Obtained NIR down-conversion emissions are suitable for improving the conversion efficiency of c-Si solar cells.
6. The origin of the dual ferroic properties in quasi-centrosymmetrical SrFe12-xInxO19 hexaferrites

people Auteurs: Trukhanov, AV; Turchenko, VA; Kostishin, VG; Damay, F; Porcher, F; Lupu, N; Bozzo, B; Fina, I; Polosan, S; Silibin, MV; Salem, MM; Tishkevich, DI; Trukhanov, SV
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Publié: DEC 15 2021, JOURNAL OF ALLOYS AND COMPOUNDS, 886, DOI: 10.1016/j.jallcom.2021.161249

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    The local crystal/magnetic structures of the SrFe12-xInxO19 solid solutions (x = 0.1; 0.3; 0.6 and 1.2) were investigated using neutron powder diffraction. The measurements of the electric polarization for all investigated samples were carried out as a function of the external electric field. The presence of the ferroelectric and ferromagnetic ordering (dual ferroic ordering) in the SrFe12-xInxO19 hexaferrites at 300 K was found. This appearance contradicts to the conventional opinion describing their crystal structure (centrosymmetric space group P6(3)/mmc (No. 194)). The reason for the existence of a spontaneous polarization (nonzero dipole moment) in the SrFe12-xInxO19 hexaferrites continues controversial. The crystal structure of the hexaferrites was considered both the centrosymmetric P6(3)/mmc and non-centrosymmetric P6(3)mc space groups. This fact made it possible to find a connection between the emerging dipole moment and not equal distortions of the neighbor oxygen polyhedral. The nature description of the nonzero dipole moment formation in a quasi-centrosymmetrical system of the In-substituted SrFe12-xInxO19 hexaferrites was presented based on the neutron diffraction data. (C) 2021 Elsevier B.V. All rights reserved.
7. Arylenevinylene Oligomer-Based Heterostructures on Flexible AZO Electrodes

people Auteurs: Stanculescu, A; Socol, M; Rasoga, O; Breazu, C; Preda, N; Stanculescu, F; Socol, G; Vacareanu, L; Girtan, M; Doroshkevich, AS
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Publié: DEC 2021, MATERIALS, 14, DOI: 10.3390/ma14247688

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    We investigated the optical and electrical properties of flexible single and bi-layer organic heterostructures prepared by vacuum evaporation with a p-type layer of arylenevinylene oligomers, based on carbazole, 3,3 ' bis(N hexylcarbazole)vinylbenzene = L13, or triphenylamine, 1,4 bis [4 (N,N' diphenylamino)phenylvinyl] benzene = L78, and an n-type layer of 5,10,15,20-tetra(4-pyrydil)21H,23H-porphyne = TPyP. Transparent conductor films of Al-doped ZnO (AZO) with high transparency, >90% for wavelengths > 400 nm, and low resistivity, between 6.9 x 10(-4) ohm center dot cm and 23 x 10(-4) ohm center dot cm, were deposited by pulsed laser deposition on flexible substrates of polyethylene terephthalate (PET). The properties of the heterostructures based on oligomers and zinc phthalocyanine (ZnPc) were compared, emphasizing the effect of the surface morphology. The measurements revealed a good absorption in the visible range of the PET/AZO/arylenevinylene oligomer/TPyP heterostructures and a typical injection contact behavior with linear (ZnPc, L78) or non-linear (L13) J-V characteristics in the dark, at voltages < 0.4 V. The heterostructure PET/AZO/L78/TPyP/Al showed a current density of ~1 mA/cm(2) at a voltage of 0.3 V. The correlation between the roughness exponent, evaluated from the height-height correlation function, grain shape, and electrical behavior was analyzed. Consequently, the oligomer based on triphenylamine could be a promising replacement of donor ZnPc in flexible electronic applications.
8. Fabrication and Physico-Chemical Properties of Antifungal Samarium Doped Hydroxyapatite Thin Films

people Auteurs: Ciobanu, CS; Predoi, D; Chapon, P; Predoi, MV; Iconaru, SL
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Publié: DEC 2021, COATINGS, 11, DOI: 10.3390/coatings11121466

