National Institute Of Materials Physics - Romania

We report on a combined experimental and modelling approach towards the design and fabrication of efficient bulk shields for low-frequency magnetic fields. To this aim, MgB2 is a promising material when its growing technique allows the fabrication of suitably shaped products and a realistic numerical modelling can be exploited to guide the shield design. Here, we report the shielding properties of an MgB2 tube grown by a novel technique that produces fully machinable bulks, which can match specific shape requirements. Despite a height/radius aspect ratio of only 1.75, shielding factors higher than 175 and 55 were measured at temperature T = 20 K and in axially-applied magnetic fields mu H-0(appl) = 0.1 and 1.0 T, respectively, by means of cryogenic Hall probes placed on the tube's axis. The magnetic behaviour of the superconductor was then modelled as follows: first we used a two-step procedure to reconstruct the macroscopic critical current density dependence on magnetic field, J(c)(B), at different temperatures from the local magnetic induction cycles measured by the Hall probes. Next, using these J(c)(B) characteristics, by means of finite-element calculations we reproduced the experimental cycles remarkably well at all the investigated temperatures and positions along the tube's axis. Finally, this validated model was exploited to study the influence both of the tube's wall thickness and of a cap addition on the shield performance. In the latter case, assuming the working temperature of 25 K, shielding factors of 10(5) and 10(4) are predicted in axial applied fields it mu H-0(appl) = 0.1 and 1.0 T, respectively.

o-Alkenyl N-triflylanilides underwent rhodium(III)-catalyzed oxidative annulations with alkynes to produce different types of naphthylamides in a process which involves the cleavage of two C-H bonds. Remarkably, besides formal dehydrogenative (4C+2C) cycloadducts, the reaction also produces variable amounts of isomeric naphthylamides, whose formation requires a formal migration of the alkenyl moiety from the ortho to the meta position of the anilide. The annulation reaction can be efficiently carried out in the absence of external oxidants, such as Cu(OAc)(2).

The work demonstrates growth by the Verneuil method of SrTiO3 single crystals of 30 mm in diameter. Experiments are performed under an industrial environment. Growth was for 4.75 h, i.e., within one production shift. The optimum growth conditions for which the length of the region with bubbles D is zero and the effective length EL (i.e., the crystal length of commercial value) is maximized are for the amount of SrCO3 additive of similar to 3 wt % and for H-2 outer flow rate of similar to 35 L/min. These two parameters show the strongest influence on the bubble-free growth, but other growth parameters (H-2 inner flow rate, O-2 flow rate increase, rotation speed) were also optimized. Selected crystals are characterized from the structural, microstructural, optical, and THz spectroscopy viewpoints, and they are compared with a commercial substrate and with crystals reported in the literature. This work opens the possibility for the industrial growth of large SrTiO3 single crystals and commercialization of large area substrates.

This research work is dedicated to study structural, morphological, optical and photoluminescence properties of samarium doped calcium sulfate (CaSO4) thin films after exposure to high gamma radiations. Polycrystalline doped CaSO4 thin films have been grown on glass substrates by spray pyrolysis technique and irradiated at different high gamma doses 3, 7, 15 and 40 kGy. Physical characterization of irradiated thin films has been made by X-ray diffraction, Spectrophotometer, Scanning Electron Microscope, Energy Dispersive Spectroscopy, Fluorescence Spectrometer and Thermoluminescence. The most remarkable result, as shown by structural analysis, is the increase of grain size from 52 to a maximum value of 93 nm for 15 kGy gamma dose which indicates a clear enhancement in crystal structure by gamma irradiation. Moreover, the preferred orientation has been immediately changed from (102) plan to (100) just after the first 3 kGy gamma dose. SEM micrographs of irradiated thin layers show deep modifications in surface morphology. Optical transmission spectra have shown a sharp and intense peak at 350 nm wavelength. Band gap energy has been slightly decreased from 3.9 eV before irradiation to 3.6 eV for 40 kGy. A new and strong energy level noted E r , has been emerged and created due to high gamma irradiations in addition to the principal one relative to band gap energy. Other parameters like absorption and extinction coefficients and refractive index have been determined. Thermoluminescence data show a high sensibility to gamma radiations doses which offer opportunities for dosimetry applications. These experimental results suggest the use of irradiated samarium doped calcium sulfate as optical window for space photovoltaic devices where gamma rays are abundant. These results are also helpful for researchers using CaSO4 thin films near nuclear apparatus (nuclear reactors and particle accelerators) or those interested in studying interaction between radiations and condensed matter.

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.

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.

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.

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.

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.

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).