National Institute Of Materials Physics - Romania

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892 articles found

221. Complex exchange coupling mechanisms in SRO/BFO/Fe heterostructures

Authors: Greculeasa, SG; Schinteie, G; Hrib, LM; Stancu, V; Pasuk, I; Kuncser, A; Kuncser, V

Published: JAN 30 2019, JOURNAL OF ALLOYS AND COMPOUNDS, 773, 345, DOI: 10.1016/j.jallcom.2018.09.208

Temperature dependent interfacial coupling mechanisms in SRO/BFO/Fe layered structures were investigated. The BFO/Fe heterostructures were prepared by PLD and sputtering, respectively, on the STO(0 0 1) substrate with a 20 nm SRO buffer layer. An annealing treatment in external magnetic field was further applied. Complex characterizations with X-ray diffraction, atomic force microscopy, Transmission Electron Microscopy, Mossbauer spectroscopy, magneto-optic Kerr effect and SQUID magnetometry were performed. Before annealing, the films show good crystallization and epitaxy of the SRO and BFO layers with smooth interfaces. Two coupling mechanisms of the ferromagnetic layers (top Fe and bottom SRO, respectively) to the epitaxial BFO film with mainly antiferromagnetic structure were evidenced in the as deposited samples at low temperatures. Negative exchange bias fields of up to 67(10) Oe and 37(5) Oe at low temperatures were observed for the two ferromagnetic components, respectively, depending on the thickness of the Fe layer. The field annealing treatments induce a specific morphology and magnetic spin structure at both interfaces of the BFO spacer layer, giving rise to a long range magnetostatic coupling between the two ferromagnetic films, in addition to the interfacial couplings. Moreover, the experimentally evidenced Fe clusters penetrating the BFO/Fe interface toward the BFO layer give support for this interaction. As an additional consequence, a considerable enhancement of both uniaxial and unidirectional anisotropies as well as an increased blocking temperature of exchange bias were obtained. The involved exchange coupling mechanisms were discussed in detail. (C) 2018 Elsevier B.V. All rights reserved.

222. Addition of carbon fibers into B4C infiltrated with molten silicon

Authors: Solodkyi, I; Bezdorozhev, O; Vterkovskiy, M; Bogomol, I; Bolbut, V; Kruger, M; Badica, P; Loboda, P

Published: JAN 2019, CERAMICS INTERNATIONAL, 45, 174, DOI: 10.1016/j.ceramint.2018.09.148

Boron carbide added with 0-20 wt% carbon fibers was subject to Si infiltration. Samples mainly consist of B13C2, beta-SiC and unreacted Si. Some amount of SiB6 and alpha-SiC was also detected, while formation of B-12(B,C,Si)(3) phase was suppressed due to short infiltration time. The carbon fibers react with Si and result in formation of a composite core-shell fiber with SiC-shell and C-core. Theoretical estimations suggest that these composite fibers have a strong influence on the enhancement of the bending strength. Although apparently in good agreement with experimental data showing an increase of bending strength up to 510 +/- 27 MPa in the sample with 10 wt% carbon fiber, the implications of phase changes with the carbon fiber amount has to be carefully considered. At higher amounts of carbon fibers, bending strength decreases.

223. Extrinsic absorption in spark plasma sintered Zr0.8Sn0.2TiO4 ceramics investigated by terahertz time-domain spectroscopy

Authors: Nedelcu, L; Burdusel, M; Grigoroscuta, MA; Geambasu, CD; Banciu, MG; Badica, P

Published: 2019, 2019 44TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ), DOI:

Solid-state reaction synthesized Zr0.8Sn0.2TiO3 powders have been compacted by spark plasma sintering. In order to reduce the oxygen vacancies, the sintered samples were annealed ex-situ in air. Single-phase ceramics with different amount of oxygen vacancies and, consequently, different extrinsic absorption were investigated by terahertz time-domain spectroscopy. The results showed that the terahertz spectroscopy could be a suitable technique for tailoring the absorption properties of the spark plasma sintered materials.

