National Institute Of Materials Physics - Romania

YBa2Cu3O7-delta films were deposited on CeO2-buffered nickel substrates, with different buffer thickness. Full width at half maximum of rocking curve, Delta(omega), of CeO2 and yttrium barium copper oxide (YBCO), as well as the critical temperature, T-c, of YBCO were shown to be strongly dependent on buffer thickness. They behave similarly but not proportional to the buffer thickness increase. This and the fact that Delta(omega) vs. buffer thickness and T-c vs. buffer thickness for YBCO behave similar with RMS roughness vs. thickness of CeO2 indicates that the surface peculiarity of buffers is responsible for YBCO properties. More precisely, the surface of CeO2 films prepared by the chemical solution route based on propionic acid is prone to agglomerate (de-wet) and the degree of agglomeration depends in an intricate way on buffer thickness. We showed that nor RMS roughness neither (00l) texture degree can define alone the surface suitable for c-axis YBCO nucleation. The {111} faceted grains (even in the case of high (00l) texture) and other defects generated by agglomeration supply a low fraction of (00l) flat terminations of buffer that affect the nucleation of c-axis-oriented YBCO phase. Moreover, the thermal instability of the surface morphology of CeO2 buffers (further development of de-wetting process, {111} faceted grains, etc. during superconducting layer deposition) influence the quality of YBCO films.

Cobalt chromites (CoCr2O4) powders have been synthesized via thermal decomposition of oxalate precursor compounds (NH4)(4)[CoCr2(C2O4)(4)(OH)(4)]center dot 7H(2)O (I) and (Na-4)(12)[CoCr2(C2O4)(8)(OH)(4)]center dot(NH4)(2)C2O4 center dot 12H(2)O (II). The oxalate precursors have been characterized by infrared (IR) and ultraviolet visible spectroscopy (UV-vis), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis. The structure, morphology and magnetic properties of CoCr2O4 powders have been investigated by XRD, SEM, IR and Raman spectroscopy (RS), UV-vis and magnetic measurements. XRD patterns confirmed the formation of spinel-type cobalt chromite, with average crystallite sizes calculated from XRD patterns of 38 nm and 58 nm, for spinel chromites from precursors I and II, respectively. SEM micrographs revealed particle sizes between 30 and 130 nm, with a low degree of aggregation of primary nanocrystallites. Both CoCr2O4 powders presented ferrimagnetic ordering below the Currie temperature (T-c) and a phase transition at T-s similar to 20 K attributed to the onset of long-range spiral magnetic order. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Superconducting bulk discs, S, of 20 mm in diameter and 3.5 or 3.3 mm thickness of MgB2 (pristine or added with cubic BN, respectively) with density above 97% were prepared by Spark Plasma Sintering. Discs were combined in a pair-type sandwich-like arrangement with a permanent NdFeB axially magnetised magnet, PM (similar to 0.5 T). Measurement of the trapped field, B-tr, with temperature, time, and the reduction rate of the applied magnetic field was performed using a Hall sensor positioned at the centre between the superconductor and the permanent magnet. It is shown that the permanent magnet with certain polarity favors higher trapped field of the superconductor owing to suppression of flux jumps specific for high density MgB2 samples. The B-tr of the PM-S pair was 2.45 T (20 K) and 3.3 T (12 K). (C) 2014 Elsevier B.V. All rights reserved.

Nano- or micropowders of Eu2O3 were added to MgB2, resulting in a composition of (MgB2)(0.975)(EuO1.5)(0.025). Pristine and doped samples were prepared using spark plasma sintering and tested for (i) Vickers hardness, (ii) pH evolution in phosphate-buffered saline solution, (iii) corrosion resistance (Tafel polarization curves), (iv) cytotoxicity (in vitro tests), and (v) antibacterial activity. Eu2O3 addition influenced the investigated properties. Solutions of MgB2-based samples show a relatively high saturation pH of 8.5. This value is lower than that of solutions incubated with Mg or other Mg-based biodegradable alloys reported in the literature. MgB2-based samples have lower electro-corrosion rates than Mg. Their Vickers hardness is 6.8-10.2 GPa, and these values are higher than those of biodegradable Mg-based alloys. MgB2 has low in vitro biocompatibility, good antibacterial activity against Escherichia coli, and mild activity against Staphylococcus aureus. Our results suggest that MgB2-based materials deserve attention in biomedical applications, such as implants or sterile medical instruments. (C) 2014 Elsevier B.V. All rights reserved.

High density (above 93%) superconducting bulks of MgB2 with addition of hexagonal BN (h-BN) and cubic BN (c-BN) with compositions ((MgB2) + (BN)(x), x = 0.01, 0.03, 0.05) were obtained by ex situ spark plasma sintering. All the investigated samples have the critical temperature T-c = 38.8 K. The variation of the critical current density J(c) with the external magnetic field H for h-BN added sample is almost overlapping the J(c)(H) dependence for the pristine MgB2 sample. On the other hand, J(c) for the samples added with c-BN is larger at high magnetic fields, while the decrease of J(c) at low H is very small. At T = 20 K, a J(c) of 10(2) A cm(-2) is determined for the sample with x(c-BN) = 0.005 at H = 58 kOe, and for the sample with x(c-BN) = 0.01 at 54 kOe. Magnetic relaxation measurements indicate a significant flux pinning enhancement in MgB2 samples added with c-BN. It is proposed that the disorder at the interface caused by the convenient lattice matching relationship between the lateral plane of the MgB2 crystal prism and the face of the c-BN crystal cube is responsible for the observed vortex pinning increase.

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface, or point) and on their character (normal cores or Delta kappa cores). We have used the Dew Hughes approach to determine the types of pinning centres present in various samples, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. Results show that normal surface pinning centres are present throughout almost all the samples, as dominant pinning mechanism. Such 2D extended pinning centres are mainly due to dislocations, grain boundaries, nanorods. Strong normal point pinning centres were found to be common in BZO doped YBCO samples. Other types of pinning centres, in various (minor) concentrations were also found in some of the samples. (C) 2014 Elsevier B.V. All rights reserved.

Two different types of Ho2O3 powders (showing a much different morphology) were added to MgB2 in the ex-situ Spark Plasma Sintering (SPS). In the 5-25 K range, the first Ho2O3 powder type does not significantly suppress the critical current density J(c) at low magnetic fields and the second one enhances it at high fields, while their mixture simultaneously controls J(c) at both small and high magnetic fields so that the decrease is small at low fields and there is a notable enhancement at high fields when compared to pristine sample. The control of J(c)(H) is discussed versus specific characteristics of the raw powders, the resulting microstructure of the added SPS-ed samples and pinning details from magnetic relaxation measurements. (C) 2014 Elsevier B.V. All rights reserved.

We present the effect of single molecular magnet ([Fe(bpca) Dy-2(NO (3))(4)] a <...H (2) Oa <...4.5 CH (3) NO (2) and [Mn (2)(naften) (2)(CH 3COO)(CH 3OH)] a <...BPh (4)) thermolysis on the superconducting properties of polycrystalline MgB (2) samples when inserted and sintered by spark plasma sintering technique. As a result of pyrolysis, Fe- and Dy-magnetic compounds grow within the MgB (2) matrix and give a paramagnetic behavior in normal state, whereas the Mn-based compounds display negligible magnetic properties. We find that the magnetic critical current density of the as-obtained superconducting composites, as well as the pinning mechanism, is dependent on the magnetic background, with a noticeable advantage for the sample with magnetic compounds.