National Institute Of Materials Physics - Romania

Magnetism and Superconductivity Laboratory

Publications

Year:

1016 articles found

1. Structural distortion dependence of thermoelectric properties in CoFeZrSi Heusler material

Authors: Birsan, A

Published: APR 25 2020, DOI: 10.1016/j.jallcom.2019.153492

Abstract: The effect of potential tetragonal and triclinic distortions of the energetic favorable cubic crystalline structure in CoFeZrSi Heusler compound, was investigated, using semiclassic Boltzmann theory on the thermoelectric functionalities. Chemical potential dependence of the conductivity integral sigma/tau for relaxed cubic and tetragonal structures confirms p-type thermoelectric characteristics. When triclinic deformation is investigated, the electrical conductivity response indicates that the material's ability to conduct electric current decreases. The calculated Seebeck coefficients exhibit positive values for the crystalline structures whose angles are equal to 90 degrees (cubic and tetragonal), over the 300-1200K temperature range. The figures of merit ZT, for relaxed cubic and tetragonal structures, at optimum unit cell volume or higher, present around room temperature, promising features as a potential thermoelectric material (i.e. ZT = 0.94 for 350K in optimum cubic structure). (C) 2019 Elsevier B.V. All rights reserved.

2. Unexpected magneto-functionalities of amorphous Fe-Gd thin films crossing the magnetization compensation point

Authors: Stanciu, AE; Schinteie, G; Kuncser, A; Iacob, N; Trupina, L; Ionita, I; Crisan, O; Kuncser, V

Published: MAR 15 2020, DOI: 10.1016/j.jmmm.2019.166173

Abstract: A complex investigation of the morpho-structural, magnetic, magneto-optical and magneto-transport properties of amorphous Fe-Gd thin films crossing the magnetization compensation point is reported and the unexpected observed magneto-functionalities are discussed. A tendency of magnetic domain formation with increasing the Fe content over the compensation concentration is observed. The switch from a reversed Magneto-optical Kerr Effect loop to a direct loop when increasing the Fe content over the compensation point is explained via the specific contribution to the rotation of the polarization vector from each magnetic sublattice, belonging to Fe and Gd, respectively. Local atomic configurations and magnetic interactions ascertained the amorphous character and revealed an out-ofplane orientation of the magnetic moment of Fe above the compensation point. The thermomagnetic curves prove a concentration dependent behavior, explained by weakly coupled magnetization relaxation processes of the two magnetic sub-lattices. On the other hand, the magnetic hysteresis loops gave evidence of two exchange coupled magnetic phases with different coercive fields. According to structural and Fe-57 Mossbauer Spectroscopy results, the two phases correspond to definite nanosized volumes of two different average concentrations (one of them closer to the compensation point) which are randomly distributed in the film. The unexpected single step-like behavior of the magneto-resistivity curves was explained by dissimilar switching of the spins in these two magnetic phases distributed in nano-sized volumes.

3. Magnetic properties and thermal stability of polyvinylidene fluoride-Fe2O3 nanocomposites

Authors: Kuncser, V; Chipara, D; Martirosyan, KS; Schinteie, GA; Ibrahim, E; Chipara, M

Published: JAN 28 2020, DOI: 10.1557/jmr.2019.375

Abstract: Nanocomposites of polyvinylidene fluoride loaded with various amounts of gamma-Fe2O nanoparticles, with an average size ranging between 20 and 40 nm, have been obtained by melt mixing and investigated using various experimental techniques [Superconducting Quantum Interference Device, Mossbauer, and Thermogravimetric Analysis]. Magnetic and Mossbauer measurements confirmed the presence of maghemite and a trace of a paramagnetic iron compound. Magnetic data are consistent with a blocking temperature close to room temperature (RT), showing a decrease in the coercive field as the temperature is increased. A weak exchange bias was noticed in all nanocomposites investigated at all temperatures and tentatively ascribed to surface spin disorder. The temperature dependence of the coercive field obeys the Kneller law. The nanocomposites exhibit superparamagnetic behavior near RT. Most magnetic measurements have been performed below the blocking temperature, revealing thus a complex behavior. The dependence of the mass loss derivative versus temperature, as obtained by thermogravimetric analysis, exhibits a single peak due to the thermal degradation of the polymeric matrix. A weak increase in the thermal stability of the polymeric matrix upon loading with maghemite is reported.