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    Samarium doped hydroxyapatite (Ca10-xSmx(PO4)(6)(OH)(2), x(Sm) = 0.5, 50SmHAp) is a very promising candidate to be used for different coatings in various dental and orthopedic implants. We report, for the first time, the obtaining of 50SmHAp thin films by a cost-effective method, namely spin coating. Thin films of 50SmHAp on silicon substrate have been analyzed by various techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), Metallographic microscopy and Glow Discharge Optical Emission Spectroscopy (GDOES). The stability of 50SmHAp suspension was evaluated by ultrasound measurements. Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were also used to evaluate the 50SmHAp suspension. The antifungal activity of 50SmHAp suspension and coatings was assessed using Candida albicans ATCC 10231 fungal strain (C. albicans). The results of the antifungal assays depicted that both 50SmHAp suspensions and coatings were effective in inhibiting the development of C. albicans fungal cells, thus making them ideal candidates for the development of novel antifungal agents. The obtained results give new perspective for possible applications of 50SmHAp thin films in various medical applications due to their antifungal properties.
9. Nitrogen and Bromide Co-Doped Hydroxyapatite Thin Films with Antimicrobial Properties

people Auteurs: Iconaru, SL; Ciobanu, CS; Predoi, D; Motelica-Heino, M; Negrila, CC; Badea, ML; Predoi, MV; Chifiriuc, CM; Popa, M
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Publié: DEC 2021, COATINGS, 11, DOI: 10.3390/coatings11121505

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    Hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAp), due to its high biocompatibility, is widely used as biomaterial. Doping with various ions of hydroxyapatite is performed to acquire properties as close as possible to the biological apatite present in bones and teeth. In this research the results of a study performed on thin films of hydroxyapatite co-doped with nitrogen and bromine (NBrHAp) are presented for the first time. The NBrHAp suspension was obtained by performing the adapted co-precipitation method using cetyltrimethylammonium bromide (CTAB). The thin layers of NBrHAp were obtained by spin-coating. The stability of the NBrHAp suspension was examined by ultrasound measurements. The thin layers obtained by the spin-coating method were examined by scanning electron microscopy (SEM), optical microscopy (OM), and metallographic microscopy (MM). The presence of nitrogen and bromine were highlighted by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) studies. Fourier transform infrared spectroscopy (FTIR) was used to highlight the chemical status of nitrogen and bromine. In addition, the powder obtained from the NBrHAp suspension was analyzed by XRD. Moreover, the in vitro antimicrobial activity of the NBrHAp suspensions and coatings was investigated using the reference microbial strains Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, and Candida albicans ATCC 10231. The results highlighted the successful obtainment of N and Br co-doped hydroxyapatite suspension for the first time by an adapted co-precipitation method. The obtained suspension was used to produce pure NBrHAp composite thin films with superior morphological properties. The NBrHAp suspensions and coatings exhibited in vitro antimicrobial activity against bacterial and fungal strains and revealed their good antimicrobial activity.
10. New Phenotype and Mineralization of Biogenic Iron Oxide in Magnetotactic Bacteria

people Auteurs: Baaziz, W; Ghica, C; Cypriano, J; Abreu, F; Anselme, K; Ersen, O; Farina, M; Werckmann, J
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Publié: DEC 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11123189

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    Many magnetotactic bacteria (MTB) biomineralize magnetite crystals that nucleate and grow inside intracellular membranous vesicles originating from invaginations of the cytoplasmic membrane. The crystals together with their surrounding membranes are referred to as magnetosomes. Magnetosome magnetite crystals nucleate and grow using iron transported inside the vesicle by specific proteins. Here, we tackle the question of the organization of magnetosomes, which are always described as constituted by linear chains of nanocrystals. In addition, it is commonly accepted that the iron oxide nanocrystals are in the magnetite-based phase. We show, in the case of a wild species of coccus-type bacterium, that there is a double organization of the magnetosomes, relatively perpendicular to each other, and that the nanocrystals are in fact maghemite. These findings were obtained, respectively, by using electron tomography of whole mounts of cells directly from the environment and high-resolution transmission electron microscopy and diffraction. Structure simulations were performed with the MacTempas software. This study opens new perspectives on the diversity of phenotypes within MTBs and allows to envisage other mechanisms of nucleation and formation of biogenic iron oxide crystals.