224. STRUCTURAL EVOLUTION OF THE NiTi/NiFeGa SMART HYBRID MATERIAL DURING SEVERE PLASTIC DEFORMATION

Authors: Gurau, C; Gurau, G; Tolea, F; Sampath, V

Published: JUL-SEP 2019, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 14, 546, DOI:

High speed high pressure torsion (HSHPT) a patented new approach is proposed to fabricate nanocomposites. The goal of this work is to investigate the NiTi/NiFeGa bilayer hybrid material with nano- and submicrocrystalline structure under the influence of HSHPT. Apart from the grain refinement, the effectiveness of the joint are revealed scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanical properties of the composite layers in bulk and after severe plastic deformation are investigated. Bi-layered composite disks consisting of NiTi shape memory alloy and NiFeGa - Heusler type alloy, exhibit thermoelastic structural martensitic transformation. Submicrocrystalline structure is formed in the both layers of the hybrid material. It is also ascertained significant hardening of each layer of the hybrid as a result of HSHPT. The results highlight market differences between the bulk and the hybrid and the role of severe plastic deformation on martensitic transformation.

225. NiTi coated with oxide and polymer films in the in vivo healing processes

Authors: Batalu, D; Nastase, F; Militaru, M; Gherghiceanu, M; Badica, P

Published: JAN-MAR 2019, JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T, 8, 922, DOI: 10.1016/j.jmrt.2018.06.015

Plates of NiTi chemically etched, electro-polished, and sol gel coated with XO2 (X =Ti, Si, Zr), or coated with oxides and dip-coated polymers of Dextro-Levo-lactide-co-glycolide (DL-PLG, 0.4 mu m thickness), Dextro-Levo-lactic acid (DL-PLA, 1.3 mu m) or poly methyl methacrylate polymer (PMMA, 1.7 mu m) were obtained. Smooth and uniform NiTi surfaces without significant pitting, as revealed by AFM, were prepared for chemical etching of 120 s in HF:HNO3:H2O = 1:5:4, followed by electropolishing 120 s in H2SO4:CH3OH:H2O = 1:4:5 electrolyte and using a potential of 9 V. Dip-coated layer of PMMA has shown cracks and large pores and was eliminated from further experiments. Samples of pristine and coated NiTi were in vivo implanted into rabbits and extracted after 10 and 60 days. Clinically, all implants are biocompatible; all rabbits survived and a recovery process was observed for all cases. NiTi covered with SiO2, DL-PLG and SiO2/DL-PLG have shown the best healing evolution. For 10 and 60 days good recovery was found also for NiTi coated with TiO2. Coatings of ZrO2 and ZrO2/DL-PLG have shown the poorest results. The oxide coating and the roughness R-ZJIS that contains information on the 'deep' large areas in the coatings show the strongest influence on the healing processes. Work indicates the possibility of space- and time- scale controlled variation of the functional properties. (C) 2018 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.

226. AC magnetic response of superconducting single crystals exhibiting a second peak on the DC magnetization curves

Authors: Miu, L; Ionescu, AM; Miu, D; Petrisor, T; Park, A; Tamegai, T; Crisan, A

Published: DEC 15 2018, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 555, 6, DOI: 10.1016/j.physc.2018.10.002

The strong modulation of the AC magnetic response in the magnetic field-temperature region of the DC magnetization peak effect (PE) occurring just below the DC irreversibility line (IL) of weakly pinned superconducting single crystals is well known. However, it is not yet clear if the AC magnetic signal registered at usual frequencies and amplitudes is significantly affected by the presence of the second magnetization peak (SMP) in the DC magnetic hysteresis curves, since the SMP appears in specimens with stronger pinning and is located well below the IL. We investigated the AC magnetic response of several La2 - xSrxCuO4 and 122-type pnictide superconducting single crystals exhibiting a pronounced DC SMP. It was found that in the case of small demagnetization effects, the AC magnetic signal across the SMP remains in the linear regime, with no detectable distortions in the SMP range (within the high sensitivity of a Physical Property Measurement System). This indicates reduced values of the Campbell penetration depth compared with the London magnetic penetration depth. A clearly nonmonotonic, SMP-related variation has only been obtained for the DC field dependence of the out of phase component of the AC magnetic moment in the nonlinear regime. The nonlinear AC Bean regime far below the IL has been attained in the conditions of enhanced demagnetization effects (plate-like specimens in perpendicular magnetic fields).