4. Beneficial effects of a WC addition in FAST-densified tungsten

Authors: Novak, S; Kocen, M; Zavagnik, AS; Galatanu, A; Galatanu, M; Tarancon, S; Tejado, E; Pastor, JY; Jenus, P

Published: JAN 20 2020, DOI: 10.1016/j.msea.2019.138666

Abstract: The particle reinforcement of fusion-relevant tungsten through the incorporation of tungsten sub-carbide W2C particles at the grain boundaries is demonstrated as an effective way of eliminating the harmful W oxide, enhancing densification and stabilising the composite's microstructure and flexural strength at room and high temperatures. The W2C particles are formed in situ during the sintering by carbon diffusion from WC nano-particles added as a precursor to the W matrix. Even in an extremely fast sintering process using Field-Assisted Sintering Technology (FAST, 1900 degrees C, 5 min), the added WC completely transforms to W2C, resulting in a W-W2C composite. While at least 5 vol % of WC nanoparticles are needed to eliminate the oxide, approximately 10 vol % result in a W-W2C composite with favourable characteristics: high density, high flexural strength at RT (>1200 MPa) as well as at elevated temperatures, and high thermal conductivity, which remains above 100 W/mK up to 1000 degrees C.

5. Magnetic Fe@Y Composites as Efficient Recoverable Catalysts for the Valorization of the Recalcitrant Marine Sulfated Polysaccharide Ulvan

Authors: Prech, J; Ioannou, E; Roussis, V; Kuncser, V; Podolean, I; Coman, SM; Valtchev, V; Parvulescu, VI

Published: JAN 13 2020, DOI: 10.1021/acssuschemeng.9b05406

Abstract: Magnetic Fe@Y composites (carbon-coated magnetic iron nanoparticles incorporated in zeolite Y) with 5-8 wt % Fe were synthesized and characterized. Overall acidity of the samples ranged between 2.0 and 2.47 mmol/g and is mostly attributed to Lewis acid sites. The obtained materials were proven to catalyze the hydrolysis of the marine sulfated polysaccharide ulvan with high conversion rates. The distribution of the reaction products depended on the reaction conditions and the concentration of ulvan. The catalytic property catalytic performance correlations clearly showed that the acid zeolite Y is the active phase for the hydrolysis of ulvan, while the iron nanoparticles enable the catalyst separation in a magnetic field. Under oxygen pressure, the selectivity was completely changed to favor succinic acid production. All Fe@Y composites were recycled 10 times with no change in their catalytic performance after recovery via a simple magnetic separation and washing with water.

6. Soft-magnetic coatings as possible sensors for magnetic imaging of superconductors

Authors: Ionescu, AM; Simmendinger, J; Bihler, M; Miksch, C; Fischer, P; Soltan, S; Schutz, G; Albrecht, J

Published: JAN 2020, DOI: 10.1088/1361-6668/ab5984

Abstract: Magnetic imaging of superconductors typically requires a soft-magnetic material placed on top of the superconductor to probe local magnetic fields. For reasonable results the influence of the magnet onto the superconductor has to be small. Thin YBCO films with soft-magnetic coatings are investigated using SQUID magnetometry. Detailed measurements of the magnetic moment as a function of temperature, magnetic field and time have been performed for different heterostructures. It is found that the modification of the superconducting transport in these heterostructures strongly depends on the magnetic and structural properties of the soft-magnetic material. This effect is especially pronounced for an inhomogeneous coating consisting of ferromagnetic nanoparticles.

7. Synthesis and characterization of conducting aniline and o-anisidine nanocomposites based on montmorillonite modified clay

Authors: Kenane, A; Galca, AC; Matei, E; Yahiaoui, A; Hachemaoui, A; Benkouider, AM; Bartha, C; Istrate, MC; Galatanu, M; Rasoga, O; Stanculescu, A

Published: JAN 2020, DOI: 10.1016/j.clay.2019.105395

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.

