National Institute Of Materials Physics - Romania

Ge, GeO2 and Ge2C6H10O7 additions to MgB2 obtained by ex situ spark plasma sintering significantly enhance the critical current density J(c) in high magnetic fields. A J(c)(T = 20 K) of 102 A cm(-2) is obtained at 3.9 T in the pristine sample and at 5.8 T in the MgB2(Ge2C6H10O7)(0.0014) sample. The decrease in the critical temperature for added samples is less than 1 K and T-c(20 K, H = 0) shows a small decrease from 5.5 x 10(5) A cm(-2) in the pristine sample to 3.9 x 10(5) A cm(-2) in MgB2(Ge2C6H10O7)(0.0014) sample. Ge does not substitute into the MgB2 lattice. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

EuFeO3 perovskite was considered as a case study of the cooperative effects of the rare earth and transition metal elements in the total catalytic oxidation of aromatic hydrocarbons. For this purpose, a EuFeO3 perovskite was prepared using the citrate route. Extensive characterization was performed via ex situ and in situ methods. EXAFS revealed that oxygen vacancies are formed in the nearest neighborhood of Fe and next-nearest neighborhood of Eu. Combining Eu-151 and Fe-57 Mossbauer experiments also suggested local oxygen defects in the proximity of the Eu cations during the reaction, as well as cooperative electron delocalization effects between Eu and Fe. XPS has shown the +3 oxidation state of both Eu and Fe cations and the relatively strong ionicity of the Fe-O chemical bonds, as suggested by the weakened 2p(3/2) satellite in the Fe2p spectrum. A systematic change of this satellite with the A cation in AFeO(3) perovskites was also discussed. The thermal treatment of the catalyst at 623 K, in air or toluene, was accompanied by a partial reduction of Fe and a partial oxidation of Eu, respectively. The interaction between the two elements appeared to be influenced mainly by the temperature and only slightly by the sample environment during further treatments. Catalytic oxidation of toluene on this perovskite led only to the production of carbon dioxide and water with no side-product formation from partial oxidation reactions. All the characterization and catalytic results preferably agree with a Volkenstein mechanism. (C) 2014 Elsevier Inc. All rights reserved.

We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 A degrees C)/4 h to 1253 K (980 A degrees C)/12 h and 1223 K (950 A degrees C)/6 h to 1273 K (1000 A degrees C)/12 h in a solid powder mixture of charcoal/BaCO3 and ferrochromium/alumina/NH4Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 mu m. Besides the host gamma-phase, the layers are mainly constituted of carbides (Fe7C3, Cr23C6, Cr7C3, and Fe3C) and traces of alpha'-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.

We report the continuous, single step synthesis of titania/iron-based magnetic nanocomposites in a single step using gas-phase laser pyrolysis technique by separately and simultaneously introducing the precursors (together with C2H4 sensitizer) in the reaction zone: Fe(CO)(5) on the central flow and, using air as carrier, TiCl4 on the annular coflow. The laser power and, for the last experiment, the injection geometry were modified in order to change the Fe/Ti ratio in the resulted nanopowders. Due to the specific geometry, the reaction zone (visible as a flame) have a reductive inner central zone surrounded by and oxidative environment, allowing the formation of the metallic-carbidic iron and/or iron-doped titania and iron oxide nanophases. The raw Fe-containing nanopowders have a ferromagnetic behavior, those synthesized at higher laser power and gas velocities show significant saturation magnetization M-s values (10-12 emu/g), whereas those obtained (with higher yield and carbon content) at lower laser power and gas velocities (using wider central nozzle cross-section) have a very weak magnetization (M-s similar to 0.05 emu/g) in spite of the higher ethylene carried Fe(CO)(5) flow. The powders were annealed in air at 400 degrees C show lower carbon content and, for those highly Fe-doped, the hematite phase formation. Preliminary tests using UV light confirm the photocatalytic action of the annealed nanopowders in salicylic acid degradation process in solution. (C) 2013 Elsevier B.V. All rights reserved.