227. Flexible Delivery Patch Systems based on Thermoresponsive Hydrogels and Submicronic Fiber Heaters

Authors: Evanghelidis, A; Beregoi, M; Diculescu, VC; Galatanu, A; Ganea, P; Enculescu, I

Published: DEC 3 2018, SCIENTIFIC REPORTS, 8, DOI: 10.1038/s41598-018-35914-2

This paper proposes a novel, flexible, low cost administration patch which could be used as a non-invasive, controlled transdermal drug delivery system. The fabricated device consists in a flexible microfiber architecture heater covered with a thermoresponsive hydrogel, namely poly(N-isopropylacrylamide), as a matrix for the incorporation of active molecules. The manufacturing process consists of two main steps. First, the electrospun poly(methyl methacrylate) fiber networks are sputter coated with a thin gold layer and attached to flexible poly(ethylene terephthalate) substrates to obtain the heating platforms. Second, the heaters are encapsulated in poly(ethylene terephthalate) foils and covered with poly(N-isopropylacrylamide) hydrogel sheets. In order to illustrate the functionality of the fabricated patch, the hydrogel layer is loaded with methylene blue aqueous solution and is afterwards heated via Joule effect, by applying a voltage on the metalized fibers. The methylene blue releasing profiles of the heated patch are compared with those of the unheated one and the influence of parameters such as hydrogel composition and morphology, as well as the applied voltage values for microheating are investigated. The results indicate that the fabricated patch can be used as a drug administration instrument, while its performance can be tuned depending on the targeted application.

228. (Fe, Nd) codoped ZnO micro- and nanostructures with multifunctional characteristics like photocatalytic activity, optical and ferromagnetic properties

Authors: Diamandescu, L; Cernea, M; Tolea, F; Secu, EC; Trusca, R; Secu, M; Enculescu, M

Published: DEC 1 2018, CERAMICS INTERNATIONAL, 44, 21975, DOI: 10.1016/j.ceramint.2018.08.310

Zn1-x-yFexNdyO, x = 0.02 and y = 0.00-0.05 were successfully synthesized via a hydrothermal method at 200 degrees C. XRD and Mossbauer spectroscopy indicated that (Fe, Nd) codoped ZnO, calcined at 700 degrees C, 3 h in air, contains hexagonal ZnO phase as main phase and traces of ZnFe2O4; no phase containing neodymium was observed. In this study, a strong powder morphology dependence on Nd concentration and calcination temperature was evidenced by SEM. The photocatalytic properties were found to depend on the sample morphology and Nd content. Thermoluminescence signal is weak compared to the undoped ZnO and the curves shape was almost independent on the doping level and dependent on the calcination temperature and was related to the structural defects induced by the synthesis and calcination. Photoluminescence spectra showed that the green-yellow band due to the oxygen related defects, decreases by Fe and Nd-codoping. Thermo-magnetization curves measured in low applied fields and the magnetic hysteresis loops recorded in the temperature range 5-380 K, indicate two superimposed magnetic regimes, one predominant above 100 K the other below 100 K. The observed magnetic order is not directly related to the dopant Fe or Nd ions, but to different type of defects.

229. Production and Structural Characterization of Some Magnesium Matrix Composites Reinforced with Amorphous/Nanocrystalline NiTi Particulates

Authors: Ciurdas, M; Necsulescu, DA; Pantilimon, CM; Ion, V; Galatanu, M; Ruiu, G; Dumitrescu, RE

Published: DEC 2018, REVISTA DE CHIMIE, 69, 3507, DOI:

Two mixtures of elemental nickel and titanium powders in atomic proportions of 50% Ni + 50% Ti and 32% Ni + 68% Ti, respectively, were ground for 40 hours in a high energy planetary mill. In the case of the first mixture, the mechanical alloying was totally produced, while for the second, the alloying was partial. In both mixtures, qualitative X-ray diffraction phase analysis revealed the presence of metastable phases, such as Ni HC and NiTi- R-phase. Also, the equiatomic mixture is characterized by a partially amorphous structure. 10% of each type of mixture submitted to milling was used as reinforcing element in the form of particulates for two magnesium matrix composites. They were obtained by sintering in the plasma at 590 degrees C. In the case of the reinforced with the second mixture composite, the production of new phases other than the matrix and those present in the mixture of nickel and titanium powders after milling were recorded. The electron microscopy images of the two composites have resistant, free of micropores or microcracks matrix / particulates interfaces. The Mg-10% (32 at% Ni + 68 at% Ti) composite is characterized by Vickers hardness higher than that of the composite reinforced with the equiatomic mixture.

230. Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB2 Tapes with Open and Closed Ends

Authors: Aldica, G; Burdusel, M; Popa, S; Pasuk, I; Ionescu, AM; Kuncser, A; Badica, P

Published: NOV 2018, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 31, 3432, DOI: 10.1007/s10948-018-4616-9

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.