8. Bulks of Al-B-C obtained by reactively spark plasma sintering and impact properties by Split Hopkinson Pressure Bar

Authors: Vasylkiv, O; Borodianska, H; Demirskyi, D; Li, P; Suzuki, TS; Grigoroscuta, MA; Pasuk, I; Kuncser, A; Badica, P

Published: DEC 20 2019, DOI: 10.1038/s41598-019-55888-z

Abstract: Mixtures of B4C, alpha-AlB12 and B powders were reactively spark plasma sintered at 1800 degrees C. Crystalline and amorphous boron powders were used. Samples were tested for their impact behavior by the Split Hopkinson Pressure Bar method. When the ratio R = B4C/alpha-AlB12 >= 1.3 for a constant B-amount, the major phase in the samples was the orthorhombic AlB24C4, and when R < 1 the amount of AlB24C4 significantly decreased. Predictions that AlB24C4 has the best mechanical impact properties since it is the most compact and close to the ideal cubic packing among the Al-B-C phases containing B-12-type icosahedra were partially confirmed. Namely, the highest values of the Vickers hardness (32.4 GPa), dynamic strength (1323 MPa), strain and toughness were determined for the samples with R = 1.3, i.e., for the samples with a high amount of AlB24C4. However, the existence of a maximum, detectable especially in the dynamic strength vs. R, indicated the additional influence of the phases and the composite's microstructure in the samples. The type of boron does not influence the dependencies of the indicated mechanical parameters with R, but the curves are shifted to slightly higher values for the samples in which amorphous boron was used.

9. Superconducting MgB2 textured bulk obtained by ex situ spark plasma sintering from green compacts processed by slip casting under a 12T magnetic field

Authors: Grigoroscuta, MA; Sandu, V; Kuncser, A; Pasuk, I; Aldica, G; Suzuki, TS; Vasylkiv, O; Badica, P

Published: DEC 2019, DOI: 10.1088/1361-6668/ab4620

Abstract: MgB2 green bodies were prepared by magnetic field slip casting in ethyl alcohol with added polyethyleneimine dispersing agent under a high magnetic field, mu H-0(0) = 12 T. Samples were further processed by spark plasma sintering (SPS) and characterized for superconducting properties. Slip casting provides texturing of MgB2 (the degree of c-axis orientation is approximately 3.5%), which is further increased significantly (to about 21%) in the SPSed sample. The critical current density (J(c)) displays anisotropy relative to the orientation of the measuring magnetic field. Specific features of J(c)(H, T) and of the pinning force extracted from magnetic measurements with the field parallel and perpendicular to H-0 are discussed.

10. Designing functional ferroelectric interfaces from first-principles: dipoles and band bending at oxide heterojunctions

Authors: Dorin, R; Filip, LD; Pintilie, L; Butler, KT; Plugaru, N

Published: NOV 2019, DOI: 10.1088/1367-2630/ab4d8b

Abstract: The fundamental phenomena at ferroelectric interfaces have been the subject of thorough theoretical and computational studies due to their usefulness in a large variety of emergent electronic devices, solar cells and catalysts. Ferroelectricity determines interface band-bending and shifts in electron energies, which can be beneficial or detrimental to device performance. However, the underlying mechanisms are still the subject of debate and investigation, as a deeper understanding of the electrochemistry is required to develop bona fide design principles for functional ferroelectric surfaces and interfaces. Here, using first principles calculations within the GGA + U formalism, we investigate the problem of band alignment in non-defective, asymmetric SrRuO3/PbTiO3/SrRuO3 capacitors with ultra-thin ferroelectric layers. The effects of the dielectric size on the polar distortion stability and interface-specific properties are analyzed. It is shown that the critical size of the dielectric for polarization switching is approximate to 2 nm (5 PbTiO3 u.c.). Below this limit there is no bulk-like region in the dielectric, the space charge accumulated at interfaces leads to the presence of gap states in the whole PbTiO3 layer and ferroelectricity vanishes. We draw the band alignment diagrams as given by the band line-up and band structure terms, as well as by taking Ti 3s semi-core states as reference. In the ferroelectric structures, both approaches predict a strong effect of band-bending on the type of contact, Schottky or Ohmic, at the asymmetric interfaces. The effect of interface states on the interface dipole amplitude and band alignment is discussed.