Vortex excitations and creep regimes in YBa2Cu3O7 (YBCO) films with nonsuperconducting nanorods (columnar defects) and/or nanoparticles in a low external magnetic field H oriented perpendicular to the film surface have been identified using standard DC magnetization relaxation measurements. It was found that by increasing the pinning energy dispersion through nanoparticle addition the relevant changes in the temperature T variation of the normalized magnetization relaxation rate S appear at relatively high temperatures (T similar to 65K for H = 2 kOe), owing to the inhibition of double vortex- kink formation. This indicates that the often observed S(T) maximum at low T (around 30 K) is not related to the excitation of double vortex kinks and super-kinks in specimens with columnar defects, as previously believed. The low-T S(T) maximum is present in the case of strongly pinned specimens without nanorods, as well, and depends on the film thickness, which definitely shows that it is caused by thermo-magnetic instabilities, as recently suggested. (C) 2014 Elsevier B.V.All rights reserved.

xLi(2)O-(1 x)alpha-Fe2O3 (x=0.1, 0.3, 0.5, and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of Li2O and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of xLi(2)O-(1 x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size as function of composition and milling times and indicated the presence of Li-substituted hematite and tetra lithium iron oxide LiFeO2, along with the formation of multiple phases for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with lithium oxide. Magnetic measurements recorded at 5 K to room temperature (RT) in an applied magnetic field of 50,000 Oe showed that the magnetization of the milled samples is larger at low temperatures than at RT and increases with decreasing particle size. Zero field cooling measurements made possible the determination of the blocking temperatures of the specimens as function of ball milling time and evidenced the occurrence of superparamagnetism in the studied samples. This result correlates well with the observed presence of a quadrupole-split doublet in the Mossbauer spectra. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Sintered bulks require new approaches for the control and improvement of their functional characteristics. Nanostructuring, synergy effects at interfaces in a composite, and technology specific features are essential in this regard. In this work, for a better understanding of challenges, complex relationships between materials and technology and different practical concepts such as "transferability", "composite within a composite", and "multilevel design", and "multifunctionality", a comparative analysis is proposed for very different nano structured materials such as La0.9Sr0.1Ga0.8Mg0.2O3-x x solid electrolyte, hard B4C-based materials, and doped MgB2 superconductors. Samples were obtained by reactive spark plasma sintering (SPS). SPS is reconfirmed as a powerful processing technique that generates high-density unique bulk materials impossible to fabricate by other methods. However, it is shown that SPS is not universal and its suitability should be carefully considered depending on materials and targeted applications. (C) 2014 The Japan Society of Applied Physics

A mixture of B2O3 and amorphous B with the mole ratio of 1:14 is shown to react and form B6O ceramic under in-situ spark plasma sintering (SPS) conditions. The optimum SPS temperature and time to obtain phase B6O are 1250 C and 30 min, respectively, and Rietveld refinement of the XRD patterns indicates that the oxygen occupancy of B6Ox is reasonably high at x = 0.89(99). However, to reach high density above 98%, SPS temperatures of 1700-1800 degrees C are necessary and a one-step twotemperature in-situ reactive SPS was designed and applied. As-prepared B6O dense ceramic has Vickers hardness (36.7 +/- 1.2 GPa) and fracture toughness (K-1c = 4.2 +/- 0.15 MPa. m(1/2)) comparable with the highest values reported in literature for the bulks obtained by processing routes of already reacted B6O powders (ex-situ routes).

Dense boron carbide (above 95%) was achieved through high pressure (300 MPa) and low temperature (1600 degrees C) Spark Plasma Sintering (SPS). This approach resulted in improvement of fracture toughness and of dynamic toughness when compared to corresponding toughness values of the sample sintered by conventional SPS (2100 degrees C, 50 MPa). Dynamic toughness was extracted from Split Hopkinson Pressure Bar measurements. Results are understood based on microstructure and on very different behaviour of the samples in respect to residual B2O3 and carbon available in the raw B4C powder. (C) 2014 The Ceramic Society of Japan. All rights reserved.

Boron carbide ceramic was prepared by reactive Spark Plasma Sintering under N-2-atmosphere and for different heating times and maximum pressure regimes. Split-Hopkinson Pressure Bar (SHPB), indentation, XRD and microscopy measurements were performed for samples characterization. It is shown that SHPB toughness control depending on SPS regime is possible and the main reason is introduction of nitrogen into B4C ceramic. Complex relationships between processing conditions, sintering mechanism, material's specifics, static and dynamic mechanical properties are discussed. Improvement of dynamic toughness is through mechanisms resembling those working for static load conditions such as cracks deflection and pull out, but there are also significant differences. (c) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.